DIABETIC NEUROPATHY: CLINICAL MANAGEMENT - PART 7 pdf

ARIs for the Treatment of Diabetic Neuropathy 311Handelsman 1981 sb, nonrmd, co 4 months Symptomatic improvementFagious 1981 db, rmd 12 weeks Improvement of symptoms, VPT and ulnar mcv S

facilitate the recruitment of homogeneous study subject populations Second, correction

or partial correction of abnormalities in regeneration is a concrete target with a clearclinical interpretation Furthermore, an improvement in the regeneration rate will pre-cede an improvement in nerve fiber density and is likely to be a more sensitive meas-ure Finally, such a study designed to detect a 50% normalization of regeneration with80% power in nonneuropathic subjects with diabetes would require about 65 subjectsper treatment arm A recent trial of Timcodar dimesylate in healthy control subjectsused such measures of collateral and regenerative sprouting as outcome measures.Although the compound did not accelerate regeneration by either measure, the trial diddemonstrate that such an approach was feasible and the measures were reproducible and

robust (42).

In conclusion, skin biopsy with determination of epidermal nerve fiber density is apowerful tool that provides investigators insight into a population of nerve fibers that isprominently affected in diabetes and yet has been relatively under investigated The super-ficial nature of epidermal nerve fibers allows repeated sampling of these nerves in a

Fig 7 For each subject, a regression line from postcapsaicin time-points is generated and the

slope of this line is used as the rate of regeneration The mean line for each group is shown as athick solid line The rate of regeneration following denervation is 0.177 ± 0.075 fibers permm/day for control subjects (red), 0.10 ± 0.07 fibers per mm/day (p = 0.03) for subjects with

diabetes but no neuropathy (green), and 0.04 ± 0.03 fibers per mm/day (p = 0.03) for subjects

with diabetes and neuropathy (blue)

relatively non invasive fashion, and in sites that cannot be assessed through conventionalelectrodiagnostical techniques These features have allowed investigators to diagnoseneuropathy earlier and to define an association between neuropathy and impaired glucosetolerance Finally, the ability to injure these fibers in a standardized fashion has led tonovel measures of human axonal regeneration that may provide a more sensitive scale

by which to assess promising regenerative compounds

tary spastic paraplegia Neurology 1966;16(1):7–10.

3 Dalsgaard CJ, Rydh M, Haegerstrand A Cutaneous innervation in man visualized with

protein gene product 9.5 (PGP 9.5) antibodies Histochemistry 1989;92(5):385–390.

4 Kennedy WR, Wendelschafer-Crabb G, Brelje TC Innervation and vasculature of humansweat glands: an immunohistochemistry-laser scanning confocal fluorescence microscopy

study J Neurosci 1994;14(11 Pt 2):6825–6833.

5 Nolano M, Provitera V, Crisci C, et al Quantification of myelinated endings and

mechanoreceptors in human digital skin Ann Neurol 2003;54(2):197–205.

6 Lombardi R, Erne B, Lauria G, et al IgM deposits on skin nerves in anti-myelin-associated

glycoprotein neuropathy Ann Neurol 2005;57(2):180–187.

7 McArthur JC, Stocks EA, Hauer P, Cornblath DR, Griffin JW Epidermal nerve fiber

density:normative reference range and diagnostic efficiency Arch Neurol 1998;55(12):

1513–1520

8 Kennedy WR, Wendelschafer-Crabb G, Polydefkis M, McArthur JC Pathology and

quan-titation of cutaneous innervation, in Peripheral Neuropathy (Dyck PJ, Thomas PK, eds.)

Elsevier Saunders, Philadelphia, 2005, pp 869–897

9 Holland NR, Stocks A, Hauer P, Cornblath DR, Griffin JW, McArthur JC Intraepidermal nerve

fiber density in patients with painful sensory neuropathy Neurology 1997;48(3):708–711.

10 Polydefkis M, Yiannoutsos C, Cohen B, et al Reduced intraepidermal nerve fiber density

in HIV-associated sensory neuropathy Neurology 2002; in press.

11 Rowbotham MC, Yosipovitch G, Connolly MK, Finlay D, Forde G, Fields HL Cutaneous

innervation density in the allodynic form of postherpetic neuralgia Neurobiol Dis

1996;3(3):205–214

12 Kennedy WR, Wendelschafer-Crabb G, Johnson T Quantitation of epidermal nerves in

diabetic neuropathy Neurology 1996;47(4):1042–1048.

13 Silos-Santiago I, Molliver DC, Ozaki S, et al Non-TrkA-expressing small DRG neurons

are lost in TrkA deficient mice J Neurosci 1995;15(9):5929–5942.

14 Kennedy WR, Nolano M, Wendelschafer-Crabb G, Johnson TL, Tamura E A skin blister

method to study epidermal nerves in peripheral nerve disease Muscle Nerve

1999;22(3):360–371

15 Kennedy WR, Wendelschafer-Crabb G The innervation of human epidermis J Neurol Sci

1993;115(2):184–190

16 McCarthy BG, Hsieh ST, Stocks A, et al Cutaneous innervation in sensory neuropathies:

evaluation by skin biopsy Neurology 1995;45(10):1848–1855.

17 Lauria G, Borgna M, Morbin M, et al Tubule and neurofilament immunoreactivity in

human hairy skin: markers for intraepidermal nerve fibers Muscle Nerve 2004;30(3):

310–316

18 Polydefkis M, Hauer P, Sheth S, Sirdofsky M, Griffin JW, McArthur JC The time course

of epidermal nerve fibre regeneration: studies in normal controls and in people with

dia-betes, with and without neuropathy Brain 2004;127(Pt 7):1606–1615.

19 Lauria G, Morbin M, Borgna M, et al Vanilloid receptor (VR1) expression in human

peripheral nervous system J Periph Nerv Sys 2003;8(S1):1–78.

20 Herrmann DN, Griffin JW, Hauer P, Cornblath DR, McArthur JC Epidermal nerve fiber

density and sural nerve morphometry in peripheral neuropathies Neurology 1999;53(8):

logical function Diabetologia 1992;35(9):889–897.

23 Brown MJ, Martin JR, Asbury AK Painful diabetic neuropathy A morphometric study

Arch Neurol 1976;33(3):164–171.

24 Hsieh ST, Chiang HY, Lin WM Pathology of nerve terminal degeneration in the skin

J Neuropathol Exp Neurol 2000;59(4):297–307.

25 Pan CL, Tseng TJ, Lin YH, Chiang MC, Lin WM, Hsieh ST Cutaneous innervation in

Guillain-Barre syndrome: pathology and clinical correlations Brain 2003;126(Pt 2):

386–397

26 Lauria G, Morbin M, Lombardi R, et al Axonal swellings predict the degeneration of

epidermal nerve fibers in painful neuropathies Neurology 2003;61(5):631–636.

27 Shun CT, Chang YC, Wu HP, et al Skin denervation in type 2 diabetes: correlations with

diabetic duration and functional impairments Brain 2004;127(Pt 7):1593–1605.

28 Sustained effect of intensive treatment of type 1 diabetes mellitus on development and gression of diabetic nephropathy:the Epidemiology of Diabetes Interventions and

pro-Complications (EDIC) study Jama 2003;290(16):2159–2167.

29 Singleton JR, Smith AG, Bromberg MB Increased prevalence of impaired glucose

tolerance in patients with painful sensory neuropathy Diabetes Care 2001;24(8):

1448–1453

30 Singleton JR, Smith AG, Bromberg MB Painful sensory polyneuropathy associated with

impaired glucose tolerance Muscle Nerve 2001;24(9):1225–1228.

31 Sumner CJ, Sheth S, Griffin JW, Cornblath DR, Polydefkis M The spectrum of

neuropa-thy in diabetes and impaired glucose tolerance Neurology 2003;60(1):108–111.

32 Hughes RA, Umapathi T, Gray IA, et al A controlled investigation of the cause of chronic

idiopathic axonal polyneuropathy Brain 2004;127(Pt 8):1723–1730.

33 Smith AG, Singleton JR Peripheral neuropathy and the metabolic syndrome Annals of Neurology 2005;58(S9):S31.

34 Knowler WC, Barrett-Connor E, Fowler SE, et al Reduction in the incidence of type 2

dia-betes with lifestyle intervention or metformin N Engl J Med 2002;346(6):393–403.

