Metabolism Of Halothane Research Articles

Effect of polyI

Drug-induced liver injury (DILI) is a challenging problem in drug development and clinical practice. Patient susceptibility to DILI is multifactorial, making these reactions difficult to predict and prevent. Clinical observations have suggested that concurrent bacterial and viral infections represent an important risk factor in determining patient susceptibility to developing adverse drug reactions, although the underlying mechanism is not clear. In the present study, we employed the viral RNA mimetic (polyinosinic-polycytidylic acid [polyI:C]) to emulate viral infection and examined its effect on halothane-induced liver injury. Although pretreatment of mice with polyI:C attenuated halothane hepatotoxicity due to its inhibitory effect on halothane metabolism, posttreatment significantly exacerbated liver injury with hepatocellular apoptosis being significantly higher than that in mice treated with polyI:C alone or halothane alone. The pan-caspase inhibitor z-VAD-fmk suppressed liver injury induced by polyI:C/posthalothane cotreatment, suggesting that the increased hepatocyte apoptosis contributes to the exacerbation of liver injury. Posttreatment with polyI:C also caused activation of hepatic Kupffer cells (KCs) and natural killer (NK) cells and upregulated multiple proapoptotic factors, including tumor necrosis factor-alpha (TNF-alpha), NK receptor group 2, member D (NKG2D), and Fas ligand (FasL). These factors may play important roles in mediating polyI:C-induced hepatocyte apoptosis. This is the first study to provide evidence that concurrent viral infection can inhibit cytochrome (CYP)450 activities and activate the hepatic innate immune system to proapoptotic factors. DILI may be attenuated or exacerbated by pathogens depending on the time of infection.

Hepatotoxicity after Desflurane Anesthesia in a 15-month-old Child with Mobius Syndrome after Previous Exposure to Isoflurane

HALOGENATED anesthetic agents have been associated with perioperative liver dysfunction and injury. Most cases of hepatotoxicity reported involve halothane exposure. Halothane produces two types of hepatotoxicity. A mild, transient syndrome with modest increases in transaminase enzymes occurs in 20% of patients given halothane. 1 A more severe and fulminant hepatitis with marked liver dysfunction, jaundice, encephalopathy, and even death is observed in 1 in 20,000 anesthetic exposures to halothane. 2 Halothane toxicity occurs in children as well as in adults.3'4 Cross-sensitization between halothane and other volatile agents has been previously reported with isoflurane,5 enflurane, 6-8 and desflurane. 9 ' 10 To date, only four cases of hepatotoxicity in adults were related to desflurane exposure. 9-12 We report the case of a young child who developed acute hepatotoxicity after desflurane anesthesia in the context of two previous isoflurane exposures.

Modelo experimental de indução de lesão oxidativa hepática em ratos por halotano

The anesthetic halothane can be reductively metabolized to reactives intermediates that may initiate lipid peroxidation accompanied by hepatic injury. Hypoxia and phenobarbital pretreatment in rats increases metabolism of halothane, the oxidative stress, cause liver antioxidant enzymes changes and tissue damage. We investigated the effect of halothane on hepatic lipid peroxidation and on hepatic histology after increases reductive metabolism of halothane caused by hypoxia and phenobarbital pretreatment. Twenty-five male wistar rats were divided in five equals groups: CO (Control), HO14 (Halothane/Hypoxia), F (fenobarbital alone), O14 (Hypoxia alone) and H (Halothane alone). After 24 hours the rats were killed, their livers removed to determine chemoluminescence, thiobarbituric acid-reactive substances, catalase, superoxide dismutase, and blood samples were taken to determine AST and ALT. The histopathologic evaluation was performed with hematoxylin and eosin staining. Histopathologic scores are presented as 25th-75th percentile/range values and median +/- range. Halothane-hypoxic exposure resulted in a significant changes in the activities of antioxidant enzymes, and induced hepatic lipoperoxidation. Moreover it resulted in histopathologic liver injury as well as significant increase of serum activity of AST and ALT.

A model which can be used to improve the treatment of acute hepatic failure.

