Advances in molecular toxicology-towards understanding idiosyncratic drug toxicity

Use of intravenous iron and risk of anaphylaxis: A multinational observational post-authorisation safety study in Europe

PURPOSE

This post-authorisation safety study estimated the risk of anaphylaxis in patients receiving intravenous (IV) iron in Europe, with interest in iron dextran and iron non-dextrans. Studies conducted in the United States have reported risk of anaphylaxis to IV iron ranging from 2.0 to 6.8 per 10 000 first treatments.

METHODS

Cohort study of IV iron new users, captured mostly through pharmacy ambulatory dispensing, from populations covered by health and administrative data sources in five European countries from 1999 to 2017. Anaphylaxis events were identified through an algorithm that used parenteral penicillin as a positive control.

RESULTS

A total of 304 210 patients with a first IV iron treatment (6367 iron dextran), among whom 13-16 anaphylaxis cases were identified and reported as a range to comply with data protection regulations. The pooled unadjusted incidence proportion (IP) ranged from 0.4 (95% confidence interval [CI], 0.2-0.9) to 0.5 (95% CI, 0.3-1.0) per 10 000 first treatments. No events were identified at first dextran treatments. There were 231 294 first penicillin treatments with 30 potential cases of anaphylaxis (IP = 1.2; 95% CI, 0.8-1.7 per 10 000 treatments).

CONCLUSION

We found an IP of anaphylaxis from 0.4 to 0.5 per 10 000 first IV iron treatments. The study captured only a fraction of IV iron treatments administered in hospitals, where most first treatments are likely to happen. Due to this limitation, the study could not exclude a differential risk of anaphylaxis between iron dextran and iron non-dextrans. The IP of anaphylaxis in users of penicillin was consistent with incidences reported in the literature.

Treating Through Drug-Associated Exanthems in Drug Allergy Management: Current Evidence and Clinical Aspects

In the setting of an acute cutaneous adverse drug reaction there is increasing interest in selected phenotypes and hosts to continue drug therapy, especially in settings in which there are limited therapeutic options. This concept of "treating through," defined as the continued use of a drug in the setting of, in particular maculopapular exanthema, potentially avoids unnecessary drug discontinuation. A review of the recent literature, historical viewpoints, and expert opinion are provided within to form recommendations and algorithms for a "treating-through" approach.

Today's Challenges to De-Risk and Predict Drug Safety in Human "Mind-the-Gap"

Current gaps in drug safety sciences can result from the inability (1) to identify hazard across multiple target organs, (2) to predict and risk assess with certainty against drug safety liabilities for the major target organs, (3) to optimally manage and mitigate against drug safety liabilities, and (4) to apply principles of governance on the generation, integration, and use of experimental data. Translational safety assessment to evaluate several target-organ drug toxicities can only be partially achieved by use of current in vitro and in vivo test systems. What remains to be tackled necessitates the deployment of in vitro-human-relevant test systems to address human specific or selective forms of toxicities. Nevertheless, such models may only address in part some of the requirements in today's armament of biomedical tools essential for improving the discovery of drug candidates. Refinement of in silico tools, Target Safety Assessment and a greater understanding of mechanistic insights of toxicities might provide future opportunities to better identify drug safety liabilities. The increasing diversity of drug modalities present further challenges for nonclinical and clinical development requiring further research to develop suitable test systems and technologies. Our ability to optimally manage and mitigate safety risk will come from the greater refinement of safety margin estimates, provision and use of human-relevant safety biomarkers, and understanding of the translation from in silico, in vitro, and in vivo studies to human. An improvement of governance frameworks and standards at all levels within organizations, national, and international, can only help facilitate drug discovery and development programs.

The Metabolism of Methazolamide in Immortalized Human Keratinocytes, HaCaT Cells

OBJECTIVE

Drug therapy is occasionally accompanied by an idiosyncratic severe toxicity, which occurs very rarely, but can lead to patient mortality. Methazolamide, an anti-glaucomatous agent, could cause severe skin eruptions called Stevens-Johnson syndrome/toxic epidermal necrolyis (SJS/TEN). Its precise etiology is still uncertain. In this study, the metabolism of methazolamide was investigated in immortalized human keratinocytes to reveal the possible mechanism which causes SJS/TEN.

METHODS

The metabolism of methazolamide was studied using immortalized human keratinocytes, HaCaT cells. HPLC was used to isolate a metabolite from the culture medium. Mass spectrometry (LCMS/ MS) was employed for its characterization. Three typical chemical inducers were assessed for the inducibility of cytochrome P450, and methimazole was used as the inhibitor of flavin-containing monooxygenase (FMO).

RESULTS

A sulfonic acid, N-[3-methyl-5-sulfo-1,3,4-thiadiazol-2(3H)-ylidene]acetamide (MSO) was identified as the final metabolite. Dexamethasone and β-naphthoflavone behaved as an inducer of cytochrome P450 in the metabolism, but isoniazid did not. The effect of methimazole was not consistent. We did not detect any glucuronide nor any mercapturic acid (N-acetylcysteine conjugate).

CONCLUSION

N-[3-methyl-5-sulfo-1,3,4-thiadiazol-2(3H)-ylidene]acetamide (MSO) is not considered to be a direct product of an enzymatic reaction, but rather an auto-oxidation product of N-[3-methyl-5- sulfe-1,3,4-thiadiazol-2(3H)-ylidene]acetamide, a chemically unstable sulfenic acid, which is produced by cytochrome P450 from the β-lyase product of cysteine conjugate of methazolamide. MSO is considered to be susceptible to glutathione and to return to glutathione conjugate of methazolamide, forming a futile cycle. A hypothetical scenario is presented as to the onset of the disease.

