Structural Modification and Biological Evaluation of 2,8-Disubstituted Adenine and Its Nucleosides as A2AAdenosine Receptor Antagonists: Exploring the Roles of Ribose at Adenosine Receptors

Building on the preceding structural analysis and a structure–activity relationship (SAR) of 8-aryl-2-hexynyl nucleoside hA2AAR antagonist 2a, we strategically inverted C2/C8 substituents and eliminated the ribose moiety. These modifications aimed to mitigate potential steric interactions between ribose and adenosine receptors. The SAR findings indicated that such inversions significantly modulated hA3AR binding affinities depending on the type of ribose, whereas removal of ribose altered the functional efficacy via hA2AAR. Among the synthesized derivatives, 2-aryl-8-hexynyl adenine 4ademonstrated the highest selectivity for hA2AAR (Ki,hA2A= 5.0 ± 0.5 nM, Ki,hA3/Ki,hA2A= 86) and effectively blocked cAMP production and restored IL-2 secretion in PBMCs. Favorable pharmacokinetic properties and a notable enhancement of anticancer effects in combination with an mAb immune checkpoint blockade were observed upon oral administration of 4a. These findings establish 4aas a viable immune-oncology therapeutic candidate.