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## **Meet the editor**

Dr Efraín Gaxiola is a Chief of Cardiac Catheterization Laboratory at Jardines Hospital de Especialidades in Guadalajara, México. He completed medical school at the Universidad Autónoma de Guadalajara and did his Internal Medicine training at Centro Médico La Raza in México City. After completing his cardiology fellowship training at Instituto Nacional de Cardiología "Ignacio

Chávez" in México City, he was an Interventional Cardiology Fellow at the Univeristy of Florida in Gainesville, Florida. He is a former President of the Mexican Interventional Cardiology Society and the Latin American Interventional Cardiology Society. Dr. Gaxiola has been a member of TCT Faculty for more than 8 years and a member of the International Scientific Committee of the American College of Cardiology. He has published more than 80 manuscripts, mainly related to atherothrombosis, coronary artery disease, oral antiplatelet agents and has written more than 15 book chapters. His early research involved risk stratification and trials of antiplatelet and antithrombotic therapy in acute coronary syndromes.

Contents

**Preface IX** 

Chapter 3 **Roles of Serotonin in** 

Po-Hsun Huang

Chapter 6 **In Search for Novel Biomarkers** 

Chapter 1 **Pathology and Pathophysiology of** 

Atsushi Yamashita and Yujiro Asada

Chapter 2 **Biomarkers of Atherosclerosis and Acute** 

Takuya Watanabe and Shinji Koba

Yahye Merhi and Walid Mourad

**of Acute Coronary Syndrome 97**  Kavita K. Shalia and Vinod K. Shah

Chapter 7 **Lower Extremity Peripheral Arterial Disease 119**  Aditya M. Sharma and Herbert D. Aronow

**Atherothrombosis: Virchow's Triad Revisited 1** 

**Coronary Syndromes – A Clinical Perspective 21**  Richard Body, Mark Slevin and Garry McDowell

**Atherothrombosis and Related Diseases 57** 

Chapter 4 **Endothelial Progenitor Cell in Cardiovascular Diseases 71** 

Chapter 5 **CD40 Ligand and Its Receptors in Atherothrombosis 79**  Daniel Yacoub, Ghada S. Hassan, Nada Alaadine,

### Contents

#### **Preface XI**


Aditya M. Sharma and Herbert D. Aronow

## Preface

Atherothrombosis has reached pandemic proportions worldwide. It is the underlying condition that results in events leading to myocardial infarction, ischemic stroke and vascular death. As such, it is the leading cause of death worldwide manifested mainly as cardiovascular/cerebrovascular death.

As the population of many countries becomes more aged, so the burden of atherothrombosis increases. The burden of atherothrombosis is felt in numerous ways: shortened life expectancy, increased morbidity and mortality and future risk of consequences in multiple systems.

Although therapeutic improvements and public health policies for risk factors control have brought about a reduction in atherothrombosis among the general population, this success has not been extended to some group populations as diabetics.

The complex and intimate relationship between atherothrombosis and traditional and novel risk factors is discussed in the following chapters of *Traditional and Novel Risk Factors in Atherothrombosis* – from basic science to clinical and therapeutic concerns. Beginning with pathology and pathophysiology of atherothrombosis, plaque rupture/disruption, this book continues with molecular, biochemical, inflammatory, cellular aspects and finally analyzes several aspects of clinical pharmacology.

This book is made up of seven chapters. In the first, Yamashita and Asada delineate the pathophysiologic mechanisms of plaque disruption and thrombus formation as critical steps for the onset of cardiovascular events, and that simultaneous activation of coagulation cascade and platelets play an important role in thrombus formation after plaque disruption. Next, Body, Slevin and McDowell discuss current methods for assessment of the presence, degree of severity and 'plaque composition' in patients with atherosclerosis, incuding current and novel imaging technology and measurement of circulating biomarkers of atherosclerosis. Subsequently, Watanabe and Koba clarify the roles of Serotonin in atherothrombosis and its related diseases, and how serotonin plays a crucial role in the formation of thrombosis and atherosclerotic lesions through 5-HT2A receptors. Po-Hsun Huang analyzes the therapeutic use of endothelial progenitor cell in cardiovascular diseases. Yacoub, Hassan, Alaadine, Merhi, and Mourad discuss the role of CD40 Ligand and its

#### X Preface

receptors in atherothrombosis. They show that besides its pivotal role in humoral immunity, CD40L is now regarded as a key player to all major phases of atherothrombosis, a concept supported in part by the strong relationship between its circulating soluble levels and the occurrence of cardiovascular diseases. The last two chapters are dedicated to diagnostic and therapeutic issues. Shalia and Shah describe the current use of diagnostic biomarkers in ACS, as well as novel cardiac biomarkers of ACS. Sharma and Aronow talk about the optimal diagnosis and management of lower extremity peripheral arterial disease, detailing both the classical and modern therapeutic options.

I would like to pay tribute and express our appreciation to the distinguished and internationally renowned collaborators of this book for their outstanding contribution. Despite their many commitments and busy time schedules, all of them enthusiastically stated their acquiescence to cooperate. This book could not have become a reality were it not for their dedicated efforts.

> **Efraín Gaxiola, MD, FACC** Cardiology Chief Jardínes Hospital de Especialidades Guadalajara, México

X Preface

therapeutic options.

it not for their dedicated efforts.

receptors in atherothrombosis. They show that besides its pivotal role in humoral immunity, CD40L is now regarded as a key player to all major phases of atherothrombosis, a concept supported in part by the strong relationship between its circulating soluble levels and the occurrence of cardiovascular diseases. The last two chapters are dedicated to diagnostic and therapeutic issues. Shalia and Shah describe the current use of diagnostic biomarkers in ACS, as well as novel cardiac biomarkers of ACS. Sharma and Aronow talk about the optimal diagnosis and management of lower extremity peripheral arterial disease, detailing both the classical and modern

I would like to pay tribute and express our appreciation to the distinguished and internationally renowned collaborators of this book for their outstanding contribution. Despite their many commitments and busy time schedules, all of them enthusiastically stated their acquiescence to cooperate. This book could not have become a reality were

**Efraín Gaxiola, MD, FACC**

Jardínes Hospital de Especialidades

Cardiology Chief

Guadalajara, México

**1** 

*Japan* 

**Pathology and Pathophysiology of** 

Atsushi Yamashita and Yujiro Asada

*University of Miyazaki,* 

**Atherothrombosis: Virchow's Triad Revisited** 

In 1856, Rudolf Virchow published "Cellular pathology" based on macroscopic and microscopic observation of diseases, and described a triad of factors on thrombosis. The three components were vascular change, blood flow alteration, and abnormalities of blood constituents. Although Virchow originally referred to venous thrombosis, the theory can also be applied to arterial thrombosis, and it is considered that atherothrombus formation is regulated by the thrombogenicity of exposed plaque contents, local hemorheology, and blood factors. Thrombus formation on a disrupted atherosclerotic plaque is a critical event that leads to atherothrombosis. However, it does not always result in complete thrombotic occlusion with subsequent acute symptomatic events (Sato et al., 2009). Therefore, thrombus growth is also critical to the onset of clinical events. In spite of intensive investigation on the mechanisms of thrombus formation, little is known about the mechanisms involved in thrombogenesis or thrombus growth after plaque disruption, because thrombus is assessed with chemical or physical injury of "normal" arteries in most animal models of thrombosis. Vascular change is an essential factor of atherothrombosis. Atherothrombosis is initiated by disruption of atherosclerotic plaque. The plaque disruption is morphologically characterized, however, the triggers of plaque disruption have not been completely understood. Tissue factor (TF) is an initiator of the coagulation cascade, is normally expressed in adventitia and variably in the media of normal artery (Drake et al., 1989). Because the atherosclerotic lesion expresses active TF, it is considered that TF in atherosclerotic lesion is a major determinant of vascular wall thrombogenicity (Owens & Mackman, 2010). Therefore, atherosclerotic lesions with TF expression are indispensable for studying atherothrombosis. To examine thrombus formation on TF-expressing atherosclerotic lesions, we established a rabbit model of atherothrombosis (Yamashita et al., 2003, 2009). This allowed us to investigate the "Virchow's triad" on atherothrombosis.

Blood flow is a key modulator of the development of atherosclerosis and thrombus formation. The areas of disturbed flow or low shear stress are susceptible for atherogenesis, whereas areas under steady flow and physiologically high shear stress are resistant to atherogenesis (Malek et al., 1999). The transcription of thrombogenic or anti-thrombogenic genes is also regulated by shear stress (Cunningham & Gotlieb, 2005). The blood flow can be altered by vascular stenosis, acute luminal change after plaque disruption, and micovascular constriction induced by distal embolism (Topol & Yadav, 2003). The blood flow alteration

after plaque disruption may affect thrombus formation.

**1. Introduction** 

## **Pathology and Pathophysiology of Atherothrombosis: Virchow's Triad Revisited**

Atsushi Yamashita and Yujiro Asada *University of Miyazaki, Japan* 

#### **1. Introduction**

In 1856, Rudolf Virchow published "Cellular pathology" based on macroscopic and microscopic observation of diseases, and described a triad of factors on thrombosis. The three components were vascular change, blood flow alteration, and abnormalities of blood constituents. Although Virchow originally referred to venous thrombosis, the theory can also be applied to arterial thrombosis, and it is considered that atherothrombus formation is regulated by the thrombogenicity of exposed plaque contents, local hemorheology, and blood factors. Thrombus formation on a disrupted atherosclerotic plaque is a critical event that leads to atherothrombosis. However, it does not always result in complete thrombotic occlusion with subsequent acute symptomatic events (Sato et al., 2009). Therefore, thrombus growth is also critical to the onset of clinical events. In spite of intensive investigation on the mechanisms of thrombus formation, little is known about the mechanisms involved in thrombogenesis or thrombus growth after plaque disruption, because thrombus is assessed with chemical or physical injury of "normal" arteries in most animal models of thrombosis.

Vascular change is an essential factor of atherothrombosis. Atherothrombosis is initiated by disruption of atherosclerotic plaque. The plaque disruption is morphologically characterized, however, the triggers of plaque disruption have not been completely understood. Tissue factor (TF) is an initiator of the coagulation cascade, is normally expressed in adventitia and variably in the media of normal artery (Drake et al., 1989). Because the atherosclerotic lesion expresses active TF, it is considered that TF in atherosclerotic lesion is a major determinant of vascular wall thrombogenicity (Owens & Mackman, 2010). Therefore, atherosclerotic lesions with TF expression are indispensable for studying atherothrombosis. To examine thrombus formation on TF-expressing atherosclerotic lesions, we established a rabbit model of atherothrombosis (Yamashita et al., 2003, 2009). This allowed us to investigate the "Virchow's triad" on atherothrombosis.

Blood flow is a key modulator of the development of atherosclerosis and thrombus formation. The areas of disturbed flow or low shear stress are susceptible for atherogenesis, whereas areas under steady flow and physiologically high shear stress are resistant to atherogenesis (Malek et al., 1999). The transcription of thrombogenic or anti-thrombogenic genes is also regulated by shear stress (Cunningham & Gotlieb, 2005). The blood flow can be altered by vascular stenosis, acute luminal change after plaque disruption, and micovascular constriction induced by distal embolism (Topol & Yadav, 2003). The blood flow alteration after plaque disruption may affect thrombus formation.

Pathology and Pathophysiology of Atherothrombosis: Virchow's Triad Revisited 3

account in the current American Heart Association classification of atherosclerosis (Stary et al., 1995). Plaque erosion is characterized by a denuded plaque surface and thrombus formation, and defined by the lack of surface disruption of the fibrous cap. Compared with plaque rupture, patients with plaque erosion are younger, no male predominance. Angiographycally, there is less narrowing and irregularity of the luminal surface in erosion. The morphologic characteristics include an abundance of SMCs and proteoglycan matrix, expecially versican and hyaluronan, and disruption of surface endothelium. Necrotic core is often absent. Plaque erosion contains relatively few macrophages and T cells compared with plaque rupture (Virmani et al., 2000). Thrombotic occlusion is less common with plaque erosion than plaque rupture, whereas microembolization in distal small vessels is more common with plaque erosion than plaque rupture (Schwartz et al., 2009). The proportions of fibrin and platelets differ in coronary thrombi on ruptured and eroded plaques. Thrombi on ruptured plaque are fibrin-rich, but those on eroded plaque are platelet-rich. TF and C reactive protein (CRP) are abundantly present in ruptured plaque, compared with eroded plaques (Sato et al., 2005). These distinct morphologic features suggest the different

mechanisms in plaque rupture and erosion.

HE

rupture

erosion

infarction.

500μm

100μm

GPIIb/IIIa Fibrin

100μm

100μm

100μm

500μm

eosin stain (from Sato et al. 2005, with permission).

**3. Pathology of asymptomatic atherothrombus** 

Fig. 1. Human coronary plaque rupture and erosion in patients with acute myocardial

Large necrotic core and disrupted thin fibrous cap is accompanied by thrombus formation in ruptured plaque. Eroded plaque has superficial injury of SMC-rich atherosclerotic lesion with thrombus formation. Both thrombi comprise platelets and fibrin. HE, Hematoxylin

On the other hands, the disruption of atherosclerotic plaque does not always result in complete thrombotic occlusion with subsequent acute symptomatic events. The clinical studies using angioscopy have revealed that multiple plaque rupture is a frequent complication in patients with coronary atherothrombosis (Okada et al., 2011). Healed stages

Blood circulates in the vessel as a liquid. This property suddenly changes after plaque disruption. The exposure of matrix proteins and TF induce platelet adhesion, aggregation and activation of coagulation cascade, resulted in platelet-fibrin thrombus formation. Clinical studies revealed increased platelet reactivity, coagulation factors, and reduced fibrinolytic activity in patients with atherothrombosis (Feinbloom & Bauer, 2005), and that risk factors for atherothrombosis can affect these blood factors (Lemkes et al., 2010, Rosito et al., 2004). In addition, recent evidences suggest that white blood cells can influence arterial thrombus formation. It seems that abnormalities on blood factors affect thrombus growth rather than initiation of thrombus formation.

This article focuses on pathology and pathophysiology of coronary atherothrombosis. Because mechanisms of atherothrombus formation are highly complicated, we separately discuss the "Virchow's triad" on atherothrombogenesis and thrombus growth.

#### **2. Pathology of atherothrombosis**

Traditionally, it is considered that arterial thrombi are mainly composed of aggregated platelets because of rapid blood flow condition, and the development of platelet-rich thrombi has been regarded as a cause of atherothrombosis. However, recent evidences indicate that atherothrombi are composed of aggregated platelets and fibrin, along erythrocytes and white blood cells, and constitutively immunopositive for GPIIb/IIIa (a platelet integrin), fibrin, glycophorin A (a membrane protein expressed on erythrocytes), von Willbrand factor (VWF, a blood adhesion molecule). And neutrophils are major white blood cells in coronary atherothrombus (Nishihira et al., 2010, Yamashita et al., 2006a). GPIIb/IIIa colocalized with VWF. TF was closely associated with fibrin (Yamashita et al., 2006a). The findings suggest that VWF and/or TF contribute thrombus growth and obstructive thrombus formation on atherosclerotic lesions, and that the enhanced platelet aggregation and fibrin formation indicate excess thrombin generation mediated by TF.

Overexpression of TF and its procoagulant activity have been found in human atherosclerotic plaque, and TF-expressing cells are identified as macrophages and smooth muscle cells (SMC) in the intima (Wilcox et al., 1989). The TF activity is more prominent in fatty streaks and atheromatous plaque than in the diffuse intimal thickening in aorta (Hatakeyama et al., 1997). Thus, atherosclerotic plaque has a potential to initiate coagulation cascade after plaque disruption, and TF in the plaque is thought to play an important role in thrombus formation after plaque disruption. Interestingly, TF pathway inhibitor (TFPI), a major down regulator of TF-factor VIIa (FVIIa) complex, is also upregulated in atherosclerotic lesions (Crawley et al., 2000). In addition to endothelial cells, macrophages, medial and intimal SMCs express TFPI. These evidence suggest that imbalance between TF and TFPI contribute to vascular wall thrombogenicity.

Two major patterns of plaque disruption are plaque rupture and plaque erosion (Figure 1). Plaque rupture is caused by fibrous cap disruption, allowing blood to come in contact with the thrombogenic necrotized core, resulting in thrombus formation. Ruptured plaque is characterized by disruption of thin fibrous caps, usually less than 65 μm in thickness, rich in macrophages and lymphocytes, and poor in SMCs (Virmani et al., 2000). Thus, the thinning of the fibrous cap is though to be a state vulnerable to rupture, the so-called thin-cap fibroatheroma (Kolodgie et al., 2001). However, the thin-cap fibroatheroma is not taken into

Blood circulates in the vessel as a liquid. This property suddenly changes after plaque disruption. The exposure of matrix proteins and TF induce platelet adhesion, aggregation and activation of coagulation cascade, resulted in platelet-fibrin thrombus formation. Clinical studies revealed increased platelet reactivity, coagulation factors, and reduced fibrinolytic activity in patients with atherothrombosis (Feinbloom & Bauer, 2005), and that risk factors for atherothrombosis can affect these blood factors (Lemkes et al., 2010, Rosito et al., 2004). In addition, recent evidences suggest that white blood cells can influence arterial thrombus formation. It seems that abnormalities on blood factors affect thrombus growth

This article focuses on pathology and pathophysiology of coronary atherothrombosis. Because mechanisms of atherothrombus formation are highly complicated, we separately

Traditionally, it is considered that arterial thrombi are mainly composed of aggregated platelets because of rapid blood flow condition, and the development of platelet-rich thrombi has been regarded as a cause of atherothrombosis. However, recent evidences indicate that atherothrombi are composed of aggregated platelets and fibrin, along erythrocytes and white blood cells, and constitutively immunopositive for GPIIb/IIIa (a platelet integrin), fibrin, glycophorin A (a membrane protein expressed on erythrocytes), von Willbrand factor (VWF, a blood adhesion molecule). And neutrophils are major white blood cells in coronary atherothrombus (Nishihira et al., 2010, Yamashita et al., 2006a). GPIIb/IIIa colocalized with VWF. TF was closely associated with fibrin (Yamashita et al., 2006a). The findings suggest that VWF and/or TF contribute thrombus growth and obstructive thrombus formation on atherosclerotic lesions, and that the enhanced platelet aggregation and fibrin formation indicate excess thrombin generation mediated by TF.

Overexpression of TF and its procoagulant activity have been found in human atherosclerotic plaque, and TF-expressing cells are identified as macrophages and smooth muscle cells (SMC) in the intima (Wilcox et al., 1989). The TF activity is more prominent in fatty streaks and atheromatous plaque than in the diffuse intimal thickening in aorta (Hatakeyama et al., 1997). Thus, atherosclerotic plaque has a potential to initiate coagulation cascade after plaque disruption, and TF in the plaque is thought to play an important role in thrombus formation after plaque disruption. Interestingly, TF pathway inhibitor (TFPI), a major down regulator of TF-factor VIIa (FVIIa) complex, is also upregulated in atherosclerotic lesions (Crawley et al., 2000). In addition to endothelial cells, macrophages, medial and intimal SMCs express TFPI. These evidence suggest that imbalance between TF

Two major patterns of plaque disruption are plaque rupture and plaque erosion (Figure 1). Plaque rupture is caused by fibrous cap disruption, allowing blood to come in contact with the thrombogenic necrotized core, resulting in thrombus formation. Ruptured plaque is characterized by disruption of thin fibrous caps, usually less than 65 μm in thickness, rich in macrophages and lymphocytes, and poor in SMCs (Virmani et al., 2000). Thus, the thinning of the fibrous cap is though to be a state vulnerable to rupture, the so-called thin-cap fibroatheroma (Kolodgie et al., 2001). However, the thin-cap fibroatheroma is not taken into

discuss the "Virchow's triad" on atherothrombogenesis and thrombus growth.

rather than initiation of thrombus formation.

**2. Pathology of atherothrombosis** 

and TFPI contribute to vascular wall thrombogenicity.

account in the current American Heart Association classification of atherosclerosis (Stary et al., 1995). Plaque erosion is characterized by a denuded plaque surface and thrombus formation, and defined by the lack of surface disruption of the fibrous cap. Compared with plaque rupture, patients with plaque erosion are younger, no male predominance. Angiographycally, there is less narrowing and irregularity of the luminal surface in erosion. The morphologic characteristics include an abundance of SMCs and proteoglycan matrix, expecially versican and hyaluronan, and disruption of surface endothelium. Necrotic core is often absent. Plaque erosion contains relatively few macrophages and T cells compared with plaque rupture (Virmani et al., 2000). Thrombotic occlusion is less common with plaque erosion than plaque rupture, whereas microembolization in distal small vessels is more common with plaque erosion than plaque rupture (Schwartz et al., 2009). The proportions of fibrin and platelets differ in coronary thrombi on ruptured and eroded plaques. Thrombi on ruptured plaque are fibrin-rich, but those on eroded plaque are platelet-rich. TF and C reactive protein (CRP) are abundantly present in ruptured plaque, compared with eroded plaques (Sato et al., 2005). These distinct morphologic features suggest the different mechanisms in plaque rupture and erosion.

Fig. 1. Human coronary plaque rupture and erosion in patients with acute myocardial infarction.

Large necrotic core and disrupted thin fibrous cap is accompanied by thrombus formation in ruptured plaque. Eroded plaque has superficial injury of SMC-rich atherosclerotic lesion with thrombus formation. Both thrombi comprise platelets and fibrin. HE, Hematoxylin eosin stain (from Sato et al. 2005, with permission).

#### **3. Pathology of asymptomatic atherothrombus**

On the other hands, the disruption of atherosclerotic plaque does not always result in complete thrombotic occlusion with subsequent acute symptomatic events. The clinical studies using angioscopy have revealed that multiple plaque rupture is a frequent complication in patients with coronary atherothrombosis (Okada et al., 2011). Healed stages

Pathology and Pathophysiology of Atherothrombosis: Virchow's Triad Revisited 5

The atherosclerotic lesion shows superficial erosive injury with mural thrombus (arrows). The thrombus is too small to obstruct coronary lumen and induce symptomatic event

As described above, atherothrombosis is initiated by plaque rupture or plaque erosion. The plaque disruption is probably affected by vascular wall change and local blood flow. Our recent study revealed that disturbed blood flow could trigger plaque erosion in rabbit femoral artery with SMC-rich plaque. We separately discuss possible factors that affect

The thinning and disruption of fibrous cap by metalloproteases together with local rheological forces and emotional status is likely to be involved in plaque rupture. Accumulating evidence supports a key role for inflammation in the pathogenesis of plaque rupture. The inflammatory cells that appear quite numerous in rupture-prone atherosclerotic plaques can produce enzymes degrading the extracellular matrix of the fibrous cap. Macrophages in human atheroma overexpress interstitial collagenases and gelatinases, and elastolytic enzymes. Activated T lymphocytes and macrophages can secrete interferon γ (INF-γ), which inhibits collagen synthesis and induces apoptotic death of SMC (Shah, 2003). Moreover, INF-γ can induce interleukine (IL)-18, an accelerator of inflammation. IL-18 is colocalized with INF-γ in macrophage located at shoulder region, but not at necrotic core, and is associated with coronary thrombus formation in patients with ischemic heart disease (Nishihira et al., 2007). IL-10, an important anti-inflammatory cytokine, also is upregulated in macrophage in atherosclerotic lesion from patients with unstable angina compared with stable angina (Nishihira et al., 2006b). Heterogeneity of macrophages in atherosclerotic plaque could explain the paradoxical findings (Waldo et al., 2008). These evidences indicate that the imbalance of inflammatory pathway appear to participate in the destabilization of the plaque that triggers

Other possible trigger of plaque rupture is intraplaque hemorrhage. The frequency of previous hemorrhages is greater in coronary atherosclerotic lesions with late necrosis and thin fibrous cap than those lesions with early necrosis or intimal thickening (Kolodgie et al., 2003). Plaque hemorrhage is present in majority (>75%) of acute ruptures, and in 40% of fibrous cap and thin-fibrous cap atheromas. In addition, intraplaque hemorrhage is more frequently seen in patients with AMI compared to patients with healed myocardial infarction or non-cardiac death (Virmani et al., 2003). In coronary culprit lesions obtained by directional coronary atherectomy, intraplaque hemorrhage and iron deposition were more prominent in patients with unstable angina pectoris than with stable angina pectoris. The iron deposition correlated with oxidized low density lipoprotein and thioredoxin, an antioxidant protein, and was also associated with thrombus formation (Nishihira et al., 2008b). The pathological findings imply a possible relationship among intraplaque hemorrhage, oxidative stress, and plaque instability. However, the direct evidence that links intraplaque

(hematoxyline eosin stain, from Sato et al. 2009, with permission).

plaque rupture or plaque erosion in atherosclerotic vessels.

**4. Pathophysiology of atherothrombosis** 

**4.1 Triggers on plaque disruption** 

**4.1.1 Vascular change in plaque rupture** 

thrombosis in fibrous cap rupture.

hemorrhage to plaque instability is still lacking.

of plaque disruption are also occasionally observed in autopsy cases with or without coronary atherothrombosis (Burke et al., 2001). To evaluate the incidence and morphological characteristics of thrombi and plaque disruption in patients with non-cardiac death, Sato et al. (2009) examined 102 hearts from non-cardiac death autopsy cases and 19 from those who died of acute myocardical infarction (AMI). They found coronary thrombi in 16% of cases with non-cardiac death, and most of them developed on plaque erosion, and the thrombi were too small to affect coronary lumen (Figure 2, Table 1). The disrupted plaques in noncardiac death case had smaller lipid areas, thicker fibrous caps, and more modest luminal narrowing than those in cases with AMI. A few autopsy studies have examined the incidence of coronary thrombus in non-cardiac death. Davies et al. (1989) and Arbustini et al. (1993) found 3 (4%) mural thrombi in 69, and 10 (7%) thrombi in 132 autopsy cases with non-cardiac death. The all coronary thrombi from non-cardiac death were associated with plaque erosion (Arbustini et al., 1993). Although the precise mechanisms of plaque erosion remain unknown, it is possible that the superficial erosive injury is a common mechanism of coronary thrombus formation. The results suggest that plaque disruption does not always result in complete thrombotic occlusion with subsequent acute symptomatic events, that thrombus growth is critical step for the onset of clinical events, and that at least the regional factors influence the size of coronary thrombus after plaque disruption.

Fig. 2. Human coronary plaque erosion in patient with non-cardiac death.


(From Sato et al. 2009, with permission)

Table 1. Incidence of thrombosis in non-cardiac death and acute myocardial infarction.

The atherosclerotic lesion shows superficial erosive injury with mural thrombus (arrows). The thrombus is too small to obstruct coronary lumen and induce symptomatic event (hematoxyline eosin stain, from Sato et al. 2009, with permission).

#### **4. Pathophysiology of atherothrombosis**

#### **4.1 Triggers on plaque disruption**

4 Traditional and Novel Risk Factors in Atherothrombosis

of plaque disruption are also occasionally observed in autopsy cases with or without coronary atherothrombosis (Burke et al., 2001). To evaluate the incidence and morphological characteristics of thrombi and plaque disruption in patients with non-cardiac death, Sato et al. (2009) examined 102 hearts from non-cardiac death autopsy cases and 19 from those who died of acute myocardical infarction (AMI). They found coronary thrombi in 16% of cases with non-cardiac death, and most of them developed on plaque erosion, and the thrombi were too small to affect coronary lumen (Figure 2, Table 1). The disrupted plaques in noncardiac death case had smaller lipid areas, thicker fibrous caps, and more modest luminal narrowing than those in cases with AMI. A few autopsy studies have examined the incidence of coronary thrombus in non-cardiac death. Davies et al. (1989) and Arbustini et al. (1993) found 3 (4%) mural thrombi in 69, and 10 (7%) thrombi in 132 autopsy cases with non-cardiac death. The all coronary thrombi from non-cardiac death were associated with plaque erosion (Arbustini et al., 1993). Although the precise mechanisms of plaque erosion remain unknown, it is possible that the superficial erosive injury is a common mechanism of coronary thrombus formation. The results suggest that plaque disruption does not always result in complete thrombotic occlusion with subsequent acute symptomatic events, that thrombus growth is critical step for the onset of clinical events, and that at least the regional

factors influence the size of coronary thrombus after plaque disruption.

Fig. 2. Human coronary plaque erosion in patient with non-cardiac death.

Fresh thrombus 10 (10%) 14 (74%) <0.001 erosion 7 (7%) 4 (21%) 0.07 rupture 3 (3%) 10 (53%) <0.001 Old thrombus 6 (6%) 5 (26%) <0.05

Table 1. Incidence of thrombosis in non-cardiac death and acute myocardial infarction.

Acute myocardial infarction

(n=19) P value

Non-cardiac death (n=102)

(From Sato et al. 2009, with permission)

As described above, atherothrombosis is initiated by plaque rupture or plaque erosion. The plaque disruption is probably affected by vascular wall change and local blood flow. Our recent study revealed that disturbed blood flow could trigger plaque erosion in rabbit femoral artery with SMC-rich plaque. We separately discuss possible factors that affect plaque rupture or plaque erosion in atherosclerotic vessels.

#### **4.1.1 Vascular change in plaque rupture**

The thinning and disruption of fibrous cap by metalloproteases together with local rheological forces and emotional status is likely to be involved in plaque rupture. Accumulating evidence supports a key role for inflammation in the pathogenesis of plaque rupture. The inflammatory cells that appear quite numerous in rupture-prone atherosclerotic plaques can produce enzymes degrading the extracellular matrix of the fibrous cap. Macrophages in human atheroma overexpress interstitial collagenases and gelatinases, and elastolytic enzymes. Activated T lymphocytes and macrophages can secrete interferon γ (INF-γ), which inhibits collagen synthesis and induces apoptotic death of SMC (Shah, 2003). Moreover, INF-γ can induce interleukine (IL)-18, an accelerator of inflammation. IL-18 is colocalized with INF-γ in macrophage located at shoulder region, but not at necrotic core, and is associated with coronary thrombus formation in patients with ischemic heart disease (Nishihira et al., 2007). IL-10, an important anti-inflammatory cytokine, also is upregulated in macrophage in atherosclerotic lesion from patients with unstable angina compared with stable angina (Nishihira et al., 2006b). Heterogeneity of macrophages in atherosclerotic plaque could explain the paradoxical findings (Waldo et al., 2008). These evidences indicate that the imbalance of inflammatory pathway appear to participate in the destabilization of the plaque that triggers thrombosis in fibrous cap rupture.

Other possible trigger of plaque rupture is intraplaque hemorrhage. The frequency of previous hemorrhages is greater in coronary atherosclerotic lesions with late necrosis and thin fibrous cap than those lesions with early necrosis or intimal thickening (Kolodgie et al., 2003). Plaque hemorrhage is present in majority (>75%) of acute ruptures, and in 40% of fibrous cap and thin-fibrous cap atheromas. In addition, intraplaque hemorrhage is more frequently seen in patients with AMI compared to patients with healed myocardial infarction or non-cardiac death (Virmani et al., 2003). In coronary culprit lesions obtained by directional coronary atherectomy, intraplaque hemorrhage and iron deposition were more prominent in patients with unstable angina pectoris than with stable angina pectoris. The iron deposition correlated with oxidized low density lipoprotein and thioredoxin, an antioxidant protein, and was also associated with thrombus formation (Nishihira et al., 2008b). The pathological findings imply a possible relationship among intraplaque hemorrhage, oxidative stress, and plaque instability. However, the direct evidence that links intraplaque hemorrhage to plaque instability is still lacking.

Pathology and Pathophysiology of Atherothrombosis: Virchow's Triad Revisited 7

Experimental aortic stenosis can induce acute endothelial change or damage of the normal aorta (Fry, 1968). Therefore, hemodynamic force, particularly disturbed blood flow induced by stenosis or vasoconstriction, could be a crucial factor in generating surface vascular damage and thrombosis. To address the relation between disturbed blood flow and plaque erosion, we investigated the pathological change after acute luminal narrowing in SMC-rich plaque in rabbit. The SMC-rich plaque was induced by a balloon injury of rabbit femoral artery, and expressed TF as human atherosclerotic plaques. Actually, the disturbed blood by acute vascular narrowing induced superficial erosive injury to the SMC-rich plaque at post stenotic regions in rabbit femoral arteries. Figure 3 shows microscopic images of the longitudinal section of the neointima at the post- stenotic region 15 min after vascular narrowing. The endothelial cells and SMCs at this region were broadly detached with time, and associated with platelet adhesion to the sub-endothelium. Apoptosis of endothelial cells

Fig. 3. Representative images of superficial erosive injury of SMC-rich plaque and thrombus

SMC-rich plaque 15 min after vascular narrowing shows endothelial detachement (small arrows) accompanies platelet adhesion (arrow heads) at 1mm form vascular narrowing (A, hematoxyline eosin stain). Detachment of endothelial cells and exposure of subendothelial matrix is accompanied by platelet aggregation on the left side, and residual endothelial cell layer is present on right side (inset, high magnification of aggregated platelets) (B. scanning electron microscopy). Immunohistochemistry for VWF (C, a marker of endothelium) or smooth muscle actin (D, a marker of SMC) confirm detachment of endothelial cells and

SMCs at post stenotic region (from Sumi et al. 2010, with permission).

formation at the post-stenotic region.

#### **4.1.2 Blood flow-induced mechanical stress on plaque rupture**

Blood flow-induced mechanical stress is an essential factor of development of atherosclerosis and atherothrombosis. The low shear stress and oscillatory shear stress are both important stimuli for induction of atherosclerosis. Using a perivascular shear stress modifier in mice, Cheng et al. (2006) revealed that low shear stress induces larger lesions with vulnerable plaque phenotype (more lipids, more proteolytic enzymes, less SMCs, and less collagen) whereas vortices with oscillatory shear stress induce stable lesions. Chatzizisis et al. (2011) reported development of thin fibrous cap atheroma in coronary artery with low shear stress in pigs. In addition, the shear stress-induced changes in atherosclerotic plaque composition are modulated by chemokines. Inhibition of fractalkine, which is exclusively expressed in the low shear stress-induced atherosclerotic plaque, was reduced lipid and macrophage accumulation in the brachiocephalic arteries in mice (Cheng et al., 2007). Therefore, lower shear stress can induce atherosclerotic lesion prone to plaque rupture. Although it is well recognized that a mechanical stress triggers the disruption of fibrous cap, it remains unclear which factor is mainly responsible for the disruption of the thin fibrous cap. A variety of mechanical factors have been postulated to play a role in plaque rupture, including hemodynamic shear stress, turbulent pressure fluctuation (Loree et al., 1991), sudden increases in intraluminal pressure (Muller et al., 1989), and tensile stress concentration within the wall of the lesion. To investigate the relationship between shear stress distribution and coronary plaque rupture, Fukumoto et al. (2008) analyzed 3 dimmensional intravascular ultrasound images in patients with acute coronary thrombosis by a program for calculating the fluid dynamics. The ruptured sites were located in the proximal or top portion of the plaques, and the localized high shear stress is frequently correlated with the rupture sites. This finding is inconsistent with role of low shear stress on atherogenesis. It is possible that the process of initiating plaque rupture is quite different form that of atherogenesis. On the other hand, an excessive concentration of tensile stress within the plaque may be one of the triggers of plaque rupture. When the tensile stress becomes greater than the fragility of the fibrous cap, a plaque disruption may be initiated. The tensile stress is increased by development of a lipid core, thinning of the fibrous cap (Loree et al., 1992). Cheng et al. (1993) analyzed the distribution of circumferential stress in human coronary arteries. The maximum circumferential stress in ruptured plaques was significantly higher than that in stable plaques, although plaque rupture does not always occur at the region of highest stress. These results suggest that a mechanical factor that triggers plaque rupture differ in each case and lesion.

#### **4.1.3 Disturbed blood flow on plaque erosion**

Although it has been postulated that erosions result from coronary vasospasm of SMC-rich plaque, the mechanisms of plaque erosion are poorly understood. Approximately 80% thrombi of plaque erosion are nonocclusive in spite of sudden coronary death (Virmani et al., 2000). Platelet rich emboli are found in 74% of patients dying suddenly with plaque erosion compared with plaque rupture (40%). Because activated platelets release vasoconstrictive agents, such as 5-hydroxytriptamine (5-HT, serotonin) and thromboxane A2, these emboli might increase peripheral resistance leading to alteration of coronary blood flow. 5-HT can induce vasoconstriction and reduce coronary blood flow in human atherosclerotic vessels but not in normal arteries (Golino et al., 1991).

Blood flow-induced mechanical stress is an essential factor of development of atherosclerosis and atherothrombosis. The low shear stress and oscillatory shear stress are both important stimuli for induction of atherosclerosis. Using a perivascular shear stress modifier in mice, Cheng et al. (2006) revealed that low shear stress induces larger lesions with vulnerable plaque phenotype (more lipids, more proteolytic enzymes, less SMCs, and less collagen) whereas vortices with oscillatory shear stress induce stable lesions. Chatzizisis et al. (2011) reported development of thin fibrous cap atheroma in coronary artery with low shear stress in pigs. In addition, the shear stress-induced changes in atherosclerotic plaque composition are modulated by chemokines. Inhibition of fractalkine, which is exclusively expressed in the low shear stress-induced atherosclerotic plaque, was reduced lipid and macrophage accumulation in the brachiocephalic arteries in mice (Cheng et al., 2007). Therefore, lower shear stress can induce atherosclerotic lesion prone to plaque rupture. Although it is well recognized that a mechanical stress triggers the disruption of fibrous cap, it remains unclear which factor is mainly responsible for the disruption of the thin fibrous cap. A variety of mechanical factors have been postulated to play a role in plaque rupture, including hemodynamic shear stress, turbulent pressure fluctuation (Loree et al., 1991), sudden increases in intraluminal pressure (Muller et al., 1989), and tensile stress concentration within the wall of the lesion. To investigate the relationship between shear stress distribution and coronary plaque rupture, Fukumoto et al. (2008) analyzed 3 dimmensional intravascular ultrasound images in patients with acute coronary thrombosis by a program for calculating the fluid dynamics. The ruptured sites were located in the proximal or top portion of the plaques, and the localized high shear stress is frequently correlated with the rupture sites. This finding is inconsistent with role of low shear stress on atherogenesis. It is possible that the process of initiating plaque rupture is quite different form that of atherogenesis. On the other hand, an excessive concentration of tensile stress within the plaque may be one of the triggers of plaque rupture. When the tensile stress becomes greater than the fragility of the fibrous cap, a plaque disruption may be initiated. The tensile stress is increased by development of a lipid core, thinning of the fibrous cap (Loree et al., 1992). Cheng et al. (1993) analyzed the distribution of circumferential stress in human coronary arteries. The maximum circumferential stress in ruptured plaques was significantly higher than that in stable plaques, although plaque rupture does not always occur at the region of highest stress. These results suggest that a mechanical factor that

Although it has been postulated that erosions result from coronary vasospasm of SMC-rich plaque, the mechanisms of plaque erosion are poorly understood. Approximately 80% thrombi of plaque erosion are nonocclusive in spite of sudden coronary death (Virmani et al., 2000). Platelet rich emboli are found in 74% of patients dying suddenly with plaque erosion compared with plaque rupture (40%). Because activated platelets release vasoconstrictive agents, such as 5-hydroxytriptamine (5-HT, serotonin) and thromboxane A2, these emboli might increase peripheral resistance leading to alteration of coronary blood flow. 5-HT can induce vasoconstriction and reduce coronary blood flow in human

**4.1.2 Blood flow-induced mechanical stress on plaque rupture** 

triggers plaque rupture differ in each case and lesion.

atherosclerotic vessels but not in normal arteries (Golino et al., 1991).

**4.1.3 Disturbed blood flow on plaque erosion** 

Experimental aortic stenosis can induce acute endothelial change or damage of the normal aorta (Fry, 1968). Therefore, hemodynamic force, particularly disturbed blood flow induced by stenosis or vasoconstriction, could be a crucial factor in generating surface vascular damage and thrombosis. To address the relation between disturbed blood flow and plaque erosion, we investigated the pathological change after acute luminal narrowing in SMC-rich plaque in rabbit. The SMC-rich plaque was induced by a balloon injury of rabbit femoral artery, and expressed TF as human atherosclerotic plaques. Actually, the disturbed blood by acute vascular narrowing induced superficial erosive injury to the SMC-rich plaque at post stenotic regions in rabbit femoral arteries. Figure 3 shows microscopic images of the longitudinal section of the neointima at the post- stenotic region 15 min after vascular narrowing. The endothelial cells and SMCs at this region were broadly detached with time, and associated with platelet adhesion to the sub-endothelium. Apoptosis of endothelial cells

Fig. 3. Representative images of superficial erosive injury of SMC-rich plaque and thrombus formation at the post-stenotic region.

SMC-rich plaque 15 min after vascular narrowing shows endothelial detachement (small arrows) accompanies platelet adhesion (arrow heads) at 1mm form vascular narrowing (A, hematoxyline eosin stain). Detachment of endothelial cells and exposure of subendothelial matrix is accompanied by platelet aggregation on the left side, and residual endothelial cell layer is present on right side (inset, high magnification of aggregated platelets) (B. scanning electron microscopy). Immunohistochemistry for VWF (C, a marker of endothelium) or smooth muscle actin (D, a marker of SMC) confirm detachment of endothelial cells and SMCs at post stenotic region (from Sumi et al. 2010, with permission).

Pathology and Pathophysiology of Atherothrombosis: Virchow's Triad Revisited 9

Representative immunohistochemical microphotographs of normal (A) and balloon-injured femoral artery at 3 weeks after injury under 0.5% cholesterol diet (B). Atherosclerotic lesion composed of SMCs and macrophages develops in injured artery. TF expression is present in the lesion and adventitia of both arteries. HE/VB, hematoxyline eosin/Victoria blue stain

**HE/VB SMC Macrophage TF** 

100μm

Fig. 4. Histological images of rabbit femoral arteries.

(From Yamashita et al. 2009, with permission).

**M** 

**I** 

**I** 

Fig. 5. Immunofluorescence images of thrombus on rabbit femoral artery.

IEL, internal elastic lamina. (From Yamashita et al. 2009, with permission).

Representative immunofluorescent microphotographs of thrombi 15 minutes after balloon injury of normal femoral artery and of atherosclerotic plaque under 0.5% cholesterol diet. Rows show differential interference contrast images, images stained with fluorescein isothiocyanate-labeled GPIIb/IIIa (green), Cy3-labeled fibrin (red), and merged

immunofluorescent images. Areas with colocalized factors are stained yellow. The thrombi on normal intima is composed of small aggregated platelet (A), while the thrombi on atherosclerotic plaque is large, and composed of platelet and fibrin (B). I, intima; M, media;

<sup>100</sup>μ<sup>m</sup>**A** 

**B** 

**A** 

**IEL** 

**B** 

and superficial SMCs was also observed at the post- stenotic region within 15 minutes (Sumi et al., 2010). The vascular narrowing induced large mural thrombi which composed of platelets and fibrin, as human plaque erosion. Thus, disturbed blood flow can induce superficial erosive injury to SMC-rich plaque and thrombus formation at post stenotic region. Computational fluid simulation analysis indicated that oscillatory shear stress contributes to the development of the erosive damage to the plaque (Sumi et al., 2010). Although direct clinical evidence has not yet supported the notion that coronary artery vasospasm plays a role in plaque erosion, the superficial erosive injury of SMC-rich plaque by disturbed blood flow is similar to those of human plaque erosion (Sato et al., 2005). And, platelet and blood coagulation in coronary circulation are activated after vasospastic angina (Miyamoto et al. 2001, Oshima et al., 1990). Therefore, these evidence suggest that an acuteonset disturbed blood flow due to vasoconstriction could trigger plaque erosion. Hemodynamic factors could play an important role in development of plaque erosion.

#### **4.2 Virchow's triads on thrombus growth**

As described above, plaque disruption does not always result in complete thrombotic occlusion. Thrombus growth is considered critical to the onset of clinical events. Although thrombus formation is regulated by the vascular wall thrombogenicity, local blood flow, and blood contents, their contribution to thrombus growth has not been clearly defined. We separately discuss three factors that affect thrombus growth in atherosclerotic vessels.

#### **4.2.1 Vascular factors on thrombus growth**

Most fundamental difference between normal artery and atherosclerotic artery is presence of abundant active TF in atherosclerotic lesions (Hatakeyama et al., 1997, Wilcox et al., 1989). It seems that vascular wall TF contribute to thrombus size/propagation on atherosclerotic lesions. However, recent studies indicate that a small amount of TF is detectable in the blood and is capable of supporting clot formation in vitro. Plasma TF levels are elevated in patients with unstable angina and AMI and correlate with adverse outcomes (Mackman, 2004). Therefore, it is still controversial whether vascular wall and/or blood-derived TF support thrombus propagation. Hematopoietic cell-derived, TF-positive microparticles contributed to laser injury-induced thrombosis in the microvasculature of mouse cremaster muscle (Chou et al. 2004). In contrast, vascular smooth muscle-derived TF contributed to FeCl3 induced thrombosis in mouse carotid artery (Wang et al., 2009). We investigated whether plaque and/or blood TF contribute to thrombus formation in rabbit femoral artery with or without atherosclerotic lesions. The atherosclerotic lesions in rabbit femoral arteries were induced by a 0.5% cholesterol diet and balloon injury, and showed TF expression and increased procoagulant activity compared with normal femoral arteries (Figure 4). Balloon injury of the atherosclerotic plaque induced thrombin-dependent large platelet-fibrin thrombi. In contrast, balloon injury of normal femoral artery induced thrombin-independent small platelet thrombi (Figure 5). Moreover, whole blood coagulation in the rabbits was not affected by blood TF inhibition with a TF antibody even in hyperlipidemic condition (Yamashita et al., 2009). Therefore, at least, atherosclerotic plaque-derived TF might contribute to activation of intravascular coagulation cascade and thrombus size/propagation on atherosclerotic lesions.

and superficial SMCs was also observed at the post- stenotic region within 15 minutes (Sumi et al., 2010). The vascular narrowing induced large mural thrombi which composed of platelets and fibrin, as human plaque erosion. Thus, disturbed blood flow can induce superficial erosive injury to SMC-rich plaque and thrombus formation at post stenotic region. Computational fluid simulation analysis indicated that oscillatory shear stress contributes to the development of the erosive damage to the plaque (Sumi et al., 2010). Although direct clinical evidence has not yet supported the notion that coronary artery vasospasm plays a role in plaque erosion, the superficial erosive injury of SMC-rich plaque by disturbed blood flow is similar to those of human plaque erosion (Sato et al., 2005). And, platelet and blood coagulation in coronary circulation are activated after vasospastic angina (Miyamoto et al. 2001, Oshima et al., 1990). Therefore, these evidence suggest that an acuteonset disturbed blood flow due to vasoconstriction could trigger plaque erosion. Hemodynamic factors could play an important role in development of plaque erosion.

As described above, plaque disruption does not always result in complete thrombotic occlusion. Thrombus growth is considered critical to the onset of clinical events. Although thrombus formation is regulated by the vascular wall thrombogenicity, local blood flow, and blood contents, their contribution to thrombus growth has not been clearly defined. We separately discuss three factors that affect thrombus growth in atherosclerotic vessels.

Most fundamental difference between normal artery and atherosclerotic artery is presence of abundant active TF in atherosclerotic lesions (Hatakeyama et al., 1997, Wilcox et al., 1989). It seems that vascular wall TF contribute to thrombus size/propagation on atherosclerotic lesions. However, recent studies indicate that a small amount of TF is detectable in the blood and is capable of supporting clot formation in vitro. Plasma TF levels are elevated in patients with unstable angina and AMI and correlate with adverse outcomes (Mackman, 2004). Therefore, it is still controversial whether vascular wall and/or blood-derived TF support thrombus propagation. Hematopoietic cell-derived, TF-positive microparticles contributed to laser injury-induced thrombosis in the microvasculature of mouse cremaster muscle (Chou et al. 2004). In contrast, vascular smooth muscle-derived TF contributed to FeCl3 induced thrombosis in mouse carotid artery (Wang et al., 2009). We investigated whether plaque and/or blood TF contribute to thrombus formation in rabbit femoral artery with or without atherosclerotic lesions. The atherosclerotic lesions in rabbit femoral arteries were induced by a 0.5% cholesterol diet and balloon injury, and showed TF expression and increased procoagulant activity compared with normal femoral arteries (Figure 4). Balloon injury of the atherosclerotic plaque induced thrombin-dependent large platelet-fibrin thrombi. In contrast, balloon injury of normal femoral artery induced thrombin-independent small platelet thrombi (Figure 5). Moreover, whole blood coagulation in the rabbits was not affected by blood TF inhibition with a TF antibody even in hyperlipidemic condition (Yamashita et al., 2009). Therefore, at least, atherosclerotic plaque-derived TF might contribute to activation of intravascular coagulation cascade and thrombus

**4.2 Virchow's triads on thrombus growth** 

**4.2.1 Vascular factors on thrombus growth** 

size/propagation on atherosclerotic lesions.

Fig. 4. Histological images of rabbit femoral arteries.

Representative immunohistochemical microphotographs of normal (A) and balloon-injured femoral artery at 3 weeks after injury under 0.5% cholesterol diet (B). Atherosclerotic lesion composed of SMCs and macrophages develops in injured artery. TF expression is present in the lesion and adventitia of both arteries. HE/VB, hematoxyline eosin/Victoria blue stain (From Yamashita et al. 2009, with permission).

Fig. 5. Immunofluorescence images of thrombus on rabbit femoral artery. Representative immunofluorescent microphotographs of thrombi 15 minutes after balloon injury of normal femoral artery and of atherosclerotic plaque under 0.5% cholesterol diet. Rows show differential interference contrast images, images stained with fluorescein isothiocyanate-labeled GPIIb/IIIa (green), Cy3-labeled fibrin (red), and merged immunofluorescent images. Areas with colocalized factors are stained yellow. The thrombi on normal intima is composed of small aggregated platelet (A), while the thrombi on atherosclerotic plaque is large, and composed of platelet and fibrin (B). I, intima; M, media; IEL, internal elastic lamina. (From Yamashita et al. 2009, with permission).

Pathology and Pathophysiology of Atherothrombosis: Virchow's Triad Revisited 11

other hand, the CRP overexpression did not enhance atherosclerosis induced by hyperchoresterol diets (Koike et al., 2009). CRP localized in atherosclerotic plaques might enhance vascular wall thrombogenicity and thrombus formation after plaque disruption

Blood flow is a key modulator of thrombus growth. Clinical studies revealed an alteration of coronary blood flow in patients with ischemic heart diseases. Marzilli et al. (2000) reported an approximate 80% reduction in coronary blood flow during ischemia in patients with unstable angina. An autopsy study reported that intramyocardial microemboli were frequently present in sudden coronary death patients (Schwartz et al. 2009). Distal microvascular embolism and/or vasoconstriction could affect blood flow alteration and thrombus formation and growth at the culprit lesions (Erbel & Heusch, 2000). To assess the issue, we examined the effects of the blood flow reduction to thrombus formation in our animal model. Blood flow reduction (>75%) promoted the growth of thrombus, a mixture of platelets and fibrin, on atherosclerotic lesion, which grew to occlusive one. The flow reduction also induced thrombus formation on normal arteries, but the thrombi were very small and composed only of platelets (Yamashita et al. 2004). Therefore, blood flow reduction associated with increased vascular wall thrombogenecity is considered to contribute thrombus growth. We also demonstrated an important role of 5-HT2A receptor on platelets and SMCs in this process via platelet aggregation and thrombogenic

In addition to distal vascular resistance, disturbed blood flow by acute vascular narrowing promotes thrombus growth at post stenotic regions. As described above, vascular narrowing of rabbit femoral artery induced superficial erosive injury to SMC-rich plaque at post stenotic regions. The thrombi consisted of a mixture of aggregated platelets and a considerable amount of fibrin. The whole blood hemostatic parameters in the rabbits was not changed after vascular narrowing or anti-rabbit TF antibody treatment, which evidence indicates that TF derived from eroded plaque rather than circulating TF plays an important

The rheological effect on thrombus growth may be partly explained by a shear gradientdependent platelet aggregation mechanism. Using in vitro and in vivo stenotic microvessels and imaging systems, Nesbitt et al. (2009) revealed a shear gradient-dependent platelet aggregation process which is preceded by soluble agonist-dependent aggregation. Shear microgradient at post stenosis region or down stream face of thrombi induced stable platelets aggregates, and the shear microgradients directly influenced the platelet aggregation size. This process required ligand binding to integrin αIIbβ3, transient Ca2+ flux, but did not required global platelet shape change or soluble agonists. The findings suggest that platelets principally use a biomechanical platelet aggregation mechanism in early phase of platelet adhesion and aggregation. Vessel and/or thrombus geometry itself may promote

As described above, platelet is a major cellular component in coronary thrombus, and platelets play an important role in growing phase of thrombus formation, as well as initial

rather than atherogenesis.

thrombus formation.

**4.2.3 Blood factors on thrombus growth** 

**4.2.2 Altered blood flow on thrombus growth** 

vasoconstriction (Nishihira et al., 2006a, 2008a).

role in fibrin generation and thrombus growth (Sumi et al. 2010).

Several factors can influence TF expression in plaques and atherothrombus formation after plaque disruption. CRP is an inflammatory acute-phase reactant that has emerged as a powerful predictor of cardiovascular disease (Ridker, 2007). CRP is localized in atherosclerotic plaques and is more in thrombotic plaques than non-thrombotic ones (Ishikawa et al., 2003, Sun et al., 2005). The findings imply that CRP is implicated in atherothrombogenesis. To address this issue, CRP-transgenic rabbits were generated, because as human CRP, CRP in rabbits but not in mice works as an acute-phase reactant during inflammation (Koike et al., 2009). In the rabbits, CRP was overexpressed in livers and circulated in blood and deposited in the both SMC-rich and macrophage-rich atherosclerotic lesions. The thrombus size on SMC-rich plaque or macrophage-rich plaque after balloon injury was significantly increased in CRP-transgenic rabbits as compared with wild nontransgenic rabbits (Figure 6). TF expression and its acivity in the plaques were significantly increased in CRP-transgenic rabbits. The degree of CRP deposition correlated with TF expression in plaques and thrombus size on injured plaques (Matsuda et al., 2011). On the

> 100μm HE GP IIb/IIIa Fibrin 100μm Non-transgenic Rb CRP-transgenic Rb

Fig. 6. Thrombus formation on SMC-rich plaque in CRP-transgenic or non-transgenic rabbit femoral artery.

The images show thrombus formation on SMC-rich plaque (arrows) 15 min after balloon injury of rabbit femoral arteries. The thrombus size is increased in CRP-transgenic rabbits as compared with non-transgenic rabbit. Immunopositive areas for GPIIb/IIIa and fibrin also increase in CRP-transgenic rabbit (from Matsuda et al. 2011, with permission).

other hand, the CRP overexpression did not enhance atherosclerosis induced by hyperchoresterol diets (Koike et al., 2009). CRP localized in atherosclerotic plaques might enhance vascular wall thrombogenicity and thrombus formation after plaque disruption rather than atherogenesis.

#### **4.2.2 Altered blood flow on thrombus growth**

10 Traditional and Novel Risk Factors in Atherothrombosis

Several factors can influence TF expression in plaques and atherothrombus formation after plaque disruption. CRP is an inflammatory acute-phase reactant that has emerged as a powerful predictor of cardiovascular disease (Ridker, 2007). CRP is localized in atherosclerotic plaques and is more in thrombotic plaques than non-thrombotic ones (Ishikawa et al., 2003, Sun et al., 2005). The findings imply that CRP is implicated in atherothrombogenesis. To address this issue, CRP-transgenic rabbits were generated, because as human CRP, CRP in rabbits but not in mice works as an acute-phase reactant during inflammation (Koike et al., 2009). In the rabbits, CRP was overexpressed in livers and circulated in blood and deposited in the both SMC-rich and macrophage-rich atherosclerotic lesions. The thrombus size on SMC-rich plaque or macrophage-rich plaque after balloon injury was significantly increased in CRP-transgenic rabbits as compared with wild nontransgenic rabbits (Figure 6). TF expression and its acivity in the plaques were significantly increased in CRP-transgenic rabbits. The degree of CRP deposition correlated with TF expression in plaques and thrombus size on injured plaques (Matsuda et al., 2011). On the

Non-transgenic Rb CRP-transgenic Rb

Fig. 6. Thrombus formation on SMC-rich plaque in CRP-transgenic or non-transgenic rabbit

The images show thrombus formation on SMC-rich plaque (arrows) 15 min after balloon injury of rabbit femoral arteries. The thrombus size is increased in CRP-transgenic rabbits as compared with non-transgenic rabbit. Immunopositive areas for GPIIb/IIIa and fibrin also

increase in CRP-transgenic rabbit (from Matsuda et al. 2011, with permission).

100μm

100μm

HE

GP IIb/IIIa

Fibrin

femoral artery.

Blood flow is a key modulator of thrombus growth. Clinical studies revealed an alteration of coronary blood flow in patients with ischemic heart diseases. Marzilli et al. (2000) reported an approximate 80% reduction in coronary blood flow during ischemia in patients with unstable angina. An autopsy study reported that intramyocardial microemboli were frequently present in sudden coronary death patients (Schwartz et al. 2009). Distal microvascular embolism and/or vasoconstriction could affect blood flow alteration and thrombus formation and growth at the culprit lesions (Erbel & Heusch, 2000). To assess the issue, we examined the effects of the blood flow reduction to thrombus formation in our animal model. Blood flow reduction (>75%) promoted the growth of thrombus, a mixture of platelets and fibrin, on atherosclerotic lesion, which grew to occlusive one. The flow reduction also induced thrombus formation on normal arteries, but the thrombi were very small and composed only of platelets (Yamashita et al. 2004). Therefore, blood flow reduction associated with increased vascular wall thrombogenecity is considered to contribute thrombus growth. We also demonstrated an important role of 5-HT2A receptor on platelets and SMCs in this process via platelet aggregation and thrombogenic vasoconstriction (Nishihira et al., 2006a, 2008a).

In addition to distal vascular resistance, disturbed blood flow by acute vascular narrowing promotes thrombus growth at post stenotic regions. As described above, vascular narrowing of rabbit femoral artery induced superficial erosive injury to SMC-rich plaque at post stenotic regions. The thrombi consisted of a mixture of aggregated platelets and a considerable amount of fibrin. The whole blood hemostatic parameters in the rabbits was not changed after vascular narrowing or anti-rabbit TF antibody treatment, which evidence indicates that TF derived from eroded plaque rather than circulating TF plays an important role in fibrin generation and thrombus growth (Sumi et al. 2010).

The rheological effect on thrombus growth may be partly explained by a shear gradientdependent platelet aggregation mechanism. Using in vitro and in vivo stenotic microvessels and imaging systems, Nesbitt et al. (2009) revealed a shear gradient-dependent platelet aggregation process which is preceded by soluble agonist-dependent aggregation. Shear microgradient at post stenosis region or down stream face of thrombi induced stable platelets aggregates, and the shear microgradients directly influenced the platelet aggregation size. This process required ligand binding to integrin αIIbβ3, transient Ca2+ flux, but did not required global platelet shape change or soluble agonists. The findings suggest that platelets principally use a biomechanical platelet aggregation mechanism in early phase of platelet adhesion and aggregation. Vessel and/or thrombus geometry itself may promote thrombus formation.

