Robin D. Allan

(+)‐ and (‐)‐cis‐2‐Aminomethylcyclopropanecarboxylic Acids Show Opposite Pharmacology at Recombinant ρ1 and ρ2 GABAC Receptors

Abstract: The effects of the enantiomers of (±)‐CAMP and(±)‐TAMP [(±)‐cis‐ and(±)‐trans‐2‐aminomethylcyclopropanecarboxylic acids,respectively], which are cyclopropane analogues of GABA, were tested onGABAA and GABAC receptors expressed in Xenopuslaevis oocytes using two‐electrode voltage clamp methods. (+)‐CAMP wasfound to be a potent and full agonist at homooligomeric GABACreceptors (KD∼40 μM andImax∼100% at ρ1;KD∼17 μM and Imax∼100% at ρ2) but a very weak antagonist atα1β2γ2L GABAAreceptors. In contrast, (‐)‐CAMP was a very weak antagonist at bothα1β2γ2L GABAAreceptors and homooligomeric GABAC receptors (IC50∼900 μM at ρ1 and ∼400 μM atρ2). Furthermore, (+)‐CAMP appears to be a superior agonist tothe widely used GABAC receptor partial agonistcis‐4‐aminocrotonic acid (KD∼74μM and Imax∼78% at ρ1;KD∼70 μM and Imax∼82% at ρ2). (‐)‐TAMP was the most potent of thecyclopropane analogues on GABAC receptors (KD∼9 μM and Imax∼40% atρ1; KD∼3 μM andImax∼50‐60% at ρ2), but it was also amoderately potent GABAA receptor partial agonist(KD∼50‐60 μM and Imax∼50% at α1β2γ2LGABAA receptors). (+)‐TAMP was a less potent partial agonist atGABAC receptors (KD∼60 μM andImax∼40% at ρ1; KD∼30 μM and Imax∼60% atρ2) and a weak partial agonist atα1β2γ2L GABAAreceptors (KD∼500 μM andImax∼50%). None of the isomers of (±)‐CAMP and(±)‐TAMP displayed any interaction with GABA transport at theconcentrations tested. Molecular modeling based on the present resultsprovided new insights into the chiral preferences for either agonism orantagonism at GABAC receptors.

A new synthesis resolution and in vitro activities of (R)- and (S)-β-Phenyl-Gaba

Abstract β-Phenyl-GABA (2) was resolved by separation by crystallization and/or h.p.l.c. of the diastereoisomeric (R)-(-)-pantolactone esters of the N-phthalimido protected β-phenyl-GABA. The absolute stereochemical assignments obtained from chiroptical studies of the enantiomers(8a) and(8b) and an X-ray crystallographic study of the diastereoisomer(7a) were supported by the activities of the enantiomers(8a) and(8b) in binding and electrophysiological studies. Details of synthesis, binding, electrophysiological, chiroptical and X-ray crystallographic studies are reported.

Isothiouronium compounds as γ-aminobutyric acid agonists

1 Analogues of γ‐aminobutyric acid (GABA) incorporating an isothiouronium salt as a replacement for a protonated amino functional group have been investigated for activity on: GABA receptors in the guinea‐pig ileum; [3H]‐GABA and [3H]‐diazepam binding to rat brain membranes; and GABA uptake and transamination. 2 For the homologous series of α‐isothiouronium alkanoic acids, maximum GABA‐mimetic activity was found at 3‐[(aminoiminomethyl)thio]propanoic acid. 3 Introduction of unsaturation into this compound gave two isomeric conformationally restricted analogues. The trans isomer was inactive at GABA receptors while the cis compound ((Z)‐3‐[(aminoiminomethyl)thio]prop‐2‐enoic acid (ZAPA)) was more potent than muscimol and GABA as a GABA agonist with respect to low affinity GABA receptor sites. 4 Both isomers were moderately potent at inhibiting the uptake of [3H]‐GABA into rat brain slices. 5 Comparison of possible conformations of the two unsaturated isomers by interactive computer graphics modelling and comparison with muscimol has led to a plausible active conformation of ZAPA, which may be a selective and potent agonist for low affinity GABA binding sites.

