Assay Application

ICCVAM and its member agencies conduct, support, and evaluate studies investigating the suitability of new laboratory methods for specific purposes. Methods evaluated during 2020 and 2021 addressed endpoints including inhalation toxicity, skin sensitization, eye irritation, and developmental toxicity.

Electrophilic Allergen Screening Assay Validation Study

Binding of a chemical to skin proteins is the first step in the development of allergic contact dermatitis. The electrophilic allergen screening assay is a chemical assay that measures light absorbance or a fluorescent signal in proportion to a chemical’s tendency to bind to proteins.

The electrophilic allergen screening assay, developed by scientists at NIOSH, was nominated to ICCVAM to evaluate its usefulness for identifying potential skin sensitizers. Five ICCVAM member agencies are participating in an ongoing validation study of the assay. Testing of 10 chemicals during 2018 showed that the method has sufficiently good reproducibility and accuracy rates to support further evaluation. In 2019, scientists at CPSC and NIST modified the assay to a 96-well format to increase throughput and accessibility of the assay; details of this process are described in an abstract (Gordon et al.) accepted for presentation at the Society of Toxicology 2020 annual meeting. Testing of 20 chemicals will be completed during 2022 using the 96-well assay.

OptiSafe Validation Study

NICEATM coordinated a multi-laboratory validation study to determine the reliability and relevance of the OptiSafe test method. In this method, a test substance is applied to a semi-permeable membrane to assess the substance’s potential to cause eye irritation.

The study was completed in 2018 and a report on the study has been published (Choksi et al. 2020). The study demonstrated that the OptiSafe method is useful for identifying non-surfactant substances that do not require classification for ocular irritancy and thus can reduce the use of animals for this type of testing.

Developing a Defined Approach for Eye Irritation Testing

NICEATM, PETA Science Consortium International e.V., EPA, and CropLife America member companies are collaborating to develop an in vitro defined approach for hazard classification of eye irritation potential of agrochemical formulations. A three-phased prospective evaluation was designed to (1) assess the applicability of seven in vitro eye irritation/corrosion protocols to agrochemical formulations and (2) develop a defined approach for agrochemical formulations testing for prediction of U.S. and international irritancy classifications. Sixteen formulations were tested in the bovine corneal opacity and permeability, neutral red release, isolated chicken eye, EpiOcular, and porcine cornea reversibility test methods. Results were compared to the hazard classification assigned based on the in vivo rabbit test (Choksi et al. 2021). For each test method, at least one formulation was discordant with the in vivo rabbit classification, but none of the methods yielded discordant results for all tested formulations. Initial results indicate that certain test method combinations may be used to predict in vivo outcomes. Additional analyses will focus on physicochemical properties and composition of tested formulations to determine if there are any common features that impact in vitro test method performance. Analysis results were presented in a poster (Choksi et al.) at the 11th World Congress on Alternatives and Animal Use in the Life Sciences. These data will be considered in the context of a recent review of eye anatomy and mechanisms of chemically induced eye irritation in humans and other species (Clippinger et al. 2021), which supported reduced reliance on comparisons to rabbit data to show the validity of other methods in favor of a greater focus on human relevance and assay reliability.

Interlaboratory Study to Examine Effects of Key Protocol Elements for Zebrafish Developmental Toxicity Studies

To enable broader adoption of zebrafish for toxicological screening, NTP established the Systematic Evaluation of the Application of Zebrafish in Toxicology (SEAZIT) program.

An initial information-gathering phase of the SEAZIT program identified a need for an interlaboratory study to more closely examine the effects of variation in key protocol elements in developmental toxicity studies. The study was designed to determine the effect of chorion removal and exposure media renewal on study outcomes. Participating laboratories will use in-house protocols to test a defined chemical set while varying these two protocol elements. The chemical set, which was designed to provide overlap with other NTP studies, includes chemicals with a range of physicochemical properties and developmental effects. Many of the chemicals have in vivo reference data available from rodent and other zebrafish studies. The interlaboratory study also includes a pilot study of chemical kinetics in support of future studies of ADME in zebrafish. Dose range-finding experiments study began in 2019, although progress over the last 2 years has been delayed due to COVID-related laboratory closures.

In addition to advancing broader adoption of zebrafish for toxicological screening, SEAZIT is also supporting development of best practices for data analysis. To this end, the data generated in this study will be made publicly available, so that all study data may be used by investigators to estimate consensus toxicity values for each chemical.

