An Optimized Human Erythroblast Differentiation System Reveals Cholesterol-Dependency of Robust Production of Cultured Red Blood Cells Ex Vivo

The generation of cultured red blood cells (cRBCs) ex vivo represents a potentially unlimited source for RBC transfusion and other cell therapies. Human cRBCs can be generated from the terminal differentiation of proliferating erythroblasts derived from hematopoietic stem/progenitor cells or erythroid precursors in peripheral blood mononuclear cells. Efficient differentiation and maturation into cRBCs highly depend on replenishing human plasma, which exhibits variable potency across donors or batches and complicates the consistent cRBC production required for clinical translation. Hence, the role of human plasma in erythroblast terminal maturation is investigated and uncovered that 1) a newly developed cell culture basal medium mimicking the metabolic profile of human plasma enhances cell growth and increases cRBC yield upon erythroblast terminal differentiation and 2) LDL-carried cholesterol, as a substitute for human plasma, is sufficient to support erythroid survival and terminal differentiation ex vivo. Consequently, a chemically-defined optimized medium (COM) is developed, enabling robust generation of cRBCs from erythroblasts of multiple origins, with improved enucleation efficiency and higher reticulocyte yield, without the need for supplementing human plasma or serum. In addition, the results reveal the crucial role of lipid metabolism during human terminal erythropoiesis.

A decellularized matrix enriched collagen microscaffold for a 3D in vitro liver model

The development of liver scaffolds retaining their three-dimensional (3D) structure and extra-cellular matrix (ECM) composition is essential for the advancement of liver tissue engineering. We report the design and validation of an alginate-based platform using a combination of decellularized matrices and collagen to preserve the functionality of liver cells. The scaffolds were characterized using SEM and fluorescence microscopy techniques. The proliferation and functional behaviours of hepatocellular carcinoma HuH7 cells were observed. It was found that the decellularized skin scaffold with collagen was better for maintaining the growth of cells in comparison to other decellularized matrices. In addition, we observed a significant increase in the functional profile once exogenous collagen was added to the liver matrix. Our study also suggests that a cirrhotic liver model should have a different matrix composition as compared to a healthy liver model. When primary rat hepatocytes were used for developing a healthy liver model, the proliferation studies with hepatocytes showed a decellularized skin matrix as the better option, but the functionality was only maintained in a decellularized liver matrix with addition of exogenous collagen. We further checked if these platforms can be used for studying drug induced toxicity observed in the liver by studying the activation of cytochrome P450 upon drug exposure of the cells growing in our model. We observed a significant induction of the CYP1A1 gene on administering the drugs for 6 days. Thus, this platform could be used for drug-toxicity screening studies using primary hepatocytes in a short span of time. Being a microscaffold based system, this platform offers some advantages, such as smaller volumes of samples, analysing multiple samples simultaneously and a minimal amount of decellularized matrix in the matrix composition, making it an economical option compared to a completely dECM based platform.

Coculture model of a liver sinusoidal endothelial cell barrier and HepG2/C3a spheroids-on-chip in an advanced fluidic platform

The liver is one of the main organs involved in the metabolism of xenobiotics and a key organ in toxicity studies. Prior to accessing the hepatocytes, xenobiotics pass through the hepatic sinusoid formed by liver sinusoidal endothelial cells (LSECs). The LSECs barrier regulates the kinetics and concentrations of the xenobiotics before their metabolic processing by the hepatocytes. To mimic this physiological situation, we developed an in vitro model reproducing an LSECs barrier in coculture with a hepatocyte biochip, using a fluidic platform. This technology made dynamic coculture and tissue crosstalk possible. SK-HEP-1 and HepG2/C3a cells were used as LSECs and as hepatocyte models, respectively. We confirmed the LSECs phenotype by measuring PECAM-1 and stabilin-2 expression levels and the barrier's permeability/transport properties with various molecules. The tightness of the SK-HEP-1 barrier was enhanced in the dynamic coculture. The morphology, albumin secretion, and gene expression levels of markers of HepG2/C3a were not modified by coculture with the LSECs barrier. Using acetaminophen, a well-known hepatotoxic drug, to study tissue crosstalk, there was a reduction in the expression levels of the LSECs markers stabilin-2 and PECAM-1, and a modification of those of CLEC4M and KDR. No HepG2/C3a toxicity was observed. The metabolisation of acetaminophen by HepG2/C3a monocultures and cocultures was confirmed. Although primary cells are required to propose a fully relevant model, the present approach highlights the potential of our system for investigating xenobiotic metabolism and toxicity.

