Selection of test chemicals for the ECVAM international validation study on in vitro embryotoxicity tests. European Centre for the Validation of Alternative Methods

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.

Pluripotent Stem Cell Assays: Modalities and Applications For Predictive Developmental Toxicity

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A systematic scoping review of the literature evaluated the embryonic stem cell test (EST).

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1533 publications included 18 publications testing 10 or more compounds in human or mouse EST.

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Selected case examples included 5-fluorouracil, thalidomide, and caffeine.

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Applicability, limitations, and recommendations for further work are discussed.

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.

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).

Integrating in vitro chemical transplacental passage into a generic PBK model: A QIVIVE approach

With the increasing application of cell culture models as primary tools for predicting chemical safety, the quantitative extrapolation of the effective dose from in vitro to in vivo (QIVIVE) is of increasing importance. For developmental toxicity this requires scaling the in vitro observed dose-response characteristics to in vivo fetal exposure, while integrating maternal in vivo kinetics during pregnancy, in particular transplacental transfer. Here the transfer of substances across the placental barrier, has been studied using the in vitro BeWo cell assay and six embryotoxic compounds of different kinetic complexity. The BeWo assay results were incorporated in an existing generic Physiologically Based Kinetic (PBK) model which for this purpose was extended with rat pregnancy. Finally, as a "proof of principle", the BeWo PBK model was used to perform a QIVIVE based on developmental toxicity as observed in various different in vitro toxicity assays. The BeWo results illustrated different transport profiles of the chemicals across the BeWo monolayer, allocating the substances into two distinct groups: the 'quickly-transported' and the 'slowly-transported'. BeWo PBK exposure simulations during gestation were compared to experimentally measured maternal blood and fetal concentrations and a reverse dosimetry approach was applied to translate in vitro observed embryotoxicity into equivalent in vivo dose-response curves. This approach allowed for a direct comparison of the in vitro dose-response characteristics as observed in the Whole Embryo Culture (WEC), and the Embryonic Stem Cell test (cardiac:ESTc and neural:ESTn) with in vivo rat developmental toxicity data. Overall, the in vitro to in vivo comparisons suggest a promising future for the application of such QIVIVE methodologies for screening and prioritization purposes of developmental toxicants. Nevertheless, the clear need for further improvements is acknowledged for a wider application of the approach in chemical safety assessment.

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.

Evaluation of in vitro embryotoxicity tests for Chinese herbal medicines

Chinese herbal medicines (CHMs) have been widely used during pregnancy, but feto-embryo safety tests are lacking. Here we evaluated in vitro embryotoxicity tests (IVTs) as alternative methods in assessing developmental toxicity of CHMs. Ten CHMs were selected and classified as strongly, weakly and non-embryotoxic. Three well validated IVTs and prediction models (PMs), including embryonic stem cell test (EST), micromass (MM) and whole embryo culture (WEC), were compared. All strongly embryotoxic CHMs were predicted by MM and WEC PM2. While all weakly embryotoxic CHMs were predicted by MM and WEC PM1. All non-embryotoxic CHMs were classified by EST, MM, but over-classified as weakly embryotoxic by WEC PM1. Overall predictivity, precision and accuracy of WEC determined by PM2 were better than EST and MM tests. Compared with validated chemicals, performance of IVTs for CHMs was comparable. So IVTs are adequate to identify and exclude embryotoxic potential of CHMs in this training set.

An engineered mouse embryonic stem cell model with survivin as a molecular marker and EGFP as the reporter for high throughput screening of embryotoxic chemicals in vitro

Embryonic stem cell test (EST) is the only generally accepted in vitro method for assessing embryotoxicity without animal sacrifice. However, the implementation and application of EST for regulatory embryotoxicity screening are impeded by its technical complexity, long testing period, and limited endpoint data. In this study, a high throughput embryotoxicity screening based on mouse embryonic stem cells (mESCs) expressing enhanced green fluorescent protein (EGFP) driven by a human survivin promoter and a human cytomegalovirus promoter, respectively, was developed. These EGFP expressing mESCs were cultured in three-dimensional (3D) fibrous scaffolds in microbioreactors on a multiwell plate with EGFP fluorescence signals as cell responses to chemicals monitored noninvasively in a high throughput manner. Nine chemicals with known developmental toxicity were used to validate the survivin-based embryotoxicity assay, which showed that strongly embryotoxic compounds such as 5-fluorouracil, retinoic acid, and methotrexate downregulated survivin expression by more than 50% in 3 days, while weakly embryotoxic compounds such as boric acid, methoxyacetic acid, and tetracyclin showed modest downregulation effect and nonembryotoxic saccharin, penicillin G, and acrylamide had negligible downregulation effect on survivin expression, confirming that survivin can be used as a molecular endpoint for high throughput screening of embryotoxicants. The potential developmental toxicity of three Chinese herbal medicines were also evaluated using this assay, demonstrating its application in in vitro developmental toxicity test for drug safety assessment.

