Abstracts

Rationale: Padsevonil (PSL; UCB0942) is a novel antiepileptic drug candidate which has recently completed a Phase II proof-of-concept study in patients with drug-resistant focal seizures (Muglia et al., AES 2017). PSL has a dual mode of action, showing high and moderate affinity binding to synaptic vesicle 2 (SV2) proteins and the benzodiazepine (BZD) site on GABAA receptors, respectively (Wood et al., AES 2017). Here we report in vitro functional effects of PSL on recombinant and native GABAA receptors. Methods: The efficacy and potency of PSL was measured in patch clamp experiments (PatchXpress 7000A; Axon instruments), using human recombinant α1β2γ2 GABAA receptors stably expressed in CHO cells. Its relative efficacy was evaluated in comparison to a reference BZD ligand, zolpidem. The GABAA receptor subtype selectivity of PSL was studied in Xenopus oocytes using two-electrode voltage clamp recordings (Roboocyte; Multi Channel Systems). Oocytes were injected with mRNA for the human GABAA receptors encoding the α1,α2,α3,α4,α5 subunits in combination with the β2γ2 receptor subunits. The concentration-response of PSL on the different receptor subunits was measured in the presence of GABA at EC20 and its relative efficacy was compared to chlordiazepoxide. The effects of PSL on native GABAA receptors were also studied in primary cultures of rat cortical neurons (10-14 DIV) using manual patch-clamp. Results: Padsevonil (1nM-30µM) potentiated the effects of GABA at its EC20 in recombinant α1β2γ2 GABAA receptors expressed in CHO cells, and showed a relative efficacy of 44% when compared to zolpidem. PSL showed activity in a range of human GABAA receptors expressed in oocytes, including α1β2γ2 (EC50 290nM), α2β2γ2 (EC50 1480nM), α3β2γ2 (EC50 1970nM), and α5β2γ2 (EC50 374nM), but was inactive on the BZD-insensitive α4β2γ2 receptors. PSL was also inactive on receptors lacking the γ2 subunit (α1β2 receptors) further confirming its interaction with the BZD binding site. When compared to chlordiazepoxide, the relative efficacy of PSL to potentiate GABA EC20 on the different GABAA receptors was equal to 60% (α1β2γ2), 25% (α2β2γ2), 56% (α3β2γ2) and 42% (α5β2γ2). PSL (100nM-30µM) demonstrated GABA-potentiating activity in native GABAA receptors (EC50 200nM) in cultured rat cortical neurons. Conclusions: Present studies confirmed functional activity of PSL on different GABAA receptors, which was consistent with its moderate binding affinity to the BZD site (Wood et al., AES 2017). Importantly, PSL displayed partial agonist profile when compared to reference BZD site ligands, which may suggest reduced potential for tolerance induction. Funding: UCB Pharma-sponsored.