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

Abstract

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 A(3) 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) A(1) and A(2A) and at human (h) A(1) and A(3) adenosine receptors, showed high hA(3) 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 hA(3) affinity (K(i) = 0.60 nM) and strong selectivity vs hA(1) 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 hA(3) receptor. Structure-activity relationships have been explained analyzing the three-dimensional structure of the antagonist-receptor models obtained by molecular docking simulation.

Selective Human Adenosine A3 Antagonists based on Pyrido[2,1‐f]purine‐2,4‐diones: Novel Features of hA3 Antagonist Binding

“…[28] The putative binding site of the ligands was located using CastP analysis, [29] and included the residues that are known to be crucial for activity located in transmembrane (TM) segments 3, 5, 6, and 7, [18] and the second extracellular loop (EL2). From visual inspection, the main interactions within the putative binding site agree with those proposed by Moro and co-workers for their triazoloquinoxaline [30] and pyrazolotriazolopyrimidine derivatives. [31] The ligands were placed inside the TM bundle, with the tricyclic ring system stacked between Phe 168 and the hydrogen-bonded Gln 167-Asn 250 pair, the condensed pyridine ring pointing toward EL2 (situated on top acting as a lid), and the CO group at position 4 oriented toward the NH group of Phe 168 ( Figure 2).…”

“…Molecular modeling studies performed with several of our pyridoA C H T U N G T R E N N U N G [2,1-f]purine-2,4-diones in the proposed model for the human A 3 receptor, [18] resulted in the ligands being located in the putative binding site already proposed by other authors. [30,31] The Gln 167-Asn 250 pair, maintained during the MD simulation, together with the highly conserved Phe 168, delimits the cavity where the pyridoA C H T U N G T R E N N U N G [2,1-f]purine-2,4-diones are located, and could be an important feature related to the hA 3 subtype binding site. Van der Waals and electrostatic contributions of crucial residues to the binding of the pyridopurinediones are in good agreement with SAR data.…”

Docking studies of agonists and antagonists suggest an activation pathway of the A3 adenosine receptor

“…Most of the current potent and selective nonnucleoside A 3 AR antagonists, including pyridines (3 MRS1523), dihydropyridines (5 MRS1334), triazoloquinazolines (6 MRS1220), and pyrazolotriazolopyrimidines (7 MRE-3008F20), have a high C log P (>3.7) and thus are highly lipophilic and display a very low degree of water-solubility. More water-soluble A 3 AR antagonists, 2 [24] and 4 [25], are considered to be more tractable pharmacological tools for in vitro and in vivo studies. As displayed in Fig.…”

A New Series of 2-Alkoxy(aralkoxy)-[1,2,4]triazolo[1,5-a]quinazolin-5-ones as Adenosine Receptor Antagonists

“…1) those containing a 1,2,4-triazoloquinoxaline scaffold have been extensively explored, showing to be an extremely versatile motif during the identification of new valuable and selective AR ligands. [23][24][25][26][27][28][29][30][31][32][33] In our previous papers, [34][35][36] we have described the synthetic methodology to obtain a new series of 2-alkoxy-[1,2,4]triazolo[1,5-a]quinazolin-5-ones and their derivatives 1-11, Chart 1. In this scenario, and as a part of our interest in the search for novel adenosine receptor antagonists, we herein report the pharmacological characterization of our compounds (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11) at all adenosine receptor subtypes.…”