About two-thirds of women with breast cancer have tumors that contain hormone receptors. This means a tumor has receptors for the hormone estrogen (called ER-positive) or the hormone progesterone (PR-positive) or both. Hormone therapy blocks these hormones and fight the cancer's growth.
Women who are ER-positive are more likely to respond to hormone treatment than women who are ER-negative.
Research shows that taking tamoxifen for 5 years lowers the chance of breast cancer recurrence and new breast cancers in women with ER-positive or ER-unknown breast tumors. Doctors also use tamoxifen to treat breast cancer that has spread to other parts of the body. And they use it to prevent breast cancer in healthy women with high odds of developing the disease.
Tamoxifen also helps prevent osteoporosis, or weak bones.
But women who take tamoxifen are more likely to develop cancer of the uterus (endometrial cancer) than other women. You should get regular pelvic exams and tell your doctor about any abnormal uterine bleeding.
Other problems that may happen when you take tamoxifen include blood clots in your legs and lungs (deep vein thrombosis and pulmonary embolism), stroke, and cataracts. Minor side effects include hot flashes and mood swings.
Aromatase inhibitors are medications that treat breast cancer in both early stages and advanced ones. They prevent your body from making estrogen. They only work in women who are past menopause, though.
Anastrozole (Arimidex), exemestane (Aromasin), and letrozole (Femara) are aromatase inhibitors. Doctors prescribe them to treat ER-positive breast cancer, either following tamoxifen treatment or by themselves.
There are other hormone therapy drugs used to treat breast cancer, too. Most, such as fulvestrant (Faslodex) and toremifene (Fareston), are used to treat metastatic breast cancer. Toremifene, like tamoxifen, blocks certain effects of estrogen and is used as a treatment for advanced breast cancer in postmenopausal women. Fulvestrant blocks estrogen receptors temporarily.
Palbociclib (Ibrance), and ribociclib (Kisqali) are targeted therapies sometimes used in combination with an aromatase inhibitor or fulvestrant in women who have hormone receptor positive. Alpelisib (Piqray) is a targeted therapy used with fulvestrant in some women who have gone through menopause who have advanced hormone receptor positive, HER2 – negative breast cancer. Abemaciclib (Verzenio), has been approved for early stage, ER+ Her2- breast cancer with high risk of recurrence.
If you haven't yet gone through menopause and have ER-positive cancer, your doctor may want to stop your ovaries from making estrogen. This can be done by:
- Aiming radiation at the ovaries
- Surgically removing the ovaries
- Taking a drug called luteinizing hormone-releasing hormone (LHRH) agonist
Your treatment may include both ovarian ablation and hormone therapy. Studies show that giving women an LHRH agonist alone or with tamoxifen is at least as effective as the chemotherapy combination used in hormone-sensitive, early breast cancer and in metastatic breast cancer of premenopausal women.