Population Pharmacokinetics of Busulfan in Saudi Pediatric Patients Undergoing Hematopoietic Stem Cell Transplantation

Background Busulfan is an antineoplastic drug that is used widely as part of a conditioning regimen in pediatric patients undergoing hematopoietic stem cell transplantation. It has a narrow therapeutic index and highly variable pharmacokinetics; therefore therapeutic drug monitoring is recommended to optimize busulfan dosing. Objective To study the population pharmacokinetics of busulfan in Saudi pediatric patients to optimize its dosing. Settings King Abdullah Specialist Children's Hospital in Riyadh, Saudi Arabia. Methods This pharmacokinetic observational study was conducted between January 2016 and December 2018. All pediatric patients receiving IV busulfan and undergoing routine therapeutic drug monitoring were included. Population pharmacokinetics modeling was conducted using Monolix2019R1. Pharmacokinetic data of busulfan in children. Results The study included 59 patients and 513 samples. The mean ± SD age was 6.10 ± 3.17 years, and the dose administered was 0.994 ± 0.15 mg/kg. The mean ± SD Cmax and area under the curve (AUC) were 900.60 ± 402.8 ng/mL and 1031.14 ± 300.75 µM min, respectively. Based on our simulations, the European Medicines Agency recommended dose were adequate for most patient's groups to achieve the conventional target of an AUC of 900-1350 µM min. For patients in the lower weight group < 9 kg, higher doses were need at 1.2 mg/kg. With regards to the newly proposed target of AUC 78-101 mg h/mL, all of the doses we tested had low probability of achieving it. Conclusions Most of our patients had less than a proportional increase in busulfan concentration suggesting autoinduction. The high interindividual variability and autoinduction make dose adjustments challenging and AUC at steady state difficult to predict from the first dose. One approach to improve dose predictions is to use Bayesian dosing software. Based on our simulations, the European Medicines Agency recommended doses were adequate for most patient groups, except those in the lower (< 9 kg) and higher weight groups (> 34 kg).