Additional Study Information: This is a Phase 1/2 multicenter study of VX15/2503 in pediatric patients and young adults with solid tumors which have come back or have not responded to standard therapy. The study treatment is considered experimental because VX15/2503 is not approved by the United States (US) Food and drug Administration (FDA) for treating pediatric patients and young adults with refractory solid tumors. The purpose of this study is to learn what kind of side effects VX15/2503 can cause and to find the highest safe dose of VX15/2503 that can be given without causing severe side effects. Another goal is to determine whether VX15/2503 is a beneficial treatment for osteosarcoma which has come back or has not responded to standard therapy. VX15/2503 will be administered as an intravenous infusion on Days 1 and 15 of a 28-day period. This entire 28-day period is called a cycle. Patients may continue to receive VX15/2503 for up to about 12 months. Blood samples will be collected to determine how much VX15/2503 is in the blood during the study (pharmacokinetics). Disease assessment will be completed before beginning treatment, at the end of Cycle 1 and then every other cycle.