Evidence of an immune-mediated mechanism for an idiosyncratic nevirapine-induced reaction in the female Brown Norway rat

The use of PD-1 functional knockout rats to study idiosyncratic adverse reactions to nevirapine

Idiosyncratic drug reactions (IDRs) are associated with significant patient morbidity/mortality and lead to considerable drug candidate attrition in drug development. Their idiosyncratic nature makes the study of IDRs difficult. In particular, nevirapine is associated with a relatively high risk of serious skin rash and liver injury. We previously found that nevirapine causes a similar skin rash in female Brown Norway rats, but these animals do not develop significant liver injury. Programmed cell death protein-1 (PD-1) is an immune checkpoint involved in immune tolerance, and anti-PD-1 antibodies have been used to treat cancer. However, they increase the risk of liver injury caused by co-administered drugs. We found that PD-1-/- mice are more susceptible to drug-induced liver injury, but PD-1-/- mice are not a good model for all drugs. In particular, they do not develop a skin rash when treated with nevirapine, at least in part because they lack the sulfotransferase in their skin that forms the reactive metabolite responsible for the rash. Therefore, we developed a PD-1 mutant (PD-1m/m) rat, with an excision in the ligand-binding domain of PD-1, to test whether nevirapine would cause a more serious skin rash in these animals. The PD-1m/m rat was based on a Sprague Dawley background, which has a lower incidence of skin rash than Brown Norway rats. The treated PD-1m/m rats developed more severe liver injury than PD-1-/- mice, but in contrast to expectations, they did not develop a skin rash. Functional knockouts provide a unique tool to study the mechanisms of IDRs.

Understanding Idiosyncratic Toxicity: Lessons Learned from Drug-Induced Liver Injury

Idiosyncratic adverse drug reactions (IADRs) encompass a diverse group of toxicities that can vary by drug and patient. The complex and unpredictable nature of IADRs combined with the fact that they are rare makes them particularly difficult to predict, diagnose, and treat. Common clinical characteristics, the identification of human leukocyte antigen risk alleles, and drug-induced proliferation of lymphocytes isolated from patients support a role for the adaptive immune system in the pathogenesis of IADRs. Significant evidence also suggests a requirement for direct, drug-induced stress, neoantigen formation, and stimulation of an innate response, which can be influenced by properties intrinsic to both the drug and the patient. This Perspective will provide an overview of the clinical profile, mechanisms, and risk factors underlying IADRs as well as new approaches to study these reactions, focusing on idiosyncratic drug-induced liver injury.

Singularities of nevirapine metabolism: from sex-dependent differences to idiosyncratic toxicity

Nevirapine (NVP) is a first-generation non-nucleoside reverse transcriptase inhibitor widely used for the treatment and prophylaxis of human immunodeficiency virus infection. The drug is taken throughout the patient's life and, due to the availability of an extended-release formulation, it is administered once daily. This antiretroviral is one of the scarce examples of drugs with prescription criteria based on sex, in order to prevent adverse reactions. The therapy with NVP has been associated with potentially life-threatening liver and idiosyncratic skin toxicity. Multiple evidence has emerged regarding the formation of electrophilic NVP metabolites as crucial for adverse idiosyncratic reactions. The formation of reactive metabolites that yield covalent adducts with proteins has been demonstrated in patients under NVP-based treatment. Interestingly, several pharmacogenetic- and sex-related factors associated with NVP toxicity can be mechanistically explained by an imbalance toward increased formation of NVP-derived reactive metabolites and/or impaired detoxification capability. Moreover, the haptenation of self-proteins by these reactive species provides a plausible link between NVP bioactivation and immunotoxicity, further supporting the relevance of this toxicokinetics hypothesis. In the current paper, we review the existing knowledge and recent developments on NVP metabolism and their relation to NVP toxicity.

The skin as a metabolic and immune-competent organ: Implications for drug-induced skin rash

Current advances in the study of cutaneous adverse drug reactions can be attributed to the recent understanding that the skin is both a metabolically and immunologically competent organ. The ability of the skin to serve as a protective barrier with limited drug biotransformation ability, yet highly active immune function, has provided insights into its biological capability. While the immune response of the skin to drugs is vastly different from that of the liver due to evolutionary conditioning, it frequently occurs in response to various drug classes and manifests as a spectrum of hypersensitivity reactions. The skin is a common site of adverse and idiosyncratic drug reactions; drug-specific T-cells, as well as involvement of an innate immune response, appear to be key mechanistic drivers in such scenarios. Association of other factors such as human leukocyte antigen (HLA) polymorphisms may play a significant role for particular drugs. This review aims to integrate emerging findings into proposed mechanisms of drug metabolism and immunity in the skin that are likely responsible for rashes and other local allergic responses. These unique biological aspects of the skin, and their translation into implications for drug development and the use of animal models, will be discussed.

