New developments in the evolution and application of the WHO/IPCS framework on mode of action/species concordance analysis

Incorporating a weight-of-evidence approach into a tiered assessment for chemicals management, with emphasis on program development and applications in developing countries and emerging economies

A flexible approach is described for incorporating a weight-of-evidence (WoE) methodology into a tiered ecological risk assessment (ERA)/management framework for chemicals. The approach is oriented toward informing decisions about chemicals. Communication is regarded as a critical component of the risk assessment process. The paper resulted from insights gained from seven ERA workshops held by SETAC (Society of Environmental Toxicology and Chemistry, www.setac.org) in the Asia-Pacific, African, and Latin American regions. Formal ERA methods are not fully developed or applied in many of these countries and assessments often begin with tables of risk values and test methods from countries where ERA is already implemented. While appropriate and sometimes necessary, workshop participants had questions about the reliability and relevance of using this information for regionally specific ecosystems with different receptors, fate processes, and exposure characteristics. The idea that an assessment of reliability and relevance of available information and the need for additional information was necessary at an early stage of the assessment process was considered. The judgment of reliability and relevance is central to WoE approaches along with the identification of information needs and the integration of such information. The need to engage in WoE considerations early and throughout the assessment process indicates that a tiered approach is appropriate for unifying the evaluation process in a consistent way from early screening-level steps to later more involved evaluations. The approach outlined in this article is complementary to WoE guidance developed by the Organization for Economic Co-operation and Development and many national guidance documents. To link assessments of risk to management decisions, emphasis is given to communications at each tier between the risk assessor (technical side) and the decision-makers (policy and regulatory side). Tools and information sources are suggested for each tier and suggestions are meant to be illustrative and not prescriptive. Integr Environ Assess Manag 2024;00:1-15. © 2024 SETAC.

A UK framework for the assessment and integration of different scientific evidence streams in chemical risk assessment

BACKGROUND

Few methods are available for transparently combining different evidence streams for chemical risk assessment to reach an integrated conclusion on the probability of causation. Hence, the UK Committees on Toxicity (COT) and on Carcinogenicity (COC) have reviewed current practice and developed guidance on how to achieve this in a transparent manner, using graphical visualisation.

METHODS/APPROACH

All lines of evidence, including toxicological, epidemiological, new approach methodologies, and mode of action should be considered, taking account of their strengths/weaknesses in their relative weighting towards a conclusion on the probability of causation. A qualitative estimate of the probability of causation is plotted for each line of evidence and a combined estimate provided.

DISCUSSION/CONCLUSIONS

Guidance is provided on integration of multiple lines of evidence for causation, based on current best practice. Qualitative estimates of probability for each line of evidence are plotted graphically. This ensures a deliberative, consensus conclusion on likelihood of causation is reached. It also ensures clear communication of the influence of the different lines of evidence on the overall conclusion on causality. Issues on which advice from the respective Committees is sought varies considerably, hence the guidance is designed to be sufficiently flexible to meet this need.

Associations between Ethylene Oxide Exposure and Liver Function in the US Adult Population

BACKGROUND

Ethylene oxide, a reactive epoxy compound, has been widely used in various industries for many years. However, evidence of the combined toxic effects of ethylene oxide exposure on the liver is still lacking.

METHODS

We analyzed the merged data from the National Health and Nutrition Examination Survey (NHANES) from 2013 to 2016. Ultimately, 4141 adults aged 18 and over were selected as the sample. We used linear regression to explore the association between blood ethylene oxide and LFT indicators.

RESULTS

The weighted linear regression model showed that HbEO is positively correlated with ALP (β = 2.61, 95% CI 1.97, 3.24, p < 0.0001), GGT (β = 5.75, 95% CI 4.46, 7/05, p < 0.0001), ALT (β = 0.50, 95% CI 0.09, 0.90, p = 0.0158), and AST (β = 0.71, 95% CI 0.44, 0.98, p < 0.0001) and negatively correlated with TBIL (β = -0.30, 95% CI -0.43, -0.16, p < 0.0001).

CONCLUSIONS

Ethylene oxide exposure is significantly associated with changes in liver function indicators among adults in the United States. Future work should further examine these relationships.

