Characterisation of flucloxacillin and 5-hydroxymethyl flucloxacillin haptenated HSA in vitro and in vivo

Immediate-type hypersensitivity drug reactions

Hypersensitivity reactions including anaphylaxis have been reported for nearly all classes of therapeutic reagents and these reactions can occur within minutes to hours of exposure. These reactions are unpredictable, not directly related to dose or the pharmacological action of the drug and have a relatively high mortality risk. This review will focus on the clinical presentation, immune mechanisms, diagnosis and prevention of the most serious form of immediate onset drug hypersensitivity reaction, anaphylaxis. The incidence of drug-induced anaphylaxis deaths appears to be increasing and our understanding of the multiple and complex reasons for the unpredictable nature of anaphylaxis to drugs is also expanding. This review highlights the importance of enhancing our understanding of the biology of the patient (i.e. immune response, genetics) as well as the pharmacology and chemistry of the drug when investigating, diagnosing and treating drug hypersensitivity. Misdiagnosis of drug hypersensitivity leads to substantial patient risk and cost. Although oral provocation is often considered the gold standard of diagnosis, it can pose a potential risk to the patient. There is an urgent need to improve and standardize diagnostic testing and desensitization protocols as other diagnostic tests currently available for assessment of immediate drug allergy are not highly predictive.

Characterization of amoxicillin- and clavulanic acid-specific T cells in patients with amoxicillin-clavulanate-induced liver injury

Drug-induced liver injury (DILI) frequently has a delayed onset with several human leukocyte antigen (HLA) genotypes affecting susceptibility, indicating a potential role for the adaptive immune system in the disease. The aim of this study was to investigate whether drug-responsive T lymphocytes are detectable in patients who developed DILI with the combination, antimicrobial amoxicillin-clavulanate. Lymphocytes from 6 of 7 patients were found to proliferate and/or secrete interferon-gamma (IFN-γ) when cultured with amoxicillin and/or clavulanic acid. Amoxicillin (n = 105) and clavulanic acid (n = 16) responsive CD4(+) and CD8(+) T-cell clones expressing CCR, chemokine (C-C motif) receptor 4, CCR9, and chemokine (C-X-C motif) receptor 3 were generated from patients with and without HLA risk alleles; no cross-reactivity was observed between the two drug antigens. Amoxicillin clones were found to secrete a heterogeneous panel of mediators, including IFN-γ, interleukin-22 and cytolytic molecules. In contrast, cytokine secretion by the clavulanic acid clones was more restricted. CD4(+) and CD8(+) clones were major histocompatability complex class II and I restricted, respectively, with the drug antigen being presented to CD4(+) clones in the context of HLA-DR molecules. Several pieces of evidence indicate that the clones were activated by a hapten mechanism: First, professional antigen-presenting cells (APCs) were required for optimal activation; second, pulsing APCs for 4-16 hours activated the clones; and third, inhibition of processing abrogated the proliferative response and cytokine release.

CONCLUSION

Both amoxicillin- and clavulanic acid-specific T cells participate in the liver injury that develops in certain patients exposed to amoxicillin-clavulanate.

Update on Advances in Research on Idiosyncratic Drug-Induced Liver Injury

Drug-induced liver injury (DILI) is a major concern for public health, as well as for drug development in the pharmaceutical industry, since it can cause liver failure and lead to drug withdrawal from the market and black box warnings. Thus, it is important to identify biomarkers for early prediction to increase our understanding of mechanisms underlying DILI that will ultimately aid in the exploration of novel therapeutic strategies to prevent or manage DILI. DILI can be subdivided into 'intrinsic' and 'idiosyncratic' categories, although the validity of this classification remains controversial. Idiosyncratic DILI occurs in a minority of susceptible individuals with a prolonged latency, while intrinsic DILI results from drug-induced direct hepatotoxicity over the course of a few days. The rare occurrence of idiosyncratic DILI requires multicenter collaborative investigations and phenotype standardization. Recent progress in research on idiosyncratic DILI is based on key developments in 3 areas: (1) newly developed high-throughput genotyping across the whole genome allowing for the identification of genetic susceptibility markers, (2) new mechanistic concepts on the pathogenesis of DILI revealing a key role of drug-responsive T lymphocytes in the immunological response, and (3) broad multidisciplinary approaches using different platform "-omics" technologies that have identified novel biomarkers for the prediction of DILI. An association of a specific human leukocyte antigen (HLA) allele with DILI has been reported for several drugs. HLA-restricted T-cell immune responses have also been investigated using lymphocytes and T-cell clones isolated from patients. A microRNA, miR-122, has been discovered as a promising biomarker for the early prediction of DILI. In this review, we summarize recent advances in research on idiosyncratic DILI with an understanding of the key role of adaptive immune systems.

