**4.1 Different types of vascular calcification**

It is generally well recognized that the prevalence of calcification increases with progressively decreasing kidney function and is greater than that in the general population. Cardiovascular calcification is associated with increased frequency of major cardiovascular diseases, and could be of predictive importance for adverse clinical outcomes, including cardiovascular events and death (Foley RN et al., 1998). There is an increased prevalence of cardiovascular calcification in patients even at early stages of CKD. Thus, an important percentage of CKD patients are at high risk of cardiovascular events from vascular calcification. Two patterns of vascular calcification have been described: namely intimal and medial calcification. In the general population, an elevated coronary artery calcium (CAC) score almost exclusively reflects the atherosclerotic disease burden. In two small autopsy studies, it became apparent that, in dialysis patients, CAC is also predominantly localized in the coronary intima, whereas the medial calcifications observed in a minority of such patients seemed to be adjacent to plaque areas just beneath the internal elastic lamina. Although the coronary vascular bed may differ considerably from other arteries with regard to the calcification process and its manifestations, the same group observed a 'pure' medial calcification in the coronary arteries during the early stages of CKD (Schwarz et al., 2000). A 'pure' medial calcification, in the absence of intimal disease, was also observed in epigastric arteries obtained from dialysis patients at the time of renal transplantation (Amann K., 2008;).

#### **4.2 Promoters and inhibitors of calcification**

Vascular calcification is the result of passive and active processes, as is bone mineralization. It has been shown that that normal extracellular phosphate concentration is required for

The New Kidney and Bone Disease:

**a)**

**c)**

Chronic Kidney Disease – Mineral and Bone Disorder (CKD–MBD) 35

**b)**

**d)**

Fig. 1A. Intima and media calcification by radiography. a) Femoral artery intimal

Iliac arteries mixed calcification. (London et al. 2003).

courtesy of Pr P. Raggi).

calcification; b) Femoral artery medial calcification; c) Pelvic artery medial calcification; d)

Fig. 1B. Coronary artery calcification by Electron beam computed tomography (EBCT), (scan

bone mineralization, while lowering this concentration prevents mineralization of any extracellular matrix. However, simply raising extracellular phosphate concentration is not sufficient to induce pathological mineralization, because of the presence in all extracellular matrices of pyrophosphate, an inhibitor of mineralization (Riser et al., 2011). They further showed that extracellular matrix mineralization normally occurs only in bone because of the exclusive coexpression in osteoblasts of Type I collagen and of tissue non-specific alkaline phosphatase (Tnap), an enzyme that cleaves pyrophosphate. Pyrophosphate probably is the most important non-protein inhibitor of vascular calcification. Its extracellular concentration is strictly regulated by several enzymes. It is generated by PC-1 nucleotide triphosphate pyrophosphohydrolase and metabolized to inorganic phosphate by nucleotide pyrophosphatase/phosphodiesterase (NPP1), in addition to Tnap. Its hydrolysis to inorganic phosphate actually transforms it from a calcification inhibitor to a promoter. In addition to pyrophosphate other inhibitors are also present locally in VSMCs, including matrix-gla protein (MGP) and Smad6 proteine (Lomashvili et al., 2008; Rutsch et al., 2001; Johnson et al., 2005).

#### **4.3 Contribution of experimental models in vascular calcification**

Arterial calcification assessed by all the available imaging studies cannot accurately differentiate calcification that is localized to the intima from calcification in the media adjacent to the internal elastic lamina, or in the medial layer (Figure 1 and 2). Thus, there is neither definitive evidence to suggest that isolated medial calcification is distinct from the calcification that occurs in the natural history of atherosclerosis nor is there definite proof against it. Experimental and ex vivo studies suggest that the vascular smooth muscle cell may be critical in the development of calcification by transforming into an osteoblast-like phenotype (Giachelli CM, 2004). Elevated phosphorus, elevated calcium, oxidized lowdensity lipoprotein cholesterol, cytokines, and elevated glucose, among others, stimulate this transformation of vascular smooth muscle cells into osteoblast-like cells in vitro using cell-culture techniques. These factors likely interact at the patient level to increase and/or accelerate calcification in CKD. Given the potential complexity of the pathogenesis and the inability of radiological techniques to differentiate the location of calcification, the approach to all patients with calcification should be to minimize atherosclerotic risk factors and control biochemical parameters of CKD–MBD. In addition, the pericyte in the media and adventitia may have a role in the secretion of vascular calcification-inducing factors (Giachelli et al., 2004). The stimulus for such a transformation may depend on the location of calcification within the artery wall (Figure 2A and 2B). For example, in intimal lesions, atherosclerosis may be the most important stimulus. However, in patients with CKD and medial calcification, there may be additional, or additive, factors potentially explaining why medial calcification of the peripheral arteries can be seen without intimal changes and is more common in CKD than in the non-CKD population (Moe et al., 2003).

