Isomerism and anaesthetic drugs

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.

Topological Isomerism in Three-Dimensional Covalent Organic Frameworks

Although isomerism is a typical and significant phenomenon in organic chemistry, it is rarely found in covalent organic framework (COF) materials. Herein, for the first time, we report a controllable synthesis of topological isomers in three-dimensional COFs via a distinctive tetrahedral building unit under different solvents. Based on this strategy, both isomers with a dia or qtz net (termed JUC-620 and JUC-621) have been obtained, and their structures are determined by combining powder X-ray diffraction and transmission electron microscopy. Remarkably, these architectures show a distinct difference in their porous features; for example, JUC-621 with a qtz net exhibits permanent mesopores (up to ∼23 Å) and high surface area (∼2060 m² g^-1), which far surpasses those of JUC-620 with a dia net (pore size of ∼12 Å and surface area of 980 m² g^-1). Furthermore, mesoporous JUC-621 can remove dye molecules efficiently and achieves excellent iodine adsorption (up to 6.7 g g^-1), which is 2.3 times that of microporous JUC-620 (∼2.9 g g^-1). This work thus provides a new way for constructing COF isomers and promotes structural diversity and promising applications of COF materials.

Risk assessment of occupational exposure to anesthesia Isoflurane in the hospital and veterinary settings

Despite the modern ventilation and waste anesthetic gas (WAG) scavenging systems, occupational exposure to common volatile anesthesia, isoflurane, can occur in the hospital and veterinary settings, but limited information exists on potential exposure and health risk of isoflurane. We assessed exposure dose rates and risks among clinicians and veterinary professionals from occupational exposure to isoflurane. Through a critical review of open literature (1965 to 2020), we summarized potential adverse effects and exposure scenarios of isoflurane among the professional groups, including anesthetists, nurses, operating room personnel, researchers, and/or veterinarians. Deterministic United States National Research Council/Environmental Protection Agency's risk assessment framework (hazard identification, dose-response relationship, exposure assessment and risk characterization) was used to compute inhalation Reference Doses (RfDs), Average Daily Doses (ADDs), and Hazard Quotient (HQ) values-an established measure of non-carcinogenic (systemic) risks-from exposure to isoflurane to workers in hospital and veterinary settings. We identified the central nervous system as the main target for isoflurane, and that isoflurane has dose-dependent effects on cardiac hemodynamics, can impair pulmonary functions and potentially cross the utero-placental barrier leading to congenital malformation in fetus. Based on the modelled RfDs (range 0.8003-7.55 mg/kg-day) and ADDs (range 0.071-1.9617 mg/kg-day), we estimated 56 different HQ values, of which 5 HQs were higher than 1 (range 1.099-2.4512) under high exposure scenarios. Our results suggest a significant non-carcinogenic risk from isoflurane exposures among workers in the occupational settings. The findings underscore the need to significantly minimize isoflurane release to protect workers' health in the hospital and veterinary environments.

Molecular determinants of pro-arrhythmia proclivity of d- and l-sotalol via a multi-scale modeling pipeline

Drug isomers may differ in their proarrhythmia risk. An interesting example is the drug sotalol, an antiarrhythmic drug comprising d- and l- enantiomers that both block the hERG cardiac potassium channel and confer differing degrees of proarrhythmic risk. We developed a multi-scale in silico pipeline focusing on hERG channel – drug interactions and used it to probe and predict the mechanisms of pro-arrhythmia risks of the two enantiomers of sotalol. Molecular dynamics (MD) simulations predicted comparable hERG channel binding affinities for d- and l-sotalol, which were validated with electrophysiology experiments. MD derived thermodynamic and kinetic parameters were used to build multi-scale functional computational models of cardiac electrophysiology at the cell and tissue scales. Functional models were used to predict inactivated state binding affinities to recapitulate electrocardiogram (ECG) QT interval prolongation observed in clinical data. Our study demonstrates how modeling and simulation can be applied to predict drug effects from the atom to the rhythm for dl-sotalol and also increased proarrhythmia proclivity of d- vs. l-sotalol when accounting for stereospecific beta-adrenergic receptor blocking.

