Anti-estrogens such as femara and tamoxifen, often prescribed for breast cancer, are substances that block the production of estrogens – the hormones that promote, develop and maintain female sex characteristics – or inhibit their effects. These drugs are “estrogen-receptor modulators” that block the binding of estrogen to estrogen binding sites. They are often used as adjuvant therapy to help reduce the risk of the cancer returning after surgery, or they can be used to treat cancer if it returns or spreads.
According to the American Cancer Society (ACS), estrogen promotes the growth of about two out of three breast cancers, especially those that have receptors for the hormones estrogen and progesterone. Tamoxifen and toremifene temporarily block estrogen receptors on breast cancer cells. Tamoxifen is used to treat metastatic breast cancer, as well as to reduce the risk of developing breast cancer in women at high risk. For women with hormone receptor-positive cancers, taking tamoxifen after surgery for five years reduces the chances of the cancer coming back by about half. There are side effects associated with these drugs, including fatigue, hot flashes, vaginal dryness or discharge and mood swings. Blood clots are other possible, potentially dangerous side effects.
Femara® (letrozole) is an aromatase inhibitor (AI) that blocks estrogen production in post-menopausal women. Along with anastrozole (Arimidex®) and exemestane (Aromasin®), it has been approved to treat both early and advanced breast cancer. None of these drugs are effective in stopping the ovaries of pre-menopausal women from making estrogen. Using these drugs, either alone or after tamoxifen, has been shown to better reduce the risk of the cancer coming back later than using tamoxifen alone for five years, according to the ACS. Most doctors now recommend that post-menopausal women whose cancers are hormone receptor-positive use an AI at some time during adjuvant therapy. The AIs appear to have fewer serious side effects than tamoxifen, although they may cause muscle and joint pain and stiffness. AIs remove all estrogens from women after menopause, so possible side effects include bone thinning, osteoporosis and even fractures.
The National Comprehensive Cancer Network says ongoing studies are attempting to determine if one AI is better than another. Studies are also looking at whether more than five years of an aromatase inhibitor is safe and whether more prolonged treatment can reduce breast cancer recurrence rates.