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Anastrozole (INN) (marketed under the trade name Arimidex by AstraZeneca) is a non-steroidal aromatase-inhibiting drug approved for treatment of breast cancer after surgery, as well as for metastasis in both pre and post-menopausal women. The severity of breast cancer can be increased by estrogen, as sex hormones cause hyperplasia, and differentiation at estrogen receptor sites. Anastrozole works by inhibiting the synthesis of estrogen.
Many breast cancers also contain aromatase; the importance of tumor-generated estrogens is uncertain. Treatment of breast cancer has included efforts to decrease estrogen levels by ovariectomy premenopausally and by use of anti-estrogens and progestational agents both pre- and post-menopausally, and these interventions lead to decreased tumor mass or delayed progression of tumor growth in some women. Anastrozole is a potent and selective non-steroidal aromatase inhibitor. It significantly lowers serum estradiol concentrations and has no detectable effect on formation of adrenal corticosteroids or aldosterone.
The ATAC (Arimidex, Tamoxifen, Alone or in Combination) trial was an international randomised controlled trial of 9366 women with localized breast cancer who received either anastrozole, tamoxifen, or both for five years, followed by five years of follow-up. After more than 5 years the group that received anastrozole had significantly better clinical results than the tamoxifen group. The trial suggested that anastrozole is the preferred medical therapy for postmenopausal women with localized breast cancer, which is estrogen receptor (ER) positive. Another study found that the risk of recurrence was reduced 40%, but was associated with an increased risk of bone fractures. The study concluded that ER positive patients benefited from switching from tamoxifen to anastrozole in patients who have completed 2 years' adjuvant tamoxifen. A more recent trial found that anastrozole significantly reduced the incidence of breast cancer in postmenopausal women relative to placebo, and while there were side effects related to estrogen deprivation observed, the researchers concluded that this was probably not related to the treatment.