[Recent findings regarding the mechanism of idiosyncratic drug toxicity]

Animal experiments cannot predict the probability of idiosyncratic drug toxicity; consequently, an important goal of the pharmaceutical industry is to develop a new methodology for preventing this form of drug reaction. Although the mechanism remains unclear, immune reactions are likely involved in the toxic processes underlying idiosyncratic drug toxicity: the drug is first activated into a chemically reactive metabolite that binds covalently to proteins and then acts as an immunogen. Therefore, screening tests to detect chemically reactive metabolites are conducted early during drug development and typically involve trapping with glutathione. More quantitative methods are then used in a later stage of drug development and frequently employ (14)Cor (3)H-labeled compounds. It has recently been demonstrated that a zone classification system can be used to separate risky drugs from likely safe drugs: by plotting the amount of each protein-bound reactive metabolite in vitro against the dose levels in vivo, the risk associated with each drug candidate can be assessed. A mechanism for idiosyncratic drug-induced hepatotoxicity was proposed by analogy to virus-induced hepatitis, in which cytotoxic T lymphocytes play an important role. This mechanism suggests that polymorphism in human leukocyte antigens is involved in idiosyncrasy, and a strong correlation with a specific genotype of human leukocyte antigens has been found in many cases of idiosyncratic drug toxicity. Therefore, gene biomarkers hold promise for reducing the clinical risk and prolonging the life cycle of otherwise useful drugs.

HLA Class I restricted CD8+ and Class II restricted CD4+ T cells are implicated in the pathogenesis of nevirapine hypersensitivity

OBJECTIVES

This study sought to examine nevirapine hypersensitivity (NVP HSR) phenotypes and their relationship with differing major histocompatibility complex (MHC) Class I and Class II alleles and the associated CD4 and CD8 T-cell NVP-specific responses and their durability over time.

METHODS

A retrospective cohort study compared HIV-positive patients with NVP HSR, defined by fever and hepatitis and/or rash, with those tolerant of NVP for more than 3 months. Covariates included class I (HLA-A, B, C) and class II (HLA-DR) alleles. Cellular studies examined NVP-specific CD4 and CD8 T-cell responses by interferon-gamma (IFNγ) ELISpot assay and intracellular cytokine staining (ICS).

RESULTS

NVP HSR occurred in 19 out of 451 (4%) NVP-exposed individuals between March 1993 and December 2011. HLA associations were phenotype dependent with HLA-DRB1*01 : 01 associated with hepatitis (P = 0.02); HLA-B*35 : 01 and HLA-Cw4 associated with cutaneous NVP HSR (P = 0.001, P = 0.01), and HLA-Cw*08 was associated with NVP HSR with eosinophilia (P = 0.04) and multisystemic NVP HSR (P = 0.02). NVP-specific INFγ responses waned significantly more than 3 months from the original reaction and were diminished or completely abrogated when either CD4 or CD8 T cells were depleted from the peripheral blood mononuclear cells culture.

CONCLUSION

The association of specific class I and II allele pairings with specific phenotypes of NVP HSR, and cellular studies showing both CD4 and CD8 T-cell NVP-specific responses suggest that specific combinations of NVP reactive class I restricted CD8 and class II restricted CD4 T cells contribute to the immunopathogenesis of NVP HSR.

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.

Bioactivation of nevirapine to a reactive quinone methide: implications for liver injury

Nevirapine (NVP) treatment is associated with a significant incidence of liver injury. We developed an anti-NVP antiserum to determine the presence and pattern of covalent binding of NVP to mouse, rat, and human hepatic tissues. Covalent binding to hepatic microsomes from male C57BL/6 mice and male Brown Norway rats was detected on Western blots; the major protein had a mass of ~55 kDa. Incubation of NVP with rat CYP3A1 and 2C11 or human CYP3A4 also led to covalent binding. Treatment of female Brown Norway rats or C57BL/6 mice with NVP led to extensive covalent binding to a wide range of proteins. Co-treatment with 1-aminobenzotriazole dramatically changed the pattern of binding. The covalent binding of 12-hydroxy-NVP, the pathway that leads to a skin rash, was much less than that of NVP, both in vitro and in vivo. An analogue of NVP in which the methyl hydrogens were replaced by deuterium also produced less covalent binding than NVP. These data provide strong evidence that covalent binding of NVP in the liver is due to a quinone methide formed by oxidation of the methyl group. Attempts were made to develop an animal model of NVP-induced liver injury in mice. There was a small increase in ALT in some NVP-treated male C57BL/6 mice at 3 weeks that resolved despite continued treatment. Male Cbl-b(-/-) mice dosed with NVP had an increase in ALT of >200 U/L, which also resolved despite continued treatment. Liver histology in these animals showed focal areas of complete necrosis, while most of the liver appeared normal. This is a different pattern from the histology of NVP-induced liver injury in humans. This is the first study to report hepatic covalent binding of NVP and also liver injury in mice. It is likely that the quinone methide metabolite is responsible for NVP-induced liver injury.

