Abstract
The A(3) adenosine receptor (A(3)AR) is a promising therapeutic target for inflammatory diseases, cancer, and chronic neuropathic pain, with agonists already in advanced clinical trials. Here we report an in-depth comparison of the pharmacological properties and structure-activity relationships of existing and expanded compound libraries of 2-substituted 1H-imidazo[4,5-c]quinolin-4-amine and 4-amino-substituted quinoline derivatives that function as A(3)AR positive allosteric modulators (PAMs). We also show that our lead compound from each series enhances adenosine-induced A(3)AR signaling preferentially toward activation of Gα(i3) and Gα(oA) isoproteins, which are coexpressed with the A(3)AR in immune cells and spinal cord neurons. Finally, utilizing an extracellular/intracellular chimeric A(3)AR approach composed of sequences from a responding (human) and a nonresponding (mouse) species, we provide evidence in support of the idea that the imidazoquinolin-4-amine class of PAMs variably interacts dually with the orthosteric ligand binding site as well as with a separate allosteric site located within the inner/intracellular regions of the receptor. This study has advanced both structural and pharmacological understanding of these two classes of A(3)AR PAMs, which includes leads for future pharmaceutical development.