SCH-202676: An allosteric modulator of both agonist and antagonist binding to G protein-coupled receptors

A novel thiadiazole compound, SCH-202676 (N-(2,3-diphenyl-1,2,4- thiadiazol-5-(2H)-ylidene)methanamine), has been identified as an inhibitor of both agonist and antagonist binding to G protein-coupled receptors (GPCRs). SCH-202676 inhibited radioligand binding to a number of structurally distinct, heterologously expressed GPCRs, including the human mu-, delta-, and kappa- opioid, alpha- and beta -adrenergic, muscarinic M-1 and M-2, and dopaminergic D-1 and D-2 receptors, but not to the tyrosine kinase epidermal growth factor receptor. SCH-202676 had no direct effect on G protein activity as assessed by [S-35] guanosine-5'-O-(gamma -thio) triphosphate binding to purified recombinant G(o alpha)- or G(beta gamma)-stimulated ADP-ribosylation of G(o alpha) by pertussis toxin. In addition, SCH-202676 inhibited antagonist binding to the beta (2)-adrenergic receptor expressed in Escherichia coli, a system devoid of classical heterotrimeric G proteins. SCH-202676 inhibited radiolabeled agonist and antagonist binding to the alpha (2a)-adrenergic receptor with an IC50 value of 0.5 muM, decreased the B-max value of the binding sites with a slight increase in the K-D value, and inhibited agonist-induced activation of the receptor. The effects of SCH-202676 were reversible. Incubation of plasma membranes with 10 muM SCH-202676 did not alter subsequent radioligand binding to the alpha (2a)-adrenergic receptor and the dopaminergic D-1 receptor. Taken together, our data suggest that SCH-202676 has the unique ability to allosterically regulate agonist and antagonist binding to GPCRs in a manner that is both selective and reversible. The scope of the data presented suggests this occurs by direct interaction with a structural motif common to a large number of GPCRs or by activation/inhibition of an unidentified accessory protein that regulates GPCR function.