Epitope mapping of human CYP1A2 in dihydralazine-induced autoimmune hepatitis

Setting up criteria for drug-induced autoimmune-like hepatitis through a systematic analysis of published reports

Nitrofurantoin, minocycline, methyldopa and infliximab, have been found to induce autoimmune-like hepatitis (DI-AILH). Evidence for other drugs and herbal and dietary supplements (HDS) is unclear. The aims of the study were to establish criteria to define and review the published evidence of suspected DI-AILH. Search was undertaken in Pubmed using search terms "drug-induced liver injury," "autoimmune hepatitis," and "drug-induced autoimmune hepatitis." DI-AILH was defined as (1) drug as a potential trigger of liver injury with autoimmune features and histological findings compatible with AIH; (2) no or incomplete recovery or worsening of liver tests after discontinuation of the drug; (3) corticosteroids requirement or spontaneous recovery; (4) follow-up without immunosuppression (IS) and no relapse of AIH at least 6 months after discontinuation of IS; and (5) drugs potentially inducing AILH with a chronic course. Cases fulfilling the first four criteria were considered probable DI-AILH with three possible DI-AILH. A total of 186 case reports were identified for conventional drugs (n = 148; females 79%; latency 2.6 months) and HDS (n = 38; females 50%). The most commonly reported agents of DI-AILH were interferons (n = 37), statins (n = 24), methylprednisolone (MPS) (n = 16), adalimumab (n = 10), imatinib (n = 8), and diclofenac (n = 7). Tinospora cordifolia and Khat were the only HDS with probable DI-AILH cases. No relapses of AIH were observed when IS was stopped after interferons, imatinib, diclofenac, and methylprednisolone. Conclusion: Beyond well-recognized nitrofurantoin, methyldopa, hydralazine, minocycline, and infliximab as causes of DI-AILH, interferons, imatinib, adalimumab, and MPS were the best-documented agents leading to probable DI-AILH. Khat and Tinospora cordifolia were the only HDS found to be able to induce DI-AILH. Long-term immunosuppression appears to be rarely required in patients with DI-AILH due to these drugs.

Recent Progress in Prediction Systems for Drug-induced Liver Injury Using in vitro Cell Culture

BACKGROUND

In order to avoid drug-induced liver injury (DILI), in vitro assays, which enable the assessment of both metabolic activation and immune reaction processes that ultimately result in DILI, are needed.

OBJECTIVE

In this study, the recent progress in the application of in vitro assays using cell culture systems is reviewed for potential DILI-causing drugs/xenobiotics and a mechanistic study on DILI, as well as for the limitations of in vitro cell culture systems for DILI research.

METHODS

Information related to DILI was collected through a literature search of the PubMed database.

RESULTS

The initial biological event for the onset of DILI is the formation of cellular protein adducts after drugs have been metabolically activated by drug metabolizing enzymes. The damaged peptides derived from protein adducts lead to the activation of CD4+ helper T lymphocytes and recognition by CD8+ cytotoxic T lymphocytes, which destroy hepatocytes through immunological reactions. Because DILI is a major cause of drug attrition and drug withdrawal, numerous in vitro systems consisting of hepatocytes and immune/inflammatory cells, or spheroids of human primary hepatocytes containing non-parenchymal cells have been developed. These cellular-based systems have identified DILIinducing drugs with approximately 50% sensitivity and 90% specificity.

CONCLUSION

Different co-culture systems consisting of human hepatocyte-derived cells and other immune/inflammatory cells have enabled the identification of DILI-causing drugs and of the actual mechanisms of action.

