Heterocyclic analogues of GABA: chemistry, molecular pharmacology and therapeutic aspects

Design, synthesis, In-vitro, In-silico and DFT studies of novel functionalized isoxazoles as antibacterial and antioxidant agents

A series of new isoxazolederivatives incorporating the sulfonate ester function has been synthesized from 2-benzylidenebenzofuran-3(2 H)-one, known as aurone. The synthesis of the target compounds was carried out following an efficient methodology that allows access to the desired products in a reproducible way and with good yield. The structures of the synthesized compounds were established using NMR (1H and 13C) spectroscopy and mass spectrometry. A theoretical study was performed to optimize the geometrical structures and to calculate the structural and electronic parameters of the synthesized compounds. The calculations were also carried out to understand the influence and the effect of substitutions on the chemical reactivity of the studied compounds. The synthesized isoxazoles were screened for their antioxidant and antibacterial activities. The findings demonstrate that the studied compounds exhibit good to moderate antibacterial activity against the tested bacteria (Staphylococcus aureus, Bacillus subtilis, and Escherichia coli). Moreover, a number of the tested isoxazole derivatives exhibit high effectiveness against DPPH free radicals. Besides that, molecular docking studies were carried out to predict binding affinity and identify the most likely binding interactions between the active molecules and the target microorganisms' proteins. A 100 ns molecular dynamics study was then conducted to examine the dynamic behavior and stability of the highly potent isoxazole 4e in complex with the target bacterial proteins. Finally, the ADMET analyses suggest that all the synthesized isoxazoles have good pharmacokinetic profiles and non-toxicity and non-carcinogenicity in biological systems.

Using heteroaryl-lithium reagents as hydroxycarbonyl anion equivalents in conjugate addition reactions with (S,S)-(+)-pseudoephedrine as chiral auxiliary; enantioselective synthesis of 3-substituted pyrrolidines

We have developed an efficient protocol for carrying out the stereocontrolled formal conjugate addition of hydroxycarbonyl anion equivalents to α,β-unsaturated carboxylic acid derivatives using (S,S)-(+)-pseudoephedrine as chiral auxiliary, making use of the synthetic equivalence between the heteroaryl moieties and the carboxylate group. This protocol has been applied as key step in the enantioselective synthesis of 3-substituted pyrrolidines in which, after removing the chiral auxiliary, the heteroaryl moiety is converted into a carboxylate group followed by reduction and double nucleophilic displacement. Alternatively, the access to the same type of heterocyclic scaffold but with opposite absolute configuration has also been accomplished by making use of the regio- and diastereoselective conjugate addition of organolithium reagents to α,β,γ,δ-unsaturated amides derived from the same chiral auxiliary followed by chiral auxiliary removal, ozonolysis, and reductive amination/intramolecular nucleophilic displacement sequence.

Plasma and CNS concentrations of Gaboxadol in rats following subcutaneous administration

Gaboxadol has been suggested to be a selective extrasynaptic GABA(A) receptor agonist. However, there is little information on Gaboxadol concentrations in the central nervous system (CNS) at therapeutically relevant doses. In order to investigate this, rats were injected subcutaneously with Gaboxadol and plasma and CNS concentrations were determined using the dynamic-no-net-flux and ultraslow microdialysis methods. Results using the 2 methods were similar and showed that Gaboxadol rapidly entered the brain and that peak CNS concentrations after 2.5, 5 and 10 mg/kg were in the range of 0.7 to 3 microM. Furthermore, a very short half-life (28 min) in both plasma and CNS was observed. It is concluded that concentrations of Gaboxadol in the CNS are in a range, which are likely to activate only extrasynaptic (nongamma subunit containing) GABA(A) receptors.

