Structural determinants for AMPA agonist activity of aryl or heteroaryl substituted AMPA analogues. Synthesis and...

We have previously reported the synthesis and pharmacological characterization of analogues of 2-amino-3-(3-hydroxy-5-methyl-4-isoxazolyl)propionic acid (AMPA, 1a), in which the methyl group was replaced by a phenyl group (APPA, 1b) or heteroaryl groups. While 2b and its 3-pyridyl analogue 2-amino-3-[3-hydroxy-5-(3-pyridyl)-4-isoxazolyl]propionic acid (3-Py-AMPA, 3) show very low affinity for AMPA receptors, introduction of heteroaryl substituents containing heteroatom in the 2-position provides potent AMPA receptor agonists. We here report the synthesis and pharmacology of 2-amino-3-(3-hydroxy-5-pyrazinyl-4-isoxazolyl)propionic acid (7) (IC50 = 1.2 microM), which is weaker as an AMPA agonist than AMPA (IC50 = 0.040 microM; EC50 = 3.5 microM) but comparable in potency with 2-Py-AMPA (4) (IC50 = 0.57 microM; EC50 = 7.4 microM), as determined in radioligand binding and electrophysiological experiments, respectively. The AMPA analogues 8a-c, containing 2-, 3-, or 4-methoxyphenyl substituents, respectively, and the corresponding hydroxyphenyl analogues, 9a-c, were also synthesized and evaluated pharmacologically. With the exception of 2-amino-3-[3-hydroxy-5-(2-hydroxyphenyl)-4-isoxazolyl]propionic acid (9a), which is a very weak AMPA agonist (IC50 = 45 microM; EC50 = 324 microM), none of these compounds showed detectable effect at AMPA receptors.