35 McArthur JC, Yiannoutsos C, Simpson DM, et al A phase II trial of nerve growth factorfor sensory neuropathy associated with HIV infection AIDS Clinical Trials Group Team

291 Neurology 2000;54(5):1080–1088.

36 Hart AM, Wilson AD, Montovani C, et al Acetyl-l-carnitine: a pathogenesis based

treat-ment for HIV-associated antiretroviral toxic neuropathy Aids 2004;18(11):1549-1560.

37 Pittenger GL, Simmons K, Anandacoomaraswamy D, Rice A, Barlow P, Vinik A.Topiramate improves intraepidermal nerve fiber morphology and quantitative measures in

diabetic neuropathy patients J Periph Nerv Sys 2005;10(S1):73.

38 Rajan B, Polydefkis M, Hauer P, Griffin JW, McArthur JC Epidermal reinnervation after

intracutaneous axotomy in man J Comp Neurol 2003;457(1):24–36.

39 Hahn K, Brown A, Hauer P, McArthur J, Polydefkis M Epidermal reinnervation aftermechanical intracutanous axotomy in skin biopsies in normal controls and in people with

HIV Neurology 2005;64(6 S1):A245–A246.

40 Simone DA, Nolano M, Johnson T, Wendelschafer-Crabb G, Kennedy WR Intradermalinjection of capsaicin in humans produces degeneration and subsequent reinnervation

of epidermal nerve fibers: correlation with sensory function J Neurosci 1998;18(21):

8947–8959

41 Nolano M, Simone DA, Wendelschafer-Crabb G, Johnson T, Hazen E, Kennedy WR.Topical capsaicin in humans: parallel loss of epidermal nerve fibers and pain sensation

Pain 1999;81(1–2):135–145.

42 Polydefkis M, Sirdofsky M, Hauer P, Petty BG, Murinson BB, McArthur JC Factors

influ-encing nerve regeneration in a trial of Timcodar dimesylate Neurology 2006; 66(2):259–261.

43 Polydefkis M, Hauer P, Griffin JW, McArthur JC Skin biopsy as a tool to assess distal

small fiber innervation in diabetic neuropathy Diabetes Technol Ther 2001;3:23–28.

44 Polydefkis M, Griffin JW, McArthur J New insights into diabetic polyneuropathy JAMA

2003;290:1371–1376

It has been more than 30 years since the first aldose reductase inhibitor (ARI) was tested

in diabetic and galactosemic rats and found to control the polyol accumulation Since then, aconsiderable number of ARIs have been tested in experimental and human diabetes Despitethe initial encouraging results from tests that were conducted for the past 20 years, ARIs havenot been established for the treatment of diabetic neuropathy yet The main reasons for this areinconsistent results and the unacceptable high rate of side-effects associated with the initiallytested compounds The lack of well-defined end points and the inability to produce an inhibitorthat achieves satisfactory tissue penetration and enzyme inhibition are other major contributingfactors for this failure This chapter focuses on the clinical trials that have examined the effect ofall tested ARIs on human diabetic neuropathy

Key Words: Aldose reductase inhibitors; clinical trials; human diabetic neuropathy; efficacy;

side effects; clinical use

INTRODUCTION

It has been more than 30 years since the first aldose reductase inhibitor (ARI) wastested in diabetic and galactosemic rats and found to control the polyol accumulation

(1) Since then, a considerable number of ARIs have been tested in experimental and

human diabetes However, very few new information have become available since thelast edition of this book, probably an indication that either interest in these compounds

is waning down in the scientific community or that despite all the intensive efforts, theideal compound that will offer satisfactory enzyme inhibition with minimal side-effecthas not been discovered as yet A thorough review of work on experimental diabeteswould be out of the spirit of this chapter; however, more information is provided inchapters of this edition The following chapter will focus on the results from clinical

trials in diabetic neuropathy (2).

END POINTS FOR CLINICAL TRIALS IN DIABETIC NEUROPATHY

Painful symptoms and foot ulceration are the two most important clinical problemsrelated to peripheral somatic diabetic neuropathy The conduction of clinical trials,which test the efficacy of new therapies for painful neuropathy is straight forward:

From: Contemporary Diabetes: Diabetic Neuropathy: Clinical Management, Second Edition

Edited by: A Veves and R Malik © Humana Press Inc., Totowa, NJ

309

patients with this condition are provided trial medication and the primary end point isthe reduction of the symptoms, which is expected to occur during a reasonable periodafter the treatment has been initiated In contrast, foot ulcers develop long after theinitiation of events which lead to nerve damage and, by this time, the possibility ofrestoring the nerve lesions, or halting their progression, is close to nonexisting.Therefore, if a study was to be conducted having as primary end point the prevention

of foot ulceration it should involve patients who have diabetic neuropathy in the earlystages and follow them until they reach the very late stages of the disease This wouldmean that a large number of patients should be followed for prolonged periods oftime, even decades, before any conclusion would be reached

It is obvious from the aforementioned that more practical end points should be used

in order to conduct clinical therapeutic trials, which will be financially supported by thepharmaceutical industry where efficient development of new medications are of para-mount importance In addition, these end points should give an accurate and moredetailed picture about the effects of the treatment on the progression of the disease,mainly to what extent it can restore the already established lesions

Sural nerve biopsies were initially considered to be the best method for evaluatingnew medications However, the interpretation of the biopsy results was found to be moredifficult than was originally believed and as a result, nerve biopsies fall out of favor.Most recent studies have used surrogate measurements, mainly nerve electrophysiolog-ical measurements and quantitative sensory testing Regarding the electrophysiologicalmeasurements, Dyck and O’Brian, based on epidemiological data, initially suggestedthat a mean change of 2.9 m per second in the combined conduction velocities of theulnar, median, and peroneal nerves, or a change of 2.2 m per second in the peronealnerve alone should be achieved in order that the results can have a meaningful clinical

significance (4) However, it should be emphasized that the selection of end points is not

the only issue with the design of new clinical trials Thus, the current prevailing ion is that future studies will have to include large number of patients and be of longenough duration (probably around 18–24 months), pay particular attention to the vari-ability of the end point measurements, and have rigorous quality control in order to allowthe drawing of definite conclusions regarding the efficacy of ARIs in treating diabetic neuropathy

opin-CLINICAL TRIALS WITH ARIS

Alrestatin

Alrestatin was the first ARI to be tried in human diabetic neuropathy In the first,uncontrolled study conducted in 1981, 10 patients with symptomatic neuropathy were

treated with intravenous infusions of alrestatin for 5 days (5) Although, symptomatic

improvement was noticed in seven patients, objective measurements failed to improve.Therefore, as the trial was not controlled, a placebo effect accounting for the sympto-matic improvement cannot be excluded No adverse effects of alrestatin were noticed inthis trial (Table 1)

The next trial included nine patients with diabetes with severe symptomatic

neuro-pathy, which had necessitated at least one hospital admission before the study (6) The

trial was a single-blind, nonrandomized, placebo-crossover, which lasted for 4 months.Each patient received the maximum tolerated oral dose for 2 months and was onplacebo for the other two Subjective improvement was noted by most of the patients

ARIs for the Treatment of Diabetic Neuropathy 311

Handelsman (1981) sb, nonrmd, co 4 months Symptomatic improvementFagious (1981) db, rmd 12 weeks Improvement of symptoms,

VPT and ulnar mcv

Sorbinil

Judzewitsch (1983) db, rmd 9 weeks Improvement of peroneal mcv

and median mcv and scv

Young (1983) db, rmd, co 4 weeks Improvement of symptoms

and sural sap

Fagious (1985) db, rmd 6 months Improvement of posterior

tibial mcv and ulnar nevre

F wl and dsl

sural sap and mfd

Ponalrestat

Tolrestat

Boulton (1990) db, rmd 12 months Improvement of paraesthetic

symptoms and peroneal mcvMacleod (1992) db, rmd 6 months Improvement of VPT, median

and ulnar mcvBoulton (1992) db, rmd, 12 months Improvement of symptoms,

Giugliano (1993) db, rmd 12 months Improvement of autonomic

measurements and VPTGiugliano (1995) db, rmd 12 months Improvement of autonomic

measurements and VPTDidangelos (1999) db, rmd 24 months Improvement of autonomic

measurementsGreene (1999) db, rmd 12 months Increase in small diameter

myelinated fibersHotta (2001) db, rmd 12 months Improvement of symptoms,

median fcv and median F-wave latency

Johnson (2004) db, rmd 12 months Exercise LVEF and cardiac

stroke volume

sb, single blind; db, double-blind; Uncntr, uncontrolled; nonrmd, nonrandomized; rmd, randomized;

co, crossover; mcv, motor nerve conduction velocity; scv, sensory nerve conduction velocity; sap, sensory action potential; wl, wave latency; dsl, distal sensory latency; VPT, vibration perception threshold; mfd, myelinated fibre density.

