New Approach Methodologies for the Endocrine Activity Toolbox: Environmental Assessment for Fish and Amphibians

Cross-species comparisons of plasma binding and considerations for data evaluation

The US Environmental Protection Agency is increasingly employing new approach methods (NAMs), including in vitro plasma binding and hepatocyte clearance experiments to collect chemical-species specific data. This paper presents data from plasma binding experiments using rapid equilibrium dialysis (RED) devices and plasma from humans, rats, and rainbow trout with a 4-h incubation time. A total of 54 chemicals, utilizing two concentrations, were tested across the three species resulting in 238 chemical-species specific datasets. Mass balance controls for chemical plasma stability and dialysis system recovery were used to evaluate the datasets and almost 40 % of the datasets (92/238 datasets) produced quantitative measurements. Cross-species comparisons and evaluations of the impact of physicochemical properties on chemical-assay performance were also evaluated. Comparisons of human-rat plasma binding revealed rat plasma generally demonstrated higher fup values for chemicals than human. While fup values in trout plasma were frequently lower than rat or human plasma. A comparison with literature data was performed and correlations between plasma binding, expressed as fraction unbound in plasma (fup), and log Kow across all three species indicate that the strongest relationship occurs at log Kow values between 1.5 and 4. The obtained datasets exhibited a wide range of behaviors, emphasizing the need for a robust approach to data quality assessment. The broader analysis of fup values indicates that chemicals with log Kow > 4.5 will be highly bound (fup ≤ 0.0001), difficult to measure, and have low reproducibility across laboratories, suggesting that use of different methods may be needed across different physicochemical properties.

Screening for Endocrine Bioactivity Potential of Tobacco Product Chemicals Including Flavor Chemicals

Adolescence and pregnancy involve elevated levels of hormones (e.g., estrogen, androgen) during which exposure to endocrine disruptors could have long-term developmental and reproductive toxicity (DART) effects. Therefore, the use prevalence and abuse liability of electronic nicotine delivery systems (ENDS) among adolescents and youth, and during pregnancy, raises concerns about possible exposure to endocrine disruptors. In addition, endocrine disruptors have adverse effects on wildlife and environmental health. While many studies focus on carcinogenicity and mutagenicity of tobacco products, research efforts screening chemicals in tobacco products for endocrine disruption potential are few. In this study, we curated 5179 chemicals in tobacco and tobacco smoke, 2803 flavor chemicals, and 156 e-liquid chemicals from literature or openly available databases. We screened the chemicals for endocrine bioactivity using new approach methodologies (NAMs) developed through US Environmental Protection Agency's Endocrine Disruptor Screening Program. The specific NAMs, estrogenic and androgenic pathway models, identified 137 tobacco chemicals, 34 flavor chemicals, and three e-liquid chemicals (Veratraldehyde, (2E)-3-Phenylprop-2-enal, and 2'-Acetonaphthone) as "active," indicating potential endocrine bioactivity. Further, among the tobacco chemicals with endocrine bioactivity potential, 48 were environmentally persistent, 29 bioaccumulative, and 19 both persistent and bioaccumulative. Our findings document many chemicals in tobacco products with potential endocrine bioactivity, which raises concerns for both human and environmental health. These results also underscore the importance of DART potential of tobacco products and flavors. Overall, our study characterizes the endocrine bioactivity potential of tobacco and flavor chemicals and provides a list of chemicals to consider in future ecological and health risk assessments.

Cross-taxa extrapolation: Is there a role for thyroid hormone conjugating liver enzymes during amphibian metamorphosis?

Chemical safety assessment includes evaluating the potential to disrupt the endocrine system in humans and wildlife. The thyroid hormone system shows high complexity which is conserved across vertebrates, allowing biological read-across between regulatory important taxa, namely mammals and amphibians. Potential thyroid disruption in aquatic vertebrates is typically investigated by activity assays (Amphibian Metamorphosis Assay (AMA), Xenopus Eleutheroembryo Thyroid Assay). Since neither assay is designed to provide detailed mechanistic information, mode of action analyses often rely on mammalian data, assuming overall cross-vertebrate conservation. This manuscript elaborates on the imperative that, despite overall conservation, the T-modality in metamorphosing amphibians needs to be understood in detail to justify biological read-across between mammals and amphibians. To this end, we revisit the AMA regarding amphibian developmental physiology, and the T-modality regarding mechanistic cross-vertebrate conservation. The importance of a mechanistic understanding for read-across is showcased based on the AMA's apparent insensitivity to at least one category of prototypical liver enzyme inducers. From a regulatory perspective, deeper mechanistic understanding is needed, not only to strengthen the scientific basis for designing testing strategies and interpreting study results, but also to allow the identification of data gaps and thus development of New Approach Methodologies (NAMs) to minimize vertebrate testing.

