Pharmacokinetics/Pharmacodynamic Relationship in Busulfan Conditioning Regimen: Results from a Large Pediatric Cohort Undergoing Hematopoietic Stem-Cell Transplantation

Background : Busulfan (Bu) is the corner stone of hematopoietic stem-cell transplantation (HSCT) regimens with a narrow therapeutic window (TW). Graft rejection or toxicities are reported according to plasmatic exposure. In very young children, Bu exhibits large pharmacokinetic (PK) variability. Bu clearance was demonstrated to be non-linearly related to body weight (BW) and thus BW is used to optimally define the Bu dosage in children [Vassal et al., CCP 2006]. Search for additional data to confirm the TW and to describe clinical outcomes is still a topical question to better understand Pk/Pd relationship and to safely use Bu in young children and infants. Objective : To study the Pk/Pd relationship of Bu in pediatric recipients of bone marrow transplantation (BMT) receiving Bu-based conditioning regimens (CR). To correlate early toxicities (mainly hepatic veno-occlusive disease (VOD), non infectious pulmonary disease (niPD) and outcome i.e. overall survival (OS) at last follow up with type of CR, the underlying diseases, age at transplantation and Pk of busulfan Patients and Method : This multicenter prospective observational study included 307 pts transplanted between 2006 and 2013 from 14 French Pediatric BMT units; median age at transplantation was 18.4 months [1.3-289], with a median BW of 11.3 kg [3.4-82]. 100 pts were younger than 1 year, 71 pts PK was assessed on plasma samples with area under the curve (AUC) evaluation from the 1 st (D1), 9 th (D9) and 13 th (D13) dose in 150 patients, D1 and D9 in 40 patients, D1 and D13 in 46 patients while 62 patients were monitored on D1 only (684 PK datasets). PK analysis was performed using Non linear Mixed Effect Modelling. A one-compartment PK model suitably fitted the concentrations vs time data. Median follow up is 27 months (0.33 to 96 months post BMT). Results : At D1, 67% of patients were within the therapeutic window (TW) [900-1500 µM.min] and 66% of patients reached the cumulative TW [14400-24000 µM.min]. Incidence of VOD and niPD were respectively 35% and 14%. Both toxicities correlated with type of CR and age 900 µM.min. OS was 69.1% at last follow up and was not significantly associated with CR and age at transplantation. Conclusion : In this large series of pediatric BMT recipients, we found that toxicities as VOD and niPD correlated with type of CR and age at transplantation in univariate analysis. Combination of 2 alkylating agents with Bu or BuFlu and transplantation Disclosures No relevant conflicts of interest to declare.