Immunological principles of adverse drug reactions: the initiation and propagation of immune responses elicited by drug treatment

Mechanisms of drug-induced allergy

We identified English-language publications on hypersensitivity reactions to xenobiotics through the PubMed database, using the search terms drug and/or xenobiotic, hypersensitivity reaction, mechanism, and immune mediated. We analyzed articles pertaining to the mechanism and the role of T cells. Immune hypersensitivity reactions to drugs are mediated predominantly by IgE antibodies or T cells. The mechanism of IgE-mediated reactions is well investigated, but the mechanisms of T-cell-mediated drug hypersensitivity are not well understood. The literature describes 2 concepts: the hapten/prohapten concept and the concept of pharmacological interactions of drugs with immune receptors. In T-cell-mediated allergic drug reactions, the specificity of the T-cell receptor that is stimulated by the drug may often be directed to a cross-reactive major histocompatibility complex-peptide compound. Thus, previous contact with the causative drug is not obligatory, and an immune mechanism should be considered as the cause of hypersensitivity, even in reactions that occur on primary exposure. Indeed, immune-mediated reactions to xenobiotics in patients without prior exposure to the agent have been described recently for radiocontrast media and neuromuscular blocking agents. Thus, the "allergenic" potential of a drug under development should be evaluated not only by screening its haptenlike characteristics but also by assessing its direct immunostimulatory potential.

Mechanisms of drug-induced allergy

We identified English-language publications on hypersensitivity reactions to xenobiotics through the PubMed database, using the search terms drug and/or xenobiotic, hypersensitivity reaction, mechanism, and immune mediated. We analyzed articles pertaining to the mechanism and the role of T cells. Immune hypersensitivity reactions to drugs are mediated predominantly by IgE antibodies or T cells. The mechanism of IgE-mediated reactions is well investigated, but the mechanisms of T-cell-mediated drug hypersensitivity are not well understood. The literature describes 2 concepts: the hapten/prohapten concept and the concept of pharmacological interactions of drugs with immune receptors. In T-cell-mediated allergic drug reactions, the specificity of the T-cell receptor that is stimulated by the drug may often be directed to a cross-reactive major histocompatibility complex-peptide compound. Thus, previous contact with the causative drug is not obligatory, and an immune mechanism should be considered as the cause of hypersensitivity, even in reactions that occur on primary exposure. Indeed, immune-mediated reactions to xenobiotics in patients without prior exposure to the agent have been described recently for radiocontrast media and neuromuscular blocking agents. Thus, the "allergenic" potential of a drug under development should be evaluated not only by screening its haptenlike characteristics but also by assessing its direct immunostimulatory potential.

Triptan drugs, natural killer cell cytotoxicity, and neutrophils pro-matrix metalloproteinase-9 secretion

OBJECTIVE

To study the effect of various triptan-like drugs, eg, avitriptan, naratriptan, and sumatriptan, as well as the benzopyran alnitidan, on the natural killer cell (NKC) activity of peripheral blood mononuclear cell (PBMC) samples and highly purified NKC (HPNKC) preparations. We also examined the possible role of these agents as immunomodulators by studying their effect upon the in vitro secretion of pro-matrix metalloproteinase-9 (pMMP-9) from whole blood and purified neutrophils samples.

BACKGROUND

The pharmacological profile of a large number of triptan-like compounds has been extensively studied. However, relatively little is known of their interactions with cellular components of the immune system.

METHODS

Blood was obtained from nonsmoking, drug-free healthy individuals from the Blood Bank of the University of Chile main Clinical Hospital (J.J.A.). PBMC were separated by centrifugation and HPNKC acquired by an immunomagnetic isolation procedure. NKC cytotoxicity was assayed using (51)Cr-labeled K-562 cells as target. Addition of drugs and of effector cells (30 : 1, 50 : 1, and 70 : 1 ratio for PBMCs, and 5 : 1 for HPNKCs) was followed by incubation. Paired Student's t-test (2-tailed) was used to determine the significance of the specific (51)Cr release in controls vs drug-treated samples. Aliquots of whole blood or purified neutrophils were added test drug, incubated, centrifuged, and the supernatant analyzed by gelatine zymography. Gelatinolytic activity was visualized, and a digested zone at MW 92 kD indicated presence of pMMP-9. Area of proteolysis was estimated by densitometry; prestained standards were used to assess pMMP-9 molecular weight.

RESULTS

Peripheral blood mononuclear cell's NKC cytotoxicity was consistently decreased after incubation with each and every one of the drugs tested. This result, observed for the 3 effector : target (E : T) cell ratios used, was relatively similar among the various compounds studied, and reached statistical significance only at E : T 70 : 1. Similar drug treatment failed, however, to produce significant changes in the cytotoxicity of HPNKC preparations, suggesting that modulation of the PBMC's NKC activity and that of HPNKC samples require different kinds of cell's derived signal. Incubation with either of the drugs tested failed to significantly alter (basal) nonstimulated pMPP-9 secretion by whole blood samples. However, basal pMMP-9 secretion by purified neutrophil preparations was significantly inhibited by alnitidan and sumatriptan, and not affected by naratriptan.

