Inclusion Criteria:  Adult women (≥ 18 years of age) with proven diagnosis of adenocarcinoma of the breast with evidence of locoregionally recurrent or metastatic disease not amenable to resection or radiation therapy with curative intent and for whom chemotherapy is not clinically indicated. Documentation of histologically or cytologically confirmed diagnosis of estrogen-receptor positive (ER+, > 10%) breast cancer based on local laboratory results. Previously untreated with any systemic anti-cancer therapy for their locoregionally recurrent or metastatic ER+ disease. Postmenopausal women. At least one measurable lesion as defined per RECIST v.1.1. Eastern Cooperative Oncology Group (ECOG) performance status (PS) 0-2. Adequate organ and marrow function. Willingness and ability to comply with scheduled visits, treatment plan, laboratory tests, and other study procedures.  Exclusion Criteria:  Patients with advanced, symptomatic, visceral spread, that are at risk of life-threatening complications including any of the following:  massive uncontrolled effusions [pleural, pericardial, peritoneal] pulmonary lymphangitis, over 50% liver involvement Known active uncontrolled or symptomatic CNS metastases, carcinomatous meningitis,or leptomeningeal disease as indicated by clinical symptoms, cerebral edema, and/or progressive growth. Prior neoadjuvant or adjuvant treatment with a non-steroidal aromatase inhibitor (ie, anastrozole or letrozole) with disease recurrence while on or within 12 months of completing treatment. Prior treatment with any CDK4/6 inhibitor. QTc >480 msec (based on the mean value of the triplicate ECGs), family or personal history of long or short QT syndrome, Brugada syndrome or known history of QTc prolongation, or Torsade de Pointes (TdP). Female patients who are pregnant or nursing.