Vol. 26, Issue 10, 970-976, October 1998

Identification of the Human Liver Cytochrome P450 Enzymes Involved in the In Vitro Metabolism of a Novel 5-Lipoxygenase Inhibitor

Joseph M. Machinist, Michael D. Mayer, Ellen M. Roberts, Bruce W. Surber, and A. David Rodrigues¹

Drug Metabolism Department, Pharmaceutical Products Division, Abbott Laboratories

In vitro studies were conducted to identify the hepatic cytochrome P450 (CYP) forms involved in the oxidative metabolism of [¹⁴C]ABT-761 and its N-dehydroxylated metabolite, [¹⁴C]ABT-438, by human liver microsomes. The two compounds were metabolized by parallel pathways, to form the corresponding methylene bridge hydroxy metabolites. There was no evidence of sulfoxidation and/or ring hydroxylation. Over the ABT-761 and ABT-438 concentration ranges studied (1-300 µM), the rate of NADPH-dependent hydroxylation was linear with respect to substrate concentration ([S]) and did not conform to saturable Michaelis-Menten kinetics. Under these conditions ([S] < K_M), the intrinsic clearance (V_max/K_M) of ABT-438 was 10-fold higher than that of ABT-761 (1.7 ± 0.8 vs. 0.17 ± 0.06 µl/min/mg, mean ± SD, N = 3 livers). The hydroxylation of both compounds was shown to be highly correlated (r = 0.83, p < 0.01, N = 11 different human livers) with CYP3A-selective erythromycin N-demethylase activity, and the correlation between ABT-761 hydroxylation and tolbutamide hydroxylase (CYP2C9-selective) activity (r = 0.63, p < 0.05, N = 10) was also statistically significant. Ketoconazole (2.0 µM), a CYP3A-selective inhibitor, inhibited the hydroxylation of both compounds by 53-67%, and sulfaphenazole (CYP2C9-selective) decreased activity by 10-20%. By comparison, -naphthoflavone, a known activator of CYP3A, stimulated the hydroxylation of ABT-761 (8-fold) and ABT-438 (4-fold). In addition, the abundance-normalized rates of cDNA-expressed CYP-dependent metabolism indicated that hydroxylation was largely mediated (66-86%) by CYP3A(4). Therefore, it is concluded that the hydroxylation of ABT-761 and ABT-438 (10 µM) is primarily mediated by CYP3A, although CYP2C9 may play an ancillary role.