Synthesis and biological evaluation of 2,3,5-substituted [1,2,4]thiadiazoles as allosteric modulators of adenosine receptors.

A number of 2,3,5-substituted [1,2,4]thiadiazole analogues of SCH-202676 (N-(2,3-diphenyl[1,2,4]thiadiazole-5(2H)-ylidene)methanamine, 7a) were synthesized and tested as potential allosteric modulators of adenosine receptors. All compounds were capable of displacing the binding of the radiolabeled agonist [(3)H]CCPA to human A(1) adenosine receptors, whereas modest and varying effects were observed on the binding of [(3)H]DPCPX, a radiolabeled antagonist for this receptor subtype. Four compounds, 7a (SCH-202676), 7k (LUF5792), 7l (LUF5794), and 8e (LUF5789), were selected for more detailed characterization. They all proved allosteric inhibitors of agonist binding, with 7k being most potent, whereas their effects on antagonist binding were more ambiguous. Subsequently, experiments were done on human adenosine A(2A) and A(3) receptors. Compounds 7a and 7l displayed peculiar displacement characteristics of both radiolabeled agonist and antagonist binding to A(2A) receptors, whereas 7a showed some activity on A(3) receptors.