Enantiomers Of Baclofen Research Articles

GABA-B agonists: enantiomeric resolution of 4-amino-3-(5-chlorothien-2-yl)butyric acid and analogues on chiral crown ether stationary phase

The enantiomeric resolution of 4-amino-3-(5-chlorothien-2-yl)butyric acid, an analogue of baclofen [4-amino-3-(4-chlorophenyl)butyric acid], was examined by HPLC using a chiral stationary phase consisting of a crown ether and perchloric acid-methanol as the mobile phase. Optimization of the separation was achieved by variation of temperature, pH and eluent composition. The absolute configuration may be assigned by comparison with authentic enantiomers of baclofen of known absolute configuration. The best results obtained were α = 1.15 and R s = 2.72 at 20°C with HClO 4 (pH 1.3)CH 3OH (90:10). The study was extended to analogues, viz., 3-(substituted thienyl) and 3-(substituted furanyl)-4-amino acids.

Direct chromatographic separation of the enantiomers of phaclofen, saclofen and hydroxysaclofen: Influence of the anionic moiety

The direct resolution of three analogues of baclofen (phaclofen, saclofen and hydroxysaclofen), potent -γ-aminobutyric acid B antagonists, is achieved by HPLC on an enantioselective crown ether column. The effects of the anionic group are discussed. Perchloric acid and methanol as organic modifier is used as mobile phase. The absolute configuration is proposed by comparison with enantiomers of baclofen (β- p-chlorophenyl-γ-aminobutyric acid). The above-mentioned compounds are easily and completely resolved.

ChemInform Abstract: Chemoenzymatic Synthesis of Both Enantiomers of Baclofen (I).

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Effects of phaclofen and the enantiomers of baclofen on cardiovascular responses to intrathecal administration of L- and D-baclofen in the rat

In a previous study it was found that i.t. administration of L-baclofen decreased arterial pressure and heart rate while D-baclofen differentially increased arterial pressure. The objective of the present study was to determine which of these effects was blocked by prior administration of the GABA B receptor antagonist, phaclofen, and whether the effect of one enantiomer of baclofen could be blocked by prior administration of the other. The decreases in systolic and diastolic arterial pressures and in heart rate produced by i.t. administration of 70 nmol of L-baclofen were unaffected by i.t. administration of 7, 70 or 700 nmol of D-baclofen 10 min prior to administration of L-baclofen, but were blocked by administration of 5 μmol of phaclofen given 3–5 min prior to L-baclofen. On the other hand, the increases in systolic and diastolic arterial pressures induced by i.t. administration of 700 nmol of D-baclofen were blocked by 70 nmol but not by 7 nmol of L-baclofen, as well as by 2.5 μmol of phaclofen; the effect of L-baclofen cannot be attributed to a desensitization of D-baclofen-sensitive receptors as two successive doses of D-baclofen given 7 min apart had quantitatively similar effects. Phaclofen alone increased systolic and diastolic arterial pressures and heart rate. The results are interpreted as indicating that D-baclofen is not an antagonist of L-baclofen in this paradigm; rather, they suggest that L-baclofen reduces the effects of D-baclofen. In addition, the increases in arterial pressure and heart rate induced by phaclofen alone suggest that it is blocking a tonic depression of sympathetic activity to the vessels and heart. Finally, the results indicate that phaclofen cannot distinguish between the effects of L- and D-baclofen.

Chemoenzymatic synthesis of both enantiomers of baclofen

Both enantiomers of baclofen have been synthesized in five steps from 4-chlorocinnamic acid. The key step is the highly stereoselective enzymatic hydrolysis of dimethyl 3-(4-chlorophenyl)glutarate by chymotrypsin.

GABA terminal autoreceptors in the pars compacta and in the pars reticulata of the rat substantia nigra are GABA B

The depolarization-evoked release of γ-aminobutyric acid (GABA) and its modulation mediated by autoreceptors were studied in superfused synaptosomes prepared from the pars compacta and from the pars reticulata of the rat substantia nigra. The release of [ 3H]GABA evoked by 9 mM KCl was almost totally calcium-dependent in both nigral subregions. In the presence of SK&F 89976A (N-(4,4-diphenyl-3-butenyl)nipecotic acid), a GABA uptake inhibitor added to minimize carrier-mediated homoexchange, GABA (0.3–10 μM) inhibited, in a concentration-dependent way, the K +-evoked overflow of [ 3H]GABA from both pars compacta and pars reticulata synaptosomes. Similarly to GABA, (−)-baclofen (0.3–10 μM) reduced the [ 3H]GABA overflow, being roughly equipotent to GABA in both nigral subregions. The (+) enantiomer of baclofen was ineffective. The overflow of [ 3H]GABA was not consistently affected by muscimol in either the pars compacta or the pars reticulata. The effects of GABA were bicuculline- and picrotoxin-insensitive. However, the inhibition by GABA of the [ 3H]GABA overflow was antagonized by phaclofen. It is concluded that (a) GABA autoreceptors are sited on GABAergic nerve endings in both the pars compacta and pars reticulata of the rat substantia nigra; (b) these autoreceptors belong to the GABA B type.

