Anastrozole inhibits CYPs 1A2, 2C8/9 and 3A4 in vitro. Clinical studies with antipyrine and warfarin showed that anastrozole at a 1 mg dose did not significantly inhibit the metabolism of antipyrine and R- and S-warfarin indicating the co-administration of anastrozole with other medicinal products is unlikely to result in clinically significant medicinal product interactions mediated by CYP enzymes.
The enzymes mediating metabolism of anastrozole have not been identified. Cimetidine, a weak, unspecific inhibitor of CYP enzymes, did not affect the plasma concentrations of anastrozole. The effect of potent CYP inhibitors is unknown.
A review of the clinical trial safety database did not reveal evidence of clinically significant interaction in patients treated with anastrozole who also received other commonly prescribed medicinal products.
There were no clinically significant interactions with bisphosphonates.
Co-administration of tamoxifen or estrogen-containing therapies with anastrozole is not recommended as this may diminish its pharmacological action (see previous text).