Isoflurane anesthesia is stereoselective

General Anesthetics: Aspects of Chirality, Pharmacodynamics, and Pharmacokinetics

The introduction of general anesthetics in the mid-19th century is considered one of the greatest contributions to medical practice. It was the first time that complicated surgical interventions became feasible, without putting an excessive strain on the patient. The first general anesthetics-diethyl ether, chloroform, and nitrous oxide-were limited by often severe adverse reactions and a narrow therapeutic window. They were later succeeded by modern anesthetics, with high anesthetic effect along with diminished toxicity. As with other medical drugs, many anesthetic compounds contain chiral centers in their molecules. Although currently used as racemates, the pharmacological activity of the respective enantiomerically pure antipodes can vary considerably, as can their adverse effects. Herein, we report on the available studies into the differences in bioactivity and toxicity between the enantiomers of chiral anesthetic agents. Both inhalational and intravenous anesthetics are discussed. Aspects of pharmacodynamics and pharmacokinetics are surveyed as well. The results could stimulate further research into the potential application of single-enantiomer anesthetics in clinical practice.

General anaesthetics as 'awakening agents'? Re-appraising the evidence for suggested 'pressure reversal' of anaesthesia

Increasing ambient pressure has been suggested to reverse general anaesthesia and provides support for the 'lipid theory'. Anaesthetic dissolution into cell membranes is said to cause their expansion to a critical volume. This triggers a sequence of events as basis of a unitary theory of anaesthestic mechanism. Pressure is argued to restore membrane volume to below critical level, reversing this process. We wished to review the original literature to assess internal consistency within and across papers, and to consider if alternative interpretations were possible. A literature search yielded 31 relevant 'pressure reversal' papers for narrative review, and 8 papers that allowed us to re-plot original data more consistently as 'dose-response' curves for the anaesthetics examined. Original studies were heterogenous for end-points, pressure ranges, species, and agents. Pressure effects were inconsistent, with narcosis at certain pressures and excitation at others, influenced by carrier gas (e.g., nitrogen vs helium). Pressure reversal (a right- or downward-shift on the re-plotted dose-response curves) was evident, but only in some species and at certain pressures and anaesthetic concentrations. However, even more striking was a novel 'awakening' effect of anaesthetics: i.e., anaesthetics reversed the narcotic effect of pressure, but this was limited to certain pressures at generally low anaesthetic concentrations. Contrary to the established view, 'pressure reversal' is not a universal phenomenon. The awakening effect of anaesthetics - described here for the first time - has equal evidence to support it, within the same literature, and is something that cannot be fully explained. Pressure cannot meaningfully be used to gain insight into anaesthetic mechanisms because of its heterogenous, non-specific and unpredictable effects on biological systems.

Towards Quantum-Chemical Modeling of the Activity of Anesthetic Compounds

The modeling of the activity of anesthetics is a real challenge because of their unique electronic and structural characteristics. Microscopic approaches relevant to the typical features of these systems have been developed based on the advancements in the theory of intermolecular interactions. By stressing the quantum chemical point of view, here, we review the advances in the field highlighting differences and similarities among the chemicals within this group. The binding of the anesthetics to their partners has been analyzed by Symmetry-Adapted Perturbation Theory to provide insight into the nature of the interaction and the modeling of the adducts/complexes allows us to rationalize their anesthetic properties. A new approach in the frame of microtubule concept and the importance of lipid rafts and channels in membranes is also discussed.

Electrophysiological-mechanical coupling in the neuronal membrane and its role in ultrasound neuromodulation and general anaesthesia

The current understanding of the role of the cell membrane is in a state of flux. Recent experiments show that conventional models, considering only electrophysiological properties of a passive membrane, are incomplete. The neuronal membrane is an active structure with mechanical properties that modulate electrophysiology. Protein transport, lipid bilayer phase, membrane pressure and stiffness can all influence membrane capacitance and action potential propagation. A mounting body of evidence indicates that neuronal mechanics and electrophysiology are coupled, and together shape the membrane potential in tight coordination with other physical properties. In this review, we summarise recent updates concerning electrophysiological-mechanical coupling in neuronal function. In particular, we aim at making the link with two relevant yet often disconnected fields with strong clinical potential: the use of mechanical vibrations-ultrasound-to alter the electrophysiogical state of neurons, e.g., in neuromodulation, and the theories attempting to explain the action of general anaesthetics. STATEMENT OF SIGNIFICANCE: General anaesthetics revolutionised medical practice; now an apparently unrelated technique, ultrasound neuromodulation-aimed at controlling neuronal activity by means of ultrasound-is poised to achieve a similar level of impact. While both technologies are known to alter the electrophysiology of neurons, the way they achieve it is still largely unknown. In this review, we argue that in order to explain their mechanisms/effects, the neuronal membrane must be considered as a coupled mechano-electrophysiological system that consists of multiple physical processes occurring concurrently and collaboratively, as opposed to sequentially and independently. In this framework the behaviour of the cell membrane is not the result of stereotypical mechanisms in isolation but instead emerges from the integrative behaviour of a complexly coupled multiphysics system.

