Structure-activity studies on benzhydrol-containing nipecotic acid and guvacine derivatives as potent, orally-active inhibitors of GABA uptake

Bioactive Components of Areca Nut: An Overview of Their Positive Impacts Targeting Different Organs

Areca catechu L. is a widely cultivated tropical crop in Southeast Asia, and its fruit, areca nut, has been consumed as a traditional Chinese medicinal material for more than 10,000 years, although it has recently attracted widespread attention due to potential hazards. Areca nut holds a significant position in traditional medicine in many areas and ranks first among the four southern medicines in China. Numerous bioactive compounds have been identified in areca nuts, including alkaloids, polyphenols, polysaccharides, and fatty acids, which exhibit diverse bioactive functions, such as anti-bacterial, deworming, anti-viral, anti-oxidant, anti-inflammatory, and anti-tumor effects. Furthermore, they also display beneficial impacts targeting the nervous, digestive, and endocrine systems. This review summarizes the pharmacological functions and underlying mechanisms of the bioactive ingredients in areca nut. This helps to ascertain the beneficial components of areca nut, discover its medicinal potential, and guide the utilization of the areca nut.

γ-Aminobutyric acid transporters as relevant biological target: Their function, structure, inhibitors and role in the therapy of different diseases

γ-Aminobutyric acid (GABA) is a major inhibitory neurotransmitter in the nervous system. It plays a crucial role in many physiological processes. Upon release from the presynaptic element, it is removed from the synaptic cleft by reuptake due to the action of GABA transporters (GATs). GATs belong to a large SLC6 protein family whose characteristic feature is sodium-dependent relocation of neurotransmitters through the cell membrane. GABA transporters are characterized in many contexts, but their spatial structure is not fully known. They are divided into four types, which differ in occurrence and role. Herein, the special attention was paid to these transporting proteins. This comprehensive review presents the current knowledge about GABA transporters. Their distribution in the body, physiological functions and possible utilization in the therapy of different diseases were fully discussed. The important structural features were described based on published data, including sequence analysis, mutagenesis studies, and comparison with known SLC6 transporters for leucine (LeuT), dopamine (DAT) and serotonin (SERT). Moreover, the most important inhibitors of GABA transporters of various basic scaffolds, diverse selectivity and potency were presented.

Structure, Function, and Modulation of γ-Aminobutyric Acid Transporter 1 (GAT1) in Neurological Disorders: A Pharmacoinformatic Prospective

γ-Aminobutyric acid (GABA) Transporters (GATs) belong to sodium and chloride dependent-transporter family and are widely expressed throughout the brain. Notably, GAT1 is accountable for sustaining 75% of the synaptic GABA concentration and entails its transport to the GABAA receptors to initiate the receptor-mediated inhibition of post-synaptic neurons. Imbalance in ion homeostasis has been associated with several neurological disorders related to the GABAergic system. However, inhibition of the GABA uptake by these transporters has been accepted as an effective approach to enhance GABAergic inhibitory neurotransmission in the treatment of seizures in epileptic and other neurological disorders. Here, we reviewed computational methodologies including molecular modeling, docking, and molecular dynamic simulations studies to underscore the structure and function of GAT1 in the GABAergic system. Additionally, various SAR and QSAR methodologies have been reviewed to probe the 3D structural features of inhibitors required to modulate GATs activity. Overall, present review provides an overview of crucial role of GAT1 in GABAergic system and its modulation to evade neurological disorders.

GABA transporters as targets for new drugs

GABA, the principal inhibitory neurotransmitter in the cerebral cortex, maintains the inhibitory tone that counterbalances neuronal excitation. The identification and subsequent development of GABA-transport inhibitors has shown the important role that GABA transporters play in the control of the CNS. To date, four GABA transporters have been cloned (GAT1-4). Compounds that inhibit GABA uptake are targets for epilepsy treatment. Currently, they are also being investigated for other possible indications such as the treatment of psychosis, general anxiety and sleep disorders, drug addiction, acute and chronic pain. These and other issues are discussed in this article.

