This study investigates the use of an investigational medication in children and adolescents with certain types of cancers that have come back or have not responded to treatment. These cancers include rhabdomyosarcoma (RMS), non-rhabdomyosarcoma soft tissue sarcoma (NRSTS), and Ewing sarcoma (EWS). The purpose is to assess the safety and preliminary effects of the investigational medication to see if further study is worthwhile.

Participants in this study will receive the investigational medication, and researchers will monitor their health and any changes in their cancer. The study involves regular health checks and tests to see how the body responds to the treatment. A placebo, which is an inactive substance that looks like the investigational medicine but does not contain any medicine, is not used in this study.

• Who can participate: Children and adolescents with relapsed or refractory rhabdomyosarcoma, non-rhabdomyosarcoma soft tissue sarcoma, or Ewing sarcoma can participate. Participants must have no other curative options, have recovered from prior therapies, and have adequate kidney and liver function.
• Study details: Participants will receive the investigational medication and undergo regular health assessments to monitor their response to the treatment. No placebo will be used, meaning all participants receive the investigational medication.