Advances in Engineered Liver Models for Investigating Drug-Induced Liver Injury

Mechanobiological stimulation in organ-on-a-chip systems reduces hepatic drug metabolic capacity in favor of regenerative specialization

Hepatic physiology depends on the liver's complex structural composition which among others, provides high oxygen supply rates, locally differential oxygen tension, endothelial paracrine signaling, as well as residual hemodynamic shear stress to resident hepatocytes. While functional improvements were shown by implementing these factors into hepatic culture systems, direct cause-effect relationships are often not well characterized-obfuscating their individual contribution in more complex microphysiological systems. By comparing increasingly complex hepatic in vitro culture systems that gradually implement these parameters, we investigate the influence of the cellular microenvironment to overall hepatic functionality in pharmacological applications. Here, hepatocytes were modulated in terms of oxygen tension and supplementation, endothelial coculture, and exposure to fluid shear stress delineated from oxygen influx. Results from transcriptomic and metabolomic evaluation indicate that particularly oxygen supply rates are critical to enhance cellular functionality-with cellular drug metabolism remaining comparable to physiological conditions after prolonged static culture. Endothelial signaling was found to be a major contributor to differential phenotype formation known as metabolic zonation, indicated by WNT pathway activity. Lastly, oxygen-delineated shear stress was identified to direct cellular fate towards increased hepatic plasticity and regenerative phenotypes at the cost of drug metabolic functionality - in line with regenerative effects observed in vivo. With these results, we provide a systematic evaluation of critical parameters and their impact in hepatic systems. Given their adherence to physiological effects in vivo, this highlights the importance of their implementation in biomimetic devices, such as organ-on-a-chip systems. Considering recent advances in basic liver biology, direct translation of physiological structures into in vitro models is a promising strategy to expand the capabilities of pharmacological models.

Compound Absorption in Polymer Devices Impairs the Translatability of Preclinical Safety Assessments

Organotypic and microphysiological systems (MPS) that can emulate the molecular phenotype and function of human tissues, such as liver, are increasingly used in preclinical drug development. However, despite their improved predictivity, drug development success rates have remained low with most compounds failing in clinical phases despite promising preclinical data. Here, it is tested whether absorption of small molecules to polymers commonly used for MPS fabrication can impact preclinical pharmacological and toxicological assessments and contribute to the high clinical failure rates. To this end, identical devices are fabricated from eight different MPS polymers and absorption of prototypic compounds with different physicochemical properties are analyzed. It is found that overall absorption is primarily driven by compound hydrophobicity and the number of rotatable bonds. However, absorption can differ by >1000-fold between polymers with polydimethyl siloxane (PDMS) being most absorptive, whereas polytetrafluoroethylene (PTFE) and thiol-ene epoxy (TEE) absorbed the least. Strikingly, organotypic primary human liver cultures successfully flagged hydrophobic hepatotoxins in lowly absorbing TEE devices at therapeutically relevant concentrations, whereas isogenic cultures in PDMS devices are resistant, resulting in false negative safety signals. Combined, these results can guide the selection of MPS materials and facilitate the development of preclinical assays with improved translatability.

Meeting report of the second European Biotransformation Workshop

Challenges and opportunities in the field of biotransformation were presented and discussed at the 2nd European Biotransformation workshop which was conducted virtually in collaboration with the DMDG on November 24/25, 2021. Here we summarise the presentations and discussions from this workshop.The following topics were covered:Regulatory requirements and biotransformation studies for antibody drug conjugates (ADCs) and antisense oligonucleotides (ASOs)Solutions for mass spectral data processing of peptides and oligonucleotidesFuture outsourcing needs in biotransformation for new modalitiesEstablished quantitative and qualitative workflows for metabolite identificationNew in vitro systems to study new chemical entities (NCEs) with low metabolic turnoverNew strategies on the timing of the human ADME (absorption, distribution, metabolism, excretion) study and to investigate the impact of human microbiome on drug development.

Microfluidic Organ-on-a-Chip Devices for Liver Disease Modeling In Vitro

Mortality from liver disease conditions continues to be very high. As liver diseases manifest and progress silently, prompt measures after diagnosis are essential in the treatment of these conditions. Microfluidic organs-on-chip platforms have significant potential for the study of the pathophysiology of liver diseases in vitro. Different liver-on-a-chip microphysiological platforms have been reported to study cell-signaling pathways such as those activating stellate cells within liver diseases. Moreover, the drug efficacy for liver conditions might be evaluated on a cellular metabolic level. Here, we present a comprehensive review of microphysiological platforms used for modelling liver diseases. First, we briefly introduce the concept and importance of organs-on-a-chip in studying liver diseases in vitro, reflecting on existing reviews of healthy liver-on-a-chip platforms. Second, the techniques of cell cultures used in the microfluidic devices, including 2D, 3D, and spheroid cells, are explained. Next, the types of liver diseases (NAFLD, ALD, hepatitis infections, and drug injury) on-chip are explained for a further comprehensive overview of the design and methods of developing liver diseases in vitro. Finally, some challenges in design and existing solutions to them are reviewed

Influence of Liver Extracellular Matrix in Predicting Drug-Induced Liver Injury: An Alternate Paradigm

In vitro drug-induced liver injury (DILI) models are promising tools for drug development to predict adverse events during clinical usage. However, the currently available DILI models are not specific or not able to predict the injury accurately. This is believed to be mainly because of failure to conserve the hepatocyte phenotype, lack of longevity, and difficulty in maintaining the tissue-specific microenvironment. In this study, we have assessed the potential of decellularized liver extracellular matrix (DLM) in retaining the hepatic cellular phenotype and functionality in the presence of a tissue-specific microenvironment along with its role in influencing the effect of the drug on hepatic cells. We show that DLM helps maintain the phenotype of the hepatic cell line HepG2, a well-known cell line for secretion of human proteins that is easily available. Also, the DLM enhanced the expression of a metabolic marker carbamoyl phosphate synthetase I (CPS1), a regulator of urea cycle, and bile salt export pump (BSEP), a marker of hepatocyte polarity. We further validated the DLM for its influence on the sensitivity of cells toward different classes of drugs. Interestingly, the coculture model, in the presence of endothelial cells and stellate cells, exhibited a higher sensitivity for both acetaminophen and trovafloxacin, a toxic compound that does not show any toxicity on preclinical screening. Thus, our results demonstrate for the first time that a multicellular combination along with DLM can be a potential and reliable DILI model to screen multiple drugs.

