1,2,4-Triazolo[4,3-a]quinoxalin-1-one Moiety as an Attractive Scaffold To Develop New Potent and Selective Human A3 Adenosine Receptor Antagonists:  Synthesis, Pharmacological, and Ligand−Receptor Modeling Studies

In the past few years much effort in our laboratory has been directed toward the study of adenosine receptor antagonists, and recently we focused our attention on 2-aryl-1,2,4-triazolo[4,3-a]quinoxaline-1,4-diones and 2-aryl-1,2,4-triazolo[4,3-a]quinoxalin-4-amino-1-ones, some of which were potent and/or selective A3 receptor antagonists. In the present paper, a new series of triazoloquinoxaline derivatives is described. Most of the new compounds, biologically evaluated in radioligand binding assays at bovine (b) A1 and A2A and at human (h) A1 and A3 adenosine receptors, showed high hA3 adenosine receptor affinity and selectivity. In particular, 2-(4-nitrophenyl)-1,2,4,5-tetrahydro-1,2,4-triazolo[4,3-a]quinoxaline-1,4-dione (1), also tested at the hA2A ARs, shows the best binding profile with a high hA3 affinity (K i = 0.60 nM) and strong selectivity vs hA1 and vs hA2A receptors (both selectivity ratios greater than 16 600). To interpret our experimental results, we decided to theoretically depict the putative transmembrane binding motif of our triazoloquinoxaline analogues on hA3 receptor. Structure−activity relationships have been explained analyzing the three-dimensional structure of the antagonist−receptor models obtained by molecular docking simulation.