Niraparib and Dostarlimab for the Treatment of Small Cell Lung Cancer and Other High-Grade Neuroendocrine Carcinomas

Inclusion Criteria: * Participant must have unresected or locally advanced small cell lung cancer (Cohort 1) or high-grade neuroendocrine carcinoma (Cohort 2) confirmed by staff pathologist. High-grade neuroendocrine carcinoma of prostate (e.g. aggressive variant prostate cancer, small cell of prostate, etc.) are excluded * Patients must have had at least one prior line of systemic therapy directed at their malignancy * Participant must have an Eastern Cooperative Oncology Group (ECOG) performance status of =\< 1 * Participant must be \>= 18 years of age * Absolute neutrophil count \>= 1,500/uL * Platelets \>= 100,000/uL * Hemoglobin \>= 9 g/dL * Serum creatinine =\< 1.5 x upper limit of normal (ULN) or calculated creatinine clearance \>= 60 mL/min using the Cockcroft-Gault equation * Total bilirubin =\< 1.5 x ULN (=\< 2.0 in patients with known Gilberts syndrome) OR direct bilirubin =\< 1 x ULN * Aspartate aminotransferase and alanine aminotransferase =\< 2.5 x ULN unless liver metastases are present, in which case they must be =\< 5 x ULN * International normalized ratio (INR) or prothrombin time (PT) =\< 1.5 x ULN unless patient is receiving anticoagulant therapy as long as PT or partial thromboplastin (PTT) is within therapeutic range of intended use of anticoagulants. Activated partial thromboplastin time (aPTT) =\< 1.5 x ULN unless patient is receiving anticoagulant therapy as long as PT or PTT is within therapeutic range of intended use of anticoagulants * Participant…