Gas chromatographic separation of the diastereomers of labetalol as n-butylboronate

Abstract

Labetalol (LA) is a new kind of antihypertensive agent that consists of two racemic mixtures, LA-1 (R·S+S·R) and LA-2 (R·R+S·S). These racemic mixtures show a difference in their pharmacological activity. We developed the separative determination of LA-1 and LA-2 by gas chromatography in order to clarify the difference in their excretion and metabolism in human subject. LA was heated with n-butylboronic acid in pyridine at 60°C for 2 h and the reaction mixture injected into gas chromatograph (1.5% OV-1, 285°C). LA-2 boronate had a retention time of 4 min and LA-1 boronate of 5 min, and the resolution of these peaks was almost complete. Linear relationship between peak height ration and weight ratio of LA-1 to LA-2 was observed. By this method, urinary excretion of LA-1 and LA-2 was examined after single oral administration of LA·HCl (100mg/body) to three healthy volunteers. Urine was collected for 24h after administration. To the urine sample was added prochlorperazine maleate as an internal standard, and extracted with ethyl acetate before or after treatment with β-glucuronidase. The percentage of the dose (50 mg as each racemic mixture) excreted as LA-1, its phenolic glucuronide, LA-2 and its phenolic glucuronide were in the range of 1.2-2.4, 11.2-24.6, 0.6-1.3 and 4.7-11.2, respectively.