Cysteine conjugate beta-lyase-dependent metabolism of compound A (2-[fluoromethoxy]-1,1,3,3,3-pentafluoro-1-propene) in human subjects anesthetized with sevoflurane and in rats given compound A

Metabolic and Pharmaceutical Aspects of Fluorinated Compounds

The applications of fluorine in drug design continue to expand, facilitated by an improved understanding of its effects on physicochemical properties and the development of synthetic methodologies that are providing access to new fluorinated motifs. In turn, studies of fluorinated molecules are providing deeper insights into the effects of fluorine on metabolic pathways, distribution, and disposition. Despite the high strength of the C-F bond, the departure of fluoride from metabolic intermediates can be facile. This reactivity has been leveraged in the design of mechanism-based enzyme inhibitors and has influenced the metabolic fate of fluorinated compounds. In this Perspective, we summarize the literature associated with the metabolism of fluorinated molecules, focusing on examples where the presence of fluorine influences the metabolic profile. These studies have revealed potentially problematic outcomes with some fluorinated motifs and are enhancing our understanding of how fluorine should be deployed.

Glutathione-dependent bioactivation

The classical view of the glutathione (GSH) conjugation pathway involves GSH S-transferase (GST)-dependent formation of thioether conjugates between GSH and an electrophilic substrate, processing to yield the corresponding cysteine S-conjugate, which is then converted to an N-acetylcysteine conjugate (or mercapturate). Mercapturates of most GST substrates are rendered more polar and thus readily excreted in urine. In contrast, there is a growing number of GST substrates that, rather than being detoxified, are bioactivated. These substrates include several halogenated solvents, many of which are nephrotoxic because of the tissue distribution of GSH conjugation pathway enzymes and membrane transporters, and prodrugs of certain chemotherapeutic agents. Although the initiating steps are the same regardless of whether the substrate is detoxified or bioactivated, the cysteine conjugate functions as a branch point. Bioactivated cysteine S-conjugates are metabolized in the kidneys by either cysteine conjugate β-lyase or flavin-containing monooxygenase to produce a reactive intermediate.

Urinary lipid and protein oxidation products upon halothane, isoflurane, or sevoflurane anesthesia in humans: potential biomarkers for a subclinical nephrotoxicity

OBJECTIVE

To investigate whether lipid and protein oxidation products are elevated and correlated with routine clinical markers of hepatic and renal function in patients anesthetized with halothane, isoflurane, or sevoflurane.

METHODS

Urine and blood samples were collected from patient groups. Excretion of aldehydes, acetone, and o,o'-dityrosine was measured before and after anesthesia. Blood samples were analysed for clinical markers.

RESULTS

Urinary concentrations of aldehydes, acetone, o,o'-dityrosine and glucose were significantly increased after anesthesia in halothane and sevoflurane groups earlier than clinical markers. Significant correlations were found in sevoflurane group.

CONCLUSION

Lipid and protein oxidation contributes to subclinical sevoflurane nephrotoxicity. Oxidation products may serve as early biomarkers.

Chemical Toxicology of Reactive Intermediates Formed by the Glutathione-Dependent Bioactivation of Halogen-Containing Compounds

The concept that reactive intermediate formation during the biotransformation of drugs and chemicals is an important bioactivation mechanism was proposed in the 1970s and is now accepted as a major mechanism for xenobiotic-induced toxicity. The enzymology of reactive intermediate formation as well as the characterization of the formation and fate of reactive intermediates are now well-established. The mechanism by which reactive intermediates cause cell damage and death is, however, still poorly understood. Although most xenobiotic-metabolizing enzymes catalyze the bioactivation of chemicals, glutathione-dependent biotransformation has been largely associated with detoxication processes, particularly mercapturic acid formation. Abundant evidence now shows that glutathione-dependent biotransformation constitutes an important bioactivation mechanism for halogen-containing drugs and chemicals and has for many compounds been implicated in their organ-selective toxicity and in their mutagenic and carcinogenic potential. The glutathione-dependent biotransformation of haloalkenes is the first step in the cysteine S-conjugate beta-lyase pathway for the bioactivation of nephrotoxic haloalkenes. This pathway has been a rich source of reactive intermediates, including thioacyl halides, alpha-chloroalkenethiolates, 3-halo-alpha-thiolactones, 2,2,3-trihalothiiranes, halothioketenes, and vinylic sulfoxides. Glutathione-dependent bioactivation of gem-dihalomethanes and 1,2-, 1,3-, and 1,4-dihaloalkanes leads to the formation of alpha-chlorosulfides, thiiranium ions, sulfenate esters, and tetrahydrothiophenium ions, respectively, and these reactions lead to reactive intermediate formation.

Compound A, formaldehyde and methanol concentrations during low-flow sevoflurane anaesthesia: comparison of three carbon dioxide absorbers

AIM

To determine compound A, formaldehyde and methanol concentrations in low-flow anaesthesia using different carbon dioxide absorbers.