Abstract A model using the pig from wich the liver has been removed for isolated perfusion, as a subject showing some of the features of acute hepatic failure for later treatment by cross-circulation with its own perfused liver, is desceribed. Effects of such a procedure on such a procedure on the glucose and halothane metabolism and intravascular coagulation and bleeding are detailed.

EFFECTS OF TAIL FAT ON HALOTHANE BIOTRANSFORMATION IN FAT-TAILED SHEEP

1. The aim of the present study was to evaluate the effects of tail fat on halothane biotransformation following similar anaesthetic exposure in intact sheep and sheep with a ligated median sacral artery. 2. A prospective randomized experimental study was performed using 12 healthy, 10-12-month-old female sheep. 3. Sheep were randomly divided into two groups of six animals each and were anaesthetized twice at 2 weekly intervals. After mask induction with halothane in 100% oxygen, sheep were intubated and anaesthesia was maintained for 3 h using a rebreathing system. Serum fluoride concentration (SFC) was measured at 0, 1, 3, 6, 12, 24, 48 and 72 h following the induction of anaesthesia. Serum biochemistry was also evaluated at baseline and 72 h after anaesthesia. Induction and extubation times and time to sternal recumbency were also recorded during anaesthetic induction and recovery. Prior to the second anaesthesia (2 weeks later), the median sacral artery (MSA) was ligated under epidural anaesthesia in the experimental group. Sheep in the control group underwent sham operation. All sheep were anaesthetized as before. 4. Following the first halothane anaesthesia, SFC was significantly increased from 3 to 48 h compared with baseline. In the second stage of the experiment, the increases in SFC in the control group were similar to those seen in the first stage of the experiment. However, in MSA-ligated sheep, the increases in SFC were only significant between 3 and 12 h compared with baseline. The SFC was significantly higher in intact sheep from 3 to 72 h compared with the MSA-ligated group. Extubation and sternal recumbency times were significantly longer in intact sheep. 5. Ligation of the MSA in fat-tailed sheep induced a significant reduction in SFC, suggesting that the presence of tail fat substantially affects halothane metabolism during the peri-anaesthetic period in sheep. The greater extent of halothane biotransformation may be clinically important in, otherwise normal, obese patients.

Biotransformation of halothane in humans. 1967.

Biotransformation of halothane in humans. 1967.

Concordance between trifluoroacetic acid and hepatic protein trifluoroacetylation after disulfiram inhibition of halothane metabolism in rats.

Cytochrome P4502E1(CYP2E1)-mediated oxidation of halothane to a reactive intermediate (trifluoroacyl chloride) that covalently binds to hepatic proteins forming trifluoroacetylated neoantigens is believed to be the initiating event in a complex immunologic cascade culminating in antibody formation and severe hepatic necrosis ('halothane hepatitis') in susceptible patients. Trifluoroacyl chloride may also hydrolyze to the stable metabolite trifluoroacetic acid (TFA). CYP2E1 inactivation by disulfiram or its primary metabolite, diethyldithiocarbamate, inhibits human halothane oxidation to TFA in vitro and in vivo. Nevertheless, disulfiram effects on hepatic protein trifluoroacetylation by halothane in vivo are unknown. This investigation tested the hypotheses that disulfiram prevents halothane-dependent protein trifluoroacetylation in vivo, and that TFA represents a biomarker for hepatic protein trifluoroacetylation. Rats were pretreated with isoniazid (CYP2E1 induction), isoniazid followed by disulfiram (CYP2E1 inhibition), or nothing (controls), then anesthetized with halothane or nothing (controls). Plasma and urine TFA were quantified by ion HPLC; hepatic microsomal TFA-proteins were analyzed by Western blot. CYP2E1 induction increased both TFA and TFA-protein formation compared with uninduced halothane-treated rats. Disulfiram, even after CYP2E1 induction, nearly abolished both TFA and TFA-protein formation. Pretreatments similarly affected both TFA and TFA-protein formation across all groups. Disulfiram inhibition of CYP2E1-mediated halothane oxidation prevents hepatic protein trifluoroacetylation. Based on the concordance between TFA and TFA-protein formation, TFA appears to be a valid biomarker for TFA-protein formation. Disulfiram inhibition of human halothane oxidation in vivo, previously assessed by diminished TFA formation, probably also confers inhibition of hepatic TFA-protein formation.

