Aldose reductase inhibition: studies with alrestatin

CADD Studies in the Discovery of Potential ARI (Aldose Reductase Inhibitors) Agents for the Treatment of Diabetic Complications

The lack of currently available drugs for treating diabetes complications has stimulated our interest in finding new Aldose Reductase inhibitors (ARIs) with more beneficial biological properties. One metabolic method uses aldose reductase inhibitors in the first step of the polyol pathway to control excess glucose flux in diabetic tissues. Computer-aided drug discovery (CADD) is key in finding and optimizing potential lead substances. AR inhibitors (ARI) have been widely discussed in the literature. For example, Epalrestat is currently the only ARI used to treat patients with diabetic neuropathy in Japan, India, and China. Inhibiting R in patients with severe to moderate diabetic autonomic neuropathy benefits heart rate variability. AT-001, an AR inhibitor, is now being tested in COVID-19 to see how safe and effective it reduces inflammation and cardiac damage. In summary, these results from animal and human studies strongly indicate that AR can cause cardiovascular complications in diabetes. The current multi-center, large-scale randomized human study of the newly developed powerful ARI may prove its role in diabetic cardiovascular disease to establish therapeutic potential. During the recent coronavirus disease (COVID-19) outbreak in 2019, diabetes and cardiovascular disease were risk factors for severely negative clinical outcomes in patients with COVID19. New data shows that diabetes and obesity are among the strongest predictors of COVID-19 hospitalization. Patients and risk factors for severe morbidity and mortality of COVID- 19.

Physiological and Pathological Roles of Aldose Reductase

Aldose reductase (AR) is an aldo-keto reductase that catalyzes the first step in the polyol pathway which converts glucose to sorbitol. Under normal glucose homeostasis the pathway represents a minor route of glucose metabolism that operates in parallel with glycolysis. However, during hyperglycemia the flux of glucose via the polyol pathway increases significantly, leading to excessive formation of sorbitol. The polyol pathway-driven accumulation of osmotically active sorbitol has been implicated in the development of secondary diabetic complications such as retinopathy, nephropathy, and neuropathy. Based on the notion that inhibition of AR could prevent these complications a range of AR inhibitors have been developed and tested; however, their clinical efficacy has been found to be marginal at best. Moreover, recent work has shown that AR participates in the detoxification of aldehydes that are derived from lipid peroxidation and their glutathione conjugates. Although in some contexts this antioxidant function of AR helps protect against tissue injury and dysfunction, the metabolic transformation of the glutathione conjugates of lipid peroxidation-derived aldehydes could also lead to the generation of reactive metabolites that can stimulate mitogenic or inflammatory signaling events. Thus, inhibition of AR could have both salutary and injurious outcomes. Nevertheless, accumulating evidence suggests that inhibition of AR could modify the effects of cardiovascular disease, asthma, neuropathy, sepsis, and cancer; therefore, additional work is required to selectively target AR inhibitors to specific disease states. Despite past challenges, we opine that a more gainful consideration of therapeutic modulation of AR activity awaits clearer identification of the specific role(s) of the AR enzyme in health and disease.

Development of Aldose Reductase Inhibitors for the Treatment of Inflammatory Disorders and Cancer: Current Drug Design Strategies and Future Directions

Aldose Reductase (AR) is an enzyme that converts glucose to sorbitol during the polyol pathway of glucose metabolism. AR has been shown to be involved in the development of secondary diabetic complications due to its involvement in causing osmotic as well as oxidative stress. Various AR inhibitors have been tested for their use to treat secondary diabetic complications, such as retinopathy, neuropathy, and nephropathy in clinical studies. Recent studies also suggest the potential role of AR in mediating various inflammatory complications. Therefore, the studies on the development and potential use of AR inhibitors to treat inflammatory complications and cancer besides diabetes are currently on the rise. Further, genetic mutagenesis studies, computer modeling, and molecular dynamics studies have helped design novel and potent AR inhibitors. This review discussed the potential new therapeutic use of AR inhibitors in targeting inflammatory disorders and cancer besides diabetic complications. Further, we summarized studies on how AR inhibitors have been designed and developed for therapeutic purposes in the last few decades.

Addressing selectivity issues of aldose reductase 2 inhibitors for the management of diabetic complications

Aldose Reductase 2 (ALR2), the rate-limiting enzyme of the polyol pathway, plays an important role in detoxification of some toxic aldehydes. Under hyperglycemia, this enzyme overactivates and causes diabetic complications (DC). Therefore, ALR2 inhibition has been established as a potential approach to manage these complications. Several ALR2 inhibitors have been reported, but none of them could reach US FDA approval. One of the main reasons is their poor selectivity over ALR1, which leads to the toxicity. The current review underlines the molecular connectivity of ALR2 with DC and comparative analysis of the catalytic domains of ALR2 and ALR1, to better understand the selectivity issues. This report also discusses the key features required for ALR2 inhibition and to limit toxicity due to off-target activity.

