Effects of sorbinil therapy in diabetic patients with painful peripheral neuropathy and autonomic neuropathy

The time to develop treatments for diabetic neuropathy

BACKGROUND

Diabetic neuropathy is a multifaceted condition affecting up to 50% of individuals with long standing diabetes. The most common presentation is peripheral diabetic sensory neuropathy (DPN).

METHODS

We carried out a systematic review of papers dealing with diabetic neuropathy on Pubmed in addition to a targeted Google search.Search terms included small fiber neuropathy,diffuse peripheral neuropathy, quantitative sensory testing, nerve conduction testing, intra-epidermal nerve fiber density, corneal confocal reflectance microscopy, aldose reductase inhbitors, nerve growth factor, alpha-lipoic acid, ruboxistaurin, nerve growth factor antibody, and cibinetide.

RESULTS

Over the past half century, there have been a number of agents undergoing unsuccessful trials for treatment of DPN.There are several approved agents for relief of pain caused by diabetic neuropathy, but these do not affect the pathologic process.

EXPERT OPINION

The failure to find treatments for diabetic neuropathy can be ascribed to (1) the complexity of design of studies and (2) the slow progression of the condition, necessitating long duration trials to prove efficacy.We propose a modification of the regulatory process to permit early introduction of agents with demonstrated safety and suggestion of benefit as well as prolongation of marketing exclusivity while long term trials are in progress to prove efficacy.

Therapy for diabetic neuropathy: an overview

Neuropathy is a highly prevalent complication of diabetes that is only likely to increase as the diabetic epidemic continues. Unfortunately, the only disease-modifying treatment is to address the underlying diabetes with enhanced glucose control. In patients with type 1 diabetes, improved glycemic control dramatically reduces the incidence of neuropathy. In contrast, in patients with type 2 diabetes, better glucose control has only a marginal effect on the prevention of neuropathy. However, recognition and treatment of neuropathic pain is also important. An ever expanding number of randomized, controlled clinical trials support multiple medications for the reduction of pain. This includes medications such as calcium channel agonists, tricyclic antidepressants, and selective serotonin/norepinephrine reuptake inhibitors. However, the precise order and combination of these medications remains unclear. Furthermore, several new promising medications are being developed. Overall, the cornerstones of the treatment of diabetic neuropathy are improved glycemic control and initiation of a neuropathic pain medication with high levels of evidence to support its use when pain is present.

Systemic and ocular pharmacokinetics of N-4-benzoylaminophenylsulfonylglycine (BAPSG), a novel aldose reductase inhibitor

To better develop N-[4-(benzoylamino)phenylsulfonyl]glycine (BAPSG), a potent and selective aldose reductase inhibitor capable of delaying the progression of ocular diabetic complications, the objective of this study was to assess its pharmacokinetics. The plasma pharmacokinetics of BASPG was assessed in male Sprague-Dawley rats following intravenous, intraperitoneal and oral routes of administration and its distribution to various tissues including those of the eye was studied following intraperitoneal administration. In addition, rat plasma protein binding of BAPSG was studied using ultracentrifugation method and its ocular tissue disposition was assessed following topical administration in rabbits. Plasma and tissue levels of BAPSG were analysed using an HPLC assay. BAPSG exhibited dose-proportionate AUC0 --> infinity (area under the plasma concentration-time curve) following both intravenous and intraperitoneal administration over the dose range (5-50 mg kg(-1)) studied and an erratic oral absorption profile with low oral bioavailability. The fraction bioavailability following oral and intraperitoneal administration was 0.06 and 0.7-1, respectively. BAPSG exhibited short plasma elimination half-lives in the range 0.5-1.5 h. BAPSG was bound to rat plasma proteins and the percent protein binding ranged from 83 to 99.8%. BAPSG was better distributed to cornea, lens and retina than to brain, following intraperitoneal administration in rats. However, the distribution was lower compared with kidney and liver. Following topical administration in rabbits, BAPSG delivery to the surface ocular tissues, cornea and conjunctiva was higher compared with intraocular tissues, aqueous humour, iris-ciliary body and lens. Thus, BAPSG was distributed to ocular tissues following systemic and topical modes of administration.