Testing to Expand the Applicability Domain of Three In Vitro Skin Sensitization Assays

In 2021, OECD issued Guideline 497, Defined Approaches on Skin Sensitisation. Drafted and sponsored by ICCVAM agency scientists and international partners, Guideline 497 is the first internationally harmonized guideline to describe a non-animal approach that can be used to replace an animal test to identify skin sensitizers.

To assess and expand the potential applicability of these defined approaches and those accepted by EPA to a broader range of chemical types, ICCVAM agencies nominated more than 200 chemicals for additional testing in the direct peptide reactivity assay, the KeratinoSens assay, and the human cell line activation test. Chemicals tested included pesticide ingredients and formulations, industrial chemicals, and personal care product ingredients. NTP completed this testing in 2020; reports are being prepared for publication in 2022. A poster describing performance of these test methods individually and in the defined approach for testing of 27 agrochemical formulations (Strickland et al.) was presented at the 11th World Congress on Alternatives and Animal Use in the Life Sciences.

Application of the Monocyte Activation Test for Medical Device Pyrogen Testing

Pyrogens are substances that can produce fever when present as contaminants in a drug or medical device. Most pyrogens are biological substances derived from bacteria, fungi, and viruses; material-mediated pyrogens, while less common, may also be present. Drugs for injection and medical device products for implantation or other systemic exposure should meet pyrogen limit specifications before they are marketed.

Animal-based pyrogen tests are often conducted to investigate the presence of pyrogens. Non-animal monocyte activation tests (MAT) are widely available but infrequently used for pyrogen testing. To review MAT and discuss ongoing challenges to its widespread implementation for medical device testing, NICEATM and the PETA Science Consortium International e.V. co-organized a September 2018 workshop. A workshop report has been published in ALTEX (Brown et al. 2021).

Workshop participants explored how the FDA Medical Device Development Tools Program could be used to qualify MAT as a standalone pyrogen test for specific medical device contexts of use. Attendees discussed practical aspects of pyrogen testing and the evidence needed to support qualification of MAT as a replacement for animal-based pyrogen tests. There was general agreement that MAT could be qualified as acceptable for batch-release testing for microbial-based pyrogens. However, additional studies were recommended to demonstrate its ability to detect known material-mediated pyrogens. This testing would determine whether the assay can be used for both biocompatibility and sterility or if other information on material-mediated pyrogens would be needed to address biocompatibility. Participants also discussed information gaps on material-mediated pyrogens, potential test controls, and other challenges and opportunities for implementing the use of MAT as a comprehensive pyrogen test.

Human-relevant Approaches to Assess Eye Corrosion/irritation Potential

Although multiple internationally harmonized test guidelines describe in vitro and ex vivo eye irritation and corrosion test methods for regulatory use, these methods have not been widely adopted for testing agrochemical formulations due to a lack of concordance with parallel results from the rabbit eye test. The inherent variability of the rabbit test, differences in the anatomy of the rabbit and human eyes, and differences in modeling exposures in rabbit eyes relative to human eyes contribute to this lack of concordance. Because the regulatory purpose for these tests is protection of human health, there is a need for a testing approach based on human biology. A paper coauthored by EPA and NICEATM (Clippinger et al. 2021) reviews the available in vivo, in vitro, and ex vivo test methods with respect to their relevance to human ocular anatomy, anticipated exposure scenarios, and the mechanisms of eye irritation/corrosion in humans. Consideration of the mechanisms of eye irritation and the strengths and limitations of the in vivo, in vitro, and ex vivo test methods show that the in vitro and ex vivo methods are as or more reflective of human biology and less variable than the currently used rabbit approach. The paper suggests approaches to further optimize the most promising methods to distinguish between severe (corrosive), moderate, mild, and non-irritants and provide information about the reversibility of effects. It also considers the utility of including additional information such as physicochemical properties in a hazard assessment, consistent with accepted guidance (OECD 2019) on integrated approaches to testing and assessment for potential eye irritation.

