Abstracts

Rationale: Cenobamate (YKP3089) is a newly developed antiepileptic drug (AED) being investigated for use in patients with partial-onset seizures. Cenobamate modulates several properties of voltage-gated sodium ion channels, with preferential inhibition of the persistent current. In a series of studies, the effects of cenobamate on GABA_A receptors and GABA-mediated currents were investigated to further elucidate the mechanism of action of cenobamate. Methods: Radioligand binding displacement assays were conducted to assess the binding of cenobamate on GABA_A receptor sites. Relative activities on human GABA_A receptor subtypes were studied using electrophysiological analysis on six human GABA_A ion channel subtypes expressed in heterologous cells. Potentiation of GABA-induced currents, as well as effects on both phasic and tonic GABA_A currents, were assessed in rat hippocampal CA3 neurons, rat dentate gyrus granule cells (DGGC), and mouse and rat hippocampal CA1 neurons. Conventional whole-cell patch clamp assays were used to obtain electrophysiological recordings. Results: Cenobamate did not significantly displace the binding of GABA, muscimol, flunitrazepam, or Ro-15-1788 (flumazenil) to GABA_A receptors. However, cenobamate significantly displaced the binding of TBPS radioligand to GABA-gated Cl^- channel (K_i=228-250 µM), consistent with its positive allosteric modulation of human GABA_A receptor subtypes (EC₅₀ range: 42-194 µM for a1ß2?2, a2ß3?2, a3ß3?2, a4ß3?2, a5ß3?2, a6ß3?2). In rat hippocampal CA3 neurons, cenobamate (=30 µM) significantly enhanced GABA-induced current (EC₅₀=164 µM) in a concentration-dependent manner. In contrast to diazepam, the potentiation of GABA-induced currents by cenobamate was not affected in the presence of flumazenil, a selective benzodiazepine receptor antagonist. In mouse CA1 neurons, bath application of cenobamate significantly delayed the decay of evoked inhibitory postsynaptic currents without altering the peak amplitude. Cenobamate also enhanced tonic GABA_A currents in a concentration-dependent manner in rat CA1 neurons and DGGCs, although the effect was more prominent in CA1 neurons. Conclusions: Cenobamate was shown to be a non-benzodiazepine-type positive allosteric modulator of the GABA_A receptor. Enhancement of GABA_A receptor-mediated currents occurred in both the phasic and tonic modality in rodent hippocampal neurons. The modulation of GABA_A receptor-mediated currents, along with the previously shown sodium ion channel inhibition, may contribute to the antiepileptic activity of cenobamate. Funding: SK Life Science, Inc.; SK Biopharmaceuticals, Co., Ltd.