A Phase 0/1 Study of cDNA for TP53, Checkpoint Inhibition and Radiation in Children With Recurrent, Progressive or Refractory CNS Malignancies.

This clinical trial is studying a drug called SGT-53 along with radiation and another drug called Nivolumab. It's for children with brain tumors that have come back, gotten worse, or didn't get better with earlier treatments. The main questions it aims to answer are: What is the right dose of SGT-53 that children can safely receive when it is used with radiation and Nivolumab? This dose will be used in the second phase of the trial. What side effects are there of SGT-53 when it is used with radiation and Nivolumab? How does SGT-53 move through the body when given with radiation and Nivolumab? How much of the SGT-53 drug is found in the tumor tissue? This will be tested in a small group of patients? Participants will: For the first treatment cycle: Get SGT-53 twice per week Get Nivolumab every 2 weeks Receive radiation therapy during week 2 For Cycles 2-6: Get SGT-53 once per week during even cycles and twice per week during odd cycles Get Nivolumab every 2 weeks For Cycles 7+ Get both SGT-53 and Nivolumab every 2 weeks In the phase 0 part of the study, 4 participants will have genetic testing performed on their tumor tissue after receiving SGT-53. These samples will be compared to another sample taken from the skin.