New antiepileptic drugs: review on drug interactions

During the Past decade, nine new antiepileptic drugs (AEDs) namely, Felbamate, Gabapentin, Levetiracetam, Lamotrigine, Oxcarbazepine, Tiagabine, Topiramate, Vigabatrin and Zonisamide have been marketed worldwide. The introduction of these drugs increased appreciably the number of therapeutic combinations used in the treatment of epilepsy and with it, the risk of drug interactions. In general, these newer antiepileptic drugs exhibit a lower potential for drug interactions than the classic AEDs, like phenytoin, carbamazepine and valproic acid, mostly because of their pharmacokinetic characteristics. For example, vigabatrin, levetiracetam and gabapentin, exhibit few or no interactions with other AEDs. Felbamate, tiagabine, topiramate and zonisamide are sensitive to induction by known anticonvulsants with inducing effects but are less vulnerable to inhibition by common drug inhibitors. Felbamate, topiramate and oxcarbazepine are mild inducers and may affect the disposition of oral contraceptives with a risk of failure of contraception. These drugs also inhibit CYP2C19 and may affect the disposition of phenytoin. Lamotrigine is eliminated mostly by glucuronidation and is susceptible to inhibition by valproic acid and induction by classic AEDs such as phenytoin, carbamazepine, phenobarbital and primidone.