Selective A₃ Adenosine Receptor Antagonist Radioligand for Human and Rodent Species

The
A3 adenosine receptor (A3AR) 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 A3AR antagonist DPTN 9 (N-[4-(3,5-dimethylphenyl)-5-(4-pyridyl)-1,3-thiazol-2-yl]­nicotinamide)
for radiolabeling, including 3-halo derivatives (3-iodo, MRS7907),
and characterized 9 as a high -affinity radioligand [3H]­MRS7799. A3AR Kd values
were (nM): 0.55 (human), 3.74 (mouse), and 2.80 (rat). An extended
methyl acrylate (MRS8074, 19) maintained higher affinity
(18.9 nM) than a 3-((5-chlorothiophen-2-yl)­ethynyl) derivative 20. Compound 9 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.