The GABAA receptor agonist THIP is neuroprotective in organotypic hippocampal slice cultures

The potential neuroprotective effects of the GABA(A) receptor agonists THIP (4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol) and muscimol, and the selective GluR5 kainate receptor agonist ATPA ((RS)-2-amino-3-(3-hydroxy-5-tert-butylisoxazol-4-yl)propanoic acid), which activates GABAergic interneurons, were examined in hippocampal slice cultures exposed to N-methyl-D-aspartate (NMDA). The NMDA-induced excitotoxicity was quantified by densitometric measurements of propidium iodide (PI) uptake. THIP (100-1000 microM) was neuroprotective in slice cultures co-exposed to NMDA (10 microM) for 48 h, while muscimol (100-1000 microM) and ATPA (1-3 microM) were without effect. The results demonstrate that direct GABA(A) agonism can mediate neuroprotection in the hippocampus in vitro as previously suggested in vivo.

Discovery of non-zwitterionic GABA(A) receptor full agonists and a superagonist

Numerous previous studies of GABA(A) receptor ligands have suggested that GABA(A) receptor agonists must be zwitterionic and feature an intercharge separation similar to that of GABA (approx. 4.7-6A). In this communication we demonstrate that appropriately functionalized GABA amides are partial, full, or superagonists, despite their non-zwitterionic structure.

Superimposition-based protocol as a tool for determining bioactive conformations. II. Application to the GABA(A) receptor

The natural templates (NT) superimposition method is used to determine the pharmacophoric requirements of the A subtype of the gamma-aminobutyric acid (GABA) receptor. Bioactive conformations for antagonists and agonists are found by superimposing them on a relatively rigid alkaloid bicuculline, which itself is a competitive antagonist at this ligand-gated ion channel receptor. As has been usual in the application of this modeling method, consideration of available experimental data is the cornerstone for obtaining realistic models. The identification of two substructural fragments of bicuculline permitted classification of the ligands. Analysis of the antagonists and agonists with respect to the two substructural fragments revealed two bioactive conformations of the highly flexible GABA molecule, one of which is extended with the nonhydrogenic atoms roughly coplanar torsional angles of -37 and -179 degrees at N-C-C-C and C-C-C-C (carboxyl), respectively. The second bioactive compound is clearly non planar (torsional angles of -81 and -109 degrees at N-C-C-C and C-C-C-C (carboxyl), respectively).

GABAA agonists: resolution and pharmacology of (+)- and (-)-isoguvacine oxide

(3SR,4RS)-3,4-Epoxypiperidine-4-carboxylic acid (isoguvacine oxide) is a potent and specific GABAA receptor agonist. Isoguvacine oxide, originally designed as a potentially alkylating agonist, turned out to interact with the GABAA receptor in a fully reversible manner. The protected form of isoguvacine oxide, benzyl (3SR,4RS)-1-(benzyl-oxycarbonyl)-3,4-epoxypiperidin e-4-carboxylate (1) (Scheme 1), has now been resolved by chiral chromatography using cellulose triacetate as a chiral stationary phase. The enantiomers of 1 (ee > or = 98.8%) were subsequently deprotected by hydrogenolysis. Whereas both enantiomers of isoguvacine oxide were inactive as inhibitors of the binding of [3H]GABA to GABAB receptor sites (IC50 > 100 microM), (+)-isoguvacine oxide (IC50 = 0.20 +/- 0.03 microM) and (-)-isoguvacine oxide (IC50 = 0.32 +/- 0.05 microM) showed comparable potencies as inhibitors of the binding of [3H]GABA to GABAA receptor sites. Furthermore, (+)-isoguvacine oxide (EC50 = 6 microM; 33% relative efficacy) and (-)-isoguvacine oxide (EC50 = 5 microM; 38% efficacy relative to 10 microM muscimol) were approximately equipotent and equiefficacious as stimulators of the binding of [3H]diazepam to the GABAA receptor-associated benzodiazepine site. This latter effect is an in vitro estimate of GABAA agonist efficacy. These pharmacological data for isoguvacine oxide and its enantiomers do not seem to support our earlier conception of the topography of the GABAA recognition site(s), derived from extensive structure-activity studies on GABAA agonists.(ABSTRACT TRUNCATED AT 250 WORDS)

