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 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. METHODS: 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. 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% (EC(50) 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 (EC(50) 208 nmol/L) in cultured rat cortical neurons. Padsevonil also potentiated GABA (EC(20) ) responses in GABA(A) Rs expressed in oocytes, with higher potency at α1- and α5-containing receptors (EC(50) 295 and 281 nmol/L) than at α2- and α3-containing receptors (EC(50) 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. 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.