title: Drug interactions with calcium channel blockers: possible involvement of metabolite-intermediate complexation with CYP3A.
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 IC(50) values for the inhibition of CYP2D6 and CYP2C9 for nicardipine were 3 to 9 microM, whereas those for all others ranged from 14 to >150 microM. Except for nifedipine, all calcium channel blockers showed increased inhibitory potency toward CYP3A activities (testosterone 6beta-hydroxylation and midazolam 1'-hydroxylation) after 30-min preincubation with NADPH. IC(50) values for the inhibition of testosterone 6beta-hydroxylase obtained in the NADPH-preincubation experiment for nicardipine (1 microM), verapamil (2 microM), and diltiazem (5 microM) were within 10-fold, whereas those for amlodipine (5 microM) and felodipine (13 microM) 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 ( approximately 30% at 100 microM), and was a much faster-acting inhibitor (maximal inhibition rate constant approximately 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.
authors.affiliation: Department of Drug Metabolism, Merck Research Laboratories, West Point, Pennsylvania 19486, USA.
authors.forename: B
authors.initials: B
authors.lastname: Ma
authors.forename: T
authors.initials: T
authors.lastname: Prueksaritanont
authors.forename: J H
authors.initials: JH
authors.lastname: Lin
mesh_terms: D001189:Aryl Hydrocarbon Hydroxylases; D002121:Calcium Channel Blockers / Q000378:metabolism / Q000494:pharmacology*; D051544:Cytochrome P-450 CYP3A; D065607:Cytochrome P-450 Enzyme Inhibitors; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism*; D004347:Drug Interactions; D006801:Humans; D007501:Iron / Q000378:metabolism; D007527:Isoenzymes / Q000037:antagonists & inhibitors / Q000378:metabolism; D020748:Mibefradil / Q000494:pharmacology; D017551:Microdialysis; D008862:Microsomes, Liver / Q000187:drug effects / Q000201:enzymology; D010089:Oxidoreductases, N-Demethylating / Q000037:antagonists & inhibitors / Q000378:metabolism*; D011994:Recombinant Proteins / Q000378:metabolism
chemical_list: D002121:Calcium Channel Blockers; D065607:Cytochrome P-450 Enzyme Inhibitors; D007527:Isoenzymes; D011994:Recombinant Proteins; D020748:Mibefradil; D003577:Cytochrome P-450 Enzyme System; D007501:Iron; D001189:Aryl Hydrocarbon Hydroxylases; D051544:Cytochrome P-450 CYP3A; D010089:Oxidoreductases, N-Demethylating