Over the past decade, several animal studies have provided evidence for an accelerated progression of atherosclerosis in association with the uremic state. We and others have used the apolipoprotein e knockout (*Apoe*–/–) mouse with superimposed CKD and observed that in this experimental model of severe hypercholesterolemia the development of atheromatous lesions was greatly enhanced compared with the rate of lesion development in nonuremic *Apoe*–/– mice (Massy et al., 2005; Ivanovski et al., 2005). Additionally, in our

bone mineralization, while lowering this concentration prevents mineralization of any extracellular matrix. However, simply raising extracellular phosphate concentration is not sufficient to induce pathological mineralization, because of the presence in all extracellular matrices of pyrophosphate, an inhibitor of mineralization (Riser et al., 2011). They further showed that extracellular matrix mineralization normally occurs only in bone because of the exclusive coexpression in osteoblasts of Type I collagen and of tissue non-specific alkaline phosphatase (Tnap), an enzyme that cleaves pyrophosphate. Pyrophosphate probably is the most important non-protein inhibitor of vascular calcification. Its extracellular concentration is strictly regulated by several enzymes. It is generated by PC-1 nucleotide triphosphate pyrophosphohydrolase and metabolized to inorganic phosphate by nucleotide pyrophosphatase/phosphodiesterase (NPP1), in addition to Tnap. Its hydrolysis to inorganic phosphate actually transforms it from a calcification inhibitor to a promoter. In addition to pyrophosphate other inhibitors are also present locally in VSMCs, including matrix-gla protein (MGP) and Smad6 proteine (Lomashvili et al., 2008; Rutsch et al., 2001;

Arterial calcification assessed by all the available imaging studies cannot accurately differentiate calcification that is localized to the intima from calcification in the media adjacent to the internal elastic lamina, or in the medial layer (Figure 1 and 2). Thus, there is neither definitive evidence to suggest that isolated medial calcification is distinct from the calcification that occurs in the natural history of atherosclerosis nor is there definite proof against it. Experimental and ex vivo studies suggest that the vascular smooth muscle cell may be critical in the development of calcification by transforming into an osteoblast-like phenotype (Giachelli CM, 2004). Elevated phosphorus, elevated calcium, oxidized lowdensity lipoprotein cholesterol, cytokines, and elevated glucose, among others, stimulate this transformation of vascular smooth muscle cells into osteoblast-like cells in vitro using cell-culture techniques. These factors likely interact at the patient level to increase and/or accelerate calcification in CKD. Given the potential complexity of the pathogenesis and the inability of radiological techniques to differentiate the location of calcification, the approach to all patients with calcification should be to minimize atherosclerotic risk factors and control biochemical parameters of CKD–MBD. In addition, the pericyte in the media and adventitia may have a role in the secretion of vascular calcification-inducing factors (Giachelli et al., 2004). The stimulus for such a transformation may depend on the location of calcification within the artery wall (Figure 2A and 2B). For example, in intimal lesions, atherosclerosis may be the most important stimulus. However, in patients with CKD and medial calcification, there may be additional, or additive, factors potentially explaining why medial calcification of the peripheral arteries can be seen without intimal changes and is

**4.3 Contribution of experimental models in vascular calcification** 

more common in CKD than in the non-CKD population (Moe et al., 2003).

Over the past decade, several animal studies have provided evidence for an accelerated progression of atherosclerosis in association with the uremic state. We and others have used the apolipoprotein e knockout (*Apoe*–/–) mouse with superimposed CKD and observed that in this experimental model of severe hypercholesterolemia the development of atheromatous lesions was greatly enhanced compared with the rate of lesion development in nonuremic *Apoe*–/– mice (Massy et al., 2005; Ivanovski et al., 2005). Additionally, in our

Johnson et al., 2005).