The role of the immune system in nevirapine-induced subclinical liver injury of a rat model

In this study, the role of the immune system in nevirapine- (NVP-) induced subclinical liver injury was investigated by observing for changes of some immune parameters during the initial stages of NVP-induced hepatotoxicity in a rat model. In the acute phase, two test-groups of 10 Sprague-Dawley rats each were administered with bacterial lipopolysaccharide (LPS) or saline (S) intraperitoneally, followed by oral NVP, after which 5 rats from each group were sacrificed at 6 and 24 hours. For the chronic phase, two groups of 15 rats each received daily NVP, and on days 7, 14, and 21, five rats from each group were administered with either LPS or S, followed by that day's NVP dose, and were sacrificed 24 hours later. NVP caused liver injury up to seven days and progressively increased IL-2 and IFN-γ levels and lymphocyte count over the 21 days. NVP-induced liver injury was characterized by apoptosis and degeneration changes, while, for LPS, it was cell swelling, leukostasis, and portal inflammation. Coadministration of NVP and LPS attenuated NVP-induced liver injury. In conclusion, the immune system is involved in NVP toxicity, and the LPS effects may lay the clue to development of therapeutic strategies against NVP-induced hepatotoxicity.

Protein adducts as prospective biomarkers of nevirapine toxicity

Nevirapine (NVP) is a non-nucleoside reverse transcriptase inhibitor used against human immunodeficiency virus type-1 (HIV-1), mostly to prevent mother-to-child HIV-1 transmission in developing countries. Despite its clinical efficacy, NVP administration is associated with a variety of toxic responses that include hepatotoxicity and skin rash. Although the reasons for the adverse effects of NVP administration are still unclear, increasing evidence supports the involvement of metabolic activation to reactive electrophiles. In particular, Phase II activation of the NVP metabolite 12-hydroxy-NVP is thought to mediate NVP binding to bionucleophiles, which may be at the onset of toxicity. In the present study, we investigated the nature and specific locations of the covalent adducts produced in human serum albumin and human hemoglobin by reaction in vitro with the synthetic model electrophile 12-mesyloxy-NVP, used as a surrogate for the Phase II metabolite 12-sulfoxy-NVP. Multiple sites of modification were identified by two different mass spectrometry-based methodologies, liquid chromatography-electrospray ionization tandem mass spectrometry (LC-ESI-MS/MS) and matrix-assisted laser desorption ionization tandem mass spectrometry (MALDI-TOF-TOF-MS). These two distinct methodologies, which in some instances afforded complementary information, allowed the identification of multiple adducts involving cysteine, lysine, tryptophan, histidine, serine, and the N-terminal valine of hemoglobin. Tryptophan, which is not a common site of covalent protein modification, was the NVP-modified amino acid residue detected in the two proteins and consistently identified by both LC-ESI-MS/MS and MALDI-TOF-TOF-MS. The propensity of tryptophan to react with the NVP-derived electrophile is further emphasized by the fact that human serum albumin possesses a single tryptophan residue, which suggests a remarkable selectivity that may be useful for biomonitoring purposes. Likewise, the NVP adduct with the terminal valine of hemoglobin, detected by LC-ESI-MS/MS after N-alkyl Edman degradation, appears as an easily assessed marker of NVP binding to proteins. Our results demonstrate the merits and complementarity of the two MS-based methodologies for the characterization of protein binding by NVP and suggest a series of plausible biomarkers of NVP toxicity that should be useful in the monitoring of toxicity effects in patients administered NVP.

Amino acid adduct formation by the nevirapine metabolite, 12-hydroxynevirapine--a possible factor in nevirapine toxicity

