Pharmacological characterization of novel A(3) adenosine receptor-selective antagonists

The effects of putative A(3) adenosine receptor antagonists of three diverse chemical classes (the flavonoid MRS 1067, the 6-phenyl-1,4-dihydropyridines MRS 1097 and MRS 1191, and the triazoloquinazoline MRS 1220) were characterized in receptor binding and functional assays. MRS 1067, MRS 1191 and MRS 1220 were found to be competitive in saturation binding studies using the agonist radioligand [I-125]AB-MECA (N-6-(4-amino-3-iodobenzyl)adenosine-5'-N-methyluronamide) at cloned human brain A(3) receptors expressed in HEK-293 cells. Antagonism was demonstrated in functional assays consisting of agonist-induced inhibition of adenylate cyclase and the stimulation of binding of [S-35]guanosine 5'-O-(3-thiotriphosphate) ([S-35]GTP-gamma-S) to the associated G-proteins. MRS 1220 and MRS 1191, with K-B values of 1.7 and 92 nM, respectively, proved to be highly selective for human A(3) receptor vs human A(1) receptor-mediated effects on adenylate cyclase. In addition, MRS 1220 reversed the effect of A(3) agonist-elicited inhibition of tumor necrosis factor-alpha formation in the human macrophage U-937 cell line, with an IC50 value of 0.3 mu M. Published by Elsevier Science Ltd.