Selecting a Minimal Set of Androgen Receptor Assays for Screening Chemicals

Screening certain environmental chemicals for their ability to interact with endocrine targets, including the androgen receptor, is an important global concern. EPA and NICEATM previously developed a model using a battery of 11 in vitro androgen receptor assays to predict in vivo androgen receptor activity (Kleinstreuer et al. 2017). Follow-up work completed in 2020 (Judson et al. 2020) revised the modeling approach to incorporate data from newly available assays and demonstrate that subsets of assays can provide close to the same level of predictivity. These subset models were evaluated against the full 11-assay model using 1820 chemicals, as well as in vitro and in vivo reference chemicals from the literature. Agonist batteries of as few as six assays and antagonist batteries of as few as five assays can yield balanced accuracies of 95% or better relative to the full model. Balanced accuracy of the subset batteries for predicting reference chemicals was 100%. The paper also outlines an approach for researchers to develop their own subset batteries to accurately detect androgen receptor activity using assays that map to the pathway of key molecular and cellular events involved in chemical-mediated androgen receptor activation and transcriptional activity. This work indicates that in vitro bioactivity and in silico predictions mapping to the androgen receptor pathway could be used in an IATA for identifying chemicals that interact directly with the mammalian androgen receptors.

An Evaluation Framework for NAMs for Human Health Safety Assessment

Increased interest in using NAMs for safety assessment has resulted in an explosion of initiatives by numerous organizations. For the most part, these have been carried out independently and are not coordinated in any meaningful way. To help remedy this situation, a multistakeholder group of industry, academic, and regulatory experts including FDA and NIEHS scientists developed a framework that presents a consistent set of criteria, universal across initiatives, to evaluate a NAM's fit-for-purpose (Parish et al. 2020). This framework will provide a structure to collect relevant, confidence-building information that will accelerate, facilitate, and encourage development of new NAMs for use within the appropriate regulatory contexts. In addition, this framework provides a systematic approach to evaluate currently available NAMs and determine their suitability for potential regulatory application. This three-step evaluation framework, along with the demonstrated application with case studies, will help build confidence in the scientific understanding of these methods and their value for chemical assessment and regulatory decision-making.

Agencies Partner with NASA to Provide Support for MPS Research

In May 2021, NASA announced a multiagency initiative, “Extended Longevity of 3D Tissues and Microphysiological Systems for Modeling of Acute and Chronic Exposures to Stressors.” The initiative is focused on adapting existing 3D tissues and MPS to extend their longevity to at least 6 months. Among the sponsors of the initiative are several ICCVAM agencies: NIH (NCATS and National Institute of Allergy and Infectious Diseases), NCI, and FDA. Proposals for projects to be funded under the initiative were accepted through September 2021, with awardees announced in March 2022.

FDA Pilot Program to Support Novel Approaches to Drug Development

In November 2020, FDA announced establishment of the Innovative Science and Technology Approaches for New Drugs pilot program. This program is designed to encourage innovation of drug development tools that are out of scope for existing qualification programs but may still be useful for drug development. Drug development tools are methods, materials, or measures that have the potential to facilitate drug development. Approaches that could be considered under the pilot program include using MPS to assess safety or efficacy questions; developing novel nonclinical pharmacology or toxicology assays; or using artificial intelligence-based algorithms to evaluate patients, develop novel endpoints, or inform study design.

Optimization and Validation of an In Vitro Botulinum Neurotoxin Assay

Tests to detect and measure botulinum neurotoxin are required by multiple federal agencies for a variety of purposes, such as detecting toxin in possibly contaminated food or wildlife. Currently, the standard test for these endpoints is a mouse lethality assay that can use large numbers of animals. NIEHS supported the optimization and validation of ELISAs that replace animal-based methods for diagnosing suspected avian botulism samples. Methods were developed for determining the presence or absence of botulinum neurotoxin serotypes C-D (chimeric toxin) and E in field-collected samples from a wide range of bird species. There was good correlation between botulinum intoxication diagnoses and the ELISAs in the optimization phases, but occurrences of false positives and false negatives in specific scenarios were identified as opportunities for improvement. To address these potentially confounding factors, the USGS National Wildlife Health Center (DOI) is accumulating data and setting sample quality standards. Progress on testing these samples in the ELISA assay has been hampered over the last 2 years due to the pandemic but will progress in 2022 as conditions permit.

Application of Defined Approaches to Evaluating Skin Sensitization Potential of Agrochemicals

Skin sensitization testing is a regulatory requirement for safety evaluations of pesticides in multiple countries. Globally harmonized test guidelines that include in chemico and in vitro methods reduce animal use, but no single assay is recommended as a complete replacement for animal tests. Defined approaches that integrate data from multiple non-animal methods are internationally accepted, specifically via OECD Guideline 497. However, these defined approaches were evaluated with mono-constituent substances, which may limit their applicability to multi-constituent substances such as pesticides. An analysis by NIEHS scientists and collaborators evaluated rule-based defined approaches for hazard and/or potency categorization of skin sensitization for agrochemical formulations. The data set for the analysis included 27 formulations, each tested using the direct peptide reactivity assay, the KeratinoSens™ assay, and the human cell line activation test. The KeratinoSens assay had the highest performance for predicting in vivo hazard outcomes and performed better than any of the defined approaches. These results were presented in a poster (Strickland et al.) at the 2021 annual meeting of the American Society for Cellular and Computational Toxicology. The analysis demonstrates that non-animal test methods have utility for evaluating the skin sensitization potential of agrochemical formulations. Further investigation will be required to determine whether defined approaches can outperform individual assays for predicting in vivo sensitization hazard of pesticide formulations in general.

