MRS1220 is a putative A3 adenosine receptor antagonist. MRS 1220 was found to be competitive in saturation binding studies using the agonist radioligand ¹²⁵I AB-MECA at cloned human brain A3 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 ³⁵S guanosine 5′-O-(3-thiotriphosphate (³⁵S GTP-gamma-S) to the associated G-proteins. MRS 1220 and MRS 1191, with KB values of 1.7 and 92 nM, respectively, proved to be highly selective for human A3 receptor vs human A1 receptor-mediated effects on adenylate cyclase. In addition, MRS 1220 reversed the effect of A3 agonist-elicited inhibition of tumor necrosis factor-alpha formation in the human macrophage U-937 cell line, with an IC₅₀ value of 0.3 μM.

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