%0 Journal Article %A A. David Rodrigues %A Ellen M. Roberts %A Darcy J. Mulford %A Ye Yao %A Daniele Ouellet %T Oxidative Metabolism of Clarithromycin in the Presence of Human Liver Microsomes %B Major Role for the Cytochrome P4503A (CYP3A) Subfamily %D 1997 %J Drug Metabolism and Disposition %P 623-630 %V 25 %N 5 %X In vitro studies were conducted to identify the hepatic cytochrome P450 (CYP) protein(s) involved in the oxidative metabolism of [14C]clarithromycin (CLAR) in the presence of native human liver microsomes. The identity of the two major CLAR metabolites present in microsome incubates, 14-(R)-hydroxy-CLAR andN-desmethyl-CLAR, was confirmed by MS. Over the CLAR concentration range of 1.0–140 μM, the rate of CLAR 14-(R)-hydroxylation (KM = 48.7 ± 17.7 μM; Vmax = 206 ± 76 pmol/min/mg protein;Vmax/KM = 4.2 ± 0.21 μl/min/mg; mean ± SD, N = 3 livers) andN-demethylation (KM = 59.1 ± 24.0 μM; Vmax = 189 ± 52.0 pmol/min/mg protein; Vmax/Km = 3.3 ± 0.53 μl/min/mg) conformed to monophasic (saturable) Michaelis-Menten kinetics and was highly correlated (r= 0.90–0.92; p < 0.001; N = 11) with CYP3A-selective erythromycin N-demethylase activity. Ketoconazole (≤2.0 μM) or troleandomycin, CYP3A-selective inhibitors, markedly decreased (≥99%) the formation of both metabolites, whereas inhibitors selective of other CYP forms were relatively ineffective (≤10% inhibition). In agreement with chemical inhibitor studies, CLAR metabolism was only detectable with human B-lymphoblastoid microsomes containing cDNA-expressed CYP3A4 (vs. CYP2C19, CYP2C9, CYP2D6, CYP1A2, CYP2E1, or CYP2A6). Furthermore, the apparent KM characterizing the 14-(R)-hydroxylation and N-demethylation of CLAR in the presence of insect cell microsomes containing cDNA-expressed CYP3A4 (KM = 18–63 μM) was similar to that obtained with native human liver microsomes. Based on the results of this study, it is concluded that the 14-(R)-hydroxylation and N-demethylation of CLAR is primarily mediated by one or more members of the human liver CYP3A subfamily. The American Society for Pharmacology and Experimental Therapeutics %U https://dmd.aspetjournals.org/content/dmd/25/5/623.full.pdf