Involvement of Pazopanib and Sunitinib Aldehyde Reactive Metabolites in Toxicity and Drug-Drug Interactions in Vitro and in Patient Samples

Tyrosine kinase inhibitors (TKI) are targeted anticancer drugs that have been successfully developed over the past 2 decades. To date, many of them (around 70%) require warnings for liver injury and five of them, including pazopanib and sunitinib, have Black Box Warning (BBW) labels. Although TKI-induced hepatotoxicity is the first cause of drug failures in clinical trials, BBW labels, and market withdrawals, the underlying mechanisms remain unclear. However, the recent discovery of new reactive metabolites (RM) with aldehyde structures during pazopanib and sunitinib metabolism offers new perspectives for investigating their involvement in the toxicity of these two TKI. These hard electrophiles have a high reactivity potential toward proteins and are thought to be responsible for cytochrome P450 inactivation, drug-drug interactions (DDI), and liver toxicity. We report here, for the first time, the presence of these aldehyde RM in human plasma samples obtained during drug monitoring. Docking experiments in the CYP3A4 active site were performed and showed that pazopanib and sunitinib fitting in the catalytic site are in accordance with their regioselective oxidation to aldehydes. They also suggested that aldehyde RM may react with lysine and arginine residues. Based on these results, we studied the reactivity of the aldehyde RM toward lysine and arginine residues as potential targets on the protein framework to better understand how these RM could be involved in liver toxicity and drug-drug interactions. Adduct formation with different hepatic and plasma proteins was investigated by LC-MS/MS, and adducts between pazopanib or sunitinib aldehyde derivatives and lysine residues on both CYP3A4 and plasma proteins were indeed shown for the first time.

On the Origin of Urinary Renin: A Translational Approach

Urinary angiotensinogen excretion parallels albumin excretion, which is not the case for renin, while renin's precursor, prorenin, is undetectable in urine. We hypothesized that renin and prorenin, given their smaller size, are filtered through the glomerulus in larger amounts than albumin and angiotensinogen, and that differences in excretion rate are because of a difference in reabsorption in the proximal tubule. To address this, we determined the glomerular sieving coefficient of renin and prorenin and measured urinary renin/prorenin 1) after inducing prorenin in Cyp1a1-Ren2 rats and 2) in patients with Dent disease or Lowe syndrome, disorders characterized by defective proximal tubular reabsorption. Glomerular sieving coefficients followed molecular size (renin>prorenin>albumin). The induction of prorenin in rats resulted in a >300-fold increase in plasma prorenin and doubling of blood pressure but did not lead to the appearance of prorenin in urine. It did cause parallel rises in urinary renin and albumin, which losartan but not hydralazine prevented. Defective proximal tubular reabsorption increased urinary renin and albumin 20- to 40-fold, and allowed prorenin detection in urine, at ≈50% of its levels in plasma. Taken together, these data indicate that circulating renin and prorenin are filtered into urine in larger amounts than albumin. All 3 proteins are subsequently reabsorbed in the proximal tubule. For prorenin, such reabsorption is ≈100%. Minimal variation in tubular reabsorption (in the order of a few %) is sufficient to explain why urinary renin and albumin excretion do not correlate. Urinary renin does not reflect prorenin that is converted to renin in tubular fluid.

CYP2E1 autoantibodies in liver diseases

Autoimmune reactions involving cytochrome P4502E1 (CYP2E1) are a feature of idiosyncratic liver injury induced by halogenated hydrocarbons and isoniazid, but are also detectable in about one third of the patients with advanced alcoholic liver disease (ALD) and chronic hepatitis C (CHC). In these latter the presence of anti-CYP2E1 auto-antibodies is an independent predictor of extensive necro-inflammation and fibrosis and worsens the recurrence of hepatitis following liver transplantation, indicating that CYP2E1-directed autoimmunity can contribute to hepatic injury. The molecular characterization of the antigens recognized by anti-CYP2E1 auto-antibodies in ALD and CHC has shown that the targeted conformational epitopes are located in close proximity on the molecular surface. Furthermore, these epitopes can be recognized on CYP2E1 expressed on hepatocyte plasma membranes where they can trigger antibody-mediated cytotoxicity. This does not exclude that T cell-mediated responses against CYP2E1 might also be involved in causing hepatocyte damage. CYP2E1 structural modifications by reactive metabolites and molecular mimicry represent important factors in the breaking of self-tolerance against CYP2E1 in, respectively, ALD and CHC. However, genetic or acquired interferences with the mechanisms controlling the homeostasis of the immune system are also likely to contribute. More studies are needed to better characterize the impact of anti-CYP2E1 autoimmunity in liver diseases particularly in relation to the fact that common metabolic alterations such as obesity and diabetes stimulates hepatic CYP2E1 expression.

