Effects of a new aldose reductase inhibitor, (2S, 4S)-6-fluoro-2',5'-dioxospiro[chroman-4,4'-imidazolidine]-2-ca rboxamid e (SNK-860), on the slowing of motor nerve conduction velocity and metabolic abnormalities in the peripheral nerve in acute streptozotocin-induced diabetic rats

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.

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.

Addition of the Aldose Reductase Inhibitor Benzofuroxane Derivative BF-5m to Prolonged and Moderate Exercise Training Enhanced Protection of the Rat Heart From Type-1 Diabetes

Moderate exercise training may not be sufficient to exert beneficial effects on the cardiovascular system because of the long-term multifactorial etiology of diabetic complications. The addition of a proper pharmacological tool to the physical exercise should improve the outcomes of the diabetic damage. Here it is shown that 8 weeks exercise training of type 1 diabetic Sprague-Dawley (SD) rats resulted in a significantly increased heart rate, a 14% increase in the left ventricular ejection fraction (LVEF) increased plasma insulin levels and a 13% decrease in plasma glucose with respect to sedentary animals. The training also resulted in a 22% reduction in cardiac QT interval from a diabetic sedentary value of 185 ± 19 ms. Treatment of trained rats with the new antioxidant and NO-releasing aldose reductase 2 inhibitor 5(6)-(benzo[d]thiazol-2-ylmethoxy) benzofuroxane BF-5m, 20 mg/kg/day, added a further and significant (P < 0.01 vs. sedentary) increase of the LVEF up to 38% at 8 week time point. The long QT interval recorded in trained rats was reduced to further 12% by addition to the training of pharmacological treatment with 20 mg/kg/day BF-5m. At this time, the association of the two treatments improved the expression into the cardiac tissue of sarcoplasmic reticulum Ca²⁺ ATPase 2 (SERCA2) and manganese superoxide dismutase (MnSOD), and reduced the fibrosis.

Litsea japonica Extract Inhibits Aldose Reductase Activity and Hyperglycemia-Induced Lenticular Sorbitol Accumulation in db/db Mice

Aldose reductase (AR) is the first and rate-limiting enzyme of the polyol pathway. AR-dependent synthesis of excess polyols leads to lens opacification in diabetic cataract. The purpose of this study is to investigate the protective effect of Litsea japonica extract (LJE) on diabetes-induced lens opacification and its protective mechanism in db/db mice. Seven-week-old male db/db mice were treated with LJE (100 and 250 mg/kg body weight) once a day orally for 12 weeks. LJE dose dependently inhibited rat lens aldose reductase activity in vitro (IC₅₀ = 13.53 ± 0.74 µg/mL). In db/db mice, lens was slightly opacified, and lens fiber cells were swollen and ruptured. In addition, lenticular sorbitol accumulation was increased in db/db mice. However, the administration of LJE inhibited these lenticular sorbitol accumulation and lens architectural changes in db/db mice. Our results suggest that LJE might be beneficial for the treatment of diabetes-induced lens opacification. The ability of LJE to suppress lenticular sorbitol accumulation may be mediated by the inhibition of AR activity.

KIOM-79 inhibits aldose reductase activity and cataractogenesis in Zucker diabetic fatty rats

Objectives

KIOM-79, a combination of four plant extracts, has a preventive effect on diabetic nephropathy and retinopathy in diabetic animal models. In this study, we have investigated the inhibitory effects of KIOM-79 on diabetic cataractogenesis.

Methods

We evaluated aldose reductase activity during cataractogenesis using Zucker diabetic fatty (ZDF) rat, an animal model of type 2 diabetes. ZDF rats were treated orally with KIOM-79 (50 mg/kg body weight) once a day for 13 weeks.

Key findings

In vehicle-treated ZDF rats, lens opacity was increased, and lens fibre swelling and membrane rupture were observed. In addition, aldose reductase activity and aldose reductase protein expression in diabetic lens were markedly enhanced. However, the administration of KIOM-79 inhibited the development of diabetic cataract through the inhibition of aldose reductase activity and protein expression in diabetic lenses.

Conclusions

These observations suggested that KIOM-79 was useful against the treatment of diabetic cataractogenesis.

Inhibitory effects of chlorogenic acid on aldose reductase activity in vitro and cataractogenesis in galactose-fed rats

Chlorogenic acid (5-O-caffeoylquinic acid, CA), a phenolic compound found ubiquitously in plants, has antidiabetic effect in diabetic animal models. In this study, we investigated the inhibitory effect of CA on diabetic cataractogenesis. We evaluated the aldose reductase (AR) activity during cataract development in 50% galactose-fed rats, an animal model of sugar cataract. Galactose-fed rats were treated orally with CA (10 and 50 mg/kg body weight) once a day for 2 weeks. In vehicle-treated galactose-fed rats, lens opacity was increased, and lens fiber swelling and membrane rupture were observed. In addition, AR protein was highly expressed in lens epithelial cells and lens cortical fibers of galactose-fed rats. However, CA inhibited the rat AR activity in vitro, and the administration of CA prevented the development of sugar cataract through the inhibition of AR activity. These observations suggest that CA is useful for the treatment of sugar cataract.