Organotypic and Microphysiological Human Tissue Models for Drug Discovery and Development-Current State-of-the-Art and Future Perspectives

The number of successful drug development projects has been stagnant for decades despite major breakthroughs in chemistry, molecular biology, and genetics. Unreliable target identification and poor translatability of preclinical models have been identified as major causes of failure. To improve predictions of clinical efficacy and safety, interest has shifted to three-dimensional culture methods in which human cells can retain many physiologically and functionally relevant phenotypes for extended periods of time. Here, we review the state of the art of available organotypic culture techniques and critically review emerging models of human tissues with key importance for pharmacokinetics, pharmacodynamics, and toxicity. In addition, developments in bioprinting and microfluidic multiorgan cultures to emulate systemic drug disposition are summarized. We close by highlighting important trends regarding the fabrication of organotypic culture platforms and the choice of platform material to limit drug absorption and polymer leaching while supporting the phenotypic maintenance of cultured cells and allowing for scalable device fabrication. We conclude that organotypic and microphysiological human tissue models constitute promising systems to promote drug discovery and development by facilitating drug target identification and improving the preclinical evaluation of drug toxicity and pharmacokinetics. There is, however, a critical need for further validation, benchmarking, and consolidation efforts ideally conducted in intersectoral multicenter settings to accelerate acceptance of these novel models as reliable tools for translational pharmacology and toxicology. SIGNIFICANCE STATEMENT: Organotypic and microphysiological culture of human cells has emerged as a promising tool for preclinical drug discovery and development that might be able to narrow the translation gap. This review discusses recent technological and methodological advancements and the use of these systems for hit discovery and the evaluation of toxicity, clearance, and absorption of lead compounds.

Efficient Generation of Functional Hepatocytes from Human Induced Pluripotent Stem Cells for Disease Modeling and Disease Gene Discovery

In vitro hepatocyte cell models are being used to study the pathogenesis of liver disease and in the discovery and preclinical stages of drug development. The culture of hepatic cell lines and primary hepatocytes as in vitro cell models has been carried out for several decades. However, hepatic cell lines (hepatic carcinoma generated or immortalized) have limited accuracy when recapitulating complex physiological functions of the liver. Additionally, primary hepatocytes sourced from human cadavers or medical biopsies are difficult to obtain due to sourcing limitations, particularly for large-scale population studies or in applications requiring large number of cells. Hepatocyte cultures differentiated from human embryonic stem cells (ESCs) and induced pluripotent stem cell (iPSCs) overcome in large part the limitations of traditional hepatocyte in vitro models. In this chapter, we described an efficient protocol routinely used in our laboratory to differentiate human iPSCs into functional hepatocyte cultures for in vitro modeling of liver function and disease. The protocol uses a three-stage differentiation strategy to generate functional hepatocytes from human iPSCs. The differentiated cells show characteristic hepatocyte morphology including flat and polygonal shape, distinct round nuclei, and presence of biliary canaliculi and they express hepatic markers alpha-fetoprotein (AFP), albumin (ALB), E-cadherin (CHD1), hepatocyte nuclear factor 4 alpha (HNF4α), and actin.

Bioprinting of Human Liver-Derived Epithelial Organoids for Toxicity Studies

There is a need for long-lived hepatic in vitro models to better predict drug induced liver injury (DILI). Human liver-derived epithelial organoids are a promising cell source for advanced in vitro models. Here, organoid technology is combined with biofabrication techniques, which holds great potential for the design of in vitro models with complex and customizable architectures. Here, porous constructs with human hepatocyte-like cells derived from organoids are generated using extrusion-based printing technology. Cell viability of bioprinted organoids remains stable for up to ten days (88-107% cell viability compared to the day of printing). The expression of hepatic markers, transporters, and phase I enzymes increased compared to undifferentiated controls, and is comparable to non-printed controls. Exposure to acetaminophen, a well-known hepatotoxic compound, decreases cell viability of bioprinted liver organoids to 21-51% (p < 0.05) compared to the start of exposure, and elevated levels of damage marker miR-122 are observed in the culture medium, indicating the potential use of the bioprinted constructs for toxicity testing. In conclusion, human liver-derived epithelial organoids can be combined with a biofabrication approach, thereby paving the way to create perfusable, complex constructs which can be used as toxicology- and disease-models.

Human Three-Dimensional Hepatic Models: Cell Type Variety and Corresponding Applications

Owing to retained hepatic phenotypes and functions, human three-dimensional (3D) hepatic models established with diverse hepatic cell types are thought to recoup the gaps in drug development and disease modeling limited by a conventional two-dimensional (2D) cell culture system and species-specific variability in drug metabolizing enzymes and transporters. Primary human hepatocytes, human hepatic cancer cell lines, and human stem cell-derived hepatocyte-like cells are three main hepatic cell types used in current models and exhibit divergent hepatic phenotypes. Primary human hepatocytes derived from healthy hepatic parenchyma resemble in vivo-like genetic and metabolic profiling. Human hepatic cancer cell lines are unlimitedly reproducible and tumorigenic. Stem cell-derived hepatocyte-like cells derived from patients are promising to retain the donor's genetic background. It has been suggested in some studies that unique properties of cell types endue them with benefits in different research fields of in vitro 3D modeling paradigm. For instance, the primary human hepatocyte was thought to be the gold standard for hepatotoxicity study, and stem cell-derived hepatocyte-like cells have taken a main role in personalized medicine and regenerative medicine. However, the comprehensive review focuses on the hepatic cell type variety, and corresponding applications in 3D models are sparse. Therefore, this review summarizes the characteristics of different cell types and discusses opportunities of different cell types in drug development, liver disease modeling, and liver transplantation.