CONCLUSIONS

Various drugs with a triptan-like chemical structure interact with cellular components of the innate immune system, resulting in an apparent indirect inhibition of NKC activity and direct inhibition of neutrophils pMMP-9 secretion. These results suggest that they may play a positive role in decreasing the severity of inflammatory processes. Whether this effect is part of triptans antimigraine mechanism of action, or just an added beneficial effect of their use for the reversal treatment of migraine headaches remains to be explored.

Toxicity as a result of immunostimulation by biologics

The immune system has evolved highly effective mechanisms of surveillance and defense against foreign pathogens, and is also thought to act in surveillance and suppression of cancer. Thus, a predominant goal of immune system-based therapies is to normalize or enhance the host immune response in the areas of infectious disease and oncology. This review considers general approaches used for therapeutic immunostimulation, alterations in immune response mechanisms that occur with these treatments and the syndromes that commonly arise as a result of these changes. Because nonclinical studies of these therapies are conducted in animal models as the basis for predicting potential human toxicities, this review also considers the value of nonclinical testing to predict human toxicity.

Delayed drug hypersensitivity reactions ? new concepts

Immune reactions to small molecular compounds such as drugs can cause a variety of diseases mainly involving skin, but also liver, kidney, lungs and other organs. In addition to the well-known immediate, IgE-mediated reactions to drugs, many drug-induced hypersensitivity reactions appear delayed. Recent data have shown that in these delayed reactions drug-specific CD4(+) and CD8(+) T cells recognize drugs through their T cell receptors (TCR) in an MHC-dependent way. Immunohistochemical and functional studies of drug-reactive T cells in patients with distinct forms of exanthems revealed that distinct T cell functions lead to different clinical phenotypes. Taken together, these data allow delayed hypersensitivity reactions (type IV) to be further subclassified into T cell reactions, which by releasing certain cytokines and chemokines preferentially activate and recruit monocytes (type IVa), eosinophils (type IVb), or neutrophils (type IVd). Moreover, cytotoxic functions by either CD4(+) or CD8(+) T cells (type IVc) seem to participate in all type IV reactions. Drugs are not only immunogenic because of their chemical reactivity, but also because they may bind in a labile way to available TCRs and possibly MHC-molecules. This seems to be sufficient to stimulate certain, probably preactivated T cells. The drug seems to bind first to the fitting TCR, which already exerts some activation. For full activation, an additional interaction of the TCR with the MHC molecules is needed. The drug binding to the receptor structures is reminiscent of a pharmacological interaction between a drug and its (immune) receptor and was thus termed the p-i concept. In some patients with drug hypersensitivity, such a response occurs within hours even upon the first exposure to the drug. The T cell reaction to the drug might thus not be due to a classical, primary response, but is due to peptide-specific T cells which happen to be stimulated by a drug. This new concept has major implications for understanding clinical and immunological features of drug hypersensitivity and a model to explain the frequent skin symptoms in drug hypersensitivity is proposed.

Adverse cutaneous drug eruptions and HIV: a clinician's global perspective

Adverse drug reactions (ADRs) are common and mostly avoidable. Some ADRs cannot as yet be predicted, but at-risk populations/patients and high-risk drugs are identifiable. HIV-infected patients are at risk of developing cutaneous ADRs, especially Stevens-Johnson syndrome, toxic epidermal necrolysis, and drug hypersensitivity syndrome. Multiple factors of causation variably present in patients with HIV infection best explain the pathogenesis of these cutaneous ADRs. When no effective alternate therapy is available, drug rechallenge in HIV-infected patients can be attempted with little morbidity or mortality if done according to rationalized protocols.

Adverse side-effects to biological agents

Biological agents-like cytokines, monoclonal antibodies and fusion proteins are widely used in anti-inflammatory and tumour therapy. They are highly efficient in certain diseases, but can cause a great variety of adverse side-effects. Based on the peculiar features of biological agents a new classification of these adverse side-effects of biological agents is proposed - related but clearly distinct from the classification of side-effects observed with chemicals and drugs. This classification differentiates five distinct types, namely clinical reactions because of high cytokine levels (type alpha), hypersensitivity because of an immune reaction against the biological agent (beta), immune or cytokine imbalance syndromes (gamma), symptoms because of cross-reactivity (delta) and symptoms not directly affecting the immune system (epsilon). This classification could help to better deal with the clinical features of these side-effects, to identify possible individual and general risk factors and to direct research in this novel area of medicine.

Association of drug-serum protein adducts and anti-drug antibodies in dogs with sulphonamide hypersensitivity: A naturally occurring model of idiosyncratic drug toxicity

BACKGROUND

Sulphonamide antimicrobials, such as sulphamethoxazole (SMX), provide effective infection prophylaxis in immunocompromised patients, but can lead to drug hypersensitivity (HS) reactions. These reactions also occur in dogs, with a similar time course and clinical presentation as seen in humans.

OBJECTIVES

Drug-serum adducts and anti-drug antibodies have been identified in sulphonamide HS humans. The aim of this study was to determine whether similar markers were present in dogs with sulphonamide HS.

METHODS

Thirty-four privately owned sulphonamide HS dogs, 10 sulphonamide-'tolerant' dogs, 18 sulphonamide-naïve dogs, and four dogs experimentally dosed with SMX and the oxidative metabolite SMX-nitroso, were tested for drug-serum adducts by immunoblotting, and anti-drug antibodies by ELISA.