#### **4.2.3 Blood factors on thrombus growth**

As described above, platelet is a major cellular component in coronary thrombus, and platelets play an important role in growing phase of thrombus formation, as well as initial

Pathology and Pathophysiology of Atherothrombosis: Virchow's Triad Revisited 13

formation by degradation of coagulation or fibrinolysis factors and promoting platelet function (Kornecki et al., 1988, Moir et al., 2002). Inhibition of interaction between p-selectin and p-selectin glycoprotein ligand 1 reduced fibrin formation in vivo (Palabrica et al., 1992). These adhesion molecules have been implicated in recruitment of leukocytes and leukocyte MPs to thrombi (Vandendries et al., 2004). To reveal the neutrophil-mediated procoagulant mechanisms, Massberg et al. (2010) investigated thrombus formation using neutrophil elastase and cathepsin G deficient mice. Proteolysis of TFPI by these proteases enhanced fibrin and thrombus formation after FeCl3-induced vessel injury. In addition, activated platelets by collagen accelerated nucleosome externalization by neutrophils. The neutrophilderived externalized nucleosomes can form neutrophil extracellular traps that provide a scaffold for platelets and red blood cells and histone 3/4 can induce platelet aggregation (Fuchs et al., 2010). On the other hands, neutrophil elastase has fibriolytic potential, and there is significant correlation between neutrophil elastase-digested fibrin and leukocyte content in human atherothrombi (Rábai et al., 2010). Zeng et al. (2002) investigated contribution of polymorphonuclear leukocytes (PMNs) to fibrinolysis in vivo using plasminogen deficient mice. The PMNs accumulated within the thrombi by 6 hours after FeCl3-induced vessel injury and peaked at 24 hours. There were no significant differences between the PMNs from plasminogen deficient mice and wild type mice within the 6 hour after thrombus formation, whereas there was significant greater retention of PMNs within the thrombus over 24 hours after thrombus formation. PMNs from both mice showed fibrinolytic activity, but the degradation products were a distinct pattern. Therefore, it is possible that neutrophils works as positive or negative regulator of early or late phase of

Endothelial progenitor cells (EPC) contributes to angiogenesis and wound healing (Asahara et al., 1997), and the number of EPCs in blood is associated with cardiovascular risk (Hill et al., 2003). The mechanisms that regulate mobilization, migration, and differentiation of EPCs and their homing to sites of vascular injury are complex and involve several mediators and receptors, such as P-selectin glycoprotein ligand-1, CXC chemokine, and integrins (Chavakis et al., 2005, Massberg et al., 2006). Interaction of thrombus contents and EPCs influences their mobilization and differentiation to mature endothelial cells during vascular injury (de Boer HC et al., 2006). Abou-Saleh et al. (2009) reported that human peripheral blood mononuclear cell derived EPCs bound platelets via p-selectin and inhibit platelet activation, aggregation, and adhesion to collagen in vitro, and that injection of these EPCs reduced

Other possible mechanism contributing thrombus propagation in vivo is intrinsic coagulation pathway. The intrinsic coagulation pathway is initiated when coagulation factor XII (FXII) comes into contact with negatively charged surfaces in a reaction involving the plasma proteins, high molecular mass kininogen and plasma kallikrein. Factor XI (FXI) is activated by activated FXII, thrombin, and activated XI. Feedback activation of FXI by thrombin promotes further thrombin generation in vitro (Gailani & Broze, 1991). FXI was present in platelet-fibrin thrombus induced balloon injury of atherosclerotic lesion in rabbits, and anti-FXI antibody reduced thrombus growth without prolonging bleeding (Yamashita et al., 2006b). FXI plays an important role in thrombus growth via further thrombin generation. On the other hand, there are conflicts of evidence that FXII supports arterial thrombus growth. FXII deficient mice were resistant to thrombotic occlusion after FeCl3 induced vessel injury of carotid arteries (Cheng Q et al., 2010). However, a clinical study demonstrated an inverse relationship between FXII level and risk of myocardial

thrombus formation after FeCl3-induced vessel injury of mouse carotid arteries.

thrombus formation, respectively.

phase of thrombus formation. Adhesion molecules and its receptors on platelets are essential for thrombus formation, because these molecules support platelet tethering, firm adhesion, aggregation and platelet recruitment to thrombus surface. VWF is a large, multimeric, plasma protein that undergoes a conformational change when bound to matrix under permit its binding to GPIbα. Recent studies in vitro and in vivo showed that platelet recruitment on thrombus surface was primary mediated by VWF and GPIbα on flowing platelets (Bergmeier et al. 2006, Kulkuni et al. 2000). We demonstrated that a large amount of VWF was localized in coronary thrombi in patients with AMI (Nishihira et al., 2010, Yamashita et al., 2006a), and that monoclonal antibody against VWF A1 domain, which interacts platelet GPIbα, significantly suppressed formation of platelet-fibrin thrombi and completely inhibited occlusive thrombus formation in rabbit atherosclerotic lesions (Yamashita et al., 2003, 2004). These findings indicated a crucial role of VWF in thrombus growth via platelet recruitment. The multimer size of VWF can affect thrombus size and is regulated by a plasma protease, a disintegrin and metalloprotease with a thrombospondin type 1 motif 13 (ADAMTS-13). A deficiency of ADAMTS-13 activity causes an increased level of circulating ultralarge VWF multimers, and correlates with the onset of the general thrombotic disease, thrombotic thrombocytopenic purpura (TTP). A clinical evidence suggested dysregulation of VWF multimer size in AMI patient. The ratio of VWF/ADAMTS-13 antigen was higher in patients with AMI than in those with stable angina pectris, and there was a inverse correlation between plasma VWF antigen and ADAMTS-13 activity in AMI patients (Kaikita et al. 2006). The ADAMTS-13 closely localized with VWF in fresh coronary thrombi from AMI patients (Moriguchi-Goto et al., 2009). A reducing ADAMTS-13 activity by monoclonal antibody against distintegrin-like domain enhanced platelet thrombus growth on immobilized type I collagen at a high shear rate (1500S-1) and platelet-fibrin thrombus formation on injured atherosclerotic lesion of rabbit femoral arteries (Moriguchi-Goto et al., 2009). The study also showed cleavage of large sized VWF multimer during platelet thrombus formation under a high shear rate. The VWFcleaving site by ADAMTS-13 localized on the surface of platelet thrombus, and the ADAMTS-13 activity was shear dependent manner (Shida et al. 2008). Thus, ADAMTS-13 may work at the site of ongoing thrombus generation and limit thrombus growth.

The recent studies in vitro showed various blood cells, not only monocytes but also neutrophils, eosinophils, and even if platelets, can synthesize TF. Although there is much on debate on the TF expression in blood cells, it is likely that monocytes are the only blood cells that synthesize and express TF (Østerud, 2010). A related topic is contribution of microparticles (MPs) to thrombus formation. MPs are small fragments of membrane-bound cytoplasm that are shed from the surface of an activated or apoptotic cells (Blann et al. 2009). The procoagulant activity of MPs is increased with the exposure of phosphatidylserine and the presence of TF. In fact, MPs have significantly elevated in acute coronary syndrome and ischemic strokes (Geiser et al. 1998, Singh et al. 1995). However, it is still unclear whether the elevated levels of MPs are a cause or consequence of atherothrombosis. Moreover, our animal studies did not support the role of blood-derived TF in atherothrombus formation as described above. Future studies are required to clarify contribution of blood derived TF and/or MPs to thrombus propagation on atherosclerotic lesions.

Among the white blood cells, neutrophils are mostly found in coronary thrombus in patients with AMI, and CD34 positive leukocytes are also found in the thrombus (Nishihira et al., 2010). Recent evidences revealed neutrophils and endothelial progenitor cells influence thrombus growth. Neutrophils can positively or negatively affect thrombus

phase of thrombus formation. Adhesion molecules and its receptors on platelets are essential for thrombus formation, because these molecules support platelet tethering, firm adhesion, aggregation and platelet recruitment to thrombus surface. VWF is a large, multimeric, plasma protein that undergoes a conformational change when bound to matrix under permit its binding to GPIbα. Recent studies in vitro and in vivo showed that platelet recruitment on thrombus surface was primary mediated by VWF and GPIbα on flowing platelets (Bergmeier et al. 2006, Kulkuni et al. 2000). We demonstrated that a large amount of VWF was localized in coronary thrombi in patients with AMI (Nishihira et al., 2010, Yamashita et al., 2006a), and that monoclonal antibody against VWF A1 domain, which interacts platelet GPIbα, significantly suppressed formation of platelet-fibrin thrombi and completely inhibited occlusive thrombus formation in rabbit atherosclerotic lesions (Yamashita et al., 2003, 2004). These findings indicated a crucial role of VWF in thrombus growth via platelet recruitment. The multimer size of VWF can affect thrombus size and is regulated by a plasma protease, a disintegrin and metalloprotease with a thrombospondin type 1 motif 13 (ADAMTS-13). A deficiency of ADAMTS-13 activity causes an increased level of circulating ultralarge VWF multimers, and correlates with the onset of the general thrombotic disease, thrombotic thrombocytopenic purpura (TTP). A clinical evidence suggested dysregulation of VWF multimer size in AMI patient. The ratio of VWF/ADAMTS-13 antigen was higher in patients with AMI than in those with stable angina pectris, and there was a inverse correlation between plasma VWF antigen and ADAMTS-13 activity in AMI patients (Kaikita et al. 2006). The ADAMTS-13 closely localized with VWF in fresh coronary thrombi from AMI patients (Moriguchi-Goto et al., 2009). A reducing ADAMTS-13 activity by monoclonal antibody against distintegrin-like domain enhanced platelet thrombus growth on immobilized type I collagen at a high shear rate (1500S-1) and platelet-fibrin thrombus formation on injured atherosclerotic lesion of rabbit femoral arteries (Moriguchi-Goto et al., 2009). The study also showed cleavage of large sized VWF multimer during platelet thrombus formation under a high shear rate. The VWFcleaving site by ADAMTS-13 localized on the surface of platelet thrombus, and the ADAMTS-13 activity was shear dependent manner (Shida et al. 2008). Thus, ADAMTS-13

may work at the site of ongoing thrombus generation and limit thrombus growth.

and/or MPs to thrombus propagation on atherosclerotic lesions.

The recent studies in vitro showed various blood cells, not only monocytes but also neutrophils, eosinophils, and even if platelets, can synthesize TF. Although there is much on debate on the TF expression in blood cells, it is likely that monocytes are the only blood cells that synthesize and express TF (Østerud, 2010). A related topic is contribution of microparticles (MPs) to thrombus formation. MPs are small fragments of membrane-bound cytoplasm that are shed from the surface of an activated or apoptotic cells (Blann et al. 2009). The procoagulant activity of MPs is increased with the exposure of phosphatidylserine and the presence of TF. In fact, MPs have significantly elevated in acute coronary syndrome and ischemic strokes (Geiser et al. 1998, Singh et al. 1995). However, it is still unclear whether the elevated levels of MPs are a cause or consequence of atherothrombosis. Moreover, our animal studies did not support the role of blood-derived TF in atherothrombus formation as described above. Future studies are required to clarify contribution of blood derived TF

Among the white blood cells, neutrophils are mostly found in coronary thrombus in patients with AMI, and CD34 positive leukocytes are also found in the thrombus (Nishihira et al., 2010). Recent evidences revealed neutrophils and endothelial progenitor cells influence thrombus growth. Neutrophils can positively or negatively affect thrombus formation by degradation of coagulation or fibrinolysis factors and promoting platelet function (Kornecki et al., 1988, Moir et al., 2002). Inhibition of interaction between p-selectin and p-selectin glycoprotein ligand 1 reduced fibrin formation in vivo (Palabrica et al., 1992). These adhesion molecules have been implicated in recruitment of leukocytes and leukocyte MPs to thrombi (Vandendries et al., 2004). To reveal the neutrophil-mediated procoagulant mechanisms, Massberg et al. (2010) investigated thrombus formation using neutrophil elastase and cathepsin G deficient mice. Proteolysis of TFPI by these proteases enhanced fibrin and thrombus formation after FeCl3-induced vessel injury. In addition, activated platelets by collagen accelerated nucleosome externalization by neutrophils. The neutrophilderived externalized nucleosomes can form neutrophil extracellular traps that provide a scaffold for platelets and red blood cells and histone 3/4 can induce platelet aggregation (Fuchs et al., 2010). On the other hands, neutrophil elastase has fibriolytic potential, and there is significant correlation between neutrophil elastase-digested fibrin and leukocyte content in human atherothrombi (Rábai et al., 2010). Zeng et al. (2002) investigated contribution of polymorphonuclear leukocytes (PMNs) to fibrinolysis in vivo using plasminogen deficient mice. The PMNs accumulated within the thrombi by 6 hours after FeCl3-induced vessel injury and peaked at 24 hours. There were no significant differences between the PMNs from plasminogen deficient mice and wild type mice within the 6 hour after thrombus formation, whereas there was significant greater retention of PMNs within the thrombus over 24 hours after thrombus formation. PMNs from both mice showed fibrinolytic activity, but the degradation products were a distinct pattern. Therefore, it is possible that neutrophils works as positive or negative regulator of early or late phase of thrombus formation, respectively.

Endothelial progenitor cells (EPC) contributes to angiogenesis and wound healing (Asahara et al., 1997), and the number of EPCs in blood is associated with cardiovascular risk (Hill et al., 2003). The mechanisms that regulate mobilization, migration, and differentiation of EPCs and their homing to sites of vascular injury are complex and involve several mediators and receptors, such as P-selectin glycoprotein ligand-1, CXC chemokine, and integrins (Chavakis et al., 2005, Massberg et al., 2006). Interaction of thrombus contents and EPCs influences their mobilization and differentiation to mature endothelial cells during vascular injury (de Boer HC et al., 2006). Abou-Saleh et al. (2009) reported that human peripheral blood mononuclear cell derived EPCs bound platelets via p-selectin and inhibit platelet activation, aggregation, and adhesion to collagen in vitro, and that injection of these EPCs reduced thrombus formation after FeCl3-induced vessel injury of mouse carotid arteries.

Other possible mechanism contributing thrombus propagation in vivo is intrinsic coagulation pathway. The intrinsic coagulation pathway is initiated when coagulation factor XII (FXII) comes into contact with negatively charged surfaces in a reaction involving the plasma proteins, high molecular mass kininogen and plasma kallikrein. Factor XI (FXI) is activated by activated FXII, thrombin, and activated XI. Feedback activation of FXI by thrombin promotes further thrombin generation in vitro (Gailani & Broze, 1991). FXI was present in platelet-fibrin thrombus induced balloon injury of atherosclerotic lesion in rabbits, and anti-FXI antibody reduced thrombus growth without prolonging bleeding (Yamashita et al., 2006b). FXI plays an important role in thrombus growth via further thrombin generation. On the other hand, there are conflicts of evidence that FXII supports arterial thrombus growth. FXII deficient mice were resistant to thrombotic occlusion after FeCl3 induced vessel injury of carotid arteries (Cheng Q et al., 2010). However, a clinical study demonstrated an inverse relationship between FXII level and risk of myocardial

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#### **5. Conclusion**

More than 150 years ago, Virchow described the mechanims of thrombus formation. It has still remained as a fundamental theory of thrombus formation. To date, pathological and experimental studies have clarified the mechanisms of atherothrombus formation. The thrombus formation is initiated by plaque rupture and plaque erosion. Among the Virchow's triad, vascular and rheological factors are responsible for plaque rupture. Disruption of thin fibrous cap atheroma triggers plaque rupture. On the other hand, disturbed blood by acute luminal change can trigger plaque erosion to SMC-rich plaque. Pathological findings of human atherothrombosis suggest that thrombus growth rather than plaque disruption is a critical step for the onset of cardiovascular events, and that simultaneous activation of coagulation cascade and platelets play an important role in thrombus formation after plaque disruption. All three factors contribute to atherothrombus growth. Our rabbit model of atherothrombosis revealed that excess thrombin generation mediated by plaque TF contribute to large plate-fibrin thrombus formation on atherosclerotic lesion, and that disturbed flow condition after plaque disruption promote thrombus growth. Recent evidence suggests that leukocytes influence arterial thrombus formation as well as platelet and coagulation/fibrinolysis factors. Differences between hemostasis and thrombus growth may shed light on a novel anti-atherothrombogic drug with a wide safety margin.

#### **6. Acknowledgement**

The work is supported in part by Grants-in-Aid for Scientific Research in Japan (No.23790410), Mitsubishi Pharma Research Foundation, and Integrated Research Project for Human and Veterinary Medicine.

#### **7. References**


infarction (Doggen et al., 2006). Moreover, inhibition of FXII did not change platelet aggregation and fibrin formation on atherosclerotic plaque surface under flow in vitro. The effect of FXII on coagulation became obvious only absence of TF (Reininger et al., 2010).

More than 150 years ago, Virchow described the mechanims of thrombus formation. It has still remained as a fundamental theory of thrombus formation. To date, pathological and experimental studies have clarified the mechanisms of atherothrombus formation. The thrombus formation is initiated by plaque rupture and plaque erosion. Among the Virchow's triad, vascular and rheological factors are responsible for plaque rupture. Disruption of thin fibrous cap atheroma triggers plaque rupture. On the other hand, disturbed blood by acute luminal change can trigger plaque erosion to SMC-rich plaque. Pathological findings of human atherothrombosis suggest that thrombus growth rather than plaque disruption is a critical step for the onset of cardiovascular events, and that simultaneous activation of coagulation cascade and platelets play an important role in thrombus formation after plaque disruption. All three factors contribute to atherothrombus growth. Our rabbit model of atherothrombosis revealed that excess thrombin generation mediated by plaque TF contribute to large plate-fibrin thrombus formation on atherosclerotic lesion, and that disturbed flow condition after plaque disruption promote thrombus growth. Recent evidence suggests that leukocytes influence arterial thrombus formation as well as platelet and coagulation/fibrinolysis factors. Differences between hemostasis and thrombus growth may shed light on a novel anti-atherothrombogic drug

The work is supported in part by Grants-in-Aid for Scientific Research in Japan (No.23790410), Mitsubishi Pharma Research Foundation, and Integrated Research Project for

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**2** 

*1,2,4,5UK 3Spain* 

**Biomarkers of Atherosclerosis and Acute** 

Richard Body1, Mark Slevin2,3 and Garry McDowell4,5

 *Manchester Metropolitan University, Manchester, 3Cardiovascular Research Centre, CSIC-ICCC, Hospital de la Santa Creu i Sant Pau, Barcelona, 4Faculty of Health, Edge Hill University, Ormskirk,* 

**Coronary Syndromes – A Clinical Perspective** 

*1Cardiovascular Sciences Research Group, University of Manchester, Manchester,* 

Coronary heart disease remains the single biggest killer in the United Kingdom, accounting for around one in five deaths in men and one in six deaths in women (1). In 2003 the total annual cost of coronary heart disease in the United Kingdom was around £3.5 billion (£60 per capita), with the cost of inpatient care accounting for around 79% of these costs (2). Approximately 3% of patients who attend the ED have chest pain that the treating physician suspects may be cardiac in origin (3). 74-88% of these patients are admitted to hospital, making up one in five of all medical admissions (3-5). Ultimately only a quarter of these patients will be diagnosed with an acute coronary syndrome (ACS), which implies that a very cautious approach to the problem has been adopted. Despite this fact, up to 6% of the patients with chest pain who are discharged from the ED actually have myocardial damage that has prognostic significance (6). These patients are up to three times as likely to die as

Over the past century tremendous advances have been made in our understanding of coronary heart disease and its pathophysiological evolution. In 1910 a Russian physician first described the clinical presentation of acute myocardial infarction (AMI) (8). Two years later, an association was drawn between AMI and acute thrombotic coronary occlusion (9). By 1913 it had been hypothesised that atherosclerosis developed as a result of gradual lipid accumulation within the arterial wall (10). The advent of coronary revascularisation procedures in the latter half of the 20th century allowed the observation that restoring blood flow beyond significant coronary stenotic lesions often led to alleviation of anginal symptoms. This helped to propagate the widespread belief that the greater the coronary

**1. Introduction** 

similar patients who were admitted to hospital (7).

**2. The pathophysiology of coronary heart disease** 

*2School of Biology, Chemistry and Health Science, John Dalton Building,* 

*5School of Translational Medicine, University of Manchester, Manchester,* 

protein in atherosclerotic lesions: its origin and pathophysiological significance. *Am J Pathol*, Vol.167, No.4, pp.1139-1148.


## **Biomarkers of Atherosclerosis and Acute Coronary Syndromes – A Clinical Perspective**

Richard Body1, Mark Slevin2,3 and Garry McDowell4,5

*1Cardiovascular Sciences Research Group, University of Manchester, Manchester, 2School of Biology, Chemistry and Health Science, John Dalton Building, Manchester Metropolitan University, Manchester, 3Cardiovascular Research Centre, CSIC-ICCC, Hospital de la Santa Creu i Sant Pau, Barcelona, 4Faculty of Health, Edge Hill University, Ormskirk, 5School of Translational Medicine, University of Manchester, Manchester, 1,2,4,5UK 3Spain* 

#### **1. Introduction**

20 Traditional and Novel Risk Factors in Atherothrombosis

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Yamashita, A., Sumi, T., Goto, S., Hoshiba, Y., Nishihira, K., Kawamoto R, et al. (2006a).

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*Atherosclerosis*. Vol.206, No.2, pp.418-426.

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pp.2420-2424.

No.5, pp.805-810.

1501.

protein in atherosclerotic lesions: its origin and pathophysiological significance. *Am* 

fibroatheroma: a type of vulnerable plaque. *J Interv Cardiol*. Vol.16.No.3, pp.267-272.

sudden coronary death: a comprehensive morphological classification scheme for atherosclerotic lesions. *Arterioscler Thromb Vasc Biol,* Vol.20, No.5, pp.1262-1275. Waldo, SW., Li, Y., Buono, C., Zhao, B., Billings, EM., Chang, J., Kruth, HS. (2008).

Heterogeneity of human macrophages in culture and in atherosclerotic plaques.

Vascular smooth muscle-derived tissue factor is critical for arterial thrombosis after

the normal vessel wall and in the atherosclerotic plaque. *Proc Natl Acad Sci U S A*,

Tamura, S., Ikeda, Y., Sumiyoshi, A. (2003). Contribution of von Willebrand factor to thrombus formation on neointima of rabbit stenotic iliac artery under high blood-flow velocity. *Arterioscler Thromb Vasc Biol*, Vol.23, No.6, pp.1105-1110. Yamashita, A., Furukoji, E., Marutsuka, K., Hatakeyama, K., Yamamoto, H., Tamura, S.,

Ikeda, Y., Sumiyoshi, A., Asada, Y. (2004). Increased vascular wall thrombogenicity combined with reduced blood flow promotes occlusive thrombus formation in rabbit femoral artery. *Arterioscler Thromb Vasc Biol*, Vol.24, No.12,

Detection of von Willebrand factor and tissue factor in platelets-fibrin rich coronary

Hattori, K., Asada, Y. (2006b). Factor XI contributes to thrombus propagation on injured neointima of the rabbit iliac artery. *J Thromb Haemost*, Vol.4, No.7, pp.1496-

Sumi, T., Imamura, T., Shima, M., Kitamura, K., Asada, Y. (2009). Thrombin generation by intimal tissue factor contributes to thrombus formation on macrophage-rich neointima but not normal intima of hyperlipidemic rabbits.

plasminogen deficiency on the contribution of polymorphonuclear leucocytes to fibrin/ogenolysis: studies in plasminogen knock-out mice. *Thromb Haemost*. Vol.88, Coronary heart disease remains the single biggest killer in the United Kingdom, accounting for around one in five deaths in men and one in six deaths in women (1). In 2003 the total annual cost of coronary heart disease in the United Kingdom was around £3.5 billion (£60 per capita), with the cost of inpatient care accounting for around 79% of these costs (2). Approximately 3% of patients who attend the ED have chest pain that the treating physician suspects may be cardiac in origin (3). 74-88% of these patients are admitted to hospital, making up one in five of all medical admissions (3-5). Ultimately only a quarter of these patients will be diagnosed with an acute coronary syndrome (ACS), which implies that a very cautious approach to the problem has been adopted. Despite this fact, up to 6% of the patients with chest pain who are discharged from the ED actually have myocardial damage that has prognostic significance (6). These patients are up to three times as likely to die as similar patients who were admitted to hospital (7).

#### **2. The pathophysiology of coronary heart disease**

Over the past century tremendous advances have been made in our understanding of coronary heart disease and its pathophysiological evolution. In 1910 a Russian physician first described the clinical presentation of acute myocardial infarction (AMI) (8). Two years later, an association was drawn between AMI and acute thrombotic coronary occlusion (9). By 1913 it had been hypothesised that atherosclerosis developed as a result of gradual lipid accumulation within the arterial wall (10). The advent of coronary revascularisation procedures in the latter half of the 20th century allowed the observation that restoring blood flow beyond significant coronary stenotic lesions often led to alleviation of anginal symptoms. This helped to propagate the widespread belief that the greater the coronary

Biomarkers of Atherosclerosis and Acute Coronary Syndromes – A Clinical Perspective 23

than 50% stenosis on angiography (14). The explanation for these phenomena resides in the understanding that there are, in basic terms, two kinds of coronary atheromatous plaques: those which are stable and those which are unstable. While stable plaques may be responsible for stable anginal symptoms (such as exertional chest pain relieved by rest), they are less likely to rupture and cause the clinical manifestations that we recognise as ACS. Meanwhile, unstable plaques are vulnerable and highly likely to rupture with the ensuing risk of developing ACS. There are notable pathological differences between these two types of plaque. Stable plaques are more likely to cause coronary stenosis, presenting a fixed obstruction to blood flow and therefore often being responsible for causing stable anginal symptoms such as exertional chest pain. Unstable plaques, however, may cause little arterial stenosis, thus explaining the observation that the majority of AMIs are caused by lesions that are only mildly stenotic. What is more, they may cause little in the way of clinical symptoms until they rupture, leading to the often dramatic and frequently fatal clinical

Pathologically, stable plaques are likely to be more enriched with smooth muscle cells than those which are prone to rupture. They are likely to contain a dense fibrous cap consisting of collagen and extracellular matrix, which give the plaque tensile strength. On the contrary, plaques that are vulnerable to rupture are likely to have thin, friable fibrous caps, contain abundant inflammatory cells including macrophages and they are rich in extracellular lipid, often with a lipid core containing pro-inflammatory oxygen free radicals, pro-thrombotic

material such as tissue factor and necrotic cellular debris (Figure 2) (15;16).

Fig. 2. Stability and instability: The two varieties of coronary atheroma.

manifestations of ACS.

stenosis the greater the risk of a clinically significant event such as AMI or unstable angina pectoris. This axiom underpins much of modern practice in cardiology. Figure 1 illustrates the traditional model of the evolution of coronary atheroma (11).

Fig. 1. Drawings of cross sections of the most proximal parts of six left anterior descending coronary arteries, illustrated to depict the traditional concept of the evolution of coronary atheroma. From Stary *et al*, 1995 (11).

In recent years this whole concept has been challenged. Far from a bland disease of cholesterol storage characterised by a passive accumulation of lipid within the vessel wall, a growing body of research and a progression in current thinking suggest that coronary atherosclerosis is in fact a dynamic inflammatory disease, dependent upon complex interactions between the immune, coagulation and humoral systems. It would seem that progression of coronary atherosclerosis is not so much a gradual process as a stepwise one, often characterised by swift and sudden increases in plaque size. Atherosclerotic plaque rupture or endothelial damage may lead to haemorrhage into the plaque or thrombus formation with subsequent organisation. This leads to rapid expansion of the plaque (12). Further, the severity of coronary stenosis on angiography does not predict the development of subsequent AMI (13). Indeed, two thirds of AMIs are provoked by plaques that cause less

stenosis the greater the risk of a clinically significant event such as AMI or unstable angina pectoris. This axiom underpins much of modern practice in cardiology. Figure 1 illustrates

Fig. 1. Drawings of cross sections of the most proximal parts of six left anterior descending coronary arteries, illustrated to depict the traditional concept of the evolution of coronary

In recent years this whole concept has been challenged. Far from a bland disease of cholesterol storage characterised by a passive accumulation of lipid within the vessel wall, a growing body of research and a progression in current thinking suggest that coronary atherosclerosis is in fact a dynamic inflammatory disease, dependent upon complex interactions between the immune, coagulation and humoral systems. It would seem that progression of coronary atherosclerosis is not so much a gradual process as a stepwise one, often characterised by swift and sudden increases in plaque size. Atherosclerotic plaque rupture or endothelial damage may lead to haemorrhage into the plaque or thrombus formation with subsequent organisation. This leads to rapid expansion of the plaque (12). Further, the severity of coronary stenosis on angiography does not predict the development of subsequent AMI (13). Indeed, two thirds of AMIs are provoked by plaques that cause less

atheroma. From Stary *et al*, 1995 (11).

the traditional model of the evolution of coronary atheroma (11).

than 50% stenosis on angiography (14). The explanation for these phenomena resides in the understanding that there are, in basic terms, two kinds of coronary atheromatous plaques: those which are stable and those which are unstable. While stable plaques may be responsible for stable anginal symptoms (such as exertional chest pain relieved by rest), they are less likely to rupture and cause the clinical manifestations that we recognise as ACS. Meanwhile, unstable plaques are vulnerable and highly likely to rupture with the ensuing risk of developing ACS. There are notable pathological differences between these two types of plaque. Stable plaques are more likely to cause coronary stenosis, presenting a fixed obstruction to blood flow and therefore often being responsible for causing stable anginal symptoms such as exertional chest pain. Unstable plaques, however, may cause little arterial stenosis, thus explaining the observation that the majority of AMIs are caused by lesions that are only mildly stenotic. What is more, they may cause little in the way of clinical symptoms until they rupture, leading to the often dramatic and frequently fatal clinical manifestations of ACS.

Pathologically, stable plaques are likely to be more enriched with smooth muscle cells than those which are prone to rupture. They are likely to contain a dense fibrous cap consisting of collagen and extracellular matrix, which give the plaque tensile strength. On the contrary, plaques that are vulnerable to rupture are likely to have thin, friable fibrous caps, contain abundant inflammatory cells including macrophages and they are rich in extracellular lipid, often with a lipid core containing pro-inflammatory oxygen free radicals, pro-thrombotic material such as tissue factor and necrotic cellular debris (Figure 2) (15;16).

Fig. 2. Stability and instability: The two varieties of coronary atheroma.

Biomarkers of Atherosclerosis and Acute Coronary Syndromes – A Clinical Perspective 25

surface, changing them from a low-affinity to a high-affinity state (35). These activated integrins may then bind firmly to two further adhesion molecules that are expressed by activated endothelium: intercellular adhesion molecule-1 (ICAM-1) and vascular cellular adhesion molecule-1 (VCAM-1). This strong adhesion brings the rolling leukocytes to a halt.

Fig. 3. The pivotal anti-atherogenic role of nitric oxide on a molecular level. Abbreviations: LDL, low density lipoprotein; CRP, C-reactive protein; CV, cardiovascular; TNF- α, tumour necrosis factor α; oxLDL, oxidised LDL; ROS, reactive oxygen species; SMC, smooth muscle cell; NO, nitric oxide; LOX-1, oxidised LDL receptor-1; eNOS, endothelial nitric oxide synthase.

In the presence of further activating signals from within the arterial intima, the leukocytes may subsequently undergo a cytoskeletal change, enabling them to squeeze between the tight cell-cell junctions of the endothelium via interactions with the PECAM-1 (CD31) receptor. Again, under normal circumstances PECAM-1 binds endothelial cells strongly together, preventing leukocyte migration into the arterial intima. However, substances such as thrombin and histamine that are expressed during periods of localised inflammation loosen this binding, promoting cellular retraction and vascular permeability. This enables glycoproteins on the cell surface of the activated leukocytes to bind to PECAM-1, allowing them to pass through the endothelial layer into the arterial intima in a process labelled diapedesis (29;36). Within the arterial intima, activated leukocytes will then migrate towards chemokines (including monocyte chemotactic protein, MCP-1) expressed within foci of

inflammation where they participate in inflammatory processes (Figure 4) (29;37;38).

process will continue indefinitely (27).

Circulating low-density lipoprotein (LDL) cholesterol can also bind to endothelial receptors and is subsequently modified or oxidised by the endothelial cells. Within the arterial intima, oxidised LDL acts as a strong stimulus for further migration and localisation of inflammatory cells (16). Following migration, monocytes mature into macrophages and, via scavenger receptors, ingest oxidised LDL to become foam cells (24). Together with T lymphocytes and activated endothelial cells, these cells secrete an array of pro-inflammatory cytokines, forming a positive feedback loop which enhances the inflammatory reaction within the arterial intima. If the inflammatory stimuli are not removed or neutralised, this

While an unstable plaque often causes little or no arterial stenosis, it does not follow that unstable plaques are necessarily smaller in size than their stable counterparts. It has become apparent that the arterial wall is not a static and rigid structure but rather is capable of socalled 'outward remodelling', increasing its external diameter without narrowing the lumen. An unstable plaque may therefore be comparatively large in size while causing little arterial stenosis (15;17-21).

#### **2.1 The pathophysiological evolution of an acute coronary syndrome**

In order to fully comprehend the limitations to current diagnostic strategies and to attempt the development of effective new strategies for the diagnosis of ACS it is important to have a reasonable understanding of the initiation and progression of the disease from a molecular level upwards. If we can recognise the precise disease processes we are trying to accurately identify, we stand a much better chance of understanding our current problems and of developing effective novel diagnostic strategies that can be applied in clinical practice.

Coronary atherosclerosis is an inflammatory disease whose origins can only be adequately understood through a sound appreciation of vascular biology (17;22-27). We no longer regard the blood vessel wall as simply an inert tubular conduit for flowing blood but rather as a complex living structure that plays a pivotal role in maintaining vascular homeostasis and integrity. Of particular importance in this regard is the endothelium, a monolayer of cells forming a barrier between flowing blood and tissue. The human endothelium has a total surface area of approximately 1000m2 (16) and constitutes around 16% of the myocardium (28). It plays a key role in modulating vascular tone, responding to neural, humoral and mechanical stimuli by synthesising and releasing vasoactive substances. By sending activating signals to circulating inflammatory cells, the endothelium orchestrates complex fluid and cellular movements designed to neutralise and eliminate foreign elements. While these mechanisms are usually beneficial, under certain circumstances these processes can become extreme and counter-productive (29;30).

The endothelium is an active player in the protection against and development of coronary disease, being the guardian of the integrity of the vessel wall. A functional endothelium produces a healthy balance of vascular constricting and relaxing factors. In this respect, the role of endothelium-derived nitric oxide is particularly crucial. In addition to its important vasodilator effect, nitric oxide protects against vascular injury, inflammation and thrombosis. It inhibits leukocyte adhesion to the endothelium, smooth muscle cell proliferation and migration and platelet aggregation (31-34). In the presence of traditional cardiac risk factors such as hyperlipidaemia, smoking, diabetes and hypertension and where there is local or systemic inflammation or reduced shear stress (such as at the branch points of coronary arteries), nitric oxide production is inhibited and its degradation enhanced (Figure 3) (23). Under these conditions, many of the protective inhibitory effects of nitric oxide are lost. Cell adhesion molecules (CAMs) including P-selectin and E-selectin are expressed by the endothelium, where they mediate leukocyte binding. P-selectin and Eselectin bind to carbohydrates that are constitutively expressed on the surface of circulating leukocytes, causing the leukocytes to bind loosely to the endothelial surface and to literally roll across it, scanning the endothelium for further activating signals. Chemoattractant cytokines or chemokines that are also expressed by activated endothelial cells can then induce a conformational change in integrin molecules expressed at the leukocyte cell

While an unstable plaque often causes little or no arterial stenosis, it does not follow that unstable plaques are necessarily smaller in size than their stable counterparts. It has become apparent that the arterial wall is not a static and rigid structure but rather is capable of socalled 'outward remodelling', increasing its external diameter without narrowing the lumen. An unstable plaque may therefore be comparatively large in size while causing little arterial

In order to fully comprehend the limitations to current diagnostic strategies and to attempt the development of effective new strategies for the diagnosis of ACS it is important to have a reasonable understanding of the initiation and progression of the disease from a molecular level upwards. If we can recognise the precise disease processes we are trying to accurately identify, we stand a much better chance of understanding our current problems and of developing effective novel diagnostic strategies that can be applied in clinical practice.

Coronary atherosclerosis is an inflammatory disease whose origins can only be adequately understood through a sound appreciation of vascular biology (17;22-27). We no longer regard the blood vessel wall as simply an inert tubular conduit for flowing blood but rather as a complex living structure that plays a pivotal role in maintaining vascular homeostasis and integrity. Of particular importance in this regard is the endothelium, a monolayer of cells forming a barrier between flowing blood and tissue. The human endothelium has a total surface area of approximately 1000m2 (16) and constitutes around 16% of the myocardium (28). It plays a key role in modulating vascular tone, responding to neural, humoral and mechanical stimuli by synthesising and releasing vasoactive substances. By sending activating signals to circulating inflammatory cells, the endothelium orchestrates complex fluid and cellular movements designed to neutralise and eliminate foreign elements. While these mechanisms are usually beneficial, under certain circumstances these

The endothelium is an active player in the protection against and development of coronary disease, being the guardian of the integrity of the vessel wall. A functional endothelium produces a healthy balance of vascular constricting and relaxing factors. In this respect, the role of endothelium-derived nitric oxide is particularly crucial. In addition to its important vasodilator effect, nitric oxide protects against vascular injury, inflammation and thrombosis. It inhibits leukocyte adhesion to the endothelium, smooth muscle cell proliferation and migration and platelet aggregation (31-34). In the presence of traditional cardiac risk factors such as hyperlipidaemia, smoking, diabetes and hypertension and where there is local or systemic inflammation or reduced shear stress (such as at the branch points of coronary arteries), nitric oxide production is inhibited and its degradation enhanced (Figure 3) (23). Under these conditions, many of the protective inhibitory effects of nitric oxide are lost. Cell adhesion molecules (CAMs) including P-selectin and E-selectin are expressed by the endothelium, where they mediate leukocyte binding. P-selectin and Eselectin bind to carbohydrates that are constitutively expressed on the surface of circulating leukocytes, causing the leukocytes to bind loosely to the endothelial surface and to literally roll across it, scanning the endothelium for further activating signals. Chemoattractant cytokines or chemokines that are also expressed by activated endothelial cells can then induce a conformational change in integrin molecules expressed at the leukocyte cell

**2.1 The pathophysiological evolution of an acute coronary syndrome** 

processes can become extreme and counter-productive (29;30).

stenosis (15;17-21).

surface, changing them from a low-affinity to a high-affinity state (35). These activated integrins may then bind firmly to two further adhesion molecules that are expressed by activated endothelium: intercellular adhesion molecule-1 (ICAM-1) and vascular cellular adhesion molecule-1 (VCAM-1). This strong adhesion brings the rolling leukocytes to a halt.

Fig. 3. The pivotal anti-atherogenic role of nitric oxide on a molecular level. Abbreviations: LDL, low density lipoprotein; CRP, C-reactive protein; CV, cardiovascular; TNF- α, tumour necrosis factor α; oxLDL, oxidised LDL; ROS, reactive oxygen species; SMC, smooth muscle cell; NO, nitric oxide; LOX-1, oxidised LDL receptor-1; eNOS, endothelial nitric oxide synthase.

In the presence of further activating signals from within the arterial intima, the leukocytes may subsequently undergo a cytoskeletal change, enabling them to squeeze between the tight cell-cell junctions of the endothelium via interactions with the PECAM-1 (CD31) receptor. Again, under normal circumstances PECAM-1 binds endothelial cells strongly together, preventing leukocyte migration into the arterial intima. However, substances such as thrombin and histamine that are expressed during periods of localised inflammation loosen this binding, promoting cellular retraction and vascular permeability. This enables glycoproteins on the cell surface of the activated leukocytes to bind to PECAM-1, allowing them to pass through the endothelial layer into the arterial intima in a process labelled diapedesis (29;36). Within the arterial intima, activated leukocytes will then migrate towards chemokines (including monocyte chemotactic protein, MCP-1) expressed within foci of inflammation where they participate in inflammatory processes (Figure 4) (29;37;38).

Circulating low-density lipoprotein (LDL) cholesterol can also bind to endothelial receptors and is subsequently modified or oxidised by the endothelial cells. Within the arterial intima, oxidised LDL acts as a strong stimulus for further migration and localisation of inflammatory cells (16). Following migration, monocytes mature into macrophages and, via scavenger receptors, ingest oxidised LDL to become foam cells (24). Together with T lymphocytes and activated endothelial cells, these cells secrete an array of pro-inflammatory cytokines, forming a positive feedback loop which enhances the inflammatory reaction within the arterial intima. If the inflammatory stimuli are not removed or neutralised, this process will continue indefinitely (27).

Biomarkers of Atherosclerosis and Acute Coronary Syndromes – A Clinical Perspective 27

Fig. 5. Progression to organised atheroma. Following migration into the arterial intima, monocytes mature into tissue macrophages and, via receptors including LOX-1 and CD36, take up extracellular lipid including oxidised LDL cholesterol (oxLDL) to become foam cells. Together with T helper cells (Th), foam cells secrete an array of pro-inflammatory cytokines (interleukin-1 (IL-1), interferon- (IFN-), interleukin-6 (IL-6), monocyte colony stimulating factor (MCSF), tumour necrosis factor- α (TNF- α)), which lead to migration of vascular smooth muscle cells from the arterial media. Following migration, these smooth muscle cells secrete a dense extracellular matrix (ECM) and collagen fibres, which form a tough fibrous cap.

Fig. 6. CD40/L interactions within coronary atheroma. CD40/40L interactions lead to enhanced inflammation, impaired capacity for endothelial repair and regeneration, secretion of pro-coagulant tissue factor, MMPs and upregulation of myeloperoxidase (MPO) secretion.

MPO produces reactive oxygen species (ROS) and oxidised LDL (oxLDL), enhancing upregulation and leading to degradation of the fibrous cap by activating the precursors of MMPs (pro-MMPS) and inhibiting tissue inhibitors of metalloproteinases (TIMPs).

Fig. 4. The multistep model of leukocyte migration. 1. Leukocytes bind to selectins expressed by activated endothelium, causing them to roll, scanning the endothelium for activating signals. 2. In the presence of activating signals, integrins on the cell surface of the leukocyte undergo a structural change and can bind firmly to ICAM-1 and VCAM-1. 3. Leukocytes can then migrate through to the arterial intima by binding to PECAM-1 at the cell junction. 4. Leukocytes migrate along a chemokine gradient (illustrated as MCP-1), which helps to localise the inflammatory response within the intima. Cell adhesion molecules are subsequently released into the circulation in soluble form.

In addition to enhancing inflammation, cytokines stimulate differentiation and migration of smooth muscle cells from the arterial media into the intima (39). While this may ultimately lead to mechanical expansion of the plaque, smooth muscle cells actually play a vital role in maintaining the stability of the atherosclerotic plaque by secreting a dense, fibrous extracellular matrix and substances that prevent its degradation (tissue inhibitors of metalloproteinases, TIMPs) (16) (Figure 5).

Enhanced inflammatory activity within the plaque ultimately renders the plaque vulnerable to rupture by destabilising this fibrous cap. Activated macrophages and neutrophils within atheroma secrete myeloperoxidase (MPO), an enzyme which enhances consumption of nitric oxide, generating highly reactive and pro-inflammatory oxygen free radicals and oxidised LDL, thus perpetuating and enhancing both endothelial dysfunction and the formation of foam cells (40;41). MPO inactivates TIMPs, paving the way for degradation of the fibrous cap. Further, MPO activates matrix metalloproteinases (MMPs), enzymes responsible for actively degrading the fibrous cap (42) (Figure 6) (43).

Atheroma is rendered even more vulnerable to rupture by interactions between the CD40 receptor (which is expressed by endothelial cells, monocytes and B lymphocytes) and its ligand CD40L, which is expressed by activated T helper cells, smooth muscle cells, macrophages, basophils and activated platelets (44;45). This interaction leads to the formation of another positive feedback loop that enhances endothelial dysfunction and inflammation within the plaque and stimulates the release of both the procoagulant tissue factor and MMPs into the lipid core (46-50). The latter further enhance degradation of the fibrous cap (Figure 6).

Fig. 4. The multistep model of leukocyte migration. 1. Leukocytes bind to selectins expressed by activated endothelium, causing them to roll, scanning the endothelium for activating signals. 2. In the presence of activating signals, integrins on the cell surface of the leukocyte undergo a structural change and can bind firmly to ICAM-1 and VCAM-1. 3. Leukocytes can then migrate through to the arterial intima by binding to PECAM-1 at the cell junction. 4. Leukocytes migrate along a chemokine gradient (illustrated as MCP-1), which helps to localise the inflammatory response within the intima. Cell adhesion

In addition to enhancing inflammation, cytokines stimulate differentiation and migration of smooth muscle cells from the arterial media into the intima (39). While this may ultimately lead to mechanical expansion of the plaque, smooth muscle cells actually play a vital role in maintaining the stability of the atherosclerotic plaque by secreting a dense, fibrous extracellular matrix and substances that prevent its degradation (tissue inhibitors of

Enhanced inflammatory activity within the plaque ultimately renders the plaque vulnerable to rupture by destabilising this fibrous cap. Activated macrophages and neutrophils within atheroma secrete myeloperoxidase (MPO), an enzyme which enhances consumption of nitric oxide, generating highly reactive and pro-inflammatory oxygen free radicals and oxidised LDL, thus perpetuating and enhancing both endothelial dysfunction and the formation of foam cells (40;41). MPO inactivates TIMPs, paving the way for degradation of the fibrous cap. Further, MPO activates matrix metalloproteinases (MMPs), enzymes

Atheroma is rendered even more vulnerable to rupture by interactions between the CD40 receptor (which is expressed by endothelial cells, monocytes and B lymphocytes) and its ligand CD40L, which is expressed by activated T helper cells, smooth muscle cells, macrophages, basophils and activated platelets (44;45). This interaction leads to the formation of another positive feedback loop that enhances endothelial dysfunction and inflammation within the plaque and stimulates the release of both the procoagulant tissue factor and MMPs into the lipid core (46-50). The latter further enhance degradation of the fibrous cap (Figure 6).

molecules are subsequently released into the circulation in soluble form.

responsible for actively degrading the fibrous cap (42) (Figure 6) (43).

metalloproteinases, TIMPs) (16) (Figure 5).

Fig. 5. Progression to organised atheroma. Following migration into the arterial intima, monocytes mature into tissue macrophages and, via receptors including LOX-1 and CD36, take up extracellular lipid including oxidised LDL cholesterol (oxLDL) to become foam cells. Together with T helper cells (Th), foam cells secrete an array of pro-inflammatory cytokines (interleukin-1 (IL-1), interferon- (IFN-), interleukin-6 (IL-6), monocyte colony stimulating factor (MCSF), tumour necrosis factor- α (TNF- α)), which lead to migration of vascular smooth muscle cells from the arterial media. Following migration, these smooth muscle cells secrete a dense extracellular matrix (ECM) and collagen fibres, which form a tough fibrous cap.

Fig. 6. CD40/L interactions within coronary atheroma. CD40/40L interactions lead to enhanced inflammation, impaired capacity for endothelial repair and regeneration, secretion of pro-coagulant tissue factor, MMPs and upregulation of myeloperoxidase (MPO) secretion. MPO produces reactive oxygen species (ROS) and oxidised LDL (oxLDL), enhancing upregulation and leading to degradation of the fibrous cap by activating the precursors of MMPs (pro-MMPS) and inhibiting tissue inhibitors of metalloproteinases (TIMPs).

Biomarkers of Atherosclerosis and Acute Coronary Syndromes – A Clinical Perspective 29

Fig. 8. Plaque rupture. There is haemorrhage into the plaque, causing rapid expansion and, as the contents of the lipid core are highly prothrombotic, thrombus formation ensues.

Circulating blood is exposed to the prothrombotic lipid core. Tissue factor activates factor VIIa, which ultimately leads to the cleavage of thrombin from prothrombin and further activation of the coagulation cascade (61). Several substances from within the plaque, including thrombin, CD40L and P-selectin, activate circulating platelets by inducing a conformational change in the glycoprotein receptors and enabling cross-linking or adhesion via fibrinogen and other adhesive ligands. During this process, activated platelets themselves express P-selectin and CD40L, which appear to be necessary for the formation of a stable arterial thrombus (62-64). Both P-selectin and CD40L are later enzymatically cleaved

When plaque rupture is small, intraplaque haemorrhage may lead to rapid expansion with platelet activation and adhesion but the thrombus does not extend into the arterial lumen (12). The thrombus subsequently undergoes organisation, the endothelial layer regenerates and the episode is clinically silent. Among patients with coronary atheroma who died of non-vascular causes such as motor vehicle accidents and subsequently underwent postmortem examination, up to 8% were noted to have had a recent plaque disruption with intra-plaque thrombi (67). Indeed, in pathological studies of subjects who died of ischaemic heart disease each patient had on average two to three plaque disruptions, although in each case one culprit thrombus was identified that was apparently responsible for causing death (68-70). In the presence of a large plaque rupture or, indeed, when the rupture is not large but the patient is in a pro-thrombotic state (for example during periods of stress or systemic

from the platelet surface and released into the circulation in soluble form (65;66).

Abbreviations: RBC, red blood cell.

Where there is abundant intimal inflammation, pro-inflammatory cytokines may prime cells within the plaque for apoptotic death upon engagement with activated T lymphocytes (22;51). Stimulated apoptosis of smooth muscle cells impedes maintenance of the fibrous cap, favouring its breakdown. Apoptosis of endothelial cells may lead to erosions of the endothelial layer, enabling circulating blood to come into contact with the pro-thrombotic contents of the plaque (Figure 7). Circulating platelets are activated upon contact, binding to the arterial wall and to each other (52). When these areas of endothelial erosion are small, this platelet aggregation occurs only on a microscopic level and is clinically insignificant, serving only to stimulate endothelial regeneration and smooth muscle growth. The new endothelial cells may be dysfunctional, however, predisposing to vasoconstriction (15).

Fig. 7. Positive feedback loops within unstable coronary atheroma and processes leading to endothelial erosion.

In the presence of larger endothelial erosions there may be a rapid increase in intimal inflammation (53) and sufficient platelet aggregation and subsequent fibrin deposition to produce a large thrombus with symptomatic luminal obstruction (15;17;54;55). In itself, this process accounts for approximately 25% of all major thrombi that lead to acute coronary syndromes (56) and may have even greater importance in women and young people (57). Of even greater importance, however, is the high tensile stress that a vulnerable plaque must withstand. As the lipid core is soft and deformable, it cannot bear circumferential stress. This stress is therefore borne by the fibrous cap, made of tough collagen and extracellular matrix. Depending upon the shape of the plaque and its position within the artery, the fibrous cap must withstand focal concentrations of load up to seven or eight times normal systolic wall stress (58;59). This is particularly significant in unstable plaques where the fibrous cap may be thin and friable.

Ultimately, this may lead to sudden rupture of the plaque with endothelial disruption, causing haemorrhage of circulating blood into the core of the plaque (Figure 8). This may be particularly likely to occur following a trigger such as unaccustomed physical activity or emotional stress, which leads to a rapid increase in systolic blood pressure and thus increased circumferential stress on an already vulnerable plaque (60).

Where there is abundant intimal inflammation, pro-inflammatory cytokines may prime cells within the plaque for apoptotic death upon engagement with activated T lymphocytes (22;51). Stimulated apoptosis of smooth muscle cells impedes maintenance of the fibrous cap, favouring its breakdown. Apoptosis of endothelial cells may lead to erosions of the endothelial layer, enabling circulating blood to come into contact with the pro-thrombotic contents of the plaque (Figure 7). Circulating platelets are activated upon contact, binding to the arterial wall and to each other (52). When these areas of endothelial erosion are small, this platelet aggregation occurs only on a microscopic level and is clinically insignificant, serving only to stimulate endothelial regeneration and smooth muscle growth. The new endothelial cells may be dysfunctional, however, predisposing to vasoconstriction (15).

Fig. 7. Positive feedback loops within unstable coronary atheroma and processes leading to

In the presence of larger endothelial erosions there may be a rapid increase in intimal inflammation (53) and sufficient platelet aggregation and subsequent fibrin deposition to produce a large thrombus with symptomatic luminal obstruction (15;17;54;55). In itself, this process accounts for approximately 25% of all major thrombi that lead to acute coronary syndromes (56) and may have even greater importance in women and young people (57). Of even greater importance, however, is the high tensile stress that a vulnerable plaque must withstand. As the lipid core is soft and deformable, it cannot bear circumferential stress. This stress is therefore borne by the fibrous cap, made of tough collagen and extracellular matrix. Depending upon the shape of the plaque and its position within the artery, the fibrous cap must withstand focal concentrations of load up to seven or eight times normal systolic wall stress (58;59). This is particularly significant in unstable plaques where the

Ultimately, this may lead to sudden rupture of the plaque with endothelial disruption, causing haemorrhage of circulating blood into the core of the plaque (Figure 8). This may be particularly likely to occur following a trigger such as unaccustomed physical activity or emotional stress, which leads to a rapid increase in systolic blood pressure and thus

increased circumferential stress on an already vulnerable plaque (60).

endothelial erosion.

fibrous cap may be thin and friable.

Fig. 8. Plaque rupture. There is haemorrhage into the plaque, causing rapid expansion and, as the contents of the lipid core are highly prothrombotic, thrombus formation ensues. Abbreviations: RBC, red blood cell.

Circulating blood is exposed to the prothrombotic lipid core. Tissue factor activates factor VIIa, which ultimately leads to the cleavage of thrombin from prothrombin and further activation of the coagulation cascade (61). Several substances from within the plaque, including thrombin, CD40L and P-selectin, activate circulating platelets by inducing a conformational change in the glycoprotein receptors and enabling cross-linking or adhesion via fibrinogen and other adhesive ligands. During this process, activated platelets themselves express P-selectin and CD40L, which appear to be necessary for the formation of a stable arterial thrombus (62-64). Both P-selectin and CD40L are later enzymatically cleaved from the platelet surface and released into the circulation in soluble form (65;66).

When plaque rupture is small, intraplaque haemorrhage may lead to rapid expansion with platelet activation and adhesion but the thrombus does not extend into the arterial lumen (12). The thrombus subsequently undergoes organisation, the endothelial layer regenerates and the episode is clinically silent. Among patients with coronary atheroma who died of non-vascular causes such as motor vehicle accidents and subsequently underwent postmortem examination, up to 8% were noted to have had a recent plaque disruption with intra-plaque thrombi (67). Indeed, in pathological studies of subjects who died of ischaemic heart disease each patient had on average two to three plaque disruptions, although in each case one culprit thrombus was identified that was apparently responsible for causing death (68-70). In the presence of a large plaque rupture or, indeed, when the rupture is not large but the patient is in a pro-thrombotic state (for example during periods of stress or systemic

Biomarkers of Atherosclerosis and Acute Coronary Syndromes – A Clinical Perspective 31

(73,74), with expression being particularly marked in patients with unstable angina (75). Pselectin is also expressed by activated platelets and has been used as a marker of platelet activation (76). Several investigators have demonstrated significantly raised soluble Pselectin levels in patients with AMI (77-84), unstable angina (85-88) and cohorts of patients with any ACS (89,90). However conflicting results have also been reported, with two reports that P-selectin does not help to predict adverse events in patients with ACS and two studies that did not detect any elevation of plasma P-selectin levels in patients with ACS compared

Five studies have investigated the utility of P-selectin for diagnosis of ACS in the ED population. One small study of 44 patients found no different in plasma soluble P-selectin levels between patients diagnosed with ACS and non-cardiac pain (94). Although the same group also reported that P-selectin was not an independent predictor for a diagnosis of ACS (95), another group reported that P-selectin was an independent predictor for the occurrence of serious cardiac events within three months of presentation to the ED with chest pain (96). Other groups have reported sensitivities of 35 and 55.8% and NPVs of 53 and 71% for the diagnosis of ACS (97-98). The data suggests that the use of soluble P-selectin as a sole ruleout strategy for ACS in the ED is likely to lead to an unacceptably high false negative rate. Our own data however in 713 patients presenting to the ED with suspected cardiac chest pain demonstrated P-selectin had early diagnostic value for AMI and prognostic

E-selectin has also been investigated in this regard. Plasma levels of E-selectin have been shown to correlate with the severity of coronary atherosclerosis (87,100). A number of studies have reported elevated plasma E-selectin levels in patients with AMI (101-108). Eselectin elevations have also been reported in patients with unstable angina (85,109). Other studies have reported raised E-selectin in all ACS (80,82,110-112). Plasma E-selectin levels in patients with AMI may be higher among patients who experienced a prodrome of unstable angina (105). Raised E-selectin levels have also been reported following attacks of variant angina (113), although there may be no difference in E-selectin levels during episodes of stable angina (114). A reduction in plasma E-selectin levels has been described in patients with AMI following successful reperfusion (101,108). Further, plasma E-selectin levels may be useful for predicting the risk of death among patients with AMI (103). However not all reports have been consistent. Three groups have failed to find elevated E-selectin levels in ACS (115-117). There is no clear explanation for the discrepancy in the results, although one group measured E-selectin in serum rather than plasma, which may have introduced an important bias. Only one study has investigated E-selectin levels in the ED population with undifferentiated chest pain (118). This study failed to demonstrate a difference in E-selectin levels between patients with AMI, unstable angina and controls. However, the study had significant limitations, including small numbers, suboptimal gold standards and no clinical follow-up. The study was not designed to appraise the performance of E-selectin as a diagnostic test for use in the ED. The available research suggests that E-selectin has promising characteristics for use as a marker of ACS. A large prospective observational cohort study is necessary to evaluate its performance as a diagnostic test. Incorporation into

significance independent of troponin T and ECG findings (99)

with controls (91-93).

**3.1.2 E-selectin** 

infection), platelet activation and aggregation may extend into the arterial lumen. Activation of the coagulation cascade leads to fibrin deposition, which increases the size of the thrombus. Again, thrombus formation may be arrested without causing significant luminal stenosis. However, as the thrombus is exposed to flowing blood distal emboli may occur, potentially causing myocardial necrosis on a microscopic level and recognisable symptoms. As activated platelets aggregate to form a platelet-rich arterial thrombus, they release mediators such as serotonin and thromboxane A2, which cause vasoconstriction. This may lead to localised coronary arterial spasm, which even in the absence of an obstructive coronary thrombus, may lead to transmural myocardial ischaemia and a clinically apparent ACS (71). When thrombus formation continues unchecked, total arterial occlusion may occur. If such occlusion occurs suddenly in a previously uncompromised artery without a well-developed collateral circulation, significant downstream myocardial necrosis will occur with the clinically recognisable signs of acute myocardial infarction (AMI). The cell membranes of the necrosed myocytes are breached and their intracellular constituents are washed out into the circulation. These constituents include myoglobin, creatine kinase, the cardiac troponins and human fatty acid binding protein.

