Sacituzumab Govitecan +/- Pembrolizumab In HR+ / HER2 - MBC

The research study procedures include: screening for eligibility, research blood collections, at least two research biopsies, paired research stool collections, and study treatment including evaluations and follow up visits. The names of the study interventions involved in this study are: * Sacituzumab govitecan (IMMU-132) * Pembrolizumab (Keytruda®; MK-3475) * Questionnaires/Data Collection/Sample Collection Participants will be randomized into one of two groups. Group A: Sacituzumab govitecan (IMMU-132) and Pembrolizumab Group B: Sacituzumab govitecan (IMMU-132) Participants will receive study treatment for as long they are benefiting from the therapy. Participants will be followed for the rest of their lives. It is expected that about 110 people will take part in this research study This research study is a Phase II clinical trial. Phase II clinical trials test the safety and effectiveness of an investigational drug or a combination of investigational drugs to learn whether the drug or drug combination works in treating a specific disease. "Investigational" means that the drug or drug combination is being studied. The U.S. Food and Drug Administration (FDA) has not approved Sacituzumab Govitecan for your specific disease, but it has been approved for other uses. The U.S. Food and Drug Administration (FDA) has not approved Pembrolizumab for your specific disease but it has been approved for other uses.