**1. Introduction**

#### **1.1. Overview of DPN**

In diabetes mellitus, nerves and their supporting cells are subjected to prolonged hypergly‐ cemia and metabolic disturbances and this culminates in reversible/irreversible nervous system dysfunction and damage, namely diabetic peripheral neuropathy (DPN). Due to the varying compositions and extents of neurological involvements, it is difficult to obtain accurate and thorough prevalence estimates of DPN, rendering this microvascular complication vastly underdiagnosed and undertreated [1-4]. According to American Diabetes Association, DPN occurs to 60-70% of diabetic individuals [5] and represents the leading cause of peripheral neuropathies among all cases [6, 7]. As the incidence of diabetes is approaching global epidemic level, its neurological consequences are estimated to affect some \$300 million people worldwide [8] and costs 15 billion dollars on annual health‐ care expenditures in the U.S. alone [9].

#### *1.1.1. A Complex natural history*

Because diverse anatomic distributions and fiber types may be differentially affected in patients with diabetes, the disease manifestations, courses and pathologies of clinical and subclinical DPN are rather heterogeneous and encompass a broad spectrum [1, 10, 11]. Additionally, dietary influences, risk covariates, genetic and phenotypic multiplicity further perplex the definition, diagnosis, classification and natural history of DPN [6, 10, 12, 13]. Current consensus divides diabetes-associated somatic neuropathic syndromes into the

focal/multifocal and diffuse/generalized neuropathies [6, 14]. The first category comprises a group of asymmetrical, acute-in-onset and self-limited single lesion(s) of nerve injury or impairment largely resulting from the increased vulnerability of diabetic nerves to mechanical insults (Carpal Tunnel Syndrome) (reviewed in 15). Such mononeuropathies occur idiopathically and only become a clinical problem in association with aging in 5-10% of those affected. Therefore, focal neuropathies are not extensively covered in this chap‐ ter [16]. The rest of the patients frequently develop diffuse neuropathies characterized by symmetrical distribution, insidious onset and chronic progression. In particular, a distal symmetrical sensorimotor polyneuropathy accounts for 90% of all DPN diagnoses in type 1 and type 2 diabetics and affects all types of peripheral sensory and motor fibers in a temporally non-uniform manner [6, 17].

diabetic foot ulcers [30] and increased toe vibration perception threshold (VPT) [31] are all independent risk factors for mortality. Overall, neuropathic pain, foot complication as well as various associated psychosocial comorbidities inflict a significant diminution on the quality and duration of life of individuals affected by DPN, which in turn is raising an escalating health, social and economic problem in both developed and developing countries [4, 14].

From Animal Models to Clinical Practicality: Lessons Learned from Current Translational Progress of…

http://dx.doi.org/10.5772/55364

31

Unfortunately, current therapy for DPN is far from effective and at best only delays the onset and/or progression of the disease via tight glucose control, the only established means for managing diabetic complications in the U.S. Several large-scale, multicenter and landmark clinical studies, including Diabetes Control and Complication Trial, provided irrefutable evidence that chronic hyperglycemia is a leading factor in the etiology and treatment of DPN [32-36]. However, euglycemia cannot always be achieved through aggressive insulin therapy or other anti-diabetic agents. Even with near normoglycemic control, a substantial proportion of patients still suffer the debilitating neurotoxic consequences of diabetes [34]. On the other hand, some with poor glucose control are spared from clinically evident signs and symptoms of neuropathy for a long time after diagnosis [37-39]. Thus, other etiological factors independ‐ ent of hyperglycemia are likely to be involved in the development of DPN. Data from a number of prospective, observational studies suggested that older age, longer diabetes duration, genetic polymorphism, presence of cardiovascular disease markers, malnutrition, presence of other microvascular complications, alcohol and tobacco consumption, and higher constitu‐ tional indexes (e.g. weight and height) interact with diabetes and make for strong predictors of neurological decline [13, 32, 40-42]. Targeting some of these modifiable risk factors in

