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Antiestrogens
Information about Antiestrogens
The aromatase inhibitors block estrogen synthesis and are used as therapy of estrogen receptor positive breast cancer, usually after resection and as a first line treatment or after failure of tamoxifen (another antiestrogen that acts by blocking the estrogen receptor).
Liver toxicity of Antiestrogens
The aromatase inhibitors include anastrozole, letrozole and exemestane, some of which have been implicated in causing rare instances of clinically apparent liver injury.
Mechanism of action of Antiestrogens
Aromatase is the enzyme responsible for the conversion of testosterone to estrone (E1) and androstenedione to estradiol (E2). Highest levels of aromatase are found in the ovary and placenta, which are the major sources of estrogen in premenopausal women. However, aromatase is also found in other tissues, such as liver, kidney, adrenals, brain, muscle and subcutaneous fat where it is also active in producing estrogens, although at low levels. These tissues are the major source of estrogen after menopause or oopherectomy. Inhibitors of aromatase were developed to block the synthesis of estrogen in the peripheral tissues and, thus, as antiestrogen therapy of estrogen receptor positive breast cancer in postmenopausal women. The first aromatase inhibitor used in clinical medicine was aminoglutethimide, which was initially developed as an anticonvulsant, but later found to inhibit adrenocorticoid steroid synthesis.
More specific aromatase inhibitors with antiestrogen effects only were subsequently developed, the current agents being considered third generation inhibitors. These inhibitors include anastrozole and letrozole which are nonsteroidal, and exemestane which is a steroidal aromatase inhibitor. These agents have little or no effect on adrenal glucocorticoid or mineralocorticoid synthesis.
FDA approval information for Antiestrogens
The commercial names and year of approval in the United States are: letrozole (Femara, 1995), anastrozole (Arimidex, 1997), and exemestane (Aromasin, 1999). All three agents are now available in generic forms as well. They are used largely as adjuvant therapy in postmenopausal women with estrogen sensitive breast cancer, generally given in daily oral doses for up to five years.
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