A Study of Niraparib (GSK3985771) Maintenance Treatment in Participants With Advanced Ovarian Cancer Following Response on Front-Line Platinum-Based Chemotherapy

Open to females ages 18 years and up

• Participants must have histologically diagnosed high-grade serous or endometrioid, or high-grade predominantly serous or endometrioid ovarian cancer, fallopian tube cancer, or primary peritoneal cancer that is Stage III or IV according to Federation Internationale de Gynécologie et d'Obstétrique (FIGO) criteria.
• Participants with inoperable Stage III and IV disease; All Stage IV participants with operable disease; Participants with stage III or IV disease treated with neoadjuvant chemotherapy and interval debulking surgery; and Participants with stage III disease who have visible residual disease after primary debulking surgery.
• Participants who have received intraperitoneal chemotherapy; All participants must have had more than or equal to (>=)6 and less than or equal to (<=)9 cycles of platinum-based therapy; Participants must have had >=2 post-operative cycles of platinum-based therapy following interval debulking surgery; Participants must have physician assessed Complete response (CR) or Partial response (PR) after >=3 cycles of therapy; and Participants must have either Cancer antigen 125 (CA-125) in the normal range or CA-125 decrease by more than 90 percent(%) during their front-line therapy that is stable for at least 7 days (no increase more than (>)15% from nadir).
• Participants must be randomized within 12 weeks of the first day of the last cycle of chemotherapy.
• All participants must agree to undergo central tumor HRD testing.
• Participants of childbearing potential must have a negative serum or urine pregnancy test (beta human chorionic gonadotropin [hCG]) within 7 days prior to receiving the first dose of study treatment.

• Participant has mucinous or clear cell subtypes of epithelial ovarian cancer, carcinosarcoma or undifferentiated ovarian cancer.
• Participants with Stage III disease who have had complete cytoreduction (no visible residual disease) after primary debulking surgery.
• Participant has undergone more than two debulking surgeries for the study disease.
• Participant is pregnant, breastfeeding, or expecting to conceive children while receiving study treatment and for up to 180 days after the last dose of study treatment.
• Participant has a known hypersensitivity to the components of niraparib or its excipients.
• Participant has received prior treatment with a known PARP inhibitor or has participated in a study where any treatment arm included administration of a known PARP inhibitor.
• Participant is to receive bevacizumab as maintenance treatment.
• Participant has had investigational 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 the first scheduled day of dosing in this study.
• Participant has had any known >=Grade 3 anemia, neutropenia or thrombocytopenia due to prior chemotherapy that persisted >4 weeks.
• Participant has a condition (such as transfusion dependent anemia or thrombocytopenia), therapy, or laboratory abnormality that might confound the study results or interfere with the participation for the full duration of the study treatment, including:
• Participant received a transfusion (platelets or red blood cells) within 2 weeks of the first dose of study treatment.
• Participant received colony-stimulating factors (e.g., granulocyte colony stimulating factor [G-CSF], granulocyte macrophage colony-stimulating factor [GM-CSF] or recombinant erythropoietin) within 2 weeks prior to the first dose of study treatment.
• Participant has been diagnosed and/or treated for invasive cancer less than 5 years prior to study enrollment.