Safety and Efficacy of BMS-986504 in Unresectable Malignant Peripheral Nerve Sheath Tumor

People who have a cancer called MPNST, or Malignant Peripheral Nerve Sheath Tumor, may be eligible for this study. The purpose of this study is to see if a new medicine called BMS-986504 may work better than other available medicines for people with MPNST. Methylthioadenosine Phosphorylase (MTAP) loss is a gene mutation that some people have. MTAP loss seems to increase the chance that BMS-986504 can kill MPNST cancer cells. People who are missing MTAP from their tumor may be able to enroll in this study. Treating MPNST based on MTAP loss is considered experimental and is not approved by the US Food and Drug Administration (FDA) for determining whether BMS-98650 will be active against cancer. The purpose of this study is to evaluate the safety and effectiveness of BMS-986504 in participants with MPNST.

Inclusion Criteria: * Male or female participants ≥ 12 years of age at the time of screening. * Ability to understand and willingness to sign documentation of informed consent if ≥ 18 years of age or documentation of assent if 12-17 years of age. * Pathohistological verification of MPNST. * Measurable disease (size of primary tumor and metastatic lesions can be trended by CT or MRI scans). * Unresectable (locally advanced or metastatic) disease. * Confirmation of homozygous MTAP deletion by next generation sequencing * Recovery from the adverse effects of prior therapy at the time of enrollment to baseline or ≤ Grade 1 (excluding alopecia, peripheral neuropathy, and parameters superseded by other eligibility criteria \[eg, hematology parameters\]). Note: Participants with prior endocrine adverse effects are permitted to enroll if they are stably maintained on appropriate replacement therapy and are asymptomatic. * Recovery from the acute toxic effects (≤ grade 1 as defined by the National Cancer Institute Common Terminology Criteria for Adverse Events v5.0) of all prior chemotherapy prior to the entering study (exceptions: alopecia, anorexia, mass pain). * Normal marrow function and recovery of blood cell counts from any myelosuppressive chemotherapy prior to entering study: * Peripheral absolute neutrophil count (ANC) ≥ 1500/mcL (microliter). * Hemoglobin ≥ 9 g/dL (packed red blood cell transfusion is not allowed up to 14 days prior to starting BMS-986504 treatment to meet eligibility). * Platelet count ≥ 100,000/mcL (microliter) (platelet transfusion is not allowed up to 14 days prior to starting BMS-986504 treatment to meet eligibility). * Adequate organ function, including: * Liver function: * Total bilirubin ≤ 1.5 times the upper limit of normal (ULN) for age, or ≤ 3 x ULN if associated with liver metastatic disease or Gilbert's disease). * ALT (alanine transaminase) and AST (aspartate aminotransferase) \< 3 x ULN for age, or \< 5 x ULN if associated with liver metastatic disease. * Serum albumin ≥ 2.0 g/dL. * PT (prothrombin time) and/or INR (International Normalized Ratio) ≤ 1.5 x ULN, or within therapeutic range if receiving anticoagulant therapy. * Renal function: * Creatinine \< 1.5 times institutional ULN for age. * Performance status at time of screening: * ECOG (Eastern Cooperative Oncology Group) performance status 0-1 for participants ≥ 18 years old. * Lansky performance status (ages 12-15) or Karnofsky performance status (ages 16-17) ≥ 50. * Individuals of childbearing potential (IOCBP) must practice effective contraception during the trial. Exclusion Criteria: * Participants who have been treated previously with a PRMT5 inhibitor. * Participants who are unable to swallow tablets. * History of gastrointestinal disease, inflammatory bowel disease, major gastric surgery or other gastrointestinal conditions (eg, uncontrolled nausea, vomiting, malabsorption syndrome) likely to alter absorption of study treatment or result in inability to swallow oral medications. * Participants with active drug use. * Any botanical preparation (e.g., herbal supplements or traditional Chinese medicines) intended to treat the disease under study received within 4 weeks prior to randomization. The concurrent use of any botanical preparation is not permitted while on study. * Ongoing need for a medication known as a strong inhibitor or strong inducer of CYP3A4 and/or P-gp or a PPI (proton pump inhibitor) that cannot be switched to an alternative treatment prior to randomization.