Exposure‐Based Validation List for Developmental Toxicity Screening Assays

New approach methodologies to confirm developmental toxicity of pharmaceuticals based on weight of evidence

The aim of embryo-fetal developmental toxicity assessments for pharmaceuticals is to inform potential risk of adverse pregnancy outcome, which has traditionally relied on studies in pregnant animals. Recent updates to international safety guidelines (ICH S5R3) have incorporated information on how to use weight of evidence and alternative assays to reduce animal use while still informing risk of fetal harm. Uptake of these alternative approaches has been slow due to limitations in understanding how alternative assays translate to in vivo effects and then relevance to human exposure. To understand the predictivity of new approach methodologies for developmental toxicity (DevTox NAMs), we used two pharmaceutical examples (glasdegib and lorlatinib) to illustrate the value of DevTox NAMs to complement weight of evidence (WoE) assessments while considering the relationship of concentration-effect levels in NAMs to in vivo studies. The in vitro results generated in a battery of assays (mEST, rWEC, zebrafish, and human based stem cells) confirmed the WoE based on literature and further confirmed by preliminary embryo-fetal development data. The data generated for these two compounds supports integrating DevTox NAMs into the developmental toxicity assessment for advanced cancer indications.

RosetteArray® Platform for Quantitative High-Throughput Screening of Human Neurodevelopmental Risk

Neural organoids have revolutionized how human neurodevelopmental disorders (NDDs) are studied. Yet, their utility for screening complex NDD etiologies and in drug discovery is limited by a lack of scalable and quantifiable derivation formats. Here, we describe the RosetteArray® platform's ability to be used as an off-the-shelf, 96-well plate assay that standardizes incipient forebrain and spinal cord organoid morphogenesis as micropatterned, 3-D, singularly polarized neural rosette tissues (>9000 per plate). RosetteArrays are seeded from cryopreserved human pluripotent stem cells, cultured over 6-8 days, and immunostained images can be quantified using artificial intelligence-based software. We demonstrate the platform's suitability for screening developmental neurotoxicity and genetic and environmental factors known to cause neural tube defect risk. Given the presence of rosette morphogenesis perturbation in neural organoid models of NDDs and neurodegenerative disorders, the RosetteArray platform could enable quantitative high-throughput screening (qHTS) of human neurodevelopmental risk across regulatory and precision medicine applications.

Validation of a mouse 3D gastruloid-based embryotoxicity assay in reference to the ICH S5(R3) guideline chemical exposure list

There is growing interest in establishing alternative methods in place of conventional animal tests to assess the developmental and reproductive toxicity (DART) of chemicals. Gastruloids are 3D aggregates of pluripotent stem cells that spontaneously exhibit axial elongation morphogenesis similar to gastrulation. They have been explored as in vitro embryogenesis models for developmental and toxicological studies. Here, a mouse gastruloid-based assay was validated for DART assessment in accordance with the ICH S5(R3) guideline, which provides the plasma concentration data of various reference drugs in rodents, specifically Cmax and AUC for NOAEL and LOAEL. First, adverse effect concentrations of the reference drugs and their known metabolites on gastruloid development were determined based on morphological impact, namely reduced growth or aberrant elongation. Then, the NOAEL to LOAEL concentration range obtained from the gastruloid assay was compared with that in rodents to examine similarities in sensitivity between the in vitro and in vivo assays for each chemical. For 18 out of the 24 reference drugs that have both NOAEL and LOAEL information in rodents, the sensitivity of the gastruloid assay was comparable to the in vivo assay within an 8-fold concentration margin. For 7 out of the 8 additional reference drugs that have only NOAEL or LOAEL information in rodents, the gastruloid assay was in line with the in vivo data. Altogether, these results support the effectiveness of the gastruloid assay, which may be exploited as a non-animal alternative method for DART assessment.

Standardizing Extracted Data Using Automated Application of Controlled Vocabularies

BACKGROUND

Extraction of toxicological end points from primary sources is a central component of systematic reviews and human health risk assessments. To ensure optimal use of these data, consistent language should be used for end point descriptions. However, primary source language describing treatment-related end points can vary greatly, resulting in large labor efforts to manually standardize extractions before data are fit for use.

OBJECTIVES

To minimize these labor efforts, we applied an augmented intelligence approach and developed automated tools to support standardization of extracted information via application of preexisting controlled vocabularies.

METHODS

We created and applied a harmonized controlled vocabulary crosswalk, consisting of Unified Medical Language System (UMLS) codes, German Federal Institute for Risk Assessment (BfR) DevTox harmonized terms, and The Organization for Economic Co-operation and Development (OECD) end point vocabularies, to roughly 34,000 extractions from prenatal developmental toxicology studies conducted by the National Toxicology Program (NTP) and 6,400 extractions from European Chemicals Agency (ECHA) prenatal developmental toxicology studies, all recorded based on the original study report language.

RESULTS

We automatically applied standardized controlled vocabulary terms to 75% of the NTP extracted end points and 57% of the ECHA extracted end points. Of all the standardized extracted end points, about half (51%) required manual review for potential extraneous matches or inaccuracies. Extracted end points that were not mapped to standardized terms tended to be too general or required human logic to find a good match. We estimate that this augmented intelligence approach saved > 350 hours of manual effort and yielded valuable resources including a controlled vocabulary crosswalk, organized related terms lists, code for implementing an automated mapping workflow, and a computationally accessible dataset.

DISCUSSION

Augmenting manual efforts with automation tools increased the efficiency of producing a findable, accessible, interoperable, and reusable (FAIR) dataset of regulatory guideline studies. This open-source approach can be readily applied to other legacy developmental toxicology datasets, and the code design is customizable for other study types. https://doi.org/10.1289/EHP13215.