Transcriptomics Points-of-Departure (tPODs) to Support Hazard Assessment of Benzo[a]pyrene in Early-Life-Stage Rainbow Trout

New approach methods (NAMs) are urgently needed to address the significant ethical and economic concerns associated with live animal testing as well as the low throughput associated with current toxicity testing frameworks. NAMs such as rapid mechanistic early-life-stage fish assays are promising alternatives to current hazard assessment approaches, as they can be used to derive toxicity thresholds and guide decision-makers on identifying or prioritizing chemicals of concern. This study aimed to derive benchmark concentrations from RNaseq data (transcriptomic points-of-departure; tPOD) from a short-term exposure study with early life stages of rainbow trout (RBT; Oncorhynchus mykiss) using benzo[a]pyrene (B[a]P) as the model compound. tPODs were then calibrated with higher organizational-level responses observed during an extended 28 day exposure period. RBT were exposed from 1 to 28 days post-hatch (dph) to 0.079, 0.35, 1.5, 7.4, and 29 μg/L (28 d time weighted average measured) B[a]P, as well as 0.05% dimethyl sulfoxide and water only controls. Benchmark concentration analysis of transcriptomic responses at 4 dph, based on the most sensitive transcriptomic features, yielded tPODs between 0.028 and 0.47 μg/L B[a]P. At 28 dph, Cyp1a1 exhibited significantly increased catalytic activity, with biochemical POD, bPODEROD,28dph of 0.599 μg/L B[a]P, while morphometric analysis showed significant growth inhibition in terms of length, with apical POD, aPODlength,28dph of 1.77 μg/L B[a]P, with a notable decreasing trend in body weight. A toxicity pathway model constructed from genes and apical end points exhibiting concentration-dependent responses provided further evidence supporting the utility of tPODs from short-term RBT early-life-stage assay to support chemical risk assessment to guide decision-makers in chemical testing prioritization.

Control performance of Amphibian Metamorphosis Assays with Xenopus laevis

The amphibian metamorphosis assay (AMA) is an in vivo screen to assess potential interactions of chemicals with the amphibian thyroid system. Tadpoles are exposed for 21-days, then assessed for development and growth after 7 days and at test termination. This paper presents data from studies performed to satisfy test orders from the US EPA's Endocrine Disruptor Screening Program. Data Evaluation Records were used to collate the control variability and performance of biological endpoints in AMAs conducted in different laboratories, then supplemented with recent studies. We examine the statistical power of AMA endpoint analysis and assess whether historical control data (HCD) can assist evidence-based interpretation of the endpoints, with 52 studies from 7 different laboratories. HCD can be used to understand assay performance post validation. The analysis identifies some need for flexibility in the interpretation of the Test Guidelines' performance criteria, including latitude with analytical variability and statistical analysis of late-stage animals. Additionally, more guidance is suggested for feed regiments and the selection criteria for batches of animals to initiate the assay. Potential Guideline refinements that improve interpretation of the data and have potential to reduce the number of vertebrate animals used in the conduct of AMAs are identified and discussed.

Use of alternatives to animal testing for Environmental Safety Assessment (ESA): Report from the 2023 EPAA partners' forum

