PHARMACOKINETICS AND DISPOSITION Population pharmacokinetic analysis of tacrolimus in the first year after pediatric liver transplantation

Purposes Tacrolimus (TAC) is the most widely used immu-nosuppressant for the prevention of acute rejection after solid organ transplantation. Its pharmacokinetics (PK) show considerable variability, making TAC a good candidate for therapeutic drug monitoring (TDM). The principal aim of the study was to describe the PK of TAC in pediatric patients during the first year after transplantation. Methods Routine TDM trough levels of TAC were obtained from 42 pediatric liver allograft recipients during the first year after transplantation. A population PK model was de-veloped using nonlinear mixed-effects modeling to describe TAC PK during this period and to explain the observed variability by means of patients ’ demographics, biochemical test results and physiological characteristics. Results The PK of TAC were best described by a two-compartment model with first-order elimination. Apparent volumes of the central compartment, intercomparmental clearance and maximum blood clearance estimates were 253 L, 115 L/day and 314 L/day, respectively. The absorp-tion first-order rate and volume of peripheral compartment were fixed to 4.5 h-1 and 100 L, respectively. While hemat-ocrit levels, time after transplantation and bodyweight influenced TAC clearance, bodyweight was the only covar-iate retained on volume of distribution. Conclusions We developed a TAC population PK model in pediatrics covering the first year after liver transplantation that may serve as a tool for TAC dose individualization as part of TDM