Stereochemistry in anaesthesia

Mechanisms underlying neonate-specific metabolic effects of volatile anesthetics

Volatile anesthetics (VAs) are widely used in medicine, but the mechanisms underlying their effects remain ill-defined. Though routine anesthesia is safe in healthy individuals, instances of sensitivity are well documented, and there has been significant concern regarding the impact of VAs on neonatal brain development. Evidence indicates that VAs have multiple targets, with anesthetic and non-anesthetic effects mediated by neuroreceptors, ion channels, and the mitochondrial electron transport chain. Here, we characterize an unexpected metabolic effect of VAs in neonatal mice. Neonatal blood β-hydroxybutarate (β-HB) is rapidly depleted by VAs at concentrations well below those necessary for anesthesia. β-HB in adults, including animals in dietary ketosis, is unaffected. Depletion of β-HB is mediated by citrate accumulation, malonyl-CoA production by acetyl-CoA carboxylase, and inhibition of fatty acid oxidation. Adults show similar significant changes to citrate and malonyl-CoA, but are insensitive to malonyl-CoA, displaying reduced metabolic flexibility compared to younger animals.

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.

Pharmacokinetics and Placental Transfer of Bupivacaine Enantiomers in HIV-Infected Parturient Women on Antiretroviral Therapy

Bupivacaine, a local anesthetic, is commercialized as a racemic mixture of R-bupivacaine and S-bupivacaine enantiomers. HIV infection increases the expression of placental P-glycoprotein (P-gp), and antiretroviral (ARV) therapy inhibits cytochrome P450 3A and P-gp. The present study evaluates the kinetic disposition of bupivacaine enantiomers in HIV-infected parturient women on ARV therapy. In this study, HIV-infected parturient women (n = 10) treated with zidovudine, lamivudine, lopinavir, and ritonavir were investigated. Anesthesia and/or analgesia was achieved by the administration of 0.5% racemic bupivacaine hydrochloride with 1:200000 epinephrine in the epidural space at doses of 2.5 to 22.5 mg. Maternal serial blood samples were obtained at the time immediately before and 5, 15, 30, 45, and 60 minutes and 2, 4, 6, 8, 10, 12, and 14 hours after administration of the bupivacaine. At the time of delivery, samples of maternal and umbilical cord vein blood were also collected. The results suggest that bupivacaine pharmacokinetics are enantioselective, revealing higher maternal plasma concentrations of the R-bupivacaine enantiomer (area under the total plasma concentration-time curve was calculated by the trapezoid method with extrapolation to infinity/dose_(S)/(R) = 0.91; P < .05). The plasma unbound fraction of the drug (0.09 vs 0.06) and the umbilical cord vein/maternal plasma ratio (0.47 vs 0.39) were higher for the R-bupivacaine enantiomer than the S-bupivacaine enantiomer (P < .05). ARV therapy with ritonavir confers an enantioselective interaction between the enantiomers of bupivacaine and placental P-gp, producing greater inhibition of efflux transport of the R-bupivacaine enantiomer. Possible changes in the well-being of the fetuses of mothers under analgesia may be a consequence of the increased placental transfer of bupivacaine enantiomers to the fetal circulation.

Neuraxial drug administration: a review of treatment options for anaesthesia and analgesia

Neuraxial drug administration describes techniques that deliver drugs in close proximity to the spinal cord, i.e. intrathecally into the CSF or epidurally into the fatty tissues surrounding the dura, by injection or infusion. This approach was initially developed in the form of spinal anaesthesia over 100 years ago. Since then, neuraxial drug administration has evolved and now includes a wide range of techniques to administer a large number of different drugs to provide anaesthesia, but also analgesia and treatment of spasticity in a variety of acute and chronic settings. This review concentrates on the pharmacological agents used and the clinical basis behind currently utilised approaches to neuraxial drug administration. With regard to local anaesthetics, the main focus is on the development of the enantiomer-specific compounds ropivacaine and levobupivacaine, which provide similar efficacy to bupivacaine with a reduced risk of severe cardiotoxicity. Opioids are the other group of drugs widely used neuraxially, in particular to provide analgesia alone or more commonly in combination with other agents. The physicochemical properties of the various opioids explain the main differences in efficacy and safety between these drugs when used intrathecally, of which morphine, fentanyl and sufentanil are most commonly used. Another group of drugs including clonidine, dexmedetomidine and epinephrine (adrenaline) provide neuraxial analgesia via alpha-adrenergic receptors and are used mainly as adjuvants to local anaesthetics and opioids. Furthermore, intrathecal baclofen is in routine clinical use to treat spasticity in a number of neurological conditions. Beside these established approaches, a wide range of other drugs have been assessed for neuraxial administration to provide analgesia; however, most are in various early stages of investigation and are not used routinely. These drugs include neostigmine, ketamine, midazolam and adenosine, and the conotoxin ziconotide. The latter is possibly the most unusual compound here; it has recently gained registration for intrathecal use in specific chronic pain conditions.

