Niraparib + Dostarlimab In BRCA Mutated Breast Cancer

The research study procedures include screening for eligibility and study treatment including laboratory evaluations, two mandatory research biopsies, imaging assessments, and follow up visits. Participants will receive treatment for 18 weeks. After 18 weeks, participants will be evaluated to determine if a candidate for surgery or if additional treatment outside of the study. Participants with triple negative breast cancer will be randomized to one of two treatment arms. * Arm A: Niraparib with Dostarlimab for 18 weeks * Arm B: Niraparib alone for 3 weeks, followed by Niraparib with Dostarlimab for 15 weeks Participants with estrogen receptor positive breast cancer will be placed directly into Arm C. There is no randomization for these participants. \- Arm C: Niraparib with dostarlimab for 18 weeks It is expected that about 62 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 drug combination to learn whether the drug combination works in treating a specific disease. "Investigational" means that the study drugs, Niraparib and Dostarlimab, are being studied for use in this setting and the research doctors are trying to learn more about the drug combination-the side effects the combination may cause and if it is effective in treating this type of cancer. The U.S. Food and Drug Administration (FDA) has not yet approved either of the drugs in this study for your type of cancer. Niraparib has been approved by the FDA for treatment of advanced ovarian cancer in BRCA mutation carriers. The use of Dostarlimab in this research study is experimental, which means that it is not approved by any regulatory auit is not approved by any regulatory authority, including the FDA, for treatment of breast cancer, or any other disease.