Benzodiazepine-like discriminative stimulus effects of toluene vapor

In vitro studies show that the abused inhalant toluene affects a number of ligand-gated ion channels.The two most consistently implicated of these are γ-aminobutyric acid type A(GABAA) receptors which are positively modulated by toluene and N-methyl-D-aspartate(NMDA) receptors which are negatively modulated by toluene. Behavioral studies also suggest an interaction of toluene with GABAA and/or NMDA receptors but it is unclear if these receptors underlie the abuse-related intoxicating effects of toluene. Seventeen B6SJLF1/J mice were trained using a two-choice operant drug discrimination procedure to discriminate 10 min of exposure to 2000 ppm toluene vapor from 10 min of exposure to air. The discrimination was acquired in a mean of 65 training sessions. The stimulus effects of 2000 ppm toluene vapor were exposure concentration-dependent but rapidly diminished following the cessation of vapor exposure. The stimulus effects of toluene generalized to the chlorinated hydrocarbon vapor perchloroethylene but not 1,1,2-trichloroethane nor the volatile anesthetic isoflurane. The competitive NMDA antagonist CGS-19755, the uncompetitive antagonist dizocilpine and the glycine-site antagonist L701,324 all failed to substitute for toluene. The classical nonselective benzodiazepines midazolam and chlordiazepoxide produced toluene-like stimulus effects but the alpha 1 subunit preferring positive GABAA modulator zaleplon failed to substitute for toluene. The barbiturates pentobarbital and methohexital and the GABAA positive modulator neurosteroid allopregnanolone did not substitute for toluene. These data suggest that the stimulus effects of toluene may be at least partially mediated by benzodiazepine-like positive allosteric modulation of GABAA receptors containing alpha 2, 3 or 5 subunits.

Stereoselectivity of isoflurane in adhesion molecule leukocyte function-associated antigen-1

BACKGROUND

Isoflurane in clinical use is a racemate of S- and R-isoflurane. Previous studies have demonstrated that the effects of S-isoflurane on relevant anesthetic targets might be modestly stronger (less than 2-fold) than R-isoflurane. The X-ray crystallographic structure of the immunological target, leukocyte function-associated antigen-1 (LFA-1) with racemic isoflurane suggested that only S-isoflurane bound specifically to this protein. If so, the use of specific isoflurane enantiomers may have advantage in the surgical settings where a wide range of inflammatory responses is expected to occur. Here, we have further tested the hypothesis that isoflurane enantioselectivity is apparent in solution binding and functional studies.

METHODS

First, binding of isoflurane enantiomers to LFA-1 was studied using 1-aminoanthracene (1-AMA) displacement assays. The binding site of each enantiomer on LFA-1 was studied using the docking program GLIDE. Functional studies employed the flow-cytometry based ICAM binding assay.

RESULTS

Both enantiomers decreased 1-AMA fluorescence signal (at 520 nm), indicating that both competed with 1-AMA and bound to the αL I domain. The docking simulation demonstrated that both enantiomers bound to the LFA-1 "lovastatin site." ICAM binding assays showed that S-isoflurane inhibited more potently than R-isoflurane, consistent with the result of 1-AMA competition assay.

CONCLUSIONS

In contrast with the x-ray crystallography, both enantiomers bound to and inhibited LFA-1. S-isoflurane showed slight preference over R-isoflurane.

