Niraparib for the Treatment of Leiomyosarcoma

Inclusion Criteria: * Participant must have histologically documented LMS. Pathology review and confirmation of diagnosis will occur at the site enrolling the patient on this study. * Patients must have had a recent, within six months of screening, tumor biopsy testing positive for pathogenic mutation or homozygous loss of either BARD1, BRCA1, BRCA2, BRIP1, PALB2, RAD51, RAD51B, RAD51C, or RAD51D using a clinically validated next-generation (NGS) sequencing panel. * Variant pathogenicity will be arbitrated by the primary investigative team * Patients must have locally advanced and unresectable or metastatic disease * Patients must have disease which is measurable at study entry according to Response Evaluation Criteria in Solid Tumors (RECIST) version 1.1. Additionally, patients must have a site of disease deemed accessible for biopsy at no or minimal risk to the patient (including through the use of image-guidance). If there are questions regarding the feasibility of biopsy, the case should be reviewed with interventional radiology or the appropriate department at the study site * Patients must have had prior progression on, or intolerance to, at least one line of systemic therapy for advanced LMS. Adjuvant therapy administered after curative resection will not qualify as prior treatment. There is no upper limit on the number of prior therapies received * Participant must have an Eastern Cooperative Oncology Group (ECOG) performance status of =\< 2 * Participant must be …