RESULTS

Sulphonamide-serum adducts were found in 10/20 HS dogs tested (50%), but in no tolerant dogs. Anti-sulphonamide IgG antibodies were detected in 17/34 HS dogs (50%), but in only one tolerant dog; antibody absorbance values were significantly higher in HS dogs. There was a significant association between the presence of sulphonamide-serum adducts and anti-sulphonamide antibodies (P = 0.009). Anti-drug antibodies were also found in dogs experimentally dosed with SMX-nitroso followed by SMX, but not in a dog dosed with drug vehicle, followed by SMX.

CONCLUSION

Similar humoral markers are present in dogs and humans with sulphonamide HS, supporting the use of dogs as a naturally occurring model for this syndrome in humans. These data suggest the potential use of drug-serum adducts and anti-drug antibodies as markers for sulphonamide HS. Preliminary data indicate that anti-sulphonamide antibodies may be triggered by the SMX-nitroso metabolite, not by the parent drug, in dogs.

Transfection of drug-specific T-cell receptors into hybridoma cells: tools to monitor drug interaction with T-cell receptors and evaluate cross-reactivity to related compounds

In the context of drug hypersensitivity, our group has recently proposed a new model based on the structural features of drugs (pharmacological interaction with immune receptors; p-i concept) to explain their recognition by T cells. According to this concept, even chemically inert drugs can stimulate T cells because certain drugs interact in a direct way with T-cell receptors (TCR) and possibly major histocompatibility complex molecules without the need for metabolism and covalent binding to a carrier. In this study, we investigated whether mouse T-cell hybridomas transfected with drug-specific human TCR can be used as an alternative to drug-specific T-cell clones (TCC). Indeed, they behaved like TCC and, in accordance with the p-i concept, the TCR recognize their specific drugs in a direct, processing-independent, and dose-dependent way. The presence of antigen-presenting cells was a prerequisite for interleukin-2 production by the TCR-transfected cells. The analysis of cross-reactivity confirmed the fine specificity of the TCR and also showed that TCR transfectants might provide a tool to evaluate the potential of new drugs to cause hypersensitivity due to cross-reactivity. Recombining the alpha- and beta-chains of sulfanilamide- and quinolone-specific TCR abrogated drug reactivity, suggesting that both original alpha- and beta-chains were involved in drug binding. The TCR-transfected hybridoma system showed that the recognition of two important classes of drugs (sulfanilamides and quinolones) by TCR occurred according to the p-i concept and provides an interesting tool to study drug-TCR interactions and their biological consequences and to evaluate the cross-reactivity potential of new drugs of the same class.

Roles of endogenous ascorbate and glutathione in the cellular reduction and cytotoxicity of sulfamethoxazole-nitroso

Sulfamethoxazole (SMX) is an effective drug for the management of opportunistic infections, but its use is limited by hypersensitivity reactions, particularly in HIV-infected patients. The oxidative metabolite SMX-nitroso (SMX-NO), is thought to be a proximate mediator of SMX hypersensitivity, and can be reduced in vitro by ascorbate or glutathione. Leukocytes from patients with SMX hypersensitivity show enhanced cytotoxicity from SMX metabolites in vitro; this finding has been attributed to a possible "detoxification defect" in some individuals. The purpose of this study was to determine whether variability in endogenous ascorbate or glutathione could be associated with individual differences in SMX-NO cytotoxicity. Thirty HIV-positive patients and 23 healthy control subjects were studied. Both antioxidants were significantly correlated with the reduction of SMX-NO to its hydroxylamine, SMX-HA, by mononuclear leukocytes, and both were linearly depleted during reduction. Controlled ascorbate supplementation in three healthy subjects increased leukocyte ascorbate with no change in glutathione, and significantly enhanced SMX-NO reduction. Ascorbate supplementation also decreased SMX-NO cytotoxicity compared to pre-supplementation values. Rapid reduction of SMX-NO to SMX-HA was associated with enhanced direct cytotoxicity from SMX-NO. When forward oxidation of SMX-HA back to SMX-NO was driven by the superoxide dismutase mimetic, Tempol, SMX-NO cytotoxicity was increased, without enhancement of adduct formation. This suggests that SMX-NO cytotoxicity may be mediated, at least in part, by redox cycling between SMX-HA and SMX-NO. Overall, these data indicate that endogenous ascorbate and glutathione are important for the intracellular reduction of SMX-NO, a proposed mediator of SMX hypersensitivity, and that redox cycling of SMX-HA to SMX-NO may contribute to the cytotoxicity of these metabolites in vitro.

Noncovalent interactions of drugs with immune receptors may mediate drug-induced hypersensitivity reactions

Drug-induced hypersensitivity reactions are instructive examples of immune reactions against low molecular weight compounds. Classically, such reactions have been explained by the hapten concept, according to which the small antigen covalently modifies an endogenous protein; recent studies show strong associations of several HLA molecules with hypersensitivity. In recent years, however, evidence has become stronger that not all drugs need to bind covalently to the major histocompatibility complex (MHC)-peptide complex in order to trigger an immune response. Rather, some drugs may bind reversibly to the MHC or possibly to the T-cell receptor (TCR), eliciting immune reactions akin to the pharmacological activation of other receptors. While the exact mechanism is still a matter of debate, noncovalent drug presentation clearly leads to the activation of drug-specific T cells. In some patients with hypersensitivity, such a response may occur within hours of even the first exposure to the drug. Thus, the reaction to the drug may not be the result of a classical, primary response but rather be mediated by existing, preactivated T cells that display cross-reactivity for the drug and have additional (peptide) specificity as well. In this way, certain drugs may circumvent the checkpoints for immune activation imposed by the classical antigen processing and presentation mechanisms, which may help to explain the idiosyncratic nature of many drug hypersensitivity reactions.

