Palbociclib After CDK and Endocrine Therapy (PACE)

This research study is a Phase II clinical trial. Phase II clinical trials test 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 and the researchers are trying to find out more about it- for example, the side effects it may cause, and the activity of a drug, or combination of drugs, against a cancer. In this research study, the investigators are evaluating the activity of fulvestrant alone, fulvestrant and palbociclib, or fulvestrant, palbociclib, and avelumab combined, in participants with metastatic hormone receptor positive HER2 negative breast cancer that has previously stopped responding to prior palbociclib and endocrine therapy. The FDA (the U.S. Food and Drug Administration) has approved both palbociclib and fulvestrant as treatment options for this disease, however the use of palbociclib has not been studied in people who have previously been treated with palbociclib. The FDA has not approved avelumab as a treatment for any disease. 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 Dependent Kinases 4 and 6 (CDK 4/6). These proteins are part of a pathway, or a sequence of steps, which is known to promote cancer cell growth. Laboratory testing has shown that palbociclib may stop the growth of hormone receptor positive breast cancer. Palbociclib is FDA-approved as therapy for metastatic hormone receptor positive HER2 negative breast cancer in combination with endocrine therapy in the first line setting, and in combination with fulvestrant for pre-treated disease. It is not known whether cancers that have grown despite prior palbociclib treatment are still sensitive to palbociclib. Endocrine therapy prevents growth of hormone receptor positive breast cancer by blocking stimulation of cancer cells by estrogen. During this study, the endocrine therapy will be fulvestrant. Fulvestrant is a drug that is approved by the FDA for treatment of metastatic hormone receptor positive breast cancer. The immune system is the body's natural defense against disease. The immune system sends a type of cells called T cells throughout the body to detect and fight infections and diseases-including cancers. One way the immune system controls the activity of T cells is through the PD-1 (programmed cell death protein-1) pathway. However, some cancer cells hide from T-cell attack by taking control of the PD-1 pathway and this stops T cells from attacking cancer cells. Avelumab is an antibody designed to block the PD-1 pathway and helps the immune system in detecting and fighting cancer cells. An antibody is a protein produced by the body's immune system when it detects harmful substances. Previous studies show that the administration of antibodies which block the PD-1 pathway can lead to tumor destruction. In the laboratory, adding avelumab to fulvestrant and palbociclib appears to improve effectiveness. It is not known whether this is true in humans