In Vitro Strategy for Screening Environmental Chemicals and Mixtures

USGS (DOI) is collaborating with McGill University and Environment and Climate Change Canada to evaluate and validate the use of an avian embryo early life stage test in conjunction with transcriptomic assessment using the avian EcoToxChip (Farhat et al. 2019) for screening environmental chemicals and mixtures. The early life stage test is a more biologically realistic alternative to cell-based in vitro testing methods because metabolism and multi-organ effects can be studied. Other advantages of the early life stage protocol are its low cost and rapidity, its reduced animal usage, and its ability to be applied to non-model or wildlife species to improve cross-species toxicity assessments. The avian EcoToxChip, developed initially for Japanese quail, is a targeted quantitative polymerase chain reaction array of about 360 genes. It includes genes representing about 20 biological pathways of regulatory relevance, including immune and endocrine systems and xenobiotic metabolism. In tandem with its partnered data evaluation tool, this focused transcriptomic approach will permit more rapid and less expensive characterization, prioritization, and management of environmental chemicals, reducing animal use.

Use of Cause-and-effect Analysis to Optimize Reliability of In Vitro Inhalation Toxicity Assays

Human-relevant in vitro inhalation toxicology methods are increasingly being used to replace animal testing for research and regulatory purposes. However, these methods use a variety of biological test systems, exposure platforms and conditions, substances tested, and endpoints. These differences represent a major challenge for use of the methods in regulatory testing. Additionally, there is a need to systematically account for variability and maximize the reliability of these methods, especially methods that use cells cultured at an air–liquid interface. One tool that has been used to evaluate the robustness of in vitro test methods is cause-and-effect analysis, a conceptual approach to analyze key sources of potential variability in a test method. Identified sources of variability can then be evaluated using robustness testing and potentially incorporated into in-process control measurements in the assay protocol. A paper by CPSC and NIST scientists and collaborators (Petersen et al. 2021) describes how cause-and-effect analysis can be applied using a modular approach, based on the idea that shared components of different test methods have similar sources of variability even though other components may differ. Cause-and-effect analyses of different in vitro inhalation methods revealed a common set of recommended exposure systems and biological in-process control measurements. This approach, when applied in conjunction with Good Laboratory Practice criteria, should help improve the inter- and intralaboratory agreement of in vitro inhalation test results, leading to increased confidence in these methods for regulatory and research purposes.

Quantitative Analysis of In Vitro Assay Data to Rank Mutagenic Potency of Cigarette Products

Short-term in vitro genotoxicity assays are useful tools to assess whether new and emerging tobacco products potentially have reduced toxicity. Scientists at the FDA Center for Tobacco Products and the FDA National Center for Toxicological Research used quantitative analysis of data from two types of in vitro tests to rank mutagenic potency of cigarette whole-smoke solutions.

Meng et al. (2021) used the bacterial reverse mutation (Ames) test to analyze whole-smoke solution samples generated from two commercial cigarette brands under different smoking machine regimens. Benchmark dose modeling analysis was used to rank the mutagenic potency of the products. The quantitative approaches resulted in a similar rank order of mutagenic potency for the Ames test for both frameshift mutations and base-pair substitution. Under the conditions of this study, these results indicate that quantitative analysis of the Ames test data can discriminate between the mutagenic potencies of whole-smoke solution samples on the basis of smoking machine regimen and differences in smoke chemistry.

