Mechanism of cytochrome P450-3A inhibition by ketoconazole.

Greenblatt, David J.; Zhao, Yanli; Venkatakrishnan, Karthik; Duan, Su X.; Harmatz, Jerold S.; Parent, Sarah J.; Court, Michael H.; von Moltke, Lisa L.

Objectives Ketoconazole is extensively used as an index inhibitor of cytochrome P450-3A (CYP3A) activity in vitro and in vivo, but the mechanism of ketoconazole inhibition of CYP3A still is not clearly established. Methods Inhibition of metabolite formation by ketoconazole (seven concentrations from 0.01 to 1.0 µ m) was studied in human liver microsomes ( n = 4) at six to seven substrate concentrations for triazolam, midazolam, and testosterone, and at two substrate concentrations for nifedipine. Key findings Analysis of multiple data points per liver sample based on a mixed competitive-noncompetitive model yielded mean inhibition constant Ki values in the range of 0.011 to 0.045 µ m. Ketoconazole IC50 increased at higher substrate concentrations, thereby excluding pure noncompetitive inhibition. For triazolam, testosterone, and midazolam α-hydroxylation, mean values of α (indicating the 'mix' of competitive and noncompetitive inhibition) ranged from 2.1 to 6.3. However, inhibition of midazolam 4-hydroxylation was consistent with a competitive process. Determination of Ki and α based on the relation between 50% inhibitory concentration values and substrate concentration yielded similar values. Pre-incubation of ketoconazole with microsomes before addition of substrate did not enhance inhibition, whereas inhibition by troleandomycin was significantly enhanced by pre-incubation. Conclusions Ketoconazole inhibition of triazolam α- and 4-hydroxylation, midazolam α-hydroxylation, testosterone 6 β-hydroxylation, and nifedipine oxidation appeared to be a mixed competitive-noncompetitive process, with the noncompetitive component being dominant but not exclusive. Quantitative estimates of Ki were in the low nanomolar range for all four substrates.

KETOCONAZOLE; ANTIFUNGAL agents; PIPERAZINE; CYTOCHROME P-450; CYTOCHROMES

Journal of Pharmacy & Pharmacology, 2011, Vol 63, Issue 2, p214

0022-3573

Academic Journal

10.1111/j.2042-7158.2010.01202.x