Clinical pharmacology and therapeutic drug monitoring of zonisamide

Zonisamide (1,2-benzisoxazole-3-methanesulfonamide) is a new antiepileptic drug developed in Japan. This compound is insoluble in water, and it is available in tablet and powder form. In experimental animals, this compound has been found to have a strong inhibitory effect on convulsions of cortical origin because it suppresses focal spiking and the spread of secondary generalized seizures. In humans, a series of double-blind, placebo-controlled studies revealed the efficacy of zonisamide for patients with refractory partial seizures and for selected patients with infantile spasms. Its antiepileptic mechanism of action remains unclear, but it is likely to involve blockade of both sodium and T-type calcium channels. Oral bioavailability of zonisamide is excellent in healthy human volunteers. Zonisamide is slowly absorbed and has a mean t(max) of 5 to 6 hours. Almost 100% of it is absorbed; there is no difference in bioavailability between tablets and powder. Zonisamide concentrations are highest in erythrocytes and then in whole blood and plasma. It is approximately 40% to 60% bound to plasma proteins, primarily albumin. Its volume distribution is 0.9 to 1.4 L/kg. In adults, the elimination half-life is between 50 and 62 hours, and it takes as long as 2 weeks to reach steady state. The dose-serum level correlation is linear up to doses of 10 to 15 mg/kg per day, and the therapeutic range is 10 to 40 mu g/ml. However, the relationship between serum zonisamide levels, clinical response, and adverse effects appears weak. Concurrent enzyme-inducing anticonvulsants such as phenytoin, carbamazepine, or barbiturates stimulate zonisamide metabolism and decrease serum zonisamide levels at steady state. Although zonisamide has been reported to increase the serum levels of phenytoin and carbamazepine in some patients, the interactions of zonisamide with other antiepileptic drugs seem to be of minor clinical relevance. A pilot study of zonisamide suppositories revealed that it is beneficial for patients with neurologic disorders in whom antiepileptic drugs cannot be administered by mouth.