Nevirapine (NVP) is a non-nucleoside reverse transcriptase inhibitor used against the human immunodeficiency virus type-1 (HIV-1), mostly to prevent mother-to-child transmission of the virus in developing countries. However, reports of severe NVP-induced hepatotoxicity and serious adverse cutaneous effects have raised concerns about its use. NVP metabolism involves oxidation of the 4-methyl substituent to 4-hydroxymethyl-NVP (12-hydroxy-NVP) and the formation of phenolic derivatives. Further metabolism, through either oxidation to quinoid derivatives or phase II esterification, may produce electrophilic derivatives capable of reacting with bionucleophiles to yield covalent adducts. These adducts could potentially be involved in the initiation of toxic responses. To gain insight into potentially reactive sites in proteins and prepare reliable and fully characterized NVP-amino acid adduct standards for subsequent assessment as biomarkers of NVP toxicity, we have used the model electrophile, 12-mesyloxy-NVP, as a synthetic surrogate for the NVP metabolite, 12-sulfoxy-NVP. Reactions of this model ester were conducted with glutathione and the nucleophilic amino acids arginine, cysteine, histidine, and tryptophan. Moreover, because adducts through the N-terminal valine of hemoglobin are convenient biomarkers of exposure to electrophilic toxicants, we also investigated the reaction with valine. We obtained very efficient (>80%) binding through the sulfur of both glutathione and N-acetylcysteine and moderate yields (10-14%) for binding through C2 of the indole ring of tryptophan and N1 of the imidazole ring of histidine. Reaction with arginine occurred through the alpha-amino group, possibly due to the high basicity of the guanidino group in the side chain. Reaction at the alpha-amino group of valine occurred to a significant extent (33%); the resulting adduct was converted to a thiohydantoin derivative, to obtain a standard useful for prospective biomonitoring studies. All adducts were characterized by a combination of (1)H and (13)C NMR spectroscopy and mass spectrometry techniques. The NVP conjugates with glutathione and N-acetylcysteine identified in this work were previously reported to be formed in vivo, although the corresponding structures were not fully characterized. Our results support the validity of 12-mesyloxy-NVP as a surrogate for 12-sulfoxy-NVP and suggest that NVP metabolism to 12-hydroxy-NVP, and subsequent esterification, could potentially be a factor in NVP toxicity. They further imply that multiple sites in proteins may be targets for modification by 12-hydroxy-NVP-derived electrophiles in vivo. Additionally, we obtained reliable, fully characterized standards for the assessment of protein modification by NVP in vivo, which should help clarify the potential role of metabolism in NVP-induced toxicity.

The role of cytokines in the mechanism of adverse drug reactions

Cytokines are thought to play a role in acute and/or immune-mediated adverse drug reactions (ADRs) due to their ability to regulate the innate and adaptive immune systems. This role is highly complex owing to the pluripotent nature of cytokines, which enables the same cytokine to play multiple roles depending on target organ(s) involved. As a result, the discussion of cytokine involvement in ADRs is organized according to target organ(s); specifically, ADRs targeting skin and liver, as well as ADRs targeting multiple organs, such as drug-induced autoimmunity and infusion-related reactions. In addition to discussing the mechanism(s) by which cytokines contribute to the initiation, propagation, and resolution of ADRs, we also discuss the usefulness and limitations of current methodologies available to conduct such mechanistic studies. While animal models appear to hold the most promise for uncovering additional mechanisms, this field is plagued by a lack of good animal models and, as a result, the mechanism of cytokine involvement in ADRs is often studied using less informative in vitro studies. The recent formation of the Drug-Induced Liver Injury Network, whose goal is collect thousands of samples from drug-induced liver injury patients, has enormous potential to advance knowledge in this field, by enabling large-scale cytokine polymorphism studies. In conclusion, we discuss how further advances in this field could be of significant benefit to patients in terms of preventing, predicting, and treating ADRs.

Nevirapine hypersensitivity

Treatment of HIV-1 infections with nevirapine is associated with skin and liver toxicity. These two organ toxicities range from mild to severe, in rare cases resulting in life-threatening liver failure or toxic epidermal necrolysis. The study of the mechanistic steps leading to nevirapine-induced skin rash has been facilitated by the discovery of an animal model in which nevirapine causes a skin rash in rats that closely mimics the rash reported in patients. The similarity in characteristics of the rash between humans and rats strongly suggests that the basic mechanism is the same in both. The rash is clearly immune-mediated in rats, and partial depletion of CD4(+) T cells, but not CD8(+) T cells, is protective. We have demonstrated that the rash is related to the 12-hydroxylation of nevirapine rather than to the parent drug. This is presumably because the 12-hydroxy metabolite can be converted to a reactive quinone methide in skin, but that remains to be demonstrated. Although the rash is clearly related to the 12-hydroxy metabolite rather than the parent drug, cells from rechallenged animals respond ex vivo to the parent drug by producing cytokines such as interferon-gamma with little response to the 12-hydroxy metabolite, even when the rash was induced by treatment with the metabolite rather than the parent drug. This indicates that the response of T cells in vitro cannot be used to determine what caused an immune response. We are now studying the detailed steps by which the 12-hydroxy metabolite induces an immune response and skin rash. This animal model provides a unique tool to study the mechanistic details of an idiosyncratic drug reaction; however, it is likely that there are significant differences in the mechanisms of different idiosyncratic drug reactions, and therefore the results of these studies cannot safely be generalized to all idiosyncratic drug reactions.