Chirality and anaesthetic drugs: A review and an update

Many molecules can exist as right-handed and left-handed forms that are non-superimposable mirror images of each other. They are known as enantiomers or substances of opposite shape. Such compounds are also said to be chiral (Greek chiros meaning ‘hand’). Such chiral molecules are of great relevance to anaesthetic theory and practice. This review summarizes the basic concepts, pharmacokinetic and pharmacodynamic aspects of chirality, and some specific examples of their application in anaesthesia, along with recent advances to elucidate the anaesthetic mechanisms. Chirality is relevant to anaesthesia, simply because more than half of the synthetic agents used in anaesthesia practice are chiral drugs. Almost all these synthetic chiral drugs are administered as racemic mixture, rather than as single pure enantiomers. These mixtures are not drug formulations containing two or more therapeutic substances, but combination of isomeric substances, with the therapeutic activity residing mainly in one of the enantiomer. The other enantiomer can have undesirable properties, have different therapeutic activities or be pharmacologically inert. Specific examples of application of chirality in anaesthetic drugs include inhalational general anaesthetics (e.g. isoflurane), intravenous anaesthetics (e.g. etomidate, thiopentone), neuromuscular blocking agents (e.g. cisatracurium), local anaesthetics (e.g. ropivacaine and levobupivacaine) and other agents (e.g. levosimendan, dexmedetomidine, L-cysteine). In the recent advances, chirality study has not only helped new drug development as mentioned above, but has also contributed in a more profound way to the understanding of the mechanism of anaesthesia and anaesthetic drugs.

GABA(A) positive modulator and NMDA antagonist-like discriminative stimulus effects of isoflurane vapor in mice

RATIONALE

Several neurotransmitter systems have been hypothesized to be involved in the in vivo effects of volatile anesthetics. Drug discrimination may represent a novel procedure to explore the neurochemical systems underlying the sub-anesthetic behavioral effects of these compounds.

OBJECTIVES

The purpose of the present study was to examine the contribution of GABA(A) and NMDA receptors to the discriminative stimulus effects of a behaviorally active sub-anesthetic concentration of isoflurane vapor.

METHODS

Sixteen B6SJLF1/J mice were trained to discriminate 10 min of exposure to 6,000 ppm isoflurane vapor from air. Substitution tests were conducted with volatile anesthetics, abused vapors, GABA(A) positive modulators, NMDA antagonists, and nitrous oxide.

RESULTS

The volatile anesthetics, enflurane and halothane as well as the abused vapors toluene and 1,1,1-trichloroethane fully substituted for isoflurane. The GABA(A) positive modulators, pentobarbital, midazolam, and zaleplon but not the direct GABA(A) agonist, muscimol, produced high levels of partial substitution for isoflurane. The anticonvulsant, valproic acid fully substituted for isoflurane but a second, tiagabine, did not substitute. The competitive NMDA antagonist, CGS-19755, fully and the non-competitive NMDA antagonist, dizocilpine, partially substituted for isoflurane. The glycine-site NMDA antagonist, L-701,324 did not substitute for isoflurane. Gamma-hydroxybutric acid and nitrous oxide gas also failed to substitute for isoflurane.

CONCLUSIONS

The discriminative stimulus effects of sub-anesthetic concentrations of isoflurane vapor are shared by other vapor anesthetics and abused inhalants. The discriminative stimulus effects of isoflurane vapor appear to be mediated by both positive allosteric modulation of GABA(A) receptors as well as antagonism of NMDA receptors.

New insights into the molecular mechanisms of general anaesthetics

This paper provides new insights of how general anaesthetic research should be carried out in the future by an analysis of what we know, what we do not know and what we would like to know. I describe previous hypotheses on the mechanism of action of general anaesthetics (GAs) involving membranes and protein receptors. I provide the reasons why the GABA type A receptor, the NMDA receptor and the glycine receptor are strong candidates for the sites of action of GAs. I follow with a review on attempts to provide a mechanism of action, and how future research should be conducted with the help of physical and chemical methods.

Species-Specific Differences in Response to Anesthetics and Other Modulators by the K2P Channel TRESK

TRESK (TWIK-related spinal cord K+ channel) is the most recently characterized member of the tandem-pore domain potassium channel (K2P) family. Human TRESK is potently activated by halothane, isoflurane, sevoflurane, and desflurane, making it the most sensitive volatile anesthetic-activated K2P channel yet described. Herein, we compare the anesthetic sensitivity and pharmacologic modulation of rodent versions of TRESK to their human orthologue. Currents passed by mouse and rat TRESK were enhanced by isoflurane at clinical concentrations but with significantly lower efficacy than human TRESK. Unlike human TRESK, the rodent TRESKs are strongly inhibited by acidic extracellular pH in the physiologic range. Zinc inhibited currents passed by both rodent TRESK in the low micromolar range but was without effect on human TRESK. Enantiomers of isoflurane that have stereoselective anesthetic potency in vivo produced stereospecific enhancement of the rodent TRESKs in vitro. Amide local anesthetics inhibited the rodent TRESKs at almost 10-fold smaller concentrations than that which inhibit human TRESK. These results identified interspecies differences and similarities in the pharmacology of TRESK. Further characterization of TRESK expression patterns is needed to understand their role in anesthetic mechanisms.

