Niraparib and TSR-042 for the Treatment of BRCA-Mutated Unresectable or Metastatic Breast, Pancreas, Ovary, Fallopian Tube, or Primary Peritoneal Cancer

Inclusion Criteria: * Participant must have breast, pancreas, ovary, fallopian tube or primary peritoneal cancer that is unresectable or metastatic, with a pathogenic mutation in BRCA1 or BRCA2 (either germline or somatic) as confirmed by next generation gene sequencing such as University of Washington (UW) OncoPlex assay or equivalent, and who have experienced progression or been intolerant to standard therapies for their disease. * Breast cancer patients with or without HER2+, estrogen receptor (ER)+, and/or progesterone receptor (PR)+ disease, as determined by pathological report, are allowed * Participant must be able and willing to undergo pre-treatment and on-treatment biopsy * Participant must have life expectancy of 4 months or greater * Tumor must be measurable according to Response Evaluation Criteria in Solid Tumors (RECIST)1.1 criteria * Participant must have an Eastern Cooperative Oncology Group (ECOG) performance status of =\< 1 * Participant must be \>= 18 years of age * Patient must be able to tolerate oral medication * 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 ar…