Exploring the Directionality of 5-Substitutions in a New Series of 5-Alkylaminopyrazolo[4,3-e]1,2,4-triazolo[1,5-c]pyrimidine as a Strategy to Design Novel Human A3 Adenosine Receptor Antagonists.

The structure−activity relationship (SAR) of new 5-alkylaminopyrazolo-[4,3-e]1,2,4-triazolo[1,5-c]pyrimidines as antagonists of the A3 adenosine receptor (AR) was explored with the principal aim to establish the directionality of 5-substitutions inside the orthosteric binding site of the A3 AR. All the synthesized compounds showed affinity for the hA3 AR from nanomolar to subnanomolar range. In particular, the most potent and selective antagonist presents an (S) α-phenylethylamino moiety at the 5 position (26, Ki hA3 = 0.3 nM). Using an in silico receptor-driven approach, we have determined the most favorable orientation of the substitutions at the 5 position of the pyrazolo[4,3-e]1,2,4-triazolo[1,5-c]-pyrimidine (PTP) scaffold, opening the possibility for further derivatizations aimed at directing the N5 position toward the extracellular environment.