1,2,4-Triazolo[1,5-a]quinoxaline derivatives and their simplified analogues as adenosine A3 receptor antagonists. Synthesis, structure-affinity relationships and molecular modeling studies.

The 1,2,4-triazolo[1,5-a]quinoxaline (TQX) scaffold was extensively investigated in our previously reported studies and recently, our attention was focused at position 5 of the tricyclic nucleus where different acyl and carboxylate moieties were introduced (compounds 2-15). This study produced some interesting compounds endowed with good hA(3) receptor affinity and selectivity. In addition, to find new insights about the structural requirements for hA(3) receptor-ligand interaction, the tricyclic TQX ring was destroyed yielding some 1,2,4-triazole derivatives (compounds 16-23). These simplified compounds, though maintaining the crucial structural requirements for adenosine receptor-ligand interaction, have a very low hA(3) adenosine receptor affinity, the only exception being compound 23 (1-[3-(4-methoxyphenyl)-1-phenyl-1H-1,2,4-triazol-5-yl]-3-phenylurea) endowed with a K-i value in the micro-molar range and high hA(3) selectivity versus both hA(1) and hA(2A) AR subtypes. Evaluation of the side products obtained in the herein reported synthetic pathways led to the identification of some new triazolo[1,5-a]quinoxalines as hA(3)AR antagonists (compounds 24-27). These derivatives, though lacking the classical structural requirements for the anchoring at the hA(3) receptor site, show high hA(3) affinity and in some case selectivity versus hA(1) and hA(2A) subtypes. Molecular docking of the herein reported tricyclic and simplified derivatives was carried out to depict their hypothetical binding mode to our model of hA(3) receptor. (C) 2014 Elsevier Ltd. All rights reserved.