Enantioselective analysis of metoprolol in plasma using high-performance liquid chromatographic direct and indirect separations: applications in pharmacokinetics.

Direct enantioselective separation on chiral stationary phases and indirect separation based on the formation of diastereomeric derivatives were developed and compared for the HPLC analysis of R(+) and S(-)-metoprolol in human plasma. Plasma samples prepared using solid-phase extraction columns or liquid-liquid extraction were directly analyzed on a Chiralpack AD or on a Chiralcel OD-H columns, respectively. S-(-)-menthyl choroformate was also used to yield diastereomeric derivatives resolved on a RP-8 column. The methods were employed to determine plasma concentrations of metoprolol enantiomers in a pharmacokinetic study of single dose administration of racemic metoprolol to a healthy Caucasian volunteer phenotyped as extensive metabolizer of debrisoquine. The correlation coefficients among enantioselective metoprolol plasma concentrations (5-223 ng/ml) obtained by the three methods were equal or higher than 0.99. The direct method that employed the chiral column Chiralpak AD may be considered the most sensitive, although the three methods demonstrated interchangeable use in the pharmacokinetic investigation.