Differentiation between isomeric oxidative metabolites of a piperidine‐containing drug by liquid chromatography–tandem mass spectrometry with stable‐isotope tracer technique

This report describes the application of a liquid chromatography–tandem mass spectrometry (LC‐MS/MS) method to differentiation of hydroxylations and N ‐oxidations and of two different aliphatic hydroxylations in the investigation of the metabolism of pibutidine hydrochloride, a novel H 2 antagonist, the structure of which includes a piperidine ring. Pibutidine metabolites in urine samples from adult male volunteers after oral administration of pibutidine hydrochloride were separated by reversed‐phase LC and ionized using an electrospray ionization (ESI) interface. A hydroxylated form of pibutidine was distinguished from the N ‐oxide by comparison of their product ion spectra, although their mass‐to‐charge ratios of protonated molecules were identical. Further, two hydroxylated compounds were present in rat microsomal incubation mixtures with pibutidine. The distinction between their positions of hydroxylation (β‐ and γ‐carbon hydroxylation) on the piperidine ring was studied using [piperidine‐ 2 H 10 ] pibutidine as incubation substrate. The production of the β‐hydroxylated form was accompanied by the elimination of three 2 H, resulting from a mechanism including the formation of iminium/enamine. The participation of the iminium ion intermediate in the β‐hydroxylation was confirmed by the observation that a cyanide adduct of pibutidine was formed instead of the β‐hydroxylated form when another incubation was performed in the presence of cyanide. Copyright © 2001 John Wiley & Sons, Ltd.