#### **3. Biomarkers of unstable coronary disease**

Current diagnostic strategies incorporate biomarkers of myocardial necrosis, the end-point in the pathophysiological evolution of ACS. The measurement of cardiac troponins in the bloodstream has revolutionised the diagnosis of AMI in this regard, enabling the detection of microscopic amounts of myocardial necrosis that could not have previously been identified (72). As described in detail earlier in this chapter, however, a whole host of pathophysiological processes have occurred before myocardial necrosis, none of which are detectable using current diagnostic technology. In fact myocardial necrosis is merely a surrogate marker of the disease process, which occurs within the coronary artery and not the cardiac myocyte. As it is possible to use biomarkers to detect myocardial necrosis with high sensitivity and specificity this raises the additional possibility that other biomarkers may be able to detect evidence of the disease process itself within the coronary arteries. A number of novel biomarkers have been investigated in this regard in recent years.

#### **3.1 Soluble cell adhesion molecules**

Cell adhesion molecules (CAMs) mediate the interactions between the endothelium and blood cells, enabling the localised inflammatory response that is essential for the initiation and propagation of coronary atherosclerosis. Their upregulation enhances this inflammatory response, which ultimately renders the atherosclerotic plaque vulnerable to rupture. Following their expression, CAMs are shed from the cell surface. As these soluble CAMs are detectable in peripheral blood, they are promising candidates for use as early markers of vascular activation (37). CAMs that have attracted interest as potential biomarkers of ACS include the molecules P-selectin, E-selectin, ICAM-1 and VCAM-1.

#### **3.1.1 P-selectin**

P-selectin mediates the interaction of platelets and endothelial cells with neutrophils and monocytes (65). It is expressed by endothelial cells in atherosclerotic, but not normal, vessels (73,74), with expression being particularly marked in patients with unstable angina (75). Pselectin is also expressed by activated platelets and has been used as a marker of platelet activation (76). Several investigators have demonstrated significantly raised soluble Pselectin levels in patients with AMI (77-84), unstable angina (85-88) and cohorts of patients with any ACS (89,90). However conflicting results have also been reported, with two reports that P-selectin does not help to predict adverse events in patients with ACS and two studies that did not detect any elevation of plasma P-selectin levels in patients with ACS compared with controls (91-93).

Five studies have investigated the utility of P-selectin for diagnosis of ACS in the ED population. One small study of 44 patients found no different in plasma soluble P-selectin levels between patients diagnosed with ACS and non-cardiac pain (94). Although the same group also reported that P-selectin was not an independent predictor for a diagnosis of ACS (95), another group reported that P-selectin was an independent predictor for the occurrence of serious cardiac events within three months of presentation to the ED with chest pain (96). Other groups have reported sensitivities of 35 and 55.8% and NPVs of 53 and 71% for the diagnosis of ACS (97-98). The data suggests that the use of soluble P-selectin as a sole ruleout strategy for ACS in the ED is likely to lead to an unacceptably high false negative rate. Our own data however in 713 patients presenting to the ED with suspected cardiac chest pain demonstrated P-selectin had early diagnostic value for AMI and prognostic significance independent of troponin T and ECG findings (99)

#### **3.1.2 E-selectin**

30 Traditional and Novel Risk Factors in Atherothrombosis

infection), platelet activation and aggregation may extend into the arterial lumen. Activation of the coagulation cascade leads to fibrin deposition, which increases the size of the thrombus. Again, thrombus formation may be arrested without causing significant luminal stenosis. However, as the thrombus is exposed to flowing blood distal emboli may occur, potentially causing myocardial necrosis on a microscopic level and recognisable symptoms. As activated platelets aggregate to form a platelet-rich arterial thrombus, they release mediators such as serotonin and thromboxane A2, which cause vasoconstriction. This may lead to localised coronary arterial spasm, which even in the absence of an obstructive coronary thrombus, may lead to transmural myocardial ischaemia and a clinically apparent ACS (71). When thrombus formation continues unchecked, total arterial occlusion may occur. If such occlusion occurs suddenly in a previously uncompromised artery without a well-developed collateral circulation, significant downstream myocardial necrosis will occur with the clinically recognisable signs of acute myocardial infarction (AMI). The cell membranes of the necrosed myocytes are breached and their intracellular constituents are washed out into the circulation. These constituents include myoglobin, creatine kinase, the

Current diagnostic strategies incorporate biomarkers of myocardial necrosis, the end-point in the pathophysiological evolution of ACS. The measurement of cardiac troponins in the bloodstream has revolutionised the diagnosis of AMI in this regard, enabling the detection of microscopic amounts of myocardial necrosis that could not have previously been identified (72). As described in detail earlier in this chapter, however, a whole host of pathophysiological processes have occurred before myocardial necrosis, none of which are detectable using current diagnostic technology. In fact myocardial necrosis is merely a surrogate marker of the disease process, which occurs within the coronary artery and not the cardiac myocyte. As it is possible to use biomarkers to detect myocardial necrosis with high sensitivity and specificity this raises the additional possibility that other biomarkers may be able to detect evidence of the disease process itself within the coronary arteries. A

number of novel biomarkers have been investigated in this regard in recent years.

include the molecules P-selectin, E-selectin, ICAM-1 and VCAM-1.

Cell adhesion molecules (CAMs) mediate the interactions between the endothelium and blood cells, enabling the localised inflammatory response that is essential for the initiation and propagation of coronary atherosclerosis. Their upregulation enhances this inflammatory response, which ultimately renders the atherosclerotic plaque vulnerable to rupture. Following their expression, CAMs are shed from the cell surface. As these soluble CAMs are detectable in peripheral blood, they are promising candidates for use as early markers of vascular activation (37). CAMs that have attracted interest as potential biomarkers of ACS

P-selectin mediates the interaction of platelets and endothelial cells with neutrophils and monocytes (65). It is expressed by endothelial cells in atherosclerotic, but not normal, vessels

cardiac troponins and human fatty acid binding protein.

**3. Biomarkers of unstable coronary disease** 

**3.1 Soluble cell adhesion molecules** 

**3.1.1 P-selectin** 

E-selectin has also been investigated in this regard. Plasma levels of E-selectin have been shown to correlate with the severity of coronary atherosclerosis (87,100). A number of studies have reported elevated plasma E-selectin levels in patients with AMI (101-108). Eselectin elevations have also been reported in patients with unstable angina (85,109). Other studies have reported raised E-selectin in all ACS (80,82,110-112). Plasma E-selectin levels in patients with AMI may be higher among patients who experienced a prodrome of unstable angina (105). Raised E-selectin levels have also been reported following attacks of variant angina (113), although there may be no difference in E-selectin levels during episodes of stable angina (114). A reduction in plasma E-selectin levels has been described in patients with AMI following successful reperfusion (101,108). Further, plasma E-selectin levels may be useful for predicting the risk of death among patients with AMI (103). However not all reports have been consistent. Three groups have failed to find elevated E-selectin levels in ACS (115-117). There is no clear explanation for the discrepancy in the results, although one group measured E-selectin in serum rather than plasma, which may have introduced an important bias. Only one study has investigated E-selectin levels in the ED population with undifferentiated chest pain (118). This study failed to demonstrate a difference in E-selectin levels between patients with AMI, unstable angina and controls. However, the study had significant limitations, including small numbers, suboptimal gold standards and no clinical follow-up. The study was not designed to appraise the performance of E-selectin as a diagnostic test for use in the ED. The available research suggests that E-selectin has promising characteristics for use as a marker of ACS. A large prospective observational cohort study is necessary to evaluate its performance as a diagnostic test. Incorporation into

Biomarkers of Atherosclerosis and Acute Coronary Syndromes – A Clinical Perspective 33

upper reference limit as a diagnostic cut-off sCD40L had a sensitivity of only 56.5% for the

MPO is an enzyme secreted by phagocytic cells. It utilises hydrogen peroxidise to generate oxygen free radicals. In health this leads to the generation of hypochlorous acid, which has bactericidal and viricidal properties (155). Neutrophils and foam cells within coronary atheroma also produce MPO, where the generation of highly reactive oxygen free radicals leads to the generation of oxidised LDL cholesterol, which enhances the formation of foam cells propagating inflammation (156). It also perpetuates the endothelial dysfunction by enhancing the breakdown of nitric oxide (155). Further, MPO activates MMPs from their precursors and inactivates their physiological inhibitors, TIMPS (42). This enables breakdown of the fibrous cap, rendering the plaque vulnerable to rupture. MPO is abundantly expressed by macrophages in eroded or ruptured coronary plaques, although it has not been identified in fatty streaks (43). While expression is enhanced in unstable angina and AMI, it is not enhanced in variant angina or in response to ischaemia in chronic stable angina (157). These findings suggest that increased MPO expression is associated with the ongoing inflammatory process rather than indicating reperfusion injury or a tissue response to ischaemia. Blood levels of MPO have been shown to correlate strongly with the presence of coronary artery disease. When divided into quartiles, patients with MPO levels in the fourth quartile had an adjusted odds ratio for the presence of coronary artery disease of 20.4 compared to patients in the first quartile. MPO levels were more predictive of risk of

A case control study involving 874 patients demonstrated elevated MPO levels in patients with ACS compared with controls who had normal coronary angiograms (159). Two separate analyses of data from randomised controlled trials, involving a total of 2,614 patients, have reported that MPO levels help to predict the occurrence of adverse events in patients with confirmed ACS. Interestingly MPO was found to add additional prognostic information to cardiac troponins. However the rate of major adverse events within 30 days in patients with MPO levels below selected cut-offs remained around 5% in both studies (160,161). Other studies have also demonstrated that MPO levels in patients with confirmed ACS help to predict prognosis (162-165) , although one study reported that MPO levels did not help to predict mortality among 325 male patients who had been admitted to hospital with chest pain and were awaiting coronary angiography (166). Several studies have investigated the use of MPO in the ED population. The largest study, of 604 consecutive patients presenting to the ED with suspected cardiac chest pain, found that MPO levels predicted a diagnosis of ACS with sensitivity 65.7%, specificity 60.7%, PPV 53.3%, and NPV 72.2%. MPO levels also predicted adverse events, although 14.8% of patients with normal MPO and troponin levels still had a major adverse event within 30 days (167). A second study of 414 low risk patients who presented to the ED with suspected ACS found that MPO had a sensitivity of 71%, specificity 32% and negative likelihood ratio 0.89 (95% CI 0.26 – 2.05), suggesting that MPO was not a useful diagnostic test for AMI. However the study was underpowered as only seven patients were diagnosed with ACS (168). Among 140 consecutive ED patients with chest pain, MPO helped to diagnose AMI with sensitivity 92.3% (CI 95% 66.7% - 99.6%), specificity 40.2% (CI 95% 32.0% - 48.9%), PPV 13.6% (CI 95%

diagnosis of ACS in the patients with acute chest pain (141).

coronary artery disease than Framingham risk score (158).

**3.2 Myeloperoxidase (MPO)** 

a multimarker strategy with markers that may reflect other aspects of the pathophysiological evolution of ACS may be necessary to obtain sufficient sensitivity.

#### **3.1.3 Intercellular Adhesion Molecule-1 (ICAM-1) and Vascular Cell Adhesion Molecule-1 (VCAM-1)**

ICAM-1 and VCAM-1 are responsible for mediating firm adhesion of leukocytes to the endothelium, enabling their subsequent migration. ICAM-1 (but not VCAM-1) levels have been shown to predict adverse cardiac events in apparently healthy men (119-121) and women (122) and may help to predict the development (121,123) and progression of coronary atherosclerosis (124). In addition, ICAM-1 (but not VCAM-1) levels are raised in patients with stable angina and levels may correlate with disease severity (125-127). Levels of both VCAM-1 and ICAM-1 have been shown to be elevated in patients with ACS (128- 129), although conflicting results have also been reported (130). Interestingly, levels of VCAM-1 and ICAM-1 in patients with unstable angina who had demonstrable ruptured plaque on coronary intravascular ultrasound were significantly higher than in patients with stable angina who had no evidence of plaque rupture, although neither biomarker was an independent predictor of plaque rupture on multivariate analysis (131). Finally, VCAM-1 and ICAM-1 levels have both been shown to predict prognosis and complications in patients with confirmed ACS (91,103,128,132-137). Evidence for the use of ICAM-1 and VCAM-1 in the ED population with undifferentiated chest pain is disappointing, however. A study of 241 men who presented to the ED with chest pain failed to find a significant difference in ICAM-1 and VCAM-1 levels between patients with AMI and patients with (presumed) noncardiac chest pain, although the gold standards for diagnosis of non-cardiac pain were suboptimal (138). Other studies have demonstrated no correlation between ICAM-1 and VCAM-1 levels and the occurrence of adverse events within three months of presentation (96,139). One study demonstrated that ICAM-1 predicted in-hospital adverse events with a sensitivity of 63.3%, a specificity of 47.2% and a NPV of 79.3%, which is clearly not sufficient for ICAM-1 to be used in the clinical environment (140).

#### **3.1.4 Soluble CD40 Ligand (sCD40L)**

As described earlier in this chapter sCD40L plays a pivotal role in mediating interactions between inflammatory cells within coronary atheroma that ultimately render the plaque vulnerable to rupture. In addition, sCD40L is expressed by activated platelets and plasma levels have been shown to correlate with platelet activation (141). Case control studies have consistently demonstrated elevated levels of sCD40L in patients with ACS when compared with controls (48;141-151). sCD40L levels have also been shown to stratify patients with confirmed ACS according to their risk of developing adverse events (152), although conflicting results have also been reported (153,154). An analysis of data from 1088 patients with confirmed ACS who had been enrolled in a randomised controlled trial (the c7E3 Fab antiplatelet therapy in unstable refractory angina (CAPTURE) trial) and 626 patients who were admitted to hospital with acute chest pain, found that sCD40L was a powerful independent predictor of adverse events at 72 hours, 30 days and six months. Levels correlated poorly with troponin T and may thus identify a separate at-risk group. However sCD40L may be more useful for prognostication than diagnosis. Using the 97.5th percentile upper reference limit as a diagnostic cut-off sCD40L had a sensitivity of only 56.5% for the diagnosis of ACS in the patients with acute chest pain (141).

#### **3.2 Myeloperoxidase (MPO)**

32 Traditional and Novel Risk Factors in Atherothrombosis

a multimarker strategy with markers that may reflect other aspects of the pathophysiological evolution of ACS may be necessary to obtain sufficient sensitivity.

ICAM-1 and VCAM-1 are responsible for mediating firm adhesion of leukocytes to the endothelium, enabling their subsequent migration. ICAM-1 (but not VCAM-1) levels have been shown to predict adverse cardiac events in apparently healthy men (119-121) and women (122) and may help to predict the development (121,123) and progression of coronary atherosclerosis (124). In addition, ICAM-1 (but not VCAM-1) levels are raised in patients with stable angina and levels may correlate with disease severity (125-127). Levels of both VCAM-1 and ICAM-1 have been shown to be elevated in patients with ACS (128- 129), although conflicting results have also been reported (130). Interestingly, levels of VCAM-1 and ICAM-1 in patients with unstable angina who had demonstrable ruptured plaque on coronary intravascular ultrasound were significantly higher than in patients with stable angina who had no evidence of plaque rupture, although neither biomarker was an independent predictor of plaque rupture on multivariate analysis (131). Finally, VCAM-1 and ICAM-1 levels have both been shown to predict prognosis and complications in patients with confirmed ACS (91,103,128,132-137). Evidence for the use of ICAM-1 and VCAM-1 in the ED population with undifferentiated chest pain is disappointing, however. A study of 241 men who presented to the ED with chest pain failed to find a significant difference in ICAM-1 and VCAM-1 levels between patients with AMI and patients with (presumed) noncardiac chest pain, although the gold standards for diagnosis of non-cardiac pain were suboptimal (138). Other studies have demonstrated no correlation between ICAM-1 and VCAM-1 levels and the occurrence of adverse events within three months of presentation (96,139). One study demonstrated that ICAM-1 predicted in-hospital adverse events with a sensitivity of 63.3%, a specificity of 47.2% and a NPV of 79.3%, which is clearly not sufficient

As described earlier in this chapter sCD40L plays a pivotal role in mediating interactions between inflammatory cells within coronary atheroma that ultimately render the plaque vulnerable to rupture. In addition, sCD40L is expressed by activated platelets and plasma levels have been shown to correlate with platelet activation (141). Case control studies have consistently demonstrated elevated levels of sCD40L in patients with ACS when compared with controls (48;141-151). sCD40L levels have also been shown to stratify patients with confirmed ACS according to their risk of developing adverse events (152), although conflicting results have also been reported (153,154). An analysis of data from 1088 patients with confirmed ACS who had been enrolled in a randomised controlled trial (the c7E3 Fab antiplatelet therapy in unstable refractory angina (CAPTURE) trial) and 626 patients who were admitted to hospital with acute chest pain, found that sCD40L was a powerful independent predictor of adverse events at 72 hours, 30 days and six months. Levels correlated poorly with troponin T and may thus identify a separate at-risk group. However sCD40L may be more useful for prognostication than diagnosis. Using the 97.5th percentile

**3.1.3 Intercellular Adhesion Molecule-1 (ICAM-1) and Vascular Cell Adhesion** 

for ICAM-1 to be used in the clinical environment (140).

**3.1.4 Soluble CD40 Ligand (sCD40L)** 

**Molecule-1 (VCAM-1)** 

MPO is an enzyme secreted by phagocytic cells. It utilises hydrogen peroxidise to generate oxygen free radicals. In health this leads to the generation of hypochlorous acid, which has bactericidal and viricidal properties (155). Neutrophils and foam cells within coronary atheroma also produce MPO, where the generation of highly reactive oxygen free radicals leads to the generation of oxidised LDL cholesterol, which enhances the formation of foam cells propagating inflammation (156). It also perpetuates the endothelial dysfunction by enhancing the breakdown of nitric oxide (155). Further, MPO activates MMPs from their precursors and inactivates their physiological inhibitors, TIMPS (42). This enables breakdown of the fibrous cap, rendering the plaque vulnerable to rupture. MPO is abundantly expressed by macrophages in eroded or ruptured coronary plaques, although it has not been identified in fatty streaks (43). While expression is enhanced in unstable angina and AMI, it is not enhanced in variant angina or in response to ischaemia in chronic stable angina (157). These findings suggest that increased MPO expression is associated with the ongoing inflammatory process rather than indicating reperfusion injury or a tissue response to ischaemia. Blood levels of MPO have been shown to correlate strongly with the presence of coronary artery disease. When divided into quartiles, patients with MPO levels in the fourth quartile had an adjusted odds ratio for the presence of coronary artery disease of 20.4 compared to patients in the first quartile. MPO levels were more predictive of risk of coronary artery disease than Framingham risk score (158).

A case control study involving 874 patients demonstrated elevated MPO levels in patients with ACS compared with controls who had normal coronary angiograms (159). Two separate analyses of data from randomised controlled trials, involving a total of 2,614 patients, have reported that MPO levels help to predict the occurrence of adverse events in patients with confirmed ACS. Interestingly MPO was found to add additional prognostic information to cardiac troponins. However the rate of major adverse events within 30 days in patients with MPO levels below selected cut-offs remained around 5% in both studies (160,161). Other studies have also demonstrated that MPO levels in patients with confirmed ACS help to predict prognosis (162-165) , although one study reported that MPO levels did not help to predict mortality among 325 male patients who had been admitted to hospital with chest pain and were awaiting coronary angiography (166). Several studies have investigated the use of MPO in the ED population. The largest study, of 604 consecutive patients presenting to the ED with suspected cardiac chest pain, found that MPO levels predicted a diagnosis of ACS with sensitivity 65.7%, specificity 60.7%, PPV 53.3%, and NPV 72.2%. MPO levels also predicted adverse events, although 14.8% of patients with normal MPO and troponin levels still had a major adverse event within 30 days (167). A second study of 414 low risk patients who presented to the ED with suspected ACS found that MPO had a sensitivity of 71%, specificity 32% and negative likelihood ratio 0.89 (95% CI 0.26 – 2.05), suggesting that MPO was not a useful diagnostic test for AMI. However the study was underpowered as only seven patients were diagnosed with ACS (168). Among 140 consecutive ED patients with chest pain, MPO helped to diagnose AMI with sensitivity 92.3% (CI 95% 66.7% - 99.6%), specificity 40.2% (CI 95% 32.0% - 48.9%), PPV 13.6% (CI 95%

Biomarkers of Atherosclerosis and Acute Coronary Syndromes – A Clinical Perspective 35

events within 30 days (175). Finally, among 422 patients who presented to the ED with chest pain but had neither troponin elevations nor ECG abnormalities, PAPP-A was found to be a significant predictor of adverse events after a median of 60 weeks follow up, although this was not significant once other factors had been taken into account (including a clinical risk score, exercise tolerance testing and plasma levels of other biomarkers) (181). The available evidence suggests that PAPP-A levels alone are unlikely to be sufficient to enable early diagnosis of AMI in the ED or to accurately identify a population of patients who are at

D-dimer is a degradation product of cross-linked fibrin. Its presence indicates both thrombus formation and subsequent endogenous fibrinolysis, thus confirming that both thrombin and plasmin have been generated (182). It is a sensitive tool for exclusion of venous thromboembolism in the low risk group (183). In ACS plaque rupture or erosion is followed by exposure of the procoagulant lipid core to circulating blood with ensuing thrombus formation. As coronary thrombus precedes myocardial necrosis, it is possible that coagulation markers such as D-dimer are sensitive markers of ACS, potentially rising earlier than markers of myocardial necrosis including troponins. Elevated D-dimer levels in apparently healthy males have been shown to predict the future occurrence of AMI, ACS and coronary heart disease (121,184,185). Further, patients in whom the first presentation of coronary heart disease is with AMI may have higher D-dimer levels than patients who first present with stable angina (186). A weak but statistically significant correlation has been demonstrated between plasma D-dimer levels and severity of coronary disease on angiography in patients with unstable angina (187,188). In a cohort of 54 patients who were diagnosed with unstable angina and underwent coronary angiography, D-dimer levels (cutoff 270ng/ml) predicted significant coronary disease on angiography with sensitivity 70%, specificity 50%, PPV 86%, NPV 72%. By lowering the cut-off to 200ng/ml sensitivity

Several studies have demonstrated that patients with ACS have elevated levels of D-dimer when compared to controls with stable angina or no coronary disease (139,190,193). Plasma D-dimer level has also been shown to be a significant predictor of long-term mortality (after a median of 29 months follow up) in 320 patients with a diagnosis of NSTE-ACS (194), although a separate study of 358 patients with NSTE-ACS found that D-dimer did not predict the occurrence of adverse events (death, AMI, revascularisation or hospital admission for acute heart failure) within six months (hazard ratio 1.26, 95% CI 0.79 – 2.02) (195). Among 257 patients D-dimer levels (cut-off 500ng/l) at the time of admission (mean 160 minutes from symptom onset) diagnosed AMI with sensitivity 65%, specificity 80%, PPV 36% and NPV 93%, although the study utilised an outdated gold standard (incorporating CK-MB levels) for the diagnosis of AMI. D-dimer levels were found to be significantly higher in patients who were diagnosed with ischaemic pain, AMI and unstable angina (196). Another study of 184 patients who presented to the ED with suspected cardiac chest pain showed that D-dimer levels taken at the time of presentation were on average 111% higher in patients who were diagnosed with ACS compared to those who were not. Ddimer (at a cut-off of 1mg/l) had a sensitivity of 18% in order to achieve a set specificity of

sufficiently low risk of adverse events to affect clinical practice.

increased to 95% but specificity dropped to 20% (189).

**3.4 Coagulation markers** 

**3.4.1 D-dimer** 

8.0% - 22.3%) and NPV 98.1% (CI 95% 89.9% - 99.9%). Again, however, the study was underpowered, with only 13 patients being diagnosed with AMI (169). Finally, MPO levels measured in 148 ED patients with chest pain were found to be significantly higher among those diagnosed with AMI. However MPO was both insensitive (13.9% of patients with MPO levels in the bottom quartile had AMI) and non-specific as a diagnostic marker for AMI (only 38.4% of patients with values in the highest quartile had AMI). MPO levels were found to be significant predictors of adverse events within 30 days (167) The available data suggest that MPO is unlikely to have sufficient sensitivity or specificity to be useful as an early diagnostic marker of ACS in the ED. However it may have a role for risk stratification and prediction of prognosis, particularly in troponin negative patients.

#### **3.3 Pregnancy-Associated Plasma Protein A (PAPP-A)**

PAPP-A is a matrix metalloproteinase (MMP), one of a family of at least 25 proteases, of which 14 have been characterised in vascular cells. They are secreted by a variety of cells that are involved in the atherosclerotic process including foam cells, endothelial cells, T lymphocytes, mast cells and smooth muscle cells (170). They are upregulated in atherosclerotic plaque and play a pivotal role in the degradation of the fibrous cap that renders the plaque vulnerable to rupture (171,172). PAPP-A was originally detected in the serum in late pregnancy and has been used in first trimester screening for Down's syndrome (173). It is also abundantly expressed in unstable but not stable atherosclerotic plaques (54) and raised levels have been shown to correlate with complex coronary stenoses on angiography (174). A small case control study involving a total of 69 patients found that PAPP-A levels were significantly higher in patients with AMI or unstable angina compared to patients with stable angina and healthy controls without coronary disease (54). PAPP-A has been investigated as a potential early marker of AMI, with mixed results. A study of 346 patients who presented to the ED with chest pain found that PAPP-A levels were significantly higher in those patients who were diagnosed with AMI (175). In a second study that included 415 patients admitted to a cardiology unit with suspected ACS, PAPP-A levels were also found to be significantly higher in those patients with AMI although the AUC was only 0.56, suggesting that PAPP-A is unlikely to be useful as a lone diagnostic investigation for AMI (176). Further, a case control study found no significant difference in PAPP-A levels between 80 patients with STEMI and 80 healthy controls (177). Finally, among 59 patients who presented to the ED with suspected ACS and were deemed to be at intermediate risk for having a significant coronary event, PAPP-A was found to be an independent predictor of a diagnosis of ACS (odds ratio 2.09), following adjustment for other clinical factors (178).

When tested at the time of presentation in the ED population, PAPP-A may help to predict cardiac events in the near future. In a subgroup of a large study involving 626 ED patients with chest pain, Heeschen et al found that PAPP-A predicted adverse events with an adjusted odds ratio of 2.32 (179). In an ED population of 136 patients with suspected ACS but negative troponin I, Lund et al found PAPP-A to be an independent predictor of adverse cardiac events at six months, albeit with a sensitivity of only 54%, specificity 75%, PPV 30% and NPV 15% (180). In a study of 364 ED patients with suspected ACS, Laterza et al reported that PAPP-A predicted adverse events at 30 days with a sensitivity of 66.7%, specificity 51.5%, PPV 12.6% and NPV 93.6%. Thus for every 100 patients discharged and reassured on the basis of a negative PAPP-A level, three would have an adverse cardiac events within 30 days (175). Finally, among 422 patients who presented to the ED with chest pain but had neither troponin elevations nor ECG abnormalities, PAPP-A was found to be a significant predictor of adverse events after a median of 60 weeks follow up, although this was not significant once other factors had been taken into account (including a clinical risk score, exercise tolerance testing and plasma levels of other biomarkers) (181). The available evidence suggests that PAPP-A levels alone are unlikely to be sufficient to enable early diagnosis of AMI in the ED or to accurately identify a population of patients who are at sufficiently low risk of adverse events to affect clinical practice.

#### **3.4 Coagulation markers**

#### **3.4.1 D-dimer**

34 Traditional and Novel Risk Factors in Atherothrombosis

8.0% - 22.3%) and NPV 98.1% (CI 95% 89.9% - 99.9%). Again, however, the study was underpowered, with only 13 patients being diagnosed with AMI (169). Finally, MPO levels measured in 148 ED patients with chest pain were found to be significantly higher among those diagnosed with AMI. However MPO was both insensitive (13.9% of patients with MPO levels in the bottom quartile had AMI) and non-specific as a diagnostic marker for AMI (only 38.4% of patients with values in the highest quartile had AMI). MPO levels were found to be significant predictors of adverse events within 30 days (167) The available data suggest that MPO is unlikely to have sufficient sensitivity or specificity to be useful as an early diagnostic marker of ACS in the ED. However it may have a role for risk stratification

PAPP-A is a matrix metalloproteinase (MMP), one of a family of at least 25 proteases, of which 14 have been characterised in vascular cells. They are secreted by a variety of cells that are involved in the atherosclerotic process including foam cells, endothelial cells, T lymphocytes, mast cells and smooth muscle cells (170). They are upregulated in atherosclerotic plaque and play a pivotal role in the degradation of the fibrous cap that renders the plaque vulnerable to rupture (171,172). PAPP-A was originally detected in the serum in late pregnancy and has been used in first trimester screening for Down's syndrome (173). It is also abundantly expressed in unstable but not stable atherosclerotic plaques (54) and raised levels have been shown to correlate with complex coronary stenoses on angiography (174). A small case control study involving a total of 69 patients found that PAPP-A levels were significantly higher in patients with AMI or unstable angina compared to patients with stable angina and healthy controls without coronary disease (54). PAPP-A has been investigated as a potential early marker of AMI, with mixed results. A study of 346 patients who presented to the ED with chest pain found that PAPP-A levels were significantly higher in those patients who were diagnosed with AMI (175). In a second study that included 415 patients admitted to a cardiology unit with suspected ACS, PAPP-A levels were also found to be significantly higher in those patients with AMI although the AUC was only 0.56, suggesting that PAPP-A is unlikely to be useful as a lone diagnostic investigation for AMI (176). Further, a case control study found no significant difference in PAPP-A levels between 80 patients with STEMI and 80 healthy controls (177). Finally, among 59 patients who presented to the ED with suspected ACS and were deemed to be at intermediate risk for having a significant coronary event, PAPP-A was found to be an independent predictor of a diagnosis of ACS (odds ratio 2.09), following adjustment for other clinical factors (178). When tested at the time of presentation in the ED population, PAPP-A may help to predict cardiac events in the near future. In a subgroup of a large study involving 626 ED patients with chest pain, Heeschen et al found that PAPP-A predicted adverse events with an adjusted odds ratio of 2.32 (179). In an ED population of 136 patients with suspected ACS but negative troponin I, Lund et al found PAPP-A to be an independent predictor of adverse cardiac events at six months, albeit with a sensitivity of only 54%, specificity 75%, PPV 30% and NPV 15% (180). In a study of 364 ED patients with suspected ACS, Laterza et al reported that PAPP-A predicted adverse events at 30 days with a sensitivity of 66.7%, specificity 51.5%, PPV 12.6% and NPV 93.6%. Thus for every 100 patients discharged and reassured on the basis of a negative PAPP-A level, three would have an adverse cardiac

and prediction of prognosis, particularly in troponin negative patients.

**3.3 Pregnancy-Associated Plasma Protein A (PAPP-A)** 

D-dimer is a degradation product of cross-linked fibrin. Its presence indicates both thrombus formation and subsequent endogenous fibrinolysis, thus confirming that both thrombin and plasmin have been generated (182). It is a sensitive tool for exclusion of venous thromboembolism in the low risk group (183). In ACS plaque rupture or erosion is followed by exposure of the procoagulant lipid core to circulating blood with ensuing thrombus formation. As coronary thrombus precedes myocardial necrosis, it is possible that coagulation markers such as D-dimer are sensitive markers of ACS, potentially rising earlier than markers of myocardial necrosis including troponins. Elevated D-dimer levels in apparently healthy males have been shown to predict the future occurrence of AMI, ACS and coronary heart disease (121,184,185). Further, patients in whom the first presentation of coronary heart disease is with AMI may have higher D-dimer levels than patients who first present with stable angina (186). A weak but statistically significant correlation has been demonstrated between plasma D-dimer levels and severity of coronary disease on angiography in patients with unstable angina (187,188). In a cohort of 54 patients who were diagnosed with unstable angina and underwent coronary angiography, D-dimer levels (cutoff 270ng/ml) predicted significant coronary disease on angiography with sensitivity 70%, specificity 50%, PPV 86%, NPV 72%. By lowering the cut-off to 200ng/ml sensitivity increased to 95% but specificity dropped to 20% (189).

Several studies have demonstrated that patients with ACS have elevated levels of D-dimer when compared to controls with stable angina or no coronary disease (139,190,193). Plasma D-dimer level has also been shown to be a significant predictor of long-term mortality (after a median of 29 months follow up) in 320 patients with a diagnosis of NSTE-ACS (194), although a separate study of 358 patients with NSTE-ACS found that D-dimer did not predict the occurrence of adverse events (death, AMI, revascularisation or hospital admission for acute heart failure) within six months (hazard ratio 1.26, 95% CI 0.79 – 2.02) (195). Among 257 patients D-dimer levels (cut-off 500ng/l) at the time of admission (mean 160 minutes from symptom onset) diagnosed AMI with sensitivity 65%, specificity 80%, PPV 36% and NPV 93%, although the study utilised an outdated gold standard (incorporating CK-MB levels) for the diagnosis of AMI. D-dimer levels were found to be significantly higher in patients who were diagnosed with ischaemic pain, AMI and unstable angina (196). Another study of 184 patients who presented to the ED with suspected cardiac chest pain showed that D-dimer levels taken at the time of presentation were on average 111% higher in patients who were diagnosed with ACS compared to those who were not. Ddimer (at a cut-off of 1mg/l) had a sensitivity of 18% in order to achieve a set specificity of

Biomarkers of Atherosclerosis and Acute Coronary Syndromes – A Clinical Perspective 37

patients with NSTEMI had BNP levels above 80pg/ml, suggesting that BNP, at least at the

In a study of 100 patients who were admitted to a Medical Admissions Unit with suspected cardiac chest pain, BNP (diagnostic cut-off 5pg/ml) helped to diagnose AMI with sensitivity 88.6%, specificity 78.6%, PPV 75%, NPV 89.6% with an AUC of 0.868. BNP was significantly more sensitive but less specific than troponin T when used at the time of admission to the unit. By combining BNP and troponin T performance improved (sensitivity 95.4%, specificity 76.8%). Unfortunately, however, this study had significant weaknesses as the primary outcome (discharge diagnosis of cardiac pain) could not be objectively verified through use of a gold standard, the study was retrospective and no follow up data was provided (226). Several studies have investigated the diagnostic and prognostic value of BNP levels in the ED population. Among 631 consecutive patients who presented to the ED with suspected cardiac chest pain with symptom onset <12 hours, BNP levels at the time of admission were found to be significantly higher among patients who were ultimately diagnosed with AMI (227). For predicting a diagnosis of AMI, BNP had an AUC of 0.710. Using a cut-off of 100pg/ml BNP predicted AMI with sensitivity 70.8%, specificity 68.9%, PPV 22.7%, NPV 94.8%, positive likelihood ratio 2.28 and negative likelihood ratio 0.42. When combined with CK-MB and troponin I, the presence of any raised biomarker for a diagnosis of AMI performed with sensitivity 87.3%, specificity 65.7%, PPV 27.0%, NPV 97.3%, positive likelihood ratio 2.55 and negative likelihood ratio 0.19. This suggests that, had BNP been introduced into clinical practice, this would have enabled the early detection of an additional 22 AMIs that could not otherwise have been recognised at the time of admission. However this would come at a cost of 163 false positive diagnoses (227). In a retrospective analysis of 546 patients who presented to the ED with suspected cardiac chest pain, a point-of-care BNP test was found to have an AUC of 0.755 for a diagnosis of AMI. At a cut-off of 100ng/l, BNP sensitivity was 66.7%, specificity 71.3%, PPV 17.1%, NPV 96.0%, positive likelihood ratio 2.32 and negative likelihood ratio 0.47. However the study had significant weaknesses. Clinicians were not blinded to BNP results, the study was subject to significant verification bias as only a minority of patients with normal point of care tests underwent subsequent gold standard troponin testing and, for those who did undergo troponin testing, an outdated troponin cut-off was used to diagnose AMI (228). Another study prospectively recruited 306 patients who presented to the ED with suspected cardiac chest pain. BNP was measured using two separate assays at the time of admission. The AUC of each assay for a diagnosis of ACS was found to be less than 0.6. BNP levels were found to be significant predictors of adverse events after 30 and 90 days but, again, the AUC was less

Finally, in another prospective cohort study, 426 patients who presented to the ED with suspected cardiac chest pain had BNP levels measured at the time of presentation. The AUC of BNP for diagnosis of AMI, diagnosis of ACS and occurrence of adverse events (death, AMI or coronary revascularisation) within 30 days was 0.766, 0.691 and 0.675 respectively. The authors incorporated BNP into a multimarker strategy that also included CK-MB, myoglobin and troponin I. Using serial estimations at the time of ED presentation and 90 minutes later, this multimarker panel had a sensitivity of 97.4% (95% CI 86.5 – 100.0%), specificity 47.8% (42.7 – 52.9%), PPV 15.8% (11.5 – 21.1%) and NPV 99.5% (97.0 – 100.0%) for diagnosis of AMI. For a diagnosis of ACS performance was slightly worse, with a sensitivity

stated diagnostic cut-off, may have limited sensitivity for these diagnoses (211).

than 0.7 for each assay (229).

92%, although the implications of accepted a more conventional, lower D-dimer cut-off were not evaluated (197). In 102 patients who presented to a Brazilian ED, D-dimer levels at the time of ED presentation were significantly higher in patients who had a troponin T >0.01ng/ml at the time of presentation compared with patients whose troponin T was <0.01ng/ml. Unfortunately the results of 12-hour troponin testing were not available for analysis in this study, precluding evaluation of true diagnostic performance for AMI (198). The largest study to have investigated the use of D-dimer for the diagnosis of AMI in ED patients included a total of 741 patients who presented to the ED with suspected AMI. In that study, plasma D-dimer levels measured 12-24 hours after arrival at the ED had an AUC of 0.734 (95% CI 0.715 – 0.753) for predicting a troponin T result of >0.03ng/ml. At a cut-off of 500μg/l D-dimer had a sensitivity of 95%, specificity 27%, PPV 92% and NPV 41% (199). Finally, in a study of 432 patients who presented to the ED with suspected ACS D-dimer levels measured at the time of ED presentation did not help to predict the occurrence of adverse events (death, AMI, revascularisation, recurrent ACS or hospital admission with congestive heart failure) after 42 days of follow up (odds ratio 1.3, 95% CI 0.4 – 4.5, at a cutoff of 500μg/l).

The evidence suggests that D-dimer is unlikely to be useful as an early marker of AMI when used alone in the ED population and at present the evidence for the use of D-dimer as a prognostic marker is also sparse. Future research into this biomarker must focus upon evaluating its potential value as part of a multimarker strategy.

#### **3.5 Markers of ventricular stress**

#### **3.5.1 Brain Natriuretic Peptide (BNP)**

BNP was first isolated from porcine brains but it has since been recognised as a cardiac hormone synthesised predominantly by the ventricles in response to ventricular wall stress. Together with atrial natriuretic peptide, which is secreted primarily by the atria, BNP belongs to the natriuretic peptide family that is involved in cardiac homeostasis. Biological effects include diuresis, vasodilatation, inhibition of the renin-aldosterone system and of cardiac and vascular myocyte growth (200). BNP is known to be a marker of acute and chronic left ventricular dysfunction and may be useful for the ED diagnosis of the former (201,202). It has been used as a marker of left ventricular systolic dysfunction following AMI, where it provides prognostic information (203). BNP is also expressed in ischaemic human myocardium and plasma levels may rise during periods of ischaemia (204-208). A number of studies, that together have included a total of 5159 patients, have demonstrated that BNP level acts as a strong predictor of mortality at seven days, 30 days, six months and 10 months in patients with confirmed ACS (208-214). Other studies have shown that BNP levels help to predict all adverse cardiac events, both during the index hospital admission and at follow up after up to 1 year (215-217). BNP levels have also been shown to help predict the development of congestive heart failure when measured in patients with both STEMI and NSTE-ACS (218-219). In addition to having prognostic value, there is evidence that BNP may assist in the diagnosis of ACS. Several small case control studies have demonstrated higher BNP levels in patients with AMI (220-222) and unstable angina (223,224) when compared with controls. There is some evidence to suggest that BNP levels may, in fact, correlate with infarct size (225). However, among 1676 patients with confirmed NSTE-ACS only 15.6% of patients had BNP levels above 80pg/ml. Indeed only 25.2% of

92%, although the implications of accepted a more conventional, lower D-dimer cut-off were not evaluated (197). In 102 patients who presented to a Brazilian ED, D-dimer levels at the time of ED presentation were significantly higher in patients who had a troponin T >0.01ng/ml at the time of presentation compared with patients whose troponin T was <0.01ng/ml. Unfortunately the results of 12-hour troponin testing were not available for analysis in this study, precluding evaluation of true diagnostic performance for AMI (198). The largest study to have investigated the use of D-dimer for the diagnosis of AMI in ED patients included a total of 741 patients who presented to the ED with suspected AMI. In that study, plasma D-dimer levels measured 12-24 hours after arrival at the ED had an AUC of 0.734 (95% CI 0.715 – 0.753) for predicting a troponin T result of >0.03ng/ml. At a cut-off of 500μg/l D-dimer had a sensitivity of 95%, specificity 27%, PPV 92% and NPV 41% (199). Finally, in a study of 432 patients who presented to the ED with suspected ACS D-dimer levels measured at the time of ED presentation did not help to predict the occurrence of adverse events (death, AMI, revascularisation, recurrent ACS or hospital admission with congestive heart failure) after 42 days of follow up (odds ratio 1.3, 95% CI 0.4 – 4.5, at a cut-

The evidence suggests that D-dimer is unlikely to be useful as an early marker of AMI when used alone in the ED population and at present the evidence for the use of D-dimer as a prognostic marker is also sparse. Future research into this biomarker must focus upon

BNP was first isolated from porcine brains but it has since been recognised as a cardiac hormone synthesised predominantly by the ventricles in response to ventricular wall stress. Together with atrial natriuretic peptide, which is secreted primarily by the atria, BNP belongs to the natriuretic peptide family that is involved in cardiac homeostasis. Biological effects include diuresis, vasodilatation, inhibition of the renin-aldosterone system and of cardiac and vascular myocyte growth (200). BNP is known to be a marker of acute and chronic left ventricular dysfunction and may be useful for the ED diagnosis of the former (201,202). It has been used as a marker of left ventricular systolic dysfunction following AMI, where it provides prognostic information (203). BNP is also expressed in ischaemic human myocardium and plasma levels may rise during periods of ischaemia (204-208). A number of studies, that together have included a total of 5159 patients, have demonstrated that BNP level acts as a strong predictor of mortality at seven days, 30 days, six months and 10 months in patients with confirmed ACS (208-214). Other studies have shown that BNP levels help to predict all adverse cardiac events, both during the index hospital admission and at follow up after up to 1 year (215-217). BNP levels have also been shown to help predict the development of congestive heart failure when measured in patients with both STEMI and NSTE-ACS (218-219). In addition to having prognostic value, there is evidence that BNP may assist in the diagnosis of ACS. Several small case control studies have demonstrated higher BNP levels in patients with AMI (220-222) and unstable angina (223,224) when compared with controls. There is some evidence to suggest that BNP levels may, in fact, correlate with infarct size (225). However, among 1676 patients with confirmed NSTE-ACS only 15.6% of patients had BNP levels above 80pg/ml. Indeed only 25.2% of

evaluating its potential value as part of a multimarker strategy.

off of 500μg/l).

**3.5 Markers of ventricular stress** 

**3.5.1 Brain Natriuretic Peptide (BNP)** 

patients with NSTEMI had BNP levels above 80pg/ml, suggesting that BNP, at least at the stated diagnostic cut-off, may have limited sensitivity for these diagnoses (211).

In a study of 100 patients who were admitted to a Medical Admissions Unit with suspected cardiac chest pain, BNP (diagnostic cut-off 5pg/ml) helped to diagnose AMI with sensitivity 88.6%, specificity 78.6%, PPV 75%, NPV 89.6% with an AUC of 0.868. BNP was significantly more sensitive but less specific than troponin T when used at the time of admission to the unit. By combining BNP and troponin T performance improved (sensitivity 95.4%, specificity 76.8%). Unfortunately, however, this study had significant weaknesses as the primary outcome (discharge diagnosis of cardiac pain) could not be objectively verified through use of a gold standard, the study was retrospective and no follow up data was provided (226). Several studies have investigated the diagnostic and prognostic value of BNP levels in the ED population. Among 631 consecutive patients who presented to the ED with suspected cardiac chest pain with symptom onset <12 hours, BNP levels at the time of admission were found to be significantly higher among patients who were ultimately diagnosed with AMI (227). For predicting a diagnosis of AMI, BNP had an AUC of 0.710. Using a cut-off of 100pg/ml BNP predicted AMI with sensitivity 70.8%, specificity 68.9%, PPV 22.7%, NPV 94.8%, positive likelihood ratio 2.28 and negative likelihood ratio 0.42. When combined with CK-MB and troponin I, the presence of any raised biomarker for a diagnosis of AMI performed with sensitivity 87.3%, specificity 65.7%, PPV 27.0%, NPV 97.3%, positive likelihood ratio 2.55 and negative likelihood ratio 0.19. This suggests that, had BNP been introduced into clinical practice, this would have enabled the early detection of an additional 22 AMIs that could not otherwise have been recognised at the time of admission. However this would come at a cost of 163 false positive diagnoses (227). In a retrospective analysis of 546 patients who presented to the ED with suspected cardiac chest pain, a point-of-care BNP test was found to have an AUC of 0.755 for a diagnosis of AMI. At a cut-off of 100ng/l, BNP sensitivity was 66.7%, specificity 71.3%, PPV 17.1%, NPV 96.0%, positive likelihood ratio 2.32 and negative likelihood ratio 0.47. However the study had significant weaknesses. Clinicians were not blinded to BNP results, the study was subject to significant verification bias as only a minority of patients with normal point of care tests underwent subsequent gold standard troponin testing and, for those who did undergo troponin testing, an outdated troponin cut-off was used to diagnose AMI (228). Another study prospectively recruited 306 patients who presented to the ED with suspected cardiac chest pain. BNP was measured using two separate assays at the time of admission. The AUC of each assay for a diagnosis of ACS was found to be less than 0.6. BNP levels were found to be significant predictors of adverse events after 30 and 90 days but, again, the AUC was less than 0.7 for each assay (229).

Finally, in another prospective cohort study, 426 patients who presented to the ED with suspected cardiac chest pain had BNP levels measured at the time of presentation. The AUC of BNP for diagnosis of AMI, diagnosis of ACS and occurrence of adverse events (death, AMI or coronary revascularisation) within 30 days was 0.766, 0.691 and 0.675 respectively. The authors incorporated BNP into a multimarker strategy that also included CK-MB, myoglobin and troponin I. Using serial estimations at the time of ED presentation and 90 minutes later, this multimarker panel had a sensitivity of 97.4% (95% CI 86.5 – 100.0%), specificity 47.8% (42.7 – 52.9%), PPV 15.8% (11.5 – 21.1%) and NPV 99.5% (97.0 – 100.0%) for diagnosis of AMI. For a diagnosis of ACS performance was slightly worse, with a sensitivity

Biomarkers of Atherosclerosis and Acute Coronary Syndromes – A Clinical Perspective 39

The utility of a multimarker strategy must also be considered in the light of developments in assays. We have evaluated a high sensitivity troponin T assay in 915 patients, where the

In recent years there has been substantial and growing interest in a number of novel biomarkers that may facilitate early diagnosis of AMI and enhanced risk stratification of patients who present to the ED with suspected ACS. Promising markers of each step in the pathophysiological evolution of an acute coronary syndrome have been identified, each of which may be detected in the peripheral circulation. Unfortunately it is unlikely that any of these biomarkers will be as cardio-specific as the cardiac troponins and, despite considerable research, there is at present no single biomarker that can be used to confirm or exclude a diagnosis of ACS in the ED. If there is to be a future for novel biomarkers in the ED diagnosis of ACS, therefore, future research must focus on incorporating levels of multiple biomarkers and available clinical information into a risk score or clinical decision rule, in order that the predictive value of individual biomarkers and clinical features may be

[1] British Heart Foundation. Mortality statistics 2004. www.heartstats.com . 2005. Ref Type:

[2] Allender S, Peto V, Scarborough P, Boxer A, Rayner M. Coronary Heart Disease

[3] Fothergill NJ, Hunt MT, Touquet R. Audit of patients with chest pain presenting to an

[4] Ekelund U, Nilsson HJ, Frigyesi A, Torffvit O. Patients with suspected acute coronary

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results demonstrated a negative predictive value of 99.4%. [241]

**5. Conclusion** 

combined and enhanced.

Generic

**6. References** 

of 88.1% and NPV 92.9% and for predicting adverse events within 30 days the panel performed with sensitivity 88.5%, specificity 43.9%, PPV 18.0% and NPV 96.5% (230). The available evidence suggests that BNP may have value as a diagnostic and prognostic marker in patients who present to the ED with suspected ACS. However it is readily apparent that BNP is unsuitable for use as a lone biomarker in this situation. Future research is still necessary in order to define the potential role of BNP as part of a multimarker strategy.

#### **3.6 Novel markers of myocardial necrosis**

#### **3.6.1 Heart-type Fatty Acid Binding Protein (H-FABP)**

H-FABP is a cytoplasmic protein that is abundantly expressed in human myocardial cells. It is also found in much lower concentration in skeletal muscle, kidney and brain tissue (231). Experimental data first suggested that H-FABP may be a potential novel biomarker of AMI as early as 1988 (232). In 1991 Tanaka et al reported elevated H-FABP levels in patients with AMI, with levels peaking earlier than CK-MB (233). Despite interest in H-FABP as an early marker of AMI for many years it has never gained widespread acceptance for use in clinical practice.

Five studies have investigated the diagnostic utility of H-FABP when used for the diagnosis of AMI at the time of presentation to the ED (234-238). Four of these studies utilised qualitative assays that are available as point of care tests. All five studies had significant weaknesses, with most studies employing now outdated gold standards for AMI diagnosis and being subject to significant verification bias. The data reporting in the small study by Alashemi et al precludes calculation of total sensitivity and specificity (235). If the remainder of the results are pooled this would give H-FABP a total sensitivity of 70.0% (95% CI 66.0 – 73.7%) and a total specificity of 80.7% (78.1 – 83.0). Excluding the study by Ghani et al, in which a quantitative assay was used, the pooled sensitivity is 76.8% (72.6 – 80.5%), pooled specificity 72.5% (68.9 – 75.8%), pooled PPV 65.8% (61.6 – 69.8%) and pooled NPV 82.0% (78.6 – 85.0%). The positive likelihood ratio would be 2.79 and negative likelihood ratio 0.32. It should be acknowledged that pooling results in this manner does not take account of heterogeneity and is inferior to a formal meta-analysis. However, assuming that these statistics are a true reflection of the performance of H-FABP, if we were to apply the test in a typical United Kingdom ED population with suspected cardiac chest pain who have a prevalence of AMI of approximately 18%, the post-test probability of AMI given a normal H-FABP test would be 6.6%. This provides similar predictive value to a normal ECG in this cohort (239) but is still far from excluding the diagnosis.

#### **4. Multimarker strategy**

Previous work from our own group, investigating heart fatty acid binding protein (H-FABP), CK-MB, myoglobin, cTnI, BNP, D-dimer, neutrophil gelatinase associated lipocalin (NGAL) and myeloperoxidase, in 705 patients presenting to the emergency department demonstrated that no single biomarker could exclude AMI. However multivariate analysis identified cTnI and H-FABP as an optimal biomarker combination. When combined with clinical risk stratification, the strategy exhibited a sensitivity of 96.9%, specificity of 54% and negative predictive value of 98%. [240].

The utility of a multimarker strategy must also be considered in the light of developments in assays. We have evaluated a high sensitivity troponin T assay in 915 patients, where the results demonstrated a negative predictive value of 99.4%. [241]

#### **5. Conclusion**

38 Traditional and Novel Risk Factors in Atherothrombosis

of 88.1% and NPV 92.9% and for predicting adverse events within 30 days the panel performed with sensitivity 88.5%, specificity 43.9%, PPV 18.0% and NPV 96.5% (230). The available evidence suggests that BNP may have value as a diagnostic and prognostic marker in patients who present to the ED with suspected ACS. However it is readily apparent that BNP is unsuitable for use as a lone biomarker in this situation. Future research is still necessary in order to define the potential role of BNP as part of a multimarker strategy.

H-FABP is a cytoplasmic protein that is abundantly expressed in human myocardial cells. It is also found in much lower concentration in skeletal muscle, kidney and brain tissue (231). Experimental data first suggested that H-FABP may be a potential novel biomarker of AMI as early as 1988 (232). In 1991 Tanaka et al reported elevated H-FABP levels in patients with AMI, with levels peaking earlier than CK-MB (233). Despite interest in H-FABP as an early marker of AMI for many years it has never gained widespread acceptance for use in clinical

Five studies have investigated the diagnostic utility of H-FABP when used for the diagnosis of AMI at the time of presentation to the ED (234-238). Four of these studies utilised qualitative assays that are available as point of care tests. All five studies had significant weaknesses, with most studies employing now outdated gold standards for AMI diagnosis and being subject to significant verification bias. The data reporting in the small study by Alashemi et al precludes calculation of total sensitivity and specificity (235). If the remainder of the results are pooled this would give H-FABP a total sensitivity of 70.0% (95% CI 66.0 – 73.7%) and a total specificity of 80.7% (78.1 – 83.0). Excluding the study by Ghani et al, in which a quantitative assay was used, the pooled sensitivity is 76.8% (72.6 – 80.5%), pooled specificity 72.5% (68.9 – 75.8%), pooled PPV 65.8% (61.6 – 69.8%) and pooled NPV 82.0% (78.6 – 85.0%). The positive likelihood ratio would be 2.79 and negative likelihood ratio 0.32. It should be acknowledged that pooling results in this manner does not take account of heterogeneity and is inferior to a formal meta-analysis. However, assuming that these statistics are a true reflection of the performance of H-FABP, if we were to apply the test in a typical United Kingdom ED population with suspected cardiac chest pain who have a prevalence of AMI of approximately 18%, the post-test probability of AMI given a normal H-FABP test would be 6.6%. This provides similar predictive value to a normal ECG in this

Previous work from our own group, investigating heart fatty acid binding protein (H-FABP), CK-MB, myoglobin, cTnI, BNP, D-dimer, neutrophil gelatinase associated lipocalin (NGAL) and myeloperoxidase, in 705 patients presenting to the emergency department demonstrated that no single biomarker could exclude AMI. However multivariate analysis identified cTnI and H-FABP as an optimal biomarker combination. When combined with clinical risk stratification, the strategy exhibited a sensitivity of 96.9%, specificity of 54% and

**3.6 Novel markers of myocardial necrosis** 

practice.

**3.6.1 Heart-type Fatty Acid Binding Protein (H-FABP)** 

cohort (239) but is still far from excluding the diagnosis.

**4. Multimarker strategy** 

negative predictive value of 98%. [240].

In recent years there has been substantial and growing interest in a number of novel biomarkers that may facilitate early diagnosis of AMI and enhanced risk stratification of patients who present to the ED with suspected ACS. Promising markers of each step in the pathophysiological evolution of an acute coronary syndrome have been identified, each of which may be detected in the peripheral circulation. Unfortunately it is unlikely that any of these biomarkers will be as cardio-specific as the cardiac troponins and, despite considerable research, there is at present no single biomarker that can be used to confirm or exclude a diagnosis of ACS in the ED. If there is to be a future for novel biomarkers in the ED diagnosis of ACS, therefore, future research must focus on incorporating levels of multiple biomarkers and available clinical information into a risk score or clinical decision rule, in order that the predictive value of individual biomarkers and clinical features may be combined and enhanced.

#### **6. References**


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**3** 

*Japan* 

**Roles of Serotonin in Atherothrombosis** 

Serotonin (5-hydroxytryptamine, 5-HT) was first identified as a powerful vasoconstrictor over a century ago (Rapport et al., 1948), and in the past 20 years has been recognized as an arterial smooth muscle mitogen (Nemecek et al., 1986). Serotonin is also known to act as a monoaminergic neurotransmitter in the brain and gastrointestinal tract, and is involved in a variety of functions, such as mood regulation, urine storage and voiding, the regulation of sleep and body temperature, food intake, and intestinal motility (Ni & Watts, 2006). Serotonin is predominantly synthesized and secreted into the blood stream by enterochromaffin cells in the gastrointestinal tract and is rapidly taken up and stored in small dense granules in platelets (Fanburg & Lee, 1997). In humans, 90% of the body's 5-HT is located in the intestines, and the rest is present primarily in platelets (89%) and the central nervous system (12%) (Fanburg & Lee, 1997). When platelets adhere and aggregate at sites of vessel injury, 5-HT is secreted and directly accelerates platelet aggregation (De

The first step in the synthesis of 5-HT from tryptophan is the enzyme tryptophan hydroxylase (TPH), which is also the rate-limiting enzyme in its biosynthesis. TPH is known to have two isoforms, TPH-1 and TPH-2, which share an overall identity of approximately 70% (Walther et al., 2003). TPH-1 is mainly present in the pineal gland, thymus, spleen, and enterochromaffin cells of the gastrointestinal tract. TPH-2 is expressed solely in neuronal cells, such as the raphe nuclei of the brainstem. Finally, 5-HT is metabolized by monoamine oxidase A to form the metabolite 5-hydroxyindole acetic acid. Monoamine oxidase A is an intracellular enzyme and 5-HT must first be taken up into the cell prior to metabolism, and

Serotonin is an extracellular mediator recognized by the 5-HT transporter and seven different receptors (5-HT15-HT7), giving rise to pleiotropic intracellular responses. All 5-HT receptors, with the exception of 5-HT6, are involved in cardiovascular regulation. Central 5- HT1A, 5-HT3, and 5-HT7 receptors play physiological roles in the regulation of cardiovascular reflexes, controlling changes in parasympathetic drive to the heart (Ramage

**1. Introduction** 

Clerck, 1990; Wester et al., 1992).

this achieved via the 5-HT transporter (Ni & Watts, 2006).

 **and Related Diseases** 

Takuya Watanabe1 and Shinji Koba2 *1Laboratory of Cardiovascular Medicine,* 

 *Tokyo University of Pharmacy and Life Sciences, Tokyo, 2Department of Medicine, Division of Cardiology, Showa University School of Medicine, Tokyo,* 


## **Roles of Serotonin in Atherothrombosis and Related Diseases**

Takuya Watanabe1 and Shinji Koba2

*1Laboratory of Cardiovascular Medicine, Tokyo University of Pharmacy and Life Sciences, Tokyo, 2Department of Medicine, Division of Cardiology, Showa University School of Medicine, Tokyo, Japan* 

#### **1. Introduction**

56 Traditional and Novel Risk Factors in Atherothrombosis

[236] Seino Y, Ogata K, Takano T, Ishii J, Hishida H, Morita H, et al. Use of a whole blood

[237] Ghani F, Wu AH, Graff L, Petry C, Armstrong G, Prigent F, et al. Role of heart-type

[238] Valle HA, Riesgo LGC, Bel MS, Gonazalo FE, Sanchez MS, Oliva LI. Clinical

exclusion of acute myocardial infarction. Resuscitation 2011; 82: 1041-6. [241] Body R, Carley S, McDowell G, Jaffe AS, France M, Cruickshant K, Wibberley C,

American Journal of Medicine 115(3):185-90, 2003 Aug 15.