The danger hypothesis applied to idiosyncratic drug reactions

The danger hypothesis has had a profound effect on the way immunologists view the immune response. This hypothesis proposes that the major determinant of whether an immune response is mounted against some agent is determined by whether that agent causes some type of cell damage. Assuming that most idiosyncratic drug reactions (IDRs) are immune-mediated, this hypothesis also has the potential to explain many aspects of the mechanism of these adverse drug reactions. For example, most IDRs appear to be caused by chemical metabolites rather than the parent drug, but not all drugs that form reactive metabolites are associated with a significant incidence of IDRs. Therefore, using the danger hypothesis, one feature of a drug candidate that may predict whether it causes an IDR is whether the drug, or more likely its reactive metabolites, cause cell damage. Although the range of molecules that can act as danger signals is unknown, the most attractive candidates are high mobility group box 1 protein (HMGB1), heat shock proteins, and S100 proteins. These molecules act through the same receptors (toll-like receptors) as pathogen-associated molecules that stimulate the immune system. Therefore, other environmental factors such as infections or trauma might determine which patients would be at increased risk for IDRs. Although there are examples where this appears to be the case, in most cases there are no obvious environmental factors that determine IDR risk. In addition, in animal models of immune-mediated reactions, stimulation of toll-like receptors often does not increase the immune response, and depending on the timing, it can actually be protective. Therefore, there may be additional unknown control mechanisms that are involved. A better understanding of these fundamental immune mechanisms has the potential to have a significant impact on many areas of medicine.

5-lipoxygenase-activating protein inhibitors: development of 3-[3-tert-butylsulfanyl-1-[4-(6-methoxy-pyridin-3-yl)-benzyl]-5-(pyridin-2-ylmethoxy)-1H-indol-2-yl]-2,2-dimethyl-propionic acid (AM103)

The potent and selective 5-lipoxygenase-activating protein leukotriene synthesis inhibitor 3-[3-tert-butylsulfanyl-1-[4-(6-methoxy-pyridin-3-yl)-benzyl]-5-(pyridin-2-ylmethoxy)-1H-indol-2-yl]-2,2-dimethyl-propionic acid (11j) is described. Lead optimization was designed to afford compounds with superior in vitro and in vivo inhibition of leukotriene synthesis in addition to having excellent pharmacokinetics and safety in rats and dogs. The key structural features of these new compounds are incorporation of heterocycles on the indole N-benzyl substituent and replacement of the quinoline group resulting in compounds with excellent in vitro and in vivo activities, superior pharmacokinetics, and improved physical properties. The methoxypyridine derivative 11j has an IC(50) of 4.2 nM in a 5-lipoxygenase-activating protein (FLAP) binding assay, an IC(50) of 349 nM in the human blood LTB(4) inhibition assay, and is efficacious in a murine ovalbumin model of allergen-induced asthma. Compound 11j was selected for clinical development and has successfully completed phase 1 trials in healthy volunteers.

A study of the specificity of lymphocytes in nevirapine-induced skin rash

Nevirapine treatment can cause a skin rash. We developed an animal model of this rash and determined that the 12-hydroxylation metabolic pathway is responsible for the rash, and treatment of animals with 12-OH-nevirapine also leads to a rash. In the present study, we investigated the specificity of lymphocytes in nevirapine-induced skin rash. Brown Norway rats were treated with nevirapine or 12-OH-nevirapine to induce a rash. Lymph nodes were removed, and the response of lymphocytes to nevirapine and its metabolites/analogs was determined by cytokine production (enzyme-linked immunosorbent assay, enzyme-linked immunosorbent spot assay, and Luminex) and proliferation (alamar blue assay). Subsets of lymphocytes were depleted to determine which cells were responsible for cytokine production. Lymphocytes from animals rechallenged with nevirapine proliferated to nevirapine, but not to 12-OH-nevirapine or 4-chloro-nevirapine. They also produced interferon-gamma (IFN-gamma) when exposed to nevirapine, significantly less when exposed to 4-chloro-nevirapine, and very little when exposed to 12-OH-nevirapine, even though oxidation to 12-OH-nevirapine is required to induce the rash. Moreover, the specificity of lymphocytes from 12-OH-nevirapine-treated rats was the same, i.e., responding to nevirapine more than to 12-OH-nevirapine, even though these animals had never been exposed to nevirapine. A Luminex immunoassay showed that a variety of other cytokines/chemokines were also produced by nevirapine-stimulated lymphocytes. CD4(+) cells were the major source of IFN-gamma. The specificity of lymphocytes in activation assays cannot be used to determine what initiated an immune response. This has significant implications for understanding the evolution of an immune response and the basis of the pharmacological interaction hypothesis.

Risk factors for treatment-limiting toxicities in patients starting nevirapine-containing antiretroviral therapy

BACKGROUND

This collaboration of seven observational clinical cohorts investigated risk factors for treatment-limiting toxicities in both antiretroviral-naive and experienced patients starting nevirapine-based combination antiretroviral therapy (NVPc).

