Abstracts

Rationale: T-type calcium channels (TTCCs or Cav3) are low threshold voltage-gated channels that regulate neuronal excitability and gain-of-function mutants have been identified patients with absence epilepsy (AE) and other generalized epilepsy syndromes. TTCCs are highly expressed in the thalamocortical axis which is implicated in the pathophysiology of idiopathic generalized epilepsy. In pre-clinical rodent models, overexpression of Cav3.1 results in an AE-like phenotype and mice lacking Cav3.1 show resistance to pharmacologically induced absence seizures. Currently approved drugs including zonisamide and ethosuximide, have been reported to have non-selective TTCC antagonist activity and while both drugs have shown evidence of efficacy in clinical studies, many patients experience lack of efficacy or intolerable side effects resulting in treatment discontinuation. Thus, selective targeting of TTCCs with potent, brain penetrant and state dependent small molecules has the potential to improve efficacy and tolerability. The objective of the study was to evaluate the efficacy of selective TTCC antagonists in the Genetic Absence Epilepsy Rat from Strasbourg (GAERS), a model of spontaneous AE with enhanced Cav3 currents and burst-firing during seizures. Methods: CX-8998 and its clinical stage back-up analogue CX-5395 were evaluated in a cohort of 10 GAERS rats at doses of 1, 3 and 10 mg/kg or vehicle by oral gavage. Ethosuximide administered at 100 mg/kg by oral gavage was used as a positive control reference compound.  EEG recordings using stereotactically implanted electrodes were made on freely moving animals for 20 minutes (baseline) prior to vehicle or test compound administration and for 90 minutes post administration. Animals were treated in a cross-over design with at least 3-4 days between administrations and maintained in a quiet state of wakefullness throughout the recording period. EEG recordings were scored by a blinded reviewer to identify spike wave discharges (SWD). Data were summarized by number and time spent in SWD. Following completion of the EEG recording phase, compounds were administered, and blood samples collected to quantify test compound plasma levels. Time course data was analyzed by two-way ANOVA for repeated measures and dose-response data were analyzed using one-way ANOVA with a significance level of p<0.05. When significant, paired comparisons vs. baseline, vehicle and reference compound were conducted. Results: The number and cumulated duration of SWD was stable after vehicle administration. Ethosuxmide induced a persistent and stable reduction in number and cumulative duration of SWD (60%) that was maintained through the post-dose recording period (p<0.0001 vs. vehicle).  CX-8998 showed a dose dependent reduction of both number and cumulative duration of SWD with equivalent effects to ethosuximide at 3 mg/kg (p<0.0001 vs. vehicle) and near complete suppression (99%) of seizure activity during the post-dose recording period at 10 mg/kg (p<0.0001 vs. vehicle; p<0.001 vs. ethosuximide).  CX-5395 showed similar results, consistent with its shorter half-life in rodents. Conclusions: TTCC are genetically and pharmacologically validated targets for absence seizures and these results demonstrate that potent, highly selective TTCC antagonists CX-8998 and CX-5395 show dose dependent efficacy in rodent models of AE. These results highlight the potential for selective TTCC antagonists as a therapeutic option in AE and support their further investigation in a clinical setting. CX-8998 is currently being evaluated in a Phase 2 clinical study in medically-resistant generalized epilepsy syndromes with absences seizures (NCT03406702). Funding: Funding by Cavion, Inc.