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

Inclusion Criteria: * Participants must have histologically or cytologically confirmed invasive breast cancer with unresectable locally advanced or metastatic disease. Participants without pathologic or cytologic confirmation of metastatic disease should have unequivocal evidence of metastasis from physical examination or radiologic evaluation, i.e. visible chest wall disease or metastases on imaging meeting standard radiology criteria (i.e. lymph nodes larger than 1 cm in the short axis diameter). * Participants must have HR-positive, HER2-negative breast cancer (ER\>1% and/or, PR\>1%, HER2-negative per ASCO CAP guidelines) on local pathology review. If a patient has more than one histological result, the most recent sample will be considered for inclusion. * Participants must have either progressed on or within 12 months of adjuvant endocrine therapy or have progressed on at least one line of endocrine therapy for metastatic disease, and be considered appropriate candidates for chemotherapy. * Participants must have evaluable or measurable disease per RECIST 1.1. For instance, patients with bone only disease will be allowed to participate. * Participants must agree to undergo a research biopsy, if tumor is safely accessible, at baseline. Previously collected archival tissue will also be obtained on all participants. Tissue needs to be located and availability confirmed at time of registration. Participants must agree to a mandatory repeat biopsy 3-6 weeks after starting tr…