Mittelstaedt et al. (2021) evaluated dose-response modeling of in vitro micronucleus test data to determine if this test can discriminate among different tobacco products. Micronucleus responses were generated in mouse lymphoma and human lymphoblastoid cells from a series of whole-smoke solutions expected to have different levels of genotoxicity based on differences in their machine-generated smoke constituents. Eight whole-smoke solutions were prepared by machine-smoking different numbers of two commercial cigarettes under two different smoking machine regimens and tested in the two cell lines with and without rat liver S9 activation. The S9-mediated dose-response data were evaluated with PROAST software (developed by the Netherlands National Institute for Public Health and the Environment) and benchmark doses and upper and lower confidence intervals generated. The response differed based on the number and type of cigarettes smoked and smoking machine regimen. Responses produced in mouse lymphoma cells generally were greater than in the human lymphoblastoid cells, but the ability of the two cell types to differentiate between whole-smoke solutions was similar. The results indicate that benchmark dose potency ranking is useful for differentiating between in vitro micronucleus test responses.

Chemical and In Vitro Bioactivity Analysis of Cigarillos

There has been limited toxicity testing of cigarillos, including comparisons to cigarettes. A study by scientists at the FDA Center for Tobacco Products and collaborators (Crosby et al. 2021) compared the smoke chemistry and the cytotoxic and genotoxic potential of 10 conventional cigarettes and 10 cigarillos based on the greatest market share. Tobacco-specific nitrosamines, carbonyls, and polycyclic aromatic hydrocarbons were measured using gas chromatography-mass spectrometry. Bioactivity of total particulate matter smoke extracts was evaluated using several in vitro assays. Cytotoxicity was assessed in continuously cultured human bronchial epithelial cell lines using the neutral red uptake assay. Genotoxic potential was assessed using the micronucleus (continuously cultured human lung adenocarcinoma cell line), Ames, and thymidine kinase assays. The study found that the tested U.S.-marketed cigarillos have greater tobacco constituent levels, cytotoxicity, and genotoxicity than the compared cigarettes. These findings are important for understanding the human health toxicity from the use of cigarillos relative to cigarettes and for building upon knowledge regarding harm from cigarillos to inform risk mitigation strategies.

Evaluation of Farnesoid X Receptor (FXR)-active Chemicals Identified from Tox21 Screening

One of the primary goals of Tox21 is using in vitro HTS assays to prioritize toxicity evaluations for large numbers of chemicals in commercial use for which little or no toxicity data are available. Chemicals that can disrupt nuclear receptor signaling are a particular area of interest for Tox21 because such activity can have profound biological impacts. The farnesoid X receptor (FXR) is a nuclear receptor that can affect bile acid homeostasis, glucose metabolism, lipid homeostasis, and hepatic regeneration. NIEHS scientists and collaborators evaluated FXR agonists and antagonists using in vitro cell-based assays, in silico modeling approaches, and in vivo assessments using a fish model. These studies demonstrated the molecular complexity of FXR–ligand interactions and confirmed the ability of diverse ligands to modulate FXR, facilitate differential coregulator recruitment, and activate or repress receptor-mediated transcription. A paper describing these studies will be published in 2022.

Optimization and Prequalification of a Splenocyte-based Assay for Potency Testing of Vaccines

Potency assays for vaccines are often limited to animal-based methods in which animals are immunized, challenged with the disease vector, and evaluated for symptoms or mortality. NIEHS has supported the evaluation of a novel relative potency assay that measures immunological response in vitro. This method uses splenocytes from vaccine-immunized animals to determine the ability of increasing doses of a vaccine to elicit a reactivation response in vitro. Therefore, this method has potential to both reduce and refine animal use for vaccine potency testing. The test method developer has also established a cryopreservation procedure to allow long-term storage of splenocytes. The goals of this study are to (1) determine whether a splenocyte-based assay can reliably determine the relative potency of a test sample, and (2) demonstrate that cryopreserved cells can be used in the proposed potency method.

Use of Fish Embryo Toxicity Tests for Prioritizing Testing of Environmental Contaminants

As part of ongoing assessments of wildlife health, DOI is investigating potential cardiovascular effects on fish from pesticides and pharmaceuticals frequently detected in surface waters and fish tissues. The USGS Columbia Environmental Research Center conducts high-content screening of compounds to formulate hypotheses and prioritize compounds for further toxicity testing. This approach reduces animal use, test compound needed, and waste by utilizing pre-feeding fish embryos in a microtiter plate format. This approach is also being used to better characterize toxicity of polycyclic aromatic hydrocarbons (PAHs) and oxygenated PAHs from a subsurface oil spill by assaying groundwater samples from different trophic levels. These assays can provide evidence to justify larger-scale studies to determine actual risk versus perceived risk of contaminants.