Fig. 1A. Intima and media calcification by radiography. a) Femoral artery intimal calcification; b) Femoral artery medial calcification; c) Pelvic artery medial calcification; d) Iliac arteries mixed calcification. (London et al. 2003).

Fig. 1B. Coronary artery calcification by Electron beam computed tomography (EBCT), (scan courtesy of Pr P. Raggi).

The New Kidney and Bone Disease:

be recommended.

Chronic Kidney Disease – Mineral and Bone Disorder (CKD–MBD) 37

model, accelerated calcification of the aortic wall both in the intima and the media, (Figure 3A and 3B, respectively) occurred in the absence of hypertension, and fetuin a deficiency greatly enhanced intimal calcification. Similar observations have been made using another hyper cholesterolemic animal model of severe atherosclerosis with superimposed CKD, namely the LDL receptor knockout mouse model (Mathew et al. 2007; Davies et al. 2005). Of note, the first cardiovascular changes observed in early stages of CKD in *Apoe–/–* mice as well as in wild type mice were left ventricular hypertrophy, altered left ventricular relaxation and increased aortic stiffness in the absence of identifiable morphological changes of the vessel wall. The observed cardiac and aortic abnormalities were not associated with the degree of aortic calcification or the level of serum total cholesterol, but with the extent of subendothelial dysfunction and the severity of CKD. Our findings have revealed that the cardio vascular lesions observed in early stage of acute kidney injury are likely functional. Although the above experimental findings need to be confirmed by additional studies in the clinical setting, they open up the possibility of attenuation of atherosclerosis and even reversal by adequate therapeutic strategies. Findings from experimental observations favor the existence of two different types of vascular disease linked to CKD, namely early arteriosclerosis, in the absence of atherosclerosis, and the acceleration of already existing or subsequently developing atherosclerosis by the uremic state (Drueke and Massy, 2010).

Finally, a rare but very severe form of medial calcification of small (cutaneous) arteries is calciphylaxis, also called calcific uremic arteriolopathy. This complication is strongly associated with CKD-related disturbances of mineral metabolism, including secondary HPT, in approximately one-third of cases. It is characterized by ischemic, painful skin ulcerations followed by superinfections, and is associated with high mortality. Relationships with dysregulated calcification inhibitors (fetuin-A and matrix Gla protein) have been implicated in the pathogenesis of calciphylaxis (Schoppet et al., 2008; Suliman et al., 2008), but because of the relatively low incidence of the disease, no conclusive data are available to firmly comment on the nature of the disease process or to allow generalizable treatment options to

Recently it has been confirmed that cardiovascular calcification development and progression can be influenced by treatment. Given that vascular calcification is associated with increased cardiovascular risk, and that the pathogenesis seems to be related to CKD– MBD abnormalities and atherosclerosis, it is appropriate to evaluate and modify both. CKD– MBD longitudinal studies have also shown that the progression of vascular calcification to be modifiable by the choice of phosphate binders. Aluminum-containing phosphate binders have been widely abandoned because of serious adverse effects including adynamic bone disease, microcytic anemia, dementia, and death (Alfrey et al., 1976). They were initially replaced by calcium-containing, aluminum-free phosphate binders. Subsequently, several studies showed that the high amounts of calcium ingested with these binders were associated with vascular calcification whose progression could be slowed by the calciumfree, aluminum-free binder sevelamer (Block et al., 2005; Chertow et al., 2002; London et al., 2008). The Treat-to-Goal study compared sevelamer-HCl to calcium-containing phosphate binders, analyzing the progression of coronary artery and aortic calcification (by EBCT) in prevalent HD patients over 1 year. Although calcification scores progressed with calcium-

**4.4 Management of patients with vascular/valvular calcification** 

B

Fig. 2. Localization of different types of vascular calcification in humans. A) Intimal; B) Medial; (London et al. 2003).