Meanwhile, enormous efforts have been devoted to understanding and intervening with the molecular and biochemical processes linking the metabolic disturbances to sensorimotor deficits by studying diabetic animal models. In return, nearly 2,200 articles were published in PubMed central and at least 100 clinical trials were reported evaluating the efficacy of a number of pharmacological agents; the majority of them are designed to inhibit specific pathogenic mechanisms identified by these experimental approaches. Candidate agents have included aldose reductase inhibitors, AGE inhibitors, γ-linolenic acid, α-lipoic acid, vasodilators, nerve growth factor, protein kinase Cβ inhibitors, and vascular endothelial growth factor. Notwith‐ standing a fruitful of knowledge and promising results in animals, none has translated into definitive clinical success (Figure 1). While the notorious biochemical heterogeneity and temporal non-uniformity of the disease processes among and even within individuals can take much of the blame, investigators must take into serious consideration the marked differences between animals and humans, which may substantially impair the application of experimental data to clinical settings. The following sections of this chapter describe the clinical outcomes of these pathogenetic treatments that put previous observations generated by animal studies into perspective, and discuss the molecular, cellular and physiological roots underlying the

addition to glycemia may improve the management of DPN.

**1.2. A medical challenge**

limited translation.

Symptoms begin with prickling, tingling, numbness, paresthesia, dysesthesia and various qualities of pain associated with small sensory fibers at the very distal end (toes) of lower extremities [1, 18]. Presence of the above symptoms together with abnormal nociceptive response of epidermal C and A-δ fibers to pain/temperature (as revealed by clinical examina‐ tion) constitute the diagnosis of small fiber sensory neuropathy, which produces both painful and insensate phenotypes [19]. Painful diabetic neuropathy is a prominent, distressing and chronic experience in at least 10-30% of DPN populations [20, 21]. Its occurrence does not necessarily correlate with impairment in electrophysiological or quantitative sensory testing (QST). Some have suggested pain to reflect the pathobiological changes of serum glucose level at least in individuals with pre- or recent diagnosis. Consistent with this notion, severe neuropathic pain often presents as a typical feature in acute reversible sensory/hyperglycemic neuropathy and its onset and remission following glycemic control can be indicative of spontaneous repair of nerve damage in the early phase of DPN [1, 10, 22, 23]. Pain in many diabetics may persist, however, only to be alleviated as progressive and irreversible nerve deterioration and loss of thermal sensitivity take place [10, 21]. Large myelinated sensory fibers that innervate the dermis, such as Aβ, also become involved later on, leading to impaired proprioception, vibration and tactile detection, and mechanical hypoalgesia [19]. Following this "stocking-glove", length-dependent and dying-back evolvement, neurodegeneration gradually proceeds to proximal muscle sensory and motor nerves. Its presence manifests in neurological testings as reduced nerve impulse conductions, diminished ankle tendon reflex, unsteadiness and muscle weakness [1, 24].

Both the absence of protective sensory response and motor coordination predispose neuro‐ pathic foot to impaired wound healing and gangrenous ulceration—often ensued by limb amputation in severe and/or advanced cases [25, 26]. This traumatic procedure is performed on approximately 100,000 Americans every year and is a major attributing factor for diabetesrelated hospital bed occupancy and medical expenses [27]. Although symptomatic motor deficits only appear in later stages of DPN [25], motor denervation and distal atrophy can increase the rate of fractures by causing repetitive minor trauma or falls [24, 28]. Other unusual but highly disabling late sequelae of DPN include limb ischemia and joint deformity [6]; the latter also being termed Charcot's neuroarthropathy or Charcot's joints [1]. In addition to significant morbidities, several separate cohort studies provided evidence that DPN [29], diabetic foot ulcers [30] and increased toe vibration perception threshold (VPT) [31] are all independent risk factors for mortality. Overall, neuropathic pain, foot complication as well as various associated psychosocial comorbidities inflict a significant diminution on the quality and duration of life of individuals affected by DPN, which in turn is raising an escalating health, social and economic problem in both developed and developing countries [4, 14].