Species-specific developmental toxicity in rats and rabbits: Generation of a reference compound list for development of alternative testing approaches

For regulatory information requirements, developmental toxicity testing is often conducted in two mammalian species. In order to provide a set of reference compounds that could be used to explore alternative approaches to supersede testing in a second species, a retrospective data analysis was conducted. The aim was to identify compounds for which species sensitivity differences between rats and rabbits are not caused by maternal toxicity or toxicokinetic differences. A total of 330 compounds were analysed and classified according to their species-specific differences. A lack of concordance between rat and rabbit was observed in 24% of the compounds, of which 10% were found to be selective developmental toxicants in one of the species. In contrast to previously published analyses the presented comparison is based entirely on publically data allowing validating and comparing alternative approaches for developmental toxicity testing. Furthermore, this list could be useful to identify mechanisms leading to species differences.

Editor's Highlight: Development of Novel Neural Embryonic Stem CellTests for High-Throughput Screening of Embryotoxic Chemicals

There is a great demand for appropriate alternative methods to rapidly evaluate the developmental and reproductive toxicity of a wide variety of chemicals. We used the differentiation of mouse embryonic stem cells (mESCs) into cardiomyocytes as a basis for establishing a rapid and highly reproducible invitro embryotoxicity test known as the Hand1-Luc Embryonic Stem Cell Test (Hand1-Luc EST). In this study, we developed novel neural-Luc ESTs using two marker genes for neural development, tubulin beta-3 (Tubb3) and Reelin (Reln), and evaluated the capacity of these tests to predict developmental toxicity. In addition, we tested whether an integrated approach (a combination of neural-Luc ESTs and the Hand1-Luc EST) improved developmental toxicant detection. To perform our neural-Luc ESTs, we needed to generate stable transgenic mESCs with individual promoters linked to the luciferase gene, and to establish that similar changes in promoter activities and mRNA expression levels occur during neural differentiation. Based on the concentration-response curves of 15 developmental toxicants and 17 non-developmental toxic chemicals, we derived a prediction formula and assessed the capacity of this formula to predict developmental toxicity. Although both were highly sensitive and specific for predicting developmental toxicity, neural-Luc ESTs had similar predictive capacities. In contrast, neural-Luc ESTs and Hand1-Luc EST had significantly different predictive powers. As expected, the combination of these ESTs increased the sensitivity of developmental toxicant detection. These results demonstrate the convenience and the usefulness of this combination of ESTs as an alternative assay system for future toxicological and mechanistic studies of developmental toxicity.

Editor's Highlight: Identification and Characterization of Teratogenic Chemicals Using Embryonic Stem Cells Isolated From a Wnt/β-Catenin-Reporter Transgenic Mouse Line

Embryonic stem cells (ESCs) are commonly used for the analysis of gene function in embryonic development and provide valuable models for human diseases. In recent years, ESCs have also become an attractive tool for toxicological testing, in particular for the identification of teratogenic compounds. We have recently described a Bmp-reporter ESC line as a new tool to identify teratogenic compounds and to characterize the molecular mechanisms mediating embryonic toxicity. Here we describe the use of a Wnt/β-Catenin-reporter ESC line isolated from a previously described mouse line that carries the LacZ reporter gene under the control of a β-Catenin responsive promoter. The reporter ESC line stably differentiates into cardiomyocytes within 12 days. The reporter was endogenously induced between day 3-5 of differentiation reminiscent of its expression in vivo, in which strong LacZ activity is detected around gastrulation. Subsequently its expression becomes restricted to mesodermal cells and cells undergoing an epithelial to mesenchymal transition. The Wnt/β-Catenin-dependent expression of the reporter protein allowed quantification of dose- and time-dependent effects of teratogenic chemicals. In particular, valproic acid reduced reporter activity on day 7 whereas retinoic acid induced reporter activity on day 5 at concentrations comparable to the ones inhibiting the formation of functional cardiomyocytes, the classical read-out of the embryonic stem cell test (EST). In addition, we were also able to show distinct effects of teratogenic chemicals on the Wnt/β-Catenin-reporter compared with the previously described Bmp-reporter ESCs. Thus, different reporter cell lines provide complementary tools for the identification and analysis of potentially teratogenic compounds.

A multi-laboratory evaluation of microelectrode array-based measurements of neural network activity for acute neurotoxicity testing

There is a need for methods to screen and prioritize chemicals for potential hazard, including neurotoxicity. Microelectrode array (MEA) systems enable simultaneous extracellular recordings from multiple sites in neural networks in real time and thereby provide a robust measure of network activity. In this study, spontaneous activity measurements from primary neuronal cultures treated with three neurotoxic or three non-neurotoxic compounds was evaluated across four different laboratories. All four individual laboratories correctly identifed the neurotoxic compounds chlorpyrifos oxon (an organophosphate insecticide), deltamethrin (a pyrethroid insecticide) and domoic acid (an excitotoxicant). By contrast, the other three compounds (glyphosate, dimethyl phthalate and acetaminophen) considered to be non-neurotoxic ("negative controls"), produced only sporadic changes of the measured parameters. The results were consistent across the different laboratories, as all three neurotoxic compounds caused concentration-dependent inhibition of mean firing rate (MFR). Further, MFR appeared to be the most sensitive parameter for effects of neurotoxic compounds, as changes in electrical activity measured by mean frequency intra burst (MFIB), and mean burst duration (MBD) did not result in concentration-response relationships for some of the positive compounds, or required higher concentrations for an effect to be observed. However, greater numbers of compounds need to be tested to confirm this. The results obtained indicate that measurement of spontaneous electrical activity using MEAs provides a robust assessment of compound effects on neural network function.

