Purine derivatives as ligands for A3 adenosine receptors

Selective agonists and antagonists for A3 adenosine receptors (ARs) are being explored for the treatment of a variety of disorders, including brain and heart ischemic conditions, cancer, and rheumatoid arthritis. This review covers both the structure activity relationships of nucleoside agonist ligands and selected antagonists acting at this receptor and the routes of synthesis. Highly selective agonists have been designed, using both empirical approaches and a semi-rational approach based on molecular modeling. The prototypical A3 agonists IB-MECA 10 and the more receptor-subtype-selective Cl-IB-MECA 11, both of which have affinity in binding to the receptor of approximately 1 nM, have been used widely as pharmacological probes in the elucidation of the physiological role of this receptor. In addition to the exploration of the effects of structural modification of the adenine and ribose moieties on A3AR affinity, the effects of these structural changes on the intrinsic efficacy have also been studied in a systematic fashion. Key structural features determining A3AR interaction include the N6-benzyl group, 2-position substitution such as halo, substitution of ribose (e.g., the (N)-methanocarba ring system, various 2'- and 3'-substitutions and 4'-thio substitution of oxygen). Conformational studies of the ribose moiety and its equivalents indicate that the ring oxygen is not required and the North (N) ring conformation is preferred in binding to the A3AR. Using these observations, a series of ring constrained (N)-methanocarba 5'-uronamide derivatives was recently reported to be highly selective A3AR agonists, the most notable amongst them was MRS3558 113 having a Ki value in binding to the human A3 receptor of 0.3 nM.