On Dr Dick Climie and Dr Jack Thomas, and the genesis of chemical-clinical pharmacology in Australian anaesthesia research

Pharmacokinetic and pharmacodynamic research is regularly reported in most contemporary anaesthesia-oriented journals. This sub-specialty area of pharmacology grew rapidly from the 1960s as various essential concepts and tools-laboratory analysis of drug/metabolite concentrations in biofluids, physiological signal collection, and methods for analysing/presenting relevant pharmacokinetic and pharmacodynamic data-started coming together. For Australia, such research began in Sydney in the mid-1960s with collaboration between anaesthetist Dr C.R. Climie (1923-2013) at the Royal Hospital for Women and medicinal chemist Dr J. Thomas OAM (1928-2017), and was achieved through a succession of postgraduate research student projects in the Department of Pharmacy of The University of Sydney, initially supervised by Dr Thomas. These consisted of studies concerned with the systemic absorption and placental transmission of drugs being used in parturients. By the late 1960s, Sydney anaesthetists Drs G.J. Long and C.A. Shanks (1936-1998) were also participating, and the projects were becoming more complex, including studies of the metabolism of local anaesthetics and other drugs by mothers and neonates. Between the mid-1970s and early-1980s, with additional anaesthetists, postgraduate research students and their academic supervisors participating, the projects focussed mainly on the pharmacokinetics and pharmacodynamics of neuromuscular blocking agents. This form of chemical-clinical-pharmacologically-based anaesthesia-oriented research that started in Sydney with the collaboration of Drs Climie and Thomas led to many challenging higher degree projects for pharmaceutical scientists, and access to unprecedented research capabilities for anaesthetists. Most significantly, it established a permanent place for multidisciplinary pharmacokinetic- and pharmacodynamic-based research within Australian academic departments of anaesthesia.

Dr Snow killed a bird: The genesis of pharmacokinetics and pharmacodynamics in anaesthesia

This essay presents a pharmacologist's perspective of what would be now called 'preclinical research' and 'uncontrolled clinical trials' surrounding the first public demonstration by William Thomas Green Morton of painless surgery achieved by the inhalation of ether in a patient at the Massachusetts General Hospital on 16 October 1846. Of the many people who made history in those earliest days of surgical anaesthesia in both the United States and Great Britain, John Snow stands out for his personal research that spanned basic science and clinical medicine. Primarily, Snow used the relationship between the vapour pressure of a volatile liquid and temperature to design a vaporiser. This allowed control of the inspired concentration of the volatile liquid epitomised by diethyl ether, and thus the time-course and depth of anaesthesia. In an era when developments in anaesthesia were almost exclusively based on empirical modifications to apparatus and technique, Snow, and to a lesser extent his contemporary Andrew Buchanan, stood out from all others in advancing the quantitative basis of anaesthesia. Both described the physiological basis of control over gas uptake whereby they related that gas moved across concentration gradients in the body: alveolar to arterial to tissue to venous gas tensions, and Snow devised a progressional semi-quantitative scale of five 'stages' of ether anaesthesia. They thereby introduced the elements of what would be referred to 'pharmacokinetics' and 'pharmacodynamics', a century later. This essay attempts to place them and their scientific insights into context with contemporaneous principal personae and knowledge.

Postoperative course of plasma protein binding of lignocaine, ropivacaine and bupivacaine in sheep

The plasma protein binding of the 2,6-xylidide local anaesthetic agents lignocaine, ropivacaine and bupivacaine enantiomers was determined by equilibrium dialysis in plasma obtained from chronically catheterized sheep before and up to 21 days after surgery. Three concentrations (1, 5 and 10 mg L−1), were used for each agent. Concentration-dependent binding was evident for each agent throughout the study period. R(+)-Bupivacaine was more extensively bound than S(–)-bupivacaine at the higher concentrations. Compared with pre-surgery, binding of each agent was less on the first postoperative day but did not differ significantly from days 8 to 21.

Ventilatory responses of healthy subjects to intravenous combinations of morphine and oxycodone under imposed hypercapnic and hypoxaemic conditions

AIMS

Previous isobolographic analysis revealed that coadministration of morphine and oxycodone produces synergistic antinociception in laboratory rodents. As both opioids can produce ventilatory depression, this study was designed to determine whether their ventilatory effects were synergistic when coadministered to healthy human subjects.

METHODS

A placebo-controlled, randomized, crossover study was performed in 12 male volunteers. Ventilatory responses to hypoxaemia and hypercapnia were determined from 1-h intravenous infusions of saline ('placebo'), 15 mg morphine sulphate (M), 15 mg oxycodone hydrochloride (O), and their combination in the dose ratios of 1:2, 1:1, 2:1. Drug and metabolite concentrations in serial peripheral venous blood samples were measured by high-performance liquid chromatography-MS/MS.

RESULTS

'Placebo' treatment was without significant ventilatory effects. There were no systematic differences between active drug treatments on either the slopes or intercepts of the hypoxaemic and hypercapnia ventilation responses. During drug treatment, the mean minute ventilation at PetCO(2) = 55 mmHg (V(E55)) decreased to 74% of the subjects' before treatment values (95% confidence interval 62, 87), 68% (57, 80), 69% (59, 79), 68% (63, 73), and 61% (52, 69) for M15, M10/O5, M7.5/O7.5, M5/O10 and O15, respectively. Recovery was more prolonged with increasing oxycodone doses, corresponding to its greater potency and lower clearance compared with morphine.

CONCLUSIONS

Although adverse ventilatory effects of these drugs were found as expected, no unexpected or disproportionate effects of any of the morphine and oxycodone treatments were found that might impede their use in combination for pain management.

Stereochemistry in anaesthetic and analgetic drugs

Isomers are molecules that have the same number of the same kind of atoms arranged in different ways. There are two major categories of isomers: constitutional (or structural) isomers and stereoisomers. Stereoisomers have identical sets of atoms that are configured in the same positions but are arranged differently spatially. Enantiomers are stereoisomers bearing a mirror image relationship. The pharmacological complication caused by drug racemates is that their component enantiomers usually have different pharmacodynamic effects and different pharmacokinetic properties. Enantioselective pharmacology can occur at any site in the body where a drug interacts with an endogenous chiral centre. The purpose of this presentation is to give some examples where drug chirality is pharmacologically significant to potency, uptake, distribution and elimination. The chosen examples were the anesthetic drugs, thiopentone and bupivacaine.

Plasma levobupivacaine concentrations following scalp block in patients undergoing awake craniotomy

BACKGROUND

Levobupivacaine is an effective local anaesthetic agent for nerve blockade with less systemic toxicity than racemic bupivacaine. The safety and efficacy of levobupivacaine for scalp blockade during awake craniotomy have not been addressed previously.

METHODS

Serial arterial plasma levobupivacaine concentrations following scalp blockade were measured to 2 h in 10 patients booked for awake craniotomy for epilepsy or tumour surgery. Bilateral scalp blockade providing surgical anaesthesia was achieved with a mean dose of 177 mg (2.5 mg kg(-1), range 1.6-3.2 mg kg(-1)) of levobupivacaine (0.5%, 5 mg ml(-1)) with epinephrine (5 microg ml(-1)) added immediately before the block insertion.

RESULTS

The maximum measured plasma levobupivacaine concentration was 1.58 (0.44) microg ml(-1) [mean (SD)] with a mean time to peak plasma concentration of 12 (4) min. There were no episodes in any of the 10 patients of symptoms or signs suggestive of either CNS or CVS toxicity.

CONCLUSIONS

This study demonstrated a relatively rapid rise of plasma levobupivacaine concentration without evidence of cardiovascular or central nervous system sequelae in a sample population of patients who may be particularly prone to perioperative seizures.

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.

Anatomical-physiological approaches in pharmacokinetics and pharmacodynamics

Use of an anatomical-physiological approach allows an investigator an alternative to regarding the whole body as a 'black box' producing biofluid specimens for drug assay, and then blindly applying a formula-driven mathematical approach to determine the pharmacokinetics and pharmacodynamics of the drug of interest. Instead, it means the investigator can consider that the body is the sum of interacting parts or regions connected anatomically by blood flow carrying the drug of interest, that the regions as well as the carrier blood are not homogeneous because each has a physiological role, and that the parts or regions are connected neurally and humorally so that the response in any region or part of the system may be modified by and/or modulate effects at another region or part. Such an approach is difficult to institute experimentally because a complicated (and often expensive) preparation is usually required in animal studies, and is rarely possible in research with humans because of ethical constraints. Despite these restrictions, there are many examples of the use of an anatomical-physiological approach allowing greater insight into pharmacological problems than would have been possible with a conventional 'whole body' approach alone. This paper takes a number of examples from the discipline of anaesthesia and pain management and groups them to illustrate the principles of the approach regarding drug arterio-venous equality and tissue distribution, multiple sites of clearance and multiple sites of action.