Organoid engineering with microfluidics and biomaterials for liver, lung disease, and cancer modeling

As life expectancy improves and the number of people suffering from various diseases increases, the need for developing effective personalized disease models is rapidly rising. The development of organoid technology has led to better recapitulation of the in vivo environment of organs, and can overcome the constraints of existing disease models. However, for more precise disease modeling, engineering approaches such as microfluidics and biomaterials, that aid in mimicking human physiology, need to be integrated with the organoid models. In this review, we introduce key elements for disease modeling and recent engineering advances using both liver and lung organoids. Due to the importance of personalized medicine, we also emphasize patient-derived cancer organoid models and their engineering approaches. These organoid-based disease models combined with microfluidics, biomaterials, and co-culture systems will provide a powerful research platform for understanding disease mechanisms and developing precision medicine; enabling preclinical drug screening and drug development. STATEMENT OF SIGNIFICANCE: The development of organoid technology has led to better recapitulation of the in vivo environment of organs, and can overcome the constraints of existing disease models. However, for more precise disease modeling, engineering approaches such as microfluidics and biomaterials, that aid in mimicking human physiology, need to be integrated with the organoid models. In this review, we introduce liver, lung, and cancer organoids integrated with various engineering approaches as a novel platform for personalized disease modeling. These engineered organoid-based disease models will provide a powerful research platform for understanding disease mechanisms and developing precision medicine.

Insulin-dependent glucose consumption dynamics in 3D primary human liver cultures measured by a sensitive and specific glucose sensor with nanoliter input volume

The liver plays a central role in glucose homeostasis and hepatic insulin resistance constitutes a key feature of type 2 diabetes. However, platforms that accurately mimic human hepatic glucose disposition and allow for rapid and scalable quantification of glucose consumption dynamics are lacking. Here, we developed and optimized a colorimetric glucose assay based on the glucose oxidase-peroxidase system and demonstrate that the system can monitor glucose consumption in 3D primary human liver cell cultures over multiple days. The system was highly sensitive (limit of detection of 3.5 µM) and exceptionally accurate (R²  = 0.999) while requiring only nanoliter input volumes (250 nL), enabling longitudinal profiling of individual liver microtissues. By utilizing a novel polymer, off-stoichiometric thiol-ene (OSTE), and click-chemistry based on thiol-Michael additions, we furthermore show that the assay can be covalently bound to custom-build chips, facilitating the integration of the sensor into microfluidic devices. Using this system, we find that glucose uptake of our 3D human liver cultures closely resembles human hepatic glucose uptake in vivo as measured by euglycemic-hyperinsulinemic clamp. By comparing isogenic insulin-resistant and insulin-sensitive liver cultures we furthermore show that insulin and extracellular glucose levels account for 55% and 45% of hepatic glucose consumption, respectively. In conclusion, the presented data show that the integration of accurate and scalable nanoliter glucose sensors with physiologically relevant organotypic human liver models enables longitudinal profiling of hepatic glucose consumption dynamics that will facilitate studies into the biology and pathobiology of glycemic control, as well as antidiabetic drug screening.

Latest impact of engineered human liver platforms on drug development

Drug-induced liver injury (DILI) is a leading cause of drug attrition, which is partly due to differences between preclinical animals and humans in metabolic pathways. Therefore, in vitro human liver models are utilized in biopharmaceutical practice to mitigate DILI risk and assess related mechanisms of drug transport and metabolism. However, liver cells lose phenotypic functions within 1–3 days in two-dimensional monocultures on collagen-coated polystyrene/glass, which precludes their use to model the chronic effects of drugs and disease stimuli. To mitigate such a limitation, bioengineers have adapted tools from the semiconductor industry and additive manufacturing to precisely control the microenvironment of liver cells. Such tools have led to the fabrication of advanced two-dimensional and three-dimensional human liver platforms for different throughput needs and assay endpoints (e.g., micropatterned cocultures, spheroids, organoids, bioprinted tissues, and microfluidic devices); such platforms have significantly enhanced liver functions closer to physiologic levels and improved functional lifetime to >4 weeks, which has translated to higher sensitivity for predicting drug outcomes and enabling modeling of diseased phenotypes for novel drug discovery. Here, we focus on commercialized engineered liver platforms and case studies from the biopharmaceutical industry showcasing their impact on drug development. We also discuss emerging multi-organ microfluidic devices containing a liver compartment that allow modeling of inter-tissue crosstalk following drug exposure. Finally, we end with key requirements for engineered liver platforms to become routine fixtures in the biopharmaceutical industry toward reducing animal usage and providing patients with safe and efficacious drugs with unprecedented speed and reduced cost.

Recent developments in in vitro and in vivo models for improved translation of preclinical pharmacokinetics and pharmacodynamics data

Improved pharmacokinetics/pharmacodynamics (PK/PD) prediction in the early stages of drug development is essential to inform lead optimization strategies and reduce attrition rates. Recently, there have been significant advancements in the development of new in vitro and in vivo strategies to better characterize pharmacokinetic properties and efficacy of drug leads. Herein, we review advances in experimental and mathematical models for clearance predictions, advancements in developing novel tools to capture slowly metabolized drugs, in vivo model developments to capture human etiology for supporting drug development, limitations and gaps in these efforts, and a perspective on the future in the field.

3D human liver spheroids for translational pharmacology and toxicology

Drug development is a failure-prone endeavour, and more than 85% of drugs fail during clinical development, showcasing that current preclinical systems for compound selection are clearly inadequate. Liver toxicity remains a major reason for safety failures. Furthermore, all efforts to develop pharmacological therapies for a variety of chronic liver diseases, such as non-alcoholic steatohepatitis (NASH) and fibrosis, remain unsuccessful. Considering the time and expense of clinical trials, as well as the substantial burden on patients, new strategies are thus of paramount importance to increase clinical success rates. To this end, human liver spheroids are becoming increasingly utilized as they allow to preserve patient-specific phenotypes and functions for multiple weeks in culture. We here review the recent application of such systems for i) predictive and mechanistic analyses of drug hepatotoxicity, ii) the evaluation of hepatic disposition and metabolite formation of low clearance drugs and iii) the development of drugs for metabolic and infectious liver diseases, including NASH, fibrosis, malaria and viral hepatitis. We envision that with increasing dissemination, liver spheroids might become the new gold standard for such applications in translational pharmacology and toxicology.

