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Does Exemestane Reduce the Risk of Breast Cancer?

A recent, well-conducted placebo-controlled trial (NCIC CTG MAP.3) involving 4560 healthy postmenopausal women found that the aromatase inhibitor exemestane reduced the incidence of a diagnosis of invasive breast cancer by 65%. Women in this trial (median age, 62 years) were at moderately increased risk for breast cancer, based on established criteria. Importantly, exemestane also reduced the risk of known breast-cancer precursor lesions.

This trial might prompt health professionals and patients to ask where we are in breast cancer chemoprevention, as opposed to treatment of diagnosed cases. Tamoxifen and its sister drug, raloxifene, have shown efficacy for chemoprevention, but are rarely used. Less than 0.1% of all women in the US who are 40-79 years of age have accepted the use of tamoxifen for prevention of breast cancer. Such limited use is in part related to fears of severe toxicity; such as venous thromboembolism; however, serious adverse effects are rare.

Of the aromatase inhibitors (anastrozole, letrozole, exemestane), only exemestane has proven clinical-trial efficacy for breast-cancer prevention in postmenopausal women, and is the preferred aromatase inhibitor for this indication. Indirect comparisons of randomized prevention trials suggest the greater efficacy of exemestane compared to either tamoxifen or raloxifene. In the NCIC CTG MAP.3 trial, no serious toxic effects were observed with exemestane over 3 years of follow-up.

Breast cancer is the second most frequent cause of death from cancer. An editorial accompanying the exemestane trial aptly states: "We have the knowledge and tools to reduce its (breast cancer) incidence today.  We have run out of excuses. What are we waiting for?