Integration of in vitro neurotoxicity data with biokinetic modelling for the estimation of in vivo neurotoxicity

The application of human-derived cell lines in neurotoxicity studies of environmental pollutants

As industrial and societal advancements progress, an increasing number of environmental pollutants linked to human existence have been substantiated to elicit neurotoxicity and developmental neural toxicity. For research in this field, human-derived neural cell lines have become excellent in vitro models. This study examines the utilization of immortalized cell lines, specifically the SH-SY5Y human neuroblastoma cell line, and neural cells derived from human pluripotent stem cells, in the investigation of neurotoxicity and developmental neural toxicity caused by environmental pollutants. The study also explores the culturing techniques employed for these cell lines and provides an overview of the standardized assays used to assess various biological endpoints. The environmental pollutants involved include a variety of organic compounds, heavy metals, and microplastics. The utilization of cell lines derived from human sources holds significant significance in elucidating the neurotoxic effects of environmental pollutants and the underlying mechanisms. Finally, we propose the possibility of improving the in vitro model of the human nervous system and the toxicity detection methods.

An integrated in vitro approach to identifying chemically induced oxidative stress and toxicity in mitochondria

Growing concerns exist about increasing chemical usage and the potential health risks. Developing an efficient strategy to evaluate or predict the toxicity of chemicals is necessary. The mitochondria are essential organelles for cell maintenance and survival but also serve as one of the main targets of toxic chemicals. Mitochondria play an important role in the pathology of respiratory disease, and many environmental chemicals may induce impairment of the respiratory system through mitochondrial damage. This study aimed to develop integrated in vitro approaches to identify chemicals that could induce adverse health effects by increasing mitochondria-mediated oxidative stress using the H441 cells, which have a club-cell-like phenotype. Twenty-six environmental toxicants (biocides, phthalates, bisphenols, and particles) were tested, and each parameter was compared with eleven reference compounds. The inhibitory concentrations (IC20 and IC50) and benchmark doses (BMD) of the tested compounds were estimated from three in vitro assays, and the toxic concentration was determined. At the lowest IC20, the effects of compounds on mitochondrial reactive oxygen species (ROS) production and mitochondrial membrane potential (MMP) were compared. Principal component analysis and k-mean clustering were performed to cluster the chemicals that had comparable effects on the cells. Chemicals that induce mitochondrial damage at different concentrations were used for an in-depth high-tier assessment and classification as electron transport system (ETS) uncoupling or inhibiting agents. Additionally, using in vitro to in vivo extrapolation (IVIVE) tools, equivalent administration doses and maximum plasma concentrations of tested compounds in human were estimated. This study suggests an in vitro approach to identifying mitochondrial damage by integrating several in vitro toxicity tests and calculation modeling.

Advancing New Approach Methodologies (NAMs) for Tobacco Harm Reduction: Synopsis from the 2021 CORESTA SSPT-NAMs Symposium

New approach methodologies (NAMs) are emerging chemical safety assessment tools consisting of in vitro and in silico (computational) methodologies intended to reduce, refine, or replace (3R) various in vivo animal testing methods traditionally used for risk assessment. Significant progress has been made toward the adoption of NAMs for human health and environmental toxicity assessment. However, additional efforts are needed to expand their development and their use in regulatory decision making. A virtual symposium was held during the 2021 Cooperation Centre for Scientific Research Relative to Tobacco (CORESTA) Smoke Science and Product Technology (SSPT) conference (titled "Advancing New Alternative Methods for Tobacco Harm Reduction"), with the goals of introducing the concepts and potential application of NAMs in the evaluation of potentially reduced-risk (PRR) tobacco products. At the symposium, experts from regulatory agencies, research organizations, and NGOs shared insights on the status of available tools, strengths, limitations, and opportunities in the application of NAMs using case examples from safety assessments of chemicals and tobacco products. Following seven presentations providing background and application of NAMs, a discussion was held where the presenters and audience discussed the outlook for extending the NAMs toxicological applications for tobacco products. The symposium, endorsed by the CORESTA In Vitro Tox Subgroup, Biomarker Subgroup, and NextG Tox Task Force, illustrated common ground and interest in science-based engagement across the scientific community and stakeholders in support of tobacco regulatory science. Highlights of the symposium are summarized in this paper.

The SH-SY5Y human neuroblastoma cell line, a relevant in vitro cell model for investigating neurotoxicology in human: focus on organic pollutants

Investigation of the toxicity triggered by chemicals on the human brain has traditionally relied on approaches using rodent in vivo models and in vitro cell models including primary neuronal cultures and cell lines from rodents. The issues of species differences between humans and rodents, the animal ethical concerns and the time and cost required for neurotoxicity studies on in vivo animal models, do limit the use of animal-based models in neurotoxicology. In this context, human cell models appear relevant in elucidating cellular and molecular impacts of neurotoxicants and facilitating prioritization of in vivo testing. The SH-SY5Y human neuroblastoma cell line (ATCC® CRL-2266^TM) is one of the most used cell lines in neurosciences, either undifferentiated or differentiated into neuron-like cells. This review presents the characteristics of the SH-SY5Y cell line and proposes the results of a systematic review of literature on the use of this in vitro cell model for neurotoxicity research by focusing on organic environmental pollutants including pesticides, 2, 3, 7, 8-tetrachlorodibenzo-p-dioxin (TCDD), flame retardants, PFASs, parabens, bisphenols, phthalates, and PAHs. Organic environmental pollutants are widely present in the environment and increasingly known to cause clinical neurotoxic effects during fetal & child development and adulthood. Their effects on cultured SH-SY5Y cells include autophagy, cell death (apoptosis, pyroptosis, necroptosis, or necrosis), increased oxidative stress, mitochondrial dysfunction, disruption of neurotransmitter homeostasis, and alteration of neuritic length. Finally, the inherent advantages and limitations of the SH-SY5Y cell model are discussed in the context of chemical testing.

Current status and future directions for a neurotoxicity hazard assessment framework that integrates in silico approaches

Neurotoxicology is the study of adverse effects on the structure or function of the developing or mature adult nervous system following exposure to chemical, biological, or physical agents. The development of more informative alternative methods to assess developmental (DNT) and adult (NT) neurotoxicity induced by xenobiotics is critically needed. The use of such alternative methods including in silico approaches that predict DNT or NT from chemical structure (e.g., statistical-based and expert rule-based systems) is ideally based on a comprehensive understanding of the relevant biological mechanisms. This paper discusses known mechanisms alongside the current state of the art in DNT/NT testing. In silico approaches available today that support the assessment of neurotoxicity based on knowledge of chemical structure are reviewed, and a conceptual framework for the integration of in silico methods with experimental information is presented. Establishing this framework is essential for the development of protocols, namely standardized approaches, to ensure that assessments of NT and DNT based on chemical structures are generated in a transparent, consistent, and defendable manner.

