Characterization of (R,S)‐5,7‐di‐tert‐butyl‐3‐hydroxy‐3‐trifluoromethyl‐3H‐benzofuran‐2‐one as a positive allosteric modulator of GABAB receptors

Background and purpose: As baclofen is active in patients with anxiety disorders, GABAB receptors have been implicated in the modulation of anxiety. To avoid the side effects of baclofen, allosteric enhancers of GABAB receptors have been studied to provide an alternative therapeutic avenue for modulation of GABAB receptors. The aim of this study was to characterize derivatives of (R,S)‐5,7‐di‐tert‐butyl‐3‐hydroxy‐3‐trifluoromethyl‐3H‐benzofuran‐2‐one (rac‐BHFF) as enhancers of GABAB receptors. Experimental approach: Enhancing properties of rac‐BHFF were assessed in the Chinese hamster ovary (CHO)‐Gα16‐hGABAB(1a,2a) cells by Fluorometric Imaging Plate Reader and GTPγ[35S]‐binding assays, and in rat hippocampal slices by population spike (PS) recordings. In vivo activities of rac‐BHFF were assessed using the loss of righting reflex (LRR) and stress‐induced hyperthermia (SIH) models. Key results: In GTPγ[35S]‐binding assays, 0.3 μM rac‐BHFF or its pure enantiomer (+)‐BHFF shifted the GABA concentration–response curve to the left, an effect that resulted in a large increase in both GABA potency (by 15.3‐ and 87.3‐fold) and efficacy (149% and 181%), respectively. In hippocampal slices, rac‐BHFF enhanced baclofen‐induced inhibition of PS of CA1 pyramidal cells. In an in vivo mechanism‐based model in mice, rac‐BHFF increased dose‐dependently the LRR induced by baclofen with a minimum effective dose of 3 mg kg−1 p.o. rac‐BHFF (100 mg kg−1 p.o.) tested alone had no effect on LRR nor on spontaneous locomotor activity, but exhibited anxiolytic‐like activity in the SIH model in mice. Conclusions and implications: rac‐BHFF derivatives may serve as valuable pharmacological tools to elucidate the pathophysiological roles played by GABAB receptors in the central and peripheral nervous systems. British Journal of Pharmacology (2008) 154, 797–811; doi:10.1038/bjp.2008.135; published online 21 April 2008