Evolving the Principles and Practice of Validation for New Alternative Approaches to Toxicity Testing

Validation is essential for the translation of newly developed alternative approaches to animal testing into tools and solutions suitable for regulatory applications. Formal approaches to validation have emerged over the past 20 years or so and although they have helped greatly to progress the field, it is essential that the principles and practice underpinning validation continue to evolve to keep pace with scientific progress. The modular approach to validation should be exploited to encourage more innovation and flexibility in study design and to increase efficiency in filling data gaps. With the focus now on integrated approaches to testing and assessment that are based on toxicological knowledge captured as adverse outcome pathways, and which incorporate the latest in vitro and computational methods, validation needs to adapt to ensure it adds value rather than hinders progress. Validation needs to be pursued both at the method level, to characterise the performance of in vitro methods in relation their ability to detect any association of a chemical with a particular pathway or key toxicological event, and at the methodological level, to assess how integrated approaches can predict toxicological endpoints relevant for regulatory decision making. To facilitate this, more emphasis needs to be given to the development of performance standards that can be applied to classes of methods and integrated approaches that provide similar information. Moreover, the challenge of selecting the right reference chemicals to support validation needs to be addressed more systematically, consistently and in a manner that better reflects the state of the science. Above all however, validation requires true partnership between the development and user communities of alternative methods and the appropriate investment of resources.

Cord Blood Cells for Developmental Toxicology and Environmental Health

The Tox21 program initiated a shift in toxicology toward in vitro testing with a focus on the biological mechanisms responsible for toxicological response. We discuss the applications of these initiatives to developmental toxicology. Specifically, we briefly review current approaches that are widely used in developmental toxicology to demonstrate the gap in relevance to human populations. An important aspect of human relevance is the wide variability of cellular responses to toxicants. We discuss how this gap can be addressed by using cells isolated from umbilical cord blood, an entirely non-invasive source of fetal/newborn cells. Extension of toxicological testing to collections of human fetal/newborn cells would be useful for better understanding the effect of toxicants on fetal development in human populations. By presenting this perspective, we aim to initiate a discussion about the use of cord blood donor-specific cells to capture the variability of cellular toxicological responses during this vulnerable stage of human development.

A toxicology study to evaluate the embryotoxicity of metformin compared with the hypoglycemic drugs, the anticancer drug, the anti-epileptic drug, the antibiotic, and the cyclo-oxygenase (COX)-2 inhibitor

BACKGROUND

The safe use of medications in pregnant females, their embryos and in offspring is important. The aim of the present study was to evaluate embryotoxicity of metformin (MET) compared with other hypoglycemic drugs (rosiglitazone [RSG] and glimepiride [GLIM]), the anticancer drug 5-fluorouracil (5-FU), the anti-epileptic drug diphenylhydantoin (DPH), the antibiotic penicillin G (PenG), and the cyclo-oxygenase (COX)-2 inhibitor nimesulide (NIM) in an embryonic stem cell test (EST).

METHODS

Differences in the expression of developmental marker genes following treatment with the test compounds during the course of differentiation (from embryonic stem cell D3 (D3 cells) to myocardial cells) were determined using real-time quantitative polymerase chain reaction. In these studies, 5-FU was used as a positive control and PenG was used as a negative control. The cytotoxicity of these drugs against D3 cells and 3T3 fibroblasts was determined by the 3-(4,5-dimethyl-2 thiazoyl)-2,5-diphenyl-2H-tetrazolium bromide (MTT) assay. Embryotoxicity was classified according to the prediction model of EST.

RESULTS

At concentrations >800 μg/mL MET had a greater cytotoxic effect on D3 cells than 3T3 fibroblasts. At the highest concentration of MET (5 mg/mL), the cell viability of D3 cells and 3T3 fibroblasts was <10% and >30%, respectively. The size of the embryonic body (EB) differentiation area was almost the same over the concentration range 50-200 μg/mL MET, and there was no significant difference in EB differentiation area until a concentration of 400 μg/mL MET. At a concentration of 800 μg/mL MET, the size of EB outgrowth was significantly reduced. The same assays revealed GLIM, RSG, and NIM to be weakly embryotoxic substances.

CONCLUSIONS

Based on the EST, MET can be classified as a weakly embryotoxic substance, which suggests that it should be prescribed with caution to pregnant women with gestational diabetes.

