Antiestrogens
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.)
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