Chiral Aspects of Local Anesthetics

Thanks to the progress made in chemical technology (particularly in the methodologies of stereoselective syntheses and analyses) along with regulatory measures, the number of new chiral drugs registered in the form of pure enantiomers has increased over the past decade. In addition, the pharmacological and pharmacokinetic properties of the individual enantiomers of already-introduced racemic drugs are being re-examined. The use of the pure enantiomer of a drug that has been used to date in the form of a racemate is called a "chiral switch". A re-examination of the properties of the pure enantiomers of racemates has taken place for local anesthetics, which represent a group of drugs which have long been used. Differences in (R) and (S)-enantiomers were found in terms of pharmacodynamic and pharmacokinetic activity as well as in toxicity. Levobupivacaine and robivacaine were introduced into practice as pure (S)-(-)-enantiomers, exhibiting more favorable properties than their (R)-(+)-stereoisomers or racemates. This overview focuses on the influence of chirality on the pharmacological and toxicological activity of local anesthetics as well as on individual HPLC and capillary electrophoresis (CE) methods used for enantioseparation and the pharmacokinetic study of individual local anesthetics with a chiral center.

Indistinguishable odour enantiomers: Differences between peripheral and central-nervous electrophysiological responses

The ability of humans to discriminate enantiomeric odour pairs is substance -specific. Current literature suggests that psychophysical discrimination of odour enantiomers mainly depends on the peripheral processing at the level of the olfactory sensory neurons (OSN). To study the influence of central processing in discrimination, we investigated differences in the electrophysiological responses to psychophysically indistinguishable (+)- and (-)- rose oxide enantiomers at peripheral and central-nervous levels in humans. We recorded the electro-olfactogram (EOG) from the olfactory epithelium and the EEG-derived olfactory event-related potentials (OERP). Results from a psychophysical three alternative forced choice test indicated indistinguishability of the two odour enantiomers. In a total of 19 young participants EOG could be recorded in 74 and OERP in 95% of subjects. Significantly different EOG amplitudes and latencies were recorded in response to the 2 stimuli. However, no such differences in amplitude or latency emerged for the OERP. In conclusion, although the pair of enantiomer could be discriminated at a peripheral level this did not lead to a central-nervous/cognitive differentiation of the two stimuli.

Ketamine enantiomers in the rapid and sustained antidepressant effects

Recent evidence has suggested that the N-methyl-D-aspartate receptor antagonist ketamine shows significant therapeutic effects in major depression and bipolar disorder. This effect is especially important in treatment-resistant depression and depression with suicidal ideation. In this review we explain the mechanism of action, drug efficacy, and the side effects of ketamine; the antidepressive effects of ketamine; the individual effects of ketamine isomers, R(-) ketamine and S(+) ketamine; the effects of the combination of ketamine with electroconvulsive therapy; and the possible use of ketamine in treating depression.

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.