Synthesis and biological evaluation of (R)-N-(diarylmethylthio/sulfinyl)ethyl/propyl-piperidine-3-carboxylic acid hydrochlorides as novel GABA uptake inhibitors

A series of new (R)-1-(2-diarylmethylthio/sulfinyl)ethyl-piperidine-3-carboxylic acid hydrochlorides 5a-d/6a-d and (R)-1-(3-diarylmethylthio)propyl-piperidine-3-carboxylic acid hydrochlorides 5'a-d were synthesized and evaluated as gamma-aminobutyric acid uptake inhibitors through cultured cell lines expressing mouse GAT1. Biological screening results demonstrated that the compounds 6a-d with diarylmethylsulfinyl ethyl side chain show more potent GAT1 inhibitory activities than 5a-d/5'a-d with diarylmethylthio ethyl/propyl moieties. Some of them, such as 6a, exhibited excellent inhibitions of [(3)H]-GABA uptake in cultured cells, which is 496-fold higher than (R)-nipecotic acid and 11.5 times less than tiagabine. The synthesis and structure-activity relationships are discussed.

Selective inhibitors of GABA uptake: synthesis and molecular pharmacology of 4-N-methylamino-4,5,6,7-tetrahydrobenzo[d]isoxazol-3-ol analogues

A series of lipophilic diaromatic derivatives of the glia-selective GABA uptake inhibitor (R)-4-amino-4,5,6,7-tetrahydrobenzo[d]isoxazol-3-ol [(R)-exo-THPO, 4] were synthesized via reductive amination of 3-ethoxy-4,5,6,7-tetrahydrobenzo[d]isoxazol-4-one (9) or via N-alkylation of O-alkylatedracemic 4. The effects of the target compounds on GABA uptake mechanisms in vitro were measured using a rat brain synaptosomal preparation or primary cultures of mouse cortical neurons and glia cells (astrocytes), as well as HEK cells transfected with cloned mouse GABA transporter subtypes (GAT1-4). The activity against isoniazid-induced convulsions in mice after subcutaneous administration of the compounds was determined. All of the compounds were potent inhibitors of synaptosomal uptake the most potent compound being (RS)-4-[N-(1,1-diphenylbut-1-en-4-yl)amino]-4,5,6,7-tetrahydrobenzo[d]isoxazol-3-ol (17a, IC50 = 0.14 microM). The majority of the compounds showed a weak preference for glial, as compared to neuronal, GABA uptake. The highest degree of selectivity was 10-fold corresponding to the glia selectivity of (R)-N-methyl-exo-THPO (5). All derivatives showed a preference for the GAT1 transporter, as compared with GAT2-4, with the exception of (RS)-4-[N-[1,1-bis(3-methyl-2-thienyl)but-1-en-4-yl]-N-methylamino]-4,5,6,7-tetrahydrobenzo[d]isoxazol-3-ol (28d), which quite surprisingly turned out to be more potent than GABA at both GAT1 and GAT2 subtypes. The GAT1 activity was shown to reside in (R)-28d whereas (R)-28d and (S)-28d contributed equally to GAT2 activity. This makes (S)-28d a GAT2 selective compound, and (R)-28d equally effective in inhibition of GAT1 and GAT2 mediated GABA transport. All compounds tested were effective as anticonvulsant reflecting that these compounds have blood-brain barrier permeating ability.

Efficient and mild room temperature reduction of benzophenones under ultrasound irradiation

Benzophenones without strong electron-releasing substituents were reduced into the corresponding benzhydrols at room temperature in moderate to good yields with Zn-NaOH-95% ethanol under ultrasound irradiation. This method is also propitious to the synthesis of 3-phenylphthalide and its derivates.