Electrospun Nanofibers Loaded with Quercetin Promote the Recovery of Focal Entrapment Neuropathy in a Rat Model of Streptozotocin-Induced Diabetes

In this study, quercetin-loaded zein-based nanofibers were developed using electrospinning technique. The therapeutic effect of these quercetin-loaded nanofibers on neuropathy in streptozotocin- (STZ-) induced diabetes in rats was assessed. Diabetic condition was induced in male Wistar rats by STZ, after which a crush injury of the right sciatic nerve was performed to induce mononeuropathy. Functional recovery was assessed using walking track analysis, measurements of foot withdrawal reflex, nerve conduction velocity, and morphological analysis. The oxidative stress status and the ratio of phosphorylated extracellular recognition kinase (pERK)/extracellular recognition kinase (ERK) expression in the nerve lesion were also assessed in order to elucidate the potential mechanisms involved. Results showed that quercetin-loaded zein-based nanofibers slightly enhanced functional recovery from neuropathy in STZ-diabetic rats. The potential mechanism might partially involve improvements in oxidative stress status and the ratio of pERK/ERK expression in the nerve lesion.

Epalrestat Upregulates Heme Oxygenase-1, Superoxide Dismutase, and Catalase in Cells of the Nervous System

Heme oxygenase (HO)-1 has potent antioxidant and anti-inflammatory functions. Recent studies have shown that the upregulation of HO-1 is beneficial to counteract neuroinflammation, making HO-1 a new therapeutic target for neurological diseases. We have reported that epalrestat (EPS), which is currently used for the treatment of diabetic neuropathy, increases HO-1 levels through the activation of nuclear factor erythroid 2-related factor 2 (Nrf2) in bovine aortic endothelial cells. In this study, we tested the hypothesis that EPS upregulates HO-1 via Nrf2 activation in the component cells of the nervous system, by using rat Schwann cells and human SH-SY5Y cells. Treatment of Schwann cells with EPS at near-plasma concentration led to a dramatic increase in HO-1 levels. Nrf2 knockdown by small interfering RNA (siRNA) suppressed the EPS-induced HO-1 expression. EPS did not promote the intracellular accumulation of free ferrous ion and reactive oxygen species, by increasing ferritin via Nrf2 during HO-1 induction. Moreover, EPS stimulated the expression of superoxide dismutase 1 and catalase, which also are Nrf2 target gene products. It also markedly increased HO-1 levels in SH-SY5Y cells through the activation of Nrf2. We demonstrated for the first time that EPS upregulates HO-1, superoxide dismutase, and catalase by activating Nrf2. We suggest that EPS has the potential to prevent several neurological diseases.

Epalrestat increases glutathione, thioredoxin, and heme oxygenase-1 by stimulating Nrf2 pathway in endothelial cells

Epalrestat (EPS) is the only aldose reductase inhibitor that is currently available for the treatment of diabetic neuropathy. Recently, we found that EPS at near-plasma concentration increases the intracellular levels of glutathione (GSH) in rat Schwann cells. GSH plays a crucial role in protecting endothelial cells from oxidative stress, thereby preventing vascular diseases. Here we show that EPS increases GSH levels in not only Schwann cells but also endothelial cells. Treatment of bovine aortic endothelial cells (BAECs), an in vitro model of the vascular endothelium, with EPS caused a dramatic increase in intracellular GSH levels. This was concomitant with the up-regulation of glutamate cysteine ligase, an enzyme catalyzing the first and rate-limiting step in de novo GSH synthesis. Moreover, EPS stimulated the expression of thioredoxin and heme oxygenase-1, which have important redox regulatory functions in endothelial cells. Nuclear factor erythroid 2-related factor 2 (Nrf2) is a key transcription factor that regulates the expression of antioxidant genes. EPS increased nuclear Nrf2 levels in BAECs. Nrf2 knockdown by siRNA suppressed the EPS-induced glutamate cysteine ligase, thioredoxin-1, and heme oxygenase-1 expression. Interestingly, LY294002, an inhibitor of phosphatidylinositol 3-kinase, abolished the EPS-stimulated GSH synthesis, suggesting that the kinase is associated with Nrf2 activation induced by EPS. Furthermore, EPS reduced the cytotoxicity induced by H2O2 and tert-butylhydroperoxide, indicating that EPS plays a role in protecting cells from oxidative stress. Taken together, the results provide evidence that EPS exerts new beneficial effects on endothelial cells by increasing GSH, thioredoxin, and heme oxygenase-1 levels through the activation of Nrf2. We suggest that EPS has the potential to prevent several vascular diseases caused by oxidative stress.

