Ranirestat (AS-3201; SX-3201) is a novel, oral and potent aldose reductase inhibitor with the potential for the treatment of diabetic neuropathy. It inhibits aldose reductase (AR) with IC50s of 11 nM and 15 nM for rat lens AR and recombinant human AR, respectively, and a Ki of 0.38 nM for recombinant human AR. Ranirestat also has a neuroprotective effect on diabetic retinas. Aldose reductase is an enzyme that catalyzes one of the steps in sorbitol (polyol) pathway which is responsible for formation of fructose from glucose. Aldose reductase activity is increased, parallel to glucose blood levels, in tissues that are not insulin sensitive, including lenses, peripheral nerves and renal glomeruli. Sorbitol does not diffuse through cell membranes easily and therefore accumulates in these tissues, causing osmotic damage, leading to retinopathy and neuropathy.