TY - JOUR T1 - Prediction of the Effect of Erythromycin, Diltiazem, and Their Metabolites, Alone and in Combination, on CYP3A4 Inhibition JF - Drug Metabolism and Disposition JO - Drug Metab Dispos SP - 150 LP - 160 DO - 10.1124/dmd.108.022178 VL - 37 IS - 1 AU - Xin Zhang AU - David R. Jones AU - Stephen D. Hall Y1 - 2009/01/01 UR - http://dmd.aspetjournals.org/content/37/1/150.abstract N2 - Predictive models of complex drug-drug interactions between multiple inhibitors and their metabolites have not been evaluated. The purpose of this study was to evaluate an interaction model for cytochrome P450 3A4 (CYP3A4) that incorporated the simultaneous reversible and irreversible inhibition by multiple inhibitors. Erythromycin (ERY) and diltiazem (DTZ), and their major metabolites, N-desmethylerythromycin (nd-ERY) and N-desmethyldiltiazem (nd-DTZ), were chosen to evaluate the model. kinact (rate constant for maximal inactivation), KI (inhibitor concentration at 50% maximal inactivation), and Ki (reversible inhibition constant) were estimated for ERY, DTZ, nd-ERY, and nd-DTZ, respectively, using cDNA-expressed CYP3A4 and human liver microsomes under optimal experimental conditions. To evaluate the interaction model, combinations of inhibitors and metabolites were incubated at concentrations equal to KI, ½KI, and 2KI of each inhibitor for specified durations in both enzyme systems. The models were further evaluated by the incubation of combinations of inhibitors with the substrate testosterone for 10 min. CYP3A4 inhibition in the presence of drug mixtures was predicted from the inhibition parameters determined for each drug or metabolite alone. The CYP3A4 activity in the presence of multiple inhibitors was well predicted by the model incorporating additive irreversible inhibition as modified by mutual competitive inhibition (percent mean error and percent mean absolute error ranged from –0.06 to 0.04 and from 0.03 to 0.09, respectively). In conclusion, the additive model predicted the combined effect of multiple inhibitors on CYP3A inhibition in vitro. However, simultaneous reversible and irreversible inhibition effects should be taken into account in a reaction mixture of substrate and multiple inhibitors of CYP3A4. The American Society for Pharmacology and Experimental Therapeutics ER -