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 as pathobiochemical probes

In tissues susceptible to damage from chronic diabetes, excess glucose is metabolized by aldose reductase (AR) to sorbitol. Originally, AR-catalyzed sorbitol formation (and accumulation) was found in the diabetic lens; the cataractogenicity of this process was proven by preventing cataract formation with an AR inhibitor (ARI). These findings were extended to the hypothesis that, in diabetic tissues, excessive intracellular sorbitol formation initiates a cascade of metabolic abnormalities which gradually progress to loss of functional and structural integrity. The pivotal role of AR as a trigger for such abnormalities was established by preventing their occurrence in diabetic animals treated with an ARI. By inference, this led to the concept that inhibition of AR should prevent, arrest, and, possibly, reverse the development of late diabetic sequelae. In addition to motivating drug-oriented research, the ARI concept provided a rationale for the use of ARIs as experimental tools to probe the pathogenesis of diabetic complications. By helping to elucidate the metabolic, functional, and structural ramifications of the AR-catalyzed disposal of excess glucose in diabetic schemes, and in addition, by helping to define the applicability of animal models for the study of early functional pathogenic alterations occurring in diabetic subjects, ARIs may enable the discrimination in diabetic tissue of arrestible and reversible from the irreversible abnormalities.

One-year treatment with the aldose reductase inhibitor, ponalrestat, in diabetic neuropathy

A double blind placebo controlled trial was performed to evaluate the effects of the aldose reductase inhibitor, ponalrestat, on symptomatic diabetic neuropathy. After a 4-week placebo run-in phase, 60 patients were 2:1 randomized to receive either 600 mg ponalrestat or placebo once daily over 12 months. Forty-six patients, 30 of whom were treated with ponalrestat and 16 with placebo, completed the study. Motor and sensory nerve conduction, thermal and vibration sensation thresholds, heart rate variation at rest, E/I ratio, pupillary dilation velocity and pupillary reflex latency were determined at baseline and after 6 and 12 months. Neuropathic symptom scores were assessed every 3 months. Among the fifteen nerve function parameters studied, only trends in favour of ponalrestat were noted for heart rate variation and E/I ratio after 6 months (P = 0.06), but no significant differences between the groups could be demonstrated during the study. No adverse reactions were observed. It is concluded that one-year treatment with ponalrestat has no beneficial effects on symptoms or electrophysiological parameters in diabetic neuropathy.

Antidiabetic and aldose reductase activities of biflavanones of Garcinia kola

Kolaviron, a mixture of C-3/C-8 linked biflavonoids obtained from Garcinia kola produces significant hypoglycaemic effects when administered intraperitoneally to normal and alloxan diabetic rabbits at a dose of 100 mg kg-1. The fasting blood sugar in normoglycaemic rabbits was reduced from 115 mg/100 mL to 65 mg/100 mL after 4 h. In alloxan diabetic rabbits, the blood sugar was lowered from 506 mg/100 mL to 285 mg/100 mL at 12 h. The hypoglycaemic effects have been compared with those of tolbutamide. Kolaviron inhibited rat lens aldose reductase (RLAR) activity, with an IC50 value of 5.4 x 10(-6). The significance of these findings in the potential use of kolaviron as an antidiabetic agent is discussed.

A comparison of nerve conduction velocities and current perception thresholds as correlates of clinical severity of diabetic sensory neuropathy

Nerve conduction velocities (NCVs) are the standard measurements used to confirm the presence or absence of diabetic neuropathy. NCVs were contrasted with the newer technique of measurement of alternating current perception thresholds (CPTs) in assessing the quantitative level of correlation with severity of diabetic sensory neuropathy. A very detailed, scored neurological history (symptoms) and physical examination, emphasising sensory assessment, was conducted on 71 individuals with diabetic neuropathy of varying degrees of severity. Sensory and motor NCVs and CPTs at 5, 250, and 2000 Hz of the upper and lower extremities were determined for these individuals. In addition, vibration thresholds (VTs) were measured as a third modality. Twenty eight individuals underwent repeated evaluations at 2, 6, 10 and 12 months after the initial procedures. Using the results of 169 complete evaluations, correlations were determined between physical scores (PS) and symptoms scores (SS) and NCVs. NCV correlations with the SS were weaker than with the PS. The strongest of the correlations were found between the PS and motor NCVs of the median nerve (rho = 0.29) and the tibial nerve (rho = 0.38). Normal NCVs were present in the face of very significant historical and physical abnormality. Correlations of the SS and PS with both VTs and CPTs were higher than with the NCVs. CPTs proved the more effective as predictors of both symptomatic and physical impairment. NCVs appear to lack the resolving power necessary to evaluate subtle differences in clinical state of diabetic sensory neuropathy. The supplementary use of current perception testing may improve the quantitative assessment of this condition.

