Pharmacokinetic and Safety Study of MRX-2843 in Adolescents and Adults With Relapsed/Refractory AML, ALL, or MPAL

This study tests how the body processes a new drug called MRX-2843 in teenagers and adults with acute leukemia (AML, ALL, or mixed-type leukemia) that has relapsed or not responded to at least one round of chemotherapy. Researchers also want to understand its safety profile in this younger population. **You may be eligible if...** - You are at least 12 years old and weigh at least 40 kg - You have been diagnosed with AML, ALL, or mixed-phenotype acute leukemia that has relapsed at least twice or has not responded to at least one chemotherapy regimen - You are able to swallow oral medication - You have white blood cell counts below 25,000/mm3 at screening - If female, you are not pregnant or breastfeeding and agree to use contraception **You may NOT be eligible if...** - Your leukemia has never been treated before - You have only relapsed once and that relapse responded to treatment - You cannot swallow pills - You are pregnant or breastfeeding - You have significant organ problems affecting eligibility Talk to your doctor to see if this trial is right for you.