Γ-aminobutyric Acid Analogues Research Articles

Gabapentin may inhibit synaptic transmission in the mouse spinal cord dorsal horn through a preferential block of P/Q-type Ca 2+ channels

Gabapentin is a lipophilic analog of γ-amino butyric acid (GABA) with therapeutic activity against certain forms of epilepsy and neuropathic pain. Despite its structural similarity to GABA, it does not bind GABA A or GABA B receptors and the mechanism, especially of its analgesic action, has remained elusive. Here, we have studied its effects on synaptic transmission mediated by the major spinal fast excitatory and inhibitory neurotransmitters, l-glutamate and glycine, in the superficial layers of the spinal cord dorsal horn, a CNS area, which is critically involved in nociception. Gabapentin reversibly reduced evoked excitatory postsynaptic currents mediated by α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA-EPSCs) and inhibitory postsynaptic currents mediated by glycine (gly-IPSCs). Inhibition of AMPA-EPSCs and gly-IPSCs occurred with similar potencies (∼10–50 nM) and by about the same degree (∼40% at 1 μM). Gabapentin did not affect membrane currents elicited by exogenously applied glutamate or glycine arguing against a postsynaptic site of action. Selective blockade of N-type Ca 2+ channels with ω-conotoxin GVIA dramatically increased and blockade of P/Q-type channels with ω-agatoxin IVA strongly attenuated inhibition of evoked synaptic transmission by gabapentin. These results show that gabapentin affects both excitatory and inhibitory spinal neurotransmission via a presynaptic mechanism which preferentially involves P/Q-type Ca 2+ channels.

Endogenous GABA Does Not Mediate the Inhibitory Effects of Gabapentin on Spinal Reflexes in Rats

The novel antiepileptic drug gabapentin was designed as a structural analog of γ-aminobutyric acid (GABA). However, its mechanism of action remains unclear. In the present study, we investigated the effect of gabapentin on spinal reflexes in anesthetized rats. The mono- and polysynaptic reflex potentials were recorded from the ipsilateral L5 ventral root after stimulation of the L5 dorsal root. The dorsal root reflex potential, an index of presynaptic inhibition, was recorded from the ipsilateral L4 dorsal root. In non-spinalized (intact) and spinalized rats, intravenously administered gabapentin reduced the mono- and polysynaptic reflex potentials in a dose-dependent manner. These inhibitory effects of gabapentin were not suppressed by the GABAA antagonist picrotoxin. Moreover, gabapentin also decreased spinal reflexes in spinalized rats depleted of spinal GABA with semicarbazide, an inhibitor of the GABA-synthesizing enzyme. The dorsal root reflex potentials were not affected by gabapentin. These results suggest that endogenous GABA does not mediate the inhibitory effects of gabapentin on spinal reflexes.

Gabapentin versus Ropinirole in the Treatment of Idiopathic Restless Legs Syndrome

Dopaminergic agents such as ropinirole are the drugs of first choice in treating restless legs syndrome (RLS). Recently, gabapentin, a structural analogue of γ-aminobutyric acid, has also been shown to improve sensorimotor symptoms in RLS. Therefore, the tolerability and efficacy of randomized treatment with either gabapentin or ropinirole in patients with idiopathic RLS was evaluated in this 4-week open clinical trial. Patients with idiopathic RLS were treated with either 300 mg of gabapentin (n = 8) or 0.5 mg of ropinirole (n = 8) as the initial dose, and the dose was up-titrated until relief of symptoms was achieved (gabapentin mean dosage 800 ± 397 mg, range 300–1,200 mg; ropinirole mean dosage 0.78 ± 0.47 mg, range 0.25–1.50 mg). In both groups, International Restless Legs Syndrome Study Group questionnaire scores improved significantly (p ≤ 0.018), whereas the scores of the Epworth sleepiness scale remained unchanged within normal limits. Polysomnographic data showed a reduction of periodic leg movements during sleep (PLMS; p < 0.03) and PLMS index (p < 0.02) in both groups. Side effects were only mild and mostly transient. After 6–10 months of follow-up, in most patients, RLS symptoms were still improved. We conclude that gabapentin and ropinirole provide a similarly well-tolerated and effective treatment of PLMS and sensorimotor symptoms in patients with idiopathic RLS.

