Interactions:

NOTE: High concentrations of anastrozole inhibited metabolic reactions catalyzed by cytochromes P450 (CYP) 1A2, 2C8/9, and 3A4. Anastrozole did not inhibit CYP2A6 or the polymorphic CYP2D6 in human liver microsomes. Per the manufacturer, it is unlikely that anastrozole administered at the recommended dose will inhibit the metabolism of cytochrome P450-mediated drugs given concomitantly (13). In a study in male volunteers (n=16), anastrozole did not alter the warfarin pharmacokinetics (Cmax or AUC), and did not alter warfarin anticoagulant activity as measured by prothrombin time, activated partial thromboplastin time, and thrombin time of both R- and S-warfarin (13). Anastrozole and tamoxifen should not be administered together. Clinical and pharmacokinetic results from the ATAC study (14) demonstrate that concurrent administration of anastrozole and tamoxifen results in a reduction of anastrozole plasma levels by 27% compared to those achieved with anastrozole alone.(13) However, coadministration did not affect the pharmacokinetics of tamoxifen or N-desmethyltamoxifen.(13). The goal of anastrozole therapy is to decrease circulating estrogen concentrations and inhibit the growth of hormonally-responsive cancers.(13) Anastrozole should not be given concurrently with any estrogens or estrogen-containing products, including combined oral contraceptives, as these could interfere with the pharmacologic action of anastrozole. In addition, in women receiving long-term aromatase inhibitor therapy, atrophic vaginitis due to estrogen suppression is common; atrophic vaginitis due to aromatase inhibitor therapy is sometimes treated with vaginal estrogen as the systemic exposure of estrogen from vaginal preparations is thought to be low. In a recent study of 7 women on aromatase inhibitor therapy, estrogen concentrations rose significantly after the addition of vaginally administered estrogen for atrophic vaginitis. Estrogen concentrations increased from a mean baseline level of < 5 pmol/l to 72 pmol/l at 2 weeks and to < 35 pmol/l at 4 weeks. Although the study was small, estrogen concentrations rose significantly in 6/7 patients. Clinicians should be aware that serum estrogen concentrations may increase with the use of vaginal estrogen preparations; alternative treatments for atrophic vaginitis in patients taking aromatase inhibitors should be considered(15). Androstenedione is an important metabolic precursor for androgens and estrogens in both males and females. Androstenedione supplements should not be given concurrently with any aromatase inhibitors, as androstenedione could interfere with the pharmacologic action of the aromatase inhibitor. Conversely, aromatase inhibitors (e.g., aminoglutethimide, anastrozole, exemestane, letrozole, testolactone, vorozole) could interfere with biotransformation of androstenedione in both males and females; the enzyme aromatase converts androstenedione to estriol. Prasterone, dehydroepiandrosterone, DHEA is converted via hydrosteroid dehydrogenases and aromatase into androstenedione, testosterone, and estradiol by peripheral tissues.(16) Prasterone or DHEA supplements should not be given concurrently with any aromatase inhibitors, as DHEA could interfere with the pharmacologic action of the aromatase inhibitor and compromise aromatase inhibitor effectiveness. Conversely, aromatase inhibitors (e.g., aminoglutethimide, anastrozole,(13) exemestane,(17) letrozole,(18) testolactone,(19) vorozole could interfere with biotransformation of DHEA.