Pharmacologically active compounds in the environment and their chirality

Pharmacologically active compounds including both legally used pharmaceuticals and illicit drugs are potent environmental contaminants. Extensive research has been undertaken over the recent years to understand their environmental fate and toxicity. The one very important phenomenon that has been overlooked by environmental researchers studying the fate of pharmacologically active compounds in the environment is their chirality. Chiral drugs can exist in the form of enantiomers, which have similar physicochemical properties but differ in their biological properties such as distribution, metabolism and excretion, as these processes (due to stereospecific interactions of enantiomers with biological systems) usually favour one enantiomer over the other. Additionally, due to different pharmacological activity, enantiomers of chiral drugs can differ in toxicity. Furthermore, degradation of chiral drugs during wastewater treatment and in the environment can be stereoselective and can lead to chiral products of varied toxicity. The distribution of different enantiomers of the same chiral drug in the aquatic environment and biota can also be stereoselective. Biological processes can lead to stereoselective enrichment or depletion of the enantiomeric composition of chiral drugs. As a result the very same drug might reveal different activity and toxicity and this will depend on its origin and exposure to several factors governing its fate in the environment. In this critical review a discussion of the importance of chirality of pharmacologically active compounds in the environmental context is undertaken and suggestions for directions in further research are made. Several groups of chiral drugs of major environmental relevance are discussed and their pharmacological action and disposition in the body is also outlined as it is a key factor in developing a full understanding of their environmental occurrence, fate and toxicity. This review will be of interest to environmental scientists, especially those interested in issues associated with environmental contamination with pharmacologically active compounds and chiral pollutants. As the review will outline current state of knowledge on chiral drugs, it will be of value to anyone interested in the phenomenon of chirality, chiral drugs, their stereoselective disposition in the body and environmental fate (212 references).

Total intravenous anaesthesia with propofol-racemic ketamine and propofol-S-ketamine: a comparative study and haemodynamic evaluation in dogs undergoing ovariohysterectomy

Total intravenous anaesthesia (TIVA) with propofol and ketamine proved to be very satisfactory from a clinical point of view. This blind randomised controlled trial was designed to compare induction and maintenance of anaesthesia under continuous infusion of propofol-racemic ketamine (PRK) with that of propofol-S-ketamine (PSK) and evaluate their haemodynamic, metabolic and ventilatory effects. Seven female dogs undergoing ovariohysterectomy were involved in each group. Anaesthesia was induced: in Group PRK, with propofol (4.0mg kg-1) and racemic ketamine (2.0mg kg-1) intravenous (i.v.), followed by i.v. infusion of propofol (initial dose of 0.5mg kg-1 min-1) and racemic ketamine (0.2mg kg-1 min-1); in Group PSK, with propofol (4.0mg kg-1) and S-ketamine (1.0 mg kg¹) i.v., followed by i.v. infusion of propofol (initial dose of 0.5mg kg-1 min-1) and S-ketamine (0.1mg kg-1 min-1). Parameters were assessed before anaesthesia and in 6 time points after induction. In both groups, heart rate increased significantly at all time points. There was a slight decrease in systemic blood pressure, cardiac output and cardiac index in both groups. The systolic index decrease significantly and intense respiratory depression was observed in all groups, making assisted ventilation necessary.

Comparison of ketamine and S(+)-ketamine, with romifidine and diazepam, for total intravenous anesthesia in horses

OBJECTIVE

To compare the quality of induction and recovery, degree of muscle relaxation, clinically apparent potency and cardiopulmonary effects of racemic ketamine or S(+)-ketamine when used for total intravenous anesthesia in horses.

STUDY DESIGN

Prospective randomized clinical trial

ANIMALS

Sixteen healthy stallions (323 +/- 99 kg), with a mean age of 6.2 years, undergoing castration.

METHODS

Horses were pre-medicated with romifidine IV, 15 minutes before induction of anesthesia. Each animal was then randomly allocated to receive either diazepam and ketamine (DK) or diazepam and S(+)-ketamine (DKS) at similar doses to induce anesthesia. For maintenance of anesthesia, 1/4 of the initial bolus of ketamine alone or S(+)-ketamine alone was administered, as required. Heart rate (HR), respiratory rate (RR) and systolic blood pressure were measured before and at 10-minute intervals during recumbency. Time from induction to lateral recumbency, time from induction to first additional dose, time from last additional dose to return to sternal posture and time from last additional dose to standing were recorded, and a subjective evaluation of quality of induction, endotracheal intubation, muscle relaxation and quality of recovery was recorded.

RESULTS

The quality of the induction and duration of anesthesia were similar in both groups. HR, RR and systolic blood pressure were not significantly different between groups. Although some animals which received DKS showed some minor excitatory effects (25% of them) during the induction of anesthesia, these animals received 32% fewer doses for the maintenance of anesthesia and the recovery scores were better.

