Neratinib +/- Fulvestrant in HER2+, ER+ Metastatic Breast Cancer

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. The FDA (the U.S. Food and Drug Administration) has approved Neratinib as a treatment for breast cancer. Fulvestrant has been FDA approved for treatment of metastatic hormone receptor positive breast cancer. The purpose of this research study is to determine how well neratinib, by itself or together with Fulvestrant, works in treating breast cancer that has spread to other parts of the body. Neratinib is a recently discovered oral drug that may stop breast cancer cells from growing abnormally by inhibiting (or blocking) members of a family of proteins that include Human Epidermal Growth Factor Receptor 2 (HER2). Neratinib has been used in other research studies and information from those other research studies suggests that neratinib may help to shrink or stabilize HER2-positive breast cancer in this research study.