Pregabalin: a new anxiolytic

Pregabalin (S-[+]-3-isobutylgaba) was designed as a lipophilic GABA (γ-aminobutyric acid) analogue substituted at the 3′-position in order to facilitate diffusion across the blood–brain barrier. It was originally developed as an anticonvulsant agent, however it has been shown to be effective in the treatment of several disorders including hyperalgesia and behavioural disorders. Although its exact mode of action remains unclear, pregabalin interacts with the same binding site and has a similar pharmacological profile as its predecessor, gabapentin (1-[aminomethyl] cyclohexane acetic acid). Its main site of action appears to be on the α2δ subunit of voltage-dependent calcium channels, widely distributed throughout the peripheral and central nervous system. Pregabalin appears to produce an inhibitory modulation of neuronal excitability. In healthy volunteers, it is rapidly absorbed with peak blood concentrations within 1 h and it has a bioavailability of ∼ 90%. In preclinical trials of anticonvulsant activity, pregabalin is three to ten times more potent than gabapentin. It is well-tolerated and associated with dose-dependent adverse effects (ataxia, dizziness, headache and somnolence) that are mild-to-moderate and usually transient. There are no known pharmacokinetic drug–drug interactions reported to date. Preliminary animal and human studies showed beneficial effects in both ethological and conflict models of anxiety, as well as having some sleep-modulating properties. In Phase II and III trials, pregabalin shows promising anxiolytic action when compared to placebo in generalised anxiety disorder, social phobia and panic disorder.

Pregabalin: a new anxiolytic

Pregabalin (S-[+]-3-isobutylgaba) was designed as a lipophilic GABA (γ-aminobutyric acid) analogue substituted at the 3′-position in order to facilitate diffusion across the blood–brain barrier. It was originally developed as an anticonvulsant agent, however it has been shown to be effective in the treatment of several disorders including hyperalgesia and behavioural disorders. Although its exact mode of action remains unclear, pregabalin interacts with the same binding site and has a similar pharmacological profile as its predecessor, gabapentin (1-[aminomethyl] cyclohexane acetic acid). Its main site of action appears to be on the α2δ subunit of voltage-dependent calcium channels, widely distributed throughout the peripheral and central nervous system. Pregabalin appears to produce an inhibitory modulation of neuronal excitability. In healthy volunteers, it is rapidly absorbed with peak blood concentrations within 1 h and it has a bioavailability of ∼ 90%. In preclinical trials of anticonvulsant activity, pregabalin is three to ten times more potent than gabapentin. It is well-tolerated and associated with dose-dependent adverse effects (ataxia, dizziness, headache and somnolence) that are mild-to-moderate and usually transient. There are no known pharmacokinetic drug–drug interactions reported to date. Preliminary animal and human studies showed beneficial effects in both ethological and conflict models of anxiety, as well as having some sleep-modulating properties. In Phase II and III trials, pregabalin shows promising anxiolytic action when compared to placebo in generalised anxiety disorder, social phobia and panic disorder.

Enantioselective Syntheses of (-)-(R)-Rolipram, (-)-(R)-Baclofen and Other GABA Analogues via Rhodium-Catalyzed Conjugate Addition of Arylboronic Acids

Highly enantioselective syntheses of two important γ-aminobutyric acid (GABA) analogues, the antispastic drug (-)-(R)-Baclofen and the antidepressant agent (-)-(R)-Rolipram, are reported. Key-steps in both syntheses are the Rh-catalyzed asymmetric 1,4-additions of arylboronic acids to 4-aminobut-2,3-enoic acid derivatives.

