Editor's Highlight: An Impaired Immune Tolerance Animal Model Distinguishes the Potential of Troglitazone/Pioglitazone and Tolcapone/Entacapone to Cause IDILI

Assessing immune hepatotoxicity of troglitazone with a versatile liver-immune-microphysiological-system

Drug-induced liver injury is a prevalent adverse event associated with pharmaceutical agents. More significantly, there are certain drugs that present severe hepatotoxicity only during the clinical phase, consequently leading to the termination of drug development during clinical trials or the withdrawal from the market after approval. The establishment of an evaluation model that can sensitively manifest such hepatotoxicity has always been a challenging aspect in drug development. In this study, we build a liver-immune-microphysiological-system (LIMPS) to fully demonstrate the liver injury triggered by troglitazone (TGZ), a drug that was withdrawn from the market due to hepatotoxicity. Leveraging the capabilities of organ-on-chip technology allows for the dynamic modulation of cellular immune milieu, as well as the synergistic effects between drugs, hepatocytes and multiple immune cells. Through the LIMPS, we discovered that 1) TGZ can promote neutrophils to adhered hepatocytes, 2) the presence of TGZ enhances the crosstalk between macrophages and neutrophils, 3) the induction of damage in hepatocytes by TGZ at clinically relevant blood concentrations not observed in other in vitro experiments, 4) no hepatotoxicity was observed in LIMPS when exposed to rosiglitazone and pioglitazone, structurally similar analogs of TGZ, even at the higher multiples of blood drug concentration levels. As an immune-mediated liver toxicity assessment method, LIMPS is simple to operate and can be used to test multiple drug candidates to detect whether they will cause severe liver toxicity in clinical settings as early as possible.

Exploiting the predictive power of educated spheroids to detect immune-mediated idiosyncratic drug-induced liver injury: the case of troglitazone

Idiosyncratic drug-induced liver injury (iDILI) is a major concern in drug development because its occurrence is unpredictable. Presently, iDILI prediction is a challenge, and cell toxicity is observed only at concentrations that are much higher than the therapeutic doses in preclinical models. Applying a proprietary cell educating technology, we developed a person-dependent spheroid system that contains autologous educated immune cells that can detect iDILI risk at therapeutic concentrations. Integrating this system into a high-throughput screening platform will help pharmaceutical companies accurately detect the iDILI risk of new molecules de-risking drug development.

The development and characterization of a CRISPR/Cas9-mediated PD-1 functional knockout rat as a tool to study idiosyncratic drug reactions

Idiosyncratic drug reactions are rare but serious adverse drug reactions unrelated to the known therapeutic properties of the drug and manifest in only a small percentage of the treated population. Animal models play an important role in advancing mechanistic studies examining idiosyncratic drug reactions. However, to be useful, they must possess similarities to those seen clinically. Although mice currently represent the dominant mammalian genetic model, rats are advantageous in many areas of pharmacologic study where their physiology can be examined in greater detail and is more akin to that seen in humans. In the area of immunology, this includes autoimmune responses and susceptibility to diabetes, in which rats more accurately mimic disease states in humans compared with mice. For example, oral nevirapine treatment can induce an immune-mediated skin rash in humans and rats, but not in mice due to the absence of the sulfotransferase required to form reactive metabolites of nevirapine within the skin. Using CRISPR-mediated gene editing, we developed a modified line of transgenic rats in which a segment of IgG-like ectodomain containing the core PD-1 interaction motif containing the native ligand and therapeutic antibody domain in exon 2 was deleted. Removal of this region critical for mediating PD-1/PD-L1 interactions resulted in animals with an increased immune response resulting in liver injury when treated with amodiaquine.

Recent Advances in Models of Immune-Mediated Drug-Induced Liver Injury

Hepatic inflammation is a key feature of a variety of liver diseases including drug-induced liver injury (DILI), orchestrated by the innate immune response (Kupffer cells, monocytes, neutrophils, dendritic cells) and the adaptive immune system (T cells and natural killer T cells). In contrast to acute DILI, prediction of immune-mediated DILI (im-DILI) has been more challenging due to complex disease pathogenesis, lack of reliable models and limited knowledge of underlying mechanisms. This review summarizes in vivo and in vitro systems that have been used to model im-DILI. In particular, the review focuses on state-of-the-art in vitro human-based multicellular models which have been developed to supplement the use of in vivo models due to interspecies variation and increasing ethical concerns regarding animal use. Advantages of the co-cultures in maintaining hepatocyte functions and importantly, introducing heterotypic cell-cell interactions to mimic inflammatory hepatic microenvironment are discussed. Challenges regarding cell source and incorporation of different cells with physical cell-cell contact are outlined and potential solutions are proposed. It is likely that better understanding of the interplay of immune cells in liver models will allow for the development of more accurate systems to better predict hepatotoxicity and stratification of drugs that can cause immune-mediated effects.