Predictive Performance of Next Generation Physiologically Based Kinetic (PBK) Model Predictions in Rats Based on In Vitro and In Silico Input Data

The goal of the present study was to assess the predictive performance of a minimal generic rat physiologically based kinetic (PBK) model based on in vitro and in silico input data to predict peak plasma concentrations (Cmax) upon single oral dosing. To this purpose, a dataset was generated of 3960 Cmax predictions for 44 compounds, applying different combinations of in vitro and in silico approaches for chemical parameterization, and comparison of the predictions to reported in vivo data. Best performance was obtained when (1) the hepatic clearance was parameterized based on in vitro measured intrinsic clearance values, (2) the method of Rodgers and Rowland for calculating partition coefficients, and (3) in silico calculated fraction unbound plasma and Papp values (the latter especially for very lipophilic compounds). Based on these input data, the median Cmax of 32 compounds could be predicted within 10-fold of the observed Cmax, with 22 out of these 32 compounds being predicted within 5-fold, and 8 compounds within 2-fold. Overestimations of more than 10-fold were observed for 12 compounds, whereas no underestimations of more than 10-fold occurred. Median Cmax predictions were frequently found to be within 10-fold of the observed Cmax when the scaled unbound hepatic intrinsic clearance (Clint,u) was either higher than 20 l/h or lower than 1 l/h. Similar findings were obtained with a test set of 5 in-house BASF compounds. Overall, this study provides relevant insights in the predictive performance of a minimal PBK model based on in vitro and in silico input data.

Prediction of the dose range for adverse neurological effects of amiodarone in patients from an in vitro toxicity test by in vitro-in vivo extrapolation

Amiodarone is an antiarrhythmic agent inducing adverse effects on the nervous system, among others. We applied physiologically based pharmacokinetic (PBPK) modeling combined with benchmark dose modeling to predict, based on published in vitro data, the in vivo dose of amiodarone which may lead to adverse neurological effects in patients. We performed in vitro–in vivo extrapolation (IVIVE) from concentrations measured in the cell lysate of a rat brain 3D cell model using a validated human PBPK model. Among the observed in vitro effects, inhibition of choline acetyl transferase (ChAT) was selected as a marker for neurotoxicity. By reverse dosimetry, we transformed the in vitro concentration–effect relationship into in vivo effective human doses, using the calculated in vitro area under the curve (AUC) of amiodarone as the pharmacokinetic metric. The upper benchmark dose (BMDU) was calculated and compared with clinical doses eliciting neurological adverse effects in patients. The AUCs in the in vitro brain cell culture after 14-day repeated dosing of nominal concentration equal to 1.25 and 2.5 µM amiodarone were 1.00 and 1.99 µg*h/mL, respectively. The BMDU was 385.4 mg for intravenous converted to 593 mg for oral application using the bioavailability factor of 0.65 as reported in the literature. The predicted dose compares well with neurotoxic doses in patients supporting the hypothesis that impaired ChAT activity may be related to the molecular/cellular mechanisms of amiodarone neurotoxicity. Our study shows that predicting effects from in vitro data together with IVIVE can be used at the initial stage for the evaluation of potential adverse drug reactions and safety assessment in humans.

An integrated pathway based on in vitro data for the human hazard assessment of nanomaterials

In line with the 3R concept, nanotoxicology is shifting from a phenomenological to a mechanistic approach based on in vitro and in silico methods, with a consequent reduction in animal testing. Risk Assessment (RA) and Life Cycle Assessment (LCA) methodologies, which traditionally rely on in vivo toxicity studies, will not be able to keep up with the pace of development of new nanomaterials unless they adapt to use this new type of data. While tools and models are already available and show a great potential for future use in RA and LCA, currently none is able alone to quantitatively assess human hazards (i.e. calculate chronic NOAEL or ED₅₀ values). By highlighting which models and approaches can be used in a quantitative way with the available knowledge and data, we propose an integrated pathway for the use of in vitro data in RA and LCA. Starting with the characterization of nanoparticles' properties, the pathway then investigates how to select relevant in vitro human data, and how to bridge in vitro dose-response relationships to in vivo effects. If verified, this approach would allow RA and LCA to stir up the development of nanotoxicology by giving indications about the data and quality requirements needed in risk methodologies.

Investigating the state of physiologically based kinetic modelling practices and challenges associated with gaining regulatory acceptance of model applications

Physiologically based kinetic (PBK) models are used widely throughout a number of working sectors, including academia and industry, to provide insight into the dosimetry related to observed adverse health effects in humans and other species. Use of these models has increased over the last several decades, especially in conjunction with emerging alternative methods to animal testing, such as in vitro studies and data-driven in silico quantitative-structure-activity-relationship (QSAR) predictions. Experimental information derived from these new approach methods can be used as input for model parameters and allows for increased confidence in models for chemicals that did not have in vivo data for model calibration. Despite significant advancements in good modelling practice (GMP) for model development and evaluation, there remains some reluctance among regulatory agencies to use such models during the risk assessment process. Here, the results of a survey disseminated to the modelling community are presented in order to inform the frequency of use and applications of PBK models in science and regulatory submission. Additionally, the survey was designed to identify a network of investigators involved in PBK modelling and knowledgeable of GMP so that they might be contacted in the future for peer review of PBK models, especially in regards to vetting the models to such a degree as to gain a greater acceptance for regulatory purposes.

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Physiologically Based kinetic (PBK) models are used widely in academia, industry, and government.

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Good modelling practice (GMP) for model development and evaluation continues to expand.

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Further guidance for establishing GMP is called for.

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There remains some reluctance among regulatory agencies to use PBK models.

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The next generation of PBK models could be developed using only data from in vitro and in silico methods.