Irreversible binding of an anticancer compound (BI-94) to plasma proteins

1. We investigated the mechanisms responsible for the in vivo instability of a benzofurazan compound BI-94 (NSC228148) with potent anti-cancer activity. 2. BI-94 was stable in MeOH, water, and in various buffers at pHs 2.5-5, regardless of the buffer composition. In contrast, BI-94 was unstable in NaOH and at pHs 7-9, regardless of the buffer composition. BI-94 disappeared immediately after spiking into mice, rat, monkey, and human plasma. BI-94 stability in plasma can be only partially restored by acidifying it, which indicated other mechanisms in addition to pH for BI-94 instability in plasma. 3. BI-94 formed adducts with the trapping agents, glutathione (GSH) and N-acetylcysteine (NAC), in vivo and in vitro via nucleophilic aromatic substitution reaction. The kinetics of adduct formation showed that neutral or physiological pHs enhanced and accelerated GSH and NAC adduct formation with BI-94, whereas acidic pHs prevented it. Therefore, physiological pHs not only altered BI-94 chemical stability but also enhanced adduct formation with endogenous nucleophiles. In addition, adduct formation with human serum albumin-peptide 3 (HSA-T3) at the Cys34 position was demonstrated. 4. In conclusion, BI-94 was unstable at physiological conditions due to chemical instability and irreversible binding to plasma proteins.

In vitro diagnosis of delayed-type drug hypersensitivity: mechanistic aspects and unmet needs

Several laboratories use the lymphocyte transformation test for the diagnosis of delayed-type drug hypersensitivity reactions. Recently, the availability of multiple readouts has improved our ability to diagnose reactions. It is important to note that most published studies characterizing the usefulness of diagnostic tests utilize blood samples from well-defined test and control patient groups. The purpose of this article is to briefly summarize the cellular and chemical basis of delayed-type drug hypersensitivity reactions and to review in vitro assays that are available for drug hypersensitivity diagnosis.

The importance of hapten-protein complex formation in the development of drug allergy

PURPOSE OF REVIEW

Drug allergy is an adverse drug reaction that is immune-mediated. Immune activation can occur when drugs or haptens bind covalently to proteins and then act as antigens. The purpose of this review is to summarize the recent data on the formation of hapten-protein complexes and to assess the importance of these complexes in the generation of drug allergy.

RECENT FINDINGS

The formation of hapten-protein complexes by drugs and their reactive metabolites has largely been investigated using model proteins such as human serum albumin. Precise identification of the structure of the hapten and the resulting modified residue(s) in the protein has been undertaken for a small number of drugs, such as p-phenylenediamine, nevirapine, carbamazepine, β-lactams and abacavir. Some progress has also been made in identifying hapten-protein complexes in the serum of patients with allergy.

SUMMARY

Drug-specific T cells have been isolated from different patients with allergy. Formation of hapten-protein complexes, their processing and antigen presentation have been implicated in the development of drug allergy to p-phenylenediamine, sulfonamides and β-lactams. However, evidence also supports the pi mechanism of immune activation wherein drugs interact directly with immune receptors. Thus, multiple mechanisms of immune activation may occur for the same drug.

Human leukocyte antigen genetic risk factors of drug-induced liver toxicology

INTRODUCTION

Drug-induced liver injury (DILI) is a rare adverse drug reaction, which impacts significantly on patients. Human leukocyte antigen (HLA) risk alleles have been found to be associated with DILI supporting an immunological basis to DILI pathogenesis.

AREAS COVERED

HLA alleles associated with risk of liver injury induced by specific therapeutic drugs are described. The evidence for a role of the adaptive immune system in DILI is presented; case-control studies showing an association between DILI and HLA alleles are reviewed. Clinical applications of pharmacogenomics are considered.

EXPERT OPINION

Increasing evidence points to a crucial role for the adaptive immune system in the pathogenesis of DILI. Identification of specific HLA alleles as risk factors through large genome-wide association studies has been instrumental in this and in vitro analyses have facilitated improved understanding of the molecular mechanisms. This provides the basis for developing clinical pharmacogenomic applications. Already, genotyping for hypersensitivity HLA risk alleles has been implemented and opportunities for pre-prescription testing in DILI identified. However, although associations are strong, the rarity of DILI means routine testing has not been formally evaluated. Nevertheless, enhanced understanding of how HLA alleles contribute to injury risk is valuable for drug development. Translation of this research into effective pre-emption and primary prevention remains the goal.