Trends in the pharmacology of opioids: implications for the pharmacotherapy of pain

Some 25 years ago, it was popular to write a figurative equation for drug treatment: Patient + drug --> effect. Clearly, this was too simplistic for the reliable pharmacotherapy of pain. The 'patient' with pain is a complex individual and the 'drug' is not the same in effect to every patient, regardless of being an opioid analgesic. Besides, the 'effect' needs specification: it really means an acceptable balance between therapeutic and side-effect--to that patient. These days, neural plasticity and neuromodulation associated with nociception are well-known. Psychological involvement in the physical domain of nociception is similarly well-known. Given these complexities, it is surprising that the pharmacological control of pain through the application of relatively simple analgesics can be so effective. Opioid analgesics have been administered by every possible route. Because the degree of invasion of the patient differs between routes, the balance between simplicity, aesthetic appeal, and efficacy of the various routes needs to be considered. The effectiveness of opioid analgesics depends in large measure on the application of the right agent, in the right dose, by the right route, at the right time for that patient. To assist in this task, researchers have produced patient-controlled analgesia and 'pharmacologically engineered' analgetic molecules to achieve receptor selectivity and pharmacokinetic predictability. Also, much relevant data concerning the time courses of analgetic drug (+/- metabolite) concentrations in the body and in the ways in which these properties are modified by different normal and pathophysiological variations have been gathered, and the philosophy of 'opioid rotation' has arisen to maintain therapeutic benefits when tolerance or metabolite-induced side-effects prevail. This essay discusses some of the trends in opioid pharmacotherapy over the past 25 years.

Concurrent ketamine and alfentanil administration: pharmacokinetic considerations

BACKGROUND

A ketamine-alfentanil combination has been suggested for total i.v. anaesthesia. We determined the pharmacokinetics of ketamine and alfentanil, alone and together, in three groups of adult male rats, to assess any pharmacokinetic interaction.

METHODS

Group 1 animals were infused with i.v. ketamine for 5 min; in group 2, constant low plasma concentrations of alfentanil were maintained by computer-controlled infusion; in group 3, the treatments were combined. Serial plasma and terminal tissue concentrations were measured by high performance liquid chromatography or gas chromatography-mass spectrometry.

RESULTS

In the presence of alfentanil, the mean plasma ketamine concentration-time area under the curve (AUC) value was significantly lower (by 13%, P<0.05), while clearance (CIT) and volume of distribution (Vss) were significantly higher (by 16 and 28%, respectively, both P<0.05). Tissue:plasma distribution coefficients for ketamine in the presence of alfentanil were significantly higher in forebrain (by 128%, P<0.005), hindbrain (by 207%, P<0.01), gut (by 254%, P<0.005), and fat (by 344%, P<0.0001). Mean AUC values for alfentanil did not differ significantly in the presence of ketamine, but alfentanil tissue concentrations were significantly lower in forebrain (by 77%, P<0.0001), hindbrain (by 28%, P<0.01), heart (by 33%, P<0.01), lung (30%, P<0.05), and gut (by 21%, P<0.05). Corresponding tissue:plasma distribution coefficients were significantly lower for forebrain (by 69%, P<0.0001) alone.

CONCLUSIONS

The finding that the distribution of ketamine into the brain was increased by low plasma concentrations of alfentanil could have important clinical applications for pain management.

Tissue uptake of ketamine and norketamine enantiomers in the rat: indirect evidence for extrahepatic metabolic inversion

Ketamine, used clinically as an intravenous analgetic and dissociative anaesthetic agent, is a racemate with both pharmacokinetic and pharmacodynamic enantioselectivity. S-ketamine has been found have a higher clearance and greater potency than R-ketamine as well as a greater therapeutic index. We performed a study in rats with two complementary paradigms: (i) constant rate "washin" infusion until fatal, (ii) brief infusion then "washout". These, respectively, allowed examination of ketamine and norketamine serial plasma enantiomer concentrations and tissue distribution at maximal and minimal drug effects. Both paradigms found plasma concentrations of R-ketamine>S-ketamine; however, tissue distribution coefficients for S-ketamine>R-ketamine. For paradigm (i), plasma concentrations of R-norketamine>S-norketamine; for paradigm (ii), R-norketamine>>S-norketamine initially, but S-norketamine>>R-norketamine later. Comparison of distribution coefficients of ketamine and norketamine enantiomers for the two paradigms provided indirect evidence for metabolic inversion. During washin, when circulating concentrations of ketamine enantiomers were high, uptake and metabolism occurred predominantly in the kidney and to a lesser extent in liver, lung and gut, with formation of R-norketamine by a (presumed) first-order process predominating. However, following washout, when circulating concentrations of ketamine enantiomers were low, uptake and metabolism was dominated by the kidney and gut. Under these conditions inversion of R- to S-ketamine appeared to predominate with subsequent metabolism to S-norketamine by (presumed) zero-order processes. In summary, different profiles for the uptake and metabolism of ketamine enantiomers were apparent following constant rate washin, and brief infusion washout, paradigms with i.v. rac-ketamine. Uptake into most tissues, and metabolism in some tissues, was enantioselective.

Prolonged thiopentone infusion for neurosurgical emergencies: usefulness of therapeutic drug monitoring

Serial serum thiopentone concentrations were measured during and following completion of an intravenous infusion of thiopentone in 20 patients with neurosurgical emergencies. The concentration data from a further 55 patients who had had some such measurements were reviewed retrospectively. The patients received an infusion for longer than 24 hours at a rate adjusted to maintain EEG burst suppression. The data were interpreted in terms of thiopentone pharmacokinetics and used to produce statistical models relating to clinical outcomes. In these patients, the one-month mortality rate following commencement of thiopentone treatment was 20%; the mean durations of pupillary and motor unresponsiveness following cessation of an infusion were 22 and 91 hours, respectively. Predictors of a prolonged duration of motor unresponsiveness included a prolonged duration of pupillary unresponsiveness, a low thiopentone clearance and a high maximum serum concentration of thiopentone. From pooled logistic regression, median effective serum thiopentone concentrations (EC50) were found to be 50 mg x l(-1) for recovery of pupillary responsiveness and 12 mg x l(-1) for the recovery of motor responsiveness. Because prolonged high-dose thiopentone leads to prolonged residual serum concentrations, it is difficult to distinguish the residual pharmacological effects of thiopentone from the clinical condition. This study suggests that, based on EC50 values for responses, monitoring of post-infusion serum thiopentone concentrations may help determine whether a patient's clinical state is due to residual thiopentone pharmacological effects.

Cardiotoxicity with modern local anaesthetics: is there a safer choice?

The recognition that long-acting local anaesthetics, particularly bupivacaine the de facto standard long-acting local anaesthetic, were disproportionately more cardiotoxic than their shorter-acting counterparts stimulated the development of the bupivacaine congeners, ropivacaine and levobupivacaine. These agents, like all local anaesthetics, can produce cardiotoxic sequelae by direct and indirect mechanisms that derive from their mode of local anaesthetic actions, i.e. inhibition of voltage-gated ion channels. While all local anaesthetics can cause direct negative inotropic effects, ropivacaine and levobupivacaine are less cardiotoxic than bupivacaine judging by the larger doses tolerated in laboratory animal preparations before the onset of serious cardiotoxicity (particularly electro-mechanical dissociation or malignant ventricular arrhythmias). Additionally, they are less toxic to the CNS than bupivacaine judging by the larger doses tolerated before the onset of seizures. This may be clinically important because CNS effects may be involved in the production of serious cardiotoxicity. Preclinical studies in humans are a 'blunt instrument' in their ability to distinguish significant differences between these drugs because of the relatively small doses that can be used. Nevertheless, available evidence from human studies corroborates the preclinical laboratory animal studies. Because clinically significant differences between these drugs are more quantitative than qualitative, i.e. toleration of a larger dose before manifestation of toxicity, we have concluded that these newer agents have a lower risk of causing serious cardiotoxicity than bupivacaine. Thus, compared with bupivacaine, the newer agents may be seen as 'safer', but they must not be regarded as 'safe'.

