Hybrid ortho/allosteric ligands for the adenosine A1 receptor

Many G protein-coupled receptors (GPCRs), including the adenosine A1 receptor (A1AR), have been shown to be allosterically modulated by small molecule ligands. So far, in the absence of structural information, the exact location of the allosteric site on the A1AR is not known. We synthesized a series of bivalent ligands (4) with an increasing linker length between the orthosteric and allosteric pharmacophores and used these as tools to search for the allosteric site on the A1AR. The compounds were tested in both equilibrium radioligand displacement and functional assays in the absence and presence of a reference allosteric enhancer, (2-amino-4,5-dimethyl-3-thienyl)-[3-(trifluoromethyl)phenyl]methanone, PD81,723 (1). Bivalent ligand N6-[2-amino-3-(3,4-dichlorobenzoyl)-4,5,6,7-tetrahydrothieno[2,3-c]pyridin-6-yl-9-nonyloxy-4-phenyl]-adenosine 4h (LUF6258) with a 9 carbon atom spacer did not show significant changes in affinity or potency in the presence of 1, indicating that this ligand bridged both sites on the receptor. Furthermore, 4h displayed an increase in efficacy, but not potency, compared to the parent, monovalent agonist 2. From molecular modeling studies, we speculate that the allosteric site of the A1AR is located in the proximity of the orthosteric site, possibly within the boundaries of the second extracellular loop of the receptor.