Progress in research of drug-induced autoimmune hepatitis

Drug-induced autoimmune hepatitis (DIAIH) has been reported to be caused by many drugs, which possesses the characteristics of both drug-induced liver injury (DILI) and autoimmune hepatitis (AIH). A better understanding of the epidemiology, pathogenesis, pathology and clinical symptoms of DIAIH can help us better diagnose and treat this disease.

Drug-induced autoimmune-like hepatitis

The clinical phenotype of classical autoimmune hepatitis can be mimicked by idiosyncratic drug-induced liver injury, and differentiation can be difficult. The goals of this review are to enumerate the major agents of drug-induced autoimmune-like hepatitis, describe the clinical findings and risk factors associated with it, detail the clinical tools by which to assess causality, discuss putative pathogenic mechanisms, and describe treatment and outcome. The frequency of drug-induced autoimmune-like hepatitis among patients with classical features of autoimmune hepatitis is 9%. Minocycline and nitrofurantoin are implicated in 90% of cases. Female predominance, acute onset, and absence of cirrhosis at presentation are important clinical manifestations. Genetic factors affecting phase I and phase II transformations of the drug, polymorphisms that protect against cellular oxidative stress, and human leukocyte antigens that modulate the immune response may be important pathogenic components. Clinical judgment is the mainstay of diagnosis as structured diagnostic methods for drug-induced liver injury are imperfect. The covalent binding of a reactive drug metabolite to a hepatocyte surface protein (commonly a phase I or phase II enzyme), formation of a neoantigen, activation of CD8 T lymphocytes with nonselective antigen receptors, and deficient immune regulatory mechanisms are the main bases for a transient loss of self-tolerance. Discontinuation of the offending drug is the essential treatment. Spontaneous improvement usually ensues within 1 month. Corticosteroid therapy is warranted for symptomatic severe disease, and it is almost invariably effective. Relapse after corticosteroid withdrawal probably does not occur, and its absence distinguishes drug-induced disease from classical autoimmune hepatitis.

Toxicological Significance of Mechanism-Based Inactivation of Cytochrome P450 Enzymes by Drugs

Cytochrome P450 (P450) enzymes oxidize xenobiotics into chemically reactive metabolites or intermediates as well as into stable metabolites. If the reactivity of the product is very high, it binds to a catalytic site or sites of the enzyme itself and inactivates it. This phenomenon is referred to as mechanism-based inactivation. Many clinically important drugs are mechanism-based inactivators that include macrolide antibiotics, calcium channel blockers, and selective serotonin uptake inhibitors, but are not always structurally and pharmacologically related. The inactivation of P450s during drug therapy results in serious drug interactions, since irreversibility of the binding allows enzyme inhibition to be prolonged after elimination of the causal drug. The inhibition of the metabolism of drugs with narrow therapeutic indexes, such as terfenadine and astemizole, leads to toxicities. On the other hand, the fate of P450s after the inactivation and the toxicological consequences remains to be elucidated, while it has been suggested that P450s modified and degraded are involved in some forms of tissue toxicity. Porphyrinogenic drugs, such as griseofulvin, cause mechanism-based heme inactivation, leading to formation of ferrochelatase-inhibitory N-alkylated protoporphyrins and resulting in porphyria. Involvement of P450-derived free heme in halothane-induced hepatotoxicity and catalytic iron in cisplatin-induced nephrotoxicity has also been suggested. Autoantibodies against P450s have been found in hepatitis following administration of tienilic acid and dihydralazine. Tienilic acid is activated by and covalently bound to CYP2C9, and the neoantigens thus formed activate immune systems, resulting in the formation of an autoantibodydirected against CYP2C9, named anti-liver/kidney microsomal autoantibody type 2, whereas the pathological role of the autoantibodies in drug-induced hepatitis remains largely unknown.

