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Aim The narrow therapeutic index and large interpatient variability in sirolimus pharmacokinetics(PK) make therapeutic drug monitoring necessary.Factors responsible for PK variability are not well understood, and published PK studies do not include pediatric patients with immune cytopenia. The objective of this study was to characterize the PK of sirolimus in paediatric patients with immune cytopenia and to develop a population pharmacokinetic model in Chinese children,and evaluated its utility for dose individualization. Methods A total of 27 children with either acquired or congenital immune cytopenia aged 8.16±3.60 years(range1-15 years) were included. Therapeutic drug monitoring(TDM) data for sirolimus were collected. The population pharmacokinetic model of sirolimus was described using the nonlinear mixed-effects modeling(Phoenix NLME 1.3 software) approach.Covariate analysis was applied to select candidate factors associated with pharmacokinetic parameters. The final model was validated using bootstrap(1000 runs) and visual predictive check(VPC) method. Results A one-compartment model with first-order absorption and elimination was developed. The outcome parameters were as follows: apparent clearance(CL/F) 5.63 L/h, apparent distribution volume(V/F) 144.16 L. Interindividual variabilities for CL/F, V/F were 3.53%, 7.27%,respectively. The intra-individual variability of proportional error model was 22.45%.The covariate test found that body weight and total bilirubin was strongly associated with clearance, however, we didn’t find the relevance between the covariate and volume of distribution of sirolimus. Personalized dosage regimens were provided based on Bayesian method. The oral dose should be adjusted according to weight and total bilirubin. Conclusion This study is the first to describe a population PK model of sirolimus and disposition in pediatric patients with immune cytopenia. The developed model will facilitate PK model-based dose individualization of sirolimus and the design of future clinical studies in children.