The Opportunities of Metabolomics in Drug Safety Evaluation

Although safety of drug candidates is carefully monitored in preclinical and clinical studies using a variety of approaches, drug toxicity may still occur in clinical practice. Therefore, novel approaches are needed to complement the current drug safety evaluation system. Metabolomics comprehensively analyzes the metabolites altered by drug exposure, which can therefore be used to profile drug metabolism, endobiotic metabolism, and drug-microbiota interactions. The information from metabolomic analysis can be used to determine the off-targets of a drug candidate, and thus provide a mechanistic understanding of drug toxicity. We herein discuss the opportunities of metabolomics in drug safety evaluation.

Identification of novel glutathione adducts of benzbromarone in human liver microsomes

Benzbromarone (BBR) is a potent uricosuric drug that can cause serious liver injury. Our recent study suggested that 1'-hydroxy BBR, one of major metabolites of BBR, is metabolized to a cytotoxic metabolite that could be detoxified by glutathione (GSH). The aim of this study was to clarify whether GSH adducts are formed from 1'-hydroxy BBR in human liver microsomes (HLM). Incubation of 1'-hydroxy BBR with GSH in HLM did not result in the formation of GSH adducts, but 1',6-dihydroxy BBR was formed. In addition, incubation of 1',6-dihydroxy BBR with GSH in HLM resulted in the formation of three novel GSH adducts (M1, M2 and M3). The structures of M1 and M2 were estimated to be GSH adducts in which the 1-hydroxyethyl group at the C-2 position and the hydroxyl group at the C-1' position of 1',6-dihydroxy BBR were substituted by GSH, respectively. We also found that the 6-hydroxylation of 1'-hydroxy BBR is mainly catalyzed by CYP2C9 and that several CYPs and/or non-enzymatic reaction are involved in the formation of GSH adducts from 1',6-dihydroxy BBR. The results indicate that 1'-hydroxy BBR is metabolized to reactive metabolites via 1',6-dihydroxy BBR formation, suggesting that these reactive metabolites are responsible for BBR-induced liver injury.

Do Preclinical Testing Strategies Help Predict Human Hepatotoxic Potentials?

Overt hepatotoxicity due to drug administration is a real and present issue in drug development and regulatory circles. Preclinical drug development is intended to identify potential risks and target tissues prior to introduction of new molecular entities into the human population. The standard regimen is testing at various multiples of the intended human therapeutic dose in at least 2 species of animals, one rodent (rats or mice), one non-rodent (dogs, nonhuman primates, minipigs, and rabbits, as examples) for at least two weeks of repeated dosing. Experience has shown that this regimen “works” most of the time. However, preclinical models are not infallible and are not always predictive. Whether the lack of predictivity is due to individual human genetic sensitivities, immunologically mediated phenomena, disease mediation or idiosyncratic reactions, the animal models are limited in detecting these characteristics and other low incidence phenomena. While it is uncommon for drug developers to continue development with products that elicit overt hepatic toxicity early in the animal testing, some products have made it through the approval process and then shown significant adverse effects. Some of the drugs (acetaminophen, isoniazid, trovafloxacin, troglitazone, bromfenac, clarithromycin, telithromycin) that have shown this propensity will be discussed in detail from early preclinical development to marketing and, in some instances, to limitations to usage or removal from the U.S. marketplace.

The Use of Minipig in Drug Discovery and Development: Pros and Cons of Minipig Selection and Strategies to Use as a Preferred Nonrodent Species

The pig was introduced more than 20 years ago in drug development following attempts of finding a species that shares better homology with human than the dog, based on biophysiological parameters. However, miniaturization, standardized breeding, and health status control were required before the pig could find a broader than niche application in pharmaceutical industry. During the years of experience with minipigs in pharmaceutical research and the science evolving rapidly, the selection of a nonrodent animal species for preclinical safety testing became primarily driven by pharmacological (target expression homologous function), pharmacokinetic, and biophysiological considerations. This offered a broad field of application for the minipig, besides the well-established use in dermal projects in all areas of drug development but also in novel approaches including genetically modified animals. In this article, we look at recent approaches and requirements in the optimal selection of a nonrodent model in pharmaceutical development and critically ask how good a choice the minipig offers for the scientist, how did the testing environment evolve, and what are the key requirements for a broader use of the minipig compared to the other well-established nonrodent species like dog or monkey.

Drug-induced liver injury from antiepileptic drugs

Drug-induced liver injury is a potential complication of innumerable medications. Most cases do not occur in a predictable, dose-dependent manner, leading to delayed recognition of a drug's hepatotoxic potential until after its release into the market. The estimated occurrence is 1 in 10,000 to 100,000 patients. However, the rates are likely higher because many cases go unrecognized owing to lack of reporting or missed diagnosis. This article reviews the most commonly associated antiepileptic drugs.

Drug-induced acute myocardial infarction: identifying 'prime suspects' from electronic healthcare records-based surveillance system

BACKGROUND

Drug-related adverse events remain an important cause of morbidity and mortality and impose huge burden on healthcare costs. Routinely collected electronic healthcare data give a good snapshot of how drugs are being used in 'real-world' settings.

OBJECTIVE

To describe a strategy that identifies potentially drug-induced acute myocardial infarction (AMI) from a large international healthcare data network.