METHODS

Patients starting NVPc after 1 January 1998 were included. CD4 cell count at starting NVPc was classified as high (>400/microl/>250/microl for men/women, respectively) or low. Cox models were used to investigate risk factors for discontinuations due to hypersensitivity reactions (HSR, n = 6547) and discontinuation of NVPc due to treatment-limiting toxicities and/or patient/physician choice (TOXPC, n = 10,186). Patients were classified according to prior antiretroviral treatment experience and CD4 cell count/viral load at start NVPc. Models were stratified by cohort and adjusted for age, sex, nadir CD4 cell count, calendar year of starting NVPc and mode of transmission.

RESULTS

Median time from starting NVPc to TOXPC and HSR were 162 days [interquartile range (IQR) 31-737] and 30 days (IQR 17-60), respectively. In adjusted Cox analyses, compared to naive patients with a low CD4 cell count, treatment-experienced patients with high CD4 cell count and viral load more than 400 had a significantly increased risk for HSR [hazard ratio 1.45, confidence interval (CI) 1.03-2.03] and TOXPC within 18 weeks (hazard ratio 1.34, CI 1.08-1.67). In contrast, treatment-experienced patients with high CD4 cell count and viral load less than 400 had no increased risk for HSR 1.10 (0.82-1.46) or TOXPC within 18 weeks (hazard ratio 0.94, CI 0.78-1.13).

CONCLUSION

Our results suggest it may be relatively well tolerated to initiate NVPc in antiretroviral-experienced patients with high CD4 cell counts provided there is no detectable viremia.

Evaluation of Which Reactive Metabolite, If Any, Is Responsible for a Specific Idiosyncratic Reaction

Reactive metabolites are believed to be responsible for most idiosyncratic drug reactions. It is often assumed that if a reactive metabolite is found, it must be responsible for the idiosyncratic reactions associated with that drug. However, the evidence linking reactive metabolites and idiosyncratic reactions is circumstantial at best, and in many cases we have virtually no evidence. Furthermore, it is common for a drug to form several reactive metabolites, so it can be difficult to determine which, if any, is responsible for a given idiosyncratic reaction. Although the reactive metabolite hypothesis is logical, it has important implications for drug development, and we need to develop ways to test the hypothesis for specific drugs rigorously. Valid animal models are a powerful tool for testing whether a specific reactive metabolite is responsible for a specific adverse reaction and for studying further the mechanism by which it may induce such reactions; however, such models are rare.

Demonstration of the metabolic pathway responsible for nevirapine-induced skin rash

The reverse transcriptase inhibitor, nevirapine (NVP), causes skin rashes and hepatotoxicity. We used a rat model to determine if the rash is caused by the parent drug or a reactive metabolite. By manipulation of metabolic pathways and testing analogues, we eliminated all but one pathway, 12-hydroxylation, which involves the oxidation of an exocyclic methyl group, as being responsible for the rash. Treatment with 12-OH-NVP caused a rash, and an analogue in which the methyl hydrogens were replaced by deuterium to inhibit the 12-OH pathway did not cause a rash; however, quite unexpectedly, blood levels of the deuterated analogue were very low. This is due to partitioning of the benzylic free radial intermediate between oxygen rebound to form 12-OH-NVP and loss of another hydrogen atom to form a reactive quinone methide, which inactivates P450. Cotreatment with the P450 inhibitor, 1-aminobenzotriazole, led to comparable levels of NVP and the deuterated analogue, and the deuterated analogue still caused a lower rash incidence. These data clearly point to the 12-hydroxy pathway being responsible for NVP skin rash. We propose that the hepatotoxicity of NVP in humans is due to the quinone methide formed by P450 in the liver, while the skin rash may be due to the quinone methide formed in the skin by sulfation of 12-OH metabolite followed by loss of sulfate. This is the first example in which a valid animal model of an idiosyncratic drug reaction was used to determine the metabolic pathway responsible for the reaction.

Uncomplicated outcome after an accidental overdose of nevirapine in a newborn

We report the first case of a massive accidental overdose of nevirapine in a 1-week newborn, due to confusion between nevirapine (Viramune) and nelfinavir (Viracept). The drug was eliminated spontaneously and quickly. We only observed mild neutropenia and hyperlactatemia, which regressed on its own without any clinical complication. Despite the good evolution of this massive overdose, physicians should be aware of confusion risks between some antiretroviral drugs.

HLA-DRB1*01 associated with cutaneous hypersensitivity induced by nevirapine and efavirenz

HLA typing, demographic and immunological risk factors for nevirapine and efavirenz reactions were studied in a French cohort of HIV patients. Cases with isolated rash were significantly associated with HLA-DRB101 allele. No liver toxicity was observed and no association was detected with the percentage of CD4 T-cells. This study suggests that HLA-DRB101 allele plays an important role in susceptibility to cutaneous reactions associated with nevirapine and efavirenz in HIV patients.

