Japan Phase 2 Study of Niraparib (Maintenance Therapy) in Participants With Relapsed Ovarian Cancer

Inclusion Criteria 1. Japanese female participants aged 20 years or older on the day of signing informed consent. 2. Voluntary written consent must be given before performance of any study related procedure not part of standard medical care, with the understanding that consent may be withdrawn by the participant at any time without prejudice to future medical care. 3. Participant must have a histologically diagnosed ovarian cancer, fallopian tube cancer, or primary peritoneal cancer. 4. Participant must have a high-grade (or Grade 3) serous or high-grade predominantly serous histology or known to have germline breast cancer gene mutation (gBRCAmut). 5. Participants must have completed at least 2 previous lines of platinum-containing therapy (eg, carboplatin, oxaliplatin, or cisplatin): Note: The last platinum regimen did not necessarily have to immediately follow the next-to-last (penultimate) platinum regimen. For example, if a participant received a non-platinum regimen between the penultimate platinum regimen and last platinum regimen, she could have been eligible as long as she met all entry criteria. 1. For the penultimate platinum-based chemotherapy prior to study enrollment, participants must have had platinum-sensitive disease after this treatment, defined as achieving a response (CR or PR) and disease progression \>6 months after completion of her last dose of platinum therapy (documented 6 to 12 months or \>12 months). Source documentation was required. 2. For the last line of platinum-based chemotherapy prior to study enrollment: * Participants must have received a platinum-containing regimen for a minimum of 4 cycles. * Participants must have achieved a partial or complete tumor response. * Following the last regimen, participants must have had either: CA-125 in the normal range, OR; CA-125 decrease by more than 90% during the last platinum regimen, and which was stable for at least 7 days (ie, no increase \>15%). * Following the last regimen, participants could not have had any measurable lesion \>2 cm at the time of study enrollment. 3. Participants must have been enrolled within 8 weeks after completion of their final dose of the platinum-containing regimen. 6. Participants must have performance status of ≤1 on the Eastern Cooperative Oncology Group (ECOG) Performance Status Scale. 7. Participants must have adequate organ function as indicated by the following laboratory values: 1. Absolute neutrophil count (ANC) ≥1,500/μL. 2. Platelet count ≥100,000/μL. 3. Hemoglobin ≥9 g/dL. 4. Serum creatinine ≤1.5× institutional upper limit of normal (ULN) OR calculated creatinine clearance ≥50 mL/minute, using the Cockcroft-Gault equation. 5. Total bilirubin ≤1.5×ULN OR direct bilirubin ≤1×ULN. 6. Aspartate aminotransferase (AST) and alanine aminotransferase (ALT) ≤2.5×ULN unless liver metastases were present, in which case they had to be ≤5×ULN. 8. Participants must be able to take oral medications. 9. Female participants of childbearing potential must be negative for pregnancy test (beta-human chorionic gonadotropin \[β-hCG\]) within 7 days prior to receiving the first dose of study treatment. 10. Female participants who: 1. Are postmenopausal for at least 1 year before the screening visit, OR 2. Are surgically sterile, OR 3. If they are of childbearing potential, agree to practice 1 highly effective method of contraception and 1 additional effective (barrier) method at the same time, from the time of signing the informed consent through 180 days after the last dose of study drug, OR 4. Agree to practice true abstinence, when this is in line with the preferred and usual lifestyle of the participant. (Periodic abstinence \[eg, calendar, ovulation, symptothermal, postovulation methods\], condoms only, withdrawal, spermicides only, and lactational amenorrhea are not acceptable methods of contraception. Female and male condoms should not be used together.) Exclusion Criteria 1. Participants who have had drainage of ascites during last 2 cycles of last chemotherapy. 2. Participants who have had palliative radiotherapy encompassing \>20% of the bone marrow within 1 week of the first dose of study treatment. 3. Participants who have any, persistent, Grade ≥3 toxicity from last cancer therapy. 4. Participants who have symptomatic, uncontrolled brain or leptomeningeal metastases. To be considered "controlled," central nervous system (CNS) disease must have undergone treatment (eg, radiation or chemotherapy) at least 1 month prior to study enrollment. The participant must not have had any new or progressive signs or symptoms related to the CNS disease and must have been taking a stable dose of steroids or no steroids (as long as these were started at least 4 weeks prior to enrollment\] or no steroids). A scan to confirm the absence of brain metastases at baseline was not required. Participants with spinal cord compression might have been considered if they had received definitive treatment for this and evidence of clinically stable disease for 28 days. 5. Participants who have known hypersensitivity to the components of niraparib. 6. Participants who have had prior treatment with a known poly (adenosine diphosphate \[ADP\]-ribose) polymerase (PARP) inhibitor. 7. Participant who have had treatment with any investigational products within 28 days or 5 half-lives (whichever was longer) before the first dose. 8. Participants who have had major surgery per Investigator judgment within 3 weeks of the first dose. Participant must have recovered from any effects of any major surgery. 9. Participants who have diagnosis, detection, or treatment of invasive second primary malignancy other than ovarian cancer ≤24 months prior to study enrollment (except basal or squamous cell carcinoma of the skin that was definitively treated). Note: Participants must not have any known history or current diagnosis of myelodysplastic syndrome (MDS) or acute myeloid leukemia (AML), irrespective of the time for disease history. 10. Participants who are considered a poor medical risk due to a serious, uncontrolled medical disorder, non-malignant systemic disease, or active, uncontrolled infection. Examples include, but are not limited to, uncontrolled ventricular arrhythmia, recent (within 90 days of the first dose) myocardial infarction, uncontrolled major seizure disorder, unstable spinal cord compression, superior vena cava syndrome, small bowel obstruction or other serious gastrointestinal disorder, or any psychiatric disorder that prohibits obtaining informed consent. 11. Participants who have received a transfusion (platelets or red blood cells) within 4 weeks of the first dose of study treatment. 12. Participants who have received a live virus or bacterial vaccines within 4 weeks of the first dose of study treatment. 13. Participants who have a history or current evidence of any condition, therapy, or lab abnormality (including active or uncontrolled myelosuppression \[ie, anemia, leukopenia, neutropenia, thrombocytopenia\]) that might confound the results of the study, interfere with the participant's participation throughout the study period, or study participation is not in the best interest of the participant. 14. Participants who are regular user (including "recreational use") of any illicit drugs at the time of signing informed consent or have a recent history (within the past year) of drug or alcohol abuse. 15. Participants who are pregnant or breast-feeding, or expecting to conceive within the planned duration of the study. NOTE: If a breast-feeding woman discontinue breast-feeding, she may be enrolled in the study. 16. Participants who are immunocompromised (participants with splenectomy are allowed). 17. Participants who have known human immunodeficiency virus (HIV) positive. 18. Participants who have known hepatitis B surface antigen (HBsAg) positive, or known or suspected active hepatitis C virus (HCV) infection. NOTE: Participants who are positive for hepatitis B core antibody (HBcAb) or hepatitis B surface antibody (HBsAb) can be enrolled but must have an undetectable hepatitis B virus (HBV) viral load. Participants who have positive hepatitis C virus antibody (HCVAb) must have an undetectable HCV viral load.