Pembrolizumab and Capecitabine in Treating Patients With Locally Advanced or Metastatic Triple Negative or Hormone-Refractory Breast Cancer That Cannot Be Removed by Surgery

The purpose of this study is to see whether a combination of two different drugs - pembrolizumab and capecitabine - is safe, and if it might be effective in treating triple negative and hormone-refractory breast cancer. Pembrolizumab is a type of drug that contains an antibody. Antibodies are the part of your immune system that finds things that don't belong in your body, such as bacteria or viruses. The antibody in pembrolizumab finds and blocks a protein, which allows your immune system to target and destroy cancer cells. Pembrolizumab is Food and Drug Administration (FDA) approved for other types of cancer. It is not approved for breast cancer, meaning that it is an "experimental" or "investigational" treatment. Capecitabine is a type of chemotherapy pill that is a standard treatment and FDA-approved for breast cancer. It stops the cancer cells from being able to multiply.