Physiologically inspired culture medium prolongs the lifetime and insulin sensitivity of human hepatocytes in micropatterned co-cultures

Given significant species-specific differences in liver functions, cultures of primary human hepatocytes (PHHs) are useful for assessing drug metabolism and to mitigate the risk of drug-induced hepatotoxicity in humans. While significant advances have been made to keep PHHs highly functional for 2-4 weeks in vitro, especially upon co-culture with both liver- and non-liver-derived non-parenchymal cells (NPCs), the functional lifespan of PHHs is 200-400 days in vivo. Therefore, it is desirable to determine culture conditions that can further prolong PHHs functions in vitro for modeling chronic drug exposure, disease pathogenesis, and to provide flexibility to the end-user for staggering drug incubations across multiple culture batches. Most PHH culture platforms utilize supraphysiologic levels of glucose and insulin and bovine-derived serum when including NPCs, which can alter PHH functions. Therefore, here we developed a culture medium containing physiologic levels of glucose (5 mM), insulin (500 pM), and human serum (10 % v/v) and tested its effects on micropatterned co-cultures (MPCCs) in which PHHs are organized onto collagen domains of empirically optimized dimensions and surrounded by 3T3-J2 murine fibroblasts that express liver-like molecules and induce higher PHH functions than liver-derived NPCs. Our physiologically-inspired culture medium allowed better retention of PHH morphology, polarity, and functions (albumin and urea, cytochrome-P450 activities, and sensitivity to insulin-mediated inhibition of gluconeogenesis) for up to 10 weeks relative to the traditional medium. Finally, PHHs in the physiologic medium displayed clinically-relevant responses to prototypical drugs for hepatoxicity and cytochrome-P450 induction. Ultimately, our physiologic culture medium could find broader utility for the continued development of PHH-NPC co-cultures for drug development, investigating the effects of patient-derived sera on PHH functions and disease phenotypes, and for use in cell-based therapies.

Application of the Extended Clearance Classification System (ECCS) in Drug Discovery and Development: Selection of Appropriate In Vitro Tools and Clearance Prediction

In vitro to in vivo extrapolation (IVIVE) to predict human hepatic clearance, including metabolism and transport, requires extensive experimental resources. In addition, there may be technical challenges to measure low clearance values. Therefore, prospective identification of rate-determining step(s) in hepatic clearance through application of the Extended Clearance Classification System (ECCS) could be beneficial for optimal compound characterization. IVIVE for hepatic intrinsic clearance (CLint,h) prediction is conducted for a set of 36 marketed drugs with low-to-high in vivo clearance, which are substrates of metabolic enzymes and active uptake transporters in the liver. The compounds were assigned to the ECCS classes, and CLint,h, estimated with HepatoPac (a micropatterned hepatocyte coculture system), was compared with values calculated based on suspended hepatocyte incubates. An apparent permeability threshold (apical to basal) of 50 nm/s in LLC-PK1 cells proved optimal for ECCS classification. A reasonable performance of the IVIVE for compounds across multiple classes using HepatoPac was achieved (with 2-3-fold error), except for substrates of uptake transporters (class 3b), for which scaling of uptake clearance using plated hepatocytes is more appropriate. Irrespective of the ECCS assignment, metabolic clearance can be estimated well using HepatoPac. The validation and approach elaborated in the present study can result in proposed decision trees for the selection of the optimal in vitro assays guided by ECCS class assignment, to support compound optimization and candidate selection. SIGNIFICANCE STATEMENT: Characterization of the rate-determining step(s) in hepatic elimination could be on the critical path of compound optimization during drug discovery. This study demonstrated that HepatoPac and plated hepatocytes are suitable tools for the estimation of metabolic and active uptake clearance, respectively, for a larger set of marketed drugs, supporting a comprehensive strategy to select optimal in vitro tools and to achieve Extended Clearance Classification System-dependent in vitro to in vivo extrapolation for human clearance prediction.

Application of zebrafish to safety evaluation in drug discovery

Traditionally, safety evaluation at the early stage of drug discovery research has been done using in silico, in vitro, and in vivo systems in this order because of limitations on the amount of compounds available and the throughput ability of the assay systems. While these in vitro assays are very effective tools for detecting particular tissue-specific toxicity phenotypes, it is difficult to detect toxicity based on complex mechanisms involving multiple organs and tissues. Therefore, the development of novel high throughput in vivo evaluation systems has been expected for a long time. The zebrafish (Danio rerio) is a vertebrate with many attractive characteristics for use in drug discovery, such as a small size, transparency, gene and protein similarity with mammals (80% or more), and ease of genetic modification to establish human disease models. Actually, in recent years, the zebrafish has attracted interest as a novel experimental animal. In this article, the author summarized the features of zebrafish that make it a suitable laboratory animal, and introduced and discussed the applications of zebrafish to preclinical toxicity testing, including evaluations of teratogenicity, hepatotoxicity, and nephrotoxicity based on morphological findings, evaluation of cardiotoxicity using functional endpoints, and assessment of seizure and drug abuse liability.

Liver Bioreactor Design Issues of Fluid Flow and Zonation, Fibrosis, and Mechanics: A Computational Perspective

Tissue engineering, with the goal of repairing or replacing damaged tissue and organs, has continued to make dramatic science-based advances since its origins in the late 1980’s and early 1990’s. Such advances are always multi-disciplinary in nature, from basic biology and chemistry through physics and mathematics to various engineering and computer fields. This review will focus its attention on two topics critical for tissue engineering liver development: (a) fluid flow, zonation, and drug screening, and (b) biomechanics, tissue stiffness, and fibrosis, all within the context of 3D structures. First, a general overview of various bioreactor designs developed to investigate fluid transport and tissue biomechanics is given. This includes a mention of computational fluid dynamic methods used to optimize and validate these designs. Thereafter, the perspective provided by computer simulations of flow, reactive transport, and biomechanics responses at the scale of the liver lobule and liver tissue is outlined, in addition to how bioreactor-measured properties can be utilized in these models. Here, the fundamental issues of tortuosity and upscaling are highlighted, as well as the role of disease and fibrosis in these issues. Some idealized simulations of the effects of fibrosis on lobule drug transport and mechanics responses are provided to further illustrate these concepts. This review concludes with an outline of some practical applications of tissue engineering advances and how efficient computational upscaling techniques, such as dual continuum modeling, might be used to quantify the transition of bioreactor results to the full liver scale.