#### **1.2. A medical challenge**

focal/multifocal and diffuse/generalized neuropathies [6, 14]. The first category comprises a group of asymmetrical, acute-in-onset and self-limited single lesion(s) of nerve injury or impairment largely resulting from the increased vulnerability of diabetic nerves to mechanical insults (Carpal Tunnel Syndrome) (reviewed in 15). Such mononeuropathies occur idiopathically and only become a clinical problem in association with aging in 5-10% of those affected. Therefore, focal neuropathies are not extensively covered in this chap‐ ter [16]. The rest of the patients frequently develop diffuse neuropathies characterized by symmetrical distribution, insidious onset and chronic progression. In particular, a distal symmetrical sensorimotor polyneuropathy accounts for 90% of all DPN diagnoses in type 1 and type 2 diabetics and affects all types of peripheral sensory and motor fibers in a

30 Peripheral Neuropathy - A New Insight into the Mechanism, Evaluation and Management of a Complex Disorder

Symptoms begin with prickling, tingling, numbness, paresthesia, dysesthesia and various qualities of pain associated with small sensory fibers at the very distal end (toes) of lower extremities [1, 18]. Presence of the above symptoms together with abnormal nociceptive response of epidermal C and A-δ fibers to pain/temperature (as revealed by clinical examina‐ tion) constitute the diagnosis of small fiber sensory neuropathy, which produces both painful and insensate phenotypes [19]. Painful diabetic neuropathy is a prominent, distressing and chronic experience in at least 10-30% of DPN populations [20, 21]. Its occurrence does not necessarily correlate with impairment in electrophysiological or quantitative sensory testing (QST). Some have suggested pain to reflect the pathobiological changes of serum glucose level at least in individuals with pre- or recent diagnosis. Consistent with this notion, severe neuropathic pain often presents as a typical feature in acute reversible sensory/hyperglycemic neuropathy and its onset and remission following glycemic control can be indicative of spontaneous repair of nerve damage in the early phase of DPN [1, 10, 22, 23]. Pain in many diabetics may persist, however, only to be alleviated as progressive and irreversible nerve deterioration and loss of thermal sensitivity take place [10, 21]. Large myelinated sensory fibers that innervate the dermis, such as Aβ, also become involved later on, leading to impaired proprioception, vibration and tactile detection, and mechanical hypoalgesia [19]. Following this "stocking-glove", length-dependent and dying-back evolvement, neurodegeneration gradually proceeds to proximal muscle sensory and motor nerves. Its presence manifests in neurological testings as reduced nerve impulse conductions, diminished ankle tendon reflex,

Both the absence of protective sensory response and motor coordination predispose neuro‐ pathic foot to impaired wound healing and gangrenous ulceration—often ensued by limb amputation in severe and/or advanced cases [25, 26]. This traumatic procedure is performed on approximately 100,000 Americans every year and is a major attributing factor for diabetesrelated hospital bed occupancy and medical expenses [27]. Although symptomatic motor deficits only appear in later stages of DPN [25], motor denervation and distal atrophy can increase the rate of fractures by causing repetitive minor trauma or falls [24, 28]. Other unusual but highly disabling late sequelae of DPN include limb ischemia and joint deformity [6]; the latter also being termed Charcot's neuroarthropathy or Charcot's joints [1]. In addition to significant morbidities, several separate cohort studies provided evidence that DPN [29],

temporally non-uniform manner [6, 17].

unsteadiness and muscle weakness [1, 24].

Unfortunately, current therapy for DPN is far from effective and at best only delays the onset and/or progression of the disease via tight glucose control, the only established means for managing diabetic complications in the U.S. Several large-scale, multicenter and landmark clinical studies, including Diabetes Control and Complication Trial, provided irrefutable evidence that chronic hyperglycemia is a leading factor in the etiology and treatment of DPN [32-36]. However, euglycemia cannot always be achieved through aggressive insulin therapy or other anti-diabetic agents. Even with near normoglycemic control, a substantial proportion of patients still suffer the debilitating neurotoxic consequences of diabetes [34]. On the other hand, some with poor glucose control are spared from clinically evident signs and symptoms of neuropathy for a long time after diagnosis [37-39]. Thus, other etiological factors independ‐ ent of hyperglycemia are likely to be involved in the development of DPN. Data from a number of prospective, observational studies suggested that older age, longer diabetes duration, genetic polymorphism, presence of cardiovascular disease markers, malnutrition, presence of other microvascular complications, alcohol and tobacco consumption, and higher constitu‐ tional indexes (e.g. weight and height) interact with diabetes and make for strong predictors of neurological decline [13, 32, 40-42]. Targeting some of these modifiable risk factors in addition to glycemia may improve the management of DPN.

