A Phase II Randomized, Open Label Non-inferiority Study of NiraParib Maintenance After 3 vs. 6 Cycles of Platinum-based Chemotherapy in completeLy debUlked Advanced HRDpositive High-grade Ovarian Cancer patientS in First Line Therapy

Inclusion Criteria: 1. Written informed consent and obtained from the subject prior to performing any protocol-related procedures, including screening evaluations. 2. Female patient, age ≥ 18 years. 3. FIGO Stage III-IV high-grade ovarian cancer (all histological types, except mucinous histology) 4. Complete primary debulked patients (without any macroscopic residuals), confirmed by CT-Scan postoperatively. 5. Patients must have formalin-fixed, paraffin-embedded tumor samples available from the primary cancer for central NGS analysis and must be HRDpositive defined as BRCAmut independent of NOGGO GIS Score OR NOGGO GIS Score \>83 independent of BRCA status, based on these results. 6. Eastern Cooperative Oncology Group (ECOG) performance status 0 or 1. 7. Patients must be able to take oral medications. 8. Synchronous and secondary malignancies are allowed if the prognosis of the ovarian cancer is not affected. The investigator must contact the medical monitoring team before enrolling the patient in the clinical trial. 9. Patients must have normal organ and bone marrow function: 1. Hemoglobin ≥ 10.0 g/dL independent of transfusion ≤ 14 days prior to screening hemoglobin assessment 2. Absolute neutrophil count (ANC) ≥ 1.5 x 109/L 3. Platelet count ≥ 100 x 109/L 4. Total bilirubin ≤ 1.5 x institutional upper limit of normal (ULN); \< 2 × ULN if hyperbilirubinemia is due to Gilbert's syndrome 5. Aspartate aminotransferase /Serum Glutamic Oxaloacetic Transaminase (ASAT/SGOT)) and Alanine aminotransferase /Serum Glutamic Pyruvate Transaminase (ALAT/SGPT)) ≤ 2,5 x ULN 6. Serum creatinine ≤ 1.5 x institutional ULN and creatinine clearance \> 30 mL/min. 10. Postmenopausal or evidence of non-childbearing status for women of childbearing potential prior to the first dose of study treatment. Female patients of childbearing potential must have a negative serum pregnancy test result ≤3 days prior to administration of the first dose of study treatment. Patients are considered to be of childbearing potential unless 1 of the following applies: 1. Considered to be permanently sterile. Permanent sterilization includes hysterectomy, bilateral salpingectomy, and/or bilateral oophorectomy; or 2. Is postmenopausal, defined as no menses for at least 12 months without an alternative medical cause. A high follicle-stimulating hormone (FSH) level consistently in the postmenopausal range (30 mIU/mL or higher) may be used to confirm a postmenopausal state in women not using hormonal contraception or hormonal replacement therapy; however, in the absence of 12 months of amenorrhea, a single FSH measurement is insufficient to confirm a postmenopausal state. Female patients of reproductive potential must practice highly effective methods (failure rate \< 1% per year) of contraception with their partners, if of reproductive potential, during treatment and for 6 months following the last dose of chemotherapy or the last dose of niraparib, whichever occurs later, or longer if requested by local authorities. Highly effective contraception includes: Ongoing use of progesterone only injectable or implantable contraceptives; Placement of an intrauterine device (IUD) or intrauterine system (IUS); Bilateral tubal occlusion; Sexual abstinence as defined as complete or true abstinence, acceptable only when it is the usual and preferred lifestyle of the patient; periodic abstinence (e.g., calendar, symptothermal, post-ovulation methods) is not acceptable; or Sterilization of the male partner, with appropriate post-vasectomy documentation of absence of sperm in ejaculate. Exclusion Criteria: 1. Non-epithelial origin of the ovary, the fallopian tube or the peritoneum (i.e., germ cell tumors) and Ovarian tumors of low malignant potential (e.g., borderline tumors), or mucinous carcinoma of the ovary. 2. Low-grade ovarian, fallopian tube or peritoneal cancer. 3. Has known hypersensitivity to any of the study drugs or any of the excipients of any of the study drugs. 4. Has known hypersensitivity to platin-containing compounds other than carboplatin. 5. Patients posttransplant, including previous allogeneic bone marrow transplant. 6. Has undergone interval debulking of the tumor. 7. Has received any anti-cancer therapy for ovarian cancer other than primary surgery. 8. Administration of other simultaneous chemotherapy drugs, any other anti-cancer therapy or anti-neoplastic hormonal therapy, or simultaneous radiotherapy during the trial treatment period (hormonal replacement therapy is permitted as are steroidal antiemetics). 9. Has received prior treatment with a PARP inhibitor or has participated in a trial where any treatment arm included the administration of a PARP inhibitor. 10. Bevacizumab is planned to be given together with first line chemotherapy or as maintenance. 11. Clinically significant cardiovascular disease: 1. Cerebrovascular accident or myocardial infarction or unstable angina ≤6 months before start of study treatment 2. Severe cardiac arrhythmia (recent event or active or uncontrolled) 3. New York Heart Association grade ≥2 congestive heart failure 4. Uncontrolled hypertension (defined as systolic blood pressure \>140 mmHg and/or diastolic blood pressure \>90 mmHg), or history of hypertensive crisis, or hypertensive encephalopathy or posterior reversible encephalopathy syndrome 5. History of stroke or transient ischemic attack ≤6 months before start of study treatment 6. Coronary/peripheral artery bypass graft ≤6 months before start of study treatment 7. Deep vein thrombosis or thromboembolic events ≤1 month before start of study treatment 12. History or evidence of brain metastases or spinal cord compression. 13. Known history of MDS or a pre-treatment cytogenetic testing result at risk for a diagnosis of MDS/AML. 14. Current, clinically relevant bowel obstruction at the time of randomization. 15. Patients with gastrointestinal disorders likely to interfere with absorption of the study medication. 16. Pregnant or lactating women, women of child-bearing potential who do not agree to the usage of highly effective contraception methods (see inclusion criteria) starting with the screening visit through at least 6 months after the last dose of chemotherapy treatment or through at least 1 month after the last dose of niraparib, whichever occurs later. 17. Participation in another clinical study with an investigational product immediately prior to randomization. Earliest time point for randomization is after the time required for the investigational product to undergo 5 half-lives has passed. 18. Has a known history of Human Immunodeficiency Virus (HIV) infection (known HIV1/HIV2 antibodies positive) or acquired immunodeficiency syndrome (AIDS) related illness. 19. Has a known history of Hepatitis B (defined as Hepatitis B surface antigen \[HBsAg\] reactive) or known active Hepatitis C virus (defined as HCV RNA \[qualitative\] has been detected) infection. 20. Has active infection with SARS-CoV-2 (antigen test). 21. Has received a live vaccine or live-attenuated vaccine within 30 days prior to the first dose of chemotherapy treatment and while and 28 days after the last dose of trial treatment. Examples of live vaccines include, but are not limited to, the following: measles, mumps, rubella, varicella/zoster, yellow fever, rabies, BCG, and typhoid vaccine. Administration of inactivated vaccines is allowed. 22. Patient has contraindications listed in the most recent SmPC. 23. Patient who might be dependent on the sponsor, CRO, site or the investigator. 22\. In Germany: Patient who has been incarcerated or involuntarily institutionalized by court order or by the authorities § 40a S. 1 Nr. 2 AMG.