GABAB antagonists: resolution, absolute stereochemistry, and pharmacology of (R)- and (S)-phaclofen

Structural mechanism of ligand activation in human GABA(B) receptor

Human GABA(B) (γ-aminobutyric acid class B) receptor is a G-protein-coupled receptor central to inhibitory neurotransmission in the brain. It functions as an obligatory heterodimer of the subunits GBR1 and GBR2. Here we present the crystal structures of a heterodimeric complex between the extracellular domains of GBR1 and GBR2 in the apo, agonist-bound and antagonist-bound forms. The apo and antagonist-bound structures represent the resting state of the receptor; the agonist-bound complex corresponds to the active state. Both subunits adopt an open conformation at rest, and only GBR1 closes on agonist-induced receptor activation. The agonists and antagonists are anchored in the interdomain crevice of GBR1 by an overlapping set of residues. An antagonist confines GBR1 to the open conformation of the inactive state, whereas an agonist induces its domain closure for activation. Our data reveal a unique activation mechanism for GABA(B) receptor that involves the formation of a novel heterodimer interface between subunits.

Enantiomeric analysis of baclofen analogs by capillary zone electrophoresis, using highly sulfated cyclodextrins: Inclusion ionization constant pKa determination

Using cyclodextrin-capillary zone electrophoresis (CD-CZE), baseline separation of baclofen phaclofen, saclofen, and hydroxy-saclofen, potent gamma-aminobutyric acid(B) (GABA(B)) agonist or antagonists was achieved. A method for the enantioresolution of those analogs of GABA was developed using anionic cyclodextrins (highly sulfated CD or highly S-CD) as chiral selectors and capillaries dynamically coated with polyethylene oxide (PEO). With charged CDs we observed good resolutions due to the large electrophoretic mobility of these chiral selectors opposite to the mobility of the solutes. The highly S-alpha-CD and S-beta-CD were found to be complementary and the most effective complexing agent, allowing good enantiomeric resolution in short runtimes. The complete resolution was obtained using 25 mM phosphate buffer at pH 2.5 containing 3% w/v of highly S-alpha-CD or S-beta-CD at 25 degrees C with an applied field of 0.30 kV/cm. The apparent binding constants of the inclusion complexes were evaluated and the migration order was determined. A comparison was possible to investigate the importance of the anionic group of the molecules in the separations. The pK(a) values were determined for all four compounds in order to explain relative electrophoretic migration of the solutes.

Synthesis and pharmacology of the baclofen homologues 5-amino-4-(4-chlorophenyl)pentanoic acid and the R- and S-enantiomers of 5-amino-3-(4-chlorophenyl)pentanoic acid

(RS)-5-Amino-4-(4-chlorophenyl)pentanoic acid (10) and the R-form (11) and S-form (12) of (RS)-5-amino-3-(4-chlorophenyl)pentanoic acid, which are homologues of the 4-aminobutanoic acidB (GABAB) receptor agonist (RS)-4-amino-3-(4-chlorophenyl)butanoic acid (baclofen), were synthesized. Compound 10 was synthesized by homologation at the carboxyl end of baclofen using a seven-step reaction sequence. N-Boc-protected (4R, 5R)-4-(4-chlorophenyl)-5-hydroxy-2-piperidone (18) was deoxygenated via a modified Barton-McCombie reaction to give N-Boc-protected (R)-4-(4-chlorophenyl)-2-piperidone (20), which was ring opened and deprotected to give 11.HCl. The corresponding S-enantiomer, 12.HCl, was synthesized analogously from the 4S,5S-enantiomer of 18, compound 21. The enantiomeric purities of 11.HCl (ee = 99.8%) and 12. HCl (ee = 99.3%) were determined by chiral HPLC. Compound 10 did not show detectable affinity for GABAA or GABAB receptor sites and was inactive as an agonist or an antagonist at GABAB receptors in the guinea pig ileum. Like the enantiomers of baclofen, neither 11 nor 12 showed detectable affinity for GABAA receptor sites, and in agreement with the findings for (S)-baclofen, 12 did not interact significantly with GABAB receptor sites. Compound 11 (IC50 = 7.4 +/- 0.6 microM), a homologue of (R)-baclofen (2), was shown to be some 50 times weaker than 2 (IC50 = 0.14 +/- 0.01 microM) as an inhibitor of GABAB binding. Accordingly, 11 (EC50 = 150 +/- 23 microM) was shown to be weaker than 2 (EC50 = 11 +/- 1 microM) as an inhibitor of electrically induced contractions of the guinea pig ileum. However, whereas this effect of 2 was sensitive to the GABAB antagonist, CGP35348 (4), the inhibition by 11 was not significantly affected. Furthermore, 12 (EC50 = 310 +/- 16 microM) was shown to be one-half as potent as 11 in this test system, and this effect of 12 also was insensitive to 4. The dissimilarities of the pharmacological effects of 2 and compounds 11 and 12 were emphasized by the observation that whereas 2 only inhibits the ileum contraction by 59 +/- 5%, 11 as well as 12 were shown to inhibit this response by approximately 94%. Neither 11 nor 12 appeared to affect significantly cholinergic mechanisms in the ileum, and their mechanism(s) of action remain enigmatic.

GABAB receptor antagonism by resolved (R)-saclofen in the guinea-pig ileum

The GABAB receptor antagonist saclofen (3-amino-2-(4-chlorophenyl)propylsulphonic acid) has been resolved by chiral high-performance liquid chromatography. The enantiomer (R)-saclofen, but not (S)-saclofen, reversibly antagonised the (R,S)-baclofen-induced depression of cholinergic twitch contractions in the guinea-pig ileum with an apparent pA2 of 5.3. Also, 2-hydroxy-saclofen was resolved by the same method, its (S)-enantiomer yielding an apparent pA2 of 5.0. This method provides a convenient resolution of these antagonists.

The (S)-enantiomer of 2-hydroxysaclofen is the active GABAB receptor antagonist in central and peripheral preparations

In the guinea-pig isolated ileum, (RS)-(+/-)-baclofen induced a depression of cholinergic twitch contractions, reversibly and competitively antagonised by (S)-2-hydroxysaclofen (pA2 = 5.2 +/- 0.2), but not by (R)-2-hyroxysaclofen. The depression of excitatory field potentials by baclofen ( 5 mu M) in rat CA1 hippocampal slices was antagonised by (S)-2-hydroxysaclofen (100 mu m) (pA2 = 4.3), whilst in rat neocortex, (S)-2-hyroxysaclofen (50-500 mu M) antagonised the baclofen (10 mu M)-induced suppression of spontaneous discharges, the (R)-enantiomer being inactive. These results show that (S)-2-hydroxysaclofen is the active antagonist at central and peripheral GABAB receptors.

GABAB receptors

GABAB receptors are a distinct subclass of receptors for the major inhibitory transmitter 4-aminobutanoic acid (GABA) that mediate depression of synaptic transmission and contribute to the inhibition controlling neuronal excitability. The development of specific agonists and antagonists for these receptors has led to a better understanding of their physiology and pharmacology, highlighting their diverse coupling to different intracellular effectors through Gi/G(o) proteins. This review emphasises our current knowledge of the neurophysiology and neurochemistry of GABAB receptors, including their heterogeneity, as well as the therapeutic potential of drugs acting at these sites.