METHODS

Fifteen patients scheduled for general or urological surgery were exposed to low-flow (500 ml/min) anaesthesia with sevoflurane. The patients were randomly allocated to three groups: soda lime, DrägerSorb Free or Amsorb Plus. The concentrations of compound A, formaldehyde and methanol were sampled and analysed from the limbs of the anaesthesia circuit at T30 (30 min after the start of low-flow sevoflurane anaesthesia), T90 (90 min) and T150 (150 min). The temperatures of the absorbers were measured at the same time.

RESULTS

Statistically significant differences (P < 0.05) were found in the production of compound A from soda lime (with the highest values), DrägerSorb Free and Amsorb Plus at each measurement time. Only traces of methanol (ranging from < 0.131 to 3.799 mg/m(3)) were measured, higher with Amsorb Plus (statistically significant differences were found only at T90). The formaldehyde values (ranging from < 0.1227 to 17.79 mcg/m(3) p.p.b.) were higher with soda lime, and the difference was statistically significant at T150 and, in the inspiratory limb only, at T90. The temperatures of the absorbers were higher for soda lime and lower for Amsorb Plus; the difference was statistically significant at T0 in the upper canister and at T30 in both canisters.

CONCLUSION

The concentrations of harmful products in the circuit were negligible and were lower using the new-generation absorbers. Using Amsorb Plus, the temperatures in the canisters were lower than with the other two absorbers.

Analyses of representative biomarkers of exposure and effect by chromatographic, mass spectrometric, and nuclear magnetic resonance techniques: method development and application in life sciences

Biomarkers are essential tools in monitoring studies, which include environmental monitoring, biological monitoring, biological effect monitoring, and health surveillance, as well as drug development processes. Their discovery, validation, and analysis require highly sensitive and selective analytical technologies. In this regard, gas and liquid chromatography, mass spectrometry, and nuclear magnetic resonance spectroscopy have facilitated great achievements in all these areas. In addition and closely related to biomarkers, the ongoing developments in these techniques promise a better understanding of the nature and mechanisms of toxic effects originating from various chemical, biological, or physical sources. This Review compiles studies performed on selected biomarkers with respect to both method development and application. Section 1 summarizes the concept of biomarkers; their application in various industrial/occupational, agricultural, drug developmental, and medical/clinical platforms. This section also focuses on biotransformation studies in close relation to biomarker discovery and validation, and on major techniques utilized in this area. In Section 2, biotransformation of volatile anesthetics in humans with a focus on mercapturic acid derivatives as potential biomarkers of effect is reviewed. The use of GC-ECD, GC/MS, and 19F-NMR in these studies is described. Section 3 focuses on the analysis of aldehydic lipid peroxidation degradation products by GC-ECD in mammalian cells in which oxidative stress induced chemically, and in humans after various challenges; anesthetic exposure, ischemia-reperfusion, and controlled endurance exercise. In Section 4, method development for protein and DNA oxidation products by LC-tandem MS and its application in mammalian cells and in humans are summarized. Possibilities, limitations, and future perspectives are discussed in Section 5.

Gene expression profiling of nephrotoxicity from the sevoflurane degradation product fluoromethyl-2,2-difluoro-1-(trifluoromethyl)vinyl ether ("compound A") in rats

The major degradation product of the volatile anesthetic sevoflurane, the haloalkene fluoromethyl-2,2-difluoro-1-(trifluoromethyl)vinyl ether (FDVE or "compound A"), is nephrotoxic in rats. FDVE undergoes complex metabolism and bioactivation, which mediates the nephrotoxicity. Nevertheless, the molecular and cellular mechanisms of FDVE toxification are unknown. This investigation evaluated the gene expression profile of kidneys in rats administered a nephrotoxic dose of FDVE. Male Fischer 344 rats (five per group) received 0.25 mmol/kg intraperitoneal FDVE or corn oil (controls) and were sacrificed after 24 or 72 h. Urine output and kidney histological changes were quantified. Kidney RNA was extracted for microarray analysis using Affymetrix GeneChip Rat Expression Array 230A arrays. Quantitative real-time PCR confirmed the modulation of several genes. FDVE caused significant diuresis and necrosis at 24 h, with normal urine output and evidence of tubular regeneration at 72 h. There were 517 informative genes that were differentially expressed >1.5-fold (p < 0.05) versus control at 24 h, of which 283 and 234 were upregulated and downregulated, respectively. Major classes of upregulated genes included those involved in apoptosis, oxidative stress, and inflammatory response (mostly at 24 h), and regeneration and repair; downregulated genes were generally associated with transporters and intermediary metabolism. Among the quantitatively most upregulated genes were kidney injury molecule, osteopontin, clusterin, tissue inhibitor of metalloproteinase 1, and TNF receptor 12, which have been associated with other forms of nephrotoxicity, and angiopoietin-like protein 4, glycoprotein nmb, ubiquitin hydrolase, and HSP70. Microarray results were confirmed by quantitative real-time PCR. FDVE causes rapid and brisk changes in gene expression, providing potential insights into the mechanism of FDVE toxification, and potential biomarkers for FDVE nephrotoxicity which are more sensitive than conventional measures of renal function.