Hydromorphone-3-glucuronide: a more potent neuro-excitant than its structural analogue, morphine-3-glucuronide

In humans, hydromorphone (HMOR) is metabolised principally by conjugation with glucuronic acid to form hydromorphone-3-glucuronide (H3G), a close structural analogue of morphine-3-glucuronide (M3G), the major metabolite of morphine. In a previous study we described the biochemical synthesis of H3G together with a preliminary evaluation of its pharmacology which revealed that it is a neuroexcitant in rats in a manner analogous to M3G. Thus the aims of the current study were to quantify the neuro-excitatory behaviours evoked by intracerebroventricular (icv) H3G in the rat and to define its potency relative to M3G. Groups of adult male Sprague-Dawley rats received icv injections (1 microL) of H3G (1 - 3 microg), M3G (2 - 7 microg) or vehicle via a stainless steel guide cannula that had been implanted stereotaxically seven days prior to drug administration. Behavioural excitation was monitored by scoring fifteen different behaviours (myoclonic jerks, chewing, wet-dog-shakes, rearing, tonic-clonic-convulsions, explosive motor behaviour, grooming, exploring, general activity, eating, staring, ataxia, righting reflex, body posture, touch evoked agitation) immediately prior to icv injection and at the following post-dosing times: 5, 15, 25, 35, 50, 65 and 80 min. H3G produced dose-dependent behavioural excitation in a manner analogous to that reported previously for M3G by our laboratory and reproduced herein. H3G was found to be approximately 2.5-fold more potent than M3G, such that the mean (+/- S.D.) ED50 values were 2.3 (+/- 0.1) microg and 6.1 (+/- 0.6) microg respectively. Thus, our data clearly imply that if H3G crosses the BBB with equivalent efficiency to M3G, then the myoclonus, allodynia and seizures observed in some patients dosed chronically with large systemic doses of HMOR, are almost certainly due to the accumulation of sufficient H3G in the central nervous system, to evoke behavioural excitation.

High-performance liquid chromatographic determination of bradykinin in saliva: a critical review and a new method

Because of difficulties or dubious results with previously published methodologies, a new semi-automated HPLC method with UV absorbance detection was developed and applied to the determination of bradykinin (BK) in human saliva. The new method consisted of an uncomplicated sample preparation involving the addition to saliva of an equal volume of 0.1 M orthophosphoric acid to stabilize BK, vortex-mixing, centrifugation, and separation, followed by chromatography of the supernatant phase on a C8, 150x3.9-mm (I.D.) stainless steel column. The mobile phase was composed of 19% acetonitrile/0.1% trifluoroacetic acid at flow-rate of 0.4 ml/min. Using UV detection at 220 nm, the detection limit was 1 ng/ml for the BK standard, and 7 ng/ml for the assay of endogenous salivary BK. The orthophosphoric acid initially added to the saliva allowed BK to be stabilized from enzymic degradation at 20 degrees C for 5 days and at 4 degrees C for 60 days. Assignment made to the peak with the chromatographic properties of salivary BK was confirmed by HPLC-MS with an electrospray interface. This paper presents a new method that is reproducible, reliable and allows kinetic studies of salivary BK to be performed for clinical investigations.

Thiopental is a competitive inhibitor at the human alpha7 nicotinic acetylcholine receptor

The nicotinic acetylcholine receptors (nAChRs) in the central nervous system may be a potential target for the anesthetic effects of thiopental. We evaluated the mechanism of action of thiopental on the human alpha7 nAChR by using 2-electrode voltage clamp methodology. Concentration response curves for agonist were prepared in the presence of 25-250 microM of thiopental. Inhibition by the S- and R-thiopental enantiomers was compared with inhibition by racemic thiopental. We found that thiopental acts as a competitive inhibitor at the human alpha7 nAChR. Inhibition is independent of membrane potential and the K(i(apparent)) is 13 microM of thiopental. The clinical 50% effective concentration for thiopental in humans is 25 microM. Thus, with a K(i(apparent)) of 13 microM, inhibition of the human alpha7 nAChR is within a clinically relevant range. The S- and R-enantiomers of thiopental cause inhibition indistinguishable from the inhibition caused by racemic thiopental. This discordance makes it unlikely that the alpha7 nAChR plays a role in loss of righting reflex induced by thiopental in mice, although nicotinic inhibition by thiopental may mediate other anesthetic effects and side effects.

IMPLICATIONS

The receptors for nicotine in the brain may be involved in the mechanism of general anesthetics. We have shown that a human receptor for nicotine is inhibited by the anesthetic barbiturate thiopental, at concentrations used clinically. The nicotinic receptor thus may mediate some of the actions of this drug.

Sex-related differences in antinociception and tolerance development following chronic intravenous infusion of morphine in the rat: modulatory role of testosterone via morphine clearance

This study investigated possible sex-related differences in levels of antinociception and the rate of development of tolerance to the antinociceptive effects following prolonged (48 h) intravenous (i.v.) morphine administration in the rat. Groups of adult intact male, castrated male, female, and testosterone-pretreated female Sprague-Dawley rats received prolonged (48 h) infusions of i.v. morphine (5 or 10 mg/day) plus intra-arterial (i.a.) saline or i.v. morphine (5 mg/day) plus i.a. chloramphenicol (300 mg/day). Antinociception was quantified using the hotplate test. Serum concentrations of morphine and morphine-3-glucuronide (M3G) were assayed using high performance liquid chromatography with electrochemical detection, whereas the serum testosterone concentrations were quantified using an enzyme-linked immunosorbent assay method. Consistent with our previous findings in intact male rats, prolonged coinfusion of chloramphenicol with morphine produced a marked increase in the extent and duration of morphine antinociception in all experimental groups. Additionally, female and castrated male rats developed tolerance more slowly than either intact male or testosterone-pretreated female rats, when coinfused with parenteral morphine plus chloramphenicol. However, mean levels of antinociception were not significantly correlated with either the mean serum morphine or M3G concentrations, but were significantly inversely correlated with the mean values of the M3G/morphine serum molar concentration ratio. Testosterone pretreatment of female rats for 1 week before chronic morphine infusion abolished antinociception and markedly reduced both the serum morphine and M3G concentrations. These latter findings imply that testosterone modulates antinociception evoked by prolonged morphine infusion in rats via a mechanism that appears to involve modulation of morphine metabolism.

Effects of diclofenac in the rat tail ischaemia--reperfusion injury model of acute hyperalgesia

The rat tail ischaemia--reperfusion model of acute hyperalgesia described by Gelgor et al. (Pain 24 (1986) 251) has been investigated pharmacologically and electrophysiologically. Despite the advantages of this reusable animal model, biochemical changes associated with the behavioural response have not been determined. After injury+/-subcutaneous diclofenac pretreatment, we investigated the behavioural response (changes to thermally-induced tail flick latency) and measured diclofenac, prostaglandin E(2), 6-keto-prostaglandin F(1 alpha) and thromboxane B(2) concentrations in the tail, spinal cord and brain. Subcutaneous injection of 40 mg kg(-1) diclofenac sodium abolished the hyperalgesic response, suppressed the increased eicosanoid production in the tail, inhibited eicosanoid synthesis in the brain, but gave equivocal effects on eicosanoid concentrations in the spinal cord. Injection of 10 and 20 mg kg(-1) diclofenac reduced the duration of hyperalgesia but did not abolish the behavioural response. Diclofenac concentrations in all three tissues were similar, being approximately 5--10% of the corresponding plasma concentrations. We propose that both central and peripheral mechanisms are associated with the hyperalgesia and that the findings lend indirect support to a central action for non-steroidal anti-inflammatory drugs.

The efficacy of intrathecal morphine and clonidine in the treatment of pain after spinal cord injury

We performed a double-blinded, randomized, controlled trial in 15 patients to determine the efficacy of intrathecal morphine or clonidine, alone or combined, in the treatment of neuropathic pain after spinal cord injury. The combination of morphine and clonidine produced significantly more pain relief than placebo 4 h after administration; either morphine or clonidine alone did not produce as much pain relief. In addition, lumbar and cervical cerebrospinal fluid (CSF) concentrations, sampled at these levels at different times after administration were examined for a relationship between pain relief and CSF drug concentration. Lumbar CSF drug concentrations were initially several orders of magnitude larger than those in cervical CSF. After 1-2 h, the concentrations of morphine in cervical CSF markedly exceeded those of clonidine. The concentration of morphine in the cervical CSF and the degree of pain relief correlated significantly. We conclude that intrathecal administration of a mixture of clonidine and morphine is more effective than either drug administered alone and is related to the CSF-borne drug concentration above the level of spinal cord injury. If there is pathology that may restrict CSF flow, consideration should be given to intrathecal administration above the level of spinal cord damage to provide an adequate drug concentration in this region.

