Metabolism of the antimammary cancer antiestrogenic agent tamoxifen. I. Cytochrome P-450-catalyzed N-demethylation and 4-hydroxylation.Drug Metab Dispos. 1993 Jul-Aug; 21(4):645-56.DM
Previous studies suggested that the therapeutic effect of the antimammary cancer agent tamoxifen might be related to its metabolism. This study examined the cytochrome P-450 enzymes in rat and human liver catalyzing the metabolism of tamoxifen. Incubations of tamoxifen with rat liver microsomes yielded three major polar metabolites identified as the N-oxide, N-desmethyl, and 4-hydroxy derivatives. N-Oxide formation was catalyzed by the flavin-containing monooxygenase (see part II). Carbon monoxide, SKF-525A, metyrapone, and benzylimidazole strongly inhibited N-demethylation and 4-hydroxylation, indicating the participation of P-450 monooxygenase in these reactions. Antibodies to NADPH-P450 reductase inhibited N-demethylation and 4-hydroxylation. Comparison of the metabolism of tamoxifen in untreated male and female rats demonstrated some sexual dimorphism. N-Demethylation was higher in the male rat and 4-hydroxylation was higher in the female. Treatment of rats with phenobarbital (PB), pregnenolone-16 alpha-carbonitrile (PCN), and methylcholanthrene (MC) enhanced N-demethylation, demonstrating the potential participation of multiple P-450s in N-demethylation. Evidence strongly indicates that CYP3A enzyme(s) catalyzes N-demethylation in liver microsomes of PB- and PCN-treated rats (PB and PCN microsomes, respectively): i) N-demethylation was inhibited by cortisol and erythromycin (alternate substrates) and a time-dependent inhibition was observed with troleandomycin (TAO) in vitro; ii) treatment of female rats with TAO, followed by dissociation of the microsomal TAO-P-450 complex, elevated N-demethylation; iii) treatment of PCN-induced female rats with chloramphenicol inhibited N-demethylation; and iv) polyclonal antibodies (PAbs) to CYP3A1 inhibited N-demethylation in PCN- and PB-treated female rats. Although we were unable to reconstitute the N-demethylation activity with purified CYP3A1, which is difficult to reconstitute, collectively the evidence demonstrated that CYP3A enzymes catalyze N-demethylation in PB and PCN microsomes. By contrast, antibodies against CYP2B1/B2 did not inhibit N-demethylation and reconstituted 2B1 did not catalyze N-demethylation of tamoxifen, indicating that 2B1 was not involved. The increase in N-demethylation by MC treatment appears to be due to elevation of CYP1A1/1A2 (P-450c/d). Alternate substrates of CYP1A1/1A2 inhibited N-demethylation and reconstituted rat CYP 1A1-catalyzed N-demethylation. Surprisingly, monoclonal antibodies (MAbs) against CYP1A1/1A2 only partially inhibited, and PAbs against CYP1A1 did not inhibit N-demethylation in MC microsomes, indicating that in MC microsomes, 1A1 does not contribute significantly to that reaction. Mab anti-CYP2C11/2C6 (P-450h/k) inhibited N-demethylation in PB, PCN, and control male rat liver microsomes, suggesting that CYP2C11 and/or CYP2C6 catalyze this reaction to some extent.(