Formation and toxicity of anesthetic degradation products

Toxic degradation products are formed from a range of old and modern anesthetic agents. The common element in the formation of degradation products is the reaction of the anesthetic agent with the bases in the carbon dioxide absorbents in the anesthesia circuit. This reaction results in the conversion of trichloroethylene to dichloroacetylene, halothane to 2-bromo-2-chloro-1,1-difluoroethylene, sevoflurane to 2-(fluoromethoxy)-1,1,3,3,3-pentafluoro-1-propene (Compound A), and desflurane, isoflurane, and enflurane to carbon monoxide. Dichloroacetylene, 2-bromo-2-chloro-1,1-difluoroethylene, and Compound A form glutathione S-conjugates that undergo hydrolysis to cysteine S-conjugates and bioactivation of the cysteine S-conjugates by renal cysteine conjugate beta-lyase to give nephrotoxic metabolites. The elucidation of the mechanisms of formation and bioactivation of degradation products has allowed for the safe use of anesthetics that may undergo degradation in the anesthesia circuit.

Use of 19F-nuclear magnetic resonance and gas chromatography-electron capture detection in the quantitative analysis of fluorine-containing metabolites in urine of sevoflurane-anaesthetized patients

1: The use of fluorine-19 nuclear magnetic resonance (19F-NMR) and gas chromatography-electron capture detection (GC-ECD) in the analysis of fluorine-containing products in the urine of sevoflurane-exposed patients was explored. 2: Ten patients were anaesthetized by sevoflurane for 135-660 min at a flow rate of 6 l min(-1). Urine samples were collected before, directly after and 24 h after discontinuation of anaesthesia. 3: 19F-NMR analysis of the urines showed the presence of several fluorine-containing metabolites. The main oxidative metabolite, hexafluoroisopropanol (HFIP)-glucuronide, showed two strong quartet signals in the 19F-NMR spectrum. HFIP concentrations after beta-glucuronidase treatment were quantified by (19)F-nuclear magnetic resonance. Concentrations directly after and 24 h after discontinuation of anaesthesia were 131 +/- 41 (mean +/- SEM) and 61 +/- 19 mol mg(-1) creatinine, respectively. Urinary HFIP excretions correlated with sevoflurane exposure. 4: Longer scanning times enabled the measurement of signals from two compound A-derived metabolites, i.e. compound A mercapturic acid I (CAMA-I) and compound A mercapturic acid II (CAMA-II), as well as products from beta-lyase activation of the respective cysteine conjugates of compound A. The signals of the mercapturic acids, 3,3,3-trifluoro-2-(fluoromethoxy)-propanoic acid and 3,3,3-trifluorolactic acid were visible after combining and concentrating the patient urines. CAMA-I and -II excretions in patients were completed after 24 h. 5: Since 19F-nuclear magnetic resonance is not sensitive enough, urinary mercapturic acids concentrations were quantified by gas chromatography-electron capture detection. CAMA-I and -II urinary concentrations were 2.3 +/- 0.7 and 1.4 +/- 0.4 mol mg(-1) creatinine, respectively. Urinary excretion of CAMA-I showed a correlation with sevoflurane exposure, whereas CAMA-II did not. 6. The results show that 19F-nuclear magnetic resonance is a very selective and convenient technique to detect and quantify HFIP in non-concentrated human urine. 19F-nuclear magnetic resonance can also be used to monitor the oxidative biotransformation of sevoflurane in anaesthetized patients. Compound A-derived mercapturic acids and 3,3,3-trifluoro-2-(fluoromethoxy)-propanoic acid and 3,3,3-trifluorolactic acid, however, require more sensitive techniques such as gas chromatography-electron capture detection and/or gas chromatography-mass spectrometry for quantification.

Role of Cytochrome P4503A in Cysteine S-Conjugates Sulfoxidatıon and the Nephrotoxicity of the Sevoflurane Degradatıon Product Fluoromethyl-2,2-difluoro-1-(trifluoromethyl)vinyl Ether (Compound A) in Rats

