Aldose reductase (AR) is an NADPH-dependent enzyme implicated in diabetic complications. AS-3201 [(R)-(-)-2-(4-bromo-2-fluorobenzyl)-1,2,3,4-tetrahydropyrrolo[1,2-a]pyrazine-4-spiro-3'-pyrrolidine-1,2',3,5'-tetrone] is a structurally novel and potent ARI with an inhibitor constant (K(i) = 10(-)(10) M) 2000-fold lower than that of its optical antipode (S-isomer). To elucidate the inhibition modes and the stereochemical differences in their inhibitory potencies, we examined the interaction of these R- and S-isomers with AR under physiological conditions. Enzyme kinetic analysis, which was performed by using physiological substrates at 37 degrees C, showed that both isomers selectively act on the E-NADP(+) complex in both the forward and reverse reactions of AR. However, fluorometric titration analysis demonstrated that the affinities of the isomers for the E-NADP(+) complex are about the same as those for the E-NADPH complex and the apoenzyme. These results suggested that the selective binding to the E-NADP(+) complex arises from the predominance of this enzyme form during steady-state turnover rather than from binding specificity. Both the competition with a known active site-directed ARI and the protective effect on AR inactivation by N-bromosuccinimide showed that the isomers bind to the active site of the enzyme, but the thermodynamic parameters for the binding to AR indicated that additional hydrogen bonds and/or van der Waals interactions contribute to the energetic stabilization in the E-R-isomer complex. Molecular modeling, together with the deductions from spectroscopic studies, suggested that the succinimide ring and the 4-bromo-2-fluorobenzyl group of the R-isomer are optimally located for formation of a hydrogen-bonding network with AR, and that the latter benzyl group is also effective for the differentiation between AR and aldehyde reductase (a closely related enzyme).

Abstract. Ranirestat (AS-3201) is a novel aldose reductase (AR) inhibitor with potentially beneficial effects on diabetic sensorimotor polyneuropathy. In this study, we performed a kinetic analysis to determine the mode of inhibition of ranirestat on AR and investigated the effects of ranirestat on sorbitol levels in the sciatic nerves and lens of streptozotocin (STZ)-diabetic rats. We also evaluated the effects on motor nerve conduction velocity (MNCV) in STZ-diabetic rats. Kinetic analyses revealed that the ranirestat inhibition of AR is uncompetitive and reversible. In the sciatic nerve and lens of STZ-diabetic rats, single oral administration of ranirestat slightly reduced sorbitol levels. However, repeated oral administration of ranirestat for 5, 21, or 60 days enhanced the reducing effect of the ranirestat on sorbitol levels in the sciatic nerves and lens of STZ-diabetic rats with maximum effects after 21 days of treatment. Finally, repeated oral administration of ranirestat for 21 or 42 days dose-dependently improved the STZ-induced decrease in MNCV in STZ-diabetic rats. These findings demonstrate that repeated oral administration of ranirestat reduces sorbitol accumulation and improves MNCV in STZ-diabetic rats, indicating that ranirestat is an agent for the management of diabetic sensorimotor polyneuropathy.

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Abstract. 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 IC 50 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.

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