Synthesis, Biological Properties, and Molecular Modeling Investigation of the First Potent, Selective, and Water-Soluble Human A<sub>3</sub> Adenosine Receptor Antagonist

A new, highly potent, selective, and water-soluble antagonist of the hA<sub>3</sub> adenosine receptor was synthesized and tested in binding and functional assays. Compound <b>4</b> (5-[[(4-pyridyl)amino]carbonyl]amino-8-methyl-2-(2-furyl)-pyrazolo[4,3-<i>e</i>]1,2,4-triazolo[1,5-<i>c</i>]pyrimidine hydrochloride) displayed high water solubility (15 mM) and the highest affinity (<i>K</i><sub>i</sub> = 0.01 nM) and selectivity for the hA<sub>3</sub> versus A<sub>1</sub>, A<sub>2A</sub>, and A<sub>2B</sub> receptors (>10000-fold) ever reported. A Schild analysis of the antagonism by <b>4 </b>of agonist-induced inhibition of cAMP production in CHO cells expressing the hA<sub>3</sub> receptor indicated a <i>K</i><sub>B</sub> value of 0.20 nM.