7 DRUG INTERACTIONS
7.1 Cytochrome P450 (CYP)
In vitro and in vivo studies have shown that estrogens are metabolized partially by cytochrome P450 3A4 (CYP3A4). Concomitant administration of itraconazole, a strong CYP3A4 inhibitor, with DUAVEE, resulted in increases in bazedoxifene exposure (40%) and, to a lesser extent, conjugated estrogens exposure (9% for baseline-adjusted total estrone, 5% for total equilin), compared to DUAVEE alone [see Pharmacokinetics (12.3)]. Inducers of CYP3A4, such as St. John's Wort (Hypericum perforatum) preparations, phenobarbital, carbamazepine, and rifampin, may reduce plasma concentrations of some estrogens, possibly resulting in a decrease in therapeutic effects and/or changes in the uterine bleeding profile.
Bazedoxifene does not induce or inhibit the activities of major CYP isoenzymes. In vitro data suggest that bazedoxifene is unlikely to interact with co-administered drugs via CYP-mediated metabolism.
7.2 Uridine Diphosphate Glucuronosyltransferase (UGT)
Bazedoxifene undergoes metabolism by UGT enzymes in the intestinal tract and liver. The metabolism of bazedoxifene may be increased by concomitant use of substances known to induce UGTs, such as rifampin, phenobarbital, carbamazepine, and phenytoin. A reduction in bazedoxifene exposure may be associated with an increase risk of endometrial hyperplasia. Adequate diagnostic measures, including directed or random endometrial sampling when indicated, should be undertaken to rule out malignancy in postmenopausal women with undiagnosed persistent or recurring abnormal genital bleeding.