Influence of coadministered antiepileptic drugs on serum antiepileptic drug concentrations in epileptic patients - Quantitative analysis based on suitable transforming factor

Abstract

We conducted a study to clarify the most suitable transforming factor related to the daily dose of antiepileptic drugs (D) providing a steady-state serum concentration (Ct) and analyzed the influences of the concomitant use of antiepileptic drugs on Ct quantitatively. Data obtained by routine therapeutic drug monitoring from epileptic patients treated with the multiple oral administration of valproic acid (VPA), carbamazepine (CBZ), zonisamide (ZNS), phenobarbital (PB), and phenytoin (PHT) were used for the analysis. Employing the ideal body weight or the extracellular water volume as a transforming factor, allowed the level/dose (L/D) ratio to be independent of the patient's age and gender for monotherapy with VPA or CBZ, ZNS, PB, and PHT, respectively. Each Ct was revealed to be dependent on only one variable in terms of the transformed daily dose (D′). Ct was proportional to the power function of D′ for VPA and CBZ and was linearly proportional to D′ for ZNS and PB. The L/D ratio is expressed as a linear function of Ct for PHT. For a detailed analysis of the influences of the coadministered antiepileptic drugs, we defined the parameter as an alteration ratio, representing the influence of each antiepileptic drug on the Ct of VPA and CBZ alone, and on the L/D ratio of ZNS and PB alone, respectively. A model based on the assumption that each value of an alteration ratio was independent from one other and multiplicative for VPA, CBZ, and ZNS, and that the coadministered drug inhibited the drug-metabolizing enzyme competitively for PB, was adopted. The Michaelis-Menten kinetic model was adopted for PHT. The analysis clarified that CBZ, PB, and PHT significantly lowered (P<0.05) Ct to 0.81, 0.88, and 0.83 compared with the value of VPA alone, that PB and PHT significantly lowered Ct to 0.77 and 0.71 compared with the value of CBZ alone, and that VPA, CBZ, PB, and PHT significantly lowered the L/D ratio of ZNS alone to 0.87, 0.85, 0.85, and 0.80, respectively. VPA, CBZ, and PHT significantly increased (P<0.05) the L/D ratio of PB to 1.47, 1.18, and 1.19, respectively. The daily PHT dose was decreased to 0.89, 0.91, 0.90, and 0.84 the dose of PHT alone to maintain C t in the therapeutic range when VPA, CBZ, ZNS, and PB were coadministered, respectively. In the case of the addition or discontinuance of concomitant treatment with antiepileptic drugs in the same patient, the estimated Ct values were calculated using the value of each alteration ratio and compared with the measured ones. Each mean of prediction error was about 20%. Our results appear valid and these alteration ratios should be available for clinical use. © 2004 The Pharmaceutical Society of Japan.