IMPLICATIONS

Mouse and rat TRESK (TWIK-related spinal cord K+ channel) have different pharmacologic responses compared with human TRESK. In particular, we found stereospecific differences in response to isoflurane by the rodent TRESKs but not by human TRESK. TRESK may be a target site for the mechanism of action of volatile anesthetics.

Chirality: a blueprint for the future

The chirality that is inherent in the enzyme systems of living organisms results in an abundance of enantiopure organic molecules in the living world. In addition to the optical properties first noticed by Pasteur, stereospecific interactions at recognition sites result in differences in both biological and toxicological effects. This fact underlies the continuing growth in chiral chemistry, rooted as it is in fundamental biochemistry. The pharmaceutical industry has undergone a strategic shift and embraced the wide spectrum of asymmetrical synthetic methods now available. The use of these processes in developmental synthesis and large-scale manufacturing has provided new challenges in drug discovery, motivated by a desire to improve industrial efficacy and decrease the time from the conception of a new drug to the market. The economic impact of the industrial production of chiral drugs is now huge--more than 50% of the 500 top-selling drugs were single-enantiomers in 1997. Sales have continued to increase by more than 20% for the past 6 yr and worldwide annual sales of enantiomeric drugs exceeded US$100 billion for the first time in the year 2000, chiral drugs representing close to one-third of all sales worldwide. While some 'chiral switches' may be of less apparent benefit, or indeed detrimental in some cases, encouragement by the regulatory agencies and the ability to extend the life cycle of a drug coming off patent promotes the trend. However, it may turn out to be the ability to provide chiral templates, and thereby attack the key targets of selectivity and specificity, that will lead to the greatest benefits. Research into new chemical entities that can interact specifically with enzyme families may potentially lead to new therapies for complex disease processes. As Richards has stated, the approach is designed to create a made to measure product, rather than one off the peg.

Anesthetics and ion channels: molecular models and sites of action

The mechanisms of general anesthesia in the central nervous system are finally yielding to molecular examination. As a result of research during the past several decades, a group of ligand-gated ion channels have emerged as plausible targets for general anesthetics. Molecular biology techniques have greatly accelerated attempts to classify ligand-gated ion channel sensitivity to general anesthetics, and have identified the sites of receptor subunits critical for anesthetic modulation using chimeric and mutated receptors. The experimental data have facilitated the construction of tenable molecular models for anesthetic binding sites, which in turn allows structural predictions to be tested. In vivo significance of a putative anesthetic target can now be examined by targeted gene manipulations in mice. In this review, we summarize from a molecular perspective recent advances in our understanding of mechanisms of action of general anesthetics on ligand-gated ion channels.

Is anesthesia caused by potentiation of synaptic or intrinsic inhibition? Recent insights into the mechanisms of volatile anesthetics

Volatile anesthetics modulate synaptic (GABAA receptor-mediated) and intrinsic (K+ channel-controlled) neuronal inhibition. GABAA receptor activity is enhanced, leading to increased charge transfer and prolonged synaptic inhibition, and members of the two pore domain family of potassium channels are activated, leading to neuronal hyperpolarization and reduced excitability. These effects may underlie different components of the complex anesthetic state.

Headspace gas chromatography-mass spectrometry analysis of isoflurane enantiomers in blood samples after anesthesia with the racemic mixture

