Eliminating estrogen. Letrozole like anastrazole and Exemestane is an aromatase inhibitor. In postmenopausal pts, the ovaries no longer produce estrogen but the body creates low levels of estrogen via the enzyme aromatase. The ais block this enzyme thus eliminating any estrogen. In premenopausal pts, this actually causes a feedback that stimulates the ovaries to make more estrogen thus it can't be used to treat er+.
Blocks hormones. It blocks some hormones that have been found to increase the risk of breast cancer.