Inhibition of high affinity l-glutamic acid uptake into rat cortical synaptosomes by the conformationally restricted analogue of glutamic acid, cis-1-aminocyclobutane-1,3-dicarboxylic acid

The action of two cyclobutane derivatives of L-glutamic acid on the high affinity uptake of L-glutamic acid was investigated using a preparation of synaptosomes from rat cerebral cortex. cis-1-Aminocyclobutane-1,3-dicarboxylic acid (also known as trans-2,4-methanoglutamic acid) potently inhibited L-glutamic acid uptake (IC50 30 microM), whereas trans-1-aminocyclobutane-1,3-dicarboxylic acid (also known as cis-2,4-methanoglutamic acid), a potent N-methyl-D-aspartate (NMDA) agonist, was inactive. Analysis of the kinetics of L-glutamic acid uptake in the presence and absence of cis-1-aminocyclobutane-1,3-dicarboxylic acid (CACB) suggests that it may act as a competitive inhibitor (Ki 8 microM). CACB may be substrate for the L-glutamic acid high-affinity uptake carrier since preincubation of CACB with the synaptosomal preparation increased its potency in inhibiting L-glutamic acid uptake. The conformationally restricted structure of CACB may be indicative of the conformations of L-glutamic acid that interact with the high affinity uptake carrier.

The effects of cyclopentane and cyclopentene analogues of GABA at recombinant GABAC 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 rho(1) and rho(2) GABA(C) receptors expressed in Xenopus oocytes using two-electrode voltage clamp methods. These compounds are conformationally restricted analogues of gamma-aminobutyric acid (GABA) held in a five-membered ring. (+)-TACP (EC(50) (rho(1))=2.7+/-0.2 microM; EC(50) (rho(2))=1.45+/-0.22 microM), (+)-CACP (EC(50) (rho(1))=26.1+/-1.1 microM; EC(50) (rho(2))=20.1+/-2.1 microM) and (-)-CACP (EC(50) (rho(1))=78.5+/-3.5 microM; EC(50) (rho(2))=63.8+/-23.3 microM) were moderately potent partial agonists at rho(1) and rho(2) GABA(C) receptors, while (-)-TACP (100 microM inhibited 56% and 62% of the current produced by 1 microM GABA at rho(1) and rho(2) receptors, respectively) was a weak partial agonist with low intrinsic activity at these receptors. In contrast, (+)-4-ACPCA (K(i) (rho(1))=6.0+/-0.1 microM; K(i) (rho(2))=4.7+/-0.3 microM) did not activate GABA(C) rho(1) and rho(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) rho(1) and rho(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.

Evidence that antagonism by σ-aminovaleric acid of GABAB receptors in the guinea-pig ileum may be due to an interaction between GABAA and GABAB receptors

The antagonism by delta-aminovaleric acid has been investigated using electrically stimulated segments of isolated guinea-pig ileum. The depression of the twitch response by baclofen was antagonized by delta-aminovaleric acid and this antagonism is also mimicked by the selective GABAA agonists, 3-aminopropanesulphonic acid and isoguvacine. The GABAA antagonist bicuculline reversed the antagonism of the baclofen response. These results indicate that this antagonism of GABAB receptors may be due to GABAA receptor modulation of GABAB receptors.

