Modulation of adenosine receptor affinity and intrinsic efficacy in adenine nucleosides substituted at the 2-position

We have studied the structural determinants of binding affinity and efficacy of adenosine receptor (AR) agonists. Substituents at the 2-position of adenosine have been combined with N 6-substitutions known to enhance human A 3AR affinity. The environment surrounding the 2-position within the putative A 3AR binding site was explored using rhodopsin-based homology modeling and ligand docking. We studied the structural determinants of binding affinity and efficacy of adenosine receptor (AR) agonists. Substituents at the 2-position of adenosine were combined with N 6-substitutions known to enhance human A 3AR affinity. Selectivity of binding of the analogues and their functional effects on cAMP production were studied using recombinant human A 1, A 2A, A 2B, and A 3ARs. Mainly sterically small substituents at the 2-position modulated both the affinity and intrinsic efficacy at all subtypes. The 2-cyano group decreased hA 3AR affinity and efficacy in the cases of N 6-(3-iodobenzyl) and N 6-( trans-2-phenyl-1-cyclopropyl), for which a full A 3AR agonist was converted into a selective antagonist; the 2-cyano- N 6-methyl analogue was a full A 3AR agonist. The combination of N 6-benzyl and various 2-substitutions (chloro, trifluoromethyl, and cyano) resulted in reduced efficacy at the A 1AR. The environment surrounding the 2-position within the putative A 3AR binding site was explored using rhodopsin-based homology modeling and ligand docking.