Halogenated anesthetics form liver adducts and antigens that cross-react with halothane-induced antibodies

Species differences in developmental toxicity of epoxiconazole and its relevance to humans

Epoxiconazole, a triazole-based fungicide, was tested in toxicokinetic, prenatal and pre-postnatal toxicity studies in guinea pigs, following oral (gavage) administration at several dose levels (high dose: 90 mg/kg body weight per day). Maternal toxicity was evidenced by slightly increased abortion rates and by histopathological changes in adrenal glands, suggesting maternal stress. No compound-related increase in the incidence of malformations or variations was observed in the prenatal study. In the pre-postnatal study, epoxiconazole did not adversely affect gestation length, parturition, or postnatal growth and development. Administration of epoxiconazole did not alter circulating estradiol levels. Histopathological examination of the placentas did not reveal compound-related effects. The results in guinea pigs are strikingly different to those observed in pregnant rats, in which maternal estrogen depletion, pathological alteration of placentas, increased gestation length, late fetal death, and dystocia were observed after administration of epoxiconazole. In the studies reported here, analysis of maternal plasma concentrations and metabolism after administration of radiolabeled epoxiconazole demonstrated that the different results in rats and guinea pigs were not due to different exposures of the animals. A comprehensive comparison of hormonal regulation of pregnancy and birth in murid rodents and primates indicates that the effects on pregnancy and parturition observed in rats are not applicable to humans. In contrast, the pregnant guinea pig shares many similarities to pregnant humans regarding hormonal regulation and is therefore considered to be a suitable species for extrapolation of related effects to humans.

Mechanisms, chemical structures and drug metabolism

From the studies that have been done by many laboratories over the last 2 decades, it is now clear that the toxicities produced by many drugs are due to their reactive metabolites. It is though that, in many cases, reactive metabolites cause toxicity by binding covalently to tissue proteins. However, until recently it was difficult to identify these protein targets. Due to the development of an immunochemical approach, this problem has been overcome, as is illustrated here by studies that have been conducted on the metabolic basis of the idiosyncratic hepatitis caused by the inhalation anaesthetic halothane. The major problem to solve in the future will be to determine how protein adduct formation leads to toxicity. It is possible that protein adduct formation may alter an important cellular function or may lead to immunopathology, as is thought to occur in the case of halothane hepatitis. If an allergic reaction is suspected, purified protein targets of reactive metabolites can serve as antigens for identifying sensitized individuals. This information can be used to prevent not only an allergic reaction to the drug, but possible cross-reactions to other drugs that are structurally related. Another important application of these studies is the design of safer alternative drugs that will not produce structurally similar toxic reactive metabolites.

Monitoring drug-protein interaction

A variety of therapeutic drugs can undergo biotransformation via Phase I and Phase II enzymes to reactive metabolites that have intrinsic chemical reactivity toward proteins and cause potential organ toxicity. A drug-protein adduct is a protein complex that forms when electrophilic drugs or their reactive metabolite(s) covalently bind to a protein molecule. Formation of such drug-protein adducts eliciting cellular damages and immune responses has been a major hypothesis for the mechanism of toxicity caused by numerous drugs. The monitoring of protein-drug adducts is important in the kinetic and mechanistic studies of drug-protein adducts and establishment of dose-toxicity relationships. The determination of drug-protein adducts can also provide supportive evidence for diagnosis of drug-induced diseases associated with protein-drug adduct formation in patients. The plasma is the most commonly used matrix for monitoring drug-protein adducts due to its convenience and safety. Measurement of circulating antibodies against drug-protein adducts may be used as a useful surrogate marker in the monitoring of drug-protein adducts. The determination of plasma protein adducts and/or relevant antibodies following administration of several drugs including acetaminophen, dapsone, diclofenac and halothane has been conducted in clinical settings for characterizing drug toxicity associated with drug-protein adduct formation. The monitoring of drug-protein adducts often involves multi-step laboratory procedure including sample collection and preliminary preparation, separation to isolate or extract the target compound from a mixture, identification and determination. However, the monitoring of drug-protein adducts is often difficult because of short half-lives of the protein adducts, sampling problem and lack of sensitive analytical techniques for the protein adducts. Currently, chromatographic (e.g. high performance liquid chromatography) and immunological methods (e.g. enzyme-linked immunosorbent assay) are two major techniques used to determine protein adducts of drugs in patients. The present review highlights the importance for clinical monitoring of drug-protein adducts, with an emphasis on methodology and with a further discussion of the application of these techniques to individual drugs and their target proteins.