Hand1-Luc embryonic stem cell test (Hand1-Luc EST): a novel rapid and highly reproducible in vitro test for embryotoxicity by measuring cytotoxicity and differentiation toxicity using engineered mouse ES cells

The embryonic stem cell test (EST) is a promising alternative method for evaluating embryotoxicity of test chemicals by measuring cytotoxicity and differentiation toxicity using mouse ES cells. Differentiation toxicity is analyzed by microscopically counting the beating of embryonic bodies after 10 days of culture. However, improvements are necessary to reduce the laborious manipulations involved and the time required to obtain results. We have previously reported the successful stable transfection of ES cells (ES-D3) with the heart and neural crest derivatives expressed transcript 1 (Hand1) gene and the establishment of a 96-well multi-plate-based new EST with luciferase reporter assay 6 days after treatment with test chemicals. Now, we propose an even more rapid and easier EST, named Hand1-Luc EST. We established another cell line to monitor the Hand1 gene expression via a luciferase reporter gene. By mRNA analysis and luciferase assay, we examined in detail the luciferase activity during cell differentiation, which allowed us to reduce the time of measurement from day 6 to day 5 (120 hr). Furthermore, the protocol was improved, with, among others, the measurement of cytotoxicity and differentiation toxicity taking place in the same 96-well round bottom plate instead of two different plates. With the positive control, 5-fluorouracil (5-FU), and 9 test chemicals, data with high reproducibility and very low variation (CV < 50%) in the relevant endpoints were obtained. This study shows that the Hand1-Luc EST could provide an accurate and sensitive short-term test for prediction of embryotoxicants by measuring cytotoxicity and differentiation toxicity from the same sample.

Developing osteoblasts as an endpoint for the mouse embryonic stem cell test

The mouse Embryonic Stem cell Test (EST) using cardiomyocyte differentiation is a promising in vitro assay for detecting potential embryotoxicity; however, the addition of another differentiation endpoint, such as osteoblasts, may improve the predictive value of the test. A number of variables such as culture conditions and starting cell number were investigated. A 14 day direct plating method of D3 mouse embryonic stem cells (mESCs) was used to test the predictivity of osteoblast differentiation as an endpoint in the EST. Twelve compounds were tested using the prediction model developed in the ECVAM validation study. Eight of the compounds selected from the EST validation study served as model compounds; four additional compounds known to produce skeletal defects were also tested. Our results indicate comparable chemical classification between the validated cardiomyocyte endpoint and the osteoblast endpoint. These results suggest that differentiation to osteoblasts may provide confirmatory information in predicting embryotoxicity.

Non-human primate and rodent embryonic stem cells are differentially sensitive to embryotoxic compounds

Many industrial chemicals and their respective by-products need to be comprehensively evaluated for toxicity using reliable and efficient assays. In terms of teratogenicity evaluations, the murine-based embryonic stem cell test (EST) offers a promising solution to screen for multiple tissue endpoints. However, use of a mouse model in the EST can yield only a limited understanding of human development, anatomy, and physiology. Non-human primate or human in vitro models have been suggested to be a pharmacologically and pathophysiologically desirable alternative to murine in vitro models. Here, we comparatively evaluated the sensitivity of embryonic stem cells (ESCs) of a non-human primate to skeletal teratogens with mouse ESCs hypothesizing that inclusion of non-human primate cells in in vitro tests would increase the reliability of safety predictions for humans.

First, osteogenic capacity was compared between ESCs from the mouse and a New World monkey, the common marmoset. Then, cells were treated with compounds that have been previously reported to induce bone teratogenicity. Calcification and MTT assays evaluated effects on osteogenesis and cell viability, respectively. Our data indicated that marmoset ESCs responded differently than mouse ESCs in such embryotoxicity screens with no obvious dependency on chemical or compound classes and thus suggest that embryotoxicity screening results could be affected by species-driven response variation. In addition, ESCs derived from rhesus monkey, an Old World monkey, and phylogenetically closer to humans than the marmoset, were observed to respond differently to test compounds than marmoset ESCs. Together these results indicate that there are significant differences in the responses of non-human primate and mouse ESC to embryotoxic agents.

Organophosphorus pesticide chlorpyrifos and its metabolites alter the expression of biomarker genes of differentiation in D3 mouse embryonic stem cells in a comparable way to other model neurodevelopmental toxicants

There are discrepancies about whether chlorpyrifos is able to induce neurodevelopmental toxicity or not. We previously reported alterations in the pattern of expression of biomarker genes of differentiation in D3 mouse embryonic stem cells caused by chlorpyrifos and its metabolites chlorpyrifos-oxon and 3,5,6-trichloro-2-pyridinol. Now, we reanalyze these data comparing the effects on these genes with those caused in the same genes by retinoic acid, valproic acid, and penicillin-G (model compounds considered as strong, weak, and non-neurodevelopmental toxicants, respectively). We also compare the effects of chlorpyrifos and its metabolites on the cell viability of D3 cells and 3T3 mouse fibroblasts with the effects caused in the same cells by the three model compounds. We conclude that chlorpyrifos and its metabolites act, regarding these end-points, as the weak neurodevelopmental toxicant valproic acid, and consequently, a principle of caution should be applied avoiding occupational exposures in pregnant women. A second independent experiment run with different cellular batches coming from the same clone obtained the same result as the first one.

