Discovery of New Human A(2A) Adenosine Receptor Agonists: Design, Synthesis, and Binding Mode of Truncated 2-Hexynyl-4'-thioadenosine.

The truncated C2- and C8-substituted-4'-thioadenosine derivatives 4a-d were synthesized from D-mannose, using palladium-catalyzed cross coupling reactions as key steps. In this study, an A(3) adenosine receptor (AR) antagonist, truncated 4'-thioadenosine derivative 3 was successfully converted into a potent A(2A)AR agonist 4a (K(i) = 7.19 ± 0.6 nM) by appending a 2-hexynyl group at the C2-position of a derivative of 3 that was N(6)-substituted. However, C8-substitution greatly reduced binding affinity at the human A(2A)AR. All synthesized compounds 4a-d maintained their affinity at the human A(3)AR, but 4a was found to be a competitive A(3)AR antagonist/A(2A)AR agonist in cyclic AMP assays. This study indicates that the truncated C2-substituted-4'-thioadenosine derivatives 4a and 4b can serve as a novel template for the development of new A(2A)AR ligands.