The volatile anesthetic sevoflurane is degraded to fluoromethyl-2,2-difluoro-1-(trifluoromethyl)vinyl ether (FDVE) in anesthesia machines. FDVE is nephrotoxic in rats. FDVE undergoes glutathione conjugation, subsequent conversion to cysteine and mercapturic acid conjugates, and cysteine conjugate metabolism by renal beta-lyase, which is a bioactivation pathway mediating nephrotoxicity in rats. Recent in vitro studies revealed cytochrome P4503A-catalyzed formation of novel sulfoxide metabolites of FDVE cysteine-S and mercapturic acid conjugates in rat liver and kidney microsomes. FDVE-mercapturic acid sulfoxides were more toxic than other FDVE conjugates to renal proximal tubular cells in culture. Nevertheless, the occurrence and toxicological significance of FDVE sulfoxides formation in vivo remain unknown. This investigation determined, in rats in vivo, the existence, role of P4503A, and nephrotoxic consequence of FDVE conjugates sulfoxidation. Rats were pretreated with dexamethasone, phenobarbital, troleandomycin, or nothing (controls) before FDVE, and then, nephrotoxicity, FDVE-mercapturate sulfoxide urinary excretion, and FDVE-mercapturate sulfoxidation by liver microsomes were assessed. The formation of FDVE-mercapturic acid sulfoxide metabolites in vivo and their urinary excretion were unambiguously established by mass spectrometry. Dexamethasone and phenobarbital increased, and troleandomycin decreased (i) liver microsomal FDVE-mercapturic acid sulfoxidation in vitro, (ii) FDVE-mercapturic acid sulfoxide urinary excretion in vivo, and (iii) FDVE nephrotoxicity in vivo assessed by renal histology, blood urea nitrogen concentrations, and urine volume and protein excretion. Urine 3,3,3-trifluoro-2-(fluoromethoxy)propanoic acid, reflecting beta-lyase-dependent FDVE-cysteine S-conjugates metabolism, was minimally affected by the pretreatments. These results demonstrate that FDVE S-conjugates undergo P4503A-catalyzed sulfoxidation in rats in vivo, and this sulfoxidation pathway contributes to nephrotoxicity. FDVE S-conjugates sulfoxidation constitutes a newly discovered mechanism of FDVE bioactivation and toxicification in rats, in addition to beta-lyase-catalyzed metabolism of FDVE-cysteine S-conjugates.

Sulfoxidation of Cysteine and Mercapturic Acid Conjugates of the Sevoflurane Degradation Product Fluoromethyl-2,2-difluoro-1-(trifluoromethyl)vinyl Ether (Compound A)

The volatile anesthetic sevoflurane is degraded in anesthesia machines to the haloalkene fluoromethyl-2,2-difluoro-1-(trifluoromethyl)vinyl ether (FDVE), which can cause renal and hepatic toxicity in rats. FDVE is metabolized to S-[1,1-difluoro-2-fluoromethoxy-2-(trifluoromethyl)ethyl]-L-cysteine (DFEC) and (E) and (Z)-S-[1-fluoro-2-fluoromethoxy-2-(trifluoromethyl)vinyl]-L-cysteine [(E,Z)-FFVC], which are N-acetylated to N-Ac-DFEC and (E,Z)-N-Ac-FFVC S-conjugates. Some haloalkene S-conjugates undergo sulfoxidation. This investigation tested the hypothesis that FDVE S-conjugates can also undergo sulfoxidation, by evaluating sulfoxide formation by human and rat liver and kidney microsomes and expressed P450s and flavin monooxygenases. Rat, and at lower rates human, liver microsomes oxidized (Z)-N-Ac-FFVC and N-Ac-DFEC to the corresponding sulfoxides. Much lower rates of (Z)-N-Ac-FFVC, but not N-Ac-DFEC, sulfoxidation occurred with rat and human kidney microsomes. In human liver microsomes, the P450 inhibitor 1-aminobenzotriazole completely inhibited S-oxidation, while heating to inactivate FMO decreased (Z)-N-Ac-FFVC and N-Ac-DFEC sulfoxidation only 0 and 30%, respectively. Of the various cytochrome P450s examined, P450s 3A4 and 3A5 had the highest S-oxidase activity toward (Z)-N-Ac-FFVC; P450 3A4 was the predominant enzyme forming N-Ac-DFEC-SO. The P450 3A inhibitors troleandomycin and ketoconazole inhibited >95% of (Z)-N-Ac-FFVC sulfoxidation by P450 3A4 and 3A5 and 40-100% of (Z)-N-Ac-FFVC sulfoxidation by human liver microsomes and 15-85% of N-Ac-DFEC sulfoxidation by human liver microsomes. Sulfoxidation of DFEC was also examined in human liver microsomes. Substantial amounts of sulfoxide were observed, even in the absence of NADPH or protein, while enzymatic formation was comparatively minimal. These results show that FDVE S-conjugates undergo P450-catalyzed and nonenzymatic sulfoxidation and that enzymatic sulfoxidation of (Z)-N-Ac-FFVC and N-Ac-DFEC is catalyzed predominantly by P450 3A. The extent of FDVE sulfoxidation in vivo and the toxicologic significance of FDVE sulfoxides remain unknown and merit further investigation.

