Recent Advances in GABA Agonists, Antagonists and Uptake Inhibitors: Structure–Activity Relationships and Therapeutic Potential

Bioisoteres for carboxylic acids: From ionized isosteres to novel unionized replacements

Carboxylic acids are key pharmacophoric elements in many molecules. They can be seen as a problem by some, due to perceived permeability challenges, potential for high plasma protein binding and the risk of forming reactive metabolites due to acyl-glucuronidation. By others they are viewed more favorably as they can decrease lipophilicity by adding an ionizable center which can be beneficial for solubility, and can add enthalpic interactions with the target protein. However, there are many instances where the replacement of a carboxylic acid with a bioisosteric group is required. This has led to the development of a number of ionizable groups which sufficiently mimic the carboxylic acid functionality whilst improving, for example, the metabolic profile of the molecule in question. An alternative strategy involves replacement of the carboxylate by neutral functional groups. This review initially details carefully selected examples whereby tetrazoles, acyl sulfonamides or isoxazolols have been beneficially utilized as carboxylic acid bioisosteres altering physicohemical properties, interactions with the target and metabolism and/or pharmacokinetics, before delving further into the binding mode of carboxylic acid derivatives with their target proteins. This analysis highlights new ways to consider the replacement of carboxylic acids by neutral bioisosteric groups which either rely on hydrogen bonds or cation-π interactions. It should serve as a useful guide for scientists working in drug discovery.

Novel-Substituted Heterocyclic GABA Analogues. Enzymatic Activity against the GABA-AT Enzyme from Pseudomonas fluorescens and In Silico Molecular Modeling

γ-Aminobutyric acid (GABA) is the most important inhibitory neurotransmitter in the central nervous system, and a deficiency of GABA is associated with serious neurological disorders. Due to its low lipophilicity, there has been an intensive search for new molecules with increased lipophilicity to cross the blood-brain barrier to raise GABA concentrations. We have designed and evaluated in vitro and in silico some new analogues of GABA, where the nitrogen atom at the γ-position is embedded in heterocyclic scaffolds and determined their inhibitory potential over the GABA-AT enzyme from Pseudomonas fluorescens. These modifications lead to compounds with inhibitory activity as it occurs with compounds 18a and 19a. The construction of Pseudomonas fluorescens and human GABA-AT models were carried out by homology modeling. Docking assays were done for these compounds over the GABA-AT enzyme models where 19a showed a strong interaction with both GABA-AT enzymes.

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.

GABA(A) agonists and partial agonists: THIP (Gaboxadol) as a non-opioid analgesic and a novel type of hypnotic

The GABA(A) receptor system is implicated in a number of central nervous system (CNS) disorders, making GABA(A) receptor ligands interesting as potential therapeutic agents. Only a few different classes of structures are currently known as ligands for the GABA recognition site on the hetero-pentameric GABA(A) receptor complex, reflecting the very strict structural requirements for GABA(A) receptor recognition and activation. A large number of the compounds showing agonist activity at the GABA(A) receptor site are structurally derived from the GABA(A) agonists muscimol, THIP (Gaboxadol), or isoguvacine, which we developed at the initial stage of the project. Using recombinant GABA(A) receptors, functional selectivity has been shown for a number of compounds, including THIP, showing subunit-dependent potency and maximal response. The pharmacological and clinical activities of THIP probably reflect its potent effects at extrasynaptic GABA(A) receptors insensitive to benzodiazepines and containing alpha(4)beta(3)delta subunits. The results of ongoing clinical studies on the effect of the partial GABA(A) agonist THIP on human sleep pattern show that the functional consequences of a directly acting agonist are distinctly different from those seen after administration of GABA(A) receptor modulators, such as benzodiazepines. In the light of the interest in partial GABA(A) receptor agonists as potential therapeutics, structure-activity studies of a number of analogues of 4-PIOL, a low-efficacy partial GABA(A) agonist derived from THIP, have been performed. In this connection, a series of GABA(A) ligands has been developed showing pharmacological profiles ranging from low-efficacy partial GABA(A) agonist activity to selective antagonist effect.

