Abstract S3-01: Breast Cancer Prevention Using Anastrozole in Postmenopausal Women at High Risk

Abstract Background: Third generation aromatase inhibitors are the most effective endocrine treatment for hormone receptor positive breast cancer in postmenopausal women. Here, we assess the efficacy of anastrozole in postmenopausal women who do not have breast cancer, but are at high risk of developing the disease. Methods: A multi-centre randomised placebo-controlled trial of 1mg/day oral anastrozole vs. matching placebo for five years was conducted in 3864 postmenopausal women at increased risk of breast cancer. The primary endpoint was the incidence of breast cancer (including ductal carcinoma in-situ (DCIS) and differences were assessed by the proportional hazards model. Detailed information on adverse events was collected. Results: After a median follow up of 5.03 years, 125 breast cancers were recorded. A 53% reduction (95% CI (32-68%), P<0.0001) was seen in the anastrozole arm. Significant reductions were seen for all invasive (50%), oestrogen receptor positive invasive (58%) and in situ tumours (70%). Fractures were non-significantly higher (8.5% vs. 7.7%, P = 0.3) and musculoskeletal events were significantly higher in the anastrozole arm (1226 vs. 1124, RR = 1.10 (1.05-1.16)) but were very common in both arms (63.9% vs. 57.8%). Vasomotor symptoms were also increased with anastrozole (RR = 1.15 (1.08-1.22)). Cancers at other sites were significantly decreased (40 vs. 70, RR = 0.58 (0.39-0.85)). Deaths from breast cancer and other causes were similar in both arms. Conclusions: Anastrozole is an effective agent for reducing breast cancer incidence in postmenopausal women at high risk. Anastrozole was well tolerated and side effects associated with oestrogen deprivation were only slightly higher than for placebo. Citation Information: Cancer Res 2013;73(24 Suppl): Abstract nr S3-01.