Cytotoxicity of S-conjugates of the sevoflurane degradation product fluoromethyl-2,2-difluoro-1-(trifluoromethyl) vinyl ether (Compound A) in a human proximal tubular cell line

Fluoromethyl-2,2-difluoro-1-(trifluoromethyl)vinyl ether (FDVE) is a fluorinated alkene formed by degradation of the volatile anesthetic sevoflurane in anesthesia machines. FDVE is nephrotoxic in rats but not humans. Rat FDVE nephrotoxicity is attributed to FDVE glutathione conjugation and bioactivation of subsequent FDVE-cysteine S-conjugates, in part by renal beta-lyase. Although FDVE conjugation and metabolism occur in both rats and humans, the mechanism for selective toxicity in rats and lack of effect in humans is incompletely elucidated. This investigation measured FDVE S-conjugate cytotoxicity in cultured human proximal tubular HK-2 cells, and compared this with known cytotoxic S-conjugates. HK-2 cells were incubated with FDVE and its GSH, cysteine S-mercapturic acid, cysteine S-sulfoxide, and mercapturic acid sulfoxide conjugates (0.1-2.7 mM) for 24 h. Cytotoxicity was determined by lactate dehydrogenase (LDH) release, total LDH, and the ability of viable cells to reduce a tetrazolium-based compound (MTT). FDVE was cytotoxic only at concentrations >/=0.9 mM. No increase in LDH release was observed with either FDVE-GSH conjugate. The FDVE-cysteine conjugates S-(1,1-difluoro-2-fluoromethoxy-2-(trifluoromethyl) ethyl)-L-cysteine (DFEC) and (Z)-S-(1-fluoro-2-fluoromethoxy-2-(trifluoromethyl) vinyl)-L-cysteine ((Z)-FFVC) caused significant differences in LDH release and MTT reduction only at 2.7 mM; (Z)-FFVC was slightly more cytotoxic. Both S-(1,1-difluoro-2-fluoromethoxy-2-(trifluoromethyl) ethyl)-L-cysteine sulfoxide (DFEC-SO) and (Z)-N-acetyl-S-(1-fluoro-2-fluoromethoxy-2-(trifluoromethyl) vinyl)-L-cysteine sulfoxide ((Z)-N-Ac-FFVC-SO) caused slightly greater changes in LDH release or total LDH than the corresponding equimolar DFEC and (Z)-N-acetyl-S-(1-fluoro-2-fluoromethoxy-2-(trifluoromethyl) vinyl)-L-cysteine ((Z)-N-Ac-FFVC) conjugates. In contrast to FDVE S-conjugates, S-(1,2-dichlorovinyl)-L-cysteine was markedly cytotoxic, at concentrations as low as 0.1 mM. These results show that human proximal tubular cells are relatively resistant to FDVE and FDVE S-conjugate cytotoxicity. This may partially explain the lack of FDVE nephrotoxicity in humans.

Halogenated inhalational anaesthetics

The halogenated inhalational anaesthetics halothane, enflurane, isoflurane and desflurane can produce metabolic hepatocellular injury in humans to a variable extent. During metabolism of these anaesthetics, tissue acetylation occurs due to the formation of reactive intermediates. Proteins modified by acetylation may constitute neo-antigens with a potential for triggering an antibody-mediated immune response. The likelihood of suffering post-operative immune hepatitis depends on the amount of the anaesthetic metabolized and is thereby considerably less with enflurane, isoflurane or desflurane compared with halothane. Plasma inorganic fluoride concentrations are regularly increased after sevoflurane. Elevated inorganic fluoride concentrations have been associated with nephrotoxicity following methoxyflurane anaesthesia but not after sevoflurane. Another source of concern is the products of degradation from reactions with carbon dioxide absorbents. Most important is compound A, which has been shown to exhibit nephrotoxicity in rodents. However, no significant changes in renal function parameters have been reported in surgical patients.

Sevoflurane low-flow anaesthesia: best strategy to reduce Compound A concentration

BACKGROUND

To define the best strategy to reduce Compound A production in Sevoflurane low-flow anaesthesia by experiments in vitro and in vivo of different absorbers and different anaesthesia machines.

METHODS

In vitro Compound A has been measured at 45 degrees C in vitro following Sevoflurane interactions with potassium hydroxide, sodium hydroxide, soda lime, Dragersorb 800 Plus and Amsorb, a new absorber that does not contain sodium or potassium hydroxide. In vivo Compound A concentration in the anaesthesia circuit (inspiratory branch) has been measured using an indirect sampling method through absorber vials (SKC) with active coal granules, during low flows (500 ml/min) general anaesthesia using soda lime, Dragersorb 800 Plus or Amsorb as absorber. Compound A was also measured during low flows (500 ml/min) general anaesthesia using as carbon dioxide absorber soda lime with different anaesthesia machines.

RESULTS

In vitro at 45 degrees C Compound A concentration with soda lime and Dragersorb 800 Plus was about 10 times higher than with Amsorb. In vivo the Compound A concentrations in the inspiratory branch of the circuit were lower in the group with Amsorb.

CONCLUSION

The Compound A production is minimal with Amsorb as carbon dioxide absorber.

