Initiation of exemestane in two warfarin-treated patients leading to elevation and variability of INR

Aromatase inhibitors are sometimes chosen for adjuvant therapy in post-menopausal breast cancer patients with a history of venous thromboembolism over an antiestrogen due to the lower risk of venous thromboembolism associated with aromatase inhibitors compared to antiestrogens. We report two cases where patients on warfarin therapy had an increase in their international normalized ratio with the initiation of exemestane therapy. Initially, the patients also showed international normalized ratio variability possibly due to variable absorption of exemestane. We suggest patients being treated with warfarin and exemestane concomitantly need close monitoring and education in order to decrease the risk of adverse events that could be associated with this possible interaction. To our knowledge, there are no similar reported cases in the literature.