Several in vivo and in vitro studies on the stereoselective potency of isoflurane enantiomers suggest beneficial effects of the (+)-(S)-enantiomer. In order to detect possible differences in the pharmacokinetics of isoflurane enantiomers, a clinical study of 41 patients undergoing general anesthesia maintained with racemic isoflurane was performed. The isoflurane enantiomers were analyzed in blood samples drawn before induction, at cessation (day 0), and up to eight days after isoflurane anesthesia (day 1-8). A multipurpose sampler (Gerstel MPS) was used for the headspace gas chromatography-mass spectrometry (GC/MS) analysis, and it was combined with a cold injection system (Gerstel CIS 3) for coldtrapping, enrichment, and focusing of the analyte. The enantiomer separation was achieved by using a capillary column coated with octakis(3-O-butanoyl-2,6-di-O-pentyl)-gamma-cyclodextrin (Lipodex E) dissolved in the polysiloxane PS 255. Detection was done in the selected ion monitoring mode with ions m/z 117 and m/z 149. An enrichment of (+)-(S)-isoflurane in all blood samples drawn after anesthesia was found. The highest enantiomer bias, up to 52-54% (+)-(S)-isoflurane as compared to 50% for the racemate, was observed on day 2 for most of the patients. Furthermore, quantification of isoflurane in blood samples of five patients was done by enantiomer labeling, employing enantiomerically pure (+)-(S)-isoflurane as internal standard. The isoflurane concentration decreased rapidly from 383 nmol/ml to 0.6 nmol/ml (mean values) eight days after anesthesia. The present study shows differences in the pharmacokinetics of isoflurane enantiomers in man. However, it is not possible to distinguish between enantioselective distribution and enantioselective metabolism, if any.

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.

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.

Neuronal nicotinic acetylcholine receptors: a new target site of ethanol

Whereas a variety of neuroreceptors and ion channels have been demonstrated to be affected by ethanol including GABAA receptors, NMDA receptors, non-NMDA glutamate receptors, 5-HT3 receptors and voltage-gated calcium channels, neuronal nicotinic acetylcholine receptors (nnAChRs) have recently emerged as a new target site of ethanol. The nnAChRs are different from the muscle type nicotinic AChRs with respect to their molecular architecture and pharmacology. This article briefly reviews the structure, distribution and function of nnAChRs for which a considerable amount of information has been rapidly accumulated during the past 5-10 years. The potent and unique action of ethanol on nnAChRs has been unveiled only during the past few years. Most recent developments along this line of ethanol action are discussed in this paper.

Sigmoidal compression rate-dependence of inert gas narcotic potency in rats: implication for lipid vs. protein theories of inert gas action in the central nervous system

Inert gases at raised pressure exert anaesthetic effects. It is assumed that anaesthesia by the inert gases is fundamentally similar to anaesthesia produced by general anaesthetics. However, do general anaesthetics bind directly to proteins or influence activity by indirectly perturbing membrane lipids still remains a major question. Although the pressure required to achieve anaesthesia with inert gases has been suggested to exert potentially some pressure antagonism per se, this has not been studied yet to our knowledge. We investigated this possibility using nitrogen, argon, and nitrous oxide. Whatever the narcotic agent used, our results showed that the pressure of narcotic required to induce anaesthetic effects increased, as compression rate increased, in a sigmoid fashion rather than in a linear fashion. Evidence for sigmoïdal responses vs. linear responses depended of the narcotic potency of the anaesthetic agent used (nitrogen: r2=0.973 vs. r2=0.941; argon: r2=0. 971 vs. r2=0.866; nitrous oxide: r2=0.995 vs. r2=0.879). Since a linear antagonism is predicted by lipid theories, we think it likely that these findings indicate that inert gases bind to a modulatory site of a protein receptor and act as allosteric modulators. Since other workers provided evidence for binding processes using volatile anaesthetics, the present findings could indicate that all classes of general anaesthetics, including inert gases, could act by binding directly to proteins rather than by dissolving in some lipids of the cellular membrane.

Desflurane and the nonimmobilizer 1,2-dichlorohexafluorocyclobutane suppress learning by a mechanism independent of the level of unconditioned stimulation

We previously demonstrated that anesthetics and non-immobilizers suppress learning and memory in rats. In the training portion of the test, rats received a light plus a footshock and learned to associate the two, as evidenced by subsequent potentiation of the response (jumping) to light plus a noise (fear-potentiated startle). However, anesthetics and nonimmobilizers also decreased the response of animals receiving footshocks during training, which suggests that the reduction in fear-potentiated startle might reflect analgesia, rather than an impairment of learning and memory. Furthermore, although we previously demonstrated that the nonimmobilizer 2,3-dichlorohexafluorocyclobutane (2N) could completely abolish learning, we did not demonstrate the minimal dose required. In the present study, we eliminated analgesia as a confounding factor by training rats breathing desflurane and 2N with footshock intensities that produced responses at least equal to those produced in control animals. Both desflurane and 2N suppressed learning at 0.2 times the minimum alveolar anesthetic concentration (MAC) or the MAC predicted from lipid solubility, despite the increased footshock intensity. This partial pressure of desflurane equals that previously shown to suppress learning at lower footshock intensities. We conclude that suppression of learning and memory by desflurane and 2N does not result from decreased sensitivity to the unconditioned stimulus (the footshock) and that the potency of 2N is consistent with its lipophilicity.

