ABSTRACT:The cytochrome P450 (P450)-mediated biotransformation of tamoxifen is important in determining both the clearance of the drug and its conversion to the active metabolite, trans-4-hydroxytamoxifen. Biotransformation by P450 forms expressed extrahepatically, such as in the breast and endometrium, may be particularly important in determining tissue-specific effects of tamoxifen. Moreover, tamoxifen may serve as a useful probe drug to examine the regioselectivity of different forms. Tamoxifen metabolism was investigated in vitro using recombinant human P450s. Forms CYP1A1, 1A2, 1B1, 2A6, 2B6, 2C9, 2C19, 2D6, 2E1, 3A4, 3A5, and 3A7 were coexpressed in Escherichia coli with recombinant human NADPHcytochrome P450 reductase. Bacterial membranes were harvested and incubated with tamoxifen or trans-4-hydroxytamoxifen under conditions supporting P450-mediated catalysis. CYP2D6 was the major catalyst of 4-hydroxylation at low tamoxifen concentrations (170 ؎ 20 pmol/40 min/0.2 nmol P450 using 18 M tamoxifen), but CYP2B6 showed significant activity at high substrate concentrations (28.1 ؎ 0.8 and 3.1 ؎ 0.5 nmol/120 min/0.2 nmol P450 for CYP2D6 and CYP2B6, respectively, using 250 M tamoxifen). These two forms also catalyzed 4-hydroxylation (13.0 ؎ 1.9 and 1.4 ؎ 0.1 nmol/120 min/0.2 nmol P450, respectively, for CYP2B6 and CYP2D6 at 250 M tamoxifen; 0.51 ؎ 0.08 pmol/40 min/0.2 nmol P450 for CYP2B6 at 18 M tamoxifen). Tamoxifen N-demethylation was mediated by CYP2D6, 1A1, 1A2, and 3A4, at low substrate concentrations, with contributions by CYP1B1, 2C9, 2C19 and 3A5 at high concentrations. CYP1B1 was the principal catalyst of 4-hydroxytamoxifen trans-cis isomerization but CYP2B6 and CYP2C19 also contributed.
