Synthesis and biological evaluation of analogues of 7-chloro-4,5-dihydro-4-oxo-8-(1,2,4-triazol-4-yl)-1,2,4-triazolo[1,5-a]quinoxaline-2-carboxylic acid (TQX-173) as novel selective AMPA receptor antagonists

In recent papers (Catarzi, D.; et al. J. Med. Chem. 2000, 43, 3824-3826; 2001, 44, 31573165) we reported chemical and biological studies on 4,5-dihydro-4-oxo-1,2,4-triazolo[1,5-a]-quinoxaline-2-carboxylates (TQXs) bearing different nitrogen-containing heterocycles at position-8. In particular, from these studies it emerged that both the 7-chloro-4,5-dihydro-4-oxo-8-(1,2,4-triazol-4-yl)-1,2,4-triazolo[1,5-a] quinoxaline-2-carboxylic acid TQX-173 (compound B) and its corresponding ethyl ester (compound A) were the most active and selective compounds of this series. In pursuing our investigation on the structure-activity relationships of these TQX derivatives, different electron-withdrawing groups (CF3, NO2) were introduced at position 7 on the TQX ring system, replacing the 7-chloro substituent of B and of other selected 8-heteroaryltriazoloquinoxaline-2-carboxylates previously described. All the newly synthesized compounds were biologically evaluated for their binding at the Gly/NMDA, AMPA, and KA high-affinity receptors. Gly/NMDA binding assays were performed to assess the selectivity of the reported compounds toward the AMPA receptor. Compounds endowed with micromolar binding affinity for the KA high-affinity binding site were also evaluated for their binding at the KA low-affinity receptor. Some selected compounds were also tested for their functional antagonist activity at the AMPA and NMDA receptor-ion channel complex. The results obtained in this study have pointed out that 4,5-dihydro-7-nitro-4-oxo-8-(3-carboxypyrrol-1-yl)-1,2,4-triazolo[1,5-alquinoxaline-2-carboxylic acid (9b) and its corresponding ethyl ester (9a) are the most potent and selective AMPA receptor antagonists reported to date among the TQX series.