1,2,4-triazolo[1,5-a]quinoxaline as a versatile tool for the design of selective human A3 adenosine receptor antagonists:: Synthesis, biological evaluation, and molecular modeling studies of 2-(hetero)aryl- and 2-carboxy-substitued derivatives

A number of 4-oxo-substituted 1,2,4-triazolo[1,5-alquinoxaline derivatives bearing at position-2 the claimed (hetero)aryl moiety (compounds 1-15) but also a carboxylate group (16-28, 3236) or a hydrogen atom (29-31) were designed as human A(3) (hA(3)) adenosine receptor (AR) antagonists. This study produced some interesting compounds and among them the 2-(4methoxyphenyl)-1,2,4-triazolo[1,5-alquinoxalin-4-one (8), which can be considered one of the most potent and selective hA(3) adenosine receptor antagonists reported till now. Moreover, as a new finding, replacement of the classical 2-(hetero)aryl moiety with a 2-carboxylate function (compounds 16-28 and 32-36) maintained good hA(3) AR binding activity but, most importantly and interestingly, produced a large increase in bA(3) versus hA(1) selectivity. A receptor-based SAR analysis provided new interesting insights about the steric and electrostatic requirements that are important for the anchoring of these derivatives at the hA(3) receptor recognition site, thus highlighting the versatility of the triazoloquinoxaline scaffold for obtaining potent and selective hA(3) AR antagonists.