Environmental Safety Assessments (ESA) are mandatory for several regulatory purposes and are an important component of stewardship/sustainability initiatives. Fish testing is used for assessing chemical toxicity and bioaccumulation potential; amphibians are included in some jurisdictions and their use is increasing to assess endocrine disruption. Alternative methods are becoming more available, covering the principles of the 3Rs (i.e., replacing, reducing and refining animal tests), but their regulatory incorporation is still limited. A cross-sector review by the European Partnership for Alternative Approaches to Animal Testing (EPAA), discussed the status and priorities for accelerating the adoption of non-animal approaches in ESA. The lack of an internationally agreed definition for "animal testing" was recognized as a challenge. For example, testing with vertebrate embryos up to specific developmental stages is a suitable refinement alternative only in some jurisdictions. Invertebrate testing offers refinement alternatives to develop tiered approaches using vertebrate testing as a last resort. Aquatic ESA was identified as a common need by all sectors and regulatory areas, while terrestrial ESA is particularly relevant for agrochemicals. The standardization and validation of some alternative methods as OECD test guidelines (TGs) for fish acute toxicity and fish bioaccumulation have not yet triggered the expected replacement in regulatory settings. Priority actions in these areas are needed to generate confidence in the regulatory use of the available OECD TGs designed as alternatives, including the identification of applicability domains and guidance/decision-trees for integrating different lines of evidence. Case studies under the OECD Integrated Approaches to Testing and Assessment (IATA) program could facilitate further global regulatory uptake. Replacement of fish chronic toxicity testing is more complex and less advanced. A dual approach was suggested, in the short-term, exploring lines of evidence that, alone or in combination, could identify when further fish testing is not needed. The second phase should focus on the application of the 3Rs in those cases where chronic information is needed. Another area of increasing interest is endocrine disruption. It represents a challenge but also an opportunity for implementing mechanistic non-animal methods, in addition to integrate human and ESA. This requires a step-by-step approach with continuous dialogue to ensure that technical developments will address regulatory needs. The review also agreed that the long-term aspiration is a new ESA paradigm, mapping the protection goals and providing connectivity between the chemical legislation and environmental protection policies.

In silico models for the screening of human transthyretin disruptors

The use of New Approach Methodologies (NAMs), such as Quantitative Structure-Activity Relationship (QSAR) models, is highly recommended by international regulations to speed up hazard and risk assessment of Endocrine Disruptors, which are known to be linked to a wide spectrum of severe diseases on humans and wildlife. A very sensitive target for these chemicals is the thyroid hormone system, which plays a key role in regulating metabolic and cognitive functions. Several chemicals have been demonstrated to compete with the thyroid hormone thyroxine (T4) for binding to human thyroid hormone distributor protein transthyretin (hTTR). In this work, we generated three new datasets composed by T4-hTTR competing potencies of more than 200 heterogeneous chemicals measured by three different in vitro assays. These datasets were used for the development of new regression QSAR models. The best models were thoroughly validated by internal and external validation procedures. The mechanistic interpretation of the selected molecular descriptors provided information on structural features which are relevant to characterise hTTR binders, such as the presence of hydroxylated and halogenated aromatic rings. PCA analysis was used to rank the studied chemicals according to their increasing T4-hTTR competing potency. Hydroxylated and halogenated bicyclic aromatic compounds are ranked as the strongest hTTR binders. The new QSARs are useful to screen potential Thyroid Hormone System-Disrupting Chemicals (THSDCs), and to support the identification of sustainable alternatives to hazardous chemicals.

Assessing plasmatic transport inhibitors of thyroid hormone in mammals in the Xenopus Eleutheroembryonic Thyroid Assay (XETA)

The Xenopus Eleutheroembryonic Thyroid Assay (XETA, OECD TG 248) was established as an alternative to the Amphibian Metamorphosis Assay (AMA, OECD TG 231) for the analysis of (anti-)thyroid activity of chemicals. The XETA is a New Approach Method (NAM) since the embryonic life stages used in the assay are not yet feeding independently, which renders the assay to be considered a non-animal test under many national laws. Physiologically, the used embryos are not fully developed yet, and thus there are limitations to the XETA for detecting certain mechanisms along the hypothalamic-pituitary-thyroid (HPT) axis. However, the plasmatic transport inhibition of thyroid hormone should physiologically be detectable in the XETA but has not yet been sufficiently investigated. Here, we tested three substances that are known, amongst others, to inhibit thyroid hormone transport by competitive binding to transthyretin in mammalian studies, namely pentachlorophenol (PCP), tetrabromo bisphenol A (TBBPA), and mefenamic acid. Following the test guideline, X. laevis eleutheroembryos of Nieuwkoop-Faber stage 45 were exposed for 72 h at 21 °C in 6-well plates to different concentrations of the test substances. For PCP and TBBPA, the XETA showed a decrease in fluorescence intensity, which would be expected for thyroid hormone transport inhibition amongst other, similar modes of action, while for the lower potency substance mefenamic acid, a trend was visible, but no statistically significant inhibition was detected. Overall, the results indicate that in the XETA, the plasmatic transport inhibition of thyroid hormone should be detectable alongside other inhibitory modes of action on the HPT axis.