(eight out of nine), but electrophysiological measurements remained virtually unchanged.The most notable side-effects were nausea, and photosensitivity, which was severe in two cases.

Around the same time, the most comprehensive trial of alrestatin was conducted.Thirty patients with long-standing diabetes and mild to moderate neuropathy were stud-

ied in a double-blind, randomized, placebo-controlled trial, which lasted 12 weeks (7).

Symptomatic improvement, reduction of the sensory impairment score, and ment of vibration perception threshold and ulnar nerve conduction velocity werenoticed, but the rest of the electrophysiological measurements in the median, peroneal,and sural nerves did not show any significant difference

improve-The earlier-mentioned studies indicated that treatment with ARIs might be helpful intreating diabetic neuropathy and also highlighted the need for well-conducted long-termtrials in order to fully explore the potential of this new therapeutic approach On thedown side, the high incidence of side-effects of alrestatin prohibited its further devel-opment This led the way for using some newly discovered compounds such as sorbiniland tolrestat

thy (8) The design of the study was randomized, double-blind, crossover and each

patient received active treatment for 9 weeks The results showed a small but tically significant increase of the conduction velocity of the peroneal motor nerve(0.70 m per second), the median motor nerve (0.66 m per second), and the mediansensory nerve (1.16 m per second) during the treatment with the active drug Anotherimportant finding was that the increase declined rapidly after cessation of the treat-ment so that the nerve conduction velocity was similar to pretreatment levels 3 weekslater Five patients were withdrawn from the study because of fever and rash, whichwere attributed to sorbinil

statis-In contrast with the previous trial, the ones which followed included mainly patientswith diabetes with symptomatic neuropathy The first one studied 11 patients withseverely painful neuropathy who failed to respond to conventional treatment with anal-

gesics or tricyclic antidepressants (10) In a single blind design the patients were treated

with sorbinil for 3–5 weeks and the pain relief was measured using a graphic scale.Marked to moderate pain relief was noted in eight patients usually 3–4 days after being

on treatment, whereas the pain returned to pretreatment levels in seven of the ders when they stopped taking the drug The motor and sensory conduction velocities

respon-of the median nerve improved in four patients, whereas the peroneal motor conductionvelocity improved in two patients It is of interest however, that in four patients whoresponded to the treatment the pain was related to proximal motor neuropathy, a condi-tion, which is thought to be caused by mechanisms not related to polyol accumulation

No significant side-effects were noted in the 11 patients who finished the study whereas12th patient who started the study was withdrawn because of rash

The next study had a double-blind, randomized, placebo-controlled crossover design,and included 15 patients with painful symptoms, which were present for more than

1 year (10) The patients were observed for 16 weeks but they were on active treatment

for only 4 weeks, either from week 5 to 8 or from 9 to 12 Painful symptoms wereassessed using a standardized symptom score, whereas other measurements includedneurological findings on clinical examination, vibration perception threshold, motor andsensory nerve conduction velocities, and autonomic system function tests A significantnumber of patients reported improvement of painful symptoms while on the active treat-ment, but when the pain score was calculated using their diaries no difference was foundbetween sorbinil and placebo treatment Significant improvement was also noticed inthe sural sensory potential action, whereas the rest of the electrophysiological measure-ments remained unchanged The number of patients who withdrew because of side-effects (mainly rash and fever) had increased in comparison with the previous study;four patients in total had an idiosyncratic reaction which resolved rapidly after the dis-continuation of the drug

The next trial used the same layout, i.e., double-blind, placebo-controlled crossover,and included 13 patients with diabetes with chronic symptomatic neuropathy (mean

duration of symptoms 6 years) (11) The duration of treatment with sorbinil was the

same as in the previous trial, 4 weeks out of a total study period of 16 weeks The painintensity was measured using a 100-mm visual analog scale whereas other measure-ments included vibration perception threshold, motor and sensory conduction veloci-ties, autonomic function tests, and duration of sleep In contrast to the previous study,

no difference was found in any parameter, including the severity of neuropathic toms and the objective measurements of peripheral nerve function Side-effects werepresent only in one patient who took sorbinil in the form of a febrile rash necessitatinghis withdrawal from the study

symp-The aforementioned short-term trials were followed by long-term ones, which ined the effects of aldose reductase inhibition for periods of 6–12 months The firstlong-term study studied 55 male patients with diabetes with symptomatic neuropathy

exam-for 6 months in a double-blind placebo-controlled parallel group design (12) To avoid

a possible long-term effect of the drug, the authors elected to randomize their patients

to active- and placebo-treatment and to avoid the crossover design Patients assessmentincluded clinical examination, neurophysiological measurements, thermal and vibrationperception thresholds, and autonomic system function tests

No significant improvement was found in the sorbinil-treated group when it wascompared with the placebo group, although three sorbinil-treated patients reported amarked overall improvement compared with none from the placebo group When thesethree patients were compared with the whole sorbinil-treated group their age was lowerthan the mean group age and the neuropathy assessed by electrophysiology was lesssevere All three patients worsened to pretreatment levels when sorbinil was discontinued

No significant changes were found in the vibration and thermal discrimination old From the electrophysiological measurements improvement was noticed in themotor posterior tibial nerve conduction velocity (approximately 1.5 m per second),F-wave latency of the ulnar nerve, and the distal sensory latency of the ulnar nerve Fromthe autonomic tests, a significant improvement in the R–R interval variation during deep

breathing was found in the sorbinil-treated group The number of patients with seriousside-effects was smaller in this study; only two patients had to be withdrawn from thestudy because of rash and lymphadenopathy.

The next long-term study included 31 patients with mild to moderate neuropathy and

lasted for 14 months (including a 2-month run-in period) (13) The study was designed

as double-blind, randomized, placebo-controlled, and two-third of patients were treatedwith sorbinil whereas one-third received placebo Assessments of the patients responsewere performed every 3 months and included the measurement of symptoms such

as pain, tingling, and temperature insensitivity using a 100 mm visual analog scale, ical examination, vibration perception thresholds, electrophysiology, and autonomicfunction tests The results indicated no benefit for the sorbinil-treated patients in any ofthe measured parameters In addition, as similar doses of the drug were used in this trialand the previous ones, and was accompanied by serum sorbinil levels measurements,inadequate drug dosage or poor patient compliance could not be held responsible for the observed discrepancies Hypersensitivity reactions with fever, rash, and myalgiaoccurred in two patients who recovered completely after the drug was discontinued

clin-No improvement was also found in another double-blind, randomized trial, whichlasted for 12 months and included patients with severe neuropathy with or without

symptoms (14) Thirty nine patients took part in this study and the severity of

neuropa-thy is indicated by the fact that a history of foot ulceration was present in 21 patients.Efficacy assessments included clinical evaluation, vibration and thermal perceptionthresholds, nerve conduction velocities in 12 nerves, and somatosensory-evoked poten-tials The results showed no difference in any of the above measurements betweensorbinil and placebo-treated patients, both for the lower and upper extremities, despitethe fact that the arms were less severely affected

As it can be seen from the aforementioned studies, the beneficial results, which wereinitially reported failed to be confirmed in subsequent, better designed, long-term trials

