Characterization of COR627 and COR628, Two Novel Positive Allosteric Modulators of the GABA B Receptor

The potential efficacy of GABAB receptor agonists in the treatment of pain, drug addiction, epilepsy, cognitive dysfunctions, and anxiety disorders is supported by extensive preclinical and clinical evidence. However, the numerous side effects produced by the GABAB receptor agonist baclofen considerably limit the therapeutic use of this compound. The identification of positive allosteric modulators (PAMs) of the GABAB receptor may constitute a novel approach in the pharmacological manipulation of the GABAB receptor, leading to fewer side effects. The present study reports the identification of two novel compounds, methyl 2-(1-adamantanecarboxamido)-4-ethyl-5-methylthiophene-3-carboxylate (COR627) and methyl 2-(cyclohexanecarboxamido)-4-ethyl-5-methylthiophene-3-carboxylate (COR628), which act as GABAB PAMs in 1) rat cortical membranes and 2) in vivo assay. Both compounds potentiated GABA- and baclofen-stimulated guanosine 5′- O -(3-[35S]thio)-triphosphate binding to native GABAB receptors, while producing no effect when given alone. GABA concentration-response curves in the presence of fixed concentrations of COR627 and COR628 revealed an increase of potency of GABA rather than its maximal efficacy. In radioligand binding experiments [displacement of the GABAB receptor antagonist, 3- N -[1-(( S )-3,4dichlorophenyl)-ethylaminol]-2-( S )hydroxypropyl cyclo-hexylmethyl phosphinic acid ([3H]CGP54626)], both COR627 and COR628 increased the affinity of high- and low-affinity binding sites for GABA, producing no effect when administered alone up to a concentration of 1 mM. In vivo experiments indicated that pretreatment with per se ineffective doses of COR627 and COR628 potentiated the sedative/hypnotic effect of baclofen. In conclusion, COR627 and COR628 may represent two additional tools for use in investigating the roles and functions of positive allosteric modulatory binding sites of the GABAB receptor.