Diabetic neuropathy--a continuing enigma

In this article we will review the clinical signs and symptoms of diabetic somatic polyneuropathy (DPN), its prevalence and clinical management. Staging and classification of DPN will be exemplified by various staging paradigms of varied sophistication. The results of therapeutic clinical trials will be summarized. The pathogenesis of diabetic neuropathy reviews an extremely complex issue that is still not fully understood. Various recent advances in the understanding of the disease will be discussed, particularly with respect to the differences between neuropathy in the two major types of diabetes. The neuropathology and natural history of diabetic neuropathy will be discussed pointing out the heterogeneities of the disease. Finally, the various prospective therapeutic avenues will be dealt with and discussed.

Aldose reductase inhibitors for the prevention and treatment of diabetic peripheral neuropathy

OBJECTIVES

To assess the efficacy of aldose reductase inhibitors in the prevention, reversal or delay in the progression of diabetic peripheral neuropathy.

SEARCH STRATEGY

The Cochrane Diabetes Group's database was searched and the citation lists of identified trials and previous reviews checked. Investigators identified as active in the field were approached for overlooked studies.

SELECTION CRITERIA

Randomised controlled trials of aldose reductase inhibitors versus placebo, no treatment or other treatment in diabetic patients with or without clinical neuropathy.

DATA COLLECTION AND ANALYSIS

Nerve conduction velocity was the only end point measured in all trials. Treatment effect was evaluated in terms of nerve conduction velocity mean difference in median and peroneal motor and median and sural sensory nerves.

MAIN RESULTS

19 trials, testing 4 different aldose reductase inhibitors for between 4 to 208 weeks duration (median 24 weeks), met the inclusion criteria for the meta-analysis. A small but statistically significant reduction in decline of median and peroneal motor nerve conduction velocities was present in the treated group when compared to the control group (weighted mean 0.66 m/s 95% CI 0.18-1.14 m/s and 0.53 m/s 95% CI 0.02-1.04m/s respectively). No clear benefit of aldose reductase inhibitor treatment was observed in either of the sensory nerves.

REVIEWER'S CONCLUSIONS

Although aldose reductase inhibitor treatment has been demonstrated to diminsh the reduction in motor nerve conduction velocity, the clinical relevance of such a change in this outcome measure is uncertain. There was no effect in terms of this outcome measure in the smaller sensory fibres, degeneration of which is primarily responsible for the most common neuropathic syndrome associated with diabetes, that of severe pain and loss of sensation in the extremity leading in some cases to ulceration and eventual amputation.

Long-term effect of epalrestat on cardiac autonomic neuropathy in subjects with non-insulin dependent diabetes mellitus

To evaluate the effect of long-term administration of an aldose reductase inhibitor on diabetic cardiovascular autonomic neuropathy, 22 subjects with non-insulin dependent diabetes mellitus (NIDDM, 11 men and 11 women, mean age; 64.8 +/- 7.8 years, duration of diabetes; 18.3 +/- 5.6 years) were administered epalrestat, one type of aldose reductase inhibitor, for 36 months. The changes in the coefficient of variation of the R-R interval (CV(R R)) during rest and the QTc interval were compared with 43 age-matched NIDDM (controls). During the study, the CV(R R) value gradually decreased in the controls, while it slightly increased in subjects treated with epalrestat. After 36 months, the CV(R R) value (2.31 +/- 1.09%) in subjects treated with epalrestat was significantly (P < 0.05) higher than that (1.84 +/- 0.75%) in the controls. There were no significant differences in QTc intervals in both groups. These results suggest that long-term administration of an aldose reductase inhibitor may be available for cardiac autonomic neuropathy in even relatively older diabetic subjects with long duration.