Role of caspase-8 and/or -9 as biomarkers that can distinguish the potential to cause toxic- and immune related-adverse event, for the progress of acetaminophen-induced liver injury

AIMS

Acetaminophen (APAP) overdose can cause acute liver failure. Although it is well known that APAP-induced liver injury (AILI) is caused by toxic mechanism, recently it is also reported to be immune related. However, the detail of the mechanism has been unclear. Therefore, elucidation of the pathophysiology is required.

MAIN METHODS

In AILI model rats (800 mg/kg), the levels of AST, ALT and Caspase (C)-3/-8/-9 levels were measured. In in vitro study using human hepatocyte cells (FLC-4) and THP-1 cells, APAP (1.0 mM) were added to FLC-4 and the cell viability, C-9, cytochrome c, mitochondria membrane potential, and glutathione levels of FLC-4 and inflammasome activation of THP-1 were evaluated.

KEY FINDINGS

In AILI model rats, the levels of AST and ALT were increased only at 12-24 h. C-3/-9 levels rose at 6-9 h, whereas C-8 level rose hours later, moreover, 24 h after; C-3/-8/-9 levels re-rose. In FLC-4 cells, cytochrome c was released from the mitochondria which is promoted by oxidative stress due to drug metabolism and C-9 was activated. Thus, AILI was caused mitochondrial damage by NAPQI as early reaction (first stage). In the next stage, inflammasomes of human antigen presenting cells, which released inflammatory cytokines were activated by damage-associated molecular patterns (DAMPs) released from damaged hepatocyte by APAP.

SIGNIFICANCE

It is confirmed that AILI includes immune related mechanism. Thereby, in case of N-acetylcysteine refractory, additional administration of steroid hormones should be effective and recommended as a novel strategy for AILI with immune related adverse event (irAE).

Preclinical models of idiosyncratic drug-induced liver injury (iDILI): Moving towards prediction

Idiosyncratic drug-induced liver injury (iDILI) encompasses the unexpected harms that prescription and non-prescription drugs, herbal and dietary supplements can cause to the liver. iDILI remains a major public health problem and a major cause of drug attrition. Given the lack of biomarkers for iDILI prediction, diagnosis and prognosis, searching new models to predict and study mechanisms of iDILI is necessary. One of the major limitations of iDILI preclinical assessment has been the lack of correlation between the markers of hepatotoxicity in animal toxicological studies and clinically significant iDILI. Thus, major advances in the understanding of iDILI susceptibility and pathogenesis have come from the study of well-phenotyped iDILI patients. However, there are many gaps for explaining all the complexity of iDILI susceptibility and mechanisms. Therefore, there is a need to optimize preclinical human in vitro models to reduce the risk of iDILI during drug development. Here, the current experimental models and the future directions in iDILI modelling are thoroughly discussed, focusing on the human cellular models available to study the pathophysiological mechanisms of the disease and the most used in vivo animal iDILI models. We also comment about in silico approaches and the increasing relevance of patient-derived cellular models.

Permeabilized cryopreserved human hepatocytes as an exogenous metabolic system in a novelmetabolism-dependent cytotoxicity assay (MDCA) for the evaluation of metabolic activation anddetoxification of drugs associated with drug induced liver injuries: Results with acetaminophen,amiodarone, cyclophosphamide, ketoconazole, nefazodone, and troglitazone

We report here a novel in vitro experimental system, the metabolism-dependent cytotoxicity assay (MDCA), for the definition of the roles of hepatic drug metabolism in toxicity. MDCA employs permeabilized cofactor-supplemented cryopreserved human hepatocytes (MetMax{trade mark, serif} human hepatocytes, MMHH), as an exogenous metabolic activating system, and HEK-293 cells, a cell line devoid of drug metabolizing enzyme activity, as target cells for the quantification of drug toxicity. The assay was performed in the presence and absence of cofactors for key drug metabolism pathways known to play key roles in drug toxicity: NADPH/NAD+ for phase 1 oxidation, UDPGA for UGT mediated glucuronidation, PAPS for SULT mediated sulfation, and GSH for GST mediated GSH conjugation. Six drugs with clinically significant hepatoxicity, resulting in liver failure or a need for liver transplantation: acetaminophen, amiodarone, cyclophosphamide, ketoconazole, nefazodone and troglitazone were evaluated. All six drugs exhibited cytotoxicity enhancement by NADPH, suggesting metabolic activation via phase 1 oxidation. Attenuation of cytotoxicity by UDPGA was observed for acetaminophen, ketoconazole and troglitazone, by PAPS for acetaminophen, ketoconazole and troglitazone, and by GSH for all six drugs. Our results suggest that MDCA can be applied towards the elucidation of metabolic activation and detoxification pathways, providing information that can be applied in drug development to guide structure optimization to reduce toxicity and to aid the assessment of metabolism-based risk factors for drug toxicity. GSH detoxification represents an endpoint for the identification of drugs forming cytotoxic reactive metabolites, a key property of drugs with idiosyncratic hepatotoxicity. Significance Statement Application of the metabolism-dependent cytotoxicity assay (MDCA) for the elucidation of the roles of metabolic activation and detoxification pathways in drug toxicity may provide information to guide structure optimization in drug development to reduce hepatotoxic potential, and to aid the assessment of metabolism-based risk factors. GSH detoxification represents an endpoint for the identification of drugs forming cytotoxic reactive metabolites may be applied towards the evaluation of idiosyncratic hepatotoxicity.

