Study to Evaluate Sacituzumab Govitecan in Combination With Talazoparib in Patients With Metastatic Breast Cancer.

This is a Phase I/II clinical trial. You are being asked to participate in the Phase I portion of the study. A Phase I clinical trial tests the safety of an investigational drug and also tries to define the appropriate dose of the investigational drug to use for further studies. "Investigational" means that the drug is being studied. The U.S. Food and Drug Administration (FDA) has not approved sacituzumab govitecan as a treatment for any disease. The FDA has not approved talazoparib for this specific disease, but it has been approved for other uses in breast cancer. Sacituzumab govitecan is an antibody-drug conjugate which means it's made up of an antibody attached to an anticancer drug. An antibody is a protein normally made by the immune system (the system in the body that fights off diseases). Sacituzumab govitecan is believed to work by binding the antibody portion of the drug to the tumor(s) while the anticancer drug portion works to prevent the cancer cells from growing/spreading. Talazoparib belongs to a group of drugs called PARP inhibitors. PARP is a protein that is involved with repairing damaged DNA (the genetic material of cells). Talazoparib is believed to work by inhibiting (stopping) the PARP proteins from working in the cancer cells so that the cancer cannot fix its damaged DNA. The investigators believe that the combination of sacituzumab govitecan and talazoparib may help stop the cancer from growing and spreading by administering an anticancer drug directly to the cancerous tumor(s) through sacituzumab govitecan and by stopping the cancer's cells from fixing its damaged DNA through talazoparib.