Tissue distribution of bupivacaine enantiomers in sheep

Analgesia for Sheep in Commercial Production: Where to Next?

Simple Summary

Increasing societal and customer pressure to provide animals with ‘a life worth living’ continues to apply pressure on industry to alleviate pain associated with husbandry practices, injury and illness. Although a number of analgesic solutions are now available for sheep, providing some amelioration of the acute pain responses, this review has highlighted a number of potential areas for further research.

Abstract

Increasing societal and customer pressure to provide animals with ‘a life worth living’ continues to apply pressure on livestock production industries to alleviate pain associated with husbandry practices, injury and illness. Over the past 15–20 years, there has been considerable research effort to understand and develop mitigation strategies for painful husbandry procedures in sheep, leading to the successful launch of analgesic approaches specific to sheep in a number of countries. However, even with multi-modal approaches to analgesia, using both local anaesthetic and non-steroidal anti-inflammatory drugs (NSAID), pain is not obliterated, and the challenge of pain mitigation and phasing out of painful husbandry practices remains. It is timely to review and reflect on progress to date in order to strategically focus on the most important challenges, and the avenues which offer the greatest potential to be incorporated into industry practice in a process of continuous improvement. A structured, systematic literature search was carried out, incorporating peer-reviewed scientific literature in the period 2000–2019. An enormous volume of research is underway, testament to the fact that we have not solved the pain and analgesia challenge for any species, including our own. This review has highlighted a number of potential areas for further research.

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.

Chirality of Central Nervous System (CNS) Acting Drugs: A Formidable Therapeutic Hurdle Against CNS Diseases

BACKGROUND

Over fifty percent of drugs being used clinically are chiral and 90% of them are racemates. Unfortunately, they have both adverse and beneficial effects on body systems.

METHODS

Because of the erratic effects of chiral compounds on body functional systems, literature search was carried out with a view to identify CNS chiral drugs, their clinical advantages and disadvantages, unique physicochemical properties and structural modifications into safer drugs.

RESULTS

Findings have shown that majority of CNS and non-CNS acting drugs have chiral functional groups that may occur as either dextrorotatory (clockwise) or levorotatory (anticlockwise) or racemates which are inert. Sometimes, the enantiomers (optical isomers) could undergo keto-enol tautomerism, appearing in either acidic or basic or inert form. Chiral CNS acting drugs have agonistic and antagonistic effects, clinical advantages, disadvantages, and special clinical applications, possible modifications for better therapeutic effects and possible synthesis of more potent drugs from racemates. Clockwise chirality may be more effective and safer than the drugs with anticlockwise chirality. When chiral drugs are in racemate state they become inert and may be safer than when they are single. Also, diastereoisomers may be more dangerous than stereoisomers.

CONCLUSION

Therefore, chiral compounds should be adequately studied in lab rodents and primates, and their mechanisms of actions should be comprehensively understood before being used in clinical setting. Since many of them are toxic, their use should be based on principle of individualized medicine. Their molecular weights, functional groups, metabolites, polymers and stereoisomers could be valuable tools for their modifications.

Tissue distribution, metabolism and hepatic tissue injury in Chinese lizards (Eremias argus) after a single oral administration of lambda-cyhalothrin

Lambda-cyhalothrin (LCT) is a widely used pyrethroid with neurotoxicity. However, little is known about the toxicokinetics of LCT in reptiles. In this study, the absorption, distribution, metabolism and excretion of LCT in Chinese lizards (Eremias Argus) were determined following a single dose (10 mg kg^-1) treatment. In the liver, brain, gonads and skin, LCT levels peaked within several hours and then decreased rapidly. However, the concentration of LCT gradually increased in the fat tissue. More than 90% of the LCT dose was excreted in the faeces. One LCT metabolite, 3-phenoxybenzoic acid (PBA), was detected in lizard plasma and tissues. PBA preferentially accumulates in the brain and plasma. The half-life of PBA in the brain was 3.2 days, which was 35.4-fold greater than that of LCT. In the plasma, the concentration of PBA was significantly higher than that of LCT. The bioaccumulation of LCT in tissues was enantioselective, and the enantiomeric fractions (EF) ranged from 0.72 to 0.26. The preferential accumulation of enantiomers changed according to exposure time, but the reasons behind this phenomenon were not clear. For pathological analysis, vacuolation of the cytoplasm and large areas of necrosis were observed in the liver sections after 168 h of dosing. The liver tissues exhibited both decreases in the hepatosomatic index and histopathological lesions during the exposure period. In this study, the effect concentration of LCT in lizards was 200-fold lower than its LD₅₀ value used in risk assessments for birds. These results may provide additional information for the risk assessment of LCT for reptiles and indicate that birds may not be an ideal surrogate for reptile toxicity evaluation.