Specific GABA(A) agonists and partial agonists

The GABA(A) receptor system is implicated in a number of neurological and psychiatric diseases, making GABA(A) receptor ligands interesting as potential therapeutic agents. Only a few different classes of structures are currently known as ligands for the GABA recognition site on the hetero-pentameric GABA(A) receptor complex, reflecting the very strict structural requirements for GABA(A) receptor recognition and activation. Within the series of compounds showing agonist activity at the GABA(A) receptor site that have been developed, most of the ligands are structurally derived from the GABA(A) agonists muscimol, THIP, or isoguvacine, which we developed in the initial stages of the project. Using recombinant GABA(A) receptors, functional selectivity was demonstrated for a number of compounds, including THIP, showing highly subunit-dependent potency and maximal response. In light of the interest in partial GABA(A) receptor agonists as potential therapeutics, structure-activity studies of a number of analogs of 4-PIOL, a low-efficacy partial GABA(A) agonist derived from THIP, have been performed. In this connection, a series of GABA(A) ligands has been developed that exhibit pharmacological profiles from moderately potent low-efficacy partial GABA(A) agonist activity to potent and selective antagonist effects. Very little information is available on direct-acting GABA(A) receptor agonists in clinical studies. However, the results of clinical studies on the effect of the partial GABA(A) agonist THIP on human sleep patterns show that the functional consequences of a direct-acting agonist are different from those seen after the administration of GABA(A) receptor modulators, such as benzodiazepines and barbiturates.

Highly water-soluble derivatives of the anesthetic agent propofol: in vitro and in vivo evaluation of cyclic amino acid esters

Cyclic amino acid esters of propofol were synthesized in an attempt to develop new water-soluble anesthetic agents. Their solubility and stability in aqueous solution, and their ability to release propofol in vitro under physiological conditions were determined. L-Proline (6a) and racemic nipecotic acid (6c) esters were found to be highly soluble in water. Sufficiently stable at physiological pH (half-lives >6 h), the alpha-amino acid esters, 6a and 6b, were found to be quantitatively hydrolyzed in plasma and liver esterase solutions within a few minutes, showing prodrug behavior. The in vitro activity of the esters, determined either by the [(35)S]tert-butylbicyclophosphorothionate ([(35)S]TBPS) binding assay or electrophysiological measurements of the action at cloned human receptors, proved to be a mechanism involving allosteric modulation of GABA(A) receptors. Indeed, L-proline (6a), and racemic pipecolinate (6b) and nipecotate (6c), like propofol, reduced [(35)S]TBPS binding, whereas isonipecotate (6d) showed bicuculline-like behavior, increasing [(35)S]TBPS binding. A nonlinear relation between GABA(A) receptor binding affinity and lipophilicity, as assessed by reversed-phase high-performance liquid chromatography, emerged as a trend. The in vivo anticonvulsant and anesthetic activities of prolinate 6a, intraperitoneally administered in water solution, showed that is a water-soluble propofol prodrug candidate for developing formulations useful for parenteral administration.

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.

Synthesis of novel GABA uptake inhibitors. Part 6: preparation and evaluation of N-Omega asymmetrically substituted nipecotic acid derivatives

In a previous series of potent GABA uptake inhibitors published from this laboratory, we noticed that asymmetry in the substitution pattern of the bis-aromatic moiety in known GABA uptake inhibitors such as 4 [1-(4,4-diphenyl-3-butenyl)-3-piperidinecarboxylic acid] and 5 [(R)-1-(4,4-bis(3-methyl-2-thienyl)-3-butenyl)-3-piperidinecarboxylic acid] was beneficial for high affinity. This led us to investigate asymmetric analogues of known symmetric GABA uptake inhibitors in which one of the aryl groups has been exchanged with an alkyl, alkylene or cycloalkylene moiety as well as other modifications in the lipophilic part. The in vitro values for inhibition of [(3)H]-GABA uptake in rat synaptosomes was determined for each compound, and it was found that several of the novel compounds inhibit GABA uptake as potently as their known symmetrical reference analogues. Several of the novel compounds were also evaluated for their ability to inhibit clonic seizures induced by a 15 mg/kg (ip) dose of methyl 6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM) in vivo. Some of the compounds, for example 18 [(R)-1-(2-(((1,2-bis(2-fluorophenyl)ethylidene)amino)oxy)ethyl)-3-piperidinecarboxylic acid], show a high in vivo potency and protective index comparable with that of our recently launched anticonvulsant product, 5 [(R)-1-(4,4-bis(3-methyl-2-thienyl)-3-butenyl)-3-piperidinecarboxylic acid], and may therefore serve as second-generation drug candidates.

Bioisosteric determinants for subtype selectivity of ligands for heteromeric GABA(A) receptors

The potency and efficacy of a series of bioisosterically modified GABA analogues were determined electrophysiologically using heteromeric GABA(A) receptors expressed in Xenopus oocytes. These agonist parameters were shown to be strongly dependent on the receptor subunit combination. On the other hand, the antagonist potencies of the classical GABA(A) antagonists SR 95531 (7) and BMC (8) and also of 5g and the phosphinic acid bioisosteres of 5a, compounds 5f and 6, were essentially independent of the receptor subunit combinations.

