Abstract

Amlodipine is a commonly prescribed calcium channel blocker for the treatment of hypertension and ischemic heart disease. The drug is slowly cleared in human primarily via dehydrogenation of its dihydropyridine moiety to a pyridine derivative (M9) followed by M9 further oxidation. Results from clinical drug-drug interaction studies suggest that CYP3A4/5 mediates metabolism of amlodipine. However, attempts to identify role of CYP3A5 in amlodipine metabolism in human between the pharmacokinetic data of CYP3A5 expressers and non-expressers failed. Objectives of this study were to determine metabolite profile of amlodipine (a racemic mixture and S-isomer) in human liver microsomes (HLM), and to identify the CYP enzyme(s) involved. Liquid chromatography/mass spectrometry (LC/MS) analysis of the amlodipine samples after HLM incubation showed that amlodipine was converted to a major metabolite, M9, which underwent further O-demethylation, O-dealkylation and oxidative deamination to various pyridine derivatives. This observation is consistent with amlodipine metabolism in human. Incubations of amlodipine with HLM in the presence of selective CYP inhibitors showed that both ketoconazole (an inhibitor of CYP3A4 and CYP3A5) and CYP3cide (an inhibitor of CYP3A4) completely blocked M9 degeneration, while other chemical inhibitors had little effect. Furthermore, metabolism of amlodipine in recombinant human CYP enzymes showed that only CYP3A4 had significant activity in amlodipine dehydrogenation. Metabolite profiles and CYP reaction phenotyping data of a racemic mixture and S-isomer of amlodipine were very similar. The results from this study suggest that CYP3A4 rather than CYP3A5 plays a key role in metabolic clearance of amlodipine in human.