Drug bioactivation, covalent binding to target proteins and toxicity relevance

A number of therapeutic drugs with different structures and mechanisms of action have been reported to undergo metabolic activation by Phase I or Phase II drug-metabolizing enzymes. The bioactivation gives rise to reactive metabolites/intermediates, which readily confer covalent binding to various target proteins by nucleophilic substitution and/or Schiff's base mechanism. These drugs include analgesics (e.g., acetaminophen), antibacterial agents (e.g., sulfonamides and macrolide antibiotics), anticancer drugs (e.g., irinotecan), antiepileptic drugs (e.g., carbamazepine), anti-HIV agents (e.g., ritonavir), antipsychotics (e.g., clozapine), cardiovascular drugs (e.g., procainamide and hydralazine), immunosupressants (e.g., cyclosporine A), inhalational anesthetics (e.g., halothane), nonsteroidal anti-inflammatory drugs (NSAIDSs) (e.g., diclofenac), and steroids and their receptor modulators (e.g., estrogens and tamoxifen). Some herbal and dietary constituents are also bioactivated to reactive metabolites capable of binding covalently and inactivating cytochrome P450s (CYPs). A number of important target proteins of drugs have been identified by mass spectrometric techniques and proteomic approaches. The covalent binding and formation of drug-protein adducts are generally considered to be related to drug toxicity, and selective protein covalent binding by drug metabolites may lead to selective organ toxicity. However, the mechanisms involved in the protein adduct-induced toxicity are largely undefined, although it has been suggested that drug-protein adducts may cause toxicity either through impairing physiological functions of the modified proteins or through immune-mediated mechanisms. In addition, mechanism-based inhibition of CYPs may result in toxic drug-drug interactions. The clinical consequences of drug bioactivation and covalent binding to proteins are unpredictable, depending on many factors that are associated with the administered drugs and patients. Further studies using proteomic and genomic approaches with high throughput capacity are needed to identify the protein targets of reactive drug metabolites, and to elucidate the structure-activity relationships of drug's covalent binding to proteins and their clinical outcomes.

Anaphylaxis during the perioperative period

Anesthesiologists use a myriad of drugs during the provision of an anesthetic. Many of these drugs have side effects that are dose related, and some lead to severe immune-mediated adverse reactions. Anaphylaxis is the most severe immune-mediated reaction; it generally occurs on reexposure to a specific antigen and requires the release of proinflammatory mediators. Anaphylactoid reactions occur through a direct non-immunoglobulin E-mediated release of mediators from mast cells or from complement activation. Muscle relaxants and latex account for most cases of anaphylaxis during the perioperative period. Symptoms may include all organ systems and present with bronchospasm and cardiovascular collapse in the most severe cases. Management of anaphylaxis includes discontinuation of the presumptive drug (or latex) and anesthetic, aggressive pulmonary and cardiovascular support, and epinephrine. Although a serum tryptase confirms the diagnosis of an anaphylactic reaction, the offending drug can be identified by skin-prick, intradermal testing, or serologic testing. Prevention of recurrences is critical to avoid mortality and morbidity.

Separation and detection methods for covalent drug–protein adducts

Covalent binding of reactive metabolites of drugs to proteins has been a predominant hypothesis for the mechanism of toxicity caused by numerous drugs. The development of efficient and sensitive analytical methods for the separation, identification, quantification of drug-protein adducts have important clinical and toxicological implications. In the last few decades, continuous progress in analytical methodology has been achieved with substantial increase in the number of new, more specific and more sensitive methods for drug-protein adducts. The methods used for drug-protein adduct studies include those for separation and for subsequent detection and identification. Various chromatographic (e.g., affinity chromatography, ion-exchange chromatography, and high-performance liquid chromatography) and electrophoretic techniques [e.g., sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), two-dimensional SDS-PAGE, and capillary electrophoresis], used alone or in combination, offer an opportunity to purify proteins adducted by reactive drug metabolites. Conventionally, mass spectrometric (MS), nuclear magnetic resonance, and immunological and radioisotope methods are used to detect and identify protein targets for reactive drug metabolites. However, these methods are labor-intensive, and have provided very limited sequence information on the target proteins adducted, and thus the identities of the protein targets are usually unknown. Moreover, the antibody-based methods are limited by the availability, quality, and specificity of antibodies to protein adducts, which greatly hindered the identification of specific protein targets of drugs and their clinical applications. Recently, the use of powerful MS technologies (e.g., matrix-assisted laser desorption/ionization time-of-flight) together with analytical proteomics have enabled one to separate, identify unknown protein adducts, and establish the sequence context of specific adducts by offering the opportunity to search for adducts in proteomes containing a large number of proteins with protein adducts and unmodified proteins. The present review highlights the separation and detection technologies for drug-protein adducts, with an emphasis on methodology, advantages and limitations to these techniques. Furthermore, a brief discussion of the application of these techniques to individual drugs and their target proteins will be outlined.