Pharmacological inactivation in the analysis of the central control of movement

In this review, we describe how pharmacological inactivation can be used to elucidate the central control of skilled limb movement. Local anesthetics and tetrodotoxin block neuronal cell bodies and passing fibers while gamma-aminobutyric acid (GABA) and muscimol only block cell bodies. Blockade induction time is short (several minutes) for all the agents. Blockade duration produced by local anesthetics and GABA is 15-60 min, while that of tetrodotoxin and muscimol is up to several days. We describe our drug injection system, with an integrated microelectrode and a viewing port for visually monitoring drug flow into the injection cannula. We used glucose metabolism to assess the extent of inactivation. Intracortical lidocaine or muscimol injection produces a central core of maximal hypometabolism (1 mm radius), which could be due to drug spread, surrounded by an extensive region (several millimeters) of reduced hypometabolism, possibly due to reduced synaptic activity of neurons receiving projections from the core region. Drug injection only depresses neuronal activity, which contrasts with cooling, where there can be neuronal hyperexcitability at the periphery of the inactivation site. Our experiments in behaving animals show how pharmacological inactivation is an effective analytical tool for dissecting the differential functional contributions of subcortical and cortical forelimb representations to limb movement control.

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)

Effects of GABAA receptors activation on brain glucose metabolism in normal subjects and temporal lobe epilepsy (TLE) patients. A positron emission tomography (PET) study. Part I: Brain glucose metabolism is increased after GABAA receptors activation

Though gamma-aminobutyric acid (GABA) is the major inhibitory neurotransmitter in the human central nervous system, the metabolic response to GABA system activation remains imperfectly known. We studied in vivo with positron emission tomography (PET) the variations of glucose metabolism in the human brain after stimulation of the GABAA receptors by systemic administration of the specific GABAA agonist, 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol (THIP). These investigations were performed in three normal volunteers and as part of presurgical evaluation for temporal lobe epilepsy in six patients. While clinical and electroencephalographic (EEG) monitoring showed a sedative effect and sleepiness after THIP administration, glucose metabolism was paradoxically increased in grey matter structures, which are known to have a high density of GABAA receptors. These findings suggest that the pharmacological activation of GABA pathways, although inhibitory and producing a decrease of vigilance, increases the energetic demand at least during a phase of GABA agonist action, probably at the synaptic or at the glial cell level.

The pharmacology and function of central GABAB receptors

In conclusion, GABAB receptors enable GABA to modulate neuronal function in a manner not possible through GABAA receptors alone. These receptors are present at both pre- and postsynaptic sites and can exert both inhibitory and disinhibitory effects. In particular, GABAB receptors are important in regulating NMDA receptor-mediated responses, including the induction of LTP. They also can regulate the filtering properties of neural networks, allowing peak transmission in the frequency range of theta rhythm. Finally, GABAB receptors are G protein-coupled to a variety of intracellular effector systems, and thereby have the potential to produce long-term changes in the state of neuronal activity, through actions such as protein phosphorylation. Although the majority of the effects of GABAB receptors have been reported in vitro, recent studies have also demonstrated that GABAB receptors exert electrophysiological actions in vivo. For example, GABAB receptor antagonists reduce the late IPSP in vivo and consequently can decrease inhibition of spontaneous neuronal firing following a stimulus (Lingenhöhl and Olpe, 1993). In addition, blockade of GABAB receptors can increase spontaneous activity of central neurons, suggesting the presence of GABAB receptor-mediated tonic inhibition (Andre et al., 1992; Lingenhöhl and Olpe, 1993). Despite these electrophysiological effects, antagonism of GABAB receptors has generally been reported to produce few behavioral actions. This lack of overt behavioral effects most likely reflects the modulatory nature of the receptor action. Nevertheless, two separate behavioral studies have recently reported an enhancement of cognitive performance in several different animal species following blockade of GABAB receptors (Mondadori et al., 1992; Carletti et al., 1993). Because of their small number of side effects, GABAB receptor antagonists may represent effective therapeutic tools for modulation of cognition. Alternatively, the lack of overt behavioral effects of GABAB receptors may indicate that these receptors are more important in pathologic rather than normal physiological states (Wojcik et al., 1989). For example, a change in receptor affinity or receptor number brought on by the pathology could enhance the effectiveness of GABAB receptors. Of significance, CGP 35348 has been shown to block absence seizures in genetically seizure prone animals, while inducing no seizures in control animals (Hosford et al., 1992; Liu et al., 1992). Thus, GABAB receptors may represent effective sites for pharmacological regulation of absence seizures. Perhaps further behavioral effects of these receptors will become apparent only after additional studies have been performed using the highly potent antagonists that have been recently introduced.(ABSTRACT TRUNCATED AT 400 WORDS)

Hydroxylated analogues of 5-aminovaleric acid as 4-aminobutyric acidB receptor antagonists: stereostructure-activity relationships