Chemistry 2000;46(5):718-9.

Cardiol 2011; 58: 1333-9.

rapid panel test for heart-type fatty acid-binding protein in patients with acute chest pain: comparison with rapid troponin T and myoglobin tests.[see comment].

fatty acid-binding protein in early detection of acute myocardial infarction. Clinical

assessment of heart-type fatty acid-binding protein in early diagnosis of acute coronary syndrome. European Journal of Emergency Medicine 2008;15(3):140-4. [239] Panju AA, Hemmelgam BR, Guyatt GH, Simei DL. The rational clinical examination: Is this patient having a myocardial infarction. JAMA 1998;280(14):1256-63. [240] Body R, McDowell G, Carley S, Wibberley C, Gerguson J, Mackway-Jones K. A FABP-

ulous rule out strategy? Heart fatty acid binding protein and troponin for rapid

Nuttall M, Mackway-Jones K. Rapid exclusion of acute myocardial infarction in patients with undetectable troponin using a high sensitivity assay. J Am Coll

Serotonin (5-hydroxytryptamine, 5-HT) was first identified as a powerful vasoconstrictor over a century ago (Rapport et al., 1948), and in the past 20 years has been recognized as an arterial smooth muscle mitogen (Nemecek et al., 1986). Serotonin is also known to act as a monoaminergic neurotransmitter in the brain and gastrointestinal tract, and is involved in a variety of functions, such as mood regulation, urine storage and voiding, the regulation of sleep and body temperature, food intake, and intestinal motility (Ni & Watts, 2006). Serotonin is predominantly synthesized and secreted into the blood stream by enterochromaffin cells in the gastrointestinal tract and is rapidly taken up and stored in small dense granules in platelets (Fanburg & Lee, 1997). In humans, 90% of the body's 5-HT is located in the intestines, and the rest is present primarily in platelets (89%) and the central nervous system (12%) (Fanburg & Lee, 1997). When platelets adhere and aggregate at sites of vessel injury, 5-HT is secreted and directly accelerates platelet aggregation (De Clerck, 1990; Wester et al., 1992).

The first step in the synthesis of 5-HT from tryptophan is the enzyme tryptophan hydroxylase (TPH), which is also the rate-limiting enzyme in its biosynthesis. TPH is known to have two isoforms, TPH-1 and TPH-2, which share an overall identity of approximately 70% (Walther et al., 2003). TPH-1 is mainly present in the pineal gland, thymus, spleen, and enterochromaffin cells of the gastrointestinal tract. TPH-2 is expressed solely in neuronal cells, such as the raphe nuclei of the brainstem. Finally, 5-HT is metabolized by monoamine oxidase A to form the metabolite 5-hydroxyindole acetic acid. Monoamine oxidase A is an intracellular enzyme and 5-HT must first be taken up into the cell prior to metabolism, and this achieved via the 5-HT transporter (Ni & Watts, 2006).

Serotonin is an extracellular mediator recognized by the 5-HT transporter and seven different receptors (5-HT15-HT7), giving rise to pleiotropic intracellular responses. All 5-HT receptors, with the exception of 5-HT6, are involved in cardiovascular regulation. Central 5- HT1A, 5-HT3, and 5-HT7 receptors play physiological roles in the regulation of cardiovascular reflexes, controlling changes in parasympathetic drive to the heart (Ramage

Roles of Serotonin in Atherothrombosis and Related Diseases 59

effects of other vasoactive agents, such as endothelin-1, angiotensin II, urotensin II, and thromboxane A2 (Watanabe et al., 2001c, 2001d, 2001e; Pakala et al. 1997), platelet-derived microparticles (Pakala, 2004), coagulation factors, such as thrombin and coagulation factor Xa (Pakala & Benedict, 1999; Pakala, 2003), and monocyte chemoattractant protein-1 on VSMCs (Watanabe et al., 2001f). In addition, 5-HT stimulates the expression of interleukin-6

Serotonin stimulates the expression of tissue factor and plasminogen activator inhibitor-1 in endothelial cells through 5-HT2A receptors (Kawano et al., 2001). Serotonin-stimulated endothelial cells secrete a T lymphocyte-specific chemotactic cytokine with competence growth factor activity (Katz et al., 1994). Serotonin, alone and combined with thromboxane A2, potently induces endothelial cell proliferation (Pakala et al., 1994, 1999b). However, there is still controversy regarding the effects of 5-HT on endothelial cell proliferation (Ruiz-

Serotonin stimulates monocyte adhesion (Lorenowicz et al., 2006), and enhances macrophage foam cell formation associated with increased uptake of oxidized LDL (Aviram et al., 1992) and up-regulation of acyl-coenzyme A:cholesterol acyltransferase-1 (ACAT-1)

The roles of 5-HT in the pathogenesis of atherothrombotic diseases are revealed by the results of pharmacological interventions involving 5-HT2A receptors. Functional analyses of the roles of 5-HT in the cardiovascular system using 5-HT2A receptor knockout mice have not been performed. Several studies performed before the discovery of specific and/or selective 5-HT2A receptor antagonists indicated that 5-HT2 receptor blockers inhibit angioplasty-induced vasospasm and microvascular constriction following atherosclerotic

Sarpogrelate, a selective 5-HT2A receptor antagonist, inhibits responses to 5-HT mediated by 5-HT2A receptors, such as platelet aggregation and thrombus formation (H Hara et al., 1991a; Nishihira et al., 2006), and prevents the development of atherosclerotic lesions (H Hara et al., 1991b; Hayashi et al., 2003), vasospasm (Miyata et al., 2000), and intimal hyperplasia in vein grafts after bypass grafting (Kodama et al., 2009). This drug suppresses ACAT-1 expression in macrophages (Suguro et al., 2006), vascular oxidative stress and VSMC proliferation (Watanabae et al., 2001d; Sun et al., 2011), up-regulates endothelial nitric oxide synthase (Hayashi et al., 2003), and reduces the expression of matrix metalloproteinase-1 that degrades the arterial extracellular matrix (Hayashi et al., 2003), contributing to

In a previous study involving the measurement of washed platelet-bound 5-HT concentration in three groups based on the presence and absence of thrombotic diseases,

plaque rupture in atherosclerotic rabbit models (Sigal et al., 1991; Taylor et al., 2004).

and cyclooxygenase-2 in VSMCs (Ito et al., 2000; Machida et al., 2011).

**5. Endothelial cell function** 

**6. Macrophage foam cell formation** 

through 5-HT2A receptors (Suguro et al., 2006).

**7. 5-HT2A receptor blockade** 

stabilization of vulnerable plaque.

**8. 5-HT concentration and Cardiovascular Disease** 

Perez et al., 2011).

& Villalon, 2008). These reflexes also affect the activity of the sympathetic nervous system, which itself can be inhibited by stimulation of central 5-HT1A receptors causing a drop in blood pressure and excited by 5-HT2 receptor stimulation resulting in an increase in blood pressure. Acute vascular constriction by 5-HT is usually mediated by 5-HT1B and 5-HT2A receptors, except in the intracranial arteries in which constriction is mediated only through 5-HT1B receptors (Kaumann & Levy, 2006). Both 5-HT1B and 5-HT2A receptors can mediate coronary artery spasm and pulmonary hypertension.

Serotonin promotes platelet aggregation and the proliferation, migration, and contraction of vascular smooth muscle cells (VSMCs). In addition to physiological hemostasis, these vascular responses play pivotal roles in the development and progression of atherothrombotic diseases.

#### **2. Platelet aggregation**

When platelets aggregate, 5-HT is released into the extracellular environment from the dense granules of activated platelets. The 5-HT thus released further activates other platelets by binding to 5-HT2A receptors on the platelet membrane, contributing to thrombus formation (Satoh et al., 2006). Serotonin promotes further platelet recruitment and activates the coagulation pathway. The blood vessels in which platelets aggregate are exposed to high concentrations of 5-HT (Benedict et al., 1986).

#### **3. Vasoconstriction**

Serotonin is well known to act as a potent vasoconstrictor and has been shown to cause both vasoconstriction and vasodilation by interacting with receptors expressed on VSMCs, endothelial cells, or adrenergic nerve endings. In systemic arterial smooth muscle, 5-HT induces contractions only at sites of endothelial damage where platelet aggregation occurs, and this effect is antagonized by 5-HT2 receptor antagonists (Sigal et al., 1991). Serotonin also amplifies the effects of other vasoconstrictors, such as histamine, angiotensin II, prostaglandin F2, and noradrenaline (O'Rourke et al., 2006). The potent contractile effect of 5-HT may contribute to the vasoconstriction of coronary collateral vessels developed by reduction of coronary blood flow (Wright et al., 1992) and vasospastic disorders in arteries covered with regenerated endothelium and in atherosclerotic arteries (Sobey et al., 1991). The blood vessel wall chronically exposed to abnormally high blood pressure is characterized by increased vascular responsiveness to 5-HT. Chronic blockade of 5-HT2A receptors reduces the development of hypertension in spontaneously hypertensive rats (Gradin et al., 1991).

### **4. VSMC proliferation**

Serotonin stimulates the migration and proliferation of VSMCs through 5-HT2A receptors (Tamura et al., 1997; Pakala et al., 1997, 1999a). Serotonin interacts synergistically with atherogenic lipoproteins (low-density lipoprotein [LDL] and β-very low density lipoprotein) (Koba et al., 1999, 2000), oxidized LDL and its major components, such as lysophosphatidylcholine, 4-hydroxy-2-nonenal, and reactive oxygen species (Watanabe et al., 2001a, 2001b) in inducing VSMC proliferation. Serotonin also potentiates the mitogenic effects of other vasoactive agents, such as endothelin-1, angiotensin II, urotensin II, and thromboxane A2 (Watanabe et al., 2001c, 2001d, 2001e; Pakala et al. 1997), platelet-derived microparticles (Pakala, 2004), coagulation factors, such as thrombin and coagulation factor Xa (Pakala & Benedict, 1999; Pakala, 2003), and monocyte chemoattractant protein-1 on VSMCs (Watanabe et al., 2001f). In addition, 5-HT stimulates the expression of interleukin-6 and cyclooxygenase-2 in VSMCs (Ito et al., 2000; Machida et al., 2011).

#### **5. Endothelial cell function**

58 Traditional and Novel Risk Factors in Atherothrombosis

& Villalon, 2008). These reflexes also affect the activity of the sympathetic nervous system, which itself can be inhibited by stimulation of central 5-HT1A receptors causing a drop in blood pressure and excited by 5-HT2 receptor stimulation resulting in an increase in blood pressure. Acute vascular constriction by 5-HT is usually mediated by 5-HT1B and 5-HT2A receptors, except in the intracranial arteries in which constriction is mediated only through 5-HT1B receptors (Kaumann & Levy, 2006). Both 5-HT1B and 5-HT2A receptors can mediate

Serotonin promotes platelet aggregation and the proliferation, migration, and contraction of vascular smooth muscle cells (VSMCs). In addition to physiological hemostasis, these vascular responses play pivotal roles in the development and progression of

When platelets aggregate, 5-HT is released into the extracellular environment from the dense granules of activated platelets. The 5-HT thus released further activates other platelets by binding to 5-HT2A receptors on the platelet membrane, contributing to thrombus formation (Satoh et al., 2006). Serotonin promotes further platelet recruitment and activates the coagulation pathway. The blood vessels in which platelets aggregate are exposed to high

Serotonin is well known to act as a potent vasoconstrictor and has been shown to cause both vasoconstriction and vasodilation by interacting with receptors expressed on VSMCs, endothelial cells, or adrenergic nerve endings. In systemic arterial smooth muscle, 5-HT induces contractions only at sites of endothelial damage where platelet aggregation occurs, and this effect is antagonized by 5-HT2 receptor antagonists (Sigal et al., 1991). Serotonin also amplifies the effects of other vasoconstrictors, such as histamine, angiotensin II, prostaglandin F2, and noradrenaline (O'Rourke et al., 2006). The potent contractile effect of 5-HT may contribute to the vasoconstriction of coronary collateral vessels developed by reduction of coronary blood flow (Wright et al., 1992) and vasospastic disorders in arteries covered with regenerated endothelium and in atherosclerotic arteries (Sobey et al., 1991). The blood vessel wall chronically exposed to abnormally high blood pressure is characterized by increased vascular responsiveness to 5-HT. Chronic blockade of 5-HT2A receptors reduces the development of hypertension in spontaneously hypertensive rats

Serotonin stimulates the migration and proliferation of VSMCs through 5-HT2A receptors (Tamura et al., 1997; Pakala et al., 1997, 1999a). Serotonin interacts synergistically with atherogenic lipoproteins (low-density lipoprotein [LDL] and β-very low density lipoprotein) (Koba et al., 1999, 2000), oxidized LDL and its major components, such as lysophosphatidylcholine, 4-hydroxy-2-nonenal, and reactive oxygen species (Watanabe et al., 2001a, 2001b) in inducing VSMC proliferation. Serotonin also potentiates the mitogenic

coronary artery spasm and pulmonary hypertension.

concentrations of 5-HT (Benedict et al., 1986).

atherothrombotic diseases.

**2. Platelet aggregation** 

**3. Vasoconstriction** 

(Gradin et al., 1991).

**4. VSMC proliferation** 

Serotonin stimulates the expression of tissue factor and plasminogen activator inhibitor-1 in endothelial cells through 5-HT2A receptors (Kawano et al., 2001). Serotonin-stimulated endothelial cells secrete a T lymphocyte-specific chemotactic cytokine with competence growth factor activity (Katz et al., 1994). Serotonin, alone and combined with thromboxane A2, potently induces endothelial cell proliferation (Pakala et al., 1994, 1999b). However, there is still controversy regarding the effects of 5-HT on endothelial cell proliferation (Ruiz-Perez et al., 2011).

#### **6. Macrophage foam cell formation**

Serotonin stimulates monocyte adhesion (Lorenowicz et al., 2006), and enhances macrophage foam cell formation associated with increased uptake of oxidized LDL (Aviram et al., 1992) and up-regulation of acyl-coenzyme A:cholesterol acyltransferase-1 (ACAT-1) through 5-HT2A receptors (Suguro et al., 2006).

### **7. 5-HT2A receptor blockade**

The roles of 5-HT in the pathogenesis of atherothrombotic diseases are revealed by the results of pharmacological interventions involving 5-HT2A receptors. Functional analyses of the roles of 5-HT in the cardiovascular system using 5-HT2A receptor knockout mice have not been performed. Several studies performed before the discovery of specific and/or selective 5-HT2A receptor antagonists indicated that 5-HT2 receptor blockers inhibit angioplasty-induced vasospasm and microvascular constriction following atherosclerotic plaque rupture in atherosclerotic rabbit models (Sigal et al., 1991; Taylor et al., 2004).

Sarpogrelate, a selective 5-HT2A receptor antagonist, inhibits responses to 5-HT mediated by 5-HT2A receptors, such as platelet aggregation and thrombus formation (H Hara et al., 1991a; Nishihira et al., 2006), and prevents the development of atherosclerotic lesions (H Hara et al., 1991b; Hayashi et al., 2003), vasospasm (Miyata et al., 2000), and intimal hyperplasia in vein grafts after bypass grafting (Kodama et al., 2009). This drug suppresses ACAT-1 expression in macrophages (Suguro et al., 2006), vascular oxidative stress and VSMC proliferation (Watanabae et al., 2001d; Sun et al., 2011), up-regulates endothelial nitric oxide synthase (Hayashi et al., 2003), and reduces the expression of matrix metalloproteinase-1 that degrades the arterial extracellular matrix (Hayashi et al., 2003), contributing to stabilization of vulnerable plaque.

#### **8. 5-HT concentration and Cardiovascular Disease**

In a previous study involving the measurement of washed platelet-bound 5-HT concentration in three groups based on the presence and absence of thrombotic diseases,

Roles of Serotonin in Atherothrombosis and Related Diseases 61

CAD patients aged 70 years old indicated a better prognosis with regard to cardiac events for patients with low 5-HT (< 1 mol/l) (log rank test, p < 0.05). Venous plasma 5-HT concentration measured by radioimmunoassay was reported to be significantly higher in patients with variant angina pectoris than in those with healed myocardial infarction or controls (Figueras et al., 2005). On the other hand, comparison of 5-HT concentration in platelet-poor plasma and whole blood indicated that plasma 5-HT concentration tended to increase with age, while its concentration in whole blood decreased (K Hara et al., 2004). The ratio of plasma to whole-blood concentration of 5-HT was significantly higher in various types of CAD, such as variant angina pectoris, acute coronary syndrome (ACS), and prior myocardial infarction, compared with healthy controls, whereas whole-blood 5-HT levels were somewhat higher in healthy controls than in patients with effort angina. The ratio of plasma to whole-blood concentration of 5-HT was recently demonstrated to be positively correlated with Framingham 10-year risk scores for CAD (Y Hirowatari et al., 2011). These clinical studies suggested that high levels of 5-HT are significantly associated with atherosclerotic cardiovascular diseases and the occurrence of cardiovascular events. Thus, 5-

Several clinical studies with 5-HT2A receptor blockers have supported the experimental results demonstrating that 5-HT plays an important role in the development of CAD due to platelet aggregation, VSMC constriction, and migration and proliferation of VSMCs. In a study of 22 patients with stable effort angina, oral administration of 200 mg of sarpogrelate 1 hour prior to treadmill exercise test was shown to improve exercise capacity and the severity score determined by myocardial perfusion scintigraphy in 12 patients with welldeveloped collateral flow evaluated by coronary angiography, whereas sarpogrelate affected neither exercise time nor severity score in other patients without collateral flow (Tanaka et al., 1998). This was confirmed in another study involving 2 weeks of treatment with 300 mg of sarpogrelate in 20 patients with stable angina pectoris (Kinugawa et al., 2002); treatment with sarpogrelate significantly increased the specific activity scale score, increased exercise time and rate-pressure product, an index of myocardial oxygen consumption, at onset of ischemic ST depression 0.1 mV on electrocardiogram, and decreased the number of anginal attacks only in patients with angiographically proven welldeveloped collateral flow. On the other hand, sarpogrelate showed no effects in the patients without well-developed collateral flow. Similar results were obtained in another study with intravenous injection of another 5-HT2A receptor antagonist, ketanserin (Kyriakides et al., 1999). In a study of stable angina pectoris patients with single-vessel disease, ketanserin increased coronary collateral blood flow and decreased myocardial ischemia during POBA. In a study of 15 CAD patients without significant stenosis (< 75% diameter stenosis) in the left anterior descending coronary artery, oral administration of 200 mg of sarpogrelate increased the coronary blood flow velocity at both baseline and hyperemia evaluated by intracoronary Doppler guidewire without any effects on systemic blood pressure or cardiac output (Satomura et al., 2002). On the other hand, there were no significant differences in baseline or hyperemic coronary blood flow velocity in the control group. These results suggested that sarpogrelate augments coronary flow reserve by inhibiting 5-HT-induced coronary vasoconstriction and platelet aggregation in collateral vessels in CAD patients.

HT plays a key role in the pathogenesis of atherothrombosis.

**9. 5-HT2A receptor blocker and treatment of CAD** 

platelet 5-HT levels were highest in patients with deep-vein thrombosis and pulmonary embolism prior to death from thrombotic events. The lowest levels were detected in subjects without thrombosis, and intermediate levels were seen in patients with cerebral thrombosis (Misra et al., 1975). These findings suggested that 5-HT plays an important role in the initiation of thrombus formation.

With regard to the association between 5-HT and coronary artery disease (CAD), it has been reported that coronary sinus plasma samples from CAD patients evoked vasoconstriction, whereas systemic artery and venous samples from patients without CAD did not (Rubanyi et al., 1987). In addition, the vasoactive activity of the coronary sinus plasma showed a positive correlation with the severity and extent of coronary artery narrowing, and among various pharmacological interventions only methiothepin, a non-selective 5-HT receptor antagonist, prevented the vasoconstriction induced by these coronary sinus plasma samples. Although this study did not measure 5-HT concentration directly, these results suggested that the amount of 5-HT released into the coronary sinus plays an important role in vasoconstriction in the coronary circulation. The first direct measurement of 5-HT concentration in human coronary circulation was reported by van den Berg and co-workers (van den Berg et al., 1989), who measured 5-HT concentration by modified radioenzymatic assay (Benedict et al., 1986; Hussain & Sole, 1981) in platelet-poor plasma obtained from the central aorta and coronary sinus of 52 patients referred for cardiac catheterization. The 5-HT concentration in the coronary circulation determined by subtracting the levels in the aorta from those in the coronary sinus is significantly higher in patients with CAD compared with those without CAD (0.6 ± 6.6 ng/ml vs. -5.6 ± 10.3 ng/ml, mean ± SD, p < 0.05). These concentrations were significantly higher in CAD patients with complex coronary lesions compared with those with smooth concentric lesions (3.1 ± 5.5 ng/ml vs. -1.9 ± 6.6 ng/ml, p < 0.02). A similar method was used to measure 5-HT concentration in coronary circulation in 8 patients with CAD undergoing plain old balloon angioplasty (POBA) (Golino et al., 1994). The 5-HT levels in the coronary sinus increased significantly after POBA, while those in the aorta did not change. Coronary constriction distal to the site of dilation observed after POBA was positively correlated with the 5-HT concentration in the coronary circulation, and this coronary constriction was inhibited by pretreatment with the 5-HT2A receptor antagonist, ketanserin. Other studies using similar techniques have shown that the transcardiac 5-HT concentration is significantly higher in patients with variant angina pectoris compared with non-CAD controls (Murakami et al., 1996, 1998). These studies demonstrated that 5-HT released from activated platelets plays an important role in the pathogenesis of CAD in humans. However, the methods used in these studies required invasive procedures.

Vikenes and co-workers measured 5-HT concentrations in platelet rich plasma from venous blood using high-performance liquid chromatography (HPLC) in 122 men undergoing coronary angiography (Vikenes et al., 1999). Their data indicated that total 5-HT concentration was positively correlated with platelet count (r = 0.552, p < 0.001), and both total 5-HT concentration and platelet counts were significantly higher in patients with CAD compared with those without CAD. The difference in 5-HT level was greatest in men aged 60 years old, and the difference reduced steadily with age. The high 5-HT concentration ≥ 1 mol/l was significantly associated with CAD, with an odds ratio (OR) of 3.84 (95% confidence interval [CI] 1.1213.11), independently of age and smoking. During a mean follow-up period of 44 ± 15 months, Kaplan-Meier cardiac event-free survival curves for

platelet 5-HT levels were highest in patients with deep-vein thrombosis and pulmonary embolism prior to death from thrombotic events. The lowest levels were detected in subjects without thrombosis, and intermediate levels were seen in patients with cerebral thrombosis (Misra et al., 1975). These findings suggested that 5-HT plays an important role in the

With regard to the association between 5-HT and coronary artery disease (CAD), it has been reported that coronary sinus plasma samples from CAD patients evoked vasoconstriction, whereas systemic artery and venous samples from patients without CAD did not (Rubanyi et al., 1987). In addition, the vasoactive activity of the coronary sinus plasma showed a positive correlation with the severity and extent of coronary artery narrowing, and among various pharmacological interventions only methiothepin, a non-selective 5-HT receptor antagonist, prevented the vasoconstriction induced by these coronary sinus plasma samples. Although this study did not measure 5-HT concentration directly, these results suggested that the amount of 5-HT released into the coronary sinus plays an important role in vasoconstriction in the coronary circulation. The first direct measurement of 5-HT concentration in human coronary circulation was reported by van den Berg and co-workers (van den Berg et al., 1989), who measured 5-HT concentration by modified radioenzymatic assay (Benedict et al., 1986; Hussain & Sole, 1981) in platelet-poor plasma obtained from the central aorta and coronary sinus of 52 patients referred for cardiac catheterization. The 5-HT concentration in the coronary circulation determined by subtracting the levels in the aorta from those in the coronary sinus is significantly higher in patients with CAD compared with those without CAD (0.6 ± 6.6 ng/ml vs. -5.6 ± 10.3 ng/ml, mean ± SD, p < 0.05). These concentrations were significantly higher in CAD patients with complex coronary lesions compared with those with smooth concentric lesions (3.1 ± 5.5 ng/ml vs. -1.9 ± 6.6 ng/ml, p < 0.02). A similar method was used to measure 5-HT concentration in coronary circulation in 8 patients with CAD undergoing plain old balloon angioplasty (POBA) (Golino et al., 1994). The 5-HT levels in the coronary sinus increased significantly after POBA, while those in the aorta did not change. Coronary constriction distal to the site of dilation observed after POBA was positively correlated with the 5-HT concentration in the coronary circulation, and this coronary constriction was inhibited by pretreatment with the 5-HT2A receptor antagonist, ketanserin. Other studies using similar techniques have shown that the transcardiac 5-HT concentration is significantly higher in patients with variant angina pectoris compared with non-CAD controls (Murakami et al., 1996, 1998). These studies demonstrated that 5-HT released from activated platelets plays an important role in the pathogenesis of CAD in humans. However, the

initiation of thrombus formation.

methods used in these studies required invasive procedures.

Vikenes and co-workers measured 5-HT concentrations in platelet rich plasma from venous blood using high-performance liquid chromatography (HPLC) in 122 men undergoing coronary angiography (Vikenes et al., 1999). Their data indicated that total 5-HT concentration was positively correlated with platelet count (r = 0.552, p < 0.001), and both total 5-HT concentration and platelet counts were significantly higher in patients with CAD compared with those without CAD. The difference in 5-HT level was greatest in men aged 60 years old, and the difference reduced steadily with age. The high 5-HT concentration ≥ 1 mol/l was significantly associated with CAD, with an odds ratio (OR) of 3.84 (95% confidence interval [CI] 1.1213.11), independently of age and smoking. During a mean follow-up period of 44 ± 15 months, Kaplan-Meier cardiac event-free survival curves for CAD patients aged 70 years old indicated a better prognosis with regard to cardiac events for patients with low 5-HT (< 1 mol/l) (log rank test, p < 0.05). Venous plasma 5-HT concentration measured by radioimmunoassay was reported to be significantly higher in patients with variant angina pectoris than in those with healed myocardial infarction or controls (Figueras et al., 2005). On the other hand, comparison of 5-HT concentration in platelet-poor plasma and whole blood indicated that plasma 5-HT concentration tended to increase with age, while its concentration in whole blood decreased (K Hara et al., 2004). The ratio of plasma to whole-blood concentration of 5-HT was significantly higher in various types of CAD, such as variant angina pectoris, acute coronary syndrome (ACS), and prior myocardial infarction, compared with healthy controls, whereas whole-blood 5-HT levels were somewhat higher in healthy controls than in patients with effort angina. The ratio of plasma to whole-blood concentration of 5-HT was recently demonstrated to be positively correlated with Framingham 10-year risk scores for CAD (Y Hirowatari et al., 2011). These clinical studies suggested that high levels of 5-HT are significantly associated with atherosclerotic cardiovascular diseases and the occurrence of cardiovascular events. Thus, 5- HT plays a key role in the pathogenesis of atherothrombosis.

#### **9. 5-HT2A receptor blocker and treatment of CAD**

Several clinical studies with 5-HT2A receptor blockers have supported the experimental results demonstrating that 5-HT plays an important role in the development of CAD due to platelet aggregation, VSMC constriction, and migration and proliferation of VSMCs. In a study of 22 patients with stable effort angina, oral administration of 200 mg of sarpogrelate 1 hour prior to treadmill exercise test was shown to improve exercise capacity and the severity score determined by myocardial perfusion scintigraphy in 12 patients with welldeveloped collateral flow evaluated by coronary angiography, whereas sarpogrelate affected neither exercise time nor severity score in other patients without collateral flow (Tanaka et al., 1998). This was confirmed in another study involving 2 weeks of treatment with 300 mg of sarpogrelate in 20 patients with stable angina pectoris (Kinugawa et al., 2002); treatment with sarpogrelate significantly increased the specific activity scale score, increased exercise time and rate-pressure product, an index of myocardial oxygen consumption, at onset of ischemic ST depression 0.1 mV on electrocardiogram, and decreased the number of anginal attacks only in patients with angiographically proven welldeveloped collateral flow. On the other hand, sarpogrelate showed no effects in the patients without well-developed collateral flow. Similar results were obtained in another study with intravenous injection of another 5-HT2A receptor antagonist, ketanserin (Kyriakides et al., 1999). In a study of stable angina pectoris patients with single-vessel disease, ketanserin increased coronary collateral blood flow and decreased myocardial ischemia during POBA. In a study of 15 CAD patients without significant stenosis (< 75% diameter stenosis) in the left anterior descending coronary artery, oral administration of 200 mg of sarpogrelate increased the coronary blood flow velocity at both baseline and hyperemia evaluated by intracoronary Doppler guidewire without any effects on systemic blood pressure or cardiac output (Satomura et al., 2002). On the other hand, there were no significant differences in baseline or hyperemic coronary blood flow velocity in the control group. These results suggested that sarpogrelate augments coronary flow reserve by inhibiting 5-HT-induced coronary vasoconstriction and platelet aggregation in collateral vessels in CAD patients.

Roles of Serotonin in Atherothrombosis and Related Diseases 63

Miyazaki and colleagues (Miyazaki et al., 2007), 22 patients with PAD received either sarpogrelate at a dose of 300 mg orally or conventional therapy for 12 weeks. Both forearm and leg endothelium-dependent vasodilation were improved and maintained for 24 weeks in patients treated with sarpogrelate whereas no improvement was observed in patients treated with conventional therapy. In addition, endothelium-nondependent vasodilation was similar between the two treatment groups. These results suggest that 12 weeks of treatment with sarpogrelate improved vascular endothelial function in PAD patients. Further they investigated the effects of a combination of bone marrow mononuclear cell implantation and sarpogrelate on endothelial function in 16 PAD patients (Higashi et al., 2010). They performed the evaluations before and after bone marrow mononuclear cell implantation in 16 patients with critical limb ischemia. A 12-week course of sarpogrelate treatment amplified the increased leg blood flow responses to acetylcholine evaluated by plethysmography induced by bone marrow mononuclear cell implantation compared with conventional treatment, whereas bone marrow mononuclear cell implantation improved limb ischemic symptoms in the sarpogrealate group as well as in the conventional treatment group. These two studies showed that a treatment with sarpogrelate for at least 12 weeks has a beneficial effect on vascular endothelial function in PAD patients treated with

There have been several small studies of the effects of sarpogrelate on various biomarkers without controls in PAD patients. In a study of 13 patients with ASO, treatment with sarpogrelate for 1 week decreased adenosine diphosphate (ADP)- or collagen-induced platelet aggregation and reduced the releases of platelet-derived growth factor, soluble Pselectin, and transforming growth factor-β1 from platelets stimulated by ADP or collagen (Nakamura et al., 2001). In a study of 24 non-diabetic and non-medicated diabetic patients with PAD (Fontaine grades 1 and 2), 300 mg of sarpogrelate decreased insulin resistance at 2 weeks and 3 months after treatment, and increased plasma levels of adiponectin at 3 months after treatment (Kokubu, 2006). Similarly, 300 mg of sarpogrelate increased adiponectin levels at 2 and 3 months after treatment in 8 diabetic patients with ASO (Yamakawa et al., 2003). Treatment with 300 mg of sarpogrelate improved limb ischemic symptoms and decreased interleukin-18 levels in 8 diabetic patients with ASO (Yamakawa et al., 2004). In a study of 10 patients with Buerger's disease, 8 weeks of treatment with 300 mg of sarpogrelate was well-tolerated. However, platelet aggregation induced by 5-HT increased significantly after 2 and 4 weeks, and whole-blood 5-HT concentration increased

significantly after 2 weeks of treatment (Rydzewski et al, 1996).

**11. 5-HT2A receptor blocker and treatment of ischemic cerebrovascular** 

There have been no reports of high plasma 5-HT levels in patients with stroke at the acute phase. In a study of elderly subjects, plasma 5-HT concentration measured by enzyme immunoassay was significantly higher in patients with vascular dementia caused by stroke or atherosclerotic small vessel disease compared with age-matched controls (Ban et al.,

In a double-blind, controlled, clinical-pharmacological study, 47 patients with ischemic stroke who discontinued any antiplatelet agents and anticoagulants or fibrinolytic agents,

conventional therapy.

**disease** 

2007).

In a comparative study of the effects of oral administration of sarpogrelate administration (200 mg) or placebo in addition to aspirin and ticlopidine 60 minutes before POBA in 20 patients with stable effort angina with a de novo single stenotic lesion of 75%90%, length < 20 mm in the proximal left anterior descending coronary artery, sarpogrelate significantly reduced the ischemic ST changes after coronary angioplasty compared with the placebo group with no changes in collateral blood flow, blood pressure, or heart rate (Horibe et al., 2004). These observations suggested that sarpogrelate improves myocardial ischemic injury by pharmacological ischemic preconditioning rather than by stimulating collateral development.

Studies investigating the effects of 5-HT2A receptor blockers on prevention of restenosis after coronary angioplasty yielded conflicting results between ketanserin and sarpogrelate. In a small placebo-controlled study, 24-hour infusion of ketanserin following POBA prevented the early restenosis evaluated at 24 hours after POBA but failed to prevent restenosis at 4 to 9 months after POBA (Klein et al., 1990). The Post-Angioplasty Restenosis Ketanserin (PARK) study was a randomized, double-blind, placebo-controlled trial to assess the effects of ketanserin in prevention of restenosis after POBA (Serruys et al., 1993). A total of 658 patients with stable or unstable angina pectoris who were scheduled to undergo elective POBA received either ketanserin (loading dose, 40 mg 1 hour before POBA; maintenance dose, 40 mg bid for 6 months) or placebo. All patients received aspirin for 6 months. The primary clinical end points were defined as any one of the following: cardiac death, myocardial infarction, or the need for repeat angioplasty or bypass surgery of the previously dilated sites between the first POBA and 6 months after POBA. The relative risk of the primary end points for the ketanserin group compared with the placebo group was 0.89 (95% CI 0.701.13). The restenosis rate according to > 50% stenosis and the quantitative angiographic findings were similar between the two groups. The PARK study failed to show that ketanserin could prevent restenosis and improve clinical outcome after POBA. On the other hand, in an investigation of the effects of sarpogrelate in prevention of restenosis after coronary stenting in Japanese patients with stable angina pectoris, pretreatment with sarpogrelate for 3 days before coronary stenting and continuation of 300 mg of sarpogrelate for 6 months in addition to 81 mg of aspirin and 200 mg of ticlopidine markedly reduced restenosis rate at 6 months after coronary stenting compared with the non-sarpogrelate treatment group (4.3% vs. 28.6%, p < 0.005) (Fujita et al., 2003). The results of multivariate logistic regression analysis showed that treatment with sarpogrelate was a significant predictor for angiographic restenosis, independent of the findings of quantitative coronary angiography, stent characteristics, and the presence of diabetes. These two studies differed in 5-HT2A receptor blocker drug characteristics and pretreatment period as well as the angioplasty procedure. The latter study supported the suggestion that sarpogrelate may prevent the development of intimal hyperplasia due to VSMC proliferation. Further randomized controlled trials to investigate the effects of sarpogrelate on prevention of restenosis after placement of drug-eluting stents are required.

#### **10. 5-HT2A receptor blocker and treatment of Peripheral Artery Disease**

Sarpogrelate is widely used clinically as an anti-platelet drug for prevention of thrombosis and treatment of critical limb ischemic symptoms in patients with peripheral artery disease (PAD), such as arteriosclerosis obliterans (ASO) and Buerger's disease. In a study by

In a comparative study of the effects of oral administration of sarpogrelate administration (200 mg) or placebo in addition to aspirin and ticlopidine 60 minutes before POBA in 20 patients with stable effort angina with a de novo single stenotic lesion of 75%90%, length < 20 mm in the proximal left anterior descending coronary artery, sarpogrelate significantly reduced the ischemic ST changes after coronary angioplasty compared with the placebo group with no changes in collateral blood flow, blood pressure, or heart rate (Horibe et al., 2004). These observations suggested that sarpogrelate improves myocardial ischemic injury by pharmacological ischemic preconditioning rather than by stimulating collateral

Studies investigating the effects of 5-HT2A receptor blockers on prevention of restenosis after coronary angioplasty yielded conflicting results between ketanserin and sarpogrelate. In a small placebo-controlled study, 24-hour infusion of ketanserin following POBA prevented the early restenosis evaluated at 24 hours after POBA but failed to prevent restenosis at 4 to 9 months after POBA (Klein et al., 1990). The Post-Angioplasty Restenosis Ketanserin (PARK) study was a randomized, double-blind, placebo-controlled trial to assess the effects of ketanserin in prevention of restenosis after POBA (Serruys et al., 1993). A total of 658 patients with stable or unstable angina pectoris who were scheduled to undergo elective POBA received either ketanserin (loading dose, 40 mg 1 hour before POBA; maintenance dose, 40 mg bid for 6 months) or placebo. All patients received aspirin for 6 months. The primary clinical end points were defined as any one of the following: cardiac death, myocardial infarction, or the need for repeat angioplasty or bypass surgery of the previously dilated sites between the first POBA and 6 months after POBA. The relative risk of the primary end points for the ketanserin group compared with the placebo group was 0.89 (95% CI 0.701.13). The restenosis rate according to > 50% stenosis and the quantitative angiographic findings were similar between the two groups. The PARK study failed to show that ketanserin could prevent restenosis and improve clinical outcome after POBA. On the other hand, in an investigation of the effects of sarpogrelate in prevention of restenosis after coronary stenting in Japanese patients with stable angina pectoris, pretreatment with sarpogrelate for 3 days before coronary stenting and continuation of 300 mg of sarpogrelate for 6 months in addition to 81 mg of aspirin and 200 mg of ticlopidine markedly reduced restenosis rate at 6 months after coronary stenting compared with the non-sarpogrelate treatment group (4.3% vs. 28.6%, p < 0.005) (Fujita et al., 2003). The results of multivariate logistic regression analysis showed that treatment with sarpogrelate was a significant predictor for angiographic restenosis, independent of the findings of quantitative coronary angiography, stent characteristics, and the presence of diabetes. These two studies differed in 5-HT2A receptor blocker drug characteristics and pretreatment period as well as the angioplasty procedure. The latter study supported the suggestion that sarpogrelate may prevent the development of intimal hyperplasia due to VSMC proliferation. Further randomized controlled trials to investigate the effects of sarpogrelate on prevention of

restenosis after placement of drug-eluting stents are required.

**10. 5-HT2A receptor blocker and treatment of Peripheral Artery Disease** 

Sarpogrelate is widely used clinically as an anti-platelet drug for prevention of thrombosis and treatment of critical limb ischemic symptoms in patients with peripheral artery disease (PAD), such as arteriosclerosis obliterans (ASO) and Buerger's disease. In a study by

development.

Miyazaki and colleagues (Miyazaki et al., 2007), 22 patients with PAD received either sarpogrelate at a dose of 300 mg orally or conventional therapy for 12 weeks. Both forearm and leg endothelium-dependent vasodilation were improved and maintained for 24 weeks in patients treated with sarpogrelate whereas no improvement was observed in patients treated with conventional therapy. In addition, endothelium-nondependent vasodilation was similar between the two treatment groups. These results suggest that 12 weeks of treatment with sarpogrelate improved vascular endothelial function in PAD patients. Further they investigated the effects of a combination of bone marrow mononuclear cell implantation and sarpogrelate on endothelial function in 16 PAD patients (Higashi et al., 2010). They performed the evaluations before and after bone marrow mononuclear cell implantation in 16 patients with critical limb ischemia. A 12-week course of sarpogrelate treatment amplified the increased leg blood flow responses to acetylcholine evaluated by plethysmography induced by bone marrow mononuclear cell implantation compared with conventional treatment, whereas bone marrow mononuclear cell implantation improved limb ischemic symptoms in the sarpogrealate group as well as in the conventional treatment group. These two studies showed that a treatment with sarpogrelate for at least 12 weeks has a beneficial effect on vascular endothelial function in PAD patients treated with conventional therapy.

There have been several small studies of the effects of sarpogrelate on various biomarkers without controls in PAD patients. In a study of 13 patients with ASO, treatment with sarpogrelate for 1 week decreased adenosine diphosphate (ADP)- or collagen-induced platelet aggregation and reduced the releases of platelet-derived growth factor, soluble Pselectin, and transforming growth factor-β1 from platelets stimulated by ADP or collagen (Nakamura et al., 2001). In a study of 24 non-diabetic and non-medicated diabetic patients with PAD (Fontaine grades 1 and 2), 300 mg of sarpogrelate decreased insulin resistance at 2 weeks and 3 months after treatment, and increased plasma levels of adiponectin at 3 months after treatment (Kokubu, 2006). Similarly, 300 mg of sarpogrelate increased adiponectin levels at 2 and 3 months after treatment in 8 diabetic patients with ASO (Yamakawa et al., 2003). Treatment with 300 mg of sarpogrelate improved limb ischemic symptoms and decreased interleukin-18 levels in 8 diabetic patients with ASO (Yamakawa et al., 2004). In a study of 10 patients with Buerger's disease, 8 weeks of treatment with 300 mg of sarpogrelate was well-tolerated. However, platelet aggregation induced by 5-HT increased significantly after 2 and 4 weeks, and whole-blood 5-HT concentration increased significantly after 2 weeks of treatment (Rydzewski et al, 1996).

#### **11. 5-HT2A receptor blocker and treatment of ischemic cerebrovascular disease**

There have been no reports of high plasma 5-HT levels in patients with stroke at the acute phase. In a study of elderly subjects, plasma 5-HT concentration measured by enzyme immunoassay was significantly higher in patients with vascular dementia caused by stroke or atherosclerotic small vessel disease compared with age-matched controls (Ban et al., 2007).

In a double-blind, controlled, clinical-pharmacological study, 47 patients with ischemic stroke who discontinued any antiplatelet agents and anticoagulants or fibrinolytic agents,

Roles of Serotonin in Atherothrombosis and Related Diseases 65

Ban, Y.; Watanabe, T.; Miyazaki, A.; Nakano, Y.; Tobe, T.; Idei, T.; Iguchi, T.; Ban, Y. &

spontaneous occlusive coronary thrombus formation. *Circ Res* 58, pp. 58-67 De Clerck, F. (1990). The role of serotonin in thrombogenesis. *Clin Physiol Biochem* 8 (Suppl

Fanburg, B.L. & Lee, S.L. (1997). A new role for an old molecule: serotonin as mitogen. *Am J* 

Figueras, J.; Domingo, E.; Cortadellas, J.; Padilla, F.; Dorado, D.G.; Segura, R.; Galard, R. &

Golino, P.; Piscione, F.; Benedict, C.R.; Anderson, H.V.; Cappelli-Bigazzi, M.; Indolfi, C.;

released during coronary angioplasty. *N Engl J Med* 330, pp. 523-528 Gradin, K.; Hedner, T. & Persson, B. (1991). Antihypertensive effects of chronic 5-

spontaneously hypertensive rat. *J Neural Transm Gen Sect* 83, pp. 227-233 Hara, H.; Kitajima, A.; Shimada, H. & Tamao, Y. (1991a). Antithrombotic effect of MCI-9042,

Hara, H.; Shimada, H.; Kitajima, A. & Tamao, Y. (1991b). Effect of (+/-)-2-(dimethylamino)-

Hara, K.; Hirowatari, Y.; Yoshika, M.; Komiyama, Y.; Tsuka, Y. & Takahashi, H. (2004). The

Hayashi, T.; Sumi, D.; Matsui-Hirai, H.; Fukatsu, A.; Arockia, J.; Rani, P.J.A.; Kano, H.;

Higashi, Y.; Miyazaki, M.; Goto, C.; Sanada, H.; Sueda, T. & Chayama, K. (2010).

Hirowatari, Y.; Hara, K.; Shimura, Y. & Takahashi, H. (2011). Serotonin levels in platelet-

cardiovascular disease. *J Lab Clin Med* 144, pp. 31-37

ischemia. *J Cardiovasc Pharmacol* 55, pp. 56-61

*Atherosclerosis* 168, pp. 23-31

atherosclerosis on vascular dementia. *Atherosclerosis* 195, pp. 153-159 Benedict, C.R.; Mathew, B.; Rex, K.A.; Cartwright, J. & Sordahl, L.A. (1986). Correlation of

3), pp. 40-49

*Physiol* 272, pp. L795-806

*Heart J* 145, pp. E16

pp. 484-488

616-620

Katagiri T. (2007). Impact of increased plasma serotonin levels and carotid

plasma serotonin changes with platelet aggregation in an in vivo dog model of

Soler, J.S. (2005). Comparison of plasma serotonin levels in patients with variant angina pectoris versus healed myocardial infarction. *Am J Cardiol* 96, pp. 204-207 Fujita, M.; Mizuno, K.; Ho, M.; Tsukahara, R.; Miyamoto, A.; Miki, O.; Ishii, K. & Miwa, K.

(2003). Sarpogrelate treatment reduces restenosis after coronary stenting. *Am J* 

Condorelli, M.; Chiariello, M. & Willerson, J.T. (1994). Local effect of serotonin

hydroxytryptamine (5-HT2) receptor blockade with irindalone in the

a new antiplatelet agent on experimental thrombosis models. *Thromb Haemost* 66,

1-[[o-(m-methoxyphenethyl)phenoxy] methyl]ethyl hydrogen succinate on experimental models of peripheral obstructive disease. *Arzneimittelforschung* 41, pp.

ratio of plasma to whole-blood serotonin may be a novel marker of atherosclerotic

Tsunekawa, T. & Iguchi, A. (2003). Sarpogrelate HCl, a selective 5-HT2A antagonist, retards the progression of atherosclerosis through a novel mechanism.

Sarpogrelate hydrochloride, a selective 5-hydroxytryptamine(2A) antagonist, augments autologous bone marrow mononuclear cell implantation-induced improvement in endothelium-dependent vasodilation in patients with critical limb

poor plasma and whole blood from healthy subjects: relationship with lipid markers and coronary heart disease risk score. *J Atheroscler Thromb* 18, pp. 874-882

were randomly assigned to receive one of three daily doses of sarpogrelate, i.e., 75, 225, or 300 mg, for 7 days (Uchiyama et al., 2007). Sarpogrelate treatment inhibited platelet aggregation induced by 5-HT plus adrenaline in a dose-dependent manner. The Sarpogrelate-Aspirin Comparative Clinical Study for Efficacy and Safety in Secondary Prevention of Cerebral Infarction (S-ACCESS) trial was a randomized, double-blind, controlled trial to evaluate and compare the efficacy and safety of sarpogrelate with those of aspirin for prevention of recurrence in patients with ischemic stroke (Shinohara et al., 2008). A total of 1050 patients with recent ischemic stroke (1 week to 6 months after onset) were randomly allocated to receive either 300 mg of sarpogrelate or 81 mg of aspirin with a mean duration of follow-up of 1.59 years (maximum: 3.37 years). The annual recurrence rates of cerebral infarction were 6.09% (95% CI 4.837.67) with sarpogrelate and 4.86% (3.756.28) with aspirin. The hazard ratio (HR) was 1.25 (95% CI 0.891.77); the upper limit of 95% CI of the HR exceeded 1.33, indicating that sarpogrelate was slightly inferior to aspirin in preventing the recurrence of cerebral infarction. On the other hand, the incidence rates of serious vascular events, defined as stroke, ACS, or vascular event-related death, were similar between the sarpogrelate and aspirin group. There were significantly fewer bleeding events in the sarpogrelate group compared with the aspirin group (11.9%, 95% CI 9.614.4 vs. 17.3%, 14.720.2, respectively, p < 0.005). In subgroup analysis in the S-ACCESS trial, sarpogrelate was shown to be inferior to aspirin in most subgroups except diabetic patients (Shinohara & Nishimaru, 2009). Thus, sarpogrelate may be a useful treatment option for Japanese stroke patients with diabetes.

#### **12. Conclusion**

This review presented a discussion of the potential involvement of 5-HT mediated through 5-HT2A receptors in the development of atherothrombotic cardiovascular diseases, including platelet aggregation, thrombus formation, VSMC contraction, and arterial intimal hyperplasia. These responses are synergistically augmented with other vasoactive compounds, atherogenic lipids, and various inflammatory cytokines. The 5-HT2A receptor antagonists inhibit the 5-HT-mediated atherothrombotic process. Although ketanserin inhibits not only 5-HT2A receptors but also 1-adrenergic and histamine H1 receptors, it was withdrawn due to its tendency to induce proarrhythmia. Sarpogrelate is a specific 5-HT2A receptor antagonist that has been reported to have various beneficial effects especially in patients with CAD and/or atherosclerotic cardiovascular disease with diabetes, and to have fewer adverse effects compared with other anti-platelet agents. However, larger randomized controlled trials of sarpogrelate in CAD, PAD, stroke, and diabetes are required.

#### **13. Acknowledgments**

We thank Dr. Claude R. Benedict and Dr. Rajbabu Pakala for their many insightful comments and kind support in our research.

#### **14. References**

Aviram, M.; Fuhrman, B.; Maor, I. & Brook, G.J. (1992). Serotonin increases macrophage uptake of oxidized low density lipoprotein. *Eur J Clin Chem Clin Biochem* 30, pp. 55-61

were randomly assigned to receive one of three daily doses of sarpogrelate, i.e., 75, 225, or 300 mg, for 7 days (Uchiyama et al., 2007). Sarpogrelate treatment inhibited platelet aggregation induced by 5-HT plus adrenaline in a dose-dependent manner. The Sarpogrelate-Aspirin Comparative Clinical Study for Efficacy and Safety in Secondary Prevention of Cerebral Infarction (S-ACCESS) trial was a randomized, double-blind, controlled trial to evaluate and compare the efficacy and safety of sarpogrelate with those of aspirin for prevention of recurrence in patients with ischemic stroke (Shinohara et al., 2008). A total of 1050 patients with recent ischemic stroke (1 week to 6 months after onset) were randomly allocated to receive either 300 mg of sarpogrelate or 81 mg of aspirin with a mean duration of follow-up of 1.59 years (maximum: 3.37 years). The annual recurrence rates of cerebral infarction were 6.09% (95% CI 4.837.67) with sarpogrelate and 4.86% (3.756.28) with aspirin. The hazard ratio (HR) was 1.25 (95% CI 0.891.77); the upper limit of 95% CI of the HR exceeded 1.33, indicating that sarpogrelate was slightly inferior to aspirin in preventing the recurrence of cerebral infarction. On the other hand, the incidence rates of serious vascular events, defined as stroke, ACS, or vascular event-related death, were similar between the sarpogrelate and aspirin group. There were significantly fewer bleeding events in the sarpogrelate group compared with the aspirin group (11.9%, 95% CI 9.614.4 vs. 17.3%, 14.720.2, respectively, p < 0.005). In subgroup analysis in the S-ACCESS trial, sarpogrelate was shown to be inferior to aspirin in most subgroups except diabetic patients (Shinohara & Nishimaru, 2009). Thus, sarpogrelate may be a useful treatment option for

This review presented a discussion of the potential involvement of 5-HT mediated through 5-HT2A receptors in the development of atherothrombotic cardiovascular diseases, including platelet aggregation, thrombus formation, VSMC contraction, and arterial intimal hyperplasia. These responses are synergistically augmented with other vasoactive compounds, atherogenic lipids, and various inflammatory cytokines. The 5-HT2A receptor antagonists inhibit the 5-HT-mediated atherothrombotic process. Although ketanserin inhibits not only 5-HT2A receptors but also 1-adrenergic and histamine H1 receptors, it was withdrawn due to its tendency to induce proarrhythmia. Sarpogrelate is a specific 5-HT2A receptor antagonist that has been reported to have various beneficial effects especially in patients with CAD and/or atherosclerotic cardiovascular disease with diabetes, and to have fewer adverse effects compared with other anti-platelet agents. However, larger randomized

controlled trials of sarpogrelate in CAD, PAD, stroke, and diabetes are required.

We thank Dr. Claude R. Benedict and Dr. Rajbabu Pakala for their many insightful

Aviram, M.; Fuhrman, B.; Maor, I. & Brook, G.J. (1992). Serotonin increases macrophage

uptake of oxidized low density lipoprotein. *Eur J Clin Chem Clin Biochem* 30, pp. 55-61

Japanese stroke patients with diabetes.

**12. Conclusion** 

**13. Acknowledgments** 

**14. References** 

comments and kind support in our research.


Roles of Serotonin in Atherothrombosis and Related Diseases 67

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**4** 

Po-Hsun Huang

*Taiwan* 

**Endothelial Progenitor Cell** 

 **in Cardiovascular Diseases** 

*Division of Cardiology, Taipei Veterans General Hospital,* 

 *Cardiovascular Research Center, National Yang-Ming University, Taipei,* 

The last decade has seen a huge interest in the field of regenerative biology, with particular emphasis on the use of isolated or purified stem and progenitor cells to restore structure and function to damaged organs. Circulating endothelial progenitor cells (EPCs) have been studied as a potential cell source that contributes to neovascularization via postnatal vasculogenesis (Asahara et al., 1997). EPCs are reported to naturally home and integrate into sites of physiological vessel formation *in vivo* and incorporate into the vasculature of tumors, ischemic skeletal and cardiac muscle (Asahara et al., 1999). Furthermore, Accumulating evidence demonstrates a relationship between the frequency of circulating EPCs and cardiovascular disease risk (Hill et al., 2003). In the following, we will review the putative role of EPCs in endothelial repair and provides evidence for their influence on

Despite the availability of effective preventive measures, coronary artery disease (CAD) remains a leading cause of morbidity and mortality in most industrialized countries. Convincing evidence indicates that the integrity and functional activity of the endothelial monolayer play an important role in atherogenesis. Traditional view suggests that endothelium integrity is maintained by neighboring mature endothelial cells which migrate and proliferate to restore injured endothelial cells. However, a series of clinical and basic studies prompted by the discovery of bone marrow-derived EPCs have provided new insights into these processes and demonstrate that the injured endothelial monolayer is regenerated partly by circulating EPCs. Putative circulating endothelial progenitors were first described in the adult human by Asahara *et al.* (Asahara et al., 1997) in 1997. They used the presence of CD34 to sort cells from the adult peripheral blood mononuclear component, based on the knowledge that this antigen is carried by both the angioblasts and haemopoietic stem cells responsible for vasculogenesis in embryonic life. By culturing MNCs (mononuclear cells) enriched or depleted in these CD34+ cells, they showed that the CD34+ component is able to give rise to spindle-shaped cells after 3 days, which become attached to fibronectin. Such culture led to an up-regulation of endothelial lineage markers such as CD31, Flk-1 and Tie2, and loss of the pan-leucocyte CD45 antigen, in these attaching cells. Asahara *et al.* (Asahara et

**1. Introduction** 

atherosclerosis.

**2. Identification of EPCs** 


## **Endothelial Progenitor Cell in Cardiovascular Diseases**

Po-Hsun Huang

*Division of Cardiology, Taipei Veterans General Hospital, Cardiovascular Research Center, National Yang-Ming University, Taipei, Taiwan* 

#### **1. Introduction**

70 Traditional and Novel Risk Factors in Atherothrombosis

Watanabe, T.; Pakala, R.; Katagiri, T. & Benedict, CR. (2001e). Synergistic effect of urotensin

Watanabe, T.; Pakala, R.; Katagiri, T. & Benedict, C.R. (2001f). Monocyte chemotactic protein

Wester, P.; Dietrich, W.D.; Prado, R.; Watson, B.D. & Globus, M.Y.T. (1992). Serotonin

Wright L.; Homans, D.C.; Laxson, D.D.; Dai, X.Z. & Bache, R.J. (1992). Effect of serotonin

Yamakawa, J.; Takahashi, T.; Itoh, T.; Kusaka, K.; Kawaura, K.; Wang, X.Q. & Kanda, T.

Yamakawa, J.; Takahashi, T.; Saegusa, S.; Moriya, J.; Itoh, T.; Kusaka, K.; Kawaura, K.;

collateral vessels. *J Am Coll Cardiol* 19, pp.687-693

obliterans. *J Int Med Res* 32, pp. 166-169

2191-2196

870-875

2478

38, pp. 341-349

II with serotonin on vascular smooth muscle cell proliferation. *J Hypertens* 19, pp.

1 amplifies serotonin-induced vascular smooth muscle cell proliferation. *J Vasc Res*

release into plasma during common carotid artery thrombosis in rats. *Stroke* 23, pp.

and thromboxane A2 on blood flow through moderately well developed coronary

(2003). A novel serotonin blocker, sarpogrelate, increases circulating adiponectin levels in diabetic patients with arteriosclerosis obliterans. *Diabetes Care* 26, pp. 2477-

Wang, X.Q. & Kanda, T. (2004). Effect of the serotonin blocker sarpogrelate on circulating interleukin-18 levels in patients with diabetes and arteriosclerosis The last decade has seen a huge interest in the field of regenerative biology, with particular emphasis on the use of isolated or purified stem and progenitor cells to restore structure and function to damaged organs. Circulating endothelial progenitor cells (EPCs) have been studied as a potential cell source that contributes to neovascularization via postnatal vasculogenesis (Asahara et al., 1997). EPCs are reported to naturally home and integrate into sites of physiological vessel formation *in vivo* and incorporate into the vasculature of tumors, ischemic skeletal and cardiac muscle (Asahara et al., 1999). Furthermore, Accumulating evidence demonstrates a relationship between the frequency of circulating EPCs and cardiovascular disease risk (Hill et al., 2003). In the following, we will review the putative role of EPCs in endothelial repair and provides evidence for their influence on atherosclerosis.