Effects of fidarestat, an aldose reductase inhibitor, on nerve conduction velocity and bladder function in streptozotocin-treated female rats

The effects of fidarestat, an aldose reductase inhibitor (ARI), were assessed on nerve conduction velocity (NCV) in somatic nerves and on multiple measures of bladder function in rats made hyperglycemic with streptozotocin (STZ) and in age-matched controls. Nerve conduction velocity was recorded at baseline and at 10, 20, 30, and 50 days after confirmation of the STZ-induced hyperglycemia in all rats (N=47); bladder function was assessed in a representative subset of rats (N=20) at Day 50. Caudal NCV was markedly slowed by STZ, and this effect was significantly reversed by fidarestat. The initial deficit and treatment-related improvement were especially evident for responses driven by high-frequency repetitive stimulation. Of the 11 parameters of bladder activity assessed, four measures-bladder capacity, micturition volume, micturition frequency, and bladder weight-were significantly different in the control and STZ-treated groups. These deficits were not affected by fidarestat. At Day 50, the induced deficits in bladder function were highly correlated with caudal NCV (r values ranging from 0.70 to 0.96; P values ranging from .02 to <.0001). These results suggested that fidarestat improved the slowing of somatic nerve NCV in hyperglycemic rats, but it was not effective in reversing associated bladder dysfunction, in spite of the highly significant correlation between these two diabetes-induced deficits. Possible explanations for this dissociation are discussed.

The role of polyol pathway in high glucose-induced endothelial cell damages

To clarify the mechanism by which hyperglycemia in diabetes mellitus causes endothelial cell damages, the effects of high glucose on DNA fragmentation and caspase-3 activity of cultured endothelial cells and on the generation of 8-hydroxy-2'-deoxyguanosine (8-OHdG) were studied. Furthermore, the involvement of the polyol pathway in this process was investigated using aldose reductase inhibitor (SNK-860). Human umbilical vein endothelial cells (HUVECs) were incubated with 5.5mmol/L (low glucose medium) or 28mmol/L (high glucose medium) of glucose. The amounts of fragmented DNA, caspase-3 activity and 8-OHdG in the medium increased in significantly greater extent in high glucose-incubated HUVECs than in low glucose-incubated HUVECs. No significant increase in fragmented DNA or 8-OHdG was observed when HUVECs were incubated with mannitol (500mg/mL). The concentration of intracellular sorbitol was significantly higher in HUVECs incubated in high glucose medium than that in low glucose medium. Addition of the aldose reductase inhibitor SNK-860 dose-dependently decreased the intracellular sorbitol concentration in HUVECs incubated in high glucose medium, and also significantly suppressed the increases in fragmented DNA, caspase-3 activity and 8-OHdG by conditioning with high glucose medium. These results suggest that high glucose-induced endothelial cell damages may be mediated by activation of the polyol pathway accompanied by augmented oxidative stress.

Neurotrophin-3-induced production of nerve growth factor is suppressed in Schwann cells exposed to high glucose: involvement of the polyol pathway

Development of hypesthesia, a loss of sensitivity to stimulation, is associated with impaired regeneration of peripheral sensory fibers, in which Schwann cells play a key role by secreting nerve growth factor (NGF). Recent clinical trials indicated that an inhibitor of aldose reductase (AR), the rate-limiting enzyme in the polyol pathway, significantly improved hypesthesia in diabetic patients. The fact that AR is localized in Schwann cells led us to investigate the role of the polyol pathway in NGF production of isolated Schwann cells. Among various endogenous factors examined, increased production of NGF was demonstrated in the cells treated with neurotrophin-3 (NT-3) for 24 h. NT-3-induced NGF production was significantly suppressed when cells were cultured in the medium containing high glucose. In these cells, the levels of glutathione (GSH) and cAMP-response element binding protein (CREB) were reduced, whereas the level of activated nuclear factor-kappaB (NF-kappaB) was elevated. These changes were abolished when an AR inhibitor fidarestat was included in the medium. NT-3-induced NGF production was further attenuated in the cells treated with an inhibitor of GSH synthesis. Together, the enhanced polyol pathway activity under high-glucose conditions seems to elicit reduced NT-3-induced NGF production in Schwann cells. Enhanced oxidative stress linked to the polyol pathway activity may mediate this process.

Polyol pathway and protein kinase C activity of rat Schwannoma cells

BACKGROUND

Polyol pathway hyperactivity-induced decreases in protein kinase C (PKC) activities have been proposed as a pathogenic mechanism of diabetic neuropathy. Increased PKC activities have recently been invoked in the pathogenesis of other diabetic complications, especially retinopathy, nephropathy, and macroangiopathy. However, it remains unclear whether PKC activities in neural cells such as Schwann cells are increased, decreased, or unchanged. This study investigated the effects of high glucose and increased polyol pathway activity on neural cell growth and PKC activities.

METHODS

Rat Schwannoma cells were cultured in 5.5 or 20 mM glucose in the presence or absence of an aldose reductase inhibitor, epalrestat (1 microM) for 14 days. Proliferation activities, PKC activities, and the protein expression of PKC isoforms were measured.

