Hormonal therapy, also called endocrine therapy, is an effective treatment for most tumors that test positive for either estrogen or progesterone receptors (ER-positive or PR-positive, in both early-stage and metastatic cancer. This type of tumor uses hormones to fuel its growth. Blocking the hormones can help prevent a cancer recurrence and death from breast cancer when used for early-stage disease either by itself or after adjuvant or neoadjuvant chemotherapy. Hormonal therapy is also effective as treatment for metastatic breast cancer, shrinking the cancer and improving cancer-related symptoms.
Aromatase inhibitors (AIs) decrease the amount of estrogen made by tissues other than the ovaries in postmenopausal women by blocking the aromatase enzyme, which changes weak male hormones called androgens into estrogen when the ovaries have stopped making estrogen during menopause. These drugs include anastrozole , letrozole and exemestane. All of the AIs are tablets taken daily by mouth. Treatment with AIs, either alone or following tamoxifen, is more effective than tamoxifen alone at reducing the risk of recurrence from early-stage breast cancer in post-menopausal women. AIs are also an effective treatment for metastatic hormone receptor positive breast cancer.
Research shows that all three AI drugs work equally well and have similar side effects. Women who have not gone through menopause should not take AIs, as they do not block the effects of estrogen made by the ovaries. Women who have gone through menopause and are given hormonal therapy have several options: take tamoxifen or an AI for five years, begin treatment with tamoxifen for two to three years and then switch to an AI for the rest of the five-year period, or take tamoxifen for five years then switch to an AI for what is called extended hormonal therapy. Tamoxifen, ovarian suppression and AIs are also commonly used to treat metastatic breast cancer.
Reference: http://www.cancer.net/cancer-types/breast-cancer/treatment-options