Interlaboratory Study on Zebrafish in Toxicology: Systematic Evaluation of the Application of Zebrafish in Toxicology's (SEAZIT's) Evaluation of Developmental Toxicity

Embryonic zebrafish represent a useful test system to screen substances for their ability to perturb development. The exposure scenarios, endpoints captured, and data analysis vary among the laboratories who conduct screening. A lack of harmonization impedes the comparison of the substance potency and toxicity outcomes across laboratories and may hinder the broader adoption of this model for regulatory use. The Systematic Evaluation of the Application of Zebrafish in Toxicology (SEAZIT) initiative was developed to investigate the sources of variability in toxicity testing. This initiative involved an interlaboratory study to determine whether experimental parameters altered the developmental toxicity of a set of 42 substances (3 tested in duplicate) in three diverse laboratories. An initial dose-range-finding study using in-house protocols was followed by a definitive study using four experimental conditions: chorion-on and chorion-off using both static and static renewal exposures. We observed reasonable agreement across the three laboratories as 33 of 42 test substances (78.6%) had the same activity call. However, the differences in potency seen using variable in-house protocols emphasizes the importance of harmonization of the exposure variables under evaluation in the second phase of this study. The outcome of the Def will facilitate future practical discussions on harmonization within the zebrafish research community.

The ever-growing world of gastruloids: autogenous models of mammalian embryogenesis

During early development, extrinsic cues prompt a collection of pluripotent cells to begin the extensive process of cellular differentiation that gives rise to all tissues in the mammalian embryo, a process known as gastrulation. Advances in stem cell biology have resulted in the generation of stem cell-based in vitro models of mammalian gastrulation called gastruloids. Gastruloids and subsequent gastruloid-based models are tractable, scalable and more accessible than mammalian embryos. As such, they have opened an unprecedented avenue for modelling in vitro self-organisation, patterning and fate specification. This review focuses on discussing the recent advances of this rapidly moving research area, clarifying what structures they model and the underlying signal hierarchy. We highlight the exciting potential of these models and where the field might be heading.

Toward better assessments of developmental toxicity using stem cell-based in vitro embryogenesis models

In the past few decades, pluripotent stem cells have been explored as nonanimal alternatives to assess the developmental toxicity of chemicals. To date, numerous versions of stem cell-based assays have been reported that are allegedly effective. Nonetheless, none of the assays has become the gold standard in developmental toxicity assessment. Why? This article discusses several issues in the hope of facilitating the refinement of stem cell assays and their acceptance as the cornerstone in predictive developmental toxicology. Each stem cell assay is built on a limited representation of embryogenesis, so that multiple assays are needed to detect the diverse effects of various chemicals. To validate and compare the strengths and weaknesses of individual assays, standardized lists of reference chemicals should be established. Reference lists should consist of exposures defined by toxicokinetic data, namely maternal plasma concentrations that cause embryonic death or malformations, and also by the effects on the molecular machineries that control embryogenesis. Although not entirely replacing human or animal tests, carefully selected stem cell assays should serve as practical and ethical alternatives to proactively identify chemical exposures that disturb embryogenesis. To achieve this goal, unprecedented levels of coordination and conviction are required among research and regulatory communities.

The DevTox Germ Layer Reporter Platform: An Assay Adaptation of the Human Pluripotent Stem Cell Test

Environmental chemical exposures are a contributing factor to birth defects affecting infant morbidity and mortality. The USA EPA is committed to developing new approach methods (NAMs) to detect chemical risks to susceptible populations, including pregnant women. NAM-based coverage for cellular mechanisms associated with early human development could enhance identification of potential developmental toxicants (DevTox) for new and existing data-poor chemicals. The human pluripotent stem cell test (hPST) is an in vitro test method for rapidly identifying potential human developmental toxicants that employs directed differentiation of embryonic stem cells to measure reductions in SOX17 biomarker expression and nuclear localization. The objective of this study was to expand on the hPST principles to develop a model platform (DevTox GLR) that utilizes the transgenic RUES2-GLR cell line expressing fluorescent reporter fusion protein biomarkers for SOX17 (endoderm marker), BRA (mesoderm marker), and SOX2 (ectoderm and pluripotency marker). Initial assay adaption to definitive endoderm (DevTox GLR-Endo) was performed to emulate the hPST SOX17 endpoint and enable comparative evaluation of concordant chemical effects. Assay duration was reduced to two days and screening throughput scaled to 384-well format for enhanced speed and efficiency. Assay performance for 66 chemicals derived from reference and training set data resulted in a balanced accuracy of 72% (79% sensitivity and 65% specificity). The DevTox GLR-Endo assay demonstrates successful adaptation of the hPST concept with increased throughput, shorter assay duration, and minimal endpoint processing. The DevTox GLR model platform expands the in vitro NAM toolbox to rapidly identify potential developmental hazards and mechanistically characterize toxicant effects on pathways and processes associated with early human development.

Pluripotent stem cell assays: Modalities and applications for predictive developmental toxicity

This manuscript provides a review focused on embryonic stem cell-based models and their place within the landscape of alternative developmental toxicity assays. Against the background of the principles of developmental toxicology, the wide diversity of alternative methods using pluripotent stem cells developed in this area over the past half century is reviewed. In order to provide an overview of available models, a systematic scoping review was conducted following a published protocol with inclusion criteria, which were applied to select the assays. Critical aspects including biological domain, readout endpoint, availability of standardized protocols, chemical domain, reproducibility and predictive power of each assay are described in detail, in order to review the applicability and limitations of the platform in general and progress moving forward to implementation. The horizon of innovative routes of promoting regulatory implementation of alternative methods is scanned, and recommendations for further work are given.

