Abstracts

Rationale: PRAX-330 (GS967), is a highly potent and selective blocker of persistent sodium current that is being developed for the treatment of seizures and comorbidities in patients with SCN8A epileptic encephalopathy (EE). SCN8A-EE is caused by de novo gain of function mutations in the SCN8A gene which encodes the voltage gated sodium channel Na_V1.6. These mutations commonly lead to an increase in persistent current in Na_V1.6. PRAX-330 blocks hNa_V1.6 with an IC50 of 38 nM. In line with its in vitro profile, PRAX-330 has previously been shown to block spontaneous seizures and improve survival in an animal model of SCN8A-EE, the SCN8A-N1768D/+ mouse model. The goal of the current study was to determine the broader anticonvulsant profile of PRAX-330 in preclinical seizure models and determine its protective index. Methods: PRAX-330 was evaluated in CD1 mice in the maximal electroshock seizure model (MES), the pentylenetetrazol (PTZ) seizure model, the 6 HZ seizure model and a spontaneous locomotor assay (sLMA). Animals were treated with PRAX-330 at doses of 0.1 to 1 mg/kg (subcutaneously), 0.03 to 1.5 mg/kg (orally), 0.5 to 3 mg/kg (orally) and 1.5 to 5 mg/kg (orally) in the MES, PTZ, 6HZ and sLMA assay, respectively. Plasma and brain samples were collected from study animals at the end of testing and PRAX-330 levels were determined by mass spectrometry. EC₅₀ and TC₅₀ brain levels were used to determine the protective index which was calculated based on the following formula: protective index (PI) = TC₅₀/EC₅₀. Results: PRAX-330 demonstrated potent and dose-dependent inhibition of seizures in all three seizure models. In the MES model, PRAX-330 increased the latency to tonic extension seizures with an ED₅₀ of 0.26 mg/kg (n=12 mice per group). The estimated EC₅₀ plasma and brain levels at the ED₅₀ were 54.1 ng/ml and 98.5 ng/g, respectively. In the PTZ model, PRAX-330 significantly increased the latency to tonic extension seizures at 1.5 mg/kg but not at 0.3 mg/kg (n=12, p50 of 0.61 mg/kg (n=8-10 mice per group). The estimated EC₅₀ plasma and brain levels at the ED₅₀ were 80.9 ng/ml and 113.9 ng/g, respectively. PRAX-330 reduced locomotor activity at higher doses in the sLMA assay. The TD₅₀ was 4.9 mg/kg (n=10 mice per group). The estimated TC₅₀ plasma and brain levels at the TD₅₀ were 881.9 ng/ml and 1471.0 ng/g, respectively. The protective index for PRAX-330 was 15 and 13 in the MES and 6 Hz model, respectively. Conclusions: The novel persistent sodium current blocker PRAX-330 is highly potent in vivo and has a broad anticonvulsant profile. PRAX-330 is well tolerated at efficacious doses and exposures and has a high protective index. These data suggest that PRAX-330 could be well tolerated and have a broad therapeutic potential in patients with epilepsy. Funding: All studies were funded by Praxis Precisions Medicines, Inc.