Amlodipine inhibits rat microsomal cytochrome P450-mediated drug biotransformation

Calcium channel antagonists have been shown to inhibit cytochrome P-450-mediated metabolism both in vitro and in vivo. The purpose of the present study was to examine the effect of amlodipine on a suite of rat hepatic microsomal cytochrome P-450 activities to determine the potential for drug interactions. In this study, amlodipine (0.05 and 0.5 mM) decreased CYP1A-mediated ethoxyresorufin O-deethylase activity in microsomes prepared from noninduced (56 and 73% inhibition) and pyridine-induced (30 and 51% inhibition) rats. Amlodipine reduced pentoxyresorufin O-deethylase activity (a marker for CYP2B) to 15% of control in incubations utilizing microsomes from phenobarbital-treated rats, but had no effect on this enzyme reaction in noninduced microsomes. The para-nitrophenol hydroxylase, erythromycin N-demethylase, and lauric acid omega and omega-1 hydroxylase activities were significantly inhibited by 1 mM amlodipine in both noninduced and induced microsomes. These results suggest that amlodipine inhibits a number of different P450 forms and therefore has the potential to inhibit the metabolism of a large number of drugs.