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In order to investigate the 4-hydroxy-1,2,5-oxadiazol-3-yl moiety as a carboxylic acid bioisoster at ionotropic glutamate receptors (iGluRs), a series of acidic alpha-aminocarboxylic acids in which the distal carboxy group was replaced by the 4-hydroxy-1,2,5-oxadiazol-3-yl group was synthesized. Ionization constants were determined. All target compounds, except the Asp analogue 12, were resolved using chiral HPLC. Whereas 12 showed good affinity exclusively at NMDA receptors, the Glu analogue, (+)-10, was an unselective, though potent AMPA receptor preferring agonist (EC(50) = 10 microM at iGluR2) showing only low stereoselectivity. The two higher Glu homologues, (+)-15 and (+)-18, turned out to be weak agonists at iGluR2 as well as weak antagonists at NR1/NR2A, whereas the corresponding (-)-isomers were selective NR1/NR2A antagonists with somewhat higher potency. The results proved the 4-hydroxy-1,2,5-oxadiazol-3-yl moiety to be a useful bioisoster at all three classes of iGluRs, capable of being integrated into agonists as well as antagonists.

Although the function of the kainate receptors in the brain is still not clear, they are increasingly defined as targets in the development of new classes of anti-epileptics. The thienopyrimidines described in this report were tested for their antagonistic effect at the kainate receptor subtypes GluR5 and GluR6. The highest effectiveness was obtained by a 4-ethoxy-thieno[2,3-d]pyrimidin with an IC50 = 68 microM at the GluR6 receptor.

Recently we identified (R,S)-2-acetyl-1-(4'-chlorophenyl)-6,7-dimethoxy-1,2,3,4-tetrahydroisoquinoline (6) as a potent non-competitive AMPA receptor antagonist able to prevent epileptic seizures. We report here the optimized synthesis of compound 6, its resolution by chiral preparative HPLC, and the absolute configuration of (R)-enantiomer established by X-ray diffractometry. The biological tests of the single enantiomers revealed that higher anticonvulsant and antagonistic effects reside in (R)-enantiomer as also suggested by molecular modeling studies.

Two 3-(5-tetrazolylmethoxy) analogues, 1a and 1b, of (RS)-2-amino-3-(3-hydroxy-5-methyl-4-isoxazolyl)propionic acid (AMPA), a selective AMPA receptor agonist, and (RS)-2-amino-3-(5-tert-butyl-3-hydroxy-4-isoxazolyl)propionic acid (ATPA), a GluR5-preferring agonist, were synthesized. Compounds 1a and 1b were pharmacologically characterized in receptor binding assays, and electrophysiologically on homomeric AMPA receptors (GluR1-4), homomeric (GluR5 and GluR6) and heteromeric (GluR6/KA2) kainic acid receptors, using two-electrode voltage-clamped Xenopus laevis oocytes expressing these receptors. Both analogues proved to be antagonists at all AMPA receptor subtypes, showing potencies (Kb=38-161 microM) similar to that of the AMPA receptor antagonist (RS)-2-amino-3-[3-(carboxymethoxy)-5-methyl-4-isoxazolyl]propionic acid (AMOA) (Kb=43-76 microM). Furthermore, the AMOA analogue, 1a, blocked two kainic acid receptor subtypes (GluR5 and GluR6/KA2), showing sevenfold preference for GluR6/KA2 (Kb=19 microM). Unlike the iGluR antagonist (S)-2-amino-3-[5-tert-butyl-3-(phosphonomethoxy)-4-isoxazolyl]propionic acid [(S)-ATPO], the corresponding tetrazolyl analogue, 1b, lacks kainic acid receptor effects. On the basis of docking to a crystal structure of the isolated extracellular ligand-binding core of the AMPA receptor subunit GluR2 and a homology model of the kainic acid receptor subunit GluR5, we were able to rationalize the observed structure-activity relationships.

The synthesis and biological evaluation at the Gly/NMDA, AMPA and Kainate receptors of new oxazolo[4,5-c]quinolin-4-one derivatives are reported. Different substituents were introduced at the 2-position (mercapto, carbonyl and methyl groups) and on the fused benzo ring (chlorine atom(s) and trifluoromethyl group). Among the herein reported compounds, the 2-mercapto-derivatives 1-4 showed the highest Gly/NMDA affinities, comparable to that of 5,7-dichlorokynurenic acid. The most active compound was the 7-chloro-substituted derivative 1 (Ki=0.082 microM) which possesses a Gly/NMDA selectivity of 50- and 500-fold with respect to AMPA and KA receptors, respectively. Functional antagonism studies performed on some selected 2-mercapto compounds, at both AMPA and NMDA receptor-ion channels, assessed the antagonistic properties of these derivatives. SAR studies pointed out the importance of the concurrent presence of electron-rich moieties at both the 2- and 3-positions of the oxazolo[4,5-c]quinolin-4-one framework. In fact, the 3-sp2-nitrogen atom plays a significant role in reinforcing the hydrogen bond that the 4-carbonyl oxygen probably forms with the arginine residue (R523) of the Gly/NMDA receptor site. The presence of 2-substituent able to form a hydrogen bonding interaction was also proved to be important for a good Gly/NMDA receptor affinity.