Toxic, halogenated cysteine S-conjugates and targeting of mitochondrial enzymes of energy metabolism

Several haloalkenes are metabolized in part to nephrotoxic cysteine S-conjugates; for example, trichloroethylene and tetrafluoroethylene are converted to S-(1,2-dichlorovinyl)-L-cysteine (DCVC) and S-(1,1,2,2-tetrafluoroethyl)-L-cysteine (TFEC), respectively. Although DCVC-induced toxicity has been investigated since the 1950s, the toxicity of TFEC and other haloalkene-derived cysteine S-conjugates has been studied more recently. Some segments of the US population are exposed to haloalkenes either through drinking water or in the workplace. Therefore, it is important to define the toxicological consequences of such exposures. Most halogenated cysteine S-conjugates are metabolized by cysteine S-conjugate beta-lyases to pyruvate, ammonia, and an alpha-chloroenethiolate (with DCVC) or an alpha-difluoroalkylthiolate (with TFEC) that may eliminate halide to give a thioacyl halide, which reacts with epsilon-amino groups of lysine residues in proteins. Nine mammalian pyridoxal 5'-phosphate (PLP)-containing enzymes catalyze cysteine S-conjugate beta-lyase reactions, including mitochondrial aspartate aminotransferase (mitAspAT), and mitochondrial branched-chain amino acid aminotransferase (BCAT(m)). Most of the cysteine S-conjugate beta-lyases are syncatalytically inactivated. TFEC-induced toxicity is associated with covalent modification of several mitochondrial enzymes of energy metabolism. Interestingly, the alpha-ketoglutarate- and branched-chain alpha-keto acid dehydrogenase complexes (KGDHC and BCDHC), but not the pyruvate dehydrogenase complex (PDHC), are susceptible to inactivation. mitAspAT and BCAT(m) may form metabolons with KGDHC and BCDHC, respectively, but no PLP enzyme is known to associate with PDHC. Consequently, we hypothesize that not only do these metabolons facilitate substrate channeling, but they also facilitate toxicant channeling, thereby promoting the inactivation of proximate mitochondrial enzymes and the induction of mitochondrial dysfunction.

Reactive intermediate formation from the 2-(Fluoromethoxy)-1,1,3,3,3-pentafluoro-1-propene (compound A)-derived cysteine S-conjugate S-[2-(Fluoromethoxy)-1,1,3,3,3-pentafluoropropyl]-L-cysteine in pyridoxal model systems

2-(Fluoromethoxy)-1,1,3,3,3-pentafluoro-1-propene (compound A) is a degradation product of the anesthetic sevoflurane and undergoes cysteine conjugate beta-lyase-dependent bioactivation to nephrotoxic metabolites in rats. The present experiments were designed to identify reactive intermediates formed from S-[2-(fluoromethoxy)-1,1,3,3,3-pentafluoropropyl]-L-cysteine, a compound A-derived cysteine S-conjugate, in two pyridoxal model systems, namely Cu2+/pyridoxal and N-dodecylpyridoxal in cetyltrimethylammonium micelles. S-[2-(Fluoromethoxy)-1,1,3,3,3-pentafluoropropyl]-L-cysteine was incubated in the model systems with benzyl bromide, pentafluorobenzyl bromide, aniline, and o-phenylenediamine as trapping agents. The products were purified by TLC and identified by 19F and 1H NMR spectroscopy and by GC/MS. In the absence of trapping agents, 2-(fluoromethoxy)-3,3,3-trifluoropropanoic acid and 3,3,3-trifluorolactic acid, which have been identified previously in biotransformation studies, were formed. With the chemical models, 2-(fluoromethoxy)-1,1,3,3,3-pentafluoropropanethiolate, the expected first intermediate, was not trapped with benzyl bromide. Rather, the dehydrofluorination product 2-(fluoromethoxy)-1,3,3,3-tetrafluoro-1-propenylthiolate was trapped with benzyl bromide to give benzyl 2-(fluoromethoxy)-3,3,3-trifluoropropanethioate, which was formed in both chemical models. When pentafluorobenzyl bromide was used as a trapping agent, GC/MS analysis showed that the expected thiolate was trapped to give pentafluorobenzyl 2-(fluoromethoxy)-1,1,3,3,3-pentafluoropropyl sulfide in the N-dodecylpyridoxal model. In both chemical models, 2-(fluoromethoxy)-3,3,3-trifluorothioacyl fluoride was trapped with aniline to give N-phenyl 2-(fluoromethoxyl)-3,3,3-trifluoropropanethioamide, which cyclized to give 3-phenyl-4-thiono-5-(trifluoromethyl)-1,3-oxazolane. The results demonstrate that most of the reactive intermediates and products formed by the beta-lyase-catalyzed biotransformation of compound A-derived cysteine S-conjugates are also formed in the two chemical systems studied. Some products were, however, formed in chemical systems that have not been observed in previous in vivo and in vitro studies; it is not known whether these products are formed in biological systems and whether they contribute to the observed nephrotoxicity of cysteine S-conjugates.