VI. Synthesis of heterocyclic analogs of γ-aminobutyric acid from 3,5-dinitrobenzoic acid

A series of 7-carboxy-3-R-1,5-dinitro-3-azabicyclo[3.3.1]non-6-enes was synthesized by reduction of 3,5-dinitrobenzoic acid with sodium borohydride followed by Mannich reaction with formaldehyde and primary amines. The mechanism of decomposition under electron impact of the 3-azabicyclo[3.3.1]nonane was established. Enthalpies of formation of compounds synthesized were calculated by semiempirical PM3 method.

Application of Polymer-Supported Enzymes and Reagents in the Synthesis of γ-Aminobutyric Acid (GABA) Analogues

Polymer-supported pig liver esterase was used for the resolution of meso-diesters. The enzyme can be recovered quantita- tively from the reaction mixture by filtration and reused without sig- nificant loss of activity. Further transformation of the resulting enantiomerically enriched carboxylic acids through the application of polymer-supported reagents and scavengers provides a number of GABA-analogues. Enzymes have become valuable tools in asymmetric or- ganic synthesis, notably in the desymmetrisation of meso- compounds. 1 In this way, achiral starting materials can be converted into enantiomerically enriched compounds in up to quantitative yield. However, the high cost of enzyme preparations have made recycling of the biocatalyst an im- portant issue. Since filtration is a simple way of recover- ing a catalyst from a reaction mixture, enzymes have been covalently linked to a multitude of insoluble supports and adsorbed onto various materials. However, immobilising enzymes can modify their properties and care has to be taken not to alter the selectivity and the activity of the na- tive catalyst. 2 With the need for automated parallel synthesis of large numbers of pharmaceutically interesting compounds, polymer-supported reagents have been developed that display high degree of versatility and applicability in re- cent years. 3 Compared to traditional solution phase and solid phase chemistry, supported reagents offer many ad- vantages for example the reactions can be monitored by conventional techniques (TLC, LC-MS, GC, NMR) and no additional steps are required in order to attach and sub- sequently remove the substrate from the support. As yet, polymer-supported enzymes have not been effectively integrated into the toolbox of reagents for multistep syn-

The effects of cyclopentane and cyclopentene analogues of GABA at recombinant GABA C receptors

The pharmacological effects of the enantiomers of cis-3-aminocyclopentanecarboxylic acids ((+)- and (−)-CACP), the enantiomers of trans-3-aminocyclopentanecarboxylic acids ((+)- and (−)-TACP), and the enantiomers of 4-aminocyclopent-1-ene-1-carboxylic acids ((+)- and (−)-4-ACPCA) were studied on human homomeric ρ 1 and ρ 2 GABA C receptors expressed in Xenopus oocytes using two-electrode voltage clamp methods. These compounds are conformationally restricted analogues of γ-aminobutyric acid (GABA) held in a five-membered ring. (+)-TACP (EC 50 (ρ 1)=2.7±0.2 μM; EC 50 (ρ 2)=1.45±0.22 μM), (+)-CACP (EC 50 (ρ 1)=26.1±1.1 μM; EC 50 (ρ 2)=20.1±2.1 μM) and (−)-CACP (EC 50 (ρ 1)=78.5±3.5 μM; EC 50 (ρ 2)=63.8±23.3 μM) were moderately potent partial agonists at ρ 1 and ρ 2 GABA C receptors, while (−)-TACP (100 μM inhibited 56% and 62% of the current produced by 1 μM GABA at ρ 1 and ρ 2 receptors, respectively) was a weak partial agonist with low intrinsic activity at these receptors. In contrast, (+)-4-ACPCA ( K i (ρ 1)=6.0±0.1 μM; K i (ρ 2)=4.7±0.3 μM) did not activate GABA C ρ 1 and ρ 2 receptors but potently inhibited the action of GABA at these receptors, while (−)-4-ACPCA had little effect as either an agonist or an antagonist. The affinity order at both GABA C ρ 1 and ρ 2 receptors was (+)-TACP>(+)-4-ACPCA≫(+)-CACP>(−)-CACP≫(−)-TACP≫(−)-4-ACPCA. This study shows that the cyclopentane and cyclopentene analogues of GABA affect GABA C receptors in a unique manner, defining a preferred stereochemical orientation of the amine and carboxylic acid groups when binding to GABA C receptors. This is exemplified by the partial agonist, (+)-TACP, and the antagonist, (+)-4-ACPCA.