#### **2. Identification of EPCs**

Despite the availability of effective preventive measures, coronary artery disease (CAD) remains a leading cause of morbidity and mortality in most industrialized countries. Convincing evidence indicates that the integrity and functional activity of the endothelial monolayer play an important role in atherogenesis. Traditional view suggests that endothelium integrity is maintained by neighboring mature endothelial cells which migrate and proliferate to restore injured endothelial cells. However, a series of clinical and basic studies prompted by the discovery of bone marrow-derived EPCs have provided new insights into these processes and demonstrate that the injured endothelial monolayer is regenerated partly by circulating EPCs. Putative circulating endothelial progenitors were first described in the adult human by Asahara *et al.* (Asahara et al., 1997) in 1997. They used the presence of CD34 to sort cells from the adult peripheral blood mononuclear component, based on the knowledge that this antigen is carried by both the angioblasts and haemopoietic stem cells responsible for vasculogenesis in embryonic life. By culturing MNCs (mononuclear cells) enriched or depleted in these CD34+ cells, they showed that the CD34+ component is able to give rise to spindle-shaped cells after 3 days, which become attached to fibronectin. Such culture led to an up-regulation of endothelial lineage markers such as CD31, Flk-1 and Tie2, and loss of the pan-leucocyte CD45 antigen, in these attaching cells. Asahara *et al.* (Asahara et

Endothelial Progenitor Cells, Coronary Artery Disease, Atherosclerosis 73

1992). Various culture preparations have been used to encourage endothelial lineage proliferation from human blood-derived MNCs. There has been a considerable variation in the details of techniques used: for example, some have replated the adherent cells after 2 days initial culture, whereas others have used the non-adherent cells at this time (Vasa, 2001). Then, endothelial cell lineage was confirmed by indirect immunostaining with the use of DiI-acLDL and co-staining with BS-1 lectin. However, controversy exists with respect to the identification and the origin of EPCs, which are isolated from peripheral blood

EPCs can be isolated, cultured, and differentiated ex vivo from the circulating mononuclear cells (MNCs) and exhibit characteristic endothelial properties and markers. Currently, two types of EPCs, namely early and late outgrowth EPCs, can be derived and identified from peripheral blood. The early EPCs appear after 3-5 days of culture, are spindle-shaped, have peak growth at approximately 2 weeks and die by 4 weeks. These have been variously termed 'early EPCs' by Gulati et al. (Gulati et al., 2003) and Hur et al. (Hur et al., 2004), 'attaching cells' by Asahara et al. (Asahara et al., 1997) and CACs (circulating angiogenic cells) by Rehman et al. (Rehman et al., 2003). The second type of EPCs appears only after longer culture, of approximately 2-3 weeks, forming a cobblestone monolayer with near-complete confluence, and can show exponential population growth without senescence over 4-8 weeks and live for up to 12 weeks. These were termed 'late EPCs' by Hur *et al.* (Hur et al., 2004) or OECs by Lin *et al.* (Lin et al., 2000) and Gulati *et al*. (Gulati et al., 2003). Early and late outgrowth EPCs (OECs) share some endothelial phenotype similarities but show different morphology, proliferation rate, survival features, and functions in neovascularization. For clarity, we will use the terms

Early EPCs, in contrast, do not participate in tube-forming assays, have only weak invasive ability on gels and produce only low levels of NO. They do, however, demonstrate some features in keeping with an endothelial lineage such as acetylated LDL uptake and lectin binding. In addition, early EPCs do not develop into OECs upon prolonged culture. Among antigenic markers, CD14 (a monocytic marker) has been found by several groups on early EPCs (Romagnani et al., 2005; Urbich et al., 2003). Early EPCs lack the impressive replicative ability of OECs, but are prolific producers of several growth factors, cytokines and chemokines, including VEGF, HGF (hepatocyte growth factor), G-CSF (granulocyte colonystimulating factor) and GM-CSF (granulocyte/macrophage colony-stimulating factor). The lineage origin of these two culture-derived endothelial-type cells has been examined. Expression of the pan-leucocyte antigen CD45 is relatively greatest in MNCs, lower in early EPCs and lowest in OECs. It appears that early EPCs are mostly derived from a CD14+ population of MNCs, implying a monocytic, rather than true endothelial, lineage (Yoon et al., 2005). In contrast, OECs derive exclusively, or almost exclusively, from the CD14<sup>−</sup> population of MNCs (Yoon et al., 2005). It has been suggested that the MNCs from which OECs are derived may represent a 'true' circulating endothelial precursor (angioblasts).

OECs have many similarities to mature ECs, in terms of surface antigens (including KDR, vWF, and VE-cadherin) and high levels of NO (nitric oxide) production by eNOS (endothelial NO synthase). They are able to participate effectively in tube-forming assays *in vitro*. However, OECs differ from mature ECs in having far greater proliferative ability *in* 

mononuclear cells by cultivation in medium favoring endothelial differentiation.

**4. Early and Late outgrowth EPCs** 

early EPCs and OECs in the present review.

al., 1999) went on to deliver labelled, CD34+-enriched, MNCs into mouse and rabbit models of hindlimb ischemia and demonstrated neovascularization in the relevant limb with apparent incorporation of labelled cells into capillary walls. In separate experiments, they delivered murine labelled-MNCs enriched for Flk-1 and similarly found incorporation into capillaries and small arteries in the mouse hindlimb ischemia model. Carriage of CD31 and lectin binding was observed in these incorporated cells. These landmark studies suggest that circulating EPCs in adult peripheral blood could differentiate into cells of endothelial lineage and enhance revascularization through vasculogenesis.

#### **3. Issues of definition for EPCs**

Since these important findings, an enormous amount of research has been undertaken into EPCs; however, in attempting to collate and interpret these results, a major limiting factor is that no simple definition of EPCs exists at the present time, and various methods to define EPC have been reported. This pertains to the unresolved issue of how EPCs should best be defined.

#### **3.1 Antigen-based definitions of EPC**

The first method of classification of EPC is based on expression of cell-surface antigens, typically using flow cytometry to quantify relevant populations. Endothelial cells (EC) display a characteristic combination of such antigens, including CD34, KDR (kinase insert domain-containing receptor, a type of VEGFR2), VE-cadherin, vWF (von Willebrand factor) and E-selectin. In order to distinguish mature endothelial cells from circulating endothelial progenitors, some groups have additionally used other antigens which are lost during maturation of endothelial lineage cells, most commonly CD133 (also termed AC133) (Hristov & Weber, 2004). The combination of CD34, KDR and CD133 has been used by several investigators, although many others have used only two of these three. Unfortunately, even with use of all three, this phenotype is not entirely specific, since this same cluster of antigens may also be found on haemopoietic stem cells (Adams & Scadden, 2006; Verfaillie, 2002). This relates to the probable origin of haemopoietic and EC lines from a common precursor, termed the haemangioblast. As haemopoietic stem cells differentiate, CD34, KDR and CD133 antigens are down-regulated and disappear. Furthermore, the use of CD133 to make the distinction from mature ECs will also lead to the exclusion of 'more mature' EPCs which may have lost this marker, while not yet being terminally differentiated. To complicate matters further, while the use of antigenic combinations may have logical appeal, whether this approach actually identifies a group of precursors capable of producing ECs has recently been challenged (Case et al., 2007).

#### **3.2 Culture-based definitions of EPC**

The second commonly employed definition for EPCs derives from *in vitro* culture work. Asahara *et al.* described *in vitro* culture of CD34-enriched MNCs leading to the formation of spindle-shaped attaching cells within 3 days (Asahara et al., 1997). Co-culture of CD34 enriched and CD34-depleted cells gave rise, within 12 hours, to multiple clusters, containing round cells centrally and sprouts of spindle-shaped cells at the periphery. This cluster appearance was reminiscent of the blood islands previously described, wherein angioblasts surround hematopoeitic stem cells as the initial stage of vasculogenesis (Flamme & Risau, 1992). Various culture preparations have been used to encourage endothelial lineage proliferation from human blood-derived MNCs. There has been a considerable variation in the details of techniques used: for example, some have replated the adherent cells after 2 days initial culture, whereas others have used the non-adherent cells at this time (Vasa, 2001). Then, endothelial cell lineage was confirmed by indirect immunostaining with the use of DiI-acLDL and co-staining with BS-1 lectin. However, controversy exists with respect to the identification and the origin of EPCs, which are isolated from peripheral blood mononuclear cells by cultivation in medium favoring endothelial differentiation.

#### **4. Early and Late outgrowth EPCs**

72 Traditional and Novel Risk Factors in Atherothrombosis

al., 1999) went on to deliver labelled, CD34+-enriched, MNCs into mouse and rabbit models of hindlimb ischemia and demonstrated neovascularization in the relevant limb with apparent incorporation of labelled cells into capillary walls. In separate experiments, they delivered murine labelled-MNCs enriched for Flk-1 and similarly found incorporation into capillaries and small arteries in the mouse hindlimb ischemia model. Carriage of CD31 and lectin binding was observed in these incorporated cells. These landmark studies suggest that circulating EPCs in adult peripheral blood could differentiate into cells of endothelial lineage and enhance

Since these important findings, an enormous amount of research has been undertaken into EPCs; however, in attempting to collate and interpret these results, a major limiting factor is that no simple definition of EPCs exists at the present time, and various methods to define EPC have been reported. This pertains to the unresolved issue of how EPCs should best be

The first method of classification of EPC is based on expression of cell-surface antigens, typically using flow cytometry to quantify relevant populations. Endothelial cells (EC) display a characteristic combination of such antigens, including CD34, KDR (kinase insert domain-containing receptor, a type of VEGFR2), VE-cadherin, vWF (von Willebrand factor) and E-selectin. In order to distinguish mature endothelial cells from circulating endothelial progenitors, some groups have additionally used other antigens which are lost during maturation of endothelial lineage cells, most commonly CD133 (also termed AC133) (Hristov & Weber, 2004). The combination of CD34, KDR and CD133 has been used by several investigators, although many others have used only two of these three. Unfortunately, even with use of all three, this phenotype is not entirely specific, since this same cluster of antigens may also be found on haemopoietic stem cells (Adams & Scadden, 2006; Verfaillie, 2002). This relates to the probable origin of haemopoietic and EC lines from a common precursor, termed the haemangioblast. As haemopoietic stem cells differentiate, CD34, KDR and CD133 antigens are down-regulated and disappear. Furthermore, the use of CD133 to make the distinction from mature ECs will also lead to the exclusion of 'more mature' EPCs which may have lost this marker, while not yet being terminally differentiated. To complicate matters further, while the use of antigenic combinations may have logical appeal, whether this approach actually identifies a group of precursors capable

The second commonly employed definition for EPCs derives from *in vitro* culture work. Asahara *et al.* described *in vitro* culture of CD34-enriched MNCs leading to the formation of spindle-shaped attaching cells within 3 days (Asahara et al., 1997). Co-culture of CD34 enriched and CD34-depleted cells gave rise, within 12 hours, to multiple clusters, containing round cells centrally and sprouts of spindle-shaped cells at the periphery. This cluster appearance was reminiscent of the blood islands previously described, wherein angioblasts surround hematopoeitic stem cells as the initial stage of vasculogenesis (Flamme & Risau,

revascularization through vasculogenesis.

**3. Issues of definition for EPCs** 

**3.1 Antigen-based definitions of EPC** 

**3.2 Culture-based definitions of EPC** 

of producing ECs has recently been challenged (Case et al., 2007).

defined.

EPCs can be isolated, cultured, and differentiated ex vivo from the circulating mononuclear cells (MNCs) and exhibit characteristic endothelial properties and markers. Currently, two types of EPCs, namely early and late outgrowth EPCs, can be derived and identified from peripheral blood. The early EPCs appear after 3-5 days of culture, are spindle-shaped, have peak growth at approximately 2 weeks and die by 4 weeks. These have been variously termed 'early EPCs' by Gulati et al. (Gulati et al., 2003) and Hur et al. (Hur et al., 2004), 'attaching cells' by Asahara et al. (Asahara et al., 1997) and CACs (circulating angiogenic cells) by Rehman et al. (Rehman et al., 2003). The second type of EPCs appears only after longer culture, of approximately 2-3 weeks, forming a cobblestone monolayer with near-complete confluence, and can show exponential population growth without senescence over 4-8 weeks and live for up to 12 weeks. These were termed 'late EPCs' by Hur *et al.* (Hur et al., 2004) or OECs by Lin *et al.* (Lin et al., 2000) and Gulati *et al*. (Gulati et al., 2003). Early and late outgrowth EPCs (OECs) share some endothelial phenotype similarities but show different morphology, proliferation rate, survival features, and functions in neovascularization. For clarity, we will use the terms early EPCs and OECs in the present review.

Early EPCs, in contrast, do not participate in tube-forming assays, have only weak invasive ability on gels and produce only low levels of NO. They do, however, demonstrate some features in keeping with an endothelial lineage such as acetylated LDL uptake and lectin binding. In addition, early EPCs do not develop into OECs upon prolonged culture. Among antigenic markers, CD14 (a monocytic marker) has been found by several groups on early EPCs (Romagnani et al., 2005; Urbich et al., 2003). Early EPCs lack the impressive replicative ability of OECs, but are prolific producers of several growth factors, cytokines and chemokines, including VEGF, HGF (hepatocyte growth factor), G-CSF (granulocyte colonystimulating factor) and GM-CSF (granulocyte/macrophage colony-stimulating factor). The lineage origin of these two culture-derived endothelial-type cells has been examined. Expression of the pan-leucocyte antigen CD45 is relatively greatest in MNCs, lower in early EPCs and lowest in OECs. It appears that early EPCs are mostly derived from a CD14+ population of MNCs, implying a monocytic, rather than true endothelial, lineage (Yoon et al., 2005). In contrast, OECs derive exclusively, or almost exclusively, from the CD14<sup>−</sup> population of MNCs (Yoon et al., 2005). It has been suggested that the MNCs from which OECs are derived may represent a 'true' circulating endothelial precursor (angioblasts).

OECs have many similarities to mature ECs, in terms of surface antigens (including KDR, vWF, and VE-cadherin) and high levels of NO (nitric oxide) production by eNOS (endothelial NO synthase). They are able to participate effectively in tube-forming assays *in vitro*. However, OECs differ from mature ECs in having far greater proliferative ability *in* 

Endothelial Progenitor Cells, Coronary Artery Disease, Atherosclerosis 75

Rapid and complete restoration of endothelial integrity and function prevents development and growth of a neointimal lesion; however, inadequate response to injury will instead allow the formation of an atheromatous lesion. The discovery of circulating endothelial progenitors has led to the theory that they are important mediators of this repair arm, and hence that a depletion or dysfunction in these cells would result in an imbalance between endothelial injury and repair, favoring atherosclerosis. Schmidt-Lucke *et al.* (Schmidt-Lucke et al., 2005) followed up a group of 120 individuals, comprising normal subjects and also patients with either stable or unstable coronary artery disease. They found that major cardiovascular events, CABG (coronary artery bypass grafting) or ischemic stroke were significantly more frequent in the subgroup with lower levels of circulating CD34/KDR double-positive cells at baseline. This association persisted after accounting for conventional cardiovascular risk factors. Werner *et al.* (Werner et al., 2005) studied CD34/KDR doublepositive cell numbers in a cohort of 519 patients diagnosed with coronary artery disease by angiography. After adjustment for confounding variables, higher levels of EPCs were associated with a reduced risk of death from cardiovascular causes and of occurrence of a first cardiovascular event at 12 months follow-up. The authors followed up the outcomes when patients were grouped by baseline levels of CFUs (i.e. a culture-based definition of colony formation). Higher CFU formation was associated with a reduced occurrence of a first major cardiovascular event and reduced revascularization at follow-up. However, as discussed above, recent work on the CFU assay suggests that it is assessing the *in vitro* activity of cells which may be relevant to vascular function, but which are not actually EPCs

Moreover, there is relevant animal-based work in this area of progenitor cells and endothelial function. Wassmann *et al.* (Wassmann et al., 2006) studied endothelial function in ApoE-knockout mice, on a high-cholesterol diet, with atherosclerotic plaques and demonstrable endothelial dysfunction of aortic rings *ex vivo*. They showed that the intravenous administration of spleen-derived MNCs improved endothelium-dependent vasodilation. In addition, Gulati *et al.* (Gulati et al., 2003). used a rabbit model of balloon injury to the carotid arteries. They cultured peripheral blood MNCs in endothelial growth medium for 2 weeks, producing endothelial-phenotype cells carrying CD31 and eNOS, and delivered these culture-modified cells immediately after balloon injury. They found that, compared with saline-treated controls, local treatment with EPCs led to accelerated reendothelialization and improved endothelial function. Whether the improvement in endothelial function is directly due to increased numbers of new ECs or an indirect effect on pre-existing cells or a papacrine effect by implantation of EPCs remains unclear; however, an increase in vascular NOS activity was documented and is likely to mediate the effect.

A crucial target in the treatment or prevention of atherosclerosis is to promote and maintain the integrity and health of endothelium. Since EPCs play a role in maintaining an intact and functional endothelium, decreased and dysfunctional EPCs may contribute to endothelial dysfunction and susceptibility to atherosclerosis. Enhancement of the regenerative capacity of the injured endothelium seems one way to reduce the incidence of atherosclerotic lesions (Hristov & Weber, 2007). Transplantation of human cord blood-derived EPCs was reported

**6. Anti-atherosclerotic actions of EPC** 

themselves (Rohde et al., 2007; Hur et al., 2007).

**7. Therapeutic implications and perspective** 

*vitro* and greater angiogenic potential *in vivo*. A small population of OECs with the highest proliferative potential was able to produce more than 200 progeny per replated cell. Based on these findings, these features make OECs attractive candidates for therapeutic use in ischemia-related neovascularization.

#### **5. Endothelial progenitor cells and atherosclerosis**

The discovery of endothelial progenitors within adult peripheral blood presents another possible means of vascular maintenance, namely a reservoir of circulating cells which can home to sites of injury and restore endothelial integrity thus allowing continued normal function. Hill *et al.* (Hill et al., 2003) studied men without known cardiovascular disease but with varying degrees of estimated cardiovascular risk. Endothelial function was determined by using brachial artery flow-mediated vasodilation, and EPC numbers were measured using their CFU assay in study subjects. An inverse correlation was found between numbers of CFUs and the overall Framingham risk score of the participants. Furthermore, they found a positive correlation between the number of EPCs and endothelial function as assessed by brachial artery reactivity of the subjects. These findings are compatible with the hypothesis that an adequate pool of EPCs in the blood may be a key requirement for appropriate endothelial function. It appears that bone marrow-derived EPCs play a pivotal role in the maintenance of adult vascular endothelium. However, the basis of this correlation between EPC levels and endothelial function remains to be determined.

Although the critical role of circulating EPCs in the pathogenesis of atherosclerotic diseases is substantiated by several observations, the relationship between circulating EPCs and coronary artery disease (CAD) remains a subject of debate. Several studies have examined the association between circulating EPCs and CAD or risk factors predisposing to coronary artery disease. Vasa *et al.* reported that the circulating EPCs levels were significantly reduced in patients with CAD compared to those without CAD (Vasa et al., 2001). Wang and coworkers indicated that decreased number and activity of EPCs were observed in patients with stable CAD, and EPC levels were negatively correlated with the severity of coronary stenosis assessed by Gensini score (Wang et al., 2007). Fadini *et al.* also reported that EPCs were significantly reduced in subjects with increased intimamedia thickness (Fadini et al., 2006), implying that depletion of EPCs may be an independent predictor of subclinical atherosclerosis. However, Guven *et al.* showed that increased EPCs levels were associated with the presence of significant CAD, and EPC numbers correlated with maximum angiographic stenosis severity (Guven et al., 2006). The apparent conflicting results between different studies may have many explanations, including fundamental differences in the methodologies used to identify circulating EPCs in different studies; heterogeneity of patient population, and effect of the disease stage on biological properties of circulating EPC levels. Based on the angiographic classifications by Syntax score, our recent work has shown that severe CAD patients (with higher Syntax Score) have decreased circulating EPCs numbers than mild CAD patients and subjects with normal angiographic results (unpublished data). Moreover, circulating EPC levels were shown to be negatively correlated with the SXscore in patients with angiographic evidence of CAD. These findings are consistent with a recent study showing that lower level of circulating EPCs predicts CAD progression (Briguori et al., 2010), suggesting the critical role of EPCs in the pathogenesis of CAD.

#### **6. Anti-atherosclerotic actions of EPC**

74 Traditional and Novel Risk Factors in Atherothrombosis

*vitro* and greater angiogenic potential *in vivo*. A small population of OECs with the highest proliferative potential was able to produce more than 200 progeny per replated cell. Based on these findings, these features make OECs attractive candidates for therapeutic use in

The discovery of endothelial progenitors within adult peripheral blood presents another possible means of vascular maintenance, namely a reservoir of circulating cells which can home to sites of injury and restore endothelial integrity thus allowing continued normal function. Hill *et al.* (Hill et al., 2003) studied men without known cardiovascular disease but with varying degrees of estimated cardiovascular risk. Endothelial function was determined by using brachial artery flow-mediated vasodilation, and EPC numbers were measured using their CFU assay in study subjects. An inverse correlation was found between numbers of CFUs and the overall Framingham risk score of the participants. Furthermore, they found a positive correlation between the number of EPCs and endothelial function as assessed by brachial artery reactivity of the subjects. These findings are compatible with the hypothesis that an adequate pool of EPCs in the blood may be a key requirement for appropriate endothelial function. It appears that bone marrow-derived EPCs play a pivotal role in the maintenance of adult vascular endothelium. However, the basis of this correlation between

Although the critical role of circulating EPCs in the pathogenesis of atherosclerotic diseases is substantiated by several observations, the relationship between circulating EPCs and coronary artery disease (CAD) remains a subject of debate. Several studies have examined the association between circulating EPCs and CAD or risk factors predisposing to coronary artery disease. Vasa *et al.* reported that the circulating EPCs levels were significantly reduced in patients with CAD compared to those without CAD (Vasa et al., 2001). Wang and coworkers indicated that decreased number and activity of EPCs were observed in patients with stable CAD, and EPC levels were negatively correlated with the severity of coronary stenosis assessed by Gensini score (Wang et al., 2007). Fadini *et al.* also reported that EPCs were significantly reduced in subjects with increased intimamedia thickness (Fadini et al., 2006), implying that depletion of EPCs may be an independent predictor of subclinical atherosclerosis. However, Guven *et al.* showed that increased EPCs levels were associated with the presence of significant CAD, and EPC numbers correlated with maximum angiographic stenosis severity (Guven et al., 2006). The apparent conflicting results between different studies may have many explanations, including fundamental differences in the methodologies used to identify circulating EPCs in different studies; heterogeneity of patient population, and effect of the disease stage on biological properties of circulating EPC levels. Based on the angiographic classifications by Syntax score, our recent work has shown that severe CAD patients (with higher Syntax Score) have decreased circulating EPCs numbers than mild CAD patients and subjects with normal angiographic results (unpublished data). Moreover, circulating EPC levels were shown to be negatively correlated with the SXscore in patients with angiographic evidence of CAD. These findings are consistent with a recent study showing that lower level of circulating EPCs predicts CAD progression (Briguori et al., 2010), suggesting the

ischemia-related neovascularization.

**5. Endothelial progenitor cells and atherosclerosis** 

EPC levels and endothelial function remains to be determined.

critical role of EPCs in the pathogenesis of CAD.

Rapid and complete restoration of endothelial integrity and function prevents development and growth of a neointimal lesion; however, inadequate response to injury will instead allow the formation of an atheromatous lesion. The discovery of circulating endothelial progenitors has led to the theory that they are important mediators of this repair arm, and hence that a depletion or dysfunction in these cells would result in an imbalance between endothelial injury and repair, favoring atherosclerosis. Schmidt-Lucke *et al.* (Schmidt-Lucke et al., 2005) followed up a group of 120 individuals, comprising normal subjects and also patients with either stable or unstable coronary artery disease. They found that major cardiovascular events, CABG (coronary artery bypass grafting) or ischemic stroke were significantly more frequent in the subgroup with lower levels of circulating CD34/KDR double-positive cells at baseline. This association persisted after accounting for conventional cardiovascular risk factors. Werner *et al.* (Werner et al., 2005) studied CD34/KDR doublepositive cell numbers in a cohort of 519 patients diagnosed with coronary artery disease by angiography. After adjustment for confounding variables, higher levels of EPCs were associated with a reduced risk of death from cardiovascular causes and of occurrence of a first cardiovascular event at 12 months follow-up. The authors followed up the outcomes when patients were grouped by baseline levels of CFUs (i.e. a culture-based definition of colony formation). Higher CFU formation was associated with a reduced occurrence of a first major cardiovascular event and reduced revascularization at follow-up. However, as discussed above, recent work on the CFU assay suggests that it is assessing the *in vitro* activity of cells which may be relevant to vascular function, but which are not actually EPCs themselves (Rohde et al., 2007; Hur et al., 2007).

Moreover, there is relevant animal-based work in this area of progenitor cells and endothelial function. Wassmann *et al.* (Wassmann et al., 2006) studied endothelial function in ApoE-knockout mice, on a high-cholesterol diet, with atherosclerotic plaques and demonstrable endothelial dysfunction of aortic rings *ex vivo*. They showed that the intravenous administration of spleen-derived MNCs improved endothelium-dependent vasodilation. In addition, Gulati *et al.* (Gulati et al., 2003). used a rabbit model of balloon injury to the carotid arteries. They cultured peripheral blood MNCs in endothelial growth medium for 2 weeks, producing endothelial-phenotype cells carrying CD31 and eNOS, and delivered these culture-modified cells immediately after balloon injury. They found that, compared with saline-treated controls, local treatment with EPCs led to accelerated reendothelialization and improved endothelial function. Whether the improvement in endothelial function is directly due to increased numbers of new ECs or an indirect effect on pre-existing cells or a papacrine effect by implantation of EPCs remains unclear; however, an increase in vascular NOS activity was documented and is likely to mediate the effect.

#### **7. Therapeutic implications and perspective**

A crucial target in the treatment or prevention of atherosclerosis is to promote and maintain the integrity and health of endothelium. Since EPCs play a role in maintaining an intact and functional endothelium, decreased and dysfunctional EPCs may contribute to endothelial dysfunction and susceptibility to atherosclerosis. Enhancement of the regenerative capacity of the injured endothelium seems one way to reduce the incidence of atherosclerotic lesions (Hristov & Weber, 2007). Transplantation of human cord blood-derived EPCs was reported

Endothelial Progenitor Cells, Coronary Artery Disease, Atherosclerosis 77

Adams, G. & Scadden, D. (2006). The hematopoietic stem cell in its place. *Nat Immunol,*

Asahara, T., et al., (1997). Isolation of putative progenitor endothelial cells for angiogenesis.

Asahara, T., et al., (1999). Bone marrow origin of endothelial progenitor cells responsible for

Assmus, B., et al. (2002). Transplantation of Progenitor Cells and Regeneration Enhancement

Bahlmann, F., et al. (2005). Stimulation of endothelial progenitor cells: a new putative

Briguori, C., et al. (2010). Correlations between progression of coronary artery disease and circulating endothelial progenitor cells. *FASEB J.* Vol.24; No.6, pp. 1981-1988, Case, J., et al. (2007). Human CD34+AC133+VEGFR-2+ cells are not endothelial progenitor

Di, S., et al. (2009). p66ShcA modulates oxidative stress and survival of endothelial progenitor cells in response to high glucose. *Cardiovasc Res.* Vol.82; No.3, pp. 421-429, Fadini, G., et al. (2006). Peripheral blood cd34+kdr+ endothelial progenitor cells are

Flamme, I. & Risau, W. (1992). Induction of vasculogenesis and hematopoiesis in vitro.

Gulati, R., et al. (2003). Diverse origin and function of cells with endothelial phenotype obtained from adult human blood. *Circ Res.* Vol.93; No.11, pp. 1023-1025, Gulati, R., et al. (2003). Autologous culture-modified mononuclear cells confer vascular protection after arterial injury. Circulation. Vol.108; No.12, pp. 1520-1526, Guven, H., et al. (2006). The number of endothelial progenitor cell colonies in the blood is

Hill, J., et al., (2003). Circulating Endothelial Progenitor Cells, Vascular Function, and

Hristov, M. & Weber, C. (2004). Endothelial progenitor cells: characterization,

Hristov, M. & Weber, C. (2007). Endothelial progenitor cells: Cellular biomarkers in vascular

Huang, P., et al. (2010). Intake of red wine increases the number and functional capacity of

Hur, J., et al. (2004). Characterization of two types of endothelial progenitor cells and their

Cardiovascular Risk. *N Engl J Med,* Vol.348; No.7, pp. 593-600,

postnatal vasculogenesis in physiological and pathological neovascularization. *Circ* 

in Acute Myocardial Infarction (TOPCARE-AMI). *Circulation.* Vol.106; No.24, pp.

therapeutic effect of angiotensin II receptor antagonists. *Hypertension.* Vol. 45; No.4,

cells but distinct, primitive hematopoietic progenitors. *Exp Hematol,* Vol.35; No.7,

determinants of subclinical atherosclerosis in a middle-aged general population.

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pathophysiology, and possible clinical relevance. *J Cell Mol Med,* Vol.8; No.4, pp.

circulating endothelial progenitor cells by enhancing nitric oxide bioavailability.

different contributions to neovasculogenesis. *Arterioscler Thromb Vasc Biol.* Vol.24;

**10. References** 

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to contribute to neovascularization in various ischemic diseases, and EPC transplantation on diabetic wounds has a beneficial effect, mainly achieved by their direct paracrine action on keratinocytes, fibroblasts, and endothelial cells, rather than through their physical engraftment into host tissues (vasculogenesis). In the TOPCARE-AMI (i.e., "Transplantation of Progenitor Cells and Regeneration Enhancement in Acute Myocardial Infarction") trial (Assmus et al., 2002), intracoronary infusion of cultured human EPCs in patients with recent myocardial infarction was associated with improvements in global left ventricular function and microvascular function. In addition, an EPC-conditioned medium was shown to be therapeutically equivalent to EPCs, at least for the treatment of diabetic dermal wounds (Kim et al., 2010).

There are several ways to increase levels of circulating EPCs and improve their function by pharmacological strategies and lifestyle modification. Notably, it was shown that the angiotensin-converting enzyme (ACE) inhibitors such as ramipril (Min et al., 2004), and angiotensin II (AT II) inhibitors, like valsartan (Bahlmann et al., 2005) increased EPC levels in patients, probably interfering with the CD26/dipeptidylpeptidase IV system. Our recent data showed that moderate intake of red wine significantly enhanced circulating EPC levels and improved EPC functions by modifying NO bioavailability (Huang et al., 2010). Other studies indicated that either the phosphatidylinositol 3 kinase/Akt/endothelial nitric oxide synthase/NO (*PI3K/Akt*/*eNOS*/*NO*) signaling pathway or the interaction between hyperglycemia and hyperlipidemia in diabetic patients who have vascular diseases, are potential therapeutic targets for abolishing the impaired function of EPCs (Wang et al., 2011). Neutralization of the p66ShcAgene, which regulates the apoptotic response to oxidative stress, prevented high glucose-induced EPC impairment *in vitro* (Di et al., 2009). The existence of molecules acting on EPCs can be used to positively condition cultured EPCs before therapeutic transplantation. Thus, because it is known that chemokine SDF-1α is able to mobilize EPCs, and because EPCs are known to have receptors for SDF-1α, it was demonstrated that SDF-1α - primed EPCs exhibit increased adhesion to HUVEC, resulting in more efficient incorporation of EPCs into sites of neovascularization (Zemani et al., 2008).

#### **8. Conclusions**

In conclusion, EPCs are biomarkers of endothelial repair with therapeutic potential, since low EPC levels predict endothelial dysfunction and a poor clinical outcome. Various studies have focused on the important role of EPCs in vasculogenesis and angiogenesis of ischemic tissue in peripheral artery disease as well as acute myocardial infarction, but only a few studies have concentrated on the role of EPCs in the prevention and therapy of atherosclerosis.

#### **9. Acknowledgements**

This study was supported in part by research grants from the UST-UCSD International Center of Excellence in Advanced Bio-engineering NSC-99-2911-I-009-101 from the National Science Council; VGH-V98B1-003 and VGH-V100E2-002 from Taipei Veterans General Hospital, and also a grant from the Ministry of Education "Aim for the Top University" Plan.

#### **10. References**

76 Traditional and Novel Risk Factors in Atherothrombosis

to contribute to neovascularization in various ischemic diseases, and EPC transplantation on diabetic wounds has a beneficial effect, mainly achieved by their direct paracrine action on keratinocytes, fibroblasts, and endothelial cells, rather than through their physical engraftment into host tissues (vasculogenesis). In the TOPCARE-AMI (i.e., "Transplantation of Progenitor Cells and Regeneration Enhancement in Acute Myocardial Infarction") trial (Assmus et al., 2002), intracoronary infusion of cultured human EPCs in patients with recent myocardial infarction was associated with improvements in global left ventricular function and microvascular function. In addition, an EPC-conditioned medium was shown to be therapeutically equivalent to EPCs, at least for the treatment of diabetic dermal wounds

There are several ways to increase levels of circulating EPCs and improve their function by pharmacological strategies and lifestyle modification. Notably, it was shown that the angiotensin-converting enzyme (ACE) inhibitors such as ramipril (Min et al., 2004), and angiotensin II (AT II) inhibitors, like valsartan (Bahlmann et al., 2005) increased EPC levels in patients, probably interfering with the CD26/dipeptidylpeptidase IV system. Our recent data showed that moderate intake of red wine significantly enhanced circulating EPC levels and improved EPC functions by modifying NO bioavailability (Huang et al., 2010). Other studies indicated that either the phosphatidylinositol 3 kinase/Akt/endothelial nitric oxide synthase/NO (*PI3K/Akt*/*eNOS*/*NO*) signaling pathway or the interaction between hyperglycemia and hyperlipidemia in diabetic patients who have vascular diseases, are potential therapeutic targets for abolishing the impaired function of EPCs (Wang et al., 2011). Neutralization of the p66ShcAgene, which regulates the apoptotic response to oxidative stress, prevented high glucose-induced EPC impairment *in vitro* (Di et al., 2009). The existence of molecules acting on EPCs can be used to positively condition cultured EPCs before therapeutic transplantation. Thus, because it is known that chemokine SDF-1α is able to mobilize EPCs, and because EPCs are known to have receptors for SDF-1α, it was demonstrated that SDF-1α - primed EPCs exhibit increased adhesion to HUVEC, resulting in more efficient incorporation of EPCs

In conclusion, EPCs are biomarkers of endothelial repair with therapeutic potential, since low EPC levels predict endothelial dysfunction and a poor clinical outcome. Various studies have focused on the important role of EPCs in vasculogenesis and angiogenesis of ischemic tissue in peripheral artery disease as well as acute myocardial infarction, but only a few studies have concentrated on the role of EPCs in the prevention and therapy of

This study was supported in part by research grants from the UST-UCSD International Center of Excellence in Advanced Bio-engineering NSC-99-2911-I-009-101 from the National Science Council; VGH-V98B1-003 and VGH-V100E2-002 from Taipei Veterans General Hospital, and also a grant from the Ministry of Education "Aim for the Top University"

(Kim et al., 2010).

**8. Conclusions** 

atherosclerosis.

Plan.

**9. Acknowledgements** 

into sites of neovascularization (Zemani et al., 2008).


**5** 

 *Canada* 

**CD40 Ligand and Its** 

Yahye Merhi2 and Walid Mourad1\*

*Hôpital Saint-Luc, Montréal,* 

**Receptors in Atherothrombosis** 

*1Laboratoire d'Immunologie Cellulaire et Moléculaire, Centre Hospitalier de l'Université de Montréal,* 

Daniel Yacoub1, Ghada S. Hassan1, Nada Alaadine1,

*2Institut de Cardiologie de Montréal, Université de Montréal, Montréal,* 

Atherothrombosis is the main underlying determinant of cardiovascular diseases, which remain the leading cause of death in developed countries. Multiple lines of evidence now support the concept of atherothrombosis as a chronic inflammatory disease of the arterial wall.1, 2 This process involves a complex interplay between modified lipids and cells of the immune and vascular system, which usually evolves into the formation of atherosclerotic lesions yielding a stable necrotic plaque. If left untreated, plaque rupture and thrombosis may ensue, leading to important clinical manifestations, such as acute coronary syndromes

As the incorporation of modified low-density lipoproteins in the arterial wall represents a important step in the onset of atherothrombosis, the subsequent recruitment and activation of inflammatory cells, including monocytes, B- and T-lymphocytes, neutrophils and platelets play a critical role in the pathogenesis of this disease.4 These cells exhibit proatherogenic functions through multiple co-stimulatory and immune molecules present on their cell surface. Among these, the CD40L/CD40 receptor-ligand pair has been the focus of much attention, such that this dyad is now regarded as a pivotal contributor to all underlying phases of atherothrombosis.5-7 Indeed, the CD40L/CD40 interaction exerts a wide array of biological functions at the forefront of the pathophysiology of this disease and disruption of this cascade by both pharmacological and genetic approaches have shown beneficial results in animal and clinical studies.8-11 While CD40L is known to mainly interact with its classical receptor CD40, additional binding partners have been described, namely the integrins αIIbβ3, αMβ2 and α5β1. This chapter discusses the role of CD40L and its receptors in the pathophysiology of atherothrombosis, while highlighting its therapeutic potentials in

**1. Introduction** 

and sudden death.3

\* Corresponding Author

the treatment of this chronic inflammatory disease.


## **CD40 Ligand and Its Receptors in Atherothrombosis**

Daniel Yacoub1, Ghada S. Hassan1, Nada Alaadine1, Yahye Merhi2 and Walid Mourad1\*

*1Laboratoire d'Immunologie Cellulaire et Moléculaire, Centre Hospitalier de l'Université de Montréal, Hôpital Saint-Luc, Montréal, 2Institut de Cardiologie de Montréal, Université de Montréal, Montréal, Canada* 

#### **1. Introduction**

78 Traditional and Novel Risk Factors in Atherothrombosis

Hur, J., et al. (2007). Identification of a novel role of T cells in postnatal vasculogenesis:

Kim, J., et al. (2010). Human cord blood-derived endothelial progenitor cells and their

Lin, Y., et al. (2000). Origins of circulating endothelial cells and endothelial outgrowth from

Min, T., et al. (2004). Improvement in endothelial progenitor cells from peripheral blood by

Rehman, J., et al. (2003). Peripheral blood 'endothelial progenitor cells' are derived from

Rohde, E., et al. (2007). Immune cells mimic the morphology of endothelial progenitor

Romagnani, P., et al. (2005). CD14+CD34-low cells with stem cell phenotypic and functional

Schmidt-Lucke, C., et al. (2005). Reduced number of circulating endothelial progenitor cells

of endogenous vascular repair. *Circulation.* Vol.111; No.22, pp. 2981-2987, Urbich, C., et al. (2003). Relevance of monocytic features for neovascularization capacity of circulating endothelial progenitor cells. *Circulation.* Vol.108; No.20, pp. 2511-2516, Vasa, M., et al. (2001). Number and migratory activity of circulating endothelial progenitor

Verfaillie, C. (2002). Hematopoietic stem cells for transplantation. *Nat Immunol,* Vol.3; No.4,

Wang, H., et al. (2007). Circulating endothelial progenitor cells, c-reactive protein and

Wang, R., et al. (2011). Activation of vascular BK channels by docosahexaenoic acid is

Wassmann, S., et al. (2006). Improvement of endothelial function by systemic transfusion of

Werner, N., et al. (2005). Circulating endothelial progenitor cells and cardiovascular

Yoon, C., et al. (2005). Synergistic neovascularization by mixed transplantation of early

Zemani, F., et al. (2008). Ex vivo priming of endothelial progenitor cells with SDF-1 before

vascular progenitor cells. *Circ Res.* Vol.99; No.8, pp. 74-83,

outcomes. *N Engl J Med.* Vol.353; No.10, pp. 999-1007,

colonies in vitro. *Stem Cells*. Vol.25; No.7, pp. 1746-1752,

pp. 1671-1682,

*Transplant.* Vol.19, No.12, pp. 1635-1644,

*Ther.* Vol.18, No.3, pp. 203-209,

Vol.107; No.8, pp. 1164-1169,

No.4, pp. 314-322,

Vol.89; No.1, pp. 1-7,

*Hypertens Res.* Vol.30, No.2, pp. 133-141,

*Vasc Biol.* Vol.28; No.4, pp. 644-650,

pp. 314-317,

pp. 344-352,

pp. 1618-1627,

blood. *J Clin Invest.* Vol.105; No.1, pp. 71-77,

characterization of endothelial progenitor cell colonies. *Circulation.* Vol.116; No.15,

conditioned media exhibit therapeutic equivalence for diabetic wound healing. *Cell* 

ramipril therapy in patients with stable coronary artery disease. *Cardiovasc Drugs* 

monocyte/macrophages and secrete angiogenic growth factors. *Circulation.*

features are the major source of circulating endothelial progenitors. *Circ Res.* Vol.97;

predicts future cardiovascular events: proof of concept for the clinical importance

cells inversely correlate with risk factors for coronary artery disease. *Circ Res.*

severity of coronary stenosis in chinese patients with coronary artery disease.

dependent on cytochrome P450 epoxygenase activity. *Cardiovasc Res.* Vol.90; No.2,

endothelial progenitor cells and late outgrowth endothelial cells: the role of angiogenic cytokines and matrix metalloproteinases. *Circulation.* Vol.112, No.11,

transplantation could increase their proangiogenic potential. *Arterioscler Thromb* 

Atherothrombosis is the main underlying determinant of cardiovascular diseases, which remain the leading cause of death in developed countries. Multiple lines of evidence now support the concept of atherothrombosis as a chronic inflammatory disease of the arterial wall.1, 2 This process involves a complex interplay between modified lipids and cells of the immune and vascular system, which usually evolves into the formation of atherosclerotic lesions yielding a stable necrotic plaque. If left untreated, plaque rupture and thrombosis may ensue, leading to important clinical manifestations, such as acute coronary syndromes and sudden death.3

As the incorporation of modified low-density lipoproteins in the arterial wall represents a important step in the onset of atherothrombosis, the subsequent recruitment and activation of inflammatory cells, including monocytes, B- and T-lymphocytes, neutrophils and platelets play a critical role in the pathogenesis of this disease.4 These cells exhibit proatherogenic functions through multiple co-stimulatory and immune molecules present on their cell surface. Among these, the CD40L/CD40 receptor-ligand pair has been the focus of much attention, such that this dyad is now regarded as a pivotal contributor to all underlying phases of atherothrombosis.5-7 Indeed, the CD40L/CD40 interaction exerts a wide array of biological functions at the forefront of the pathophysiology of this disease and disruption of this cascade by both pharmacological and genetic approaches have shown beneficial results in animal and clinical studies.8-11 While CD40L is known to mainly interact with its classical receptor CD40, additional binding partners have been described, namely the integrins αIIbβ3, αMβ2 and α5β1. This chapter discusses the role of CD40L and its receptors in the pathophysiology of atherothrombosis, while highlighting its therapeutic potentials in the treatment of this chronic inflammatory disease.

<sup>\*</sup> Corresponding Author

CD40 Ligand and Its Receptors in Atherothrombosis 81

The engagement of CD40 by CD40L promotes the clustering of CD40 and induces the association of TRAFs to the cytoplasmic domain of CD40.30 These adapter proteins are essential for the activation of different signaling pathways including the canonical and noncanonical nuclear factor κB (NF-κB)-signaling pathways and the activation of mitogenactivated protein kinases (MAPKs).30 The TRAF family comprises six known members, among which TRAF-1, -2, -3, -5 and -6 have been shown to drive CD40-dependent cellular responses. TRAF-1 can only bind weakly to the cytoplasmic tail of CD40 and therefore regulates the signaling of others TRAF members, in particular TRAF-2.30, 31 Indeed, TRAF-1 deficiency in antigen presenting cells and B-lymphocytes leads to a significant reduction in the recruitment of TRAF-2 to CD40, indicating that TRAF-1 facilitates the association of TRAF-2 to the cytoplasmic domain of CD40.32, 33 In agreement with these results, it has been shown that the recruitment of both TRAF-1 and TRAF-2 are required for complete activation of NFκB in B-cells, since the knockout of both genes results in a greater inhibition of the NF-κB

TRAF-2 is an important contributor to CD40 signaling and its major role resides in the activation of the NF-κB signaling pathway, as well as the activation of the p38, Akt and JNK MAPKs. CD40 bears a direct binding site for TRAF-2 and blockage of this interaction leads to immune deficiencies such as B-cell proliferation and isotype switching.24, 34 Despite its significant implications in CD40 signaling, TRAF-2 deficiency may be overcome by TRAF-6 activation. This was confirmed by data showing that binding of either TRAF-2 or TRAF-6 alone may activate the NF-κB pathway, while inhibition of both these members completely abolishes CD40-dependant B-cell activation, suggesting that both members collaborate for

TRAF-3 functions as a negative regulator of CD40 signaling through its constitutive association with TRAF-2.38, 39 In absence of stimulation, TRAF-3 interacts with TRAF-2, which allows the degradation of the NF-κB inducing kinase (NIK) protein, a critical stimulator of NF-κB.40 TRAF-3 deficiency in B cells exacerbates NF-κB and JNK activation, primarily through cytosolic accumulation of NIK, thus confirming the negative regulatory

Very little information is available regarding the role of TRAF-5 in CD40 signaling. Nevertheless, it appears that TRAF-5 can interact with TRAF-3 to modulate NF-κB activation in B cells. This was shown by experiments in which TRAF-5 deficiency diminishes NF-κB activation, causing a reduction in cell activation, expression of co-stimulatory

TRAF-6 plays a significant role in the activation of key CD40-dependent signaling pathways, such as NF-κB, p38, JNK and Akt.44 As discussed above, TRAF-6 synergizes with TRAF-2 in order to regulate the activation of NF-κB. Although TRAF-6 contains a direct binding site for CD40, specific inhibition of this domain shows lesser inhibitory effects than ablation of the complete protein, indicating that TRAF-6 may still have a functional role in CD40 signaling without binding directly to CD40.30, 45 Indeed, one of the main functions of TRAF-6 resides in its ability to interact with TRAF-2, which is already bound to CD40, and

**CD40-TRAF dependent signaling** 

signaling pathway, in comparison to single knockouts.33

the activation of this critical signaling cascade.35-37

functions of TRAF-3 in B cells.41

molecules and antibody production.42, 43

facilitate the activation of downstream targets.

#### **2. The CD40L system**

#### **2.1 CD40L**

CD40L, also known as CD154, is a 39 kDa transmembrane protein belonging to the tumor necrosis factor (TNF) superfamily originally identified on cells of the immune system.12, 13 The interaction of CD40L with its respective receptor on B cells, CD40, a glycoprotein also from the TNF receptor (TNFR) family, is of critical importance for immunoglobulin isotype switching during the immune response.14 The importance of this interaction is highlighted by the pathophysiological manifestations seen in patients suffering from the X-linked hyperimmunoglobulin-M syndrome, in which B-cells fail to produce the immunoglobulin's IgG, IgA and IgE as a consequence of a genetic mutation in the CD40L gene.15 Because of its wide distribution across cells of the vascular system (endothelium, B and T lymphocytes, neutrophils, platelets, monocytes, dentritic cells and smooth muscle cells), the CD40L/CD40 dyad also shares important implications in cell-mediated immunity. CD40L-induced signaling in these cells leads to the up-regulation of adhesion and co-stimulatory molecules, and the production of pro-inflammatory cytokines, chemokines, growth factors, matrix metalloproteinases (MMPs) and procoagulants.16-19 These cellular events are also the main mechanisms by which CD40L regulates numerous inflammatory disorders, in particular atherothrombosis and its related complications. In fact, circulating levels of soluble CD40L (sCD40L), which originate from the proteolytic cleavage of membrane-bound CD40L at the surface of activated platelets, have now emerged as strong indicators of cardiovascular events such as atherothrombosis and acute coronary syndromes.20-22

#### **2.2 CD40L receptors**

#### **CD40**

CD40 is the classical high affinity receptor for CD40L. It is constitutively or inducibly expressed by most cells of the vascular system (hematopoietic and non-hematopoietic cells) and represents the main signaling molecule in the CD40L/CD40 receptor-ligand pair.23 The cytoplasmic domain of CD40 bears signaling domains required for the association of binding proteins termed TNF receptor-associated factors (TRAFs).24 During humoral immunity, a tight interplay between dendritic cells, T-lymphocytes and B-lymphocytes occurs, throughout which the activation of CD40 provides a crucial signal for the activation, differentiation and secretion of immunoglobulins by B cells.25 Moreover, CD40 activation in these cells induces an important anti-apoptotic signal that facilitates cell survival and differentiation, primarily through activation of the anti-apoptotic proteins Bcl-XL, A20, Bfl-1 and Mcl-1, which protect against Fas ligand and TNF-induced cell death.26 As discussed above, CD40 signaling plays a significant role in cell-mediated immunity, an important aspect by which the CD40L/CD40 dyad initiates and exacerbates atherosclerotic lesions. For instance, CD40 activation on endothelial cells induces the up-regulation of a plethora of proinflammatory adhesion molecules, cytokines, chemokines, matrix metalloproteinases and procoagulants.16 In addition, it has been demonstrated that upon CD40L binding CD40 activation on platelets can enhance platelet function and promote the secretion of inflammatory cytokines involved in plaque formation (this aspect will be discussed in greater detail bellow).27-29

#### **CD40-TRAF dependent signaling**

80 Traditional and Novel Risk Factors in Atherothrombosis

CD40L, also known as CD154, is a 39 kDa transmembrane protein belonging to the tumor necrosis factor (TNF) superfamily originally identified on cells of the immune system.12, 13 The interaction of CD40L with its respective receptor on B cells, CD40, a glycoprotein also from the TNF receptor (TNFR) family, is of critical importance for immunoglobulin isotype switching during the immune response.14 The importance of this interaction is highlighted by the pathophysiological manifestations seen in patients suffering from the X-linked hyperimmunoglobulin-M syndrome, in which B-cells fail to produce the immunoglobulin's IgG, IgA and IgE as a consequence of a genetic mutation in the CD40L gene.15 Because of its wide distribution across cells of the vascular system (endothelium, B and T lymphocytes, neutrophils, platelets, monocytes, dentritic cells and smooth muscle cells), the CD40L/CD40 dyad also shares important implications in cell-mediated immunity. CD40L-induced signaling in these cells leads to the up-regulation of adhesion and co-stimulatory molecules, and the production of pro-inflammatory cytokines, chemokines, growth factors, matrix metalloproteinases (MMPs) and procoagulants.16-19 These cellular events are also the main mechanisms by which CD40L regulates numerous inflammatory disorders, in particular atherothrombosis and its related complications. In fact, circulating levels of soluble CD40L (sCD40L), which originate from the proteolytic cleavage of membrane-bound CD40L at the surface of activated platelets, have now emerged as strong indicators of cardiovascular

CD40 is the classical high affinity receptor for CD40L. It is constitutively or inducibly expressed by most cells of the vascular system (hematopoietic and non-hematopoietic cells) and represents the main signaling molecule in the CD40L/CD40 receptor-ligand pair.23 The cytoplasmic domain of CD40 bears signaling domains required for the association of binding proteins termed TNF receptor-associated factors (TRAFs).24 During humoral immunity, a tight interplay between dendritic cells, T-lymphocytes and B-lymphocytes occurs, throughout which the activation of CD40 provides a crucial signal for the activation, differentiation and secretion of immunoglobulins by B cells.25 Moreover, CD40 activation in these cells induces an important anti-apoptotic signal that facilitates cell survival and differentiation, primarily through activation of the anti-apoptotic proteins Bcl-XL, A20, Bfl-1 and Mcl-1, which protect against Fas ligand and TNF-induced cell death.26 As discussed above, CD40 signaling plays a significant role in cell-mediated immunity, an important aspect by which the CD40L/CD40 dyad initiates and exacerbates atherosclerotic lesions. For instance, CD40 activation on endothelial cells induces the up-regulation of a plethora of proinflammatory adhesion molecules, cytokines, chemokines, matrix metalloproteinases and procoagulants.16 In addition, it has been demonstrated that upon CD40L binding CD40 activation on platelets can enhance platelet function and promote the secretion of inflammatory cytokines involved in plaque formation (this aspect will be discussed in

events such as atherothrombosis and acute coronary syndromes.20-22

**2. The CD40L system** 

**2.2 CD40L receptors** 

greater detail bellow).27-29

**CD40** 

**2.1 CD40L** 

The engagement of CD40 by CD40L promotes the clustering of CD40 and induces the association of TRAFs to the cytoplasmic domain of CD40.30 These adapter proteins are essential for the activation of different signaling pathways including the canonical and noncanonical nuclear factor κB (NF-κB)-signaling pathways and the activation of mitogenactivated protein kinases (MAPKs).30 The TRAF family comprises six known members, among which TRAF-1, -2, -3, -5 and -6 have been shown to drive CD40-dependent cellular responses.

TRAF-1 can only bind weakly to the cytoplasmic tail of CD40 and therefore regulates the signaling of others TRAF members, in particular TRAF-2.30, 31 Indeed, TRAF-1 deficiency in antigen presenting cells and B-lymphocytes leads to a significant reduction in the recruitment of TRAF-2 to CD40, indicating that TRAF-1 facilitates the association of TRAF-2 to the cytoplasmic domain of CD40.32, 33 In agreement with these results, it has been shown that the recruitment of both TRAF-1 and TRAF-2 are required for complete activation of NFκB in B-cells, since the knockout of both genes results in a greater inhibition of the NF-κB signaling pathway, in comparison to single knockouts.33

TRAF-2 is an important contributor to CD40 signaling and its major role resides in the activation of the NF-κB signaling pathway, as well as the activation of the p38, Akt and JNK MAPKs. CD40 bears a direct binding site for TRAF-2 and blockage of this interaction leads to immune deficiencies such as B-cell proliferation and isotype switching.24, 34 Despite its significant implications in CD40 signaling, TRAF-2 deficiency may be overcome by TRAF-6 activation. This was confirmed by data showing that binding of either TRAF-2 or TRAF-6 alone may activate the NF-κB pathway, while inhibition of both these members completely abolishes CD40-dependant B-cell activation, suggesting that both members collaborate for the activation of this critical signaling cascade.35-37

TRAF-3 functions as a negative regulator of CD40 signaling through its constitutive association with TRAF-2.38, 39 In absence of stimulation, TRAF-3 interacts with TRAF-2, which allows the degradation of the NF-κB inducing kinase (NIK) protein, a critical stimulator of NF-κB.40 TRAF-3 deficiency in B cells exacerbates NF-κB and JNK activation, primarily through cytosolic accumulation of NIK, thus confirming the negative regulatory functions of TRAF-3 in B cells.41

Very little information is available regarding the role of TRAF-5 in CD40 signaling. Nevertheless, it appears that TRAF-5 can interact with TRAF-3 to modulate NF-κB activation in B cells. This was shown by experiments in which TRAF-5 deficiency diminishes NF-κB activation, causing a reduction in cell activation, expression of co-stimulatory molecules and antibody production.42, 43

TRAF-6 plays a significant role in the activation of key CD40-dependent signaling pathways, such as NF-κB, p38, JNK and Akt.44 As discussed above, TRAF-6 synergizes with TRAF-2 in order to regulate the activation of NF-κB. Although TRAF-6 contains a direct binding site for CD40, specific inhibition of this domain shows lesser inhibitory effects than ablation of the complete protein, indicating that TRAF-6 may still have a functional role in CD40 signaling without binding directly to CD40.30, 45 Indeed, one of the main functions of TRAF-6 resides in its ability to interact with TRAF-2, which is already bound to CD40, and facilitate the activation of downstream targets.

CD40 Ligand and Its Receptors in Atherothrombosis 83

binding is mediated through the KGD recognition domain present on the αIIbβ3 molecule. Interestingly, CD40L also contains a KGD sequence making its interaction with αIIbβ3 possible. Binding of CD40L to αIIbβ3 was shown to induce phosphorylation of tyrosine residues within the cytoplasmic domain of the β3 subunit and appears essential for thrombus stabilization *in vivo*.48, 49 Indeed, CD40L-/- mice exhibit unstable thrombi, which can be overcome by infusion of wild type recombinant human CD40L and not CD40L

The *α5β1* integrin was also shown to act as a CD40L receptor.50 Indeed, sCD40L can bind and activate cells of the undifferentiated human monocytic U937 cell line in a CD40- and αIIbβ3-independent manner. Binding to this cell line was reversed by an anti-*α5β1* antibody, as well as in the presence of soluble *α5β1*, thus confirming *α5β1* specificity. Moreover, this interaction is unaffected by pre-treatment of CD40L with soluble CD40, indicating that CD40L can bind both CD40 and *α5β1* concomitantly.50 Interestingly, CD40L binds to inactive *α5β1*, contrary to most ligands of the integrin family. However, the physiological relevance of this interaction remains unexplored and additional studies are needed to fully characterize the interplay that might take place between CD40L and *α5β1* in inflammatory

The αMβ2 (Mac-1) integrin mediates firm adhesion of leukocytes to inflamed vessels by interacting with its endothelial cell counterreceptor intercellular adhesion molecule 1 (ICAM-1).51 CD40L was also recently shown to bind to active αMβ2 and this interaction may represent an alternative pathway for CD40L-mediated inflammation.52 Indeed, inhibition of this novel CD40L binding partner significantly attenuates leukocyte accumulation at sites of inflammation and reduces atherogenesis, indicating that CD40L may promote, at least in part, atherosclerotic lesions in a αMβ2-dependent manner. Again, the relative contribution of this CD40L receptor (in comparison to CD40) in the development of inflammatory disorders such as atherothrombosis remains unknown. Perhaps, each of these CD40L receptors may interfere at different stages of the disease, thus contributing to

The involvement of CD40L in the pathogenesis of atherothrombosis is supported by numerous studies. Targeting of CD40L by both pharmacological and genetic approaches has highlighted the importance of this molecule in all stages of the disease. In 1998, Mach et al. showed that treatment of hyperlipidemic LDLR-/- mice with an anti-CD40L antibody significantly ameliorates the size and lipid contents of atherosclerotic lesions.53 These results were further confirmed by a genetic approach, which showed that CD40L-/-/ApoE-/- mice exhibit considerably smaller plaque area than control ApoE-/- mice.8 Moreover, these animals display enhanced collagen fibrils within the fibrous cap of lesions, a key component of plaque stability. In an additional study, the administration of an anti-CD40L antibody in ApoE-/- mice, at the onset of lesions or once atherosclerotic lesions are fully established, reduces lipid contents and increases plaque stability.54 Taken together, these studies support

the contribution of CD40L in plaque initiation, progression and stability (Figure 2).

specifically mutated at the site of interaction with αIIbβ3.48

proinflammatory reactions and atherogenesis in their own way.

**3. CD40L in atherothrombosis** 

**α5β1** 

disorders.

**αMβ2** 

Recently, a study aiming at evaluating the implication of TRAF members in neointima formation, a critical step of atherothrombosis, was conducted. Using a CD40 transgenic mouse model, in which mutations at the TRAF2/3/5, TRAF6 or TRAF2/3/5/6 binding sites were carried out, the authors conclude that the CD40-TRAF6 axis is a key regulator of inflammatory cell infiltration and neointima formation at sites of vascular injury.46

Although most vascular complications associated to CD40L, including atherothrombosis, have been largely attributed to its interaction with CD40, recently identified additional receptors merit attention. These include the integrins αIIbβ3, α5β1 and αMβ2 and (Figure 1).

