Selective A 3 Adenosine Receptor Antagonist Radioligand for Human and Rodent Species

The A adenosine receptor (A AR) is a target for pain, ischemia, and inflammatory disease therapy. Among the ligand tools available are selective agonists and antagonists, including radioligands, but most high-affinity non-nucleoside antagonists are limited in selectivity to primate species. We have explored the structure-activity relationship of a previously reported A AR antagonist DPTN ( -[4-(3,5-dimethylphenyl)-5-(4-pyridyl)-1,3-thiazol-2-yl]nicotinamide) for radiolabeling, including 3-halo derivatives (3-iodo, MRS7907), and characterized as a high -affinity radioligand [ H]MRS7799. A AR values were (nM): 0.55 (human), 3.74 (mouse), and 2.80 (rat). An extended methyl acrylate (MRS8074, ) maintained higher affinity (18.9 nM) than a 3-((5-chlorothiophen-2-yl)ethynyl) derivative . Compound had an excellent brain distribution in rats (brain/plasma ratio ∼1). Receptor docking predicted its orthosteric site binding by engaging residues that were previously found to be essential for AR binding. Thus the new radioligand promises to be a useful species-general antagonist tracer for receptor characterization and drug discovery.