Functional characterization of the antiepileptic drug candidate, padsevonil, on GABA A receptors.

Objective 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 gamma-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. Methods The effect of padsevonil on GABA-mediated Cl- currents was determined by patch clamp on recombinant human GABA(A)Rs (alpha 1 beta 2 gamma 2) 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 (alpha 1-5/beta 2/gamma 2) in Xenopus oocytes. Results 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 alpha 1- and alpha 5-containing receptors (EC50 295 and 281 nmol/L) than at alpha 2- and alpha 3-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 alpha 1 beta 2 gamma 2, 26% for alpha 2 beta 2 gamma 2, 56% for alpha 3 beta 2 gamma 2, and 41% for alpha 5 beta 2 gamma 2; no activity was observed at benzodiazepine-insensitive alpha 4 beta 2 gamma 2 receptors. Significance 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.