Clinical aspects of diabetic neuropathies

Diabetic neuropathy is a common complication of diabetes that may be associated both with considerable morbidity (painful polyneuropathy, neuropathic ulceration) and mortality (autonomic neuropathy). The epidemiology and natural history of diabetic neuropathy is clouded with uncertainty, largely due to confusion in the definition and measurement of this disorder. We have reviewed a variety of the clinical manifestations associated with somatic and autonomic neuropathy and discussed current views related to the management of the different abnormalities. Although unproven, the best evidence suggests that near normal control of blood glucose in the early years following onset of diabetes may help delay the development of clinically significant nerve impairment. Intensive therapy to achieve normalization of blood glucose may also lead to reversibility of early diabetic neuropathy, but again this is unproven. Our ability to manage successfully the many different manifestations of diabetic neuropathy depends ultimately on our success in uncovering the pathogenic processes underlying this disorder. The recent resurgence of interest in the vascular hypothesis, for example, has opened up new avenues of investigation for therapeutic intervention. Paralleling our increased understanding of the pathogenesis of diabetic neuropathy, there must be refinements in our ability to measure quantitatively the different types of defects that occur in this disorder. These tests must be validated and standardized to allow comparability between studies and more meaningful interpretation of study results.

Hyperglycemia-induced activation of human erythrocyte aldose reductase and alterations in kinetic properties

Incubation of human erythrocytes with varying concentrations of glucose resulted in a several-fold increase in aldose reductase (alditol:NADP+ 1-oxidoreductase, EC 1.1.1.21) activity as determined by the rate of NADPH oxidation and the rate of sorbitol formation. As compared to aldose reductase from human erythrocytes not incubated with glucose (native enzyme), aldose reductase from 30 mM glucose-incubated erythrocytes (activated enzyme) exhibited altered kinetic and inhibition properties. Native enzyme showed biphasic kinetics with substrates (glucose and glyceraldehyde), was strongly inhibited by 15 microM ADP, 1,3-diphosphoglycerate, 2,3-diphosphoglycerate and 3-phosphoglycerate, and aldose reductase inhibitors such as sorbinil and alrestatin. The activated enzyme, on the other hand, exhibited monophasic kinetics, low Km for substrates, was not inhibited by the phosphorylated intermediates, and was less susceptible to inhibition by aldose reductase inhibitors. In erythrocytes of the diabetic subjects, we have found an excellent correlation between aldose reductase activity and plasma glucose levels and have observed that whenever the blood glucose level was higher than 15 mM, all of the erythrocyte aldose reductase was present in the activated form and exhibited properties similar to those observed with aldose reductase obtained from 30 mM glucose-incubated erythrocytes.

Chicken muscle aldose reductase: purification, properties and relationship to other chicken aldo/keto reductases

An enzyme that catalyzes the NADPH-dependent reduction of a wide range of aromatic and hydroxy-aliphatic aldehydes was purified from chicken breast muscle. This enzyme shares many properties with mammalian aldose reductases including molecular weight, relative substrate specificity, Michaelis constants, an inhibitor specificity. Therefore, it seems appropriate to call this enzyme an aldose reductase (EC 1.1.1.21). Chicken muscle aldose reductase appears to be kinetically identical to an aldose reductase that has been purified from chicken kidney (Hara et al., Eur. J. Biochem. 133, 207-214) and to hen muscle L-glycol dehydrogenase (Bernado et al., Biochim. biophys. Acta 659, 189-198). The association of this aldose reductase with muscular dystrophy in the chick is discussed.

Activated and unactivated forms of human erythrocyte aldose reductase

Aldose reductase (alditol:NADP+ 1-oxidoreductase, EC 1.1.1.21) has been partially purified from human erythrocytes by DEAE-cellulose (DE-52) column chromatography. This enzyme is activated severalfold upon incubation with 10 microM each glucose 6-phosphate, NADPH, and glucose. The activation of the enzyme was confirmed by following the oxidation of NADPH as well as the formation of sorbitol with glucose as substrate. The activated form of aldose reductase exhibited monophasic kinetics with both glyceraldehyde and glucose (Km of glucose = 0.68 mM and Km of glyceraldehyde = 0.096 mM), whereas the native (unactivated) enzyme exhibited biphasic kinetics (Km of glucose = 9.0 and 0.9 mM and Km of glyceraldehyde = 1.1 and 0.14 mM). The unactivated enzyme was strongly inhibited by aldose reductase inhibitors such as sorbinil, alrestatin, and quercetrin, and by phosphorylated intermediates such as ADP, glycerate 3-phosphate, glycerate 1,3-bisphosphate, and glycerate 2,3-trisphosphate. The activated form of the enzyme was less susceptible to inhibition by aldose reductase inhibitors and phosphorylated intermediates.