Pharmacological interaction of drugs with immune receptors: the p-i concept

Drug-induced hypersensitivity reactions have been explained by the hapten concept, according to which a small chemical compound is too small to be recognized by the immune system. Only after covalently binding to an endogenous protein the immune system reacts to this so called hapten-carrier complex, as the larger molecule (protein) is modified, and thus immunogenic for B and T cells. Consequently, a B and T cell immune response might develop to the drug with very heterogeneous clinical manifestations. In recent years, however, evidence has become stronger that not all drugs need to bind covalently to the MHC-peptide complex in order to trigger an immune response. Rather, some drugs may bind directly and reversibly to immune receptors like the major histocompatibility complex (MHC) or the T cell receptor (TCR), thereby stimulating the cells similar to a pharmacological activation of other receptors. This concept has been termed pharmacological interaction with immune receptors the (p-i) concept. While the exact mechanism is still a matter of debate, non-covalent drug presentation clearly leads to the activation of drug-specific T cells as documented for various drugs (lidocaine, sulfamethoxazole (SMX), lamotrigine, carbamazepine, p-phenylendiamine, etc.). In some patients with drug hypersensitivity, such a response may occur within hours even upon the first exposure to the drug. Thus, the reaction to the drug may not be due to a classical, primary response, but rather be mediated by stimulating existing, pre-activated, peptide-specific T cells that are cross specific for the drug. In this way, certain drugs may circumvent the checkpoints for immune activation imposed by the classical antigen processing and presentation mechanisms, which may help to explain the peculiar nature of many drug hypersensitivity reactions.

Role of bioactivation in drug-induced hypersensitivity reactions

Drug-induced hypersensitivity reactions are a major problem in both clinical treatment and drug development. This review covers recent developments in our understanding of the pathogenic mechanisms involved, with special focus on the potential role of metabolism and bioactivation in generating a chemical signal for activation of the immune system. The possible role of haptenation and neoantigen formation is discussed, alongside recent findings that challenge this paradigm. Additionally, the essential role of costimulation is examined, as are the potential points whereby costimulation may be driven by reactive metabolites. The relevance of local generation of metabolites in determining the location and character of a reaction is also covered.

Kinetics of tienilic acid bioactivation and functional generation of drug–protein adducts in intact rat hepatocytes

Drug-induced autoimmune hepatitis is among the most severe hepatic idiosyncratic adverse drug reactions. Considered multifactorial, the disease combines immunological and metabolic aspects, the latter being to date much better known. As for many other model drugs, studies on tienilic acid (TA)-induced hepatitis have evidenced the existence of bioactivation during the hepatic oxidation of the drug, allowing the identification of the neoantigen of anti-LKM2 autoantibodies and the pathway responsible for its formation. However, most of these results are based on the use of microsomal fractions whose relevance to the liver in vivo still needs to be established. In the more complex intact cell environment, several endogenous processes may play a significant role on triggering the reaction and should therefore be considered. In this work we have characterised the kinetics of TA biotransformation in metabolically competent hepatocytes, the influence of TA bioactivation on physiological GSH levels, and the qualitative and quantitative profile of drug-protein conjugates generated in situ, as a function of exposure time. Results confirm that intact hepatocytes reproduce in vitro the metabolic sequence that leads to the functional generation of drug-protein adducts, in conditions that simulate clinical human exposure to TA. Metabolically competent cultured hepatocytes appear as a very promising approach to investigate the early preimmunological events of drug-induced autoimmune hepatitis, adequate to identify the conditions that may modulate the formation and specificity of drug-protein adducts in vivo, to study the hepatic disposition of the TA-protein targets, and to define the specific role of the hepatocyte in the origin of this adverse reaction.

Direct T-cell stimulations by drugs--bypassing the innate immune system

Some drug induced immune mediated side effects appear rapidly (6-<48hrs) and are hard to reconcile with the generation of a new immune response to the drug. By extending the previously presented p-i concept (pharmacological interaction with immune receptors), I propose that drugs might stimulate memory T-cells via their T-cell receptors, which happen to react not only with a peptide antigen, but also with a (chemically inert) drug. In this model, the generation of a new, drug specific immune response is actually bypassed: the innate immune system must not be stimulated, as no naive T-cells are stimulated. Only previously activated memory T-cells are re-activated, as they have a lower threshold of reactivity to a T-cell receptor transmitted signal, whereby certain cofactors like an ongoing immune response to a virus might facilitate reactivity to the drug. This concept has major implications for preclinical testing of the allergenic potential of a drug, which normally is measured by the ability of the drug to cause a new immune response.

Cellular mechanisms of T cell mediated drug hypersensitivity

Noncovalent drug presentation leads to the activation of drug-specific T cells. In some patients with hypersensitivity, such a response occurs within hours even upon the first exposure to the drug. Thus, the reaction to the drug might not be due to a classical, primary response, but rather mediated by existing, preactivated T cells that are cross specific for the drug, and have an additional (peptide) specificity as well.

