Determination of the R-(-) and S-(+) enantiomers of the monohydroxylated metabolite of oxcarbazepine in human plasma by enantioselective high-performance liquid chromatography.

An enantioselective liquid chromatographic assay for the simultaneous determination of the S-(+) and R-(-) enantiomers of the monohydroxylated metabolite of oxcarbazepine in human plasma is described. The metabolite is the active principle. The method is based on the extraction of plasma with diethyl ether-dichloromethane (2:1, v/v), separation of the organic phase, evaporation of the solvent and dissolution of the residue in the mobile phase. The two enantiomers were resolved on a Chiralcel OD (250 mm x 4.6 mm I.D.) high-performance liquid chromatographic column. The separation was achieved by isocratic elution with n-hexane-2-propanol (77:23, v/v). The flow-rate of the mobile phase was 1.0 ml/min and the two enantiomers were detected by ultraviolet absorbance at 210 nm. The analytical method is suitable for the quantitative and simultaneous determination of the two enantiomers in plasma at concentrations down to 0.4 mumol/l after administration of oxcarbazepine.