Long-term effects of pancreatic transplantation on diabetic neuropathy

Restoration of a long-lasting euglycemic state by a functioning pancreatic transplantation (PTx) is the most logical treatment for insulin-dependent diabetes mellitus and for amelioration of secondary complications, including neuropathy. We evaluated neurological function by clinical examination, nerve conduction studies, and autonomic function tests in 115 patients with a functioning PTx and in 92 control patients treated with insulin, at baseline and 1, 2, 3.5, 5, 7, and 10 years later. In control patients, neuropathy progressively worsened during follow-up. The clinical examination score and composite indices of abnormality of motor and sensory nerve conduction decreased significantly at all intervals tested. Autonomic function indices also decreased, but significantly only after 1 year. In patients who received a successful PTx the neuropathy improved. The motor and sensory nerve conduction indices increased significantly at all intervals after transplantation, whereas the clinical examination and autonomic tests improved only slightly. Patients who received either a PTx alone, a PTx after a kidney graft, or simultaneous pancreatic and kidney transplantations improved similarly over the follow-up. These results indicate that a functioning PTx halts the progression and improves the signs of diabetic polyneuropathy by restoration of a normoglycemic state.

C-peptide improves autonomic nerve function in IDDM patients

In order to determine the possible influence of C-peptide on nerve function, 12 insulin-dependent diabetic (IDDM) patients with symptoms of diabetic polyneuropathy were studied twice under euglycaemic conditions. Tests of autonomic nerve function (respiratory heart rate variability, acceleration and brake index during tilting), quantitative sensory threshold determinations, nerve conduction studies and clinical neurological examination were carried out before and during a 3-h i.v. infusion of either C-peptide (6 pmol.kg-1.min-1) or physiological saline solution in a double-blind study. Plasma C-peptide concentrations increased from 0.11 +/- 0.02 to 1.73 +/- 0.04 nmol/l during C-peptide infusion. Clinical neurological examination quantitative sensory threshold evaluations and nerve conduction measurements failed to detect significant changes between C-peptide and saline study periods. Respiratory heart rate variability increased significantly from 13 +/- 1 to 20 +/- 2% during C-peptide infusion (p < 0.001), reaching normal values in five of the subjects; control studies with saline infusion did not alter the heart rate variability (basal, 14 +/- 2; saline, 15 +/- 2%). A reduced brake index value was found in seven patients and increased significantly during the C-peptide infusion period (4.6 +/- 1.0 to 10.3 +/- 2.2%, p < 0.05) but not during saline infusion (5.9 +/- 2 to 4.1 +/- 1.1%, NS). It is concluded that short-term (3-h) infusion of C-peptide in physiological amounts may improve autonomic nerve function in patients with IDDM.

Experimental and clinical studies on the reduction of erythrocyte sorbitol-glucose ratios by ascorbic acid in diabetes mellitus

In order to confirm the effect of ascorbic acid (AA) on human erythrocyte sorbitol accumulation and explore its mechanism of action, the effects of ascorbic acid in vitro on the sorbitol (S) and glucose (EG) content of human erythrocytes and in particular on the S/EG ratio as a marker of aldose reductase (AR) activity were carefully observed. The results showed that both the accumulation of erythrocyte sorbitol and the S/EG ratio were strongly reduced by the addition of AA. The sorbitol content in the erythrocyte and the S/EG ratio were reduced by a maximum of 87.3% and 83.4% and 93.8% and 63.9% when the medium's AA concentration was at its peak with 5.6 mmol/l and 28 mmol/l glucose in the medium, respectively. The contents of erythrocyte glucose measured coincidentally revealed a positive correlation with the ascorbic acid concentration in the medium during incubation in 5.6 mmol/l glucose while at a higher glucose level (28 mmol/l) in the medium the correlation became negative. These results suggested that the polyol pathway could be inhibited effectively by AA through its direct action on AR. The results of a double-blind cross-over trial using AA tablets or inositol tablets in eight diabetic patients showed that the supplementation of 1000 mg AA/day for 2 weeks resulted in reductions of 12.2% and 21.8% in erythrocyte sorbitol and red cell sorbitol/plasma glucose (S/PG) ratio, respectively (P < 0.05), whereas the fasting plasma glucose levels measured coincidentally revealed no changes (P > 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

The efficacy of aldose reductase inhibitors in the management of diabetic complications. Comparison with intensive insulin treatment and pancreatic transplantation