The Endoplasmic Reticulum in Xenobiotic Toxicity

The endoplasmic reticulum (ER) is involved in an array of cellular functions that play important roles in xenobiotic toxicity. The ER contains the majority of cytochrome P450 enzymes involved in xenobiotic metabolism, as well as a number of conjugating enzymes. In addition to its role in drug bioactivation and detoxification, the ER can be a target for damage by reactive intermediates leading to cell death or immune-mediated toxicity. The ER contains a set of luminal proteins referred to as ER stress proteins (including GRP78, GRP94, protein disulfide isomerase, and calreticulin). These proteins help regulate protein processing and folding of membrane and secretory proteins in the ER, calcium homeostasis, and ER-associated apoptotic pathways. They are induced in response to ER stress. This review discusses the importance of the ER in molecular events leading to cell death following xenobiotic exposure. Data showing that the ER is important in both renal and hepatic toxicity will be discussed.

Selection of new chemical entities with decreased potential for adverse drug reactions

Adverse drug reactions, such as hepatotoxicity, blood dyscrasias and hypersensitivity are a major obstacle for the use and the development of new medicines. Many forms of organ-directed toxicity can arise from the bioactivation of drugs to so-called chemically reactive metabolites, which can modify tissue macromolecules. It is well established that the toxicities of model hepatotoxins, such as acetaminophen, furosemide, bromobenzene and methapyrilene can be correlated with the generation of chemically reactive metabolites, which can be detected by measurement of the irreversible binding of radiolabelled material to hepatic protein and/or the detection of stable phase II metabolites such as glutathione conjugates. The basic chemistry of the reaction of such metabolites with model nucleophiles is relatively well understood. A major challenge is to define how certain reactive intermediates may chemically modify critical proteins and how modification of specific amino acids may alter protein function which in turn may affect cell signalling, regulation, defence, function and viability. This in turn will determine whether or not bioactivation will result in a particular form of drug-induced injury. It is now clear that even relatively simple reactive intermediates can react in a discriminative manner with particular cellular proteins and even with specific amino acids within those proteins. Therefore both non-covalent, as well as covalent bonds will be important determinants of the target protein for a particular reactive metabolite. Mammalian cells have evolved numerous defence systems against reactive intermediates. Sensitive redox proteins such as Nrf-2 recognize oxidative stress and electrophilic agents. This is achieved by chemical modification of cysteine groups within keap-1, which normally forms an inactive heterodimer with Nrf-2. Modification of keap-1 releases Nrf-2 that translocates to the nucleus and effects gene transcription of a number of genes involved in the detoxication of chemically reactive metabolites. Diminution of protein function can occur by either covalent modification of nucleophilic amino acids (e.g. cysteine, lysine, histidine etc.) or oxidation of thiols, which can be reversible or irreversible. In the case of acetaminophen, more than 30 target proteins have been identified and for several of them, corresponding alterations in protein function have been defined in the context of tissue necrosis. Alternatively, protein modification may induce signalling systems which initiate cell death, an immune response or to an altered tissue genotype.

The role of metabolic activation in drug-induced hepatotoxicity

The importance of reactive metabolites in the pathogenesis of drug-induced toxicity has been a focus of research interest since pioneering investigations in the 1950s revealed the link between toxic metabolites and chemical carcinogenesis. There is now a great deal of evidence that shows that reactive metabolites are formed from drugs known to cause hepatotoxicity, but how these toxic species initiate and propagate tissue damage is still poorly understood. This review summarizes the evidence for reactive metabolite formation from hepatotoxic drugs, such as acetaminophen, tamoxifen, diclofenac, and troglitazone, and the current hypotheses of how this leads to liver injury. Several hepatic proteins can be modified by reactive metabolites, but this in general equates poorly with the extent of toxicity. Much more important may be the identification of the critical proteins modified by these toxic species and how this alters their function. It is also important to note that the toxicity of reactive metabolites may be mediated by noncovalent binding mechanisms, which may also have profound effects on normal liver physiology. Technological developments in the wake of the genomic revolution now provide unprecedented power to characterize and quantify covalent modification of individual target proteins and their functional consequences; such information should dramatically improve our understanding of drug-induced hepatotoxic reactions.