Effects of ethylene glycol monomethyl ether and its metabolite, 2-methoxyacetic acid, on organogenesis stage mouse limbs in vitro

Exposure to ethylene glycol monomethyl ether (EGME), a glycol ether compound found in numerous industrial products, or to its active metabolite, 2-methoxyacetic acid (2-MAA), increases the incidence of developmental defects. Using an in vitro limb bud culture system, we tested the hypothesis that the effects of EGME on limb development are mediated by 2-MAA-induced alterations in acetylation programming. Murine gestation day 12 embryonic forelimbs were exposed to 3, 10, or 30 mM EGME or 2-MAA in culture for 6 days to examine effects on limb morphology; limbs were cultured for 1 to 24 hr to monitor effects on the acetylation of histones (H3K9 and H4K12), a nonhistone protein, p53 (p53K379), and markers for cell cycle arrest (p21) and apoptosis (cleaved caspase-3). EGME had little effect on limb morphology and no significant effects on the acetylation of histones or p53 or on biomarkers for cell cycle arrest or apoptosis. In contrast, 2-MAA exposure resulted in a significant concentration-dependent increase in limb abnormalities. 2-MAA induced the hyperacetylation of histones H3K9Ac and H4K12Ac at all concentrations tested (3, 10, and 30 mM). Exposure to 10 or 30 mM 2-MAA significantly increased acetylation of p53 at K379, p21 expression, and caspase-3 cleavage. Thus, 2-MAA, the proximate metabolite of EGME, disrupts limb development in vitro, modifies acetylation programming, and induces biomarkers of cell cycle arrest and apoptosis

Establishment and assessment of a new human embryonic stem cell-based biomarker assay for developmental toxicity screening

A metabolic biomarker-based in vitro assay utilizing human embryonic stem (hES) cells was developed to identify the concentration of test compounds that perturbs cellular metabolism in a manner indicative of teratogenicity. This assay is designed to aid the early discovery-phase detection of potential human developmental toxicants. In this study, metabolomic data from hES cell culture media were used to assess potential biomarkers for development of a rapid in vitro teratogenicity assay. hES cells were treated with pharmaceuticals of known human teratogenicity at a concentration equivalent to their published human peak therapeutic plasma concentration. Two metabolite biomarkers (ornithine and cystine) were identified as indicators of developmental toxicity. A targeted exposure-based biomarker assay using these metabolites, along with a cytotoxicity endpoint, was then developed using a 9-point dose-response curve. The predictivity of the new assay was evaluated using a separate set of test compounds. To illustrate how the assay could be applied to compounds of unknown potential for developmental toxicity, an additional 10 compounds were evaluated that do not have data on human exposure during pregnancy, but have shown positive results in animal developmental toxicity studies. The new assay identified the potential developmental toxicants in the test set with 77% accuracy (57% sensitivity, 100% specificity). The assay had a high concordance (≥75%) with existing in vivo models, demonstrating that the new assay can predict the developmental toxicity potential of new compounds as part of discovery phase testing and provide a signal as to the likely outcome of required in vivo tests.

Validation of alternative tests for developmental and reproductive toxicology testing: an ILSI Health and Environmental Sciences Institute perspective

The current political and societal climate is driving the science of toxicology towards developing non-animal testing methodologies. Though alternative and in vitro tests have always been a mainstay for toxicological testing, technological advances in the last decade have allowed toxicologists to move rapidly towards a better understanding of the relevance of in vitro endpoints for traditional apical endpoints. Non-animal research using new technologies have illuminated toxicologists on the mechanisms of protection and adverse health outcomes. In this context, the "validation" of alternative and in vitro tests has taken on significant importance, particularly in regard to satisfying safety concerns of drugs and chemicals in a regulatory setting. The purpose of this chapter is to briefly review the impetus for the development of alternative and in vitro tests, discuss the projects underway at the ILSI Health and Environmental Sciences Institute (HESI) that are oriented towards this topic, and summarize the processes for formal validation. It should be noted that though there are validated assays and tests, these are under constant evaluation by scientific researchers as our understanding of the underlying biological processes continues to evolve.