Idiosyncratic drug reactions: past, present, and future

Although the major working hypothesis for the mechanism of idiosyncratic drug reactions (IDRs), the hapten hypothesis, has not changed since 1987, several hypotheses have been added, for example, the danger hypothesis and the pharmaceutical interaction hypothesis. Genetic studies have found that several IDRs are linked to specific HLA genes, providing additional evidence that they are immune-mediated. Evidence that most IDRs are caused by reactive metabolites has led pharmaceutical companies to avoid drug candidates that form significant amounts of reactive metabolites; however, at least one IDR, ximelagatran-induced liver toxicity, does not appear to be caused by a reactive metabolite. It is possible that there are biomarkers such as those related to cell stress that would predict that a drug candidate would cause a significant incidence of IDRs; however, there has been no systematic study of the changes in gene expression induced by drugs known to cause IDRs. A major impediment to the study of the mechanisms of IDRs is the paucity of valid animal models, and if we had a better mechanistic understanding, it should be easier to develop such models. There is growing evidence that these adverse reactions are more varied and complex than previously recognized, and it is unlikely that a quick fix will be achieved. However, IDRs are an important cause of patient morbidity and mortality and markedly increase the uncertainty of drug development; therefore, continued basic research in this area is essential.

Is it safe to switch between efavirenz and nevirapine in the event of toxicity?

The non-nucleoside reverse transcriptase inhibitors (NNRTIs) efavirenz and nevirapine are chemically distinct, but both may cause cutaneous hypersensitivity and hepatotoxicity. We reviewed the literature to assess the evidence for cross-reactivity between nevirapine and efavirenz. All papers, abstracts, or presentations, regardless of study design, that made reference to the response of patients who were switched from one NNRTI to another as a result of an adverse drug reaction were included. Most of the studies were retrospective. Recurrent reactions occurred in 30 (12.6%) of 239 reported patients with rash who were switched from nevirapine to efavirenz, compared with eight (50%) of 16 patients switched from efavirenz to nevirapine. Hepatitis did not recur in either the 11 reported patients switched from nevirapine to efavirenz, or in the single reported patient who was switched from efavirenz to nevirapine. Substituting efavirenz for nevirapine following hepatotoxicity or cutaneous hypersensitivity appears to be reasonable, providing that the adverse reaction to nevirapine was not life-threatening. There is insufficient evidence to recommend substituting nevirapine for efavirenz following either hepatotoxicity or cutaneous hypersensitivity.

Drug hypersensitivity in HIV

PURPOSE OF REVIEW

Drug hypersensitivity has been reported to occur 100 times more commonly in those living with HIV. In the first decade of HIV treatment, this mainly involved drugs used to treat HIV-related infections but now primarily includes drugs used to treat HIV. This review focuses on the current knowledge of the epidemiology, pathophysiology and clinical features of drug hypersensitivity reactions of drugs used in the management of the HIV-infected patient.

RECENT FINDINGS

Our understanding of the immunogenetics and host predisposition to drug hypersensitivity has been advanced considerably by the antiretroviral drugs abacavir and nevirapine. The association of abacavir hypersensitivity reaction with HLA-B*5701 has been particularly important and provides a basis for genetic screening in the clinic setting.

SUMMARY

The increased predisposition of drug hypersensitivity disease in HIV will continue to provide a fertile ground for study of the diverse and complex processes that drive its pathophysiology. Our knowledge of drug hypersensitivity will also increase as the expanding armentarium of antiretroviral therapy is applied to more diverse populations in the developing world. The potential for widespread implementation of HLA-B*5701 screening for abacavir hypersensitivity will set an important precedent for bringing individualized medicine to the clinic and the use of genetic testing to improve drug safety.

Idiosyncratic drug reactions: current understanding

Clinical characteristics and circumstantial evidence suggest that idiosyncratic drug reactions are caused by reactive metabolites and are immune-mediated; however, there are few definitive data and there are likely exceptions. There are three principal hypotheses for how reactive metabolites might induce an immune-mediated idiosyncratic reaction: the hapten hypothesis, the danger hypothesis, and the PI hypothesis. It has been proposed that some idiosyncratic reactions, especially those involving the liver, represent metabolic idiosyncrasy; however, there are even less data to support this hypothesis. The unpredictable nature of these reactions makes mechanistic studies difficult. There is a very strong association with specific human leukocyte antigen (HLA) genes for certain reactions, but this has only been demonstrated for very few drugs. Animal models represent a very powerful tool for mechanistic studies, but the number of valid models is also limited. There may be biomarkers of risk; however, much more work needs to be done.

Immunological principles of T-cell-mediated adverse drug reactions in skin

Drug hypersensitivity reactions in skin are an immune-mediated phenomenon associated with significant patient mortality and morbidity. Antigen-specific T cells, which have been isolated from the peripheral circulation and target organs of hypersensitive patients, are thought to propagate and regulate the development of clinical symptoms. The investigation of clinical cases with respect to the basic cellular and chemical mechanisms that underpin drug hypersensitivity has resulted in: i) the need to redress some aspects of present immunological dogma; and ii) additional fundamental immunological questions. Thus, the aim of this review article is to summarise present opinion on how and why drugs initiate a pathogenic T-cell response in a small section of the population and subsequently reflect on gaps in basic immunology and where future research might lead.