IMPLICATIONS

General anesthesia eliminates recall of intraoperative events, including pain. Using an animal model, we refuted the hypothesis that lack of recall results from the analgesia (i.e., the reduced response to painful stimuli produced by inhaled drugs) rather than from a direct effect on learning.

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.

Minimum alveolar anesthetic concentration values for the enantiomers of isoflurane differ minimally

Results of in vivo and in vitro studies of the anesthetic potencies of the enantiomers (optical isomers) of isoflurane provide various results ranging from no difference to differences of nearly two fold. A finding of a difference in anesthetic requirement in the whole animal has particular relevance to theories of anesthetic mechanisms of action because it suggests that anesthesia may result from a specific anesthetic-receptor interaction. This led to our decision to redetermine the minimum alveolar anesthetic concentration (MAC) of (+)-S and (-)-R enantiomers of isoflurane in 12 Sprague-Dawley rats (six per group). The (+)-S enantiomer gave a MAC of 0.0144 +/- 0.0012 atm (i.e., 1.44% +/- 0.12% at 1 atm pressure; mean +/- SD) and the (-)-R enantiomer gave a MAC of 0.0169 +/- 0.0020 atm. Although the 17% greater value for the (-)-R enantiomer is qualitatively consistent with previous results the difference is not significant (P = 0.06), and the absolute difference is smaller than that found by a previous study. However, given the small sample size, our power to define a small significant difference is limited. Regardless of statistical significance, our results do not confirm the conclusion that interaction with a specific receptor is important to the mechanism of action of inhaled anesthetics.

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.

[Esters and stereoisomers]

This review discusses concepts of isomers, stereoisomers, chirality, and enantiomers as applied to drugs used in anaesthesia. The inhalational anaesthetics enflurane and isoflurane are examples of stereoisomers. A chiral centre is formed when a carbon or quaternary nitrogen atom is connected to four different atoms. A molecule with one chiral centre is then present in one of two possible configurations termed enantiomers. A racemate is a mixture of both enantiomers in equal proportions. Many of the drugs used in anaesthesia are racemic mixtures (the inhalation anaesthetics, local anaesthetics, ketamine, and others). The shape of the atracurium molecule is comparable to that of a dumb-bell:the two isoquinoline groups representing the two bulky ends connected by an aliphatic chain. In each isoquinoline group there are two chiral centres, one formed by a carbon and the other by a quaternary nitrogen atom. From a geometric point of view, the connections from the carbon atom to a substituted benzene ring and from the quaternary nitrogen to the aliphatic chain may point in the same direction (cis configuration) or in opposite directions (trans configuration). The two isoquinoline groups in atracurium are paired in three geometric configurations: cis-cis, trans-trans, or cis-trans. However, the two chiral centres allow each isoquinoline group to exist in one of four stereoisometric configurations. In the symmetrical atracurium molecule, the number of possible stereoisomers is limited to ten. Among these, 1 R-cis, 1'R-cis atracurium was isolated and its pharmacologic properties studied. This isomer, named cis-atracurium, offers clinical advantages over the atracurium mixture, principally due to the lack of histamine-releasing propensity and the higher neuromuscular blocking potency. The ester groups appear in one of two steric configurations true and reverse esters. In the true esters, oxygen is positioned between the nitrogen atom and the carbonyl group, while in the reverse esters in its positioned on the other side of the carbonyl group. True esters, suxamethonium and mivacurium, are hydrolysed by the enzyme plasma cholinesterase (butyrylcholinesterase), albeit at different rates. The more rapid degradation of suxamethonium is responsible for its fast onset and short duration of action in comparison with mivacurium. The reverse esters, atracurium, cisatracurium, and remifentanil, are hydrolysed by nonspecific esterases in plasma (carboxyesterases). Remifentanil is hydrolysed rapidly; the degradation leads to its inactivation and short duration of action. Cis-atracurium is preferentially degraded and inactivated by a process known as Hofmann elimination. In a second step, one of the degradation products, the monoester acrylate, is hydrolysed by a nonspecific esterase.

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.