The USGS Center’s current high-content screening assay is a developmental cardiotoxicity assay that assesses total body length, pericardial area, intersegmental vessel area, circulation, and heart rate after a 72-hour exposure to a test substance. This array of endpoints allows for a targeted assessment of toxicity. In addition to an LC50 estimate, the assay provides a rapid approach to inform on mode of action, allowing formation of hypotheses on sublethal impacts of contaminants. Data derived from these studies on acute toxicity and mode of action for pesticides, pharmaceuticals, PAHs, and oxygenated PAHs will support a better understanding of potential effects on wildlife species.

Alternative Approaches to Evaluating Bioactivity in Surface Waters

Chemical contaminants are introduced to environmental waters via many sources, and many of these contaminants have the potential to adversely affect organisms living in these waters. Recognized adverse effects include the induction of cancer via genotoxic mechanisms, endocrine disruption via the derailment of normal hormone signaling pathways, and outright toxicity leading to disease or death. Over the past decade, USGS scientists (DOI) have established or adapted water collection, extraction, and in vitro screening assays to evaluate the bioactivity of surface water samples. These approaches circumvent the need to utilize vertebrates and minimize endpoint variability in bioactivity measures. Data from these assays are incorporated into predictive modeling analyses to identify land uses associated with predicted biological disruption. They are also applied to responsibly inform site selection for comprehensive environmental sampling. Notably, it has been found that these approaches are better predictors of the impact of various land use scenarios on wildlife health than sentinel vertebrates sampled at the same locations. During 2020-2021, these assays were applied to augment USGS and other collaborator data sets collected from environmental surface and well waters collected in the eastern and midwestern United States.

Integrated Selection and Screening Process for Potential Fish Toxicants

Fisheries managers use pesticides, specifically referred to as management chemicals, to control invasive and undesirable fish species. Effective management chemicals should show heightened toxicity to target invasive or undesirable species while posing a minimal hazard to native species. The USGS Upper Midwest Environmental Science Center (DOI) has developed a two-phase screening process to minimize the use of animal testing during the development of new management chemicals. Although in vivo testing continues to be utilized in the development of new management chemicals, the screening process minimizes the number of animal toxicity studies necessary during research and development.

In the first phase, an in silico analysis is conducted using ecotoxicity QSAR modeling that incorporates species sensitivity distributions and the EPA tool Sequence Alignment to Predict Across Species Susceptibility (SeqAPASS). The ecotoxicity modeling determines species-specific responses to chemical exposures using existing toxicity data and chemical physicochemical properties. These in silico assessment methods can be used to prioritize candidate compounds and estimate cytotoxicity. Models utilize more than 3,000 endpoints from over 600 taxa that are reported for more than 9,000 chemicals, and a publicly available interactive model prototype is in development.

In the second phase, promising chemical candidates identified by ecotoxicity modeling are tested using fish cell line cytotoxicity assays to examine target versus non-target species sensitivity. Cell lines developed from multiple tissue types (e.g., gill tissue) were established at the U.S. Fish and Wildlife Service Midwest Fishery Center; species represented include bluegill, fathead minnow, lake sturgeon, paddlefish, gizzard shad, silver carp, bighead carp, grass carp, and trout. Multiple species-specific chemical candidates have been identified and screened through cytotoxicity assays, with only the most promising candidates evaluated using in vivo toxicity studies.

Activities of the Developmental Neurotoxicity Health Effects Innovation Program

Exposure to environmental chemicals is a contributing factor to the significant increase in prevalence of neurodevelopmental disorders in children. However, there is limited developmental neurotoxicity information on chemicals in use due to challenges and limitations with current in vivo guidelines. To address these challenges, an in vitro battery of promising NAMs was developed through an international collaboration (Crofton and Mundy 2021). An OECD guidance document with specific IATA case studies is being developed that informs on the application and interpretation of the testing battery, and will be published in 2022.

The Developmental Neurotoxicity Health Effects Innovation Program (DNT HEI) is a program within the NIEHS DNTP that aims to generate and analyze screening-level information on chemicals using the in vitro battery. DNTP developed a unified data analysis pipeline to combine data from the individual assays in the in vitro battery (Behl et al. 2019). In 2021, a set of 115 chemicals (Phase 1 chemicals) was selected and distributed for testing in the in vitro battery. Data have been received from most of the assay developers and are currently being analyzed by the DNT HEI, with results to be published in early 2023. A second set of chemicals has been selected by the DNT HEI based on nominations from various stakeholders.

In addition, DNTP led the development of a case study focused on organophosphorus flame retardants for use of a developmental neurotoxicity IATA to prioritize chemicals within a class of compounds. The case study will be included in the final OECD guidance document described above.