Role of drug metabolism for breaking tolerance and the localization of drug hypersensitivity

There are three major working hypotheses for the mechanism of drug hypersensitivity reactions: the hapten hypotheses, the danger hypothesis and the PI hypothesis. These hypotheses are difficult to test because of the idiosyncratic nature of hypersensitivity reactions. There is evidence that reactive metabolites are involved in many hypersensitivity reactions, and the reactive metabolite is often formed in the target organ of toxicity, presumably because the half-life of most reactive metabolites is too short to allow them to reach distant sites. In the case of less reactive species that freely circulate the pattern of hypersensitivity usually fits that expected of an extracellular antigen, specifically, an antibody-mediated reaction. We have used two animal models: penicillamine-induced autoimmunity and nevirapine-induced skin rash in Brown Norway rats to test hypotheses. We have found that tolerance is readily induced with a lower dose of the drug, although the nature of tolerance is different in the two models. In the penicillamine model, tolerance is immune-mediated and can be overcome by agents that act as a danger signal. Reactive metabolites may also act as a danger signal. The models can also be used to test the role of reactive species in the mechanism of hypersensitivity reactions.

Genetic aspects of immune-mediated adverse drug effects

Adverse drug effects (ADEs) are of great importance in medicine and account for up to 5% of all hospital admissions. ADEs can arise from several mechanisms and a wide range of drugs can cause immune-mediated ADEs (IMADEs). For a drug to elicit an IMADE, it must be both immunogenic (that is, able to sensitize the immune system) and antigenic (that is, able to evoke a response from a sensitized immune system). Unlike protein therapeutics, small-molecule drugs (or xenobiotics) are usually neither immunogenic nor antigenic. IMADEs are therefore the result of complex interactions between drug-metabolizing enzymes, immune sensitization and immune effectors. The genetic aspects of this interplay are discussed in this review.

Cytokine release does not improve the sensitivity and specificity of the direct popliteal lymph node assay

The popliteal lymph node assay (PLNA) is being considered as a tool to predict the potential of drugs for inducing systemic autoimmune and hypersensitivity reactions. Despite the use of different technical approaches and the evaluation of over 130 compounds, the sensitivity and specificity of the PLNA are still debatable due to many false positive and negative responses. In this study, cytokine production was assessed as a possible endpoint to improve the direct (primary) PLNA. Diclofenac, imipramine, hydralazine, glafenin and minocycline were tested using the classical procedure. TH1 cytokines (IL-2 and IFN-gamma), TH2 cytokines (IL-4 and IL-5) and pro-inflammatory cytokines (IL-6, TNF-alpha, monocyte chemoattractant protein-1 (MCP-1), IL-12p70 and IL-10) were measured in the serum and in suspensions of popliteal lymph node cells of female Balb/c mice by flow cytometry 7 days after drug administration. Only diclofenac and imipramine induced a cellularity index above 5 (considered as a positive response). Of the five tested drugs, only diclofenac induced a slight increase in TH1 cytokines, but there were no effects on TH2 cytokine production whatever the drug tested. Diclofenac increased the production of pro-inflammatory cytokines, whereas the production of MCP-1 was increased by minocycline and decreased by imipramine. No changes in serum cytokine levels were evident. These results suggest that measuring cytokine release is unlikely to improve the sensitivity and specificity of the direct PLNA.

Immune mechanism of drug hypersensitivity

Drug hypersensitivity reactions can lead to a great variety of different diseases. The main cause is a specific interaction of antibodies or T cells with a drug. In addition to the hapten concept, some drugs can bind directly to T-cell receptors and stimulate them. Based on recent investigation on different exanthemas, an extended classification of the Gell and Coombs type IV reaction is proposed.

The lymphocyte transformation test in the diagnosis of drug hypersensitivity

Diagnosis of drug hypersensitivity is difficult, as an enormous amount of different drugs can elicit various immune-mediated diseases with distinct pathomechanism. The lymphocyte transformation test (LTT) measures the proliferation of T cells to a drug in vitro--from which one concludes to a previous in vivo reaction due to a sensitization. This concept of the LTT has been confirmed by the generation of drug-specific T-cell clones and the finding that drugs can directly interact with the T-cell receptor, without previous metabolism or need to bind to proteins. In this review, technical aspects and usefulness of this test for the diagnosis of drug hypersensitivity are discussed. The main advantage of this test is its applicability with many different drugs in different immune reactions, as drug-specific T cell are almost always involved in drug hypersensitivity reactions. Its main disadvantages are that an in vitro proliferation of T cells to a drug is difficult to transfer to the clinical situation and that the test per se is rather cumbersome and technically demanding. In addition, its sensitivity is limited (for beta-lactam allergy it is in the range of 60-70%), - although at least in our hands - it is higher than of other tests for drug hypersensitivity diagnosis. Consequently, drug hypersensitivity diagnosis needs to rely on a combination of history and different tests, as none of the single tests available has per se a sufficiently good sensitivity. Within this setting, the LTT has proven to be a useful test for the diagnosis of drug hypersensitivity reactions and helped to better understand these reactions. Further work on the simplification of this test and systematic evaluation of its sensitivity and specificity in some main groups of drugs are necessary to make this test more widely available.