A

B

Fig. 2. Localization of different types of vascular calcification in humans. A) Intimal; B)

Medial; (London et al. 2003).

model, accelerated calcification of the aortic wall both in the intima and the media, (Figure 3A and 3B, respectively) occurred in the absence of hypertension, and fetuin a deficiency greatly enhanced intimal calcification. Similar observations have been made using another hyper cholesterolemic animal model of severe atherosclerosis with superimposed CKD, namely the LDL receptor knockout mouse model (Mathew et al. 2007; Davies et al. 2005). Of note, the first cardiovascular changes observed in early stages of CKD in *Apoe–/–* mice as well as in wild type mice were left ventricular hypertrophy, altered left ventricular relaxation and increased aortic stiffness in the absence of identifiable morphological changes of the vessel wall. The observed cardiac and aortic abnormalities were not associated with the degree of aortic calcification or the level of serum total cholesterol, but with the extent of subendothelial dysfunction and the severity of CKD. Our findings have revealed that the cardio vascular lesions observed in early stage of acute kidney injury are likely functional. Although the above experimental findings need to be confirmed by additional studies in the clinical setting, they open up the possibility of attenuation of atherosclerosis and even reversal by adequate therapeutic strategies. Findings from experimental observations favor the existence of two different types of vascular disease linked to CKD, namely early arteriosclerosis, in the absence of atherosclerosis, and the acceleration of already existing or subsequently developing atherosclerosis by the uremic state (Drueke and Massy, 2010).

Finally, a rare but very severe form of medial calcification of small (cutaneous) arteries is calciphylaxis, also called calcific uremic arteriolopathy. This complication is strongly associated with CKD-related disturbances of mineral metabolism, including secondary HPT, in approximately one-third of cases. It is characterized by ischemic, painful skin ulcerations followed by superinfections, and is associated with high mortality. Relationships with dysregulated calcification inhibitors (fetuin-A and matrix Gla protein) have been implicated in the pathogenesis of calciphylaxis (Schoppet et al., 2008; Suliman et al., 2008), but because of the relatively low incidence of the disease, no conclusive data are available to firmly comment on the nature of the disease process or to allow generalizable treatment options to be recommended.

#### **4.4 Management of patients with vascular/valvular calcification**

Recently it has been confirmed that cardiovascular calcification development and progression can be influenced by treatment. Given that vascular calcification is associated with increased cardiovascular risk, and that the pathogenesis seems to be related to CKD– MBD abnormalities and atherosclerosis, it is appropriate to evaluate and modify both. CKD– MBD longitudinal studies have also shown that the progression of vascular calcification to be modifiable by the choice of phosphate binders. Aluminum-containing phosphate binders have been widely abandoned because of serious adverse effects including adynamic bone disease, microcytic anemia, dementia, and death (Alfrey et al., 1976). They were initially replaced by calcium-containing, aluminum-free phosphate binders. Subsequently, several studies showed that the high amounts of calcium ingested with these binders were associated with vascular calcification whose progression could be slowed by the calciumfree, aluminum-free binder sevelamer (Block et al., 2005; Chertow et al., 2002; London et al., 2008). The Treat-to-Goal study compared sevelamer-HCl to calcium-containing phosphate binders, analyzing the progression of coronary artery and aortic calcification (by EBCT) in prevalent HD patients over 1 year. Although calcification scores progressed with calcium-

The New Kidney and Bone Disease:

Chronic Kidney Disease – Mineral and Bone Disorder (CKD–MBD) 39

containing phosphate binders, treatment with sevelamer-HCl was associated with a lack of calcification progression (Chertow et al., 2002). A similar design was used, and the results showed more calcification progression in patients treated with calcium based binders compared with sevelamer-HCl in the Renagel in New Dialysis Patients study, which studied incident HD patients who were randomized within 90 days after starting dialysis treatment (Block et al., 2005). The Calcium Acetate Renagel Evaluation-2 study showed that the use of sevelamer-HCl and calcium acetate was associated with equal progression of CAC when statins were used to achieve a similar control of the serum low-density lipoprotein cholesterol in the two study arms (Qunibi et al., 2008). Interestingly, in Calcium Acetate Renagel Evaluation-2, the combination of sevelamer- HCl and atorvastatin was actually associated with a higher progression rate of CAC than that in Treat-to-Goal, instead of showing an amelioration of CAC progression with the combination of calcium acetate and statin. It is difficult to reconcile these differences, although one potential explanation is that the Calcium Acetate Renagel Evaluation- 2 study patient population had a higher number of

cardiovascular risk factors than did that of the Treat-to-Goal study (Floege J., 2008).