Optimization of the TeraTox assay for preclinical teratogenicity assessment

Current animal-free methods to assess teratogenicity of drugs under development still deliver high numbers of false negatives. To improve the sensitivity of human teratogenicity prediction, we characterized the TeraTox test, a newly developed multilineage differentiation assay using 3D human-induced pluripotent stem cells. TeraTox produces primary output concentration-dependent cytotoxicity and altered gene expression induced by each test compound. These data are fed into an interpretable machine-learning model to perform prediction, which relates to the concentration-dependent human teratogenicity potential of drug candidates. We applied TeraTox to profile 33 approved pharmaceuticals and 12 proprietary drug candidates with known in vivo data. Comparing TeraTox predictions with known human or animal toxicity, we report an accuracy of 69% (specificity: 53%, sensitivity: 79%). TeraTox performed better than 2 quantitative structure-activity relationship models and had a higher sensitivity than the murine embryonic stem cell test (accuracy: 58%, specificity: 76%, and sensitivity: 46%) run in the same laboratory. The overall prediction accuracy could be further improved by combining TeraTox and mouse embryonic stem cell test results. Furthermore, patterns of altered gene expression revealed by TeraTox may help grouping toxicologically similar compounds and possibly deducing common modes of action. The TeraTox assay and the dataset described here therefore represent a new tool and a valuable resource for drug teratogenicity assessment.

A novel human stem cell-based biomarker assay for in vitro assessment of developmental toxicity

BACKGROUND

Testing for developmental toxicity according to the current regulatory guidelines requires large numbers of animals, making these tests very resource intensive, time-consuming, and ethically debatable. Over the past decades, several alternative in vitro assays have been developed, but these often suffered from low predictability and the inability to provide a mechanistic understanding of developmental toxicity.

METHODS

To identify embryotoxic compounds, we developed a human induced pluripotent stem cells (hiPSCs)-based biomarker assay. The assay is based on the differentiation of hiPSCs into functional cardiomyocytes and hepatocytes. Proper stem cell differentiation is investigated by morphological profiling and assessment of time-dependent expression patterns of cell-specific biomarkers. In this system, a decrease in the expression of the biomarker genes and morphology disruption of the differentiated cells following compound treatment indicated teratogenicity.

RESULTS

The hiPSCs-based biomarker assay was validated with 21 well-established in vivo animal teratogenic and non-teratogenic compounds during cardiomyocyte and hepatocyte differentiation. The in vivo teratogenic compounds (e.g., thalidomide and valproic acid) markedly disrupted morphology, functionality, and the expression pattern of the biomarker genes in either one or both cell types. Non-teratogenic chemicals generally had no effect on the morphology of differentiated cells, nor on the expression of the biomarker genes. Compared to the in vivo classification, the assay achieved high accuracy (91%), sensitivity (91%), and specificity (90%).

CONCLUSION

The assay, which we named ReproTracker®, is a state-of-the-art in vitro method that can identify the teratogenicity potential of new pharmaceuticals and chemicals and signify the outcome of in vivo test systems.

A large scale mass spectrometry-based histone screening for assessing epigenetic developmental toxicity

Toxicoepigenetics is an emerging field that studies the toxicological impact of compounds on protein expression through heritable, non-genetic mechanisms, such as histone post-translational modifications (hPTMs). Due to substantial progress in the large-scale study of hPTMs, integration into the field of toxicology is promising and offers the opportunity to gain novel insights into toxicological phenomena. Moreover, there is a growing demand for high-throughput human-based in vitro assays for toxicity testing, especially for developmental toxicity. Consequently, we developed a mass spectrometry-based proof-of-concept to assess a histone code screening assay capable of simultaneously detecting multiple hPTM-changes in human embryonic stem cells. We first validated the untargeted workflow with valproic acid (VPA), a histone deacetylase inhibitor. These results demonstrate the capability of mapping the hPTM-dynamics, with a general increase in acetylations as an internal control. To illustrate the scalability, a dose–response study was performed on a proof-of-concept library of ten compounds (1) with a known effect on the hPTMs (BIX-01294, 3-Deazaneplanocin A, Trichostatin A, and VPA), (2) classified as highly embryotoxic by the European Centre for the Validation of Alternative Methods (ECVAM) (Methotrexate, and All-trans retinoic acid), (3) classified as non-embryotoxic by ECVAM (Penicillin G), and (4) compounds of abuse with a presumed developmental toxicity (ethanol, caffeine, and nicotine).

Dolutegravir impairs stem cell-based 3D morphogenesis models in a manner dependent on dose and timing of exposure: an implication for its developmental toxicity

Dolutegravir is an anti-retroviral drug of the integrase strand transfer inhibitor class used to treat HIV infection. It is the recommended first-line regimen for most people, including women of childbearing age. However, some human and animal studies have suggested that dolutegravir causes birth defects, although its developmental toxicity remains controversial. Here, we investigated the adverse effects of dolutegravir using pluripotent stem cell-based in vitro morphogenesis models that have previously been validated as effective tools to assess the developmental toxicity of various chemicals. Dolutegravir diminished the growth and axial elongation of the morphogenesis model of mouse pluripotent stem cells at exposures of 2 μM and above in a concentration-dependent manner. Concomitantly, dolutegravir altered the expression profiles of developmental regulator genes involved in embryonic patterning. The adverse effects were observed when the morphogenesis model was exposed to dolutegravir at early stages of development, but not at later stages. The potency and molecular impact of dolutegravir on the morphogenesis model were distinct from other integrase strand transfer inhibitors. Lastly, dolutegravir altered the growth and gene expression profiles of the morphogenesis model of human embryonic stem cells at 1 μM and above. These studies demonstrate that dolutegravir impairs morphological and molecular aspects of the in vitro morphogenesis models in a manner dependent on dose and timing of exposure through mechanisms that are unrelated to its action as an integrase strand transfer inhibitor. This finding will be useful for interpreting the conflicting outcomes regarding the developmental toxicity of dolutegravir in human and animal studies.

