Impact of busulfan exposure stratification on transplant outcomes and immune reconstitution in children with severe aplastic anemia.

BACKGROUND: Severe aplastic anemia (SAA) is a life-threatening bone marrow failure disorder. Allogeneic hematopoietic stem cell transplantation (allo-HSCT) is an established curative treatment for pediatric patients with SAA. Busulfan, a key component of conditioning regimens, has a narrow therapeutic index and marked interindividual pharmacokinetic variability. However, the impact of different first-dose busulfan exposure levels on long-term outcomes, particularly immune reconstitution, remains unclear in pediatric patients receiving a 2-day conditioning regimen. We evaluated the clinical significance of busulfan exposure stratification in pediatric patients with SAA undergoing allo-HSCT. METHODS: This retrospective study included 40 pediatric patients with SAA who underwent allo-HSCT. Patients were stratified into a low-exposure group (first-dose area under the concentration-time curve [AUC0-t] <900 μmol min/L, n = 24, 1 mg h/L ≈ 243.6 μmol min/L) and a standard-exposure group (AUC0-t ≥900 μmol min/L, n = 16). Engraftment kinetics, transplant-related complications, and survival outcomes were compared between the groups. Longitudinal immune reconstitution (T-cell, NK-cell, B-cell counts, and immunoglobulin levels) was analyzed using linear mixed-effects models with Holm adjustment for multiple testing. RESULTS: No significant differences were observed between the groups in neutrophil or platelet engraftment, incidence of graft-versus-host disease, hemorrhagic cystitis, engraftment syndrome, or short-term survival outcomes. All nine immune outcomes changed significantly over the first 12 months after transplantation, documenting pronounced temporal immune reconstitution (All adjusted P < 0.001 except IgM: P = 0.002). However, exposure-related differences in these trajectories were not confirmed after Holm adjustment (all adjusted P > 0.05). CONCLUSION: First-dose busulfan AUC0-t < 900 μmol min/L (3.7 mg h/L) was not associated with impaired engraftment or increased early toxicity in pediatric patients with SAA undergoing allo-HSCT. The predominant longitudinal immune finding was time-dependent immune reconstitution, without confirmatory evidence that the two exposure groups followed different trajectories. These findings are limited by the modest sample size and retrospective design; therefore, prospective validation is needed to inform dosing strategies.