First Demonstration of a Functional Role for Central Nervous System Betaine/γ-Aminobutyric Acid Transporter (mGAT2) Based on Synergistic Anticonvulsant Action among Inhibitors of mGAT1 and mGAT2

In a recent study, EF1502 [N-[4,4-bis(3-methyl-2-thienyl)-3-butenyl]-3-hydroxy-4-(methylamino)-4,5,6,7-tetrahydrobenzo [d]isoxazol-3-ol], which is an N-substituted analog of the GAT1-selective GABA uptake inhibitor exo-THPO (4-amino-4,5,6,7-tetrahydrobenzo[d]isoxazol-3-ol), was found to inhibit GABA transport mediated by both GAT1 and GAT2 in human embryonic kidney (HEK) cells expressing the mouse GABA transporters GAT1 to 4 (mGAT1-4). In the present study, EF1502 was found to possess a broad-spectrum anticonvulsant profile in animal models of generalized and partial epilepsy. When EF1502 was tested in combination with the clinically effective GAT1-selective inhibitor tiagabine [(R)-N-[4,4-bis(3-methyl-2-thienyl)-3-butenyl]nipecotic acid] or LU-32-176B [N-[4,4-bis(4-fluorophenyl)-butyl]-3-hydroxy-4-amino-4,5,6,7-tetrahydrobenzo[d]isoxazol-3-ol], another GAT1-selective N-substituted analog of exo-THPO, a synergistic rather than additive anticonvulsant interaction was observed in the Frings audiogenic seizure-susceptible mouse and the pentylenetetrazol seizure threshold test. In contrast, combination of the two mGAT1-selective inhibitors, tiagabine and LU-32-176B, resulted in only an additive anticonvulsant effect. Importantly, the combination of EF1502 and tiagabine did not result in a greater than additive effect in the rotarod behavioral impairment test. In subsequent in vitro studies conducted in HEK-293 cells expressing the cloned mouse GAT transporters mGAT1 and mGAT2, EF1502 was found to noncompetitively inhibit both mGAT1 and the betaine/GABA transporter mGAT2 (K(i) of 4 and 5 muM, respectively). Furthermore, in a GABA release study conducted in neocortical neurons, EF1502 did not act as a substrate for the GABA carrier. Collectively, these findings support a functional role for mGAT2 in the control of neuronal excitability and suggest a possible utility for mGAT2-selective inhibitors in the treatment of epilepsy.

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.

Nipecotic acid directly activates GABA(A)-like ion channels

The GABA-related compound nipecotic acid is commonly used to inhibit GABA uptake. This report shows that nipecotic acid can also directly activate GABA(A)-like chloride channels. When applied to outside-out patches of paraventricular neurones, nipecotic acid (1 mM) activated inward unitary currents (approximately 3 pA at a holding potential of -60 mV, E(Cl)+44 mV). The EC(50) for ion channel activation was approximately 300 microM, 3 fold greater than that found for GABA itself in this preparation. The nipecotic acid activated channels had similar conductance and kinetic properties to those of GABA activated channels in the same patches, reversed near E(Cl) and were inhibited by bicuculline (3 microM). This study indicates that for experiments in which relatively high concentrations of nipecotic acid are used, possible direct GABA(A) receptor agonist properties should be considered.

Synthesis of novel gamma-aminobutyric acid (GABA) uptake inhibitors. 5.(1) Preparation and structure-activity studies of tricyclic analogues of known GABA uptake inhibitors

On the basis of the SAR of a series of known gamma-aminobutyric acid (GABA) uptake inhibitors, including 4 (SKF 89976), new tricyclic analogues have been prepared. These novel compounds are derivatives of nipecotic acid, guvacine, and homo-beta-proline, substituted at the nitrogen of these amino acids by various lipophilic moieties such as (10,11-dihydro-5H-dibenz[b,f]azepin-5-yl)alkoxyalkyl or (10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5-ylidene)alkoxyalkyl. The in vitro values for inhibition of [(3)H]-GABA uptake in rat synaptosomes was determined for each compound in this new series, and it was found that several of the novel compounds showed a high potency comparable with that of the reference compounds 4, 5 (tiagabine), and 6 (CI-966). Several of the novel compounds were also evaluated for their ability in vivo to inhibit clonic seizures induced by a 15 mg/kg (ip) dose of methyl 6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM). One compound, (R)-1-(2-(2-(10,11-dihydro-5H-dibenz[b,f]azepin-5-yl)ethoxy)ethyl)-3-piperidinecarboxylic acid (23), was selected for further biological investigations and showed a protective index comparable to or slightly better than that of the recently launched anticonvulsant product 5 ((R)-1-(4,4-bis(3-methyl-2-thienyl)-3-butenyl)-3-piperidinecarboxylic acid).