Fig. 1. CD40L and its receptors. The binding of CD40L to its classical CD40 counterreceptor regulates numerous critical biological responses. These mainly include B-cell dependent isotype switching, cell-mediated immunity (production of cytokines, chemokines, adhesion molecules, growth factors, MMPs and procoagulants) and apoptosis. The CD40L/CD40 interaction is at the forefront of the pathogenesis of multiple inflammatory disorders, including atherothrombosis. The interaction of CD40L with the αIIbβ3 platelet integrin is involved in thrombus stabilization and may provide a novel outside-in signaling pathway by which platelets can be activated. CD40L can also bind to the inactive conformation α5β1 and this interaction was shown to induce activation of the human monocytic U937 cell line. Finally, αMβ2 can mediate CD40L-dependent inflammatory responses, in particular leukocyte adhesion and neointimal formation. The pathophysiological relevance of these novel CD40L-mediated interactions in inflammation remains elusive and additional studies will be required to address this issue.

#### **αIIbβ3**

The αIIbβ3 integrin is the most abundant receptor of the surface of platelets and mediates platelet adhesion and aggregation. Like all molecules of the integrin family, it will change conformation upon inside-out cellular activation, thereby allowing binding to its natural ligands (fibrinogen, fibronectin, vWF...).47 These ligands contain KGD sequences and binding is mediated through the KGD recognition domain present on the αIIbβ3 molecule. Interestingly, CD40L also contains a KGD sequence making its interaction with αIIbβ3 possible. Binding of CD40L to αIIbβ3 was shown to induce phosphorylation of tyrosine residues within the cytoplasmic domain of the β3 subunit and appears essential for thrombus stabilization *in vivo*.48, 49 Indeed, CD40L-/- mice exhibit unstable thrombi, which can be overcome by infusion of wild type recombinant human CD40L and not CD40L specifically mutated at the site of interaction with αIIbβ3.48

#### **α5β1**

82 Traditional and Novel Risk Factors in Atherothrombosis

Recently, a study aiming at evaluating the implication of TRAF members in neointima formation, a critical step of atherothrombosis, was conducted. Using a CD40 transgenic mouse model, in which mutations at the TRAF2/3/5, TRAF6 or TRAF2/3/5/6 binding sites were carried out, the authors conclude that the CD40-TRAF6 axis is a key regulator of inflammatory

Although most vascular complications associated to CD40L, including atherothrombosis, have been largely attributed to its interaction with CD40, recently identified additional receptors merit attention. These include the integrins αIIbβ3, α5β1 and αMβ2 and (Figure 1).

Fig. 1. CD40L and its receptors. The binding of CD40L to its classical CD40 counterreceptor regulates numerous critical biological responses. These mainly include B-cell dependent isotype switching, cell-mediated immunity (production of cytokines, chemokines, adhesion molecules, growth factors, MMPs and procoagulants) and apoptosis. The CD40L/CD40 interaction is at the forefront of the pathogenesis of multiple inflammatory disorders, including atherothrombosis. The interaction of CD40L with the αIIbβ3 platelet integrin is involved in thrombus stabilization and may provide a novel outside-in signaling pathway by which platelets can be activated. CD40L can also bind to the inactive conformation α5β1 and this interaction was shown to induce activation of the human monocytic U937 cell line. Finally, αMβ2 can mediate CD40L-dependent inflammatory responses, in particular leukocyte adhesion and neointimal formation. The pathophysiological relevance of these novel CD40L-mediated interactions in inflammation remains elusive and additional studies

The αIIbβ3 integrin is the most abundant receptor of the surface of platelets and mediates platelet adhesion and aggregation. Like all molecules of the integrin family, it will change conformation upon inside-out cellular activation, thereby allowing binding to its natural ligands (fibrinogen, fibronectin, vWF...).47 These ligands contain KGD sequences and

will be required to address this issue.

**αIIbβ3** 

cell infiltration and neointima formation at sites of vascular injury.46

The *α5β1* integrin was also shown to act as a CD40L receptor.50 Indeed, sCD40L can bind and activate cells of the undifferentiated human monocytic U937 cell line in a CD40- and αIIbβ3-independent manner. Binding to this cell line was reversed by an anti-*α5β1* antibody, as well as in the presence of soluble *α5β1*, thus confirming *α5β1* specificity. Moreover, this interaction is unaffected by pre-treatment of CD40L with soluble CD40, indicating that CD40L can bind both CD40 and *α5β1* concomitantly.50 Interestingly, CD40L binds to inactive *α5β1*, contrary to most ligands of the integrin family. However, the physiological relevance of this interaction remains unexplored and additional studies are needed to fully characterize the interplay that might take place between CD40L and *α5β1* in inflammatory disorders.

#### **αMβ2**

The αMβ2 (Mac-1) integrin mediates firm adhesion of leukocytes to inflamed vessels by interacting with its endothelial cell counterreceptor intercellular adhesion molecule 1 (ICAM-1).51 CD40L was also recently shown to bind to active αMβ2 and this interaction may represent an alternative pathway for CD40L-mediated inflammation.52 Indeed, inhibition of this novel CD40L binding partner significantly attenuates leukocyte accumulation at sites of inflammation and reduces atherogenesis, indicating that CD40L may promote, at least in part, atherosclerotic lesions in a αMβ2-dependent manner. Again, the relative contribution of this CD40L receptor (in comparison to CD40) in the development of inflammatory disorders such as atherothrombosis remains unknown. Perhaps, each of these CD40L receptors may interfere at different stages of the disease, thus contributing to proinflammatory reactions and atherogenesis in their own way.

#### **3. CD40L in atherothrombosis**

The involvement of CD40L in the pathogenesis of atherothrombosis is supported by numerous studies. Targeting of CD40L by both pharmacological and genetic approaches has highlighted the importance of this molecule in all stages of the disease. In 1998, Mach et al. showed that treatment of hyperlipidemic LDLR-/- mice with an anti-CD40L antibody significantly ameliorates the size and lipid contents of atherosclerotic lesions.53 These results were further confirmed by a genetic approach, which showed that CD40L-/-/ApoE-/- mice exhibit considerably smaller plaque area than control ApoE-/- mice.8 Moreover, these animals display enhanced collagen fibrils within the fibrous cap of lesions, a key component of plaque stability. In an additional study, the administration of an anti-CD40L antibody in ApoE-/- mice, at the onset of lesions or once atherosclerotic lesions are fully established, reduces lipid contents and increases plaque stability.54 Taken together, these studies support the contribution of CD40L in plaque initiation, progression and stability (Figure 2).

CD40 Ligand and Its Receptors in Atherothrombosis 85

Plaque initiation is normally characterized by the accumulation of low-density lipoproteins (LDL) in the arterial wall and the subsequent recruitment and transmigration of leukocytes within the sub-endothelial space.1 The initial trigger of CD40L (and CD40) expression within cells of the developing atherosclerotic plaque (endothelial cells, lymphocytes, platelets, monocytes/macrophages, and smooth muscle cells) remains elusive. Possible candidates include oxidized LDL, infectious pathogens and alterations in vascular hemodynamic forces.55- 57 For instance it has been demonstrated that lipid lowering reduces CD40L expression in atheroma.55 In addition, oxidized LDL were reported to induce the expression of CD40 on endothelial cells, which can then bind CD40L from activated T-lymphocytes adherent to the site of injury.58 CD40 activation on endothelial cells provides a critical proinflammatory signal for the initiation of lesions. Indeed, the CD40L/CD40 interaction favors the up-regulation of adhesion molecules (E-selectin, P-selectin, vascular cell adhesion molecule-1 [VCAM-1] and ICAM-1) and leads to the secretion of proinflammatory cytokines (IL-6, IL-8, IL-15, monocytes chemotactic protein-1 [MCP-1], macrophage inflammatory protein-1 [MIP-1 α/β] and regulated on activation normal T cell expressed and secreted [RANTES]) by the endothelium.6, 7, 59-62 These reactions induce in turn the incorporation and accumulation of

additional leukocytes, in particular monocytes, at the sites of developing lesions.

As discussed above, αMβ2, an integrin expressed on neutrophils and monocytes/ macrophages, has been identified as a receptor for CD40L. This interaction may also mediate adhesion and migration of inflammatory cells at sites of plaque initiation. In agreement with this hypothesis, αMβ2 deficiency attenuates lesion development and reduces lesional macrophage accumulation in LDLR-/- mice, supporting the implication of this integrin in atherothrombosis.52 However, additional studies will be required to specifically establish the

In addition, platelets have been shown to play a crucial role in the initiation of atherothrombosis. Platelets are among the first inflammatory cells at the site of injury and their adhesion to the endothelium provides a fundamental mechanism by which leukocytes are recruited.63, 64 Because the surface of activated platelets contains a higher density of Pselectin than activated endothelial cells, significantly more leukocytes will incorporate at the sites of injury in their presence.65 Interestingly, CD40L from activated platelets can also induce a proinflammatory response on endothelial cells, in a similar fashion to that of Tlymphocytes. Henn et al. demonstrated that CD40 ligation on endothelial cells by CD40L from activated platelets induces the expression of numerous adhesion molecules, cytokines,

and matrix metalloproteinases involved in the initiation of inflammatory reactions.16

The progression of the atherosclerotic plaque is typically highlighted by the proliferation and migration of smooth muscle cells into the intima, as well as the formation neovessels (angiogenesis), which supports the growth of lesions. This process will eventually yield a stable lipid-enriched necrotic plaque surrounded by a fibrous cap. Once in the subendothelial space, macrophages (originally monocytes) undergo phagocytosis of oxidized LDL particles, leading to the formation of foam cells.66 Thereafter, CD40L from infiltrated T-

**3.1 Initiation of lesions and leukocyte recruitment** 

importance of the CD40L/αMβ2 in plaque initiation.

**3.2 Plaque development and progression** 

Fig. 2. Role of CD40L in atherothrombosis. The incorporation of oxidized LDLs, among other factors, may upregulate the expression of the CD40L system on the endothelium, thereby promoting the recruitment of platelets and leukocytes at the sites of injury. The CD40L-dependent adhesion of T-lymphocytes and platelets to the endothelium induces an important inflammatory response characterized by the secretion of various cytokines and the upregulation of additional endothelial adhesion molecules. This favors in turn the incorporation and transmigration of additional leukocytes, in particular monocytes. Once in the sub-endothelial space, CD40L/CD40 interactions between foam cells (macrophages which have undergone phagocytosis of oxidized LDL particules), T-lymphocytes and smooth muscle cells take place. These cross talks ultimately lead in part to the proliferation of smooth muscle cells into the intima and promote vascular angiogenesis, primarily through the secretion of key inflammatory and angiogenic cytokines and chemokines. This process eventually yields a stable lipid-enriched atherosclerotic plaque surrounded by a fibrous cap. Plaque stability is threatened by the production of MMPs, which are directly responsible for collagen degradation and rupture of the fibrous cap. The binding of CD40 on endothelial cells, macrophages and smooth muscle cells can provoke the secretion of a long list of MMPs. Following rupture, platelets rapidly adhere to the surface of the highly prothrombotic contents of the atherosclerotic plaque, thereby leading to thrombus formation and arterial occlusion. CD40L may also be involved in thrombus formation and procoagulant activity. CD40L binding to the endothelium promotes tissue factor expression, while the binding of CD40L (soluble and membrane-bound forms) to CD40 and αIIbβ3 on platelets enhances platelets aggregation and thrombus stabilization, respectively.

Fig. 2. Role of CD40L in atherothrombosis. The incorporation of oxidized LDLs, among other factors, may upregulate the expression of the CD40L system on the endothelium, thereby promoting the recruitment of platelets and leukocytes at the sites of injury. The CD40L-dependent adhesion of T-lymphocytes and platelets to the endothelium induces an important inflammatory response characterized by the secretion of various cytokines and the upregulation of additional endothelial adhesion molecules. This favors in turn the incorporation and transmigration of additional leukocytes, in particular monocytes. Once in the sub-endothelial space, CD40L/CD40 interactions between foam cells (macrophages which have undergone phagocytosis of oxidized LDL particules), T-lymphocytes and smooth muscle cells take place. These cross talks ultimately lead in part to the proliferation of smooth muscle cells into the intima and promote vascular angiogenesis, primarily through the secretion of key inflammatory and angiogenic cytokines and chemokines. This process eventually yields a stable lipid-enriched atherosclerotic plaque surrounded by a fibrous cap. Plaque stability is threatened by the production of MMPs, which are directly responsible for collagen degradation and rupture of the fibrous cap. The binding of CD40 on endothelial cells, macrophages and smooth muscle cells can provoke the secretion of a long list of MMPs. Following rupture, platelets rapidly adhere to the surface of the highly prothrombotic contents of the atherosclerotic plaque, thereby leading to thrombus formation

and arterial occlusion. CD40L may also be involved in thrombus formation and

platelets enhances platelets aggregation and thrombus stabilization, respectively.

procoagulant activity. CD40L binding to the endothelium promotes tissue factor expression, while the binding of CD40L (soluble and membrane-bound forms) to CD40 and αIIbβ3 on

#### **3.1 Initiation of lesions and leukocyte recruitment**

Plaque initiation is normally characterized by the accumulation of low-density lipoproteins (LDL) in the arterial wall and the subsequent recruitment and transmigration of leukocytes within the sub-endothelial space.1 The initial trigger of CD40L (and CD40) expression within cells of the developing atherosclerotic plaque (endothelial cells, lymphocytes, platelets, monocytes/macrophages, and smooth muscle cells) remains elusive. Possible candidates include oxidized LDL, infectious pathogens and alterations in vascular hemodynamic forces.55- 57 For instance it has been demonstrated that lipid lowering reduces CD40L expression in atheroma.55 In addition, oxidized LDL were reported to induce the expression of CD40 on endothelial cells, which can then bind CD40L from activated T-lymphocytes adherent to the site of injury.58 CD40 activation on endothelial cells provides a critical proinflammatory signal for the initiation of lesions. Indeed, the CD40L/CD40 interaction favors the up-regulation of adhesion molecules (E-selectin, P-selectin, vascular cell adhesion molecule-1 [VCAM-1] and ICAM-1) and leads to the secretion of proinflammatory cytokines (IL-6, IL-8, IL-15, monocytes chemotactic protein-1 [MCP-1], macrophage inflammatory protein-1 [MIP-1 α/β] and regulated on activation normal T cell expressed and secreted [RANTES]) by the endothelium.6, 7, 59-62 These reactions induce in turn the incorporation and accumulation of additional leukocytes, in particular monocytes, at the sites of developing lesions.

As discussed above, αMβ2, an integrin expressed on neutrophils and monocytes/ macrophages, has been identified as a receptor for CD40L. This interaction may also mediate adhesion and migration of inflammatory cells at sites of plaque initiation. In agreement with this hypothesis, αMβ2 deficiency attenuates lesion development and reduces lesional macrophage accumulation in LDLR-/- mice, supporting the implication of this integrin in atherothrombosis.52 However, additional studies will be required to specifically establish the importance of the CD40L/αMβ2 in plaque initiation.

In addition, platelets have been shown to play a crucial role in the initiation of atherothrombosis. Platelets are among the first inflammatory cells at the site of injury and their adhesion to the endothelium provides a fundamental mechanism by which leukocytes are recruited.63, 64 Because the surface of activated platelets contains a higher density of Pselectin than activated endothelial cells, significantly more leukocytes will incorporate at the sites of injury in their presence.65 Interestingly, CD40L from activated platelets can also induce a proinflammatory response on endothelial cells, in a similar fashion to that of Tlymphocytes. Henn et al. demonstrated that CD40 ligation on endothelial cells by CD40L from activated platelets induces the expression of numerous adhesion molecules, cytokines, and matrix metalloproteinases involved in the initiation of inflammatory reactions.16

#### **3.2 Plaque development and progression**

The progression of the atherosclerotic plaque is typically highlighted by the proliferation and migration of smooth muscle cells into the intima, as well as the formation neovessels (angiogenesis), which supports the growth of lesions. This process will eventually yield a stable lipid-enriched necrotic plaque surrounded by a fibrous cap. Once in the subendothelial space, macrophages (originally monocytes) undergo phagocytosis of oxidized LDL particles, leading to the formation of foam cells.66 Thereafter, CD40L from infiltrated T-

CD40 Ligand and Its Receptors in Atherothrombosis 87

Following rupture, platelets rapidly adhere to the surface of the highly pro-thrombotic contents of the atherosclerotic plaque, thereby leading to thrombus formation and arterial occlusion.83, 84 Accumulating evidence also support a role for CD40L in platelet function and thrombus formation, albeit some of the data remain conflicting. For instance, Andre et al. have shown that CD40L plays a role in thrombus stabilization by interacting with αIIbβ3, while we and others have demonstrated that CD40L enhances platelet aggregation and thrombus formation through a CD40-mediated TRAF-2/Rac1/p38 signaling pathway.27, 28, 48 Indeed, enhanced levels of circulating sCD40L exacerbate thrombus formation *in vivo,* also in a CD40-dependent fashion.27 Nevertheless, these studies all support the concept of CD40L as a pro-thrombotic agent, predisposing platelets to enhanced cell function. CD40L may also enhance the coagulation system through the induction of tissue factor release from various vascular cells. CD40 engagement on endothelial cells, macrophages and smooth muscle cells by CD40L from activated platelets or T cells induces tissue factor expression and activity.85-87 Besides its important role in the induction of the extrinsic coagulation cascade, tissue factor

Given the pivotal contribution of the CD40L system in atherothrombosis, multiple clinical studies have evaluated the association between levels of circulating sCD40L and cardiovascular risk, in particular acute coronary syndrome (ACS) such as acute myocardial infarction (AMI) and unstable angina (UA). These studies can be divided into two main categories. The first type of clinical studies has investigated the link between levels of sCD40L and ACS, while the second has determined the link between levels of sCD40L and

For the most part, clinical studies demonstrate that circulating sCD40L levels are significantly higher in patients with ACS and stable coronary artery disease (CAD), compared with control subjects.88-92 Indeed, it appears that a gradual increase in sCD40L levels occurs with ACS progression, with peaks as early as 9 hours following the onset of AMI or UA.89, 93, 94 For instance, patients suffering from AMI or UA present with levels ranging from 5-25 ng/mL, depending on the study.88 Moreover, sCD40L levels are independent from other important inflammatory markers, such as IL-6, sICAM-1, sVCAM-1, C-reactive protein and troponin, indicating that sCD40L may represent a more reliable risk factor, as compared to others.92 Because sCD40L in circulation almost exclusively originates from the shedding of membrane-bound CD40L at the surface of activated platelets, its measuring levels may reflect a state of platelet activation rather than an

More importantly, some clinical studies have evaluated the relationship between sCD40L levels and disease prognosis. In the CAPTURE (c7E3 Fab Anti-Platelet Therapy in unstable Refractory Angina) trial, patients with high levels of sCD40L were 3-fold more at risk of developing cardiovascular death or AMI.21 Moreover, in the MIRACL (Myocardial Ischemia Reduction with Aggressive Cholesterol Lowering) study, sCD40L levels were an independent risk factor for recurrent cardiovascular events, such as death, nonfatal myocardial infarction, cardiac arrest and worsening angina requiring rehospitalization.95 Interestingly, individuals carrying the -3459A>G polymorphism on the CD40L gene, are

also represents a powerful platelet agonist.

prognosis and risk prediction.

inflammatory condition.

more at risk of developing AMI.96

**4. Soluble CD40L and coronary syndromes** 

lymphocytes will bind to CD40 on the surface of differentiated foam cells, favoring the release of further proinflammatory cytokines (IL-1, IL-6 and IL-12), growth factors (vascular endothelial growth factor [VEGF]) and MMPs (MMP-1 and MMP-3).7, 67 These responses are intimately involved in the proliferation and migration of smooth muscle cells into the intima layer. In parallel, cross talks between smooth muscle cells and T-lymphocytes may also take place, in which CD40 activation on the former initiates a positive feedback loop enhancing the inflammatory reactions already in place.6 Indeed, CD40 signaling in smooth muscle cells has been shown to induce the secretion of the cytokines IL-8 and MCP-1.67, 68 Moreover, the accumulation of migrating fibroblasts within the intima layer exacerbates the atherosclerotic lesions in development and the CD40L/CD40 axis might also takes part in this process.69 Stimulation of fibroblasts with CD40L was reported to up-regulate the expression of cell surface adhesion molecules, thus facilitating their interaction with immune cells at the site of lesions.70 This interaction also induces their proliferation and secretion of chemoattractant cytokines such as IL-6 and IL-8.71-73 Hence, the CD40L/CD40 interaction is at the forefront of a plethora of key inflammatory reactions involved in neointima formation and plaque accumulation. Lesions with eventually develop into the formation of a stable necrotic core consisting of infiltrated leukocytes, foam cells, proliferating smooth muscle cells, extracellular matrix proteins and lipids.

The formation of neovessels or angiogenesis plays an integral part in plaque progression and several reports have highlighted the importance of CD40L in this process. For instance, CD40 ligation on endothelial cells and macrophages was shown to upregulate the expression of potent angiogenic factors such as VEGF, fibroblast growth factor and plateletactivating factor, in addition to inducing the synthesis and proteinase activity of various MMPs such as MMP-1, MMP-2, MMP-3 and MMP-9.18, 74-76 These responses are tightly linked to tubule formation and angiogenesis, essential elements of plaque support and growth. Interestingly, the α5β1 integrin is upregulated on angiogenesis-prone endothelial cells and could also provide a novel mechanism by which CD40L modulates pathological angiogenesis.77 It would be worthwhile investigating this issue in further details.

#### **3.3 Plaque instability and thrombosis**

Plaque stability is regulated by a tight balance between extracellular matrix proteins such as collagen fibers and MMP production. A thin fibrous cap protects the highly thrombotic components of the atherosclerotic plaque. However, upon secretion of MMPs by macrophages and other inflammatory cells present, plaque rupture may ensue following digestion of the collagen fibers within the fibrous cap.78, 79 This process leads to thrombus formation and may cause complete obstruction of the artery. CD40L mediates several of the processes that set the stage for plaque rupture and its clinical sequelae. CD40L stimulation on endothelial cells, macrophages and smooth muscle cells can provoke the secretion of a long list of MMPs (MMP-1, MMP-2, MMP-3, MMP-8, MMP-9 and MMP-13), the main digestive enzymes of the collagen-rich fibrous cap.18, 80, 81 Platelets, in addition to their pivotal role in thrombosis, also participate in this process. Indeed, membrane-bound CD154 expressed on the surface of activated platelets can induce MMP upregulation in endothelial cells.82 MMP secretion and proteolytic activity can be abrogated by physical hindrance of platelet-endothelial contacts, αIIbβ3 interfering agents or anti-CD40L antibodies, thus highlighting in part the importance of platelets and CD40L in this phenomenon.

lymphocytes will bind to CD40 on the surface of differentiated foam cells, favoring the release of further proinflammatory cytokines (IL-1, IL-6 and IL-12), growth factors (vascular endothelial growth factor [VEGF]) and MMPs (MMP-1 and MMP-3).7, 67 These responses are intimately involved in the proliferation and migration of smooth muscle cells into the intima layer. In parallel, cross talks between smooth muscle cells and T-lymphocytes may also take place, in which CD40 activation on the former initiates a positive feedback loop enhancing the inflammatory reactions already in place.6 Indeed, CD40 signaling in smooth muscle cells has been shown to induce the secretion of the cytokines IL-8 and MCP-1.67, 68 Moreover, the accumulation of migrating fibroblasts within the intima layer exacerbates the atherosclerotic lesions in development and the CD40L/CD40 axis might also takes part in this process.69 Stimulation of fibroblasts with CD40L was reported to up-regulate the expression of cell surface adhesion molecules, thus facilitating their interaction with immune cells at the site of lesions.70 This interaction also induces their proliferation and secretion of chemoattractant cytokines such as IL-6 and IL-8.71-73 Hence, the CD40L/CD40 interaction is at the forefront of a plethora of key inflammatory reactions involved in neointima formation and plaque accumulation. Lesions with eventually develop into the formation of a stable necrotic core consisting of infiltrated leukocytes, foam cells, proliferating smooth muscle cells, extra-

The formation of neovessels or angiogenesis plays an integral part in plaque progression and several reports have highlighted the importance of CD40L in this process. For instance, CD40 ligation on endothelial cells and macrophages was shown to upregulate the expression of potent angiogenic factors such as VEGF, fibroblast growth factor and plateletactivating factor, in addition to inducing the synthesis and proteinase activity of various MMPs such as MMP-1, MMP-2, MMP-3 and MMP-9.18, 74-76 These responses are tightly linked to tubule formation and angiogenesis, essential elements of plaque support and growth. Interestingly, the α5β1 integrin is upregulated on angiogenesis-prone endothelial cells and could also provide a novel mechanism by which CD40L modulates pathological

Plaque stability is regulated by a tight balance between extracellular matrix proteins such as collagen fibers and MMP production. A thin fibrous cap protects the highly thrombotic components of the atherosclerotic plaque. However, upon secretion of MMPs by macrophages and other inflammatory cells present, plaque rupture may ensue following digestion of the collagen fibers within the fibrous cap.78, 79 This process leads to thrombus formation and may cause complete obstruction of the artery. CD40L mediates several of the processes that set the stage for plaque rupture and its clinical sequelae. CD40L stimulation on endothelial cells, macrophages and smooth muscle cells can provoke the secretion of a long list of MMPs (MMP-1, MMP-2, MMP-3, MMP-8, MMP-9 and MMP-13), the main digestive enzymes of the collagen-rich fibrous cap.18, 80, 81 Platelets, in addition to their pivotal role in thrombosis, also participate in this process. Indeed, membrane-bound CD154 expressed on the surface of activated platelets can induce MMP upregulation in endothelial cells.82 MMP secretion and proteolytic activity can be abrogated by physical hindrance of platelet-endothelial contacts, αIIbβ3 interfering agents or anti-CD40L antibodies, thus

angiogenesis.77 It would be worthwhile investigating this issue in further details.

highlighting in part the importance of platelets and CD40L in this phenomenon.

cellular matrix proteins and lipids.

**3.3 Plaque instability and thrombosis** 

Following rupture, platelets rapidly adhere to the surface of the highly pro-thrombotic contents of the atherosclerotic plaque, thereby leading to thrombus formation and arterial occlusion.83, 84 Accumulating evidence also support a role for CD40L in platelet function and thrombus formation, albeit some of the data remain conflicting. For instance, Andre et al. have shown that CD40L plays a role in thrombus stabilization by interacting with αIIbβ3, while we and others have demonstrated that CD40L enhances platelet aggregation and thrombus formation through a CD40-mediated TRAF-2/Rac1/p38 signaling pathway.27, 28, 48 Indeed, enhanced levels of circulating sCD40L exacerbate thrombus formation *in vivo,* also in a CD40-dependent fashion.27 Nevertheless, these studies all support the concept of CD40L as a pro-thrombotic agent, predisposing platelets to enhanced cell function. CD40L may also enhance the coagulation system through the induction of tissue factor release from various vascular cells. CD40 engagement on endothelial cells, macrophages and smooth muscle cells by CD40L from activated platelets or T cells induces tissue factor expression and activity.85-87 Besides its important role in the induction of the extrinsic coagulation cascade, tissue factor also represents a powerful platelet agonist.

#### **4. Soluble CD40L and coronary syndromes**

Given the pivotal contribution of the CD40L system in atherothrombosis, multiple clinical studies have evaluated the association between levels of circulating sCD40L and cardiovascular risk, in particular acute coronary syndrome (ACS) such as acute myocardial infarction (AMI) and unstable angina (UA). These studies can be divided into two main categories. The first type of clinical studies has investigated the link between levels of sCD40L and ACS, while the second has determined the link between levels of sCD40L and prognosis and risk prediction.

For the most part, clinical studies demonstrate that circulating sCD40L levels are significantly higher in patients with ACS and stable coronary artery disease (CAD), compared with control subjects.88-92 Indeed, it appears that a gradual increase in sCD40L levels occurs with ACS progression, with peaks as early as 9 hours following the onset of AMI or UA.89, 93, 94 For instance, patients suffering from AMI or UA present with levels ranging from 5-25 ng/mL, depending on the study.88 Moreover, sCD40L levels are independent from other important inflammatory markers, such as IL-6, sICAM-1, sVCAM-1, C-reactive protein and troponin, indicating that sCD40L may represent a more reliable risk factor, as compared to others.92 Because sCD40L in circulation almost exclusively originates from the shedding of membrane-bound CD40L at the surface of activated platelets, its measuring levels may reflect a state of platelet activation rather than an inflammatory condition.

More importantly, some clinical studies have evaluated the relationship between sCD40L levels and disease prognosis. In the CAPTURE (c7E3 Fab Anti-Platelet Therapy in unstable Refractory Angina) trial, patients with high levels of sCD40L were 3-fold more at risk of developing cardiovascular death or AMI.21 Moreover, in the MIRACL (Myocardial Ischemia Reduction with Aggressive Cholesterol Lowering) study, sCD40L levels were an independent risk factor for recurrent cardiovascular events, such as death, nonfatal myocardial infarction, cardiac arrest and worsening angina requiring rehospitalization.95 Interestingly, individuals carrying the -3459A>G polymorphism on the CD40L gene, are more at risk of developing AMI.96

CD40 Ligand and Its Receptors in Atherothrombosis 89

Research over the years overwhelmingly supports the notion of atherothrombosis as a chronic inflammatory disease. Despite the plethora of inflammatory mediators identified thus far as potential contributors to this complication, the CD40L system has attracted a great deal of interest. Besides its pivotal role in humoral immunity, CD40L is now regarded as a key player to all major phases of atherothrombosis, a concept supported in part by the strong relationship between its circulating soluble levels and the occurrence of cardiovascular diseases. In addition to its well-established CD40 counterreceptor, CD40L can also interact with novel binding partners, namely the integrin receptors αIIbβ3, αMβ2 and α5β1. Although most CD40L-mediated functions have been attributed to its interaction with CD40, these novel receptors add complexity to the diverse interplays that might take place during inflammation. The elucidation of the exact physiopathological relevance of these interactions in inflammatory disorders might pave the way for the development of novel anti-CD40L therapeutic targets for the treatment of atherothrombosis and cardiovascular

[1] Hansson GK, Libby P. The immune response in atherosclerosis: A double-edged sword.

[4] Weber C, Zernecke A, Libby P. The multifaceted contributions of leukocyte subsets to atherosclerosis: Lessons from mouse models. *Nat Rev Immunol*. 2008;8:802-815 [5] Lievens D, Eijgelaar WJ, Biessen EA, Daemen MJ, Lutgens E. The multi-functionality of cd40l and its receptor cd40 in atherosclerosis. *Thromb Haemost*. 2009;102:206-214 [6] Schonbeck U, Libby P. Cd40 signaling and plaque instability. *Circ Res*. 2001;89:1092-1103 [7] Hassan GS, Merhi Y, Mourad WM. Cd154 and its receptors in inflammatory vascular

[8] Lutgens E, Gorelik L, Daemen MJ, de Muinck ED, Grewal IS, Koteliansky VE, Flavell

[9] Schonbeck U, Sukhova GK, Shimizu K, Mach F, Libby P. Inhibition of cd40 signaling

[10] Davis JC, Jr., Totoritis MC, Rosenberg J, Sklenar TA, Wofsy D. Phase i clinical trial of a

[11] Schuler W, Bigaud M, Brinkmann V, Di Padova F, Geisse S, Gram H, Hungerford V,

[12] Armitage RJ, Fanslow WC, Strockbine L, Sato TA, Clifford KN, Macduff BM, Anderson

characterization of a murine ligand for cd40. *Nature*. 1992;357:80-82

RA. Requirement for cd154 in the progression of atherosclerosis. *Nat Med*.

limits evolution of established atherosclerosis in mice. *Proc Natl Acad Sci U S A*.

monoclonal antibody against cd40-ligand (idec-131) in patients with systemic lupus

Kleuser B, Kristofic C, Menninger K, Tees R, Wieczorek G, Wilt C, Wioland C, Zurini M. Efficacy and safety of abi793, a novel human anti-human cd154 monoclonal antibody, in cynomolgus monkey renal allotransplantation.

DM, Gimpel SD, Davis-Smith T, Maliszewski CR, et al. Molecular and biological

[2] Ross R. Atherosclerosis--an inflammatory disease. *N Engl J Med*. 1999;340:115-126

**6. Conclusions** 

diseases.

**7. References** 

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1999;5:1313-1316

2000;97:7458-7463

[3] Lusis AJ. Atherosclerosis. *Nature*. 2000;407:233-241

pathologies. *Trends Immunol*. 2009;30:165-172

erythematosus. *J Rheumatol*. 2001;28:95-101

*Transplantation*. 2004;77:717-726

Whether enhanced levels of sCD40L seen in patients with ACS are a consequence of increased platelet activation or a predetermining cause of these complications (or perhaps both) is still unknown. Recently, we have shown that enhancing levels of circulating sCD40L in mice to approximately 45 ng/mL exacerbates thrombus formation in a CD40-dependent manner.27 This observation supports the idea that increased levels of sCD40L in patients may drive, at least in part, the development of certain cardiovascular complications. It would be tempting to speculate the existence of a positive feedback loop taking place in these patients, where disease initiation correlates with platelet activation and release of sCD40L in the circulation. This in turn could further exacerbate pre-existing complications through enhancement of platelet function and thrombus formation.

#### **5. Disruption of the CD40L system as a therapeutic target in atherothrombosis**

In light of all the aforementioned data supporting the contribution of CD40L in inflammation, disruption of this system as a therapeutic strategy for the treatment of atherothrombosis and its clinical manifestations has been investigated. Unfortunately, clinical trials using an anti-CD40L antibody were put on hold due to thromboembolic complications.97, 98 Interactions between CD40 and CD40L-immune complexes at the surface of platelets have been suggested as a possible mechanism by which CD40L therapy induces these complications.97

Since circulating levels of sCD40L result from platelet activation, indirect targeting of the CD40L system through anti-platelet therapy may represent an alternative approach to suppress this important component. Clopidogrel, a potent inhibitor of the platelet adenosine diphosphate (ADP) receptor, has been reported to block sCD40L release from ADPstimulated platelets.99 Interestingly, clopidogrel regiment significantly reduces platelet CD40L expression and sCD40L levels in patients with stable CAD.100 Moreover, αIIbβ3 inhibitors, such as abciximab, inhibit platelet aggregation and sCD40L release from activated platelets.101 In the CAPTURE trial, abciximab significantly reduces sCD40L levels and cardiovascular risk in high-risk ACS patients, confirming a link between αIIbβ3 signaling and platelet sCD40L release.21

Statins exert multiple pleiotropic anti-inflammatory effects, in addition to their lipid lowering properties. Several reports have investigated the effects of these drugs on inflammatory markers, including CD40L. Particularly, they have been shown to reduce cytokine-induced CD40L expression on endothelial cells, smooth muscle cells and macrophages.102 Notably, atorvastatin treatment in the MIRACL trial reduces the risk of recurrent cardiovascular events, which are associated with sCD40L levels.95

Most of these agents indirectly target the CD40L system, perhaps through inhibition of platelet activation and the subsequent release of sCD40L. Specific disruption of CD40L or its receptors remains a promising approach for the treatment of atherothrombosis. Although clinical studies using anti-CD40L antibodies have been unsuccessful, alternative targets of this system may render better clinical outcomes. For example, novel anti-CD40L agents that specifically target the interaction of CD40L with its different receptors or inhibition of critical intracellular signaling elements, such as TRAFs, represent valuable approaches.

#### **6. Conclusions**

88 Traditional and Novel Risk Factors in Atherothrombosis

Whether enhanced levels of sCD40L seen in patients with ACS are a consequence of increased platelet activation or a predetermining cause of these complications (or perhaps both) is still unknown. Recently, we have shown that enhancing levels of circulating sCD40L in mice to approximately 45 ng/mL exacerbates thrombus formation in a CD40-dependent manner.27 This observation supports the idea that increased levels of sCD40L in patients may drive, at least in part, the development of certain cardiovascular complications. It would be tempting to speculate the existence of a positive feedback loop taking place in these patients, where disease initiation correlates with platelet activation and release of sCD40L in the circulation. This in turn could further exacerbate pre-existing complications

In light of all the aforementioned data supporting the contribution of CD40L in inflammation, disruption of this system as a therapeutic strategy for the treatment of atherothrombosis and its clinical manifestations has been investigated. Unfortunately, clinical trials using an anti-CD40L antibody were put on hold due to thromboembolic complications.97, 98 Interactions between CD40 and CD40L-immune complexes at the surface of platelets have been suggested as a possible mechanism by which CD40L therapy induces

Since circulating levels of sCD40L result from platelet activation, indirect targeting of the CD40L system through anti-platelet therapy may represent an alternative approach to suppress this important component. Clopidogrel, a potent inhibitor of the platelet adenosine diphosphate (ADP) receptor, has been reported to block sCD40L release from ADPstimulated platelets.99 Interestingly, clopidogrel regiment significantly reduces platelet CD40L expression and sCD40L levels in patients with stable CAD.100 Moreover, αIIbβ3 inhibitors, such as abciximab, inhibit platelet aggregation and sCD40L release from activated platelets.101 In the CAPTURE trial, abciximab significantly reduces sCD40L levels and cardiovascular risk in high-risk ACS patients, confirming a link between αIIbβ3

Statins exert multiple pleiotropic anti-inflammatory effects, in addition to their lipid lowering properties. Several reports have investigated the effects of these drugs on inflammatory markers, including CD40L. Particularly, they have been shown to reduce cytokine-induced CD40L expression on endothelial cells, smooth muscle cells and macrophages.102 Notably, atorvastatin treatment in the MIRACL trial reduces the risk of

Most of these agents indirectly target the CD40L system, perhaps through inhibition of platelet activation and the subsequent release of sCD40L. Specific disruption of CD40L or its receptors remains a promising approach for the treatment of atherothrombosis. Although clinical studies using anti-CD40L antibodies have been unsuccessful, alternative targets of this system may render better clinical outcomes. For example, novel anti-CD40L agents that specifically target the interaction of CD40L with its different receptors or inhibition of critical intracellular signaling elements, such as TRAFs, represent valuable approaches.

recurrent cardiovascular events, which are associated with sCD40L levels.95

through enhancement of platelet function and thrombus formation.

**atherothrombosis** 

these complications.97

signaling and platelet sCD40L release.21

**5. Disruption of the CD40L system as a therapeutic target in** 

Research over the years overwhelmingly supports the notion of atherothrombosis as a chronic inflammatory disease. Despite the plethora of inflammatory mediators identified thus far as potential contributors to this complication, the CD40L system has attracted a great deal of interest. Besides its pivotal role in humoral immunity, CD40L is now regarded as a key player to all major phases of atherothrombosis, a concept supported in part by the strong relationship between its circulating soluble levels and the occurrence of cardiovascular diseases. In addition to its well-established CD40 counterreceptor, CD40L can also interact with novel binding partners, namely the integrin receptors αIIbβ3, αMβ2 and α5β1. Although most CD40L-mediated functions have been attributed to its interaction with CD40, these novel receptors add complexity to the diverse interplays that might take place during inflammation. The elucidation of the exact physiopathological relevance of these interactions in inflammatory disorders might pave the way for the development of novel anti-CD40L therapeutic targets for the treatment of atherothrombosis and cardiovascular diseases.

#### **7. References**


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**6** 

*India* 

**In Search for Novel Biomarkers of Acute** 

*Sir H. N. Medical Research Society, Sir H. N. Hospital and Research Centre,* 

Coronary artery disease (CAD) and its one of the severe clinical manifestation, Acute Myocardial Infarction (AMI) continue to be significant cause of morbidity and mortality in both men and women around the world. The extent of myocardial damage after an acute coronary event of atherothrombosis determines the prognosis. The diagnosis of acute coronary syndrome (ACS) encompassing unstable angina, non ST elevated myocardial infarction (NSTEMI) (Bertrand et al., 2002; Braunwald et al., 2000; Hamm et al., 2001) to STEMI (AMI) (Alpert et al., 2000), is based on a combination of symptoms, electrocardiographic changes and

The physical examination can be inadequate in identifying atypical chest pain from chest pain of cardiac origin. On one hand 33% of patients with ACS have no chest pain. On the other hand approximately half of patients with acute chest pain, who have the initial diagnostic findings of ACS and are admitted to the hospital, are later found not to suffer from ACS. In the majority of patients with chest pain, the electrocardiogram (ECG) is the most readily available tool for identifying patients with ACS. However, the ECG is also often not diagnostic for acute chest pain and in fact; the sensitivity of borderline ECG for

Over the last 50 years, the contribution of laboratory Medicine to the management of cardiac diseases has become increasingly sophisticated. In 1950s, Karmen et al first reported that enzyme released from necrotic cardiac myocytes could be detected in the serum and could be used in the diagnosis of MI. The ensuing years witnessed progressive improvement in the type of cardiac tissue specific biochemical markers and a corresponding enhancement in

Today, markers of myocardial necrosis at the down stream of the pathophysiology of ACS; some specific to myocardial necrosis have gained their mark under routine diagnosis of ACS (Table 1).

The main advantage of myoglobin is early detection of patients with AMI (Gibler et al., 1987, Roxin et al., 1984). The NACB Laboratory Medicine Practice Guidelines have recommended

detecting ACS is only 60% (Canto et al., 2000, Panteghini, 2002).

the clinical sensitivity and specificity of their use.

**1.1 Current practice of diagnostic biomarkers in ACS** 

**1. Introduction** 

biomarkers.

**1.1.1 Myoglobin** 

**Coronary Syndrome** 

Kavita K. Shalia and Vinod K. Shah


## **In Search for Novel Biomarkers of Acute Coronary Syndrome**

Kavita K. Shalia and Vinod K. Shah *Sir H. N. Medical Research Society, Sir H. N. Hospital and Research Centre, India* 

#### **1. Introduction**

96 Traditional and Novel Risk Factors in Atherothrombosis

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Ganz P, Kinlay S, Libby P. Oxidized low-density lipoprotein augments and 3 hydroxy-3-methylglutaryl coenzyme a reductase inhibitors limit cd40 and cd40l

> Coronary artery disease (CAD) and its one of the severe clinical manifestation, Acute Myocardial Infarction (AMI) continue to be significant cause of morbidity and mortality in both men and women around the world. The extent of myocardial damage after an acute coronary event of atherothrombosis determines the prognosis. The diagnosis of acute coronary syndrome (ACS) encompassing unstable angina, non ST elevated myocardial infarction (NSTEMI) (Bertrand et al., 2002; Braunwald et al., 2000; Hamm et al., 2001) to STEMI (AMI) (Alpert et al., 2000), is based on a combination of symptoms, electrocardiographic changes and biomarkers.

> The physical examination can be inadequate in identifying atypical chest pain from chest pain of cardiac origin. On one hand 33% of patients with ACS have no chest pain. On the other hand approximately half of patients with acute chest pain, who have the initial diagnostic findings of ACS and are admitted to the hospital, are later found not to suffer from ACS. In the majority of patients with chest pain, the electrocardiogram (ECG) is the most readily available tool for identifying patients with ACS. However, the ECG is also often not diagnostic for acute chest pain and in fact; the sensitivity of borderline ECG for detecting ACS is only 60% (Canto et al., 2000, Panteghini, 2002).

> Over the last 50 years, the contribution of laboratory Medicine to the management of cardiac diseases has become increasingly sophisticated. In 1950s, Karmen et al first reported that enzyme released from necrotic cardiac myocytes could be detected in the serum and could be used in the diagnosis of MI. The ensuing years witnessed progressive improvement in the type of cardiac tissue specific biochemical markers and a corresponding enhancement in the clinical sensitivity and specificity of their use.

#### **1.1 Current practice of diagnostic biomarkers in ACS**

Today, markers of myocardial necrosis at the down stream of the pathophysiology of ACS; some specific to myocardial necrosis have gained their mark under routine diagnosis of ACS (Table 1).

#### **1.1.1 Myoglobin**

The main advantage of myoglobin is early detection of patients with AMI (Gibler et al., 1987, Roxin et al., 1984). The NACB Laboratory Medicine Practice Guidelines have recommended

In Search for Novel Biomarkers of Acute Coronary Syndrome 99

B –Natriuretic peptide (BNP) and its prohormone N-terminal pro BNP (Nt-proBNP) are neurohormones secreted from cardiac ventricles in response to ventricular wall stress (de Bold, 1985, Nakako et al., 1992). BNP, an established biomarker for patients with heart failure, and NT-pro BNP are elevated in ACS and can identify patients at very high risk for adverse cardiovascular events including death (de Lemos &Marrow, 2002, Ishibashi, 2002). The utility of BNP and NT-pro BNP as markers is based on the finding that it increases in the left ventricle during remodeling after a transmural infarction or as a consequence of previous ischemic damage (Lorgis et al., 2007). However these peptides have poor specificity for the diagnosis of ACS since elevated levels can also be seen in patients with

renal failure, primary aldosteronism, congestive heart failure and thyroid disease.

Despite the success of these biomarkers, there is still a need for the development of early markers that can reliably rule out ACS in the emergency room at presentation and also detect myocardial ischemia in the absence of reversible myocyte injury. Misdiagnosis has been reported to be the main cause of treatment delays. Undetected infarctions remain a serious public health issue and represent the leading cause of malpractice cases in the emergency settings. These imperfect strategies resulting in costly and inappropriate management decisions have forced us to search new non-invasive quick strategies in identifying the high-risk individuals. One of them is identifying novel cardiac

Biomarkers have multiple uses in the arenas of research and practice. It is defined as a characteristic that is objectively measured and evaluated as an indicator of normal biological processes, pathogenic processes, or pharmacologic responses to a therapeutic interventions. In clinical practice, a biomarker may be used to diagnose a medical problem, serve as a tool

Recent investigations have been directed towards analyzing components, involved in the pathogenesis of ACS, at upstream from biomarkers of necrosis, such as components released during Ischemia, components of plaque destabilization and rupture, factors of thrombosis, components representing oxidative stress, molecules of inflammation and acute phase reactants for earlier assessment of overall patient risk of adverse event and indexing them

The explicit goal is to maintain micro-circulatory flow to prevent even minor infarctions. Only marker that precedes necrosis and permits prevention of the consequence can meet the clinical need. Identifying markers of ischemia even if necrosis is not present may help in identifying a high risk individual who may in very near future experience the consequences of the infarct. The components that have been studied in this group are Free Fatty Acids unbound to Albumin (FFAu), Choline, Ischemia-Modified albumin (IMA) and Heart type

for staging disease, or provide an indicator of differential prognosis.

**1.2 In search for novel cardiac biomarkers of ACS** 

**1.2.1 Components released during ischemia** 

Fatty Acid Binding Protein (H-FABP).

under "Biomarkers" (Table 2).

**1.1.4 Natriuretic peptides** 

biomarkers.

myoglobin in addition to cardiac troponins (cTn) for the diagnosis of AMI patients who present within the 6 hours of onset of symptoms (Apple et al., 2007). The serum myoglobin level rises faster than Creatinine Kinase-MB (CK-MB) and cTn, reaching twice the normal values within 2 hours and peaking within 4 hours of symptom onset. The disadvantage of using myoglobin alone is that it has poor specificity for AMI in patients with concurrent trauma or renal failure.


Table 1. Current Biomarkers

#### **1.1.2 Creatinine Kinase (CK)**

CK-MB, the specific cardiac isoform of CK can be used in the diagnosis of myocardial necrosis (Mair et al., 1991). This was proposed by World Health Organization and was later extended for monitoring trends in cardiac disease (Apple et al., 2007, 2003). Elevation of CK-MB occurs 4 to 6 hours after the onset of acute MI and remains for 24 to 48 hours. CK-MB is relatively sensitive but less specific as it can be elevated in any conditions following acute muscle injury or in patients undergoing any surgical procedure. Furthermore CK is present in skeletal muscle, intestine, diaphragm, uterus and prostate and thus the specificity of CK-MB is impaired in the setting of injury to these organs. Moreover, serial analysis of CK-MB are required for quantitation as well as qualitative assessment of injury to cardiac muscle, therefore, many studies have suggested that a single cTn can be used as a convenient, cost effective and non invasive method for the diagnosis of myocardial necrosis (Apple et al., 1999, De Winter et al., 1995).

#### **1.1.3 Cardiac troponin (cTn)s**

Undisputedly troponin (cTnI and cTnT) are the most sensitive and specific biomarker of myocardial injury (Bleier et al., 1998). The kinetics of both the troponins are detectable in the serum within 4 to 12 hours after the onset of acute MI and depending upon the duration of ischemia and reperfusion status, peak values occur 12 to 48 hours from symptom onset (Apple et al., 1999). The tissue specificity and reliable detected concentration of cTn in the peripheral circulation makes it a good indicative of myocardial injury (Bleier et al., 1998). Moreover, several studies have shown that patients presenting with elevated cTns had a poor prognosis compared to those without detectable cTns (Panteghini, 2002). Because both forms of cTn remain in the circulation several days after injury, it allows for diagnosis even in patients who present very late (Apple et al., 2003). However because of long half lives, one of the disadvantages using the cTn is that neither cTnI nor cTnT can be used for detection of reinfarction after an index event. The other disadvantage of cTnT is that it is present in small amounts in skeletal muscle and is re -expressed in diseases that involve skeletal muscle degeneration. Therefore, an elevated cTn without clinical evidence of ACS should prompt for other possible myocardial injuries, including cardiac trauma, cardiac failure, and hypertension (Panteghini, 2002).

#### **1.1.4 Natriuretic peptides**

98 Traditional and Novel Risk Factors in Atherothrombosis

myoglobin in addition to cardiac troponins (cTn) for the diagnosis of AMI patients who present within the 6 hours of onset of symptoms (Apple et al., 2007). The serum myoglobin level rises faster than Creatinine Kinase-MB (CK-MB) and cTn, reaching twice the normal values within 2 hours and peaking within 4 hours of symptom onset. The disadvantage of using myoglobin alone is that it has poor specificity for AMI in patients with concurrent

> **Current Biomarkers**  Myoglobin Creatine Kinase-MB Troponins Natriuretic Peptides

CK-MB, the specific cardiac isoform of CK can be used in the diagnosis of myocardial necrosis (Mair et al., 1991). This was proposed by World Health Organization and was later extended for monitoring trends in cardiac disease (Apple et al., 2007, 2003). Elevation of CK-MB occurs 4 to 6 hours after the onset of acute MI and remains for 24 to 48 hours. CK-MB is relatively sensitive but less specific as it can be elevated in any conditions following acute muscle injury or in patients undergoing any surgical procedure. Furthermore CK is present in skeletal muscle, intestine, diaphragm, uterus and prostate and thus the specificity of CK-MB is impaired in the setting of injury to these organs. Moreover, serial analysis of CK-MB are required for quantitation as well as qualitative assessment of injury to cardiac muscle, therefore, many studies have suggested that a single cTn can be used as a convenient, cost effective and non invasive method for the diagnosis of myocardial necrosis (Apple et al.,

Undisputedly troponin (cTnI and cTnT) are the most sensitive and specific biomarker of myocardial injury (Bleier et al., 1998). The kinetics of both the troponins are detectable in the serum within 4 to 12 hours after the onset of acute MI and depending upon the duration of ischemia and reperfusion status, peak values occur 12 to 48 hours from symptom onset (Apple et al., 1999). The tissue specificity and reliable detected concentration of cTn in the peripheral circulation makes it a good indicative of myocardial injury (Bleier et al., 1998). Moreover, several studies have shown that patients presenting with elevated cTns had a poor prognosis compared to those without detectable cTns (Panteghini, 2002). Because both forms of cTn remain in the circulation several days after injury, it allows for diagnosis even in patients who present very late (Apple et al., 2003). However because of long half lives, one of the disadvantages using the cTn is that neither cTnI nor cTnT can be used for detection of reinfarction after an index event. The other disadvantage of cTnT is that it is present in small amounts in skeletal muscle and is re -expressed in diseases that involve skeletal muscle degeneration. Therefore, an elevated cTn without clinical evidence of ACS should prompt for other possible myocardial injuries, including cardiac trauma, cardiac

trauma or renal failure.

Table 1. Current Biomarkers

**1.1.2 Creatinine Kinase (CK)** 

1999, De Winter et al., 1995).

**1.1.3 Cardiac troponin (cTn)s** 

failure, and hypertension (Panteghini, 2002).

B –Natriuretic peptide (BNP) and its prohormone N-terminal pro BNP (Nt-proBNP) are neurohormones secreted from cardiac ventricles in response to ventricular wall stress (de Bold, 1985, Nakako et al., 1992). BNP, an established biomarker for patients with heart failure, and NT-pro BNP are elevated in ACS and can identify patients at very high risk for adverse cardiovascular events including death (de Lemos &Marrow, 2002, Ishibashi, 2002). The utility of BNP and NT-pro BNP as markers is based on the finding that it increases in the left ventricle during remodeling after a transmural infarction or as a consequence of previous ischemic damage (Lorgis et al., 2007). However these peptides have poor specificity for the diagnosis of ACS since elevated levels can also be seen in patients with renal failure, primary aldosteronism, congestive heart failure and thyroid disease.

Despite the success of these biomarkers, there is still a need for the development of early markers that can reliably rule out ACS in the emergency room at presentation and also detect myocardial ischemia in the absence of reversible myocyte injury. Misdiagnosis has been reported to be the main cause of treatment delays. Undetected infarctions remain a serious public health issue and represent the leading cause of malpractice cases in the emergency settings. These imperfect strategies resulting in costly and inappropriate management decisions have forced us to search new non-invasive quick strategies in identifying the high-risk individuals. One of them is identifying novel cardiac biomarkers.

Biomarkers have multiple uses in the arenas of research and practice. It is defined as a characteristic that is objectively measured and evaluated as an indicator of normal biological processes, pathogenic processes, or pharmacologic responses to a therapeutic interventions. In clinical practice, a biomarker may be used to diagnose a medical problem, serve as a tool for staging disease, or provide an indicator of differential prognosis.

#### **1.2 In search for novel cardiac biomarkers of ACS**

Recent investigations have been directed towards analyzing components, involved in the pathogenesis of ACS, at upstream from biomarkers of necrosis, such as components released during Ischemia, components of plaque destabilization and rupture, factors of thrombosis, components representing oxidative stress, molecules of inflammation and acute phase reactants for earlier assessment of overall patient risk of adverse event and indexing them under "Biomarkers" (Table 2).

#### **1.2.1 Components released during ischemia**

The explicit goal is to maintain micro-circulatory flow to prevent even minor infarctions. Only marker that precedes necrosis and permits prevention of the consequence can meet the clinical need. Identifying markers of ischemia even if necrosis is not present may help in identifying a high risk individual who may in very near future experience the consequences of the infarct. The components that have been studied in this group are Free Fatty Acids unbound to Albumin (FFAu), Choline, Ischemia-Modified albumin (IMA) and Heart type Fatty Acid Binding Protein (H-FABP).

In Search for Novel Biomarkers of Acute Coronary Syndrome 101

presence during ischemia of any other organ and in individual with inherent reduced cobalt binding giving false positive results, lost its specificity for routine use in detection of ischemia (Collinson & Gaze, 2008). However correlating with the clinical conditions and

Choline is a biomarker that is released when phospholipids are cleaved, which suggests that perhaps it could be a marker of ischemia and/or necrosis (Danne & Möckel, 2010). Experimental studies have demonstrated that phospholipase D enzyme activation and release of choline in blood are related to major processes of myocardial ischemia. Several studies suggested that the marker might improve prognostication in patients with ACS (Danne et al., 2003). In a study with troponin negative patients, choline detected high-risk unstable angina with a sensitivity and specificity of 86%. Additional studies are however

needed to fully investigate the clinical significance of this marker (Apple et al., 2005).

H-FABP is a low molecular weight protein involved in myocardial fatty-acid metabolism (Glatz et al., 1997). This protein is rapidly released immediately after infarction. H-FABP has been shown in mouse studies to be an early marker of ischaemia (Glatz et al., 1988) (before morphological evidence of myocardial necrosis) and can therefore help with diagnosis of MI earlier (Glatz et al., 1988, C. P. Chen et al., 2004, L. Chen et al., 2004). However, studies attempting to use H-FABP alone for early diagnosis of AMI have produced disappointing results. One review of six studies found that the pooled positive predictive value to be 65.8% and pooled negative predictive value to be 82.0% (Body, 2009). Also it still lacks cardiac specificity as it is found in brain, kidney and skeletal tissue and levels can go up in acute ischaemic strokes and intense exercise. Thus its role as biomarker needs further evaluation. In a recent study, Bhardwaj et al (2011) evaluated an array of established and emerging cardiac biomarkers for ACS among patients with chest discomfort in the emergency department. In their study neither IMA nor H-FABP detected or excluded ACS. Among patients with symptoms suggestive of ACS, results for NT-proBNP, hsTnI or FFA added diagnostic information to cTnT. In the context of hsTnI results, FFA measurement

Plaque disruption and thrombus formation in coronary arteries lead to a variable degree of luminal obstruction to the blood flow and can present clinically as unstable angina or AMI and lead to a sudden death. Three major determinants of thrombotic response are (a) the presence of local thrombogenic substances, (b) the local flow disturbances and, (c) the systemic thrombotic propensity. Thus apart from the local thrombogenic potential even systemic pro-coagulant status may determine the severity of the acute event of thrombosis.

The CD40 and CD40 ligand (CD40L) system is expressed on a variety of cell types including activated platelets, vascular endothelial cells, vascular smooth muscle cells, monocytes, and macrophages. CD40L is a trimeric, transmembrane protein (Hennet al., 2001). Following expression on the cell surface, CD40L is partly cleaved by proteases and subsequently

other markers may find its use in identifying high risk individuals.

**1.2.1.4 Heart type Fatty Acid Binding Protein (H-FABP)** 

significantly reclassified both false negatives and false positives.

**1.2.2 Thrombotic factors** 

**1.2.2.1 sCD40 ligand (sCD40L)** 

**1.2.1.3 Choline** 


Table 2. Emerging Biomarkers

#### **1.2.1.1 Free Fatty Acids unbound to albumin (FFAu)**

Increased blood catecholamines in association with ischemia can cause increased FFA by activating lipid hydrolysis within the heart and adipose tissue. Apart from this, reduction of FFA use after ischemia can also cause increased serum concentration of FFA. The observed increase in free fatty acids unbound to albumin (FFAu) in the blood with acute myocardial ischaemia has been evaluated for the early identification of cardiac injury (Kleinfeld et al., 1996). Two groups of investigators have preliminarily studied the sensitivity of this marker at patient presentation to the emergency room and have shown that FFAu was elevated well before other, more traditional, markers of cardiac necrosis and had at admission sensitivity of >90% (Kleifeld et al., 2002, Adams et al., 2002).

#### **1.2.1.2 Ischemia Modified Albumin (IMA)**

Due to ischemia the metal binding site on the amino terminus of albumin is damaged. The albumin of patient with myocardial ischemia exhibit lower metal binding capacity for cobalt than that of normal patients (Bar-Or et al., 2001a). IMA gained its importance as it demonstrated a good "negative predictive value." In the assay, Cobalt less bound to albumin reacts with the indicator (Bar-Or et al., 2000). Significant changes in albumin cobalt binding were documented to occur minutes after transient ischemia induced by balloon angioplasty and to return to baseline within 12 hours (Bar-Or et al., 2001b). However its presence during ischemia of any other organ and in individual with inherent reduced cobalt binding giving false positive results, lost its specificity for routine use in detection of ischemia (Collinson & Gaze, 2008). However correlating with the clinical conditions and other markers may find its use in identifying high risk individuals.

