Anticonvulsant activity of antagonists for the NMDA-associated glycine binding site

KYNA Derivatives with Modified Skeleton; Hydroxyquinolines with Potential Neuroprotective Effect

Kynurenic acid (KYNA) is an endogenous neuroprotective agent of increasing importance. Several derivatives have already been synthesized, bearing an abundance of functional groups attached to the main skeleton in different positions. Several of these compounds have already been tested in biological evaluations, with several of them targeting the same receptors and biological effects as KYNA. However, these modified compounds build upon the unmodified KYNA skeleton leaving a possible route for the synthesis of new, potentially neuroprotective derivatives with heteroatom-containing ring systems. The aim of this review is to summarize the syntheses of KYNA derivatives with altered skeletons and to pinpoint an appealing transformation for future medicinal lead molecules.

Amino Acid Trp: The Far Out Impacts of Host and Commensal Tryptophan Metabolism

Tryptophan (Trp) is an essential amino acid primarily derived from the diet for use by the host for protein synthesis. The intestinal tract is lined with cells, both host and microbial, that uptake and metabolize Trp to also generate important signaling molecules. Serotonin (5-HT), kynurenine and its downstream metabolites, and to a lesser extent other neurotransmitters are generated by the host to signal onto host receptors and elicit physiological effects. 5-HT production by neurons in the CNS regulates sleep, mood, and appetite; 5-HT production in the intestinal tract by enterochromaffin cells regulates gastric motility and inflammation in the periphery. Kynurenine can signal onto the aryl hydrocarbon receptor (AHR) to elicit pleiotropic responses from several cell types including epithelial and immune cells, or can be further metabolized into bioactive molecules to influence neurodegenerative disease. There is a remarkable amount of cross-talk with the microbiome with regard to tryptophan metabolites as well. The gut microbiome can regulate the production of host tryptophan metabolites and can use dietary or recycled trp to generate bioactive metabolites themselves. Trp derivatives like indole are able to signal onto xenobiotic receptors, including AHR, to elicit tolerogenic effects. Here, we review studies that demonstrate that tryptophan represents a key intra-kingdom signaling molecule.

Kynurenines in the CNS: from endogenous obscurity to therapeutic importance

In just under 20 years the kynurenine family of compounds has developed from a group of obscure metabolites of the essential amino acid tryptophan into a source of intensive research, with postulated roles for quinolinic acid in neurodegenerative disorders, most especially the AIDS-dementia complex and Huntington's disease. One of the kynurenines, kynurenic acid, has become a standard tool for use in the identification of glutamate-releasing synapses, and has been used as the parent for several groups of compounds now being developed as drugs for the treatment of epilepsy and stroke. The kynurenines represent a major success in translating a basic discovery into a source of clinical understanding and therapeutic application, with around 3000 papers published on quinolinic acid or kynurenic acid since the discovery of their effects in 1981 and 1982. This review concentrates on some of the recent work most directly relevant to the understanding and applications of kynurenines in medicine.

Kynurenic acid antagonists and kynurenine pathway inhibitors

The kynurenine pathway accounts for the metabolism of around 80% of non-protein tryptophan metabolism. It includes both an agonist (quinolinic acid) at NMDA receptors and an antagonist (kynurenic acid). Since their discovery, there has been a major development of kynurenic acid analogues as neuroprotectants for the treatment of stroke and neurodegenerative disease. Several prodrugs of kynurenic acid or its analogues that can be hydrolysed within the CNS are also available. More recently, the pathway itself has proved to be a valuable drug target, affected by agents which reduce the synthesis of quinolinic acid and increase the formation of kynurenic acid. The change in the balance of these, away from the excitotoxin and towards the neuroprotectant, has anticonvulsant and neuroprotective properties.

Inhibitors of the kynurenine pathway

Strokes (intracranial thomboses or haemorrhaging) cause death and disability, but effective treatments are lacking. The metabolism of tryptophan leads to the generation of quinolinic acid, an agonist potentially neurotoxic at glutamate receptors, and kynurenic acid, an antagonist at the same population of receptors. The commercial development of the kynurenine pathway has included the use of analogues of kynurenic acid as antagonists at glutamate receptors. A second has been to use prodrugs of kynurenic acid or its analogues. Alternatively, it is proving possible to interfere directly with the kynurenine pathway to block the synthesis of quinolinic acid and promote the formation of kynurenic acid. This change yields neuroprotectant and anticonvulsant compounds.

