Multicenter Observational Study of Fulvestrant 500 mg in Postmenopausal Japanese Women with Estrogen Receptor-Positive Advanced or Recurrent Breast Cancer after Prior Endocrine Treatment (SBCCSG29 Study).

Background: Fulvestrant 500 mg has been an option for endocrine therapy for advanced or recurrent breast cancer after prior endocrine treatment since November 2011 in Japan. This study aimed to clarify the effectiveness and safety of fulvestrant 500 mg in clinical settings.

Methods: This was a multicenter, both prospective and retrospective, observational study of 132 postmenopausal women (median age 66) with locally advanced or metastatic breast cancer, who had been treated with fulvestrant. Information from medical records was retrospectively obtained from 9 hospitals (Saitama Breast Cancer Clinical Study Group: SBCCSG) in Saitama prefecture, Japan, from October 2012 to April 2014. The primary end point was time to treatment failure (TTF). The secondary end points were overall survival (OS), objective response rate (ORR), clinical benefit rate (CBR), and adverse events (AE) (CTCAE ver. 4). The choice of subsequent therapy after fulvestrant was also evaluated.

Results: The median TTF was 6.1 months. Median OS was 28.5 months (the starting date was the first day of fulvestrant). ORR was 12.9% and CBR was 45.5%. The most common AEs were injection site reactions (9.1%). The rate of grade 3 AE was only 2.3% (3/132). The number of patients who underwent subsequent therapy after fulvestrant were 54 (55.7%) receiving chemotherapy, 42 (43.3%) receiving non-fulvestrant endocrine therapy, and 1 (1%) receiving mammalian target of rapamycin inhibitor (mTORi) + endocrine therapy (ET).

Conclusion: Fulvestrant 500 mg is an effective and safe treatment for patients with advanced or recurrent breast cancer after prior endocrine treatment.