CONCLUSIONS AND CLINICAL RELEVANCE

S(+)-ketamine showed some advantages over racemic ketamine, such as less anesthetic agent being required and better overall recovery from anesthesia. Further studies are needed to obtain the optimum induction dose for the S(+)-ketamine.

A double-blind comparison of intrathecal S(+) ketamine and fentanyl combined with bupivacaine 0.5% for Caesarean delivery

BACKGROUND

In this prospective, randomized, double-blind, controlled study, we investigated the sensory, motor and analgesic block characteristics of S(+) ketamine, fentanyl and saline given intrathecally (IT) in addition to 0.5% plain bupivacaine (10 mg) for spinal analgesia.

METHODS

Ninety ASA I or II adult patients undergoing Caesarean section were randomly allocated to receive 1.0 mL of 0.9% saline in Group S (n = 30), 0.05 mg kg-1 of S(+) ketamine (1.0 mL) in Group K (n =30) or 25 microg (1.0 mL) of fentanyl in Group F (n =30) following 10 mg of plain bupivacaine 0.5% IT. We recorded onset and duration of sensory and motor block, time to reach the maximal dermatomal level of sensory block and duration of spinal analgesia.

RESULTS

The onset time of sensory and motor block was significantly shorter in Groups K and F than in Group S (P < 0.014). Their duration was significantly longer in Group F than in Groups K and S (P < 0.009). The time to reach the maximal dermatomal level of sensory block was significantly shorter in Groups K and F than in Group S (P < 0.001). The duration of spinal analgesia was significantly longer in Group F than in Groups K and S (P < 0.001).

CONCLUSION

In patients undergoing Caesarean section with spinal analgesia, the addition of S(+) ketamine (0.05 mg kg-1) IT to 10 mg of spinal plain bupivacaine (0.5%) led to rapid onset of both sensory and motor blockade and enhanced the segmental spread of spinal block without prolonging the duration of spinal analgesia, whereas fentanyl provided prolonged analgesia.

Imaging pain modulation by subanesthetic S-(+)-ketamine

Little is known about the effects of low-dose S-(+)-ketamine on the cerebral processing of pain. We investigated the effects of subanesthetic IV S-(+)-ketamine doses on the perception of experimental painful heat stimuli. Healthy volunteers were evaluated with functional magnetic resonance imaging (fMRI) while receiving the painful stimuli in conjunction with placebo and increasing doses (0.05, 0.1, 0.15 mg x kg(-1) x h(-1)) of ketamine infusion. Vital variables were monitored and all subjects rated pain intensity and unpleasantness on a numerical rating scale. Alterations in consciousness were measured using a psycho-behavioral questionnaire. Pain unpleasantness declined as ketamine dosage was increased (55.1% decrease, placebo versus 0.15 mg x kg(-1) x h(-1) ketamine). Pain intensity ratings also decreased with increasing ketamine dosage but to a lesser extent (23.1% decrease). During placebo administration, a typical pain activation network (thalamus, insula, cingulate, and prefrontal cortex) was found, whereas decreased pain perception with ketamine was associated with a dose-dependent reduction of pain-induced cerebral activations. Analysis of the dose-dependent ketamine effects on pain processing showed a decreasing activation of the secondary somatosensory cortex (S2), insula and anterior cingulate cortex. This part of the anterior cingulate cortex (midcingulate cortex) has been linked with the affective pain component that underlines the potency of ketamine in modulating affective pain processing.

Role of Analgesic Adjuncts in Postoperative Pain Management

Postoperative pain remains a major problem. A multi-modal analgesic approach is recommended to optimize pain management and reduce opiate-related adverse effects. Several analgesic adjuncts have been investigated, and many have proved to have a useful analgesic effect. This article reviews the literature regarding use of analgesic adjuncts in the perioperative period.