Functionality of primary hepatic non-parenchymal cells in a 3D spheroid model and contribution to acetaminophen hepatotoxicity

In addition to hepatocytes, the liver comprises a host of specialised non-parenchymal cells which are important to consider in the development of in vitro models which are both physiologically and toxicologically relevant. We have characterized a 3D co-culture system comprising primary human hepatocytes (PHH) and non-parenchymal cells (NPC) and applied it to the investigation of acetaminophen-induced toxicity. Firstly, we titrated ratios of PHH:NPC and confirmed the presence of functional NPCs via both immunohistochemistry and activation with both LPS and TGF-β. Based on these data we selected a ratio of 2:1 PHH:NPC for further studies. We observed that spheroids supplemented with NPCs were protected against acetaminophen (APAP) toxicity as determined by ATP (up to threefold difference in EC₅₀ at day 14 compared to hepatocytes alone) and glutathione depletion, as well as miR-122 release. APAP metabolism was also altered in the presence of NPCs, with significantly lower levels of APAP-GSH detected. Expression of several CYP450 enzymes involved in the bioactivation of APAP was also lower in NPC-containing spheroids. Spheroids containing NPCs also expressed higher levels of miRNAs which have been implicated in APAP-induced hepatotoxicity, including miR-382 and miR-155 which have potential roles in liver regeneration and inflammation, respectively. These data indicate that the interaction between hepatocytes and NPCs can have significant metabolic and toxicological consequences important for the correct elucidation of hepatic safety mechanisms.

Emerging trends in modeling human liver disease in vitro

The liver executes 500+ functions, such as protein synthesis, xenobiotic metabolism, bile production, and metabolism of carbohydrates/fats/proteins. Such functions can be severely degraded by drug-induced liver injury, nonalcoholic fatty liver disease, hepatitis B and viral infections, and hepatocellular carcinoma. These liver diseases, which represent a significant global health burden, are the subject of novel drug discovery by the pharmaceutical industry via the use of in vitro models of the human liver, given significant species-specific differences in disease profiles and drug outcomes. Isolated primary human hepatocytes (PHHs) are a physiologically relevant cell source to construct such models; however, these cells display a rapid decline in the phenotypic function within conventional 2-dimensional monocultures. To address such a limitation, several engineered platforms have been developed such as high-throughput cellular microarrays, micropatterned cocultures, self-assembled spheroids, bioprinted tissues, and perfusion devices; many of these platforms are being used to coculture PHHs with liver nonparenchymal cells to model complex cell cross talk in liver pathophysiology. In this perspective, we focus on the utility of representative platforms for mimicking key features of liver dysfunction in the context of chronic liver diseases and liver cancer. We further discuss pending issues that will need to be addressed in this field moving forward. Collectively, these in vitro liver disease models are being increasingly applied toward the development of new therapeutics that display an optimal balance of safety and efficacy, with a focus on expediting development, reducing high costs, and preventing harm to patients.

Current insights in the complexities underlying drug-induced cholestasis

Drug-induced cholestasis (DIC) poses a major challenge to the pharmaceutical industry and regulatory agencies. It causes both drug attrition and post-approval withdrawal of drugs. DIC represents itself as an impaired secretion and flow of bile, leading to the pathological hepatic and/or systemic accumulation of bile acids (BAs) and their conjugate bile salts. Due to the high number of mechanisms underlying DIC, predicting a compound's cholestatic potential during early stages of drug development remains elusive. A profound understanding of the different molecular mechanisms of DIC is, therefore, of utmost importance. Although many knowledge gaps and caveats still exist, it is generally accepted that alterations of certain hepatobiliary membrane transporters and changes in hepatocellular morphology may cause DIC. Consequently, liver models, which represent most of these mechanisms, are valuable tools to predict human DIC. Some of these models, such as membrane-based in vitro models, are exceptionally well-suited to investigate specific mechanisms (i.e. transporter inhibition) of DIC, while others, such as liver slices, encompass all relevant biological processes and, therefore, offer a better representation of the in vivo situation. In the current review, we highlight the principal molecular mechanisms associated with DIC and offer an overview and critical appraisal of the different liver models that are currently being used to predict the cholestatic potential of drugs.

The application of omics-based human liver platforms for investigating the mechanism of drug-induced hepatotoxicity in vitro

Drug-induced liver injury (DILI) complicates safety assessment for new drugs and poses major threats to both patient health and drug development in the pharmaceutical industry. A number of human liver cell-based in vitro models combined with toxicogenomics methods have been developed as an alternative to animal testing for studying human DILI mechanisms. In this review, we discuss the in vitro human liver systems and their applications in omics-based drug-induced hepatotoxicity studies. We furthermore present bioinformatic approaches that are useful for analyzing toxicogenomic data generated from these models and discuss their current and potential contributions to the understanding of mechanisms of DILI. Human pluripotent stem cells, carrying donor-specific genetic information, hold great potential for advancing the study of individual-specific toxicological responses. When co-cultured with other liver-derived non-parenchymal cells in a microfluidic device, the resulting dynamic platform enables us to study immune-mediated drug hypersensitivity and accelerates personalized drug toxicology studies. A flexible microfluidic platform would also support the assembly of a more advanced organs-on-a-chip device, further bridging gap between in vitro and in vivo conditions. The standard transcriptomic analysis of these cell systems can be complemented with causality-inferring approaches to improve the understanding of DILI mechanisms. These approaches involve statistical techniques capable of elucidating regulatory interactions in parts of these mechanisms. The use of more elaborated human liver models, in harmony with causality-inferring bioinformatic approaches will pave the way for establishing a powerful methodology to systematically assess DILI mechanisms across a wide range of conditions.

Comprehensive Evaluation of Organotypic and Microphysiological Liver Models for Prediction of Drug-Induced Liver Injury

Drug-induced liver injury (DILI) is a major concern for the pharmaceutical industry and constitutes one of the most important reasons for the termination of promising drug development projects. Reliable prediction of DILI liability in preclinical stages is difficult, as current experimental model systems do not accurately reflect the molecular phenotype and functionality of the human liver. As a result, multiple drugs that passed preclinical safety evaluations failed due to liver toxicity in clinical trials or postmarketing stages in recent years. To improve the selection of molecules that are taken forward into the clinics, the development of more predictive in vitro systems that enable high-throughput screening of hepatotoxic liabilities and allow for investigative studies into DILI mechanisms has gained growing interest. Specifically, it became increasingly clear that the choice of cell types and culture method both constitute important parameters that affect the predictive power of test systems. In this review, we present current 3D culture paradigms for hepatotoxicity tests and critically evaluate their utility and performance for DILI prediction. In addition, we highlight possibilities of these emerging platforms for mechanistic evaluations of selected drug candidates and present current research directions towards the further improvement of preclinical liver safety tests. We conclude that organotypic and microphysiological liver systems have provided an important step towards more reliable DILI prediction. Furthermore, we expect that the increasing availability of comprehensive benchmarking studies will facilitate model dissemination that might eventually result in their regulatory acceptance.

