Studies of the mechanism of nephrotoxicity of compound A in rats

Sevoflurane and isoflurane genotoxicity in kidney cells of mice

The aim of this study was to evaluate the DNA damage and repair in kidney cells of Swiss albino mice after repeated exposure to sevoflurane and isoflurane and compare their detrimental effects. We used the alkaline comet assay to establish the genetic damage and measured three parameters: tail length, tail moment, and tail intensity of comets. These parameters were measured immediately after exposure to the above mentioned inhalation anaesthetics, two hours, six hours, and 24 hours later and were compared with the control group. Mean values of all three parameters were significantly higher in experimental groups compared to the control group. DNA damage in kidney cells of mice exposed to sevoflurane increased continuously before it reached its peak 24 hours after exposure. Isoflurane induced the highest DNA damage two hours after exposure. Levels of DNA damage recorded 24 h after cessation of exposure to both tested compounds suggest that sevoflurane was slightly more genotoxic than isoflurane to kidney cells of mice. According to these results, the currently used volatile anaesthetics sevoflurane and isoflurane are able to damage DNA in kidney cells of mice. Such findings suggest a possibility for similar outcomes in humans and that fact must be taken into account in everyday clinical practice.

Sevoflurane-Sulfobutylether-β-Cyclodextrin Complex: Preparation, Characterization, Cellular Toxicity, Molecular Modeling and Blood-Brain Barrier Transport Studies

The objective of the present investigation was to study the ability of sulfobutylether-β-cyclodextrin (SBEβCD) to form an inclusion complex with sevoflurane (SEV), a volatile anesthetic with poor water solubility. The inclusion complex was prepared, characterized and its cellular toxicity and blood-brain barrier (BBB) permeation potential of the formulated SEV have also been examined for the purpose of controlled drug delivery. The SEV-SBEβCD complex was nontoxic to the primary brain microvascular endothelial (pEND) cells at a clinically relevant concentration of sevoflurane. The inclusion complex exhibited significantly higher BBB permeation profiles as compared with the reference substance (propranolol) concerning calculated apparent permeability values (P_app). In addition, SEV binding affinity to SBEβCD was confirmed by a minimal Gibbs free energy of binding (ΔG_bind) value of −1.727 ± 0.042 kcal·mol⁻¹ and an average binding constant (K_b) of 53.66 ± 9.24 mM indicating rapid drug liberation from the cyclodextrin amphiphilic cavity.

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.

Glutathione‐dependent Bioactivation of Haloalkanes and Haloalkenes

Haloalkanes and haloalkenes constitute an important group of widely used chemicals that have the potential to induce toxicity and cancer. The toxicity of haloalkanes and haloalkenes may be associated with cytochromes P450- or glutathione transferase-dependent bioactivation. This review is concerned with the glutathione- and glutathione transferase-dependent bioactivation of dihalomethanes, 1,2-dihaloalkanes, and haloalkenes. Dihalomethanes, e.g., dichloromethane, and 1,2-dihaloethanes, e.g., 1,2-dichloroethane and 1,2-dibromoethane, undergo glutathione transferase-catalyzed bioactivation to give S-(halomethyl)glutathione or glutathione episulfonium ions, respectively, as reactive intermediates. Haloalkenes, e.g., trichloroethene, hexachlorobutadiene, chlorotrifluoroethene, and tetrafluoroethene, undergo cysteine conjugate beta-lyase-dependent bioactivation to thioacylating intermediates, including thioacyl halides, thioketenes, and 2,2,3-trihalothiiranes. With all of these compounds, the formation of reactive intermediates is associated with their observed toxicity.

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.

Inorganic fluoride kinetics and renal and hepatic function after repeated sevoflurane anesthesia

After repeated exposure to inhaled anesthetics, the hepatic function and metabolism of anesthetics may change. The purpose of this study was to investigate inorganic fluoride (F-) kinetics and renal and hepatic function after repeated exposure to sevoflurane. Ten patients (aged 40-70 yr) who had received sevoflurane anesthesia with a gas flow of 6 L/min for neurosurgery twice in 30-90 days were studied. Serum and urine F- concentrations were measured up to 24 h after anesthesia. Blood urea nitrogen, serum creatinine, serum and urine beta2-microglobulin, urine N-acetyl-beta-D-glucosaminidase, serum aspartate aminotransferase (AST), alanine aminotransferase (ALT), and total bilirubin concentrations were measured up to 7 days after anesthesia. The area under the curve (AUC) of serum and urine F- concentration and half-life of serum F concentration were calculated. Urine beta2-microglobulin, AST, and ALT increased to abnormal levels after both anesthesias, with no difference between anesthesias. No measured variables, AUC of serum and urine F- concentration, or half-life of serum F- concentration showed any differences between the first and second anesthesias. In conclusion, the second exposure to sevoflurane with a high gas flow of 6 L/min in 30-90 days did not change the hepatic and renal function or affect the metabolism of sevoflurane.

IMPLICATIONS

We studied the changes of metabolism of sevoflurane and hepatic and renal function after repeated sevoflurane anesthesia in 30-90 days. There were changes indicative of mild liver and kidney injury after sevoflurane anesthesia, but repeated exposure to sevoflurane did not enhance these changes.

Quantitative differences in the production and toxicity of CF2=BrCl versus CH2F-O-C(=CF2)(CF3) (compound A): the safety of halothane does not indicate the safety of sevoflurane

Carbon dioxide absorbents degrade both halothane and sevoflurane to toxic unsaturated compounds (CF2=CBrCl and CH2F-O-C[=CF2][CF3] [i.e., Compound A], respectively). Given the long history of safe administration of halothane, comparable toxicities of these degradation products would imply a similar safety of sevoflurane. We therefore examined CF2=CBrCl in the context of four issues relevant to previous studies of the toxicity of Compound A: 1) reactivity of the degradation product in vitro; 2) rate of its production in vitro; 3) its in vivo toxicity; 4) importance of the beta-lyase pathway to the toxicity in vivo. We found the following. 1) CF2=CBrCl is less reactive than Compound A, degrading in human serum albumin at one-fifth the rate of Compound A. 2) Over a 3-h period of "anesthesia," a standard circle system containing Baralyme (Allied Healthcare Products, Inc., St. Louis, MO) produces 30 times as much Compound A from a minimum alveolar anesthetic concentration (MAC) concentration of sevoflurane as CF2=CBrCl from a MAC concentration of halothane; with soda lime, the difference is 60-fold. Correcting for differences in uptake of halothane versus sevoflurane decreases the differences to 20-40 times. 3) For a 3-h administration to rats, the partial pressure of Compound A causing minimal renal injury or necrosis of half the affected tubule cells exceeds the partial pressure of CF2=CBrCl causing minimal injury or necrosis of half the affected tubule cells by a factor of approximately 4-6. Thus, the ratio of production (Item 2 above) to the partial pressure causing injury with CF2=CBrCl is approximately a quarter of that ratio for Compound A. 4) Compounds that block the beta-lyase pathway either do not change (acivicin) or decrease (aminooxyacetic acid; AOAA) renal injury from CF2=CBrCl in rats, whereas these compounds increase (acivicin) or do not change (AOAA) injury from Compound A. We conclude that the safety of halothane cannot be used to support the safety of sevoflurane.

IMPLICATIONS

Carbon dioxide absorbents degrade halothane and sevoflurane to unsaturated compounds nephrotoxic to rats. Relative to sevoflurane's degradation product, halothane's degradation product has less toxicity relative to production, less reactivity, and a different mechanism of injury. The clinical absence of halothane nephrotoxicity does not necessarily indicate a similar absence for sevoflurane.