Captopril-induced bilateral parotid and submandibular sialadenitis

Two cases of sialadenitis following treatment with captopril are described. In case 1, an upper chest and facial erythema and dryness of the mouth accompanied the swelling of the salivary glands. In case 2, a conjunctival erythema accompanied the sialadenitis. None of the patients had previously used captopril, and, in both cases, the swelling occurred within the first hour after the drug intake; the patients had a complete recovery within a few hours after captopril was withdrawn. It is possible that the reported effect was caused by a type-B idiosyncratic adverse drug reaction.

Drug hypersensitivity reactions in skin: understanding mechanisms and the development of diagnostic and predictive tests

Cutaneous manifestations of drug hypersensitivity can be serious and potentially life threatening and may prevent effective drug therapy. T cells play an important role in the pathology of drug hypersensitivity reactions. Classical studies suggest that T-cell activation requires drug bioactivation, covalent binding to protein and antigen processing to stimulate an immune response. Recent studies have shown that drugs can also be presented to T cells in the absence of antigen processing and drug metabolism. In this article, sulfamethoxazole is used as a paradigm to describe the chemical mechanisms involved in the initiation and maintenance of an aberrant drug antigen specific T-cell response. Presentation of the same drug to different individuals can cause a variety of skin diseases. Such reactions have been classified according to the phenotype and functionality of the T-cell response. This review summarises the different forms of cutaneous hypersensitivity reactions and describes how T-cell clones generated from hypersensitive patients have been used to study the cellular mechanisms of anticonvulsant hypersensitivity. Potential uses of in vitro cell culture assays for patient diagnosis and drug evaluation are also discussed.

Drug-Induced Hypersensitivity

Drug-induced hypersensitivity is an adverse reaction, characterised by damaging immune-mediated responses, initiated by medicine given at therapeutic doses for prevention, diagnosis or treatment. Immune-mediated drug hypersensitivity accounts for 6-10% of the adverse drug reactions, which rank between the fourth and sixth leading causes of death in the US. With <10% of all adverse drug reactions reported, the magnitude of the problem is significant, with estimates of costs >$US30 billion annually in the US (1995 value). In addition, the costs of not determining the potential of a drug to produce hypersensitivity in the pre-clinical phase of drug development can be substantial. It has been estimated that the pre-clinical phase and clinical phase I, phase II and phase III costs are approximately $US6 million, $US12 million, $US12 million and $US100 million per drug, respectively (1999 values). It is important that investigational drugs with the potential to produce hypersensitivity reactions be identified as early in the development process as possible. Some adverse reactions to drugs can be avoided if drug-drug interactions are known or if there is a structure-activity relationship established. However, these methods are inadequate. Appropriate animal models of drug-induced hypersensitivity are needed, especially because hypersensitivity has been cited as the leading reason for taking drugs off the market. It is of critical importance to be able to predict hypersensitivity reactions to drugs. Most anaphylactic reactions occur in atopic individuals. Similarly, patients who have experienced other hypersensitivity reactions are more likely to have recurrent reactions. Therefore, animal models should be considered that predispose the animal to the reaction, such as the use of appropriate adjuvants and species. Using known positive controls of varying strengths, the investigator can rank the reaction against the positive controls as standards. This approach might yield greater results in a shorter period of time than using novel models. For the greatest safety, use of well understood models that have been thoroughly validated is imperative.

Clinical Pharmacology

The dose of a drug is a major determinant of its safety, and establishing a safe dose of a novel drug is a prime objective during clinical development. The design of pre-marketing clinical trials precludes the representation of important subpopulations such as children, the elderly and people with co-morbidities. Therefore, postmarketing surveillance (PMS) activities are required to monitor the safety profile of drugs in real clinical practice. Furthermore, individual variations in pharmacogenetic profiles, the immune system, drug metabolic pathways and drug-drug interactions are also important factors in the occurrence of adverse drug reactions. Thus, the safety of a drug is a major clinical consideration before and after it is marketed. A multidisciplinary approach is required to enhance the safety profile of drugs at all stages of development, including PMS activities. Clinical pharmacology encompasses a range of disciplines and forms the backbone of drug safety consideration during clinical drug development. In this review we give an overview of the clinical drug development process and consider its limitations. We present a discussion of several aspects of clinical pharmacology and their application to enhancing drug safety. Pharmacokinetic-pharmacodynamic modelling provides a method of predicting a clinically safe dose; consideration of drug pharmacokinetics in special populations may enhance safe therapeutics in a wider spectrum of patients, while pharmacogenetics provides the possibility of genotype-specific therapeutics. Pharmacovigilance activities are also discussed. Given the complex nature and unpredictability of type B reactions, PMS activities are crucial in managing the risks drugs pose to the general population. The various aspects of clinical pharmacology discussed make a strong case for this field as the backbone of optimising and promoting safe development and use of drugs.

Screening for the potential of a drug candidate to cause idiosyncratic drug reactions

Toxicity testing has been ineffective in the prediction of drug candidates that will be associated with a relatively high incidence of idiosyncratic drug reactions (IDRs). Circumstantial evidence suggests the involvement of reactive metabolites in the aetiology of these reactions and this has prompted several companies to screen drug candidates for the formation of such compounds. Most drugs form at least one reactive metabolite. To develop efficient prediction methods, a better understanding of the basic mechanisms involved is essential. This review highlights the current mechanistic hypotheses of IDRs and discusses future directions in the development of better predictive tests.

