Tamoxifen is an antiestrogen (blocks the effect of estrogen on tissue). The precise mechanism of its action is unknown, but one possible mechanism is that it binds and blocks estrogen receptors on the surface of cells, preventing estrogens from binding and activating the cell. It is used in patients for treating and preventing breast cancer. Controversy currently exists as to which breast cancer patients will benefit from this treatment.
Tamoxifen is used for the treatment of invasive breast cancer, the most common type of breast cancer, following surgery and/or radiation and for preventing invasive breast cancer in women at high risk for developing it. Tamoxifen also is used for the treatment of women following surgery and radiation for a less common type of breast cancer called ductal carcinoma in situ (DCIS or intraductal carcinoma). Women who have had ductal carcinoma in situ are at high risk for developing invasive breast cancer at a later date, and tamoxifen prevents development of the invasive cancer in almost half of the women during the first five years of treatment. Occasionally, tamoxifen is used to stimulate ovulation.
Tamoxifen works by binding to the estrogen receptor in the breast and other estrogen-sensitive tissues, thereby preventing the subsequent binding of estrogen. A possible alternative mechanism of tamoxifen’s antiestrogenic effect is through the increased production of sex-hormone binding globulin, a protein that binds estrogen and blocks its action. In addition to its antiestrogenic effects, tamoxifen may also inhibit cell division in cancer cells1 and promote the production of a certain type of tumor-fighting white blood cells called "natural killer cells"2. Although tamoxifen acts as an estrogen "blocker" on the tumor, tamoxifen has an estrogen-like effect on the bone, liver, and endometrium.
The hormone, estrogen, enters many normal cells including mammary (breast) cells and signals growth and multiplication. In breast cancer, this growth is accelerated. Tamoxifen is an anti-estrogen, a compound shaped like natural estrogen, that binds to estrogens receptors in the nucleus of breast cancer cells and prevents the cell from growing. By binding in the place of estrogen, Tamoxifen slows or stops individual breast cell (tumor) growth.