Biotransformation of L-cysteine S-conjugates and N-acetyl-L-cysteine S-conjugates of the sevoflurane degradation product fluoromethyl-2,2-difluoro-1-(trifluoromethyl)vinyl ether (compound A) in human kidney in vitro: interindividual variability in N-acetylation, N-deacetylation, and beta-lyase-catalyzed metabolism

Fluoromethyl-2,2-difluoro-1-(trifluoromethyl)vinyl ether (FDVE; 1) is a fluoroalkene formed by the base-catalyzed degradation of the anesthetic sevoflurane. FDVE is nephrotoxic in rats. In both rats and humans, FDVE undergoes glutathione-dependent conjugation, cleavage to cysteine S-conjugates, and renal beta-lyase-catalyzed metabolism to reactive intermediates, which may cause nephrotoxicity. Interindividual variability in renal metabolism of FDVE is unknown. Therefore, this investigation quantified beta-lyase-catalyzed bioactivation and N-acetyltransferase-catalyzed inactivation of FDVE cysteine S-conjugates and reactivation of mercapturates by N-deacetylase in cytosol and microsomes from 20 human kidneys. In cytosol, N-acetylation ranged from 0.008 to 0.045 (0.024 +/- 0.01) nmol of mercapturate/mg/min and 0.001 to 0.07 (0.024 +/- 0.02) nmol of mercapturate/mg/min for alkane and alkene cysteine S-conjugates, respectively. Similar results for microsomal N-acetylation were obtained; N-acetylation ranged from 0.005 to 0.055 (0.025 +/- 0.02) nmol of mercapturate/mg/min and 0.001 to 0.06 (0.030 +/- 0.02) nmol of mercapturate/mg/min for alkane and alkene cysteine S-conjugates, respectively. Beta-lyase-catalyzed metabolism to pyruvate varied from 0.004 to 0.14 (0.051 +/- 0.04) nmol/mg/min and from 0.10 to 0.40 (0.26 +/- 0.08) nmol/mg/min for alkane and alkene cysteine-S-conjugates, respectively. N-deacetylation of mercapturates ranged from 0.8 to 2.5 (1.25 +/- 0.57) nmol of cysteine S-conjugate formed/mg/min and 0.05 to 0.37 (0.17 +/- 0.10) nmol of cysteine S-conjugate formed/mg/min for alkane and alkene FDVE mercapturates. Cytosolic cysteine S-conjugates metabolism by renal beta-lyase predominated over N-acetylation (ratio of activities was 0.2-6 and 3-146 for the alkane and alkene cysteine S-conjugates). N-deacetylation predominated over N-acetylation (ratio of activities was 20-205 and 2-54 for alkane and alkene S-conjugates). There was considerable (up to 50-fold) interindividual variability in rates of FDVE toxication (beta-lyase metabolism and N-deacetylation) and detoxication. This interindividual variability may effect individual susceptibility to the nephrotoxicity of FDVE and other haloalkenes.

Glutathione S-conjugation of the sevoflurane degradation product, fluoromethyl-2,2-difluoro-1-(trifluoromethyl)vinyl ether (compound A) in human liver, kidney, and blood in vitro

Fluoromethyl-2,2-difluoro-1-(trifluoromethyl)vinyl ether (FDVE) is a fluorinated alkene formed by degradation of the volatile anesthetic sevoflurane in anesthesia machines. FDVE is nephrotoxic in rats and undergoes glutathione-dependent conjugation to form two alkane (G1, G2) and two alkene glutathione S-conjugates (G3, G4), cleavage to cysteine S-conjugates, and beta-lyase-catalyzed metabolism to reactive thionoacyl fluorides, which may react with cellular macromolecules to cause nephrotoxicity. Although similar metabolites have been identified in human urine in vivo, little is known about sites and mechanisms of GSH conjugation in humans. This investigation quantified FDVE-GSH conjugates formed by human hepatic and renal microsomal and cytosolic fractions and blood in vitro. LC-MS/MS analysis identified all four GSH conjugates (G1-G4) formed in all human subcellular fractions. Quantitative analysis indicated that the relative order of formation was G2 > G1 > G4 > G3 with human liver and kidney subfractions. In blood, the order was G1 > G4 > G2 > G3. These results demostrate that FDVE undergoes GSH-dependent conjugation in human liver and kidney microsomes and cytosol as well as blood, which may account for the detection of corresponding mercapturic acids in the urine of patients exposed to FDVE.

Separation and absolute configuration of the enantiomers of a degradation product of the new inhalation anesthetic sevoflurane

In a rebreathing anesthesia circuit, the inhaled anesthetic sevoflurane degrades into at least two products, termed "compound A" and "compound B." The enantiomer separation of the chiral compound B (1,1,1,3,3-pentafluoro-2-(fluoromethoxy)-3-methoxypropane ) by capillary gas chromatography (cGC) using heptakis (2,3-di-O-acetyl-6-O-tert-butyldimethylsilyl)-beta-cyclodextrin as chiral selector was studied. With this cyclodextrin derivative diluted in the polysiloxane PS 86, an unprecedented high separation factor alpha of 4.1 (at 30 degrees C) was found. Consequently, the enantiomers of compound B were isolated by preparative GC and their specific rotations were measured. In addition, their absolute configurations were determined by X-ray crystallography. To collect the X-ray data, single crystals of both enantiomers were grown in situ on the diffractometer. The levorotatory enantiomer B(-) has the R-configuration while the dextrorotatory enantiomer B(+) has the S-configuration. The elution order of the compound B enantiomers on heptakis (2,3-di-O-acetyl-6-O-tert-butyldimethylsilyl)-beta-cyclodextrin is R before S.

