The potential for safer anaesthesia using stereoselective anaesthetics

What makes a molecule an anaesthetic? Studies on the mechanisms of anaesthesia using a physicochemical approach

Recent studies of mechanisms of anaesthesia have been mainly 'target orientated', investigating the activity of both volatile and i.v. agents at putative sites of action. An alternative approach is one that is 'ligand orientated', focusing on the properties of molecules that define their immobilizing ability and secondly define their potency. The use of conventional descriptors (such as non-polar solubility or the octanol-water partition coefficient [Log P]) are limited in their utility as predictors of potency as they represent three-dimensional molecular properties as a one-dimensional parameter. Using different computer-based molecular modelling methods (molecular similarity studies and comparative molecular field analysis [CoMFA]), we have identified the molecular bases of the activity of structurally diverse anaesthetics, such that they can be described as a single model based on the spatial distribution of molecular bulk and electrostatic potential. The same approach can also be used to model other properties of anaesthetic agents, such as cardiovascular depression. The present data suggest that, for the i.v. agents, it may be difficult to separate immobilizing (anaesthetic) activity and cardiovascular depression within a single molecule.

Lack of stereoselectivity for the antiallodynic effect of mexiletine in spinally injured rats

Systemically administered mexiletine, an antiarrhythmic, has been shown to also possess analgesic properties in some conditions of neuropathic pain. It has been suggested that the analgesic effect of mexiletine may be derived from the action of one of its optical isomers, (+)(S)-mexiletine. In the present study, we have compared the effects of systemic (-)-(R)- and (+)-(S)-mexiletine, on chronic mechanical allodynia-like behaviour in spinally injured rats, a model of central neuropathic pain in which racemic mexiletine has been shown to be active. I.p. racemic mexiletine as well as (-)-(R)- and (+)(S)-mexiletine at 25 mg/kg all produced significant, but brief, alleviation of mechanical allodynia in a similar fashion as assessed with von-Frey hair elicited vocalization in the spinally injured rats. A slight increase in motor impairment was observed in all three groups which reached statistical significance for the racemic mexiletine and (+)-(S)-mexiletine. Our results suggest that both isomers of mexiletine contribute to the antiallodynic effect in this model of central pain.

The impact of chirality of the fluorinated volatile inhalation anaesthetics on their clinical applications

This review discusses the various chromatographic enantioseparation methods on an analytical and preparative scale for fluorinated inhalation anaesthetics used clinically, namely halothane, enflurane, desflurane and isoflurane. The differences in the pharmacodynamics and pharmacokinetics between the enantiomers of those anaesthetics are presented. It can be concluded that using a single enantiomer for these fluorinated anaesthetics is advantageous over using the racemic mixture. The racemic switch to a single enantiomer for these fluorinated volatile anaesthetics offers a more effective and safe general anaesthetic.

Pharmacological receptors: a century of discovery--and more

A brief survey of the history of the development of the concept of the pharmacological receptor is presented. From the pioneering concepts of Paul Ehrlich, John Langley and others, receptors are described in terms of their recognition properties, their structures, transducing abilities and the impact of genomics and their role in contributing to genetic diseases. The receptor concept has firmly underpinned our advances in drug development and molecular medicine of the latter half of this century and it is clear that it will continue to drive pharmaceutical developments in the 21st century.

General anaesthetic actions on ligand-gated ion channels

The molecular mechanisms of general anaesthetics have remained largely obscure since their introduction into clinical practice just over 150 years ago. This review describes the actions of general anaesthetics on mammalian neurotransmitter-gated ion channels. As a result of research during the last several decades, ligand-gated ion channels have emerged as promising molecular targets for the central nervous system effects of general anaesthetics. The last 10 years have witnessed an explosion of studies of anaesthetic modulation of recombinant ligand-gated ion channels, including recent studies which utilize chimeric and mutated receptors to identify regions of ligand-gated ion channels important for the actions of general anaesthetics. Exciting future directions include structural biology and gene-targeting approaches to further the understanding of general anaesthetic molecular mechanisms.

