Effect of dose-dependent interaction between tamoxifen and antidepressants on patient adherence in breast cancer patients.

558 Background: Tamoxifen is a selective estrogen receptor modulator (SERM) that is commonly used to treat breast cancer. However, despite its established efficacy, recent studies indicate that 1) more than 50 percent of individuals will discontinue treatment prior to completing the recommended five-year therapeutic regimen and 2) the likelihood of discontinuation is moderated by the presence of polymorphisms in the gene encoding cytochrome P450 2D6 (CYP2D6), the enzyme responsible for metabolizing tamoxifen into its active form. Methods: We measured tamoxifen persistency rates in a cohort of breast cancer patients to test the hypothesis that tamoxifen adherence is modified by the dose administered and concomitant use of anti-depressant (SSRI) drugs known to interfere with CYP2D6 metabolism. Study subjects (N = 12,742) were identified in a large transactional claims database, cross-classified by tamoxifen dose (HIGH, NORMAL) and SSRI status (YES, NO), and followed for 360 days after initiation of therapy....