Influence of Coadministered Antiepileptic Drugs on Serum Zonisamide Concentrations in Epileptic Patients

We conducted a study to clarify the most suitable transforming factor related to the daily zonisamide dose (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 a total of 175 epileptic patients treated with the multiple oral administrations of zonisamide (ZNS) as a powder/tablets, were used for the analysis. Employing the extracellular water volume (VECW) as a transforming factor, led the level/dose (L/D) ratio (:Ct/(D/VECW)) to be independent of the patient's age and sex for the administration of ZNS alone. Ct was revealed to be dependent on only one variable regarding D/VECW and expressed as Ct=0.604×(D/VECW). Phenytoin (PHT) significantly lowered (p