Palbociclib and Letrozole or Fulvestrant in Treating Patients With Estrogen Receptor Positive, HER2 Negative Metastatic Breast Cancer

PRIMARY OBJECTIVES: I. To estimate the safety and tolerability (adverse event rate) of the combination of palbociclib and letrozole or fulvestrant in adults age 70 or older with estrogen receptor-positive, HER2-negative metastatic breast cancer. SECONDARY OBJECTIVES: I. To describe the full toxicity profile including all grade 2 and higher adverse events (per National Cancer Institute \[NCI\] Common Terminology Criteria for Adverse Events \[CTCAE\] version \[v.\] 5.0), specifically estimating the rate of grade 2 and higher myelosuppression (neutropenia, leukopenia, thrombocytopenia, and anemia), neutropenic fever, gastrointestinal (GI) side effects (nausea, diarrhea, decreased appetite, vomiting, mucositis-oral), fatigue, neuropathy, and thromboembolism. II. To describe rates of dose reductions, dose holds, and hospitalizations. III. To estimate median time to treatment failure, including progression free survival and overall survival. IV. To estimate the rate of adherence to palbociclib, letrozole and fulvestrant. V. To explore factors other than chronologic age that can affect toxicity rates as identified using a cancer-specific geriatric assessment. VI. To describe the results of the Overall Treatment Utility (OTU). VII. To determine the degree of agreement between patient-reported adverse events (AEs) using Patient Reported Outcomes (PRO)-CTCAE measures and those reported using traditional collections for AEs. VIII. To examine the association between sarcopenia and the development of toxicity and adverse events. OUTLINE: Patients receive palbociclib orally (PO) once daily (QD) on days 1-21. Patients also receive letrozole PO QD on days 1-28 or fulvestrant intramuscularly (IM) on days 1 and 15 of course 1 and on day 1 of subsequent courses per Doctor of Medicine (MD) discretion. Courses repeat every 28 days in the absence of disease progression or unacceptable toxicity. After completion of study treatment, patients are followed up for up to 5 years.