title: Prediction of cytochrome P450 3A inhibition by verapamil enantiomers and their metabolites.
abstract: Verapamil inhibition of CYP3A activity results in many drug-drug interactions with CYP3A substrates, but the mechanism of inhibition is unclear. The present study showed that verapamil enantiomers and their major metabolites [norverapamil and N-desalkylverapamil (D617)] inhibited CYP3A in a time- and concentration-dependent manner by using pooled human liver microsomes and the cDNA-expressed CYP3A4 (+b5). The values of the inactivation kinetic parameters kinact and KI obtained with the cDNA-expressed CYP3A4 (+b5) were 0.39 min(-1) and 6.46 microM for R-verapamil, 0.64 min(-1) and 2.97 microM for S-verapamil, 1.12 min(-1) and 5.89 microM for (+/-)-norverapamil, and 0.07 min(-1) and 7.93 microM for D617. Based on the ratio of kinact and KI, the inactivation potency of verapamil enantiomers and their metabolites was in the following order: S-norverapamil>S-verapamil>R-norverapamil>R-verapamil>D617. Using dual beam spectrophotometry, we confirmed that metabolic intermediate complex formation with CYP3A was the mechanism of inactivation for all compounds. The in vitro unbound fraction was 0.84 for S-verapamil, 0.68 for R-verapamil, and 0.84 for (+/-)-norverapamil. A mechanism-based pharmacokinetic model predicted that the oral area under the curve (AUC) of a CYP3A substrate that is eliminated completely (fm=1) by the hepatic CYP3A increased 1.6- to 2.2-fold after repeated oral administration of verapamil. For midazolam (fm=0.9), a drug that undergoes extensive intestinal wall metabolism, the predicted increase in oral AUC was 3.2- to 4.5-fold. The predicted results correlate well with the in vivo drug interaction data, suggesting that the model is suitable for predicting drug interactions by mechanism-based inhibitors.
authors.affiliation: Indiana University School of Medicine, Division of Clinical Pharmacology, Indianapolis, IN 46202, USA. yiwang@iupui.edu
authors.forename: Ying-Hong
authors.initials: YH
authors.lastname: Wang
authors.forename: David R
authors.initials: DR
authors.lastname: Jones
authors.forename: Stephen D
authors.initials: SD
authors.lastname: Hall
mesh_terms: D000284:Administration, Oral; D019540:Area Under Curve; D002121:Calcium Channel Blockers / Q000008:administration & dosage / Q000378:metabolism / Q000494:pharmacology*; D002851:Chromatography, High Pressure Liquid; D051544:Cytochrome P-450 CYP3A; D065607:Cytochrome P-450 Enzyme Inhibitors; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism; D004347:Drug Interactions; D006801:Humans; D066298:In Vitro Techniques; D008862:Microsomes, Liver / Q000201:enzymology / Q000378:metabolism; D008954:Models, Biological; D013237:Stereoisomerism; D014700:Verapamil / Q000008:administration & dosage / Q000031:analogs & derivatives* / Q000378:metabolism / Q000494:pharmacology*
chemical_list: D002121:Calcium Channel Blockers; D065607:Cytochrome P-450 Enzyme Inhibitors; D003577:Cytochrome P-450 Enzyme System; C016904:norverapamil; D014700:Verapamil; C104464:CYP3A protein, human; D051544:Cytochrome P-450 CYP3A; C510163:CYP3A4 protein, human