E-waste: mechanisms of toxicity and safety testing

Currently, information on the toxicity profile of the majority of the identified e-waste chemicals, while extensive and growing, is admittedly fragmentary, particularly at the cellular and molecular levels. Furthermore, the toxicity of the chemical mixtures likely to be encountered by humans during and after informal e-waste recycling, as well as their underlying mechanisms of action, is largely unknown. This review paper summarizes state-of-the-art knowledge of the potential underlying toxicity mechanisms associated with e-waste exposures, with a focus on toxic responses connected to specific organs, organ systems, and overall effects on the organism. To overcome the complexities associated with assessing the possible adverse outcomes from exposure to chemicals, a growing number of new approach methodologies have emerged in recent years, with the long-term objective of providing a human-based and animal-free system that is scientifically superior to animal testing, more effective, and acceptable. This encompasses a variety of techniques, typically regarded as alternative approaches for determining chemical-induced toxicities and holds greater promise for a better understanding of key events in the metabolic pathways that mediate known adverse health outcomes in e-waste exposure scenarios. This is crucial to establishing accurate scientific knowledge on mixed e-waste chemical exposures in shorter time frames and with greater efficacy, as well as supporting the need for safe management of hazardous chemicals. The present review paper discusses important gaps in knowledge and shows promising directions for mechanistically anchored effect-based monitoring strategies that will contribute to the advancement of the methods currently used in characterizing and monitoring e-waste-impacted ecosystems.

Mode of action analysis for fluxapyroxad-induced rat liver tumour formation: evidence for activation of the constitutive androstane receptor and assessment of human relevance

The fungicide fluxapyroxad (BAS 700 F) has been shown to significantly increase the incidence of liver tumours in male Wistar rats at dietary levels of 1500 and 3000 ppm and in female rats at a dietary level of 3000 ppm via a non-genotoxic mechanism. In order to elucidate the mode of action (MOA) for fluxapyroxad-induced rat liver tumour formation a series of in vivo and in vitro investigative studies were undertaken. The treatment of male and female Wistar rats with diets containing 0 (control), 50, 250, 1500 and 3000 ppm fluxapyroxad for 1, 3, 7 and 14 days resulted in a dose-dependent increases in relative weight at 1500 and 3000 ppm from day 3 onwards in both sexes, with an increase in relative liver weight being also observed in male rats given 250 ppm fluxapyroxad for 14 days. Examination of liver sections revealed a centrilobular hepatocyte hypertrophy in some fluxapyroxad treated male and female rats. Hepatocyte replicative DNA synthesis (RDS) was significantly increased in male rats given 1500 and 3000 ppm fluxapyroxad for 3 and 7 days and in female rats given 50-3000 ppm fluxapyroxad for 7 days and 250-3000 ppm fluxapyroxad for 3 and 14 days; the maximal increases in RDS in both sexes being observed after 7 days treatment. The treatment of male and female Wistar rats with 250-3000 ppm fluxapyroxad for 14 days resulted in significant increases in hepatic microsomal total cytochrome P450 (CYP) content and CYP2B subfamily-dependent enzyme activities. Male Wistar rat hepatocytes were treated with control medium and medium containing 1-100 μM fluxapyroxad or 500 μM sodium phenobarbital (NaPB) for 4 days. Treatment with fluxapyroxad and NaPB increased CYP2B and CYP3A enzyme activities and mRNA levels but had little effect on markers of CYP1A and CYP4A subfamily enzymes and of the peroxisomal fatty acid β-oxidation cycle. Hepatocyte RDS was significantly increased by treatment with fluxapyroxad, NaPB and 25 ng/ml epidermal growth factor (EGF). The treatment of hepatocytes from two male human donors with 1-100 μM fluxapyroxad or 500 μM NaPB for 4 days resulted in some increases in CYP2B and CYP3A enzyme activities and CYP mRNA levels but had no effect on hepatocyte RDS, whereas treatment with EGF resulted in significant increase in RDS in both human hepatocyte preparations. Hepatocytes from male Sprague-Dawley wild type (WT) and constitutive androstane receptor (CAR) knockout (CAR KO) rats were treated with control medium and medium containing 1-16 μM fluxapyroxad or 500 μM NaPB for 4 days. While both fluxapyroxad and NaPB increased CYP2B enzyme activities and mRNA levels in WT hepatocytes, only minor effects were observed in CAR KO rat hepatocytes. Treatment with both fluxapyroxad and NaPB only increased RDS in WT and not in CAR KO rat hepatocytes, whereas treatment with EGF increased RDS in both WT and CAR KO rat hepatocytes. In conclusion, a series of in vivo and in vitro investigative studies have demonstrated that fluxapyroxad is a CAR activator in rat liver, with similar properties to the prototypical CAR activator phenobarbital. A robust MOA for fluxapyroxad-induced rat liver tumour formation has been established. Based on the lack of effect of fluxapyroxad on RDS in human hepatocytes, it is considered that the MOA for fluxapyroxad-induced liver tumour formation is qualitatively not plausible for humans.