Motor nerve blockade potency and toxicity of non-racemic bupivacaine in rats

BACKGROUND

Racemic [RS(+/-)] bupivacaine can be associated with severe cardiotoxicity. The S(-) isomer is known to be less neuro- and cardiotoxic, but demonstrates a lower potency to block motor activity than RS(+/-) bupivacaine. Thus, the potency and toxicity of a non-racemic bupivacaine mixture were studied.

METHODS

Gastrocnemic muscle twitches induced by electrical stimulation of sciatic nerves in rats were used to compare the impact by bupivacaine solutions on motor activity. Field stimulation at 1 Hz eliciting ventricular muscle twitches was used to investigate the effects on cardiac contractility. The lethal dose of each local anesthetic agent was determined following drug infusions during general anesthesia in mechanically ventilated rats.

RESULTS

Non-racemic (75S:25R) bupivacaine was more potent (P<0.05) than S(-) or R(+) enantiomers to block motor nerve activity. The concentrations of RS(+/-), 75S:25R, R(+) and S(-) bupivacaine to inhibit nerve conduction by 50% were 0.84 (0.37- 2.20), 0.84 (0.47-2.48), 2.68 (0.98-3.42) and 2.11 mM (1.5-4.03), respectively. Pronounced reductions in ventricular muscle twitches were observed with RS(+/-) and R(+) bupivacaine at low concentrations (0.5-4 microM). Lethal doses for 75S:25R (39.9 mg kg(-1)), and S(-) (34.7 mg kg(-1)) were higher (P<0.05) than for R(+) (16.2 mg kg(-1)) and RS(+/-) bupivacaine (18.4 mg kg(-1)), respectively.

DISCUSSION

The potency of S(-) bupivacaine to block the motor activity in the sciatic nerve was enhanced when 25% of the S(-) isomer was replaced by the antipode R(+) bupivacaine. This effect was not associated with increased toxicity.

The Effects of S(+)-Ketamine and Racemic Ketamine on Uterine Blood Flow in Chronically Instrumented Pregnant Sheep

Ketamine could be a useful maternal analgesic in obstetric surgery, as it might avoid the need for opioid administration and associated side effects in the newborn. Racemic ketamine passes the placental barrier and has oxytocin-like properties but does not seem to affect uterine blood flow (UBF). S(+)-ketamine was recently approved for clinical use, but its effects on UBF have not been evaluated. Therefore, we studied the effects of S(+)-ketamine on maternal and fetal hemodynamic variables. Equianalgesic doses of S(+)-ketamine (10 mg.kg(-1).h(-1)) or racemic ketamine (20 mg.kg(-1).h(-1)) were infused in 12 chronically instrumented pregnant sheep. Maternal and fetal vital signs, blood gases, and UBF were recorded over 120 min. Neither compound affected uterine perfusion or maternal and fetal hemodynamics. Whereas racemic ketamine increased maternal (+19%) and fetal (+11%) PCO(2) significantly, S(+)-ketamine was without effect. However, both compounds significantly decreased maternal (racemic, -0.05; S(+), -0.03) and fetal (racemic, -0.06; S(+), -0.02) pH. The effects of racemic ketamine and S(+)-ketamine on uterine perfusion are similar, and because of its limited effect on hemodynamics and respiration, S(+)-ketamine might therefore be of interest as an analgesic in the obstetric setting.

IMPLICATIONS

The effects of S(+)-ketamine on uterine perfusion and maternal/fetal hemodynamics are similar to those of the racemic mixture in chronically instrumented pregnant sheep. A decreased effect of S(+)-ketamine, as compared with the racemic mixture, on maternal and fetal PCO(2) levels was noted.

