Myelogenous Leukemia, Acute

This is a multicenter, open-label study in people aged 18 and older with relapsed or refractory acute myeloid leukemia. It has two parts. In Phase 2, we are testing three radiation dose levels of 131I-apamistamab combined with fludarabine and low-dose whole-body radiation before stem cell transplant to find the safest and most effective dose. In Phase 3, patients will be randomly assigned to receive either this treatment combination or a standard of care regimen before transplant. The main goal is to see if the new approach helps people live longer. Phase 2 will enroll about 60 people, and Phase 3 will enroll about 246 people.