Haploidentical Peripheral Blood Stem Cell Transplantation with Post-Transplantation Cyclophosphamide in Primary Refractory Acute Myeloid Leukemia

In a pilot study, 75 patients with Primary Refractory (PRef) AML without matched family donors were offered post-transplantation cyclophosphamide (PTCY) based haploidentical peripheral blood stem cell (PBSC) transplantation. Twenty-seven patients (36%) opted for haploidentical transplantation with or without further chemotherapy. There was no difference in the patient or disease characteristics amongst patients undergoing transplantation or not. The conditioning regimen comprised of FluCyMel (n=5), FluBuMel (n=17) and FluTreoTBI (n=5). MMF was tapered between days 14 and 21 posttransplant in the absence of GVHD and cyclosporine A was tapered between days 60 and 90. The progression free survival at a median follow-up of 25 months was 36.6% and 0% in the transplant and the non-transplant group (p=0.0001). Prompt engraftment was noted at a median of 14 days irrespective of disease status or conditioning regimens. Cumulative incidences of acute graft-versus-host disease (GVHD) and chronic GVHD were 26.6% and 8% respectively. The overall incidence of infections remained low, with CMV reactivation and invasive aspergillosis occurring in 9 and 2 patients respectively. CMV disease was diagnosed in 2 patients. Non-relapse mortality at 1 year was 16.7%. The incidence of disease progression was 54%. Factors positively impacting progression free survival were < 15% marrow blasts at transplant and a Natural Killer Cell Ligand Mismatch (NKLMM) donor. NKLMM, Haplotype or B scores had no impact on CMV infection or GVHD. However, Bx Haplotype was associated with lower NRM (5%, 95%CI-1-9) compared to 48.6% (95%CI 28.2-69.0) in AA Haplotype (p=0.01). Disease status did not impact the overall survival (p=0.11) in the HSCT cohort. In fact, NKLMM donors with B haplotype had the greatest impact on overall survival in both the HSCT cohort (71.4%, 95%CI 54.3-88.5%) compared to 20% (95%CI 8.8-31.2, p=0.01) in those without the same. Our data suggests PTCY based Haploidentical PBSC transplantation is feasible in patients with PRef AML and donor NKLMM might improve progression free survival, provided the conditioning protocol and the post-grafting therapy offer the optimum platform for alloreactivity of NK cells.