Niraparib + Dostarlimab + RT in Pancreatic Cancer

This two-stage single arm phase II trial will evaluate the efficacy of niraparib with dostarlimab and radiation therapy in patients with metastatic pancreatic cancer The research study procedures include screening for eligibility and study treatment including evaluations and follow up visits. The names of the experimental interventions involved in this study are: * Dostarlimab * Niraparib * Radiation Therapy It is expected that about 25 people will take part in this research study. An initial 15 participants will be enrolled during the first stage and evaluated for treatment disease control, if none of the initial 15 participants achieve disease control the study will be terminated. It is expected participants will be on the research study for as long as the experimental interventions are safe, and their metastatic pancreatic cancer does not progress with up to 5 years of follow up after participants stop taking the experimental interventions. This research study is a Phase II clinical trial. Phase II clinical trials test the safety and effectiveness of an investigational intervention to learn whether the intervention works in treating a specific disease. "Investigational" means that the intervention is being studied. The U.S. Food and Drug Administration (FDA) has not approved dostarlimab as a treatment for any disease. Dostarlimab is a type of antibody (a protein that attaches to other cells to fight off infection) that is believed to work by attaching to a protein called PD-1 on Tcells. This PD-1 protein controls parts of the immune system (the system in the body that fights off infections and diseases) by shutting down certain immune responses responsible for recognizing and destroying cancer cells. The investigators believe that dostarlimab will inhibit the PD-1 protein, thus allowing the immune cells to recognize and destroy cancer cells. The FDA has not approved niraparib for metastatic pancreatic cancer, but it has been approved for other uses. 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 from occurring. It is believed that the combination of dostarlimab, niraparib, and radiation therapy may have a greater effect on metastatic pancreatic cancer cells than when these interventions are used alone.