Pharmacological characterization of (4R)-alkyl glutamate analogues at the ionotropic glutamate receptors--focus on subtypes iGlu(5-7).

The kainic acid (kainate, KA) receptors belong to the class of ionotropic glutamate (iGlu) receptors in the central nervous system. Five subtypes have been identified, which have been termed KA(1,2) and iGlu(5-7). In the search for subtype selective ligands, alpha-amino-5-tert-butyl-3-hydroxy-4-isoxazolyl)propionic acid (ATPA), (4R)-methyl Glu (1a), and E-4-neopentylidene Glu (2f) have all previously been reported as selective agonists for the iGlu(5) receptor subtype. In this paper, we present the pharmacological evaluation of a five-compound series of (4R)-alkyl Glu analogs (1b-e,g) which may be envisaged as conformationally released designs of ATPA and 4-alkylidenes 2a-h. Most notable is the pharmacological profile for (4R)-isopentyl Glu (1g) which shows a 10-fold increase in binding affinity for the iGlu(5) receptor subtype (K(i)=20.5 nM) in comparison with its E-4-alkylidene structural isomer 2g. Furthermore, 1g displays high selectivity over other KA receptor subtypes (KA(1,2) and iGlu(6,7)), AMPA-, and NMDA receptors (2050 and >5000 fold, respectively).

Chemoenzymatic synthesis of glutamic acid analogues: substrate specificity and synthetic applications of branched chain aminotransferase from Escherichia coli.

A new route to alpha-keto acids is described, based on the ozonolysis of enol acetates obtained from alpha-substituted beta-keto esters. Escherichia coli branched chain aminotransferase (BCAT) activity toward a variety of substituted 2-oxoglutaric acids was demonstrated analytically. BCAT was shown to have a broad substrate spectrum, complementary to that of aspartate aminotransferase, and to offer access to a variety of glutamic acid analogues. The usefulness of BCAT was demonstrated through the synthesis of several 3- and 4-substituted derivatives.

Chemoenzymatic synthesis of a series of 4-substituted glutamate analogues and pharmacological characterization at human glutamate transporters subtypes 1-3.

A series of nine L-2,4-syn-4-alkylglutamic acid analogues (1a-i) were synthesized in high yield and high enantiomeric excess (>99% ee) from their corresponding 4-substituted ketoglutaric acids (2a-i), using the enzyme aspartate aminotransferase (AAT) from pig heart or E. coli. The synthesized compounds were evaluated as potential ligands for the glutamate transporters EAAT1, EAAT2, and EAAT3 (excitatory amino acid transporter, subtypes 1-3) in the FLIPR membrane potential (FMP) assay. We found a distinct change in the pharmacological profile when the 4-methyl group (compound 1a, an EAAT1 substrate and EAAT2,3 inhibitor) was extended to a 4-ethyl group, compound 1b, as this analogue is an inhibitor at all three subtypes, EAAT1-3. Furthermore, we conclude that both large and bulky hydrophobic substituents in the 4-position of L-2,4-syn Glu are allowed by all three glutamate transporter subtypes EAAT1-3 while maintaining inhibitory activity.