Effects of inhalational general anaesthetics on native glycine receptors in rat medullary neurones and recombinant glycine receptors in Xenopus oocytes

1. Glycine responses were studied under voltage clamp in Xenopus oocytes injected with cDNA encoding mammalian glycine receptor subunits and in rat medullary neurones. Bath application of glycine gave strychnine-sensitive currents which reversed close to the expected equilibrium potentials for chloride ions. The peak currents for the receptors expressed in oocytes fitted a Hill equation with EC50 = 215 +/- 5 microM and Hill coefficient nH = 1.70 +/- 0.05 (means +/- s.e. means). The peak currents from the receptors in medullary neurones fitted a Hill equation with EC50 = 30 +/- 1 microM and Hill coefficient nH = 1.76 +/- 0.08. The current-voltage relationship for the receptors expressed in oocytes showed strong outward rectification (with Vrev = -21 +/- 2 mV), while that for the glycine responses from the medullary neurones in symmetrical Cl- was linear (with Vrev = 3.2 +/- 0.6 mV). 2. Inhalational general anaesthetics, at concentrations close to their human minimum alveolar concentrations (MACs), potentiated responses to low concentrations of glycine. The potentiation observed with the recombinant receptors (between 60-22%) was approximately twice that found with the medullary neurones (between 40-80%). For both the recombinant receptors and the receptors in medullary neurones, the degree of potentiation increased in the order of methoxyflurane approximately sevoflurane < halothane approximately isoflurane approximately enflurane. There was no significant difference between the potentiations observed for the two optical isomers of isoflurane. 3. For both the recombinant and native receptors, isoflurane potentiated the currents in a dose-dependent manner at low concentrations of glycine, although at high glycine concentrations the anaesthetic had no significant effect on the glycine-activated responses. The major effect of isoflurane was to cause a parallel leftward shift in the glycine concentration-response curves. The glycine EC50 concentration for the recombinant receptors decreased from a control value of 215 +/- 5 microM to 84 +/- 7 microM glycine at 610 microM isoflurane, while that for the medullary neurones decreased from a control value of 30 +/- 1 microM to 18 +/- 2 microM glycine at the same concentration of isoflurane. The potentiation was independent of membrane potential. 4. Isoflurane also potentiated responses to taurine, a partial agonist at the glycine receptor. This was observed for receptors expressed in oocytes at both low and saturating concentrations of taurine. The EC50 concentration decreased from a control value of 1.6 +/- 0.2 to 0.9 +/- 0.1 mM taurine in the presence of 305 microM isoflurane, while the maximum response to taurine increased from 47 +/- 2 to 59 +/- 2% of the maximum response to glycine. 5. Glycine receptors, like other members of the fast ligand-gated receptor superfamily, are sensitive to clinically relevant concentrations of inhalational general anaesthetics. Effects at these receptors may, therefore, play some role in the maintenance of the anaesthetic state.

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(-).

Isomerism and anaesthetic drugs

Isomers are two or more different substances with the same molecular formula (i.e., the same number of different types of atoms). There are two main types of isomerism: 1) structural isomerism, and 2) steroisomerism. Structural isomers (e.g., enflurane and isoflurane) have different molecular structures, and usually behave like different drugs. Occasionally, structural isomers are interconvertible (i.e., they are tautomers or dynamic isomers); this occurs with the barbiturates and midazolam. Steroisomers have identical structures, but a different configuration or spatial arrangement. Stereiosomerism in drugs is often due to chirality or "handedness"; i.e., the presence of right-handed (R)- and left-handed (S)- forms of drugs which are nonsuperimposable mirror images ("enantiomers"). Approximately 60% of anaesthetic agents are chiral drugs; some of these are administered as single enantiomers. However, many synthetic chiral drugs are equal mixtures of (R)- and (S)-isomers, and there are often important differences in their activity and pharmacokinetics. Halothane, enflurane, and isoflurane are chiral drugs with different anaesthetic potencies. Similar differences occur with intravenous anaesthetics; thus, (S) (+)-ketamine causes fewer psychotic emergence reactions, less agitated behaviour, and better intraoperative amnesia and analgesia than its enantiomer. Some local anaesthetics are administered as chiral mixtures; the (S)-isomers have a longer action because of enhanced vasoconstriction. (S)-prilocaine is more slowly metabolized than its enantiomer, while (S)-bupivacaine may produce less cardiotoxicity than (R)-bupivacaine. These differences suggest that some anaesthetic drugs (particularly ketamine and chiral local anaesthetics) should be administered as single enantiomers. In recent years, their synthesis has been greatly simplified, and almost all new drugs may soon be introduced in this form.(ABSTRACT TRUNCATED AT 250 WORDS)

