Development and characterization of novel derivatives of the antiepileptic drug lacosamide that exhibit far greater enhancement in slow inactivation of voltage-gated sodium channels.

The novel antiepileptic drug, (R)-N-benzyl 2-acetamido-3-methoxypropionamide ((R)-lacosamide, Vimpat(®) ((R)-1)), was recently approved in the US and Europe for adjuvant treatment of partial-onset seizures in adults. (R)-1 preferentially enhances slow inactivation of voltage-gated Na(+) currents, a pharmacological process relevant in the hyperexcitable neuron. We have advanced a strategy to identify lacosamide binding partners by attaching affinity bait (AB) and chemical reporter (CR) groups to (R)-1 to aid receptor detection and isolation. We showed that select lacosamide AB and AB&CR derivatives exhibited excellent activities similar to (R)-1 in the maximal electroshock seizure model in rodents. Here, we examined the effect of these lacosamide AB and AB&CR derivatives and compared them with (R)-1 on Na(+) channel function in CNS catecholaminergic (CAD) cells. Using whole-cell patch clamp electrophysiology, we demonstrated that the test compounds do not affect the Na(+) channel fast inactivation process, that they were far better modulators of slow inactivation than (R)-1, and that modulation of the slow inactivation process was stereospecific. The lacosamide AB agents that contained either an electrophilic isothiocyanate ((R)-5) or a photolabile azide ((R)-8) unit upon AB activation gave modest levels of permanent Na(+) channel slow inactivation, providing initial evidence that these compounds may have covalently reacted with their cognate receptor(s). Our findings support the further use of these agents to delineate the (R)-1-mediated Na(+) channel slow inactivation process.