Metabolism of bambuterol in rat liver microsomes: identification of hydroxylated and demethylated products by liquid chromatography mass spectrometry.

The oxidative metabolism of (R,S)-bambuterol in rat liver microsomes was studied. Metabolite fractions were analyzed by thermospray LC-MS. The use of an equimolar mixture of deuterium-labeled and unlabeled bambuterol facilitated the mass spectrometric identification of the metabolites. Six metabolites, formed via hydroxylation, demethylation, and hydrolytic reactions, were identified. The demethylated metabolites were found to be chemically unstable under physiological conditions. It is likely that the complex biotransformation of bambuterol into terbutaline is one factor contributing to the long duration of action of bambuterol.