A Study to Test the Safety of the Investigational Drug Selitrectinib in Children and Adults That May Treat Cancer

Inclusion Criteria: * Advanced solid tumor for which, in the opinion of the investigator, no other standard therapy offers greater benefit. * A solid tumor diagnosis in the setting of: * a) a documented NTRK fusion and a clinical history of relapse following a response to a prior TRK inhibitor * b) a documented NTRK fusion unresponsive to a prior TRK inhibitor * c) a documented NTRK fusion and a clinical history of intolerance to a prior TRK inhibitor * NTRK gene fusions will be identified in a CLIA-certified (or equivalently-accredited diagnostic) laboratory. If such a report cannot be provided, other available certifications/accreditations are required and need to be documented. Patients with infantile fibrosarcoma (IFS) or congenital mesoblastic nephroma (CMN) may be enrolled based on an ETV6+ FISH test without identifying NTRK3. * Performance Status: Eastern Cooperative Oncology Group (ECOG) score ≤ 2 in adults or Karnofsky Performance Status (KPS) Score≥50% (age ≥ 16 years) or Lansky Performance Score (LPS) ≥ 40% (age \< 16 years). * Life expectancy of at least 3 months. * Adequate hematologic, hepatic and renal function. * Patients with stable central nervous system (CNS) primary tumor, brain metastases, or treated spinal cord compression are eligible if neurological symptoms have been stable for 7 days prior to the first dose of selitrectinib. * Ability to receive study drug orally or by enteral administration Exclusion Criteria: * Prior exposure to second ge…