Abstracts

Rationale: Persistent sodium current (I_Na) is a subthreshold depolarizing current that contributes to the amplification of synaptic activity and enhancement of repetitive firing. Pathologic increases in persistent I_Na contribute to the neuronal hyperexcitability observed in severe developmental and epileptic encephalopathies (DEE) and can result from gain of function (GoF) variants in voltage-gated sodium channel (Na_v) genes, or from aberrant regulation of channel gating. We previously showed PRAX-562 inhibits persistent I_Na with improved preference over peak I_Na compared to standard of care and this profile was efficacious in mouse models for SCN8A (Na_V1.6) and SCN2A (Na_V1.2) DEE. Here we investigate the antiseizure activity of PRAX-562 in two non-Na_V DEE models: KCNQ2 and KCNC1.

Methods: Anticonvulsant activity of PRAX-562 (0.3-10 mg/kg) was assessed using the maximal electroshock seizure (MES) assay in outbred CD-1 mice. Effects of PRAX-562 (10-40 mg/kg) on spontaneous locomotor activity (sLMA) were measured to assess tolerability. The protective index (PI) of PRAX-562 was determined as the ratio of brain concentrations in sLMA (TC₅₀) to MES (EC₅₀). The effects of carbamazepine (CBZ) and lamotrigine (LTG) were also assessed using MES (3-30 mg/kg and 1-10 mg/kg, respectively) and sLMA (30-96 mg/kg and 20-63.4 mg/kg, respectively). Prevention of audiogenic seizures was assessed by exposing Scn8a^N1768D/+ mice to a 14 kHz tone after PRAX-562 (1-10 mg/kg) or vehicle treatment. Prevention of spontaneous seizures in Scn2a^Q54 mice was determined by comparing number of seizures in 30 min periods before and 60-90 min after dosing with PRAX-562 (0.3-10 mg/kg) or vehicle. Anticonvulsant activity in Kcnq2^K556E/+ and Kcnc1^R320H/+ DEE mouse models was determined using time to seizure (0-40 min) induced by PTZ (100 mg/kg) after PRAX-562 (10 mg/kg) or vehicle treatment. The concentration of PRAX-562 in terminal plasma and brain samples were measured using mass spectrometry.

Results: PRAX-562 (10 mg/kg) completely blocked MES evoked seizures (ED₅₀ 2 mg/kg, brain EC₅₀ 116 ng/g) without affecting sLMA (TD₅₀ 44 mg/kg, brain TC₅₀ 1899 ng/g; PI 16x), representing a wider PI compared to CBZ and LTG (PIs 5-6x). PRAX-562 (10 mg/kg) also completely protected Scn2a^Q54 and Scn8a^N1768D/+ mice from spontaneous or audiogenic induced seizures. PRAX-562 (10 mg/kg) delayed the appearance of PTZ-induced seizure in Kcnq2^K556E/+and Kcnc1^R320H/+ DEE models as well as the respective wild type littermates.

Conclusions: PRAX-562 exhibited robust anticonvulsant activity in multiple mouse models of non-Na_V DEE indicating broad protection regardless of the underlying genetic cause. Moreover, PRAX-562 exhibited markedly improved preclinical tolerability compared to standard of care Na_V blockers, potentially due to its preference for persistent I_Na. The profile of PRAX-562 may translate into well-tolerated efficacy in epilepsy as well as other indications caused by neuronal hyperexcitability.

Funding: Praxis Precision Medicines