Abstract

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 μ-, δ-, and κ-opioid, α- and β-adrenergic, muscarinic M₁ and M₂, and dopaminergic D₁ and D₂receptors, but not to the tyrosine kinase epidermal growth factor receptor. SCH-202676 had no direct effect on G protein activity as assessed by [³⁵S]guanosine-5′-O-(γ-thio)triphosphate binding to purified recombinant G_oα- or G_{β γ}-stimulated ADP-ribosylation of G_oα by pertussis toxin. In addition, SCH-202676 inhibited antagonist binding to the β₂-adrenergic receptor expressed in Escherichia coli, a system devoid of classical heterotrimeric G proteins. SCH-202676 inhibited radiolabeled agonist and antagonist binding to the α_2a-adrenergic receptor with an IC₅₀ value of 0.5 μM, decreased theB _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 μM SCH-202676 did not alter subsequent radioligand binding to the α_2a-adrenergic receptor and the dopaminergic D₁ 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.