Kinetic study of acamprosate absorption in rat small intestine.

Acamprosate (calcium bis acetyl-homotaurine), a homotaurine derivative, a structural analogue of gamma-aminobutyric acid (GABA) and an upper homologue of taurine, is a relatively new drug used to prevent relapse in weaned alcoholics. When administered orally as enteric-coated tablets at relatively high doses, this drug has a bioavailability of about 11%; however, the intestinal absorption mechanism has not been studied in depth. The present study was therefore planned to characterize the intestinal transport of acamprosate in the rat and the effect of chronic alcohol treatment on this process, quantifying its kinetic parameters and investigating possible inhibitors. Using an in vitro technique, acamprosate absorption was measured in the rat intestine from three different groups: alcohol group [fed a liquid diet containing 5% (w/v) ethanol for 4 weeks], isocaloric pair-fed control, and a solid diet group. Intestinal acamprosate absorption was found to occur mainly by passive diffusion with a diffusive permeability of 0.213+/-0.004 cm/h in control pair-fed animals, 0.206+/-0.001 cm/h in animals receiving chronic alcohol treatment, and 0.193+/-0.001 cm/h in the solid diet group. Inhibition studies showed that at a 10(-3) M acamprosate concentration, some compounds such as GABA, taurine, proline, and glycine at 40 mM each did not affect acamprosate transport. Nevertheless, when a lower concentration of the drug (10(-4) M) was assayed, a significant reduction of acamprosate transport in the presence of taurine or GABA 40 mM was found. These results suggest that acamprosate in the rat jejunum, could be transported, in part, by a carrier system. Further experiments using different concentrations of taurine (10, 20, and 80 mM) showed that the maximum inhibition (32%) is achieved at 20 mM of taurine. These latter results suggest that acamprosate and taurine share, at least, an intestinal carrier system in rat jejunum. From the above results, it can be concluded that there are probably two pathways involved in the intestinal absorption of acamprosate: passive diffusion and mediated transport, with the former being predominant. Moreover, neither chronic ethanol intake nor the type of diet seems to alter the intestinal absorption of the drug.

A Short Stereoselective Synthesis of a Cyclopropyl Analog of γ-Aminobutyric acid (GABA)

An efficient synthesis of (S,S)-2-(aminomethyl) cyclopropane carboxylic acid has been achieved in 7 steps (38% overall yield) starting from commercially available cinnamyl alcohol. The synthesis involves initial stereoselective formation of the functionalized cyclopropane derivative 5 and its further transformation to the desired product.

Action of bicyclic isoxazole GABA analogues on GABA transporters and its relation to anticonvulsant activity