Dose-dependent metabolism of fluoromethyl-2,2-difluoro-1-(trifluoromethyl)vinyl ether (compound A), an anesthetic degradation product, to mercapturic acids and 3,3,3-trifluoro-2-(fluoromethoxy)propanoic acid in rats

The volatile anesthetic sevoflurane is degraded in anesthesia machines to fluoromethyl-2,2-difluoro-1-(trifluoromethyl)vinyl ether (FDVE), to which humans are exposed. FDVE is metabolized in rats and humans to two alkane and two alkene glutathione S-conjugates that are hydrolyzed to the corresponding cysteine S-conjugates. The latter are N-acetylated to mercapturic acids, or bioactivated by renal cysteine conjugate beta-lyase to metabolites which may react with cellular macromolecules or hydrolyze to 3,3,3-trifluoro-2-(fluoromethoxy)propanoic acid. FDVE causes nephrotoxicity in rats, which evidence suggests is mediated by renal uptake of FDVE S-conjugates and metabolism by beta-lyase. Although pathways of FDVE metabolism have been described qualitatively, the purpose of this investigation was to quantify FDVE metabolism via mercapturic acid and beta-lyase pathways. Fischer 344 rats underwent 3-h nose-only exposure to FDVE (0 +/- 0, 46 +/- 19, 98 +/- 7, 150 +/- 29, and 220 +/- 40 ppm), and urine was collected for 24 h. Urine concentrations of the mercapturates, N-acetyl-S-(1,1,3,3, 3-pentafluoro-2-fluoromethoxypropyl)-L-cysteine and N-acetyl-S-(1-fluoro-2-fluoromethoxy-2-(trifluoromethyl)vinyl)-L- cysteine, the beta-lyase-dependent metabolite 3,3, 3-trifluoro-2-(fluoromethoxy)propanoic acid, and its degradation product trifluorolactic acid, were determined by GC/MS. There was dose-dependent urinary excretion of the alkane mercapturate N-acetyl-S-(1,1,3,3,3-pentafluoro-2-fluoromethoxypropyl)-L- cysteine and 3,3,3-trifluoro-2-(fluoromethoxy)propanoic acid, while excretion of the alkene mercapturate N-acetyl-S-(1-fluoro-2-fluoromethoxy-2-(trifluoromethyl)vinyl)-L- cysteine plateaued at higher FDVE exposures. The alkane:alkene mercapturic acid excretion ratio was between 2:1 and 4:1. Trifluorolactic acid was only rarely observed. Urine excretion of the beta-lyase-dependent metabolite 3,3, 3-trifluoro-2-(fluoromethoxy)propanoic acid was 10-fold greater than that of the combined mercapturates. Results show that FDVE cysteine S-conjugates undergo facile metabolism via renal beta-lyase, particularly in comparison with detoxication by mercapturic acid formation. The quantitative assay developed herein may provide a biomarker for FDVE exposure and relative metabolism via toxification and detoxifying pathways, applicable to animal and human investigations.

Carbon dioxide absorption: toxicity from sevoflurane and desflurane

The degradation of volatile anaesthetics by desiccated carbon dioxide absorbents can result in adverse outcomes. Desiccated carbon dioxide absorbent reacting with desflurane can cause potentially life-threatening intraoperative carbon monoxide exposure; the reaction with sevoflurane can cause the formation of several toxic breakdown products, e.g. compound A. Compound A-related renal toxicity in humans is still a matter of controversy.

[Adsorption of carbon dioxide gas]

OBJECTIVES

To analyse the various methods for carbon dioxide absorption in anaesthesia, the available absorbents and their modes of use.

DATA SOURCES

We searched the Medline and Internet databases for papers using the key words: carbon dioxide absorption, soda-lime, zeolite. We also had correspondence and contacts with soda lime manufacturers.

STUDY SELECTION

All types of articles containing data on CO2 absorption.

DATA EXTRACTION

The articles were analysed for the benefits and adverse effects of the various absorbents.

DATA SYNTHESIS

Carbon dioxide absorption enables the use of low flow anaesthesia, and a decreased consumption of medical gases and halogenated anaesthetics, as well as reduced pollution. Chemical absorbents (soda-lime and barium hydroxide lime (Baralyme) may produce toxic compounds: carbon monoxide with all halogenated anaesthetics and compound A with sevoflurane. Simple measures against desiccation of the lime prevent carbon monoxide production. The toxicity of compound A, shown in the rat, has not been proven in clinical anaesthesia. Recent improvements in manufacture processes have decreased the powdering of lime. Moreover, filters inserted between the anaesthesia circuit and the patient abolish the risk for powder inhalation.