Structural Modification and Biological Evaluation of 2,8-Disubstituted Adenine and Its Nucleosides as A_2A Adenosine...

Building on the preceding structural analysis and a structure-activity relationship (SAR) of 8-aryl-2-hexynyl nucleoside hA_2AAR 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 hA_2AAR. Among the synthesized derivatives, 2-aryl-8-hexynyl adenine 4a demonstrated the highest selectivity for hA_2AAR (K_i,hA2A = 5.0 ± 0.5 nM, K_i,hA3/K_i,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 4a as a viable immune-oncology therapeutic candidate.