METHODS

Post-marketing safety surveillance was conducted in seven population-based healthcare databases in three countries (Denmark, Italy, and the Netherlands) using anonymised demographic, clinical, and prescription/dispensing data representing 21,171,291 individuals with 154,474,063 person-years of follow-up in the period 1996-2010. Primary care physicians' medical records and administrative claims containing reimbursements for filled prescriptions, laboratory tests, and hospitalisations were evaluated using a three-tier triage system of detection, filtering, and substantiation that generated a list of drugs potentially associated with AMI. Outcome of interest was statistically significant increased risk of AMI during drug exposure that has not been previously described in current literature and is biologically plausible.

RESULTS

Overall, 163 drugs were identified to be associated with increased risk of AMI during preliminary screening. Of these, 124 drugs were eliminated after adjustment for possible bias and confounding. With subsequent application of criteria for novelty and biological plausibility, association with AMI remained for nine drugs ('prime suspects'): azithromycin; erythromycin; roxithromycin; metoclopramide; cisapride; domperidone; betamethasone; fluconazole; and megestrol acetate.

LIMITATIONS

Although global health status, co-morbidities, and time-invariant factors were adjusted for, residual confounding cannot be ruled out.

CONCLUSION

A strategy to identify potentially drug-induced AMI from electronic healthcare data has been proposed that takes into account not only statistical association, but also public health relevance, novelty, and biological plausibility. Although this strategy needs to be further evaluated using other healthcare data sources, the list of 'prime suspects' makes a good starting point for further clinical, laboratory, and epidemiologic investigation.

Increase in covalent binding of 5-hydroxydiclofenac to hepatic tissues in rats co-treated with lipopolysaccharide and diclofenac: involvement in the onset of diclofenac-induced idiosyncratic hepatotoxicity

Diclofenac (DCF), a nonsteroidal anti-inflammatory drug, is well known to induce idiosyncratic hepatotoxicity. Although there remains much to be elucidated about its onset mechanism, it is widely accepted as a hypothesis that idiosyncratic hepatotoxicity arises from a specific immune response to a hapten formed by covalent binding of drugs or their reactive metabolites to hepatic tissues. In this study, we investigated the effects of covalent binding of DCF reactive metabolites to hepatic tissues using a rat model of liver injury induced by co-treatment with lipopolysaccharide (LPS) at a non-hepatotoxic dose. In studies done in vitro using hepatic microsomes prepared from rats treated with LPS alone, 4'- and 5-hydroxylation activities on DCF metabolism and adducts of reactive metabolites to dansyl glutathione (dGSH) were markedly decreased associated with a decrease in total P450 content. However, in studies done in vivo, the LPS/DCF co-treatment significantly increased adducts of 5-hydroxydiclofenac (5-OH-DCF) to rat hepatic tissues and delayed the elimination of 5-OH-DCF from plasma. Furthermore, we investigated the effects of co-treatment on hepatic GSH level in rats. A decrease of hepatic GSH was observed with the LPS/DCF co-treatment but not with LPS or DCF alone. The results suggest that covalent binding of reactive metabolites via 5-OH-DCF to hepatic tissues may play an important role in the onset of DCF-induced idiosyncratic hepatotoxicity, especially under decreased GSH conditions.

Metabolic activation and inflammation reactions involved in carbamazepine-induced liver injury

Drug-induced liver injury is a major safety concern in drug development and clinical pharmacotherapy; however, advances in the understanding of the mechanisms of drug-induced liver injury are hampered by the lack of animal models. Carbamazepine (CBZ) is a widely used antiepileptic agent. Although the drug is generally well tolerated, only a small number of patients prescribed CBZ develop severe hepatitis. In the present study, we developed a mouse model of CBZ-induced liver injury and elucidated the mechanisms accounting for the hepatotoxicity of CBZ. Male BALB/c mice were orally administered CBZ for 5 days. The plasma levels of alanine aminotransferase and aspartate aminotransferase were prominently increased, and severe liver damage was observed via histological evaluation. The analysis of the plasma concentration of CBZ and its metabolites demonstrated that 3-hydroxy CBZ may be relevant in CBZ-induced liver injury. The hepatic glutathione levels were significantly decreased, and oxidative stress markers were significantly altered. Mechanistic investigations found that hepatic mRNA levels of toll-like receptor 4, receptor for advanced glycation end products, and their ligands were significantly increased. Moreover, the plasma concentrations of proinflammatory cytokines were also increased. Prostaglandin E(1) administration ameliorated the hepatic injury caused by CBZ. In conclusion, metabolic activation followed by the stimulation of immune responses was demonstrated to be involved in CBZ-induced liver injury in mice.

Review article: Drug-induced liver injury--its pathophysiology and evolving diagnostic tools

BACKGROUND

Drug-induced liver injury (DILI) is a significant cause of morbidity and mortality accounting for at least 13% of acute liver failure cases in the US. It is the leading cause of acute liver failure among patients referred for liver transplantation and the most common reason that drugs in development do not obtain FDA approval. The incidence of DILI has been reported to be one in 10,000 to one in 100,000 patients; however, the actual incidence is probably higher due in part to the difficulty of diagnosis.

AIM

To present a review of the current literature on DILI with a focus on its pathophysiology and evolving diagnostic modalities.

METHODS

A PubMed literature search was conducted using the terms 'drug induced liver injury', 'pathophysiology', 'causality', 'diagnosis', 'toxicogenomics' and 'pharmacogenetics'.

RESULTS

Drug-induced liver injury is an area of ongoing research. From the time it was first recognised, our understanding of the pathophysiology, its classification, diagnosis and reporting by established national networks continues to challenge and evolve. Metabonomics, pharmacogenetics, proteomics and transcriptomics are more recent areas of study that have been applied to further the understanding of DILI.