In an effort to clear the confusion, the next trial used sural nerve biopsies, which allow

more precise evaluation of the therapeutic efficacy (15) This trial included 16 patients

with established peripheral neuropathy and involved subjects undergoing fascicular

sural nerve biopsies of the same limb at the beginning and the end of the study (16) The

design of the trial was double-blind, randomized, placebo-controlled, and lasted

12 months Additional investigations included clinical neurological assessments, mal perception thresholds, and electrophysiological measurements Although bothactively- and placebo-treated groups showed some clinical improvement at the end ofthe study this was more pronounced in the sorbinil-treated group The nonbiopsised suralnerve of the sorbinil group showed an improvement of 1 µV in the action-potentialamplitude and of 2 m per second in the sensory conduction velocity (2 m per second),results that were not found in the placebo group

ther-The analysis of the sural nerve biopsies showed that the sorbitol levels in the sorbinilgroup were reduced, indicating a successful aldose reductase inhibition in the nerve tissue The myelinated fibers density, the best single histopathological criterion to quan-tify neuropathy, was similarly reduced at baseline by 50% in both the sorbinil andplacebo groups when they were compared with age-matched nondiabetic subjects After

12 months of treatment, a significant increase of 33% was found in the sorbinil group,

whereas no difference was noted in the placebo group The regeneration and nation activity in the sorbinil group was also increased, whereas no change was noticed

remyeli-in the placebo group Important changes were also noticed remyeli-in the degree of paranodaldemyelination, segmental demyelination, and myelin wrinkling The main importance ofthis study lies in the fact that it was the first to demonstrate morphological improvements

in nerve biopsies after long-term aldose reductase inhibition in humans and suggestedthat long-term treatment in properly selected patients might be the most beneficial

A second clinical trial, which used repeated sural nerve biopsies, assessed thechanges in nerve concentrations of alcohol sugars after a 12-month period with sorbinil

treatment (17) Six patients took part in this study and histochemical measurements

showed a significant decrease in nerve sorbitol and fructose levels in the follow-up visitcompared with baseline, whereas the levels of glucose and myo-inositol remainedunchanged The earlier findings were interpreted by the investigators as indicating thatsorbinil is an effective inhibitor of aldose reductase, but raised doubts about the role ofmyo-inositol in the pathogenesis of diabetic neuropathy

A common factor, present in virtually all the earlier studies which used sorbinil wasthe relatively high rate of side-effects The main adverse reactions were rash, fever, andlymphadenopathy, which subsided when the drug was discontinued Nevertheless,these adverse reactions would make the use of sorbinil for prolonged period of time

in relatively asymptomatic patients unacceptable, and therefore, the compound waswithdrawn

Ponalrestat

The main characteristic of ponalrestat compared with the previous two drugs was itssafety profile: very few adverse reaction were reported during the preliminary safety tri-als, making it ideal for long-term usage These early expectations were soon dashed as

it became apparent that the nerve-tissue concentration levels were probably insufficient

to inhibit aldose reductase Therefore, it is hardly surprising that the few properly ducted trials with this compound reported negative results, despite some modest

con-improvements, which were reported in short, preliminary trials (18,19).

An example of a published paper with ponalrestat was that by Ziegler et al (20), who

reported a randomized, double-blind, placebo-controlled trial of 60 patients withchronic symptomatic peripheral diabetic neuropathy for 12 months No difference inany peripheral nerve function measurements, including electrophysiology, were docu-mented at the end of the study As was expected, the drug was well tolerated and no sig-nificant side-effects were present during the study Similar results were subsequently

reported by Krentz et al (21) in a study with almost identical design.

Tolrestat

Tolrestat was the first ARI to be licensed for the treatment of diabetic neuropathy incertain countries all over the world including Italy, Mexico, and Ireland Given orally,tolrestat is rapidly absorbed at a rate of 60–70% Its plasma half-life is 10 hours and inclinical practice a dose of 200 mg per day is sufficient to provide satisfactory inhibition

of the aldose reductase for 24 hours Excretion is mainly through kidneys (70%),whereas a further 25% of the dose is excreted in the feces

In a multicenter, double-blind, randomized, placebo-controlled trial, which lasted for

12 months the efficacy of tolrestat on symptomatic neuropathy was studied in 556 patients

with either type 1 or type 2 diabetes (22) Inclusion criteria were stable or increasing

severity of neuropathic symptoms, and abnormal motor or sensory nerve logical measurements in at least three of six tested nerves Patients were randomized todoses from 50 to 200 mg daily and efficacy assessments included the response of thepainful and paraesthetic symptoms and electrophysiological measurements

electrophysio-The painful symptoms improved in both the tolrestat- and placebo-treated patients butthe paraesthetic symptoms improved significantly in patients treated with 200 mg tolre-stat daily over placebo From the objective measurements, a significant improvement (up to 2 m per second) was noticed for the tibial and peroneal nerve conduction veloci-ties when they were compared both with the baseline measurements and with theplacebo-treated group Improvement in both symptoms and electrophysiological meas-urements was found in 28% of tolrestat-treated patients, significantly higher when com-pared with the 5% of the placebo-treated patients who had a similar response Theadverse reaction profile of the tolrestat was also satisfactory The only symptom whichoccurred more frequently in the tolrestat group was dizziness Elevation of transaminaseswas found in 13 (2.9%) patients with diabetes treated with tolrestat on any dose, but thetransaminases returned to normal levels within 8–16 weeks after the drug was discontin-ued There was no evidence of severe liver dysfunction in any of the patients A small butsignificant drop of the blood pressure, up to 7 mmHg in the systolic and 3.4 mm in thediastolic was also noticed without any consequences No hypersensitivity reactions sim-ilar to the ones which were present with other ARIs were noticed

The same design with the previous study was adopted by a multicentre European study,

which enrolled 190 patients with symptomatic diabetic neuropathy (23) The study lasted

for 6 months and patients were randomized to take either placebo or tolrestat 200 mg perday The efficacy analysis included measurements of painful and paraesthetic symptoms,vibration perception threshold in three sites, and nerve conduction velocities of fourmotor and two sensory nerves No difference in the painful symptoms was foundbetween the placebo and tolrestat group at the end of the study, although both groupsimproved in comparison with baseline measurements In contrast, a significant improve-ment of paraesthetic symptoms was noticed in the placebo group compared both with tol-restat group and with baseline measurements Regarding the vibration perceptionthreshold measurements, a significant change in favor of tolrestat-treated patients wasfound in one of the three sites it was measured (carpal site, which was located at the dor-sum of the second metacarpal bone)

Significant increases in the motor conduction velocities in tolrestat-treated patientswere recorded at the median nerve compared both with baseline (2 m per second) andwith the placebo group, and in the ulnar nerve compared with baseline When the changes

of all motor conduction velocities were combined together, a significant improvement wasfound at the end of the study, compared with baseline measurements and with the placebogroup All the above changes were present only at the end of the study, after

24 weeks of treatment At the same time, 48% of the tolrestat-treated patients showed

an improvement in three of the four motor nerve conduction velocities, whereas in theplacebo-treated patients similar response was noticed in 28% No changes in the two

sensory nerve function measurements were present at the end of the study, although theheart rate in the tolrestat group was slower compared with baseline measurements of thesame group and with the placebo group Six tolrestat-treated and two placebo-treatedpatients were discontinued from the study because of elevated liver enzymes.

A considerable number of patients who took part in the aforementioned studies tinued to take the drug for several years after the studies were completed and were thecohort of the subsequent trial, which was designed as a randomized, double-blind,

con-placebo-controlled withdrawal study (24) Thus, 372 patients who had already received

tolrestat for a mean period of 4.2 years were randomly selected either to continuereceiving tolrestat at a dose of 200 or 400 mg or to switch to placebo for 1 year Anotherinteresting feature of the design of this trial was the fact that patients were given theoption to change treatment on one occasion after the first three months of the studywithout breaking the code and therefore, maintaining the double-blind design of thetrial The symptom score and the motor conduction velocities of four nerves were used

as end points

A significant deterioration of the symptom score was noticed at the 24th and 36thweeks in the placebo group compared with the tolrestat group However, at the end of thestudy, although a small difference still existed between the two groups, it failed to reachstatistical significance The conduction velocities of three out of the four motor nervesalso deteriorated considerably in the patients who switched to placebo, whereas nochange was noticed in the patients who continued on tolrestat Thus, in the median nervethere was a drop of 0.9 m per second, in the ulnar 1.3 m per second, and in the peroneal0.8 m per second, whereas the mean reduction of both nerves was 0.9 m per second Inaddition, in patients who switched from tolrestat to placebo during the study there was amean drop of 1.3 m per second for all four nerves, whereas in the patients who switchedfrom placebo to tolrestat an improvement of 1 m per second was recorded Therefore, asmall but significant benefit of long-term treatment with tolrestat which can disappearwhen the treatment is discontinued was the main finding of the aforementioned study

