Blast Clearance after High Dose-Melphalan Conditioning for Allogeneic Stem Cell Transplantation in Relapsed and Refractory Acute Myeloid Leukemia

Introduction: For patients (pts) with relapsed or refractory acute myeloid leukemia (r/r AML) allogeneic stem cell transplantation (ASCT) has become an established highly effective and potential curative treatment option. Even for pts with active disease, direct ASCT using sequential conditioning regimens has shown promising outcomes. In this retrospective study, we examined prognostic factors for long-term survival in r/r AML pts with active disease with special focus to the prognostic value of blast clearance during condition therapy. Patients and Methods: 186 r/r AML pts treated with sequential conditioning regimens at our center between 2014-2023 were included in our analysis. Patients underwent high-dose melphalan (100 - 140 mg/m 2, day -11 prior ASCT) followed by either fractionated total body irradiation with 8 Gy (TBI, n = 71) or non-TBI regimens (busulfan, n = 101, or treosulfan, n = 14) in combination with fludarabin (120 - 150 mg/m 2) from day -6 before ASCT. TBI-based conditioning was applied almost exclusively in patients aged < 60 years. Graft versus host-disease (GvHD) prophylaxis consisted of calcineurin inhibitors, mycophenolate and anti-T-lymphocyte globulin. In 180 out of 186 patients, blast clearance was assessed by cytologic and flow cytometric blast count via bone marrow (BM) aspirate in median 5 days after Melphalan and prior to continuation of the sequential conditioning therapy. Blast clearance was defined as cytologic BM blast count < 5% and < 0.1% BM cells with leukemia-associated immunophenotype in flow cytometry. Results: Median pts age was 67 years (range 45 - 76 years) in the non-TBI group and 51 years (range 19 - 71 years) in the TBI group, respectively. Median follow up of surviving patients was 46 months. As expected, comorbidities and disease-specific factors varied between these groups. HCTCI scores > 3 points were observed in 14% of TBI pts compared to 32% in non-TBI pts ( p .01) and 45% versus 54% harbored adverse risk genetics according to European Leukemia Net (ELN) 2017 classification ( p .41), respectively. In the non-TBI group, 38% of the pts had a secondary or therapy-related AML compared to 20% of pts in the TBI group ( p .03). HLA-mismatched (