This study is a multicenter, randomized, open-label, two treatment arm, efficacy and safety study. Subjects will be randomized to the two treatment arms in a 1:1 fashion.~The primary efficacy endpoint of the study will be the median PFS by RECIST v1.1.~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.
To demonstrate the efficacy of sabizabulin in the treatment of ER+HER2- metastatic breast cancer (MBC) as measured by progression free survival (PFS) by RECIST v1.1.