PubMed2004
We have previously used homologation of (S)-glutamic acid (Glu) and Glu analogs as an approach to the design of selective ligands for different subtypes of Glu receptors. (RS)-2-Amino-3-(3-carboxy-5-methyl-4-isoxazolyl)propionic acid (ACPA), which is an isoxazole homolog of Glu, is a very potent agonist at the (RS)-2-amino-3-(3-hydroxy-5-methyl-4-isoxazolyl)propionic acid (AMPA) subgroup of Glu receptors and a moderately potent ligand for the kainic acid (KA) subgroup of Glu receptors. The enantiomers of ACPA were previously obtained by chiral HPLC resolution. Prompted by pharmacological interest in ACPA, we have now prepared the (S)- and (R)-enantiomers of ACPA by stereocontrolled syntheses using (1R,2R,5R)- and (1S,2S,5S)-2-hydroxy-3-pinanone, respectively, as chiral auxiliaries. Furthermore, the 5-ethyl analog of ACPA, Ethyl-ACPA, was synthesized, and (S)- and (R)-Ethyl-ACPA were also prepared using this method. The absolute configurations of (S)- and (R)-ACPA were established by X-ray crystallographic analysis of a protected (1S,2S,5S)-2-hydroxy-3-pinanone imine derivative of (R)-ACPA. The absolute stereochemistry of (S)- and (R)-Ethyl-ACPA was assigned on the basis of a comparison of their properties with those of the enantiomers of ACPA, employing elution order on chiral HPLC columns, as well as circular dichroism (CD) spectroscopy in combination with time-dependent density functional theory. The structural and electronic basis for the Cotton effect observed for such analogs is examined. The lower homolog of ACPA, (RS)-2-amino-2-(3-carboxy-5-methyl-4-isoxazolyl)acetic acid (1), which is a Glu analog, was also synthesized. Affinities and neuroexcitatory effects were determined using rat brain membranes and cortical wedges, respectively, at native AMPA, KA, and N-methyl-D-aspartic acid (NMDA) receptors. The molecular pharmacology of (S)- and (R)-ACPA and (S)- and (R)-Ethyl-ACPA was evaluated at homomeric cloned subtypes of AMPA receptors (iGluR1o,3o,4o) and of KA receptors (iGluR5,6), expressed in Xenopus laevis oocytes. The cloned receptors mGluR1alpha, mGluR2, and mGluR4a, expressed in CHO cell lines, were used to study the effects of the five compounds at metabotropic Glu receptors. In accordance with ligand-receptor complexes known from X-ray crystallography, the conformationally restricted Glu analog 1 was inactive at all Glu receptors studied, and the R-forms of ACPA and Ethyl-ACPA were very weak or inactive at these receptors. At AMPA receptor subtypes, (S)-ACPA and (S)-Ethyl-ACPA showed equally potent agonist effects at iGluR1o and iGluR3o, whereas (S)-Ethyl-ACPA was 6-fold more potent than (S)-ACPA at iGluR4o. (S)-ACPA and (S)-Ethyl-ACPA were approximately an order of magnitude less potent at iGluR5 than at AMPA receptor subtypes, and neither compound showed detectable effects at iGluR6. The binding mode of (S)-Ethyl-ACPA at iGluR2 was examined by docking to the (S)-ACPA-iGluR2 complex.