Severe Hepatotoxicity due to Ibrutinib with a Review of Published Cases

Ibrutinib, an irreversible Bruton's tyrosine kinase inhibitor, is an effective treatment for Waldenström's macroglobulinemia, chronic lymphocytic leukemia, and several other types of lymphoma. Studies prior to FDA approval in 2015 failed to demonstrate any hepatotoxicity. However, since then, there have been 2 reports in the literature of severe hepatic injury. We present a third case of a 77-year-old woman presenting with nausea and jaundice after recent discontinuation of ibrutinib and compare the presentation as well as course of all 3 known cases. The sudden onset of acute hepatotoxicity is idiosyncratic, occurring weeks after starting ibrutinib treatment. Liver biopsies in all cases revealed mixed hepatocellular and cholestatic features. Improvement progressed slowly upon discontinuation of ibrutinib. Awareness of ibrutinib hepatotoxicity, periodic surveillance of liver function tests, early recognition of any abnormalities, and prompt discontinuation of the medication are recommended.

Effect of Dioxins in Milk on the 3D Cultured Primary Buffalo Hepatocyte Model System

Cow and human milk have been reported to contain dioxins ranging from 0.023 to 26.46 and 0.88 to 19 pg/g of fat, respectively. However, the toxic effects of the dioxins in the milk in this range of concentrations were not explored. Therefore, considering the outbred livestock tissues as better models than inbred laboratory animals, the present study targeted to study the effect of dioxins present in the milk on three-dimensionally (3D) cultured buffalo primary hepatocyte spheroids. The spheroids were treated with a model dioxin, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), directly and also through milk fat at different concentrations (i.e, 0.02-20 pg/mL) for 24 h. Among the liver-cell-specific (ALB, HNF4α, and AFP) genes, a similar ALB and upregulated HNF4α expression at all treatments indicated the functional and transcriptionally active hepatocyte spheroids. Supportingly, no significant difference in the antiapoptotic gene expression between the treatments of milk fat and milk fat containing dioxins indicated the survivability of the spheroids during dioxin treatments. Among the selected TCDD responsive (CYP1A1, CYP1A2, AHR, CYP1B1, and TIPARP) genes, a nonsignificant increasing trend of the CYP1A1 expression was observed from 0.2 to 10 pg/mL of TCDD concentration through milk fat. This pattern was similar to the reported insensitive response of human primary hepatocytes toward dioxins than that of rat primary hepatocytes. This may indicate that the buffalo hepatocyte spheroids could be better models than rats for TCDD hepatotoxic studies. Further, TCDD in the milk in the range of 0.02-20 pg/mL concentration may not be very hepatotoxic.

Application of hepatocyte-like cells to enhance hepatic safety risk assessment in drug discovery

Hepatic stress and injury from drugs continues to be a major concern within the pharmaceutical industry, leading to preclinical and clinical attrition precautionary warnings and post-market withdrawal of drugs. There is a requirement for more predictive and mechanistically accurate models to aid risk assessment. Primary human hepatocytes, subject to isolation stress, cryopreservation, donor-to-donor variation and a relatively short period of functional capability in two-dimensional cultures, are not suitable for high-throughput screening procedures. There are two areas within the drug discovery pipeline that the generation of a stable, metabolically functional hepatocyte-like cell with unlimited supply would have major impact. First, in routine, cell health risk-assessment assays where hepatic cell lines are typically deployed. Second, at later stages of the drug discovery pipeline approaching candidate nomination where bespoke/investigational studies refining and understanding the risk to patients use patient-derived induced pluripotent stem cell (iPSC) hepatocytes retaining characteristics from the patient, e.g. HLA susceptibility alleles, iPSC hepatocytes with defined disease phenotypes or genetic characteristics that have the potential to make the hepatocyte more sensitive to a particular stress mechanism. Functionality of patient-centric hepatocyte-like cells is likely to be enhanced when coupled with emerging culture systems, such as three-dimensional spheroids or microphysiological systems. Ultimately, the aspiration to confidently use human-relevant in vitro models to predict human-specific hepatic toxicity depends on the integration of promising emerging technologies.This article is part of the theme issue 'Designer human tissue: coming to a lab near you'.

Microfabrication of liver and heart tissues for drug development

Drug-induced liver- and cardiotoxicity remain among the leading causes of preclinical and clinical drug attrition, marketplace drug withdrawals and black-box warnings on marketed drugs. Unfortunately, animal testing has proven to be insufficient for accurately predicting drug-induced liver- and cardiotoxicity across many drug classes, likely due to significant differences in tissue functions across species. Thus, the field of in vitro human tissue engineering has gained increasing importance over the last 10 years. Technologies such as protein micropatterning, microfluidics, three-dimensional scaffolds and bioprinting have revolutionized in vitro platforms as well as increased the long-term phenotypic stability of both primary cells and stem cell-derived differentiated cells. Here, we discuss advances in engineering approaches for constructing in vitro human liver and heart models with utility for drug development. Design features and validation data of representative models are presented to highlight major trends followed by the discussion of pending issues. Overall, bioengineered liver and heart models have significantly advanced our understanding of organ function and injury, which will prove useful for mitigating the risk of drug-induced organ toxicity to human patients, reducing animal usage for preclinical drug testing, aiding in the discovery of novel therapeutics against human diseases, and ultimately for applications in regenerative medicine.This article is part of the theme issue 'Designer human tissue: coming to a lab near you'.

Technological advancements for the development of stem cell-based models for hepatotoxicity testing

Stem cells are characterized by their self-renewal capacity and their ability to differentiate into multiple cell types of the human body. Using directed differentiation strategies, stem cells can now be converted into hepatocyte-like cells (HLCs) and therefore, represent a unique cell source for toxicological applications in vitro. However, the acquired hepatic functionality of stem cell-derived HLCs is still significantly inferior to primary human hepatocytes. One of the main reasons for this is that most in vitro models use traditional two-dimensional (2D) setups where the flat substrata cannot properly mimic the physiology of the human liver. Therefore, 2D-setups are progressively being replaced by more advanced culture systems, which attempt to replicate the natural liver microenvironment, in which stem cells can better differentiate towards HLCs. This review highlights the most recent cell culture systems, including scaffold-free and scaffold-based three-dimensional (3D) technologies and microfluidics that can be employed for culture and hepatic differentiation of stem cells intended for hepatotoxicity testing. These methodologies have shown to improve in vitro liver cell functionality according to the in vivo liver physiology and allow to establish stem cell-based hepatic in vitro platforms for the accurate evaluation of xenobiotics.

