Efficacy & Safety Evaluation of Enobosarm in Combo With Abemaciclib in Treatment of ER+HER2- Metastatic Breast Cancer

STAGE 1: This is an open-label safety study of enobosarm 9 mg QD coadministered with a CDK 4/6 inhibitor (abemaciclib), 150 mg BID. STAGE 2: This study is a multicenter, randomized, open-label, two treatment arm, efficacy and safety study. Subjects will be randomized to the two treatment arms (Enobosarm Combination Group versus Control Treatment Group) in a 1:1 fashion. The determination of the treatment to be used in the control arm will be declared prior to randomization. If first line of therapy for metastatic breast cancer was a non-steroidal AI plus palbociclib, then the patient will be randomized to either enobosarm + abemaciclib OR fulvestrant. If first line of therapy for metastatic breast cancer was fulvestrant plus palbociclib, then the patient will be randomized to either enobosarm + abemaciclib OR AI (steroidal or non-steroidal). If the patient is randomized to the Control Treatment Group to receive steroidal AI, (exemestane) the patient may receive exemestane with or without everolimus. The primary efficacy endpoint of the study will be the median PFS as defined by RECIST 1.1. 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. Long term survival follow up - every 30 days after last dose of study drug for 1 year and then every 90 days thereafter.