CONCLUSIONS

Despite recent advances in our understanding of drug-induced liver injury, many aspects of its pathophysiology and clinical impact remain unclear. In addition, genomic-based studies are evolving concepts, which undoubtedly continue to contribute to our understanding of the underlying mechanisms of drug-induced liver injury.

Dasatinib, imatinib and staurosporine capture compounds - Complementary tools for the profiling of kinases by Capture Compound Mass Spectrometry (CCMS)

Capture Compound Mass Spectrometry (CCMS) is a platform technology for the functional isolation of subproteomes. Here we report the synthesis of two new kinase Capture Compounds (CCs) based on the tyrosine-kinase specific inhibitors dasatinib and imatinib and compare their interaction profiles to that of our previously reported staurosporine-CCs. CCs are tri-functional molecules: they comprise a sorting function (e.g. the small molecule or drug of interest) which interacts with target proteins, a photo-activatable reactivity function to covalently trap the interacting proteins, and a sorting function to isolate the CC-protein conjugates from complex biological samples for protein identification by liquid chromatography/mass spectrometry (LC-MS/MS). We present data of CCMS experiments from human HepG2 cells and compare the profiles of the kinases isolated with dasatinib, imatinib and staurosporine CC, respectively. Dasatinib and imatinib have a more selective kinase binding profile than staurosporine. Moreover, the new CCs allow isolation and identification of additional kinases, complementing the staurosporine CC. The family of kinase CCs will be a valuable tool for the proteomic profiling of this important protein class. Besides sets of expected kinases we identified additional specific interactors; these off-targets may be of relevance in the view of the pharmacological profile of dasatinib and imatinib.

Constituents in kava extracts potentially involved in hepatotoxicity: a review

Aqueous kava root preparations have been consumed in the South Pacific as an apparently safe ceremonial and cultural drink for centuries. However, several reports of hepatotoxicity have been linked to the consumption of kava extracts in Western countries, where mainly ethanolic or acetonic extracts are used. The mechanism of toxicity has not been established, although several theories have been put forward. The composition of the major constituents, the kava lactones, varies according to preparation method and species of kava plant, and thus, the toxicity of the individual lactones has been tested in order to establish whether a single lactone or a certain composition of lactones may be responsible for the increased prevalence of kava-induced hepatotoxicity in Western countries. However, no such conclusion has been made on the basis of current data. Inhibition or induction of the major metabolizing enzymes, which might result in drug interactions, has also gained attention, but ambiguous results have been reported. On the basis of the chemical structures of kava constituents, the formation of reactive metabolites has also been suggested as an explanation of toxicity. Furthermore, skin rash is a side effect in kava consumers, which may be indicative of the formation of reactive metabolites and covalent binding to skin proteins leading to immune-mediated responses. Reactive metabolites of kava lactones have been identified in vitro as glutathione (GSH) conjugates and in vivo as mercapturates excreted in urine. Addition of GSH to kava extracts has been shown to reduce cytotoxicity in vitro, which suggests the presence of inherently reactive constituents. Only a few studies have investigated the toxicity of the minor constituents present in kava extract, such as pipermethystine and the flavokavains, where some have been shown to display higher in vitro cytotoxicity than the lactones. To date, there remains no indisputable reason for the increased prevalence of kava-induced hepatotoxicity in Western countries.

Glutathione S-transferase polymorphisms in osteosarcoma patients

BACKGROUND

Osteosarcoma is the most common malignant bone tumor in children and adolescents. Multidrug resistance and poor clinical outcome are the problems that still affect osteosarcoma patients. The glutathione S-transferase supergene family includes several genes that encode enzymes involved in the detoxification of many xenobiotic agents, including carcinogens and anticancer drugs. The polymorphisms in these genes have already been associated both with cancer susceptibility and anticancer drugs resistance.

OBJECTIVES

This study aims to investigate the genotype frequencies of GSTM1, GSTT1 and GSTM3 genes in 80 osteosarcoma patients and 160 normal control participants, and also the influence of these polymorphisms in the clinical outcome of osteosarcoma patients.

METHODS

GSTM1 and GSTT1 deletion polymorphisms were examined through a multiplex-PCR and the GSTM3 polymorphism of three base pair-deletion at intron 6 using PCR-restriction fragments length polymorphism method.

RESULTS

We found that GSTM1 null genotype is correlated to poor clinical outcome characterized by the increased lung relapse occurrence [odds ratio (OR)=2.71, P=0.036], while the presence of at least one GSTM1 allele is associated with a good response to treatment and better survival (OR=4.28, P=0.020 and hazards ratio=4.09, P=0.0078, respectively). The GSTT1 null genotype was correlated with a better overall survival (hazards ratio=7.15, P=0.0247), whereas GSTM3*B allele was associated with metastasis at diagnosis (OR=2.83, P=0.028).

CONCLUSION

The findings of this study suggest that GST polymorphisms may have a role in treatment response and osteosarcoma progression.

Drug-induced liver injury

Many drugs and environmental chemicals are capable of evoking some degree of liver injury. The liver represents a primary target for adverse drug reactions due to its central role in biotransformation and excretion of foreign compounds, its portal location within the circulation exposing it to a wide variety of substances, and its anatomic and physiologic structure. Drug-induced liver injury (DILI) remains the single most common adverse indication leading to drug candidate failure or withdrawal from the market. However, the absolute incidence of DILI is low, and this presents a challenge to mechanistic studies. DILI remains unpredictable making prevention very difficult. In this chapter, we focus on the current understanding of DILI. We begin with an overview regarding the significance and epidemiology of DILI and then examine the clinical presentation and susceptibility factors related to DILI. This is followed by a review of the current literature regarding the proposed pathogenesis of DILI, which involves the participation of a drug, or most often a reactive metabolite of the drug, that either directly affects cellular function or elicits an immune response. It is our hope that this chapter will shed light on the major problems associated with DILI in regards to the pharmaceutical industry, drug regulatory agencies, physicians and pharmacists, and patients.