Identification and quantification of in vitro adduct formation between protein reactive xenobiotics and a lysine-containing model peptide

Formation of in vitro adducts between different classes of xenobiotics and the lysine-containing peptide Lys-Tyr was monitored by high-performance liquid chromatography and electrospray ionization mass spectrometry. The molecular structures of the main resulting products could be sensitively analyzed by mass spectrometry (flow injection analysis), enabling the detection of characteristic binding formations. Aldehydes such as formaldehyde, acetaldehyde, and benzaldehyde were shown to form stable linkages to lysine amino groups via Schiff bases. Other electrophilic substances (e.g., toluene-2,4-diisocyanate, 2,4-dinitro-1-fluorobenzene, 2,4,6-trinitrobenzene sulfonic acid, dansyl chloride, and phthalic acid anhydride) also formed covalent adducts with lysine residues. The reactivity of the compounds was quantified by measuring the amount of peptide that remained unchanged after incubation for a certain period with the xenobiotic. Although reactivity levels within this group of aldehydes varied only to a small extent, as would be expected, extreme differences were seen among the structurally heterogeneous group of nonaldehyde xenobiotics. These results support the hypothesis that simple chemical reactions may lead to the adduction of nucleophilic macromolecules such as peptides or proteins. Such reactions, in particular, Schiff base formation of aldehydes, have previously been shown to be capable of specifically interfering with costimulatory signaling on T cells. Our results suggest that electrophilic xenobiotics of other classes may also inherit the capacity to exert similar effects. Forming covalent linkage to peptides may represent a possible molecular mechanism of electrophilic xenobiotics in vivo, yielding immunotoxic effects. The model utilized in this study is appropriate for monitoring the adduction of xenobiotics to basic peptides and for analyzing the resulting molecular structures.

Task force report: future research needs for the prevention and management of immune-mediated drug hypersensitivity reactions

Immune-mediated drug hypersensitivity reactions (IDHR) have a significant impact on clinical practice, drug development, and public health. However, research to understand IDHR mechanisms and to develop diagnostic and predictive tests has been limited. To stimulate more research, a task force with representatives from the key stakeholders (research clinicians, regulatory scientists, and immunotoxicologists from the pharmaceutical industry) was assembled to identify critical data gaps and opportunities and to make recommendations on how to overcome some of the barriers to IDHR research and address research needs. It is hoped that this report will act as a springboard for future discussions and progress toward increased funding and development of organizational structures for IDHR research.

Humoral immune response to a sevoflurane degradation product in the guinea pig following inhalation exposure

Compound A (2-fluoromethoxy-1,1,3,3,3-pentafluoro-1-propene) is produced by reaction of the inhalation anesthetic, sevoflurane, with CO2 absorbents. Compound A has been reported to directly react with protein. Since adduction of proteins can transform them into antigenic material, Compound A was assessed for its ability to produce a humoral immune response. Male outbred Hartley guinea pigs (500-600 g, N = 7) were exposed via inhalation for 4 h to a subtoxic level (100 ppm) of Compound A, 3 times, at 42 day intervals. Blood samples obtained at 2, 14, 28 and 40 days after each exposure were measured for ALT, creatinine, and urea nitrogen and for the presence of antibodies to trifluoroacetylated guinea pig albumin (TFA-GSA). All indicators of liver and kidney injury remained within normal range throughout the course of the study. A humoral immune response to TFA-GSA was observed following each exposure to Compound A with a titer appearing by day 14 after exposure, peaking near day 28, and resolving to normal levels by day 40. The titer levels were approximately equivalent after each exposure and about one-third that previously seen in guinea pigs after multiple exposures to halothane. Compound A would appear to have the ability to form antigenic adducts during inhalation exposure. These findings are similar to those observed for halogenated inhalation anesthetics that have been linked to cases of immune-medicated idiosyncratic hepatitis and indicate that Compound A exposure may pose the same hazard.

Halothane-induced liver injury in outbred guinea pigs: role of trifluoroacetylated protein adducts in animal susceptibility

