EXCITATORY AMINO-ACID RECEPTOR LIGANDS - RESOLUTION, ABSOLUTE STEREOCHEMISTRY, AND ENANTIOPHARMACOLOGY OF -AMINO-3-(4-BUTYL-3-HYDROXYISOXAZOL-5-YL)PROPIONIC ACID

-Amino-3-(4-butyl-3-hydroxyisoxazol-5-yl)propionic acid (Bu-HIBO, 6) h as previously been shown to be an agonist at amino-3-(3-hydroxy-5-meth ylisoxazol-4-yl)propionic acid (AMPA) receptors and an inhibitor of Ca Cl2-dependent [H-3]-(S)-glutamic acid binding (J. Med. Chem. 1992, 35, 3512-3519). To elucidate the pharmacological significance of this lat ter binding affinity, which is also shown by quisqualic acid (3) but n ot by AMPA, we have now resolved Bu-HIBO via diastereomeric salt forma tion using the diprotected Bu-HIBO derivative 11 and the enantiomers o f 1-phenylethylamine (PEA). The absolute stereochemistry of (S)-Bu-HIB O (7) (ee = 99.0%) and (R)-Bu-HIBO (8) (ee > 99.6%) were established b y an X-ray crystallographic analysis of compound 15, a salt of (R)-PEA , and diprotected 8. Circular dichroism spectra of 7 and 8 were record ed. Whereas 7 (IC50 = 0.64 mu M) and 8 (IC50 = 0.57 mu M) were equipot ent as inhibitors of CaCl2-dependent [H-3]-(S)-glutamic acid binding, neither enantiomer showed significant affinity for the synaptosomal (S )-glutamic acid uptake system(s). AMPA receptor affinity (IC50 = 0.48 mu M) and agonism (EC50 = 17 mu M) were shown to reside exclusively in the S-enantiomer, 7. Compounds 7 and 8 did not interact detectably wi th kainic acid or N-methyl-D-aspartic acid (NMDA) receptor sites. Neit her 7 nor 8 affected the function of the metabotropic (S)-glutamic aci d receptors mGlu(2) and mGlu(4b), expressed in CHO cells. Compound 8 w as shown also to be inactive at mGlu(1 alpha), whereas 7 was determine d to be a moderately potent antagonist at mGlu(1 alpha) (K-i = 110 mu M) and mGlu(5a) (K-i = 97 mu M). Using the rat cortical wedge preparat ion, the AMPA receptor agonist effect of 7 was markedly potentiated by coadministration of 8 at 21 degrees C, but not at 2-4 degrees C. Thes e observations together indicate that the potentiation of the AMPA rec eptor agonism of 7 by 8 is not mediated by metabotropic (S)-glutamate receptors but rather by the CaCl2-dependent (S)-glutamic acid binding system, which shows the characteristics of a transport mechanism. Afte r intravenous administration in mice, 7 (ED50 = 44 mu mol/kg) was slig htly more potent than AMPA (1) (ED50 = 55 mu mol/kg and twice as poten t as Bu-HIBO (6) (ED50 = 94 mu mol/kg) as a convulsant, whereas 8 was inactive. After subcutaneous administration in mice, Bu-HIBO (ED50 = 1 10 mu mol/kg) was twice as potent as AMPA (ED50 = 220 mu mol/kg) as a convulsant. Since 7 and Bu-HIBO (EC50 = 37 mu M) are much weaker than AMPA (EC50 = 3.5 mu M) as AMPA receptor agonists in vitro, the presenc e of a butyl group in the molecules of Bu-HIBO and 7 seems to facilita te the penetration of these compounds through the blood-brain barrier.