Labetalol is metabolized oxidatively in humans

Previous studies on the metabolic fate of antihypertensive agent labetalol in humans identified only conjugated metabolites of the drug and accounted for only a portion of the dose. In this study, urine samples obtained from three patients on chronic labetalol therapy for hypertension were analyzed initially by thin layer chromatography for the presence of other metabolites. All three urine samples were found to contain 3-amino-1-phenylbutane. The identify of this metabolite in one of the urine samples was confirmed by electron capture negative-ion chemical ionization mass spectrometry. The mass spectrometry experiments also identified the presence in the urine sample of the D-hydroxy derivative of 3-amino-1-phenylbutane. The two metabolites are the result of oxidative biotransformations of labetalol. 3-Amino-1-phenylbutane has been reported to be a potent sympathomimetic agent, and the question arises whether the newly identified metabolites of labetalol contribute to its pharmacological effects.