Abemaciclib for Patients With Retinoblastoma-Positive, Triple Negative 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 not approved abemaciclib as a treatment for any disease. Some triple-negative breast cancers show expression of the Rb protein and are referred to as "Rb-positive." The Rb protein is important because it controls the way that cancer cells divide and grow. Drugs like abemaciclib work by changing the way that Rb functions. This can potentially stop cancer cells from dividing, and can also potentially lead to cancer cell death. In this research study, the investigators are are looking to see how safe abemaciclib is and how well it will work to help people with triple-negative breast cancer that is Rb-positive.