Impact of busulfan versus treosulfan dose intensity in myelofibrosis undergoing hematopoietic cell transplantation

Abstract One key aspect of allogeneic hematopoietic cell transplantation (HCT) is pretransplant conditioning, balancing risk for relapse versus non‐relapse mortality. Conditioning regimens with different alkylators at different doses can influence outcome, but data are missing for myelofibrosis, a challenging cohort of patients usually presenting at older age and with comorbidities. We evaluated in a multicenter retrospective study the comparative efficacy and safety of busulfan versus treosulfan in combination with fludarabine for myelofibrosis patients undergoing HCT. This study included 1115 patients (busulfan, n = 902; treosulfan, n = 213) receiving first HCT between 2005 and 2021. Patients were generally balanced for key patient characteristics. Overall survival at 4 years was 62% for the busulfan group versus 58% for the treosulfan group ( p = .22). Impact on outcome was dose‐dependent. Overall survival was 65% (95% CI, 61%–69%) for reduced intensity busulfan versus 69% (95% CI, 54%–84%) for reduced intensity treosulfan, 53% (95% CI, 44%–63%) for higher intensity busulfan, and 55% (95% CI, 46%–63%) for higher intensity treosulfan. Incidence of relapse was similar across intensity groups. In multivariable analysis, the hazard for death (with reduced intensity busulfan as reference) was 0.88 (95% CI, 0.39–2.01) for reduced intensity treosulfan ( p = .77), 1.42 (95% CI, 0.96–2.10) for higher intensity busulfan (0.08), and 1.61 (95% CI, 1.14–2.26) for higher intensity treosulfan ( p = .006). In terms of non‐relapse mortality, comparison was not significantly different, while the hazard ratio for higher intensity treosulfan was 1.48 (95% CI, 0.98–2.23; p = .06). Here, we showed comparable outcomes and improved survival in myelofibrosis undergoing HCT with reduced intensity busulfan or treosulfan.