Cross-reactivity with drugs at the T cell level

PURPOSE OF REVIEW

Cross-reactivity with drugs is an important clinical problem in drug hypersensitivity. Once a patient is labeled 'drug-allergic' all drugs of the same class are withheld and future therapeutic interventions are limited. Here we review cross-reactivity with drugs at the T cell level.

RECENT FINDINGS

Analysis of T cell recognition of various classes of drugs (beta-lactam antibiotics, sulfonamides, local anesthetics) using T cell clones suggests that at the T cell level the whole structure, in particular the core and to a lesser degree side chains, are recognized.

SUMMARY

It is necessary to differentiate cross-reactivity mediated by T cells and antibodies as only the latter seem to recognize side chains exclusively.

Drug-induced autoimmunity

PURPOSE OF REVIEW

Presentation of different mechanism of drug-induced autoimmunity and highlighting of new developments.

RECENT FINDINGS

Drugs can induce autoimmune diseases by their pharmacological properties. Injection of certain drugs into the thymus can alter positive selection in the thymus and elicit autoimmune reactions. Peripheral tolerance can be broken by increasing the expression of LFA-1 adhesion molecule on T cells. This can be related to the inhibition of intracellular kinases. Alternatively, a drug specific immune response might elicit autoimmunity by cross-reactivity: the drug reactive T cells might be cross-reactive with certain peptide antigens and possibly autoantigens.

SUMMARY

Drug-specific immune responses are well described. They have a great tendency to be cross-reactive with peptide antigens. This 'immunological' cause of autoimmunity elicited by drugs may occur more frequently than thought. It connects the field of drug hypersensitivity with drug-induced autoimmunity.

Characterization of drug-specific T cells in lamotrigine hypersensitivity

BACKGROUND

Lamotrigine is associated with hypersensitivity reactions, which are most commonly characterized by skin rash. An immune etiology has been postulated, though the nature of this is unclear.

OBJECTIVES

The aim of this study was to characterize the role of T cells in lamotrigine hypersensitivity.

METHODS

A lymphocyte transformation test was performed on 4 hypersensitive patients. Lymphocytes from 3 of 4 lamotrigine-hypersensitive patients proliferated when stimulated with lamotrigine. T-cell clones were generated from one patient to further characterize the nature of the T-cell involvement. Cells were characterized in terms of their phenotype, functionality, and mechanisms of antigen presentation and cytotoxicity.

RESULTS

Of the 44 drug-specific T-cell clones generated, most were CD4(+) with occasional CD8(+) cells. All clones expressed the alphabeta T-cell receptor; several Vbeta 5.1(+) or 9(+) T-cell clones were generated. All clones also expressed the skin-homing receptor cutaneous lymphocyte antigen. Lamotrigine-stimulated T cells were cytotoxic and secreted perforin, IFN-gamma, IL-5, and macrophage inflammatory protein 1alpha, macrophage inflammatory protein 1beta, RANTES, and I-309. Lamotrigine was present on HLA-DR and HLA-DQ by antigen-presenting cells in the absence of drug metabolism and processing. The T-cell receptor of certain clones could accommodate analogs of lamotrigine, but no cross-reactivity was seen with other anticonvulsants.

CONCLUSIONS

Our data provide evidence that T cells are involved in the pathogenesis of some lamotrigine-hypersensitivity reactions. The identification of drug-specific cells that express cutaneous lymphocyte antigen and type 1 cytokines after T-cell receptor activation is consistent with the clinical symptoms. Furthermore, identification of large numbers of Vbeta 5.1(+) T cells suggests that polymorphisms within T-cell receptor genes might act as determinants of susceptibility.

Addressing the metabolic activation potential of new leads in drug discovery: a case study using ion trap mass spectrometry and tritium labeling techniques

Metabolic activation of drug candidates to electrophilic reactive metabolites that can covalently modify cellular macromolecules may result in acute and/or idiosyncratic immune system-mediated toxicities in humans. This presents a significant potential liability for the future development of these compounds as safe therapeutic agents. We present here an example of an approach where sites of metabolic activation within a new drug candidate series were rapidly identified using online liquid chromatography/multi-stage mass spectrometry on an ion trap mass spectrometer. This was accomplished by trapping the reactive intermediates formed upon incubation of compounds with rat and human liver microsomes as their corresponding glutathione conjugates and mass spectral characterization of these thiol adducts. Based on the structures of the GSH adducts identified, potential sites and mechanisms of bioactivation within the chemical structure were proposed. These metabolism studies were interfaced with iterative structural modifications of the chemical series in order to block these bioactivation sites within the molecule. This strategy led to a significant reduction in the propensity of the compounds to undergo metabolic activation as evidenced by reductions in the irreversible binding of radioactivity to liver microsomal material upon incubation of tritium-labeled compounds with this in vitro system. With the efficiency and throughput achievable with such an approach, it appears feasible to identify and address the metabolic activation potential of new drug leads during routine metabolite identification studies in an early drug discovery setting.