The effects of a new aldose reductase inhibitor (tolrestat) in galactosemic and diabetic rats

Tolrestat(N-[[5-(trifluoromethyl)-6-methoxy-1-naphthalenyl] thioxomethyl]-N-methylglycine; AY-27,773; Alredase) is a potent, structurally novel inhibitor of aldose reductase (AR). In vitro, tolrestat inhibited in dose-dependent fashion the AR from bovine lenses (IC50, 3.5 X 10(-8) mol/L) and the formation of sorbitol in human RBC incubated with glucose (IC50, 3 X 10(-8) mol/L). Upon administration with the diet to rats made galactosemic or diabetic, tolrestat decreased, in a dose-related fashion, the accumulation of galactitol or sorbitol in the sciatic nerve and lens. The effectiveness of tolrestat depended upon the experimental conditions and tended to be higher in less severe galactosemia and after suitable pretreatment, particularly in galactosemic rats, resulting in ID50 of 5 mg/kg/d in the sciatic nerve and 12-15 mg/kg/d in the lens. Tolrestat also decreased, in dose-related manner, the RBC sorbitol levels in normal and in streptozotocin diabetic rats; in the latter, at less than 2 mg/kg/d, the RBC sorbitol was reduced to control levels.

Inhibition studies on chicken muscle aldose reductase

Several compounds that are known to inhibit mammalian aldose reductases were examined for their effects on chicken muscle aldose reductase (EC 1.1.1.21). Sorbinil was the most effective compound tested. Alrestatin and phenobarbital were effective inhibitors of the enzyme although their IC50 values were 10-fold more than that of Sorbinil. Indomethacin, diphenylhydantoin, phenacetin, and valproate were also inhibitors of chicken muscle aldose reductase but were less effective. These compounds are all non-competitive inhibitors with respect to substrate. Menadione bisulfite, a watersoluble analog of Vitamin K3 which is a substrate for carbonyl reductase but not aldose reductase, was a competitive inhibitor of chicken aldose reductase with respect to substrate. This observation is discussed with reference to the possible treatment of muscular dystrophy with specific inhibitor of aldose reductases.

Purification and properties of aldose reductase and aldehyde reductase II from human erythrocyte

Aldose reductase (EC 1.1.1.21) and aldehyde reductase II (L-hexonate dehydrogenase, EC 1.1.1.2) have been purified to homogeneity from human erythrocytes by using ion-exchange chromatography, chromatofocusing, affinity chromatography, and Sephadex gel filtration. Both enzymes are monomeric, Mr 32,500, by the criteria of the Sephadex gel filtration and polyacrylamide slab gel electrophoresis under denaturing conditions. The isoelectric pH's for aldose reductase and aldehyde reductase II were determined to be 5.47 and 5.06, respectively. Substrate specificity studies showed that aldose reductase, besides catalyzing the reduction of various aldehydes such as propionaldehyde, pyridine-3-aldehyde and glyceraldehyde, utilizes aldo-sugars such as glucose and galactose. Aldehyde reductase II, however, did not use aldo-sugars as substrate. Aldose reductase activity is expressed with either NADH or NADPH as cofactors, whereas aldehyde reductase II can utilize only NADPH. The pH optima for aldose reductase and aldehyde reductase II are 6.2 and 7.0, respectively. Both enzymes are susceptible to the inhibition by p-hydroxymercuribenzoate and N-ethylmaleimide. They are also inhibited to varying degrees by aldose reductase inhibitors such as sorbinil, alrestatin, quercetrin, tetramethylene glutaric acid, and sodium phenobarbital. The presence of 0.4 M lithium sulfate in the assay mixture is essential for the full expression of aldose reductase activity whereas it completely inhibits aldehyde reductase II. Amino acid compositions and immunological studies further show that erythrocyte aldose reductase is similar to human and bovine lens aldose reductase, and that aldehyde reductase II is similar to human liver and brain aldehyde reductase II.

