[Elucidation and Application of Novel Action of Therapeutic Agents for Diabetic Neuropathy]

Epalrestat is the only aldose reductase inhibitor that is currently available for diabetic peripheral neuropathy. Oxidative stress impairs endothelial cells, thereby leading to numerous pathological conditions. Increasing antioxidative ability is important to prevent cellular toxicity induced by reactive oxygen species. Epalrestat increases antioxidant defense factors such as glutathione and γ-glutamylcysteine ligase in vascular endothelial cells through activation of the transcription factor nuclear factor erythroid 2-related factor 2 (Nrf2). This increases suppression of oxidative stress-induced cellular toxicity. Cadmium is an industrial and environmental pollutant that targets the vascular endothelium. The vascular system is critically affected by cadmium toxicity. Therapeutic treatment against cadmium toxicity is chelation therapy that promotes metal excretion; however, cadmium chelators can cause renal toxicity. Therefore, safe and efficient therapeutic agents are required. Epalrestat suppresses cadmium-induced cytotoxicity in vascular endothelial cells through activation of Nrf2. In addition, epalrestat affects the intracellular levels of cadmium, cadmium transporter Zrt-Irt-like protein 8 (ZIP8), and metallothionein (MT). The upregulation of ZIP8 and MT may be involved in the suppression of cadmium-induced cytotoxicity by epalrestat. Drug repurposing is a new strategy for drug discovery in which the pharmacological action of existing medicines whose safety and pharmacokinetics have already been confirmed clinically and whose use has been approved is examined comprehensively at the molecular level. The results can be applied to the development of existing drugs for use as medicines for the treatment of other diseases. This review provides useful findings for future expansion of indications as research leading to drug repurposing of epalrestat.