Metabolism of Tamoxifen by Recombinant Human Cytochrome P450 Enzymes: Formation of the 4-Hydroxy, 4′-Hydroxy and N-Desmethyl Metabolites and Isomerization of trans-4-Hydroxytamoxifen

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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 NADPH-cytochrome P450 reductase. Bacterial membranes were harvested and incubated with tamoxifen ortrans-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 μM 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 μM 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 μM tamoxifen; 0.51 ± 0.08 pmol/40 min/0.2 nmol P450 for CYP2B6 at 18 μM 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-hydroxytamoxifentrans-cis isomerization but CYP2B6 and CYP2C19 also contributed.

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