In a parallel study, sural nerve biopsies were obtained at the end of the mentioned trial from 13 patients who continued to receive tolrestat and 14 patients who

earlier-received placebo (25) Morphometric analysis showed no difference between the above

two groups, but when compared with nerve biopsies from untreated neuropathicpatients both groups showed increased nerve fiber regeneration In addition, treatmentwith tolrestat was found to ameliorate the increase in the sorbitol and fructose levels inthe nerve tissue indicating that tolrestat can achieve satisfactory concentration levels inthe peripheral nerves

The following two trials with tolrestat were performed at the University of Naplesand were both randomized, placebo-controlled, double-blind, parallel trials of 52 weeksduration The first one examined the effect of 200 mg daily tolrestat on patients withasymptomatic autonomic diabetic neuropathy, defined as at least one abnormal cardio-

vascular reflex (26) At end of the study, improvement in the tolrestat-treated group was

found in all autonomic tests, which included deep breathing (E/I ratio), lying to ing (30/15) ratio, Valsalva (L/S ratio), and postural hypertension In contrast to thisimprovement, a worsening in all the above parameters except the orthostatic hypoten-sion was observed in the placebo-treated group Similar results, namely an improvement

in the tolrestat group and a worsening in the placebo group, were found in vibration ception threshold measurements, the only reported assessment of the peripheral somaticnerve function Similar results were reported in the second study, which includedpatients with subclinical neuropathy, defined as abnormality in only one autonomic test,

per-the squatting test (27) Improvement was found in all per-the autonomic tests and per-the

vibra-tion percepvibra-tion thresholds in the tolrestat group, whereas deterioravibra-tion was observed inthe placebo group in all but the orthostatic hypotension tests

In one of the last studies that were conducted with tolrestat, patients with diabeteswith clinical autonomic neuropathy were randomized to either tolrestat (200 mg daily)

or placebo for a period of 2 years As with the earlier studies, treatment with tolrestatresulted in improvement of most standard cardiovascular reflex test, in comparison withboth the baseline measurements and also to the changes that were observed in the

placebo group (28) Despite the initially promising results, tolrestat was subsequently

withdrawn from clinical use as it was associated to serious side-effects, mainly related

to liver failure

Zenarestat

Zenarestat was an ARI that was shown to achieve very good penetration in the nervetissue Initial studies indicated that in patients who achieved more than 80% sorbitolsuppression in sural nerve biopsies after a 52-week treatment with agent, there was a

significant increase in the density of small diameter sural nerve myelinated fibers (29).

However, an unfavorable risk/benefit ratio, mainly related to kidney problems, ited the conduction of large pivotal studies, and zenarestat was withdrawn

prohib-Fidarestat

In animal models, Fidarestat was shown to be one of the most potent AR inhibitors

As a result, a large multicenter placebo-controlled randomized study that used 1 mg of

Fidarestat once a day and lasted for 52 weeks was conducted (30) A total of 279 patients

with mild diabetic neuropathy were included Fidarestat-treated patients showed animprovement in two out of eight nerve electrophysiological measures (median nerveFCV and F-wave latency) that were recorded and in subjective symptoms

Zopolrestat

Zopolrestat is an interesting ARI as there are no published clinical trials indicatingfavorable effects on diabetic neuropathy However, a recently published randomized,placebo-controlled, double-blind, parallel trial of 52 weeks duration examined the effi-cacy of 500 or 1000 mg daily in patients with low diastolic peak filling rate or impairedaugmentation of left ventricular ejection fraction (LVEF) and absence of coronary

artery disease, left ventricular hypertrophy, and valvular heart disease (30) Treatment

with either dose of zopolrestat resulted in a small improvement of the exercise LVEFand stroke volume when compared with the placebo-treated patients Although, the clin-ical significance of these results is small, they do indicate that diabetic cardiomyopathymay not be exclusively related to coronary artery disease, but it might also be associ-ated to activation of the polyol pathway in the cardiac myocytes It is also of interestthat no improvement was noticed in the peripheral somatic or autonomic neuropathy inthe patients who participated in the aforementioned study

Despite the initial encouraging results from trials that were conducted during the last

20 years, ARIs have not been established for the treatment of diabetic neuropathy yet.The main reasons for this are inconsistent results in subsequent trials and the unaccept-able high rate of side-effects associated with the initially tested compounds The lack ofwell-defined end points and the conduction of numerous small trials, instead offocussing on the most promising agents and conducting large pivotal trials is one of thereasons that are related to this outcome In addition, the inability to produce an inhibitorthat achieves satisfactory tissue penetration and enzyme inhibition, whereas at the sametime is devoid of serious side-effects have also played a significant role Currently, itseems that the interest in ARIs is significantly reduced and is doubtful if new ARIs will

be tested clinically in the near future

REFERENCES

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diabetic rats: control by an aldose reductase inhibitor Science 1973;182:1146–1148.

2 Tomlinson DR, Willars GB, Carrington AL Aldose reductase inhibitors and diabetic

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need for different trial designs? Diabetes 1997;46(Suppl 2):S82–S89.

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con-duction in controlled clinical trials of diabetic neuropathy Diabetes Care 1989;12:

649–652

5 Culebras A, Alio J, Herrera JL, Lopez-Fraile IP Effect of an aldose reductase inhibitor on

diabetic peripheral neuropathy Arch Neurol 1981;38:133–134.

6 Handelsman DJ, Turtle JR Clinical trial of an aldose reductase inhibitor in diabetic

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7 Fagius J, Jameson S Effects of aldose reductase inhibitor treatment in diabetic

polyneu-ropathy—a clinical and neurophysiological study J Neurol Neurosurg Psychiatr 1981;44:

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nerve conduction velocity in diabetic patients N Engl J Med 1983;308:119–125.

9 Jaspan J, Maselli R, Herold K, Bartkus C Treatment of severely painful diabetic thy with an aldose reductase inhibitor: relief of pain and improved somatic and autonomic

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neu-ropathy with an aldose reductase inhibitor: a 24-week controlled trial Diabetologia

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Diabetic Neuropathy Diabetes 1988;37:1000–1004.

16 Sima AAF, Brill V, Nathaniel T, et al Regeneration and repair of myelinated fibres in suralnerve biopsy specimens from patients with diabetic neuropathy treated with sorbinil

N Engl J Med 1988;319:548–555.

17 Dyck PJ, Zimmerman BR, Vilen TH, et al Nerve glucose, fructose, sorbitol, myo-inositol,

and fiber degeneration and regeneration in diabetic neuropathy N Engl J Med 1988;319:

542–548

18 Gill JS, Williams G, Ghatei MA, Hetreed AH, Mather HM, Bloom SR Effect of the aldose

reductase inhibitor, ponalrestat, on diabetic neuropathy Diabete Metab 1990;16:296–302.

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ischemic conduction block in diabetic subjects Diabetes Care 1991;14:411–413.

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inhibitor, ponalrestat, in diabetic neuropathy Diabetes Res Clin Pract 1991;14:63–73.

21 Krentz AJ, Honigsberger L, Ellis SH, Hardman M, Nattrass M A 12-month randomizedcontrolled study of the aldose reductase inhibitor ponalrestat in patients with chronic

symptomatic diabetic neuropathy Diabet Med 1992;9:463–468.

22 Boulton AJM, Levin S, Comstock J A multicentre trial of the aldose reductase inhibitor,

tolrestat, in patients with symptomatic diabetic neuropathy Diabetologia 1990;33:

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23 Macleod AF, Boulton AJM, Owens DR, et al A multicentre trial of the aldose reductase

inhibitor tolrestat in patients with symptomatic diabetic peripheral neuropathy Diabete Metab 1992;18:14–20.

24 Santiago JV, Sonksen PH, Boulton AJM, et al Withdrawal of the aldose reductase inhibitor

tolrestat in patients with diabetic neuropathy: Effect on nerve function J Diab Comp

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25 Sima AAF, Greene DA, Brown MB, et al Effect of hyperglycemia and the aldose inhibitortolrestat on sural nerve biochemistry and morphometry in advanced diabetic peripheral

polyneuropathy J Diab Comp 1993;7:157–169.

