This study investigates a new drug, MRX-2843, for people aged 12 and older with certain types of leukemia (a cancer of the blood) that have come back or not responded to other treatments. The study is Phase I, meaning it's one of the first times the drug is tested in humans. The study is open-label, so everyone knows what treatment is given, and non-randomized, meaning everyone gets the same treatment. Participants will take the drug by mouth in 28-day cycles. The study will include up to 50 patients.

• Participants need to visit regularly for check-ups and tests.
• The study involves taking the drug in 28-day cycles, with continuous monitoring.
• Participants cannot be pregnant and must use birth control during and after the study.

This is a chance to try a new treatment, but it may have unknown risks. Participants should talk with their doctor to decide if this study is right for them.