Assessment of the developmental neurotoxicity of compounds by measuring locomotor activity in zebrafish embryos and larvae

The developmental neurotoxic potential of the majority of environmental chemicals and drugs is currently undetermined. Specific in vivo studies provide useful data for hazard assessment but are not amenable to screen thousands of untested compounds. In this study, methods which use zebrafish embryos, eleutheroembryos and larvae as model organisms, were proposed as alternatives for developmental neurotoxicity (DNT) testing. The evaluation of spontaneous tail coilings in zebrafish embryos aged 24-26 hours post fertilization (hpf) and the swimming activity of eleutheroembryos at 120 and larvae at 144 hpf, i.e. parameters for locomotor activity, were investigated as potential endpoints for DNT testing, according to available standard protocols. The overall performance and predictive value of these methods was then examined by testing a training set of 10 compounds, including known developmental neurotoxicants and compounds not considered to be neurotoxic. The classification of the selected compounds as either neurotoxic or non-neurotoxic, based on the effects observed in zebrafish embryos and larvae, was compared to available mammalian data and an overall concordance of 90% was achieved. Furthermore, the specificity of the selected endpoints for DNT was evaluated as well as the potential similarities between zebrafish and mammals with regard to mechanisms of action for the selected compounds. Although further studies, including the screening of a large testing set of compounds are required, we suggest that the proposed methods with zebrafish embryos and larvae might be valuable alternatives for animal testing for the screening and prioritization of compounds for DNT.

Chlorpyrifos and its metabolites alter gene expression at non-cytotoxic concentrations in D3 mouse embryonic stem cells under in vitro differentiation: considerations for embryotoxic risk assessment

The effects of organophosphate insecticide chlorpyrifos (CPF) on development are currently under discussion. CPF and its metabolites, chlorpyrifos-oxon (CPO) and 3,5,6-trichloro-2-pyridinol (TClP), were more cytotoxic for D3 mouse embryonic stem cells than for differentiated fibroblasts 3T3 cells. Exposure to 10 μM CPF and TClP and 100 μM CPO for 12 h significantly altered the in vitro expression of biomarkers of differentiation in D3 cells. Similarly, exposure to 20 μM CPF and 25 μM CPO and TClP for 3 days also altered the expression of the biomarkers in the same model. These exposures caused no significant reduction in D3 viability with mild inhibition of acetylcholinesterase and neuropathy target esterase by CPF and severe inhibition by CPO. We conclude that certain in vivo exposure scenarios are possible, which cause inhibition of acetylcholinesterase but without clinical symptoms that reach high enough systemic CPF concentrations able to alter the expression of genes involved in cellular differentiation with potentially hazard effects on development. Conversely, the risk for embryotoxicity by CPO and TClP was very low because the required exposure would induce severe cholinergic syndrome.

Assaying embryotoxicity in the test tube: current limitations of the embryonic stem cell test (EST) challenging its applicability domain

Testing for embryotoxicity in vitro is an attractive alternative to animal experimentation. The embryonic stem cell test (EST) is such a method, and it has been formally validated by the European Centre for the Validation of Alternative Methods. A number of recent studies have underscored the potential of this method. However, the EST performed well below the 78% accuracy expected from the validation study using a new set of chemicals and pharmaceutical compounds, and also of toxicity criteria, tested to enlarge the database of the validated EST as part of the Work Package III of the ReProTect Project funded within the 6th Framework Programme of the European Union. To assess the performance and applicability domain of the EST we present a detailed review of the substances and their effects in the EST being nitrofen, ochratoxin A, D-penicillamine, methylazoxymethanol, lovastatin, papaverine, warfarin, β-aminopropionitrile, dinoseb, furosemide, doxylamine, pravastatin, and metoclopramide. By delineation of the molecular mechanisms of the substances we identify six categories of reasons for misclassifications. Some of these limitations might also affect other in vitro methods assessing embryotoxicity. Substances that fall into these categories need to be included in future validation sets and in validation guidelines for embryotoxicity testing. Most importantly, we suggest conceivable improvements and additions to the EST which will resolve most of the limitations.

Alternative models in developmental toxicology

In light of various pressures, toxicologists have been searching for alternative methods for safety testing of chemicals. According to a recent policy in the European Union (Regulation, Evaluation Authorisation and Restriction of Chemicals, REACH), it has been estimated that over the next twelve to fifteen years, approximately 30,000 chemicals may need to be tested for safety, and under current guidelines such testing would require the use of approximately 7.2 million laboratory animals [ Hofer et al. 2004 ]. It has also been estimated that over 80% of all animals used for safety testing under REACH legislation would be used for examining reproductive and developmental toxicity [Hofer et al., 2004]. In addition to REACH initiatives, it has been estimated that out of 5,000 to 10,000 new drug entities that a pharmaceutical company may start with, only one is finally approved by the Food and Drug Administration at a cost of over one billion dollars [ Garg et al. 2011 ]. A large portion of this cost is due to animal testing. Therefore, both the pharmaceutical and chemical industries are interested in using alternative models and in vitro tests for safety testing. This review will examine the current state of three alternative models - whole embryo culture (WEC), the mouse embryonic stem cell test (mEST), and zebrafish. Each of these alternatives will be reviewed, and advantages and disadvantages of each model will be discussed. These models were chosen because they are the models most commonly used and would appear to have the greatest potential for future applications in developmental toxicity screening and testing.