Effects of isoflurane and hexafluorodiethyl ether on human recombinant GABA(A) receptors expressed in Sf9 cells

Effects of volatile anesthetics and a volatile convulsant on human recombinant gamma-aminobutyric acid (GABA) type A receptor responses were studied using the whole cell configuration of the patch clamp technique. Sf9 cells were transfected with bacuroviruses carrying cDNAs of alpha1beta2, alpha1beta2gamma2s, alpha3beta2 and alpha3beta2gamma2s subunit combinations of the human GABA(A) receptor. Clinical concentrations of isoflurane (a volatile anesthetic) enhanced the GABA-induced current of the alpha1beta2gamma2s and alpha3beta2gamma2s GABA(A) subunit combinations. On the other hand, isoflurane suppressed the current of the alpha1beta2 and alpha3beta2 subunit combinations, indicating that the anesthetic effects depended upon the presence of gamma2s subunit. A high concentration (2 mM) of isoflurane generated a surge current following the washout of GABA and the anesthetic. Hexafluorodiethyl ether (a volatile convulsant) decreased the GABA-response of the both alpha3beta2gamma2s and alpha3beta2 constructs without generating a surge current. The results suggest that volatile agents affect the receptor-ionophore complex via direct interaction with proteins but not through a perturbation of the membrane lipid environment. A hypothetical sequential model for the anesthetic action is presented.

Volatile anesthetics and a volatile convulsant differentially affect GABA(A) receptor-chloride channel complex

1. General anesthetics at clinical concentrations are known to affect neurotransmitter-gated ion channels in postsynaptic membranes. 2. Volatile anesthetics suppress excitatory transmissions, whereas they potentiate inhibitory chloride currents evoked by GABA or glycine. Hexafluorodiethyl ether (a volatile convulsant) markedly depresses the GABA(A) response but not the glycine-evoked chloride current or the glutamate-induced excitatory response. 3. Molecular biology has revealed that GABA(A) receptor is a heteromeric pentamer composed of alpha, beta, gamma, delta, and/or rho subunits. In baculovirus-Sf9 expression system, the gamma subunit was crucial for potentiation of the GABA-induced chloride current by volatile anesthetics. 4. Following sustained presence of GABA and high concentrations of isoflurane, simultaneous washout of both agents evoked a slowly decaying surge current, whose nature is controversial.

Kinetics of sevoflurane action on GABA- and glycine-induced currents in acutely dissociated rat hippocampal neurons

Effects of a new kind of volatile anaesthetics, sevoflurane, on GABA- and glycine-gated chloride current (ICl) were examined in single pyramidal neurons acutely dissociated from the rat hippocampal CA1 region, using the voltage-clamp mode of the nystatin-perforated patch-clamp technique. Rapid application of sevoflurane-induced ICl by itself, with the time to peak reduced as the sevoflurane concentration was increased from 10(-3) to 3 x 10(-3) M. Although a pretreatment with 10(-3) M sevoflurane enhanced the peak amplitude of GABA (3 x 10(-6) M)-induced ICl and suppressed the peak amplitude when the GABA concentration was increased to 10(-4) M, the pretreatment decreased the time to peak of the ICl induced by any concentration of GABA (from 3 x 10(-6) to 10(-4) M). The treatment also accelerated the decay phase of the GABA-induced ICl. On the other hand, sevoflurane suppressed the peak ICl induced by 3 x 10(-5) M glycine in a concentration-dependent manner. In the presence of 3 x 10(-4) M sevoflurane, the peak amplitude of the glycine-induced ICl was decreased without changes in EC50 or Hill coefficients. Pretreatment with 10(-3) M sevoflurane did not affect the time to peak of the ICl induced by any concentration of glycine (from 3 x 10(-5) to 10(-3) M). Pretreatment with 3 x 10(-8) M strychnine markedly prolonged the time to peak of the glycine-induced ICl. These results suggest that sevoflurane modulated the amplitude of the GABA responses, depending on the balance of the accelerated activation and decay phases, and that sevoflurane suppressed the glycine-induced ICl in a non-competitive manner without noticeable effect on the kinetics. The reversible and differential modulation of GABA(A) and glycine receptors might underlie a part of the anaesthetic actions and less adverse clinical effects of sevoflurane.

Stereoselective actions of halothane at GABA(A) receptors

Isoflurane anesthesia exhibits stereoselectivity, and a corresponding stereoselectivity ((+)->(-)-isomer) has been reported at GABA(A) receptors in vitro. The objective of the present study was to determine if the positive modulatory actions of halothane at GABA(A) receptors exhibited a similar stereoselectivity. Both (R)- and (S)-halothane ((+)- and (-)- isomers, respectively) enhanced [3H]flunitrazepam binding to brain membranes in a concentration dependent manner without a significant difference in either potency (EC50) or efficacy (Emax). While both (R)- and (S)-halothane enhanced [3H]muscimol binding, the potency of the (+)-isomer was slightly greater than the corresponding (-)-isomer (0.91 +/- 0.17 versus 1.45 +/- 0.04% atmospheres, respectively (P < 0.02)). Thus, subtle structural differences between inhalational anesthetics can have a significant impact on the degree of stereoselectivity at the receptor level and may provide insights for the development of more specific drugs.

