Ranirestat (AS-3201), a Potent Aldose Reductase Inhibitor, Reduces Sorbitol Levels and Improves Motor Nerve Conduction Velocity in Streptozotocin-Diabetic Rats

Exploring aldose reductase inhibitors as promising therapeutic targets for diabetes-linked disabilities

Diabetes mellitus significantly increases mortality and morbidity rates due to complications like neuropathy and nephropathy. It also leads to retinopathy and cataract formation, which is a leading cause of vision disability. The polyol pathway emerges as a promising therapeutic target among the various pathways associated with diabetic complications. This review focuses on the development of natural and synthetic aldose reductase inhibitors (ARIs), along with recent discoveries in diabetic complication treatment. AR, pivotal in the polyol pathway converting glucose to sorbitol, plays a key role in secondary diabetes complications' pathophysiology. Understanding AR's function and structure lays the groundwork for improving ARIs to mitigate diabetic complications. New developments in ARIs open up exciting possibilities for treating diabetes-related complications. However, it is still challenging to get preclinical successes to clinical effectiveness because of things like differences in how the disease starts, drug specificity, and the complexity of the AR's structure. Addressing these challenges is crucial for developing targeted and efficient ARIs. Continued research into AR's structural features and specific ARIs is essential. Overcoming these challenges could revolutionize diabetic complication treatment, enhance patient outcomes, and reduce the global burden of diabetes-related mortality and morbidity.

Strategies in synthetic design and structure-activity relationship studies of novel heterocyclic scaffolds as aldose reductase-2 inhibitors

Heterocyclic scaffolds of natural as well as synthetic origin provide almost all categories of drugs exhibiting a wide range of pharmacological activities, such as antibiotics, antidiabetic and anticancer agents, and so on. Under normal homeostasis, aldose reductase 2 (ALR2) regulates vital metabolic functions; however, in pathological conditions like diabetes, ALR2 is unable to function and leads to secondary diabetic complications. ALR2 inhibitors are a novel target for the treatment of retinopathy (cataract) influenced by diabetes. Epalrestat (stat), an ALR2 inhibitor, is the only drug candidate that was approved in the last four decades; the other drugs from the stat class were retracted after clinical trial studies due to untoward iatrogenic effects. The present study summarizes the recent development (2014 and onwards) of this pharmacologically active ALR2 heterocyclic scaffold and illustrates the rationale behind the design, structure-activity relationships, and biological studies performed on these molecules. The aim of the current review is to pave a straight path for medicinal chemists and chemical biologists, and, in general, to the drug discovery scientists to facilitate the synthesis and development of novel ALR2 inhibitors that may serve as lead molecules for the treatment of diseases related to the ALR2 enzyme.

Treatment for Diabetic Peripheral Neuropathy: What have we Learned from Animal Models?

INTRODUCTION

Animal models have been widely used to investigate the etiology and potential treatments for diabetic peripheral neuropathy. What we have learned from these studies and the extent to which this information has been adapted for the human condition will be the subject of this review article.

METHODS

A comprehensive search of the PubMed database was performed, and relevant articles on the topic were included in this review.

RESULTS

Extensive study of diabetic animal models has shown that the etiology of diabetic peripheral neuropathy is complex, with multiple mechanisms affecting neurons, Schwann cells, and the microvasculature, which contribute to the phenotypic nature of this most common complication of diabetes. Moreover, animal studies have demonstrated that the mechanisms related to peripheral neuropathy occurring in type 1 and type 2 diabetes are likely different, with hyperglycemia being the primary factor for neuropathology in type 1 diabetes, which contributes to a lesser extent in type 2 diabetes, whereas insulin resistance, hyperlipidemia, and other factors may have a greater role. Two of the earliest mechanisms described from animal studies as a cause for diabetic peripheral neuropathy were the activation of the aldose reductase pathway and increased non-enzymatic glycation. However, continuing research has identified numerous other potential factors that may contribute to diabetic peripheral neuropathy, including oxidative and inflammatory stress, dysregulation of protein kinase C and hexosamine pathways, and decreased neurotrophic support. In addition, recent studies have demonstrated that peripheral neuropathy-like symptoms are present in animal models, representing pre-diabetes in the absence of hyperglycemia.