#### **1.2.1.3 Choline**

100 Traditional and Novel Risk Factors in Atherothrombosis

**Components Released during Ischemia**  Free Fatty Acids unbound to Albumin (FFAu) Ischemia Modified Albumin (IMA) Choline Heart type Fatty acid binding protein (H-FABP) **Thrombotic Factors** Soluble CD40 Ligand (sCD40L) P-selectin Tissue Factor (TF) Plasminogen Activator Inhibitor-1 (PAI-1) **Components involved in Plaque Rupture** Myeloperoxidase (MPO) Metalloproteinases (MMPs) Cathepsins Pregnancy-associated plasma protein-A (PAPP-A) **Components representing oxidative stress**  Oxidized LDL (Ox-LDL) Lectin-like oxidized low density lipoprotein receptor-1 (LOX-1) Lipoprotein-associated phospholipase A-2 (Lp-PLA-2) **Molecules of Inflammation** Vascular Cell Adhesion Molecule (VCAM-1), Platelet Endothelial Cell Adhesion Molecule (PECAM-1) Cystatin C High-sensitivity C - reactive protein (HsCRP)

Increased blood catecholamines in association with ischemia can cause increased FFA by activating lipid hydrolysis within the heart and adipose tissue. Apart from this, reduction of FFA use after ischemia can also cause increased serum concentration of FFA. The observed increase in free fatty acids unbound to albumin (FFAu) in the blood with acute myocardial ischaemia has been evaluated for the early identification of cardiac injury (Kleinfeld et al., 1996). Two groups of investigators have preliminarily studied the sensitivity of this marker at patient presentation to the emergency room and have shown that FFAu was elevated well before other, more traditional, markers of cardiac necrosis and had at admission sensitivity

Due to ischemia the metal binding site on the amino terminus of albumin is damaged. The albumin of patient with myocardial ischemia exhibit lower metal binding capacity for cobalt than that of normal patients (Bar-Or et al., 2001a). IMA gained its importance as it demonstrated a good "negative predictive value." In the assay, Cobalt less bound to albumin reacts with the indicator (Bar-Or et al., 2000). Significant changes in albumin cobalt binding were documented to occur minutes after transient ischemia induced by balloon angioplasty and to return to baseline within 12 hours (Bar-Or et al., 2001b). However its

Table 2. Emerging Biomarkers

**1.2.1.1 Free Fatty Acids unbound to albumin (FFAu)** 

of >90% (Kleifeld et al., 2002, Adams et al., 2002).

**1.2.1.2 Ischemia Modified Albumin (IMA)** 

Choline is a biomarker that is released when phospholipids are cleaved, which suggests that perhaps it could be a marker of ischemia and/or necrosis (Danne & Möckel, 2010). Experimental studies have demonstrated that phospholipase D enzyme activation and release of choline in blood are related to major processes of myocardial ischemia. Several studies suggested that the marker might improve prognostication in patients with ACS (Danne et al., 2003). In a study with troponin negative patients, choline detected high-risk unstable angina with a sensitivity and specificity of 86%. Additional studies are however needed to fully investigate the clinical significance of this marker (Apple et al., 2005).

#### **1.2.1.4 Heart type Fatty Acid Binding Protein (H-FABP)**

H-FABP is a low molecular weight protein involved in myocardial fatty-acid metabolism (Glatz et al., 1997). This protein is rapidly released immediately after infarction. H-FABP has been shown in mouse studies to be an early marker of ischaemia (Glatz et al., 1988) (before morphological evidence of myocardial necrosis) and can therefore help with diagnosis of MI earlier (Glatz et al., 1988, C. P. Chen et al., 2004, L. Chen et al., 2004). However, studies attempting to use H-FABP alone for early diagnosis of AMI have produced disappointing results. One review of six studies found that the pooled positive predictive value to be 65.8% and pooled negative predictive value to be 82.0% (Body, 2009). Also it still lacks cardiac specificity as it is found in brain, kidney and skeletal tissue and levels can go up in acute ischaemic strokes and intense exercise. Thus its role as biomarker needs further evaluation.

In a recent study, Bhardwaj et al (2011) evaluated an array of established and emerging cardiac biomarkers for ACS among patients with chest discomfort in the emergency department. In their study neither IMA nor H-FABP detected or excluded ACS. Among patients with symptoms suggestive of ACS, results for NT-proBNP, hsTnI or FFA added diagnostic information to cTnT. In the context of hsTnI results, FFA measurement significantly reclassified both false negatives and false positives.

#### **1.2.2 Thrombotic factors**

Plaque disruption and thrombus formation in coronary arteries lead to a variable degree of luminal obstruction to the blood flow and can present clinically as unstable angina or AMI and lead to a sudden death. Three major determinants of thrombotic response are (a) the presence of local thrombogenic substances, (b) the local flow disturbances and, (c) the systemic thrombotic propensity. Thus apart from the local thrombogenic potential even systemic pro-coagulant status may determine the severity of the acute event of thrombosis.

#### **1.2.2.1 sCD40 ligand (sCD40L)**

The CD40 and CD40 ligand (CD40L) system is expressed on a variety of cell types including activated platelets, vascular endothelial cells, vascular smooth muscle cells, monocytes, and macrophages. CD40L is a trimeric, transmembrane protein (Hennet al., 2001). Following expression on the cell surface, CD40L is partly cleaved by proteases and subsequently

In Search for Novel Biomarkers of Acute Coronary Syndrome 103

increased PAI levels before first AMI attack, was given by Thogersen et al (1998). In our study, increased levels of PAI-1 were observed in AMI patients at presentation and were also more associated with younger AMI patients (Shalia et al., 2010). Hamstein et al (1985) have also reported elevated circulating concentrations of PAI-1 in young men at increased

A growing understanding of the importance of atherosclerotic plaque rupture in the pathogenesis of coronary events has led to the identification of an expanding array of markers for plaque rupture. The enzymes that have gained importance in this aspect are

Leucocytes play a central role in atherosclerotic plaque rupture (de Servi et al., 1996). Myeloperoxidase is a degranulation product, secreted by a variety of inflammatory cells, including activated neutrophils, monocytes and macrophages such as those found in atherosclerotic plaques. It possesses proinflammatory properties and may contribute directly to tissue injury (Eiserich et al., 2002). Its systemic levels predicted future cardiovascular event independent of CD40L (Baldus et al., 2003) and gave in-vitro strong support to the role of neutrophil activation as an adjunct pathophysiological event in ACS that is directly different from platelet activation. Collectively the current evidence supports the need for further studies into the actual role of MPO. One of the important roles of Myeloperoxidase in leucocytes is to activate metalloproteinases that bring about plaque

The structural integrity of myocardial Extracellular Matrix (ECM) is dependent on endogenous zinc-dependent endopeptidases known as matrix metalloproteinases (MMP). These enzymes are regulated by tissue inhibitors of metalloproteinases (TIMPs) (Kelly et al., 2008). MMPs may degrade myocardial ECM leading to the development of LV dilatation and heart failure and their inhibition in experimental models of AMI has been associated with reduced LV dilatation and wall stress. Elevated levels of MMP-9 and its major inhibitor TIMP1 have been demonstrated to be associated with cardiovascular death, heart failure or both but not with re-infarction (Kelly et al., 2007). In our study we found that there was significant increase in circulating levels of MMP-9 as well as MMP-8 in AMI at presentation. Moreover the increase in MMP-8 was independent of High sensitive C-reactive protein (HsCRP) and MMP-9 (Shah et al., 2009). MMP-2 is also shown to be elevated post MI (Dhillon et al., 2009) and is associated with poor prognosis (Kelly et al., 2008a). In another study we observed that Serum MMP-3 levels were significantly elevated at presentation of the acute MI as compared to controls (Shalia et al., 2010b) while Kelly et al (2008b) have demonstrated that MMP-3 peaks at 72 hours of MI and plateau levels are associated with increase in LV volume and a lower ejection fraction at follow up. Amongst various MMPs, it has been suggested that MMP-9 may

be of value in evaluating patients after acute coronary events (Apple et al, 2005).

Evidence has been obtained for expression in human atherosclerotic lesion of another matrix degrading enzymes cathepsin S, B, K, D and L (Jormsjö et al., 2002). Beside protease function

risk for recurrent infarction.

**1.2.3.1 Myeloperoxidase (MPO)** 

rupture (Zhang et al., 2001).

**1.2.3.3 Cathepsins** 

**1.2.3.2 Metalloproteinases (MMPs)** 

**1.2.3 Components involved in plaque rupture** 

myeloperoxidase, matrix metalloproteinases, cathepsins, etc.

released into the circulation as sCD40L which can be detected in serum and plasma. The main source of circulating sCD40L is platelets (Hennet al., 2001). The binding of CD40L enhances the inflammatory response, acts prothrombotically, leads to plaque destabilization, and inhibits endothelial regeneration. From several clinical studies it has consistently been reported that sCD40L is elevated in patients with ACS and that it provides prognostic information with therapeutic implications independent of established cardiac markers, e.g. cardiac troponins (Heesechen et al., 2003). However, pre-analytical conditions are decisive for the assessment of sCD40L and may preclude routine clinical use (Weber et al., 2006).

#### **1.2.2.2 P-selectin**

P-selectin is a cell surface glycoprotein that plays a critical role in the migration of lymphocytes into tissues. It is found constitutively in a preformed state in the Weibel-Palade bodies of endothelial cells and in α-granules of platelets. This stored P-selectin is mobilized to the cell surface within minutes in response to a variety of inflammatory and thrombogenic agents. The mobilized P-selectin is apparently present on the cell surface for only a few minutes after which it is recycled to intracellular space. P-selectin also binds monocytes and neutrophils in addition to activated platelets and is responsible for incorporation of leukocytes into the growing thrombus (Malý et al., 2003). Thus, P-selectin is a marker of platelet activation which in turn is prerequisite for thrombosis (Serebruany et al., 1999a). Fijnheer et al (1997) have concluded that endothelial cell activation is associated with an increased P-selectin concentration per platelet. Elevated levels have been reported not only in AMI and unstable angina but also in stable angina. In our study significant negative correlation of sP-selectin with age in AMI group suggests increased pro-coagulant status in younger AMI patients (Mashru et al., 2010). Its role as biomarker requires extensive clinical evaluations.

#### **1.2.2.3 Tissue Factor (TF)**

TF at the upfront of the coagulation pathway plays a crucial role in initiating thrombus formation after plaque rupture in patients with ACS. Tissue injury disrupts vascular endothelium causing its release into circulating blood and hence activation of coagulation cascades. It activates extrinsic pathway of coagulation and act as cofactor for Factor VII (fVII) and initiates cell surface procoagulant activity. It is also known to activate factor X through intrinsic pathway by activating factor IX, leading to thrombin generation and fibrin formation. Since Suefuji et al in 1997 reported the role of TF in AMI, there have been many studies conducted to determine the status of plasma TF and AMI (Kamikura et al., 1997, Nishiyama et al., 1998, Malý et al., 2003, Morange et al., 2007, Xiong et al., 2007) with contradictory findings. We observed increased levels of TF in AMI at presentation (Shalia et al., 2010a). TF exposed from ruptured plaque is the actual trigger but systemic procoagulant status also plays important role. Independent of cellular TF, blood borne soluble TF may play a role in the propagation of thrombosis which also needs monitoring in early atherosclerotic conditions.

#### **1.2.2.4 Plasminogen Activator Inhibitor-1 (PAI-1)**

PAI-1 prevents fibrinolysis and thus accelerates thrombus formation. Immunohistochemical staining of coronary artery specimens (Shindo et al., 2001) and mRNA expression studies (F. Chen et al., 2005) have demonstrated increased expression of PAI. While the evidence of increased PAI levels before first AMI attack, was given by Thogersen et al (1998). In our study, increased levels of PAI-1 were observed in AMI patients at presentation and were also more associated with younger AMI patients (Shalia et al., 2010). Hamstein et al (1985) have also reported elevated circulating concentrations of PAI-1 in young men at increased risk for recurrent infarction.

#### **1.2.3 Components involved in plaque rupture**

A growing understanding of the importance of atherosclerotic plaque rupture in the pathogenesis of coronary events has led to the identification of an expanding array of markers for plaque rupture. The enzymes that have gained importance in this aspect are myeloperoxidase, matrix metalloproteinases, cathepsins, etc.

#### **1.2.3.1 Myeloperoxidase (MPO)**

102 Traditional and Novel Risk Factors in Atherothrombosis

released into the circulation as sCD40L which can be detected in serum and plasma. The main source of circulating sCD40L is platelets (Hennet al., 2001). The binding of CD40L enhances the inflammatory response, acts prothrombotically, leads to plaque destabilization, and inhibits endothelial regeneration. From several clinical studies it has consistently been reported that sCD40L is elevated in patients with ACS and that it provides prognostic information with therapeutic implications independent of established cardiac markers, e.g. cardiac troponins (Heesechen et al., 2003). However, pre-analytical conditions are decisive for

P-selectin is a cell surface glycoprotein that plays a critical role in the migration of lymphocytes into tissues. It is found constitutively in a preformed state in the Weibel-Palade bodies of endothelial cells and in α-granules of platelets. This stored P-selectin is mobilized to the cell surface within minutes in response to a variety of inflammatory and thrombogenic agents. The mobilized P-selectin is apparently present on the cell surface for only a few minutes after which it is recycled to intracellular space. P-selectin also binds monocytes and neutrophils in addition to activated platelets and is responsible for incorporation of leukocytes into the growing thrombus (Malý et al., 2003). Thus, P-selectin is a marker of platelet activation which in turn is prerequisite for thrombosis (Serebruany et al., 1999a). Fijnheer et al (1997) have concluded that endothelial cell activation is associated with an increased P-selectin concentration per platelet. Elevated levels have been reported not only in AMI and unstable angina but also in stable angina. In our study significant negative correlation of sP-selectin with age in AMI group suggests increased pro-coagulant status in younger AMI patients (Mashru et al., 2010). Its role as biomarker requires extensive

TF at the upfront of the coagulation pathway plays a crucial role in initiating thrombus formation after plaque rupture in patients with ACS. Tissue injury disrupts vascular endothelium causing its release into circulating blood and hence activation of coagulation cascades. It activates extrinsic pathway of coagulation and act as cofactor for Factor VII (fVII) and initiates cell surface procoagulant activity. It is also known to activate factor X through intrinsic pathway by activating factor IX, leading to thrombin generation and fibrin formation. Since Suefuji et al in 1997 reported the role of TF in AMI, there have been many studies conducted to determine the status of plasma TF and AMI (Kamikura et al., 1997, Nishiyama et al., 1998, Malý et al., 2003, Morange et al., 2007, Xiong et al., 2007) with contradictory findings. We observed increased levels of TF in AMI at presentation (Shalia et al., 2010a). TF exposed from ruptured plaque is the actual trigger but systemic procoagulant status also plays important role. Independent of cellular TF, blood borne soluble TF may play a role in the propagation of thrombosis which also needs monitoring in early

PAI-1 prevents fibrinolysis and thus accelerates thrombus formation. Immunohistochemical staining of coronary artery specimens (Shindo et al., 2001) and mRNA expression studies (F. Chen et al., 2005) have demonstrated increased expression of PAI. While the evidence of

the assessment of sCD40L and may preclude routine clinical use (Weber et al., 2006).

**1.2.2.2 P-selectin** 

clinical evaluations.

**1.2.2.3 Tissue Factor (TF)** 

atherosclerotic conditions.

**1.2.2.4 Plasminogen Activator Inhibitor-1 (PAI-1)** 

Leucocytes play a central role in atherosclerotic plaque rupture (de Servi et al., 1996). Myeloperoxidase is a degranulation product, secreted by a variety of inflammatory cells, including activated neutrophils, monocytes and macrophages such as those found in atherosclerotic plaques. It possesses proinflammatory properties and may contribute directly to tissue injury (Eiserich et al., 2002). Its systemic levels predicted future cardiovascular event independent of CD40L (Baldus et al., 2003) and gave in-vitro strong support to the role of neutrophil activation as an adjunct pathophysiological event in ACS that is directly different from platelet activation. Collectively the current evidence supports the need for further studies into the actual role of MPO. One of the important roles of Myeloperoxidase in leucocytes is to activate metalloproteinases that bring about plaque rupture (Zhang et al., 2001).

#### **1.2.3.2 Metalloproteinases (MMPs)**

The structural integrity of myocardial Extracellular Matrix (ECM) is dependent on endogenous zinc-dependent endopeptidases known as matrix metalloproteinases (MMP). These enzymes are regulated by tissue inhibitors of metalloproteinases (TIMPs) (Kelly et al., 2008). MMPs may degrade myocardial ECM leading to the development of LV dilatation and heart failure and their inhibition in experimental models of AMI has been associated with reduced LV dilatation and wall stress. Elevated levels of MMP-9 and its major inhibitor TIMP1 have been demonstrated to be associated with cardiovascular death, heart failure or both but not with re-infarction (Kelly et al., 2007). In our study we found that there was significant increase in circulating levels of MMP-9 as well as MMP-8 in AMI at presentation. Moreover the increase in MMP-8 was independent of High sensitive C-reactive protein (HsCRP) and MMP-9 (Shah et al., 2009). MMP-2 is also shown to be elevated post MI (Dhillon et al., 2009) and is associated with poor prognosis (Kelly et al., 2008a). In another study we observed that Serum MMP-3 levels were significantly elevated at presentation of the acute MI as compared to controls (Shalia et al., 2010b) while Kelly et al (2008b) have demonstrated that MMP-3 peaks at 72 hours of MI and plateau levels are associated with increase in LV volume and a lower ejection fraction at follow up. Amongst various MMPs, it has been suggested that MMP-9 may be of value in evaluating patients after acute coronary events (Apple et al, 2005).

#### **1.2.3.3 Cathepsins**

Evidence has been obtained for expression in human atherosclerotic lesion of another matrix degrading enzymes cathepsin S, B, K, D and L (Jormsjö et al., 2002). Beside protease function

In Search for Novel Biomarkers of Acute Coronary Syndrome 105

LOX-1 is a multi-ligand receptor, whose repertoire of ligands includes oxidized low-density lipoprotein, advanced glycation endproducts, platelets, neutrophils, apoptotic/aged cells and bacteria. Sustained expression of LOX-1 by critical target cells, including endothelial cells, smooth muscle cells and macrophages in proximity to these ligands, sets the stage for chronic cellular activation and tissue damage suggesting the interaction of cellular LOX-1 with its ligands to contribute to the formation and development of atherosclerotic plaques. (Navarra et al, 2010). It was demonstrated to be elevated in ACS patients but not correlating with troponins or CK suggesting it not to be a marker of cardiac injury (Hayashida 2005). Kamezaki (2009) in an another study have shown it to be positively correlating with urinary 8-isoprostane and negatively correlating with superoxide dismutase in ACS patients suggesting that increased serum LOX-1 reflect enhanced oxidative stress in vascular wall.

Lp-PLA2, also known as the platelet activating factor acetylhydrolase, is a monomeric enzyme that catalyzes the hydrolysis of the sn-2 ester bond, preferentially when short acyl groups are at the sn-2 position, of oxidized phospholipids. The cascade of Lp-PLA2 activity may eventually lead to plaque destabilization, increasing the possibility of rupture and thrombosis (Hsieh et al., 2000). Confirming the same, circulating levels of sPLA2 were found to increase not only in various chronic inflammatory diseases but also independently predicted clinical coronary events in patients with unstable angina and documented

Although molecules of inflammation may have its primary role as the indicator of endothelial dysfunction and in development of atherosclerotic plaque, its soluble levels have

VCAM-1 is not routinely expressed under physiological conditions. Expression of VCAM-1 occurs only on activated endothelium and vascular smooth muscle cells in developing atherosclerotic lesion (Braun et al., 1999). It was demonstrated to be expressed especially in the intimal neovasculature and largely associated with leukocyte accumulation; promoting the binding of lymphocytes and monocytes which further move by diapedesis which release cytokines and enzymes important for progression of lesion as well as rupture of the plaque (O'Brien et al., 1993, 1996). Literature reports correlation of sVCAM-1 with the extent of coronary atherosclerosis with elevated levels in AMI (Bossowska et al., 2003, G ϋ ray ϋ, Erbay et al., 2004). Consistent with this finding we have observed highest levels with AMI patients and unstable angina in decreasing order as compared to controls (Mashru et al., 2010). In our study age matched analysis also demonstrated younger age group (<=40 years) of patients with AMI with highest sVCAM-1 as compared to age matched controls and in unstable angina it was more associated with females than male patients. Above observations suggest VCAM-1 to be also as an indicator towards ACS in patients with low-risk profile for

**1.2.4.2 Lectin-like Oxidized low density lipoprotein receptor-1 (LOX-1)** 

**1.2.4.3 Lipoprotein-associated Phospholipase A-2 (Lp-PLA-2)** 

coronary artery disease (Kugiyama et al., 1999, 2000).

**1.2.5.1 Vascular Cell Adhesion Molecule (VCAM-1)** 

cardiovascular risk factors such as age and gender.

been implicated in various studies to be associated with ACS.

**1.2.5 Molecules of inflammation** 

and vascular effects, protease detection and quantization in peripheral blood may help detect atheromatous disease stages and aid in clinical decision-making (Vivanco et al., 2005). Patients with coronary artery stenosis have demonstrated increased serum cathepsin L levels than those without lesions detectable by quantitative angiography (Liu et al., 2006a). Increased serum cathepsin S has been demonstrated in patients with atherosclerosis and diabetes (Liu et al., 2006b) and increased cathepsin D both in plasma and monocytes of ACS patients. In our study increased peripheral blood levels of cathepsin B and K and decrease in their inhibitor cystatin C at the acute phase of MI were observed (Shalia et al., 2011). Moreover plasma concentration of MMP-9; recently identified as a novel predictor of cardiovascular mortality in patients with CAD and also marker for plaque destabilization and rupture demonstrated strong positive correlation with cathepsin B and negative correlation with cystatin C in AMI group (Shalia et al., 2011).

#### **1.2.3.4 Pregnancy-Associated Plasma Protein-A (PAPP-A)**

It is known as high molecular weight (200kDa) glycoprotein synthesized by the syncytiotrophoblast and is typically measured during pregnancy for Down syndrome screening. It is pro-atherosclerotic molecule family of proteins; a member of the insulin-like growth factor (IGF) –dependent IGF binding protein-4 specific metalloproteinase (Bayes-Genis et al., 2000). It is thought to be released when neovascularization occurs and thus may be a marker of incipient plaque rupture which was later confirmed in studies demonstrating its increased expression in unstable plaques and their extracellular matrices (Bayes-Genis et al., 2001) and corresponding increased circulating levels in unstable angina and AMI (Bayes-Genis et al., 2001). Interestingly, it demonstrated increase in risk of cardiovascular death, MI or revascularisation even without a raised Troponin (Lund et al., 2003). Evidence for the use of this biomarker clinically remains scarce but promising. More studies and standardized assays will be needed to improve its clinical utility.

#### **1.2.4 Components representing oxidative stress**

Oxidative stress in conjunction with inflammation is the one of the important initiators of atherosclerosis. However they also play important role in increasing the severity of pathogenesis of ACS.

#### **1.2.4.1 Oxidized LDL (Ox-LDL)**

Ox LDL is involved at very early critical steps of atherosclerosis. Oxidized LDL as well as its antibody (ox-LDL Ab) have been documented to be elevated in ACS patients including AMI and Unstable Angina and were suggested to be helpful in diagnosis of ACS (Zhou e tal., 2006). Imazu et al (2008) examined the relationship among plasma levels of OxLDL, measured by an enzyme immunoassay using an antibody against OxLDL (FOH1a/DLH3) and apolipoprotein B, at the onset of ACS. Plasma levels of OxLDL were significantly higher in patients with new-onset type ACS than in those with worsening type ACS (2.98 versus 1.53 mg/dL, P = 0.002). In conclusion, plasma levels of OxLDL were demonstrated to be associated with CHD and significantly higher in patients with new-onset ACS. The findings of the study suggested that plasma OxLDL can be a marker of the development of ACS. Oxidized low-density lipoprotein (oxLDL)/beta(2)-glycoprotein I (beta2GPI) complexes implicated in atherogenesis were also demonstrated to be associated with severe coronary artery disease and a 3.5-fold increased risk for adverse outcomes (Greco et al., 2010).

and vascular effects, protease detection and quantization in peripheral blood may help detect atheromatous disease stages and aid in clinical decision-making (Vivanco et al., 2005). Patients with coronary artery stenosis have demonstrated increased serum cathepsin L levels than those without lesions detectable by quantitative angiography (Liu et al., 2006a). Increased serum cathepsin S has been demonstrated in patients with atherosclerosis and diabetes (Liu et al., 2006b) and increased cathepsin D both in plasma and monocytes of ACS patients. In our study increased peripheral blood levels of cathepsin B and K and decrease in their inhibitor cystatin C at the acute phase of MI were observed (Shalia et al., 2011). Moreover plasma concentration of MMP-9; recently identified as a novel predictor of cardiovascular mortality in patients with CAD and also marker for plaque destabilization and rupture demonstrated strong positive correlation with cathepsin B and negative

It is known as high molecular weight (200kDa) glycoprotein synthesized by the syncytiotrophoblast and is typically measured during pregnancy for Down syndrome screening. It is pro-atherosclerotic molecule family of proteins; a member of the insulin-like growth factor (IGF) –dependent IGF binding protein-4 specific metalloproteinase (Bayes-Genis et al., 2000). It is thought to be released when neovascularization occurs and thus may be a marker of incipient plaque rupture which was later confirmed in studies demonstrating its increased expression in unstable plaques and their extracellular matrices (Bayes-Genis et al., 2001) and corresponding increased circulating levels in unstable angina and AMI (Bayes-Genis et al., 2001). Interestingly, it demonstrated increase in risk of cardiovascular death, MI or revascularisation even without a raised Troponin (Lund et al., 2003). Evidence for the use of this biomarker clinically remains scarce but promising. More studies and standardized

Oxidative stress in conjunction with inflammation is the one of the important initiators of atherosclerosis. However they also play important role in increasing the severity of

Ox LDL is involved at very early critical steps of atherosclerosis. Oxidized LDL as well as its antibody (ox-LDL Ab) have been documented to be elevated in ACS patients including AMI and Unstable Angina and were suggested to be helpful in diagnosis of ACS (Zhou e tal., 2006). Imazu et al (2008) examined the relationship among plasma levels of OxLDL, measured by an enzyme immunoassay using an antibody against OxLDL (FOH1a/DLH3) and apolipoprotein B, at the onset of ACS. Plasma levels of OxLDL were significantly higher in patients with new-onset type ACS than in those with worsening type ACS (2.98 versus 1.53 mg/dL, P = 0.002). In conclusion, plasma levels of OxLDL were demonstrated to be associated with CHD and significantly higher in patients with new-onset ACS. The findings of the study suggested that plasma OxLDL can be a marker of the development of ACS. Oxidized low-density lipoprotein (oxLDL)/beta(2)-glycoprotein I (beta2GPI) complexes implicated in atherogenesis were also demonstrated to be associated with severe coronary

artery disease and a 3.5-fold increased risk for adverse outcomes (Greco et al., 2010).

correlation with cystatin C in AMI group (Shalia et al., 2011). **1.2.3.4 Pregnancy-Associated Plasma Protein-A (PAPP-A)** 

assays will be needed to improve its clinical utility.

**1.2.4 Components representing oxidative stress** 

pathogenesis of ACS.

**1.2.4.1 Oxidized LDL (Ox-LDL)** 

#### **1.2.4.2 Lectin-like Oxidized low density lipoprotein receptor-1 (LOX-1)**

LOX-1 is a multi-ligand receptor, whose repertoire of ligands includes oxidized low-density lipoprotein, advanced glycation endproducts, platelets, neutrophils, apoptotic/aged cells and bacteria. Sustained expression of LOX-1 by critical target cells, including endothelial cells, smooth muscle cells and macrophages in proximity to these ligands, sets the stage for chronic cellular activation and tissue damage suggesting the interaction of cellular LOX-1 with its ligands to contribute to the formation and development of atherosclerotic plaques. (Navarra et al, 2010). It was demonstrated to be elevated in ACS patients but not correlating with troponins or CK suggesting it not to be a marker of cardiac injury (Hayashida 2005). Kamezaki (2009) in an another study have shown it to be positively correlating with urinary 8-isoprostane and negatively correlating with superoxide dismutase in ACS patients suggesting that increased serum LOX-1 reflect enhanced oxidative stress in vascular wall.

#### **1.2.4.3 Lipoprotein-associated Phospholipase A-2 (Lp-PLA-2)**

Lp-PLA2, also known as the platelet activating factor acetylhydrolase, is a monomeric enzyme that catalyzes the hydrolysis of the sn-2 ester bond, preferentially when short acyl groups are at the sn-2 position, of oxidized phospholipids. The cascade of Lp-PLA2 activity may eventually lead to plaque destabilization, increasing the possibility of rupture and thrombosis (Hsieh et al., 2000). Confirming the same, circulating levels of sPLA2 were found to increase not only in various chronic inflammatory diseases but also independently predicted clinical coronary events in patients with unstable angina and documented coronary artery disease (Kugiyama et al., 1999, 2000).

#### **1.2.5 Molecules of inflammation**

Although molecules of inflammation may have its primary role as the indicator of endothelial dysfunction and in development of atherosclerotic plaque, its soluble levels have been implicated in various studies to be associated with ACS.

#### **1.2.5.1 Vascular Cell Adhesion Molecule (VCAM-1)**

VCAM-1 is not routinely expressed under physiological conditions. Expression of VCAM-1 occurs only on activated endothelium and vascular smooth muscle cells in developing atherosclerotic lesion (Braun et al., 1999). It was demonstrated to be expressed especially in the intimal neovasculature and largely associated with leukocyte accumulation; promoting the binding of lymphocytes and monocytes which further move by diapedesis which release cytokines and enzymes important for progression of lesion as well as rupture of the plaque (O'Brien et al., 1993, 1996). Literature reports correlation of sVCAM-1 with the extent of coronary atherosclerosis with elevated levels in AMI (Bossowska et al., 2003, G ϋ ray ϋ, Erbay et al., 2004). Consistent with this finding we have observed highest levels with AMI patients and unstable angina in decreasing order as compared to controls (Mashru et al., 2010). In our study age matched analysis also demonstrated younger age group (<=40 years) of patients with AMI with highest sVCAM-1 as compared to age matched controls and in unstable angina it was more associated with females than male patients. Above observations suggest VCAM-1 to be also as an indicator towards ACS in patients with low-risk profile for cardiovascular risk factors such as age and gender.

In Search for Novel Biomarkers of Acute Coronary Syndrome 107

IL-10 is an important anti-inflammatory molecule with so far contradictory findings in ACS patients. On one hand it was shown to demonstrate more favorable prognosis in patients with ACS (Heeschen et al., 2003) while on the other hand it reflected a proinflammatory state in patients with ACS which suggested that IL-10 is as effective biomarker for the risk prediction of future cardiovascular events as other markers of systemic inflammation (Mälarstig et al., 2008). Extensive study may be required to establish its role in the

It is a member of the IL-1 cytokine family, originally identified in macrophages and Kupffer cells as factor able to induce IFN- production by T cells which itself is a central proatherogenic factor. Both increased serum levels of IL-18 and reduced concentrations of IL-10 have been shown to have prognostic significance in ACS. Chalikias, et al (2005) sought to assess whether the ratio of serum IL-18/IL-10 levels has higher positive predictive value than the individual measurement of IL-10 and IL-18 in patients admitted to hospital with ACS. Their findings demonstrated that significantly higher odd ratios were found for IL-18/IL-10 ratio (1.74 95% CI 1.09-2.78) compared to individual IL-18 (1.46 95% CI 0.93-2.27) and 1/IL-10 (1.63 95% CI 1.04-2.56) measurements. Recently Hartford et al (2010) demonstrated that IL-18 levels were significantly related to all-cause mortality, even after adjustment for clinical confounders (hazard ratio [HR], 1.19; 95% confidence interval, 1.07 to 1.33; P=0.002). Long-term, cardiovascular mortality was univariately related to IL-18, and the adjusted relation between noncardiovascular mortality and IL-18 was highly significant (HR, 1.36; 95% confidence interval, 1.11 to 1.67; P=0.003). IL-18 independently predicted congestive heart failure, MI, and cardiovascular death/congestive heart failure/MI in both the short and long term. Measurements from day 1 of ACS and 3 months after ACS had a

The data from the PRIME Study, a prospective cohort of 9758 asymptomatic middle-aged men recruited in Northern Ireland and France between 1991 and 1993.demonstrated that higher circulating levels of Hs-CRP, intercellular CAM-1 (ICAM-1), IL6 and IL18 to be equally predictive of stable angina and ACS (all P-values of OR comparison >0.05) (Empana

It is thought that one of the driving forces causing atheromatous plaques to rupture or erode, causing a cascade of events leading to coronary artery occlusion, is inflammation in the plaques (Ridker, 2003, Shishehbor et al., 2003). CRP itself mediates atherothrombosis which is supported by a fairly large body of evidence (Pasceri et al., 2000, Nakajima et al., 2002, Nakagomi et al., 2000, Verma et al., 2002, Devaraj et al., 2003). The benefits of HsCRP testing in a primary setting to screen for ischaemic heart disease is very clear, People are risk stratified based on amount of CRP in blood. There are three groups; less than 1mg/l of CRP is low risk group, between 1 – 3mg/l is classified as the moderate risk group and more than 3mg/l is the high risk group (AHA/CDC, 2003). However, its use post-ACS or -MI is less clear. CRP is elevated post-acute coronary syndrome almost exclusively in the setting of myocardial necrosis indicating the level of myocardial inflammation. In a study carried out by us, we observed a three fold increase in the total HsCRP levels in MI patients at

**1.2.5.6 IL-10** 

**1.2.5.7 IL-18** 

et al., 2008).

pathogenesis of ACS and its utility as biomarker.

similar power to predict late outcome.

**1.2.5.8 High-sensitivity C-Reactive Protein (HsCRP)** 

presentation; as compared to controls (Shalia et al., 2011).

#### **1.2.5.2 Platelet Endothelial Cell Adhesion Molecule (PECAM-1)**

PECAM-1 of the immunoglobulin superfamily is with wide variety of functions such as platelet activation, inflammation, cell survival, in the immune response and in transendothelial migration of monocytes (TEM) (Muller et al., 1993). It has also been demonstrated to have role in plaque formation and thrombosis (Newman et al., 1990, Mahooti et al., 2000). In our study (Mashru et al., 2010) there was significant increase in sPECAM-1 in AMI patients at acute event consistent with the finding of Serebruany et al (1999b) and Soeki et al (2003).

#### **1.2.5.3 Monocyte Chemoattractant Protein-1 (MCP-1)**

MCP-1 is a chemokine responsible for the recruitment of monocytes to sites of inflammation that appears to play a critical role in the promotion of plaque instability (Szmitko et al., 2003). In case control studies, plasma MCP-1 concentrations have been shown to be associated with restenosis after coronary angioplasty (Cipollone et al., 2001). However, in a prospective study on a large cohort of ACS patients, the distribution of MCP-1 values in the healthy subjects and the study population overlapped considerably. However, subsequent studies have shown that MCP-1 plasma concentration is associated with different cardiovascular risk factors, and a greater risk of developing a cardiovascular event in the future (de Lemos et al., 2003, Deo et al., 2004).

#### **1.2.5.4 Cystatin C**

This biomarker is a low-molecular-weight basic protein (13 kDa) that is freely filtered and metabolized after tubular reabsorption. It seems that it is less influenced by age, gender, and muscle mass than serum creatinine. There is a U.S. Food and Drug Administration-cleared assay that is analytically robust. Some studies suggest that it is useful for prognostication in heart failure (Sarnak et al., 2005, Shlipak et al., 2005), and ACS (Jernberg et al., 2004). This would make sense as it is well accepted that renal function is a critical determinant of prognosis. In our study cystatin C levels did not deviate much from the controls maintaining its normal levels with normal kidney functioning and demonstrated negative correlation with MMP-9 in AMI group (Shalia et al., 2011).

#### **1.2.5.5 Interleukin 6 (IL-6)**

As the prototypical acute phase reactants, interleukin-6 (IL-6) has been the focus of investigations for the diagnosis of ACS. The Fragmin and Fast Revascularisation During Instability in Coronary Artery disease II trial (FRISC) study group showed that the circulating level of IL-6 is a strong independent marker of increased mortality among patients with unstable angina and is useful in directing subsequent care. However, the best timing for measurement of IL-6 for diagnosis and risk stratification of ACS remains uncertain (Lindmark et al., 2001). Alwi et al., (2008) concluded that the IL-6 level in ACS were higher than those in CHD. The IL-6 level 4.43 pg/mL could differentiate the acute condition (ACS) and stable condition (non-ACS) with sensitivity of 89.95% and specificity of 77.42%, and ROC of 0.87.

A study by Kavsak et al (2007) demonstrated that IL-6, MCP-1, and a known biomarker, NTproBNP were independent predictors of long-term risk of death or HF, highlighting the importance of identifying leukocyte activation and recruitment in ACS patients**.** 

#### **1.2.5.6 IL-10**

106 Traditional and Novel Risk Factors in Atherothrombosis

PECAM-1 of the immunoglobulin superfamily is with wide variety of functions such as platelet activation, inflammation, cell survival, in the immune response and in transendothelial migration of monocytes (TEM) (Muller et al., 1993). It has also been demonstrated to have role in plaque formation and thrombosis (Newman et al., 1990, Mahooti et al., 2000). In our study (Mashru et al., 2010) there was significant increase in sPECAM-1 in AMI patients at acute event consistent with the finding of Serebruany et al

MCP-1 is a chemokine responsible for the recruitment of monocytes to sites of inflammation that appears to play a critical role in the promotion of plaque instability (Szmitko et al., 2003). In case control studies, plasma MCP-1 concentrations have been shown to be associated with restenosis after coronary angioplasty (Cipollone et al., 2001). However, in a prospective study on a large cohort of ACS patients, the distribution of MCP-1 values in the healthy subjects and the study population overlapped considerably. However, subsequent studies have shown that MCP-1 plasma concentration is associated with different cardiovascular risk factors, and a greater risk of developing a cardiovascular event in the

This biomarker is a low-molecular-weight basic protein (13 kDa) that is freely filtered and metabolized after tubular reabsorption. It seems that it is less influenced by age, gender, and muscle mass than serum creatinine. There is a U.S. Food and Drug Administration-cleared assay that is analytically robust. Some studies suggest that it is useful for prognostication in heart failure (Sarnak et al., 2005, Shlipak et al., 2005), and ACS (Jernberg et al., 2004). This would make sense as it is well accepted that renal function is a critical determinant of prognosis. In our study cystatin C levels did not deviate much from the controls maintaining its normal levels with normal kidney functioning and demonstrated negative

As the prototypical acute phase reactants, interleukin-6 (IL-6) has been the focus of investigations for the diagnosis of ACS. The Fragmin and Fast Revascularisation During Instability in Coronary Artery disease II trial (FRISC) study group showed that the circulating level of IL-6 is a strong independent marker of increased mortality among patients with unstable angina and is useful in directing subsequent care. However, the best timing for measurement of IL-6 for diagnosis and risk stratification of ACS remains uncertain (Lindmark et al., 2001). Alwi et al., (2008) concluded that the IL-6 level in ACS were higher than those in CHD. The IL-6 level 4.43 pg/mL could differentiate the acute condition (ACS) and stable condition (non-ACS) with sensitivity of 89.95% and specificity of

A study by Kavsak et al (2007) demonstrated that IL-6, MCP-1, and a known biomarker, NTproBNP were independent predictors of long-term risk of death or HF, highlighting the

importance of identifying leukocyte activation and recruitment in ACS patients**.** 

**1.2.5.2 Platelet Endothelial Cell Adhesion Molecule (PECAM-1)**

**1.2.5.3 Monocyte Chemoattractant Protein-1 (MCP-1)** 

future (de Lemos et al., 2003, Deo et al., 2004).

correlation with MMP-9 in AMI group (Shalia et al., 2011).

(1999b) and Soeki et al (2003).

**1.2.5.4 Cystatin C** 

**1.2.5.5 Interleukin 6 (IL-6)** 

77.42%, and ROC of 0.87.

IL-10 is an important anti-inflammatory molecule with so far contradictory findings in ACS patients. On one hand it was shown to demonstrate more favorable prognosis in patients with ACS (Heeschen et al., 2003) while on the other hand it reflected a proinflammatory state in patients with ACS which suggested that IL-10 is as effective biomarker for the risk prediction of future cardiovascular events as other markers of systemic inflammation (Mälarstig et al., 2008). Extensive study may be required to establish its role in the pathogenesis of ACS and its utility as biomarker.

#### **1.2.5.7 IL-18**

It is a member of the IL-1 cytokine family, originally identified in macrophages and Kupffer cells as factor able to induce IFN- production by T cells which itself is a central proatherogenic factor. Both increased serum levels of IL-18 and reduced concentrations of IL-10 have been shown to have prognostic significance in ACS. Chalikias, et al (2005) sought to assess whether the ratio of serum IL-18/IL-10 levels has higher positive predictive value than the individual measurement of IL-10 and IL-18 in patients admitted to hospital with ACS. Their findings demonstrated that significantly higher odd ratios were found for IL-18/IL-10 ratio (1.74 95% CI 1.09-2.78) compared to individual IL-18 (1.46 95% CI 0.93-2.27) and 1/IL-10 (1.63 95% CI 1.04-2.56) measurements. Recently Hartford et al (2010) demonstrated that IL-18 levels were significantly related to all-cause mortality, even after adjustment for clinical confounders (hazard ratio [HR], 1.19; 95% confidence interval, 1.07 to 1.33; P=0.002). Long-term, cardiovascular mortality was univariately related to IL-18, and the adjusted relation between noncardiovascular mortality and IL-18 was highly significant (HR, 1.36; 95% confidence interval, 1.11 to 1.67; P=0.003). IL-18 independently predicted congestive heart failure, MI, and cardiovascular death/congestive heart failure/MI in both the short and long term. Measurements from day 1 of ACS and 3 months after ACS had a similar power to predict late outcome.

The data from the PRIME Study, a prospective cohort of 9758 asymptomatic middle-aged men recruited in Northern Ireland and France between 1991 and 1993.demonstrated that higher circulating levels of Hs-CRP, intercellular CAM-1 (ICAM-1), IL6 and IL18 to be equally predictive of stable angina and ACS (all P-values of OR comparison >0.05) (Empana et al., 2008).

#### **1.2.5.8 High-sensitivity C-Reactive Protein (HsCRP)**

It is thought that one of the driving forces causing atheromatous plaques to rupture or erode, causing a cascade of events leading to coronary artery occlusion, is inflammation in the plaques (Ridker, 2003, Shishehbor et al., 2003). CRP itself mediates atherothrombosis which is supported by a fairly large body of evidence (Pasceri et al., 2000, Nakajima et al., 2002, Nakagomi et al., 2000, Verma et al., 2002, Devaraj et al., 2003). The benefits of HsCRP testing in a primary setting to screen for ischaemic heart disease is very clear, People are risk stratified based on amount of CRP in blood. There are three groups; less than 1mg/l of CRP is low risk group, between 1 – 3mg/l is classified as the moderate risk group and more than 3mg/l is the high risk group (AHA/CDC, 2003). However, its use post-ACS or -MI is less clear. CRP is elevated post-acute coronary syndrome almost exclusively in the setting of myocardial necrosis indicating the level of myocardial inflammation. In a study carried out by us, we observed a three fold increase in the total HsCRP levels in MI patients at presentation; as compared to controls (Shalia et al., 2011).

In Search for Novel Biomarkers of Acute Coronary Syndrome 109

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Elevated peak CRP in the early phase of MI is related to early mechanical complications, including cardiac rupture, ventricular aneurysm and thrombus formation (Anzai et al., 1997). CRP levels post-MI peak at two to four days, then take 8 to 12 weeks to subside to baseline levels. One of the difficulties with CRP is that it is non-specific and also is elevated in the presence of other inflammatory conditions (rheumatoid arthritis, malignancy, vasculitis etc.). A new assay for Human Pentraxin 3 is now available. Human Pentraxin 3 is an isoform which is secreted exclusively in vascular endothelium and therefore may be more specific to the vascular plaque inflammatory activity (Matsui et al., 2010). It remains to be seen if this biomarker can provide incremental information.

#### **2. Conclusion**

Thus, non invasive indicators of separate pathobiologically diverse contributors to the progression of ACS, such as molecules of inflammation, components of plaque rupture and thrombosis, could add complementary information in variety of clinical settings. The role of these components in multi-marker testing, in identifying the high-risk individuals, the pathophysiologic stage of the disease and tailoring therapy needs to be established. The future of ACS management would probably shift from single to multi-marker testing leading to better characterization of each individual case by using a combination of both established and new markers for risk assessment and clinical decision making that will substantially improve the outcomes in patients with ACS.

Growing hand in hand with our contemporary fascination are the promises of personalized medicine, the discovery of novel biomarkers in cardiovascular diseases which has been embraced as a major objective of government, private and industry supported research initiatives. More than a decade of advances in our understanding of the complex mechanisms underlying the initiation, progression of atherothrombosis and its complications has stimulated efforts to identify and characterize new markers associated with this processes. In addition newer screening based discovery techniques such as metaloblomics and proteomics have revealed large numbers of candidate metabolites and proteins associated with this disease for which the function or role in pathophysiology has yet to be explained. The clinical application of cardiac biomarkers in ACS is no longer limited to establishing or refuting the diagnosis of myocardial necrosis. Cardiac biomarkers provide a convenient and non invasive means to gain insights into the underlying causes and consequences of ACS that mediate the risk of recurrent events and may be targets for specific therapeutic interventions.

#### **3. Acknowledgement**

Authors would like to acknowledge Sir H. N. Hospital and Research Centre and Rajawadi Municipal Hospital for recruitment of patients and Sir H. N. Medical Research Society for the financial support given for carrying out projects related to this topic.

#### **4. References**

Adams, JE.; Kleinfeld, A.; Roe, M.; et al. (2004) Measurement of levels of unbound free fatty acid allows the early identification of patients with acute coronary syndrome. *Circulation*, Vol.106, (Suppl II), p. 532.

Elevated peak CRP in the early phase of MI is related to early mechanical complications, including cardiac rupture, ventricular aneurysm and thrombus formation (Anzai et al., 1997). CRP levels post-MI peak at two to four days, then take 8 to 12 weeks to subside to baseline levels. One of the difficulties with CRP is that it is non-specific and also is elevated in the presence of other inflammatory conditions (rheumatoid arthritis, malignancy, vasculitis etc.). A new assay for Human Pentraxin 3 is now available. Human Pentraxin 3 is an isoform which is secreted exclusively in vascular endothelium and therefore may be more specific to the vascular plaque inflammatory activity (Matsui et al., 2010). It remains to

Thus, non invasive indicators of separate pathobiologically diverse contributors to the progression of ACS, such as molecules of inflammation, components of plaque rupture and thrombosis, could add complementary information in variety of clinical settings. The role of these components in multi-marker testing, in identifying the high-risk individuals, the pathophysiologic stage of the disease and tailoring therapy needs to be established. The future of ACS management would probably shift from single to multi-marker testing leading to better characterization of each individual case by using a combination of both established and new markers for risk assessment and clinical decision making that will

Growing hand in hand with our contemporary fascination are the promises of personalized medicine, the discovery of novel biomarkers in cardiovascular diseases which has been embraced as a major objective of government, private and industry supported research initiatives. More than a decade of advances in our understanding of the complex mechanisms underlying the initiation, progression of atherothrombosis and its complications has stimulated efforts to identify and characterize new markers associated with this processes. In addition newer screening based discovery techniques such as metaloblomics and proteomics have revealed large numbers of candidate metabolites and proteins associated with this disease for which the function or role in pathophysiology has yet to be explained. The clinical application of cardiac biomarkers in ACS is no longer limited to establishing or refuting the diagnosis of myocardial necrosis. Cardiac biomarkers provide a convenient and non invasive means to gain insights into the underlying causes and consequences of ACS that mediate the risk of recurrent events and may be targets for

Authors would like to acknowledge Sir H. N. Hospital and Research Centre and Rajawadi Municipal Hospital for recruitment of patients and Sir H. N. Medical Research Society for

Adams, JE.; Kleinfeld, A.; Roe, M.; et al. (2004) Measurement of levels of unbound free fatty

acid allows the early identification of patients with acute coronary syndrome.

the financial support given for carrying out projects related to this topic.

*Circulation*, Vol.106, (Suppl II), p. 532.

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**3. Acknowledgement** 

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**7** 

*USA* 

**Lower Extremity Peripheral Arterial Disease** 

*1Saint Joseph Mercy Hospital, Department of Internal Medicine, Ann Arbor, MI,* 

Lower extremity peripheral arterial disease (PAD) is a disorder characterized by atherosclerotic narrowing or occlusion of the lower limb arteries. In addition to its association with impaired mobility and functional status, PAD is a marker of systemic atherosclerosis and is associated with a high incidence of cardiovascular events such as

Eight to 12 million people in United States have PAD (Allison et al. 2007). Only ~10% of patients with PAD have classic intermittent claudication (IC), ~50% have atypical leg symptoms and ~40% are asymptomatic (Hiatt 2001). Its prevalence significantly increases with age; 12 – 20% of those above the age of 65 years suffer from PAD (Ostchega al. 2007). The prevalence of PAD is similar in men and women; however men are more likely to have symptomatic disease (Ness et al. 2000; Aronow et al. 2002; Selvin and Erlinger 2004;

PAD is a polyvascular disease. Sixty to 90% of patients with PAD also have significant coronary artery disease (CAD) and up to 25% have significant carotid artery stenosis (Hertzer et al. 1984; Klop et al. 1991; Valentine et al. 1994; Cheng et al. 1999; McFalls et al. 2004; Steg et al. 2007). In a study at the Cleveland Clinic, all patients undergoing vascular surgery between 1978 and 1981 had coronary angiogram. Only 10% of those patients had normal coronaries and 28% had severe 3 vessel coronary artery disease requiring surgical or catheter based intervention (Hertzer et al. 1984). Patients with PAD have a 3-fold greater 10 year risk (RR, 3.1; 95% CI, 1.9–4.9) of all-cause death and a 6-fold greater risk (RR, 6.6; 95% CI, 2.9–14.9) of cardiovascular (CV)-related death compared to patients without PAD (Criqui et al. 1992). In the REACH registry, by one year , 21% of patients with PAD had developed MI, stroke, cardiovascular death or hospitalization compared with 15% of

Risk factors for PAD are similar to those for atherosclerotic disease in other vascular beds

**1. Introduction** 

**2. Epidemiology** 

Ostchega et al. 2007).

patients with CAD (Steg et al. 2007).

with some variation in their individual attributable risks.

**3. Risk factors for PAD** 

stroke, myocardial infarction (MI) and vascular death.

Aditya M. Sharma1 and Herbert D. Aronow2

*2Michigan Heart and Vascular Institute, Ann Arbor, MI,*


## **Lower Extremity Peripheral Arterial Disease**

Aditya M. Sharma1 and Herbert D. Aronow2

*1Saint Joseph Mercy Hospital, Department of Internal Medicine, Ann Arbor, MI, 2Michigan Heart and Vascular Institute, Ann Arbor, MI, USA* 

#### **1. Introduction**

118 Traditional and Novel Risk Factors in Atherothrombosis

Zhang, R.; Brennan, ML.; Fu, X.; Aviles, RJ.; Pearce, GL.; Penn, MS.; et al. (2001). Association

Zhou, ZX.; Qiang, H.; Ma, AQ.; Chen, H.; Zhou, P. (2006). Measurement peripheral blood

286, No. 17, (November 2001), pp. 2136–2142.

Vol. 31, No. 2, (April 2006), pp.258-262.

between myeloperoxidase levels and risk of coronary artery disease. *JAMA,* Vol.

index related to inflammation and ox-LDL, ox-LDLAb in patients with coronary heart disease and its clinical significance. *Zhong Nan Da Xue Xue Bao Yi Xue Ban*,

> Lower extremity peripheral arterial disease (PAD) is a disorder characterized by atherosclerotic narrowing or occlusion of the lower limb arteries. In addition to its association with impaired mobility and functional status, PAD is a marker of systemic atherosclerosis and is associated with a high incidence of cardiovascular events such as stroke, myocardial infarction (MI) and vascular death.

#### **2. Epidemiology**

Eight to 12 million people in United States have PAD (Allison et al. 2007). Only ~10% of patients with PAD have classic intermittent claudication (IC), ~50% have atypical leg symptoms and ~40% are asymptomatic (Hiatt 2001). Its prevalence significantly increases with age; 12 – 20% of those above the age of 65 years suffer from PAD (Ostchega al. 2007). The prevalence of PAD is similar in men and women; however men are more likely to have symptomatic disease (Ness et al. 2000; Aronow et al. 2002; Selvin and Erlinger 2004; Ostchega et al. 2007).

PAD is a polyvascular disease. Sixty to 90% of patients with PAD also have significant coronary artery disease (CAD) and up to 25% have significant carotid artery stenosis (Hertzer et al. 1984; Klop et al. 1991; Valentine et al. 1994; Cheng et al. 1999; McFalls et al. 2004; Steg et al. 2007). In a study at the Cleveland Clinic, all patients undergoing vascular surgery between 1978 and 1981 had coronary angiogram. Only 10% of those patients had normal coronaries and 28% had severe 3 vessel coronary artery disease requiring surgical or catheter based intervention (Hertzer et al. 1984). Patients with PAD have a 3-fold greater 10 year risk (RR, 3.1; 95% CI, 1.9–4.9) of all-cause death and a 6-fold greater risk (RR, 6.6; 95% CI, 2.9–14.9) of cardiovascular (CV)-related death compared to patients without PAD (Criqui et al. 1992). In the REACH registry, by one year , 21% of patients with PAD had developed MI, stroke, cardiovascular death or hospitalization compared with 15% of patients with CAD (Steg et al. 2007).

#### **3. Risk factors for PAD**

Risk factors for PAD are similar to those for atherosclerotic disease in other vascular beds with some variation in their individual attributable risks.

Lower Extremity Peripheral Arterial Disease 121

A number of inflammatory markers are associated with prevalent PAD and outcomes among those with PAD. In a meta-analysis, the odds ratio for PAD in patients with increased homocysteine was 6.8 (Boushey et al. 1995). 30-40% of PAD patients have elevated homocysteine levels as compared with 3-5% of the general population (Hajjar 1993). Elevated C – reactive protein and D-dimer are associated with increased cardiovascular

The Prevalence of PAD increases with age. In the NHANES study, the prevalence of PAD increased significantly with each decade since beyond 40 years (Selvin and Erlinger 2004) (Table 1). In a German epidemiologic study, 21% of the patients above the age of 65 years had symptomatic or asymptomatic PAD yet again demonstrating increased incidence of

Table 1. In the NHANES study, prevalence of PAD increased significantly with age.

The prevalence of PAD varies by ethnicity and race. Non-hispanic blacks have the highest prevalence, followed by Mexican Americans and non-Hispanic whites, with 19.5%, 15.6% and 11.7% above the age of 60 years having PAD, respectively (Ostchega et al. 2007). It is believed that these relationships are mediated by a genetic predisposition in part secondary to variations in atherogenic and prothrombotic factors (Folsom et al. 1992; Conlan et al. 1993;

**4.1** Intermittent claudication: Exertional symptoms such as muscle pain, cramping, discomfort or fatigue in the legs which resolves with rest of 10 minutes or less. However,

**4.2** "Atypical" leg pain: Exertional leg symptoms which may not always resolve with rest or

**4.3** Asymptomatic: 40 % of patients with PAD have no leg symptoms but most will have

**4.4** Acute limb ischemia: A potentially limb threatening syndrome, classically manifest by one of more of the following five "P's" – pain, pulselessness, paralysis, paresthesias and pallor.

**3.5 Inflammation** 

**3.6 Age** 

mortality in PAD patients (Vidula et al. 2008).

PAD with age (Diehm et al. 2009).

**3.7 Race/ethnicity** 

Gerhard et al. 1999).

**4. Clinical presentation** 

are not consistently reproducible.

some other form of functional limitations.

PAD can present in the following forms (Hirsch et al. 2006):

these 'classic' symptoms are seen in only 10 % of patients with PAD.

#### **3.1 Cigarette smoking**

A strong predictor of prevalent PAD and its progression; it also predicts bypass graft and endovascular intervention failure. (Sapoval et al. 1996; Hirsch et al. 2001; Willigendael et al. 2005; Aboyans et al. 2006). Smoking increases the risk of lower extremity peripheral arterial disease by 2- to 6-fold and the risk of intermittent claudication by 3- to 10-fold (Kannel and McGee 1985; Smith et al. 1990; Bowlin et al. 1994; Meijer et al. 1998). 80% of patients with PAD have a smoking history (Smith et al. 1990; Meijer et al. 1998). Interestingly, cigarette smoking is a stronger risk factor for PAD than for CAD with smoker's having a 2 to 3 times greater likelihood of developing PAD than CAD (Hirsch et al. 2001). It is also an independent predictor of the risk for unplanned urgent revascularization of the lower extremities following initial successful treatment (Shamma et al. 2003). With an increasing number of pack years, there is an increase in severity of disease, increased risk of amputation, peripheral graft occlusion and overall mortality.

#### **3.2 Diabetes Mellitus (DM)**

The risk of prevalent PAD doubles in the setting of impaired glucose tolerance (Beckman et al. 2002) and increases by 2 to 4 fold in patients with overt diabetes mellitus. PAD presents at an earlier age in diabetics; the prevalence of PAD is ~ 20% in diabetic patients older than 40 years and increases to ~30% among those older than 50 years (Hirsch et al. 2001; 2003). Diabetics tend to have more severe degrees of stenosis and a higher preponderance toward calcification as compared to non-diabetic patients with PAD (Jude et al. 2001). PAD is more likely to remain asymptomatic in diabetics until very advanced. Finally, poor glvcemic control is associated with more rapid PAD progression, increased risk of amputation and mortality (Jude et al. 2001; Beckman et al. 2002).

#### **3.3 Hypertension and dyslipidemia**

Although the attributable risk for these factors is less than for DM or smoking, their adverse impact is clear from multiple studies. Uncontrolled higher blood pressures are associated with increased severity of PAD (Ness et al. 2000; Hirsch et al. 2001; Selvin and Erlinger 2004; Ostchega et al. 2007). Elevated total and LDL cholesterol, low HDL cholesterol and hypertriglyceridemia are associated with prevalent PAD (Kannel and Shurtleff 1973; Fowkes et al. 1992; Hiatt et al. 1995; Murabito et al. 2002). The risk of developing PAD increases by 5 – 10% with each 10 mg/dl increase in total cholesterol (Ingolfsson et al. 1994; Murabito et al. 1997).