RESULTS

Proliferation and PKC activities under the 20 mM glucose condition were significantly decreased compared to those under the 5.5 mM glucose condition and were prevented by epalrestat. Among PKC isoforms, the protein expression of PKC-alpha under the 20 mM glucose condition was significantly reduced compared to that under the 5.5 mM glucose condition. Epalrestat significantly inhibited the decreased expression of PKC-alpha protein. There were no significant changes in the protein expression of PKC-beta.

CONCLUSIONS

These results suggest that PKC, especially PKC-alpha activity, is decreased in Schwann cells exposed to high glucose and that this deficit is mediated through polyol pathway hyperactivity.

Polyol pathway and diabetic peripheral neuropathy

This chapter critically examines the concept of the polyol pathway and how it relates to the pathogenesis of diabetic peripheral neuropathy. The two enzymes of the polyol pathway, aldose reductase and sorbitol dehydrogenase, are reviewed. The structure, biochemistry, physiological role, tissue distribution, and localization in peripheral nerve of each enzyme are summarized, along with current informaiton about the location and structure of their genes, their alleles, and the possible links of each enzyme and its alleles to diabetic neuropathy. Inhibitors of pathway enzyme and results obtained to date with pathway inhibitors in experimental models and human neuropathy trials are updated and discussed. Experimental and clinical data are analyzed in the context of a newly developed metabolic odel of the in vivo relationship between nerve sorbitol concentration and metabolic flux through aldose reuctase. Overall, the data will be interpreted as supporting the hypothesis that metabolic flux through the polyol pathway, rather than nerve concentration of sorbitol, is the predominant polyol pathway-linked pathogeneic factor in diabetic preipheral nerve. Finally, key questions and future directions for bsic and clinical research in this area are considered. It is concluded that robust inhibition of metabolic flux through the polyol pathway in peripheral nerve will likely result in substantial clinical benefit in treating and preventing the currently intractable condition of diabetic peripheral neuropathy. To accomplish this, it is imperative to develop and test a new generation of "super-potent" polyol pathway inhibitors.

Transition metals and polyol pathway in the development of diabetic neuropathy in rats

BACKGROUND

The transition metal-catalyzed reaction is a major source of oxygen free radicals, which play an important role in vascular dysfunction leading to ischemia in diabetic tissues. The inhibition of polyol pathway hyperactivity has been reported to ameliorate neurovascular abnormalities in diabetic rats and has been proposed to improve the oxygen free radical scavenging capacity. The present study was conducted to compare the effect of a transition metal chelating agent, trientine (TRI), on diabetic neuropathy with that of an aldose reductase inhibitor, NZ-314 (NZ).

METHODS

Diabetic rats were divided into three groups: (1). untreated, (2). TRI-treated, and (3). NZ-treated. TRI (20 mg/kg) or NZ (100 mg/kg) was administered by gavage or chow containing NZ, respectively, for 8 weeks. Motor nerve conduction velocity (MNCV), coefficient of variation of the R - R interval on electrocardiogram (CVr-r), sciatic nerve blood flow (SNBF), platelet aggregation activities, and serum concentrations of malondialdehyde were measured.

RESULTS

Untreated diabetic rats showed delayed MNCV, decreased CV(R-R), and reduced SNBF compared to normal rats. TRI or NZ completely prevented these deficits. Platelet hyperaggregation activities in diabetic rats were prevented by NZ, but not by TRI. Increased concentrations of malondialdehyde in diabetic rats were partially but significantly ameliorated by either TRI or NZ.

CONCLUSIONS

These observations suggest that increased free radical formation through the transition metal-catalyzed reaction plays an important role in the development of diabetic neuropathy and that the preventive effect of an aldose reductase inhibitor on diabetic neuropathy may also be mediated by decreasing oxygen free radicals.

An aldose reductase inhibitor reverses early diabetes-induced changes in peripheral nerve function, metabolism, and antioxidative defense

Aldose reductase inhibitors (ARIs) prevent peripheral nerve dysfunction and morphological abnormalities in diabetic animal models. However, some experimental intervention studies and clinical trials of ARIs on diabetic neuropathy appeared disappointing because of either 1) their inadequate design and, in particular, insufficient correction of the sorbitol pathway activity or 2) the inability to reverse established functional and metabolic deficits of diabetic neuropathy by AR inhibition in general. We evaluated whether diabetes-induced changes in nerve function, metabolism, and antioxidative defense are corrected by the dose of ARI (sorbinil, 65 mg/kg/d in the diet), resulting in complete inhibition of increased sorbitol pathway activity. The groups included control rats and streptozotocin-diabetic rats treated with/without ARI for 2 weeks after 4 weeks of untreated diabetes. ARI treatment corrected diabetes-induced nerve functional changes; that is, decrease in endoneurial nutritive blood flow, motor and sensory nerve conduction velocities, and metabolic abnormalities (i.e., mitochondrial and cytosolic NAD+/NADH redox imbalances and energy deficiency). ARI restored nerve concentrations of two major non-enzymatic antioxidants, reduced glutathione (GSH) and ascorbate, and completely arrested diabetes-induced lipid peroxidation. In conclusion, treatment with adequate doses of ARIs (that is, doses that completely inhibit increased sorbitol pathway activity) is an effective approach for reversal of, at least, early diabetic neuropathy.

