A multicenter prospective study to evaluate bone fracture related to adjuvant anastrozole in Japanese postmenopausal women with breast cancer: two-year interim analysis of Saitama Breast Cancer Clinical Study Group (SBCCSG-06)

Background Because of its superior efficacy to tamoxifen, anastrozole has been widely used in Japan as an adjuvant treatment for postmenopausal, hormone-responsive breast cancer patients. However, anastrozole may affect bone in Japanese patients similar to its effects in Western patients. The aim of this study is to evaluate the rate of bone fracture and bone mineral density (BMD) during anastrozole treatment in Japanese patients. Patients and methods In this study, 350 postmenopausal women with hormone-responsive, stage I to IIIA breast cancer were enrolled and scheduled to receive adjuvant anastrozole treatment for up to 5 years. Patients underwent clinical examination for bone fractures and annual measurement of BMD during treatment. Results After a median follow-up of 33.0 months, bone fractures occurred in 1.8 %. Annual fracture rates were 0.3 and 1.2 % during the first and second year, respectively. The overall median BMD significantly decreased, measuring 87.5, 84.3, and 83.5 % at baseline and after 1 and 2 years, respectively. Musculoskeletal disorders were the most common (26.1 %), and hot flashes were the second most common adverse event (7.9 %). Severe adverse events occurred in 5.5 % of all the cases. Conclusions In this interim analysis, the bone fracture rate was lower than that in the Western population despite a significant reduction of BMD after 2 years of treatment with anastrozole. Adjuvant anastrozole treatment was well tolerated in Japanese postmenopausal women with breast cancer. Long-term follow-up data is necessary to elucidate the racial disparities of the safety profile of anastrozole.