Properties of ICI 128,436, a novel aldose reductase inhibitor, and its effects on diabetic complications in the rat

ICI 128,436 (3-(4-bromo-2-fluorobenzyl)-4-oxo-3H-phthalazin-1-ylacetic acid) is a chemically novel, potent inhibitor of aldose reductase. It inhibits partially purified aldose reductase isolated from a number of sources including human tissue (human lens - IC50 2.0 X 10(-8) mol/L). Dulcitol accumulation in erythrocytes and sciatic nerves of galactose loaded rats was inhibited by five days of treatment with ICI 128,436 (oral ED50's 2.21 mg/kg and 8.56 mg/kg, respectively). On oral administration for five days to streptozotocin diabetic rats, ICI 128,436, reduced sorbitol levels in sciatic nerve, lens, retina, and renal cortex. The ED50 for inhibition of nerve sorbitol accumulation was 5 mg/kg. The effect of a single dose of ICI 128,436 in diabetic rats was prolonged, with little increase in nerve sorbitol for 48 hours. No tolerance to the ability of ICI 128,436 to reduce nerve sorbitol was found on treatment for 74 days. ICI 128,436 was effective in rodent models of the neural and lenticular complications of diabetes. At doses as low as 25 mg/kg/d it completely prevented the development of cataracts in diabetic rats. The deterioration in motor nerve conduction velocity velocity found in diabetic rats was ameliorated by treatment with ICI 128,436 (3.125 mg/kg/d). Thus, ICI 128,436 constitutes a chemically novel aldose reductase inhibitor that is now being assessed for therapeutic value in the diabetic patient.

Aldose and aldehyde reductases in human tissues

Immunochemical characterizations of aldose reductase and aldehyde reductases I and II, partially purified by DEAE-cellulose (DE-52) column chromatography from human tissues, were carried out by immunotitration, using antisera raised against the homogenous preparations of human and bovine lens aldose reductase and human placenta aldehyde reductase I and aldehyde reductase II. Anti-aldose antiserum cross-reacted with aldehyde reductase I, anti-aldehyde reductase I antiserum cross-reacted with aldose reductase and anti-aldehyde reductase II antiserum precipitated aldehyde reductase II, but did not cross-react with aldose reductase or aldehyde reductase I from all the tissues examined. DE-52 elution profiles, substrate specificity and immunochemical characterization indicate that aldose reductase is present in human aorta, brain, erythrocyte and muscle; aldehyde reductase I is present in human kidney, liver and placenta; and aldehyde reductase II is present in human brain, erythrocyte, kidney, liver, lung and placenta. Monospecific anti-alpha and anti-beta antisera were purified from placenta anti-aldehyde reductase I antiserum, using immunoaffinity techniques. Anti-alpha antiserum precipitated both aldehyde reductase I and aldose reductase, whereas anti-beta antibodies cross-reacted with only aldehyde reductase I. Based on these studies, a three gene loci model is proposed to explain the genetic interrelationships among these enzymes. Aldose reductase is a monomer of alpha subunits, aldehyde reductase I is a dimer of alpha and beta subunits and aldehyde reductase II is a monomer of delta subunits.

A double blind placebo controlled trial of mixed gangliosides in diabetic peripheral and autonomic neuropathy

Twenty-six patients with signs or symptoms of diabetic neuropathy who also had motor or sensory neurophysiological abnormalities, were intensively studied in the first double-blind, placebo controlled trial of a ganglioside mixture in the U.K. (Cronassial, Fidia Farmaceutici, Abano Terme, Italy; 6 weeks of 20 mg daily intramuscularly). Diabetes control was good in both groups and there was no deterioration in either group during the course of the trial. Of the five neurophysiological parameters measured - the motor nerve compound action potential, motor nerve conduction velocity and minimum f wave latency of the common peroneal nerve and the sensory action potential and conduction velocity of the sural nerve - only the mean motor nerve action potential and conduction velocity increased though the changes were not significant. Four patients showed improvements of greater than 1.0 mV after treatment with Cronassial while none of the placebo group did so (p less than 0.05). Of the four patients who showed dramatic improvements in motor nerve action potentials, three also showed improvements in motor nerve conduction velocity. There were no significant changes in three tests of autonomic function after Cronassial therapy (orthostatic tilt reactions, single deep breath and Valsalva maneuver). Four patients in the Cronassial group (not those showing neurophysiological improvement) but only one in the placebo group reported marked improvements in their symptomatic complaints after treatment. We conclude that Cronassial is a promising new drug for the treatment of diabetic peripheral neuropathy with clear benefits for some patients.

