Study Evaluating Safety, Tolerability, and Metabolism of Niraparib

The purpose of this study is to identify the genetic characteristic(s), specifically degree of African ancestry, and environmental characteristic(s) that appear to be related to the effects, both good and bad, that the maintenance treatment has women with ovarian cancer. In this study, an investigational medication called niraparib is being tested for the treatment of ovarian cancer. Niraparib works by blocking the ability of cancer cells to fix their genes. Cancer cells with damaged genes have a harder time growing and spreading in the body and can even die.

Inclusion Criteria: 1. Participant must be female ≥18 years of age, able to understand study procedures, and agree to participate in the study by providing written informed consent. 2. Self-identify as Black. Please note that individuals who identify as Latino are eligible to participate so long as they also self-identify as Black. 3. Participant has completed adjuvant treatment for newly diagnosed stage III or IV ovarian, fallopian tube, or primary peritoneal cancer according to the International Federation of Gynecology and Obstetrics staging criteria. 4. Participant must have high-grade serous or high-grade endometrioid histology. 5. Participant must provide saliva and/or blood specimens for assessment of germline mutation(s) in the Fanconi Anemia pathway. 6. Participant must provide formalin-fixed, paraffin-embedded (FFPE) or fresh tumor specimen from initial cytoreductive surgery (primary debulking) or initial pre-treatment core biopsy (if neoadjuvant chemotherapy (NACT) received; tumor obtained from interval cytoreduction acceptable if pre-treatment biopsy not obtained). 7. Participant must have had a complete or partial clinical response to adjuvant treatment as confirmed by CT scan within 8 weeks after completion of the last dose of platinum-based chemotherapy. 8. Participant must have recovered to ≤ Grade 1 in terms of toxicity from prior treatments. 9. Participant must not have any known contraindication or hypersensitivity to niraparib or any of its excipients. 10. Participants must be considered candidates for maintenance niraparib therapy by their treating physician. 11. Participants should have adequate organ function as defined below: * Platelets ≥ 100 platelets × 10\^9/L * Hemoglobin ≥ 9 g/dL or 5.6 mmol/L * Aspartate transaminase (AST) and alanine transaminase (ALT) ≤2.5 × upper limit of normal (ULN), \20% of the bone marrow within 2 weeks or any radiation therapy within 1 week before Day 1 of protocol therapy. 13. Participant has leptomeningeal disease, carcinomatous meningitis, symptomatic brain metastasis, or radiographic signs of central nervous system hemorrhage. 14. Participant has current active pneumonitis or any history of pneumonitis requiring steroids (any dose) or immunomodulatory treatment within 90 days of planned start of the study. 15. Participants with active hepatitis B or C infection. 16. Patient with prior history of posterior reversible encephalopathy syndrome (PRES). 17. Patients with impaired decision-making capacity. 18. Participants have high 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) myocardial infarction, uncontrolled major seizure disorder, unstable spinal cord compression, superior vena cava syndrome, active uncontrolled coagulopathy, bleeding disorder, or any psychiatric disorder that prohibits obtaining informed consent. 19. Patient is currently receiving one or more cytotoxic, hormonal, or other medications to treat their cancer.