Recently, aldose reductase inhibitors (ARIs) have been registered in several countries for the improvement of glycaemic control. However, their efficacy is still controversial. ARIs inhibit the enhanced flux of glucose through the polyol pathway. As such, they can never be more effective than normoglycaemia, and so their potential benefits and limitations should be considered relative to the effects of prolonged euglycaemia. The clinical effects of ARIs can be put into perspective by assessing the effects of improved glycaemic control attained in randomised trials of intensive insulin treatment [such as the Diabetes Control and Complications Trial (DCCT)] and after pancreatic transplantation. Although direct comparison of these 3 interventions is hampered by differences in patient populations, duration and methods of follow-up and in the potency of ARIs, the effects of these 3 metabolic interventions and their course in time appear remarkably similar. For neuropathy, all 3 interventions induce an increase in average motor nerve conduction velocity of approximately 1 m/sec during the first months of treatment. At the same time, improvement of painful symptoms may occur. These changes probably largely represent a metabolic amelioration of the condition of the nerves. Around the second year of treatment with all 3 forms of metabolic improvement, an acceleration of nerve conduction of a similar magnitude occurs, with signs of structural nerve regeneration and some sensory recuperation. Experience with ARIs in nephropathy is still limited, but similar improvements in glomerular filtration rate and, less consistently, in urinary albumin excretion were found during short term normoglycaemia produced by all 3 forms of treatment. Comparison of a small number of studies, however, shows differences between intensive insulin regimens, pancreatic transplantation and ARIs in effects on retinopathy. Retinopathy often temporarily deteriorates in the early phases of improved glycaemic control, but this is not noted with ARIs. New microaneurysm formation was slightly reduced in a single long term study with the ARI sorbinil, but the preventive effects on the overall levels of retinopathy seemed less strong than in normoglycaemia trials of similar duration. However, the pharmacodynamic effects on inhibiting the polyol pathway differ among ARIs, and the half-life of the inhibiting effect of sorbinil may have been too short for a complete reduction of polyol pathway activity. The trials of prolonged intensive insulin therapy and pancreatic transplantation have demonstrated that very strict metabolic control must be maintained continuously for many years before a significant reduction of complications can be demonstrated.(ABSTRACT TRUNCATED AT 400 WORDS)

Aldose reductase inhibitors and diabetic complications

Aldose reductase inhibitors impede flux of glucose through the sorbitol pathway in diabetes mellitus. They therefore reduce the accumulation of the pathway metabolites, sorbitol and fructose, reduce the impact of the flux on the cofactors used by the pathway and reduce other derived phenomena, such as osmotic stress and myo-inositol depletion. As drugs, their targets are the chronic complications of diabetes--neuropathy, retinopathy, nephropathy and vasculopathy. In experimental models there is proof of activity against biochemical, functional and structural defects in all of the involved tissues, but we await full clinical verification of this potential.

Is hyperglycemia risky enough to justify the increased risk of hypoglycemia linked with tight diabetes control?

There is an ongoing debate about the possible disadvantages of human insulin use with respect to a possibly lower awareness of hypoglycemia than is associated with animal insulin usage. Participants in this debate have not, however, discussed a major contributory factor to this life-threatening acute complication of diabetes, the pressure on patients to achieve normal levels of blood glucose. This pressure stems from the view that hyperglycemia is the major causative factor in the long-term diabetic complications. However, the evidence that supranormal levels of tissue and plasma glucose contribute to the diabetic tissue damage is not as strong as the arguments on behalf of this position. Indeed, elevated glycemia may be no more than a crude index of other, unknown metabolic derangements which may be causative agents in diabetes-associated tissue damage. Intensive efforts to "normalize" glycemia lack experimental and clinical justification, distract attention from other possible mechanisms, and may impose an unnecessary risk on the insulin-dependent diabetic population since intensive "normalization" of glycemia lowers hypoglycemia awareness, and thus increases risk of hypoglycemia, irrespective of the type of insulin used.

Spirohydantoin inhibitors of aldose reductase inhibit iron- and copper-catalysed ascorbate oxidation in vitro

Transition metal-catalysed oxidations have been implicated in the complications of diabetes. We report here that some experimental inhibitors of the enzyme aldose reductase (implicated in diabetes mellitus via its ability to catalyse glucose reduction to sorbitol) are also potent inhibitors of transition metal-catalysed ascorbate oxidation. The inhibition appears to be dependent upon the presence of a spirohydantoin group. It is conceivable that the copper- and iron-binding capacity of these compounds may contribute to some of their observed biological effects and may provide a starting point for a new generation of experimental drugs for the treatment of diabetes mellitus.