2-ABT-S-oxide detoxification by glutathione S-transferases A1-1, M1-1 and P1-1: implications for toxicity associated with zileuton

Zileuton, an agent which targets the leukotriene pathway through inhibition of 5-lipoxygenase (5-LO), was approved for the treatment of asthma in 1997. Shortly after its release, its use was restricted due to the observation of hepatotoxicity in patients. Previous research from the authors' laboratory demonstrated the formation of the reactive metabolite, 2-ABT-S-oxide (M1) from zileuton, and has identified a mercapturate of 2-ABT, C1, in the urine of rats dosed with zileuton. The reaction between M1 and glutathione (GSH) has been established in vitro; however, the potential for catalysis by glutathione transferases (GSTs) was not addressed. The work presented here outlines a role for GSTs in the detoxification of M1. Non-enzymatic conjugation studies with M1 and GSH in control experiments led to a t(1/2) of 6.4 +/- 0.4 h at pH 6.5. This rate was accelerated in the presence of GSTA1-1, GSTM1-1 and GSTP1-1 providing t(1/2) values of 2.6 +/- 0.1, 0.53 +/- 0.02, and 0.3 +/- 0.04 h, respectively, at pH 6.5. The inhibition of various GST enzymes was also studied. Results show that M1 inhibits GSTM1-1 and GSTP1-1 to a greater extent as compared with GSTA1-1. In the case of GSTA1-1, the inhibition was observed to be reversible, whereas M1 inhibition of GSTM1-1 and GSTP1-1 was found to be irreversible under identical conditions. GSTM1-1 is present in liver and thus the finding of the alkylation and potential irreversible inactivation of this isoform in vivo could contribute to an understanding of the hepatotoxicity associated with zileuton.

Role of Metabolism in Drug-Induced Idiosyncratic Hepatotoxicity

Rare adverse reactions to drugs that are of unknown etiology, or idiosyncratic reactions, can produce severe medical complications or even death in patients. Current hypotheses suggest that metabolic activation of a drug to a reactive intermediate is a necessary, yet insufficient, step in the generation of an idiosyncratic reaction. We review evidence for this hypothesis with drugs that are associated with hepatotoxicity, one of the most common types of idiosyncratic reactions in humans. We identified 21 drugs that have either been withdrawn from the U.S. market due to hepatotoxicity or have a black box warning for hepatotoxicity. Evidence for the formation of reactive metabolites was found for 5 out of 6 drugs that were withdrawn, and 8 out of 15 drugs that have black box warnings. For the other drugs, either evidence was not available or suitable studies have not been carried out. We also review evidence for reactive intermediate formation from a number of additional drugs that have been associated with idiosyncratic hepatotoxicity but do not have black box warnings. Finally, we consider the potential role that high dosages may play in these adverse reactions.

Machine learning approaches for predicting compounds that interact with therapeutic and ADMET related proteins

Computational methods for predicting compounds of specific pharmacodynamic and ADMET (absorption, distribution, metabolism, excretion and toxicity) property are useful for facilitating drug discovery and evaluation. Recently, machine learning methods such as neural networks and support vector machines have been explored for predicting inhibitors, antagonists, blockers, agonists, activators and substrates of proteins related to specific therapeutic and ADMET property. These methods are particularly useful for compounds of diverse structures to complement QSAR methods, and for cases of unavailable receptor 3D structure to complement structure-based methods. A number of studies have demonstrated the potential of these methods for predicting such compounds as substrates of P-glycoprotein and cytochrome P450 CYP isoenzymes, inhibitors of protein kinases and CYP isoenzymes, and agonists of serotonin receptor and estrogen receptor. This article is intended to review the strategies, current progresses and underlying difficulties in using machine learning methods for predicting these protein binders and as potential virtual screening tools. Algorithms for proper representation of the structural and physicochemical properties of compounds are also evaluated.

Biomarkers, metabonomics, and drug development: can inborn errors of metabolism help in understanding drug toxicity?

Application of "omics" technology during drug discovery and development is rapidly evolving. This review evaluates the current status and future role of "metabonomics" as a tool in the drug development process to reduce the safety-related attrition rates and bridge the gaps between preclinical and clinical, and clinical and market. Particularly, the review looks at the knowledge gap between the pharmaceutical industry and pediatric hospitals, where metabonomics has been successfully applied to screen and treat newborn babies with inborn errors of metabolism. An attempt has been made to relate the clinical pathology associated with inborn errors of metabolism with those of drug-induced pathology. It is proposed that extending the metabonomic biomarkers used in pediatric hospitals, as "advanced clinical chemistry" for preclinical and clinical drug development, is immediately warranted for better safety assessment of drug candidates. The latest advances in mass spectrometry and nuclear magnetic resonance (NMR) spectroscopy should help replace the traditional approaches of laboratory clinical chemistry and move the safety evaluation of drug candidates into the new millennium.

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.

