Functional characterization of the antiepileptic drug candidate, padsevonil, on GABA_A receptors

The antiepileptic drug candidate, padsevonil, is the first in a novel class of drugs designed to interact with both presynaptic and postsynaptic therapeutic targets: synaptic vesicle 2 proteins and γ-aminobutyric acid type A receptors (GABA_A Rs), respectively. Functional aspects of padsevonil at the postsynaptic target, GABA_A Rs, were characterized in experiments reported here.

The effect of padsevonil on GABA-mediated Cl^- currents was determined by patch clamp on recombinant human GABA_A Rs (α1β2γ2) stably expressed in a CHO-K1 cell line and on native GABA_A Rs in cultured rat primary cortical neurons. Padsevonil selectivity for GABA_A R subtypes was evaluated using a two-electrode voltage clamp on recombinant human GABA_A Rs (α1-5/β2/γ2) in Xenopus oocytes.

In recombinant GABA_A Rs, padsevonil did not evoke Cl^- currents in the absence of the agonist GABA. However, when co-administered with GABA at effective concentration (EC)₂₀ , padsevonil potentiated GABA responses by 167% (EC₅₀ 138 nmol/L) and demonstrated a relative efficacy of 41% compared with zolpidem, a reference benzodiazepine site agonist. Similarly, padsevonil demonstrated GABA-potentiating activity at native GABA_A Rs (EC₅₀ 208 nmol/L) in cultured rat cortical neurons. Padsevonil also potentiated GABA (EC₂₀ ) responses in GABA_A Rs expressed in oocytes, with higher potency at α1- and α5-containing receptors (EC₅₀ 295 and 281 nmol/L) than at α2- and α3-containing receptors (EC₅₀ 1737 and 2089 nmol/L). Compared with chlordiazepoxide-a nonselective, full GABA_A R agonist-the relative efficacy of padsevonil was 60% for α1β2γ2, 26% for α2β2γ2, 56% for α3β2γ2, and 41% for α5β2γ2; no activity was observed at benzodiazepine-insensitive α4β2γ2 receptors.

Results of functional investigations on recombinant and native neuronal GABA_A Rs show that padsevonil acts as a positive allosteric modulator of these receptors, with a partial agonist profile at the benzodiazepine site. These properties may confer better tolerability and lower potential for tolerance development compared with classic benzodiazepines currently used in the clinic.