Structural determinants of AMPA agonist activity in analogues of 2-amino-3-(3-carboxy-5-methyl-4-isoxazolyl)propionic acid: Synthesis and pharmacology

We have previously shown that the 2-amino-3-(3-hydroxy-5-methyl-4-isoxazoly l)propionic acid acid (AMPA) receptor agonist, 2-amino-3-(3-carboxy-5-methy l-4-isoxazolyl)propionic acid (ACPA, 2), binds to AMPA receptors in a manne r different from that of AMPA (1) itself and that 2, in contrast to 1, also binds to kainic acid receptor sites. To elucidate the structural requireme nts for selective activation of the site/conformation of AMPA receptors rec ognized by 2, a number of isosteric analogues of 2 have now been synthesize d and pharmacologically characterized. The compound 2-amino-3-(5-carboxy-3- methoxy-4-isoxazolyl)propionic acid (3a) (IC50 = 0.11 muM; EC50 = 1.2 muM), which is a regioisostere of 2 with a methoxy group substituted for the met hyl group, was approximately equipotent with 2 (IC50 = 0.020 muM; EC50 = 1. 0 muM) as an inhibitor of [H-3]AMPA binding and as an AMPA agonist, respect ively, whereas the corresponding 3-ethoxy analogue 3b (IC50 = 1.0 muM; EC50 = 4.8 muM) was slightly weaker. The analogues 3c-e, containing C3 alkoxy g roups, were an order of magnitude weaker than 3b, whereas the additional st eric bulk of the alkoxy groups of 3f-i or the presence of an acidic hydroxy l group at the 3-position of the isoxazole ring of 3j prevented interaction with AMPA receptor sites. The 2-amino-3-(2-alkyl-5-carboxy-3-oxo-4-isoxazo lyl)propionic acids 4a,b,i, which are regio-isosteric analogues of 3a,b,i, showed negligible interaction with AMPA recognition sites. Similarly, repla cement of the carboxyl. group of 3b by isosteric tetrazolyl or 1,2,4-triazo lyl groups to give 5 and 6, respectively, or conversion of 3b into analogue 7, in which the diaminosquaric acid group has been bioisosterically substi tuted for the cc-aminocarboxylic acid unit, provided compounds completely d evoid of effect at AMPA receptors. In contrast to the parent compound ACPA (2) (IC50 = 6.3 muM), none of the analogues described showed detectable inh ibitory effect on [H-3]kainic acid receptor binding.