Advances in the application of 3D tumor models in precision oncology and drug screening

Traditional tumor models cannot perfectly simulate the real state of tumors in vivo, resulting in the termination of many clinical trials. 3D tumor models’ technology provides new in vitro models that bridge the gap between in vitro and in vivo findings, and organoids maintain the properties of the original tissue over a long period of culture, which enables extensive research in this area. In addition, they can be used as a substitute for animal and in vitro models, and organoids can be established from patients’ normal and malignant tissues, with unique advantages in clinical drug development and in guiding individualized therapies. 3D tumor models also provide a promising platform for high-throughput research, drug and toxicity testing, disease modeling, and regenerative medicine. This report summarizes the 3D tumor model, including evidence regarding the 3D tumor cell culture model, 3D tumor slice model, and organoid culture model. In addition, it provides evidence regarding the application of 3D tumor organoid models in precision oncology and drug screening. The aim of this report is to elucidate the value of 3D tumor models in cancer research and provide a preclinical reference for the precise treatment of cancer patients.

The hollow porous sphere cell carrier for the dynamic 3D cell culture

Large-scale mammalian cell culture is essential in stem cell-based therapy, the production of the vaccine, and the manufacturing of therapeutic protein drugs. Due to the adherent growth characteristic of most mammalian cell types, the combination of cell carrier and bioreactor is a common choice in large-scale mammalian cell culture. Cell carriers are usually developed by polymer crosslinking, lithography, and emulsion drops; however, all these methods are difficult to control the uniformed porous structure and porous interior design. Therefore, unable to optimize the dynamic culture condition for cell proliferation, matrix production, and cell differentiation. Here we use fused deposition modelling (FDM) 3D printing technology to fabricate hollow porous spheres (HPS), based on which a novel dynamic 3D culture system has been established. In the meantime, computational fluid dynamics (CFD) simulations were conducted to study liquid flow behaviour in HPS. A dynamic cell seeding was developed and refined using the 3D culture system, which increased 32% (roughly) seeding efficiency compared to the traditional static cell seeding method. The cell proliferation analysis demonstrated that HPSs could speed up cell growth in dynamic cell culture. The HPS with a honeycomb-like structure showed the highest inner pore velocity (CFD analysis) and achieved the fastest cell proliferation and the highest cell viability. Overall, our study, for the first time, developed a 3D printed HPS cell culture device with a uniformed porous structure, which can effectively facilitate cell adhesion and proliferation in the dynamic cultural environment, thereby could be considered an ideal carrier candidate for the manufacturing of cells and cell-based products. Furthermore, this study provides a novel 3D dynamic culture system that can be further applied in cell culture and research in the future.

Liver organ-on-chip models for toxicity studies and risk assessment

The liver is a key organ that plays a pivotal role in metabolism and ensures a variety of functions in the body, including homeostasis, synthesis of essential components, nutrient storage, and detoxification. As the centre of metabolism for exogenous molecules, the liver is continuously exposed to a wide range of compounds, such as drugs, pesticides, and environmental pollutants. Most of these compounds can cause hepatotoxicity and lead to severe and irreversible liver damage. To study the effects of chemicals and drugs on the liver, most commonly, animal models or in vitro 2D cell cultures are used. However, data obtained from animal models lose their relevance when extrapolated to the human metabolic situation and pose ethical concerns, while 2D static cultures are poorly predictive of human in vivo metabolism and toxicity. As a result, there is a widespread need to develop relevant in vitro liver models for toxicology studies. In recent years, progress in tissue engineering, biomaterials, microfabrication, and cell biology has created opportunities for more relevant in vitro models for toxicology studies. Of these models, the liver organ-on-chip (OoC) has shown promising results by reproducing the in vivo behaviour of the cell/organ or a group of organs, the controlled physiological micro-environment, and in vivo cellular metabolic responses. In this review, we discuss the development of liver organ-on-chip technology and its use in toxicity studies. First, we introduce the physiology of the liver and summarize the traditional experimental models for toxicity studies. We then present liver OoC technology, including the general concept, materials used, cell sources, and different approaches. We review the prominent liver OoC and multi-OoC integrating the liver for drug and chemical toxicity studies. Finally, we conclude with the future challenges and directions for developing or improving liver OoC models.

