Busulfan exposure associated with event-free survival in children after allogeneic haematopoietic stem cell transplantation: A retrospective multicenter cohort study

Busulfan exposure associated with event-free survival in children after allogeneic haematopoietic stem cell transplantation: A retrospective multicenter cohort study OBJECTIVE To determine the relationship between busulfan cumulative area under the curve (AUC] and event-free survival lEFS] in children undergoing allogeneic haematopoietic stem cell transplantation (alloHCT). DESIGN Retrospective, multicenter cohort study. METHODS Children who underwent alloHCT in 15 different centres worldwide were included in this study (2000-20131. Participants had to be on intravenous busulfan and pharmacokinetic samples had to be available. Exposure of interest was the cumulative AUC of busulfan, and primary outcome was EFS (time to graft failure, relapse or all-cause mortality). Cox regression models were used to derive relative risks (RR), and the optimal busulfan AUC level was estimated using propensity adjusted Weibull models. RESULTS A total of 674 subjects (41[%] malignant, 59[%] non- malignant) with a median age of 4.5 years (interquartile range 1.4-10.7 years) were included in the analysis. We observed a significant U-shaped relationship between busulfan cumulative AUC and EFS (P = 0.011). The optimal target was estimated at 90 mgh/L (78-101 mgh/L), and was independent of any of the investigated patient characteristics. An AUC below the target increased the risk of graft failure and relapse (relative risk 1.75, P = 0.004), while transplant-related mortality was more pronounced when the AUC was too high (relative risk 2.99, P <0.0011. CONCLUSION This is the largest study on the relationship between busulfan and clinical outcomes in children undergoing alloHCT. Our results strongly advocate the use of therapeutic drug monitoring of busulfan, using 90 mg-h/L (78-101 mg h/L) as a target