Effect of temperature on serum protein binding characteristics of phenytoin in monotherapy paediatric patients with epilepsy

Objectives: To determine the effects of temperature on binding characteristics of phenytoin to serum proteins in paediatric patients with epilepsy. Method: Serum samples examined in the study were obtained from 41 paediatric patients (23 male, 18 female) with epilepsy on phenytoin monotherapy. Their age ranged from 1 to 15 years (mean +/- SD, 10.2 +/- 4.0 years). Protein binding of phenytoin was evaluated by ultrafiltration under current laboratory routine conditions (25 +/- 3 degreesC) or at a temperature of 37 degreesC. The in vivo binding parameters of phenytoin to serum proteins were determined using a binding equation derived from the Scatchard equation for a one-site binding model. Results: Significant differences were observed in serum concentrations of unbound phenytoin at the two temperatures (P < 0.05). The mean association constant (K) of phenytoin to serum proteins is 0.016 L/mol at 25 +/- 3 degreesC and 0.009 L/mu mol at 37 degreesC, while mean total concentration of binding sites (n(Pt)) seems to be similar between the two temperatures (682 mu mol/L for 25 +/- 3 degreesC and 746 mu mol/L for 37 degreesC). Significant differences were observed in binding characteristics of phenytoin to serum proteins for the different temperature conditions of ultrafiltration (P < 0.05). Conclusion: Our study confirms that binding affinity for phenytoin-serum protein interaction is approximately 44% lower at 37 C than at 25 +/- 3 degreesC and consequently, binding potential (K.n(Pt)) is approximately 38% lower at 37 degreesC than at 25 +/- 3 degreesC.