Rapid Detection of Direct Compound Toxicity and Trailing Detection of Indirect Cell Metabolite Toxicity in a 96-Well Fluidic Culture Device for Cell-Based Screening Environments: Tactics in Six Sigma Quality Control Charts

Microfluidic screening tools, in vitro, evolve amid varied scientific disciplines. One emergent technique, simultaneously assessing cell toxicity from a primary compound and ensuing cell-generated metabolites (dual-toxicity screening), entails in-line systems having sequentially aligned culture chambers. To explore dual-tox screens, we probe the dissemination of nutrients involving 1-way transport with upstream compound dosing, midstream cascading flows, and downstream cessation. Distribution of flow gives rise to broad concentration ranges of dosing compound (0→ICcompound100) and wide-ranging concentration ranges of generated cell metabolites (0→ICmetabolites100). Innately, single-pass unidirectional flow retains 1st pass informative traits across the network, composed of nine interconnected culture wells, preserving both compound and cell-secreted byproducts as data indicators in each adjacent culture chamber. Thereafter, to assess effective compound hepatotoxicity (0→ECcompound100) and simultaneously classify for cell-metabolite toxicity (0→ECmetabolite100), we reveal utility by analyzing culture viability against ramping exposures of acetaminophen (APAP) and nefazodone (NEF), compounds of hepatic significance. We then discern metabolite generation with an emphasis on amplification across µchannel multiwell sites. Lastly, using conventional cell functions as indicator tools to assess dual toxicity, we investigate a non-drug induced liver injury (non-DILI) compound and DILI compound. The technology is for predictive evaluations of new compound formulations, new chemical entities (NCE), or drugs that have previously failed testing for unresolved reasons.

Microphysiological systems in absorption, distribution, metabolism, and elimination sciences

The use of microphysiological systems (MPS) to support absorption, distribution, metabolism, and elimination (ADME) sciences has grown substantially in the last decade, in part driven by regulatory demands to move away from traditional animal-based safety assessment studies and industry desires to develop methodologies to efficiently screen and characterize drugs in the development pipeline. The past decade of MPS development has yielded great user-driven technological advances with the collective fine-tuning of cell culture techniques, fluid delivery systems, materials engineering, and performance enhancing modifications. The rapid advances in MPS technology have now made it feasible to evaluate critical ADME parameters within a stand-alone organ system or through interconnected organ systems. This review surveys current MPS developed for liver, kidney, and intestinal systems as stand-alone or interconnected organ systems, and evaluates each system for specific performance criteria recommended by regulatory authorities and MPS leaders that would render each system suitable for evaluating drug ADME. Whereas some systems are more suitable for ADME type research than others, not all system designs were intended to meet the recently published desired performance criteria and are reported as a summary of initial proof-of-concept studies.

Three-Dimensional Cell Culture Models of Hepatocellular Carcinoma - a Review

INTRODUCTION

Three-dimensional (3D) cell culture studies are becoming extremely common because of their capability to mimic tumor architecture, such as cell-cell and cell-ECM interactions, more efficiently than 2D monolayer systems. These interactions have important roles in defining the tumor cell behaviors, such as proliferation, differentiation, and most importantly, tumor drug response.

OBJECTIVE

This review aims to provide an overview of the methods for 3D tumor spheroid formation to model human tumors, specifically concentrated on studies using hepatocellular carcinoma (HCC) cells.

METHOD

We obtained information from previously published articles. In this review, there is discussion of the scaffold and non-scaffold-based approaches, including hanging drop, bioreactors and 3D bioprinting.

RESULTS AND CONCLUSION

The mimicking of the tumor microenvironment (TME) as tumor spheroids could provide a valuable platform for studying tumor biology. Multicellular tumor spheroids are self-assembled cultures of mixed cells (tumor and stromal cells) organized in a 3D arrangement. These spheroids closely mimic the main features of human solid tumors, such as structural organization, central hypoxia, and overall oxygen and nutrient gradients. Hepatocellular carcinoma (HCC) is the most common liver malignancy, and most difficult to overcome because of its drug resistance and tumor heterogeneity. In order to mimic this highly heterogeneous environment, 3D cell culture systems are needed.

