Comparative anticonvulsant and mechanistic profile of the established and newer antiepileptic drugs

Since 1993, several new antiepileptic drugs (AEDs) have been introduced for management of partial seizures. Like the established AEDs, the new drugs are believed to exert their anticonvulsant action through enhancement of inhibitory-mediated neurotransmission, or reduction of excitatory-mediated neurotransmission, or by a combination of both. Among the new drugs, vigabatrin (VGB) and tiagabine (TGB) are unique in that they were derived from mechanistic-based drug discovery programs designed to identify effective AEDs that inhibit the metabolism and reuptake of the inhibitory neurotransmitter GABA, respectively. For many of the newer AEDs, several molecular mechanisms of action have been identified. For example, felbamate (FBM), lamotrigine (LTG), zonisamide (ZNS), topiramate (TPM), oxcarbazepine (OCBZ), and possibly gabapentin (GBP) share a similar mechanism with that defined for phenytoin (PHT) and carbamazepine (CBZ), i.e., a voltage- and use-dependent block of voltage-sensitive sodium (Na+) channels. In addition to their effects on Na+ currents, TPM, ZNS, and FBM also appear to act as allosteric modulators of the GABA, receptor, whereas GBP appears to increase brain GABA levels. GBP, ZNS, FBM, LTG, and OCBZ attenuate voltage-sensitive calcium (Ca2+) channels, albeit through different mechanisms and with different classes of Ca2+ channels. FBM and TPM differ from both the established and newer AEDs in their ability to modulate NMDA- and AMPA/kainate-mediated excitatory neurotransmission, respectively. The multiple mechanisms of action associated with FBM, TPM, ZNS, GBP, and perhaps LTG, and the unique modulation of GABA levels by VGB and TGB, are likely to account for the anticonvulsant efficacy of these newer AEDs in patients with epilepsy. For each of the new drugs, their proposed mechanisms of action are discussed in relationship to their preclinical and clinical anticonvulsant profiles.