Stereospecific gas chromatographic/mass spectrometric assay of the chiral labetalol metabolite 3-amino-1-phenylbutane.

We previously identified 3-amino-1-phenylbutane (APB) as an oxidative N-dealkylated, metabolite of the antihypertensive agent labetalol. Labetalol has two asymmetric centers and is used clinically as a mixture of the four possible stereoisomers; APB has one asymmetric center. We now report an enantiospecific gas chromatographic/mass spectrometric assay for APB in urine. After adding the internal standard 1-methyl-2-phenoxyethylamine and alkalinizing, the urine samples were extracted with ether. The extracts were derivatized with the optically active acid chloride prepared from (S)-alpha-methoxy-alpha-trifluoromethylphenylacetic acid. The derivatives were separated by capillary gas chromatography and detected by electron capture negative ion chemical ionization mass spectrometry with selected ion monitoring. The derivative of the R enantiomer eluted first, and the [M--32]- ions were monitored for both the drug and the internal standard. The method was linear in the 0.05-2.5 micrograms enantiomer-1 ml-1 range and had inter-assay and intra-assay coefficients of variation of less than 6%. The assay was used in the analysis of urine samples from a patient in labetalol therapy and no interference was found. Further studies are needed to elucidate the oxidative metabolism of labetalol and its stereochemical aspects.