Structure activity relationship of 2-arylalkynyl-adenine derivatives as human A 3 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 A1 AR and A3 AR adenine antagonists that incorporated known agonist affinity-enhancing N 6 and C2 substituents. Adenines with A1 AR-favoring N 6 -alkyl, cycloalkyl and arylalkyl substitutions combined with an A3 AR-favoring 2-((5-chlorothiophen-2-yl)ethynyl) group were human (h) A3 AR-selective, e.g. MRS7497 17 (∼1000-fold over A1 AR). In addition, binding selectivity over hA2A AR and hA2B AR and functional A3 AR antagonism were demonstrated. 17 was subjected to computational docking and molecular dynamics simulation in a hA3 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 N2506.55 H-bonding to the adenine N 3 and N 9 , instead of N 6 and N 7 as in adenosine agonists.