The inhibitory action of bicyclic isoxazole γ-aminobutyric acid (GABA) analogues and their 4,4-diphenyl-3-butenyl (DPB) substituted derivatives has been investigated in cortical neurones and astrocytes as well as in human embryonic kidney (HEK 293) cells transiently expressing either mouse GABA transporter-1 (GAT-1), GAT-2, -3 or -4. It was found that 4,5,6,7-tetrahydroisoxazolo(4,5- c)pyridin-3-ol (THPO) and 5,6,7,8-tetrahydro-4 H-isoxazolo[4,5- c]azepin-3-ol (THAO) displayed some inhibitory activity on GAT-1 and GAT-2, where the compounds exhibited a slightly lower potency on GAT-2 compared to GAT-1. DPB substituted THPO displayed higher inhibitory potency than the parent compound regarding the ability to inhibit GABA uptake via GAT-1 and GAT-2. Concerning the inhibitory mechanism, THPO, THAO and DPB–THPO were competitive inhibitors on GAT-1 transfected HEK 293 cells and the same mechanism was observed for THPO in GAT-3 transfected cells. Regarding GABA uptake into neurones and astroglia cells THAO and DPB–THAO both displayed competitive inhibitory action. The observations that THPO, THAO as well as their DPB derivatives act as competitive inhibitors together with earlier findings such as potent anticonvulsant activity, lack of proconvulsant activity and the ability of THPO to increase extracellular GABA concentration, indicate that these bicyclic isoxazole GABA analogues and their DPB derivatives may be useful lead structures in future search for new antiepileptic drugs.

Rh(II)-catalyzed asymmetric cyclopropenation of propargyl derivatives; synthesis of cyclopropene- and cis-cyclopropane-amino acids

The [Rh 2{(2 S)-mepy} 4]-catalyzed cyclopropenation of propargylamines carrying two carboxyl or sulfonyl protecting groups with ethyl diazoacetate proceeds in high yield and with enantioselectivities in the range of 90–>97% ee. Selective deprotection of the TEOC-derivative afforded ethyl 2-aminomethylcycloprop-2-ene-1-carboxylate which was converted to several analogs of γ-aminobutyric acid (GABA) containing the cyclopropene or cyclopropane ring.

Asymmetric synthesis of 3-substituted pyrrolidones via α-alkylation of a chiral non-racemic γ-lactam

3-Alkyl pyrrolidones 9 were synthesized in good yield and high diastereoselectivity by α-alkylation of the new chiral non-racemic lactam 8 derived from ( R)-(−)-phenylglycinol. After debenzylation and introduction of an electron-withdrawing group, 3-methylpyrrolidone 10 is easily hydrolyzed in a basic medium to produce γ-aminobutyric acid (GABA) analogue 13.

A novel class of potent γ-aminobutyric acid aminotransferase inhibitor, 3-(hydroxyamino)propylamine and analogues

Hydroxyamino analogues of γ-aminobutyric acid (GABA) were synthesized and evaluated for inhibitory activity toward γ-aminobutyric acid aminotransferase (GABA-T). The title compound, 3-(hydroxyamino)propylamine (HPA), showed a potent inhibitory activity. The inhibition is competitive with respect to GABA and the K i value of GABA-T for HPA is 0.4 mmol. The activity of inhibition is comparable to those of aminoxyacetic acid and valproic acid. 3-(Hydroxyaminomethyl)piperidine (3HMP), a cyclic analogue of HPA, also showed a potent inhibitory activity, whereas 3-(methoxyamino)propylamine (OMe-HPA), 3-( N-hydroxy- N-methylamino)propylamine (NMe-HPA) and 4-(hydroxyamino)piperidine (4HP) showed weak activity.

Unsaturated phosphinic analogues of γ-aminobutyric acid as GABAC receptor antagonists

Unsaturated phosphinic analogues of γ-aminobutyric acid as GABAC receptor antagonists

Unsaturated phosphinic analogues of γ-aminobutyric acid as GABA C receptor antagonists