A Systematic Review to Compare Chemical Hazard Predictions of the Zebrafish Embryotoxicity Test With Mammalian Prenatal Developmental Toxicity

Originally developed to inform the acute toxicity of chemicals on fish, the zebrafish embryotoxicity test (ZET) has also been proposed for assessing the prenatal developmental toxicity of chemicals, potentially replacing mammalian studies. Although extensively evaluated in primary studies, a comprehensive review summarizing the available evidence for the ZET's capacity is lacking. Therefore, we conducted a systematic review of how well the presence or absence of exposure-related findings in the ZET predicts prenatal development toxicity in studies with rats and rabbits. A two-tiered systematic review of the developmental toxicity literature was performed, a review of the ZET literature was followed by one of the mammalian literature. Data were extracted using DistillerSR, and study validity was assessed with an amended SYRCLE's risk-of-bias tool. Extracted data were analyzed for each species and substance, which provided the basis for comparing the 2 test methods. Although limited by the number of 24 included chemicals, our results suggest that the ZET has potential to identify chemicals that are mammalian prenatal developmental toxicants, with a tendency for overprediction. Furthermore, our analysis confirmed the need for further standardization of the ZET. In addition, we identified contextual and methodological challenges in the application of systematic review approaches to toxicological questions. One key to overcoming these challenges is a transition to more comprehensive and transparent planning, conduct and reporting of toxicological studies. The first step toward bringing about this change is to create broad awareness in the toxicological community of the need for and benefits of more evidence-based approaches.

In vitro teratogenicity testing using a 3D, embryo-like gastruloid system

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Gastruloids are a new in vitro platform for teratogenicity testing.

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Teratogens disrupt gastruloid gene expression and morphology with imaging readout.

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Medium-throughput gastruloid cultures are quantifiable with statistical robustness.

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Mouse and human gastruloids recapitulate species-specific sensitivities to teratogens.

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Proof-of-concept as a predictive assay with scope for automation.

Pharmaceuticals intended for use in patients of childbearing potential need to be tested for teratogenicity before marketing. Several pharmaceutical companies use animal-free in vitro models which allow a more rapid selection of lead compounds and contribute to 3Rs principles (‘replace, reduce and refine’) by streamlining the selection of promising compounds submitted to further regulatory studies in animals. Currently available in vitro models typically rely on adherent monolayer cultures or disorganized 3D structures, both of which lack the spatiotemporal and morphological context of the developing embryo. A newly developed 3D ‘gastruloid’ model has the potential to achieve a more reliable prediction of teratogenicity by providing a robust recapitulation of gastrulation-like events alongside morphological coordination at relatively high-throughput. In this first proof-of-concept study, we used both mouse and human gastruloids to examine a panel of seven reference compounds, with associated in vivo data and known teratogenic risk, to quantitatively assess in vitro teratogenicity. We observed several gross morphological effects, including significantly reduced elongation or decreased size of the gastruloids, upon exposure to several of the reference compounds. We also observed aberrant gene expression using fluorescent reporters, including SOX2, BRA, and SOX17, suggestive of multi-lineage differentiation defects and disrupted axial patterning. Finally, we saw that gastruloids recapitulated some of the known in vivo species-specific susceptibilities between their mouse and human counterparts. We therefore suggest that gastruloids represent a powerful tool for teratogenicity assessment by enabling relevant physiological recapitulation of early embryonic development, demonstrating their use as a novel in vitro teratogenic model system.

Dictyostelium discoideum: an alternative non-animal model for developmental toxicity testing

A critical aspect of toxicity evaluation is developmental and reproductive toxicity (DART) testing. Traditionally, DART testing has been conducted in vivo in mammalian model systems. New legislation aimed at reducing animal use and the prohibitive costs associated with DART testing, together with a need to understand the genetic pathways underlying developmental toxicity means there is a growing demand for alternative model systems for toxicity evaluation. Here we explore the potential of the eukaryotic social amoeba Dictyostelium discoideum, which is already widely used as a simple model system for cell and developmental biology, as a potential nonanimal model for DART testing. We developed assays for high-throughput screening of toxicity during D. discoideum growth and development. This allowed the toxicity of a broad range of test compounds to be characterized, which revealed that D. discoideum can broadly predict mammalian toxicity. In addition, we show that this system can be used to perform functional genomic screens to compare the molecular modes of action of different compounds. For example, genome-wide screens for mutations that affect lithium and valproic acid toxicity allowed common and unique biological targets and molecular processes mediating their toxicity to be identified. These studies illustrate that D. discoideum could represent a predictive nonanimal model for DART testing due to its amenability to high-throughput approaches and molecular genetic tractability.

Concordance of 3 alternative teratogenicity assays with results from corresponding in vivo embryo-fetal development studies: Final report from the International Consortium for Innovation and Quality in Pharmaceutical Development (IQ) DruSafe working group 2

An IQ DruSafe working group evaluated the concordance of 3 alternative teratogenicity assays (rat whole embryo culture, rWEC; zebrafish embryo culture, ZEC; and murine embryonic stem cells, mESC) with findings from rat or rabbit embryo-fetal development (EFD) studies. Data for 90 individual compounds from 9 companies were entered into a database. In vivo findings were deemed positive if malformations or embryo-fetal lethality were reported in either species. Each company used their own criteria for deciding whether the alternative assay predicted the in vivo findings. Standard concordance parameters were calculated, positive and negative predictive values (PPV and NPV) were adjusted for the aggregate portfolio prevalence of positive compounds (established by a survey of participating companies), and positive and negative likelihood ratios (LR+ and iLR-) were calculated. Of the 3 assays, only rWEC data were robustly predictive, particularly for negative predictions (NPVadj = 92%). However, both LR+ (4.92) and iLR- (4.72) were statistically significant for the rWEC assay. When analyzed separately for rats, the NPVadj and iLR-values for the rWEC assay increased to 96% and 9.75, respectively. These data suggest that a negative rWEC outcome could defer or replace a rat EFD study in certain regulatory settings.

