Selective γ8 transmembrane α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor regulatory protein negative allosteric modulators: Efficacy in preclinical antiseizure models

RAP-219, a potent selective negative allosteric modulator (NAM) of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) associated with γ8 transmembrane AMPAR regulatory protein (TARPγ8), is in phase 3 development for refractory focal epilepsy. This study was undertaken to assess efficacy and motor function following administration of AMPAR/TARPγ8 NAMs.

Single-dose RAP-219 (per os) efficacy was assessed in intravenous pentylenetetrazol and corneal kindling (CK) antiseizure mouse models. Single- and 7-day (once daily, per os) efficacy of a RAP-219 analog, RTX-1738, was assessed in the CK model. Pharmacokinetic and receptor occupancy (RO) studies were conducted to assess the plasma exposure and RO required for preclinical efficacy. Time to first twitch and onset of sustained clonus were recorded during intravenous pentylenetetrazol infusion following RAP-219 administration. Fully kindled mice received RAP-219 or RTX-1738 prior to corneal stimulation challenge. In parallel, plasma drug concentrations were measured in satellite groups. Motor impairment was assessed by rotarod. Hippocampal RO was determined via ex vivo autoradiography in mice.

In the pentylenetetrazol model, .1- and 1-mg/kg RAP-219 doses were associated with higher seizure threshold versus vehicle (p < .01). Kindled mice were fully protected from seizures by RAP-219 (median effective dose [ED₅₀] = .02 mg/kg, median effective concentration [EC₅₀] = 2.3 ng/mL) with minimal, non-dose-dependent rotarod failures (therapeutic index > 150). RTX-1738 administration (single or multiple day) protected kindled mice from seizures with similar efficacy and potency (single dose: ED₅₀ = .05 mg/kg, EC₅₀ = 4.7 ng/mL; multiple-day administration: ED₅₀ = .02 mg/kg, EC₅₀ = 3.6 ng/mL). RAP-219 RO EC₅₀ in mice was 1.5 ng/mL, showing good correspondence of plasma exposures with the CK studies.

Selective inhibition of TARPγ8-containing AMPARs provided potent, dose-dependent antiseizure effects that were maintained after 7-day dosing in a preclinical focal seizure model. Exposures producing antiseizure effects did not impair motor function, yielding a high therapeutic index. RAP-219 may provide a new, regionally selective medicine for patients with epilepsy.