This study is a multicenter, randomized, open-label, two treatment arm, efficacy and safety study, comprised of two sections, the main study and the post-study extension period. Subjects will be randomized to the two treatment arms in a 1:1 fashion (into the main study), with the opportunity for subjects initially randomized to the Control Treatment Group to cross over to receive the investigational treatment in the post-study extension period.~The primary efficacy endpoint of the study will be the median rPFS. Subjects will continue study treatment until disease progression confirmed by blinded independent central reader (BICR) is observed. A safety follow up visit will occur approximately 30 days after last dose of study drug. Thereafter, survival follow up will be completed monthly for one year. Survival follow up may be completed by phone or records review. After one year, survival follow up will be completed every 90 days.~After radiographic progression (by RECIST 1.1) is confirmed by blinded independent central reader (BICR) and have received approval from Medical Monitor, subjects in the Control Treatment Group may be crossed over to receive enobosarm treatment (9mg per day). Treatment will continue in this population (Enobosarm Post-study Group (EPG)) until radiographic progression (by RECIST 1.1), confirmed by BICR, is observed. The efficacy database for this crossover group will be completely separate from the main portion of the study and the data will be analyzed separately.
To demonstrate the efficacy of enobosarmin the treatment of androgen receptor positive (AR+) and estrogen receptor positive (ER+) metastatic breast cancer (MBC) as measured by radiographic progression free survival (rPFS).