Profiling the ToxCast Library With a Pluripotent Human (H9) Stem Cell Line-Based Biomarker Assay for Developmental Toxicity

The Stemina devTOX quickPredict platform is a human pluripotent stem cell-based assay that predicts the developmental toxicity potential based on changes in cellular metabolism following chemical exposure [Palmer, J. A., Smith, A. M., Egnash, L. A., Conard, K. R., West, P. R., Burrier, R. E., Donley, E. L. R., and Kirchner, F. R. (2013). Establishment and assessment of a new human embryonic stem cell-based biomarker assay for developmental toxicity screening. Birth Defects Res. B Dev. Reprod. Toxicol. 98, 343-363]. Using this assay, we screened 1065 ToxCast phase I and II chemicals in single-concentration or concentration-response for the targeted biomarker (ratio of ornithine to cystine secreted or consumed from the media). The dataset from the Stemina (STM) assay is annotated in the ToxCast portfolio as STM. Major findings from the analysis of ToxCast_STM dataset include (1) 19% of 1065 chemicals yielded a prediction of developmental toxicity, (2) assay performance reached 79%-82% accuracy with high specificity (> 84%) but modest sensitivity (< 67%) when compared with in vivo animal models of human prenatal developmental toxicity, (3) sensitivity improved as more stringent weights of evidence requirements were applied to the animal studies, and (4) statistical analysis of the most potent chemical hits on specific biochemical targets in ToxCast revealed positive and negative associations with the STM response, providing insights into the mechanistic underpinnings of the targeted endpoint and its biological domain. The results of this study will be useful to improving our ability to predict in vivo developmental toxicants based on in vitro data and in silico models.

Methoxyacetic acid inhibits histone deacetylase and impairs axial elongation morphogenesis of mouse gastruloids in a retinoic acid signaling-dependent manner

BACKGROUND

Teratogenic potential has been linked to various industrial compounds. Methoxyacetic acid (MAA) is a primary metabolite of the widely used organic solvent and plasticizer, methoxyethanol and dimethoxyethyl phthalate, respectively. Studies using model animals have shown that MAA acts as the proximate teratogen that causes various malformations in developing embryos. Nonetheless, the molecular mechanisms by which MAA exerts its teratogenic effects are not fully understood.

METHODS

Gastruloids of mouse P19C5 pluripotent stem cells, which recapitulate axial elongation morphogenesis of gastrulation-stage embryos, were explored as an in vitro model to investigate the teratogenic action of MAA. Morphometric parameters of gastruloids were measured to evaluate the morphogenetic effect, and transcript levels of various developmental regulator genes were examined to assess the impact on gene expression patterns. The effects of MAA on the level of retinoic acid (RA) signaling and histone deacetylase activity were also measured.

RESULTS

MAA reduced axial elongation of gastruloids at concentrations comparable to the teratogenic plasma level (5 mM) in vivo. MAA at 4 mM significantly altered the expression profiles of developmental regulator genes. In particular, it upregulated the RA signaling target genes. The concomitant suppression of RA signaling using a pharmacological agent alleviated the morphogenetic effect of MAA. MAA at 4 mM also significantly reduced the activity of purified histone deacetylase protein.

CONCLUSIONS

MAA impaired axial elongation morphogenesis in a RA signaling-dependent manner in mouse gastruloids, possibly through the inhibition of histone deacetylase.

Embryo bioassays with aquatic animals for toxicity testing and hazard assessment of emerging pollutants: A review

This review article gathers the available information on the use of embryo-tests as high-throughput tools for toxicity screening, hazard assessment and prioritization of new and existing chemical compounds. The approach is contextualized considering the new legal trends for animal experimentation, fostering the 3R policy, with reduction of experimental animals, addressing the potential of embryo-tests as high-throughput toxicity screening and prioritizing tools. Further, the current test guidelines, such as the ones provided by OECD and EPA, focus mainly in a limited number of animal lineages, particularly vertebrates and arthropods. To extrapolate hazard assessment to the ecosystem scale, a larger diversity of taxa should be tested. The use of new experimental animal models in toxicity testing, from a representative set of taxa, was thoroughly revised and discussed in this review. Here, we critically review current tools and the main advantages and drawbacks of different animal models and set researcher priorities.

Use of Zebrafish in Drug Discovery Toxicology

Unpredicted human safety events in clinical trials for new drugs are costly in terms of human health and money. The drug discovery industry attempts to minimize those events with diligent preclinical safety testing. Current standard practices are good at preventing toxic compounds from being tested in the clinic; however, false negative preclinical toxicity results are still a reality. Continual improvement must be pursued in the preclinical realm. Higher-quality therapies can be brought forward with more information about potential toxicities and associated mechanisms. The zebrafish model is a bridge between in vitro assays and mammalian in vivo studies. This model is powerful in its breadth of application and tractability for research. In the past two decades, our understanding of disease biology and drug toxicity has grown significantly owing to thousands of studies on this tiny vertebrate. This Review summarizes challenges and strengths of the model, discusses the 3Rs value that it can deliver, highlights translatable and untranslatable biology, and brings together reports from recent studies with zebrafish focusing on new drug discovery toxicology.