Investigations toward enhanced understanding of hepatic idiosyncratic drug reactions

Idiosyncratic drug reactions (IDRs) of a hepatic origin are a major health concern and a notoriously difficult challenge for the pharmaceutical industry. These types of adverse events are rare, with a typical occurrence of 1 in 100 to 1 in 100,000 patients. Typical adverse outcomes are most likely statistically impossible to predict in traditional preclinical safety studies or clinical trials. Unfortunately, these reactions can pose a significant risk to the public health, resulting in devastating consequences such as irreversible liver injury, liver transplantation and fatality. This review provides many examples of experimental efforts that are underway for a better understanding of molecular events that may be responsible for IDRs. A list of existing hypotheses for IDRs is also provided, each with current literature examples or supporting evidence. The possibilities for developing suitable animal models for the prediction and characterisation of IDRs are elaborated, especially for a drug-inflammation interaction rat model of hepatic IDR. The need for predictive biomarkers of IDR is addressed, with the exploration of some possible candidates. Finally, the use of primary human hepatocyte culture systems is explored as an in vitro system, with application for providing an increased mechanistic knowledge of IDR. Several examples of informative studies on the nature of IDRs that employ toxicogenomic and proteomic technologies are summarised.

Detection of autoimmunity by pharmaceuticals

Despite the important health and economic impact of autoimmunogenicity or allergenicity by pharmaceuticals models to detect such adverse effects are not available yet. The most important reason for this is the related complex interplay of multiple factors, for which reason these adverse effects are also referred to as idiosyncratic in nature. Moreover, clinical effects are quite diverse, and involve both organ-specific and systemic effects, including a diversity of skin diseases. Because of its complexity on the one hand and the fundamental knowledge on certain particular mechanistic effects it may be more relevant to design a rationalistic toolbox of test models from which a predictive strategy can be composed. Since one mechanistic aspect centers around T cell sensitization a straightforward lymph node assay such as the reporter antigen-popliteal lymph node assay (RA-PLNA) would fit in such a toolbox. This RA-PLNA holds a strong promise to distinguish sensitizing and/or neoantigen-forming capacity of low molecular weight pharmaceuticals. In addition, from the pharmacokinetic point of view a rationalistic toolbox should also contain oral exposure models with immunological read out parameters in normal or in genetically predisposed animal strains. This review focuses on these two categories of candidate test methods, PLNA and oral exposure models, and proposes to use these in tandem in order to predict the hazard of induction of allergy or autoimmune phenomena by new pharmaceutical candidates.

The 2nd International Drug Hypersensitivity Meeting

The 2nd International Drug Hypersensitivity Meeting was held at Liverpool, UK on 18 - 21 April, 2006. This meeting, attended by almost 200 delegates with expertises in chemistry, pharmacology, toxicology, allergy and immunology, provided an excellent environment to discuss recent advances in the understanding of the chemical, cellular, molecular and genetic basis of drug hypersensitivity. Furthermore, specific symposia addressed clinical, industrial and regulatory perspectives of drug hypersensitivity reactions. The ultimate aim of the meeting was to contribute towards a better understanding and management of this particular form of serious adverse drug reaction. The authors herein provide a synopsis of the main research and clinical findings discussed at the meeting.

Role of animal models in the study of drug-induced hypersensitivity reactions

Drug-induced hypersensitivity reactions (DHRs) are a major problem, in large part because of their unpredictable nature. If we understood the mechanisms of these reactions better, they might be predictable. Their unpredictable nature also makes mechanistic studies very difficult, especially prospective clinical studies. Animal models are vital to most biomedical research, and they are almost the only way to test basic hypotheses of DHRs, such as the involvement of reactive metabolites. However, useful animal models of DHRs are rare because DHRs are also unpredictable in animals. For example, sulfonamide-induced DHRs in large-breed dogs appear to be valid because they are very similar to the DHRs that occur in humans; however, the incidence is only approximately 0.25%, and large-breed dogs are difficult to use as an animal model. Two more practical models are penicillamine-induced autoimmunity in the Brown Norway rat and nevirapine-induced skin rash in rats. The toxicity in these models is clearly immune mediated. In other models, such as amodiaquine-induced agranulocytosis/hepatotoxicity and halothane-induced hepatotoxicity, the drug induces an immune response but there is no clinical toxicity. This finding suggests that regulatory mechanisms usually limit toxicity. Many of the basic characteristics of the penicillamine and nevirapine models, such as memory and tolerance, are quite different suggesting that the mechanisms are also significantly different. More animal models are needed to study the range of mechanisms involved in DHRs; without them, progress in understanding such reactions is likely to be slow.