Pharmacology of tamoxifen in laboratory animals

This paper reviews the recent laboratory findings about the nonsteroidal antiestrogen, tamoxifen, and its more potent major metabolite, monohydroxytamoxifen. Both compounds stimulate progesterone receptor synthesis in the rat uterus, and there is an inhibition of cell division in the uterine luminal epithelial cells. The effects of tamoxifen in vivo may be a result of the net effects of the parent compound and monohydroxytamoxifen. In rats with dimethylbenzanthracene (DMBA)-induced rat mammary carcinomata, young tumors that are estrogen receptor- and progesterone receptor-rich respond more favorably to tamoxifen that do older estrogen receptor- and progesterone receptor-poor tumors. However, the antitumor effect of tamoxifen in the DMBA-induced rat mammary carcinoma model is probably a result of the blockade of tumor estrogen receptors, a reduction in circulating gonadotropins, lower circulating estrogen levels, and lower circulating prolactin levels. The 30-days treatment of rats with tamoxifen 30 days after DMBA resulted in a dose-related decrease in the appearance and numbers of mammary tumors; however, only continuous therapy maintained animals in a tumor-free state. Monohydroxytamoxifen was a less-potent antitumor agent, probably because it is cleared from the rat more rapidly than tamoxifen. The present laboratory findings support the clinical use of tamoxifen as a treatment of endometrial carcinoma and the resultant metastases and as an adjuvant therapy after surgery for breast cancer.