Determination of the S(+)- and R(-)-enantiomers of baclofen in plasma and urine by gas chromatography using a chiral fused-silica capillary column and an electron-capture detector

A sensitive and enantiospecific gas chromatographic method for the determination of the S(+)- and R(-)-enantiomers of baclofen (I and II) in plasma and urine has been developed and validated. The method is based on the complete resolution of the derivatized enantiomers on a chiral fused-silica capillary column. The hydrochloride salt of a (-)-fluoro analogue of baclofen (III.HCl) was used as the internal standard in plasma, the hydrochloride salt of a (+)-fluoro analogue of baclofen (IV.HCl) as the internal standard in urine. Rapid and convenient isolation of the compounds was achieved using reversed-phase Bond-Elut C18 columns. After elution, the compounds were converted into isobutyl esters and purified by base-specific solvent extraction. The isobutyl esters were then N-acylated with heptafluorobutyric anhydride. The derivatives were quantitated after separation on the chiral column using electron-capture detection. The analysis of spiked plasma and urine samples demonstrated the good accuracy and precision of the method, with limits of quantitation of 25 nmol/l for I and II in plasma and of 2 mumol/l for I and II in urine. The method appears to be suitable for use in pharmacokinetic studies of the enantiomers in plasma and urine from animals and man after administration of the racemic baclofen.