Nematic Liquid Crystals for Active Matrix Displays: Molecular Design and Synthesis

Substances forming calamitic mesophases have been known for more than 100 years but only the recent, rapid advance in active matrix liquid crystal display (AM-LCD) technology helped these materials to achieve the crucial position in flat panel display technology they hold today. Due to their high contrast, large viewing angle, and rapid switching times, modern AM-LCDs offer a superior picture quality even compared to conventional cathode ray tubes. Their flatness, low weight, and low energy consumption render them the technology of choice for all kinds of portable devices. Some of the future promises of AM-LCD technology are centered around the development of liquid crystalline materials for the different subtypes of active matrix applications. This development is aimed, on the one hand, towards improved electrooptical and viscoelastic properties; on the other hand, the increasing performance of LCDs leads to extremely stringent reliability demands on the liquid crystals. Responding to these high standards of performance and quality, most liquid crystals for contemporary AM-LCD applications are multiply fluorinated compounds with very high purities, as is typical for materials used in the electronics industry. The synthesis of these superfluorinated materials (SFMs) often requires specialized methods, which, in several cases, had to be introduced for the first time into the canon of industrial production. The immense market pressure, as well as the rapid advance of AM-LCD technology on the side of the display manufacturers, urges an increasing pace of the materials development. This demand for new materials can no longer be fulfilled by conventional trial-and-error approaches. As in the pharmaceutical industry, in the search for new, superior liquid crystals, the purely empirical methods are increasingly supported by a rational design based on computational methods.

Synthesis of novel GABA uptake inhibitors. 4. Bioisosteric transformation and successive optimization of known GABA uptake inhibitors leading to a series of potent anticonvulsant drug candidates

By bioisosteric transformations and successive optimization of known GABA uptake inhibitors, several series of novel GABA uptake inhibitors have been prepared by different synthetic approaches. These compounds are derivatives of nipecotic acid and guvacine, substituted at the nitrogen of these amino acids by various lipophilic moieties such as diarylaminoalkoxyalkyl or diarylalkoxyalkyl. The in vitro values for inhibition of [(3)H]GABA uptake in rat synaptosomes was determined for each compound, and it was found that the most potent compound from this series, (R)-1-(2-(3,3-diphenyl-1-propyloxy)ethyl)-3-piperidinecarboxyli c acid hydrochloride (29), is so far the most potent parent compound inhibiting GABA uptake into synaptosomes. Structure-activity results confirm our earlier observations, that an electronegative center in the chain connecting the amino acid and diaryl moiety is very critical in order to obtain high in vitro potency. Several of the novel compounds were also evaluated for their ability in vivo to inhibit clonic seizures induced by a 15 mg/kg (ip) dose of methyl 6, 7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM). Some of the compounds tested show a high in vivo potency comparable with that of the recently launched anticonvulsant product 6 ((R)-1-(4, 4-bis(3-methyl-2-thienyl)-3-butenyl)-3-piperidinecarboxylic acid).

Synthesis of novel GABA uptake inhibitors. 3. Diaryloxime and diarylvinyl ether derivatives of nipecotic acid and guvacine as anticonvulsant agents