In vitro to in vivo extrapolation of effective dosimetry in developmental toxicity testing: Application of a generic PBK modelling approach

Incorporation of kinetics to quantitative in vitro to in vivo extrapolations (QIVIVE) is a key step for the realization of a non-animal testing paradigm, in the sphere of regulatory toxicology. The use of Physiologically-Based Kinetic (PBK) modelling for determining systemic doses of chemicals at the target site is accepted to be an indispensable element for such purposes. Nonetheless, PBK models are usually designed for a single or a group of compounds and are considered demanding, with respect to experimental data needed for model parameterization. Alternatively, we evaluate here the use of a more generic approach, i.e. the so-called IndusChemFate model, which is based on incorporated QSAR model parametrization. The model was used to simulate the in vivo kinetics of three diverse classes of developmental toxicants: triazoles, glycol ethers' alkoxyacetic acid metabolites and phthalate primary metabolites. The model required specific input per each class of compounds. These compounds were previously tested in three alternative assays: the whole-embryo culture (WEC), the zebrafish embryo test (ZET), and the mouse embryonic stem cell test (EST). Thereafter, the PBK-simulated blood levels at toxic in vivo doses were compared to the respective in vitro effective concentrations. Comparisons pertaining to relative potency and potency ranking with integration of kinetics were similar to previously obtained comparisons. Additionally, all three in vitro systems produced quite comparable results, and hence, a combination of alternative tests is still preferable for predicting the endpoint of developmental toxicity in vivo. This approach is put forward as biologically more plausible since plasma concentrations, rather than external administered doses, constitute the most direct in vivo dose metric.

Cell cultures in drug discovery and development: The need of reliable in vitro-in vivo extrapolation for pharmacodynamics and pharmacokinetics assessment

For ethical and cost-related reasons, use of animals for the assessment of mode of action, metabolism and/or toxicity of new drug candidates has been increasingly scrutinized in research and industrial applications. Implementation of the 3 "Rs"¹; rule (Reduction, Replacement, Refinement) through development of in silico or in vitro assays has become an essential element of risk assessment. Physiologically based pharmacokinetic (PBPK²) modeling is the most potent in silico tool used for extrapolation of pharmacokinetic parameters to animal or human models from results obtained in vitro. Although, many types of in vitro assays are conducted during drug development, use of cell cultures is the most reliable one. Two-dimensional (2D) cell cultures have been a part of drug development for many years. Nowadays, their role is decreasing in favor of three-dimensional (3D) cell cultures and co-cultures. 3D cultures exhibit protein expression patterns and intercellular junctions that are closer to in vivo states in comparison to classical monolayer cultures. Co-cultures allow for examinations of the mutual influence of different cell lines. However, the complexity and high costs of co-cultures and 3D equipment exclude such methods from high-throughput screening (HTS).³In vitro absorption, distribution, metabolism, and excretion assessment, as well as drug-drug interaction (DDI), are usually performed with the use of various cell culture based assays. Progress in in silico and in vitro methods can lead to better in vitro-in vivo extrapolation (IVIVE⁴) outcomes and have a potential to contribute towards a significant reduction in the number of laboratory animals needed for drug research. As such, concentrated efforts need to be spent towards the development of an HTS in vitro platform with satisfactory IVIVE features.

Use of physiologically based kinetic modeling-facilitated reverse dosimetry of in vitro toxicity data for prediction of in vivo developmental toxicity of tebuconazole in rats

Toxicological hazard and risk assessment largely rely on animal testing. For economic and ethical reasons, the development and validation of reliable alternative methods for these animal studies, such as in vitro assays, are urgently needed. In vitro concentration-response curves, however, need to be translated into in vivo dose-response curves for risk assessment purposes. In the present study, we translated in vitro concentration-response data of the antifungal compound tebuconazole, obtained in the ES-D3 cell differentiation assay, into predicted in vivo dose-response data for developmental toxicity using physiologically based kinetic (PBK) modeling-facilitated reverse dosimetry. Using the predicted in vivo dose-response data BMD(L)10 values for developmental toxicity in rat were calculated and compared with NOAEL values for developmental toxicity data in rats as reported in the literature. The results show that the BMDL10 value from predicted dose-response data are a reasonable approximation of the NOAEL values (ca. 3-fold difference). It is concluded that PBK modeling-facilitated reverse dosimetry of in vitro toxicity data is a promising tool to predict in vivo dose-response curves and may have the potential to define a point of departure for deriving safe exposure limits in risk assessment.

Use of Physiologically Based Kinetic Modeling-Based Reverse Dosimetry to Predict in Vivo Toxicity from in Vitro Data

The development of reliable nonanimal based testing strategies, such as in vitro bioassays, is the holy grail in current human safety testing of chemicals. However, the use of in vitro toxicity data in risk assessment is not straightforward. One of the main issues is that concentration-response curves from in vitro models need to be converted to in vivo dose-response curves. These dose-response curves are needed in toxicological risk assessment to obtain a point of departure to determine safe exposure levels for humans. Recent scientific developments enable this translation of in vitro concentration-response curves to in vivo dose-response curves using physiologically based kinetic (PBK) modeling-based reverse dosimetry. The present review provides an overview of the examples available in the literature on the prediction of in vivo toxicity using PBK modeling-based reverse dosimetry of in vitro toxicity data, showing that proofs-of-principle are available for toxicity end points ranging from developmental toxicity, nephrotoxicity, hepatotoxicity, and neurotoxicity to DNA adduct formation. This review also discusses the promises and pitfalls, and the future perspectives of the approach. Since proofs-of-principle available so far have been provided for the prediction of toxicity in experimental animals, future research should focus on the use of in vitro toxicity data obtained in human models to predict the human situation using human PBK models. This would facilitate human- instead of experimental animal-based approaches in risk assessment.

Definition of transcriptome-based indices for quantitative characterization of chemically disturbed stem cell development: introduction of the STOP-Toxukn and STOP-Toxukk tests