Central Effects Index--a semiquantitative method for the assessment of CNS toxicity of local anaesthetic agents in sheep

Local anaesthetic agents can cause central nervous system (CNS) and cardiovascular system toxicity. Whereas most previous work has described their behavioural CNS effects qualitatively (i.e., absence or presence of convulsions), we wished to describe their CNS effects more quantitatively. We hypothesised that early CNS excitatory or disinhibitory effects leading to convulsions represent a prodrome to the onset of chaotic dynamics in the form of a bifurcation in the chaotic map. We therefore used a chaotic map with a continuous scale to model their CNS effects. A Central Effects Index (CEI) was developed from our observation of behaviours before and after intravenous (iv) administration of local anaesthetic agents in graded doses to conscious sheep. These behaviours were ranked in severity, and modelled according to a logistic population growth equation using the onset of convulsive behaviour and death as point attractors. The behaviours, scaled to the maximum and area under the curve (AUC) CEI units, were then compared for intravenous doses of (+/-)-RS-bupivacaine and (-)-(S)-bupivacaine (or levobupivacaine), which is being evaluated as a substitute for (+/-)-RS-bupivacaine. (-)-(S)-bupivacaine produced smaller maximum and AUC CEI values at 75 and 100 mg doses, but equivalent values at 150 and 200 mg when the doses exceeded the convulsant threshold. It was concluded that the CEI provides a useful quantitative tool for evaluating these agents in subconvulsant doses, and that the CNS stimulatory potency of (-)-(S)-bupivacaine is less than that of (+/-)-RS-bupivacaine.

Tolerability of large-dose intravenous levobupivacaine in sheep

In preclinical pharmacological studies of levobupivacaine (S-bupivacaine), we determined its tolerability, cardiovascular actions, and pharmacokinetics, and we estimated its margin of safety compared with bupivacaine in conscious sheep. Levobupivacaine HCl. H(2)O was infused IV for 3 min into 10 previously instrumented ewes (approximately 50 kg). On subsequent days, the doses were increased by 50 mg from 200 or 250 mg until fatality occurred. All doses produced convulsions, QRS widening, and cardiac arrhythmias. With incremental doses, 4 of 4 animals survived 200 mg, 7 of 10 survived 250 mg, 3 of 7 survived 300 mg, but 0 of 3 survived 350 mg. Death resulted from sudden onset ventricular fibrillation (n = 3, within 2-3 min), electromechanical dissociation-pump failure (n = 5, within 4-5 min), or ventricular tachycardia-induced cardiac insufficiency (n = 2, >10 min). The estimated fatal dose (mean +/- SD) was 277 +/- 51 mg for levobupivacaine (compared with 156 +/- 31 mg found previously for bupivacaine). Pharmacokinetic analysis indicated initial and total distribution volumes = 4.5 (+/-1.6) and 97 (+/-22) L, total clearance = 1.7 (+/-0.4) L/min, and slow half life = 70 (+/-29) min; these values did not differ from those found previously with smaller doses. Heart and brain tissue levobupivacaine concentrations were approximately 3 times those in arterial blood. The doses of levobupivacaine survived were larger than found previously for bupivacaine, indicating its greater margin of safety.

IMPLICATIONS

Levobupivacaine produced fatal cardiac toxicity at doses significantly greater than those found in previous studies with bupivacaine. As the two drugs have similar potency for producing clinical nerve blocks, the data imply that levobupivacaine should provide a safer alternative to bupivacaine in practice.

Determination of defensin HNP-1, HNP-2, and HNP-3 in human saliva by using LC/MS

The mass spectral profiling of saliva by liquid chromatography mass spectrometry in relation to particular types of pain is being examined. The aim is to develop a profile that could be useful for the assessment of patients and their treatment programs, as well as identifying unknown compounds observed in saliva. Defensin human neutrophil peptide-1 (HNP-1) and defensin HNP-2 were identified and confirmed, whereas defensin HNP-3 was tentatively identified. Linear calibration range of defensin HNP-1 and HNP-2 was 0.25 to 3 microg/ml with R(2) values of > 0.99 for both. The detection limit for defensin HNP-1 and HNP-2 was estimated at 0.1 microg/ml. The healthy subjects surveyed in this study had readily measurable salivary concentrations of defensin HNP-1 (8.6 +/- SD 8.0 microg/ml) and defensin HNP-2 (5.6 +/- SD 5.2 microg/ml).

The effect of diclofenac on the expression of spinal cord c-fos-like immunoreactivity after ischemia-reperfusion-induced acute hyperalgesia in the rat tail

Ischemia-reperfusion of the rat tail for 20 min induces local acute hyperalgesia of approximately 1-h duration. We studied how this stimulus affected the expression of c-fos-like immunoreactivity (c-fos-LI) labeling of neurons of the sacral spinal cord, and how diclofenac pretreatment influenced the outcome. After ischemia, the number of c-fos-LI-labeled neurons was significantly increased when assessed at 60, 90, and 120 min after reperfusion (to 183%, 283%, and 164% of control, respectively; all P < 0.01). At 90 min, the number of regional c-fos-LI-labeled neurons was increased to 585% in laminae I-II, 183% in laminae III-IV, 270% in laminae V-X, and 286% in total, compared with respective control values (all P < 0.01). After diclofenac pretreatment (subcutaneous 40 mg/Kg, 30 min before insult) the number of c-fos-LI-labeled neurons at 90 min was increased to 424% in laminae I-II, 150% in laminae III-IV, 142% in laminae V-X, and 183% in total (all P < 0.01). Thus diclofenac pretreatment partially prevented the insult-induced increase in total and regional neuronal c-fos-LI. This acute nociceptive model involves only natural algogens. However, the results were similar to acute chemically induced or chronic adjuvant induced arthritic inflammatory pain models in which increases in c-Fos were partially inhibited by nonsteroidal antiinflammatory drugs.

The minimum effective concentration of opioids: a revisitation with patient controlled analgesia fentanyl

BACKGROUND AND OBJECTIVES

Whether patients titrate themselves to an individualized blood or plasma opioid concentration (the so-called minimum effective concentration or [MEC]) has been debated extensively. Nevertheless, there is consistent opinion that during patient controlled analgesia (PCA) patients balance acceptable pain relief against unacceptable side effects. This study sought to characterize fentanyl used by PCA with respect to MEC and factors influencing PCA use.

METHODS

An intensive study of 25 patients with observations over the first 24 hours after orthopedic surgery was planned on the premise that this approach would provide a measure of the fentanyl MEC. This necessitated repeated measurements of pain scores and plasma fentanyl concentrations before and 10 minutes after every PCA demand. In addition, a battery of psychological tests was given before and approximately 48 hours after surgery.

RESULTS

Logistic difficulties of maintaining a 24-hour study design resulted in its termination after 5 patients. The patients had convincingly distinct MECs (ranging from 0.23 to 0.99 ng/mL). The relationship between plasma fentanyl concentration and pain score was steep, such that small changes in concentration coincided with marked changes in pain relief. Despite preoperative expectations of achieving satisfaction in postoperative analgesia, not all patients titrated themselves to a pain-free state; all but one were satisfied with PCA. Surprisingly few side effects were reported. Unfortunately, the small sample size made systematic analysis of the psychological tests impossible.

CONCLUSIONS

This study found evidence to support the concepts of an individual MEC and a therapeutic window of fentanyl used with PCA.

Lack of secondary hyperalgesia and central sensitization in an acute sheep model

BACKGROUND AND OBJECTIVES

We aimed to determine the following in an experimental acute pain model in sheep: (1) whether multimodal analgesia with intravenous fentanyl and ketorolac was more effective than fentanyl alone; (2) whether secondary hyperalgesia (central sensitization) occurred in adjacent (foreleg) dermatomes after thoracic surgery; (3) whether ketorolac used preemptively influenced the development of secondary hyperalgesia after surgery.

METHODS

Changes in primary nociception were measured by increases to tolerated pressure, applied to the foreleg by a blunt pin, before foreleg withdrawal occurred. Changes to breath-to-breath interval and estimated end-tidal CO2 were used as indices of respiratory effects. Study 1 (n = 6) compared the paired responses to acute nociception after ketorolac (90 mg) or saline (control) pretreatment, followed by fentanyl (graded, 0 mg to 1.5 mg). Study 2 (n = 6) used a cross-over of ketorolac (90 mg) or saline (control) 24 hours and 1 hour, respectively, before a standardized thoracotomy incision, followed by antinociceptive testing with ketorolac (90 mg) and fentanyl (0.6 mg) daily over 4 days.

