Aromatase inhibitors are used for the treatment of breast cancer in postmenopausal or oophorectomized women.
They act by inhibiting the enzyme aromatase, blocking the conversion of androstenedione to estrogen in peripheral tissues, which is the main source of estrogens in postmenopausal women.
Commonly used aromatase inhibitors include anastrozole, exemestane, and letrozole.

Woodruff R. Palliative medicine evidence-based symptomatic and supportive care for patients with advanced cancer. Fourth edition. Oxford University Press, 2004. (p. 445)