Stem cell-based in vitro test systems can recapitulate specific phases of human development. In the UKK test system, human pluripotent stem cells (hPSCs) randomly differentiate into cells of the three germ layers and their derivatives. In the UKN1 test system, hPSCs differentiate into early neural precursor cells. During the normal differentiation period (14 days) of the UKK system, 570 genes [849 probe sets (PSs)] were regulated >fivefold; in the UKN1 system (6 days), 879 genes (1238 PSs) were regulated. We refer to these genes as 'developmental genes'. In the present study, we used genome-wide expression data of 12 test substances in the UKK and UKN1 test systems to understand the basic principles of how chemicals interfere with the spontaneous transcriptional development in both test systems. The set of test compounds included six histone deacetylase inhibitors (HDACis), six mercury-containing compounds ('mercurials') and thalidomide. All compounds were tested at the maximum non-cytotoxic concentration, while valproic acid and thalidomide were additionally tested over a wide range of concentrations. In total, 242 genes (252 PSs) in the UKK test system and 793 genes (1092 PSs) in the UKN1 test system were deregulated by the 12 test compounds. We identified sets of 'diagnostic genes' appropriate for the identification of the influence of HDACis or mercurials. Test compounds that interfered with the expression of developmental genes usually antagonized their spontaneous development, meaning that up-regulated developmental genes were suppressed and developmental genes whose expression normally decreases were induced. The fraction of compromised developmental genes varied widely between the test compounds, and it reached up to 60 %. To quantitatively describe disturbed development on a genome-wide basis, we recommend a concept of two indices, 'developmental potency' (D _p) and 'developmental index' (D _i), whereby D _p is the fraction of all developmental genes that are up- or down-regulated by a test compound, and D _i is the ratio of overrepresentation of developmental genes among all genes deregulated by a test compound. The use of D _i makes hazard identification more sensitive because some compounds compromise the expression of only a relatively small number of genes but have a high propensity to deregulate developmental genes specifically, resulting in a low D _p but a high D _i. In conclusion, the concept based on the indices D _p and D _i offers the possibility to quantitatively express the propensity of test compounds to interfere with normal development.

Stem cell-derived systems in toxicology assessment

Industrial sectors perform toxicological assessments of their potential products to ensure human safety and to fulfill regulatory requirements. These assessments often involve animal testing, but ethical, cost, and time concerns, together with a ban on it in specific sectors, make appropriate in vitro systems indispensable in toxicology. In this study, we summarize the outcome of an EPAA (European Partnership of Alternatives to Animal Testing)-organized workshop on the use of stem cell-derived (SCD) systems in toxicology, with a focus on industrial applications. SCD systems, in particular, induced pluripotent stem cell-derived, provide physiological cell culture systems of easy access and amenable to a variety of assays. They also present the opportunity to apply the vast repository of existing nonclinical data for the understanding of in vitro to in vivo translation. SCD systems from several toxicologically relevant tissues exist; they generally recapitulate many aspects of physiology and respond to toxicological and pharmacological interventions. However, focused research is necessary to accelerate implementation of SCD systems in an industrial setting and subsequent use of such systems by regulatory authorities. Research is required into the phenotypic characterization of the systems, since methods and protocols for generating terminally differentiated SCD cells are still lacking. Organotypical 3D culture systems in bioreactors and microscale tissue engineering technologies should be fostered, as they promote and maintain differentiation and support coculture systems. They need further development and validation for their successful implementation in toxicity testing in industry. Analytical measures also need to be implemented to enable compound exposure and metabolism measurements for in vitro to in vivo extrapolation. The future of SCD toxicological tests will combine advanced cell culture technologies and biokinetic measurements to support regulatory and research applications. However, scientific and technical hurdles must be overcome before SCD in vitro methods undergo appropriate validation and become accepted in the regulatory arena.

Estimation of in vivo dose of dermally applied chemicals leading to estrogen/androgen receptor-mediated toxicity from in vitro data--Illustration with four reproductive toxicants

We present a quantitative in vitro-in vivo extrapolation framework enabling the estimation of the external dermal exposure dose from in vitro experimental data relevant to a toxicity pathway of interest. The framework adapts elements of the biological pathway altering dose (BPAD) method [Judson et al. Chem Res Toxicol 2011;24:451] to the case of dermal exposure. Dermal doses of four toxicants equivalent to concentrations characterizing their effect on estrogen receptor α or androgen receptor activity in chemical-activated luciferase expression (CALUX) assays are estimated. The analysis shows that dermal BPADs, calculated from one in vitro concentration, can differ by up to a factor of 55, due to the impact applied dose and dermal exposure scenarios can have on skin permeation kinetics. These features should therefore be taken into account in risk assessment of dermally applied chemicals.

The long and winding road of progress in the use of in vitro data for risk assessment purposes: From "carnation test" to integrated testing strategies

This paper introduces the special issue on quantitative in vitro-in vivo extrapolations (QIVIVE). It highlights important issues in the development of in vitro toxicology towards its implementation in toxicological risk assessment.

Mechanism-based testing strategy using in vitro approaches for identification of thyroid hormone disrupting chemicals

The thyroid hormone (TH) system is involved in several important physiological processes, including regulation of energy metabolism, growth and differentiation, development and maintenance of brain function, thermo-regulation, osmo-regulation, and axis of regulation of other endocrine systems, sexual behaviour and fertility and cardiovascular function. Therefore, concern about TH disruption (THD) has resulted in strategies being developed to identify THD chemicals (THDCs). Information on potential of chemicals causing THD is typically derived from animal studies. For the majority of chemicals, however, this information is either limited or unavailable. It is also unlikely that animal experiments will be performed for all THD relevant chemicals in the near future for ethical, financial and practical reasons. In addition, typical animal experiments often do not provide information on the mechanism of action of THDC, making it harder to extrapolate results across species. Relevant effects may not be identified in animal studies when the effects are delayed, life stage specific, not assessed by the experimental paradigm (e.g., behaviour) or only occur when an organism has to adapt to environmental factors by modulating TH levels. Therefore, in vitro and in silico alternatives to identify THDC and quantify their potency are needed. THDC have many potential mechanisms of action, including altered hormone production, transport, metabolism, receptor activation and disruption of several feed-back mechanisms. In vitro assays are available for many of these endpoints, and the application of modern '-omics' technologies, applicable for in vivo studies can help to reveal relevant and possibly new endpoints for inclusion in a targeted THDC in vitro test battery. Within the framework of the ASAT initiative (Assuring Safety without Animal Testing), an international group consisting of experts in the areas of thyroid endocrinology, toxicology of endocrine disruption, neurotoxicology, high-throughput screening, computational biology, and regulatory affairs has reviewed the state of science for (1) known mechanisms for THD plus examples of THDC; (2) in vitro THD tests currently available or under development related to these mechanisms; and (3) in silico methods for estimating the blood levels of THDC. Based on this scientific review, the panel has recommended a battery of test methods to be able to classify chemicals as of less or high concern for further hazard and risk assessment for THD. In addition, research gaps and needs are identified to be able to optimize and validate the targeted THD in vitro test battery for a mechanism-based strategy for a decision to opt out or to proceed with further testing for THD.

