Pyrazolo4,3-e-1,2,4-triazolo1,5-cpyrimidine Derivatives as Adenosine Receptor Antagonists. Influence of the N5 Substituent on the Affinity at the Human A_3 and A_(2B) Adenosine Receptor Subtypes: A Molecular Modeling Investigation

摘要

A new series of Pyrazolo4,3-e-1,2,4-triazolo1,5-cpyrimidines bearing various substituents at both the N5-pyrimidinyl and N8-pyrazolyl positions have been synthesized, and their binding affinities at the four human adenosine receptor subtypes (hA_1, hA_(2A), hA_(2B), and hA_3) have been evaluated. All the described compounds contain arylacetyl moieties at the N5 position and arylalkyl substituents at the N8 position. Surprisingly, all the compounds present their most potent affinities at the hA_(2B) receptor was observed (K_i(hA_1) = 1100 nM; K_i(hA_(2A)) = 800 nM; K_i(hA_(2B)) = 20 nM; K_i(hA_3) = 300 nM; K_i(hA_1/A_(2B)) = 55, K_i(hA_(2A)/A_(2B)) = 40 nM; K_i(hA_3/hA_(2B)) = 15). To understand the molecular significance of these results, we compared the putative TM (transmembrane) binding motif of compound 9 on both hA_(2B) and hA_3 receptors. From our docking studies, compound 9 first neatly inside the TM region of the hA_(2B) receptor but not in the corresponding hA_3 region, illustrating significant differences between the two subtypes. The study herein presented permits an understanding of why the bioisosteric replacement of an -NH, present in previously reported hA_3 receptor antagonists, with a -CH_2 group at the N5 position induces such large differences in hA_(2B)/hA_3 affinity. In the molecular structure of the hA_3 receptor, two residues, Ser243 (TM6) and Ser271 (TM7), create a hydrophilic region, which seems to permit a better accommodation of the phenylurea series into this putative hA_3 binding site than the phenylacetyl series.