Antiestrogens bind to estrogen receptors and block estrogen ac-tion. The antiestrogens include tamoxifen citrate, toremifene cit-rate, and fulvestrant. Tamoxifen and toremifene are nonsteroidalestrogen agonist-antagonists, and fulvestrant is a pure estrogen antagonist.
After oral administration, tamoxifen is well absorbed and under-goes extensive metabolism in the liver before being excreted in stool. Serum levels of fulvestrant, when given I.M., peak in 7 to 9 days. Its half-life is 40 days. Toremifene is well absorbed, and ab-sorption isn’t influencd by food.
The exact antineoplastic action of these agents isn’t known. How-ever, they’re known to act as estrogen antagonists. Estrogen re-ceptors, found in the cancer cells of one-half of premenopausal and three-fourths of postmenopausal women with breast cancer, respond to estrogen to induce tumor growth.
The antiestrogens fulvestrant, tamoxifen, and toremifene bind to the estrogen receptors and inhibit estrogen-mediated tumor growth in breast tissue. Tamoxifen may be able to do this because it binds to receptors at the nuclear level or because the binding reduces the number of free receptors in the cytoplasm. Ultimately, DNA synthesis and cell growth are inhibited.
Tamoxifen is used alone and as adjuvant treatment with radi-ation therapy and surgery in women with negative axillary lymph nodes and in postmenopausal women with positive axillary nodes. It’s used for advanced breast cancer involv-ing estrogen receptor–positive tumors in postmenopausal women and may be used in palliative treatment of advanced or metastatic breast cancer that’s estrogen receptor–positive Tumors in postmenopausal women are more responsive to tamoxifen than those in premenopausal women. Tamoxifenmay also be used to reduce the incidence of breast cancer in women at high risk.
· Toremifene is used to treat metastatic breast cancer in post-menopausal women with estrogen receptor–positive tumors.
· Fulvestrant is used in postmenopausal women with receptor-positive metastatic breast cancer with disease progression after treatment with tamoxifen. (See Who benefits from tamoxifen?)
There are no known drug interactions for fulvestrant. However, these reactions may occur with other antiestrogens:
§ Tamoxifen and toremifene increase the effects of warfarin, in-creasing the risk of bleeding.
§ Bromocriptine increases the effects of tamoxifen.
§ Drugs that induce certain liver enzymes, such as phenytoin, ri-fampin, and carbamazepine, may increase tamoxifen metabolism, causing decreased serum levels. (See Adverse reactions to anti-estrogens.)