Recurrent Ovarian CarcinoSarcoma Anti-pd-1 Niraparib

Inclusion Criteria: 1. Progressive or recurrent uterine carcinosarcoma (Malignant Mixed Mullerian Tumor-MMMT). 2. The primary diagnosis must be histologically confirmed by pathological expert review of the initial tumor or biopsy at relapse. 3. Mandatory tumor samples: Availability of an archival FFPE tumor sample(s) from diagnosis, or if not available from relapse setting. 4. Progressive disease as defined by RECIST 1.1. 5. Failure after ≥1 prior platinum containing regimen, which may have been given in the adjuvant setting. 6. Patient must have had 1 prior chemotherapeutic regimen for management of carcinosarcoma that may have included chemotherapy, chemotherapy and radio-chemotherapy, and/or consolidation/maintenance therapy. 7. Patient must be free of active infection requiring antibiotics. 8. Any hormonal therapy directed at the malignant tumor must be discontinued at least one week prior to beginning protocol treatment; continuation of hormone replacement therapy is permitted. 9. Patient must have ECOG Performance Status ≤1. 10. Life expectancy of \> 2 months. 11. Adequate bone marrow function: * Platelet count greater than or equal to 100,000/mm3 * Absolute neutrophil count (ANC) greater than or equal to 1,500/mm3 * Hemoglobin \> 9g/dL 12. Adequate hepatic and renal function: * Total bilirubin ≤1.5x Upper Limit of Normal (ULN) unless liver metastases are present, in which case they must be ≤3x ULN (≤2.0 in patients with known Gilberts syndrome OR direct bilirubin ≤ 1 x ULN) * Serum creatinine ≤1.5x upper limit of normal (ULN) or calculated creatinine clearance ≥ 60 mL/min using Cockcroft-Gault equation * Aspartate aminotransferase (AST) and alanine aminotransferase (ALT) ≤2.5x ULN unless liver metastases are present, in which case they must be ≤5x ULN * Alkaline phosphatase \< 2.5 times ULN * Serum albumin \> 3 g/dL 13. International normalized ratio (INR) or prothrombin time (PT) ≤1.5× ULN unless patient is receiving anticoagulant therapy as long as PT or partial thromboplastin (PTT) is within therapeutic range of intended use of anticoagulants. Activated partial thromboplastin time (aPTT) ≤1.5× ULN unless patient is receiving anticoagulant therapy as long as PT or PTT is within therapeutic range of intended use of anticoagulants. 14. Patient must have normal BP or adequately treated and controlled hypertension (systolic BP≤140 mmHg and/or diastolic BP ≤90 mmHg) 15. Patient receiving corticosteroids may continue as long as their dose is stable and ≤10mg/day (prednisone equivalent) for at least 4 weeks prior to initiating protocol therapy. 16. Patient must agree to not donate blood during the study or for 90 days after the last dose of study treatment. 17. Patient has a negative urine or serum pregnancy test within 72 hours prior to taking study treatment if of childbearing potential and agrees to abstain from activities that could result in pregnancy from screening through 4 monhs after the last dose of study treatment, or is of nonchildbearing potential. * Non-childbearing potential is defined as follows: * ≥45 years of age and has not had menses for \>1 year * Patients who have been amenorrhoeic for \<2 years without history of a hysterectomy and oophorectomy must have a follicle stimulating hormone value in the postmenopausal range upon screening evaluation * Post-hysterectomy, post-bilateral oophorectomy, or post-tubal ligation. Documented hysterectomy or oophorectomy must be confirmed with medical records of the actual procedure or confirmed by an ultrasound. Tubal ligation must be confirmed with medical records of the actual procedure. * For women of childbearing potential: the patient must be willing to use a highly effective contraception measure throughout the study, starting with the screening visit through 4 months after the last dose of study treatment. See Section 4.3. for a list of highly effective contraception methods. Information must be captured appropriately within the site's source documents. Note: Abstinence is acceptable if this is the established and preferred contraception for the patient. 18. Patient must agree to not breastfeed during the study and for 4 months after the last dose of study treatment. 19. Patient able to take oral medications. 20. Female aged ≥18 years at time of signing ICF. 21. Patient must have signed an approved informed consent. 22. For France only: patient affiliated to, or a beneficiary of, a social security category. Exclusion Criteria: 1. Not enrolled in any interventional clinical trial (except to biological trials that must be validated by the sponsor) 2. Prior treatment with niraparib or other PARPi therapy or PD1/PDL-1 inhibitors. 3. Patient has had investigational therapy, immunotherapy, chemotherapy or biological therapy administered within 4 weeks or within a time interval less than at least 5 half-lives of the investigational agent, whichever is longer, prior to treatment initiation. Patient has had radiotherapy within 4 weeks prior to treatment initiation. 4. Patients must not have had major surgery ≤ 3 weeks prior to initiating protocol therapy and participant must have recovered from any surgical effects 5. Patient who has received more than 3 prior cytotoxic chemotherapies for management of uterine carcinosarcoma. 6. Patient with persistent, clinically significant \> Grade 1 toxicity. 7. Patient has clinically significant cardiovascular disease (eg, significant cardiac conduction abnormalities, uncontrolled hypertension, myocardial infarction, uncontrolled cardiac arrhythmia or unstable angina \< 6 months to enrollment, NYHA grade 2 or greater congestive heart failure, serious cardiac arrhythmia requiring medication, Grade 2 or greater peripheral vascular disease, and history of cerebrovascular accident within 6 months) 8. Patient with any other severe concurrent disease, which may increase the risk associated with study participation or study drug admin