Application of human pluripotent stem cells and pluripotent stem cell-derived cellular models for assessing drug toxicity

Introduction: Human pluripotent stem cells (hPSCs) are capable of differentiating into all types of cells in the body and so provide suitable toxicology screening systems even for hard-to-obtain human tissues. Since hPSCs can also be generated from differentiated cells and current gene editing technologies allow targeted genome modifications, hPSCs can be applied for drug toxicity screening both in normal and disease-specific models. Targeted hPSC differentiation is still a challenge but cardiac, neuronal or liver cells, and complex cellular models are already available for practical applications. Areas covered: The authors review new gene-editing and cell-biology technologies to generate sensitive toxicity screening systems based on hPSCs. Then the authors present the use of undifferentiated hPSCs for examining embryonic toxicity and discuss drug screening possibilities in hPSC-derived models. The authors focus on the application of human cardiomyocytes, hepatocytes, and neural cultures in toxicity testing, and discuss the recent possibilities for drug screening in a 'body-on-a-chip' model system. Expert opinion: hPSCs and their genetically engineered derivatives provide new possibilities to investigate drug toxicity in human tissues. The key issues in this regard are still the selection and generation of proper model systems, and the interpretation of the results in understanding in vivo drug effects.

Advances in Engineered Human Liver Platforms for Drug Metabolism Studies

Metabolism in the liver often determines the overall clearance rates of many pharmaceuticals. Furthermore, induction or inhibition of the liver drug metabolism enzymes by perpetrator drugs can influence the metabolism of victim drugs (drug-drug interactions). Therefore, determining liver-drug interactions is critical during preclinical drug development. Unfortunately, studies in animals are often of limited value because of significant differences in the metabolic pathways of the liver across different species. To mitigate such limitations, the pharmaceutical industry uses a continuum of human liver models, ranging from microsomes to transfected cell lines and cultures of primary human hepatocytes (PHHs). Of these models, PHHs provide a balance of high-throughput testing capabilities together with a physiologically relevant cell type that exhibits all the characteristic enzymes, cofactors, and transporters. However, PHH monocultures display a rapid decline in metabolic capacity. Consequently, bioengineers have developed several tools, such as cellular microarrays, micropatterned cocultures, self-assembled and bioprinted spheroids, and perfusion devices, to enhance and stabilize PHH functions for ≥2 weeks. Many of these platforms have been validated for drug studies, whereas some have been adapted to include liver nonparenchymal cells that can influence hepatic drug metabolism in health and disease. Here, we focus on the design features of such platforms and their representative drug metabolism validation datasets, while discussing emerging trends. Overall, the use of engineered human liver platforms in the pharmaceutical industry has been steadily rising over the last 10 years, and we anticipate that these platforms will become an integral part of drug development with continued commercialization and validation for routine screening use.

The utility of HepaRG cells for bioenergetic investigation and detection of drug-induced mitochondrial toxicity

The importance of mitochondrial toxicity in drug-induced liver injury is well established. The bioenergetic phenotype of the HepaRG cell line was defined in order to assess their suitability as a model of mitochondrial hepatotoxicity. Bioenergetic phenotyping categorised the HepaRG cells as less metabolically active when measured beside the more energetic HepG2 cells. However, inhibition of mitochondrial ATP synthase induced an increase in glycolytic activity of both HepaRG and HepG2 cells suggesting an active Crabtree Effect in both cell lines. The suitability of HepaRG cells for the acute metabolic modification assay as a screen for mitotoxicity was confirmed using a panel of compounds, including both positive and negative mitotoxic compounds. Seahorse respirometry studies demonstrated that a statistically significant decrease in spare respiratory capacity is the first indication of mitochondrial dysfunction. Furthermore, based upon comparing changes in respiratory parameters to those of the positive controls, rotenone and carbonyl cyanide m-chlorophenyl hydrazone, compounds were categorised into two mechanistic groups; inhibitors or uncouplers of the electron transport chain. Overall, the findings from this study have demonstrated that HepaRG cells, despite having different resting bioenergetic phenotype to HepG2 cells are a suitable model to detect drug-induced mitochondrial toxicity with similar detection rates to HepG2 cells.

Why Drugs Fail in Late Stages of Development: Case Study Analyses from the Last Decade and Recommendations

New drug development is both resource and time intensive, where later clinical stages result in significant costs. We analyze recent late-stage failures to identify drugs where failures result from inadequate scientific advances as well as drugs where we believe pitfalls could have been avoided. These can be broadly classified into two categories: 1) where science is mature and the failures can be avoided through rigorous and prospectively determined decision-making criteria, scientific curiosity, and discipline to follow up on emerging findings; and 2) where problems encountered in Phase 3 failures cannot be explained at this time, as the science is not sufficiently advanced and companies/investigators need to recognize the possibility of deficiency of our knowledge. Through these case studies, key themes critical for successful drug development emerge-understanding the therapeutic pathway including receptor and signaling biology, pharmacological responses related to safety and efficacy, pharmacokinetics of the drug and exposure at target site, optimum dose, and dosing regimen; and identification of patient sub-populations likely to respond and will have a favorable benefit-risk profile, design of clinical trials, and a quantitative framework that can guide data-driven decision making. It is essential that the right studies are conducted early in the development process to answer the key questions, with the emphasis on learning in the early stages of development, whereas Phase 3 should be reserved for confirming the safety and efficacy. Utilization of innovative technology in identifying patients based on molecular signature of their disease, rapid assessment of pharmacological response, mechanistic modeling of emerging data, seamless operational processes to reduce start-up and wind-down time for clinical trials through use of electronic health records and data mining, and development of novel and objective clinical efficacy endpoints are some concepts for improving the success rate.

