Structure-Based Design, Synthesis by Click Chemistry and in Vivo Activity of Highly Selective A₃ Adenosine Receptor Agonists

2-Arylethynyl derivatives of (N)-methanocarba adenosine 5'-uronamides are selective A₃AR (adenosine receptor) agonists. Here we substitute a 1,2,3-triazol-1-yl linker in place of the rigid, linear ethynyl group to eliminate its potential metabolic liability. Docking of nucleosides containing possible short linker moieties at the adenine C2 position using a hybrid molecular model of the A₃AR (based on the A_2AAR agonist-bound structure) correctly predicted that a triazole would maintain the A₃AR selectivity, due to its ability to fit a narrow cleft in the receptor. The analogues with various N⁶ and C2-aryltriazolyl substitution were synthesized and characterized in binding (K_i at hA₃AR 0.3 - 12 nM) and in vivo to demonstrate efficacy in controlling chronic neuropathic pain (chronic constriction injury). Among N⁶-methyl derivatives, a terminal pyrimidin-2-yl group in 9 (MRS7116) increased duration of action (36% pain protection at 3 h) in vivo. N⁶-Ethyl 5-chlorothien-2-yl analogue 15 (MRS7126) preserved in vivo efficacy (85% protection at 1 h) with short duration. Larger N⁶ groups, e.g. 17 (MRS7138, >90% protection at 1 and 3 h), greatly enhanced in vivo activity. Thus, we have combined structure-based methods and phenotypic screening to identify nucleoside derivatives having translational potential.