Meanwhile, enormous efforts have been devoted to understanding and intervening with the molecular and biochemical processes linking the metabolic disturbances to sensorimotor deficits by studying diabetic animal models. In return, nearly 2,200 articles were published in PubMed central and at least 100 clinical trials were reported evaluating the efficacy of a number of pharmacological agents; the majority of them are designed to inhibit specific pathogenic mechanisms identified by these experimental approaches. Candidate agents have included aldose reductase inhibitors, AGE inhibitors, γ-linolenic acid, α-lipoic acid, vasodilators, nerve growth factor, protein kinase Cβ inhibitors, and vascular endothelial growth factor. Notwith‐ standing a fruitful of knowledge and promising results in animals, none has translated into definitive clinical success (Figure 1). While the notorious biochemical heterogeneity and temporal non-uniformity of the disease processes among and even within individuals can take much of the blame, investigators must take into serious consideration the marked differences between animals and humans, which may substantially impair the application of experimental data to clinical settings. The following sections of this chapter describe the clinical outcomes of these pathogenetic treatments that put previous observations generated by animal studies into perspective, and discuss the molecular, cellular and physiological roots underlying the limited translation.

and discuss the molecular, cellular and physiological roots underlying the limited translation.

health, social and economic problem in both developed and developing countries [4, 14].

**1.2. A medical challenge** 

but highly disabling late sequelae of DPN include limb ischemia and joint deformity [6]; the latter also being termed Charcot's neuroarthropathy or Charcot's joints [1]. In addition to significant morbidities, several separate cohort studies provided evidence that DPN [29], diabetic foot ulcers [30] and increased toe vibration perception threshold (VPT) [31] are all independent risk factors for mortality. Overall, neuropathic pain, foot complication as well as various associated psychosocial comorbidities inflict a significant diminution on the quality and duration of life of individuals affected by DPN, which in turn is raising an escalating

Unfortunately, current therapy for DPN is far from effective and at best only delays the onset and/or progression of the disease via tight glucose control, the only established means for managing diabetic complications in the U.S. Several large-scale, multicenter and landmark clinical studies, including Diabetes Control and Complication Trial, provided irrefutable evidence that chronic hyperglycemia is a leading factor in the etiology and treatment of DPN [32-36]. However, euglycemia cannot always be achieved through aggressive insulin therapy or other anti-diabetic agents. Even with near normoglycemic control, a substantial proportion of patients still suffer the debilitating neurotoxic consequences of diabetes [34]. On the other hand, some with poor glucose control are spared from clinically evident signs and symptoms of neuropathy for a long time after diagnosis [37-39]. Thus, other etiological factors independent of hyperglycemia are likely to be involved in the development of DPN. Data from a number of prospective, observational studies suggested that older age, longer diabetes duration, genetic polymorphism, presence of cardiovascular disease markers, malnutrition, presence of other microvascular complications, alcohol and tobacco consumption, and higher constitutional indexes (e.g. weight and height) interact with diabetes and make for strong predictors of neurological decline [13, 32, 40-42].

Meanwhile, enormous efforts have been devoted to understanding and intervening with the molecular and biochemical processes linking the metabolic disturbances to sensorimotor deficits by studying diabetic animal models. In return, nearly 2,200 articles were published in PubMed central and at least 100 clinical trials were reported evaluating the efficacy of a number of pharmacological agents; the majority of them are designed to inhibit specific pathogenic mechanisms identified by these experimental approaches. Candidate agents have included aldose reductase inhibitors, AGE inhibitors, γ-linolenic acid, α-lipoic acid, vasodilators, nerve growth factor, protein kinase Cβ inhibitors, and vascular endothelial growth factor. Notwithstanding a fruitful of knowledge and promising results in animals, none has translated into definitive clinical success (Figure 1). While the notorious biochemical heterogeneity and temporal non-uniformity of the disease processes among and even within individuals can take much of the

substantially impair the application of experimental data to clinical settings. The following sections of this chapter describe the clinical outcomes of these pathogenetic treatments that put previous observations generated by animal studies into perspective,