The phosphinic and methylphosphinic analogues of γ-aminobutyric acid (GABA) are potent GABA C receptor antagonists but are even more potent as GABA B receptor agonists. Conformationally restricted unsaturated phosphinic and methylphosphinic analogues of GABA and some potent GABA B receptor phosphonoamino acid antagonists were tested on GABA C receptors in Xenopus oocytes expressing human retinal ϱ 1 mRNA. 3-Aminopropyl- n-butyl-phosphinic acid (CGP36742), an orally active GABA B receptor antagonist, was found to be a moderately potent GABA C receptor antagonist (IC 50 = 62 μM). The unsaturated methylphosphinic and phosphinic analogues of GABA were competitive antagonists of the GABA C receptors, the order of potency being [( E)-3-aminopropen-1-yl]methyl-phosphinic acid (CGP44530, IC 50 = 5.53 μM) > [( E)-3-aminopropen-1-yl]phosphinic acid (CGP38593, IC 50 = 7.68 μM) > [( Z)-3-aminopropen-1-yl]methylphosphinic acid (CGP70523, IC 50 = 38.94 μM) > [( Z)-3-aminopropen-1-yl]phosphinic acid (CGP70522, IC 50 > 100 μM). This order of potency differs from that reported for these compounds as GABA B receptor agonists, where the phosphinic acids are more potent than the corresponding methylphosphinic acids.

Chiral separations of amino acids by capillary electrophoresis and high-performance liquid chromatography employing chiral crown ethers

High-performance liquid chromatography with a chiral crown ether stationary phase and capillary electrophoresis (CE) with a chiral crown ether dissolved in the operating buffer were used for the separation of enantiomers of analogues of DOPA and tyrosine and some analogues of γ-aminobutyric acid (GABA). CE and HPLC yielded similar results for the DOPA and tyrosine analogues. However, for the analogues of GABA, three of the four compounds tested were well resolved by HPLC but only one was well resolved by CE. It was necessary to use an indirect detection scheme for the CE of GABA analogues. The influence of substituents on the different compounds on the resolution factors observed by the two methods is discussed, in addition to the advantages and disadvantages of the two methods in practical applications.

Synthesis of optically active cis- and trans-1,2-disubstituted cyclopropane derivatives by the Simmons-Smith reaction of allyl alcohol derivatives derived from (R)-2,3-O-isopropylideneglyceraldehyde

The Simmons-Smith reactions of Z- and E-allyl alcohol derivatives 6 derived from (R)-2,3-O-isopropylideneglyceraldehyde (6) were used for the synthesis of optically active cis- and trans-1,2-disubstituted cyclopropane derivatives. Reaction of 6 with diethyl zinc and diiodomethane gave cyclopropane derivatives 7 in 84% to quantitative yields with 35 to ≃100% des. Identical facial selectivities toward the double bonds, 1re-2si for Z-6 and 1re-2re for E-6, were observed in the cyclopropanations. The diastereoselectivity was dependent on the protecting group on the terminal allylic oxygen (R of 6, TBDPS>MOM>Bn) and on the stereochemistry of the double bond (Z >E). For TBDPS ethers Z-and E-6c, cis- and trans-7c were obtained as single distereomers, respectively. It was clearly demonstrated that the stereoselectivity of the cyclopropantion is controlled by the directing effect of the allylic oxygen (O-1) of the dioxolane ring, which coordinates to the reagent. The terminal allylic oxygen (O-2) lowered the diastereoselectivity. This reaction was applied to the synthesis of optically active cyclopropane analogs of γ-aminobutyric acid (GABA) 18,22, and ent-22

Uptake of [ 14C]taurine into tissues of schistocerca gregaria in vivo

Locusts ( Schistocerca americana gregaria) were injected with a tracer dose of 0.25 μCi of [ 14C]taurine into the haemolymph and its clearance from haemolymph and uptake into flight muscle, thoracic ganglia and eye were studied at times thereafter. Clearance of labelled taurine from haemolymph suggested that the amino acid was rapidly taken up by some tissues and more slowly by others. Thus uptake of [ 14C]taurine was rapid into nervous tissues and eye, but was much more gradual into flight muscle. Uptake was inhibited by cold and the structural analogues of taurine, ß-alanine, γ-aminobutyric acid and hypotaurine. Taurine was retained in all tissues over a long period, indicating a very slow rate of catabolism for this amino acid in the locust.