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 A_2A 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 hA_2A ARs, shows the best binding profile with a high hA₃ affinity (K_i = 0.60 nM) and strong selectivity vs hA₁ and vs hA_2A 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.