Radioimmunotherapy Conditioning With 131I- Apamistamab for Allogeneic Transplant in Relapse/Refractory AML

Inclusion Criteria: 1. Have active, relapsed, or refractory AML with ≥5% and ≤20% blasts in the marrow. 2. 2R/R AML is defined as one of the following: Primary induction failure after ≥2 cycles of therapy, first early relapse after remission \<6 months, relapse refractory to salvage combination therapy or second or subsequent relapse 3. Documented CD45 expression by leukemic cells via flow cytometry. 4. ≥18 years of age and not suitable for myeloablative conditioning regimen. 5. Circulating blast count \<10,000/mm³ (hydroxyurea allowed). 6. Calculated creatinine clearance (Cockcroft-Gault) \>50 mL/min. 7. Adequate hepatic function: AST/ALT ≤2 × ULN; total bilirubin ≤1.5 × ULN (≤3 × ULN if due to underlying malignancy or Gilbert's). 8. Karnofsky performance score ≥70. 9. Expected survival \>60 days. 10. Central venous catheter line in place before study treatment. 11. 8/8 HLA-matched related or unrelated donor (HLA-A, HLA-B, HLA-C, DRB1). 12. Women of childbearing potential must be surgically sterile or use acceptable contraception through 1-year post-transplant. 13. Men with partners of childbearing potential must be surgically sterile or use acceptable contraception through 12 weeks after last dose. 14. Able to understand procedures, provide informed consent, and comply with study requirements. Exclusion Criteria: 1. Positive human anti-mouse antibody (HAMA) at screening. 2. \>20% leukemic blasts in marrow. 3. Prior radiation to maximally tolerated levels of any critical organ. 4. Active CNS leukemia (blasts in CSF or CNS chloromas). 5. Prior allogeneic or autologous HSCT. 6. Candidates suitable for myeloablative conditioning. 7. Clinically significant cardiac disease, including: NYHA Class III or IV heart failure, Clinically significant arrhythmias (ventricular tachycardia, ventricular fibrillation, Torsade de Pointes), Myocardial infarction with uncontrolled angina within 6 months, Clinically significant congestive heart failure or cardiomyopathy 8. QTcF \>450 ms after correction of electrolytes (unless paced rhythm or investigator deems eligible; cardiology consult optional). 9. Positive HIV, HBV, or HCV test (exceptions: vaccinated HBV, or positive hepatitis markers with adequate organ function). 10. Active, uncontrolled infection. 11. Acute promyelocytic leukemia (t\[15;17\]). 12. Active malignancy within 2 years, except: Myelodysplastic syndrome, Treated non-melanoma skin cancer, Completely resected stage 0-1 melanoma (\>1 year from resection), Carcinoma in situ or cervical intraepithelial neoplasia, Organ-confined prostate cancer without progression 13. Inability to tolerate diagnostic or therapeutic procedures, particularly radiation isolation. 14. Received anti-leukemic therapy within 14 days prior to randomization (hydroxyurea allowed up to day of 131I-apamistamab).