CONCLUSION

This complexity complicates the successful treatment of diabetic peripheral neuropathy, and results in the poor outcome of translating successful treatments from animal studies to human clinical trials.

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.

Aldose Reductase: a cause and a potential target for the treatment of diabetic complications

Diabetes mellitus, a disorder of metabolism, results in the elevation of glucose level in the blood. In this hyperglycaemic condition, aldose reductase overexpresses and leads to further complications of diabetes through the polyol pathway. Glucose metabolism-related disorders are the accumulation of sorbitol, overproduction of NADH and fructose, reduction in NAD⁺, and excessive NADPH usage, leading to diabetic pathogenesis and its complications such as retinopathy, neuropathy, and nephropathy. Accumulation of sorbitol results in the alteration of osmotic pressure and leads to osmotic stress. The overproduction of NADH causes an increase in reactive oxygen species production which leads to oxidative stress. The overproduction of fructose causes cell death and non-alcoholic fatty liver disease. Apart from these disorders, many other complications have also been discussed in the literature. Therefore, the article overviews the aldose reductase as the causative agent and a potential target for the treatment of diabetic complications. So, aldose reductase inhibitors have gained much importance worldwide right now. Several inhibitors, like derivatives of carboxylic acid, spirohydantoin, phenolic derivatives, etc. could prevent diabetic complications are discussed in this article.

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.

Pharmacokinetics and Safety of Ranirestat in Patients With Hepatic Impairment

Ranirestat is an aldose reductase inhibitor hypothesized to improve diabetic neuropathy. An open-label, single-dose, parallel-group study was conducted to compare pharmacokinetic (PK) characteristics of an oral dose of ranirestat across subjects with normal hepatic function and patients with mild and moderate hepatic impairment because ranirestat is expected to be used by patients with diabetes mellitus, possibly including those with hepatic impairment. To evaluate the necessity for dose adjustment, PK profiles and tolerability were studied at the dose of 40 mg, the expected optimal clinical dose in patients with diabetic neuropathy and normal hepatic function. In total, 20 subjects, including 5, 10, and 5 subjects with normal hepatic function, mild hepatic impairment, and moderate hepatic impairment, respectively, completed the study. Serial PK sampling was conducted up to 504 hours, and PK parameters were calculated and compared between healthy subjects and patients with mild or moderate hepatic impairment. The geometric mean ratios of peak concentration and area under the concentration-time curve in patients with mild hepatic impairment (90%CI) were 86.7% (55.3% to 135.9%) and 84.7% (68.5% to 104.8%), respectively. The values in patients with moderate hepatic impairment were 81.3% (48.8% to 135.5%) and 91.7% (72.1% to 116.7%), respectively. These results demonstrated that plasma ranirestat exposure and the plasma protein binding of the drug were not substantially altered by normal, mild, or moderate hepatic impairment (protein binding 99.22%, 99.29%, and 99.00%, respectively). All adverse events were mild in severity. Based on these findings, no dose adjustment will be required for ranirestat in patients with mild or moderate hepatic impairment.

Regulation on SIRT1-PGC-1α/Nrf2 pathway together with selective inhibition of aldose reductase makes compound hr5F a potential agent for the treatment of diabetic complications

(R,E)-N-(3-(2-acetamido-3-(benzyloxy) propanamido)propyl)-2-cyano-3-(4-hydroxy phenyl)acrylamide (hr5F) was design-synthesized based on bioactivity focus strategy as a potential agent to treat diabetic complicates. With in vitro enzyme assay, it is confirmed that hr5F is an effective ALR2 inhibitor with IC₅₀ value of 2.60 ± 0.15 nM, and selectivity index of 86.0 over ALR1, which is a little bit better than the reference Epalrestat (Epa). hr5F inhibits the increase of ALR2 enzyme activity and expression in human lens epithelial cells (HLECs) induced by high glucose. By applying western blot, it was found that hr5F alleviates the high glucose-induced superoxide overproduction insults by regulating SIRT1-PGC-1α/Nrf2 pathway, together with regulating NRF-1, mtTFA, Bax/Bcl-2 to ameliorate cell apoptosis. The in vivo effects of hr5F on short term streptozocin (STZ)-induced diabetic mice confirm the same functions disclosed in vitro. All the evidences support that hr5F may serve as a promising agent in the treatment of diabetic complications with close efficacy and broader indication than the reference Epa.