A conformationally constrained, indole-based kainate analogue was designed based on Gouaux's X-ray structure of kainic acid bound to an iGluR2(S1S2) construct, a structural model for AMPA/kainate ionotropic glutamate receptors. In contrast to the parent kainic acid, a potent agonist, this compound, along with three structurally related analogues derived from synthetic intermediates, exhibited antagonist behavior towards KAR expressed in oocytes, a result that is rationalized by molecular modeling studies.

N1-substituted bicyclic pyrazole amino acids (S)-9a-9c and (R)-9a-9c, which are conformationally constrained analogues of glutamic acid, were prepared via a strategy based on a 1,3-dipolar cycloaddition. The new amino acids were tested for activity at ionotropic and metabotropic glutamate receptors. Some of them turned out to be selective for the NMDA receptors, where they behaved as weak antagonists. The biological activity is mainly due to the interaction with the glutamate binding site, and not with the glycine co-agonist site.

Philanthotoxin-343 (PhTX-343), a synthetic analogue of wasp toxin PhTX-433, is a noncompetitive antagonist at ionotropic receptors (e.g., AChR or iGluR). To determine possible effects of variations of the amino acid side chain, a library consisting of seventeen PhTX-343 analogues was prepared. Thus, tyrosine was replaced by either apolar, conformationally constrained, or bulky amino acids, whereas the acyl unit and the polyamine moiety were kept unchanged. Analogues with tertiary amide groups were prepared for the first time. Pentafluorophenyl esters were employed for amide bond formation, establishing general protocols for philanthotoxin solution- and solid-phase synthesis (39-90% and 42-54% overall yields, respectively). The analogues were tested for their ability to antagonize kainate-induced currents of 2-amino-3-(3-hydroxy-5-methyl-4-isoxazoyl)propanoic acid receptors (AMPAR) expressed in Xenopus oocytes from rat brain mRNA. This showed that steric bulk in the amino acid moiety is well tolerated and suggests that binding to AMPAR does not involve the alpha-NHCO group as a donor in hydrogen bonding.

The synthesis and pharmacology of two potential glutamic acid receptor ligands are described. Preparation of the bicyclic 3-hydroxy-delta2-isoxazoline-cyclopentane derivatives (+/-)-7 and (+/-)-8 was accomplished via 1,3-dipolar cycloaddition of bromonitrile oxide to suitably protected 1-amino-cyclopent-3-enecarboxylic acids. Their structure was established using a combination of 1H NMR spectroscopy and molecular mechanics calculations carried out on the intermediate cycloadducts (+/-)-11 and (+/-)-12. Amino acid derivatives (+/-)-7 and (+/-)-8 were assayed at ionotropic and metabotropic glutamic acid receptor subtypes and their activity compared with that of trans-ACPD and cis-ACPD. The results show that the replacement of the omega-carboxylic group of the model compounds with the 3-hydroxy-delta2-isoxazoline moiety abolishes or reduces drastically the activity at the metabotropic glutamate receptors. Conversely, on passing from cis-ACPD to derivative (+/-)-8, the agonist activity at NMDA receptors is almost unaffected.

A series of 4,5-dihydro-4-oxo-1,2,4-triazolo[1,5-a]quinoxaline-2-carboxylates analogues of TQX-173 (1b), bearing different nitrogen-containing heterocycles at position-8, were synthesized as AMPA receptor antagonists. All the reported compounds were also biologically evaluated for their binding at glycine/NMDA and KA receptors to better assess their selectivity toward the AMPA receptor. Structure-activity relationships (SAR) on these TQX derivatives have evidenced that the precise positioning of the nitrogen atoms and the specific electronic topography of the 8-heteroaromatic ring are both important for the anchoring to the AMPA receptor. In fact, it has been well-established that the presence of a N(3)-nitrogen-containing heterocycle at position-8 of the TQX framework is an essential feature for potent and selective AMPA receptor antagonists. Functional antagonism at both AMPA receptor and NMDA receptor-ion channel complex was evaluated by assessing the ability of some selected compounds to inhibit depolarization induced by 5 microM AMPA or NMDA in mouse cortical wedge preparations.