The Emerging Role of the Innate Immune Response in Idiosyncratic Drug Reactions

Idiosyncratic drug reactions (IDRs) range from relatively common, mild reactions to rarer, potentially life-threatening adverse effects that pose significant risks to both human health and successful drug discovery. Most frequently, IDRs target the liver, skin, and blood or bone marrow. Clinical data indicate that most IDRs are mediated by an adaptive immune response against drug-modified proteins, formed when chemically reactive species of a drug bind to self-proteins, making them appear foreign to the immune system. Although much emphasis has been placed on characterizing the clinical presentation of IDRs and noting implicated drugs, limited research has focused on the mechanisms preceding the manifestations of these severe responses. Therefore, we propose that to address the knowledge gap between drug administration and onset of a severe IDR, more research is required to understand IDR-initiating mechanisms; namely, the role of the innate immune response. In this review, we outline the immune processes involved from neoantigen formation to the result of the formation of the immunologic synapse and suggest that this framework be applied to IDR research. Using four drugs associated with severe IDRs as examples (amoxicillin, amodiaquine, clozapine, and nevirapine), we also summarize clinical and animal model data that are supportive of an early innate immune response. Finally, we discuss how understanding the early steps in innate immune activation in the development of an adaptive IDR will be fundamental in risk assessment during drug development. SIGNIFICANCE STATEMENT: Although there is some understanding that certain adaptive immune mechanisms are involved in the development of idiosyncratic drug reactions, the early phase of these immune responses remains largely uncharacterized. The presented framework refocuses the investigation of IDR pathogenesis from severe clinical manifestations to the initiating innate immune mechanisms that, in contrast, may be quite mild or clinically silent. A comprehensive understanding of these early influences on IDR onset is crucial for accurate risk prediction, IDR prevention, and therapeutic intervention.

[Once upon a time the hepatotoxicity…]

The liver ensures a large part of xenobiotics metabolism thanks to its sizeable enzymatic equipment, its anatomical localization and its abundant vascularization. However, these various characteristics also make it a privileged target for toxic compounds, particularly in the case of a toxic metabolism. Xenobiotics-induced hepatotoxicity is a major cause of liver damage and a real challenge for clinicians, pharmaceutical industry, and health agencies. Intrinsic, i.e. predictable and reproducible hepatotoxicities occurring at threshold doses are distinguished from idiosyncratic hepatotoxicities, occurring in an unpredictable manner in people with individual susceptibilities. Among them, idiosyncratic immune-mediated hepatotoxicity pathophysiology is still unclear. However, the development of tools to improve the prediction and understanding of these disorders may open avenues to the identification of risk factors and new mechanisms of toxicity.

Oxidative Stress in Drug-Induced Liver Injury (DILI): From Mechanisms to Biomarkers for Use in Clinical Practice

Idiosyncratic drug-induced liver injury (DILI) is a type of hepatic injury caused by an uncommon drug adverse reaction that can develop to conditions spanning from asymptomatic liver laboratory abnormalities to acute liver failure (ALF) and death. The cellular and molecular mechanisms involved in DILI are poorly understood. Hepatocyte damage can be caused by the metabolic activation of chemically active intermediate metabolites that covalently bind to macromolecules (e.g., proteins, DNA), forming protein adducts-neoantigens-that lead to the generation of oxidative stress, mitochondrial dysfunction, and endoplasmic reticulum (ER) stress, which can eventually lead to cell death. In parallel, damage-associated molecular patterns (DAMPs) stimulate the immune response, whereby inflammasomes play a pivotal role, and neoantigen presentation on specific human leukocyte antigen (HLA) molecules trigger the adaptive immune response. A wide array of antioxidant mechanisms exists to counterbalance the effect of oxidants, including glutathione (GSH), superoxide dismutase (SOD), catalase, and glutathione peroxidase (GPX), which are pivotal in detoxification. These get compromised during DILI, triggering an imbalance between oxidants and antioxidants defense systems, generating oxidative stress. As a result of exacerbated oxidative stress, several danger signals, including mitochondrial damage, cell death, and inflammatory markers, and microRNAs (miRNAs) related to extracellular vesicles (EVs) have already been reported as mechanistic biomarkers. Here, the status quo and the future directions in DILI are thoroughly discussed, with a special focus on the role of oxidative stress and the development of new biomarkers.

