Absence seizures are a common seizure type in children with genetic generalized epilepsy and are characterized by a temporary loss of awareness, arrest of physical activity, and accompanying spike-and-wave discharges on an electroencephalogram. They arise from abnormal, hypersynchronous neuronal firing in brain thalamocortical circuits. Currently available therapeutic agents are only partially effective and act on multiple molecular targets, including γ-aminobutyric acid (GABA) transaminase, sodium channels, and calcium (Ca(2+)) channels. We sought to develop high-affinity T-type specific Ca(2+) channel antagonists and to assess their efficacy against absence seizures in the Genetic Absence Epilepsy Rats from Strasbourg (GAERS) model. Using a rational drug design strategy that used knowledge from a previous N-type Ca(2+) channel pharmacophore and a high-throughput fluorometric Ca(2+) influx assay, we identified the T-type Ca(2+) channel blockers Z941 and Z944 as candidate agents and showed in thalamic slices that they attenuated burst firing of thalamic reticular nucleus neurons in GAERS. Upon administration to GAERS animals, Z941 and Z944 potently suppressed absence seizures by 85 to 90% via a mechanism distinct from the effects of ethosuximide and valproate, two first-line clinical drugs for absence seizures. The ability of the T-type Ca(2+) channel antagonists to inhibit absence seizures and to reduce the duration and cycle frequency of spike-and-wave discharges suggests that these agents have a unique mechanism of action on pathological thalamocortical oscillatory activity distinct from current drugs used in clinical practice.

Although it has been suggested that coexpression of minK related peptide (MiRP1) is required for reconstitution of native rapid delayed-rectifier current (I Kr ) by human ether-a-go-go related gene (HERG), currents resulting from HERG (I HERG ) and HERG plus MiRP1 expression have not been directly compared with native I Kr . We compared the pharmacological and selected biophysical properties of I HERG with and without MiRP1 coexpression in Chinese hamster ovary (CHO) cells with those of guinea-pig I Kr under comparable conditions. Comparisons were also made with HERG expressed in Xenopus oocytes. MiRP1 coexpression significantly accelerated I HERG deactivation at potentials negative to the reversal potential, but did not affect more physiologically relevant deactivation of outward I HERG , which remained slower than that of I Kr . MiRP1 shifted I HERG activation voltage dependence in the hyperpolarizing direction, whereas I Kr activated at voltages more positive than I HERG . There were major discrepancies between the sensitivity to quinidine, E-4031 and dofetilide of I HERG in Xenopus oocytes compared to I Kr , which were not substantially affected by coexpression with MiRP1. On the other hand, the pharmacological sensitivity of I HERG in CHO cells was indistinguishable from that of I Kr and was unaffected by MiRP1 coexpression. We conclude that the properties of I HERG in CHO cells are similar in many ways to those of native I Kr under the same recording conditions, and that the discrepancies that remain are not reduced by coexpression with MiRP1. These results suggest that the physiological role of MiRP1 may not be to act as an essential consituent of the HERG channel complex carrying native I Kr .

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