RAPID ENZYME IMMUNOASSAYS FOR THE DETECTION OF CARBARYL AND METHOPRENE IN GRAINS

In order for grain handlers and traders to reliably estimate residues of grain protectants in the field, antibody-based rapid tests were developed for carbaryl (1-naphthyl methylcarbamate) and methoprene [isopropyl (E,E)-(RS)-11-methoxy-3,7,11-trimethyldodeca-2,4-dienoate]. To complement the rapid analysis, a simple and rapid extraction technique was developed. In these tests, a pesticide-containing methanol extract of the grain sample and an enzyme-labeled component are added to precoated strips. After a brief incubation, the strips are washed and a substrate/chromogen for the enzyme is added. The color developed is stopped by acidification and the results are read either by eye or in a portable field photometer. The overall test time is under 20 minutes. For carbaryl, the test had a limit of detection of 4.5 ppb (1.1 ppm in grain), while the methoprene test had a limit of detection of 4 ppb (1 ppm in grain) based on the lower datum point, which is 15% inhibition, in the standard curves. Both assays can be used as a screening test for carbaryl and methoprene in animal feed grains.

Resolution of the stereoisomers of baclofen by high performance liquid chromatography.

The GABA analogue baclofen [3-(p-chlorophenyl)-4-aminobutanoic acid] has stereospecific actions on the peripheral and central nervous systems. This paper describes the resolution of tritium-labelled baclofen by high performance liquid chromatography on a reverse-phase C18 column using a chiral mobile phase. The method, which may have general application to certain other GABA analogues, affords optically pure (+)- and (-)-baclofen labelled with tritium to high specific activity suitable for ligand binding and other neurochemical studies.

Resolution and conformational analysis of diastereoisomeric esters of cis- and trans-2-(aminomethyl)-1-carboxycyclopropanes

Abstract (1 R ,2 S )-, (1 S ,2 R )-, (1 R ,2 R )- and (1 S ,2 S )-2-(Aminomethyl)-1-carboxycyclopropanes, conformationally restricted analogues of the neurotransmitter γ-aminobutyric acid (GABA), have been resolved by chromatographic separation of the corresponding diastereoisomeric esters which were formed between the cis - and trans -2-(acetamidomethyl)-1-carboxycyclopropanes with ( R )-(−)-pantolactone. 1 H NMR, semi-empirical conformational analysis, ab initio (DFT) structure and NMR shielding tensor calculations of the cis -diastereoisomers allowed the absolute configuration assignments of the cis -amino acids.

3-Hydroxypyridazine 1-oxides as carboxylate bioisosteres: A new series of subtype-selective AMPA receptor agonists

Three positional isomers (compounds 1, 2, and 3) of 1-uracilylalanine (willardiine) based on a 3-hydroxypyridazine 1-oxide scaffold with an alanine side-chain at positions 4 (1), 5 (2) or 6 (3) were tested for binding to recombinant homomeric AMPA receptor (AMPA-R) subtypes GluR1-4, as well for excitatory activity on the rat cortical wedge preparation. 1 had approximately 30 times higher affinity than willardiine while showing a similar selectivity profile, i.e. 22-fold selectivity for GluR1/2 over GluR3/4. The GluR1-4 affinities of 3 were similar to 1, however, its 31-fold selectivity for GluR1/2 over GluR3/4 is the highest yet observed among azine-based glutamate analogues. The non-isosteric congener 2 showed weaker binding to AMPA-Rs. In the cortical wedge, 1 evokes similar responses to AMPA, while 3 and 2 are 10- and 100-fold weaker, respectively. Dose-response curves on Xenopus laevis oocytes expressing GluR1-4(flip) confirmed that 1 and 3 are potent GluR1/2 receptor agonists (EC(50)s from 0.26 to 1.7microM) but are 10- to 160-fold less potent at GluR3/4. The structures, potencies and selectivities of this new class of AMPA agonists are compared with those of willardiine, 5-fluorowillardiine and azawillardiine, referring to the binding mode observed in the crystal structure of willardiine bound to GluR2-S1S2. The results indicate that the 3-hydroxypyridazine 1-oxide moiety can function as an outstanding carboxylate mimic at AMPA-Rs, leading the way to further fine-tuning of subtype selectivity. This little-explored molecular motif may find wider application in medicinal chemistry.