The (R) and (S) forms of 5-amino-2-hydroxyvaleric acid (2-OH-DAVA) and 5-amino-4-hydroxyvaleric acid (4-OH-DAVA) were designed as structural hybrids of the 4-aminobutyric acidB (GABAB) agonist (R)-(-)-4-amino-3-hydroxybutyric acid [(R)-(-)-3-OH-GABA] and the GABAB antagonist 5-aminovaleric acid (DAVA). (S)-(-)-2-OH-DAVA and (R)-(-)-4-OH-DAVA showed a moderately potent affinity for GABAB receptor sites in rat brain and showed GABAB antagonist effects in a guinea pig ileum preparation. The respective enantiomers, (R)-(+)-2-OH-DAVA and (S)-(+)-4-OH-DAVA, were markedly weaker in both test systems. All four compounds were weak inhibitors of GABAA receptor binding in rat brain, and none of them significantly affected synaptosomal GABA uptake. Based on molecular modeling studies it has been demonstrated that low-energy conformations of (R)-(-)-3-OH-GABA, (S)-(-)-2-OH-DAVA, and (R)-(-)-4-OH-DAVA can be superimposed. These conformations may reflect the shapes adopted by these conformationally flexible compounds during their interaction with GABAB receptors. The present studies emphasize the similar, but distinct, constraints imposed on agonists and antagonists for GABAB receptors.

Ligand structural specificity of GABAA receptors in guinea pig ileum

The ligand structural specificity of ileal GABAA receptors was examined using the strength and half-life of contractions in guinea-pig myenteric plexus-longitudinal muscle preparations. The agonists used differ by more than a factor of 1000 in affinity to central GABAA receptors and include both conformationally flexible and restricted molecules as well as pairs of enantiomers. The overall correlation between ileal contractile activity and rat brain receptor affinity was poor (r = 0.75), but within groups of conformationally flexible or conformationally restricted molecules a high correlation was found (r greater than 0.9999). When comparing data for ileal contractile activity with available data for agonist activity in the CNS no difference between ligand specificity of ileal and central GABAA receptors was apparent with the present range of ligands. The half-lives of ileal contractile responses were found to decrease with increasing GABAB agonist activity.

Bicuculline blocks nicotinic acetylcholine response in isolated intermediate lobe cells of the pig

1. The effect of bicuculline on nicotinic acetylcholine (ACh) responses in isolated intermediate lobe (IL) cells of the pig was investigated by use of patch-clamp techniques. Bicuculline was found to reduce ACh-evoked whole-cell currents (IACh) in all cells tested (n = 40). 2. The blocking effect of bicuculline on IACh was dose-dependent, the concentration producing half-maximal blockade being 43.8 microM. 3. The blockade of IACh by bicuculline was not voltage-dependent at membrane potentials above -60 mV, but a slight voltage-dependence was observed at holding potentials (HP) of -80 and -100 mV. 4. The inhibitory effect of bicuculline on IACh was partially competitive at a HP of -60 mV. 5. Neither SR 95531, a pyridazinyl gamma-aminobutyric acid derivative, nor t-butylbicyclophosphorothionate (TBPS) blocked IACh in IL cells. 6. It is concluded that bicuculline interacts directly with the ACh receptor-ionophore complex on porcine IL cells.

Increased gamma-aminobutyric acid receptor function in the cerebral cortex of myoclonic calves with an hereditary deficit in glycine/strychnine receptors

Inherited congenital myoclonus (ICM) of Poll Hereford cattle is a neurological disease in which there are severe alterations in spinal cord glycine-mediated neurotransmission. There is a specific and marked decrease, or defect, in glycine receptors and a significant increase in neuronal (synaptosomal) glycine uptake. Here we have examined the characteristics of the cerebral gamma-aminobutyric acid (GABA) receptor complex, and demonstrate that the malfunction of the spinal cord inhibitory system is accompanied by a change in the major inhibitory system in the cerebral cortex. In synaptic membrane preparations from ICM calves, both high-and low-affinity binding sites for the GABA agonist [3H]muscimol were found (KD = 9.3 +/- 1.5 and 227 +/- 41 nM, respectively), whereas only the high-affinity site was detectable in controls (KD = 14.0 +/- 3.1 nM). The density and affinity of benzodiazepine agonist binding sites labelled by [3H]diazepam were unchanged, but there was an increase in GABA-stimulated benzodiazepine binding. The affinity for t-[3H]butylbicyclo-o-benzoate, a ligand that binds to the GABA-activated chloride channel, was significantly increased in ICM brain membranes (KD = 148 +/- 14 nM) compared with controls (KD = 245 +/- 33 nM). Muscimol-stimulated 36Cl- uptake was 12% greater in microsacs prepared from ICM calf cerebral cortex, and the uptake was more sensitive to block by the GABA antagonist picrotoxin. The results show that the characteristics of the GABA receptor complex in ICM calf cortex differ from those in cortex from unaffected calves, a difference that is particularly apparent for the low-affinity, physiologically relevant GABA receptors.(ABSTRACT TRUNCATED AT 250 WORDS)