RESULTS

In study 1, fentanyl produced naloxone-antagonizable antinociception and respiratory depression. Ketorolac did not affect fentanyl antinociception, except for prolonging antinociception at the highest dose; it did not affect the respiratory effects. In study 2, preemptive ketorolac had no effect on the postoperative antinociceptive or respiratory effects of fentanyl. The pharmacokinetics of fentanyl were unaltered by ketorolac.

CONCLUSIONS

The results obtained in this acute pain model found no significant evidence of a fentanyl-ketorolac interaction, of central sensitization as shown by secondary hyperalgesia, or of a preemptive analgesic effect.

Electroencephalographic effects of thiopentone and its enantiomers in the rat

Electrophysiological studies with some chiral barbiturates have shown that one enantiomer can be excitant while the other is depressant. Thiopentone, a chiral barbiturate, has both differences in potency between enantiomers and biphasic effects on the electroencephalogram (EEG). This study investigated whether a differential EEG activity between the enantiomers of thiopentone could account for the biphasic effects. Rats were administered rac-, R- or S-thiopentone to determine the nature and time course of quantitative EEG effects. Two studies using computer-controlled i.v. infusions of the three drugs were performed in groups of animals previously prepared with EEG electrodes and/or arterial blood sampling cannulae. Study 1 used several stepwise increments in plasma drug concentration over 35 min, followed by washout. Study 2 used a 4 min period of constant plasma drug concentration, followed by washout. In both studies, both enantiomers and racemate caused an initial EEG activation followed by deactivation. Quantitative enantioselectivity was found for depression. The extent of depression was significantly less for R-thiopentone (P=0.008) and racthiopentone (P=0.038) than for S-thiopentone; recovery from depression appeared to be faster for R-thiopentone than either rac- or S-thiopentone. Fatality was only found with S-thiopentone (3/7 animals in Study 2). R-thiopentone plasma concentrations were approximately 8% less than those of S-thiopentone in rats treated with racthiopentone. Although small differences in clearance between enantiomers were found that may influence recovery, they were not large enough to account for the reported differences in potency between the two enantiomers.

Glutamate and kynurenate in the rat central nervous system following treatments with tail ischaemia or diclofenac

Kynurenate is an endogenous antagonist at the allosteric glycine site on the N-methyl-D-aspartate (NMDA) receptor, and may have a role in ameliorating nociceptive processes through modulation of NMDA receptor function. While antinociceptive effects of nonsteroidal anti-inflammatory drugs (NSAIDs) are mediated peripherally and possibly centrally through inhibition of prostaglandin synthesis, there is also evidence for centrally mediated prostaglandin-independent antinociceptive effects that may result from increased central nervous system (CNS) concentrations of kynurenate. We have investigated the effects of the NSAID diclofenac, (40 mg kg(-1), s.c.; administered to rats 1 h before killing) or the exposure of rats to noxious stimulation (tail ischaemia for 20 min before killing), on the concentrations of glutamate and kynurenate in discrete CNS regions. Regional CNS tissue concentrations of diclofenac were between 3.0-3.8 nmolg(-1). The corresponding regional glutamate concentrations ranged between 4.8-10.6 micromol g(-1), and were significantly lower in the ischaemia group when compared with both control (15%, P < 0.05) and diclofenac-treated (19%, P < 0.002) groups. Kynurenate concentrations in these CNS regions ranged between 3.3-45.8 pmol g(-1). Pairwise comparisons between the control and diclofenac-treated groups found significant increases in kynurenate concentrations in the diencephalon and lumbo-sacral regions of the CNS (P = 0.05). Noxious stimulation from tail ischaemia appeared to be associated with increased release of glutamate. Additionally, NSAIDs appeared to increase kynurenate concentrations in the spinal cord and diencephalon. Antagonism by kynurenate of glutamate effects at NMDA receptors may contribute to the antinociceptive effects of NSAIDs.

Lack of whole-body pharmacokinetic differences of halothane enantiomers in the rat

BACKGROUND

Halothane is made and used as a racemate (an equimolar mixture of R- and S- enantiomers). This study was initiated to determine whether there were demonstrable enantiomeric differences in the whole-body pharmacokinetics of halothane that might have significance for studies in which racemate is used.

METHODS

Adult male Wistar rats were exposed to halothane vaporized in the atmosphere of a closed constant volume chamber supplied with O2 commensurate with CO2 production. Concentrations of halothane enantiomers were measured by a specific gas chromatography-mass spectrometry method. Experiments were performed at four initial concentrations of halothane (0.1%, 0.5%, 1.0%, and 1.5% vol/vol). Enantiomeric differences in whole-body pharmacokinetics were assessed indirectly from the relative chamber atmosphere concentrations of halothane enantiomers.

RESULTS

Concentrations of halothane decreased biphasically. The initial more rapid decrease was interpreted as incorporating absorption, distribution, and clearance; the slower decrease was interpreted as principally incorporating metabolic clearance. The ratio of concentrations of the two halothane enantiomers and of the ratios of the respective areas under the concentration-time curves remained constant without differing from unity at any time at any concentration of halothane. The dose-normalized areas under the concentration-time curves for the concentrations 0.1%, 0.5%, and 1.0% did not differ; that for 1.5% was significantly greater, suggesting nonlinear clearance, but the values did not differ significantly between enantiomers at any concentration.

CONCLUSIONS

As there were no significant differences in concentrations of the two enantiomers in the chamber atmosphere, enantioselectivity was not demonstrated in the whole-body pharmacokinetics of halothane.

Microdialysis study of the blood-brain equilibration of thiopental enantiomers

Thiopental is a racemate of equimolar R- and S-thiopental enantiomers that have different potencies in laboratory experiments. We measured concentrations of R- and S-thiopental in plasma, tissues and brain microdialysate of rats after computer-controlled infusion of thiopental i.v. to a plasma concentration of 40 micrograms ml-1 for 20 min in two pharmacokinetic studies. In study 1, animals were found to maintain their target plasma concentrations, which then decayed biphasically after infusion. Brain microdialysate concentrations of both enantiomers increased from about 3% of corresponding plasma concentrations at 1 min to 9% at 20 min. In study 2, thiopental concentrations were found to be highest at 20 min in CNS tissue, at 30 min in muscle and at 60 min in fat. Tissue:plasma distribution coefficients of R-thiopental were greater than those of S-thiopental when calculated from total or unbound plasma concentrations. We found no pharmacokinetic evidence to support differences between the thiopental enantiomers in rates of equilibration across the blood-brain barrier after infusion of rac-thiopental.

Pulmonary insulin administration using the AERx system: physiological and physicochemical factors influencing insulin effectiveness in healthy fasting subjects

BACKGROUND

Orally inhaled insulin may provide a convenient and effective therapy for prandial glucose control in patients with diabetes. This study evaluated the influence of formulation pH and concentration and different respiratory maneuvers on pharmacokinetic and pharmacodynamic properties of inhaled insulin.

METHODS

Three, open-label crossover studies in a total of 23 healthy subjects were conducted in which the safety, pharmacokinetics, and pharmacodynamics of insulin inhalation were compared to subcutaneous (SC) injection into the abdomen of commercially available regular insulin. A novel, aerosol generating system (AERx Diabetes Management System, Aradigm Corporation, Hayward, CA) was used to deliver aqueous insulin bolus aerosols to the lower respiratory tract from formulations at pH 3.5 or 7.4 and concentrations of U250 (250 U/mL) or U500 (500 U/mL).

RESULTS

Time to maximum insulin concentration in serum (Tmax) after SC dosing occurred approximately 50-60 minutes with the time to minimum plasma glucose concentration (i.e., maximum hypoglycemic effect), (TGmin), occurring later, at around 100-120 minutes. In contrast, pulmonary delivery led to a significantly earlier Tmax (7-20 minutes) and TGmin (60-70 minutes), parameters that were shown to be largely unaffected by changing the pH or concentration of the insulin. However, investigation of changes in inhaled volume (achieved by different programming of the AERx system) for administration of the same sized aerosol bolus revealed significant effects. Significantly slower absorption and time to peak hypoglycemic activity occurred when aerosol delivery of insulin occurred during a shallow (approximately 40% vital capacity) as opposed to a deep (approximately 80% vital capacity) inspiration. In addition, it was shown that serum concentration of insulin increased immediately after a series of forced expiraratory maneuvers 30 minutes after inhaled delivery.

CONCLUSIONS

Pulmonary delivery of aqueous bolus aerosols of insulin in healthy subjects resulted in rapid absorption with an associated hypoglycemic effect quicker than is achieved after subcutaneous dosing of regular insulin. Inhaled insulin pharmacokinetics and pharmacodynamics were independent of formulation variables (pH, concentration) but affected by certain respiratory maneuvers.