Although abnormalities of calcium phosphate homeostasis have long been linked with dysfunction of large arteries in these patients, more recent studies have suggested a role in the pathogenesis of atherosclerosis in smaller, critical arteries, most notably the coronary arteries (London et al., 2003). Coronary artery calcification (CAC) is a strong predictor of atherosclerotic disease in the general population. It has been recognized that most population studies measuring CAC did not necessarily exclude individuals on the basis of kidney function and thus include variable numbers of CKD patients. In general, this literature evaluating the general population supports the view that CAC is part of the development of atherosclerosis and occurs almost exclusively together with atherosclerosis in human arteries. It seems that calcification occurs early in the atherosclerotic process; however, the amount of calcification per lesion has a variable relationship with the associated severity of luminal stenosis. The relationship between the degree of calcification in an individual lesion and the probability of plaque rupture is unknown. In the general population, the overall coronary calcium score can be considered as a measure of the overall burden of coronary atherosclerosis. The American College of Cardiology/American Heart Association document indicates that the relationship between CAC and cardiovascular events in the CKD population is less clear than that in the non-CKD population because of a relative lack of informative studies and the possibility that medial calcification may not be indicative of atherosclerotic disease severity. The almost exclusive relationship between magnitude of calcification and atherosclerosis burden is controversial in CKD patients (Amann, 2008), in contrast to the situation in the general population. Antiatherosclerotic strategies using statin treatment have been shown to have a beneficial impact on the atherogenic profile, atheroma progression, and cardiovascular events in patients with no known CKD (Nissen et al. 2004). In our experimental model, we have shown that statins had a beneficial effect on uremia enhanced vascular calcification in apoE knock out mice with chronic kidney disease. This effect was observed despite the absence of changes in uremia accelerated atherosclerosis progression, serum total cholesterol levels or osteopontin and alkaline phosphatase expression. This observation opened the possibility of a cholesterol independent action of statins on vascular calcification via a decrease in oxidative stress (Ivanovski et al., 2008). In CKD patients, there are no data on the effects of statins on arterial

(a)

(b)

Fig. 3. Extent and localization of different types of atherosclerotic lesion calcification in apoE−*/*− mice with CRF. von Kossa staining. a) solid type of plaque calcification, magnification ×25; b) non-plaque calcification, magnification×25. (Phan et al. 2008).

(a)

(b)

Fig. 3. Extent and localization of different types of atherosclerotic lesion calcification in apoE−*/*− mice with CRF. von Kossa staining. a) solid type of plaque calcification, magnification ×25; b) non-plaque calcification, magnification×25. (Phan et al. 2008).

containing phosphate binders, treatment with sevelamer-HCl was associated with a lack of calcification progression (Chertow et al., 2002). A similar design was used, and the results showed more calcification progression in patients treated with calcium based binders compared with sevelamer-HCl in the Renagel in New Dialysis Patients study, which studied incident HD patients who were randomized within 90 days after starting dialysis treatment (Block et al., 2005). The Calcium Acetate Renagel Evaluation-2 study showed that the use of sevelamer-HCl and calcium acetate was associated with equal progression of CAC when statins were used to achieve a similar control of the serum low-density lipoprotein cholesterol in the two study arms (Qunibi et al., 2008). Interestingly, in Calcium Acetate Renagel Evaluation-2, the combination of sevelamer- HCl and atorvastatin was actually associated with a higher progression rate of CAC than that in Treat-to-Goal, instead of showing an amelioration of CAC progression with the combination of calcium acetate and statin. It is difficult to reconcile these differences, although one potential explanation is that the Calcium Acetate Renagel Evaluation- 2 study patient population had a higher number of cardiovascular risk factors than did that of the Treat-to-Goal study (Floege J., 2008).