Halothane causes a mild form of liver injury in guinea pigs that appears to model the hepatotoxicity seen in approximately 20% of patients treated with this drug. In previous studies, it was concluded that the increased susceptibility of some outbred guinea pigs to halothane-induced liver injury is not caused by their inherent ability to metabolize halothane to form toxic levels of trifluoroacetylated protein adducts in the liver. In this study, we reevaluated the role of trifluoroacetylated protein adducts in halothane-induced liver injury in guinea pigs. Male outbred Hartley guinea pigs were treated with halothane intraperitoneally. On the basis of serum alanine aminotransferase levels and liver histology, treated animals were designated as being susceptible, mildly susceptible, or resistant to halothane. Immunoblot studies with the use of anti-trifluoroacetylated antibodies showed that susceptible guinea pigs for the most part had higher levels of trifluoroacetylated protein adducts in the liver 48 h after treatment with halothane than did less susceptible animals. In support of this finding, the level of trifluoroacetylated protein adducts detected immunochemically in the sera of treated guinea pigs correlated with sera levels of alanine aminotransferase activity. In addition, the levels of cytochrome P450 2A-related protein but not those of other cytochrome P450 isoforms, measured by immunoblot analysis with isoform-specific antibodies, correlated with the amount of trifluoroacetylated protein adducts detected in the livers of guinea pigs 8 h after halothane administration. The results of this study indicate that the susceptibility of outbred guinea pigs to halothane-induced liver injury is related to an enhanced ability to metabolize halothane in the liver to form relatively high levels of trifluoroacetylated protein adducts. They also suggest that cytochrome P450 2A-related protein might have a major role in catalyzing the formation of trifluoroacetylated protein adducts in the liver of susceptible guinea pigs. Similar mechanisms may be important in humans.

Kupffer cells from halothane-exposed guinea pigs carry trifluoroacetylated protein adducts

The anesthetic, halothane, is bioactivated by the liver cytochrome P450 system to trifluoroacetyl-chloride, which can readily acylate liver protein. Covalent binding of the trifluoroacetyl moiety may result in hapten formation leading to the induction of an immune response and ultimately halothane hepatitis. In this study the presence of trifluoroacetylated-protein adducts in Kupffer cells was investigated to learn how the immune system might come in contact with the proteins. Guinea pigs were exposed to 1.0% halothane, 40% oxygen for 4 h. Kupffer cells were isolated on days 1 through 9 post-exposure, by liver perfusion and purification by elutriation. Using gel electrophoresis and Western blotting techniques, it has been demonstrated that Kupffer cells obtained from halothane-treated guinea pigs, do carry trifluoroacetyl-protein adducts as recognized by an anti-trifluoroacetyl-rabbit serum albumin antibody. Apparent molecular weights of polypeptides bound by trifluoroacetyl were of a wide range, 25-152 kDa. Bands were most prominent in the larger Kupffer cells with more appearing at lower molecular weights. Trifluoroacetyl-protein adducts were not detected in lung, spleen, lymph node or peripheral blood macrophages. This work suggests a role for Kupffer cells in the presentation of altered proteins in the liver to cells of the immune system.

Demonstration of a cellular immune response in halothane-exposed guinea pigs

Halothane hepatitis is considered to be a result of an idiosyncratic autoimmune reaction brought about by the formation of neoantigens that have been generated by covalent binding of halothane biotransformation intermediates. The guinea pig is being examined as an animal model to investigate an immune-mediated mechanism for halothane hepatotoxicity. Male Hartley guinea pigs were exposed to 1% halothane/40% oxygen for 4 hr, three times with 40-day intervals. Kupffer cells and splenocytes were isolated from animals on various days after each halothane exposure. Splenocytes were cocultured in a lymphocyte transformation test with various concentrations of TFA(trifluoroacetylated)-antigens for 7 days and proliferation was measured by 3H-thymidine incorporation. In a second experiment, Kupffer cells were cocultured with autologous as well as allogeneic splenocytes with or without concanavalin A to determine whole cell sensitization and accessory function by Kupffer cells from treated animals. A 4-fold increase in splenocyte proliferation occurred in response to TFA-guinea pig albumin. No significant increase in proliferation could be detected with TFA-lysine or guinea pig albumin. A 14-fold increase in splenocyte proliferation also occurred in response to Kupffer cells from halothane-exposed animals. Autologous splenocytes demonstrated more of a response from treated versus control animals, indicating possible involvement of major histocompatibility complex II antigens. These results indicate recognition of TFA-antigens and Kupffer cells as antigen-presenting cells in halothane-exposed guinea pigs. This study provides good evidence that a cellular immune response is involved in the guinea pig after halothane exposure.

Protein targets of xenobiotic reactive intermediates

Many xenobiotics are metabolically activated to electrophilic intermediates that form covalent adducts with proteins; the mechanism of toxicity is either intrinsic or idiosyncratic in nature. Many intrinsic toxins covalently modify cellular proteins and somehow initiate a sequence of events that leads to toxicity. Major protein adducts of several intrinsic toxins have been identified and demonstrate significant decreases in enzymatic activity. The reactivity of intermediates and subcellular localization of major targets may be important in the toxicity. Idiosyncratic toxicities are mediated through either a metabolic or immune-mediated mechanism. Xenobiotics that cause hypersensitivity/autoimmunity appear to have a limited number of protein targets, which are localized within the subcellular fraction where the electrophile is produced, are highly substituted, and are accessible to the immune system. Metabolic idiosyncratic toxins appear to have limited targets and are localized within a specific subcellular fraction. Identification of protein targets has given us insights into mechanisms of xenobiotic toxicity.