A Study to Test the Safety and Efficacy of the Drug Larotrectinib for the Treatment of Tumors With NTRK-fusion in Children

Inclusion Criteria: * Phase 1 (Closed): * Dose escalation: Birth through 21 years of age at C1D1 with a locally advanced or metastatic solid tumor or primary CNS tumor that has relapsed, progressed or was nonresponsive to available therapies and for which no standard or available systemic curative therapy exists; OR Infants from birth and older with a diagnosis of malignancy and with a documented NTRK fusion that has progressed or was nonresponsive to available therapies, and for which no standard or available curative therapy exists; OR Patients with locally advanced infantile fibrosarcoma who would require, in the opinion of the investigator, disfiguring surgery or limb amputation to achieve a complete surgical resection. Phase I dose escalation cohorts are closed to enrollment. * Dose expansion: In addition to the above stated inclusion criteria, patients must have a malignancy with a documented NTRK gene fusion with the exception of patients with infantile fibrosarcoma, congenital mesoblastic nephroma or secretory breast cancer. Patients with infantile fibrosarcoma, congenital mesoblastic nephroma or secretory breast cancer may enroll into this cohort with documentation of an ETV6 rearrangement by FISH or RT-PCR or a documented NTRK fusion by next generation sequencing. * Phase 2: \-- Infants from birth and older at C1D1 with a locally advanced or metastatic infantile fibrosarcoma, patients with locally advanced infantile fibrosarcoma who would require, in the opinion of the investigator, disfiguring surgery or limb amputation to achieve a complete surgical resection; OR Birth through 21 years of age at C1D1 with a locally advanced or metastatic solid tumor or primary CNS tumor that has relapsed, progressed or was nonresponsive to available therapies and for which no standard or available systemic curative therapy exists with a documented NTRK gene fusion (or in the case of infantile fibrosarcoma, congenital mesoblastic nephroma or secretory breast cancer with documented ETV6 rearrangement (or NTRK3 rearrangement after discussion with the sponsor) by FISH or RT-PCR. Patients with NTRK-fusion positive benign tumors are also eligible; OR Potential patients older than 21 years of age with a tumor diagnosis with histology typical of a pediatric patient and an NTRK fusion may be considered for enrollment following discussion between the local site Investigator and the Sponsor. * Patients with primary CNS tumors or cerebral metastasis * Karnofsky (those 16 years and older) or Lansky (those younger than 16 years) performance score of at least 50. * Adequate hematologic function * Adequate hepatic and renal function Exclusion Criteria: * Major surgery within 14 days (2 weeks) prior to C1D1 * Clinically significant active cardiovascular disease or history of myocardial infarction within 6 months prior to C1D1, ongoing cardiomyopathy; current prolonged QTc interval \> 480 milliseconds * Active uncontrolled systemic bacterial, viral, or fungal infection * Current treatment with a strong CYP3A4 inhibitor or inducer. Enzyme-inducing anti-epileptic drugs (EIAEDs) and dexamethasone for CNS tumors or metastases, on a stable dose, are allowed. * Phase 2 only: * Prior progression while receiving approved or investigational tyrosine kinase inhibitors targeting TRK, including entrectinib, crizotinib and lestaurtinib. Patients who received a TRK inhibitor for less than 28 days of treatment and discontinued because of intolerance remain eligible.