Metabolism of verapamil: 24 new phase I and phase II metabolites identified in cell cultures of rat hepatocytes by liquid chromatography-tandem mass spectrometry.

Verapamil is a widely prescribed calcium antagonist, but suffers from extensive first pass metabolism. Despite its frequent use in drug metabolism a complete understanding of its metabolic pathway is still lacking. We thus investigated verapamil's metabolism in cultures of primary rat hepatocytes and isolated metabolites from cell culture media by solid phase extraction (SPE). In detail, we investigated their structure in multiple liquid chromatography-mass spectrometry (LC-MSn) experiments and found 25 phase I and 14 phase II metabolites. We showed many metabolites to be produced by oxidative dealkylation, and several yet unknown metabolites were identified that stem from hydroxylation and dealkylation reactions. Furthermore, we identified an array of glucuronides and, additionally, a glucoside. Finally, we investigated the enantioselective biotransformation of verapamil and found preferential metabolism of the S-enantiomers. In conclusion, this illustrates again the true complexity of verapamil's disposition.