Stereochemistry and molecular pharmacology of (S)-thio-ATPA, a new potent and selective GluR5 agonist

(RS)-2-Amino-3-(5-tert-butyl-3-hydroxy-4-isothiazolyl)propionic acid (thio- ATPA), a 3-isothiazolol analogue of (RS)-2-amino-3-(5-tert-butyl-3-hydroxy- 4-isoxazolyl)propionic acid (ATPA), has previously been shown to he a relat ively weak AMPA receptor agonist at native (S)-glutamic acid ((S)-Glu) rece ptors (EC50 = 14 muM). comparable in potency with ATPA (EC50 = 34 muM). Rec ent findings, that (S)-ATPA is a potent (EC50 = 0.48 muM) and selective ago nist at homomerically expressed ionotropic GluR5, prompted us to resolve th io-ATPA using chiral chromatography and pharmacologically characterize the two enantiomers at native as well as cloned ionotropic glutamate receptors. The enantiomers. (S)- and (R)-thio-ATPA. were obtained in high enantiomeri c excess, and their absolute stereochemistry established by an X-ray crysta llographic analysis. Electrophysiologically. the two enantiomers were evalu ated in the rat cortical wedge preparation, and the S-enantiomer was found to be an AMPA receptor agonist (EC50 = 8.7 muM) twice as potent as the race mate, whereas the R-enantiomer was devoid of activity. In accordance with t his, (S)-thio-ATPA proved to be an agonist at homomerically expressed recom binant AMPA receptors (GluR1o. GluR3o, and GluR4o) with EC50 values of 5, 3 2 and 20 muM. respectively, producing maximal steady state currents of 78-1 68% Of those maximally evoked by kainic acid, and 120-1600% of those maxima lly evoked by (S)-ATPA. At homomerically expressed GluR5. (S)-thio-ATPA was found to be a potent agonist (EC50 = 0.10 muM), thus being approximately f ive times more potent than (S)-ATPA. (R)-Thio-ATPA induced saturating curre nts with an estimated EC50 value of 10 muM. most likely due to a contaminat ion with (S)-thio-ATPA. At heteromerically expressed GluR6 + KA2 receptors, (S)-thio-ATPA showed relatively weak agonistic properties (EC50 = 4.9 muM) . Thus. (S)-thio-ATPA has been shown to be a very potent agonist at GluR5, and may be a valuable tool for the investigation of desensitization propert ies of AMPA receptors. (C) 2001 Elsevier Science B.V. All rights reserved.