Design, Synthesis and Biological Evaluation of N6-Substituted-C2-Alkynyl-4′-Thionucleoside and 4′-Trucatedthionucleoside Derivatives as Novel A₃ Adenosine Receptor Ligands

Adenosine receptors (ARs) play crucial roles in various physiological processes, making them significant targets for therapeutic intervention. This study focuses on the design, synthesis, and evaluation of N6-substituted-C2-alkynyl-4′-thioadenosine and truncated 4′-thioadenosine derivatives as selective ligands for the human A₃ adenosine receptor (hA₃ AR). Binding affinity assays demonstrated that modifications at the C2-alkyne and N6-amine positions significantly influenced receptor selectivity and potency. Compound 3b showed high affinity for hA₃ AR (K_i = 1.8 nM) with excellent subtype selectivity and acted as an agonist. In contrast, truncated derivative 6b was an antagonist, while 6s, which shares the same sugar structure as 6b, functioned as a partial agonist, highlighting the roles of sugar and base moieties in ligand function. Molecular docking studies provided further insight into distinct binding modes, demonstrating the impact of minor structural variations on ligand-receptor binding dynamics. These findings contribute to the development of potent and selective A₃ AR modulators by emphasizing the importance of fine structural adjustments in ligand design.