Pharmacokinetics of thiopental enantiomers during and following prolonged high-dose therapy

BACKGROUND

Thiopental is used as a racemate; however, this is not generally recognized. During conditions of prolonged high-dose therapy, the pharmacokinetics of thiopental may become nonlinear, but whether this derives from one or both enantiomers has not been evaluated. The authors determined the pharmacokinetics of R- and S-thiopental and serum concentrations of R- and S-pentobarbital from prolonged high-dose infusion of thiopental for neuroprotection.

METHODS

Twenty patients received a mean thiopental dose of 41.2 g over a mean duration of 95 h. R- and S-thiopental enantiomer serum concentration-time data from 18 patients were fitted with two models: a linear one-compartment model with first-order output, and a nonlinear one-compartment model with Michaelis-Menten output.

RESULTS

Nonlinear models were preferred in 16 of 18 patients. Paired analysis indicated that steady state clearance (Clss) and volume of distribution (Vd) were higher for R-thiopental (0.108 vs. 0.096 l/min, P < 0.0001; and 313 vs. 273 l, P < 0.0005, respectively); maximal rate of metabolism (Vm) was higher for S- than for R-thiopental (1.01 vs. 0.86 mg x l(-1) x h(-1), P = 0.02); elimination half-lives did not differ (14.6 vs. 14.7 h, P = 0.8); unbound fractions (f(u)) of R- and S-thiopental were 0.20 and 0.18, respectively, P < 0.0001). The differences in mean Clss, Vd and Vm were not significant when adjusted by f(u). Plasma concentrations of R- and S-pentobarbital were relatively small and unlikely to be of clinical significance.

CONCLUSION

The pharmacokinetics of R- and S-thiopental became nonlinear at these doses. The pharmacokinetic differences between R- and S-thiopental, although small, were statistically significant and were influenced by the higher f(u) of R-thiopental.

Renal dysfunction associated with the perioperative use of diclofenac

Postoperative renal dysfunction in rats is induced by ketorolac dosed concurrently with gentamicin. Herein, we report the renal effects of diclofenac in four groups of rats: control (C = anesthesia, surgery); diclofenac (D = anesthesia, surgery, diclofenac 18 mg x kg(-1) x d(-1)); gentamicin (G = anesthesia, surgery, gentamicin 20 mg x kg(-1) x d(-1)); and diclofenac and gentamicin (DG = anesthesia, surgery, diclofenac, gentamicin). Renal function, after three treatment days, was assessed using histology, p-aminohippurate (PAH), and iothalamate (IOT) clearances, serum and urine electrolytes, osmolality, urea, and creatinine. Urine output was increased (from 5.2 to 12.5 mL/24 h), and urine osmolality was decreased (from 2121 to 883 mOsm/kg) in the DG group. PAH and IOT clearances were decreased in the G and DG groups (PAH by 18%, IOT by 22%; PAH by 38%, IOT by 43%, respectively); there were no changes in the C and D groups. Urea and creatinine clearances were decreased by 61% and 43%, respectively, in the DG group. Kidney sections showed the most severe pathologic changes in the DG group. Our data indicate that the perioperative combination of diclofenac and gentamicin was deleterious to renal function.

IMPLICATIONS

Diclofenac alone does not result in significant perioperative renal dysfunction, but the combination of gentamicin and diclofenac is deleterious to renal function. Considering this and previous findings, the evidence suggests that treatment with aminoglycosides may be a significant risk factor for inducing perioperative renal failure during treatment with nonsteroidal antiinflammatory drugs.

Electroencephalographic effects of thiopentone and its enantiomers in the rat: correlation with drug tissue distribution

1. To better understand the pharmacology of the thiopentone enantiomers, we studied their quantitative electroencephalographic effects and their distribution into vital tissues. 2. Adult Wistar rats were infused with rac-, R- or S-thiopentone at 4 mg kg(-1)min(-1) until death ensued. The EEG signal was acquired continuously; serial arterial plasma and terminal tissue thiopentone concentrations were measured enantiospecifically. Relevant drug tissue : plasma distribution coefficients and plasma concentration-EEG effect relationships were determined. 3. Doses (mg kg(-1)) (mean+/-s.e.mean) for anaesthesia (toe pinch) and lethality (respiratory failure), respectively, decreased in the order R-thiopentone (55.8+/-2.4 and 176.2+/-11.2)> rac-thiopentone (39.3+/-2.1 and 97.5+/-3.9)> S-thiopentone (35.6+/-1.9 and 74.2+/-5.2); plasma drug concentrations (microg ml(-1)) decreased in the order R-thiopentone (66.3+/-4.5 and 89.8+/-5.2)> rac-thiopentone (56.7+/-2.0 and 77. 8+/-2.8)> S-thiopentone (55.0+/-1.9 and 64.1+/-2.8). 4. Initial EEG activation was similar for all thiopentone forms. Plasma drug concentrations for the same extent of EEG deactivation reflected the potency order. 5. After infusion of rac-thiopentone, tissue : plasma distribution coefficients were higher for R- than for S-thiopentone in brain and visceral regions, but not in fat or muscle. After infusion of the separate enantiomers, the relative heart : brain distribution ratio was for S-thiopentone was double that for R-thiopentone. 6. The therapeutic index of R-thiopentone (3.16+/-0. 14) was more advantageous than either rac-thiopentone (2.52+/-0.13) or S-thiopentone (2.10+/-0.14), possibly due to the relatively greater distribution into CNS tissues than heart. The data suggest that R-thiopentone could make a satisfactory single enantiomer substitute for rac-thiopentone.

Stereoselective interaction of thiopentone enantiomers with the GABA(A) receptor

1. As pharmacokinetic differences between the thiopentone enantiomers seem insufficient to explain the approximately 2 fold greater potency for CNS effects of (-)-S- over (+)-R-thiopentone, this study was performed to determine any enantioselectivity of thiopentone at the GABA(A) receptor, the primary receptor for barbiturate hypnotic effects. 2. Two electrode voltage clamp recording was performed on Xenopus laevis oocytes expressing human GABA(A) receptor subtype alpha1beta2gamma2 to determine relative differences in potentiation of the GABA response by rac-, (+)-R- and (-)-S-thiopentone, and rac-pentobarbitone. Changes in the cellular environment pH and in GABA concentrations were also evaluated. 3. With 3 microM GABA, the EC50 values were (-)-S-thiopentone (mean 26.0+/-s.e.mean 3.2 microM, n=9 cells) >rac-thiopentone (35.9+/-4.2 microM, n=6, P=0.1) >(+)-R-thiopentone (52.5+/-5.0 microM, n=8, P<0.02) >rac-pentobarbitone (97.0+/-11.2 microM, n=11, P<0.01). Adjustment of environment pH to 7.0 or 8.0 did not alter the EC50 values for (+)-R- or (-)-S-thiopentone. 4 Uninjected oocytes responded to >100 microM (-)-S- and R-thiopentone. This direct response was abolished by intracellular oocyte injection of 1,2-bis(2-aminophenoxy)ethane-N, N,N1,N1-tetraacetic acid (BAPTA), a Ca2+ chelating agent. With BAPTA, the EC50 values were (-)-S-thiopentone (20.6+/-3.2 microM, n=8) <(+)-R-thiopentone (36.2+/-3.2 microM, n=9, P<0.005). 5 (-)-S-thiopentone was found to be approximately 2 fold more potent than (+)-R-thiopentone in the potentiation of GABA at GABA(A) receptors expressed on Xenopus oocytes. This is consistent with the differences in potency for CNS depressant effects found in vivo.

The relationship between the myocardial kinetics of meperidine and its effect on myocardial contractility: model-independent analysis and optimal regional model

The myocardial kinetics of meperidine and the relationship between these kinetics and the effect of meperidine on myocardial contractility (maximum positive rate of change of left ventricular pressure) were examined by analysis of previously published data collected in sheep after the i.v. injection of 100 mg of meperidine over 1 s. There was significant hysteresis between reductions in myocardial contractility and the arterial concentrations of meperidine, but not the coronary sinus blood (effluent from the heart) or calculated myocardial concentrations. The peak reduction in contractility occurred after the peak arterial concentration, at the time of the peak myocardial concentration, but before the peak coronary sinus concentration, suggesting that the site of drug action in the heart was not in equilibrium with either arterial blood or effluent blood from the heart. The most appropriate form of a dynamic model (a linear model with a threshold) was determined, without the need to assume a kinetic model, by directly fitting the observed reductions in myocardial contractility to the calculated myocardial concentrations. To determine the optimal kinetic and combined kinetic-dynamic models, a variety of one-, two-, and three-compartment models of the myocardium were fitted to the coronary sinus concentrations by using hybrid modeling. These included "tank in series" models that accounted well for drug dispersion and "peripheral compartment" models that accounted well for deep distribution. The most appropriate model was a "compilation" model, which incorporated features of both these extremes and was a better fit to the observed data than either a traditional single flow-limited compartment or a traditional membrane-limited model.