Although abnormalities of calcium phosphate homeostasis have long been linked with dysfunction of large arteries in these patients, more recent studies have suggested a role in the pathogenesis of atherosclerosis in smaller, critical arteries, most notably the coronary arteries (London et al., 2003). Coronary artery calcification (CAC) is a strong predictor of atherosclerotic disease in the general population. It has been recognized that most population studies measuring CAC did not necessarily exclude individuals on the basis of kidney function and thus include variable numbers of CKD patients. In general, this literature evaluating the general population supports the view that CAC is part of the development of atherosclerosis and occurs almost exclusively together with atherosclerosis in human arteries. It seems that calcification occurs early in the atherosclerotic process; however, the amount of calcification per lesion has a variable relationship with the associated severity of luminal stenosis. The relationship between the degree of calcification in an individual lesion and the probability of plaque rupture is unknown. In the general population, the overall coronary calcium score can be considered as a measure of the overall burden of coronary atherosclerosis. The American College of Cardiology/American Heart Association document indicates that the relationship between CAC and cardiovascular events in the CKD population is less clear than that in the non-CKD population because of a relative lack of informative studies and the possibility that medial calcification may not be indicative of atherosclerotic disease severity. The almost exclusive relationship between magnitude of calcification and atherosclerosis burden is controversial in CKD patients (Amann, 2008), in contrast to the situation in the general population. Antiatherosclerotic strategies using statin treatment have been shown to have a beneficial impact on the atherogenic profile, atheroma progression, and cardiovascular events in patients with no known CKD (Nissen et al. 2004). In our experimental model, we have shown that statins had a beneficial effect on uremia enhanced vascular calcification in apoE knock out mice with chronic kidney disease. This effect was observed despite the absence of changes in uremia accelerated atherosclerosis progression, serum total cholesterol levels or osteopontin and alkaline phosphatase expression. This observation opened the possibility of a cholesterol independent action of statins on vascular calcification via a decrease in oxidative stress (Ivanovski et al., 2008). In CKD patients, there are no data on the effects of statins on arterial

The New Kidney and Bone Disease:

**5. CKD – MBD summary** 

**6. References** 

24.

Suppl.; (84): S207-10.

Chronic Kidney Disease – Mineral and Bone Disorder (CKD–MBD) 41

Mineral and bone disorders are complex abnormalities that cause morbidity and decreased quality of life in patients with CKD. To enhance communication and facilitate research, a new term has been established, CKD–Mineral and Bone Disorder (CKD-MBD), to describe the syndrome of biochemical, bone, and extraskeletal calcification abnormalities that occur in patients with CKD. Also, it has been recommended that the term renal osteodystrophy be used exclusively to define alterations in bone morphology associated with CKD. The latter can be further assessed by histomorphometry, with results reported on the basis of a classification system that includes parameters of turnover, mineralization, and volume. The international adoption of the proposed uniform terminology, definition, and classification to describe these two disorders caused by CKD enhanced communication, facilitated clinical decision making, and can promote the evolution of evidence based clinical-practice guidelines worldwide. This issue of Advances in CKD further describes the clinical manifestations and pathophysiology of CKD-MBD. The optimal management of CKD-MBD (Chronic Kidney Disease – Mineral and Bone Disorder) should be achieved without

Andress DL. (2006). "Vitamin D in chronic kidney disease: a systemic role for selective

Alem AM, Sherrard DJ, et al., (2000). "Increased risk of hip fracture among patients with

Alfrey AC, LeGendre GR, et al. (1976). "The dialysis encephalopathy syndrome. Possible

Amann K. (2008). "Media calcification and intima calcification are distinct entities in chronic

Baigent C, Landry M. (2003). "Study of Heart and Renal Protection (SHARP)." Kidney Int

Baum M, Schiavi S, et al. (2005). "Effect of fibroblast growth factor-23 on phosphate

Block GA, Hulbert-Shearon TE et al. (1998). "Association of serum phosphorus and calcium

Block GA, Spiegel DM et al. (2005). "Effects of sevelamer and calcium on coronary artery

Bucay N, Sarosi I et al. (1998). "Osteoprotegerin-deficient mice develop early onset

Chertow GM, Burke SK,. (2002). "Sevelamer attenuates the progression of coronary and aortic calcification in hemodialysis patients." Kidney Int.; 62(1): 245-52. Coco M, Rush H. (2000). Increased incidence of hip fractures in dialysis patients with low

osteoporosis and arterial calcification." *Genes Dev;* 12:1260-1268

serum parathyroid hormone. Am J Kidney Dis.; 36(6): 1115-21.