Psoralidin, a major component of Psoraleae Fructus, induces inflammasome activation and idiosyncratic liver injury

Idiosyncratic drug-induced liver injury (IDILI) is a rare but potentially fatal disease that is unpredictable and independent of the dose of the drug. Increasing evidence suggests that the majority of IDILI cases are immune-mediated, and the aberrant activation of inflammasome plays a vital role in progression. Psoraleae Fructus (PF), a tonic Chinese medicine, has been able to cause IDILI, but the precise mechanism of hepatotoxicity remains unclear. In this study, eight bioactive compounds involved in PF-induced inflammasome activation were investigated. The results demonstrated that psoralidin activated the inflammasomes followed by secreting caspase-1 and interleukin 1β (IL-1β) in a dose-dependent manner. Interestingly, MCC950, a potent inhibitor of the NOD-like receptor family pyrin domain-containing 3 (NLRP3) inflammasome, could not entirely suppress the psoralidin-induced inflammasome activation. Moreover, psoralidin significantly induced IL-1β maturation and caspase-1 activation in NLRP3-knockout bone marrow-derived macrophages (BMDMs), suggesting that psoralidin not only activates the NLRP3 inflammasome but also activates other types of inflammasomes. The results also demonstrated that psoralidin activated the inflammasomes by promoting the C-terminal caspase recruitment domain (ASC) oligomerization, and the production of mitochondrial reactive oxygen species (mtROS) is a decisive factor in psoralidin-induced inflammasome activation. Importantly, in vivo data revealed that psoralidin induced hepatic inflammation, increased aminotransferase activity and increased the production of IL-1β and tumor necrosis factor(TNF-α) in a susceptible mouse model of lipopolysaccharide (LPS)-mediated IDILI. In summary, these results confirmed that psoralidin causes IDILI by inducing inflammasome activation. The study suggests that psoralidin is a possible risk factor and is responsible for PF-induced IDILI.

The hepatotoxicity of Polygonum multiflorum: The emerging role of the immune-mediated liver injury

Herbal and dietary supplements (HDS)-induced liver injury has been a great concern all over the world. Polygonum multiflorum Thunb., a well-known Chinese herbal medicine, is recently drawn increasing attention because of its hepatotoxicity. According to the clinical and experimental studies, P. multiflorum-induced liver injury (PM-DILI) is considered to be immune-mediated idiosyncratic liver injury, but the role of immune response and the underlying mechanisms are not completely elucidated. Previous studies focused on the direct toxicity of PM-DILI by using animal models with intrinsic drug-induced liver injury (DILI). However, most epidemiological and clinical evidence demonstrate that PM-DILI is immune-mediated idiosyncratic liver injury. The aim of this review is to assess current epidemiological, clinical and experimental evidence about the possible role of innate and adaptive immunity in the idiosyncratic hepatotoxicity of P. multiflorum. The potential effects of factors associated with immune tolerance, including immune checkpoint molecules and regulatory immune cells on the individual's susceptibility to PM-DILI are also discussed. We conclude by giving our hypothesis of possible immune mechanisms of PM-DILI and providing suggestions for future studies on valuable biomarkers identification and proper immune models establishment.

Activation of inflammasomes by tyrosine kinase inhibitors of vascular endothelial growth factor receptor: Implications for VEGFR TKIs-induced immune related adverse events

Vascular endothelial growth factor (VEGF) promotes tumor angiogenesis through stimulating the proliferation and survival of endothelial cells. The severe adverse events caused by VEGF inhibitors might include immune-related ones; however, details of the mechanism have not been elucidated. We tested whether axitinib, pazopanib, sorafenib, and sunitinib, which are tyrosine kinase inhibitors (TKIs) of VEGF receptor used for the therapy of renal cell carcinoma can activate inflammasomes in differentiated THP-1 cells, a human macrophage cell line. We also performed similar studies with semaxanib. In this study, semaxanib and sorafenib activated the inflammasome of differentiated THP-1 cells. Although pazopanib increased the production of IL-1β, inflammasomes were not activated because caspase-1 was not activated in differentiated THP-1 cells. Our results support the hypothesis that activation of inflammasomes contributes to the idiosyncratic reactions associated with semaxanib and sorafenib. Although pazopanib did not activate inflammasomes, it did cause increased IL-1β production, which may facilitate the induction of idiosyncratic reactions.

Models of Idiosyncratic Drug-Induced Liver Injury

Drug-induced liver injury (DILI) is a leading cause of attrition during the early and late stages of drug development and after a drug is marketed. DILI is generally classified as either intrinsic or idiosyncratic. Intrinsic DILI is dose dependent and predictable (e.g., acetaminophen toxicity). However, predicting the occurrence of idiosyncratic DILI, which has a very low incidence and is associated with severe liver damage, is difficult because of its complex nature and the poor understanding of its mechanism. Considering drug metabolism and pharmacokinetics, we established experimental animal models of DILI for 14 clinical drugs that cause idiosyncratic DILI in humans, which is characterized by the formation of reactive metabolites and the involvement of both innate and adaptive immunity. On the basis of the biomarker data obtained from the animal models, we developed a cell-based assay system that predicts the potential risks of drugs for inducing DILI. These findings increase our understanding of the mechanisms of DILI and may help predict and prevent idiosyncratic DILI due to certain drugs.