The acute toxicity of local anesthetics

IMPORTANCE OF THE FIELD

Systemic toxicity, usually from overdose or intravascular dose, is feared because it mainly affects the heart and brain, and may be acutely life-threatening.

AREAS COVERED IN THIS REVIEW

Pharmacological studies of local anesthetic toxicity have largely been reviewed primarily relating to the evaluation of ropivacaine and levobupivacaine during the past decade. This review/opinion focuses more on the principles and concepts underlying the main models used, from chemical pharmacological and pharmacokinetic perspectives.

WHAT THE READER WILL GAIN

Research models required to produce pivotal toxicity data are discussed. The potencies for neural blockade and systemic toxicity are associated across virtually all models, with some deviations through molecular stereochemistry. These models show that all local anesthetics can produce direct cardiovascular system toxicity and CNS excitotoxicity that may further affect the cardiovascular system response. Whereas the longer-acting local anesthetics are more likely to cause cardiac death by malignant arrhythmias, the shorter-acting agents are more likely to cause cardiac contraction failure. In most models, equi-anesthetic doses of ropivacaine and levobupivacaine are less likely to produce serious toxicity than bupivacaine.

TAKE HOME MESSAGE

Of the various models, this reviewer favors a whole-body large animal preparation because of the comprehensive data collection possible. The conscious sheep preparation has contributed more than any other, and may be regarded as the de facto 'standard' experimental model for concurrent study of local anesthetic toxicity ± pharmacokinetics, using experimental designs that can reproduce the toxicity seen in clinical accidents.

Is comparative cardiotoxicity of S(-) and R(+) bupivacaine related to enantiomer-selective inhibition of L-type Ca(2+) channels?

Cardiac toxicity can occur after accidental intravascular injection of bupivacaine. Racemic bupivacaine can inhibit both cardiac Na(+) and Ca(2+) channels, but the contribution of these actions to cardiac depression is not totally understood. We tested whether the effect of R(+) bupivacaine on cardiac electrical activity in isolated hearts and on L-type Ca(2+) channels (I(Ca-L)) in isolated cardiac myocytes could be responsible for its increased cardiotoxicity compared with S(-) bupivacaine. Cardiac electrical activity of spontaneously beating isolated hearts was recorded before and after exposure to increasing concentrations of R(+) and S(-) bupivacaine. An increase of the PR interval (80%) and the QRS duration (370%) by 10microM R(+) bupivacaine (80% and 370%) was significantly higher than for S(-) bupivacaine (25% and 200%, respectively). R(+) but not S(-) bupivacaine produced severe arrhythmias at concentrations larger than 2.5microM. The intensity of I(Ca-L) inhibition did not differ between bupivacaine isomers. At 0 mV, I(Ca-L) was irreversibly reduced by 40.2% +/- 8.8% and 51.4% +/- 3.8% in the presence of 10microM R(+) and S(-) bupivacaine, respectively. The arrhythmogenic effect of R(+) bupivacaine could not be explained by stereoselectivity on the I(Ca-L) inhibition. Thus, other mechanisms could contribute to the cardiac electrical and contractile dysfunction induced by R(+) bupivacaine.

Bupivacaine versus lidocaine for third molar surgery: a double-blind, randomized, crossover study

PURPOSE

The aim of this study was to compare the clinical use of bupivacaine to lidocaine in third molar surgery.

PATIENTS AND METHODS

Twenty-three subjects underwent surgical removal of their third molars in two separate procedures. Bupivacaine was used for the third molars on one side, whereas lidocaine was used on the other side. Pain experience, analgesic consumption, cardiovascular response, blood concentrations, and systemic toxicity were evaluated.

RESULTS

Bupivacaine significantly reduced the postoperative pain experience only at the 8-hour period (P < .05). No difference in analgesic requirements or cardiovascular responses was observed with the two local anesthetics. The mean blood concentrations of both agents were considerably lower than their respective toxic threshold concentrations.

CONCLUSION

The results of this study do not show a difference in a variety of parameters, other than postoperative pain experience, when bupivacaine and lidocaine are compared in a standardized oral surgery procedure.

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.

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.

