Use of Antidepressants in Patients with Breast Cancer Taking Tamoxifen

Tamoxifen, a selective estrogen modulator has been used for more than three decades to treat all stages of estrogen receptor (ER)-positive breast cancer and to prevent the disease. Tamoxifen is a pro-drug that requires metabolic activation to 4-hydroxytamoxifen and 4-hydroxy-N-desmethyl-tamoxifen (endoxifen) to elicit its pharmacological activity. Endoxifen has identical properties and potency with 4-hydroxytamoxifen, but is present in concentrations up to 10-fold higher than 4-hydroxytamoxifen. The cytochrome P450 2D6 (CYP2D6) enzyme plays a key role in converting tamoxifen into its active metabolites with significantly greater affinity for the ER and greater ability to inhibit cell proliferation. Genetic variants in the CYP2D6 gene may result in CYP2D6 enzymes with reduced or null activity, thereby decreasing the anti-cancer effect. In addition to genetic inactivation of CYP2D6, inhibitors of CYP2D6, including some antidepressants to treat hot flashes or depression in patients with breast cancer, may also alter enzyme activity and negatively affect the outcomes of patients receiving adjuvant tamoxifen. This article reviews and discusses the following issues: tamoxifen metabolism, antiproliferative effects of tamoxifen and its metabolites, CYP2D6 genetic polymorphisms, treatment for hot flashes and depression in breast cancer, and the pharmacological interactions between tamoxifen and antidepressants via CYP2D6. Although routine CYP2D6 testing is not recommended yet, coadministration of potent or intermediate CYP2D6 inhibitors in women taking tamoxifen should be avoided.