Study of Lenvatinib in Children and Adolescents With Refractory or Relapsed Solid Malignancies and Young Adults With Osteosarcoma

The study consists of 5 cohorts: Cohort 1 (Single-Agent Dose-Finding) dose-escalation to find the recommended dose (RD) of lenvatinib using time-to-event continual reassessment method (TiTE-CRM) in children and adolescents with relapsed or refractory solid malignant tumors. When the RD is identified, Cohorts 2A, 2B, and 3A will enroll in parallel. Cohort 2 (Single-Agent Expansion) will evaluate the efficacy of lenvatinib at the RD in children, adolescents, and young adults with 1. 131 iodine-refractory differentiated thyroid cancer (DTC) \[Cohort 2A\] or 2. Relapsed or refractory osteosarcoma \[Cohort 2B\] Cohort 3A (Combination Dose-Finding) will determine the RD of lenvatinib in combination with ifosfamide and etoposide in children, adolescents, and young adults with relapsed or refractory osteosarcoma. Cohort 3B (Combination Expansion) will evaluate the efficacy of lenvatinib at the RD from Cohort 3A in combination with ifosfamide and etoposide in children, adolescents, and young adults with relapsed or refractory osteosarcoma. Participants with osteosarcoma who have enrolled into Cohort 1 or 2B and experienced progressive disease will also be candidates for enrollment in Cohort 3B. Lenvatinib will be provided as hard capsules containing 1, 4, or 10 mg lenvatinib. Lenvatinib capsules should be dissolved in water or apple juice for those who are unable to swallow capsules.