The risk of cutaneous adverse reactions among patients with the HLA-A* 31:01 allele who are given carbamazepine, oxcarbazepine or eslicarbazepine: a perspective review

Carbamazepine is a drug that is widely used for the treatment of epilepsy, trigeminal neuralgia and bipolar disorder. This drug is also known to cause cutaneous adverse drug reactions (cADRs) in up to 10% of patients. The recent progress in pharmacogenetics has revealed that human leukocyte antigen (HLA) genotypes are associated with a susceptibility to the cADRs caused by particular drugs. For carbamazepine-induced Stevens-Johnson syndrome and toxic epidermal necrolysis, very strong associations with HLA-B*15:02 have been found mainly in patients of Southeastern Asian origin. In some countries, prescreening HLA-B*15:02 allele has already been put to practical use as a biomarker to avoid the life-threatening adverse drug reactions. In this review, another risk factor for carbamazepine-induced cADRs is discussed, namely HLA-A*31:01. We compare the strength of the association between HLA-A*31:01 and carbamazepine-induced cADRs based on reports for various ethnic populations; discuss the difference between the HLA-A*31:01 and HLA-B*15:02 biomarkers and the usefulness of prescreening HLA-A*31:01 to detect patients at high risk for carbamazepine-induced cADRs; and refer to points that remain to be resolved.

Mass spectrometric characterization of circulating covalent protein adducts derived from a drug acyl glucuronide metabolite: multiple albumin adductions in diclofenac patients

Covalent protein modifications by electrophilic acyl glucuronide (AG) metabolites are hypothetical causes of hypersensitivity reactions associated with certain carboxylate drugs. The complex rearrangements and reactivities of drug AG have been defined in great detail, and protein adducts of carboxylate drugs, such as diclofenac, have been found in liver and plasma of experimental animals and humans. However, in the absence of definitive molecular characterization, and specifically, identification of signature glycation conjugates retaining the glucuronyl and carboxyl residues, it cannot be assumed any of these adducts is derived uniquely or even fractionally from AG metabolites. We have therefore undertaken targeted mass spectrometric analyses of human serum albumin (HSA) isolated from diclofenac patients to characterize drug-: derived structures and, thereby, for the first time, have deconstructed conclusively the pathways of adduct formation from a drug AG and its isomeric rearrangement products in vivo. These analyses were informed by a thorough understanding of the reactions of HSA with diclofenac AG in vitro. HSA from six patients without drug-: related hypersensitivities had either a single drug-: derived adduct or one of five combinations of 2-8 adducts from among seven diclofenac N-acylations and three AG glycations on seven of the protein's 59 lysines. Only acylations were found in every patient. We present evidence that HSA modifications by diclofenac in vivo are complicated and variable, that at least a fraction of these modifications are derived from the drug's AG metabolite, and that albumin adduction is not inevitably a causation of hypersensitivity to carboxylate drugs or a coincidental association.

Negative regulation by PD-L1 during drug-specific priming of IL-22-secreting T cells and the influence of PD-1 on effector T cell function

Activation of PD-1 on T cells is thought to inhibit Ag-specific T cell priming and regulate T cell differentiation. Thus, we sought to measure the drug-specific activation of naive T cells after perturbation of PD-L1/2/PD-1 binding and investigate whether PD-1 signaling influences the differentiation of T cells. Priming of naive CD4(+) and CD8(+) T cells against drug Ags was found to be more effective when PD-L1 signaling was blocked. Upon restimulation, T cells proliferated more vigorously and secreted increased levels of IFN-γ, IL-13, and IL-22 but not IL-17. Naive T cells expressed low levels of PD-1; however, a transient increase in PD-1 expression was observed during drug-specific T cell priming. Next, drug-specific responses from in vitro primed T cell clones and clones from hypersensitive patients were measured and correlated with PD-1 expression. All clones were found to secrete IFN-γ, IL-5, and IL-13. More detailed analysis revealed two different cytokine signatures. Clones secreted either FasL/IL-22 or granzyme B. The FasL/IL-22-secreting clones expressed the skin-homing receptors CCR4, CCR10, and CLA and migrated in response to CCL17/CCL27. PD-1 was stably expressed at different levels on clones; however, PD-1 expression did not correlate with the strength of the Ag-specific proliferative response or the secretion of cytokines/cytolytic molecules. This study shows that PD-L1/PD-1 binding negatively regulates the priming of drug-specific T cells. ELISPOT analysis uncovered an Ag-specific FasL/IL-22-secreting T cell subset with skin-homing properties.

