Excitatory amino acid receptor ligands: resolution, absolute stereochemistry, and enantiopharmacology of 2-amino-3-(4-butyl-3-hydroxyisoxazol-5-yl)propionic acid.
(RS)-2-Amino-3-(4-butyl-3-hydroxyisoxazol-5-yl)propionic acid (Bu-HIBO, 6) has previously been shown to be an agonist at (RS)-2-amino-3-(3-hydroxy-5-methylisoxazol-4-yl)propionic acid (AMPA) receptors and an inhibitor of CaCl2-dependent [3H]-(S)-glutamic acid binding (J. Med. Chem. 1992, 35, 3512-3519). To elucidate the pharmacological significance of this latter binding affinity, which is also shown by quisqualic acid (3) but not by AMPA, we have now resolved Bu-HIBO via diastereomeric salt formation using the diprotected Bu-HIBO derivative 11 and the enantiomers of 1-phenylethylamine (PEA). The absolute stereochemistry of (S)-Bu-HIBO (7) (ee = 99.0%) and (R)-Bu-HIBO (8) (ee > 99.6%) were established by an X-ray crystallographic analysis of compound 15, a salt of (R)-PEA, and diprotected 8. Circular dichroism spectra of 7 and 8 were recorded. Whereas 7 (IC50 = 0.64 microM) and 8 (IC50 = 0.57 microM) were equipotent as inhibitors of CaCl2-dependent [3H]-(S)-glutamic acid binding, neither enantiomer showed significant affinity for the synaptosomal (S)-glutamic acid uptake system(s). AMPA receptor affinity (IC50 = 0.48 microM) and agonism (EC50 = 17 microM) were shown to reside exclusively in the S-enantiomer, 7. Compounds 7 and 8 did not interact detectably with kainic acid or N-methyl-D-aspartic acid (NMDA) receptor sites. Neither 7 nor 8 affected the function of the metabotropic (S)-glutamic acid receptors mGlu2 and mGlu4a, expressed in CHO cells. Compound 8 was shown also to be inactive at mGlu1 alpha, whereas 7 was determined to be a moderately potent antagonist at mGlu1 alpha (Ki = 110 microM) and mGlu5a (Ki = 97 microM). Using the rat cortical wedge preparation, 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. These observations together indicate that the potentiation of the AMPA receptor 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. After intravenous administration in mice, 7 (ED50 = 44 mumol/kg) was slightly more potent than AMPA (1) (ED50 = 55 mumol/kg) and twice as potent as Bu-HIBO (6) (ED50 = 94 mumol/kg) as a convulsant, whereas 8 was inactive. After subcutaneous administration in mice, Bu-HIBO (ED50 = 110 mumol/kg) was twice as potent as AMPA (ED50 = 220 mumol/kg) as a convulsant. Since 7 and Bu-HIBO (EC50 = 37 microM) are much weaker than AMPA (EC50 = 3.5 microM) as AMPA receptor agonists in vitro, the presence of a butyl group in the molecules of Bu-HIBO and 7 seems to facilitate the penetration of these compounds through the blood-brain barrier.
Johansen
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TN
PharmaBiotec Research Center, Department of Medicinal Chemistry, Royal Danish School of Pharmacy, 2 Universitetsparken, Copenhagen, Denmark.
Ebert
B
B
Bräuner-Osborne
H
H
Didriksen
M
M
Christensen
I T
IT
Søby
K K
KK
Madsen
U
U
Krogsgaard-Larsen
P
P
Brehm
L
L
D000409:Alanine / Q000031:analogs & derivatives* / Q000737:chemistry / Q000378:metabolism / Q000494:pharmacology; D000818:Animals; D000927:Anticonvulsants / Q000737:chemistry / Q000378:metabolism / Q000494:pharmacology; D001921:Brain / Q000187:drug effects / Q000378:metabolism; D016466:CHO Cells; D002122:Calcium Chloride / Q000378:metabolism*; D002851:Chromatography, High Pressure Liquid; D006224:Cricetinae; D018360:Crystallography, X-Ray; D004594:Electrophysiology; D018690:Excitatory Amino Acid Agonists / Q000737:chemistry / Q000378:metabolism / Q000494:pharmacology*; D018691:Excitatory Amino Acid Antagonists / Q000737:chemistry / Q000378:metabolism / Q000494:pharmacology*; D007555:Isoxazoles / Q000737:chemistry / Q000378:metabolism / Q000494:pharmacology*; D008297:Male; D051379:Mice; D008968:Molecular Conformation; D051381:Rats; D017207:Rats, Sprague-Dawley; D018091:Receptors, AMPA / Q000378:metabolism; D017470:Receptors, Glutamate / Q000187:drug effects*; D018094:Receptors, Metabotropic Glutamate / Q000096:biosynthesis / Q000187:drug effects; D013237:Stereoisomerism
C111356:2-amino-3-(4-butyl-3-hydroxyisoxazol-5-yl)propionic acid; D000927:Anticonvulsants; D018690:Excitatory Amino Acid Agonists; D018691:Excitatory Amino Acid Antagonists; D007555:Isoxazoles; D018091:Receptors, AMPA; D017470:Receptors, Glutamate; D018094:Receptors, Metabotropic Glutamate; C108822:metabotropic glutamate receptor 2; C104077:metabotropic glutamate receptor type 1; D002122:Calcium Chloride; D000409:Alanine; C108823:metabotropic glutamate receptor 4