Comparison of outcomes of BuCy versus BuCyMel as preparation for allogeneic HSCT in infants with acute myeloid leukemia in first complete remission: a multicenter study from the EBMT PDWP

Allogeneic haematopoietic stem cell transplantation (HSCT) is an essential treatment component for high-risk paediatric acute myeloid leukaemia (AML), but the choice of the optimal conditioning regimen remains controversial. We compared outcomes of two widely used myeloablative regimens, BuCy (Busulfan, Cyclophosphamide) versus BuCyMel (Busulfan, Cyclophosphamide, Melphalan) in children aged under 2 years undergoing HSCT in 1st complete remission (CR1). This international, registry-based study was conducted by the Paediatric Diseases Working Party (PDWP) of the European Society for Blood and Marrow Transplant (EBMT) and included patients transplanted from 2000-2022. 335 patients, transplanted across 98 centres, were included, 185 (55.2%) patients received BuCy and 150 patients (44.8%) received BuCyMel. BuCyMel was associated with significantly better leukaemia-free survival (LFS) (4 y: 74.8% vs 58.8%, HR 0.52 (95%CI: 0.33-0.82), p = 0.004) and overall survival (OS) (4 y: 77.9% vs 66.2%, HR 0.55 (95%CI: 0.33-0.91), p = 0.02) compared with BuCy, and significantly lower relapse incidence (4 y: 18.1% BuCyMel vs 31.5% BuCy, HR 0.50 (95%CI 0.29-0.87), p = 0.01). Four-year non-relapse mortality (NRM) did not differ between groups. Our results suggest that BuCyMel is a better conditioning regimen compared with BuCy in children aged under 2 years undergoing HSCT for CR1 AML.