Clinical efficacy of fidarestat, a novel aldose reductase inhibitor, for diabetic peripheral neuropathy: a 52-week multicenter placebo-controlled double-blind parallel group study

OBJECTIVE

The purpose of this study was to evaluate the efficacy of fidarestat, a novel aldose reductase (AR) inhibitor, in a double-blind placebo controlled study in patients with type 1 and type 2 diabetes and associated peripheral neuropathy.

RESEARCH DESIGN AND METHODS

A total of 279 patients with diabetic neuropathy were treated with placebo or fidarestat at a daily dose of 1 mg for 52 weeks. The efficacy evaluation was based on change in electrophysiological measurements of median and tibial motor nerve conduction velocity, F-wave minimum latency, F-wave conduction velocity (FCV), and median sensory nerve conduction velocity (forearm and distal), as well as an assessment of subjective symptoms.

RESULTS

Over the course of the study, five of the eight electrophysiological measures assessed showed significant improvement from baseline in the fidarestat-treated group, whereas no measure showed significant deterioration. In contrast, in the placebo group, no electrophysiological measure was improved, and one measure significantly deteriorated (i.e., median nerve FCV). At the study conclusion, the fidarestat-treated group was significantly improved compared with the placebo group in two electrophysiological measures (i.e., median nerve FCV and minimal latency). Subjective symptoms (including numbness, spontaneous pain, sensation of rigidity, paresthesia in the sole upon walking, heaviness in the foot, and hypesthesia) benefited from fidarestat treatment, and all were significantly improved in the treated versus placebo group at the study conclusion. At the dose used, fidarestat was well tolerated, with an adverse event profile that did not significantly differ from that seen in the placebo group.

CONCLUSIONS

The effects of fidarestat-treatment on nerve conduction and the subjective symptoms of diabetic neuropathy provide evidence that this treatment alters the progression of diabetic neuropathy.

Physiological and morphometric analyses of neuropathy in sucrose-fed OLETF rats

To investigate the characteristic features of diabetic neuropathy in type 2 diabetes mellitus, Otsuka Long-Evans Tokushima fatty (OLETF) rats, an animal model of human type 2 diabetes mellitus, and non-diabetic Long-Evans Tokushima Otsuka (LETO) rats were fed with or without sucrose and/or an aldose reductase inhibitor, [5-(3-thienyl) tetrazol-1-yl] acetic acid (TAT), for 24 weeks, and physiological, biochemical and morphological assessments were performed. Sucrose administration caused remarkable hyperglycemia in OLETF rats but not in LETO rats. Sucrose-fed OLETF rats demonstrated delayed nerve conduction velocity, decreased coefficient of variation of R-R interval, reduced sciatic nerve blood flow, increased platelet aggregation activity, a lower concentration of erythrocyte 2,3-diphosphoglycerate, and decreased Na+/K+-ATPase activity in sciatic nerves, compared with the non-sucrose-fed OLETF and LETO rats. TAT prevented all these deficits except hyperglycemia. Sorbitol and fructose accumulation and myo-inositol depletion in tail nerves of sucrose-fed OLETF rats were ameliorated by TAT. Myelinated fiber size and density in sural nerves of sucrose-fed OLETF rats were decreased and increased, respectively, compared with non-sucrose-fed OLETF and LETO rats. These morphological abnormalities were normalized by TAT. These observations suggest that the sucrose-fed OLETF rat developed diabetic neuropathy not only electrophysiologically but also histologically, and that an aldose reductase inhibitor, TAT, possesses therapeutic value for the treatment of diabetic neuropathy.

Effects of 15-month aldose reductase inhibition with fidarestat on the experimental diabetic neuropathy in rats

We examined the effects of long-term treatment with an aldose reductase inhibitor (ARI) fidarestat on functional, morphological and metabolic changes in the peripheral nerve of 15-month diabetic rats induced by streptozotocin (STZ). Slowed F-wave, motor nerve and sensory nerve conduction velocities were corrected dose-dependently in fidarestat-treated diabetic rats. Morphometric analysis of myelinated fibers demonstrated that frequencies of abnormal fibers such as paranodal demyelination and axonal degeneration were reduced to the extent of normal levels by fidarestat-treatment. Axonal atrophy, distorted axon circularity and reduction of myelin sheath thickness were also inhibited. These effects were associated with normalization of increased levels of sorbitol and fructose and decreased level of myo-inositol in the peripheral nerve by fidarestat. Thus, the results demonstrated that long-term treatment with fidarestat substantially inhibited the functional and structural progression of diabetic neuropathy with inhibition of increased polyol pathway flux in diabetic rats.