Study of protein haptenation by amoxicillin through the use of a biotinylated antibiotic

Allergic reactions towards β-lactam antibiotics pose an important clinical problem. The ability of small molecules, such as a β-lactams, to bind covalently to proteins, in a process known as haptenation, is considered necessary for induction of a specific immunological response. Identification of the proteins modified by β-lactams and elucidation of the relevance of this process in allergic reactions requires sensitive tools. Here we describe the preparation and characterization of a biotinylated amoxicillin analog (AX-B) as a tool for the study of protein haptenation by amoxicillin (AX). AX-B, obtained by the inclusion of a biotin moiety at the lateral chain of AX, showed a chemical reactivity identical to AX. Covalent modification of proteins by AX-B was reduced by excess AX and vice versa, suggesting competition for binding to the same targets. From an immunological point of view, AX and AX-B behaved similarly in RAST inhibition studies with sera of patients with non-selective allergy towards β-lactams, whereas, as expected, competition by AX-B was poorer with sera of AX-selective patients, which recognize AX lateral chain. Use of AX-B followed by biotin detection allowed the observation of human serum albumin (HSA) modification by concentrations 100-fold lower that when using AX followed by immunological detection. Incubation of human serum with AX-B led to the haptenation of all of the previously identified major AX targets. In addition, some new targets could be detected. Interestingly, AX-B allowed the detection of intracellular protein adducts, which showed a cell type-specific pattern. This opens the possibility of following the formation and fate of AX-B adducts in cells. Thus, AX-B may constitute a valuable tool for the identification of AX targets with high sensitivity as well as for the elucidation of the mechanisms involved in allergy towards β-lactams.

T cell responses to drugs and drug metabolites

Understanding the chemical mechanisms by which drugs and drug metabolites interact with cells of the immune system is pivotal to our knowledge of drug hypersensitivity as a whole.In this chapter, we will discuss the currently accepted mechanisms where there is scientific and clinical evidence to support the ways in which drugs and their metabolites interact with T cells. We will also discuss bioanalytical platforms, such as mass spectrometry, and in vitro test assays such as the lymphocyte transformation test that can be used to study drug hypersensitivity; the combination of such techniques can be used to relate the chemistry of drug antigen formation to immune function. Ab initio T cell priming assays are also discussed with respect to predicting the potential of a drug to cause hypersensitivity reactions in humans in relation to the chemistry of the drug and its ability to form haptens, antigens and immunogens in patients.

Activation of the TCR complex by small chemical compounds

Small chemical compounds and certain metal ions can activate T cells, resulting in drug hypersensitivity reactions that are a main problem in pharmacology. Mostly, the drugs generate new antigenic epitopes on peptide-major histocompatibility complex (MHC) molecules that are recognized by the T-cell antigen receptor (TCR). In this review we discuss the molecular mechanisms of how the drugs alter self-peptide-MHC, so that neo-antigens are produced. This includes (1) haptens covalently bound to peptides presented by MHC, (2) metal ions and drugs that non-covalently bridge self-pMHC to the TCR, and (3) drugs that allow self-peptides to be presented by MHCs that otherwise are not presented. We also briefly discuss how a second signal-next to the TCR-that naïve T cells require to become activated is generated in the drug hypersensitivity reactions.

Identification and frequency of circulating CD4(+) T lymphocytes specific to Benzylpenicillin in healthy donors

BACKGROUND

Drug hypersensitivity is known to rely on a drug-specific T-cell response. Amplitude of antigen-specific T-cell response is partly controlled by the size of the antigen-specific naïve CD4(+) T-cell repertoire, but estimate of this repertoire has never been investigated for allergenic drugs. The purpose of this study was to evaluate the frequency of benzylpenicillin-specific CD4(+) T lymphocytes in healthy donors.

METHODS

Co-cultures were established with CD4(+) T lymphocytes from healthy donors and mature autologous dendritic cells loaded with benzylpenicillin coupled to human serum albumin. CD4(+) T lymphocytes were stimulated once a week for 4 weeks with benzylpenicillin coupled to human serum albumin. The CD4(+) T-cell response was measured using an interferon-γ ELISPOT assay. Frequency of benzylpenicillin-specific naive CD4(+) T lymphocytes was then calculated using the Poisson distribution law.