Novel quinazolinone-based 2,4-thiazolidinedione-3-acetic acid derivatives as potent aldose reductase inhibitors

Aim: Targeting aldose reductase enzyme with 2,4-thiazolidinedione-3-acetic acid derivatives having a bulky hydrophobic 3-arylquinazolinone residue. Materials & methods: All the target compounds were structurally characterized by different spectroscopic methods and microanalysis, their aldose reductase inhibitory activities were evaluated, and binding modes were studied by molecular modeling. Results: All the synthesized compounds proved to inhibit the target enzyme potently, exhibiting IC50 values in the nanomolar/low nanomolar range. Compound 5i (IC50 = 2.56 nM), the most active of the whole series, turned out to be almost 70-fold more active than the only marketed aldose reductase inhibitor epalrestat. Conclusion: This work represents a promising matrix for developing new potential therapeutic candidates for prevention of diabetic complications through targeting aldose reductase enzyme.

[Formula: see text]

Isolation, modification, and aldose reductase inhibitory activity of rosmarinic acid derivatives from the roots of Salvia grandifolia

To find aldose reductase inhibitors, two previously unreported compounds, grandifolias H and I, and five known compounds, including rosmarinic acid and rosmarinic acid derivatives, were isolated from the roots of Salvia grandifolia. A series of rosmarinic acid derivatives was obtained from rosmarinic acid using simple synthetic methods. The aldose reductase inhibitory activity of the isolated and synthesized compounds was assessed. Seven of the tested compounds showed moderate aldose reductase inhibition (IC50=0.06-0.30μM). The structure-activity relationship of aldose reductase inhibitory activity of rosmarinic acid derivatives was discussed for the first time. This study provided useful information that will facilitate the development of aldose reductase inhibitors.

A novel carboxymethylated mercaptotriazinoindole inhibitor of aldose reductase interferes with the polyol pathway in streptozotocin-induced diabetic rats

The aim of the present work was to study the effect of 3-mercapto-5H-1,2,4-triazino[5,6-b]indole-5-acetic acid (CMTI), an efficient aldose reductase inhibitor, on sorbitol accumulation in selected organs of streptozotocin-induced diabetic rats in vivo. In addition, the effect of CMTI on aldose reductase back reaction and on sorbitol dehydrogenase was determined. The model of experimental diabetes in male Wistar rats induced by streptozotocin was used. Experimental diabetes was induced by triple intraperitoneal doses of streptozotocin on three consecutive days. In diabetic rats, significant elevation of sorbitol concentration in the sciatic nerve and eye lenses was recorded. CMTI administered intragastrically (50 mg/kg/day) for five consecutive days significantly inhibited sorbitol accumulation in the sciatic nerve, yet it was without effect in eye lenses of diabetic animals. For aldose reductase back reaction, the substrate affinity of glycerol to aldose reductase was one order lower than that of glyceraldehyde in forward reaction. In addition, the back reaction was much slower, characterized by V(max) value of about 30 times lower than that of the forward reaction. Inhibition of aldose reductase by CMTI was characterized by closely related IC(50) values in submicromolar range for both forward and back reactions. No significant inhibition of the second enzyme of the polyol pathway, sorbitol dehydrogenase, by 100 microM CMTI was recorded (I=0.9+/-2.7 %, n=3). To conclude, the presented results showed the ability of CMTI to affect the polyol pathway in diabetic rats in vivo and represent thus a further step in a complex preclinical evaluation of CMTI as a potential agent for treatment of chronic diabetic complications.

Bioactivity Focus of α-Cyano-4-hydroxycinnamic acid (CHCA) Leads to Effective Multifunctional Aldose Reductase Inhibitors

Bioactivity focus on α-cyano-4-hydroxycinnamic acid (CHCA) scaffold results in a small library of novel multifunctional aldose reductase (ALR2) inhibitors. All the entities displayed good to excellent inhibition with IC50 72-405 nM. (R,E)-N-(3-(2-acetamido-3-(benzyloxy)propanamido)propyl)-2-cyano-3-(4-hydroxy phenyl)acrylamide (5f) was confirmed as the most active inhibitor (IC50 72.7 ± 1.6 nM), and the best antioxidant. 5f bound to ALR2 with new mode without affecting the aldehyde reductase (ALR1) activity, implicating high selectivity to ALR2. 5f was demonstrated as both an effective ALR2 inhibitor (ARI) and antioxidant in a chick embryo model of hyperglycemia. It attenuated hyperglycemia-induced incidence of neural tube defects (NTD) and death rate, and significantly improved the body weight and morphology of the embryos. 5f restored the expression of paired box type 3 transcription factor (Pax3), and reduced the hyperglycemia-induced increase of ALR2 activity, sorbitol accumulation, and the generation of ROS and MDA to normal levels. All the evidences support that 5f may be a potential agent to treat diabetic complications.