An enzymatic assay for myo-inositol in tissue samples

An enzymatic assay for myo-inositol (MI) was modified. The method is based on the oxidation of MI by NAD(+)-dependent MI dehydrogenase, coupled to reoxidation of NADH by iodonitrotetrazolium chloride and diaphorase. The resultant formazan is measured spectrophotometrically. In order to remove interference by glucose, preliminary phosphorylation of glucose by hexokinase was employed before the above reaction. The assay is quantitative for MI in amounts ranging from 1 to 20 nmol. This method gives a negligible blank, even in the measurement of rat serum. The reduced MI content in the sciatic nerve and lens of streptozotocin-induced diabetic rats recovered in a dose-dependent manner by treatment with a novel potent aldose reductase inhibitor, GP-1447 ¿3-[(4,5, 7-trifluorobenzothiazol-2-yl)methyl]-5-methylphenylacetic acid¿.

An aldose reductase inhibitor prevents the glucose-induced increase in PDGF-beta receptor in cultured rat aortic smooth muscle cells

To examine the role of platelet-derived growth factor (PDGF) and the polyol pathway in the growth activity of smooth muscle cells (SMCs), [(3)H]-thymidine incorporation, [(125)I]-PDGF-BB binding and expression of PDGF-beta receptor protein were measured in rat aortic SMCs cultured with 5.5 or 20 mM glucose with or without anti-PDGF antibody or an aldose reductase inhibitor, epalrestat. SMCs cultured with 20 mM glucose demonstrated an accelerated thymidine incorporation compared with SMCs cultured with 5.5 mM glucose, which was prevented by anti-PDGF antibody. This acceleration of growth activity by 20 mM glucose was accompanied by an increase in PDGF-BB binding, which was due to the increased number of PDGF-beta receptors and the overexpression of PDGF-beta receptor protein. Epalrestat prevented all these abnormalities. These observations suggest that polyol pathway hyperactivity plays an important role in the proliferation of SMCs which may be mediated through the accelerated expression of PDGF-beta receptor protein.

Continuous inhibition of excessive polyol pathway flux in peripheral nerves by aldose reductase inhibitor fidarestat leads to improvement of diabetic neuropathy

We investigated the effects of three aldose reductase (AR) inhibitors, fidarestat, epalrestat and zenarestat, on the slowing of sensory nerve conduction velocity (SNCV), motor nerve conduction velocity (MNCV), and minimal F-wave latency prolongation in streptozotocin (STZ)-induced diabetic rats. Two weeks after STZ injection, SNCV and MNCV in the diabetic rats were significantly slower than in normal rats. Fidarestat (0.25-2 mg/kg/day), epalrestat (48 to 96 mg/kg/day) or zenarestat (10-40 mg/kg/day) was administered orally for the following 2 weeks, and SNCV, MNCV and F-wave latency were measured 3 h after final administration. Significant prolongation of minimal F-wave latency, as well as slowing of SNCV and MNCV, was found in the untreated diabetic rats 4 weeks after STZ injection. At a dose of 0.5 mg/kg/day or more fidarestat showed significant effects on these nervous dysfunctions, effects that were more potent than those shown by the other inhibitors. Furthermore, following the 2-week administration of fidarestat (1 mg/kg/day), epalrestat (48 mg/kg/day) or zenarestat (20 mg/kg/day), which began 2 weeks after STZ injection, sorbitol content in the sciatic nerve, produced by AR, a rate-limiting enzyme in the polyol pathway, was determined at 3, 8, 12, and 24 h after final administration. At each point in time, sorbitol content in the untreated diabetic rats was much higher than that in the normal control rats. Fidarestat suppressed sorbitol accumulation remarkably and continuously until 24 h after administration. On the other hand, the inhibitory effect by zenarestat declined in a time-dependent manner, and epalrestat did not decrease sorbitol content. Therefore, these results suggest that continuous inhibition of increased polyol pathway flux can improve diabetic neuropathy more potently.

Novel, highly potent aldose reductase inhibitors: (R)-(-)-2-(4-bromo-2-fluorobenzyl)-1,2,3,4- tetrahydropyrrolo[1,2-a]pyrazine -4-spiro-3'-pyrrolidine-1,2',3,5'-tetrone (AS-3201) and its congeners

A series of novel tetrahydropyrrolo[1,2-a]pyrazine derivatives were synthesized and evaluated as aldose reductase inhibitors (ARIs) on the basis of their abilities to inhibit porcine lens aldose reductase (AR) in vitro and to inhibit sorbitol accumulation in the sciatic nerve of streptozotocin-induced diabetic rats in vivo. Of these compounds, spirosuccinimide-fused tetrahydropyrrolo[1, 2-a]pyrazine-1,3-dione derivatives showed significantly potent AR inhibitory activity. In the in vivo activity of these derivatives, 2-(4-bromo-2-fluorobenzyl)-1,2,3,4-tetrahydropyrrolo[1, 2-a]pyrazine-4-spiro-3'-pyrrolidine-1,2',3,5'-tetrone (23t) (SX-3030) showed the best oral activity. The enantiomers of 23t were synthesized, and the biological activities were evaluated. It was found that AR inhibitory activity resides in the (-)-enantiomer 43 (AS-3201), which was 10 times more potent in inhibition of the AR (IC50 = 1.5 x 10(-8) M) and 500 times more potent in the in vivo activity (ED50 = 0.18 mg/kg/day for 5 days) than the corresponding (+)-enantiomer 44 (SX-3202). From these results, AS-3201 was selected as the candidate for clinical development. The absolute configuration of AS-3201 was also established to be (R)-form by single-crystal X-ray analysis. In this article we report the preparation and structure-activity relationship (SAR) of tetrahydropyrrolopyrazine derivatives including a novel ARI, AS-3201.