Determination of the S(+)- and R(−)-enantiomers of baclofen in plasma and urine by gas chromatography using a chiral fused-silica capillary column and an electron-capture detector

A sensitive and enantiospecific gas chromatographic method for the determination of the S(+)- and R(−)-enantiomers of baclofen (I and II) in plasma and urine has been developed and validated. The method is based on the complete resolution of the derivatized enantiomers on a chiral fused-silica capillary column. The hydrochloride salt of a (−)-fluoro analogue of baclofen (III · HCl) was used as the internal standard in plasma, the hydrochloride salt of a (+)-fluoro analogue of baclofen (IV · HCl) as the internal standard in urine. Rapid and convenient isolation of the compounds was achieved using reversed-phase Bond-Elut C 18 columns. After elution, the compounds were converted into isobutyl esters and purified by base-specific solvent extraction. The isobutyl esters were then N-acylated with heptafluorobutyric anhydride The derivatives were quantitated after separation on the chiral column using electron-capture detection. The analysis of spiked plasma and urine samples demonstrated the good accuracy and precision of the method, with limits of quantitation of 25 nmol/l for I and II in plasma and of 2 μmol/l for I and II in urine. The method appears to be suitable for use in pharmacokinetic studies of the enantiomers in plasma and urine from animals and man after administration of the racemic baclofen.

Enantiospecific high-performance liquid chromatographic (HPLC) determination of baclofen and its fluoro analogue in biological material.

A method is described for the quantification of baclofen enantiomers in biological material (urine, plasma, and cerebrospinal fluid). The samples were extracted by liquid-solid extraction using Sep-Pak C18 cartridges. The subsequent derivatization procedure contained two separate steps. (1) The butyl esters of the enantiomers were formed using butanolic hydrochloric acid (followed by ion-pair extraction of the intermediate products). (2) A chiral derivatization was then performed using S-(+)-naproxen chloride as reagent. S-(+)-Benoxaprofen chloride can also be used. The diastereomeric amides were separated by high-performance liquid chromatography (HPLC) on a silica gel column (mobile phase, n-hexane/dichloromethane/ethanol; detection, fluorescence measurement at 335/365 nm). The described procedure was also used for the quantification of the fluoro analogue of baclofen. Urinary excretion of baclofen enantiomers was investigated in two healthy volunteers after p.o. administration.

Studies on [ 3H] GABA and endogenous GABA release in rat cerebral cortex suggest the presence of autoreceptors of the GABA B type

The presence of autoreceptors for γ-aminobutyric acid (GABA) in the CNS was reinvestigated using rat cortex synaptosomes prelabeled with [ 3H]GABA and exposed to GABA by superfusion in the presence of a new GABA uptake inhibitor, N-(4,4-diphenyl-3-butenyl)-nipecotic acid (SK&F 89976A). This compound itself did not increase the basal or the depolarization-evoked release of [ 3H]GABA. GABA reduced in a concentration-dependent way the release of [ 3H]GABA evoked by 15 mM K +. The effect was not antagonized by bicuculline, picrotoxin or by the new GABA A antagonist SR 95531. The GABA A agonist muscimol did not affect [ 3H]GABA release. This was reduced by (−)baclofen (but not by the (+) isomer) and the concentration-inhibition curve of (−)baclofen was superimposable on to that of GABA. Also the K +evoked release of endogenous GABA was stereoselectively and concentration dependently inhibited by the (−) enantiomer of baclofen. It is concluded that the release of GABA from rat cortical nerve endings may be inhibited through the activation of autoreceptors which appear to belong to the GABA B type.

Stereoselectivity of spinal neurotransmission: effects of baclofen enantiomer on tail-flick reflex in rats.

Spinal rats and rats with an intact neuraxis received an intrathecal injection of an enantiomer of baclofen. The R-enantiomer was 100-1000 times more potent than its antipode in inhibiting the tail-flick reflex, both in intact rats and in spinal rats. Spinalization enhanced the inhibitory effects of both enantiomers without altering their dose-response relations. The findings show that baclofen enantiomers have direct actions on stereo-selective spinal mechanisms and that spinalization fails to alter the stereo-selectivity of spinal mechanisms towards the enantiomers.

Effects of intravenously administered enantiomers of baclofen on functionally identified units in lumbar dorsal horn of the spinal cat

The present study investigated the effects of intravenous administration of the d- and l-enantiomers of baclofen on extracellular single unit spikes recorded from the lumbar dorsal horn of the spinalized cat, either anaesthetized with chloralose or decerebrated. Both enantiomers had two types of depressant effect. One was a transient (complete within 2 min), non-specific depression of the discharge of synaptically-elicited responses and of the postsynaptic glutamate-evoked excitation; it is reasoned that this effect was due to a postsynaptic action of baclofen and its antagonism by bicuculline suggests it is via classical GABA receptors. The other type of depression was more prolonged, lasting usually for more than 1 hr. It was observed as a preferential blocking of spontaneous activity, as well as of evoked responses to noxious stimuli or innocuous thermal stimuli applied to the skin, with little or no effect on responses to hair movement or to the iontophoretic application of glutamate. In addition, baclofen preferentially reduced the late, higher threshold component of evoked responses of wide dynamic range neurones to electrical stimulation of the cutaneous receptive field. Thus, whether tested on naturally- or electrically-evoked responses, baclofen had a preferential effect on those mediated by small diameter afferents. Doses giving approximately equal effects were 0.1 mg/kg for l-baclofen and 10 mg/kg for d-baclofen. The results of the present study support recent evidence of a prolonged antinociceptive effect of baelofen, indicate that at least some of this effect is mediated by an action at the spinal level and support the possibility that the action is presynaptic, perhaps on the terminals of primary afferent fibres. In addition, a similar mechanism may account for the depression of spinal polysynaptic reflexes and thus, perhaps, additionally for some of the antispastic properties of baclofen.