REVIEW Exemestane is an irreversible (Type 1), steroidal aromatase inhibitor. Aromatase catalyzes the final and rate-limiting step in the conversion of androgens to estrogens in peripheral tissues. This occurs mainly in adipose tissue, but also in normal and malignant breast tissues, and provides the main source of estrogen in postmenopausal women. The goal of hormone therapy in breast cancer is to deprive tumour cells of estrogens, which are implicated in the development or progression of tumours. Maximal estrogen suppression is produced by a 25 mg dose. It occurs after 2-3 days and returns to baseline in 10-14 days. Exemestane does not have estrogenic activity. Highly selective blockade of aromatase does not interfere with the production of other steroids (eg, adrenal corticosteroids1, aldosterone2, androgens). Evidence suggests that tumour progression is not due to a loss of estrogen suppression but to newly developed tumour resistance, including acquired hypersensitivity of some tumour cells to estrogens. Differences in the mechanism of action may contribute to the apparent lack of cross-resistance between steroidal (eg, exemestane) and nonsteroidal (eg, anastrozole, letrozole) aromatase inhibitors.
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