Integrated testing strategies for safety assessments

Despite the fact that toxicology uses many stand-alone tests, a systematic combination of several information sources very often is required: Examples include: when not all possible outcomes of interest (e.g., modes of action), classes of test substances (applicability domains), or severity classes of effect are covered in a single test; when the positive test result is rare (low prevalence leading to excessive false-positive results); when the gold standard test is too costly or uses too many animals, creating a need for prioritization by screening. Similarly, tests are combined when the human predictivity of a single test is not satisfactory or when existing data and evidence from various tests will be integrated. Increasingly, kinetic information also will be integrated to make an in vivo extrapolation from in vitro data. Integrated Testing Strategies (ITS) offer the solution to these problems. ITS have been discussed for more than a decade, and some attempts have been made in test guidance for regulations. Despite their obvious potential for revamping regulatory toxicology, however, we still have little guidance on the composition, validation, and adaptation of ITS for different purposes. Similarly, Weight of Evidence and Evidence-based Toxicology approaches require different pieces of evidence and test data to be weighed and combined. ITS also represent the logical way of combining pathway-based tests, as suggested in Toxicology for the 21st Century. This paper describes the state of the art of ITS and makes suggestions as to the definition, systematic combination, and quality assurance of ITS.

From QSAR to QSIIR: searching for enhanced computational toxicology models

Quantitative structure activity relationship (QSAR) is the most frequently used modeling approach to explore the dependency of biological, toxicological, or other types of activities/properties of chemicals on their molecular features. In the past two decades, QSAR modeling has been used extensively in drug discovery process. However, the predictive models resulted from QSAR studies have limited use for chemical risk assessment, especially for animal and human toxicity evaluations, due to the low predictivity of new compounds. To develop enhanced toxicity models with independently validated external prediction power, novel modeling protocols were pursued by computational toxicologists based on rapidly increasing toxicity testing data in recent years. This chapter reviews the recent effort in our laboratory to incorporate the biological testing results as descriptors in the toxicity modeling process. This effort extended the concept of QSAR to quantitative structure in vitro-in vivo relationship (QSIIR). The QSIIR study examples provided in this chapter indicate that the QSIIR models that based on the hybrid (biological and chemical) descriptors are indeed superior to the conventional QSAR models that only based on chemical descriptors for several animal toxicity endpoints. We believe that the applications introduced in this review will be of interest and value to researchers working in the field of computational drug discovery and environmental chemical risk assessment.

Human embryonic stem cell-derived test systems for developmental neurotoxicity: a transcriptomics approach

Developmental neurotoxicity (DNT) and many forms of reproductive toxicity (RT) often manifest themselves in functional deficits that are not necessarily based on cell death, but rather on minor changes relating to cell differentiation or communication. The fields of DNT/RT would greatly benefit from in vitro tests that allow the identification of toxicant-induced changes of the cellular proteostasis, or of its underlying transcriptome network. Therefore, the ‘human embryonic stem cell (hESC)-derived novel alternative test systems (ESNATS)’ European commission research project established RT tests based on defined differentiation protocols of hESC and their progeny. Valproic acid (VPA) and methylmercury (MeHg) were used as positive control compounds to address the following fundamental questions: (1) Does transcriptome analysis allow discrimination of the two compounds? (2) How does analysis of enriched transcription factor binding sites (TFBS) and of individual probe sets (PS) distinguish between test systems? (3) Can batch effects be controlled? (4) How many DNA microarrays are needed? (5) Is the highest non-cytotoxic concentration optimal and relevant for the study of transcriptome changes? VPA triggered vast transcriptional changes, whereas MeHg altered fewer transcripts. To attenuate batch effects, analysis has been focused on the 500 PS with highest variability. The test systems differed significantly in their responses (<20 % overlap). Moreover, within one test system, little overlap between the PS changed by the two compounds has been observed. However, using TFBS enrichment, a relatively large ‘common response’ to VPA and MeHg could be distinguished from ‘compound-specific’ responses. In conclusion, the ESNATS assay battery allows classification of human DNT/RT toxicants on the basis of their transcriptome profiles.

Cutting Edge PBPK Models and Analyses: Providing the Basis for Future Modeling Efforts and Bridges to Emerging Toxicology Paradigms

Physiologically based Pharmacokinetic (PBPK) models are used for predictions of internal or target dose from environmental and pharmacologic chemical exposures. Their use in human risk assessment is dependent on the nature of databases (animal or human) used to develop and test them, and includes extrapolations across species, experimental paradigms, and determination of variability of response within human populations. Integration of state-of-the science PBPK modeling with emerging computational toxicology models is critical for extrapolation between in vitro exposures, in vivo physiologic exposure, whole organism responses, and long-term health outcomes. This special issue contains papers that can provide the basis for future modeling efforts and provide bridges to emerging toxicology paradigms. In this overview paper, we present an overview of the field and introduction for these papers that includes discussions of model development, best practices, risk-assessment applications of PBPK models, and limitations and bridges of modeling approaches for future applications. Specifically, issues addressed include: (a) increased understanding of human variability of pharmacokinetics and pharmacodynamics in the population, (b) exploration of mode of action hypotheses (MOA), (c) application of biological modeling in the risk assessment of individual chemicals and chemical mixtures, and (d) identification and discussion of uncertainties in the modeling process.

Application of physiologically based pharmacokinetic models in chemical risk assessment

Post-exposure risk assessment of chemical and environmental stressors is a public health challenge. Linking exposure to health outcomes is a 4-step process: exposure assessment, hazard identification, dose response assessment, and risk characterization. This process is increasingly adopting "in silico" tools such as physiologically based pharmacokinetic (PBPK) models to fine-tune exposure assessments and determine internal doses in target organs/tissues. Many excellent PBPK models have been developed. But most, because of their scientific sophistication, have found limited field application-health assessors rarely use them. Over the years, government agencies, stakeholders/partners, and the scientific community have attempted to use these models or their underlying principles in combination with other practical procedures. During the past two decades, through cooperative agreements and contracts at several research and higher education institutions, ATSDR funded translational research has encouraged the use of various types of models. Such collaborative efforts have led to the development and use of transparent and user-friendly models. The "human PBPK model toolkit" is one such project. While not necessarily state of the art, this toolkit is sufficiently accurate for screening purposes. Highlighted in this paper are some selected examples of environmental and occupational exposure assessments of chemicals and their mixtures.

