Vol. 27, Issue 3, 389-394, March 1999

The Aromatase Inactivator 4-Hydroxyandrostenedione (4-OH-A) Inhibits Tamoxifen Metabolism by Rat Hepatic Cytochrome P-450 3A: Potential for Drug-Drug Interaction of Tamoxifen and 4-OH-A in Combined Anti-Breast Cancer Therapy

Shangara S. Dehal, Angela M. H. Brodie, and David Kupfer

Department of Pharmacology and Molecular Toxicology, University of Massachusetts Medical Center, Worcester, Massachusetts (S.S.D., D.K.) and Department of Pharmacology, School of Medicine, University of Maryland at Baltimore, Baltimore, Maryland (A.M.H.B).

Tamoxifen (tam), an anti-breast cancer agent, is metabolized into tam-N-oxide by the hepatic flavin-containing monooxygenase and into N-desmethyl- and 4-hydroxy-tam by cytochrome P-450s (CYPs). Additionally, tam is metabolically activated by hepatic CYP3A, forming a reactive intermediate that binds covalently to proteins. Tam and 4-hydroxyandrostenedione (4-OH-A) are currently used to treat breast cancer, and it has been contemplated that 4-OH-A be given concurrently with tam to contravene potential tumor resistance to tam. Because alterations in tam metabolism may influence its therapeutic efficacy, the effect of 4-OH-A on tam metabolism was examined. Incubation of tam with liver microsomes from phenobarbital-treated rats, in the presence of 4-OH-A (10-100 µM), resulted in marked inhibition of tam-N-demethylation and tam covalent binding and in decreased tam-N-oxide accumulation; however, there was no inhibition of the formation of 4-hydroxy-tam and of 3,4-dihydroxytamoxifen. These findings indicate that 4-OH-A inhibits CYP3A, but not P-450(s) that catalyze tam 4-hydroxylation. The diminished tam-N-oxide accumulation could be due to decreased N-oxide formation and/or due to increased N-oxide reduction. Incubation of tam-N-oxide with liver microsomes containing heat-inactivated flavin-containing monooxygenase demonstrated that 4-OH-A increases the accumulation of tam, possibly by diminishing its P-450-mediated metabolism. Kinetic studies indicate that 4-OH-A is a competitive inhibitor of CYP3A, but not a time-dependent inactivator. Consequently, the concurrent treatment of tam and 4-OH-A may result in increased tam half-life and thus could potentiate the therapeutic efficacy of tam and diminish the potential side effects of tam by inhibiting its covalent binding to proteins and possibly to DNA.