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.

Prediction of hepatic drug clearance with a human microfluidic four-cell liver acinus microphysiology system

Predicting human hepatic clearance remains a fundamental challenge in both pharmaceutical drug development and toxicological assessments of environmental chemicals, with concerns about both accuracy and precision of in vitro-derived estimates. Suggested sources of these issues have included differences in experimental protocols, differences in cell sourcing, and use of a single cell type, liver parenchymal cells (hepatocytes). Here we investigate the ability of human microfluidic four-cell liver acinus microphysiology system (LAMPS) to make predictions as to hepatic clearance for seven representative compounds: Caffeine, Pioglitazone, Rosiglitazone, Terfenadine, Tolcapone, Troglitazone, and Trovafloxacin. The model, whose reproducibility was recently confirmed in an inter-lab comparison, was constructed using primary human hepatocytes or human induced pluripotent stem cell (iPSC)-derived hepatocytes and 3 human cell lines for the endothelial, Kupffer and stellate cells. We calculated hepatic clearance estimates derived from experiments using LAMPS or traditional 2D cultures and compared the outcomes with both in vivo human clinical study-derived and in vitro human hepatocyte suspension culture-derived values reported in the literature. We found that, compared to in vivo clinically-derived values, the LAMPS model with iPSC-derived hepatocytes had higher precision as compared to primary cells in suspension or 2D culture, but, consistent with previous studies in other microphysiological systems, tended to underestimate in vivo clearance. Overall, these results suggest that use of LAMPS and iPSC-derived hepatocytes together with an empirical scaling factor warrants additional study with a larger set of compounds, as it has the potential to provide more accurate and precise estimates of hepatic clearance.

Identification of NQO2 As a Protein Target in Small Molecule Modulation of Hepatocellular Function

The utility of in vitro human disease models is mainly dependent on the availability and functional maturity of tissue-specific cell types. We have previously screened for and identified small molecules that can enhance hepatocyte function in vitro. Here, we characterize the functional effects of one of the hits, FH1, on primary human hepatocytes in vitro, and also in vivo on primary hepatocytes in a zebrafish model. Furthermore, we conducted an analogue screen to establish the structure-activity relationship of FH1. We performed affinity-purification proteomics that identified NQO2 to be a potential binding target for this small molecule, revealing a possible link between inflammatory signaling and hepatocellular function in zebrafish and human hepatocyte model systems.

Multi-organ-on-a-chip for pharmacokinetics and toxicokinetic study of drugs

Introduction: Accurate prediction of pharmacokinetic (PK) and toxicokinetics (TK) of drugs is imperative for successful development of new pharmaceutics. Although conventional in vitro methods for predicting the PK and TK of drugs are well established, limitations still exist and more advanced chip-based in vitro platforms combined with mathematical models can help researchers overcome the limitations. Areas covered: We will review recent progress in the development of multi-organ-on-a-chip platforms for predicting PK and TK of drugs, as well as mathematical approaches that can be combined with these platforms for experiment design, data analysis and in vitro-in vivo extrapolation (IVIVE) for application to humans. Expert opinion: Although there remain some challenges to be addressed, the remarkable progress in the area of multi-organ-on-a-chip in recent years indicate that we will see tangible outcomes that can be utilized in the pharmaceutical industry in near future.

Hepatic transporter-mediated pharmacokinetic drug-drug interactions: Recent studies and regulatory recommendations

Transporter-mediated drug-drug interactions are one of the major mechanisms in pharmacokinetic-based drug interactions and correspondingly affecting drugs' safety and efficacy. Regulatory bodies underlined the importance of the evaluation of transporter-mediated interactions as a part of the drug development process. The liver is responsible for the elimination of a wide range of endogenous and exogenous compounds via metabolism and biliary excretion. Therefore, hepatic uptake transporters, expressed on the sinusoidal membranes of hepatocytes, and efflux transporters mediating the transport from hepatocytes to the bile are determinant factors for pharmacokinetics of drugs, and hence, drug-drug interactions. In parallel with the growing research interest in this area, regulatory guidances have been updated with detailed assay models and criteria. According to well-established preclinical results, observed or expected hepatic transporter-mediated drug-drug interactions can be taken into account for clinical studies. In this paper, various methods including in vitro, in situ, in vivo, in silico approaches, and combinational concepts and several clinical studies on the assessment of transporter-mediated drug-drug interactions were reviewed. Informative and effective evaluation by preclinical tools together with the integration of pharmacokinetic modeling and simulation can reduce unexpected clinical outcomes and enhance the success rate in drug development.

