Design, synthesis, and biological activity of N6-substituted-4′-thioadenosines at the human A3 adenosine receptor Original Research Article

A large series of N6-substituted-4′-thioadenosines were synthesized starting from d-gulonic-γ-lactone, and structure–activity relationships were studied at the human A3 and other subtypes of adenosine receptors (ARs). 2-Chloro-substituted and 2-H analogues were compared. 2-Chloro-N6-methyl-4′-thioadenosine 19b was a highly potent and selective agonist (Ki = 0.8 ± 0.1 nM in binding) at the A3AR, and displayed the same relative efficacy in receptor activation as a known full agonist, Cl-IB-MECA. Most of N6-substituted-4′-thioadenosines were less potent in binding than the corresponding N6-substituted-adenosines or N6-substituted-4′-thioadenosine-5′-uronamides. N6-(3-Iodobenzyl) derivative 19g was demonstrated to be an A3AR-selective partial agonist displaying a Ki value of 3.2 nM.