INVOLVEMENT OF CYTOCHROME P4503A IN CATALYSIS OF TAMOXIFEN ACTIVATION AND COVALENT BINDING TO RAT AND HUMAN LIVER-MICROSOMES

Tamoxifen is the major therapeutic agent for the treatment of hormone-dependent breast cancer, Tamoxifen treatment appears to be associated with an increased incidence of endometrial carcinoma in humans and hepatocellular carcinoma in rats, These carcinogenic effects of tamoxifen might be induced by the formation of a tamoxifen reactive intermediate that binds covalently to macromolecules. Liver microsomal cytochrome P450s (CYPs) catalyze the metabolism of tamoxifen, forming a reactive intermediate that binds irreversibly to microsomal proteins, primarily to a 54 kDa protein (Mani,C, and Kupfer,D,, Cancer Res., 51, 6052-6058, 1991), The current study identifies the P450 enzymes that catalyze the activation of tamoxifen to a reactive intermediate in rats and humans, Among the species examined, rats, chickens and humans demonstrate low tamoxifen binding activity, ranging from 0.1 to 0.4 nmol bound/mg protein/h, In contrast, hamsters and mice exhibit high binding, 1.2 and 1.6 nmol/mg protein/h respectively, Treatment of male rats with phenobarbital or pregnenolone-16 alpha-carbonitrile (PCN) markedly elevated the binding of tamoxifen to liver microsomal proteins, Methylcholanthrene treatment had no effect on binding, These findings suggested the involvement of CYP3A in catalysis of the covalent binding, Alternate substrates of CYP3A, cortisol and erythromycin, inhibited tamoxifen binding in liver microsomes from PCN- and phenobarbital-treated rats, Treatment of rats with troleandomycin (TAO), an inducer of CYP3A, followed by the dissociation of the TAO-CYP3A complex, elevated the covalent binding to liver microsomes similar to 3-fold, Antibodies against rat CYP3A1 strongly inhibited tamoxifen binding to liver microsomes from PCN- and phenobarbital-treated rats, whereas the antibodies anti-CYP2B1/2B2 did not inhibit binding, In humans, tamoxifen binding was inhibited by the anti-rat CYP3A1 IgG and also by alternate substrates of CYP3A, These results indicate that the activation of tamoxifen to a reactive intermediate by rat and human liver microsomes is principally catalyzed by CYP3A enzymes.