Microphysiological Systems: A Pathologist's Perspective

High-throughput in vitro models lack human-relevant complexity, which undermines their ability to accurately mimic in vivo biologic and pathologic responses. The emergence of microphysiological systems (MPS) presents an opportunity to revolutionize in vitro modeling for both basic biomedical research and applied drug discovery. The MPS platform has been an area of interdisciplinary collaboration to develop new, predictive, and reliable in vitro methods for regulatory acceptance. The current MPS models have been developed to recapitulate an organ or tissue on a smaller scale. However, the complexity of these models (ie, including all cell types present in the in vivo tissue) with appropriate structural, functional, and biochemical attributes are often not fully characterized. Here, we provide an overview of the capabilities and limitations of the microfluidic MPS model (aka organs-on-chips) within the scope of drug development. We recommend the engagement of pathologists early in the MPS design, characterization, and validation phases, because this will enable development of more robust and comprehensive MPS models that can accurately replicate normal biology and pathophysiology and hence be more predictive of human responses.

Microphysiological systems for ADME-related applications: current status and recommendations for system development and characterization

Over the last decade, progress has been made on the development of microphysiological systems (MPS) for absorption, distribution, metabolism, and excretion (ADME) applications. Central to this progress has been proof of concept data generated by academic and industrial institutions followed by broader characterization studies, which provide evidence for scalability and applicability to drug discovery and development. In this review, we describe some of the advances made for specific tissue MPS and outline the desired functionality for such systems, which are likely to make them applicable for practical use in the pharmaceutical industry. Single organ MPS platforms will be valuable for modelling tissue-specific functions. However, dynamic organ crosstalk, especially in the context of disease or toxicity, can only be obtained with the use of inter-linked MPS models which will enable scientists to address questions at the intersection of pharmacokinetics (PK) and efficacy, or PK and toxicity. In the future, successful application of MPS platforms that closely mimic human physiology may ultimately reduce the need for animal models to predict ADME outcomes and decrease the overall risk and cost associated with drug development.

Liver microphysiological systems development guidelines for safety risk assessment in the pharmaceutical industry

The liver is critical to consider during drug development because of its central role in the handling of xenobiotics, a process which often leads to localized and/or downstream tissue injury. Our ability to predict human clinical safety outcomes with animal testing is limited due to species differences in drug metabolism and disposition, while traditional human in vitro liver models often lack the necessary in vivo physiological fidelity. To address this, increasing numbers of liver microphysiological systems (MPS) are being developed, however the inconsistency in their optimization and characterization often leads to models that do not possess critical levels of baseline performance that is required for many pharmaceutical industry applications. Herein we provide a guidance on best approaches to benchmark liver MPS based on 3 stages of characterization that includes key performance metrics and a 20 compound safety test set. Additionally, we give an overview of frequently used liver injury safety assays, describe the ideal MPS model, and provide a perspective on currently best suited MPS contexts of use. This pharmaceutical industry guidance has been written to help MPS developers and end users identify what could be the most valuable models for safety risk assessment.

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

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

A high-throughput microfluidic microphysiological system (PREDICT-96) to recapitulate hepatocyte function in dynamic, re-circulating flow conditions

