Phase 1 Study of the Dual MDM2/MDMX Inhibitor ALRN-6924 in Pediatric Cancer

This research study is a Phase I clinical trial, which tests the safety of an investigational intervention and also tries to define the appropriate dose of the investigational intervention to use for further studies. "Investigational" means that the intervention is being studied. In this research study, the investigators are evaluating a new drug, ALRN-6924, as a potential new treatment for children with cancer. The FDA (the U.S. Food and Drug Administration) has not approved ALRN-6924 as a treatment for any disease. This is the first time that ALRN-6924 will be studied in children. ALRN-6924 is a drug that blocks certain proteins in tumor cells called MDM2 and MDMX. These proteins may be important in the growth of some cancers. Laboratory experiments and results from studies with adults show that ALRN-6924 may stop tumor growth and, in some cases, may cause tumor cells to die. ALRN-6924 has been tested in adults with cancer to find out about side effects and dosing in adults. The purposes of this study are: * to evaluate side effects of ALRN-6924 and to find the best dose of ALRN-6924 when used in children. * to determine whether this drug may have benefits against the types of cancer seen in children * to see how the body breaks down ALRN-6924 by measuring the amount of ALRN-6924 in the blood. One part of the study is for children with solid tumors, lymphoma, and brain tumors (Cohorts A or B). Another part of this study is for children with leukemia (Cohort C).