Intravenous Busulfan: Pharmacokinetic Disposition and Clinical Outcomes in Children Undergoing Hematopoietic Stem Cell Transplant.

Background: Conditioning regimens preceding hematopoietic stem cell transplantation (HSCT) for various malignant and non-malignant diseases in children often include busulfan. Busulfan administration may be complicated by hepatic veno-occlusive disease (HVOD). A relationship has been described between high systemic exposure to busulfan after oral administration, as measured by area under the curve (Bu-AUC) and HVOD. Recently, IV busulfan (IV Bu) administration has been reported to be associated with a much lower incidence of HVOD than oral administration in adults. Objectives: To describe the pharmacokinetics of IVBu in infants (<1 year of age) and children. To determine the incidence of HVOD in children undergoing conditioning with IV Bu, and to correlate IV Bu AUC with the development of HVOD and neutrophil engraftment. Methods: Twenty-four children who underwent HSCT at The Hospital for Sick Children between April 2003 and September 2004 and received IV Bu as part of their conditioning regimen were included in this retrospective study. Diagnoses included: AML (6), metabolic storage disease (6), immune deficiency syndromes (4), histiocytosis (2), beta-thalassemia (2), WAS (1), MDS (1), CML (1), relapsed meduloblastoma (1). Initial IV Bu doses were based on actual patient weight: <9kg =0.95mg/kg/dose; 9–16kg =1.2mg/kg/dose; 16–23kg =1.1mg/kg/dose; 24–34kg =0.95mg/kg/dose; >34kg =0.8mg/kg/dose. Seven blood samples were drawn after the first IV Bu dose for determination of plasma busulfan concentrations. Pharmacokinetic parameters were calculated using 1-compartment analysis (WinNonLin 4.1). The third and subsequent IV Bu doses were adjusted to achieve an AUC of 900–1500μMol•min. HVOD (modified Baltimore criteria) and engraftment (ANC > 0.5 x 109/L) were evaluated. Results: The median patient age was 3.5 years (range 3mo–16.9yrs), including 9 infants. Mean IV Bu pharmacokinetic parameters were: Cmax=4.7±0.9μMol; Vss=0.70±0.22L/kg; ke=0.005±0.001min−1; AUC=1256±320μMol•min. The mean IV Bu AUC of infants was not different from older children (1164μ±331Mol•min vs. 1311±311μMol•min; p=0.35). The mean Vss was higher in infants than older children (0.84±0.29L/kg vs. 0.62±0.10L/kg; p=0.025), but the mean clearance was not different when corrected for body weight. Twenty-three patients (95.8%) engrafted between day +10 to +27. HVOD was diagnosed in 6 patients (25%), including 3 infants. Five patients had moderate HVOD and one had fatal HVOD. Mean IV Bu AUC was 1317±310μMol•min and 1074±299μMol•min in the non- HVOD vs. the HVOD group, respectively (p=0.10). The number of children who did not engraft precluded assessment of the relationship between IV Bu AUC and engraftment. Conclusions: Busulfan Vss differs significantly between infants and older children. A significant proportion of patients developed HVOD. No association was observed between IV Bu AUC and the development of HVOD in children where busulfan doses are adjusted to achieve a target IV Bu AUC.