The selective serotonin reuptake inhibitor paroxetine is effective in the treatment of diabetic neuropathy symptoms

The effect of the selective serotonin reuptake inhibitor paroxetine on diabetic neuropathy symptoms was examined in comparison to imipramine and placebo in a randomised, double-blind, cross-over study. Paroxetine was given as a fixed dose of 40 mg/day, while the dose of imipramine was adjusted to yield optimal plasma levels of imipramine plus desipramine of 400-600 nM. Paroxetine significantly reduced the symptoms of neuropathy as measured by both observer- and self-rating, but was somewhat less effective than imipramine. However, patients showing a weaker response to paroxetine than to imipramine had lower plasma concentrations of paroxetine than patients with similar response to the 2 drugs. On imipramine 5 patients dropped out because of intolerable side effects and 4 of 19 patients completing the study reported withdrawal symptoms after discontinuing imipramine. On paroxetine no patients dropped out due to side effects and no withdrawal symptoms were reported. Self-rating showed no depressive symptoms at baseline, and no changes during the study. Neither paroxetine nor imipramine caused changes in objective measures of peripheral nerve function. In conclusion, 40 mg paroxetine/day significantly reduced the symptoms in peripheral diabetic neuropathy, and it was suggested that by dose adjustment on the basis of drug level monitoring, paroxetine may become as effective as imipramine. Paroxetine was devoid of the often disturbing autonomic side effects limiting the use of imipramine in several patients.

Pupillary size in children and adolescents with type 1 diabetes

Pupillary adaptation to darkness was studied in 63 children and adolescents with Type 1 diabetes using a simple portable pupillometer. Results were compared with those in a group of age-related non-diabetic children and expressed as the ratio of the pupil diameter to the iris diameter (pupil diameter %). In the diabetic patients the pupil diameter % was 61.1 +/- 5.8 (44.4-71.9) % compared with 64.2 +/- 4.1 (53.2-72.6) % in the control subjects (p less than 0.001). Abnormal pupillary adaptation to darkness was found more commonly than abnormal heart rate variation in response to a variety of stimuli in the diabetic patients. Pupillary adaptation to darkness may be useful as an indicator of subclinical autonomic neuropathy in diabetic children.

Diabetic amyotrophy: reassessment of the clinical spectrum

Diabetic amyotrophy is a syndrome whose recognition may be difficult or delayed. We reviewed thirteen patients with this disorder, all of whom had significant proximal lower limb wasting and weakness as the predominant feature and eleven of whom had pain in the affected limbs. Significant weight loss was common. In nine patients the deficits were largely or totally reversible. Important variations from the classical features were observed. Only five patients displayed asymmetric proximal lower limb wasting, weakness and pain, motor deficits in the remainder being either unilateral or bilateral and symmetrical. The shoulder girdle and arms were also involved in two patients. Proximal limb pain was not invariable, a distal sensory peripheral neuropathy was common, and diabetic control at diagnosis was likely to be good. No prognostic factors were identified. Thus, not all patients with diabetic amyotrophy exhibit the classically-described features. Other than careful clinical examination, a thorough bilateral electromyographic and nerve conduction study remains the most helpful diagnostic test. Appreciation of the clinical spectrum and context of diabetic amyotrophy should facilitate its differentiation from other disorders, including other forms of diabetic neuropathy.

The association between heart rate and glycemia in adult diabetic subjects

We have studied the association between heart rate and glycemia in 221 diabetic and 109 non-diabetic adult subjects. Supine and standing heart rates were both statistically significantly higher (P less than 0.001) in the diabetic patients than in the control subjects. Among the diabetics, both the supine and standing heart rates were positively associated with hemoglobin A1c (r = 0.26, P less than 0.001 and r = 0.27, P less than 0.001 respectively). Patients with hemoglobin A1c values greater than 10.00% had a mean heart rate of 84.2 +/- 2.4, while those with hemoglobin A1c values less than 6.00% had a mean of 74.0 +/- 1.5 (P less than 0.001). The associations between heart rate and hemoglobin A1c persisted (P less than 0.001) with allowances for potential confounding variables in multiple regression analyses. The 15:30 difference, an indicator of parasympathetic function, was not associated with either heart rate or glycemia. These data suggest that the heart rate elevation in adult diabetic patients is the result of metabolic abnormalities.

