COMPARABLE OUTCOMES AFTER BUSULFAN- OR TREOSULFAN-BASED CONDITIONING FOR ALLO-HSCT IN CHILDREN WITH ALL-RESULTS OF FORUM

• Chemo conditioning with busulfan- or treosulfan-based regimens yields comparable outcomes for allo-HSCT in children with ALL.

• Either regimen can be effectively and safely used in patients > 4 years when TBI is contraindicated or unavailable.

The superiority of TBI-based versus chemotherapy-conditioning for allo-HSCT in children with ALL has been established in the international, prospective phase-III FORUM study (#NCT01949129), randomizing 417 patients ≤ 18 years at diagnosis (4-21 years at HSCT) in CR, transplanted from HLA-matched sibling or unrelated donors. Due to the unavailability of TBI in some regions and to accommodate individual contraindications, this study reports the pre-specified comparison of outcomes of patients receiving busulfan-based (BU) or treosulfan-based (TREO) regimens from 2013 to 2018. 180 and 128 patients (median age 9.9 years) received BU/THIO/FLU or TREO/THIO/FLU, respectively. Data were analysed as of 02/2023, with a median follow-up of 4.2 years (range 0.3-9.1). Three-year overall survival was 0.71 (0.64-0.77) (BU/THIO/FLU) and 0.72 (0.63-0.79) (TREO/THIO/FLU), event-free survival was 0.60 (0.53-0.67) (BU/THIO/FLU) and 0.55 (0.46-0.63) (TREO/THIO/FLU), with both p = NS. The 3-year cumulative incidence of relapse (0.31 (0.25-0.38) BU, 0.36 (0.27-0.44) TREO, p = 0.779) and non-relapse mortality (0.08 (0.05-0.13) BU, 0.09 (0.05-0.15) TREO, p = 0.831) were comparable (p = NS). One case of fatal veno-occlusive disease occurred in each group. No significant differences in acute and chronic GvHD or GvHD-free and relapse-free survival (0.48 (0.41-0.55) BU, 0.45 (0.37-0.54) TREO, p = 0.89) were recorded. Outcomes for CR1 or CR2 patients were similar irrespective of the regimen used. In conclusion, BU/THIO/FLU or TREO/THIO/FLU regimens can be an alternative to TBI for ALL patients > 4 years with contraindications or lack of access to TBI.