¹³¹I-Anti-CD45 Antibody Plus Fludarabine, Low-Dose Total Body Irradiation and Peripheral Blood Stem Cell Infusion for Elderly Patients with Advanced Acute Myeloid Leukemia (AML) or High-Risk Myelodysplastic Syndrome (MDS).

The poor survival of elderly patients with advanced AML or high-risk MDS following conventional chemotherapy, as well as their poor tolerance for high-dose regimens used in conventional myeloablative hematopoietic cell transplantation (HCT) demands innovative therapeutic approaches. Recent success achieving stable donor chimerism following infusion of allogeneic peripheral blood stem cells (PBSC) after reduced intensity (non-myeloablative) conditioning regimens affords an opportunity to safely induce a graft-vs-leukemia (GVL) effect with minimal acute morbidity. GVL effects, however, appear to be most potent in patients with low tumor burdens at the time of transplantation. We have therefore conducted a Phase I clinical trial of targeted hematopoietic irradiation delivered by an ¹³¹I-labeled anti-CD45 antibody (BC8) to determine the feasibility, safety and efficacy of this approach toward reducing the burden of disease before an established non-myeloablative regimen. In this dose escalation study designed to estimate the maximum tolerated dose of ¹³¹I-BC8 antibody that can be combined with fludarabine (FLU) and low dose total body irradiation (TBI), 33 patients over 50 years of age with advanced AML or high-risk MDS (> 5% blasts) were treated with 246 to 932 mCi ¹³¹I delivering an estimated 5.2 to 45.9 (mean 27.5) Gy to bone marrow, 17.3 to 155 (mean 81.2) Gy to spleen, and 12–24 Gy to the liver (dose-limiting organ). Patients then received FLU (30 mg/m² daily for 3 days), 2 Gy TBI, and HLA-matched related (n = 10) or unrelated (n = 23) PBSC grafts with graft-vs-host disease prophylaxis provided by cyclosporine and mycophenolate mofetil. The median age of patients was 61 (50–71) years. Twenty-four patients had AML, with 6 (13%) patients in second or third complete remission, 2 (4%) with primary refractory disease, and 16 (35%) in relapse. Nine (20%) patients had MDS with >5% blasts. Treatment with the ¹³¹I-BC8 Ab/FLU/TBI regimen produced a remission in all patients, and all had 100% donor CD3⁺ and CD33⁺ cell engraftment by day 28 post-transplant. The absolute neutrophil count surpassed 500/uL at a median of 14 (range, 10–19) days, and the self-sustained platelet count surpassed 20,000/uL at a median of 17 days (range, 15–43). Eighteen patients (55%) are surviving disease-free 2 to 16 months (median 9.5 months) post-transplant. In 9 (27%) patients, the disease relapsed 3 to 38 months after HCT. The day-100 non-relapse mortality was 12%. This study demonstrates that at least an average of 27 Gy of targeted radiotherapy can be delivered to bone marrow and an average of 81 Gy to the spleen, in addition to a standard reduced intensity transplant regimen, without a marked increase in day 100 mortality. Whether this approach will reduce post-transplant relapse rates for older patients with high-risk AML/MDS remains to be determined.