Selective inactivation of rat and bovine olfactory cytochrome P450 by three haloethanes

The effects of halothane, 1,1-dichloro-2,2,2-trifluoroethane (HCFC-123) and 1,1-dichloro-1-fluoroethane (HCFC-141b) on the P450 system in olfactory and hepatic microsomes of bovine and rat have been investigated. In the in vitro experiments, all three compounds decreased olfactory CYP-dependent activities in microsomes from both species, especially under anaerobic conditions, halothane showing the greatest effect. Hepatic activities were not affected. A selective olfactory CYP depletion was also observed in vivo after treatment with halothane, but not with HCFC-123 or HCFC-141b. A loss of olfactory ethoxycoumarin- O-deethylase activity was also found both in vitro and in vivo experiments, suggesting that a CYP2A isoform may be the main target of inactivation. The present results therefore suggest that CYP2A, the major isoform expressed in the olfactory tissue of mammals, may be particularly prone to catalyze the reductive metabolism of halothane both in anaerobic and aerobic conditions.

Prevention of halothane-induced hepatotoxicity by hemin pretreatment: Protective role of heme oxygenase-1 induction

Reductive metabolism of halothane in phenobarbital-pretreated rats is known to increase free radical formation that results in hepatotoxicity. It also is associated with a marked induction of microsomal heme oxygenase-1 (HO-1), suggesting that there is an alteration in heme metabolism. In this study, we examined heme metabolism in rats pretreated with phenobarbital, followed by exposure to halothane–hypoxia. In this model, there was a significant decrease in microsomal cytochrome P450 content in the liver, followed by a rapid increase in free heme concentration and a decrease in the level of mRNA for the nonspecific δ-aminolevulinate synthase. A transient but dramatic induction of HO-1 mRNA and a prolonged induction of heat shock protein 70 mRNA also occurred. The HO-1 protein was detected principally in the hepatocytes around the central vein. Serum alanine transaminase (ALT) activity, an indicator of hepatic dysfunction, increased continuously throughout the experiment. Hemin pretreatment induced hepatic HO-1 with abrogation of the halothane-induced hepatotoxicity in this model, as judged by ALT activity and normal histology. Our findings in this study thus indicate that halothane-induced hepatotoxicity is due not only to its reductive metabolite formation, but also to an increase in hepatic free heme concentration, which is a potent prooxidant; HO-1 induction is an important protective response against such changes. This is also the first study to demonstrate that hemin pretreatment, which induces HO-1 prior to exposure to halothane, effectively prevents halothane-induced hepatotoxicity.

Expression of Cytochrome P450 2A6 in Escherichia coli: Purification, Spectral and Catalytic Characterization, and Preparation of Polyclonal Antibodies

Cytochrome P450 (CYP) 2A6 is the principal human enzyme catalyzing coumarin 7-hydroxylation and is known to be involved in the metabolism of halothane, nicotine, and metabolic activation of butadiene and nitrosamines. In this paper expression of CYP2A6 in Escherichia coli is reported. In order to achieve expression, the N-terminus of protein was modified by PCR mutagenesis. The N-terminal variant with only a single amino acid change showed expression of 210 nmol of CYP2A6/liter of culture. Recombinant CYP2A6 protein was purified to electrophoretic homogeneity and further characterized. Absolute spectra were typical for CYP proteins and indicated low spin characteristics of isolated protein. Due to a hydrophobic segment the N-terminal amino acid sequence of recombinant CYP2A6 was blocked. The N-terminal formylmethionine block was removed by mild acid treatment. Purified CYP2A6 had good catalytic activity toward marker substrate coumarin in a reconstituted system (Km = 1.48 ± 0.37 μM, Vmax = 3.36 ± 0.18 nmol product/min/nmol CYP). Its activity in the reconstituted system was stimulated by the presence of cytochrome b5 and glutathione. CYP2A6 was shown to metabolize chlorzoxazone in the reconstituted system with activity of 0.32 nmol of product/min/nmol of CYP, and thus caution should be taken when interpretation of CYP2E1 in vivo phenotyping data is performed. Rabbit polyclonal antibodies were produced against recombinant CYP2A6 and proved to be very useful for immunoblotting and immunoinhibition studies. Availability of this expression system and specific antibodies should facilitate characterization of the role of CYP2A6 in the metabolism of chemicals and in the study biological relevance of genetic polymorphisms of this enzyme.

