Stereoselective analysis of labetalol in human plasma by LC‐MS/MS: Application to pharmacokinetics

Labetalol is clinically available as a mixture of two racemates (four stereoisomers). The stereoisomer (R,R) has as main activity the β₁‐antagonism and the stereoisomer (S,R) is highly selective for the α₁ adrenoceptor and is responsible for most of the α‐blocker activity. In the present investigation, a method for the analysis of labetalol stereoisomers in human plasma was developed and applied to pharmacokinetic studies. Plasma samples (0.5 ml) were extracted with methyl tert‐butyl ether at pH 9.5. The four labetalol stereoisomers were analyzed by LC‐MS/MS on a Chirobiotic® V column using a mobile phase consisting of methanol, acetic acid, and diethylamine, with a recovery of more than 90% for all four. The quantitation limit was 0.5 ng/ml and linearity was observed at 250 ng/ml plasma for each stereoisomer. Studies of precision and accuracy presented coefficients of variation and percentage inaccuracy of less than 15%, indicating that the method is precise and accurate. The method was applied to the study of the kinetic disposition of labetalol over a period of 12 h after oral administration of a single 100 mg dose to a hypertensive pregnant woman. The clinical study revealed stereoselectivity in the pharmacokinetics of labetalol, with a lower plasma proportion for the active stereoisomers (R,R)‐labetalol and (S,R)‐labetalol. The stereoselectivity observed after oral administration is due to the hepatic metabolism and the first pass effect, with an AUC_(R,R)/AUC_(S,S) ratio of 0.5. Chirality, 2009. © 2008 Wiley‐Liss, Inc.