Endocrine disruption assessment in aquatic vertebrates - Identification of substance-induced thyroid-mediated effect patterns

According to the World Health Organisation and European Commission definitions, substances shall be considered as having endocrine disrupting properties if they show adverse effects, have endocrine activity and the adverse effects are a consequence of the endocrine activity (using a weight-of-evidence approach based on biological plausibility), unless the adverse effects are not relevant to humans or non-target organisms at the (sub)population level. To date, there is no decision logic on how to establish endocrine disruption via the thyroid modality in non-mammalian vertebrates. This paper describes an evidence-based decision logic compliant with the integrated approach to testing and assessment (IATA) concept, to identify thyroid-mediated effect patterns in aquatic vertebrates using amphibians as relevant models for thyroid disruption assessment. The decision logic includes existing test guidelines and methods and proposes detailed considerations on how to select relevant assays and interpret the findings. If the mammalian dataset used as the starting point indicates no thyroid concern, the Xenopus Eleutheroembryonic Thyroid Assay allows checking out thyroid-mediated activity in non-mammalian vertebrates, whereas the Amphibian Metamorphosis Assay or its extended, fixed termination stage variant inform on both thyroid-mediated activity and potentially population-relevant adversity. In evaluating findings, the response patterns of all assay endpoints are considered, including the direction of changes. Thyroid-mediated effect patterns identified at the individual level in the amphibian tests are followed by mode-of-action and population relevance assessments. Finally, all data are considered in an overarching weight-of-evidence evaluation. The logic has been designed generically and can be adapted, e.g. to accommodate fish tests once available for thyroid disruption assessments. It also ensures that all scientifically relevant information is considered, and that animal testing is minimised. The proposed decision logic can be included in regulatory assessments to facilitate the conclusion on whether substances meet the endocrine disruptor definition for the thyroid modality in non-mammalian vertebrates.

Resource and animal use implications of the proposed REACH information requirements for endocrine disruptor assessment

Revised information requirements for endocrine disruptor (ED) assessment of chemicals under the European Union's Registration, Evaluation, Authorisation and Restriction of Chemicals (REACH) Regulation have been proposed. Implementation will substantially increase demands for new data to inform ED assessment. This article evaluates the potential animal use and financial resource associated with two proposed ED policy options, and highlights areas where further clarification is warranted. This evaluation demonstrates that studies potentially conducted to meet the proposed requirements could use tens of millions of animals, and that the approach is unlikely to be feasible in practice. Given the challenges with implementing either policy option and the need to minimise the reliance on animal testing, further consideration and clarification is needed on several aspects prior to implementation of the requirements. This includes how testing will be prioritised in a proportionate approach; how to harness new approach methodologies to waive higher-tier animal testing; and need for provision of clear guidance particularly in applying weight-of-evidence approaches. There is now a clear opportunity for the European Commission to lead the way in developing a robust and transparent ED assessment process for industrial chemicals which fully implements replacement, refinement, and reduction of the use of animals (the 3Rs).

Electrochemical degradation of diclofenac generates unexpected thyroidogenic transformation products: Implications for environmental risk assessment

Diclofenac (DCF) is an environmentally persistent, nonsteroidal anti-inflammatory drug (NSAID) with thyroid disrupting properties. Electrochemical advanced oxidation processes (eAOPs) can efficiently remove NSAIDs from wastewater. However, eAOPs can generate transformation products (TPs) with unknown chemical and biological characteristics. In this study, DCF was electrochemically degraded using a boron-doped diamond anode. Ultra-high performance liquid chromatography coupled with high-resolution mass spectrometry was used to analyze the TPs of DCF and elucidate its potential degradation pathways. The biological impact of DCF and its TPs was evaluated using the Xenopus Eleutheroembryo Thyroid Assay, employing a transgenic amphibian model to assess thyroid axis activity. As DCF degradation progressed, in vivo thyroid activity transitioned from anti-thyroid in non-treated samples to pro-thyroid in intermediately treated samples, implying the emergence of thyroid-active TPs with distinct modes of action compared to DCF. Molecular docking analysis revealed that certain TPs bind to the thyroid receptor, potentially triggering thyroid hormone-like responses. Moreover, acute toxicity occurred in intermediately degraded samples, indicating the generation of TPs exhibiting higher toxicity than DCF. Both acute toxicity and thyroid effects were mitigated with a prolonged degradation time. This study highlights the importance of integrating in vivo bioassays in the environmental risk assessment of novel degradation processes.

