Combination Treatment of Tamoxifen with Risperidone in Breast Cancer

Tamoxifen has long been used and still is the most commonly used endocrine therapy for treatment of both early and advanced estrogen receptor-positive breast cancer in pre- and post-menopause women. Tamoxifen exerts its cytotoxic effect primarily through cytostasis which is associated with the accumulation of cells in the G(0)/G(1) phase of the cell cycle. Apoptotic activity can also be exerted by tamoxifen which involves cleavage of caspase 9, caspase 7, caspase 3, and poly-ADP-ribose polymerase (PARP). Down-regulation of anti-apoptotic proteins Bcl-2 and Bcl-x(L) and up-regulation of pro-apoptotic proteins Bax and Bak have also been observed. In addition, stress response protein of GRP 94 and GRP 78 have also been induced by tamoxifen in our study. However, side effects occur during tamoxifen treatment in breast cancer patients. Researching into combination regimen of tamoxifen and drug(s) that relieves tamoxifen-induced hot flushes is important, because drug interactions may decrease tamoxifen efficacy. Risperidone has been shown to be effective in reducing or eliminating hot flushes on women with hormonal variations. In this present study, we demonstrated that combination of tamoxifen with risperidone did not interfered tamoxifen-induced cytotoxic effects in both in vitro and in vivo models, while fluoxetine abrogated the effects of tamoxifen. This is the first paper suggesting the possibility of combination treatment of tamoxifen with risperidone in breast cancer patients, providing a conceivable resolution of tamoxifen-induced side effects without interfering the efficacy of tamoxifen against breast cancer.