Structure activity relationship of 2-arylalkynyl-adenine derivatives as human A3 adenosine receptor antagonists

Recognition of nucleosides at adenosine receptors (ARs) is supported by multiple X-ray structures, but the structure of an adenine complex is unknown. We examined the selectivity of predicted A(1)AR and A(3)AR adenine antagonists that incorporated known agonist affinity-enhancing N-6 and C2 substituents. Adenines with A(1)AR-favoring N-6-alkyl, cycloalkyl and arylalkyl substitutions combined with an A(3)AR-favoring 2-((5-chlorothiophen-2-yl)ethynyl) group were human (h) A(3)AR-selective, e.g. MRS7497 17 (approximate to 1000-fold over A(1)AR). In addition, binding selectivity over hA(2A)AR and hA(2B)AR and functional A(3)AR antagonism were demonstrated. 17 was subjected to computational docking and molecular dynamics simulation in a hA(3)AR homology model to predict interactions. The SAR of nucleoside AR agonists was not recapitulated in adenine AR antagonists, and modeling suggested an alternative, inverted binding mode with the key N250(6.55) H-bonding to the adenine N-3 and N-9, instead of N-6 and N-7 as in adenosine agonists.