Stereoselective metabolism of metoprolol: Enantioselectivity of α-hydroxymetoprolol in plasma and urine

Direct stereoselective separation on chiral stationary phase was developed for HPLC analysis of the four stereoisomers of α‐hydroxymetoprolol in human plasma and urine. Plasma samples were prepared using solid‐phase extraction columns and urine samples were prepared by liquid–liquid extraction. The stereoisomers were separated on a Chiralpak® AD column at 24°C with fluorescence detection and a mobile phase consisting of a mixture of hexane:ethanol:isopropanol:diethylamine (88:10.2:1.8:0.2) for plasma samples and hexane:ethanol:diethylamine (88:12:0.2) for urine samples. Calibration curves for the individual stereoisomers were linear within the concentration range of 2.0–200 ng/ml plasma or 0.125–25 μg/ml urine. The methods were validated with intra‐ and interday variations less than 15%. The absolute configuration of the pure stereoisomers were assigned by circular dichroism spectra. The methods were employed to determine the concentrations of α‐hydroxymetoprolol stereoisomers in a metabolism study of multiple‐dose administration of racemic metoprolol to hypertensive patients phenotyped as extensive metabolizers of debrisoquine. We observed stereo‐selectivity in the α‐hydroxymetoprolol formation favoring the new 1′R chiral center from both metoprolol enantiomers (AUC0–241′R1′S = 3.02). The similar renal clearances (ClR) of the four stereoisomers demonstrated absence of stereoselectivity in their renal excretion. (–)‐(S)‐metoprolol was slightly more α‐hydroxylated than its antipode (AUC0–242S/2R = 1.19), suggesting that this pathway is not responsible for plasma accumulation of this enantiomer in humans. Chirality 15:542–549, 2003. © 2003 Wiley‐Liss, Inc.