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

This phase II trial studies the side effects and how well palbociclib and letrozole or fulvestrant works in treating patients aged 70 years and older with estrogen receptor positive, HER2 negative breast cancer that has spread to other places in the body. Palbociclib may stop the growth of tumor cells by blocking some of the enzymes needed for cell growth. Drugs used in chemotherapy, such as letrozole or fulvestrant, work in different ways to stop the growth of tumor cells, either by killing the cells, by stopping them from dividing, or by stopping them from spreading. Giving palbociclib and letrozole or fulvestrant may work better in treating patients with breast cancer. The trial will explore factors other than chronologic age that can affect toxicity rates as identified using a cancer-specific geriatric assessment.