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-isoxazolyl)propionic acid acid (AMPA) receptor agonist, 2-amino-3-(3-carboxy-5-methyl-4-isoxazolyl)propionic acid (ACPA, 2), binds to AMPA receptors in a manner 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 requirements for selective activation of the site/conformation of AMPA receptors recognized by 2, a number of isosteric analogues of 2 have now been synthesized 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 methyl 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, respectively, 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 groups, were an order of magnitude weaker than 3b, whereas the additional steric bulk of the alkoxy groups of 3f-i or the presence of an acidic hydroxyl 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-isoxazolyl)propionic acids 4a,b,i, which are regio-isosteric analogues of 3a,b,i, showed negligible interaction with AMPA recognition sites. Similarly, replacement of the carboxyl. group of 3b by isosteric tetrazolyl or 1,2,4-triazolyl groups to give 5 and 6, respectively, or conversion of 3b into analogue 7, in which the diaminosquaric acid group has been bioisosterically substituted for the cc-aminocarboxylic acid unit, provided compounds completely devoid of effect at AMPA receptors. In contrast to the parent compound ACPA (2) (IC50 = 6.3 muM), none of the analogues described showed detectable inhibitory effect on [H-3]kainic acid receptor binding.