Main Inclusion Criteria:  Histologically documented HR-positive and HER2-negative breast cancer by local testing Eastern Cooperative Oncology Group (ECOG) performance status of 0 to 1. advanced (loco regionally recurrent not amenable to curative therapy or metastatic) breast cancer. Availability of FFPE tumor block for biomarker analysis, obtained during metastatic period. HER2-E or Basal-like subtype as per central PAM50 analysis. Measurable disease or non-measurable disease, as defined by RECIST v1.1 Adequate hematologic and end-organ function Patient must be willing and able to comply with scheduled visits, treatment plans, laboratory tests, and other trial procedures. Women of childbearing potential must have confirmed negative serum pregnancy test within 7 days prior to randomization. Women of CBP must be willing to use highly effective methods of contraception.  Patient must have a 6-lead or 12-lead ECG with ALL of the following parameters at screening:  QTcF interval (QT interval using Fridericia's correction) at screening < 450 msec. Resting heart rate 50-90 beats per minute (determined from the ECG).  Main Exclusion Criteria:  Prior therapy with any CDK4/6 inhibitors. Patient has received prior treatment with chemotherapy for advanced/metastatic breast cancer