RESULTS

Results showed the presence of benzylpenicillin-specific CD4(+) T lymphocytes in 9 of 10 tested healthy donors irrespective of their HLA typing, with a mean frequency of 0.29 cells per million of CD4(+) T cells. Experiments performed on naive (CD45RA(+) ) and on memory (CD45RO(+) ) CD4(+) T lymphocytes showed that these benzylpenicillin-specific CD4(+) T lymphocytes belonged to the naive T-cell subpopulation.

CONCLUSION

This study showed for the first time the existence of naive CD4(+) T lymphocytes specific to benzylpenicillin in healthy donors.

Pharmacogenomics of severe cutaneous adverse reactions and drug-induced liver injury

Rare but severe adverse drug reactions (ADRs) are an important issue in drug development and in the proper usage of drugs during the post-approval phase. The ability to predict patient susceptibility to severe ADRs would prevent drug administration to high-risk patients. This would save lives and ensure the quality of life for these patients, but occurrence of idiosyncratic severe ADRs had been very difficult to predict for a long time. However, in this decade, genetic markers have been found for several ADRs, especially for severe cutaneous adverse reactions (SCARs) and drug-induced liver injury (DILI). In this review, we summarize recent progress in identifying genetic markers for SCARS and DILI, and discuss issues that remain unresolved. As for SCARs, associations of HLA-B*15:02 or HLA-A*31:01 and HLA-B*58:01 have been revealed for carbamazepine- and allopurinol-related Stevens-Johnson syndrome and toxic epidermal neclolysis, respectively. HLA-B*57:01 is strongly associated with abacavir-induced hypersensitivity syndrome. Several HLA alleles also demonstrate drug-specific associations with DILI, such as HLA-A*33:03 for ticlopidine, HLA-B*57:01 for flucloxacillin and HLA-DQA1*02:01 for lapatinib. Efforts should be continued to find other genetic markers to achieve high predictability for ADRs, with the goal being development of genetic tests for use in clinical settings.

HLA haplotype determines hapten or p-i T cell reactivity to flucloxacillin

Drug-induced liver injury (DILI) is a main cause of drug withdrawal. A particularly interesting example is flucloxacillin (FLUX)-DILI, which is associated with the HLA-B*57:01 allele. At present, the mechanism of FLUX-DILI is not understood, but the HLA association suggests a role for activated T cells in the pathomechanism of liver damage. To understand the interaction among FLUX, HLA molecules, and T cells, we generated FLUX-reacting T cells from FLUX-naive HLA-B*57:01(+) and HLA-B*57:01(-) healthy donors and investigated the mechanism of T cell stimulation. We found that FLUX stimulates CD8(+) T cells in two distinct manners. On one hand, FLUX was stably presented on various HLA molecules, resistant to extensive washing and dependent on proteasomal processing, suggesting a hapten mechanism. On the other hand, in HLA-B*57:01(+) individuals, we observed a pharmacological interaction with immune receptors (p-i)-based T cell reactivity. FLUX was presented in a labile manner that was further characterized by independence of proteasomal processing and immediate T cell clone activation upon stimulation with FLUX in solution. This p-i-based T cell stimulation was restricted to the HLA-B*57:01 allele. We conclude that the presence of HLA-B*57:01 drives CD8(+) T cell responses to the penicillin-derivative FLUX toward nonhapten mechanism.

Idiosyncratic adverse drug reactions: current concepts

Idiosyncratic drug reactions are a significant cause of morbidity and mortality for patients; they also markedly increase the uncertainty of drug development. The major targets are skin, liver, and bone marrow. Clinical characteristics suggest that IDRs are immune mediated, and there is substantive evidence that most, but not all, IDRs are caused by chemically reactive species. However, rigorous mechanistic studies are very difficult to perform, especially in the absence of valid animal models. Models to explain how drugs or reactive metabolites interact with the MHC/T-cell receptor complex include the hapten and P-I models, and most recently it was found that abacavir can interact reversibly with MHC to alter the endogenous peptides that are presented to T cells. The discovery of HLA molecules as important risk factors for some IDRs has also significantly contributed to our understanding of these adverse reactions, but it is not yet clear what fraction of IDRs have a strong HLA dependence. In addition, with the exception of abacavir, most patients who have the HLA that confers a higher IDR risk with a specific drug will not have an IDR when treated with that drug. Interindividual differences in T-cell receptors and other factors also presumably play a role in determining which patients will have an IDR. The immune response represents a delicate balance, and immune tolerance may be the dominant response to a drug that can cause IDRs.

