Enobosarm: A Targeted Therapy For Metastatic, Androgen Receptor Positive, Breast Cancer

568 Background: The androgen receptor (AR) is the most highly expressed steroid receptor in breast cancer (BC) with 75-95% of estrogen receptor positive (ER+) and 50% of ER negative BCs expressing AR. Prior studies have shown that women with metastatic BC (MBC) who are progressing on tamoxifen subsequently respond to non-tissue-selective, synthetic androgens with overall response rates of 20-60%. Unfortunately steroidal androgens have unwanted virilizing effects which limit clinical use. Enobosarm, a nonsteroidal, tissue-selective, AR modulator (SARM), provides a novel targeted approach to exploit the therapeutic benefits of AR activation without virilization or estrogenic effects. Methods: A proof of concept, phase II study is examining efficacy and safety of enobosarm 9mg daily in 22 postmenopausal women with ER+ MBC who previously responded to adjuvant and/or salvage endocrine therapy. Treatment continues until disease progression (PD). Primary endpoint is clinical benefit response (CBR) by 6 months (m...