(3R)-1-[4,4-bis(3-methyl-2-thienyl)-3-butenyl]-3-piperidinecarboxylic acid 1 (tiagabine, Gabitril) is a potent and selective gamma-aminobutyric acid (GABA) uptake inhibitor with proven anticonvulsant efficacy in humans. This drug, which has a unique mechanism of action among marketed anticonvulsant agents, has been launched for add-on treatment of partial seizures with or without secondary generalization in patients >12 years of age. Using this new agent as a benchmark, we have designed two series of novel GABA uptake inhibitors of remarkable potency, using a putative new model of ligand interaction at the GABA transporter type 1 (GAT-1) uptake site. This model involves the postulated interaction of an electronegative region in the GABA uptake inhibitor with a positively charged domain in the protein structure of the GAT-1 site. These two novel series of anticonvulsant agents contain diaryloxime or diarylvinyl ether functionalities linked to cyclic amino acid moieties and were derived utilizing the new model, via a series of design steps from the known 4,4-diarylbutenyl GABA uptake inhibitors. The new compounds are potent inhibitors of [(3)H]-GABA uptake in rat brain synaptosomes in vitro, and their antiepileptic potential was demonstrated in vivo by their ability to protect against seizures induced by the benzodiazepine receptor inverse agonist methyl 4-ethyl-6,7-dimethoxy-beta-carboline-3-carboxylate (DMCM) in mice. From structure-activity studies of these new GABA uptake inhibitors, we have shown that insertion of an ether oxygen in conjugation with the double bond in tiagabine (K(i) = 67 nM) improves in vitro potency by 5-fold to 14 nM.

Effects of GABA uptake inhibitors on posthypoxic myoclonus in rats

Male Sprague-Dawley rats developed posthypoxic myoclonus following 10-min cardiac arrest and resuscitation. Previous results showed that dysfunction of central GABAergic neurotransmission may contribute to the disease. In current studies, effects of GABA uptake inhibitors, guvacine hydrochloride (1,2,5,6-tetrahydro-3-pyridine carboxylic acid hydrochloride) and (+/-)-cis-4-hydroxynipecotic acid ([+/-]-cis-4-hydroxy-3-piperidine carboxylic acid), in the pathophysiology of posthypoxic myoclonus were investigated. Administration of guvacine (1 or 10 mg/kg, IP) or nipecotic acid (0.5 or 5 mg/kg, IP) significantly attenuated myoclonus scores of the animals. Tolerance to antimyoclonus effects of these two compounds did not develop after chronic administration (twice a day for 14 days) of guvacine (10 mg/kg, IP) or nipecotic acid (5 mg/kg, IP). On the other hand, tolerance was noticed with clonazepam (2.5 mg/kg, IP twice a day for 7 days). The results indicate that guvacine or nipecotic acid may be used in combination with (at reduced doses) or as alternatives to clonazepam to treat patients with the disease so as to reduce tolerance phenomenon usually associated with clonazepam.

Characterization of glutamate, aspartate, and GABA release from ischemic rat cerebral cortex

The purpose of this study was to evaluate potential mechanisms of ischemia-evoked amino acid transmitter release. Changes in extracellular levels of transmitter amino acids and lactic acid dehydrogenase (LDH) in rat cerebral cortex during and following four-vessel occlusion elicited global cerebral ischemia were examined using a cortical cup technique. Ischemia-evoked release of glutamate, aspartate and gamma-amino-butyric acid (GABA) was compared in control vs. drug-treated animals. Tetrodotoxin and antagonists of glutamate receptors (DNQX, MK-801, and AP-3) depressed the initial rate of increase in extracellular glutamate and aspartate without altering the total amount of these amino acids collected in the cortical superfusates. Cobalt, a calcium channel antagonist, failed to alter efflux. Acidic amino acid transport inhibitors (dihydrokainate, L-trans-PDC) depressed the rate of onset of glutamate and aspartate release and dihydrokainate depressed total release by 44%. PD 81723, an allosteric enhancer at the A1 adenosine receptor, depressed glutamate efflux, as did L-NAME, an inhibitor of nitric oxide synthase. Extracellular increases in GABA levels were depressed by tetrodotoxin and L-trans-PDC. The GABA transport inhibitor, nipecotic acid, increased the initial rate of onset of GABA release. Increases in LDH levels in the extracellular fluid became apparent during the period of ischemia and continued to increase during the subsequent 90 min of reperfusion. These results suggest that ischemia evokes a release of neurotransmitter amino acids that is only partially dependent upon Ca2+ influx activation or the reversal of amino acid transporters. Nonselective mechanisms, resulting from the disruption of plasma membrane integrity, may contribute significantly to the total ischemia-evoked release of excitatory amino acids.