Tafasitamab (MOR00208) in Pediatric Patients With Relapsed or Refractory Acute B Lineage Leukemia

This study is testing a drug called tafasitamab for children and teenagers with a type of blood cancer called acute B-cell leukemia (ALL) that has come back after a stem cell transplant or CAR-T therapy. Researchers want to know if this drug can help keep the cancer in remission. **You may be eligible if...** - You are between 3 and 17 years old - You have B-cell leukemia (ALL) that tests positive for a marker called CD19 - You have previously had a stem cell transplant or CAR-T therapy - There is evidence of cancer returning in the blood (measurable residual disease) **You may NOT be eligible if...** - You have more than 5% leukemia cells in your bone marrow (frank relapse) - You have Philadelphia chromosome-positive ALL - Your heart function is poor (ejection fraction below 25%) - Your kidney or liver function is severely impaired - You have severe graft-versus-host disease (a complication after transplant) - You are pregnant or breastfeeding - You have a severe active infection including HIV or hepatitis Talk to your doctor to see if this trial is right for you.