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52-week, randomized, placebo controlled trial Ann Int Med 1993;118:7–11.

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neuropathy Diabetes Care 1995;18:536–541.

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conduc-tion and morphometry in diabetic neuropathy Neurology 1999;53:580–591.

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448–454

Other Therapeutic Agents for the Treatment

of Diabetic Neuropathy

Gary L Pittenger PhD, Henri Pharson PhD, Jagdeesh Ullal MD,

and Aaron I Vinik MD,PhD

SUMMARY

The pathogenesis of diabetic neuropathy is complex and it is important to understandthe underlying pathology leading to the complication in order to best tailor treatment foreach individual patient It is unlikely that reversing any single mechanism will provesufficient for reversing nerve damage Several drugs, such as antioxidant, PKCinhibitors and nerve growth factors can have effects on multiple systems that are com-promised in diabetic neuropathy, yet even those may not be enough in and of themselves

to completely restore neurological function Combination therapy may prove to be thebest long term approach, and studies of those combinations should prove revealing as

to the relative roles of metabolic dysfunction, microvascular insufficiency and munity in the diabetic neuropathy patient population

autoim-Key Words: Antioxidant; PKC inhibitors and nerve growth factors; VEGF.

INTRODUCTION

Neuropathy is one of the most common complication of diabetes with a heterogeneousclinical presentation and a wide range of abnormalities As a result, there is still not asingle therapeutic agent for diabetic neuropathy that consistently gives more than mildrelief There is a plethora of studies in the literature indicating that metabolic, microvas-cular, and autoimmune dysfunction all play a role in the progression of diabetic neu-ropathy Given the potential significance of each of these different systems in diabetes,

it is not surprising that treatments targeted to specific pathways in these systems havecommonly proven disappointing in their efficacy Our “simple” concept of the variousfunctional changes leading to peripheral nerve disease in diabetes is presented in (Fig 1)

(1) Thus, although clinical studies with thousands of patients have shown that glycemia is at the center of diabetic neuropathy (2,3), finding specific biochemical

hyper-pathways to manipulate for therapy has proven difficult

From: Contemporary Diabetes: Diabetic Neuropathy: Clinical Management, Second Edition

Edited by: A Veves and R Malik © Humana Press Inc., Totowa, NJ

321

END POINTS FOR CLINICAL TRIALS IN DIABETIC NEUROPATHY

Among the challenges for studies of diabetic neuropathy is the selection of themeasures that can be used to determine the efficacy of the agents Common end pointsinclude:

1 Subjective measures of symptoms (e.g., nerve symptom scores)

2 Questionnaires that allow for quantification of any changes in the symptoms of diabeticneuropathy, or the quality of life of the patient (e.g., neuropathy symptom scores, total neu-ropathy scores)

3 Objective neurological examinations focussing on distal sensorimotor function, includingcombinations giving a single score (e.g., NTSS-6, a combination of numbness, pricklingsensation, aching pain, burning pain, lancinating pain, and allodynia scores)

4 Electrophysiology (nerve conduction velocity, amplitudes [especially sural nerve tude], F-wave latencies)

ampli-5 Quantitative tests of sensory and motor modalities that allow more precise measures ofnerve function

6 Autonomic measures (e.g., cardiac function or neurovascular measures)

7 Skin biopsy, which allows direct observation of the morphology of sensory nerve fibers

8 Instruments that entail various combinations of the above

The following summarizes the view of prospective end points in diabetic neuropathytrials:

Fig 1 Diagram of the pathologies underlying the development of diabetic neuropathy,

begin-ning with hyperglycemia, possibly exacerbated by alcohol or smoking Adapted from ref 1.

1 Symptoms (forms and questionnaires)—subjective.

2 Quality of life (questionnaire)—subjective.

3 Quantitative neurological examination focussing on distal sensorimotor function (sensation,

strength, and reflexes)—semiobjective.

4 Neurophysiology (nerve conduction velocity, amplitudes, F-wave latencies; multiple nerves,

both sensory and motor)—objective.

5 Quantitative sensory testing (vibration, thermal, pain, and NTSS-6)—semiobjective 6.

Autonomic testing (microvascular, cardiac measures)—objective.

7 Morphology—(skin biopsy—nerve density and length, branching pattern)—objective.

8 Combinations (e.g., NIS-LL + 7 = objective neurological examination of the lower limbs +

4 measures of nerve electrophysiology + QAFT + QST)

Unfortunately, subjective measures may not be sensitive enough to discern small

changes in nerve fiber functions in a small test group during a short period of time

Unless a study includes a large number of subjects and a sufficient period of time

sig-nificant changes may not be detectable, particularly in the short time periods that most

efficacy trials are designed for Thus, objective tests may be more useful for studies of

shorter duration

TREATMENTS

Therapies for diabetic neuropathy are generally directed at the 3 primary mechanisms

shown in Fig 1: metabolic dysfunction, autoimmunity, or microvascular insufficiency

Several of these agents, for example, antioxidants or neurotrophins (NTs), can affect

more than one of the pathogenetic mechanisms, as shown in Table 1

THERAPIES TARGETED TO METABOLIC PATHWAYS

Note: The aldose reductase inhibitors are discussed elsewhere (see Chapter 18).

Antioxidants

Hyperglycemia has been shown in a number of studies to cause oxidative stress in

tissues that are susceptible to the complications of diabetes, including peripheral nerves

In turn, the oxidative stress leads to the generation of free radicals that can attack the

lipids, proteins, and nucleic acids of the affected tissues directly, compromising

physi-ological function The end result is the loss of axons and disruption of the

microvascu-lature in the peripheral nervous system (Fig 2) It has been shown that there is an

increased presence of markers of oxidative stress, such as superoxide and peroxynitrite

ions, and that antioxidant moieties were reduced in patients with diabetic peripheral

Other Therapeutic Agents for the Treatment of Diabetic Neuropathy 323

Table 1

Mechanisms for Pathogenesis of Diabetic Neuropathy and Therapies

That Have Been Tested Addressing Them

Microvascular Nerve Metabolic dysfunction insufficiency Autoimmunity regeneration

ARIs PKC-β Inhibitor Steroidal anti-inflammatories Neurotrophins

Inhibitors of glycation Antioxidants Plasmaphoresis –

neuropathy (4) Therefore, it is reasonable to use therapies that are known to reduce

oxidative stress in tissues, and antioxidants Although a host of antioxidants have been

tested in animal models (for review see ref 5), those that have been tested in human

studies will be addressed

α-Lipoic Acid

α-lipoic acid is the best studied antioxidant therapy used in diabetic neuropathy.Lipoic acid (1,2-dithiolane-3-pentanoic acid), a derivative of octanoic acid, is present infood and is also synthesized by the liver It is a natural cofactor in the pyruvate dehy-drogenase complex where it binds acyl groups and transfers them from one part of thecomplex to another α-lipoic acid, also known as thioctic acid, has generated consider-able interest as a thiol replenishing and redox modulating agent In streptozotocin(STZ)-diabetic rats α-lipoic acid has been shown to prevent slowing of peripheral nerve

conduction velocity and to maintain peripheral nerve blood flow (6–8) It has also been

shown to be effective in ameliorating both the somatic and autonomic neuropathies in

diabetes (9–11) It is not clear that the positive effects are limited to the antioxidant

properties of α-lipoic acid, and studies are ongoing to determine its range of effects α-lipoic acid is licensed for use in diabetes in Germany and it is currently undergoing

Fig 2 The interactions of pathways leading to neurovascular and endothelial dysfunction and

the actions of drugs that alter those mechanisms and are being tested in models of neuropathy

extensive trials in the US as both an antidiabetic agent and for the treatment of diabeticneuropathy.

γ-Linolenic Acid

Linoleic acid is an essential fatty acid that is metabolized to di-homo-γ-linolenic acid(GLA), which in turn serves as an important constituent of neuronal membrane phos-pholipids In addition it can serve as a substrate for prostaglandin E synthesis, whichmay be important for preservation of nerve blood flow In diabetes, conversion oflinoleic acid to GLA and subsequent metabolites is impaired, possibly contributing to

the pathogenesis of diabetic neuropathy (12,13) A multicenter, double-blind

placebo-controlled trial using GLA for 1 year demonstrated significant improvements in both

clinical measures and electrophysiological testing (14).