Stereoselectivity in the placental transfer and kinetic disposition of racemic bupivacaine administered to parturients with or without a vasoconstrictor

The aim of the present study was to investigate the stereoselectivity in the kinetic disposition and the transplacental distribution of bupivacaine in term parturients during labor. Maternal age ranged from 18-37 years and fetal gestational age from 37.6-41.5 weeks. Healthy parturients (n = 23) received epidural 0.5% racemic bupivacaine alone (group A) or combined with epinephrine (group B). Maternal venous blood was sampled at regular intervals until 8 h after drug administration and umbilical venous blood was obtained at delivery. Bupivacaine enantiomers were determined in plasma samples by HPLC using a Chiralcel(R) OD-R column and a UV detector. One- or two-compartment models were fitted to data and differences between the (+)-(R) and (-)-(S) enantiomers were compared with the paired Wilcoxon test (P< 0.05). The influence of epinephrine was evaluated using the unpaired Mann-Whitney test (P< 0.05). The disposition of bupivacaine in maternal plasma was stereoselective, with higher V(d/f) (140.60 vs. 132.81 L for group A and 197.86 vs. 169.46 L for group B) and C(l/f) (29.00 vs. 25.43 L/h for group A and 33.15 vs. 26.39 L/h for group B) and lower t(1/2)beta (3.24 vs. 3.30 h for group A and 4.36 vs. 4.45 h for group B) being observed for (+)-(R)-bupivacaine. The combined administration of epinephrine resulted in higher V(d/f) (197.86 vs. 140.60 L for (+)-(R) and 169.46 vs. 132.81 L for (-)-(S)) and t(1/2)beta values (4.36 vs. 3.24 h for (+)-(R) and 4.45 vs. 3.30 h for (-)-(S)). The transplacental distribution of bupivacaine was stereoselective only when bupivacaine was administered without epinephrine (group B), with a higher cord blood/maternal blood ratio being observed for (-)-(S)-bupivacaine (0.40 vs. 0.35). Chirality 16:65-71, 2004.

Design, synthesis, and evaluation of analogues of 3,3,3-trifluoro-2-hydroxy-2-phenyl-propionamide as orally available general anesthetics

We have recently discovered a novel class of compounds that have oral general anesthetic activity, potent anticonvulsant activity, and minimal hemodynamic effects. The 3,3,3-trifluoro-2-hydroxy-2-phenyl-propionamide (1) demonstrated potent ability to reduce the minimum alveolar concentration (MAC) of isoflurane, with no effects on heart rate or blood pressure at therapeutic concentrations. Analogue 1 also had potent oral anticonvulsant activity against maximal electroshock (MES) and subcutaneous metrazol (scMET) models with a therapeutic index of 10 for MES activity. In this study, we further synthesized nine new racemic analogues and evaluated these compounds for effects on isoflurane MAC reduction and blood pressure. Preliminary data demonstrate potent reduction in the isoflurane MAC for two new compounds. Current mechanistic studies were unrevealing for effects on voltage-gated ion channels as a putative mechanism. Liposomal partitioning studies using (19)F NMR reveal that the aromatic region partitions into the core of the lipid. This partitioning correlated with general anesthetic activity of this class of compounds. Further, compound 1 was used at a concentration of 1 mM and slightly enhanced GABA(A) current in hippocampal neurons at 10 microM. Altogether, 3,3,3-trifluoro-2-hydroxy-2-phenyl-propionamide exhibited excellent oral general anesthetic activity and appears devoid of significant side effects (i.e., alterations in blood pressure or heart rate).

Inhibitory effects of S-(-) and R-(+) bupivacaine on neutrophil function

BACKGROUND

Local anesthetics inhibit migration, enzyme release and superoxide anion generation of polymorphonuclear leukocytes (PMN). Due to their ability to phagocytose and kill bacteria PMN represent a major defense mechanism in the circulating blood. In this study we determined the influence of racemic bupivacaine and its enantiomers on neutrophil phagocytic activity, oxidative burst as well as surface expression of complement and Fcgamma receptors.

METHODS

Venous blood was pre-incubated with different concentrations of either racemic bupivacaine, R-(+) or S-(-) bupivacaine. Fluoresceine isothiocyanate (FITC)-labeled antibodies against Fcgamma receptor III (CD16), complement receptor 1 (CD35) and complement receptor 3 (CD11b) were used to determine surface receptor expression. Phagocytic activity was measured by ingestion of FITC-labeled vital Staphylococcus aureus. Oxidative burst was determined by conversion of nonfluorescent dihydrorhodamine 123 into fluorescent rhodamine 123. Fluorescent intensity of each sample was determined by flow cytometry.

RESULTS

Racemic bupivacaine inhibited surface receptor expression, phagocytosis, and oxidative burst in a time- and concentration-dependent manner. Although the S-(-) enantiomer exerted significantly less inhibitory action on neutrophil function compared to R-(+) and racemic bupivacaine, these effects were small compared to the overall changes.

CONCLUSION

These findings suggest that bupivacaine impairs surface receptor expression and may thereby contribute to reduced phagocytic activity and oxidative burst. Enantiomer-specific effects of bupivacaine may play a minor role in the inhibition of these leukocyte functions.

Levobupivacaine

Each month, subscribers to The Formulary® Monograph Service receive five to six researched monographs on drugs that are newly released or are in late Phase III trials. The monographs are targeted to your Pharmacy and Therapeutics Committee. Subscribers also receive monthly one-page summary monographs on the agents that are useful for agendas and pharmacy/nursing in-services. A comprehensive target drug utilization evaluation (DUE) is also provided each month. The monographs are published in printed Form and on diskettes that allow customization. Subscribers to the The Formulary Monograph Service also receive access to a pharmacy bulletin board called The Formulary InFormation Exchange (The F.I.X). All topics pertinent to clinical pharmacy are discussed on The F.I.X.

Through the cooperation of The Formulary, Hospital Pharmacy publishes selected reviews in this column. If you would like information about The Formulary Monograph Service or The F.I.X., call The Formulary at 800-322-4349. The November 1999 Formulary monographs are on rapacuronium, rabeprazole, zaleplon, alosetron, and ganirelix acetate. The DUE is on zaleplon.

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.