Translational research to develop a human PBPK models tool kit-volatile organic compounds (VOCs)

Toxicity and exposure evaluations remain the two of the key components of human health assessment. While improvement in exposure assessment relies on a better understanding of human behavior patterns, toxicity assessment still relies to a great extent on animal toxicity testing and human epidemiological studies. Recent advances in computer modeling of the dose-response relationship and distribution of xenobiotics in humans to important target tissues have advanced our abilities to assess toxicity. In particular, physiologically based pharmacokinetic (PBPK) models are among the tools than can enhance toxicity assessment accuracy. Many PBPK models are available to the health assessor, but most are so difficult to use that health assessors rarely use them. To encourage their use these models need to have transparent and user-friendly formats. To this end the Agency for Toxic Substances and Disease Registry (ATSDR) is using translational research to increase PBPK model accessibility, understandability, and use in the site-specific health assessment arena. The agency has initiated development of a human PBPK tool-kit for certain high priority pollutants. The tool kit comprises a series of suitable models. The models are recoded in a single computer simulation language and evaluated for use by health assessors. While not necessarily being state-of-the-art code for each chemical, the models will be sufficiently accurate to use for screening purposes. This article presents a generic, seven-compartment PBPK model for six priority volatile organic compounds (VOCs): benzene (BEN), carbon tetrachloride (CCl(4)), dichloromethane (DCM), perchloroethylene (PCE), trichloroethylene (TCE), and vinyl chloride (VC). Limited comparisons of the generic and original model predictions to published kinetic data were conducted. A goodness of fit was determined by calculating the means of the sum of the squared differences (MSSDs) for simulation vs. experimental kinetic data using the generic and original models. Using simplified solvent exposure assumptions for oral ingestion and inhalation, steady-state blood concentrations of each solvent were simulated for exposures equivalent to the ATSDR Minimal Risk Levels (MRLs). The predicted blood levels were then compared to those reported in the National Health and Nutrition Examination Survey (NHANES). With the notable exception of BEN, simulations of combined oral and inhalation MRLs using our generic VOC model yielded blood concentrations well above those reported for the 95th percentile blood concentrations for the U.S. populations, suggesting no health concerns. When the PBPK tool kit is fully developed, risk assessors will have a readily accessible tool for evaluating human exposure to a variety of environmental pollutants.

Recent trends in statistical QSAR modeling of environmental chemical toxicity

Quantitative cheminformatics approaches such as QSAR modeling find growing applications in chemical risk assessment. Traditional methods rely on the use of calculated chemical descriptors of molecules and relatively small training sets. However, in recent years, there is a trend toward the increased use of in vitro biological testing approaches to reduce both the length of experimental studies and the animal use for chemical risk assessment. Furthermore, there is also much greater emphasis on model validation using external datasets to enable the reliable use of computational models as part of regulatory decision making. In this chapter, recent trends emphasizing the need for both careful curation of experimental data prior to model development and rigorous model validation are investigated. Furthermore, recent approaches to chemical toxicity prediction that employ both chemical descriptors and in vitro screening data for developing novel hybrid chemical/biological models are being reviewed. Examples of respective application studies that employ novel workflows for model developments are described and recent important efforts by several academic, nonprofit, and industrial groups to start placing both data and, especially, models in the public domain are discussed.

MEGen: A Physiologically Based Pharmacokinetic Model Generator

Physiologically based pharmacokinetic models are being used in an increasing number of different areas. However, they are perceived as complex, data hungry, resource intensive, and time consuming. In addition, model validation and verification are hindered by the relative complexity of the equations. To begin to address these issues a web application called MEGen for the rapid construction and documentation of bespoke deterministic PBPK model code is under development. MEGen comprises a parameter database and a model code generator that produces code for use in several commercial software packages and one that is freely available. Here we present an overview of the current capabilities of MEGen, and discuss future developments.

Effects of ethylene glycol ethers on cell viability in the human neuroblastoma SH-SY5Y cell line

Ethylene glycol ethers (EGEs) are a class of chemicals used extensively in the manufacture of a wide range of domestic and industrial products, which may result in human exposure and toxicity. Hematologic and reproductive toxicity of EGEs are well known whereas their action on neuronal cell viability has not been studied so far. In the present study, we investigated the effects of some EGEs on cell viability and on the hydrogen peroxide-induced damage in the human neuroblastoma (SH-SY5Y) cells. It has been found that 2-phenoxyethanol in a concentration-dependent manner (5-25 mM, 24 h) increased the basal and H(2)O(2)-induced lactate dehydrogenase (LDH) release and 3-[4,5-dimethylthiazol-2-yl]2,5-diphenyl tetrazolium bromide (MTT) reduction. 2-Butoxyethanol given alone did not affect LDH release and MTT reduction but concentration-dependently enhanced the cytotoxic effect of H(2)O(2). 2-Isopropoxyethanol significantly and concentration-dependently (1-25 mM) increased the basal LDH release and attenuated MTT reduction, but did not potentiate the cytotoxic effect of H(2)O(2). Contrary to this, 2-methoxyethanol did not show a cytotoxic effect while 2-ethoxyethanol at high concentrations intensified the hydrogen peroxide action. This study demonstrated that among the EGEs studied, 2-phenoxyethanol showed the most consistent cytotoxic effect on neurons in in vitro conditions and enhanced the hydrogen peroxide action. 2-Isopropoxyethanol had also a potent cytotoxic effect, but it did not enhance the hydrogen peroxide action, whereas 2-butoxyethanol only potentiated cytotoxic effect of H(2)O(2). It is concluded that the results of the present study should be confirmed in in vivo conditions and that some EGEs, especially 2-phenoxyethanol, 2-butoxyethanol and 2-isopropoxyethanol, may be responsible for initiation or exacerbation of neuronal cell damage.

Evaluation of research activities and research needs to increase the impact and applicability of alternative testing strategies in risk assessment practice

The present paper aims at identifying strategies to increase the impact and applicability of alternative testing strategies in risk assessment. To this end, a quantitative and qualitative literature evaluation was performed on (a) current research efforts in the development of in vitro methods aiming for alternatives to animal testing, (b) the possibilities and limitations of in vitro methods for regulatory purposes and (c) the potential of physiologically-based kinetic (PBK) modeling to improve the impact and applicability of in vitro methods in risk assessment practice. Overall, the evaluation showed that the focus of state-of-the-art research activities does not seem to be optimally directed at developing in vitro alternatives for those endpoints that are most animal-demanding, such as reproductive and developmental toxicity, and carcinogenicity. A key limitation in the application of in vitro alternatives to such systemic endpoints is that in vitro methods do not provide so-called points of departure, necessary for regulators to set safe exposure limits. PBK-modeling could contribute to overcoming this limitation by providing a method that allows extrapolation of in vitro concentration-response curves to in vivo dose-response curves. However, more proofs of principle are required.