Inhibitory effect of nitric oxide on the metabolism of halogenated volatile anesthetics by cytochrome P-450.

This study was undertaken to examine the effect of nitric oxide (NO) on the metabolism of halogenated volatile anesthetics (HVA) by cytochrome P-450 (CYP) under both aerobic and anaerobic conditions using rat hepatic microsomes. A microsomal fraction was prepared by centrifugation from normal and phenobarbital-treated male SD rats. The anaerobic metabolism of HVA by CYP was followed by measuring the formation of a halothane CYP complex spectrophotometrically. Aerobic CYP activity was determined using either the defluorination of sevoflurane or the demethylation of aminopyrine. The formation of the halothane-CYP complex was dose-dependently inhibited by NO. NO also decreased CYP defluorination of halothane in a dose-dependent manner. In phenobarbital-induced microsomes, the inhibition rates of both complex formation and the defluorination of halothane were the same as those seen in normal microsomes. Although the defluorination of sevoflurane and the demethylation of aminopyrine were inhibited by NO aerobically, the inhibition was much less than that of the metabolism of halothane under anaerobic conditions. These results suggest that NO binds the heme of CYP and inhibits the metabolism of HVA, with the effect lasting for a prolonged period of time. Furthermore, the balance between NO and O(2) is important for NO to inhibit CYP.

Deformability and Oxidant Stress in Red Blood Cells under the Influence of Halothane and Isoflurane Anesthesia

1. The effects of halothane and isoflurane anesthesia on red blood cell (RBC) deformability, lipid peroxidation and antioxidant enzymes were tested in rabbits. 2. RBC transit time was significantly increased to 2.12±0.07 msec after 1-hr halothane anesthesia preceded by 6 mg/kg pentobarbital injections from 1.98±0.07 msec preanesthesia value ( p<0.05). Thiobarbituric acid-reactive substances also were increased significantly, being 23.35±2.75 nmol/gHb and 33.11±5.34 nmol/gHb before and after anesthesia, respectively ( p<0.05). 3. Under halothane anesthesia without prior pentobarbital injection or under isoflurane anesthesia with or without pentobarbital injection, no significant alterations were observed in these parameters. 4. RBC superoxide dismutase activity was decreased in the group anesthetized with the pentobarbital–halothane combination. The impaired RBC deformability and increased oxidant damage might be related to the free radical formation during the metabolism of halothane. Pentobarbital can potentiate this effect either by inducing cytochrome P-450 or by altering antioxidant defense. 5. Alterations in RBC mechanical properties may contribute to the tissue perfusion problems that develop after surgery under general anesthesia.

Cytochrome P‐4502E1‐Dependent Formation of Trifluoroacetyl Adducts from Halothane by Transduced HepG2 Cells

E-9 cells, a HepG2 cell line that has the alcohol-inducible human cytochrome P-4502E1 (CYP2E1) cloned into its genome, was tested for its ability to produce trifluoroacetyl (TFA) adducts from the metabolism of halothane. The metabolism of halothane results in the production of TFA halide that can readily react with cellular proteins to form TFA adducts, which can be detected by antibodies directed against them. E-9 cells formed TFA adducts when incubated with halothane. The major consistent bands that were detected were of molecular weights of approximately 50, 78, and 100 kDa. The formation of these adducts was dependent on halothane concentration and time of incubation with halothane. MV-5 cells, the control cell line that has the viral vector without the P-450, did not produce any adducts. Inhibitors of CYP2E1 function, such as 4-methylpyrazole, inhibited adduct formation. Furthermore, phorbol esters, which have been shown to increase the CYP2E1 level in this cell line, increased TFA adduct formation. This HepG2 cell line may be of value in studying the metabolism and toxicity of halothane in a human cell culture model.

Quantitative trait loci controlling halothane sensitivity in Caenorhabditis elegans.

