Evidence for metabolism of fluoromethyl 2,2-difluoro-1-(trifluoromethyl)vinyl ether (compound A), a sevoflurane degradation product, by cysteine conjugate beta-lyase

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.

Acivicin-induced alterations in renal and hepatic glutathione concentrations and in γ-glutamyltransferase activities

gamma-Glutamyltransferase (gamma-GT) catalyzes the hydrolysis of glutathione, glutathione S-conjugates, and gamma-substituted l-glutamate derivatives. Acivicin is an irreversible inhibitor of gamma-GT that has been used to study the role of gamma-GT in glutathione homeostasis and glutathione-dependent bioactivation reactions. The present studies were undertaken because of reported conflicting effects of acivicin on the nephrotoxicity of some haloalkenes that undergo glutathione-dependent bioactivation. The objective of this study was to test the hypothesis that acivicin may alter renal glutathione concentrations; acivicin-induced changes in renal glutathione concentrations may alter the susceptibility of the kidney to the nephrotoxic effects of haloalkenes. Hence, diurnal and acivicin-induced changes in renal and hepatic glutathione concentrations along with renal and hepatic gamma-GT activities were investigated. The previously observed diurnal variations in hepatic glutathione concentrations in fed rats were confirmed, but no diurnal variations were observed in renal glutathione concentrations or in renal or hepatic gamma-GT activities. Renal and hepatic glutathione concentrations and gamma-GT activities were measured in tissue homogenates from rats given 0, 0.1, or 0.2 mmol acivicin/kg (i.p.) and killed 0, 2, 4, 8, 12, or 24 hr later. Renal glutathione concentrations were increased above control values in acivicin-treated rats, whereas acivicin had no effect on hepatic glutathione concentrations. Renal gamma-GT activities decreased within 2 hr after giving acivicin and remained decreased for 24 hr. Acivicin had no effect on hepatic gamma-GT activities, except at 24 hr after treatment when values in acivicin-treated rats were elevated compared with controls. Although the present studies do not afford an explanation of the mechanism whereby acivicin increases the nephrotoxicity of some haloalkenes, they do indicate that acivicin is not a reliable probe to investigate the role of gamma-GT in haloalkene-induced nephrotoxicity.

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.

Renal toxicity with sevoflurane: a storm in a teacup?

The inhaled anaesthetic sevoflurane is metabolised into two products that have the potential to produce renal injury. Fluoride ions are produced by oxidative defluorination of sevoflurane by the cytochrome P450 system in the liver. Until recently, inorganic fluoride has been thought to be the aetiological agent responsible for fluorinated anaesthetic nephrotoxicity, with a toxic concentration threshold of 50 micromol/L in serum. However, studies of sevoflurane administration in animals and humans have not shown evidence of fluoride-induced nephrotoxicity, despite serum fluoride concentrations in this range. Compound A (fluoromethyl-2,2-difluoro-1-[trifluoromethyl] vinyl ether) is a breakdown product of sevoflurane produced by its interaction with carbon dioxide absorbents in the anaesthesia machine. The patient then inhales compound A. Compound A produces evidence of transient renal injury in rats. The mechanism of compound A renal toxicity is controversial, with the debate focused on the role of the renal cysteine conjugate beta-lyase pathway in the biotransformation of compound A. The significance of this debate centres on the fact that the beta-lyase pathway is 10- to 30-fold less active in humans than in rats. Therefore, if biotransformation by this pathway is responsible for the production of nephrotoxic metabolites of compound A, humans may be less susceptible to compound A renal toxicity than are rats. In three studies in human volunteers and one in surgical patients, prolonged (8-hour) sevoflurane exposures and low fresh gas flow rates resulted in significant exposures to compound A. Transient abnormalities were found in biochemical markers of renal injury measured in urine. These studies suggested that sevoflurane can result in renal toxicity, mediated by compound A, under specific circumstances. However, other studies using prolonged sevoflurane administration at low flow rates did not find evidence of renal injury. Finally, there are substantial data to document the safety of sevoflurane administered for shorter durations or at higher fresh gas flow rates. Therefore, the United States Food and Drug Administration recommends the use of sevoflurane with fresh gas flow rates at least 1 L/min for exposures up to 1 hour and at least 2 L/min for exposures greater than 1 hour. We believe this is a rational, cautious approach based on available data. However, it is important to note that other countries have not recommended such limitations on the clinical use of sevoflurane and problems have not been noted.

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.

Toxicity of a sevoflurane degradation product incubated with rat liver and renal cortical slices

Compound A (2-fluoromethoxy-1,1,3,3,3-pentafluoro-1-propene) is a degradation product of the anesthetic sevoflurane which is created in closed-circuit anesthetic machines. Past in vivo and in vitro studies have implied that Compound A is nephrotoxic via bioactivation through the cysteine conjugate beta-lyase pathway. Although glutathione (GSH) conjugates of Compound A have been reported, it is not clear if they are formed enzymatically or via direct reaction with GSH. To determine if these metabolites are produced and toxic, a tissue slice system that first exposes male Fischer 344 rat liver slices to volatilized Compound A followed by exposure of rat kidney slices to the liver incubate was employed. Liver slices exposed to volatilized Compound A (6-12 microM medium conc.; approximately 23 ppm) exhibited a loss of K+ by 6 h, which was not seen in kidney slices exposed to Compound A. Aminobenzotriazole, a cytochrome P 450 suicide inhibitor, initially inhibits the cytotoxicity of Compound A to liver slices (at these times and concentrations). The sequential liver/kidney slice experiments using Compound A have not demonstrated nephrotoxic results. GSH conjugates were synthesized and was found to be nephrotoxic at concentrations above 91 microM (18 h), with higher concentrations showing toxicity at earlier times. Additionally, non-enzymatic reactions of Compound A with GSH or sulfhydryl-containing medium produces nephrotoxic products. These studies show that Compound A is directly toxic to the liver, possibly via P 450 activation, and Compound A can react with sulfhydryls directly to produce a nephrotoxic.

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.