Actions of general anaesthetics on a neuronal nicotinic acetylcholine receptor in isolated identified neurones of Lymnaea stagnalis

1. Completely isolated identified neurones from the right parietal ganglion of the pond snail Lymnaea stagnalis were studied under two-electrode voltage-clamp. Neuronal nicotinic acetylcholine receptor currents were studied at low acetylcholine (ACh) concentrations (< or = 200 nM). At these levels, control currents were non-desensitizing and proportional to the square of the ACh concentration. 2. IC50 concentrations were determined for the steady-state inhibition of the ACh-activated current by 31 general anaesthetics plus the non-anaesthetic alcohol n-tridecanol. The general anaesthetics included inhalational agents, n-alcohols, n-alkane-(alpha,omega)-diols, cycloalcohols and an n-alkane. 3. Anaesthetic inhibition was independent of voltage and consistent with two anaesthetic-binding sites on the receptor. 4. IC50 concentrations for inhibiting the neuronal nicotinic ACh receptor correlated well (r = 0.97) with EC50 concentrations for general anaesthesia. The maximum deviation from the line of identity was less than fourfold. The inhalational agents tended to be more potent as inhibitors of the ACh receptor than as general anaesthetics, while the alcohols and diols were less potent. 5. The inhibition of the ACh-induced current by the homologous series of n-alcohols exhibited a cutoff at the same position (just after dodecanol) as found for the induction of general anaesthesia in tadpoles. 6. Polarity profile maps of the anaesthetic-binding sites on the neuronal nicotinic ACh receptor were calculated from IC50 concentrations for the homologous series of n-alcohols and n-alkane-(alpha,omega)-diols. They reveal amphiphilic sites with apolar regions capable of accommodating the hydrocarbon chains of n-alcohols as large as decanol. A striking resemblance was found to profiles previously calculated from data for tadpole general anaesthesia.

Genetic differences affecting the potency of stereoisomers of halothane

The mechanism of action of volatile anesthetics is the subject of some debate. Much of the controversy has centered on whether the site of such actions is purely lipid in nature or may contain a protein target. This report studies the interaction of stereoisomers of halothane on the wild type and on a variety of genetic mutants of Caenorhabditis elegans. The mutants studied have previously been shown to have altered sensitivities to volatile anesthetics. In one mutant, fc34, (R)-halothane [the (+) isomer] was 3 times more potent than its S (-) isomer. Other mutants and wild-type animals displayed more modest differences in sensitivity to the enantiomers. The results indicate that a genetic pathway exists in C. elegans controlling sensitivity to halothane and that both lipid and protein targets may mediate halothane's effects.

The potential for safer anaesthesia using stereoselective anaesthetics

The molecular mechanisms by which inhalational agents produce anaesthesia remains a subject of controversy, despite a history of clinical use spanning two centuries. The demonstration of a significant difference in the anaesthetic potencies of (+)- and (-)-isoflurane provides compelling evidence for the hypothesis that proteins, rather than lipids, are the primary sites of anaesthetic action. Moreover, the optically active isomers of volatile anaesthetics provide new tools to discriminate among putative molecular targets of anaesthesia. A difference in the anaesthetic potencies of (+)- and (-)-isoflurane, together with an apparent lack of stereoselectivity in their myocardial suppression, raises the possibility that an optically active volatile agent may have clinical advantages over currently available racemic mixtures.

The stereospecific effects of isoflurane isomers in vivo

The anesthetic potency of racemic isoflurane and the optically pure stereoisomers was examined in rats. The (+) isomer was 53% more potent than the (-) isomer (minimum alveolar concentration (MAC) = 1.06 +/- 0.07% vs. 1.62 +/- 0.02%, P < 0.05). MAC for racemic isoflurane was 1.32 +/- 0.03%. Both stereoisomers and the racemic isoflurane produced similar depression of arterial pressure. However, the (+) isomer blunted the cardiovascular response to a painful stimulus to a greater extent than did an equi-MAC dose of the (-) isomer. These are the first data to describe pharmacological differences between stereoisomers of a volatile anesthetic administered in vivo by the conventional route (inhaled) and measuring the clinically relevant index of anesthesia, MAC. These data are consistent with a receptor-mediated anesthetic mechanism by volatile anesthetics.