Distribution of gacyclidine enantiomers after experimental spinal cord injury in rats: possible involvement of an active transport system

Role of stereochemistry in N-(3,5-dichlorophenyl)-2-hydroxysuccinamic acid (2-NDHSA) nephrotoxicity

The nephrotoxicity induced by the agricultural fungicide N-(3,5-dichlorophenyl)succinimide (NDPS) is mediated through oxidative metabolites of NDPS. Oxidation of the succinimide ring in NDPS yields the nephrotoxic metabolites N-(3,5-dichlorophenyl)-2-hydroxysuccinimide (NDHS) and its hydrolysis product N-(3,5-dichlorophenyl)-2-hydroxysuccinamic acid (2-NDHSA). The oxidation of NDPS on the succinimide ring also introduces an asymmetric carbon atom into these NDPS metabolites, so that R- and S- enantiomers of NDHS and 2-NDHSA are possible. The purpose of this study was to begin to explore the importance of the stereochemical orientation at the asymmetric carbon atom for the nephrotoxicity induced by NDPS metabolites. Male Fischer 344 rats were administered a single intraperitoneal (ip) injection of R-(+)- or S-(-)-2-NDHSA (0.05, 0.1 or 2.0 mmol/kg) or vehicle, and renal function was monitored for 48 h. R-2-NDHSA (0.1 mmol/kg) administration had little effect on renal function. R-2-NDHSA (0.2 mmol/kg) treatment induced mild diuresis on day 1, increased proteinuria, and a small increase in blood urea nitrogen (BUN) concentration, but no change in kidney weight or glucosuria. S-2-NDHSA (0.1 mmol/kg) induced marked nephrotoxicity as evidenced by diuresis on both post-treatment days, increased proteinuria, glucosuria, and increased kidney weight and BUN concentration. No evidence of hepatotoxicity was obtained in any treated group. Thus, the S-isomer of 2-NDHSA is a more potent nephrotoxicant than the R-isomer, and stereochemistry may play a role in NDPS metabolite-induced nephrotoxicity.

Pharmacokinetics of gacyclidine enantiomers in plasma and spinal cord after single enantiomer administration in rats

The purpose of this study was to determine the pharmacokinetics of gacyclidine, a non-competitive NMDA antagonist, in plasma and spinal cord extracellular fluid (ECF) after IV administration of single enantiomers in rats. After implantation of microdialysis probes in spinal cord, concentrations in plasma and ECF dialysates were determined by a chiral GC/MS assay over 5 h after administration of either (+)-gacyclidine or (-)-gacyclidine (1.25 mg/kg). Plasma protein binding was estimated in vitro by equilibrium dialysis. Plasma concentrations decayed in parallel in a biphasic manner (t(1/2)alpha approximately 9 min; t(1/2)beta approximately 90 min) with no significant difference between the two enantiomers. Clearance of (+)-gacyclidine and (-)-gacyclidine (291 versus 275 ml/min per kg, respectively), volume of distribution (Vdbeta: 38 versus 40 l/kg), and protein binding (90 versus 89%) were not stereoselective. Both gacyclidine enantiomers were quantifiable in spinal cord ECF 10 min after drug administration and their concentrations remained stable over the duration of the experiment in spite of changing blood concentrations. Penetration of the two enantiomers in spinal cord ECF was similar although highly variable between animals. Exposure of spinal cord ECF was comparable for both enantiomers, and not correlated with plasma AUCs. This study showed the absence of any pharmacokinetic difference between the two enantiomers when administered individually, and no enantiomeric inversion. Both gacyclidine enantiomers penetrate rapidly and extensively into spinal cord ECF, and their distribution may involve an active transport system.