Pharmacological characterisation of a novel α2/3 GABAA receptor selective positive allosteric modulator

Introduction

A number of compounds with varying levels of potentiating efficacy at α2 and α3 GABAA​ receptors, and minimal activity at α1 GABAA​ receptors, have been advanced to clinical trials for the treatment of generalised anxiety disorder (GAD). These compounds have generally been well tolerated, and post hoc analyses of two molecules, TPA023 and AZD7325, have indicated significant efficacy on the Hamilton Anxiety Rating Scale. More recent clinical candidates differentiate from these earlier compounds through additional activity at α5 GABAA​ receptors alongside α2/3 potentiation. While negative phase II results have been reported for some compounds in GAD, others within this class continue to be evaluated clinically.

Aims

Our aim was to develop a novel selective α2/3 GABAA​ receptor positive allosteric modulator (PAM) with minimal α1 and α5 activity for the treatment of anxiety disorders. We describe the preclinical pharmacological profile of a novel molecule, Compound 1, which demonstrates this functional selectivity and a non-sedative anxiolytic profile in rodent models.

Methods

Compounds were identified through iterative medicinal chemistry optimisation combined with in vitro human recombinant binding and electrophysiological assays to assess selective potentiation of GABA responses at α2 and α3 GABAA​ receptors. In vitro microsomal assays and membrane permeability assays were used to support selection for in vivo pharmacokinetic and CNS target engagement studies, including in vivo receptor occupancy evaluation. Functional selectivity was further assessed at native receptors using ex vivo electrophysiological recordings in mouse brain slices. Compounds were progressed to behavioural testing of anxiolytic efficacy in the rat elevated plus maze (EPM). Motor and sedative effects were evaluated using the rat rotarod test. Compound 1 was further assessed in nonhuman primates for behavioural signs of sedation using standardised observational categories based on paediatric anaesthesiology classifications.

Results

Compound 1 was identified as a potent α2/3 GABAA​ receptor PAM with no detectable α1 activity and low α5 activity. The compound showed excellent target engagement after oral dosing in rats, with approximately 50% receptor occupancy achieved at 0.5 mg/kg and a corresponding plasma EC50​ of 52 ng/mL. Compound 1 produced a dose-dependent increase in time spent on the open arms of the EPM, consistent with anxiolytic effects. No sedative effects were observed in the rat rotarod assay at doses up to 30-fold higher than the minimum efficacious dose in the EPM. In contrast to less selective molecules advanced in the field, Compound 1 induced no sedative motor effects in nonhuman primates at doses that blocked alprazolam-induced deep sedation and observable ataxia.

Conclusions

Compound 1 is a novel, potent and selective α2/3 GABAA​ receptor PAM with a preclinical profile characterised by anxiolytic efficacy and absence of sedation-related effects across rodent and nonhuman primate models. These findings highlight the importance of subtype selectivity in dissociating anxiolytic efficacy from sedation.

References

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