Use of molecular simulation for mapping conformational CYP2E1 epitopes

The identification of the epitopes recognized by autoantibodies against cytochrome P450s (CYPs) associated with drug-induced hepatotoxicity is difficult because of their conformational nature. In the present investigation, we used a novel approach based on the analysis of the whole molecule antigenic capacity following single amino acid substitutions to identify the conformational epitopes on CYP2E1. A molecular model of CYP2E1 was generated based on the CYP2C5 crystal structure, and potential motifs for amino acid exchanges were selected by computer simulation in the surface of alpha helices and beta sheets. Fourteen modified, apparently correctly folded CYP2E1 variants were produced in Escherichia coli and evaluated in immunoprecipitation experiments using sera with anti-CYP2E1 autoreactivity from 10 patients with halothane hepatitis and 12 patients with alcoholic liver disease. Ala substitution of Glu-248 and Lys-251 as well as of Lys-324, Lys-342, Lys-420, and Phe-421 severely decreased or abolished CYP2E1 recognition by the majority of both the halothane hepatitis and alcoholic liver disease sera, whereas the other substitutions had only minor effects. Based on the structural model, these substitutions identified two distinct epitopes on the CYP2E1 surface corresponding to the G-helix and an area formed by juxtaposition of the J' and K'' helices, respectively. The combined use of molecular modeling and single amino acid mutagenesis is thus a useful approach for the characterization of conformational epitopes recognized by autoantibodies.

Identification of epitopes on cytochrome P450 3A4/5 recognized by monoclonal antibodies

In this study we describe the mapping of epitopes on CYP3A4/5 recognized by a panel of monoclonal antibodies (MAbs). CYP3A4 and CYP3A5 cDNAs were cloned in GST expression vectors and the fusion proteins were subjected to Western blot. Eight MAbs reacted with the full-length GST-3A4 fusion protein as well as baculovirus cDNA-expressed CYP3A4, while six of these reacted with baculovirus cDNA-expressed CYP3A5. Five (MAb 347, 351, 352, 354, and 357) out of 8 MAbs were inhibitory in a metabolic assay using quinine as substrate. MAbs 352, 354, and 357 brought about a moderate inhibition of quinine metabolism (60-70%) while MAb 347 inhibited quinine 3- hydroxylation in human liver microsomes (n=6) by more than 70%. MAb 347 was a potent inhibitor of baculovirus-expressed CYP3A5-catalyzed metabolism of quinine (95%) at </=0.20 mg IgG/nmol P450 but only moderately inhibited CYP3A4 at much higher ratios of MAb to P450. This MAb was mapped to a region of 283 to 504 amino acids on CYP3A4 protein and to an identical region on CYP3A5 protein. The region that was identified on the CYP3A5 construct was further validated based on the ability of the construct harboring the epitope to reverse the inhibition of hydroxylation of quinine by MAb 347. Our experiments clearly demonstrate that a spatial antigenic determinant is responsible for the inhibitory potency of MAb 347.

Autoantibodies against cytochrome P450s in sera of children treated with immunosuppressive drugs

Treatment with the immunosuppressive drugs cyclosporin and tacrolimus, the mainstays of anti-graft rejection and autoimmune disease therapy, is limited by their hepato- and nephrotoxicity. The metabolic conversion of these compounds to more easily excretable products is catalysed mainly by hepatic cytochrome P4503A4 (CYP3A4) but also involves extrahepatic CYP3A5 and other P450 forms. We set out to study whether or not exposure to cyclosporin and FK506 in children undergoing organ transplantation leads to formation of autoantibodies against P450s. Immunoblotting analysis revealed anti-CYP reactivity in 16% of children on CyA for anti-graft rejection or treatment of nephrosis (n = 67), 31% of kidney transplant patients switched from CyA to FK506 (n = 16), and 21% of kidney and or liver transplant patients on FK506 (n = 14). In contrast, the frequency of reactive immunoblots was only 8.5% among the normal paediatric controls (n = 25) and 7% among adult kidney transplant patients on CyA or FK506 (n = 30). The CYP2C9+ sera were able to immunoprecipitate in vitro translated CYP2C9 and the immunoblot reactivity showed striking correlation to peaks in the age at onset of drug exposure. Sera were isoform selective as evidenced from Western blotting using human liver microsomes and heterologously expressed human P450s. These findings suggest that anti-cytochrome P450 autoantibodies, identified on the basis of their specific binding in immunoblots, are significantly increased among children on immunosuppressive drugs and in some cases are associated with drug toxicity and organ rejection.