Effect of SG-210, a novel aldose reductase inhibitor, on impaired polyol pathway in rats received diabetic manipulations

To investigate the effect of SG-210, a potent inhibitor selective to aldose reductase (ARI), on the impaired polyol pathway, we examined biochemically and histologically the potencies of this compound in streptozotocin-induced diabetic or galactosemic rats. The study with diabetic rats showed that SG-210 (1-10 mg x kg(-1)) dose-dependently inhibited sorbitol accumulations in erythrocytes, sciatic nerves, lens, and retina with ED50 values of 1.4, 1.3, 3.5, and 4.6 mg x kg(-1), respectively. Zenarestat, currently under clinical trials both in Japan and the United States, was about two or over five times less potent than SG-210 in suppressing sorbitol contents of erythrocytes or other tissues, respectively. Epalrestat, commercially available, was much less potent in reducing the contents with ED50 values of more than 30 mg x kg(-1) in all of the cells and the tissues examined. An extensive study using galactosemic rats indicated that SG-210 (3-30 mg x kg(-1)) inhibited galactitol accumulations in lens and retina as well as in erythrocytes, preventing the progression of histological abnormalities in lens accompanied by the reduction in galactitol contents. Epalrestat (3-30 mg x kg(-1)) failed to show any significant effects. Pharmacokinetic studies suggested that SG-210 has a high bioavailability and possesses a long half-life in rats (ca. 10 h). Taken together with its excellent pharmacokinetic profiles, the potent suppressive effects of SG-210 observed in this study may be available as a new treatment of diabetic complications.

Amelioration of neurovascular deficits in diabetic rats by a novel aldose reductase inhibitor, GP-1447: minor contribution of nitric oxide

The effects of a novel potent aldose reductase inhibitor, GP-1447 [3-[(4,5,7-trifluorobenzothiazol-2-yl)methyl]-5-methylphenylace tic acid] on the sciatic nerve blood flow in streptozotocin-induced diabetic rats were examined. Blood flow was analyzed in terms of mass, i.e. the volume of blood in tissue, and of velocity, i.e. the velocity of the blood flow. In diabetic rats, a 63% decrease in blood flow due to a decrease in velocity was observed. The blood mass in the same animals fluctuated, thereby increasing its range of values. Treatment with GP-1447 at a dose of 30 mg/kg per day for 4 consecutive weeks following a 3-week period without treatment ameliorated the reduced blood flow by 51%, and was accompanied by a recovery of velocity. The increase in the range of blood mass values was reversed by treatment with GP-1447. The restoration of the range of blood mass values, but not that of the blood flow, by GP-1447 was blocked by treatment with the nitric oxide (NO) synthase inhibitor, NG-nitro-L-arginine. Motor nerve conduction velocity (MCV) changes in parallel with blood flow values, while it is inversely proportionate to alterations in the range of blood mass values. It is suggested that the observed beneficial effect of GP-1447 on blood flow is involved in the restoration of decreased MCV in diabetes. It would appear that GP-1447-induced amelioration of neurovascular defects is not mediated solely by the improvement of the NO system.

Serial changes of sensory nerve conduction velocity and minimal F-wave latency in streptozotocin-induced diabetic rats

We studied the serial changes of sensory nerve conduction velocity (SNCV) in the caudal nerve of streptozotocin (STZ)-induced diabetic rats using a new technical method. Minimal F-wave latency was also studied by stimulating the tibial nerve. The SNCV in the diabetic rats was slower than that in the normal rats 2 weeks after STZ injection, and minimal F-wave latency was prolonged compared to normal rats 4 weeks after STZ injection. Treatment of the diabetic rats with insulin for 14 days inhibited SNCV slowing and minimal F-wave latency prolongation. This new method to measure SNCV is useful for various studies, and improvement of diabetic neuropathy with insulin treatment is indicated by recovery from SNCV slowing and minimal F-wave latency prolongation.

Effect of an aldose reductase inhibitor on abnormalities of electroretinogram and vascular factors in diabetic rats

The effect of an aldose reductase inhibitor, [5-(3-thienyl) tetrazol-1-yl] acetic acid (TAT), on the electroretinogram was determined in rats with streptozotocin-induced diabetes. Laboratory chow containing 0.05% TAT was given to rats for 2 months, while other diabetic rats were untreated. Groups of TAT-treated and untreated normal rats were also studied. Treatment with TAT produced significant improvement of the electroretinogram. TAT shortened the peak latencies of the b-wave oscillatory potentials, which were significantly prolonged in untreated diabetic rats (P < 0.0001 vs. untreated normal rats). This was accompanied by a significant decrease in the retinal sorbitol and fructose concentrations (by 46.5% and 25.7%, respectively). TAT treatment of diabetic rats also markedly reduced ADP-induced platelet aggregation and significantly increased the red blood cell 2,3-diphosphoglycerate level, accompanied by a marked reduction in sorbitol and fructose concentrations of platelet and red blood cells. There were significant correlations between the summed b-wave peak latencies and platelet aggregation or the 2,3-diphosphoglycerate level in diabetic rats. These findings suggest that an aldose reductase inhibitor, TAT, has therapeutic value for diabetic retinopathy.

