Targeted Radiation Delivery before Haploidentical HCT for High-risk Leukemia or MDS Patients Yields Long-term Survivors

Purpose: Hematopoietic cell transplantation (HCT) has curative potential for myeloid malignancies, though many patients cannot tolerate myeloablative conditioning with high-dose chemotherapy alone or with total-body irradiation (TBI). Here we report long-term outcomes from a phase I/II study using iodine-131 (¹³¹I)-anti-CD45 antibody BC8 combined with nonmyeloablative conditioning prior to HLA-haploidentical HCT in adults with high-risk relapsed/ refractory acute myeloid or lymphoid leukemia (AML or ALL), or myelodysplastic syndrome (MDS; ClinicalTrials.gov, NCT00589316). Patients and Methods: Patients received a tracer diagnostic dose before a therapeutic infusion of ¹³¹I-anti-CD45 to deliver escalating doses (12–26 Gy) to the dose-limiting organ. Patients subsequently received fludarabine, cyclophosphamide (CY), and 2 Gy TBI conditioning before haploidentical marrow HCT. GVHD prophylaxis was posttransplant CY plus tacrolimus and mycophenolate mofetil. Results: Twenty-five patients (20 with AML, 4 ALL and 1 high-risk MDS) were treated; 8 had ≥ 5% blasts by morphology (range 9%–20%), and 7 had previously failed HCT. All 25 patients achieved a morphologic remission 28 days after HCT, with only 2 patients showing minimal residual disease (0.002–1.8%) by flow cytometry. Median time to engraftment was 15 days for neutrophils and 23 days for platelets. Point estimates for overall survival and progression-free survival were 40% and 32% at 1 year, and 24% at 2 years, respectively. Point estimates of relapse and nonrelapse mortality at 1 year were 56% and 12%, respectively. Conclusions: ¹³¹I-anti-CD45 radioimmunotherapy prior to haploidentical HCT is feasible and can be curative in some patients, including those with disease, without additional toxicity.