Preparative enantiomer separation of the inhalation anesthetics enflurane, isoflurane and desflurane by gas chromatography on a derivatized gamma-cyclodextrin stationary phase

The preparative enantiomeric separation of the inhalation anesthetics enflurane (1) and isoflurane (2) in very high chemical (> 99.5%) and enantiomeric excess (ee > 99%) by gas chromatography (GC) on octakis(3-O-butanoyl-2,6-di-O-n-pentyl)-gamma-cyclodextrin (4), dissolved in the apolar polysiloxane SE-54 and coated on Chromosorb P AW DMCS, is described. Up to 1 g of each enantiomer of 1-2 can been obtained per diem. The enantiomers of the highly volatile desflurane (3) can also be separated, albeit with diminished ee. The enantiomeric excess of 1-3 was checked by analytical GC on 4 and the absolute configuration of 2 and 3 has been determined via anomalous X-ray diffraction.

Stereochemistry in anaesthesia

1. Interest in the pharmacokinetic and pharmacodynamic properties of the enantiomers of chiral drugs has greatly increased in recent years. This is particularly so for agents used in anaesthesia. 2. Chiral compounds are those that can exist in two nonsuperimposable forms. Each form is termed an enantiomer or stereoisomer. Two naming systems are in use: one uses the terms (+) and (-) to indicate the direction the compound will rotate polarized light, while the other system, based on the absolute three-dimensional structure of the enantiomers, uses the terms R and S. 3. Investigation of the stereoisomers of the volatile anaesthetic agent isoflurane is increasing our understanding of the mechanism of general anaesthesia. Current evidence suggests a protein, rather than a lipid, receptor site. 4. Investigation of the stereoisomers of local anaesthetics is increasing the safety of these drugs. 5. For bupivacaine, a widely used amide local anaesthetic, important enantiomeric differences can be found for toxicity, clinical effect and pharmacokinetics. In particular S-(-)-bupivacaine has an improved central nervous system and cardiac safety profile. This is partly explained by the pharmacokinetic differences. 6. Based on these differences, ropivacaine, a propyl homologue of bupivacaine, has been produced solely as the S-(-)-enantiomer. The available evidence suggests significantly improved safety for this agent over racemic bupivacaine.

Approach to the thermodynamics of enantiomer separation by gas chromatography. Enantioselectivity between the chiral inhalation anesthetics enflurane, isoflurane and desflurane and a diluted gamma-cyclodextrin derivative

The thermodynamics of enantioselectivity, -delta D,L(delta G), -delta D,L(delta H), delta D,L(delta S) and Tiso, have been determined by gas chromatography employing the concept of the retention increment R' for the inhalation anesthetics enflurane (1), isoflurane (2) and desflurane (3) and the selector octakis(3-O-butanoyl-2,6-di-O-n-pentyl)-gamma-cyclodextrin (4) in the polysiloxane SE-54. It is shown that the separation factor alpha is concentration-dependent. Therefore, the separation factor alpha should not be employed as a criterion for enantioselectivity in diluted systems. The -delta DL(delta G) data for 1 and 4 are corroborated by 1H NMR spectroscopic measurements.

19F nuclear magnetic resonance investigation of stereoselective binding of isoflurane to bovine serum albumin

Whether proteins or lipids are the primary target sites for general anesthetic action has engendered considerable debate. Recent in vivo studies have shown that the S(+) and R(-) enantiomers of isoflurane are not equipotent, implying involvement of proteins. Bovine serum albumin (BSA), a soluble protein devoid of lipid, contains specific binding sites for isoflurane and other anesthetics. We therefore conducted 19F nuclear magnetic resonance measurements to determine whether binding of isoflurane to BSA was stereoselective. Isoflurane chemical shifts were measured as a function of BSA concentration to determine the chemical shift differences between the free and bound isoflurane. KD was determined by measuring the 19F transverse relaxation times (T2) as a function of isoflurane concentration. The binding duration was determined by assessing increases in 1/T2 as a result of isoflurane exchanging between the free and bound states. The S(+) and R(-) enantiomers exhibited no stereoselectivity in chemical shifts and KD values (KD = 1.3 +/- 0.2 mM, mean +/- SE, for S(+), R(-), and the racemic mixture). Nonetheless, stereoselectivity was observed in dynamic binding parameters; the S(+) enantiomer bound with slower association and dissociation rates than the R(-).