Bioengineered Liver Models for Drug Testing and Cell Differentiation Studies

In vitro models of the human liver are important for the following: (1) mitigating the risk of drug-induced liver injury to human beings, (2) modeling human liver diseases, (3) elucidating the role of single and combinatorial microenvironmental cues on liver cell function, and (4) enabling cell-based therapies in the clinic. Methods to isolate and culture primary human hepatocytes (PHHs), the gold standard for building human liver models, were developed several decades ago; however, PHHs show a precipitous decline in phenotypic functions in 2-dimensional extracellular matrix-coated conventional culture formats, which does not allow chronic treatment with drugs and other stimuli. The development of several engineering tools, such as cellular microarrays, protein micropatterning, microfluidics, biomaterial scaffolds, and bioprinting, now allow precise control over the cellular microenvironment for enhancing the function of both PHHs and induced pluripotent stem cell-derived human hepatocyte-like cells; long-term (4+ weeks) stabilization of hepatocellular function typically requires co-cultivation with liver-derived or non-liver-derived nonparenchymal cell types. In addition, the recent development of liver organoid culture systems can provide a strategy for the enhanced expansion of therapeutically relevant cell types. Here, we discuss advances in engineering approaches for constructing in vitro human liver models that have utility in drug screening and for determining microenvironmental determinants of liver cell differentiation/function. Design features and validation data of representative models are presented to highlight major trends followed by the discussion of pending issues that need to be addressed. Overall, bioengineered liver models have significantly advanced our understanding of liver function and injury, which will prove useful for drug development and ultimately cell-based therapies.

Epidermal Growth Factor Represses Constitutive Androstane Receptor Expression in Primary Human Hepatocytes and Favors Regulation by Pregnane X Receptor

Growth factors have key roles in liver physiology and pathology, particularly by promoting cell proliferation and growth. Recently, it has been shown that in mouse hepatocytes, epidermal growth factor receptor (EGFR) plays a crucial role in the activation of the xenosensor constitutive androstane receptor (CAR) by the antiepileptic drug phenobarbital. Due to the species selectivity of CAR signaling, here we investigated epidermal growth factor (EGF) role in CAR signaling in primary human hepatocytes. Primary human hepatocytes were incubated with CITCO, a human CAR agonist, or with phenobarbital, an indirect CAR activator, in the presence or absence of EGF. CAR-dependent gene expression modulation and PXR involvement in these responses were assessed upon siRNA-based silencing of the genes that encode CAR and PXR. EGF significantly reduced CAR expression and prevented gene induction by CITCO and, to a lower extent, by phenobarbital. In the absence of EGF, phenobarbital and CITCO modulated the expression of 144 and 111 genes, respectively, in primary human hepatocytes. Among these genes, only 15 were regulated by CITCO and one by phenobarbital in a CAR-dependent manner. Conversely, in the presence of EGF, CITCO and phenobarbital modulated gene expression only in a CAR-independent and PXR-dependent manner. Overall, our findings suggest that in primary human hepatocytes, EGF suppresses specifically CAR signaling mainly through transcriptional regulation and drives the xenobiotic response toward a pregnane X receptor (PXR)-mediated mechanism.

3D in vitro models of liver fibrosis

Animal testing is still the most popular preclinical assessment model for liver fibrosis. To develop efficient anti-fibrotic therapies, robust and representative in vitro models are urgently needed. The most widely used in vitro fibrosis model is the culture-induced activation of primary rodent hepatic stellate cells. While these cultures have contributed greatly to the current understanding of hepatic stellate cell activation, they seem to be inadequate to cover the complexity of this regenerative response. This review summarizes recent progress towards the development of 3D culture models of liver fibrosis. Thus far, only a few hepatic culture systems have successfully implemented hepatic stellate cells (or other non-parenchymal cells) into hepatocyte cultures. Recent advances in bioprinting, spheroid- and precision-cut liver slice cultures and the use of microfluidic bioreactors will surely lead to valid 3D in vitro models of liver fibrosis in the near future.

Drug-Induced Liver Injury: Cascade of Events Leading to Cell Death, Apoptosis or Necrosis

Drug-induced liver injury (DILI) can broadly be divided into predictable and dose dependent such as acetaminophen (APAP) and unpredictable or idiosyncratic DILI (IDILI). Liver injury from drug hepatotoxicity (whether idiosyncratic or predictable) results in hepatocyte cell death and inflammation. The cascade of events leading to DILI and the cell death subroutine (apoptosis or necrosis) of the cell depend largely on the culprit drug. Direct toxins to hepatocytes likely induce oxidative organelle stress (such as endoplasmic reticulum (ER) and mitochondrial stress) leading to necrosis or apoptosis, while cell death in idiosyncratic DILI (IDILI) is usually the result of engagement of the innate and adaptive immune system (likely apoptotic), involving death receptors (DR). Here, we review the hepatocyte cell death pathways both in direct hepatotoxicity such as in APAP DILI as well as in IDILI. We examine the known signaling pathways in APAP toxicity, a model of necrotic liver cell death. We also explore what is known about the genetic basis of IDILI and the molecular pathways leading to immune activation and how these events can trigger hepatotoxicity and cell death.

Scaffold-free 3D bio-printed human liver tissue stably maintains metabolic functions useful for drug discovery

The liver plays a central role in metabolism. Although many studies have described in vitro liver models for drug discovery, to date, no model has been described that can stably maintain liver function. Here, we used a unique, scaffold-free 3D bio-printing technology to construct a small portion of liver tissue that could stably maintain drug, glucose, and lipid metabolism, in addition to bile acid secretion. This bio-printed normal human liver tissue maintained expression of several kinds of hepatic drug transporters and metabolic enzymes that functioned for several weeks. The bio-printed liver tissue displayed glucose production via cAMP/protein kinase A signaling, which could be suppressed with insulin. Bile acid secretion was also observed from the printed liver tissue, and it accumulated in the culture medium over time. We observed both bile duct and sinusoid-like structures in the bio-printed liver tissue, which suggested that bile acid secretion occurred via a sinusoid-hepatocyte-bile duct route. These results demonstrated that our bio-printed liver tissue was unique, because it exerted diverse liver metabolic functions for several weeks. In future, we expect our bio-printed liver tissue to be applied to developing new models that can be used to improve preclinical predictions of long-term toxicity in humans, generate novel targets for metabolic liver disease, and evaluate biliary excretion in drug development.