Ganglioside treatment in diabetic peripheral neuropathy: a multicenter trial

Ganglioside treatment was evaluated with a multicenter, randomized, double-blind, controlled, cross-over vs placebo trial in 140 insulin-treated diabetic subjects with peripheral neuropathy. The patients entered the study when they showed an impairment in at least two of the electroneurographic parameters, and were assigned to two protocols according to the presence and severity of their neurological symptoms. Ninety-seven diabetic subjects with no or mild symptoms were assigned to protocol I, whereas 43 symptomatic patients were assigned to protocol II. the treatment periods lasted 6 weeks with an intermediate washout period of 4 weeks. The treatment consisted in the daily i.m. administration of 20 mg gangliosides or of placebo. Electroneurographic parameters were recorded at the beginning and at the end of each treatment period, whereas clinical and metabolic data (mean daily plasma glucose, glycosuria and glycosylated hemoglobin) were evaluated every three weeks in protocol I and every two weeks in protocol II. No change in the metabolic parameters was observed throughout the trial period. However, the treatment induced a statistically significant improvement of paresthesias (protocol II) and of some electrophysiological parameters; in particular, ganglioside treatment improved MCV of peroneal nerve (p less than 0.03) in patients of protocol I, MCV o ulnar nerve (p less than 0.002) and SCV of median nerve (p less than 0.06) in patients of protocol II. Furthermore, 22 subjects of protocol II showed a 'drug preference' while 10 preferred placebo and 9 had no preference. In conclusion, ganglioside treatment seems to have a positive effect on diabetic peripheral neuropathy, improving both some symptoms and some electrophysiological parameters.

Aldose reductase inhibition improves nerve conduction velocity in diabetic patients

To assess the potential role of polyol-pathway activity in diabetic neuropathy, we measured the effects of sorbinil--a potent inhibitor of the key polyol-pathway enzyme aldose reductase--on nerve conduction velocity in 39 stable diabetics in a randomized, double-blind, cross-over trial. During nine weeks of treatment with sorbinil (250 mg per day), nerve conduction velocity was greater than during a nine-week placebo period for all three nerves tested: the peroneal motor nerve (mean increase [+/- S.E.M.], 0.70 +/- 0.24 m per second, P less than 0.008), the median motor nerve (mean increase, 0.66 +/- 0.27, P less than 0.005), and the median sensory nerve (mean increase, 1.16 +/- 0.50, P less than 0.035). Conduction velocity for all three nerves declined significantly within three weeks after cessation of the drug. These effects of sorbinil were not related to glycemic control, which was constant during the study. Although the effect of sorbinil in improving nerve conduction velocity in diabetics was small, the findings suggest that polyol-pathway activity contributes to slowed nerve conduction in diabetics. The clinical applicability of these observations remains to be determined, but they encourage further exploration of this approach to the treatment or prevention of diabetic neuropathy.

Properties of an aldose reductase from pig lens. Comparative studies of an aldehyde reductase from pig lens

Two monomeric NADPH enzymes from pig lens, an aldehyde reductase and an aldose reductase, have been characterized. The aldose reductase is obtained in a pure form. The aldehyde reductase, usually called hexonate dehydrogenase, is the same protein as that was recently isolated from pig liver [Branlant, G. and Biellmann, J.F. (1980) Eur. J. Biochem. 105, 611-621]. The aldose reductase is shown to have a number of properties in common with the aldehyde reductase, namely its physico-chemical properties, its tendency to be inhibited by quercitine derivatives and its substrate specificity. These two enzymes differ in their immunological properties. Only aldose reductase has a reactive Cys residue, localized in or near the substrate binding site. In contrast to that shown for aldehyde reductase [Branlant, G. et al. (1981) Eur. J. Biochem. 116, 505-512; Branlant, G. (1982) Eur. J. Biochem. 121, 407-411], no anion-recognition sites are in the substrate binding site of aldose reductase. The fact that also sugars are substrates for aldose reductase support the idea that this enzyme is implicated in the formation of sugar cataract as suggested by Kinoshita, J.H. et al. [J. Am. Med. Ass. 246, 257-261 (1981)]. Pig lens aldose reductase does not show homotropic cooperative effects with respect to either substrate or coenzyme.

