Human hepatocytes express trifluoroacetylated neoantigens after in vitro exposure to halothane

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

BACKGROUND

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.

METHODS

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.

RESULTS

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.

CONCLUSIONS

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.

Use of hepatocyte cultures for the study of hepatotoxic compounds

Human and animal hepatocytes in primary culture are widely used in pharmacotoxicological research. They represent a unique in vitro model since they retain both phase I and phase II enzyme activities as well as their inducibility by xenobiotics. Hepatocyte cultures are used for drug screening, identification of the lesions induced by toxic compounds and determination of mechanisms by which xenobiotics exert liver injury.

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

BACKGROUND

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) mumol, respectively (p < 0.001) while that of bromide was 1720 (290) and 160 (70) mumol (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.

The role of cytochrome P450 enzymes in hepatic and extrahepatic human drug toxicity

The human cytochrome P450 enzyme system metabolises a wide array of xenobiotics to pharmacologically inactive metabolites, and occasionally, to toxicologically active metabolites. Impairment of cytochrome P450 activity, which may be either genetic or environmental, may lead to toxicity caused by the parent compound itself. In practise, this usually only applies to drugs that have a narrow therapeutic index and when their clearance is critically dependent upon the fraction normally metabolised by that pathway. P450 enzymes may also convert the drug to a chemically reactive metabolite, which, if not detoxified, may lead to various forms of hepatic and extrahepatic toxicity, including cellular necrosis, hypersensitivity, teratogenicity, and carcinogenicity, depending on the site of formation and the relative stability of the metabolite, and the cellular macromolecule with which it reacts. Variation in the regulation and expression of the drug metabolising enzymes may play a key role in both interindividual variation in sensitivity to drug toxicity and tissue-specific damage. Avoidance of toxicity may be possible in rare instances by prediction of individual susceptibility or by designing new chemical entities that are metabolised by a range of enzymes (both cytochromes P450 and others) and do not undergo bioactivation.