In vivo binding characteristics of phenytoin to serum proteins in monotherapy pediatric patients with epilepsy.

Abstract

The aim of the present study was to determine the binding characteristics of phenytoin (PHT) to serum proteins in the pediatric population. Binding parameters of PHT to serum proteins in our study were conducted to compare with in vivo or in vitro binding parameters of PHT to serum proteins in adult subjects reported by other investigators. Serum samples in the study were obtained from 40 pediatric patients (16 male, 24 female) receiving PHT monotherapy. Their age ranged from 1 to 15 years (9.2 +/- 3.6 years, mean +/- SD). The in vivo population binding parameters of PHT to serum proteins and theoretical minimal unbound serum PHT fraction (fu) were determined using an equation derived from the Scatchard equation. The association constant (Ka) was 0.014 l/micromol, while the total concentration of binding sites (n(Pt)) was 747 micromol/l. The number of binding sites per albumin molecule (n) was 1.13, while binding ability (n x Ka) was 0.0161/micromol. The fu was 0.087. The n x Ka is approximately 1.2 times higher in PHT monotherapy adult patients of Pospisil et al. [1992] (i.e. 0.0191 l/micromol) than in all our patients. The association constant is approximately 1.3 times higher in the in vitro study of Monks et al. [1978] (i.e. 0.0186 l/micromol) than in our study, while n is similar between the two studies. The fu in our pediatric patients is similar to the unbound serum PHT fraction in adult patients receiving PHT therapy reported by Richens [1979] (i.e. 0.1). Our results suggest that there may be small differences in the binding characteristics of PHT to serum proteins between Japanese pediatric and non-Japanese adult subjects. The unbound serum fraction of PHT in pediatric patients with epilepsy can be assumed to be relatively constant in the therapeutic concentration range of PHT.