Radiation, Immunotherapy and PARP Inhibitor in Triple Negative Breast Cancer

This research study is looking to see whether the combination of Dostarlimab and Niraparib plus Radiation Therapy (RT) is safe and effective in participants with metastatic triple negative breast cancer. The names of the study treatment involved in this study are: * Dostarlimab * Niraparib * Radiation Therapy (RT), which is given per standard of care.

This is an open-label, phase II study that will evaluate how safe and well the combination of niraparib, dostarlimab, and Radiation Therapy (RT) works in metastatic triple negative breast cancer. Niraparib is a type of drug called a "PARP inhibitor", which blocks DNA (the genetic material of cells) damage from being repaired or may prevent damage from occurring in the first place. In cancer treatment, inhibiting PARP may help kill cancer cells by not allowing the cancer cells to repair its DNA damage or prevent DNA damage associated with metastatic triple negative breast cancer from occurring. Dostarlimab is a type of immunotherapy. It is believed to work by inhibiting (stopping) a protein called PD-1 from working. The PD-1 protein controls parts of the immune system (the system in a person's body that fights against diseases) by shutting down certain immune responses that are responsible for recognizing and destroying cancer cells. The investigators believe that dostarlimab may inhibit the PD-1 protein on triple negative breast cancer cells, thus allowing the immune cells to recognize and destroy cancer cells.Radiation therapy is a standard-of-care treatment that is given to stop the growth of tumors. Radiation therapy can also stimulate the immune system, which leads to the destruction of tumor cells in the treated areas. Combining radiation therapy with anti-cancer drugs like dostarlimab and niraparib may increase the ability of the immune system to control or destroy cancer cells throughout the body. The research study procedures include screening for eligibility and study treatment including evaluations and follow up visits. Participants will receive the study drugs for up to 2 years or until their triple negative breast cancer worsens Participants will then be followed for up to 5 years. It is expected that about 32 people will take part in this research study. The FDA has not approved niraparib for metastatic triple negative breast cancer, but it has been approved for other uses. The U.S. Food and Drug Administration (FDA) has not approved dostarlimab as treatment for any disease.