Enantioselectivity of thiopental distribution into the central neural tissue of rats: an interaction with halothane

Thiopental is a racemate. In this study, we examined whether thiopental total body clearance and its distribution into central nervous system (CNS) tissue of rats was enantioselective. Rats, either anesthetized with halothane or conscious and restrained, were infused to stepwise steady-state targets of 5, 10, and 20 microg/mL thiopental by computer-controlled infusions. Serial arterial plasma and steady-state samples of brain and spinal cord were assayed enantiospecifically for thiopental. In both groups, concurrent total and unbound plasma concentrations of S-thiopental were approximately 10%-20% higher than those of R-thiopental, corresponding to its higher clearance. CNS tissue concentrations of S-thiopental were approximately 20% higher than those of R-thiopental. Spinal cord to plasma distribution coefficients were approximately 2 x those in the brain, with relative distribution coefficients approximately 10% greater for R-thiopental in both tissues. Plasma concentrations and distribution coefficients of both enantiomers were approximately 10%-20% lower in the halothane-anesthetized group, with a slightly greater effect on R-thiopental distribution. We conclude that the total body clearance of R-thiopental > S-thiopental, that halothane enantioselectively reduces the relative uptake of R-thiopental into brain tissue, and that composition is important in determining the CNS tissue concentrations of thiopental. The reported higher potency of S-thiopental did not seem to be due to its greater distribution into CNS tissues.

IMPLICATIONS

Because thiopental is a mixture of two forms (termed R-and S-enantiomers), correct interpretation of its distribution into, and clearance from, the body requires knowledge about both enantiomers. In this study, performed in rats, we showed that the two enantiomers of thiopental differed significantly, with the R-enantiomer having the preferred profile.

Epidural analgesia reduces the release of amino acids from peripheral tissues in the ebb phase of the metabolic response to major upper abdominal surgery

The purpose of this prospective cohort study was to compare metabolic effects of epidural or patient controlled analgesia (PCA) in patients undergoing major upper abdominal surgery. Seventeen patients undergoing major upper abdominal surgery were included: 10 received perioperative epidural analgesia (Group I) and the remainder received morphine via a PCA device for postoperative analgesia (Group II). A number of measures compared between one day preoperatively (day 1) and day 2 postoperatively included femoral arterial and venous blood concentrations of glucose, lactate, pyruvate and amino acids. In addition, the relevant flux values were measured from the products of the respective arteriovenous substrate concentration differences and calf blood flow. The efflux of lactate from peripheral tissues was greater in Group II than in Group I (P < 0.01): glucose and pyruvate efflux did not differ between groups. There was no difference between groups in mean individual and total flux of amino acids on day-1. However increased efflux between day-1 and day 2 was found for alanine, valine, isoleucine, leucine, phenylalanine, lysine, arginine in both groups, and for serine, glycine, tyrosine and histidine in Group II (P < 0.05). The efflux of glycine, methionine, amino benzoic acid, alanine, and lysine was less in Group I than Group II on day 2 (P < 0.05). There was a significant difference in the total amino acid flux on day 2 (Group I = -1.2 mumol. (100 ml tissue)-1.min-1 cf Group II = -2.5 mumol. (100 ml tissue)-1.min-1; P = 0.04). In conclusion, perioperative epidural analgesia was associated with a reduced postoperative amino acid efflux two days following major upper abdominal surgery.

High-performance liquid chromatographic separation and nanogram quantitation of bupivacaine enantiomers in blood

Chiral separation of rac-bupivacaine extracted from blood was achieved with similar limits of detection but using a much simpler sample preparation than reported previously. The simple one-step sample preparation devised was highly robust and efficient and allowed a very high throughput of samples. The high-performance liquid chromatography (HPLC) conditions used gave baseline separation of the enantiomers with high sensitivity. R-(+)-bupivacaine and S-(-)-bupivacaine blood concentrations were determined using a chiral stationary phase (AGP, ChromTech) with diode array detection at 220 nm; this wavelength produced a stable baseline allowing semi-automated analysis. Sample preparation involved addition of internal standard (diphenhydramine), basification of blood, extraction with n-hexane, concentration of the extract to dryness and reconstitution in 0.002 M phosphoric acid. At rac-bupivacaine concentrations of 0.5, 5 and 50 microg/ml in blood, assay accuracy as estimated by coefficients of variation (C.V.s), were 3.3, 1.4, and 1.6%, respectively, for R-(+)-bupivacaine and 3.7, 2.0 and 1.5%, respectively, for S-(-)-bupivacaine. Using 0.6-ml samples, the estimated limits of detection for R-(+)-bupivacaine and S-(-)-bupivacaine were both 15 ng/ml of blood. Calibration curves (n=188) were linear from 0.1 to 50 microg/ml with all correlation coefficients being greater than 0.99. This semi-automated method was applied to studies involving whole body pharmacokinetics with intravenous doses ranging from 12.5 to 350 mg and regional myocardial pharmacokinetics with coronary arterial doses ranging from 2.5 to 12.5 mg. These studies generated approximately 12000 blood samples.

Preoperative dextromethorphan reduces intraoperative but not postoperative morphine requirements after laparotomy

N-methyl-D-aspartate (NMDA) antagonists combined with opioids are thought to be effective in the control of pain states. We evaluated morphine use and analgesia in 37 patients postlaparotomy. Patients received 60 mg of oral dextromethorphan or placebo the night before and again 1 h before surgery. Morphine was titrated intraoperatively to maintain blood pressure and heart rate within 20% of baseline and postoperatively via patient-controlled analgesia (PCA). The dextromethorphan and placebo groups were compared for morphine use intraoperatively, in recovery, via PCA in the first 4 and 24 h, and total use over the study period. Pain scores at rest and on activity for the first 4 and 24 h were also compared. Intraoperatively, the dextromethorphan group required less morphine: 13.1+/-4.3 vs 17.6+/-6.0 mg (P = 0.012). Postoperatively, there was no significant difference between the dextromethorphan and placebo groups for morphine use: in the recovery room 10.9+/-7.7 vs 12.1+/-7.7 mg; the first 4 h of PCA 15.9+/-9.3 vs 12.7+/-5.1 mg; the first 24 h of PCA 76.4+/-44.7 vs 61.8+/-27.5 mg; or in total morphine use 100.4+/-49.5 vs 91.5+/-3.1 mg. Pain scores for the two groups were not statistically different throughout the study period. We conclude that 60 mg of oral dextromethorphan given the night before and repeated an hour before surgery does not provide a postoperative morphine-sparing effect or improve analgesia after laparotomy.

IMPLICATIONS

Patients given dextromethorphan before surgery had significantly reduced intraoperative morphine requirements. However, postoperative morphine requirements were unaltered. Dextromethorphan may need to be continued postoperatively to improve postoperative analgesia.

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.

Pulmonary administration of aerosolised fentanyl: pharmacokinetic analysis of systemic delivery

AIMS

Pulmonary drug delivery is a promising noninvasive method of systemic administration. Our aim was to determine whether a novel breath-actuated, microprocessor-controlled metered dose oral inhaler (SmartMist, Aradigm Corporation) could deliver fentanyl in a way suitable for control of severe pain.

METHODS

Aersolised pulmonary fentanyl base 100-300 microg was administered to healthy volunteers using SmartMist and the resultant plasma concentration-time data were compared with those from the same doses administered by intravenous (i.v.) injection in the same subjects.

RESULTS

Plasma concentrations from SmartMist were similar to those from i.v. injection. Time-averaged bioavailability based upon nominal doses averaged approximately 100%, and was > 50% within 5 min of delivery. Fentanyl systemic pharmacokinetics were similar to those previously reported with no trends to dose-dependence from either route. Side-effects (e.g. sedation, lightheadedness) were the same from both routes.

CONCLUSIONS

Fentanyl delivery using SmartMist can provide analgetically relevant plasma drug concentrations. This, combined with its ease of noninvasive use and transportability, suggests a strong potential for field and domicilliary use, and for patient controlled analgesia without the need for i.v. cannulae.