Stereochemistry in anaesthesia

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

Myocardial uptake of thiopental enantiomers by the isolated perfused rat heart

Myocardial uptake of thiopental enantiomers by an isolated perfused rat heart preparation was examined after perfusion with protein-free perfusate. Outflow perfusate samples were collected at frequent intervals for 20 min during single-pass perfusion with 10 micrograms/ml racemic thiopental (washin phase) and for another 45 min during perfusion with drug-free perfusate (washout phase). (+)- and (-)-thiopental concentrations were assayed by chiral high-performance liquid chromatography. Heart rate, perfusion pressure, and electro-cardiogram were also monitored. During the washin phase, there was no significant difference between the mean values of the equilibration rate constants of (+)- and (-)-thiopental enantiomers (0.44 +/- 0.07 min-1 and 0.43 +/- 0.09 min-1, respectively, P > 0.05). Mean volumes of distribution of (+)- and (-)-thiopental enantiomers were similar (6.34 +/- 1.20 and 6.45 +/- 1.29 ml/g for the washin phase and 7.22 +/- 0.71 and 7.47 +/- 0.81 ml/g for the washout phase, respectively, P > 0.05). This indicates that tissue accumulation of thiopental enantiomers in the isolated perfused rat heart was not stereoselective. Uptake of thiopental by the heart was perfusion flow rate-limited and independent of capillary permeability. These findings suggest that myocardial tissue concentration of racemic thiopental should be an accurate predictor of myocardial drug effect.

Bupivacaine enantiomer pharmacokinetics after intercostal neural blockade in liver transplantation patients

Bupivacaine, being a racemic local anesthetic, exists as an equal mixture of its component enantiomers R(+)- and S(-)-bupivacaine, which behave pharmacokinetically as independent drugs after injection into the body. Intercostal neural blockade using bupivacaine was performed for postoperative analgesia in 12 patients after orthotopic liver transplantation. Arterial blood, sampled serially, was assayed by enantioselective high-performance liquid chromatography for R(+)- and S(-)-bupivacaine. The average of the simultaneous R(+):S(-) ratios of blood bupivacaine concentrations in the 12 patients was 0.74 (SD 0.11); however, the use of a population mean value or a mean value for any patient denies the time-dependence of this entity. The blood enantiomer concentration difference was reflected in the maximum measured concentrations which, after the first dose, were, respectively, 0.38 (SD 0.19) and 0.52 (SD 0.28) mg.L-1.100 mg-1 RS-bupivacaine administered (P = 0.0003). The difference in blood concentrations between the enantiomers, reflected by the R(+):S(-) ratio being less than unity, could be explained by a greater mean total body clearance and a larger apparent volume distribution of R(+)-bupivacaine. Elimination of both enantiomers was prolonged in these patients after liver transplantation compared to data from the literature, but there was no tendency for either enantiomer to accumulate selectively, even upon repeated dosing. We conclude that this demonstration of differences in pharmacokinetics (and, in laboratory studies, also in pharmacodynamics) between the bupivacaine enantiomers points to the need for future studies to recognize the enantiomeric duality of this local anesthetic.

Pharmacokinetics of bupivacaine enantiomers in sheep: influence of dosage regimen and study design

Bupivacaine is used as a racemate. In previous studies the mean total body clearance of R(+)-bupivacaine was found to be greater than S(-)-bupivacaine by 65% after iv bolus dose of separate enantiomers and by 20% after iv infusion to steady state of racemate. The present studies were performed to determine whether different study designs using different iv dosage regimens could influence the pharmacokinetic parameters determined for either bupivacaine enantiomer. rac-Bupivacaine.HCl was administered iv to 6 adult Merino ewes by bolus, brief infusion, and prolonged infusion. Arterial blood concentrations of R(+)- and S(-)-bupivacaine were measured by enantioselective HPLC. These regimens consistently produced lower arterial blood concentrations of R(+)-bupivacaine than S(-)-bupivacaine due to R(+)-bupivacaine having a greater initial dilution volume by 16 (95% CI = 3-29)%, volume of distribution at steady state equilibrium by 32 (95% CI = 17-32)% and mean total body clearance by 28 (95% CI = 21-35)%. The slow half-life of R(+)-bupivacaine, however, was found to be 15 (95% CI = 0-31)% longer than that of S(-)-bupivacaine. The difference between enantiomers in mean total body clearance thus was similar to the previous study based upon infusion to steady state of rac-bupivacaine. Differences in pharmacokinetics attributable to the dosage regimen consisted of a greater mean total body clearance for R(+)-bupivacaine along with a smaller terminal half life with the bolus regimen and a longer half-life of S(-)-bupivacaine after prolonged infusion. Differences in pharmacokinetics between the bupivacaine enantiomers occurred consistently in both distribution and clearance but the magnitude of the effect was less than 50% in each case. Systematic differences in pharmacokinetics associated with the dosage regimen were found mainly in terminal half-life. Dosage regimen, thus, was found to influence the pharmacokinetic results found experimentally and is therefore a significant variable in its own right.