Anticonvulsant activity of 4-urea-5,7-dichlorokynurenic acid derivatives that are antagonists at the NMDA-associated glycine binding site

Twelve 4-urea-5,7-dichlorokynurenic acid derivatives were synthesized by reacting the 4-tosylimino derivative of 5,7-dichlorokynurenate methyl ester first with triphosgene and then with a secondary amine. Compounds were screened in mice for anticonvulsant activity using maximal electroshock (MES), subcutaneous pentylenetetrazole (Met), and threshold tonic extension (TTE) tests. A rotorod test was used to determine neurotoxicity. Seven of the derivatives had anticonvulsant activity in TTE testing at 100 mg/kg. One compound, 2-methyl carboxylate-5,7-dichloro-4-([¿diphenylamino¿-carbonyl]amino)-quino line, had an ED50 value of 134 mg/kg (95% conf. int.: low-78.5, high-205.7; slope 1.9, SE = 0.44) in TTE testing. Two derivatives had MES activity. Only one compound, an N,N-diethylamino derivative, was neurotoxic in the rotorod test. Compounds were screened at a 10-microM concentration for activity in displacing 5,7-dichlorokynurenic acid from synaptosomal membrane fragments. Since 9 of the 12 compounds synthesized and tested have demonstrated anticonvulsant activity, this class of chemicals offers promise for the production of useful therapeutic agents.

Effects of valproate on amino acid and monoamine concentrations in striatum of audiogenic seizure-prone Balb/c mice

The effects of valproate on CNS concentrations of gamma-aminobutyric acid (GABA), glulamate (GLU), glutamine (GLN); dopamine (DA), serotonin (5-HT), and metabolites were examined in tissue extracts of caudate nucleus of genetic substrains of Balb/c mice susceptible (EP) or resistant (ER) to audiogenic seizures. Generalized tonic-clonic seizures observed in EP mice were inhibited by valproate, administered 1 h prior to testing, in a dose-response fashion. Concentrations of GABA, GLU, and GLN, which were lower in EP mice than in ER mice, were significantly increased by valproate at doses of 180 and 360 mg/kg. Concentrations of homovanillic acid (HVA) and hydroxyindoleacetic acid (5-HIAA), metabolites of DA and 5-HT, were substantially increased by valproate at these doses. The in situ activity of tyrosine hydroxylase (TH) was not significantly influenced by valproate, whereas a valproate-induced increase in tryptophan hydroxylase (TPH) activity was observed in both striatum and in midbrain tegmentum. The data are consistent with the interpretation that anti-convulsive doses of valproate influences the intraneuronal metabolism of monoamines, GABA, and glutamate concurrently. Valproate's influence on the metabolism of both major inhibitory (GABA) and excitatory (GLY amino acids in striatum could contribute to its anti-convulsive effects in genetically seizure prone mice, as well as to the accumulation of DA and 5-HT metabolites.

Felbamate modulates the strychnine-insensitive glycine receptor

Felbamate (2-phenyl-1,3-propanediol dicarbamate) is a novel anticonvulsant substance whose mechanism of action is not clearly understood. The present investigation examined its ability to modulate the strychnine-insensitive glycine receptor associated with the N-methyl-D-aspartate (NMDA) receptor. Felbamate decreased the magnitude of glycine (100 microM)-enhanced NMDA (100 microM)-induced intracellular calcium ([Ca2+]i) transients in mouse cerebellar granule cells which had been loaded with the Ca(2+)-sensitive fluorescent probe indo-1 acetoxymethyl ester (indo-1/AM). This effect of felbamate was concentration dependent, with a maximal effect observed at 300 microM (65 +/- 4% of control). In the Frings audiogenic seizure-susceptible mouse model of reflex epilepsy, the glycine agonist D-serine (150 nmol, i.c.v.) completely blocked the anticonvulsant activity of a maximally effective dose of felbamate (19 mg/kg, i.p.). This effect of D-serine could be reversed by increasing the administered dose of felbamate to 29 mg/kg. Furthermore, administration of D-serine (300 nmol, i.c.v.) to felbamate-treated Frings mice produced a parallel right shift in felbamate's anticonvulsant dose-response curve (ED50s: 9.4 mg/kg for felbamate vs. 17.7 mg/kg for felbamate + D-serine). The results obtained in this investigation suggest that the ability of felbamate to modulate the strychnine-insensitive glycine receptor may be physiologically and behaviorally relevant to its anticonvulsant mechanism of action.