Concomitant CYP2D6 inhibitor use and tamoxifen adherence in early-stage breast cancer: A pharmacoepidemiologic study

CRA509 Background: The use of cytochrome P450 2D6 inhibiting drugs (CYP2D6 inhibitors) during tamoxifen treatment leads to a decrease in plasma concentration of endoxifen, the major active tamoxifen metabolite. Concomitant use of CYP2D6 inhibitors, such as the commonly prescribed selective serotonin reuptake inhibitors, as well as low tamoxifen adherence may negatively impact tamoxifen efficacy in breast cancer. The objectives were to relate concomitant CYP2D6 inhibitor use and tamoxifen adherence to breast cancer event free time (EFT). Methods: Data were used from PHARMO, a pharmacy database, PALGA, a nationwide pathology database and the Dutch Medical Register in the Netherlands. Breast cancer patients who were treated with tamoxifen as adjuvant therapy between 1994 and 2006 were included. A Cox proportional hazards model with a time-dependent definition for the CYP2D6 inhibitor exposure was used. Results: 1,990 breast cancer patients using tamoxifen were included, among whom 215 (10.8%) used a CYP2D6 i...