Maintenance Niraparib Plus Ipilimumab in Patients With Metastatic Pancreatic Adenocarcinoma Whose Disease Has Not Progressed on Platinum-Based Chemotherapy

Inclusion Criteria: * Histologically or cytologically confirmed diagnosis of pancreatic adenocarcinoma with metastatic disease * ≥18 years of age * Patients must be able to understand the study procedures and agree to participate in the study by providing written informed consent * Participants must have received 8-12 cycles (4-6 months) of first-line FOLFIRINOX or modified FOLFIRINOX for metastatic disease with stable disease or better. Patients treated with liposomal irinotecan with oxaliplatin, 5-fluorouracil and leucovorin (NALIRIFOX) are also eligible. Patients who were initially treated with FOLFIRINOX or NALIRIFOX but stopped oxaliplatin because of toxicity are eligible for the trial. * Note: This requires at least stable imaging and a stable or decreasing tumor marker as applicable and as determined by the investigator. * Measurable disease is not a requirement for study entry. * Note: The study will require that at least 80% of enrolled patients (ie 55 of all patients) are biopsiable at enrollment. The investigators may require measurable/biopsiable disease as the study progresses in order to achieve this goal. * Participants must be willing to undergo a pre-treatment fresh tumor biopsy (if medically feasible). * Participants must be willing to undergo an on-treatment tumor biopsy (if medically feasible). * Female participant has a negative serum pregnancy test within 24 hours prior to taking study treatment if of childbearing potential and agrees to abstain from activities that could result in pregnancy from screening through 6 months (females) or 30 days (males) after the last dose of study treatment, or is of nonchildbearing potential. * Male patient agrees to use an adequate method of contraception starting with the first dose through 90 days after the last dose of study treatment. * Adequate organ function confirmed by the following laboratory values obtained ≤7 days prior to the first day of study therapy: * Absolute neutrophil count (ANC) ≥1.5 x 109/L * Platelets\>100 x 109/L * Hemoglobin ≥9g/dL * Aspartate aminotransferase (AST) and alanine aminotransferase (ALT) ≤2.5 x upper limit of normal (ULN); if liver metastases, then ≤5 x ULN * Total bilirubin ≤1.5 x ULN; if liver metastases or metabolic disorder such as Gilbert's syndrome, then ≤2.5 x ULN. * Serum creatinine ≤1.5 x ULN or estimated glomerular filtration rate (GFR) ≥45 mL/min using Cockcroft Gault formula. * Eastern Cooperative Oncology (ECOG) performance status of 0 to 1. Exclusion Criteria: * Prior treatment with a PARP inhibitor, ipilimumab, or other cytotoxic T-lymphocyte-associated-4 protein (CTLA-4) inhibitor. * Patients who have demonstrated resistance to FOLFIRINOX are not eligible to participate in this study * Patients with known pathogenic/likely pathogenic germline or somatic alteration(s) in BRCA1, BRCA2, PALB2, RAD51C, or RAD51D. * Patients with known mismatch repair deficiency or microsatellite instability-high cancer. * Clinical evidence of uncontrolled malabsorption and/or any other gastrointestinal disorder or defect that would, in the opinion of the investigator, interfere with the absorption of niraparib * Patients with uncontrolled hypertension, defined as systolic BP \>140mmHg and/or diastolic BP \>90mmHg * Patients with a prior history of posterior reversible encephalopathy syndrome (PRES) * Acute infection requiring intravenous antibiotics, intravenous antiviral or intravenous antifungal agents during the 14 days prior to first dose of study therapy * Patients will be excluded if they have a history of or active autoimmune disease, defined as: patients with a history of inflammatory bowel disease are excluded from this study, as are patients with a history of symptomatic autoimmune disease (e.g. rheumatoid arthritis, systemic progressive sclerosis (scleroderma), systemic lupus erythematosus, autoimmune vasculitis e.g. Wegener's Granulomatosis); motor neuropathy considered of autoimmune origin (e.g. Guillain-Barre Syndrome). * Note: Patients are permitted to enroll if they have vitiligo, type I diabetes mellitus, residual hypothyroidism due to autoimmune condition only requiring hormone replacement, psoriasis not requiring systemic treatment, or conditions not expected to recur in the absence of an external trigger. * Has a history of interstitial lung disease or active, non-infectious pneumonitis * Has received a live vaccine within 4 weeks prior to the first dose of trial therapy * Note: seasonal influenza vaccines for injection are generally inactivated and are allowed; however intranasal influenza vaccines (e.g. Flu-Mist®) are live attenuated vaccines and are not allowed. * For fertile patient (female able to become pregnant or male able to father a child), refusal to use effective contraception during the period of the trial and: * Female patients refusing to use effective contraception for 6 months after the last dose of study drug. * Male patients refusing to use effective contraception for 90 days after the last dose of study drug. * Received any systemic treatment for pancreatic cancer ≤14 days prior to first dose of therapy. Patients must not have had investigational therapy administered ≤ 4 weeks, or within a time interval less than at least 5 half-lives of the investigational agent, whichever is longer, prior to the first scheduled day of dosing in this study. * Patients will be excluded if they have a condition requiring systemic treatment with either corticosteroids (\>10mg daily prednisone equivalents) or other immunosuppressive medications within 14 days of study drug administration. Inhaled or topical steroids and adrenal replacement doses \>10mg daily prednisone equivalents are permitted in the absence of active autoimmune disease. * Patient has had any known Grade 3 or 4 anemia, neutropenia or thrombocytopenia due to prior chemotherapy that persisted \> 4 weeks and was related to the most recent treatment. * Non-study related minor surgical