How the Xenopus eleutheroembryonic thyroid assay compares to the amphibian metamorphosis assay for detecting thyroid active chemicals

The Xenopus Eleutheroembryonic Thyroid Assay (XETA) was recently published as an OECD Test Guideline for detecting chemicals acting on the thyroid axis. However, the OECD validation did not cover all mechanisms that can potentially be detected by the XETA. This study was therefore initiated to investigate and consolidate the applicability domain of the XETA regarding the following mechanisms: thyroid hormone receptor (THR) agonism, sodium-iodide symporter (NIS) inhibition, thyroperoxidase (TPO) inhibition, deiodinase (DIO) inhibition, glucocorticoid receptor (GR) agonism, and uridine 5'-diphospho-glucuronosyltransferase (UDPGT) induction. In total, 22 chemicals identified as thyroid-active or -inactive in Amphibian Metamorphosis Assays (AMAs) were tested using the XETA OECD Test Guideline. The comparison showed that both assays are highly concordant in identifying chemicals with mechanisms of action related to THR agonism, DIO inhibition, and GR agonism. They also consistently identified the UDPGT inducers as thyroid inactive. NIS inhibition, investigated using sodium perchlorate, was not detected in the XETA. TPO inhibition requires further mechanistic investigations as the reference chemicals tested resulted in opposing response directions in the XETA and AMA. This study contributes refining the applicability domain of the XETA, thereby helping to clarify the conditions where it can be used as an ethical alternative to the AMA.

Big Question to Developing Solutions: A Decade of Progress in the Development of Aquatic New Approach Methodologies from 2012 to 2022

In 2012, 20 key questions related to hazard and exposure assessment and environmental and health risks of pharmaceuticals and personal care products in the natural environment were identified. A decade later, this article examines the current level of knowledge around one of the lowest-ranking questions at that time, number 19: "Can nonanimal testing methods be developed that will provide equivalent or better hazard data compared with current in vivo methods?" The inclusion of alternative methods that replace, reduce, or refine animal testing within the regulatory context of risk and hazard assessment of chemicals generally faces many hurdles, although this varies both by organism (human-centric vs. other), sector, and geographical region or country. Focusing on the past 10 years, only works that might reasonably be considered to contribute to advancements in the field of aquatic environmental risk assessment are highlighted. Particular attention is paid to methods of contemporary interest and importance, representing progress in (1) the development of methods which provide equivalent or better data compared with current in vivo methods such as bioaccumulation, (2) weight of evidence, or (3) -omic-based applications. Evolution and convergence of these risk assessment areas offer the basis for fundamental frameshifts in how data are collated and used for the protection of taxa across the breadth of the aquatic environment. Looking to the future, we are at a tipping point, with a need for a global and inclusive approach to establish consensus. Bringing together these methods (both new and old) for regulatory assessment and decision-making will require a concerted effort and orchestration. Environ Toxicol Chem 2024;43:559-574. © 2023 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Are changes in vitellogenin concentrations in fish reliable indicators of chemical-induced endocrine activity?

Vitellogenin (VTG), a biomarker for endocrine activity, is a mechanistic component of the regulatory assessment of potential endocrine-disrupting properties of chemicals. This review of VTG data is based on changes reported for 106 substances in standard fish species. High intra-study and inter-laboratory variability in VTG concentrations was confirmed, as well as discrepancies in interpretation of results based on large differences between fish in the dilution water versus solvent control, or due to the presence of outlier measurements. VTG responses in fish were ranked against predictions for estrogen receptor agonist activity and aromatase inhibition from bioactivity model output and ToxCast in vitro assay results, respectively. These endocrine mechanisms explained most of the VTG responses in the absence of systemic toxicity, the magnitude of the VTG response being proportional to the in vitro potency. Interpretation of the VTG data was sometimes confounded by an alternative endocrine mechanism of action. There was evidence for both false positive and negative responses for VTG synthesis, but overall, it was rare for substances without endocrine activity in vitro to cause a concentration-dependent VTG response in fish in the absence of systemic toxicity. To increase confidence in the VTG results, we recommend improvements in the VTG measurement methodologies and greater transparency in reporting of VTG data (including quality control criteria for assay performance). This review supports the application of New Approach Methodologies (NAMs) by demonstrating that endocrine activity in vitro from mammalian cell lines is predictive for in vivo VTG response in fish, suggesting that in vitro mechanistic data could be used more broadly in decision-making to help reduce animal testing.