Genetic analysis is an essential tool for defining the molecular mechanisms whereby volatile anesthetics (VA) disrupt nervous system function. However, the degree of natural variation of the genetic determinants of VA sensitivity has not been determined nor have mutagenesis approaches been very successful at isolating significantly resistant mutant strains. Thus, a quantitative genetic approach was taken toward these goals. Recombinant-inbred strains derived from two evolutionarily distinct lineages of the nematode Caenorhabditis elegans were tested for sensitivity to clinically relevant concentrations (0.3-0.5 mM) of the VA halothane. The halothane sensitivities of coordinated movement and male mating behavior were highly variant among the recombinant-inbred strains with a range of EC50 values of 13- and 4-fold, respectively. Both traits were highly heritable (H2 = 0.82, 0.87, respectively). Several strains were found to be significantly resistant to halothane when compared with the wild-type strain N2. A major locus or loci mapping to the middle of chromosome V accounted for more than 40% of the phenotypic variance for both traits. Five weaker loci, four of which interact, explained most of the remaining variance. None of the halothane-sensitivity quantitative trait loci significantly affected behavior in the absence of halothane or halothane's potency for C. elegans immobilization, which requires 5-fold higher drug concentrations. Thus, the quantitative trait loci are unlikely to result from differences in halothane-independent (native) behavior or differences in halothane metabolism or permeability. Rather, these loci may code for targets and/or downstream effectors of halothane in the C. elegans nervous system or for modifiers of such gene products.

Determination of the halothane metabolites trifluoroacetic acid and bromide in plasma and urine by ion chromatography.

Halothane (CF3CHClBr), a widely used volatile anesthetic, undergoes extensive biotransformation in humans. Oxidative halothane metabolism yields the stable metabolites trifluoroacetic acid and bromide which can be detected in plasma and urine. To date, analytical methodologies have either required extensive sample preparation, or two separate analytical procedures to determine plasma and urine concentrations of these analytes. A rapid and sensitive method utilizing high-performance liquid chromatography-ion chromatography (HPLC-IC) with suppressed conductivity detection was developed for the simultaneous detection of both trifluoroacetic acid and bromide in plasma and urine. Sample preparation required only ultrafiltration. Standard curves were linear (r2> or =0.99) from 10 to 250 microM trifluoroacetic acid and 2 to 5000 microM bromide in plasma and 10 to 250 microM trifluoroacetic acid and 2 to 50 microM bromide in urine. The assay was applied to quantification of trifluoroacetic acid and bromide in plasma and urine of a patient undergoing halothane anesthesia.

Physiologically based pharmacokinetic analysis of the concentration- dependent metabolism of halothane

1. Previous studies with the halothane analogue and chlorofluorocarbon replacement 2,2-dichloro-1,1,1-trifluoroethane (HCFC-123) have shown that there are concentration dependent, sex-specific differences in the rate of uptake during inhalation exposure in rat. Since it is well established that there are sex-specific differences in the control of enzyme activity in drug metabolism, male and female rats were exposed by inhalation to halothane concentrations ranging from 500 to 4000 ppm. 2. A physiologically based pharmacokinetic model describing the concentration dependent reduction in uptake and metabolism of halothane in male and female rats was developed. The in vivo metabolic rate constants obtained were : for male rats, Km 0 4 mg litre (2 03 mu mol litre) and Vmaxc 9 2 mg kg h (46 6 mu mol kg h); for female rats, Km 0 4 mg litre (2 03 mu mol litre) and Vmaxc 10 2 mg kg h (51 7 mu mol kg h) . 3. An equation describing the concentration-dependent decrease of hepatic metabolism of halothane successfully simulated the gas-uptake data. Simulation of cumulative urinary excretion of the major metabolite, trifluoroacetic acid, required introduction of a proportionality constant to limit the extent of reduction of halothane metabolism to 20% of the amountofenzyme activity. Good simulation of urinary excretion data was achieved, which was interpreted to indicate that, when only 20% of the enzyme is inactivated, the rate of enzyme resynthesis was adequate to replenish enzyme activity within 24 h. 4. A rapidly reversible, non-biological inactivation mechanism called 'physical toxicity' is discussed as a possible explanation of concentration-dependent gas uptake.