Liquid Chromatography/Tandem Mass Spectrometry Detection of Covalent Binding of Acetaminophen to Human Serum Albumin

Covalent binding of reactive electrophilic intermediates to proteins is considered to play an important role in the processes leading to adverse drug reactions and idiosyncratic drug reactions. Consequently, both for the discovery and the development of new drugs, there is a great interest in sensitive methodologies that enable the detection of covalent binding of drugs and drug candidates in vivo. In this work, we present a strategy for the generation and analysis of drug adducts to human serum albumin. Our methodology is based on the isolation of albumin from blood, its digestion to peptides by pronase E, and the sensitive detection of adducts to the characteristic cysteine-proline-phenylalanine (CPF) tripeptide by liquid chromatography/tandem mass spectrometry. We chose acetaminophen (APAP) as a model compound because this drug is known to induce covalent binding to proteins when bioactivated by cytochromes P450 to its reactive N-acetyl-p-benzoquinoneimine metabolite. First, by microsomal incubations of APAP in presence of CPF and/or intact albumin, in vitro reference adducts were generated to determine the mass spectrometric characteristics of the expected CPF adducts and to confirm their formation on pronase E digestion of the alkylated protein. When applying this methodology to albumin isolated from blood of patients exposed to APAP, we were indeed able to detect the corresponding CPF adducts. Therefore, this strategy could be seen as a potential biomonitoring tool to detect in vivo reactive intermediates of drugs and drug candidates, e.g., in the preclinical and clinical development phase.

Mitochondrial abnormalities--a link to idiosyncratic drug hepatotoxicity?

Idiosyncratic drug-induced liver injury (DILI) is a major clinical problem and poses a considerable challenge for drug development as an increasing number of successfully launched drugs or new potential drugs have been implicated in causing DILI in susceptible patient subsets. Although the incidence for a particular drug is very low (yet grossly underestimated), the outcome of DILI can be serious. Unfortunately, prediction has remained poor (both for patients at risk and for new chemical entities). The underlying mechanisms and the determinants of susceptibility have largely remained ill-defined. The aim of this review is to provide both clinical and experimental evidence for a major role of mitochondria both as a target of drugs causing idiosyncratic DILI and as mediators of delayed liver injury. We develop a unifying hypothesis that involves underlying genetic or acquired mitochondrial abnormalities as a major determinant of susceptibility for a number of drugs that target mitochondria and cause DILI. The mitochondrial hypothesis, implying gradually accumulating and initially silent mitochondrial injury in heteroplasmic cells which reaches a critical threshold and abruptly triggers liver injury, is consistent with the findings that typically idiosyncratic DILI is delayed (by weeks or months), that increasing age and female gender are risk factors and that these drugs are targeted to the liver and clearly exhibit a mitochondrial hazard in vitro and in vivo. New animal models (e.g., the Sod2(+/-) mouse) provide supporting evidence for this concept. However, genetic analyses of DILI patient samples are needed to ultimately provide the proof-of-concept.

Patients with quinine-induced immune thrombocytopenia have both "drug-dependent" and "drug-specific" antibodies

Immune thrombocytopenia induced by quinine and many other drugs is caused by antibodies that bind to platelet membrane glycoproteins (GPs) only when the sensitizing drug is present in soluble form. In this disorder, drug promotes antibody binding to its target without linking covalently to either of the reacting macro-molecules by a mechanism that has not yet been defined. How drug provides the stimulus for production of such antibodies is also unknown. We studied 7 patients who experienced severe thrombocytopenia after ingestion of quinine. As expected, drug-dependent, platelet-reactive antibodies specific for GPIIb/IIIa or GPIb/IX were identified in each case. Unexpectedly, each of 6 patients with GPIIb/IIIa-specific antibodies was found to have a second antibody specific for drug alone that was not platelet reactive. Despite recognizing different targets, the 2 types of antibody were identical in requiring quinine or desmethoxy-quinine (cinchonidine) for reactivity and in failing to react with other structural analogues of quinine. On the basis of these findings and previous observations, a model is proposed to explain drug-dependent binding of antibodies to cellular targets. In addition to having implications for pathogenesis, drug-specific antibodies may provide a surrogate measure of drug sensitivity in patients with drug-induced immune cytopenia.

Relative and absolute quantitative expression profiling of cytochromes P450 using isotope-coded affinity tags

The development of a novel method for absolute quantification of proteins based on isotope-coded affinity tagging using ICAT reagents is described. The method exploits synthetic peptide standards to determine protein content at the femtomole level in biological samples. The approach is generally applicable to any subset of proteins, but is particularly appropriate for quantitative analysis of multiple, closely related isoforms, and for hydrophobic proteins that are poorly represented in 2-D gels. Relative and absolute quantification techniques are applied to an important group of microsomal metabolic enzymes, the cytochromes P450 (P450), which are critical in determining the disposition, safety and efficacy of drugs in man. Measurement of the P450 induction profile in response to chemicals is a fundamental aspect of drug safety evaluation and is currently achieved by low-throughput methods employing poorly discriminatory antibodies or substrates. Tagging technology is shown to supersede conventional methods for P450 profiling in terms of discriminatory power and throughput, exemplified by the simultaneous detection of distinct induction profiles for cyp2c subfamily members in response to phenobarbitone: cyp2c29 expression, but not cyp2c40 or cyp2c50, was induced threefold by treatment. This technology should abbreviate the drug development pathway, and provide a widely applicable, rapid means of quantifying proteins.

