2-Phenylpyrazolo[4,3-d]pyrimidin-7-one as a New Scaffold to obtain Potent and Selective Human A3 Adenosine Receptor Antagonists: New Insights into the Receptor-Antagonist Recognition†

A molecular simplification approach of previously reported 2-arylpyrazolo[3,4-c]quinolin-4-ones was applied to design 2-arylpyrazolo[4,3-d]pyrimidin-7-one derivatives as new human A3 adenosine receptor antagonists. Substituents with different lipophilicity and steric hindrance were introduced at the 5-position of the bicyclic scaffold (R5=H, Me, Et, Ph,CH2Ph) and on the 2-phenyl ring (OMe, Me). Most of the synthesized derivatives were highly potent hA3 adenosine receptor antagonists, the best being the 2-(4-methoxyphenyl)pyrazolo[4,3-d]pyrimidin-7-one (Ki=1.2 nM). The new compounds were also highly selective, being completely devoid of affinity toward hA1, hA2A, and hA2B adenosine receptors. On the basis of the recently published human A2A receptor crystallographic information, we propose a novel receptor-driven hypothesis to explain bothA3ARaffinity andA3 versusA2A selectivity profiles of these new antagonists.