Molecular basis for the transport of cytochrome P450 2E1 to the plasma membrane

Endoplasmic reticulum-resident cytochrome P450 enzymes that face the cytosol are present on the plasma membrane of hepatocytes, but the molecular origin for their transport to this compartment has until now remained unknown. The molecular basis for the transport of rat ethanol-inducible cytochrome P450 2E1 (CYP2E1) to the plasma membrane was investigated by transfection of several different mutant cDNAs into mouse H2.35 hepatoma cells. Two NH(2)-terminal CYP2E1 mutants were constructed: N(++)2E1, which carried two positive charges in the NH(2) terminus, and 2C-2E1, in which the transmembrane domain of CYP2E1 was replaced with that of CYP2C1, which was previously described to cause retention of CYP2C1 in the endoplasmic reticulum, as well as CYP2E1 COOH-terminally tagged with the vesicular stomatitis virus G protein (VSV-G) epitope (2E1-VSV-G). Immunofluorescent microscopy and cell surface biotinylation experiments revealed that all CYP2E1 variants were present on the extracellular side of the plasma membrane. The VSV-G epitope on CYP2E1 was detected on the outside of the plasma membrane using VSV-G-specific antibodies, indicating that the large COOH-terminal part of CYP2E1 is indeed exposed on the outside of the plasma membrane. The relative levels of CYP2E1, 2C-2E1, and 2E1-VSV-G on the cell surface were found to be about 2% of total cellular enzyme, whereas twice this amount of N(++)2E1 was recovered at the cell surface. Protease protection experiments performed on microsomes isolated from cDNA transfected cells revealed that a small fraction of CYP2E1 and all variant proteins was found to be located in the lumen of the endoplasmic reticulum (type II orientation), whereas the majority of the proteins were in the expected cytosolic or type I orientation. It is concluded that the NH(2)-terminal transmembrane domain of CYP2E1 plays a critical role in directing the protein to the cell surface and that topological inversion of a small fraction of CYP2E1 in the endoplasmic reticulum directs the protein to the plasma membrane.

Autoantibodies against cytochromes P-4502E1 and P-4503A in alcoholics

Autoantibodies against soluble liver enzymes have been reported among alcoholics, but the targets of self-reactivity toward membrane proteins of the liver have not been characterized. Previously, among alcoholics, we found antibodies against ethanol-derived radical protein adducts that are dependent on cytochrome P-4502E1 (CYP2E1) for their formation. To further investigate autoantibodies against cytochrome P-450s during alcohol abuse, sera of rats chronically treated with ethanol in the total enteral nutrition model and sera from alcoholics with or without alcohol liver disease and from control subjects were analyzed by enzyme-linked immunosorbent assay and Western blotting for the presence of IgG against rat and human CYP2E1, rat CYP3A1, and human CYP3A4. A time-dependent appearance of IgG against rat CYP3A1 and CYP2E1 was evident during chronic ethanol feeding of rats. Anti-CYP2E1 reactivity showed positive correlation with the levels of hepatic CYP2E1 and was inhibited by the CYP2E1 transcriptional inhibitor chlormethiazole. Screening of the human sera by enzyme-linked immunosorbent assay revealed reactivity against CYP3A4 and CYP2E1 in about 20 to 30% and 10 to 20% of the alcoholic sera, respectively. No difference were noted between sera from alcoholics with or without hepatitis C virus infection, and only very little reactivity was seen in sera from control subjects. Western blotting analysis revealed anti-human CYP2E1 reactivity in 8 of 85 alcoholic sera and 3 of 58 control sera, whereas anti-CYP3A4 reactivity was detected in 18 of 85 alcoholic sera and 4 of 58 control sera, which were different from the sera reactive with CYP2E1. Immunoblot reactivity of CYP3A4-positive alcoholic sera was found against glutathione-S-transferase fusion proteins containing truncated forms of CYP3A4, and such sera were also able to immunoprecipitate in vitro translated CYP3A4. Seven of eight sera showed reactivity toward domains C-terminal of position Ser281, and 1 of 8 sera recognized autoepitopes within the region Thr207-Ser281. These findings indicate that alcoholics develop autoantibodies against CYP2E1 and CYP3A4 that the CYP3A4 C-terminal domain is a target for the autoantibody reactions among a subset of alcoholics. The novel finding of CYP3A4 autoantibodies and their significant expression among alcoholics warrants further investigation. Attention should be given to immune toxicity associated with CYP3A4 autoantibodies and cases of alcohol abuse that are accompanied by exposure to drugs and substances that are CYP3A substrates.

