Abstracts

Rationale: SPN-817 is a novel, reversible acetylcholinesterase inhibitor in development for focal onset seizures. Its cholinergic mechanism of action is distinct from other antiseizure medications, with potential procognitive effects. We evaluated the potential of SPN‑817 to reduce hippocampal paroxysmal discharges (HPD) in the mouse model of mesial temporal lobe epilepsy (MTLE).

Methods: C57Bl/6J adult males were injected with kainate (KA) and implanted with a bipolar electrode in the right dorsal hippocampus. Four weeks later, 12 animals with validated MTLE were selected. SPN-817 (0.1, 0.25, and 0.5 mg/kg, PO), vehicle (10% HPβCD in 0.9% NaCl, PO), diazepam (DZP; 2 mg/kg; IP), or carbamazepine (CBZ; 40 mg/kg; IP) were administered in a crossover design. HPD occurrence, cumulative duration, and mean duration were evaluated from EEG recordings in 20 min bins for 40 min before and 120 min after dosing, along with qEEG analysis. Nine KA‑injected mice were used as satellites for terminal intracardiac blood and brain collections 1 hr post‑dose. Unbound concentrations of SPN-817 from plasma and brain homogenates were calculated using f_u,pasma=75% and f_u,brain=23.8%.

Results: Mice treated with SPN-817 showed a dose-dependent decrease in HPDs and cumulative seizure time. No cholinergic side effects were observed. SPN-817 ≥ 0.25 mg/kg significantly inhibited the number of focal seizures compared to vehicle‑treated mice, an effect lasting up to 70 min post‑dose. No change in mean duration was seen in remaining seizures. DZP reduced the number and total duration of HPDs compared to vehicle‑treated mice at all analyzed times. Mean duration of the remaining HPDs was not different from vehicle. In contrast, net inhibition of HPDs with CBZ over 120 min post‑dose was not different from vehicle, though a transient decrease in HPD number and cumulative duration was seen during the 30-50 min bin compared to the vehicle group. Unbound plasma and brain concentrations of SPN-817 were dose‑proportional. At 60 min, SPN-817 unbound concentrations in brain homogenates were 12 nM and 21 nM, and in plasma 71 nM and 177 nM for 0.25 and 0.5 mg/kg doses, respectively. The qEEG analysis showed no differences between treatment and vehicle groups during ictal and interictal periods. Compared to baseline, SPN-817 showed reduced interictal theta, alpha, beta, and gamma spectra power but did not show significant changes during the ictal period. DZP reduced delta, beta, and gamma spectra power in the ictal period and transiently increased beta and gamma spectra power in the interictal period.

Conclusions: SPN-817 significantly suppressed HPD numbers at 0.25 and 0.5 mg/kg in the MTLE model without changes in mean event duration. The highest dose resulted in a net HPD suppression comparable to DZP. Seizure suppression by SPN-817 was obtained with low unbound SPN‑817 brain concentrations. Also, SPN-817 had a different spectral profile than DZP, indicating that SPN-817 acts through different mechanisms for antiseizure effects.

Funding: Supernus Pharmaceuticals, Inc.