Purification and properties of human liver aldehyde reductases

Two NADPH-linked aldehyde reductases (alcohol:NADP+ oxidoreductase, EC 1.1.1.2), referred to here as aldehyde reductases I and II, have been purified to homogeneity from human liver by using ammonium sulfate precipitation, ion-exchange chromatography, affinity chromatography and gel filtration. Structural studies show that aldehyde reductase II is a monomer of about 32 000 daltons, whereas aldehyde reductase I is a dimer of two nonidentical subunits of molecular weights about 42 000 and 35 000. The isoelectric pH was determined to be 5.40 for aldehyde reductase II and 8.25 for aldehyde reductase I. Substrate specificity studies show that neither aldehyde reductase I nor II uses glucose as substrate but that both are capable of reducing various other aldehydes such as pyridine 3-aldehyde, butyraldehyde and DL-glyceraldehyde. The pH optimums for aldehyde reductases I and II are pH 6.0 and 7.0 respectively. Aldehyde reductase I uses both NADH and NADPH as cofactor, whereas aldehyde reductase II activity is dependent on NADPH. Aldehyde reductase I activity is more susceptible than aldehyde reductase II activity to inhibition by p-hydroxymercuribenzoate, as reflected by IC50 values of 7.5 microM and 40 microM for aldehyde reductases I and II, respectively. The susceptibility of human liver aldehyde reductases I and II to inhibition by the aldose reductase (EC 1.1.1.21) inhibitors 3,3'-tetramethylene glutaric acid, alrestatin, chromone and sorbinil was determined and compared with that of aldose reductase partially purified from bovine lenses. The aldose reductase inhibitors, besides inhibiting aldose reductase, also inhibit human liver aldehyde reductases I and II to varying degrees.

Inhibition of human brain aldose reductase and hexonate dehydrogenase by alrestatin and sorbinil

Human brain aldose reductase and hexonate dehydrogenase are inhibited by alrestatin (AY 22,284) and sorbinil (CP 45,634). Inhibition by alrestatin is noncompetitive for both enzymes, and slightly stronger for hexonate dehydrogenase (KI values 52-250 microM) than for aldose reductase (KI values 170-320 microM). Sorbinil inhibits hexonate dehydrogenase far more potently than aldose reductase, KI values being 5 5 microM for hexonate dehydrogenase and 150 microM for aldose reductase. The inhibition of hexonate dehydrogenase by sorbinil is noncompetitive with respect to both aldehyde and NADPH substrates, and is thus kinetically similar to the inhibition by alrestatin. However, sorbinil inhibition of aldose reductase is uncompetitive with respect to glyceraldehyde and noncompetitive with NADPH as the varied substrate. Inhibition of human brain aldose reductase by these two inhibitors is much less potent than that reported for the enzyme from other sources.

Susceptibility of aldehyde and aldose reductases of human tissues to aldose reductase inhibitors

The effect of aldose reductase inhibitors such as sorbinil, alrestatin, and quercitrin has been studied on the aldose reductase purified from human brain and lens, and aldehyde reductase I purified from human liver, and aldehyde reductase II purified from human brain, liver, and red cells. None of the aldose reductase inhibitors have been found to be specific for aldose reductase. Fifty micromolar sorbinil besides inhibiting aldose reductase, completely inhibits aldehyde reductase II from the brain, liver and red cells. Similarly, alrestatin and quercitrin also are potent inhibitors of aldehyde reductase I and aldehyde reductase II.

Effects of aldose reductase inhibitor treatment in diabetic polyneuropathy - a clinical and neurophysiological study

The efficacy of treatment with an aldose reductase inhibitor (1,3-dioxo-1 H-benz-de-isoquinoline-2(3H)-acetic acid, AY-22,284, Alrestatin) on peripheral nerve function in diabetic polyneuropathy was assessed. Thirty patients with long-standing diabetes and slight to moderate neuropathy participated in the double-blind placebo trial. Clinical examination, sensory threshold determinations for vibratory, tactile and thermal stimuli, conduction velocity measurements and studies of automatic function were performed to evaluate the treatment. Significant differences favouring Alrestatin over placebo were found for many of the measured variables, whereas no changes occurred on placebo. The apparent improvement of neuropathy occurred despite persisting hyperglycaemia. The results indicate that aldose reductase inhibitor treatment may be of value in diabetic polyneuropathy, and provide support for the sorbitol pathway hypothesis of diabetic polyneuropathy.

The effect of alrestatin on alanine-stimulated release of insulin and glucagon in man

Alrestatin, an aldose reductase inhibitor, was administered orally to 10 normal men. Its effect on basal plasma glucose, insulin and glucagon levels and on the response of glucose, insulin and glucagon to an oral alanine load was assessed and compared to that of a placebo. There was a significant suppression of both the mean basal insulin level and the mean insulin response to alanine in the group pretreated with alrestatin as compared to the placebo group (p less than 0.05 at 0 and 60 min). Glucagon levels rose slightly in both groups but tended to be lower in the alrestatin-treated subjects, and blood sugar levels fell slightly. There was a significant inverse correlation between the mean insulin and glucose levels in individual subjects. The possible significance and mechanisms of insulin suppression are discussed.