Discovery and Structure–Activity Relationship Studies of Novel Adenosine A(1) Receptor-Selective Agonists

[Image: see text] A series of benzyloxy and phenoxy derivatives of the adenosine receptor agonists N(6)-cyclopentyl adenosine (CPA) and N(6)-cyclopentyl 5′-N-ethylcarboxamidoadenosine (CP-NECA) were synthesized, and their potency and selectivity were assessed. We observed that the most potent were the compounds with a halogen in the meta position on the aromatic ring of the benzyloxy- or phenoxycyclopentyl substituent. In general, the NECA-based compounds displayed greater A(1)R selectivity than the adenosine-based compounds, with N(6)-2-(3-bromobenzyloxy)cyclopentyl-NECA and N(6)-2-(3-methoxyphenoxy)cyclopentyl-NECA showing ∼1500-fold improved A(1)R selectivity compared to NECA. In addition, we quantified the compounds’ affinity and kinetics of binding at both human and rat A(1)R using a NanoBRET binding assay and found that the halogen substituent in the benzyloxy- or phenoxycyclopentyl moiety seems to confer high affinity for the A(1)R. Molecular modeling studies suggested a hydrophobic subpocket as contributing to the A(1)R selectivity displayed. We believe that the identified selective potent A(1)R agonists are valuable tool compounds for adenosine receptor research.