Phase 3 Trials of Aromatase Inhibitors for Breast Cancer Prevention
The third-generation aromatase inhibitors (AIs), anastrozole, exemestane, and letrozole are potential agents for preventing estrogen receptor (ER)-positive breast cancer (BC) in high-risk postmenopausal women. No AI has yet been fully evaluated for prevention. Each AI is incorporated into the design of a phase 3 randomized BC prevention trial based on hypothesis-generating contralateral breast cancer (CLBC) data from a corresponding adjuvant trial. Arimidex, tamoxifen alone or in combination (ATAC) pitted anastrozole against tamoxifen for 5 years as initial adjuvant therapy, showing at 33.3 months fewer CLBCs with anastrozole versus tamoxifen (odds ratio [OR] 0.42 overall; OR 0.29 ER-positive BCs), offering the hypothesis on which IBIS-II (International Breast Cancer Intervention Study) is based. High-risk IBIS-II compares anastrozole to placebo for the primary prevention of BC in 6000 postmenopausal women. IES (Intergroup Exemestane Study) compared exemestane to tamoxifen following 23 years of adjuvant tamoxifen in 4742 postmenopausal women with ER-positive BCs. The benefit from switching to exemestane in reducing CLBCs (HR 0.44 at 30.6 months) underlies MAP.3 (Mammary Prevention 3), which compares exemestane to placebo for primary BC risk reduction in 4560 postmenopausal women. MA.17 showed reduced CLBC incidence (HR 0.57) at 2.4 years in postmenopausal women with receptor-positive tumors receiving extended adjuvant letrozole compared to placebo following 5 years of tamoxifen. The Study of Letrozole and Raloxifene (STELLAR), using as the control raloxifene, the new U.S. standard drug for BC prevention, would complete the trio of AI prevention trials, but STELLAR is currently on hold.
breast cancer prevention aromatase inhibitor (ai) contralateral breast cancer (clbc) arimidex tamoxifen alone or in combination (atac) selective estrogen receptor modulator (serm) tamoxifen (tam)