Exposure-based assessment of chemical teratogenicity using morphogenetic aggregates of human embryonic stem cells

Pluripotent stem cells recapitulate many aspects of embryogenesis in vitro. Here, we established a novel culture system to differentiate human embryonic stem cell aggregates (HESCA), and evaluated its utility for teratogenicity assessment. Culture of HESCA with modulators of developmental signals induced morphogenetic and molecular changes associated with differentiation of the paraxial mesoderm and neuroectoderm. To examine impact of teratogenic exposures on HESCA differentiation, 18 compounds were tested, for which adequate information on in vivo plasma concentrations is available. HESCA treated with each compound were examined for gross morphology and transcript levels of 15 embryogenesis regulator genes. Significant alterations in the transcript levels were observed for 94% (15/16) of the teratogenic exposures within 5-fold margin, whereas no alteration was observed for 92% (11/12) of the non-teratogenic exposures. Our study demonstrates that transcriptional changes in HESCA serve as predictive indicator of teratogenicity in a manner comparable to in vivo exposure levels.

A multi-institutional study benchmarking the zebrafish developmental assay for prediction of embryotoxic plasma concentrations from rat embryo-fetal development studies

Predicting embryotoxicity of pharmaceutical compounds or industrial chemicals is crucial for public safety. Conventional studies which monitor embryo-fetal development in rats and rabbits are costly and time consuming. Alternative assays which are simpler and less costly are being pursued. The purpose of this research was to assess the capacity for the zebrafish development assay to predict mammalian plasma levels that are embryotoxic. Previously published data on rat plasma levels associated with embryotoxicity were used to guide concentration ranges for each of 25 chemicals dissolved in the media bathing developing zebrafish embryos. Embryotoxic media concentrations were compared to embryotoxic rat plasma concentrations. Assays were conducted in parallel at multiple sites as a consortium effort through the Health and Environmental Sciences Institute (HESI). Considering results from all sites, the zebrafish embryo development assay predicted (within 1-log) the rat maternal exposure levels associated with embryotoxicity 75% of the time.

Fluoxetine Inhibits Canonical Wnt Signaling to Impair Embryoid Body Morphogenesis: Potential Teratogenic Mechanisms of a Commonly Used Antidepressant

Fluoxetine is one of the most commonly prescribed antidepressants in the selective serotonin reuptake inhibitor (SSRI) class. Epidemiologic studies have suggested a link between maternal fluoxetine use during pregnancy and an increased incidence of birth defects. However, the mechanisms by which fluoxetine adversely impacts embryonic developments are unknown. Here, we used the mouse P19C5 embryoid body (EB) as a 3D morphogenesis model to investigate the developmental toxicity of fluoxetine. Morphological and molecular changes in P19C5 EBs replicate the processes of axial elongation and patterning seen in early embryos, and these changes are specifically and sensitively altered by exposure to developmental toxicants. Treatment with fluoxetine, or its major metabolite, norfluoxetine, adversely affected EB morphogenesis at concentrations of 6 µM and above. Treatment with other serotonin reuptake inhibitors or serotonin itself did not impair EB morphogenesis, suggesting that the adverse effects of fluoxetine are independent of serotonin signaling. Gene expression analyses showed that various key developmental regulators were affected by fluoxetine, particularly those involved in mesodermal differentiation. Reporter assays demonstrated that fluoxetine inhibited canonical Wnt signaling, and that the pharmacologic activation of canonical Wnt signaling partially alleviated the morphogenetic effects of fluoxetine. Fluoxetine also exhibited cytostatic effects independently of inhibition of the serotonin transporter or canonical Wnt signaling. These results suggest that the SSRI-independent actions of fluoxetine, namely inhibition of canonical Wnt signaling and reduction of cellular proliferation, are largely responsible for the observed adverse morphogenetic impacts. This study provides mechanistic insight for further investigations on the teratogenicity of fluoxetine.

Embryoid body test with morphological and molecular endpoints implicates potential developmental toxicity of trans-resveratrol

Developmental toxicity of compounds, which women of reproductive age are exposed to, should be assessed to minimize the incidence of miscarriage and birth defects. The present study examined the potential developmental toxicity of resveratrol, a dietary supplement widely marketed with various health claims, using the P19C5 embryoid body (EB) morphogenesis assay, which evaluates adverse effects of chemical exposures on tissue growth and axial elongation. Resveratrol (trans isoform) impaired morphogenesis at 4 μM and higher, creating smaller and rounder EBs, whereas cis isoform, and glucuronated and sulfonated metabolites did not. Trans-resveratrol also altered expression levels of developmental regulator genes involved in embryonic patterning, such as Wnt3a, Tbx6, and Cyp26a1. To investigate the mechanisms of trans-resveratrol action, the roles of estrogen receptor, sirtuin 1 (SIRT1), and DNA replication in EB morphogenesis were examined. Neither activators of estrogen receptors (diethylstilbestrol [18 μM] and raloxifene [8 μM]) nor activator of SIRT1 (SRT1720 [2.4-3.2 μM]) caused morphological and molecular alterations that are comparable to trans-resveratrol (10 μM). By contrast, a reduction in the DNA replication rate with aphidicolin (0.4 μM) or hydroxyurea (40 μM) created smaller and rounder EBs and altered the expression levels of Wnt3a, Tbx6, and Cyp26a1 in a manner similar to trans-resveratrol. Consistently, trans-resveratrol significantly reduced the rate of EdU incorporation in P19C5 cells. These results suggest that a reduction in the DNA replication rate is one of the mechanisms by which trans-resveratrol impacts EB development. This study provides mechanistic insight for further investigations on the developmental toxicity of trans-resveratrol.