Bioactivation, protein haptenation, and toxicity of sulfamethoxazole and dapsone in normal human dermal fibroblasts

Cutaneous drug reactions (CDRs) associated with sulfonamides are believed to be mediated through the formation of reactive metabolites that result in cellular toxicity and protein haptenation. We evaluated the bioactivation and toxicity of sulfamethoxazole (SMX) and dapsone (DDS) in normal human dermal fibroblasts (NHDF). Incubation of cells with DDS or its metabolite (D-NOH) resulted in protein haptenation readily detected by confocal microscopy and ELISA. While the metabolite of SMX (S-NOH) haptenated intracellular proteins, adducts were not evident in incubations with SMX. Cells expressed abundant N-acetyltransferase-1 (NAT1) mRNA and activity, but little NAT2 mRNA or activity. Neither NAT1 nor NAT2 protein was detected. Incubation of NHDF with S-NOH or D-NOH increased reactive oxygen species formation and reduced glutathione content. NHDF were less susceptible to the cytotoxic effect of S-NOH and D-NOH than are keratinocytes. Our studies provide the novel observation that NHDF are able to acetylate both arylamine compounds and bioactivate the sulfone DDS, giving rise to haptenated proteins. The reactive metabolites of SMX and DDS also provoke oxidative stress in these cells in a time- and concentration-dependent fashion. Further work is needed to determine the role of the observed toxicity in mediating CDRs observed with these agents.

Mechanisms of drug-induced delayed-type hypersensitivity reactions in the skin

Cutaneous drug reactions (CDRs) are the most commonly reported adverse drug reactions. These reactions can range from mildly discomforting to life threatening. CDRs can arise either from immunological or nonimmunological mechanisms, though the preponderance of evidence suggests an important role for immunological responses. Some cutaneous eruptions appear shortly after drug intake, while others are not manifested until 7 to 10 days after initiation of therapy and are consistent with delayed-type hypersensitivity. This review discusses critical steps in the initiation of delayed-type hypersensitivity reactions in the skin, which include protein haptenation, dendritic cell activation/migration and T-cell propagation. Recently, an alternative mechanism of drug presentation has been postulated that does not require bioactivation of the parent drug or antigen processing to elicit a drug-specific T-cell response. This review also discusses the role of various immune-mediators, such as cytokines, nitric oxide, and reactive oxygen species, in the development of delayed-type drug hypersensitivity reactions in skin. As keratinocytes have been shown to play a crucial role in the initiation and propagation of cutaneous immune responses, we also discuss the means by which these cells may initiate or modulate CDRs.

Microarray Analysis of Lipopolysaccharide Potentiation of Trovafloxacin-Induced Liver Injury in Rats Suggests a Role for Proinflammatory Chemokines and Neutrophils

Idiosyncratic drug toxicity refers to toxic reactions occurring in a small subset of patients and usually cannot be predicted during preclinical or early phases of clinical trials. One hypothesis for the pathogenesis of hepatic idiosyncratic drug reactions is that, in certain individuals, underlying inflammation results in sensitization of the liver, such that injury occurs from an agent that typically would not cause hepatotoxicity at a therapeutic dose. We explored this possibility by cotreating rats with nonhepatotoxic doses of bacterial lipopolysaccharide (LPS) and trovafloxacin (TVX), a drug that caused idiosyncratic hepatotoxicity in humans. The combination of LPS and TVX resulted in hepatotoxicity in rats, as determined by increases in serum alanine aminotransferase activity and hepatocellular necrosis, which were not observed with either agent alone. In contrast, treatment with LPS and levofloxacin, a fluoroquinolone without human idiosyncratic liability, did not result in these changes. Liver gene expression analysis identified unique changes induced by the combination of TVX and LPS, including enhanced expression of chemokines, suggestive of liver neutrophil (PMN) accumulation and activation. Consistent with a role for PMN in the hepatotoxicity induced by LPS/TVX, prior depletion of PMN attenuated the liver injury. The results suggest that gene expression profiles predictive of idiosyncratic liability can be generated in rats cotreated with LPS and drug. Furthermore, they identify gene expression changes that could be explored as biomarkers for idiosyncratic toxicity and lead to enhanced understanding of the mechanism(s) underlying hepatotoxicity induced by TVX.

An in vitro approach to detect metabolite toxicity due to CYP3A4-dependent bioactivation of xenobiotics

Many adverse drug reactions are caused by the cytochrome P450 (CYP) dependent activation of drugs into reactive metabolites. In order to reduce attrition due to metabolism-mediated toxicity and to improve safety of drug candidates, we developed two in vitro cell-based assays by combining an activating system (human CYP3A4) with target cells (HepG2 cells): in the first method we incubated microsomes containing cDNA-expressed CYP3A4 together with HepG2 cells; in the second approach HepG2 cells were transiently transfected with CYP3A4. In both assay systems, CYP3A4 catalyzed metabolism was found to be comparable to the high levels reported in hepatocytes. Both assay systems were used to study ten CYP3A4 substrates known for their potential to form metabolites that exhibit higher toxicity than the parent compounds. Several endpoints of toxicity were evaluated, and the measurement of MTT reduction and intracellular ATP levels were selected to assess cell viability. Results demonstrated that both assay systems are capable to metabolize the test compounds leading to increased toxicity, compared to their respective control systems. The co-incubation with the CYP3A4 inhibitor ketoconazole confirmed that the formation of reactive metabolites was CYP3A4 dependent. To further validate the functionality of the two assay systems, they were also used as a "detoxification system" using selected compounds that can be metabolized by CYP3A4 to metabolites less toxic than their parent compounds. These results show that both assay systems can be used to screen for metabolic activation, or de-activation, which may be useful as a rapid and relatively inexpensive in vitro assay for the prediction of CYP3A4 metabolism-mediated toxicity.