Human leukocyte antigen (HLA)-B*57:01-restricted activation of drug-specific T cells provides the immunological basis for flucloxacillin-induced liver injury

The role of the adaptive immune system in adverse drug reactions that target the liver has not been defined. For flucloxacillin, a delay in the reaction onset and identification of human leukocyte antigen (HLA)-B*57:01 as a susceptibility factor are indicative of an immune pathogenesis. Thus, we characterize flucloxacillin-responsive CD4+ and CD8+ T cells from patients with liver injury and show that naive CD45RA+CD8+ T cells from volunteers expressing HLA-B*57:01 are activated with flucloxacillin when dendritic cells present the drug antigen. T-cell clones expressing CCR4 and CCR9 migrated toward CCL17 and CCL 25, and secreted interferon-gamma (IFN-γ), T helper (Th)2 cytokines, perforin, granzyme B, and FasL following drug stimulation. Flucloxacillin bound covalently to selective lysine residues on albumin in a time-dependent manner and the level of binding correlated directly with the stimulation of clones. Activation of CD8+ clones with flucloxacillin was processing-dependent and restricted by HLA-B*57:01 and the closely related HLA-B*58:01. Clones displayed additional reactivity against β-lactam antibiotics including oxacillin, cloxacillin, and dicloxacillin, but not abacavir or nitroso sulfamethoxazole.

CONCLUSION

This work defines the immune basis for flucloxacillin-induced liver injury and links the genetic association to the iatrogenic disease.

Drug allergy: causes and desensitization

Allergic drug reactions occur when a drug, usually a low molecular weight molecule, has the ability to stimulate an immune response. This can be done in one of two ways. The first is by binding covalently to a self-protein, to produce a haptenated molecule that can be processed and presented to the adaptive immune system to induce an immune response. Sometimes the drug itself cannot do this but a reactive breakdown product of the drug is able to bind covalently to the requisite self-protein or peptide. The second way in which drugs can stimulate an immune response is by binding non-covalently to antigen presenting or antigen recognition molecules such as the major histocompatibility complex (MHC) or the T cell receptor. This is known as the p-I or pharmacological interaction hypothesis. The drug binding in this situation is reversible and stimulation of the response may occur on first exposure, not requiring previous sensitization. There is probably a dependence on the presence of certain MHC alleles and T cell receptor structures for this type of reaction to occur.

Characterization of the antigen specificity of T-cell clones from piperacillin-hypersensitive patients with cystic fibrosis

β-Lactam antibiotics provide the cornerstone of treatment and reduce the rate of decline in lung function in patients with cystic fibrosis, but their use is limited by a high frequency of delayed-type allergic reactions. The objective of this study was to use cloned T-cells expressing a single T-cell receptor from five piperacillin-hypersensitive patients to characterize both the cellular pathophysiology of the reaction and antigen specificity to define the mechanism of activation of T-cells by piperacillin. More than 400 piperacillin-responsive CD4+, CD4+CD8+, or CD8+ T-cell clones were generated from lymphocyte transformation test and ELIspot-positive patients. The T-cell response (proliferation, T helper 2 cytokine secretion, and cytotoxicity) to piperacillin was concentration-dependent and highly specific. Enzyme-linked immunosorbent assay, gel electrophoresis, and mass spectrometry revealed that piperacillin bound exclusively to albumin in T-cell culture. Irreversible piperacillin binding at Lys 190, 195, 199, 432, and 541 on albumin and the stimulation of T-cells depended on incubation time. A synthetic piperacillin albumin conjugate stimulated T-cell receptors via a major histocompatibility complex- and processing-dependent pathway. Flucloxacillin competes for the same Lys residues on albumin as piperacillin, but the resulting conjugate does not stimulate T-cells, indicating that binding of the β-lactam hapten in peptide conjugates confers structural specificity on the activation of the T-cell receptors expressed on drug-specific clones. Collectively, these data describe the cellular processes that underlie the structural specificity of piperacillin antigen binding in hypersensitive patients with cystic fibrosis.