GABAA agonists as targets for drug development

1. Agents with selective actions on bicuculline-sensitive GABAA receptors have been developed by systematically restricting the conformational mobility of the GABA molecule. 2. THIP, a bicyclic isoxazole that represents GABA held in a relatively rigid and partially extended conformation, is an analgesic of potency comparable to that of morphine. THIP represents a lead compound for a novel series of analgesics acting independently of Naloxone-sensitive opiate systems. 3. ZAPA, a conformationally-restricted analogue of GABA containing an isothiouronium moiety, is a selective agonist for low affinity GABAA receptors and is a lead compound for the development of a novel series of anthelmintics. 4. (+)-TACP, a cyclopentane analogue of GABA, may activate a different subclass of GABAA receptors from THIP. 5. Pharmacological, molecular modelling and molecular biological studies provide evidence for a heterogeneity of GABAA receptors which might be exploited for drug development.

High affinity of the naturally-occurring biflavonoid, amentoflavon, to brain benzodiazepine receptors in vitro

In a search for pharmacologically-active ingredients in the plant extract Karmelitter Geist, we have isolated and identified a high-affinity benzodiazepine receptor ligand, amentoflavon. Amentoflavon binds in a mix-type competitive and non-competitive manner to brain benzodiazepine receptors with an IC50-value of 6 nM in vitro, comparable to the affinity of diazepam. Amentoflavon shows a GABA ratio of 1.4 at 0 degrees and increases 35S-TBPS binding by 12% (60 min incubation at 25 degrees), which indicates a partial agonistic effect on benzodiazepine receptors. The substance does not influence the binding of a variety of other brain receptor ligands. Amentoflavon does not inhibit 3H-flunitrazepam-binding to brain benzodiazepine receptors following i.v. administration in the mouse in vivo, and it is therefore not likely that amentoflavon is responsible for any pharmacological effect of Karmelitter Geist.

Comparative stereostructure-activity studies on GABAA and GABAB receptor sites and GABA uptake using rat brain membrane preparations

The affinities of a number of analogues of gamma-aminobutyric acid (GABA) for GABAA and GABAB receptor sites and GABA uptake were studied using rat brain membrane preparations. Studies on the (S)-(+)- and (R)-(-)-isomers of baclofen, 3-hydroxy-4-aminobutyric acid (3-OH-GABA), and 4,5-dihydromuscimol (DHM) revealed different stereoselectivities of these synaptic mechanisms in vitro. Although (S)-3-OH-GABA and, in particular, (S)-DHM were more potent than the corresponding (R)-isomers as inhibitors of GABAA binding, the opposite stereoselectivity was demonstrated for the GABAB binding sites. Thus, (R)-3-OH-GABA and (R)-baclofen were more potent than the (S)-isomers as inhibitors of GABAB binding, (R)-baclofen being some five times more potent than (R)-3-OH-GABA. These two (R)-isomers actually have opposite orientation of the substituents on the GABA backbones, suggesting that the lipophilic substituent of (R)-baclofen interacts with a structural element of the GABAB receptor site different from that that binds the very polar hydroxy group of (R)-3-OH-GABA. The O-methylated analogue of 3-OH-GABA, 3-methoxy-4-aminobutyric acid (3-OCH3-GABA), did not interact significantly with GABAB sites. The homologues of GABA, trans-4-aminocrotonic acid (trans-ACA), muscimol, and 3-OH-GABA, that is, 5-aminovaleric acid (DAVA), trans-5-aminopent-2-enoic acid, homomuscimol, and 3-hydroxy-5-aminovaleric acid (3-OH-DAVA), respectively, were generally much weaker than the parent compounds, whereas 2-hydroxy-5-aminovaleric acid (2-OH-DAVA) showed a significantly higher affinity for GABAB sites than the corresponding GABA analogue.(ABSTRACT TRUNCATED AT 250 WORDS)