New approach to busulfan dosing in infants and children based on a population pharmacokinetic analysis.

Purpose: Apply population pharmacokinetic modeling to a single institution busulfan therapeutic drug monitoring (TDM) data set from infants and children to refine dosing methods.

Methods: One-compartment pharmacokinetic model was fit to busulfan TDM data from 328 infants and children with malignant and non-malignant diseases treated with busulfan-containing transplant conditioning regimens. Age-dependence of busulfan clearance scaled to body weight and body surface area (BSA) was compared, and busulfan AUC was simulated for a BSA-scaled dose of 100 mg/m² combined with a BSA-banded dosing table for infants and children with a BSA < 0.5 m².

Results: Busulfan clearance scaled to body weight is age-dependent. Clearance in children ≤ 3 years (0.234 L/[h•kg]) is higher than the typical value for the population, (0.205 L/[h•kg]), and 48% of children < 5 years have subtherapeutic busulfan AUCs after the first dose. Busulfan clearance scaled to BSA (typical value, 5.47 L/[h•m²]) is more uniform across the pediatric age span, except for infants (≤ 1 year, 4.27 L/[h•m²]). Simulated busulfan AUCs with a dose of 100 mg/m² for patients with a BSA ≥ 0.5 m² combined with a BSA-banded dosing table for patients with a BSA < 0.5 m² achieved a therapeutic AUC after the first dose in 49% more patients than body weight scaled doses.

Conclusion: Our model predicts a greater proportion of children would achieve a therapeutic busulfan AUC after the first dose with a dose of 100 mg/m²/d combined with the infant dosing table for patients with a BSA < 0.5 m² compared to body weight-scaled dosing.