Biotransformation of halothane, enflurane, isoflurane, and desflurane to trifluoroacetylated liver proteins: association between protein acylation and hepatic injury.

In susceptible patients, halothane, enflurane, isoflurane, and desflurane can produce severe hepatic injury by an immune response directed against reactive anesthetic metabolites covalently bound to hepatic proteins. The incidence of hepatotoxicity appears to directly correlate with anesthetic metabolism catalyzed by cytochrome P450 2E1 to trifluoroacetylated hepatic proteins. In the present study, we examined whether the extent of acylation of hepatic proteins in rats by halothane, enflurane, isoflurane, and desflurane correlated with reported relative rates of metabolism. After pretreatment with the P450 2E1 inducer isoniazid, five groups of 10 rats breathed 1.25 minimum alveolar anesthetic concentration (MAC) of halothane, enflurane, isoflurane, or desflurane in oxygen, or oxygen alone, each for 8 h. Immunochemical analysis of livers harvested 18 h after anesthetic exposure showed tissue acylation (greatest to least) after exposure to halothane, enflurane, or isoflurane. Reactivity was not different between isoflurane as compared to desflurane or oxygen alone. An enzyme-linked immunosorbent assay showed halothane reactivity was significantly greater than that of enflurane, isoflurane, desflurane, or oxygen, and that enflurane reactivity was significantly greater than desflurane or oxygen. Sera from patients with a clinical diagnosis of halothane hepatitis showed antibody reactivity against hepatic proteins from rats exposed to halothane or enflurane. No reactivity was detected in rats exposed to isoflurane, desflurane, or oxygen alone. These results indicate that production of acylated proteins may be an important mediator of anesthetic-induced hepatotoxicity.

Identification of the enzyme responsible for oxidative halothane metabolism: implications for prevention of halothane hepatitis

SummaryBackground Fulminant hepatic necrosis ("halothane hepatitis") is an unusual and often fatal complication of halothane anaesthesia. It is mediated by immune sensitisation in susceptible individuals to trifluoroacetylated liver protein neoantigens, formed by oxidative halothane metabolism. The seminal event in halothane hepatitis is hepatic metabolism, yet the enzyme responsible for oxidative halothane metabolism and trifluoroacetylated neoantigen formation remains unidentified. This investigation tested the hypothesis that cytochrome P450 2E1 (CYP2E1) is responsible for human halothane metabolism in vivo.Methods 20 elective surgical patients received either disulfiram (500 mg orally, n=10) or nothing (controls, n=10) the night before surgery. Disulfiram, converted in vivo to an effective inhibitor of P450 2E1, was used as a metabolic probe for P450 2E1. All patients received standard halothane anaesthesia (1·0% end-tidal, 3 h). Blood halothane and plasma and urine trifluoroacetic acid, bromide, and fluoride concentrations were measured for up to 96 h postoperatively.Findings Total halothane dose, measured by cumulative end-tidal (3·8 SE 0·1 minimum alveolar concentration hours) and blood halothane concentrations, was similar in the two groups. Plasma concentrations and urinary excretion of trifluoroacetic acid and bromide, indicative of oxidative and total (oxidative and reductive) halothane metabolism, respectively, were significantly diminished in disulfiram-treated patients. In control and disulfiram-treated patients cumulative 96 h postoperative trifluoroacetic acid excretion was 12 900 (SE 1700) and 2010 (440) μmol, respectively (p<0·001) while that of bromide was 1720 (290) and 160 (70) μmol (p<0·001).Interpretation The substantial attenuation of trifluoroacetic acid production by disulfiram after halothane anaesthesia suggests that P450 2E1 is a predominant enzyme responsible for human oxidative halothane metabolism. Inhibition of P450 2E1 by a single preoperative oral disulfiram dose greatly diminished production of the halothane metabolite responsible for the neoantigen formation that initiates halothane hepatitis. Single-dose disulfiram may provide effective prophylaxis against halothane hepatitis.

Demonstration of halothane-induced hepatic lipid peroxidation in rats by quantification of F2-isoprostanes.