HLA transgenic mice: application in reproducing idiosyncratic drug toxicity

Various types of transgenic mice carrying either class I or II human leukocyte antigen (HLA) molecules are readily available, and reports describing their use in a variety of studies have been published for more than 30 years. Examples of their use include the discovery of HLA-specific antigens against viral infection as well as the reproduction of HLA-mediated autoimmune diseases for the development of therapeutic strategies. Recently, HLA transgenic mice have been used to reproduce HLA-mediated idiosyncratic drug toxicity (IDT), a rare and unpredictable adverse drug reaction that can result in death. For example, abacavir-induced IDT has successfully been reproduced in HLA-B*57:01 transgenic mice. Several reports using HLA transgenic mice for IDT have proven the utility of this concept for the evaluation of IDT using various HLA allele combinations and drugs. It has become apparent that such models may be a valuable tool to investigate the mechanisms underlying HLA-mediated IDT. This review summarizes the latest findings in the area of HLA transgenic mouse models and discusses the current challenges that must be overcome to maximize the potential of this unique animal model.

Hepatocyte-Derived Exosomes Promote Liver Immune Tolerance: Possible Implications for Idiosyncratic Drug-Induced Liver Injury

Most idiosyncratic drug-induced liver injury appears to result from an adaptive immune attack on the liver. Recent evidence suggests that the T-cell response may be facilitated by the loss of immune tolerance. In this study, we explored the hypothesis that constitutively released hepatocyte-derived exosomes (HDE) are important for maintaining normal liver immune tolerance. Exosomes were isolated from the conditioned medium of primary human hepatocytes via polymer precipitation. Mock controls were prepared by processing fresh medium that was not hepatocyte exposed with precipitation reagent. THP-1 monocytes were then treated with HDE or an equivalent volume of mock control for 24 h, followed by a 6-h stimulation with LPS. HDE exposure resulted in a significant decrease in the LPS-induced media levels of interleukin-1β and interleukin-8. Gene expression profiling performed in THP-1 cells just prior to LPS-induced stimulation identified a significant decrease among genes associated with innate immune response. MicroRNA (miRNA) profiling was performed on the HDE to identify exosome contents that may drive immune suppression. Many of the predicted mRNA target genes for the most abundant microRNAs in HDE were among the differentially expressed genes in THP-1 cells. Taken together, our data suggest that HDE play a role in maintaining normal liver immune tolerance. Future experiments will explore the possibility that drugs causing idiosyncratic liver injury promote the loss of homeostatic HDE signaling.

An in vitro coculture system of human peripheral blood mononuclear cells with hepatocellular carcinoma-derived cells for predicting drug-induced liver injury

Preventing clinical drug-induced liver injury (DILI) remains a major challenge, because DILI develops via multifactorial mechanisms. Immune and inflammatory reactions are considered important mechanisms of DILI; however, biomarkers from in vitro systems using immune cells have not been comprehensively studied. The aims of this study were (1) to identify promising biomarker genes for predicting DILI in an in vitro coculture model of peripheral blood mononuclear cells (PBMCs) with a human liver cell line, and (2) to evaluate these genes as predictors of DILI using a panel of drugs with different clinical DILI risk. Transcriptome-wide analysis of PBMCs cocultured with HepG2 or differentiated HepaRG cells that were treated with several drugs revealed an appropriate separation of DILI-positive and DILI-negative drugs, from which 12 putative biomarker genes were selected. To evaluate the predictive performance of these genes, PBMCs cocultured with HepG2 cells were exposed to 77 different drugs, and gene expression levels in PBMCs were determined. The MET proto-oncogene receptor tyrosine kinase (MET) showed the highest area under the receiver-operating characteristic curve (AUC) value of 0.81 among the 12 genes with a high sensitivity/specificity (85/66%). However, a stepwise logistic regression model using the 12 identified genes showed the highest AUC value of 0.94 with a high sensitivity/specificity (93/86%). Taken together, we established a coculture system using PBMCs and HepG2 cells and selected biomarkers that can predict DILI risk. The established model would be useful in detecting the DILI potential of compounds, in particular those that involve an immune mechanism.

What have we learned from animal models of idiosyncratic, drug-induced liver injury?

INTRODUCTION

Idiosyncratic, drug-induced liver injury (IDILI) continues to plague patients and restrict the use of drugs that are pharmacologically effective. Mechanisms of IDILI are incompletely understood, and a better understanding would reduce speculation and could help to identify safer drug candidates preclinically. Animal models have the potential to enhance knowledge of mechanisms of IDILI.

AREAS COVERED

Numerous hypotheses have emerged to explain IDILI pathogenesis, many of which center on the roles of the innate and/or adaptive immune systems. Animal models based on these hypotheses are reviewed in the context of their contributions to understanding of IDILI and their limitations.

EXPERT OPINION

Animal models of IDILI based on an activated adaptive immune system have to date failed to reproduce major liver injury that is of most concern clinically. The only models that have so far resulted in pronounced liver injury are based on the multiple determinant hypothesis or the inflammatory stress hypothesis. The liver pathogenesis in IDILI animal models involves various leukocytes and immune mediators such as cytokines. Insights from animal models are changing the way we view IDILI pathogenesis and are leading to better approaches to preclinical prediction of IDILI potential of new drug candidates.