Microphysiological systems (MPSs) are dynamic cell culture systems that provide micro-environmental and external cues to support physiologically relevant, organ-specific functions. Recent progresses in MPS fabrication technologies have enabled the development of advanced models to capture microenvironments with physiological relevance, while increasing throughput and reducing material-based artefacts. In addition to conventional cell culture systems, advanced MPSs are emerging as ideal contenders for disease modeling and incorporation into drug screening. Since liver is a central organ for drug metabolism, liver-on-chip models have been developed to recapitulate hepatic microenvironment with varying complexities, while allowing long-term culture. Recently, we have developed a novel thermoplastic, oxygen-permeable MPS for primary human hepatocyte (PHH) culture. Herein, we have adapted and extended the MPS to a) a 96 microfluidic array (PREDICT-96 array) and b) integrated a novel, ultra-low volume, re-circulating pumping system (PREDICT-96 pump) - collectively known as the PREDICT-96 platform. The PREDICT-96 platform conforms to the industrial standard 96-well footprint and enables media re-circulation. First, we demonstrate the introduction of PHHs into the PREDICT-96 array using standard handling procedures for multi-well plates and allow cells to stabilize in static conditions. Next, we introduce recirculating flow into the bottom channel (using PREDICT-96 pump) to mimic mass transport in vivo. Our results indicate an increase in metabolic and secretory functions of PHHs in the PREDICT-96 platform, and their maintenance over 10 days of flow. Furthermore, long-term culture with fluid flow allows for the periodic introduction of media components (e.g., fatty acids, cytokines) and capture cellular responses to chronic stimuli. The low-volume footprint of the pump and small media volume in the MPS allow for the interrogation of hepatic responses incorporating secretion feedback to a stimulus, which is essential for disease model development and drug interrogation. We envision future development of this liver model to incorporate key primary hepatic cells, multi-cellular co-cultures and adaptation, integration with high-throughput analytical tools.

Emerging Models of Drug Metabolism, Transporters, and Toxicity

This commentary summarizes expert mini-reviews and original research articles that have been assembled in a special issue on novel models of drug metabolism and disposition. The special issue consists of research articles or reviews on novel static or micro-flow based models of the intestine, liver, eye, and kidney. This issue reviews static intestinal systems like mucosal scrapings and cryopreserved intestinal enterocytes, as well as novel bioengineered or chemically engineered intestinal models derived from primary human tissue, iPSCs, enteroids, and crypts. Experts have reviewed hepatic systems like cryopermeabilized Metmax hepatocytes and longer term, hepatocyte coculture system from HµREL, yielding in vivo-like primary and secondary drug metabolite profiles. Additional liver models, including micropattern hepatocyte coculture, 3D liver spheroids, and microflow systems, applicable to the study of drug disposition and toxicology have also been reviewed. In this commentary, we have outlined expert opinions and current efforts on hepatic- and nephrotoxicity models. Ocular disposition models including corneal permeability models have been included within this special issue. This commentary provides a summary of in vivo mini-reviews of the issue, which have discussed the applications and drawbacks of pig and humanized mice models of P450, UGT, and rat organic anionic transporting polypeptide 1a4. While not extensively reviewed, novel positron emissions tomography imaging-based approaches to study the distribution of xenobiotics have been highlighted. This commentary also outlines in vitro and in vivo models of drug metabolism derived from breakthrough genetic, chromosomal, and tissue engineering techniques. The commentary concludes by providing a futuristic view of the novel models discussed in this issue.

Physiologically Relevant, Humanized Intestinal Systems to Study Metabolism and Transport of Small Molecule Therapeutics

Intestinal disposition of small molecules involves interplay of drug metabolizing enzymes (DMEs), transporters, and host-microbiome interactions, which has spurred the development of in vitro intestinal models derived from primary tissue sources. Such models have been bioengineered from intestinal crypts, mucosal extracts, induced pluripotent stem cell (iPSC)-derived organoids, and human intestinal tissue. This minireview discusses the utility and limitations of these human-derived models in support of small molecule drug metabolism and disposition. Enteroids from human intestinal crypts, organoids derived from iPSCs using growth factors or small molecule compounds, and enterocytes extracted from mucosal scrapings show key absorptive cell morphology while are limited in quantitative applications due to the lack of accessibility to the apical compartment, the lack of monolayers, or low expression of key DMEs, transporters, and nuclear hormone receptors. Despite morphogenesis to epithelial cells, similar challenges have been reported by more advanced technologies that have explored the impact of flow and mechanical stretch on proliferation and differentiation of Caco-2 cells. Most recently, bioengineered human intestinal epithelial or ileal cells have overcome many of the challenges, as the DME and transporter expression pattern resembles that of native intestinal tissue. Engineering advances may improve such models to support longer-term applications and meet end-user needs. Biochemical characterization and transcriptomic, proteomic, and functional endpoints of emerging novel intestinal models, when referenced to native human tissue, can provide greater confidence and increased utility in drug discovery and development.