A Study of Palbociclib in Combination With Bazedoxifene in Hormone Receptor Positive Breast Cancer

This research study is a Phase I/II clinical trial. Phase II clinical trial tests the safety and effectiveness of an investigational intervention to learn whether the intervention works in treating a specific disease. "Investigational" means that the intervention is being studied. It also means that the FDA (the U.S. Food and Drug Administration) has not yet approved Palbociclib in combination with Bazedoxifene for use in participants with your type of cancer, but it has been approved for other uses (metastatic breast cancer). Palbociclib is a drug that may stop cancer cells from growing. Palbociclib blocks activity of two closely related enzymes (proteins that help chemical reactions in the body occur), called Cyclin D Kinases 4 and 6 (CDK 4/6). These proteins are part of a pathway, or a sequence of steps which is known to control cell growth. Laboratory testing has shown that palbociclib may stop the growth of hormone receptor positive breast cancer. Endocrine therapy prevents breast cancer cell growth by blocking the activity of the estrogen receptor. During this study the endocrine therapy will be bazedoxifene. In this research study, the investigators are evaluating how safe palbociclib is and how well palbociclib in combination with bazedoxifene, a form of endocrine treatment, works in participants with a history of stage four breast cancer.