Stereochemistry in clinical medicine: a neurological perspective

Stereoisomers are compounds that have identical sets of atoms configured in the same positions but are arranged differently spatially. Approximately 25% of contemporary drugs are marketed and used as racemates (i.e., as equimolar mixtures of stereoisomers). This may have major clinical implications, as stereoisomers may possess qualitative and/or quantitative differences in pharmacological effects, plasma protein and tissue binding, metabolic and renal clearance. There are many examples of racemic drugs manufactured and used as single stereoisomers in the field of neurology including the anti-Parkinsonian drugs levodopa, selegiline, apomorphine and entacapone, the antiepileptic drugs tiagabine and levetiracetam, the secondary stroke prevention agent clopidogrel and the acetylcholinesterase inhibitor rivastigmine. The role of drug stereochemistry in the re-evaluation of established drugs and the production of new agents is becoming increasingly important as pharmaceutical companies endeavour to show proof of "no penalty" for the introduction of a racemic new drug over one or other of its single stereoisomers.

Conformation, action, and mechanism of action of neuromuscular blocking muscle relaxants

Since curare was introduced into clinical anaesthesia in 1942, efforts to create better neuromuscular blocking (NMB) muscle relaxants have continued. Today, muscle relaxation remains a mainstay of modern anaesthesia and intensive care. Through manipulation of the traditional structure-action relationships, many new and improved muscle relaxants have been created, and several have been brought to clinical use. However, structure-action relationship is inconsistent and has its limits. Using computer-aided molecular conformational analyses, the conformation-action relationships of NMB agents of various chemical classes have been explored. Conformation, no less than structure, of the NMB agents has shed new light on their mechanisms of action. By reflection, the conformations also suggest new details of the topology of the receptive sites of the nicotinic acetylcholine receptor modeled for the motor endplate of the skeletal muscle.

Role of chiral chromatography in therapeutic drug monitoring and in clinical and forensic toxicology

Advances in chiral chromatographic separations have given pharmacologists and toxicologists the tools to examine unexpected clinical results involving chiral drugs. The ability to unravel complex phenomena associated with drug transport and drug metabolism is presented in this manuscript. The relation between the chirality of the drug mefloquine and the intracellular concentrations of the drug cyclosporine is illustrated by examining the effect of the enantiomers of mefloquine on the transport activity of P-glycoprotein (Pgp). These studies were conducted using a liquid chromatographic column containing immobilized Pgp. The results demonstrated that (+)-mefloquine competitively displaced the Pgp substrate cyclosporine whereas (-)-mefloquine had no effect on cyclosporine-Pgp binding. The data suggest that cyclosporine cellular and CNS concentrations can be increased through the concomitant administration of (+)-mefloquine. The use of chirality in clinical and forensic situations is also illustrated by the metabolism of the enantiomers of ketamine (KET). The plasma concentrations of (+)-KET and (-)-KET and the norketamine metabolites (+)-NK and (-)-NK were measured in rat plasma using enantioselective gas chromatography. The separations were accomplished using a gas chromatography chiral stationary phase based on beta-cyclodextrin. The pharmacokinetic profiles of (+)-, (-)-KET and (+)-, (-)-NK were determined in control and protein-calorie malnourished (PCM) rats to determine the effect of PCM on ketamine metabolism and clearance. The results indicate that PCM produced a significant and stereoselective decrease in KET and NK metabolism. The data suggest that the effects of environmental factors (smoking, alcohol use, diet) and drug interactions (coadministered agents) can be measured using the changes in stereochemical metabolic and pharmacokinetic patterns of KET and similar drugs.

Use and efficacy of low-dose ketamine in the management of acute postoperative pain: a review of current techniques and outcomes