Analysis of altered gene expression specific to embryotoxic chemical treatment during embryonic stem cell differentiation into myocardiac and neural cells

Embryonic stem cells (ES cells), pluripotent cells derived from the inner cell mass of blastocysts, differentiate in vitro into a variety of cell types representing all three germ layers. They therefore constitute one of the most promising in vitro tools for developmental toxicology. To assess the developmental toxicity of chemicals using ES cells easily, identification of effective marker genes is a high priority. We report here altered gene expression during ES cell differentiation into myocardiac and neural cells on treatment with some embryotoxic and non-embryotoxic chemicals. Decreases in several undifferentiated markers such as Oct3/4 and Nanog, and elevated expression of genes associated with heart development or the central nervous system, respectively, were found on microarray analysis. Under differentiation of ES cells into myocardic cells, 107 genes were substantially up-regulated. Decrease in the expression of 13 genes of these (Hand1, Pim2, Tbx20, Myl4, Myl7, Hbb-bh1, Hba-a1, Col1a2, Hba-x, Cmya1, Pitx2, Smyd1 and Adam19) was observed specifically by embryotoxic chemicals. Of the 107 genes up-regulated under differentiation into neurons, 22 genes (Map2, Cpe, Marcks, Ptbp2, Sox11, Tubb2b, Vim, Arx, Emx2, Pax6, Basp1, Ddr1, Ndn, Sfrp, Ttc3, Ubqln2, Six3, Dcx, L1cam, Reln, Wnt1 and Nnat) showed reduced expression specifically by embryotoxic chemicals. Almost all gene sets identified in this study are known to be indispensable for differentiation and development of heart and brain tissues, and thus may serve in early detection or prediction of embryotoxicity of chemicals in vitro.

Primary cell and micromass culture in assessing developmental toxicity

Under the European Commission's New Chemical Policy both currently used and new chemicals should be tested for their toxicities in several areas, one of which was reproductive/developmental toxicity. Thousands of chemicals will need testing which will require a large number of laboratory animals. In vitro systems (as pre-screens or as validated alternatives) appear to be useful tools to reduce the number of whole animals used or refine procedures and hence decrease the cost for the chemical industry. Validated in vitro systems exist for developmental toxicity/embryotoxicity testing. Indeed, three assays have recently been validated: the whole embryo culture (WEC), the rat limb bud micromass (MM), and the embryonic stem cell test (EST). In this article, the use of primary embryonic cell culture, and in particular micromass culture, including a relatively novel chick heart micromass (MM) culture system has been described and compared to the validated D3 mouse embryonic stem cell (ESC) test.

Feasibility study of the zebrafish assay as an alternative method to screen for developmental toxicity and embryotoxicity using a training set of 27 compounds

To anticipate to increased testing needs for reproductive toxicity and 3R approaches, we studied zebrafish embryo/larva as an alternative for animal testing for developmental toxicity and embryotoxicity and evaluated a training set of 27 compounds with a standardized protocol. The classification of compounds in the zebrafish embryo/larva assay, based on a prediction model using a TI (teratogenic index) cut-off value of 2, was compared to available animal and human data. When comparing the classification of compounds in the zebrafish embryo/larva assay to available animal classification, a sensitivity of 72% and specificity of 100% were obtained. The predictive values obtained in comparison to a limited set of human data were 50, 60% respectively for teratogens, non-teratogens. Overall, we demonstrated that the zebrafish embryo/larva assay, may be used as screening tool for prioritization of compounds and could contribute to reduction of animal experiments in the field of developmental toxicology.

The embryonic stem cell test as tool to assess structure-dependent teratogenicity: the case of valproic acid

Teratogenicity can be predicted in vitro using the embryonic stem cell test (EST). The EST, which is based on the morphometric measurement of cardiomyocyte differentiation and cytotoxicity parameters, represents a scientifically validated method for the detection and classification of chemicals according to their teratogenic potency. Furthermore, an abbreviated protocol applying flow cytometry of intracellular marker proteins to determine differentiation into the cardiomyocyte lineage is available. Although valproic acid (VPA) is in worldwide clinical use as antiepileptic drug, it exhibits two severe side effects, i.e., teratogenicity and hepatotoxicity. These limitations have led to extensive research into derivatives of VPA. Here we chose VPA as model compound to test the applicability domain and to further evaluate the reliability of the EST. To this end, we study six closely related congeners of VPA and demonstrate that both the standard and the molecular flow cytometry-based EST are well suited to indicate differences in the teratogenic potency among VPA analogs that differ only in chirality or side chain length. Our data show that identical results can be obtained by using the standard EST or a shortened protocol based on flow cytometry of intracellular marker proteins. Both in vitro protocols enable to reliably determine differentiation of murine stem cells toward the cardiomyocyte lineage and to assess its chemical-mediated inhibition.