Diabetic neuropathy in sucrose-fed Otsuka Long-Evans Tokushima fatty rats: effect of an aldose reductase inhibitor, TAT

In an animal model of human non-insulin dependent diabetes mellitus (NIDDM), Otsuka Long-Evans Tokushima Fatty (OLETF) rats were fed with sucrose for 8 weeks to obtain severe hyperglycemia. The effects of sucrose administration on peripheral nerve functions, motor nerve conduction velocity (MNCV) and coefficient of variance of R-R interval (CVR-R), were investigated with concomitant measuring of sciatic nerve blood flow (SNBF), ADP-induced platelet aggregation and polyol content in the sciatic nerves. The effects of an aldose reductase inhibitor, TAT, on these parameters were also studied. Administration of sucrose to OLETF rats caused significant body weight reduction and remarkable hyperglycemia. Sucrose-fed OLETF rats demonstrated significantly delayed MNCV, decreased CVR-R, reduced SNBF and increased platelet aggregation activity to ADP. Sorbitol and fructose accumulation, and myo-inositol depletion in sciatic nerves were observed only in sucrose-fed OLETF rats. These abnormalities were all ameliorated by the treatment with TAT. These observations suggest that the sucrose-fed OLETF rat is a useful animal model for studying the pathogenesis of diabetic neuropathy in human NIDDM, and that an aldose reductase inhibitor is a useful therapeutic agent for the treatment of diabetic neuropathy.

Effects of a Novel Potent Aldose Reductase Inhibitor, GP-1447, on Aldose Reductase Activity In Vitro and on Diabetic Neuropathy and Cataract Formation in Rats

The influence of diabetes on the effects of morphine on the responses of ventrobasal (VB) thalamic neurons to mechanical noxious stimuli were studied in chloral hydrate-anesthetized rats. Animals were rendered diabetic by an injection of streptozotocin (60 mg/kg, i.v.). Morphine (0.3 mg/kg), administered i.v., produced a reduction in the responsiveness of VB thalamic neurons to noxious stimulation in control rats. This effect was reversed by naloxone. In contrast, the inhibitory effects of morphine on the nociceptive responses of VB thalamic neurons were significantly attenuated in diabetic rats, as compared with the controls. However, there were no significant differences in inhibitory potency between diabetic and control rats when morphine (30 nM) was administered intrathecally. It seems likely that these changes in the sensitivity of VB thalamic neurons to morphine are, to some extent, the source of the reduction in the analgesic efficacy of morphine in diabetic rats.

Nerve function and blood flow in Otsuka Long-Evans Tokushima Fatty rats with sucrose feeding: effect of an anticoagulant

To investigate the pathogenesis of diabetic neuropathy in non-insulin-dependent diabetes mellitus, Otsuka Long-Evans Tokushima Fatty rats, an animal model of non-insulin-dependent diabetes mellitus, and non-diabetic Long-Evans Tokushima Otsuka rats were fed with or without sucrose and/or cilostazol, an anticoagulant, for 8 weeks. Sucrose-fed diabetic rats showed a delayed motor nerve conduction velocity, decreased R-R interval variability of electrocardiogram, reduced sciatic nerve blood flow, increased platelet aggregability and a decreased erythrocyte 2,3-diphosphoglycerate concentration compared with non-sucrose-fed diabetic rats and non-diabetic rats. These abnormalities were significantly prevented by treatment with cilostazol without changes in the nerve tissue levels of polyols. These findings indicate that sucrose-fed Otsuka Long-Evans Tokushima Fatty rats may be a useful animal model of neuropathy in non-insulin-dependent diabetes mellitus, and that cilostazol may prevent the development of diabetic neuropathy by modifying vascular factors.

Reduced Na+/K+ adenosine triphosphatase activity and motor nerve conduction velocity in L-fucose-fed rats is reversible after dietary normalization

