Effects of Single Agent Niraparib and Niraparib Plus Programmed Cell Death-1 (PD-1) Inhibitors in Non-Small Cell Lung Cancer Participants

General Inclusion Criteria:

• Male or female participants at least 18 years of age.
• Histological or cytological proven advanced (unresectable) or metastatic NSCLC as defined as stage IIIB (positive supraclavicular lymph nodes) not amenable to definitive chemoradiotherapy or stage IV NSCLC.
• Measurable disease by Response Evaluation Criteria in Solid Tumors (RECIST) version (v) 1.1.
• Eastern Cooperative Oncology Group (ECOG) performance status of 0 to 1.
• Adequate organ function, defined as:
• Absolute neutrophil count (ANC) >= 1500 per microliter (/µL).
• Platelets >= 100,000/µL.
• Hemoglobin >= 9 grams per deciliter (g/dL) or >= 5.6 millimoles per liter (mmol/L).
• Serum creatinine = 50 milliliters per minute (mL/min) (as calculated using the Cockcroft Gault equation or measured using 24-hour urine creatinine clearance) for participants with creatinine levels > 1.5 times institutional ULN.
• Total bilirubin =45 years of age and has not had menses for > 1 year. ii) Amenorrheic for = 50%) per local assessment; with no known Epidermal Growth Factor Receptor (EGFR)-sensitizing mutation and/or ROS -1 or anaplastic lymphoma kinase (ALK) translocation, and no prior systemic chemotherapy or PD-1/PD-L1 inhibitor treatment for metastatic NSCLC.
• Cohorts 2 and 2A (combination of niraparib and PD-1 inhibitor): participants must have tumors with PD-L1 expression (TPS between 1% and 49%) per local assessment, with no known EGFR-sensitizing mutation and/or ROS-1 or ALK translocation, and no prior systemic chemotherapy or PD-1/PD-L1 inhibitor treatment for metastatic NSCLC.
• Cohort 3 (single agent niraparib): participants must have metastatic squamous non-small cell lung cancer (sqNSCLC) and have progressed after both prior platinum-based chemotherapy and prior PD-1 or PD-L1 inhibitor treatment.

Exclusion Criteria for Cohorts 1, 1A , 2 and 2A:

• Participant has received systemic therapy for the treatment of advanced stage NSCLC. Completion of treatment with chemotherapy and/or radiation as part of neoadjuvant/adjuvant therapy is allowed as long as therapy was completed at least 6 months prior to the diagnosis of metastatic disease.
• Prior therapy with an anti-PD-1, anti-PD-L1, or anti-PD-L2 agent.
• Known hypersensitivity to the components of niraparib, pembrolizumab, TSR-042 (Dostarlimab), or their excipients.
• Known EGFR (exon 19 and 21) mutations, ALK translocations, and/or ROS-1 translocations.
• Participant has a history or current condition (such as transfusion-dependent anemia or thrombocytopenia), therapy, or laboratory abnormality that might confound the study results, or interfere with the participant's participation for the full duration of the study treatment.
• Known diagnosis of immunodeficiency or receiving systemic steroid therapy or any other form of immunosuppressive therapy within 7 days prior to the first dose of study treatment.
• Participant is immunocompromised, in the opinion of the Investigator.
• Current participation in a treatment study or past participation in a study of an investigational agent within 4 weeks before the first dose of study treatment.
• Symptomatic uncontrolled brain or leptomeningeal metastases. (To be considered "controlled," central nervous system [CNS] disease must have undergone treatment [example, radiation or chemotherapy] at least 1 month prior to study entry. The participant must not have any new or progressive signs or symptoms related to the CNS disease and must be taking <= 10 mg of prednisone or equivalent per day or no steroids.) Participants who have untreated brain metastases and who are not symptomatic may enroll if the Investigator feels that treatment of these metastases is not indicated. A scan to confirm the absence of brain metastases is not required. Participants with spinal cord compression may be considered if they have received definitive treatment for this and evidence of clinically SD for 28 days.
• Active autoimmune disease that required systemic tre