Tocopherol (Vitamin E)

The tocopherols, especially the α-tocopherol isoform, have been promoted as effectiveantioxidant therapy for a number of neurological diseases, including Alzheimer’s disease,epilepsy, cerebellar ataxia, and diabetic neuropathy In studies of patients with diabetesvitamin E has been shown to decrease 8-isoprostane F2α (15), decrease low density

lipoprotein-C oxidation at high doses (16), and increase skin blood flow and reduce free radicals in skin with topical application (17) Human studies of vitamin E using com-

bined oral therapy with vitamin C, another well-established antioxidant have shown

improved vascular function, but only in type 1 patients (18) Thus, vitamin E appears to

exert antioxidant protective effects on neurons in diabetes although efficacy has not yetbeen demonstrated

INHIBITORS OF GLYCATION

It is apparent that advanced glycosylation end products (AGEs) contribute to nervedamage either by direct action on neurons and myelin or by enhancing oxidative stressunder hyperglycemic conditions Thus, there is a great deal of interest in agents thateither prevent the formation of AGEs or agents that reverse the nonenzymatic glycation

of proteins

Aminoguanidine

Animal studies using aminoguanidine, an inhibitor of the formation of AGEs,showed improvement in nerve conduction velocity in rats with STZ-induced diabeticneuropathy However, controlled clinical trials to determine its efficacy in humans have

been discontinued because of toxicity (19,20) However, there are compounds related

to aminoguanidine that reduced AGE formation and hold promise for this approach,

although these have not been systematically studied in humans (21–24).

THERAPIES FOR MICROVASCULAR INSUFFICIENCY

Although it is clear that there are significant alterations of blood vessels in diabetes,data thus far has been conflicting whether neuropathy promotes the changes in themicrovasculature or whether it is the changes in the microvessels of the nerves that lead

to neuropathy Whatever the case, evidence is growing that re-establishing more normalpatterns of blood flow to the nerves results in improved neurological function

Other Therapeutic Agents for the Treatment of Diabetic Neuropathy 325

Protein Kinase C Inhibitors

Protein kinase C (PKC) and diacylglycerol (DAG) are intracellular signaling cules that regulate vasculature by endothelial permeability and vasodilation The PKC

mole-isozymes are a family of 12 related serine/threonine kinases (25) whose normal

func-tion is the activafunc-tion of essential proteins and lipids in cells essential for cell survival.PKC-β is expressed in the vasculature (26,27) and belived to be involved in cell prolif-

eration, differentiation, and apoptosis PKC is activated by oxidative and osmolar stress,both of which are a consequence of the dysmetabolism of diabetes Increased polyolpathway activity and pro-oxidants bind to the catalytic domain of PKC and it is disin-hibited PKC-β overactivation is induced by hyperglycemia or fatty acids through receptor-mediated activation by phospholipase C It is hypothesized that AGEs and oxidantsproduced by nonenzymatic glycation and the polyol pathway, respectively, increase the

production of DAG (28) Increased DAG and calcium promotes the overactivation of

PKC-β (29) Activation of PKC-β activates MAP kinase and, subsequently,

phosphory-lation of transcription factors that are involved in angiogenesis, increased stress-related

genes, c-Jun kinases and heat shock proteins, all of which can damage cells and lar endothelial growth factor (VEGF) (30), which is known to play a critical role in nerve development (31) Diabetic animal models have shown high levels of PKC-β in a

vascu-number of tissues (28), including nerves and endothelium (32) Activation of PKC-βcauses vasoconstriction and tissue ischemia, whereas high levels may impair neuro-chemical regulation PKC-β hyperactivity leads to increased vascular permeability,

nitric oxide dysregulation (33), increased leukocyte adhesion (34), and altered blood flow (35) Furthermore, PKC-β hyperactivity in the neural microvessels causes vaso-constriction, which might lead to decreased blood flow, resulting in nerve dysfunction

and hypoxia (35) Nerve hypoxia, oxidative nitrosative stress, and an increase in NFκBcauses endothelial damage, leading to depletion of nerve growth factors, VEGF, andTGF-α autoimmunity and may further accelerate the loss of nerve conduction (31).

Both animal models and human clinical trials investigating complications of diabeteshave shown that blockade of PKC-β slows the progression of complications (33,36–47).

Multiple studies using a specific PKC-β inhibitor, ruboxistaurin mesylate (LY333531),have shown improvements in diabetic neuropathy One study in obese rats observed thatruboxistaurin increased resting nitric oxide concentration, and reduced nitric oxide by15%, indicating that this action is a PKC-β dependent phenomenon (33) Ruboxistaurin

has been shown to improve nitric oxide-dependent vascular and autonomic nerve

dys-function in diabetic mice (46) In addition to improving nitric oxide levels,

ruboxis-taurin improves nerve function and blood flow Ruboxisruboxis-taurin corrected the diabeticreduction in sciatic endoneurial blood flow, sciatic motor, and saphenous sensory nerve

conduction velocity in diabetic rats (40,43) In another study, the investigators measured

sciatic nerve, superior cervical ganglion blood flow, and nerve conduction velocity inSTZ treated rats After 8 weeks, the authors observed that diabetes reduced sciatic nerveand superior cervical ganglion blood flow by 50% and produced deficits in saphenous

nerve sensory conduction velocity (48) After 2 weeks of treatment with ruboxistaurin,

the sciatic nerve, and ganglion blood flow were improved Additionally, nerve dysfunction

is commonly attributed to alterations of the nerve transporters Other studies strated that a specific inhibitor of the PKC-β, (ruboxistaurin), prevents PMA-dependent

activation of Na+, K+-ATPase in rats (44,45) In addition to improvements with blood

flow nerve function and ion transport, ruboxistaurin corrected thermal hyperalgesia

(48,49).

These observations have been supported in preliminary clinical studies In healthyhumans, ruboxistaurin blocked the reduction in endothelium-dependent vasodilation

induced by acute hyperglycemia (47), suggesting that the hyperglycemic effects on

vasodilation are mediated through PKC-β More recently, a 1-year double-blind, lel clinical trial with 205 patients with type 1 or 2 diabetes and DPN was performed toassess the impact of ruboxistaurin on vibration perception in patients with DPN com-pared with placebo In patients with DPN, ruboxistaurin treatment improved symptomsand vibratory sensation with a significant correlation between the two compared with

paral-placebo group (50) Another recent report indicates that ruboxistaurin is particularly effective in neuropathy patients with intact sural nerve amplitudes (51) A phase II study

using NTSS-6 to assess the intensity and frequency of sensory neuropathy symptomsfurther suggested that ruboxistaurin slows the progression of hyperglycemia-induced

microvascular damage (52) Together, these studies support the belief underlying a role

for PKC-β in the etiology of diabetes-induced neuropathy

Vascular Endothelial Growth Factor

The most potent stimulus for angiogenesis is VEGF If the pathogenesis of diabeticneuropathy goes through loss of vasa nervorum, it is likely that appropriate application

of VEGF would reverse the dysfunction Normally, VEGF activity is induced by tissue

hypoxia (53,54) In diabetes it is just such hypoxia that results in increased VEGF ity in the retina, with subsequent pathological angiogenesis (55) Conflicting reports indicated that VEGF in diabetes goes up (56) and goes down (57) One possibility to

activ-explain this is whether the animals were treated with insulin, which can reduce VEGF

expression (56) Only recently it has been demonstrated that there is a reduction in

VEGF activity in STZ-diabetic mice that results in failure of neovascularization in

hypoxic tissue in the lower limb (57) Furthermore, in the same study it was shown that

intramuscular injection of an adenoviral vector encoding for VEGF could induce mal neovascularization in the hindlimb There have been no human studies of VEGF,and caution is the best approach given the demonstrated pathological effects of VEGF

nor-in the retnor-ina nor-in diabetes

Vasodilators

Microvascular insufficiency, endoneurial blood flow, and hemodynamic factors lead

to nerve damage in patients with DPN (58–62) Although, the sequence of events is not

well understood, investigators propose that microvascular vasoconstriction, edema, andischemia play a role in DPN development Endoneurial edema increases endoneurial

pressure (63), thereby causing capillary closure and subsequent nerve ischemia and damage (64,65) A diminished regulation of the endoneurial blood flow and ischemia may result from decreased nerve density and innervation of vessels (66), measured with laser Doppler (67) Nerve ischemia stimulates VEGF production exacerbating DPN

through overactivation of PKC-β (56,68–72) As a result, ischemia and low blood flow

reduces both endothelial dependent and nitric-oxide dependent vasorelaxation nangle