We have previously described the potent and selective (RS)-2-amino-3-(3-hydroxy-5-methyl-4-isoxazolyl)propionic acid (AMPA) receptor agonist, (RS)-2-amino-3-(3-carboxy-5-methyl-4-isoxazolyl)propionic acid (ACPA), and the AMPA receptor antagonist (RS)-2-amino-3-[3-(carboxymethoxy)-5-methyl-4-isoxazolyl]propionic acid (AMOA). Using these AMPA receptor ligands as leads, a series of compounds have been developed as tools for further elucidation of the structural requirements for activation and blockade of AMPA receptors. The synthesized compounds have been tested for activity at ionotropic excitatory amino acid (EAA) receptors using receptor binding and electrophysiological techniques, and for activity at metabotropic EAA receptors using second messenger assays. Compounds 1 and 4 were essentially inactive. (RS)-2-Amino-3-[3-(2-carboxyethyl)-5-methyl-4-isoxazolyl]propionic acid (ACMP, 2), on the other hand, was shown to be a selective AMPA receptor antagonist (IC(50) = 73 microM), more potent in electrophysiological experiments than AMOA (IC(50) = 320 microM). The isomeric analogue of 2, compound 5, did not show AMPA antagonist effects, but was a weak NMDA receptor antagonist (IC(50) = 540 microM). Finally, compound 3, which is an isomer of ACPA, turned out to be a very weak NMDA antagonist, and an AMPA receptor agonist approximately 1000 times weaker than ACPA. None of the compounds showed agonist or antagonist effects at metabotropic EAA receptors.

2-(9-Xanthylmethyl)-2-(2'-carboxycyclopropyl) glycine 6e is a novel metabotropic glutamate receptor antagonist. A series of alpha, C-3' disubstituted (carboxycyclopropyl)glycines 6f-n were prepared. Antagonist activity was observed for all these compounds at group 2 and group 3 mGluRs. Although they were slightly less active on group 2 mGluRs than non C-3' substituted 6e, the compounds 6f-n were more selective with lesser or no activity on group 1 mGluR subtypes (IC50 values greater than 100 microns).

A series of five 6,7-disubstituted 1,4-dihydro-2,3-quinoxalinediones was prepared, two of which are known microbial flavin metabolites and three of which are potential flavin metabolites. Four of the five compounds inhibited specific binding of [3H]-amino-3-hydroxy-5-methyl-4-isoxazolepropanoic acid ([3H]AMPA), [3H]kainic acid, and [3H]6-cyano-1,4-dihydro-7-nitro-2,3-quinoxalinedione ([3H]CNQX) in rat brain homogenate fractions, with IC50 values in the low micromolar range (the fifth compound competed only with [3H]CNQX). Two of the compounds were moderately potent AMPA antagonists in an in vitro functional test.

This study investigated the mode of action of conantokin-T, a 21 amino acid peptide toxin isolated from the venom of the fish-hunting cone snail Conus tulipa, on excitatory synaptic transmission in rat hippocampal slices using intracellular recording techniques. Superfusion of conantokin-T (1-500 nM) specifically and irreversibly decreased the pharmacologically isolated N-methyl-D-aspartate receptor (NMDA)-mediated excitatory postsynaptic potential (EPSPNMDA) in a concentration-dependent manner but had no effect on normal excitatory synaptic transmission (EPSP). The sensitivity of postsynaptic neurons to NMDA but not to alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid was also antagonized by conantokin-T pretreatment. In addition, the conantokin-T-induced depression of EPSPNMDA could be antagonized by prior treatment of hippocampal slices with either DL-2-amino-5-phosphonovaleate (10 microM) or ifenprodil (20 microM). However, 7-chlorokynurenic acid (1 microM) had no effect on the action of conantokin-T. These findings indicated that conantokin-T modulates the NMDA receptor by an interaction with its glutamate binding site and polyamine recognition site.

A series of mono-, di-, tri-, and tetrasubstituted 1,4-dihydroquinoxaline-2,3-diones (QXs) were synthesized and evaluated as antagonists at N-methyl-D-aspartate (NMDA)/glycine sites and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid-preferring non-NMDA receptors. Antagonist potencies were measured by electrical assays in Xenopus oocytes expressing rat whole brain poly(A)+ RNA. Trisubstituted QXs 17a (ACEA 1021), 17b (ACEA 1031), 24a, and 27, containing a nitro group in the 5 position and halogen in the 6 and 7 positions, displayed high potency (Kb approximately 6-8 nM) at the glycine site, moderate potency at non-NMDA receptors (Kb = 0.9-1.5 microM), and the highest (120-250-fold) selectivity in favor of glycine site antagonism over non-NMDA receptors. Tetrasubstituted QXs 17d,e were more than 100-fold weaker glycine site antagonists than the corresponding trisubstituted QXs with F being better tolerated than Cl as a substituent at the 8 position. Di- and monosubstituted QXs showed progressively weaker antagonism compared to trisubstituted analogues. For example, removal of the 5-nitro group of 17a results in a approximately 100-fold decrease in potency (10a,b,z), while removal of both halogens from 17a results in a approximately 3000-fold decrease in potency (10v). In terms of steady-state inhibition, most QX substitution patterns favor antagonism at NMDA/glycine sites over antagonism at non-NMDA receptors. Among the QXs tested, only 17i was slightly selective for non-NMDA receptors.