The necessity of adjusting the dosage of zonisamide when coadministered with other anti-epileptic drugs.

Zonisamide (ZNS), a new anti-epileptic drug that exhibits a wide anti-epileptic spectrum, is commonly prescribed concomitantly with other anti-epileptic drugs. The interaction between ZNS and other anti-epileptic drugs was investigated in epileptic patients. The steady state plasma levels of all the anti-epileptic drugs were measured by high-performance liquid chromatography. The concomitant administration of phenobarbital (PB), phenytoin (PHT), or carbamazepine (CBZ) with ZNS significantly decreased the ratio of the steady state plasma concentration to the administered dose (C/D ratio) of ZNS, whereas clonazepam (CZP) and valproic acid (VPA) when administered concomitantly with ZNS did not change the C/D ratio. Significant negative correlations were observed between the C/D ratio of ZNS and the doses of PB, PHT, CBZ, and VPA. On the other hand, the ratio of the plasma concentration of carbamazepine-10,11-epoxide (CBZ-E), the major active metabolite of CBZ, to the plasma concentration of CBZ was significantly decreased by concomitant administration with ZNS, indicating that ZNS inhibits the metabolism of CBZ. These findings show that when ZNS is administered concomitantly with these anti-epileptic drugs, it is necessary to monitor the plasma concentration of ZNS in order to adjust its dosage.