A Study of Palbociclib in Combination With Fulvestrant or Tamoxifen as Treatment for Metastatic Breast Cancer

This phase II trial studies the side effects of palbociclib when given together with fulvestrant or tamoxifen citrate in treating patients with hormone receptor positive breast cancer that has spread from where it started to nearby tissue or lymph nodes (locally advanced) or has spread to other places in the body (metastatic). Palbociclib may stop the growth of tumor cells by blocking some of the enzymes needed for cell growth. Hormone therapy using fulvestrant or tamoxifen citrate may fight breast cancer by blocking the use of estrogen by the tumor cells. Giving palbociclib together with fulvestrant or tamoxifen citrate may work better in treating hormone receptor positive breast cancer.