The goal of this clinical research study is to learn if Niraparib can help to control metastatic pancreatic cancer. The safety of this drug will also be studied. Niraparib is FDA approved and commercially available for the treatment of ovarian cancer. Its use in this study is investigational.

Key Inclusion Criteria:

• Screening tumor tissue analysis positive with germline or somatic mutation in genes involved in DNA repair.
• Ability of participant or Legally Authorized Representative (LAR) to understand this study, and participant or LAR willingness to sign a written informed consent.
• Able to swallow oral study drug
• Histologically or cytologically confirmed adenocarcinoma of the exocrine pancreas
• Measurable disease
• Patients with history of other, non-pancreatic cancers with no evidence of active disease are eligible.
• Participants who have had any prior chemotherapy as first line and/or second line therapy for metastatic disease are eligible to seek enrollment. Patients who refuse chemotherapy or do not tolerate chemotherapy are eligible.
• Patients must have adequate organ function
• Women must have a negative serum pregnancy test within 72 hours to taking study treatment.
• Women of child-bearing potential and men with partners of child-bearing potential must agree to practice sexual abstinence, or to use approved forms of contraception prior to study entry, for the duration of study participation, and for 180 days following completion of therapy.

Key Exclusion Criteria:

• Patients simultaneously enrolled in any therapeutic clinical trial
• Patients have had investigational therapy administered within the past 4 weeks
• Current or anticipated use of other investigational agents while participating in this study.
• Patient has had prior treatment with a known poly polymerase inhibitor
• Psychiatric illness/social situations that would limit compliance with study requirements.
• Pregnant, breast feeding or expecting to conceive children while receiving study treatment and for 180 days after the last dose of study treatment. There is a potential for congenital abnormalities and for this regimen to harm breast feeding infants.
• Patients must not have a known hypersensitivity to the components of niraparib or the excipients
• Patients must not have had major surgery within the last 3 weeks of starting the study and patient must have recovered from any effects of any major surgery
• Patients must not have had radiotherapy encompassing more than 20% of the bone marrow within 2 weeks or any radiation therapy within 1 week prior to Day 1 of protocol therapy
• Patients must not be immuno-compromised. Patients with splenectomy are allowed.
• Patients must not have received a transfusion (platelets or red blood cells) within 4 weeks of the first dose of study treatment
• Patients must not have current evidence of any condition, therapy, or laboratory abnormality that might confound the results of the study or interfere with the patient's participation for the full duration of the study treatment or that makes it not in the best interest of the patient to participate
• Patients must not have known, symptomatic brain or leptomeningeal metastases
• Patient must not have any known history of myelodysplastic syndrome (MDS) or acute myeloid leukemia (AML)