> of these were preventable and reversible by blocking AR [47-51]. In mice, transgenic overex‐ pression of the gene encoding human AR (hAR) in lens epithelia submitted these cataractresistant animals to sugar-induced polyol deposit and cataract formation, which became more acute when coupled with genetic SDH deficiency [52]. Studies of type 1 and type 2 diabetes models, including alloxan- and streptozotocin (STZ)-induced diabetic rats and leptin-deficient *ob*/*ob* mice, soon confirmed a significant elevation of sorbitol and fructose in sciatic nerves, dorsal root ganglia (DRGs) and spinal cord. This correlated with nerve/axonal conduction and transport deficiencies, loss of intraepidermal nerve fibers, increased neural and endoneurial oxidative-nitrosative stress as well as thermal hypoalgesia and tactile allodynia [43, 53-57]. A "polyol hypothesis" derived from diabetic lens was thus propelled to the pathogenesis of DPN [47]. In keeping with this notion, AR inhibitors that reduce nerve polyol levels showed remarkable preservation of nerve structure and function in rats with either spontaneous or chemical-induced diabetes [53, 58-60]. Systemic hAR overexpression combined with STZinduced diabetes led to an exacerbated but AR inhibitor-preventable peripheral nerve sorbitol and fructose buildup, electroactivity suppression and myelinated fiber atrophy [61]. A similar biochemical and electrophysiological but not morphological abnormality was obtained with Schwann cell (SC)-targeted hAR transgenic mice, indicating that SC AR hyperactivity con‐ tributes to many, though not all pathological change of DPN [62]. Conversely, AR-knockout mice showed no obvious sorbitol accumulation, conduction slowing, oxidative stress, or stress kinase activation. Additionally, there were fewer loss of sural nerve fibers in AR-deficient mice compared to wild-types (WTs) [63]. Since galactose has approximately 4 times higher affinity for AR than glucose [64] and its reduction product galactitol is poorly disposed, galactose-rich diet was used as a popular substitute for classical hyperglycemic models to exemplify and examine the role of excessive polyalcohol formation in the genesis of diabetic cataract and neuropathy [47]. Along the line with "aldo-osmotic theory", galactosemic rodents that accrue much greater level of this alternate AR metabolite also exhibit similar and sometimes more severe electrophysiological, anatomical and biochemical defects that are seen diabetic models [65-67]. However, galactosemia is a rare metabolic condition in humans (less than 0.002% of the population) [68] and the galactosemic lens and nerves often manifest functional and structural lesions resulting from acute and exaggerated galactitol intoxication that differ from those of diabetic cataract and neuropathy [47, 69-71]. Hence, galactose-fed animals are neither appropriate models for studying diabetic complications nor good replacements for character‐ izing the pathogenetic involvement of sorbitol pathway in these conditions. Other studies further revealed that neither the morphometrical [59] nor functional indices in DPN correlate with the tissue sorbitol content [72, 73]. Instead, nerve myo-inositol content is more closely related to the neurophysiological function according to most reports. Depletion of cytoplasmic

From Animal Models to Clinical Practicality: Lessons Learned from Current Translational Progress of…

http://dx.doi.org/10.5772/55364

33

myo-inositol, protein kinase C activation and tubulin/Na+

oxidative and vascular injury [63, 76, 77].

+

/K+

proposed mechanisms that mediate polyol pathway overflow-induced impairment of Na+



/K

Targeting some of these modifiable risk factors in addition to glycemia may improve the management of DPN.

Figure 1. Summary of Current Clinical Status of Anti-DPN Drugs Developed via Animal Models. Data are generated from published experimental and clinical results to date on pharmacological agents (a total of 23 drugs) targeting pathogenetic mechanisms listed in but not limited to section 2. **Figure 1.** Summary of Current Clinical Status of Anti-DPN Drugs Developed via Animal Models. Data are generated from published experimental and clinical results to date on pharmacological agents (a total of 23 drugs) targeting pathogenetic mechanisms listed in but not limited to section 2.