An imaging assay to analyze primary neurons for cellular neurotoxicity

The development of high-content screening technologies including automated immunostaining, automated image acquisition and automated image analysis have enabled higher throughput of cellular imaging-based assays. Here we used high-content imaging to thoroughly characterize the cultures of primary rat cerebellar granule neurons (CGNs). We describe procedures to isolate and cultivate the CGNs in 96-well and 384-well format, as well as a procedure to freeze and thaw the CGNs. These methods allow the use of CGNs in 96-well format analyzing 2500 samples per experiment using freshly isolated cells. Down-scaling to 384-well format and freezing and thawing of the CGNs allow even higher throughput. A cellular assay with rat CGN cultures was established to study the neurotoxicity of compounds in order to filter out toxic compounds at an early phase of drug development. The imaging-based toxicity assay was able to reveal adverse effects of compounds on primary neurons which were not detected in neuroblastoma or other cell lines tested.

Single-cell ELISA and flow cytometry as methods for highlighting potential neuronal and astrocytic toxicant specificity

The timeline imposed by recent worldwide chemical legislation is not amenable to conventional in vivo toxicity testing, requiring the development of rapid, economical in vitro screening strategies which have acceptable predictive capacities. When acquiring regulatory neurotoxicity data, distinction on whether a toxic agent affects neurons and/or astrocytes is essential. This study evaluated neurofilament (NF) and glial fibrillary acidic protein (GFAP) directed single-cell (S-C) ELISA and flow cytometry as methods for distinguishing cell-specific cytoskeletal responses, using the established human NT2 neuronal/astrocytic (NT2.N/A) co-culture model and a range of neurotoxic (acrylamide, atropine, caffeine, chloroquine, nicotine) and non-neurotoxic (chloramphenicol, rifampicin, verapamil) test chemicals. NF and GFAP directed flow cytometry was able to identify several of the test chemicals as being specifically neurotoxic (chloroquine, nicotine) or astrocytoxic (atropine, chloramphenicol) via quantification of cell death in the NT2.N/A model at cytotoxic concentrations using the resazurin cytotoxicity assay. Those neurotoxicants with low associated cytotoxicity are the most significant in terms of potential hazard to the human nervous system. The NF and GFAP directed S-C ELISA data predominantly demonstrated the known neurotoxicants only to affect the neuronal and/or astrocytic cytoskeleton in the NT2.N/A cell model at concentrations below those affecting cell viability. This report concluded that NF and GFAP directed S-C ELISA and flow cytometric methods may prove to be valuable additions to an in vitro screening strategy for differentiating cytotoxicity from specific neuronal and/or astrocytic toxicity. Further work using the NT2.N/A model and a broader array of toxicants is appropriate in order to confirm the applicability of these methods.

The network formation assay: a spatially standardized neurite outgrowth analytical display for neurotoxicity screening

We present a rapid, reproducible and sensitive neurotoxicity testing platform that combines the benefits of neurite outgrowth analysis with cell patterning. This approach involves patterning neuronal cells within a hexagonal array to standardize the distance between neighbouring cellular nodes, and thereby standardize the length of the neurite interconnections. This feature coupled with defined assay coordinates provides a streamlined display for rapid and sensitive analysis. We have termed this the network formation assay (NFA). To demonstrate the assay we have used a novel cell patterning technique involving thin film poly(dimethylsiloxane) (PDMS) microcontact printing. Differentiated human SH-SY5Y neuroblastoma cells colonized the array with high efficiency, reliably producing pattern occupancies above 70%. The neuronal array surface supported neurite outgrowth, resulting in the formation of an interconnected neuronal network. Exposure to acrylamide, a neurotoxic reference compound, inhibited network formation. A dose-response curve from the NFA was used to determine a 20% network inhibition (NI(20)) value of 260 microM. This concentration was approximately 10-fold lower than the value produced by a routine cell viability assay, and demonstrates that the NFA can distinguish network formation inhibitory effects from gross cytotoxic effects. Inhibition of the mitogen-activated protein kinase (MAPK) ERK1/2 and phosphoinositide-3-kinase (PI-3K) signaling pathways also produced a dose-dependent reduction in network formation at non-cytotoxic concentrations. To further refine the assay a simulation was developed to manage the impact of pattern occupancy variations on network formation probability. Together these developments and demonstrations highlight the potential of the NFA to meet the demands of high-throughput applications in neurotoxicology and neurodevelopmental biology.

Biokinetic modeling and in vitro-in vivo extrapolations

The introduction of in vitro methodologies in the toxicological risk assessment process requires a number of prerequisites regarding both the toxicodynamics and the biokinetics of the compounds under study. In vitro systems will need to be relevant for measuring those structural and physiological changes that are good indicators for adverse effects. Furthermore, the dose metric found to have an effect in the in vitro system should be relevant. One element in defining the appropriate dose metric is related to the kinetic behavior of the compound in the in vitro system: binding to proteins, binding to plastic, evaporation, and the interaction between the culture medium and the cells. Ways to measure and model "in vitro biokinetics" are described. Second, the appropriate dose metric in vitro, e.g., the effective concentration, will need to be extrapolated to relevant in vivo exposure scenarios. The application of physiologically based biokinetic modelling is essential in such extrapolations. The parameters needed to build these models often can be estimated based on nonanimal data, namely chemical properties (QSARs) and in vitro experiments.

Advancing exposure characterization for chemical evaluation and risk assessment

A new generation of scientific tools has emerged to rapidly measure signals from cells, tissues, and organisms following exposure to chemicals. High-visibility efforts to apply these tools for efficient toxicity testing raise important research questions in exposure science. As vast quantities of data from high-throughput screening (HTS) in vitro toxicity assays become available, this new toxicity information must be translated to assess potential risks to human health from environmental exposures. Exposure information is required to link information on potential toxicity of environmental contaminants to real-world health outcomes. In the immediate term, tools are required to characterize and classify thousands of environmental chemicals in a rapid and efficient manner to prioritize testing and assess potential for risk to human health. Rapid risk assessment requires prioritization based on both hazard and exposure dimensions of the problem. To address these immediate needs within the context of longer term objectives for chemical evaluation and risk management, a translation framework is presented for incorporating toxicity and exposure information to inform public health decisions at both the individual and population levels. Examples of required exposure science contributions are presented with a focus on early advances in tools for modeling important links across the source-to-outcome paradigm. ExpoCast, a new U.S. Environmental Protection Agency (EPA) program aimed at developing novel approaches and metrics to screen and evaluate chemicals based on the potential for biologically relevant human exposures is introduced. The goal of ExpoCast is to advance characterization of exposure required to translate findings in computational toxicology to information that can be directly used to support exposure and risk assessment for decision making and improved public health.