SEEING THROUGH THE MIST: ABUNDANCE VERSUS PERCENTAGE. COMMENTARY ON METABOLITES IN SAFETY TESTING

Recent attention has been given to the potential roles that metabolites could play in safety evaluations of new drugs. In 2002, a proposal was published on "metabolites in safety testing" ("MIST"), which suggested some guidelines regarding when it is necessary to provide greater assessment of the safety of metabolites. However, this proposal was based on relative abundance values, i.e., the percentage that a metabolite comprises of total exposure to drug-related material. In the present commentary, we propose that absolute abundance criteria be used rather than relative abundance. The absolute abundance of a metabolite in circulation or excreta in humans should be combined with other information regarding the chemical structure of the metabolite (e.g., similarity to the parent drug, presence of chemically reactive substituents) and potential mechanisms of toxicity (e.g., suprapharmacological effects, secondary pharmacological effects, nonspecific effects). Decision trees are described that can be used to address human metabolites in safety testing.

Gut microorganisms, mammalian metabolism and personalized health care

The mammalian gut microbiota interact extensively with the host through metabolic exchange and co-metabolism of substrates. Such metabolome-metabolome interactions are poorly understood, but might be implicated in the aetiology of many human diseases. In this paper, we assess the importance of the gut microbiota in influencing the disposition, fate and toxicity of drugs in the host, and conclude that appropriate consideration of individual human gut microbial activities will be a necessary part of future personalized health-care paradigms.

Troglitazone induces a rapid drop of mitochondrial membrane potential in liver HepG2 cells

Troglitazone, a thiazolidinedione containing compound, was widely used to treat non-insulin dependent-diabetes. Unfortunately, troglitazone was associated with a sporadic liver toxicity that led to a cessation of its use clinically. Here we show that troglitazone induces a rapid and dose-dependent drop of mitochondrial membrane potential in liver HepG2 cells. The decrease in mitochondrial membrane potential induced by 100 microM troglitazone was completed after 5 min and similar in magnitude to that caused by carbonyl cyanide m-chloro phenylhydrazone. The troglitazone-induced loss of mitochondrial membrane potential preceded changes in cell permeability and cell count. In addition, troglitazone-induced a rise of intracellular calcium, subsequent to the drop in mitochondrial membrane potential, which was blocked by EGTA and the Na+/Ca2+ exchange inhibitor bepridil. Finally, application of 100 microM troglitazone for 24h to HepG2 cells resulted in activation of caspase 3. The results of this study shed light on the molecular mechanisms by which troglitazone can cause cytotoxicity.

A high-throughput liquid chromatography/tandem mass spectrometry method for screening glutathione conjugates using exact mass neutral loss acquisition

Chemically reactive metabolites may cause hepatotoxicity and as a result liver failure or other adverse side reactions. Therefore, this is a vital topic of interest because early reactive metabolite screening may prevent compound failure at a later stage. In order to address this issue, a screening assay has been developed to detect the formation of reactive metabolites by using glutathione as a trapping reagent, which will allow us to search for phase I metabolites and also glutathiones during in vitro metabolite screening using liquid chromatography/tandem mass spectrometry (LC/MS/MS) with exact mass. Glutathione conjugations when fragmented by the mass spectrometer give a common loss corresponding to the pyroglutamic acid moiety, which can be monitored. Until recently, this work has been carried out with triple quadrupole technology using nominal mass. The advantage of the hybrid quadrupole time-of-flight mass spectrometer is the selectivity and sensitivity that can be achieved. Exact neutral loss detection is achieved via sequential low- and high-energy MS acquisitions. After detection of the loss of the pyroglutamic acid moiety, using a window of +/-20 mDa on the high-energy scan, MS/MS is carried out on the parent mass of interest to confirm the common neutral loss.

The Art and Science of Risk Management

The research-based pharmaceutical industry in the US strongly supports the concepts of risk management and sees formal risk management as playing a major role in the development of safe medicines for the public, as well as providing a mechanism to ensure that decisions concerning individual drug benefit and risk are made based on scientific evidence. Safe medicines refer to those drugs whose benefits have been found to outweigh their risks when they are used according to the approved labelling. Risk management is the comprehensive and proactive application of scientifically based methodologies to identify, assess, communicate and minimise risk throughout the life cycle of a drug so as to establish and maintain a favourable benefit-risk balance in patients. Although there are certainly a number of global risk management initiatives in place or being undertaken, harmonisation has yet to be achieved. Industry is faced with a variety of different risk management approaches and tools. There is a need to move the focus of risk management from the post-approval arena to earlier in the development process and tools need to be developed to support risk management throughout the lifecycle of a drug. The focus in the US on risk minimalisation strategies will also be an area for methodological development. A key factor in the success of overall risk management is the dialogue between industry and regulators throughout the development, review and marketing of the product. It is through such dialogue that appropriate, efficient and effective risk management strategies will be developed and implemented and the best decisions regarding the safe use of pharmaceutical products will be made.

Microarray analysis in human hepatocytes suggests a mechanism for hepatotoxicity induced by trovafloxacin

Idiosyncratic drug toxicity, defined as toxicity that is dose independent, host dependent, and usually cannot be predicted during preclinical or early phases of clinical trials, is a particularly confounding complication of drug development. An understanding of the mechanisms that lead to idiosyncratic liver toxicity would be extremely beneficial for the development of new compounds. We used microarray analysis on isolated human hepatocytes to understand the mechanisms underlying the idiosyncratic toxicity induced by trovafloxacin. Our results clearly distinguish trovafloxacin from other marketed quinolone agents and identify unique gene changes induced by trovafloxacin that are involved in mitochondrial damage, RNA processing, transcription, and inflammation that may suggest a mechanism for the hepatotoxicity induced by this agent. In conclusion, this work establishes the basis for future microarray analysis of new compounds to determine the presence of these expression changes and their usefulness in predicting idiosyncratic hepatotoxicity. Supplementary material for this article can be found on the HEPATOLOGY website (http://interscience. Wiley.com/jpages/0270-9139/suppmat/index.htnd).