Expression of autoantibodies to specific cytochromes P450 in a case of disulfiram hepatitis

BACKGROUND/AIMS

Immunological mechanisms are involved in many adverse drug reactions. In certain forms of drug-induced hepatitis, patients have been reported to express specific autoantibodies to hepatic drug-metabolising enzymes. The alcohol deterrent disulfiram is associated with a low frequency of severe liver toxicity, including hepatitis, but the mechanism of the toxicity is unknown. We investigated whether autoantibodies to cytochrome P450 enzymes were expressed in the serum of a 28-year-old male patient, who developed hepatitis after 7 weeks of disulfiram treatment and in whom possible causes of hepatitis other than disulfiram had been ruled out.

METHODS

Patient serum IgG reactivity was analysed by immunoblotting or ELISA against test antigens consisting of recombinant/purified human or rat liver P450 enzymes, or isolated rat liver microsomes.

RESULTS

A significant serum reactivity was found in immunoblotting against human cytochromes P450 1A2 and rat P450 3A1, using serum dilutions of up to 1:900 and 1:2400, respectively. In contrast, the reactivity against cytochromes P450 2E1, 2C9, 2D6, 3A4, and rat liver P450 reductase was either very low or undetectable. ELISA reactivity was low in general, indicating that the P450 epitopes were not surface exposed. Immunoblotting of rat liver microsomes revealed that autoantibodies recognised one major polypeptide corresponding to P450 3A. Autoantibody titres remained stable for at least 6 months after acute hepatitis. A similar reactivity was not found in any of ten control sera.

CONCLUSIONS

The expression of autoantibodies directed against specific cytochromes P450 in a case of disulfiram hepatitis suggests that immunological mechanisms are involved in this adverse drug reaction, and that these P450 proteins should be evaluated as possible diagnostic test antigens in disulfiram hepatotoxicity.

Drug-induced immunotoxicity

Immune-related drug responses are one of the most common sources of idiosyncratic toxicity. A number of organs may be the target of such reactions; however, this review concentrates mostly on the liver. Drug-induced hepatitis is generally divided into two categories: acute hepatitis in which the drug or a metabolite destroys a vital target in the cell; immunoallergic hepatitis in which the drug triggers an adverse immune response directed against the liver. Their clinical features are: a) low frequency; b) dose independence; c) typical immune system manifestations such as fever, eosinophilia; d) delay between the initiation of treatment and onset of the disease; e) a shortened delay upon rechallenge; and f) occasional presence of autoantibodies in the serum of patients. Such signs have been found in cases of hepatitis triggered by drugs such as halothane, tienilic acid, dihydralazine and anticonvulsants. They will be taken as examples to demonstrate the recent progress made in determining the mechanisms responsible for the disease. The following mechanisms have been postulated: 1) the drug is first metabolized into a reactive metabolite which binds to the enzyme that generated it; 2) this produces a neoantigen which, once presented to the immune system, might trigger an immune response characterized by 3) the production of antibodies recognizing both the native and/or the modified protein; 4) rechallenge leads to increased neoantigen production, a situation in which the presence of antibodies may induce cytolysis. Toxicity is related to the nature and amount of neoantigen and also to other factors such as the individual immune system. An effort should be made to better understand the precise mechanisms underlying this kind of disease and thereby identify the drugs at risk; and also the neoantigen processes necessary for their introduction into the immune system. An animal model would be useful in this regard.