Development of a population pharmacokinetic model for carbamazepine based on sparse therapeutic monitoring data from pediatric patients with epilepsy

Background: Population models can be important extensions of therapeutic drug monitoring (TDM), as they allow estimation of individual pharmacokinetic parameters based on a small number of measured drug concentrations. Objective: This study used a Bayesian approach to explore the utility of routinely collected and sparse TDM data (1 sample per patient) for carbamazepine (CBZ) monotherapy in developing a population pharmacokinetic (PPK) model for CBZ in pediatric patients that would allow prediction of CBZ concentrations for both immediate- and controlled-release formulations. Methods: Patient and TDM data were obtained from a pediatric neurology outpatient database. Data were analyzed using an iterative 2-stage Bayesian algorithm and a nonparametric adaptive grid algorithm. Models were compared by final log likelihood, mean error (ME) as a measure of bias, and root mean squared error (RMSE) as a measure of precision. Results: Fifty-seven entries with data on CBZ monotherapy were identified from the database and used in the analysis (36 from males, 21 from females; mean [SD] age, 9.1 [4.4] years [range, 2-21 years]). Preliminary models estimating clearance (Cl) or the elimination rate constant (K-el) gave good prediction of serum concentrations compared with measured serum concentrations, but estimates of Cl and Kel were highly correlated with estimates of volume of distribution (V-d). Different covariate models were then tested. The selected model had zero-order input and had age and body weight as covariates. Cl (L/h) was calculated as K-el center dot V-d, where K-el = [K-i - (K-s center dot age)] and V-d = [V-i + (V-s center dot body weight)]. Median parameter estimates were V-i (intercept) = 11.5 L (fixed); V-s (slope) = 0.3957 L/kg (range, 0.01200-1.5730); K-i (intercept) = 0.173 h(-1) (fixed); and K-s (slope) = 0.004487 h(-1) center dot y(-1) (range, 0.0001800-0.02969). The fit was good for estimates of steady-state serum concentrations based on prior values (population median estimates) (R = 0.468; R-2 = 0.219) but was even better for predictions based on individual Bayesian posterior values (R-2 = 0.991), with little bias (ME = -0.079) and good precision (RMSE = 0.055). Conclusions: Based on the findings of this study, sparse TDM data can be used for PPK modeling of CBZ clearance in children with epilepsy, and these models can be used to predict Cl at steady state in pediatric patients. However, to estimate additional pharmacokinetic model parameters (eg, the absorption rate constant and Vd), it would be necessary to combine sparse TDM data with additional well-timed samples. This would allow development of more informative PPK models that could be used as part of Bayesian dose-individualization strategies. Copyright (c) 2005 Excerpta Medica, Inc.