Immunopharmacology of hypersensitivity reactions to drugs

Drug hypersensitivity reactions are characterized by their unpredictability, lack of simple dose-dependency, host sensitivity, and potentially serious clinical outcome. They occur in a small proportion of patients, and usually the predisposing factors are unknown, although there is increasing evidence for genetic predisposition and disease being significant risk factors. The current understanding of the chemical basis of immune-mediated reactions is based on the hapten hypothesis, which requires drug bioactivation, covalent binding to proteins, followed by uptake, antigen processing, and a polyclonal immune response. The recently proposed "danger hypothesis" can be considered to be an essential addition to the hapten hypothesis. According to the danger hypothesis, the immune response to a drug-derived antigen requires the presence of co-stimulatory signals and cytokines, which propagate and determine the type of immune response. The "danger signal" might result from chemical, physical, or viral stress.

Modes of presentation of chemical neoantigens to the immune system

The immune system can interact with small molecular compounds like drugs. Drugs are not only immunogenic because of their chemical reactivity, but also because they may bind in a labile way to MHC-molecules and fit into available T-cell receptors. This seems to be sufficient to stimulate T-cells. Such structural features of a drug have to be considered in the evaluation of drug hypersensitivity reactions and should be taken into account in predictive drug allergy testing.

The danger hypothesis--potential role in idiosyncratic drug reactions

Idiosyncratic or type B reactions are characterised by their unpredictability and lack of simple dose-dependency. They occur in a small proportion of patients, and usually the predisposing factors are unknown. A proportion of, but not all, idiosyncratic reactions are immune-mediated. Our understanding of immune-mediated reactions is based on the hapten hypothesis, which requires drug bioactivation, covalent binding to proteins, followed by uptake, antigen processing and a polyclonal immune response. The recently proposed 'danger hypothesis' can be considered to be additive to the hapten hypothesis. The hypothesis states that the immune system only responds when it detects danger. If no danger is detected, tolerance results. Thus, stimulation of an immune response to a drug-protein conjugate (signal 1) requires the presence of co-stimulatory signals and cytokines (signals 2 and 3), which propagate and determine the type of immune response. The nature of the danger signal is poorly defined, and has been proposed to include different forms of stress including chemical, physical and viral. Indeed, there are several examples where the frequency of drug hypersensitivity is increased in the presence of a viral infection, most notably in HIV disease. Nevertheless, this clinical evidence has to be regarded as being circumstantial and more direct experimental evidence is required to understand the role of 'danger' in the overall pathogenesis of drug hypersensitivity reactions.

Pharmacological interaction of drugs with antigen-specific immune receptors: the p-i concept

PURPOSE OF REVIEW

Drug allergies are examples of immune reactions to small molecular compounds. In many drug allergies drug specific CD4+ and CD8+ T-cells can be detected, which recognize small chemicals via their alphabeta-T-cell receptor in a major histocompatibility complex dependent way. In this review a new concept of drug presentation to T-cells is presented.

RECENT FINDINGS

Drugs were stimulatory for T-cells if they bound covalently to peptides or proteins, but also if the drug had structural features allowing it to bind in a labile way (noncovalently) to the major histocompatibility peptide complex. This latter binding method has some similarities to superantigen stimulations and can explain allergies to drugs that are not metabolized. It has been described in patients with maculopapular, bullous and neutrophilic drug eruption, as well as in contact dermatitis.

SUMMARY

Noncovalent drug presentation leads to the stimulation of immune cells, namely T-cells. The drug needs two surface molecules (one inert serving as a scaffold, major histocompatibility complex, and one reactive, T-cell receptor) to exert its function. Although two receptor structures are involved, the process is reminiscent of a pharmacological interaction between a drug and its receptors and, from the phrase pharmacological interaction with immune receptors, was thus termed the p-i concept.

Do women have more adverse drug reactions?

Up to 5% of all hospital admissions are the result of adverse drug reactions (ADRs). Identifying those factors which may predispose to ADRs is essential for risk management. Amongst the known risk factors for adverse reactions are increasing age, polypharmacy, liver and renal disease as well as being female. Female patients have a 1.5- to 1.7-fold greater risk of developing an ADR, including adverse skin reactions, compared with male patients. The reasons for this increased risk are not entirely clear but include gender-related differences in pharmacokinetic, immunological and hormonal factors as well as differences in the use of medications by women compared with men. Women generally have a lower lean body mass, a reduced hepatic clearance, have differences in activity of cytochrome P450 (CYP) enzymes (40% increase in CYP3A4, varied decrease in CYP2D6, CYP2C19 and CYP1A2), and metabolize drugs at different rates compared with men. Other important factors include conjugation, absorption, protein binding and renal elimination, which may all have some gender-based differences. However, how these differences result in an increased risk of ADRs is not clear. There are pharmacodynamic differences between men and women, seen particularly with cardiac and psychotropic medications. There is no doubt that chlorpromazine, fluspirilene and various antipsychotics appear more effective in women than men for the same dosage and plasma concentration. Similarly, women are at increased risk of QT prolongation with certain anti-arrhythmic drugs compared with men even at equivalent serum concentrations. The mechanisms are unknown. Increasingly the evidence is that idiosyncratic drug reactions, particularly cutaneous reactions, appear to have an immunological etiology. It is possible that gender difference in T cell activation and proliferation account for this as well as the increased prevalence of skin diseases such as systemic lupus erythematosus and photosensitivity. Whatever the mechanism(s), it is important to be aware that gender is a significant factor in ADRs.