Twenty-First Century Challenges for In Vitro Neurotoxicity

During the last 40 years, studies incorporating in vitro methodologies have greatly advanced our understanding of human nerve cell biology. Attempts have been made to apply these to investigations of neurotoxicity. Due to the complexity of the nervous system, underpinned by an array of integrated interactions between a host of cell types, it is concluded that, at present, alternative neural models are most successful in determining the underlying mechanisms which can cause perturbation of normal functioning of the nervous system, both in adults and during the embryonic period. The use of tiered batteries of test models has been proposed in screening programmes for neurotoxicity, with the generation of much encouraging data in laboratories across the globe. This review aims to discuss the development of neural alternatives, considers the various model systems available, and highlights specific neuronal endpoints which can be tested, in addition to the cytotoxic evaluation of neuronal viability. Developments in molecular and stem cell biology, which are appropriate to neural tissue, and which offer the prospect of exciting advances for the next decade, are cited.

Evaluation of the importance of astrocytes when screening for acute toxicity in neuronal cell systems

Reliable, high throughput, in vitro preliminary screening batteries have the potential to greatly accelerate the rate at which regulatory neurotoxicity data is generated. This study evaluated the importance of astrocytes when predicting acute toxic potential using a neuronal screening battery of pure neuronal (NT2.N) and astrocytic (NT2.A) and integrated neuronal/astrocytic (NT2.N/A) cell systems derived from the human NT2.D1 cell line, using biochemical endpoints (mitochondrial membrane potential (MMP) depolarisation and ATP and GSH depletion). Following exposure for 72 h, the known acute human neurotoxicants trimethyltin-chloride, chloroquine and 6-hydroxydopamine were frequently capable of disrupting biochemical processes in all of the cell systems at non-cytotoxic concentrations. Astrocytes provide key metabolic and protective support to neurons during toxic challenge in vivo and generally the astrocyte containing cell systems showed increased tolerance to toxicant insult compared with the NT2.N mono-culture in vitro. Whilst there was no consistent relationship between MMP, ATP and GSH log IC(50) values for the NT2.N/A and NT2.A cell systems, these data did provide preliminary evidence of modulation of the acute neuronal toxic response by astrocytes. In conclusion, the suitability of NT2 neurons and astrocytes as cell systems for acute toxicity screening deserves further investigation.

ECVAM's ongoing activities in the area of acute oral toxicity

The 7th Amendment of the Cosmetics Directive (2003/15/EC) set up timelines for banning animal testing and marketing of cosmetic products and their ingredients tested on animals. For most of the human health effects, including acute toxicity, the deadline for these bans was in March 2009. Moreover, the new Regulation EC 1907/2006 on Registration, Evaluation, Authorisation and Restriction of Chemicals (REACH) provided a strong impetus towards the application of alternative approaches to reduce the number of animals used for toxicological testing. Therefore, the European Centre for the Validation of Alternative Methods (ECVAM) is currently putting considerable effort into developing and validating alternative methods in the field of acute toxicity. The main activities in this area include: (1) the Integrated Project ACuteTox, funded by the European Commission's 6th Framework Programme in 2005 with the aim to develop and pre-validate a testing strategy to fully replace acute oral toxicity testing in vivo; (2) a follow-up validation study to assess the predictive capacity of the validated BALB/3T3 Neutral Red Uptake cytotoxicity assay to discriminate between toxic/hazardous (LD(50)<2,000 mg/kg) substances and substances not classified for acute toxicity (LD(50)>2,000 mg/kg); (3) an approach to identify compounds with LD(50)>2,000 mg/kg using information from 28-days repeated dose toxicity studies.

Review of methods for the reduction of dietary content and toxicity of acrylamide

Potentially toxic acrylamide is largely derived from heat-induced reactions between the amino group of the free amino acid asparagine and carbonyl groups of glucose and fructose in cereals, potatoes, and other plant-derived foods. This overview surveys and consolidates the following dietary aspects of acrylamide: distribution in food originating from different sources; consumption by diverse populations; reduction of the acrylamide content in the diet; and suppression of adverse effects in vivo. Methods to reduce adverse effects of dietary acrylamide include (a) selecting potato, cereal, and other plant varieties for dietary use that contain low levels of the acrylamide precursors, namely, asparagine and glucose; (b) removing precursors before processing; (c) using the enzyme asparaginase to hydrolyze asparagine to aspartic acid; (d) selecting processing conditions (pH, temperature, time, processing and storage atmosphere) that minimize acrylamide formation; (e) adding food ingredients (acidulants, amino acids, antioxidants, nonreducing carbohydrates, chitosan, garlic compounds, protein hydrolysates, proteins, metal salts) that have been reported to prevent acrylamide formation; (f) removing/trapping acrylamide after it is formed with the aid of chromatography, evaporation, polymerization, or reaction with other food ingredients; and (g) reducing in vivo toxicity. Research needs are suggested that may further facilitate reducing the acrylamide burden of the diet. Researchers are challenged to (a) apply the available methods and to minimize the acrylamide content of the diet without adversely affecting the nutritional quality, safety, and sensory attributes, including color and flavor, while maintaining consumer acceptance; and (b) educate commercial and home food processors and the public about available approaches to mitigating undesirable effects of dietary acrylamide.

The contribution of in vitro toxicity data in hazard and risk assessment: current limitations and future perspectives

This paper discusses the role of in vitro toxicology in hazard and risk assessment. A short historical overview of the development of the field of in vitro toxicology is given, showing the possibilities, as well as the limitation for using in vitro methods in screening and in studying mechanisms of toxicity. Although the development of the use of in vitro methods over the last two or three decades is vast, the application in risk assessment is still limited. The need for a more integrated approach, i.e. using in vitro data not in isolation, but in combination with data on structure-activity and in silico biokinetic models is discussed. It is foreseen that the role of in vitro methods in future risk assessments will greatly enhance, also in the light of recent developments in technologies such as genomics and systems biology.