Emerging drugs for diabetic neuropathy

IMPORTANCE OF THE FIELD

Diabetic neuropathy (DN) is a very common and disabling diabetes-related complication. DN is associated with significant morbidity and mortality. Diabetic peripheral neuropathy (DPN) can be painful in the earlier stages of the disease before becoming painless. Most of the currently available therapies are symptomatic (focusing on pain relief) rather than disease-modifying. With the exception of good glycemic control, there is currently no effective treatment to slow the progression of or reverse DPN.

AREAS COVERED IN THIS REVIEW

In this article, we review the epidemiology, pathogenesis, currently available and future treatments for DPN, and the potential development issues/challenges related to such new therapies. Literature search was performed using PubMed, Medline and Pharmaprojects from 1950 onwards. Search terms include a combination of terms such as diabetic neuropathy, pathogenesis, pathophysiology, mechanisms, treatment, therapy, oxidative/nitrosative stress, anti-oxidants, serotonin, nitrotyrosine, protein kinase C, aldose reductase, sodium channels, taurine, lipoic acid and poly (ADP-ribose) polymerase.

WHAT THE READER WILL GAIN

The reader will gain an overview of the epidemiology, clinical features and risk factors of DN. In addition, the reader will have a better understanding of the mechanisms that underpin the development of DPN and their relationships to the current and future therapies. The reader will also develop an insight into the limitations of the current approach to DPN treatment and the potential avenues for future research.

TAKE HOME MESSAGE

DN is a very common and disabling complication that currently has no effective treatments other than diabetes control. The pathogenesis of DPN is complex and multi-factorial. Several disease-modifying and symptomatic treatments are currently under development. Oxidative and nitrosative stress have been identified as key pathogenic factors in the development of DPN and new treatments target these pathways and/or their downstream consequences. Gene therapy and growth factors have also emerged as potential new therapies that target particular cellular compartments as opposed to being delivered systemically. The recognition of the difficulty in reversing established DN has focused efforts on slowing its progression.

Managing highly coordinative substrates in asymmetric catalysis: a catalytic asymmetric amination with a lanthanum-based ternary catalyst

Full details of a catalytic asymmetric amination with a lanthanum/amide-based ligand catalyst system are described. A catalyst comprising La(NO(3))(3)*6H(2)O, (R)-3a and H-d-Val-O(t)Bu was identified to promote the catalytic asymmetric amination of nonprotected succinimide derivative 1 with as little as 1 mol % catalyst loading. Mechanistic studies by various spectroscopic analyses and several control and kinetic experiments suggested that the catalyst components were in equilibrium between the associated and dissociated forms, and that the reaction likely proceeded through a La(NO(3))(3)*6H(2)O/(R)-3a/H-d-Val-O(t)Bu ternary complex. This catalyst system was also effective for asymmetric amination of N-nonsubstituted alpha-alkoxycarbonyl amides 7, hitherto unprecedented substrates in asymmetric catalysis, probably due to their attenuated reactivity and difficult stereocontrol, affording the amination products in up to >99% yield and >99% ee. The high catalytic performance and enantiocontrol of the reaction with highly coordinative substrates were achieved by the activation/recognition of the substrates exerted by coordination to lanthanum and hydrogen bonding cooperatively in the transition state.

Diabetic neuropathy: therapies on the horizon

OBJECTIVES

This is a review of emerging interventions from the recent preclinical and clinical literature that demonstrate the potential for effectiveness in the therapy of diabetic neuropathy (DN). DN is the most common complication of diabetes mellitus and up to 50% of patients with type 1 and type 2 forms have some or other form of neuropathy. The pathology of DN is characterized by progressive nerve fibre loss that gives rise to positive and negative clinical signs and symptoms such as pain, paraesthesiae and loss of sensation.