Influence of long-term aldose reductase inhibitor therapy on autonomic dysfunction of urinary bladder, stomach and cardiovascular systems in diabetic patients

The effects of the aldose reductase inhibitor (ARI) sorbinil (250 mg/day) were tested in an open-label pilot study over 1 year, in eight diabetics with peripheral neuropathy, seven of whom had symptomic autonomic neuropathy (AN). Autonomic function studies of the urinary bladder, stomach and cardiovascular system were performed at baseline, 6 and 12 months. Six patients reported improvement in symptoms of AN by 6 months which was maintained or further improved by 12 months. Bladder sensation, as measured by cystometrographic parameters, improved at 6 months (P less than 0.02- less than 0.04), but by 12 months had reverted to baseline. Residual urine volume decreased at 6 months (P less than 0.06) and 12 months (P less than 0.06). Vagally mediated gastric acid secretion improved at 6 months (P less than 0.06); the subgroup of patients with subnormal secretion showed improvement to the normal range at 6 months (P less than 0.03). Gastric emptying of solid food was normal in six of eight subjects and showed no significant change at 6 months. Both patients with delayed emptying normalized. No change in beat-to-beat variability in heart rate with respiration was noted. Resting minimum heart rate decreased at 12 months (P less than 0.05). Glycohemoglobin levels showed no statistically significant changes. No toxic reactions were observed. These data suggest a beneficial effect of ARI treatment on symptomatic and asymptomatic manifestations of diabetic autonomic neuropathy and indicate a need for large controlled trials.

Clinical studies with an aldose reductase inhibitor in the autonomic and somatic neuropathies of diabetes

Clinical investigations with the aldose reductase inhibitor (ARI) sorbinil in diabetic patients with neuropathy are described. Cardiac autonomic neuropathy was studied in 36 patients, in a double-blind, placebo-controlled, randomized, noncrossover trial. Patients received sorbinil (250 mg qd) or placebo over 6 weeks after a one-week baseline period. Diabetic control did not change over the study period, as indicated by unchanged glycohemoglobin. Response was assessed by expiration/inspiration (E/I) ratios on EKG during 6 c/min respiration and resting minimum heart rate, both measures of vagal function. In the sorbinil group, E/I ratios improved from 1.074 +/- 0.012 to 1.096 +/- 0.020 (P less than 0.03) with a slight decrease in the placebo group from 1.112 +/- 0.023 to 1.105 +/- 0.023 (P = NS). The difference between the week 6 and week 0 changes in each group was significant (P less than 0.01). Resting minimum heart rate decreased in the sorbinil group from 76.4 +/- 2.3 to 66.8 +/- 2.4 beats/min (P less than 0.001), with a mean change of 10 +/- 2. In the placebo group, heart rate was unchanged (77.9 +/- 3.9 to 77.5 +/- 3.3). The two sample t tests of the within-group differences were likewise significant (P less than 0.001). These changes in both E/I ratio and resting minimum heart rate are consistent with a sorbinil-related improvement in cardiac parasympathetic nerve function. Several isolated cases with apparent sorbinil-related improvement in autonomic symptoms will also be described. Studies of somatic neuropathy have previously shown improvement in nerve conduction velocities with sorbinil. In a study of 11 patients with severely painful diabetic neuropathy treated with sorbinil for 3 weeks [placebo-controlled in single-blind fashion (n = 8)], pains (as assessed on a 0 to 20 rating scale) improved from a mean score of 16 down to 8, with deterioration following drug withdrawal. Objective improvements in sensation and strength were observed in some cases. In this group of patients, statistically significant improvements in nerve conduction velocity, E/I ratios, and resting minimal heart rate, similar to those previously discussed, were also documented. Somatosensory-evoked potentials studies in the 36-patient study showed significant improvements in peripheral conduction and cortical responses. Sorbinil toxicity in 106 patients was 11.3%, with sex incidence of 7/73 males (9.6%) and 5/33 females (15.2%).(ABSTRACT TRUNCATED AT 400 WORDS)