Abstract

The inhibitory effects of six commonly used calcium channel blockers on three major cytochrome P-450 activities were examined and characterized in human liver microsomes. All six compounds reversibly inhibited CYP2D6 (bufuralol 1′-hydroxylation) and CYP2C9 (tolbutamide methyl hydroxylation) activities. The IC₅₀ values for the inhibition of CYP2D6 and CYP2C9 for nicardipine were 3 to 9 μM, whereas those for all others ranged from 14 to >150 μM. Except for nifedipine, all calcium channel blockers showed increased inhibitory potency toward CYP3A activities (testosterone 6β-hydroxylation and midazolam 1′-hydroxylation) after 30-min preincubation with NADPH. IC₅₀ values for the inhibition of testosterone 6β-hydroxylase obtained in the NADPH-preincubation experiment for nicardipine (1 μM), verapamil (2 μM), and diltiazem (5 μM) were within 10-fold, whereas those for amlodipine (5 μM) and felodipine (13 μM) were >200-fold of their respective plasma concentrations reported after therapeutic doses. Similar results also were obtained based on midazolam 1′-hydroxylase activity. Unlike the observations with mibefradil, a potent irreversible inhibitor of CYP3A, the NADPH-dependent inhibition of CYP3A activity by nicardipine and verapamil was completely reversible on dialysis, whereas that by diltiazem was partially restored (80%). Additional experiments revealed that nicardipine, verapamil, and diltiazem formed cytochrome P-450-iron (II)-metabolite complex in both human liver microsomes and recombinant CYP3A4. Nicardipine yielded a higher extent of complex formation (∼30% at 100 μM), and was a much faster-acting inhibitor (maximal inhibition rate constant ∼2 min⁻¹) as compared with verapamil and diltiazem. These present findings that the CYP3A inhibition caused by nicardipine, verapamil, and diltiazem is, at least in part, quasi-irreversible provide a rational basis for pharmacokinetically significant interactions reported when they were coadministered with agents that are cleared primarily by CYP3A-mediated pathways.