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.

Strategies for Early Prediction and Timely Recognition of Drug-Induced Liver Injury: The Case of Cyclin-Dependent Kinase 4/6 Inhibitors

The idiosyncratic nature of drug-induced liver injury (DILI) represents a current challenge for drug developers, regulators and clinicians. The myriad of agents (including medications, herbals, and dietary supplements) with recognized DILI potential not only strengthens the importance of the post-marketing phase, when urgent withdrawal sometimes occurs for rare unanticipated liver toxicity, but also shows the imperfect predictivity of pre-clinical models and the lack of validated biomarkers beyond traditional, non-specific liver function tests. After briefly reviewing proposed key mechanisms of DILI, we will focus on drug-related risk factors (physiochemical and pharmacokinetic properties) recently proposed as predictors of DILI and use cyclin-dependent kinase 4/6 inhibitors, relatively novel oral anticancer medications approved for breast cancer, as a case study to discuss the feasibility of early detection of DILI signals during drug development: published data from pivotal clinical trials, unpublished post-marketing reports of liver adverse events, and pharmacokinetic properties will be used to provide a comparative evaluation of their liver safety and gain insight into drug-related risk factors likely to explain the observed differences.

Mechanistic Studies of Idiosyncratic DILI: Clinical Implications

The idiosyncratic nature of idiosyncratic drug-induced liver injury (IDILI) makes mechanistic studies very difficult, and little is known with certainty. However, the fact that the IDILI caused by some drugs is associated with specific HLA genotypes provides strong evidence that it is mediated by the adaptive immune system. This is also consistent with the histology and the general characteristics of IDILI. However, there are other mechanistic hypotheses. Various in vitro and in vivo systems have been used to test hypotheses. Two other hypotheses are mitochondrial injury and inhibition of the bile salt export pump. It is possible that these mechanisms are responsible for some cases of IDILI or that these mechanisms are complementary and are involved in initiating an immune response. In general, it is believed that the initiation of an immune response requires activation of antigen-presenting cells by molecules such as danger-associated molecular pattern molecules (DAMPs). An attractive hypothesis for the mechanism by which DAMPs induce an immune response is through the activation of inflammasomes. The dominant immune response in the liver is immune tolerance, and it is only when immune tolerance fails that significant liver injury occurs. Consistent with this concept, an animal model was developed in which immune checkpoint inhibition unmasked the ability of drugs to cause liver injury. Although it appears that the liver damage is mediated by the adaptive immune system, an innate immune response is required for an adaptive immune response. The innate immune response is not dependent on specific HLA genes or T cell receptors and may occur in most patients and animals treated with a drug that can cause IDILI. Studies of the subclinical innate immune response to drugs may provide important mechanistic clues and provide a method to screen drugs for their potential to cause IDILI.

Rotenone Increases Isoniazid Toxicity but Does Not Cause Significant Liver Injury: Implications for the Hypothesis that Inhibition of the Mitochondrial Electron Transport Chain Is a Common Mechanism of Idiosyncratic Drug-Induced Liver Injury

Idiosyncratic drug reactions (IDRs) significantly increase the risk of failure in drug development. The major IDR leading to drug candidate failure is idiosyncratic drug-induced liver injury (IDILI). Although most evidence suggests that IDRs are mediated by the immune system, there are other hypotheses, such as mitochondrial dysfunction. Many pharmaceutical companies routinely screen for mitochondrial toxicity in an attempt to "derisk" drug candidates. However, the basic hypothesis has never been rigorously tested. A major assay used for this screening involves measurement of inhibition of the mitochondrial electron transport chain. One study found that the combination of rotenone and isoniazid, which inhibit mitochondrial complex I and II, respectively, were synergistic in causing hepatocyte toxicity in vitro and suggested the combination of another drug that inhibited complex I would increase the risk of isoniazid-induced liver injury in patients. We tested this hypothesis in vivo where wild-type and PD-1-/- mice administered anti-CTLA-4, our impaired immune tolerance mouse model, were given 0.02% (w/v) rotenone in water or 0.1%, 0.05%, and 0.01% (w/w) rotenone alone or in combination with isoniazid in food. The cotreatment led to lethality in 100% of the animals receiving 0.1% rotenone and 0.2% isoniazid and 83% of the animals cotreated with 0.05% rotenone and 0.2% isoniazid in food. Nevertheless, there was no significant increase in GLDH or histological evidence of liver injury. No signs of toxicity were observed in any of the mice given rotenone or isoniazid alone. Even though inhibition of the mitochondrial electron transport chain did not lead to significant liver toxicity, it could provide danger signals that promote immune-mediated liver injury. However, rotenone did not significantly increase the liver injury induced by isoniazid in our impaired immune tolerance model. Overall, we conclude that inhibition of the mitochondrial electron transport chain is not a significant mechanism of IDILI.

