Adenosine A_2A receptor agonists with potent antiplatelet activity

Selected adenosine A_2A receptor agonists (PSB-15826, PSB-12404, and PSB-16301) have been evaluated as new antiplatelet agents. In addition, radioligand-binding studies and receptor-docking experiments were performed in order to explain their differential biological effects on a molecular level. Among the tested adenosine derivatives, PSB-15826 was the most potent compound to inhibit platelet aggregation (EC₅₀ 0.32 ± 0.05 µmol/L) and platelet P-selectin cell-surface localization (EC₅₀ 0.062 ± 0.2 µmol/L), and to increase intraplatelets cAMP levels (EC₅₀ 0.24 ± 0.01 µmol/L). The compound was more active than CGS21680 (EC₅₀ 0.97±0.07 µmol/L) and equipotent to NECA (EC₅₀ 0.31 ± 0.05 µmol/L) in platelet aggregation induced by ADP. In contrast to the results from cAMP assays, K_i values determined in radioligand-binding studies were not predictive of the A_2A agonists' antiplatelet activity. Docking studies revealed the key molecular determinants of this new family of adenosine A_2A receptor agonists: differences in activities are related to π-stacking interactions between the ligands and the residue His264 in the extracellular loop of the adenosine A_2A receptor which may result in increased residence times. In conclusion, these results provide an improved understanding of the requirements of antiplatelet adenosine A_2A receptor agonists.