Development of early defects in diabetic neuropathy has been linked to metabolic abnormalities and is considered reversible. To further address some of the questions concerning the contribution by metabolic derangements to the development of neural defects and reversibility, we have developed an animal model, by feeding rats a diet containing 20% L-fucose, that develops neural defects similar to those that occur in streptozotocin-induced diabetic rats. After 6 weeks on a 20% L-fucose diet, myo-inositol content and Na+/K+ adenosine triphosphatase (ATPase) activity of the sciatic nerve were significantly reduced, as was the motor nerve conduction velocity (MNCV). L-Fucose is a monosaccharide that occurs in low concentrations in normal serum but is increased in diabetic patients. In cultured cells, L-fucose, at concentrations that occur in diabetic circulation, is a competitive inhibitor of myo-inositol uptake. The purpose of the present study was to compare the sequential pattern of the reversibility of the slowing of MNCV with ouabain-inhibited sciatic nerve Na+/K+ ATPase activity and myo-inositol content in rats fed a diet containing 20% L-fucose for a period of 6 weeks followed by a normal diet lasting up to 2 weeks. Unbound L-fucose levels in the serum returned to normal in less than 24 hours of the rats being placed on the normal diet. Normalization of slowed MNCV after removing L-fucose-fed rats from the L-fucose diet followed a pattern of recovery similar to the recovery of sciatic nerve ouabain-inhibited Na+/K+ ATPase activity, with complete recovery occurring within 7 days of the rats being placed on the normal diet. In contrast, myo-inositol content of the sciatic nerve remained decreased following 3 days on the normal diet, and required 14 days for complete normalization. Results from these studies suggest that a causal relationship may exist for reduced Na+/K+ ATPase activity and MNCV in L-fucose-fed rats, and that a measurable decrease in myo-inositol content may not be necessary for the development of these defects in the sciatic nerve.

Polyol pathway, 2,3-diphosphoglycerate in erythrocytes and diabetic neuropathy in rats

The relationship between the 2,3-diphosphoglycerate concentration in red blood cells as a biological indicator of tissue hypoxia and diabetic neuropathy, and the effect of a potent aldose reductase inhibitor, (2S,4S)-6-fluoro-2'5'-dioxospiro [chroman-4,4'-imidazolidine]-2-carboxamide (SNK-860), on both were investigated in streptozotocin-induced diabetic rats. Diabetic rats demonstrated significantly delayed motor nerve conduction velocity and reduced sciatic nerve blood flow. Altered biochemical features in the sciatic nerves, including a marked accumulation of sorbitol and fructose, myo-inositol depletion and decreased Na+/K(+)-ATPase activity were also detected in diabetic rats. These defects were accompanied by a decrease in the red blood cell 2,3-diphosphoglycerate concentration. Treatment with SNK-860 partially or completely ameliorated these abnormalities. These observations suggest that a decrease in the red blood cell 2,3-diphosphoglycerate concentration is one of the factors contributing to tissue hypoxia, which results in diabetic neuropathy, and that this decrease is mediated through an aldose reductase inhibitor-sensitive pathway.

Alterations in the properties and isoform ratios of brain Na+/K(+)-ATPase in streptozotocin diabetic rats

In this study we analysed the changes in the properties of rat cerebral cortex Na+K(+)-ATPase in streptozotocin induced diabetes (STZ-diabetes). Special attempt was made to determine whether insulin treatment of diabetic animals could restore the altered parameters of this enzyme. Na+/K(+)-ATPase activity was found to be decreased by 15% after 2 weeks, and by 37% after 4 weeks in diabetic rat brains with a parallel decrease in maximal capacity of low affinity ouabain binding sites. There was no significant change in the high affinity ouabain binding sites. The Kd values did not change significantly. Western blot analysis of brain Na+/K(+)-ATPase isoforms indicated a 61 +/- 5.8% and 20 +/- 2.8% decrease of the alpha 1 and alpha 3 isoforms, respectively in 4 weeks diabetic animals. Change in the amount of the alpha 2 isoform proved to be less characteristic. Both types of beta subunit isoform showed a significant decrease in four weeks diabetic rats. Our data indicate a good correlation in diabetic rats between changes in Na-/K(+)-ATPase activity, low affinity ouabain binding capacity and the level of alpha 1 isoform. While insulin treatment of diabetic animals restored the blood glucose level to normal, a complete reversal of diabetes induced changes in Na+/K(+)-ATPase activity, ouabain binding capacity and Na+/K(+)-ATPase isoform composition could not be achieved.

Effect of long-term treatment with a new aldose reductase inhibitor, (2S,4S)-6-fluoro-2',5'-dioxospiro-[chroman-4,4'-imidazolidine]-2-carbox amide (SNK-860), on peripheral neuropathy in streptozotocin-induced diabetic rats

We studied the long-term effects of a new aldose reductase inhibitor, (2S,4S)-6-fluoro-2',5'-dioxospiro-[chroman-4,4'-imidazolidine]-2- carboxamide (SNK-860), on functional, biochemical, and structural changes in peripheral nerve of streptozotocin (STZ)-induced diabetic rats. During the experimental period of 26 weeks, the delayed motor-nerve conduction in diabetic rats was significantly prevented by SNK-860 treatment, and elevated sorbitol levels and reduced myo-inositol levels were normalized to 100% and 71% of control levels, respectively. Teased nerve fiber studies demonstrated that the frequency of abnormal fibers was significantly reduced in treated diabetic rats. Morphometric analysis of myelinated fibers also disclosed prevention of axonal atrophy, distorted axonal circularity and preservation of large-sized fibers following SNK-860 treatment. These results suggest that long-term treatment with SNK-860 has a beneficial preventive effect on the development of experimental diabetic neuropathy.