Direct evidence for the formation of diastereoisomeric benzylpenicilloyl haptens from benzylpenicillin and benzylpenicillenic acid in patients

Covalent binding to proteins to form neoantigens is thought to be central to the pathogenesis of penicillin hypersensitivity reactions. We have undertaken detailed mass spectrometric studies to define the mechanism and protein chemistry of hapten formation from benzylpenicillin (BP) and its rearrangement product, benzylpenicillenic acid (PA). Mass spectrometric analysis of human serum albumin exposed to BP and PA in vitro revealed that at low concentrations (drug protein molar ratio 0.001:1) and during short time incubations BP and PA selectively target different residues, Lys199 and Lys525, respectively. Molecular modeling showed that the selectivity was a function of noncovalent interaction before covalent modification. With increased exposure to higher concentrations of BP and PA, multiple epitopes were detected on albumin, demonstrating that the multiplicity of hapten formation is a function of time and concentration. More importantly, we have demonstrated direct evidence that PA is a hapten accounting for the diastereoisomeric BP antigen formation in albumin isolated from the blood of patients receiving penicillin. Furthermore, PA was found to be more potent than BP with respect to stimulation of T cells from patients with penicillin hypersensitivity, illustrating the functional relevance of diastereoisomeric hapten formation.

Mass spectrometric characterization of circulating and functional antigens derived from piperacillin in patients with cystic fibrosis

A mechanistic understanding of the relationship between the chemistry of drug Ag formation and immune function is lacking. Thus, mass spectrometric methods were employed to detect and fully characterize circulating Ags derived from piperacillin in patients undergoing therapy and the nature of the drug-derived epitopes on protein that can function as an Ag to stimulate T cells. Albumin modification with piperacillin in vitro resulted in the formation of two distinct haptens, one formed directly from piperacillin and a second in which the dioxopiperazine ring had undergone hydrolysis. Modification was time and concentration dependent, with selective modification of Lys(541) observed at low concentrations, whereas at higher concentrations, up to 13 out of 59 lysine residues were modified, four of which (Lys(190), Lys(195), Lys(432), and Lys(541)) were detected in patients' plasma. Piperacillin-specific T lymphocyte responses (proliferation, cytokines, and granzyme B release) were detected ex vivo with cells from hypersensitive patients, and analysis of incubation medium showed that modification of the same lysine residues in albumin occurred in situ. The antigenicity of piperacillin-modified albumin was confirmed by stimulation of T cells with characterized synthetic conjugates. Analysis of minimally modified T cell-stimulatory albumin conjugates revealed peptide sequences incorporating Lys(190), Lys(432), and Lys(541) as principal functional epitopes for T cells. This study has characterized the multiple haptenic structures on albumin in patients and showed that they constitute functional antigenic determinants for T cells.

T-cell recognition of chemicals, protein allergens and drugs: towards the development of in vitro assays

Chemicals can elicit T-cell-mediated diseases such as allergic contact dermatitis and adverse drug reactions. Therefore, testing of chemicals, drugs and protein allergens for hazard identification and risk assessment is essential in regulatory toxicology. The seventh amendment of the EU Cosmetics Directive now prohibits the testing of cosmetic ingredients in mice, guinea pigs and other animal species to assess their sensitizing potential. In addition, the EU Chemicals Directive REACh requires the retesting of more than 30,000 chemicals for different toxicological endpoints, including sensitization, requiring vast numbers of animals. Therefore, alternative methods are urgently needed to eventually replace animal testing. Here, we summarize the outcome of an expert meeting in Rome on 7 November 2009 on the development of T-cell-based in vitro assays as tools in immunotoxicology to identify hazardous chemicals and drugs. In addition, we provide an overview of the development of the field over the last two decades.

Drug antigenicity, immunogenicity, and costimulatory signaling: evidence for formation of a functional antigen through immune cell metabolism