Review of animal studies on the cardiovascular effects of caffeine

To address the safety of caffeine levels in energy drinks, we previously conducted a detailed evaluation of epidemiology studies in humans consuming coffee/caffeine, in which we assessed multiple health effects (unpublished). To further evaluate the effects of caffeine on the cardiovascular system, we turned to animal studies, which often use pure caffeine (not coffee), frequently at higher doses than those typical of human exposure. We identified key scientific studies and reviews in which effects of coffee or caffeine were evaluated in animals by conducting a comprehensive PubMed literature search and analyzing the results. We found that the human equivalent dose (HED) for the no observed adverse effect level (NOAEL) for cardiovascular effects was 260 mg caffeine (2-3 cups of coffee) for a single dose of caffeine for a 70-kg adult, while the lowest observed adverse effect level (LOAEL) was 770 mg (7-8 cups of coffee) for a 70-kg adult. Overall, the doses associated with possible adverse cardiovascular effects were more than either the amount of caffeine consumed over a 24-hour period in two regular energy shots (400 mg/day) or the amount in two extra strength energy shots (460 mg/day).

Exposure-Based Validation of an In Vitro Gastrulation Model for Developmental Toxicity Assays

Establishment of effective non-animal alternatives for developmental toxicity screening assays is desirable to ensure maternal and fetal health outcomes. Validation of such assays requires a comparison between the in vitro responses to chemical exposures and the in vivo impacts of the corresponding compounds at equivalent concentrations. Here, we investigated how the P19C5 gastrulation model responds to 24 compounds at specific concentrations, some of which are categorized as positive exposures based on previously observed detrimental effects on development in vivo, whereas others are categorized as negative exposures due to lack of effects in vivo. The P19C5 gastrulation model consists of in vitro morphogenesis of mouse stem cells aggregated into embryoid bodies (EBs), which recapitulates growth and axial elongation of early embryos during four days of three-dimensional culture. Adverse impacts of chemical exposures were defined as: death, impaired growth, and altered axial elongation of EBs. Ten out of 17 positive exposures caused adverse impacts on EBs. In contrast, only three out of 17 negative exposures adversely affected EBs, although two of the three diminished viability of somatic cell lines (NIH/3T3, HEK293, and JEG3), suggesting general cytotoxicity. Overall, the study showed that 24 out of 34 exposures impacted EB development in a manner concordant with the in vivo developmental effects. Validation of other alternative assays using the same set of chemical exposures will provide information on the strengths and weaknesses of each assay, and should help determine the most effective ensemble of assays to detect a wide range of developmentally toxic exposures.

Developmental toxicity assessment of common excipients using a stem cell-based in vitro morphogenesis model

Various chemical compounds can inflict developmental toxicity when sufficiently high concentrations are exposed to embryos at the critical stages of development. Excipients, such as coloring agents and preservatives, are pharmacologically inactive ingredients that are included in various medications, foods, and cosmetics. However, concentrations that may adversely affect embryo development are largely unknown for most excipients. Here, the lowest observed adverse effect level (LOAEL) to inflict developmental toxicity was assessed for three coloring agents (allura red, brilliant blue, and tartrazine) and three preservatives (butylated hydroxyanisole, metabisulfite, and methylparaben). Adverse impact of a compound exposure was determined using the stem cell-based in vitro morphogenesis model, in which three-dimensional cell aggregates, or embryoid bodies (EBs), recapitulate embryonic processes of body axis elongation and patterning. LOAEL to impair EB morphogenesis was 200 μM for methylparaben, 400 μM for butylated hydroxyanisole, 600 μM for allura red and brilliant blue, and 1000 μM for metabisulfite. Gene expression analyses of excipient-treated EBs revealed that butylated hydroxyanisole and methylparaben significantly altered profiles of developmental regulators involved in axial elongation and patterning of the body. The present study may provide a novel in vitro approach to investigate potential developmental toxicity of common excipients with mechanistic insights.

A human induced pluripotent stem cell-based in vitro assay predicts developmental toxicity through a retinoic acid receptor-mediated pathway for a series of related retinoid analogues

The relative developmental toxicity potency of a series of retinoid analogues was evaluated using a human induced pluripotent stem (iPS) cell assay that measures changes in the biomarkers ornithine and cystine. Analogue potency was predicted, based on the assay endpoint of the ornithine/cystine (o/c) ratio, to be all-trans-retinoic acid>TTNPB>13-cis-retinoic acid≈9-cis-retinoic acid>acitretin>etretinate>retinol. These rankings correlate with in vivo data and demonstrate successful application of the assay to rank a series of related toxic and non-toxic compounds. The retinoic acid receptor α (RARα)-selective antagonist Ro 41-5253 inhibited the cystine perturbation caused by all-trans-retinoic acid, TTNPB, 13-cis-retinoic acid, 9-cis-retinoic acid, and acitretin. Ornithine was altered independent of RARα in all retinoids except acitretin. These results suggest a role for an RARα-mediated mechanism in retinoid-induced developmental toxicity through altered cystine metabolism.

The era of 3Rs implementation in developmental and reproductive toxicity (DART) testing: Current overview and future perspectives

Since adoption of the first globally implemented guidelines for developmental and reproductive toxicity (DART) testing for pharmaceuticals, industrial chemicals and agrochemicals, many years passed without major updates. However in recent years, significant changes in these guidelines have been made or are being implemented. These changes have been guided by the ethical drive to reduce, refine and replace (3R) animal testing, as well as the addition of endocrine disruptor relevant endpoints. Recent applied improvements have focused on reduction and refinement. Ongoing scientific and technical innovations will provide the means for replacement of animal testing in the future and will improve predictivity in humans. The aim of this review is to provide an overview of ongoing global DART endeavors in respect to the 3Rs, with an outlook towards future advances in DART testing aspiring to reduce animal testing to a minimum and the supreme ambition towards animal-free hazard and risk assessment.