Factors influencing ketorolac-associated perioperative renal dysfunction

Nonsteroidal antiinflammatory drugs (NSAIDs) are useful for the treatment of postoperative pain, but there is continuing concern about adverse effects on renal function. We studied the renal effects of ketorolac in an animal model using Fischer 344 rats undergoing isoflurane anesthesia and laparotomy. Treatment groups--control (C), ketorolac (5 mg x kg(-1) x d(-1)) (K), large-dose ketorolac (15 mg x kg(-1) x d(-1)) (KH), dehydration-ketorolac (5 mg x kg(-1) x d(-1)) (DK), gentamicin (20 mg x kg(-1) x d(-1)) (G), and gentamicin (20 mg x kg(-1) x d(-1)) with ketorolac (5 mg x kg(-1) x d(-1)) (GK)--each comprised 10 animals. Renal function was assessed before laparotomy and after 3 treatment days using concurrent paraaminohippurate and iothalamate clearances, respectively, to estimate renal plasma flow and glomerular filtration rate, and by measuring serum and urine electrolytes, osmolality, urea, and creatinine. A significant increase in serum potassium was found in the GK and DK groups. There were no major changes in renal function in the C, K, KH, and DK groups. Mild renal dysfunction was found in the G group. We found severe and consistent changes in renal function, accompanied by severe, widespread histological changes of acute tubular necrosis, in the GK group. In this postoperative rat model, the combination of ketorolac and gentamicin was deleterious to renal function.

IMPLICATIONS

We examined the renal effects of the nonsteroidal antiinflammatory drug ketorolac. Renal function was measured in rats before and after surgery and 3 days' drug administration; the kidneys studied by using microscopy. Only ketorolac plus the antibiotic gentamicin produced marked changes in kidney function and structure.

Arterial and pulmonary arterial concentrations of the enantiomers of bupivacaine after epidural injection in elderly patients

Bupivacaine HCl is a 50:50 racemic mixture of the levo [S(-)] and dex [R(+)] enantiomers. The R(+) enantiomer exhibits greater cardiac tissue binding and toxicity. To determine whether the lung exhibits selective uptake of one of the enantiomers of bupivacaine, we measured pulmonary artery and radial artery blood concentrations of the two enantiomers after a lumbar epidural injection of 20 mL of 0.75% bupivacaine in 10 elderly patients undergoing one-stage bilateral total knee arthroplasty. Significantly lower concentrations of R(+) than S(-) were noted in both pulmonary artery and arterial blood. Both enantiomers were absorbed by the lung to a similar extent within the first 5 min after epidural injection (extraction ratio approximately equal to 0.1 or 10%). Mean time of maximal concentration (Tmax) was 6 min. In 3 of the 10 patients, Tmax occurred in 1-3 min. We conclude that the lung absorbs both the R(+) and S(-) enantiomers of bupivacaine to a similar extent after epidural injection and that this is of doubtful clinical significance. This study also suggests that peak concentrations of bupivacaine may occur earlier after epidural injection in certain elderly patients than previously believed.

IMPLICATIONS

In the first 5 min after epidural injection, approximately 10% of the local anesthetic bupivacaine was absorbed by the lung. Absorption of the two enantiomers (mirror images) of bupivacaine were similar. Lung absorption of bupivacaine is unlikely to influence local anesthetic toxicity.

Systemic and regional pharmacokinetics of levobupivacaine and bupivacaine enantiomers in sheep

Commercially available bupivacaine is an equimolar mixture of R(+)- and S(-)-bupivacaine. S(-)-bupivacaine (levobupivacaine) is the subject of current clinical evaluation. We conducted partial cross-over systemic and regional pharmacokinetic studies of i.v. bupivacaine (12.5-200 mg) and levobupivacaine (6.25-200 mg) in ewes. Enantiospecific analysis of blood drug concentration-time data and of regional myocardial and brain drug mass balance data indicated that (a) there was a higher mean total body clearance of R(+)-bupivacaine than of S(-)-bupivacaine (as previously reported); (b) there were no differences in the systemic pharmacokinetics of S(-)-bupivacaine whether administered alone or as a component of bupivacaine; (c) there was no evidence of dose-dependent pharmacokinetics with either enantiomer; (d) for both enantiomers, mean calculated myocardial tissue concentrations of 1%-4% dose occurred between 3 and 5 min. Mean brain concentrations of 0.2%-1% dose occurred between 2 and 4 min after the administration of bupivacaine but between 4 and 5 min after the administration of levobupivacaine. There was no evidence that systemic toxicity induced by these local anesthetics significantly modified their pharmacokinetics, and there was no evidence of an enantiomer-enantiomer pharmacokinetic interaction for bupivacaine.

IMPLICATIONS

Levobupivacaine comprises 50% of commercially available bupivacaine and is being considered for use in its own right. As a part of its preclinical evaluation, this study considered whether levobupivacaine behaved kinetically in the body in the same way as when administered as a component of bupivacaine.

Central nervous system toxicity attributable to epidural ropivacaine hydrochloride

Ropivacaine, a new local anaesthetic agent, has been demonstrated to have less potential than bupivacaine for central nervous system toxicity on the basis of human and animal studies. We report a case of a convulsion secondary to presumed inadvertent intravascular injection of 20 mg of ropivacaine in a 44 kg patient during an epidural for chronic pain. There were minimal signs of cardiovascular toxicity.

Cardiovascular and central nervous system effects of intravenous levobupivacaine and bupivacaine in sheep

Commercially available bupivacaine is an equimolar mixture of R(+)- and S(-)-bupivacaine. S(-)-bupivacaine (i.e., levobupivacaine) is currently undergoing preclinical evaluation. Cross-over studies with i.v. levobupivacaine and bupivacaine were conducted in two groups of seven conscious sheep. Doses were chosen to avoid convulsions (smaller dose 6.25-37.5 mg/min) or to be potentially toxic (larger dose 75-200 mg/3 min). In subconvulsive doses, both drugs produced similar time- and dose-dependent depression of left ventricular systolic contractility (dP/dt(max)). Convulsions occurred consistently with > or = 75 mg of bupivacaine and > or = 100 mg of levobupivacaine, producing an abrupt reversal of dP/dt(max) depression. Subconvulsive doses produced minor cardiovascular effects on heart rate and blood pressure, whereas both were increased by convulsions. Cardiac output and myocardial blood flow were decreased with larger doses of both drugs. Doses > 75 mg of bupivacaine or > 100 mg of levobupivacaine induced QRS widening and ventricular arrhythmias, but significantly fewer and less deleterious arrhythmias were induced by levobupivacaine. Three animals died after 150, 150, and 200 mg of bupivacaine from the sudden onset of ventricular fibrillation. These doses of levobupivacaine produced nonfatal arrhythmias that automatically returned to sinus rhythm. We conclude that levobupivacaine could offer a greater margin of clinical safety than bupivacaine.

IMPLICATIONS

Levobupivacaine comprises 50% of commercially available bupivacaine and is being considered for use in its own right. Local anesthetics can cause toxicity to the cardiovascular and central nervous systems. As a part of a preclinical evaluation of levobupivacaine, this study compared the toxic effects of levobupivacaine and bupivacaine in sheep.

The effect of duration of dose delivery with patient-controlled analgesia on the incidence of nausea and vomiting after hysterectomy

AIMS

Postoperative nausea and vomiting (PONV) may be exacerbated by postoperative opioid analgesics and may limit patients' successful use of these medications when used with patient controlled analgesia (PCA). We tested the hypothesis that the rapid change in blood morphine concentration associated with PCA bolus delivery contributed to PONV, and that prolonging its delivery to a brief infusion would result in decreased PONV.

METHODS

Patients, who were receiving morphine for pain relief via patient-controlled analgesia (PCA) after total abdominal hysterectomy, received 1 mg morphine sulphate incremental doses either over 40 s with a 5 min lockout interval or over 5 min delivery with a 1 min lockout interval. Episodes of nausea, retching and vomiting, along with the use of morphine and the pain relief obtained, were recorded.

RESULTS

Data from 20 patients in each group were analysed. Contrary to expectations, most patients in both groups reported nausea postoperatively. Those patients receiving morphine over 5 min experienced more episodes of emesis (36) than those receiving the dose over 40 s (17). Most patients receiving the 40 s doses vomited in the first 12 h (median time 8 h), while those receiving the 5 min doses vomited between 12 and 24 h (median time 19 h) (P = 0.01). There were no differences between groups in the visual analogue pain scores or use of morphine between groups.

CONCLUSIONS

Reasons for these unexpected findings remain speculative. The high incidence of PONV appears to be inherently high in gynaecological surgery patients and standard antiemetic medication regimens appear to be poorly efficacious. Reasons for the differences in the time-course of emetic episodes between the two groups may be related to differences in the time-course of central opioid receptor occupancy.