We have previously discussed the use of tamoxifen for the prevention of breast cancer in women. Raloxifene, a medication similar to tamoxifen, is also used as a chemopreventative for the development of breast cancer in women. Both tamoxifen and raloxifene are selective estrogen receptor modulators (SERMs), and work by antagonizing the estrogen receptor in some tissues while agonizing it in others. Physicians can now add a third drug to their armamentarium of medications that can be used to prevent breast cancer in women. The third drug is exemestane (Aromasin) and works by a different mechanism of action. Exemestane is an aromatase inhibitor and blocks the conversion of androgens into estrogens. Lower levels of estrogen have been hypothesized to help prevent the development of breast cancer by decreasing the amount of stimulation that estrogens exert on breath tissue. Data presented today by Dr. Paul Goss and his research team in Chicago at the American Society of Clinical Oncology meeting show that exemestane reduces the risk of developing breast cancer by 65% in high risk postmenopausal women. In the research study, the authors conducted a double blind randomized clinical trial of exemestane versus placebo in 4,560 women. Eligible women were considered to be high risk by having one of the following characteristics: Gail score >1.66%, prior atypical ductal hyperplasia, prior atypical lobular hyperplasia, prior lobular carcinoma in-situ, prior ductal carcinoma in-situ with mastectomy, or age over 60. At a median follow up of 35 months, there were 11 invasive breast cancers found in study participants taking exemestane versus 32 in the placebo group. This is a remarkable result and is better than both tamoxifen and raloxifene for the prevention of breast cancer. Tamoxifen has been shown to decrease breast cancer risk by 50%, and raloxifene cuts risk by 38%. The study authors noted that the much milder side effect profile of exemestane makes it a better choice, compared to tamoxifen and raloxifene. The study authors concluded, “Exemestane significantly reduced invasive and pre-invasive breast cancers in postmenopausal women at increased risk for breast cancer with no serious toxicities. Exemestane should be considered a new option for primary prevention of breast cancer.” This is exciting information and will add to the possible options for preventing breast cancer in postmenopausal women. Longer duration studies are needed to determine if exemestane can also be shown to decrease the death rate from breast cancer as well. Some critics have expressed concern regarding the short duration of the study, and the lack of data to show a mortality benefit. In any event, this is one step further in the battle against cancer.