Adverse effect of valproic acid on an in vitro gastrulation model entails activation of retinoic acid signaling

Valproic acid (VPA), an antiepileptic drug, is a teratogen that causes neural tube and axial skeletal defects, although the mechanisms are not fully understood. We previously established a gastrulation model using mouse P19C5 stem cell embryoid bodies (EBs), which exhibits axial patterning and elongation morphogenesis in vitro. Here, we investigated the effects of VPA on the EB axial morphogenesis to gain insights into its teratogenic mechanisms. Axial elongation and patterning of EBs were inhibited by VPA at therapeutic concentrations. VPA elevated expression levels of various developmental regulators, including Cdx1 and Hoxa1, known transcriptional targets of retinoic acid (RA) signaling. Co-treatment of EBs with VPA and BMS493, an RA receptor antagonist, partially rescued axial elongation as well as gene expression profiles. These results suggest that VPA requires active RA signaling to interfere with EB morphogenesis.

In vitro acute and developmental neurotoxicity screening: an overview of cellular platforms and high-throughput technical possibilities

Neurotoxicity and developmental neurotoxicity are important issues of chemical hazard assessment. Since the interpretation of animal data and their extrapolation to man is challenging, and the amount of substances with information gaps exceeds present animal testing capacities, there is a big demand for in vitro tests to provide initial information and to prioritize for further evaluation. During the last decade, many in vitro tests emerged. These are based on animal cells, human tumour cell lines, primary cells, immortalized cell lines, embryonic stem cells, or induced pluripotent stem cells. They differ in their read-outs and range from simple viability assays to complex functional endpoints such as neural crest cell migration. Monitoring of toxicological effects on differentiation often requires multiomics approaches, while the acute disturbance of neuronal functions may be analysed by assessing electrophysiological features. Extrapolation from in vitro data to humans requires a deep understanding of the test system biology, of the endpoints used, and of the applicability domains of the tests. Moreover, it is important that these be combined in the right way to assess toxicity. Therefore, knowledge on the advantages and disadvantages of all cellular platforms, endpoints, and analytical methods is essential when establishing in vitro test systems for different aspects of neurotoxicity. The elements of a test, and their evaluation, are discussed here in the context of comprehensive prediction of potential hazardous effects of a compound. We summarize the main cellular characteristics underlying neurotoxicity, present an overview of cellular platforms and read-out combinations assessing distinct parts of acute and developmental neurotoxicology, and highlight especially the use of stem cell-based test systems to close gaps in the available battery of tests.

Reference compounds for alternative test methods to indicate developmental neurotoxicity (DNT) potential of chemicals: example lists and criteria for their selection and use

There is a paucity of information concerning the developmental neurotoxicity (DNT) hazard posed by industrial and environmental chemicals. New testing approaches will most likely be based on batteries of alternative and complementary (non-animal) tests. As DNT is assumed to result from the modulation of fundamental neurodevelopmental processes (such as neuronal differentiation, precursor cell migration or neuronal network formation) by chemicals, the first generation of alternative DNT tests target these processes. The advantage of such types of assays is that they capture toxicants with multiple targets and modes-of-action. Moreover, the processes modelled by the assays can be linked to toxicity endophenotypes, i.e. alterations in neural connectivity that form the basis for neurofunctional deficits in man. The authors of this review convened in a workshop to define criteria for the selection of positive/negative controls, to prepare recommendations on their use, and to initiate the setup of a directory of reference chemicals. For initial technical optimization of tests, a set of >50 endpoint-specific control compounds was identified. For further test development, an additional “test” set of 33 chemicals considered to act directly as bona fide DNT toxicants is proposed, and each chemical is annotated to the extent it fulfills these criteria. A tabular compilation of the original literature used to select the test set chemicals provides information on statistical procedures, and toxic/non-toxic doses (both for pups and dams). Suggestions are provided on how to use the >100 compounds (including negative controls) compiled here to address specificity, adversity and use of alternative test systems.

A Developmental Toxicology Assay Platform for Screening Teratogenic Liability of Pharmaceutical Compounds

Increasing need for proactive safety optimization of pharmaceutical compounds has led to generation and/or refinement of in vitro developmental toxicology assays. Our laboratory has developed three in vitro developmental toxicology assays to assess teratogenic liability of pharmaceutical compounds. These assays included a mouse molecular embryonic stem cell assay (MESCA), a dechorionated zebrafish embryo culture (ZEC) assay, and a streamlined rat whole embryo culture (rWEC) assay. Individually, the assays presented good (73-82%) predictivity. However, it remains to be determined whether combining or tiering the assays could enhance performance. Seventy-three compounds representing a broad spectrum of pharmaceutical targets and chemistry were evaluated across the assays to generate testing strategies that optimized performance. The MESCA and ZEC assays were found to have two limitations: compound solubility and frequent misclassification of compounds with H1 receptor or GABAnergic activity. The streamlined rWEC assay was found to be a cost-effective stand-alone assay for supporting poorly soluble compounds and/or ones with H1 or GABAnergic activity. For all other compounds, a tiering strategy using the MESCA and ZEC assays additionally optimized throughput, cost, and minimized animal use. The tiered strategy resulted in improved performance achieving 88% overall predictivity and was comparable with 89% overall predictivity achieved with frequency analysis (final teratogenic classification made from most frequent teratogenic classification from each individual assay). Furthermore there were 21 compounds in the test set characterized as definitive or suspect human teratogens and the multiassay approach achieved 95 and 91% correct classification using the tiered or frequency screening approach, respectively.