Synthesis and enantiopharmacology of new AMPA-kainate receptor agonists

Regioisomeric 3-carboxyisoxazolinyl prolines [CIP-A (+/-)-6 and CIP-B (+/-) -7] and 3-hydroxyisoxazolinyl prolines [(+/-)-8 and (+/-)-9] were synthesiz ed and assayed for glutamate receptor activity. The tests were carried out in vitro by means of receptor binding techniques, second messenger assays, and the rat cortical wedge preparation. CIP-A showed a good affinity for bo th 2-amino-3-(3-hydroxy-5-methylisoxazol-4-yl) acid (AMPA) and kainic acid (KAIN) receptors. These results were confirmed in the cortical slice model where CIP-A displayed an EC50 value very close to that of AMPA. The convuls ant properties of all the compounds were evaluated in vivo on DBA/2 mice af ter icy injection. CIP-A showed a convulsant activity, measured as tonus an d clonus seizures, 18-65 times higher than that produced by AMPA. It was al so quite active after ip administration, since it induced seizures in mice at doses as low as 3.2 nmol/mouse. On the basis of the above-reported resul ts we prepared and tested the enantiomers of CIP-A and CIP-B, obtained by r eacting (S)-3,4-didehydroproline and (R)-3,4-didehydroproline, respectively , with ethoxycarbonylformonitrile oxide. In all the tests the S-form, CIP-A S [(-)-6], emerged as the eutomer evidencing common stereochemical requirem ents with the reference compounds AMPA and KAIN. Through modeling studies, carried out on CIP-A, AMPA, and KAIN, active conformations for CIP-AS and A MPA at AMPA receptors as well as for CIP-AS and KAIN at KAIN receptors are suggested.