Involvement of CCL2/CCR2 macrophage recruitment in amodiaquine-induced liver injury

Evidence suggests that macrophages may play a role in the development of idiosyncratic drug-induced liver injury (IDILI). However, there has yet to be a clear link between macrophage activation and the inflammatory infiltrate that is characteristic of IDILI. A major chemokine involved in the recruitment of macrophages into the liver is C-C motif chemokine ligand 2 (CCL2)/monocyte chemoattractant protein 1 (MCP1). Therefore, we tested the effect of this chemokine in an animal model of IDILI. Specifically, amodiaquine (AQ), which is known to cause IDILI in humans, causes mild liver injury in wild-type C57BL/6 mice that resolves despite continued AQ treatment, but it causes more severe liver injury that does not resolve in PD-1^-/- mice co-treated with anti-CTLA-4 to impair immune tolerance. CCR2^-/- mice treated with AQ were not protected from the expected AQ-induced liver injury seen in wild-type C57BL/6 mice. In contrast, anti-CCL2 antibodies attenuated the liver injury caused by AQ in the impaired immune tolerance model. The difference in response of the two models is likely due to a difference in the IDILI mechanism; the mild injury in wild-type animals is mediated by NK cells, while the more serious injury in the impaired immune tolerance model requires CD8 T-cells. The results from these experiments provide evidence that macrophage infiltration into the liver may not be involved in mild IDILI mediated by the innate immune system, but it does appear necessary in more severe IDILI involving cytotoxic T-cells.

Use of an animal model to test whether non-alcoholic fatty liver disease increases the risk of idiosyncratic drug-induced liver injury

Clinical evidence suggests that most idiosyncratic drug-induced liver injury (IDILI) is immune-mediated. The danger hypothesis suggests that liver injury and inflammation would increase the risk of an immune response leading to IDILI. Therefore, a reasonable hypothesis would be that an underlying chronic liver disease such as non-alcoholic steatohepatitis (NASH) would increase the risk of developing IDILI due to inflammation and release of danger signals from damaged cells. In order to test this hypothesis, mice were fed a methionine-/choline-deficient (MCD) diet that produces a consistent NASH phenotype, along with amodiaquine (AQ) - a drug known to cause IDILI in humans. This study employed both wild-type C57BL/6 mice and PD-1^-/- mice co-treated with anti-CTLA-4 antibodies. The PD-1^-/- + anti-CTLA-4 model produces an immune-mediated liver injury very similar to the idiosyncratic liver injury observed in humans. The liver injury observed in the present experiment was dominated by the injury caused by the MCD diet; there was no significant difference between mice treated with the MCD diet alone and those also treated with AQ, whether in wild-type mice of the PD-1^-/- model. Therefore, the MCD diet, which results in a state that mimics NASH, did not appear to increase the liver injury associated with AQ treatment. Ultimately, an animal model is just that - only a model, and cannot provide a definitive answer to clinical questions. However, given the difficulty of performing clinical studies with appropriate control populations, the present results provide important evidence to support a general clinical finding that underlying liver injury does not usually increase the risk of IDILI.

Mechanisms of Inflammatory Liver Injury and Drug-Induced Hepatotoxicity

PURPOSE OF REVIEW

This article provides a brief overview of mechanisms of inflammatory liver injury and how this applies to drug hepatotoxicity with a particular emphasis on the role of inflammation in acetaminophen-induced liver injury.

RECENT FINDINGS

Significant progress has been made in the last decade in our understanding of the initiation of sterile inflammation after necrotic cell death by the release of damage-associated molecular patterns and their recognition by toll-like receptors and others on macrophages. These events trigger the formation of cytokines and chemokines directly or with assistance of inflammasome activation thereby activating and recruiting leukocytes including neutrophils and monocyte-derived macrophages into the necrotic areas. Although this sterile inflammatory response is mainly geared towards the removal of necrotic cell debris and preparation of regeneration, there are conditions where these innate immune cells can aggravate the initial injury. The mechanisms and controversial findings of the innate immunity are being discussed in detail. In contrast, drug metabolism and formation of a reactive metabolite that binds to proteins in the absence of extensive cell death, can induce an adaptive immune response, which eventually also results in severe liver injury. However, the initiating event appears to be the formation of protein adducts, which act as haptens to activate an adaptive immune response. Overall, these mechanisms are less well understood.

SUMMARY

The past decade has revolutionized our understanding of the mechanisms that control the interplay between cell death and innate or adaptive immune responses. This report provides an update on these mechanisms.

Animal models of drug-induced liver injury

Drug-induced liver injury (DILI) presents unique challenges for consumers, clinicians, and regulators. It is the most common cause of acute liver failure in the US. It is also one of the most common reasons for termination of new drugs during pre-clinical testing and withdrawal of new drugs post-marketing. DILI is generally divided into two forms: intrinsic and idiosyncratic. Many of the challenges with DILI are due in large part to poor understanding of the mechanisms of toxicity. Although useful models of intrinsic DILI are available, they are frequently misused. Modeling idiosyncratic DILI presents greater challenges, but promising new models have recently been developed. The purpose of this manuscript is to provide a critical review of the most popular animal models of DILI, and to discuss the future of DILI research.