Halothane can be reductively metabolized to free radical intermediates that may initiate lipid peroxidation. Hypoxia and phenobarbital pretreatment in Sprague-Dawley rats increases reductive metabolism of halothane. F(2)-isoprostanes, a novel measure of lipid peroxidation in vivo, were used to quantify halothane-induced lipid peroxidation in rats. Rats were exposed to 1% halothane or 14% O(2) for 2 h. Pretreatments included phenobarbital, isoniazid, or vehicle. Rats also were exposed to halothane, enflurane, and desflurane at 21% O(2). Lipid peroxidation was assessed by mass spectrometric quantification of F(2)-isoprostanes. Exposure of phenobarbital-pretreated rats to 1% halothane at 21% O(2) for 2 h caused liver and plasma F(2)-isoprostane concentrations to increase fivefold compared to nonhalothane control rats. This halothane-induced increase was enhanced by 14% O(2), but hypoxia alone had no significant effect. Alanine aminotransferase activity at 24 h was significantly increased only in the 1% halothane/14% O(2) group. The effect of cytochrome P450 enzyme induction on halothane-induced F(2)-isoprostane production and liver injury was determined by comparing the effects of isoniazid and phenobarbital pretreatment with no pretreatment under hypoxic conditions. Halothane caused 4- and 11-fold increases in plasma and liver F(2)-isoprostanes, respectively, in non-pretreated rats, whereas isoniazid pretreatment had no effect. Phenobarbital pretreatment potentiated halothane-induced lipid peroxidation with 9- and 20-fold increases in plasma and liver F(2)-isoprostanes, respectively. Alanine aminotransferase activity was increased only in this group. At ambient oxygen concentrations, halothane but not enflurane or desflurane, caused F(2)-isoprostanes to increase. Specific halothane-induced lipid peroxidation was demonstrated in Sprague-Dawley rats using quantification of F(2)-isoprostanes and was increased by hypoxia and phenobarbital pretreatment, but not isoniazid pretreatment.

Rat to Human Extrapolation of HCFC-123 Kinetics Deduced from Halothane Kinetics: A Corollary Approach to Physiologically Based Pharmacokinetic Modeling

The goal of this study was to develop a human physiologically based pharmacokinetic (PBPK) model for the chemical HCFC-123 (2,2-dichloro-1,1,1-trifluoroethane) and its major metabolite, trifluoroacetic acid (TFA). No human kinetic data for HCFC-123 are available, thus a corollary approach was developed. HCFC-123 is a structural analog of the common anesthetic agent halothane (2-bromo-2-chloro-1,1,1-trifluoroethane) and follows a common pathway of oxidative biotransformation, resulting in the formation of the same metabolite, TFA. In this study, halothane models for rats and humans were developed and validated. Then the corollary approach was used to develop a human HCFC-123 model from a rat HCFC-123 model. This strategy was implemented by using a previously validated PBPK model for HCFC-123/TFA in the Fischer 344 rat as a template model for halothane in rats. Model predictions were then compared to, and were in good agreement with, measured values for the concentration of halothane in rat blood and fat tissue. A human PBPK model for halothane was developed. The identical model structure (with the exception of the description for the fat compartment) that was used to describe halothane and TFA in the rat was used for describing halothane and TFA in the human. Human physiological parameters for tissue volumes and flows were taken from the literature, and human tissue partition coefficients for halothane were measured in the laboratory. Based on reported similarity in metabolism of halothane by humans and rats, metabolic constants for halothane in the rat were used in the human model, and specific parameters describing the kinetics of TFA were estimated by optimization. The model was validated against human exposure data for halothane from six published studies (expired breath concentrations of halothane and serum/urine data for TFA). A similar approach was then used to derive a human HCFC-123 model for humans from the HCFC-123 rat model. The corollary approach described here illustrates the innovative use of template model structures to aid in the development and validation of models for structural analogs with similar metabolism and activity in biologic systems. Furthermore, given that the PBPK model adequately describes the kinetics of halothane in rats and humans and of HCFC-123 in rats, use of the human PBPK model is proposed for deriving dose–response estimates of human health risks in the absence of human kinetic data.