x phosphate product with mortality risk in chronic hemodialysis patients: a

calcification in patients new to hemodialysis.", Kidney Int. 2005 Oct;68(4):1815-

increasing the risk of metastatic calcification, including that of blood vessels.

vitamin D receptor activation". Kidney Int.; 69(1): 33-43.

aluminum intoxication. " N Engl J Med. 22;294(4):184-8.

transport in proximal tubules. " Kidney Int; 68: 1148–1153.

kidney disease". Clin J Am Soc Nephrol.: 3: 1599-605.

national study." Am J Kidney Dis.; 31(4): 607-17.

end-stage renal disease". Kidney Int.; 58(1): 396-9.

calcification, as compared with those of placebo. Even worse, the 4D study failed to show a benefit of atorvastatin treatment on the outcome of diabetic dialysis patients. Studies in progress like SHARP (Study of Heart and Renal Protection) and AURORA (A Study to Evaluate the Use of Rosuvastatin in Subjects on Regular Hemodialysis: An Assessment of Survival and Cardiovascular Events) failed to show a better understanding of the benefits of correcting atherosclerotic risk factors on cardiovascular events and mortality in patients with CKD stages 3–5 and 5D (Baigent et al., 2003).

An association of vascular calcification with high phosphate intake has so far not been directly demonstrated in uremic patients, probably owing to the fact that it is difficult, if not impossible, to assess phosphate (protein) intake in a quantitative manner over prolonged time periods. Indirect evidence for a role of oral phosphate, however, has recently been provided by Russo et al (Russo et al., 2007). They showed that in patients with CKD stage 3- 5, coronary artery calcification score progressed significantly over a time period of 2 years, in association with a significant increase in phosphaturia. Many pharmaco-epidemiologic studies have shown a survival benefit in CKD patients receiving active vitamin D derivatives, as compared to those who did not receive such treatments. Finally, let us not forget that association does not imply causation. We clearly need randomized prospective trials showing that active reduction of serum phosphorus, PTH, or alkaline phosphatases and normalization of serum calcium leads to an improvement in patient outcomes, and that specific treatments given to the patients improve outcome, as compared to either placebo or other treatments (Drueke and McCarron, 2003).

To date, there are no published prospective studies in humans that have evaluated the impact of calcimimetics or calcitriol and vitamin D analogs on arterial calcification. However, a recent observational study showed a U-curve type of relationship between serum 1,25(OH)2D3 and arterial calcification in children and adolescents with CKD stage 5D. No such association existed between serum 25(OH)D and arterial calcification. In one study in adult patients with CKD stage 5, no independent association of serum 25(OH)D or 1,25(OH)2D3 levels with arterial calcification was observed, (London et al., 2007). Although the authors of another report identified an association between 25(OH)D deficiency and the magnitude of vascular calcification (Matias et al., 2009). The experimental data supporting less toxicity of vitamin D analogs compared with calcitriol are not completely consistent across studies, but, in general, support the claim that there is reduced calcification with equivalent PTH lowering with different vitamin D analogs (Lopez et al., 2008**).** Experimental studies showed differential effects of calcimimetics and calcitriol on extraosseous calcification, the former being neutral or protective, the latter being a dose-dependent risk factor for calcification. In our studies, we have analysed the role of chronic renal failure (CRF) on the arterial wall changes including atherosclerosis and vascular calcifications in CRF apoE-/- mice experimental model (Massy, Ivanovski et al. 2005). Furthermore, we have studied the effect of different non-calcium (Phan et al., 2005) and calcium phosphate binders (Phan et al., 2008) and role of control of phosphatemia on vascular calcification and atherosclerosis (Ivanovski et al. 2009). We have also showed for the first time that the phosphate binder La carbonate is capable of preventing both uremia-enhanced vascular calcification and atherosclerosis in experimental model of CKD (Nikolov et al., 2011). These effects were comparable to those of sevelamer on vascular calcification and atherosclerosis, as previously reported by us for sevelamer-HCl in this model (Phan et al., 2008).