Do In Vitro Assays Predict Drug Candidate Idiosyncratic Drug-Induced Liver Injury Risk?

In vitro assays are commonly used during drug discovery to try to decrease the risk of idiosyncratic drug-induced liver injury (iDILI). But how effective are they at predicting risk? One of the most widely used methods evaluates cell cytotoxicity. Cytotoxicity assays that used cell lines that are very different from normal hepatocytes, and high concentrations of drug, were not very accurate at predicting idiosyncratic drug reaction risk. Even cytotoxicity assays that use more biologically normal cells resulted in many false-positive and false-negative results. Assays that quantify reactive metabolite formation, mitochondrial injury, and bile salt export pump (BSEP) inhibition have also been described. Although evidence suggests that reactive metabolite formation and BSEP inhibition can play a role in the mechanism of iDILI, these assays are not very accurate at predicting risk. In contrast, inhibition of the mitochondrial electron transport chain appears not to play an important role in the mechanism of iDILI, although other types of mitochondrial injury may do so. It is likely that there are many additional mechanisms by which drugs can cause iDILI. However, simply measuring more parameters is unlikely to provide better predictive assays unless those parameters are actually involved in the mechanism of iDILI. Hence, a better mechanistic understanding of iDILI is required; however, mechanistic studies of iDILI are very difficult. There is substantive evidence that most iDILI is immune mediated; therefore, the most accurate assays may involve those that determine immune responses to drugs. New methods to manipulate immune tolerance may greatly facilitate development of more suitable methods.

The Effects of Immune Modulators on Amodiaquine-Induced Liver Injury

If idiosyncratic drug-induced liver injury (IDILI) is immune mediated, then it is logical that immune modulators may be able to affect liver injury caused by a drug. We have previously shown that modulating the immune system by impairing programmed cell death protein (PD-1) and cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) signaling, both receptors involved in immune tolerance, was capable of producing an animal model of amodiaquine (AQ) IDILI with characteristics very similar to IDILI in humans. Other immune modulators may also increase liver injury caused by drugs that cause IDILI in humans. In this study, myeloid derived suppressor cells (MDSCs), transforming growth factor beta (TGF-β), and lymphocyte-activation gene 3 (LAG3) were targeted with antibodies, with and without PD-1 and CTLA-4 impairment. We found that anti-Gr1 antibodies used to deplete MDSCs led to a significant increase in AQ-induced liver injury in wild-type mice; however, the injury was actually less in PD-1^-/- mice, with or without anti-CTLA-4, and it was less than we have previously observed in PD-1^-/- mice combined with anti-CTLA-4 without anti-Gr1. Addition of anti-LAG3 or anti-TGF-β antibodies produced a small increase ALT in AQ-treated wild-type mice. There was a significant increase in ALT in PD-1^-/- mice co-treated with anti-LAG3 or anti-TGF-β relative to AQ-treated wild-type mice. In the case of TGF-β, this was further increased by the addition of anti-CTLA-4, but if anything, there appeared to be a paradoxical decrease when anti-CTLA-4 was combined with anti-LAG3. Overall, the results from this study were not always as expected, and they highlight the complexity of the immune response, in particular immune tolerance, which appears to be the dominant immune response to drugs that cause IDILI.

Hepatotoxicity of immune checkpoint inhibitors: An evolving picture of risk associated with a vital class of immunotherapy agents

Immune checkpoint inhibitors (ICIs) block CTLA-4, PD-1 and PD-L1, or other molecules that control antitumour activities of lymphocytes. These products are associated with a broad array of immune-related toxicities affecting a variety of organs, including the liver. ICI-associated immune-mediated hepatitis (IMH) ranges in severity between mild and life-threatening and is marked by findings that bear both similarities as well as differences with idiopathic autoimmune hepatitis. Hepatotoxic events are often detected in clinical trials of ICIs that are powered for efficacy. Risk levels for ICI-induced liver injury may be impacted by the specific checkpoint molecule targeted for treatment, the ICI dose levels, and the presence of a pre-existing autoimmune diathesis, chronic infection or tumour cells which infiltrate the liver parenchyma. When patients develop liver injury during ICI treatment, a prompt assessment of the cause of injury, in conjunction with the application of measures to optimally manage the adverse event, should be made. Strategies to manage the risk of IMH include the performance of pretreatment liver tests with regular monitoring during and after ICI treatment and patient education. Using Common Terminology Criteria for Adverse Events developed at the National Cancer Institute to measure the severity level of liver injury, recommended actions may include continued ICI treatment with close patient monitoring, ICI treatment suspension or discontinuation and/or administration of corticosteroids or, when necessary, a non-steroidal immunosuppressive agent. The elucidation of reliable predictors of tumour-specific ICI treatment responses, as well as an increased susceptibility for clinically serious immune-related adverse events, would help optimize treatment decisions for individual patients.