Other Therapeutic Agents for the Treatment of Diabetic Neuropathy 327

(46,73,74) Vascular defects also result in changes in endoneurial vessels Epineurial

changes include arteriolar attenuation, venous distension, arteriovenous shunting that

leads to new vessel formation (75) Neural regulation of blood flow is complicated by

arteriovenous anastomoses and shunting, which deviate the blood flow from the skin

creating an ischemic microenvironment (76) There is thickening and deposition of

sub-stances in the vessel wall associated with endothelial cell growth, pericyte loss (in eyes),

and occlusion (77) Changes in blood flow correlate with changes in oxygen saturation (60,78) and reduced sural nerve endoneurial oxygen tension (79) These changes are

followed by increased expression or action of vasoconstrictors, such as endothelin andangiotensin and decreased activity of vasodilators, such as prostacyclin, substance P,

CGRP, endothelial derived hyperpolarizing factor, and bradykinin (80,81) Based on

this theory, investigators have given oxygen and vasodilatory agents to patients,

how-ever, these therapies have not improved DPN (82,83) Additionally, methods of assessing

skin blood flow have demonstrated that diabetes disturbs microvasculature, tissue PO2,and vascular permeability In particular, in patients with DPN there is disruption in

vasomotion, the rhythmic contraction exhibited by arterioles, and small arteries (84,85).

In Type 2 diabetes, skin blood flow is abnormal and the loss of neurogenic vasodilativemechanism in hairy skin may precede lower limb microangiopathic processes and C-

fiber dysfunction (85,86) Changes in endoneurial blood flow often are reflected by changes in nerve conduction (8,87–89) In addition, impaired blood flow can predict ulceration (2,90–93) Therefore, both vascular or endoneural alterations may cause

damage over time in the peripheral nerves of patients with diabetes (Fig 3)

Fig 3 Mechanisms of peripheral vasodilation and agents being investigated for improving

peripheral neurovascular function in diabetes

THERAPIES TARGETING AUTOIMMUNITY

Traditional therapies for autoimmune neuropathies have proven beneficial for certain

types of diabetic neuropathy (94) Plasmaphoresis and steroidal anti-inflammatories

should be considered if the diagnosis is proximal diabetic neuropathy (diabetic trophy) or demyelinating neuropathy Failure of these treatments or evidence of auto-immunity in typical diabetic polyneuropathy might warrant anti-immune approaches

amyo-Human Intravenous Immunoglobulin

Immune intervention with human intravenous immunoglobulin (IVIg) has becomeappropriate in some patients with forms of peripheral diabetic neuropathy that are

associated with signs of antineuronal autoimmunity (94,95) Chronic inflammatory

demyelinating polyneuropathy associated with diabetes is particularly responsive toIVIg infusion Treatment with immunoglobulin is well tolerated and is considered safe,

especially with respect to viral transmission (96) The major toxicity of IVIg has been

an anaphylactic reaction, but the frequency of these reactions is now low and confinedmainly to patients with immunoglobulin (usually IgA) deficiency Patients may experi-ence severe headache because of aseptic meningitis, which resolves spontaneously

In some instances, it may be necessary to combine treatment with prednisone and/orazathioprine Relapses may occur requiring repeated courses of therapy

THERAPIES FOR NERVE REGENERATION

Neurotrophins

Neurotrophic factors are proteins that promote survival of neurons regulating geneexpression through second messenger systems These proteins may induce morpholog-ical changes, nerve differentiation, nerve cell proliferation, and induce neurotransmitter

expression and release Subsequently, reduction in levels of neurotrophic factors (97) can lead to neuronal loss, possibly through activation of apoptosis (98) Many proteins

have properties and characteristics of neurotrophic factors, including cytokine-likegrowth factors, TGF-β, NT3, nerve growth factor (NGF), insulin-like growth factor(IGF)-1, and VEGF Although only a few neurotrophic factors have been extensivelyinvestigated, there are number of proteins that have been identified as neurotrophic fac-

tors (31,99) Many of these proteins appear to have altered expression in nerves of patients with diabetes (77) For example, interleukin-6, a cytokine-like growth factor may play a role in cell proliferation (100) Although their function is not well under-

stood, IGFs-I and -II have been shown to regulate growth and differentiation of neurons

(101) IGF, NGF, and other neurotrophins have been shown to be members of a family

of proteins supporting the growth and regeneration of neurons Often these growth tors are associated with changes in nerve structure through apoptosis or proliferation.The laminin γ gene is upregulated in normal animals undergoing postsection sciatic

fac-nerve regeneration (102,103) This process is impaired in diabetes Other extracellular matrix proteins are also altered in neuropathic nerves (104) Therefore, understanding

the role of neurotrophic factors has been the focus of much investigation

Nerve Growth Factor

Neurons affected in diabetic neuropathy are developmentally dependent on NGF.Therefore, a decline in NGF synthesis in patients with diabetes plays a role in the

Other Therapeutic Agents for the Treatment of Diabetic Neuropathy 329

pathogenesis of neuropathy, especially small fibers (105) More specifically, NGF has

been shown to be trophic for sympathetic ganglion neurons and neural crest-derived system

(99) Neurotrophins, including NGF, bind to high-affinity receptors, Trk related kinases) (106) or low-affinity receptors p75 (107); both receptors may activate

(tropomyosin-different signaling cascades

Neurotrophic proteins are reduced in patients with diabetes NGF protein levels in the

serum of patients with diabetes are suppressed (108) Additionally, diabetes might result

in decreased serum IGF and increased IGF-I binding protein-I (109) thereby inhibiting

the protein’s downstream effects Despite increasing evidence that growth factors aresuppressed in patients with diabetes, there is no direct link between neurotrophic fac-

tors and the pathogenesis of diabetic neuropathy (99) and preliminary studies of both

NGF and NT3 treatment have met with limited success

There is now considerable evidence in animal models of diabetes that decreasedexpression of NGF and its high-affinity receptor, trk A, reduces retrograde axonal transport

of NGF and diminishes support of small unmyelinated neurons and their neuropeptides,

such as substance P and CGRP—both potent vasodilators (110–112) Furthermore,

recombinant human NGF (rhNGF) administration restores these neuropeptide levels

toward normal and prevents the manifestations of sensory neuropathy in animals (113).

In a 15 center, double-blind, placebo-controlled study of the safety and efficacy of

rhNGF in 250 subjects with symptomatic small fiber neuropathy (19), rhNGF improved

the neurological impairment score of the lower limbs, and improved small nerve fiberfunction cooling threshold (Aδ-fibers) and the ability to perceive heat pain (C-fiber)

compared with placebo (114) These results were consistent with the postulated actions

of NGF on trk A receptors present on small fiber neurons This led to two large center studies conducted in the US and the rest of the world Results of these two stud-

multi-ies were presented at the ADA meetings in June 1999 (115) Regrettably, rhNGF was

not found to have beneficial effects on and above placebo The reason for thisdichotomy has not been resolved, but this has somewhat dampened the enthusiasm forgrowth factor therapy of diabetic neuropathy

ANTICONVULSANT DRUGS

Although the anticonvulsants, including carbamazepine, phenytoin, and gabapentin,

have been effective in treating painful diabetic neuropathy (see Chapter 21), newer

classes of anticonvulsants have shown surprising promise in treating both the matic pain of diabetic neuropathy and the neuronal deficits

sympto-Topiramate

Topiramate is a fructose analog that was initially examined because of its antidiabeticpossibilities Although it is an anticonvulsant used in complex partial seizures, topira-mate was recently shown to be efficacious in the management of neuropathic pain

(116) Unfortunately, it was first examined only in normal animals and had no

hypo-glycemic properties It has now undergone extensive testing for epilepsy, migraine,involuntary movements, central nervous system injury, and neuropathic pain The firsttwo studies used a titration to 400 mg/day, which was associated with fairly severe cen-tral nervous system side effects, which were prohibitive The studies failed to establish

an effect in diabetic neuropathic pain A third study using different end points, with