Novel 1-Hydroxyazole Bioisosteres of Glutamic Acid. Synthesis, Protolytic Properties, and Pharmacology

A number of 1-hydroxyazole derivatives were synthesized as bioisosteres of (S)-glutamic acid (Glu) and as analogues of the AMPA receptor agonist (R,S)-2-amino-3-(3-hydroxy-5-methyl-4-isoxazolyl)propionic acid (AMPA, 3b). All compounds were subjected to in vitro pharmacological studies, including a series of Glu receptor binding assays, uptake studies on native as well as cloned Glu uptake systems, and the electrophysiological rat cortical slice model. Compounds 7a,b, analogues of AMPA bearing a 1-hydroxy-5-pyrazolyl moiety as the distal carboxylic functionality, showed only moderate affinity for [3H]AMPA receptor binding sites (IC50 = 2.7 ± 0.4 μM and IC50 = 2.6 ± 0.6 μM, respectively), correlating with electrophysiological data from the rat cortical wedge model (EC50 = 280 ± 48 μM and EC50 = 586 ± 41 μM, respectively). 1-Hydroxy-1,2,3-triazol-5-yl analogues of AMPA, compounds 8a,b, showed high affinity for [3H]AMPA receptor binding sites (IC50 = 0.15 ± 0.03 μM and IC50 = 0.13 ± 0.02 μM, respectively). Electrophysiological data showed that compound 8a was devoid of activity in the rat cortical wedge model (EC50 > 1000 μM), whereas the corresponding 4-methyl analogue 8b was a potent AMPA receptor agonist (EC50 = 15 ± 2 μM). In accordance with this disparity, compound 8a was found to inhibit synaptosomal [3H]d-aspartic acid uptake (IC50 = 93 ± 25 μM), as well as excitatory amino acid transporters (EAATs) EAAT1 (IC50 = 100 ± 30 μM) and EAAT2 (IC50 = 300 ± 80 μM). By contrast, compound 8b showed no appreciable affinity for Glu uptake sites, neither synaptosomal nor cloned. Compounds 9a−c and 10a,b, possessing 1-hydroxyimidazole as the terminal acidic function, were devoid of activity in all of the systems tested. Protolytic properties of compounds 7a,b, 8b, and 9b were determined by titration, and a correlation between the pK a values and the activity at AMPA receptors was apparent. Optimized structures of all the synthesized ligands were fitted to the known crystal structure of an AMPA−GluR2 construct. Where substantial reduction or abolition of affinity at AMPA receptors was observed, this could be rationalized on the basis of the ability of the ligand to fit the construct. The results presented in this article point to the utility of 1-hydroxypyrazole and 1,2,3-hydroxytriazole as bioisosteres of carboxylic acids at Glu receptors and transporters. None of the compounds showed significant activity at metabotropic Glu receptors.