#### **3.4 Renal insufficiency**

Patients with chronic kidney disease (creatinine clearance <60 ml/min) and end stage renal disease have a higher prevalence of IC and low ankle-brachial index (ABI) as compared to patients with normal renal function (O'Hare et al. 2004). They also have a higher prevalence of critical limb ischemia and increased incidence of peri and post operative mortality related to lower extremity revascularization procedures (O'Hare et al. 2003). Potential mechanisms include altered calcium-phosphorus, homocysteine, and lipoprotein (a) metabolism and altered inflammatory and coagulation pathways.

#### **3.5 Inflammation**

120 Traditional and Novel Risk Factors in Atherothrombosis

A strong predictor of prevalent PAD and its progression; it also predicts bypass graft and endovascular intervention failure. (Sapoval et al. 1996; Hirsch et al. 2001; Willigendael et al. 2005; Aboyans et al. 2006). Smoking increases the risk of lower extremity peripheral arterial disease by 2- to 6-fold and the risk of intermittent claudication by 3- to 10-fold (Kannel and McGee 1985; Smith et al. 1990; Bowlin et al. 1994; Meijer et al. 1998). 80% of patients with PAD have a smoking history (Smith et al. 1990; Meijer et al. 1998). Interestingly, cigarette smoking is a stronger risk factor for PAD than for CAD with smoker's having a 2 to 3 times greater likelihood of developing PAD than CAD (Hirsch et al. 2001). It is also an independent predictor of the risk for unplanned urgent revascularization of the lower extremities following initial successful treatment (Shamma et al. 2003). With an increasing number of pack years, there is an increase in severity of disease, increased risk of

The risk of prevalent PAD doubles in the setting of impaired glucose tolerance (Beckman et al. 2002) and increases by 2 to 4 fold in patients with overt diabetes mellitus. PAD presents at an earlier age in diabetics; the prevalence of PAD is ~ 20% in diabetic patients older than 40 years and increases to ~30% among those older than 50 years (Hirsch et al. 2001; 2003). Diabetics tend to have more severe degrees of stenosis and a higher preponderance toward calcification as compared to non-diabetic patients with PAD (Jude et al. 2001). PAD is more likely to remain asymptomatic in diabetics until very advanced. Finally, poor glvcemic control is associated with more rapid PAD progression, increased risk of amputation and

Although the attributable risk for these factors is less than for DM or smoking, their adverse impact is clear from multiple studies. Uncontrolled higher blood pressures are associated with increased severity of PAD (Ness et al. 2000; Hirsch et al. 2001; Selvin and Erlinger 2004; Ostchega et al. 2007). Elevated total and LDL cholesterol, low HDL cholesterol and hypertriglyceridemia are associated with prevalent PAD (Kannel and Shurtleff 1973; Fowkes et al. 1992; Hiatt et al. 1995; Murabito et al. 2002). The risk of developing PAD increases by 5 – 10% with each 10 mg/dl increase in total cholesterol

Patients with chronic kidney disease (creatinine clearance <60 ml/min) and end stage renal disease have a higher prevalence of IC and low ankle-brachial index (ABI) as compared to patients with normal renal function (O'Hare et al. 2004). They also have a higher prevalence of critical limb ischemia and increased incidence of peri and post operative mortality related to lower extremity revascularization procedures (O'Hare et al. 2003). Potential mechanisms include altered calcium-phosphorus, homocysteine, and lipoprotein (a) metabolism and

amputation, peripheral graft occlusion and overall mortality.

mortality (Jude et al. 2001; Beckman et al. 2002).

(Ingolfsson et al. 1994; Murabito et al. 1997).

altered inflammatory and coagulation pathways.

**3.4 Renal insufficiency** 

**3.3 Hypertension and dyslipidemia** 

**3.1 Cigarette smoking** 

**3.2 Diabetes Mellitus (DM)** 

A number of inflammatory markers are associated with prevalent PAD and outcomes among those with PAD. In a meta-analysis, the odds ratio for PAD in patients with increased homocysteine was 6.8 (Boushey et al. 1995). 30-40% of PAD patients have elevated homocysteine levels as compared with 3-5% of the general population (Hajjar 1993). Elevated C – reactive protein and D-dimer are associated with increased cardiovascular mortality in PAD patients (Vidula et al. 2008).

#### **3.6 Age**

The Prevalence of PAD increases with age. In the NHANES study, the prevalence of PAD increased significantly with each decade since beyond 40 years (Selvin and Erlinger 2004) (Table 1). In a German epidemiologic study, 21% of the patients above the age of 65 years had symptomatic or asymptomatic PAD yet again demonstrating increased incidence of PAD with age (Diehm et al. 2009).


Table 1. In the NHANES study, prevalence of PAD increased significantly with age.

#### **3.7 Race/ethnicity**

The prevalence of PAD varies by ethnicity and race. Non-hispanic blacks have the highest prevalence, followed by Mexican Americans and non-Hispanic whites, with 19.5%, 15.6% and 11.7% above the age of 60 years having PAD, respectively (Ostchega et al. 2007). It is believed that these relationships are mediated by a genetic predisposition in part secondary to variations in atherogenic and prothrombotic factors (Folsom et al. 1992; Conlan et al. 1993; Gerhard et al. 1999).

#### **4. Clinical presentation**

PAD can present in the following forms (Hirsch et al. 2006):

**4.1** Intermittent claudication: Exertional symptoms such as muscle pain, cramping, discomfort or fatigue in the legs which resolves with rest of 10 minutes or less. However, these 'classic' symptoms are seen in only 10 % of patients with PAD.

**4.2** "Atypical" leg pain: Exertional leg symptoms which may not always resolve with rest or are not consistently reproducible.

**4.3** Asymptomatic: 40 % of patients with PAD have no leg symptoms but most will have some other form of functional limitations.

**4.4** Acute limb ischemia: A potentially limb threatening syndrome, classically manifest by one of more of the following five "P's" – pain, pulselessness, paralysis, paresthesias and pallor.

Lower Extremity Peripheral Arterial Disease 123

Bilateral arm blood pressure (to screen for subclavian stenosis/upper extremity PAD)

Pulse should be graded on a scale of 0-3. 0=absent, 1=dampened, 2=normal or 3=bounding. If no pulse is palpable on exam, a Doppler exam using a hand-held continuous wave device should be performed. An absent or abnormal PT pulse has a very high specificity for diagnosis of PAD. An absent or abnormal DP is non-specific due to a high prevalence of

It is also vital to asses for vascular bruits (carotid, abdominal, femoral, and popliteal)

 Foot examinations should be performed at each visit for patients with PAD to assess for tissue loss (i.e., ulcers or gangrene) as well as for signs of other foot

In the PAD Awareness, Risk and Treatment: New Resources for Survival (PARTNERS) study, 6979 patients aged ≥ 70 years or aged 50-69 years with a history of cigarette smoking or diabetes were screened with ABI's (Hirsch et al. 2001). PAD (defined as an ABI of ≤ 0.90) was diagnosed in 29% of this cohort. This diagnosis would have been missed in 85% to 90% had physicians relied solely on patients presenting with intermittent claudication (Hirsch et al. 2001). Even patients with IC infrequently report this symptom as they attribute it to the normal aging process (Dormandy and Rutherford 2000). A German epidemiologic study which included 6880 patients > 65 years of age reported that 21% of these patients had asymptomatic or symptomatic PAD (Diehm et al. 2009). Given that up to 50% of patients with PAD may be asymptomatic, it is critical that these high risk patients are identified so that appropriate interventions may be initiated to prevent the associated morbidity and mortality. ACC /AHA guidelines recommend screening ABI's among high risk patients (class IC) defined as individuals with 1 or more of the following: exertional leg symptoms, non-healing wounds, age 65 years and older, or 50 years and older with a history of

The ADA also recommends that any diabetic patient above the age of 50 years who has a normal ABI should have ABI's repeated every 5 years (American Diabetic Association 2003).

Assessment of the abdomen for aortic aneurysmal and stenotic disease

The key components of physical exam are:

Thorough Examination of legs and feet

absent or anomalous DP in the general population.

Rapid elevation pallor or dependent rubor.

**8. Screening for asymptomatic patients** 

smoking or diabetes (Rooke et al. 2011).

Other characteristics that may be seen in lower extremity PAD:

pathology such as callous formation or neuropathy.

Cardiac examination

 Carotid Radial/ulnar Femoral Popliteal

Hair loss

Nail hypertrophy

 Dorsalis pedis (DP) Posterior tibial (PT)

Pulse

**4.5** Chronic limb ischemia (also known as 'critical limb ischemia [CLI]): Accounts for 1-2% of all patients with PAD. It presents as rest pain, non-healing ulcers and/or gangrene.

The severity of symptoms usually correlates with degree of stenosis, adequacy of collateral circulation and level of exertion. Location of the discomfort may shed light on the location of stenosis (Table 2).


Table 2. Level of disease and associated symptoms

#### **5. Natural history of PAD**

Over a 5-year period, only 1-2% of claudicants and asymptomatic patients develop CLI. Nevertheless, the overall mortality rate is 15 – 30% (75% of which is cardiovascular) and risk of non-fatal MI or stroke 20% at 5 years. Patients with CLI have much worse outcomes with an annual CV mortality of 25% and annual amputation rate of 25%.

It is noteworthy that the burden of PAD may not correlate with the presence or absence of claudication. Patients who are more physically active are more likely to present with claudication than those who lead a sedentary lifestyle. Patients who are too sedentary to claudicate may present with CLI as their first manifestation of disease.

#### **6. Classification of PAD**

The Rutherford and Fontaine classifications (Table 3) are frequently utilized to classify PAD symptom severity.


Table 3. Rutherford and Fontaine classification for PAD.

#### **7. Physical exam**

A comprehensive vascular history and physical exam is vital in the evaluation and appropriate treatment of patient with PAD or suspected PAD. Current ACC / AHA guidelines recommend a complete vascular exam (class I B) for patients with intermittent claudication (Hirsch et al. 2006).

The key components of physical exam are:


122 Traditional and Novel Risk Factors in Atherothrombosis

**4.5** Chronic limb ischemia (also known as 'critical limb ischemia [CLI]): Accounts for 1-2% of

The severity of symptoms usually correlates with degree of stenosis, adequacy of collateral circulation and level of exertion. Location of the discomfort may shed light on the location of

Over a 5-year period, only 1-2% of claudicants and asymptomatic patients develop CLI. Nevertheless, the overall mortality rate is 15 – 30% (75% of which is cardiovascular) and risk of non-fatal MI or stroke 20% at 5 years. Patients with CLI have much worse outcomes with

It is noteworthy that the burden of PAD may not correlate with the presence or absence of claudication. Patients who are more physically active are more likely to present with claudication than those who lead a sedentary lifestyle. Patients who are too sedentary to

The Rutherford and Fontaine classifications (Table 3) are frequently utilized to classify PAD

A comprehensive vascular history and physical exam is vital in the evaluation and appropriate treatment of patient with PAD or suspected PAD. Current ACC / AHA guidelines recommend a complete vascular exam (class I B) for patients with intermittent

all patients with PAD. It presents as rest pain, non-healing ulcers and/or gangrene.

stenosis (Table 2).

Table 2. Level of disease and associated symptoms

an annual CV mortality of 25% and annual amputation rate of 25%.

claudicate may present with CLI as their first manifestation of disease.

Table 3. Rutherford and Fontaine classification for PAD.

**5. Natural history of PAD** 

**6. Classification of PAD** 

symptom severity.

**7. Physical exam** 

claudication (Hirsch et al. 2006).


Pulse should be graded on a scale of 0-3. 0=absent, 1=dampened, 2=normal or 3=bounding. If no pulse is palpable on exam, a Doppler exam using a hand-held continuous wave device should be performed. An absent or abnormal PT pulse has a very high specificity for diagnosis of PAD. An absent or abnormal DP is non-specific due to a high prevalence of absent or anomalous DP in the general population.

	- Hair loss
	- Nail hypertrophy
	- Rapid elevation pallor or dependent rubor.
	- Foot examinations should be performed at each visit for patients with PAD to assess for tissue loss (i.e., ulcers or gangrene) as well as for signs of other foot pathology such as callous formation or neuropathy.

#### **8. Screening for asymptomatic patients**

In the PAD Awareness, Risk and Treatment: New Resources for Survival (PARTNERS) study, 6979 patients aged ≥ 70 years or aged 50-69 years with a history of cigarette smoking or diabetes were screened with ABI's (Hirsch et al. 2001). PAD (defined as an ABI of ≤ 0.90) was diagnosed in 29% of this cohort. This diagnosis would have been missed in 85% to 90% had physicians relied solely on patients presenting with intermittent claudication (Hirsch et al. 2001). Even patients with IC infrequently report this symptom as they attribute it to the normal aging process (Dormandy and Rutherford 2000). A German epidemiologic study which included 6880 patients > 65 years of age reported that 21% of these patients had asymptomatic or symptomatic PAD (Diehm et al. 2009). Given that up to 50% of patients with PAD may be asymptomatic, it is critical that these high risk patients are identified so that appropriate interventions may be initiated to prevent the associated morbidity and mortality. ACC /AHA guidelines recommend screening ABI's among high risk patients (class IC) defined as individuals with 1 or more of the following: exertional leg symptoms, non-healing wounds, age 65 years and older, or 50 years and older with a history of smoking or diabetes (Rooke et al. 2011).

The ADA also recommends that any diabetic patient above the age of 50 years who has a normal ABI should have ABI's repeated every 5 years (American Diabetic Association 2003).

Lower Extremity Peripheral Arterial Disease 125

(Sikkink et al. 1997). McDermott and colleagues found that poor functional outcomes were associated with lower ABI's (McDermott et al. 2002). Interestingly, functional outcomes did not correlate with leg symptoms, again confirming that leg symptoms are a poor marker for

Resting ABI may be insensitive for detecting aorto-iliac occlusive disease. ABI after exercise should be performed in these patients if PAD is suspected. If resting values in patients with intermittent claudication are normal, then ABI should be repeated after exercise. Incompressible arteries (e.g., elderly, diabetes, renal failure) may yield falsely normal or elevated ABI measurements. Toe- brachial index (TBI) measurement or pulse volume recording measurements should be performed in these patients as they are more accurate in

**10.2 Segmental Pressure Examination (SPE) and Pulse Volume Recordings (PVR)** 

assessing the location and severity of arterial stenosis (Symes et al. 1984).

SPE is a physiological test helpful in identifying location of individual arterial stenoses. It is performed by placing blood pressure cuffs at the upper thigh, the lower thigh, the upper calf and the lower calf above the ankle. Systolic blood pressure measurements are obtained at all these sites and both brachial arteries. A difference of ≥ 20 mm Hg between two adjacent segments is considered physiologically significant. For example, a significant pressure gradient between left upper thigh cuff and left lower thigh cuff would indicate a physiologically significant stenosis in the left superficial femoral artery. As with the ABI, pressures may be elevated or uninterpretable in patients with non-compressible vessels. Pulse volume recordings are performed by measuring volume changes in the limb along with segmental pressure recordings (Darling et al. 1972). Segmental pressure examination when performed with pulse volume recordings has an overall accuracy of 90 – 95% in

It is recommended by ACC / AHA guidelines (Class I) for routine surveillance after femoral-popliteal or femoral-tibial pedal bypass with a venous conduit (Hirsch et al. 2006). The 2011 ACC/AHA guidelines recommend that segmental pressures, Doppler waveform analysis, pulse volume recordings, or ABI with duplex ultrasonography (or some combination of these methods) to document the presence and location of PAD in the lower extremity can be used (Rooke et al. 2011). In general, imaging studies are reserved for patients with PAD in whom revascularization is planned, for bypass graft or percutaneous vascular intervention surveillance or for cases in which the diagnosis of PAD or etiology of PAD is unclear such as arterial aneurysm, fibromuscular dysplasia, entrapment syndrome or vasculitis. Imaging studies are also used to diagnose PAD in patients with non-

Physiological testing such as ABI and PVR are adequate for asymptomatic patients. For symptomatics (claudication or pseudoclaudication) ABI and PVR with or without exercise, duplex or dopler ultrasound are utilised. CTA, MRA or duplex / doppler ultrasound are usually used for patients considered for revascularization as well as for post endovascular

identifying PAD severity.

this setting(Rooke et al. 2011).

**10.3 Duplex ultrasound** 

compressible vessels such as those with ABI > 1.3.

interventions and follow ups as well as for post vein graft follow ups.

#### **9. Diagnostic evaluation**

It is necessary to differentiate claudication from other conditions causing leg pain when evaluating patients with PAD. Patients may have an abnormal ABI and present with leg pain unrelated to PAD. Claudication may be confused with pseudoclaudication resulting from spinal stenosis, nerve compression syndromes, arthritic pain or venous claudication. Certain clinical features help differentiate these conditions Questionnaires have also been frequently used for identifying, monitoring, assessing severity of PAD and treatment success. The Rose claudication questionnaire is a simple screening tool for claudication which can be administered by asking 2 simple questions; "Do you get pain in either leg when you walk?" and "Does the pain go away when you stop walking?" If the answer to both the question is yes then the like hood for PAD is 95% (Rose 1962). There is an updated version of the World Health Organization Rose Questionnaire and also the Edinburgh Claudication Questionnaire which are used to diagnose PAD and the Walking Impairment Questionnaire, the Peripheral Artery Questionnaire and the Medical Outcomes Study 36- Item Short Form Health Survey are used to assess severity of PAD and assess response to therapy (Criqui al. 1985; Leng and Fowkes 1992; Criqui et al. 1996; Spertus et al. 2004).

#### **10. Diagnostic tests**

Physiological studies such as the ABI, segmental limb pressures, or pulse-volume waveform analysis can help identify functional limb perfusion abnormalities. Imaging methods, such as duplex ultrasonography, CT angiography (CTA) and magnetic resonance angiography (MRA) provide detailed anatomical, but less frequently functional information on limb perfusion. Diagnostic modalities and respective clinical indications appear in Table 4.

#### **10.1 Ankle-Brachial Index**

ABI measurement is the universally accepted standard for the initial diagnostic evaluation of patients with suspected PAD and for high risk asymptomatic patients (Hirsch, Haskal et al. 2006; Mohler and Giri 2008; Rooke et al. 2011). It is simple, reliable, extremely sensitive (79 - 95%) and specific (96-100%) and can be performed in a primary care office in < 15 minutes (Fowkes 1988; Lijmer et al. 1996). The ABI is calculated by measuring the systolic blood pressure in each arm (brachial artery) and ankle (dorsalis pedis and posterior tibial arteries) using a hand held Doppler device in a patient who has been resting in the supine position for 10 minutes. The right ABI = higher right ankle pressure/higher pressure in either arm; the left ABI = higher left ankle pressure/higher pressure in either arm. An ABI ≤ 0.90 suggests PAD with ≥ 50% stenosis in at least one artery. (Fowkes 1988; Lijmer, Hunink et al. 1996).

Based on publication of the results of the Ankle Brachial Index Collaboration, a normal ABI range is defined as between 1.00 to 1.40, and abnormal values continue to be defined as those ≤0.90. ABI values of 0.91 to 0.99 are considered "borderline" and values ≥1.40 indicates noncompressible arteries (Fowkes, Murray et al. 2008; Rooke, Hirsch et al. 2011).

In addition to its diagnostic utility, the ABI also provides important prognostic and functional information. The relative risk of mortality increases by 3.1% with a decrease of 0.50 in ABI. Sikkink et al followed 150 patients who were >40 years of age and had ABI< 0.90 and found that the cumulative survival at 5 years was 63% for patients with an ABI of < 0.50, 71% for patients with an ABI of 0.50 to 0.69, and 91% for those with an ABI of ≥ 0.70

It is necessary to differentiate claudication from other conditions causing leg pain when evaluating patients with PAD. Patients may have an abnormal ABI and present with leg pain unrelated to PAD. Claudication may be confused with pseudoclaudication resulting from spinal stenosis, nerve compression syndromes, arthritic pain or venous claudication. Certain clinical features help differentiate these conditions Questionnaires have also been frequently used for identifying, monitoring, assessing severity of PAD and treatment success. The Rose claudication questionnaire is a simple screening tool for claudication which can be administered by asking 2 simple questions; "Do you get pain in either leg when you walk?" and "Does the pain go away when you stop walking?" If the answer to both the question is yes then the like hood for PAD is 95% (Rose 1962). There is an updated version of the World Health Organization Rose Questionnaire and also the Edinburgh Claudication Questionnaire which are used to diagnose PAD and the Walking Impairment Questionnaire, the Peripheral Artery Questionnaire and the Medical Outcomes Study 36- Item Short Form Health Survey are used to assess severity of PAD and assess response to therapy (Criqui al. 1985; Leng and Fowkes 1992; Criqui et al. 1996; Spertus et al. 2004).

Physiological studies such as the ABI, segmental limb pressures, or pulse-volume waveform analysis can help identify functional limb perfusion abnormalities. Imaging methods, such as duplex ultrasonography, CT angiography (CTA) and magnetic resonance angiography (MRA) provide detailed anatomical, but less frequently functional information on limb perfusion. Diagnostic modalities and respective clinical indications appear in Table 4.

ABI measurement is the universally accepted standard for the initial diagnostic evaluation of patients with suspected PAD and for high risk asymptomatic patients (Hirsch, Haskal et al. 2006; Mohler and Giri 2008; Rooke et al. 2011). It is simple, reliable, extremely sensitive (79 - 95%) and specific (96-100%) and can be performed in a primary care office in < 15 minutes (Fowkes 1988; Lijmer et al. 1996). The ABI is calculated by measuring the systolic blood pressure in each arm (brachial artery) and ankle (dorsalis pedis and posterior tibial arteries) using a hand held Doppler device in a patient who has been resting in the supine position for 10 minutes. The right ABI = higher right ankle pressure/higher pressure in either arm; the left ABI = higher left ankle pressure/higher pressure in either arm. An ABI ≤ 0.90 suggests PAD

Based on publication of the results of the Ankle Brachial Index Collaboration, a normal ABI range is defined as between 1.00 to 1.40, and abnormal values continue to be defined as those ≤0.90. ABI values of 0.91 to 0.99 are considered "borderline" and values ≥1.40 indicates

In addition to its diagnostic utility, the ABI also provides important prognostic and functional information. The relative risk of mortality increases by 3.1% with a decrease of 0.50 in ABI. Sikkink et al followed 150 patients who were >40 years of age and had ABI< 0.90 and found that the cumulative survival at 5 years was 63% for patients with an ABI of < 0.50, 71% for patients with an ABI of 0.50 to 0.69, and 91% for those with an ABI of ≥ 0.70

with ≥ 50% stenosis in at least one artery. (Fowkes 1988; Lijmer, Hunink et al. 1996).

noncompressible arteries (Fowkes, Murray et al. 2008; Rooke, Hirsch et al. 2011).

**9. Diagnostic evaluation** 

**10. Diagnostic tests** 

**10.1 Ankle-Brachial Index** 

(Sikkink et al. 1997). McDermott and colleagues found that poor functional outcomes were associated with lower ABI's (McDermott et al. 2002). Interestingly, functional outcomes did not correlate with leg symptoms, again confirming that leg symptoms are a poor marker for identifying PAD severity.

Resting ABI may be insensitive for detecting aorto-iliac occlusive disease. ABI after exercise should be performed in these patients if PAD is suspected. If resting values in patients with intermittent claudication are normal, then ABI should be repeated after exercise. Incompressible arteries (e.g., elderly, diabetes, renal failure) may yield falsely normal or elevated ABI measurements. Toe- brachial index (TBI) measurement or pulse volume recording measurements should be performed in these patients as they are more accurate in this setting(Rooke et al. 2011).

#### **10.2 Segmental Pressure Examination (SPE) and Pulse Volume Recordings (PVR)**

SPE is a physiological test helpful in identifying location of individual arterial stenoses. It is performed by placing blood pressure cuffs at the upper thigh, the lower thigh, the upper calf and the lower calf above the ankle. Systolic blood pressure measurements are obtained at all these sites and both brachial arteries. A difference of ≥ 20 mm Hg between two adjacent segments is considered physiologically significant. For example, a significant pressure gradient between left upper thigh cuff and left lower thigh cuff would indicate a physiologically significant stenosis in the left superficial femoral artery. As with the ABI, pressures may be elevated or uninterpretable in patients with non-compressible vessels. Pulse volume recordings are performed by measuring volume changes in the limb along with segmental pressure recordings (Darling et al. 1972). Segmental pressure examination when performed with pulse volume recordings has an overall accuracy of 90 – 95% in assessing the location and severity of arterial stenosis (Symes et al. 1984).

#### **10.3 Duplex ultrasound**

It is recommended by ACC / AHA guidelines (Class I) for routine surveillance after femoral-popliteal or femoral-tibial pedal bypass with a venous conduit (Hirsch et al. 2006).

The 2011 ACC/AHA guidelines recommend that segmental pressures, Doppler waveform analysis, pulse volume recordings, or ABI with duplex ultrasonography (or some combination of these methods) to document the presence and location of PAD in the lower extremity can be used (Rooke et al. 2011). In general, imaging studies are reserved for patients with PAD in whom revascularization is planned, for bypass graft or percutaneous vascular intervention surveillance or for cases in which the diagnosis of PAD or etiology of PAD is unclear such as arterial aneurysm, fibromuscular dysplasia, entrapment syndrome or vasculitis. Imaging studies are also used to diagnose PAD in patients with noncompressible vessels such as those with ABI > 1.3.

Physiological testing such as ABI and PVR are adequate for asymptomatic patients. For symptomatics (claudication or pseudoclaudication) ABI and PVR with or without exercise, duplex or dopler ultrasound are utilised. CTA, MRA or duplex / doppler ultrasound are usually used for patients considered for revascularization as well as for post endovascular interventions and follow ups as well as for post vein graft follow ups.

Lower Extremity Peripheral Arterial Disease 127

Approximately 20-30% in patients with DM have PAD (Marso and Hiatt 2006). The severity of PAD in this cohort correlates with the duration and severity of DM (Selvin et al. 2004; Wattanakit et al. 2005). With every 1% increase in glycosylated hemoglobin levels, the risk of PAD increases by 28% (Selvin et al. 2004)and risk of intermittent claudication by 3.5- and 8.6-fold in men and women, respectively (Kannel and McGee 1985). DM may also lead to peripheral neuropathy and decreased resistance to infection, which increases the risk of infected foot ulcers. Patients with DM are at a higher risk of amputation and have reduced primary patency after revascularization as compared to non-diabetics (Bild et al. 1989; DeRubertis et al. 2008). The UKPDS study showed that the overall microvascular complication rate decreased by 25% by lowering blood glucose levels in type 2 diabetes with intensive therapy, which achieved a median HbA1c of 7.0% compared with conventional therapy with a median HbA1c of 7.9%. No significant effect on cardiovascular complications was observed. A non-significant (p = 0.052) 16% reduction in the risk of combined fatal or

nonfatal myocardial infarction and sudden death was observed (UKPDS 1998).

The American Diabetic Association recommends maintaining hemoglobin A1c below 7% to reduce microvascular events (ADA 2010). This recommendation is endorsed by all three PAD guidelines. The ADA also recommends comprehensive foot care including proper footwear, regular podiatric foot and nail care, daily foot inspection, skin cleansing, and use

The Heart Protection Study (HPS) randomized 20,536 high-risk patients to 40 mg/d of simvastatin or placebo, including 6,748 patients with PAD. PAD patients taking statins had a 25% cardiovascular risk reduction at 5 years independent of baseline LDL level (HPS 2002). Statin use is also associated with reduction in the risk of new or worsening claudication (Pedersen, Kjekshus et al. 1998). A RCT comparing high dose atorvastatin (80 mg) vs. placebo irrespective of baseline LDL cholesterol showed that high dose atorvastatin improves pain-free walking distance and community-based physical activity in patients with intermittent claudication, however there was no change noted in the maximal walking time. This beneficial effect was noted over statins cardiovascular risk reduction benefits.

The AHA / ACC and TASC 2 guidelines recommend the following for dyslipidemia management in patients with PAD (Dormandy and Rutherford 2000; Smith et al. 2001;

All patients should have low-density lipoprotein (LDL)- cholesterol <2.59 mmol/L

 In patients with PAD and a history of vascular disease in other beds (e.g. coronary artery disease) it is reasonable to lower LDL cholesterol levels to <1.81 mmol/L (<70 mg/dL). In patients with elevated triglyceride levels where the LDL cannot be accurately calculated, the LDL level should be directly measured and treated to the above targets. Alternatively, the non-HDL (high-density lipoprotein) cholesterol level can be calculated with a goal of <3.36 mmol/L (<130 mg/dL), and in high risk patients <2.59

Dietary modification should be the initial intervention to control abnormal lipid levels [B].

**11.1.2 Diabetes Mellitus** 

of topical moisturizing creams (ADA 2010).

Hirsch et al. 2006; Norgren, Hiatt et al. 2007).

**11.1.3 Dyslipidemia** 

(Mohler, Hiatt et al. 2003).

(<100 mg/dL).

mmol/L (<100 mg/Dl).

#### **11. Management of Peripheral Arterial Disease**

Guidelines for the management of PAD have been published by


Each of these guidelines focuses on management of PAD as a two-tiered process. First and foremost, they recommend cardiovascular risk reduction through vascular risk factor modification and antiplatelet therapy and second, symptom-guided therapy including supervised exercise, pharmacological interventions and revascularization procedures, when needed.

#### **11.1 Cardiovascular risk factor modification**

#### **11.1.1 Smoking cessation**

Patients with PAD should be referred to a formal smoking cessation program including pharmacotherapy when appropriate. A Cochrane review of 20 prospective cohort studies showed that smoking cessation is associated with a 36% risk reduction in cardiovascular events in patients with known atherosclerotic disease (Critchley and Capewell 2004) and is recommended by all three guidelines(Dormandy and Rutherford 2000; Hirsch et al. 2006; Network 2006; Norgren et al. 2007; Rooke et al. 2011); Smoking cessation is achieved in approximately 5% of patients with physician encouragement and advice along with regular follow-ups and as compared to 0.1% without physician intervention at 1 year (Law and Tang 1995). Success rates are higher with interventions such as nicotine replacement, bupropion or varenicline. When compared with usual care, formal smoking cessation program which consisted of a strong physician message and 12 two-hour group sessions, using behavior modification and nicotine gum had a higher success rate at 5 years (22% vs. 5%) (Anthonisen et al. 2005). Abstinence rates with bupropion at 3-, 6- and 12- month follow up are 34%, 27% and 22% respectively, compared with 15%, 11% and 9%, for placebo(Tonstad et al. 2003). A combination of bupropion and nicotine replacement is superior to nicotine replacement alone, however but has similar in efficacy to only monotherapy with bupropion (Jorenby et al. 1999). Varenicline has been proven effective in smokers with cardiovascular disease including those with PAD. Rigotti et al conducted a multicenter, randomized, double-blind, placebo-controlled trial comparing the efficacy and safety of varenicline (12 weeks treatment) with placebo showed a continuous abstinence rate was higher for varenicline than placebo during weeks 9 through 12 (47.0% versus 13.9%;) and weeks 9 through 52 (19.2% versus 7.2%). The varenicline and placebo groups did not differ significantly in cardiovascular mortality, all-cause mortality, cardiovascular events, or serious adverse events. (Rigotti et al. 2010). Another RCT compared varenicline vs. bupropion vs placebo showed varenicline was more efficacious than bupropion or placebo in short term (9-12 weeks) (43.9% vs. 29.8% vs. 17.6)% as well as long term period 9 – 52 weeks) (23% vs. 14.6% vs. 10.3%) (Jorenby et al. 2006).

#### **11.1.2 Diabetes Mellitus**

126 Traditional and Novel Risk Factors in Atherothrombosis

a. American college of Cardiology / American Heart Association which was a Collaborative Report from the American Association for Vascular Surgery/ Society for Vascular Surgery, Society for Cardiovascular Angiography and Interventions, Society for Vascular Medicine and Biology, Society of Interventional Radiology, and the

c. Trans-Atlantic Inter-Society Consensus (TASC) II (Dormandy and Rutherford 2000;

Each of these guidelines focuses on management of PAD as a two-tiered process. First and foremost, they recommend cardiovascular risk reduction through vascular risk factor modification and antiplatelet therapy and second, symptom-guided therapy including supervised exercise, pharmacological interventions and revascularization procedures, when

Patients with PAD should be referred to a formal smoking cessation program including pharmacotherapy when appropriate. A Cochrane review of 20 prospective cohort studies showed that smoking cessation is associated with a 36% risk reduction in cardiovascular events in patients with known atherosclerotic disease (Critchley and Capewell 2004) and is recommended by all three guidelines(Dormandy and Rutherford 2000; Hirsch et al. 2006; Network 2006; Norgren et al. 2007; Rooke et al. 2011); Smoking cessation is achieved in approximately 5% of patients with physician encouragement and advice along with regular follow-ups and as compared to 0.1% without physician intervention at 1 year (Law and Tang 1995). Success rates are higher with interventions such as nicotine replacement, bupropion or varenicline. When compared with usual care, formal smoking cessation program which consisted of a strong physician message and 12 two-hour group sessions, using behavior modification and nicotine gum had a higher success rate at 5 years (22% vs. 5%) (Anthonisen et al. 2005). Abstinence rates with bupropion at 3-, 6- and 12- month follow up are 34%, 27% and 22% respectively, compared with 15%, 11% and 9%, for placebo(Tonstad et al. 2003). A combination of bupropion and nicotine replacement is superior to nicotine replacement alone, however but has similar in efficacy to only monotherapy with bupropion (Jorenby et al. 1999). Varenicline has been proven effective in smokers with cardiovascular disease including those with PAD. Rigotti et al conducted a multicenter, randomized, double-blind, placebo-controlled trial comparing the efficacy and safety of varenicline (12 weeks treatment) with placebo showed a continuous abstinence rate was higher for varenicline than placebo during weeks 9 through 12 (47.0% versus 13.9%;) and weeks 9 through 52 (19.2% versus 7.2%). The varenicline and placebo groups did not differ significantly in cardiovascular mortality, all-cause mortality, cardiovascular events, or serious adverse events. (Rigotti et al. 2010). Another RCT compared varenicline vs. bupropion vs placebo showed varenicline was more efficacious than bupropion or placebo in short term (9-12 weeks) (43.9% vs. 29.8% vs. 17.6)% as well as long term period 9

**11. Management of Peripheral Arterial Disease** 

ACC/AHA Task Force on Practice Guidelines b. Scottish Intercollegiate Guidelines Network (SIGN), and

**11.1 Cardiovascular risk factor modification** 

– 52 weeks) (23% vs. 14.6% vs. 10.3%) (Jorenby et al. 2006).

**11.1.1 Smoking cessation** 

needed.

Guidelines for the management of PAD have been published by

Hirsch et al. 2006; Network 2006; Norgren et al. 2007).

Approximately 20-30% in patients with DM have PAD (Marso and Hiatt 2006). The severity of PAD in this cohort correlates with the duration and severity of DM (Selvin et al. 2004; Wattanakit et al. 2005). With every 1% increase in glycosylated hemoglobin levels, the risk of PAD increases by 28% (Selvin et al. 2004)and risk of intermittent claudication by 3.5- and 8.6-fold in men and women, respectively (Kannel and McGee 1985). DM may also lead to peripheral neuropathy and decreased resistance to infection, which increases the risk of infected foot ulcers. Patients with DM are at a higher risk of amputation and have reduced primary patency after revascularization as compared to non-diabetics (Bild et al. 1989; DeRubertis et al. 2008). The UKPDS study showed that the overall microvascular complication rate decreased by 25% by lowering blood glucose levels in type 2 diabetes with intensive therapy, which achieved a median HbA1c of 7.0% compared with conventional therapy with a median HbA1c of 7.9%. No significant effect on cardiovascular complications was observed. A non-significant (p = 0.052) 16% reduction in the risk of combined fatal or nonfatal myocardial infarction and sudden death was observed (UKPDS 1998).

The American Diabetic Association recommends maintaining hemoglobin A1c below 7% to reduce microvascular events (ADA 2010). This recommendation is endorsed by all three PAD guidelines. The ADA also recommends comprehensive foot care including proper footwear, regular podiatric foot and nail care, daily foot inspection, skin cleansing, and use of topical moisturizing creams (ADA 2010).

#### **11.1.3 Dyslipidemia**

The Heart Protection Study (HPS) randomized 20,536 high-risk patients to 40 mg/d of simvastatin or placebo, including 6,748 patients with PAD. PAD patients taking statins had a 25% cardiovascular risk reduction at 5 years independent of baseline LDL level (HPS 2002). Statin use is also associated with reduction in the risk of new or worsening claudication (Pedersen, Kjekshus et al. 1998). A RCT comparing high dose atorvastatin (80 mg) vs. placebo irrespective of baseline LDL cholesterol showed that high dose atorvastatin improves pain-free walking distance and community-based physical activity in patients with intermittent claudication, however there was no change noted in the maximal walking time. This beneficial effect was noted over statins cardiovascular risk reduction benefits. (Mohler, Hiatt et al. 2003).

The AHA / ACC and TASC 2 guidelines recommend the following for dyslipidemia management in patients with PAD (Dormandy and Rutherford 2000; Smith et al. 2001; Hirsch et al. 2006; Norgren, Hiatt et al. 2007).


Lower Extremity Peripheral Arterial Disease 129

In a post hoc subgroup analysis of 6,452 PAD patients in The Clopidogrel versus Aspirin in Patients at Risk of Ischemic Events (CAPRIE) trial, the incidence of stroke, MI, or vascular death was reduced by 24% among those randomized to clopidogrel compared to aspirin

Dual antiplatelets therapy (DAPT) with aspirin and clopidogrel was not more effective than aspirin monotherapy in the Clopidogrel for High Atherothrombotic Risk and Ischemia Stabilization, Management and Avoidance (CHARISMA) study (Bhatt et al. 2006). However, post hoc analysis of the CHARISMA study revealed that DAPT was superior to aspirin monotherapy among patients with known symptomatic PAD at the time of enrollment (Bhatt et al. 2007). The combination of antiplatelet and anticoagulant therapy was evaluated in The Warfarin Anti-platelet Vascular Evaluation (WAVE) trial. This study found no incremental benefit of vitamin K antagonists when added to anti-platelet therapy for the prevention of cardiovascular events in patients with PAD (Anand et al. 2007). In fact, patients randomized to the combination of antiplatelet and anticoagulant therapy had an increase in life-threatening bleeding when compared with those randomized to anti-platelet

In general, all three PAD guidelines recommend antiplatelet therapy for PAD patients. Selection of an antiplatelet regimen (aspirin or clopidogrel or both or other antiplatelet medications) for the PAD patient should be individualized on the basis of tolerance and other clinical characteristics (i.e., bleeding risk) along with cost and guidance from regulatory agencies. (Dormandy and Rutherford 2000; Hirsch et al. 2006; Network 2006; Rooke et al. 2011). ACC / AHA guidelines strongly recommend antiplatelet therapy to reduce the risk of MI, stroke, and vascular death in individuals with symptomatic atherosclerotic lower extremity PAD, including those with intermittent claudication or critical limb ischemia, prior lower extremity revascularization (endovascular or surgical), or prior amputation for lower extremity ischemia (class IA). They feel that antiplatelet therapy can be useful to reduce the risk of MI, stroke, or vascular death in asymptomatic individuals with an ABI less than or equal to 0.90 (Class IIa C), however do not strongly recommend it

Many patients with PAD have severely impaired functional capacity, which leads to decreased quality of life (McDermott et al. 2004). A meta-analysis of 21 studies showed that patients with IC who underwent exercise training improved mean walking time by 180% and maximal walking time by 120% (Gardner and Poehlman 1995). Supervised treadmill exercises programs are also are more effective than lower extremity resistance training (McDermott et al. 2009). However, due to lack of reimbursement access to this important therapeutic intervention has been limited. In a Cochrane review of 8 RCT's evaluating supervised and unsupervised exercise among 319 participants with IC, statistically significant and clinically relevant improvements in maximal treadmill

given lack of definite evidence of any benefit (Rooke et al. 2011).

**11.2 Medical therapy for claudication 11.2.1 Supervised exercise program** 

**11.1.5.2 Clopidogrel** 

monotherapy (1996).

therapy alone.

 In symptomatic PAD patients, statins should be the primary agents to lower LDL cholesterol levels to reduce the risk of cardiovascular events [A].

The SIGN guidelines recommend statins for patients with PAD and total cholesterol level > 3.5 mmol/l (63 mg/dl) (Network 2006).

The ACC/AHA and TASC II guidelines also recommended considering niacin / fibrates for raising HDL cholesterol and lowering triglycerides in patients with PAD who have abnormalities in these lipid fractions. Recently, the randomized trial, AIM – HIGH, failed to reduce the incidence of cardiovascular events among patients with CAD at 3 years and was terminated prematurely, raising questions about the benefit of niacin in patients with atherosclerotic vascular disease (http://www.aimhigh-heart.com/ 2011).

#### **11.1.4 Management of hypertension**

Blood pressure lowering by any pharmacological means reduces cardiovascular risk (Chobanian et al. 2003); however angiotensin converting enzyme inhibitors (ACEI) have benefit in patients with PAD beyond their effect on lowering blood pressure. The Heart Outcomes Prevention Evaluation (HOPE) study showed a 22% reduction in risk of MI, stroke, or vascular death in patients randomized to ramipril compared to placebo, independent of the blood pressure–lowering effect (Yusuf et al. 2000). Although once thought to worsen claudication, the safety of beta blockers, has been demonstrated in a meta-analysis of 11 trials of patients with PAD (Radack and Deck 1991); these agents are of benefit of patients with CAD, particularly among those with a history of prior myocardial infarction.

#### **11.1.5 Anti-platelet therapy**

#### **11.1.5.1 Aspirin**

Aspirin was initially recommended mainly based on a sub-group analysis of a Antithrombotic Trialists' Collaboration meta-analysis which is a meta-analysis of 42 RCT's published in 2002 which showed that anti-platelet therapy (primarily aspirin) reduced cardiovascular events by 23% in patients with symptomatic PAD including patients with IC (23%), those with peripheral grafts (22%) and those undergoing peripheral angioplasty (29%) (2002). However, this study had a lot of heterogeneity in selection criteria and different antiplatelet drugs used at different doses. Also the benefit of aspirin in asymptomatic PAD wasn't in this meta-analysis. A more recent meta-analysis of 18 RCT's including 5,269 patients with PAD, aspirin therapy alone or in combination with dipyridamole led to a non-significant 12% reduction in the primary end point of cardiovascular events(Berger et al. 2009). Two large RCT evaluated the benefit of aspirin in patients with asymptomatic PAD and found no benefit (Belch et al. 2008; Fowkes et al. 2010). However, both of these RCT's had populations with asymptomatic PAD with very mild decrease in ABI and characterized as mild PAD in general. In the Prevention of Progression of Arterial Disease and Diabetes (POPADAD) trial enrolled patients with ABI <0.99 and the Aspirin for Asymptomatic Atherosclerosis trial enrolled patient with ABI 0.95 which calculated by using the lower pedal pressure at the ankle. In standard clinical practice and guidelines, ABI is calculated using the higher pedal pressure (Belch et al. 2008; Fowkes et al. 2010). Hence, both of these studies had significant limitations in design and enrollment and no change in recommendation regarding anti-platelet therapy in this cohort have occurred (Belch et al. 2008; Fowkes et al. 2010).

#### **11.1.5.2 Clopidogrel**

128 Traditional and Novel Risk Factors in Atherothrombosis

In symptomatic PAD patients, statins should be the primary agents to lower LDL

The SIGN guidelines recommend statins for patients with PAD and total cholesterol level >

The ACC/AHA and TASC II guidelines also recommended considering niacin / fibrates for raising HDL cholesterol and lowering triglycerides in patients with PAD who have abnormalities in these lipid fractions. Recently, the randomized trial, AIM – HIGH, failed to reduce the incidence of cardiovascular events among patients with CAD at 3 years and was terminated prematurely, raising questions about the benefit of niacin in patients with

Blood pressure lowering by any pharmacological means reduces cardiovascular risk (Chobanian et al. 2003); however angiotensin converting enzyme inhibitors (ACEI) have benefit in patients with PAD beyond their effect on lowering blood pressure. The Heart Outcomes Prevention Evaluation (HOPE) study showed a 22% reduction in risk of MI, stroke, or vascular death in patients randomized to ramipril compared to placebo, independent of the blood pressure–lowering effect (Yusuf et al. 2000). Although once thought to worsen claudication, the safety of beta blockers, has been demonstrated in a meta-analysis of 11 trials of patients with PAD (Radack and Deck 1991); these agents are of benefit of patients with

Aspirin was initially recommended mainly based on a sub-group analysis of a Antithrombotic Trialists' Collaboration meta-analysis which is a meta-analysis of 42 RCT's published in 2002 which showed that anti-platelet therapy (primarily aspirin) reduced cardiovascular events by 23% in patients with symptomatic PAD including patients with IC (23%), those with peripheral grafts (22%) and those undergoing peripheral angioplasty (29%) (2002). However, this study had a lot of heterogeneity in selection criteria and different antiplatelet drugs used at different doses. Also the benefit of aspirin in asymptomatic PAD wasn't in this meta-analysis. A more recent meta-analysis of 18 RCT's including 5,269 patients with PAD, aspirin therapy alone or in combination with dipyridamole led to a non-significant 12% reduction in the primary end point of cardiovascular events(Berger et al. 2009). Two large RCT evaluated the benefit of aspirin in patients with asymptomatic PAD and found no benefit (Belch et al. 2008; Fowkes et al. 2010). However, both of these RCT's had populations with asymptomatic PAD with very mild decrease in ABI and characterized as mild PAD in general. In the Prevention of Progression of Arterial Disease and Diabetes (POPADAD) trial enrolled patients with ABI <0.99 and the Aspirin for Asymptomatic Atherosclerosis trial enrolled patient with ABI 0.95 which calculated by using the lower pedal pressure at the ankle. In standard clinical practice and guidelines, ABI is calculated using the higher pedal pressure (Belch et al. 2008; Fowkes et al. 2010). Hence, both of these studies had significant limitations in design and enrollment and no change in recommendation regarding anti-platelet therapy in this cohort have occurred

cholesterol levels to reduce the risk of cardiovascular events [A].

atherosclerotic vascular disease (http://www.aimhigh-heart.com/ 2011).

CAD, particularly among those with a history of prior myocardial infarction.

3.5 mmol/l (63 mg/dl) (Network 2006).

**11.1.4 Management of hypertension** 

**11.1.5 Anti-platelet therapy** 

(Belch et al. 2008; Fowkes et al. 2010).

**11.1.5.1 Aspirin** 

In a post hoc subgroup analysis of 6,452 PAD patients in The Clopidogrel versus Aspirin in Patients at Risk of Ischemic Events (CAPRIE) trial, the incidence of stroke, MI, or vascular death was reduced by 24% among those randomized to clopidogrel compared to aspirin monotherapy (1996).

Dual antiplatelets therapy (DAPT) with aspirin and clopidogrel was not more effective than aspirin monotherapy in the Clopidogrel for High Atherothrombotic Risk and Ischemia Stabilization, Management and Avoidance (CHARISMA) study (Bhatt et al. 2006). However, post hoc analysis of the CHARISMA study revealed that DAPT was superior to aspirin monotherapy among patients with known symptomatic PAD at the time of enrollment (Bhatt et al. 2007). The combination of antiplatelet and anticoagulant therapy was evaluated in The Warfarin Anti-platelet Vascular Evaluation (WAVE) trial. This study found no incremental benefit of vitamin K antagonists when added to anti-platelet therapy for the prevention of cardiovascular events in patients with PAD (Anand et al. 2007). In fact, patients randomized to the combination of antiplatelet and anticoagulant therapy had an increase in life-threatening bleeding when compared with those randomized to anti-platelet therapy alone.

In general, all three PAD guidelines recommend antiplatelet therapy for PAD patients. Selection of an antiplatelet regimen (aspirin or clopidogrel or both or other antiplatelet medications) for the PAD patient should be individualized on the basis of tolerance and other clinical characteristics (i.e., bleeding risk) along with cost and guidance from regulatory agencies. (Dormandy and Rutherford 2000; Hirsch et al. 2006; Network 2006; Rooke et al. 2011). ACC / AHA guidelines strongly recommend antiplatelet therapy to reduce the risk of MI, stroke, and vascular death in individuals with symptomatic atherosclerotic lower extremity PAD, including those with intermittent claudication or critical limb ischemia, prior lower extremity revascularization (endovascular or surgical), or prior amputation for lower extremity ischemia (class IA). They feel that antiplatelet therapy can be useful to reduce the risk of MI, stroke, or vascular death in asymptomatic individuals with an ABI less than or equal to 0.90 (Class IIa C), however do not strongly recommend it given lack of definite evidence of any benefit (Rooke et al. 2011).

#### **11.2 Medical therapy for claudication**

#### **11.2.1 Supervised exercise program**

Many patients with PAD have severely impaired functional capacity, which leads to decreased quality of life (McDermott et al. 2004). A meta-analysis of 21 studies showed that patients with IC who underwent exercise training improved mean walking time by 180% and maximal walking time by 120% (Gardner and Poehlman 1995). Supervised treadmill exercises programs are also are more effective than lower extremity resistance training (McDermott et al. 2009). However, due to lack of reimbursement access to this important therapeutic intervention has been limited. In a Cochrane review of 8 RCT's evaluating supervised and unsupervised exercise among 319 participants with IC, statistically significant and clinically relevant improvements in maximal treadmill

Lower Extremity Peripheral Arterial Disease 131

revascularization), lesion characteristics, technical features (e.g., angioplasty alone vs. angioplasty + stent, type of surgical conduit used for surgical bypass). Hybrid procedures are performed for multi-level in an effort to minimize the overall operative risk of the revascularization (e.g., iliac artery stenting followed by femoral-popliteal bypass). Open surgical revascularization (e.g., bypass or endarterectomy) is typically reserved for

In table 4 we outline the grade A level of recommendation for PAD by TASC II guidelines. In table 5 we outline the class I recommendations by American College of Cardiology and American Heart association for PAD. Finally in table 6 we outline the differences in recommendations between the TASC II and American College of Cardiology /American

Table 4. This table outlines TASC II grade A level of recommendation for peripheral arterial

disease

endovascular failures or anatomy not likely to respond to an endovascular attempt.

**12. Summary of the guideline recommendations for PAD** 

Heart association guidelines for management of PAD.

walking distance occurred with supervised compared with non-supervised exercise therapy during 12 weeks to 12 months of follow up (Bendermacher et al. 2006). Unsupervised therapy may be less effective than supervised therapy due to lack of motivation, compliance with recommended exercise, lack of progression of workload in the absence of professional supervision and concern for personal safety to advance the moderate claudication discomfort severity.

#### **11.2.2 Pharmacological agents**

Cilastozol is an FDA-approved medication for the management of IC. It is a reversible phosphodiesterase inhibitor whose exact mechanism of benefit is unclear. It inhibits platelet aggregation, thrombin formation, and vascular smooth muscle proliferation and acts as a vasodilator. It also increases HDL and lowers TG levels. In an analysis of 9 RCT's of cilastazol at dose of 100 mg BID, cilostazol showed a 50.7% improvement from baseline maximal walking distance compared to placebo (24.3%) (Pande et al. 2010). A Cochrane review found similar benefit (Robless et al. 2008). Given the increased incidence of sudden cardiac death with other phosphodiesterase inhibitors (e.g., milrinone), cilostazol is contraindicated in patients with heart failure and/ or ejection fraction less than 40% (Packer et al. 1991). The ACC/AHA guidelines have given cilostazol a grade IA recommendation for patients with intermittent claudication in the absence of heart failure.

Pentoxifylline is a methylxanthine derivative that decreases blood viscosity and has hemorheologic (improves erythrocyte and leukocyte deformability), anti-inflammatory, and antiproliferative effects. Its anti-claudication effect has been inconsistent. A study randomizing 698 patients with IC to cilostazol, pentoxifylline or placebo did not observe any difference between pentoxifylline and placebo (Dawson et al. 2000). ACC/AHA guidelines recommend using pentoxifylline as an alternative in patients who cannot tolerate cilostazol or in whom cilostazol is contraindicated.

#### **11.2.3 Revascularization**

Revascularization is only appropriate for symptomatic patients and should not be undertaken as prophylactic therapy for an asymptomatic limb (Hirsch et al. 2006). Revascularization is indicated in patients' with acute limb ischemia (ALI), CLI and among those with lifestyle- or vocation-limiting claudication who have failed a trial of medical therapy or who have highly favorable anatomy for endovascular therapy such as focal aorto-iliac disease (Hirsch et al. 2006) . Data comparing medical therapy with either endovascular or surgical therapy are scant. The ongoing Claudication: exercise vs endoluminal revascularization (CLEVER) trial is comparing medical therapy alone vs. medical therapy + supervised exercise vs. medical therapy + endovascular therapy (Murphy et al. 2008).

Endovascular therapy is currently the preferred mode of revascularization in cases where anatomy is more favorable (e.g., aorto-iliac disease). Endovascular therapy is less invasive, is associated with fewer complications and with shorter hospital stay as compared to bypass procedures. The durability of endovascular or surgical procedures depends on a number of factors including anatomic location (aorto-iliac revascularization whether open surgical or endovascular has superior long term outcomes compared with infra-inguinal

walking distance occurred with supervised compared with non-supervised exercise therapy during 12 weeks to 12 months of follow up (Bendermacher et al. 2006). Unsupervised therapy may be less effective than supervised therapy due to lack of motivation, compliance with recommended exercise, lack of progression of workload in the absence of professional supervision and concern for personal safety to advance the

Cilastozol is an FDA-approved medication for the management of IC. It is a reversible phosphodiesterase inhibitor whose exact mechanism of benefit is unclear. It inhibits platelet aggregation, thrombin formation, and vascular smooth muscle proliferation and acts as a vasodilator. It also increases HDL and lowers TG levels. In an analysis of 9 RCT's of cilastazol at dose of 100 mg BID, cilostazol showed a 50.7% improvement from baseline maximal walking distance compared to placebo (24.3%) (Pande et al. 2010). A Cochrane review found similar benefit (Robless et al. 2008). Given the increased incidence of sudden cardiac death with other phosphodiesterase inhibitors (e.g., milrinone), cilostazol is contraindicated in patients with heart failure and/ or ejection fraction less than 40% (Packer et al. 1991). The ACC/AHA guidelines have given cilostazol a grade IA recommendation for

Pentoxifylline is a methylxanthine derivative that decreases blood viscosity and has hemorheologic (improves erythrocyte and leukocyte deformability), anti-inflammatory, and antiproliferative effects. Its anti-claudication effect has been inconsistent. A study randomizing 698 patients with IC to cilostazol, pentoxifylline or placebo did not observe any difference between pentoxifylline and placebo (Dawson et al. 2000). ACC/AHA guidelines recommend using pentoxifylline as an alternative in patients who cannot tolerate cilostazol

Revascularization is only appropriate for symptomatic patients and should not be undertaken as prophylactic therapy for an asymptomatic limb (Hirsch et al. 2006). Revascularization is indicated in patients' with acute limb ischemia (ALI), CLI and among those with lifestyle- or vocation-limiting claudication who have failed a trial of medical therapy or who have highly favorable anatomy for endovascular therapy such as focal aorto-iliac disease (Hirsch et al. 2006) . Data comparing medical therapy with either endovascular or surgical therapy are scant. The ongoing Claudication: exercise vs endoluminal revascularization (CLEVER) trial is comparing medical therapy alone vs. medical therapy + supervised exercise vs. medical

Endovascular therapy is currently the preferred mode of revascularization in cases where anatomy is more favorable (e.g., aorto-iliac disease). Endovascular therapy is less invasive, is associated with fewer complications and with shorter hospital stay as compared to bypass procedures. The durability of endovascular or surgical procedures depends on a number of factors including anatomic location (aorto-iliac revascularization whether open surgical or endovascular has superior long term outcomes compared with infra-inguinal

patients with intermittent claudication in the absence of heart failure.

moderate claudication discomfort severity.

or in whom cilostazol is contraindicated.

therapy + endovascular therapy (Murphy et al. 2008).

**11.2.3 Revascularization** 

**11.2.2 Pharmacological agents** 

revascularization), lesion characteristics, technical features (e.g., angioplasty alone vs. angioplasty + stent, type of surgical conduit used for surgical bypass). Hybrid procedures are performed for multi-level in an effort to minimize the overall operative risk of the revascularization (e.g., iliac artery stenting followed by femoral-popliteal bypass). Open surgical revascularization (e.g., bypass or endarterectomy) is typically reserved for endovascular failures or anatomy not likely to respond to an endovascular attempt.

#### **12. Summary of the guideline recommendations for PAD**

In table 4 we outline the grade A level of recommendation for PAD by TASC II guidelines. In table 5 we outline the class I recommendations by American College of Cardiology and American Heart association for PAD. Finally in table 6 we outline the differences in recommendations between the TASC II and American College of Cardiology /American Heart association guidelines for management of PAD.


Table 4. This table outlines TASC II grade A level of recommendation for peripheral arterial disease

Lower Extremity Peripheral Arterial Disease 133

Table 6. This table outlines the main differences between the ACC/AHA PAD guidelines

PAD is a common disease, present in more than 8 million Americans and is associated with a relatively high risk of cardiovascular events. Nevertheless, it remains under-recognized by healthcare providers and patients alike. Increased awareness, earlier identification of the disease and aggressive medical therapy and vascular risk factor modification would reduce the likelihood of fatal and non-fatal cardiovascular events and improve overall functional

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**13. Conclusions** 

status and quality of life.

1339.

3341.

61.

324(7329): 71-86.

**14. References** 

Table 5. This table outlines Class I recommendations for the Identification and Management of Peripheral artery Disease (PAD) by American Heart Association and American College of Cardiology with level of evidence (LOE)

Table 5. This table outlines Class I recommendations for the Identification and Management of Peripheral artery Disease (PAD) by American Heart Association and American College of

Cardiology with level of evidence (LOE)

Table 6. This table outlines the main differences between the ACC/AHA PAD guidelines and TASC II guidelines are summarized in this table

#### **13. Conclusions**

PAD is a common disease, present in more than 8 million Americans and is associated with a relatively high risk of cardiovascular events. Nevertheless, it remains under-recognized by healthcare providers and patients alike. Increased awareness, earlier identification of the disease and aggressive medical therapy and vascular risk factor modification would reduce the likelihood of fatal and non-fatal cardiovascular events and improve overall functional status and quality of life.

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### *Edited by Efraín Gaxiola*

Atherothrombosis has reached pandemic proportions worldwide. It is the underlying condition that results in events leading to myocardial infarction, ischemic stroke and vascular death. As such, it is the leading cause of death worldwide manifested mainly as cardiovascular/cerebrovascular death. The complex and intimate relationship between atherothrombosis and traditional and novel risk factors is discussed in the following chapters of Traditional and Novel Risk Factors in Atherothrombosis - from basic science to clinical and therapeutic concerns. Beginning with pathology and pathophysiology of atherothrombosis, plaque rupture/disruption, this book continues with molecular, biochemical, inflammatory, cellular aspects and finally analyzes several aspects of clinical pharmacology.

Traditional and Novel Risk Factors in Atherothrombosis

Traditional and Novel Risk

Factors in Atherothrombosis

*Edited by Efraín Gaxiola*

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