KEY FINDINGS

There are very few drugs available to directly treat DN. Those that are clinically indicated provide symptomatic relief but do not repair or reverse underlying nerve damage. However, some agents are in clinical development that may support adult neurons and direct reparative processes after injury stages. Several disease modifying drugs such as aldose reductase inhibitors and protein kinase C inhibitors are in phase III development. Agents on the horizon include neurotrophic factors, growth factors, gene therapy, immunotherapy, poly(ADP-ribose) polymerase inhibitors and non-immunosuppressive immunophilin ligands.

SUMMARY

Progress has been made toward understanding the biochemical mechanisms leading to diabetic neuropathy, and as a result, new treatment modalities are being explored. The pathogenesis, types and approaches for treating DN together with the newer therapeutic interventions on the horizon are discussed.

Improvement of motor nerve conduction velocity in diabetic rats requires normalization of the polyol pathway metabolites flux

Using ranirestat, an aldose reductase (AR) inhibitor, we investigated the relationship between sorbitol and fructose levels in the sciatic nerve and motor nerve conduction velocity (MNCV) in streptozotocin (STZ)-treated diabetic rats. Ranirestat inhibited rat and recombinant human AR with similar IC50 values and equipotently prevented sorbitol accumulation in rat erythrocytes and sciatic nerves in vitro. One week after STZ administration, sorbitol levels in rat erythrocytes and sciatic nerves significantly increased while MNCV decreased. Oral administration of ranirestat (0.03-1.0 mg/kg per day) for 3 weeks dose-dependently decreased the elevated sorbitol and fructose levels in the rat sciatic nerves without affecting blood glucose level. Particularly, at doses of 0.1 mg/kg per day or higher, ranirestat normalized both sorbitol and fructose levels in the sciatic nerves of STZ-treated rats. Ranirestat (0.1-1.0 mg/kg per day) also improved the STZ-induced decrease in MNCV in a dose-dependent manner. This improvement correlated with the decrease of sorbitol and fructose levels in the rat sciatic nerves. These findings indicate that ranirestat improves MNCV via normalization of sorbitol and fructose accumulation in the sciatic nerve.

Long-term treatment with ranirestat (AS-3201), a potent aldose reductase inhibitor, suppresses diabetic neuropathy and cataract formation in rats

We investigated the chronic functional and histopathological changes in the sciatic nerve and lens of streptozotocin (STZ)-diabetic rats and evaluated the preventive effects of ranirestat (AS-3201), a potent aldose reductase inhibitor, on these changes. Sorbitol levels in the sciatic nerve and lens, motor nerve conduction velocity (MNCV), and development of cataracts were measured in STZ-diabetic rats given a ranirestat-admixed diet (0.0005%) for 35 weeks. Ranirestat reduced sorbitol accumulation in the sciatic nerve and improved the decrease in MNCV of STZ-diabetic rats. Morphological and morphometric examination of changes in sural nerve revealed that treatment with ranirestat prevented both the deformity of myelinated fibers and the decrease in their axonal and myelin areas (atrophy). Ranirestat also averted the changes in the size frequency histogram of myelinated fibers. Finally, STZ-diabetic rats developed early lens opacities 8 weeks after STZ injection and had cataract by the end of the experimental period. However, in the ranirestat-treated diabetic rats, no lens opacity was observed in any rat throughout the entire experimental period. This study suggests that the polyol pathway plays an important role in the progress of diabetic neuropathy and cataract formation in STZ-diabetic rats. Ranirestat should be a promising agent for the treatment of complications associated with diabetes, especially neuropathy.

The aldo-keto reductase superfamily and its role in drug metabolism and detoxification

The aldo-keto reductase (AKR) superfamily comprises enzymes that catalyze redox transformations involved in biosynthesis, intermediary metabolism, and detoxification. Substrates of AKRs include glucose, steroids, glycosylation end-products, lipid peroxidation products, and environmental pollutants. These proteins adopt a (beta/alpha)(8) barrel structural motif interrupted by a number of extraneous loops and helixes that vary between proteins and bring structural identity to individual families. The human AKR family differs from the rodent families. Due to their broad substrate specificity, AKRs play an important role in the phase II detoxification of a large number of pharmaceuticals, drugs, and xenobiotics.