Ketamine hydrochloride is a well known general anesthetic and short acting analgesic in use for almost 3 decades. The role of the NMDA receptor in the processing of nociceptive input has led naturally to renewed clinical interest in N-methyl-D-aspartate (NMDA) receptor antagonists such as ketamine. This paper reviews the use and efficacy of low-dose ketamine in the management of acute postoperative pain. The literature was obtained from a computer search of the MEDLINE database from 1966 through December 1998. Studies were included for review if they were randomized, prospective, controlled, double-blind and reported pain scores. We evaluate the clinical literature and discuss the efficacy of low-dose ketamine in the management of acute postoperative pain when administered alone or in conjunction with other agents via the oral, intramuscular, subcutaneous, intravenous and intraspinal routes. Low-dose ketamine is defined as a bolus dose of less than 2 mg/g when given intramuscularly or less than 1 mg/kg when administered via the intravenous or epidural route. For continuous i.v. administration low-dose ketamine is defined as a rate of < or =20 microg/kg per min. We conclude that ketamine may provide clinicians with a tool to improve postoperative pain management and to reduce opioid related adverse effects. The evidence suggests that low-dose ketamine may play an important role in postoperative pain management when used as an adjunct to local anesthetics, opioids, or other analgesic agents. Further research is required in the following areas: (a) dose-finding studies for ketamine as an adjunct to opioids and local anesthetics (b) efficacy and optimal route of administration (c) the role of S(+)-ketamine; (d) the influence of ketamine on long-term outcome such as chronic pain (e) long-term physical and chemical stability of mixtures containing ketamine (f) spinal toxicity of ketamine and (g) effects of low-dose ketamine on cognitive and memory functioning after surgery.

Chirality in anaesthesia - ropivacaine, ketamine and thiopentone

Drug chirality (molecular handedness) is a source of pharmacological differences between otherwise chemically identical molecules. Specific applications to the pharmacology of ropivacaine (single enantiomer), ketamine and thiopentone (both racemates) are discussed. Ropivacaine is produced as a single S-enantiomer homologue of the more toxic bupivacaine to preclude the higher central nervous system and heart toxicity found in the R-enantiomer. S-ketamine is presently undergoing trials as a potential replacement for the racemate, on the grounds that it optimizes anaesthesia and minimizes psychotomimetic phenomena. Thiopentone, previously known to have quantitative differences in the pharmacology of its enantiomers, has recently also been shown to have pharmacokinetic differences. The evidence for these claims is discussed in this review.

Antinociceptive effect of the S(+)-enantiomer of ketamine on carrageenan hyperalgesia after intrathecal administration in rats

Ketamine exerts antinociceptive effects in many pain tests. We investigated the antinociceptive effect of intrathecally administered racemic ketamine and its S(+)- and R(-)-enantiomer on carrageenan-induced thermal hyperalgesia with a paw withdrawal test and acute pain (hot plate and tailflick) tests. Rats were prepared with a chronic lumbar intrathecal catheter to receive either saline or ketamine enantiomers in cumulative doses. None of the ketamines (10, 50, or 100 microg) had any effect on the withdrawal latency of the contralateral, noninjected paw. In the injected paw, intrathecal saline did not alter carrageenan-induced thermal hyperalgesia, whereas intrathecally applied S(+)-, R(-)-, and racemic ketamine decreased thermal hyperalgesia. However, compared with saline, racemic ketamine had a higher efficacy than S(+)-ketamine, whereas R(-)-ketamine did not achieve statistical significance. Neither S(+)- nor R(-)-ketamine had a significant effect in the tailflick test (10, 100, or 500 microg). In the hot plate test, only the largest dose of ketamine (500 microg) caused a nonstereospecific, significant increase in hot plate latency; this dose caused supraspinal effects as well. The results demonstrate that the behavioral hyperalgesia associated with carrageenan-induced hindpaw inflammation in rats is attenuated by the intrathecal administration of racemic and S(+)-ketamine, but not R(-)-ketamine, which only displayed an insignificant trend toward a dose-response relationship. This finding warrants further studies to investigate a possible clinical advantage of preservative-free S(+)-ketamine over the currently used preservative containing racemic mixture.

IMPLICATIONS

In rats, intrathecal S(+)-ketamine was effective for treating inflammatory pain. Although racemic ketamine has a greater efficacy, S(+)-ketamine is available as a preservative-free drug and might be of clinical interest for future neuraxial administration in different pain states.

[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.