Defined culture medium for stem cell differentiation: applicability of serum-free conditions in the mouse embryonic stem cell test

The embryonic stem cell test (EST) is a validated method to assess the developmental toxicity potency of chemicals. It was developed to reduce animal use and allow faster testing for hazard assessment. The cells used in this method are maintained and differentiated in media containing foetal calf serum. This animal product is of considerable variation in quality, and individual batches require extensive testing for their applicability in the EST. Moreover, its production involves a large number of foetuses and possible animal suffering. We demonstrate the serum-free medium and feeder cell-free maintenance of the mouse embryonic stem cell line D3 and investigate the use of specific growth factors for induction of cardiac differentiation. Using a combination of bone morphogenetic protein-2, bone morphogenetic protein-4, activin A and ascorbic acid, embryoid bodies efficiently differentiated into contracting myocardium. Additionally, examining levels of intracellular marker proteins by flow cytometry not only confirmed differentiation into cardiomyocytes, but demonstrated significant differentiation into neuronal cells in the same time frame. Thus, this approach might allow for simultaneous detection of developmental effects on both early mesodermal and neuroectodermal differentiation. The serum-free conditions for maintenance and differentiation of D3 cells described here enhance the transferability and standardisation and hence the performance of the EST.

Mouse embryonic stem cell adherent cell differentiation and cytotoxicity (ACDC) assay

An adherent cell differentiation and cytotoxicity (ACDC) assay was developed using pluripotent J1 mouse embryonic stem cells (mESCs). Adherent mESCs were used to evaluate chemical-induced effects on both stem cell viability and differentiation using an in-cell western technique after a 9-day culture. DRAQ5/Sapphire700 stains were used to quantify cell number. Myosin heavy chain protein was used as a marker of cardiomyocyte differentiation and was corrected for cell number, thereby separating cytotoxicity and effects on differentiation. Acetic acid, 5-fluorouracil and bromochloroacetic acid were evaluated using the embryonic stem cell test and ACDC assay. Both systems distinguish the relative potencies of these compounds. TaqMan low-density arrays were used to characterize the time course of differentiation and effects of chemical exposure on multiple differentiation gene markers. The ACDC assay is a technique that can be used to evaluate the effects of xenobiotics on mESC differentiation and cell number using a single assay.

Evaluation of developmental toxicant identification using gene expression profiling in embryonic stem cell differentiation cultures

The murine embryonic stem cell test (EST) is an alternative testing method designed to assess potential developmental toxicity of compounds. The implementation of transcriptomics in the EST has been shown to reduce the culture duration and improve endpoint evaluation and is expected to result in an enhanced predictability and definition of the applicability domain. We evaluated the identification of developmental toxicity in the EST using two gene sets ("Van_Dartel_heartdiff_24h" and "EST biomarker genes") defined in our earlier studies. Nonexposed embryonic stem cells (ESC) differentiation cultures were sampled 0, 24, and 48 h after initiation of differentiation. Additionally, cultures exposed to 12 diverse well-characterized positive and negative developmental toxicants were isolated 24 h after the onset of exposure. Inhibition of ESC differentiation was evaluated in parallel by morphological scoring on culture day 10. Transcriptomics analysis was conducted using the Affymetrix Gene Chips platform. We applied principal component analysis on the basis of the two predefined gene sets to define the "differentiation track" that represents ESC differentiation. The significance of derivations in the gene expression-based differentiation track because of compound exposures were evaluated to determine developmental toxicity of tested compounds. We successfully predicted developmental toxicity using transcriptomics for 83% (10/12) and 67% (8/12) of the compounds, respectively, using the two predefined gene sets ("Van_Dartel_heartdiff_24h" and "EST biomarker genes"). Our study suggests that the application of transcriptomics may improve the applicability of the EST for the prediction of the developmental toxicity of chemicals.

Embryonic stem cell test remastered: comparison between the validated EST and the new molecular FACS-EST for assessing developmental toxicity in vitro

The embryonic stem cell test (EST) represents a reliable, scientifically validated in vitro system for the detection and classification of compounds according to their teratogenic potency. However, some serious issues were frequently raised against the widespread implementation and practicability of the EST in its original version. Most importantly, the evaluation of the morphological endpoint of beating cell agglomerates requires extensive experimental experience and is prone to misjudgment. Also, the testing period of 10 days is too long and costly to be attractive for industries interested in high-throughput screening of potential drug candidates. These drawbacks prompted us to work out a new molecular approach based on analysis of the expression of certain marker proteins specific for developing heart tissue. We have previously reported that quantitative flow cytometry of marker proteins (i.e., sarcomeric myosin heavy chain and alpha-actinin) can be performed at day 7 in embryonic stem cells from mice and combined with concurrent cell viability analysis. In the present study, extensive investigations were performed in order to explore the predictive power and validity of the newly established EST, subsequently referred to as molecular fluorescence activated cell sorting (FACS)-EST, by applying and comparing a set of 10 well-known embryotoxicants that encompasses the full range of chemical inherent embryotoxic potencies possible. While the molecular FACS-EST offered the same sensitivity compared to the validated EST protocol, the test duration could be significantly reduced. Due to significant improvements, this new molecular method holds promise as a sensitive, more rapid and reproducible screen highly suited to predict developmental toxicity in vivo from in vitro data.