Recognition of drugs by immune cells is usually explained by the hapten model, which states that endogenous metabolites bind irreversibly to protein to stimulate immune cells. Synthetic metabolites interact directly with protein-generating antigenic determinants for T cells; however, experimental evidence relating intracellular metabolism in immune cells and the generation of physiologically relevant Ags to functional immune responses is lacking. The aim of this study was to develop an integrated approach using animal and human experimental systems to characterize sulfamethoxazole (SMX) metabolism-derived antigenic protein adduct formation in immune cells and define the relationship among adduct formation, cell death, costimulatory signaling, and stimulation of a T cell response. Formation of SMX-derived adducts in APCs was dose and time dependent, detectable at nontoxic concentrations, and dependent on drug-metabolizing enzyme activity. Adduct formation above a threshold induced necrotic cell death, dendritic cell costimulatory molecule expression, and cytokine secretion. APCs cultured with SMX for 16 h, the time needed for drug metabolism, stimulated T cells from sensitized mice and lymphocytes and T cell clones from allergic patients. Enzyme inhibition decreased SMX-derived protein adduct formation and the T cell response. Dendritic cells cultured with SMX and adoptively transferred to recipient mice initiated an immune response; however, T cells were stimulated with adducts derived from SMX metabolism in APCs, not the parent drug. This study shows that APCs metabolize SMX; subsequent protein binding generates a functional T cell Ag. Adduct formation above a threshold stimulates cell death, which provides a maturation signal for dendritic cells.

Drug bioactivation and protein adduct formation in the pathogenesis of drug-induced toxicity

Adverse drug reactions (ADRs) remain a major complication of drug therapy and can be classified as 'on-target' or 'off-target' (idiosyncratic) reactions. On-target reactions can be predicted from the known primary or secondary pharmacology of the drug and often represent an exaggeration of the pharmacological effect of the drug. In contrast, off-target adverse reactions cannot be predicted from knowledge of the basic pharmacology of the drug. The exact mechanisms of idiosyncratic drug reactions are still unclear; however it is believed that they can be initiated by chemically reactive drug metabolites. It is well known that xenobiotics can undergo metabolic bioactivation reactions which have the potential to cause cellular stress and damage. Bioactivation of drugs is thought to have the potential of initiating covalent linkages between cellular protein and drugs which can be recognised by the adaptive immune system in the absence of detectable cellular stress. This process cannot yet be predicted in pre-clinical models or discovered in clinical trials. Because of this hazard perception, the formation of chemically reactive metabolites in early drug discovery remains a serious impediment to the development of new medicines and can lead to withdrawal of an otherwise effective therapeutic agent. The fear of such reactions occurring at the post-licensing stage - when such problems first become evident - is a major contribution to drug attrition. The first step towards such methodology has been the development of chemically reactive metabolite screens. The chemical basis of drug bioactivation can usually be rationalised and synthetic strategies put in place to prevent such bioactivation. However, there is no simple correlation between drug bioactivation in vitro and adverse drug reactions in the clinic. Such a chemical approach is clearly limited by the facts that (a) not all drugs that can undergo bioactivation by human drug-metabolising enzymes are associated with hypersensitivity in the clinic and (b) drug bioactivation may not always be a mandatory step in drug hypersensitivity. To predict such reactions in early drug development, it will require an integrated understanding of the chemical, immunological and genetic basis of adverse drug reactions in patients, which in turn will depend on the development of novel in vitro experimental systems.

A role for the pregnane X receptor in flucloxacillin-induced liver injury

Drug-induced liver injury (DILI) due to flucloxacillin is a rare but serious complication of treatment. There is some evidence that flucloxacillin is a human pregnane X receptor (PXR) agonist. This study was designed to investigate the relevance of PXR to flucloxacillin toxicity and to identify genes changing in expression in response to flucloxacillin. Changes in gene expression in human hepatocytes after treatment with 500 microM flucloxacillin for 72 hours were examined by expression microarray analysis. The ability of flucloxacillin to act as a PXR agonist was investigated with reporter gene experiments. Flucloxacillin DILI cases (n = 51), drug-exposed controls without toxicity (n = 64), and community controls (n = 90) were genotyped for three common PXR polymorphisms. Luciferase reporter assays were used to assess the significance of a promoter region PXR polymorphism. Seventy-two probe sets representing 50 different genes showed significant changes in expression of 1.2-fold or higher. Most genes showing changes greater than 3-fold were known to be rifampicin-responsive, and this suggested a PXR-dependent mode of regulation. Using a luciferase-everted repeat separated by 6 base pairs element construct, we confirmed that flucloxacillin was a PXR agonist. We found a difference in the distribution of a PXR polymorphism (rs3814055; C-25385T) between flucloxacillin DILI cases and controls with the CC genotype associated with an increased risk of disease (odds ratio = 3.37, 95% confidence interval = 1.55-7.30, P = 0.0023). Reporter gene experiments showed lower promoter activity for the C allele than the T allele.

CONCLUSION

Flucloxacillin is a PXR agonist at pharmacologically relevant concentrations, and a functionally significant upstream PXR polymorphism is a risk factor for flucloxacillin-induced DILI.