Interspecies scaling of toxicity reference values in human health versus ecological risk assessments: A critical review

Risk assessments that focus on anthropogenic chemicals in environmental media-whether considering human health or ecological effects-often rely on toxicity data from experimentally studied species to estimate safe exposures for species that lack similar data. Current default extrapolation approaches used in both human health risk assessments and ecological risk assessments (ERAs) account for differences in body weight between the test organisms and the species of interest, but the two default approaches differ in important ways. Human health risk assessments currently employ a default based on body weight raised to the three-quarters power. Ecological risk assessments for wildlife (i.e., mammals and birds) are typically based directly on body weight, as measured in the test organism and receptor species. This review describes differences in the experimental data underlying these default practices and discusses the many factors that affect interspecies variability in chemical exposures. The interplay of these different factors can lead to substantial departures from default expectations. Alternative methodologies for conducting more accurate interspecies extrapolations in ERAs for wildlife are discussed, including tissue-based toxicity reference values, physiologically based toxicokinetic and/or toxicodynamic modeling, chemical read-across, and a system of categorical defaults based on route of exposure and toxic mode of action. Integr Environ Assess Manag 2024;20:749-764. © 2023 SETAC.

Evaluation of neural reflex activation as a potential mode of action for respiratory and cardiovascular effects of fine particulate matter

OBJECTIVES

Mortality from respiratory and cardiovascular health conditions contributes largely to the total mortality that has been associated with exposure to PM2.5 in epidemiology studies. A mode of action (MoA) for these underlying morbidities has not been established, but it has been proposed that some effects of PM2.5 occur through activation of neural reflexes.

MATERIALS AND METHODS

We critically reviewed the experimental studies of PM2.5 (including ambient PM2.5, diesel exhaust particles, concentrated ambient particles, diesel exhaust, and cigarette smoke) and neural reflex activation, and applied the principles of the International Programme on Chemical Safety (IPCS) MoA/human relevance framework to assess whether they support a biologically plausible and human-relevant MoA by which PM2.5 could contribute to cardiovascular and respiratory causes of death. We also considered whether the evidence from these studies supports a non-threshold MoA that operates at low, human-relevant PM2.5 exposure concentrations.

RESULTS AND DISCUSSION

We found that the proposed MoA of neural reflex activation is biologically plausible for PM2.5-induced respiratory effects at high exposure levels used in experimental studies, but further studies are needed to fill important data gaps regarding the relevance of this MoA to humans at lower PM2.5 exposure levels. A role for the proposed MoA in PM2.5-induced cardiovascular effects is plausible for some effects but not others.

CONCLUSIONS

Further studies are needed to determine whether neural reflex activation is the MoA by which PM2.5 could cause either respiratory or cardiovascular morbidities in humans, particularly at the ambient concentrations associated with total mortality in epidemiology studies.

Specific effects on the thyroid relevant for performing a dietary cumulative risk assessment of pesticide residues: 2024 update

EFSA updated its previous work on the establishment of specific effects that are considered relevant for grouping pesticide residues targeting the thyroid and for performing the retrospective assessment of dietary cumulative risk (CRA). The two specific effects already selected in 2019 leading to the two cumulative assessment groups (CAGs) 'hypothyroidism' and 'C-cell hypertrophy, hyperplasia and neoplasia' were reconfirmed. Compared to 2019, the list of indicators that can be used to identify these specific effects was refined to only include histopathological changes. In a second phase of the work, data will be extracted on indicators of the specific effects from the dossiers on active substances (a.s.) used as plant protection products. The criteria for including a.s. into CAGs were also updated, together with the hazard characterisation methodology and the lines of evidence for assessing CAG-membership probabilities. The tasks related to the data extraction and the establishment of the CAGs on hypothyroidism and on C-cell hypertrophy, hyperplasia and neoplasia are beyond the scope of this report. This part of the CRA process has been outsourced and will be the subject of a separate report.

Ten years of using key characteristics of human carcinogens to organize and evaluate mechanistic evidence in IARC Monographs on the identification of carcinogenic hazards to humans: Patterns and associations

Systematic review and evaluation of mechanistic evidence using the Key Characteristics approach was proposed by the International Agency for Research on Cancer (IARC) in 2012 and used by the IARC Monographs Working Groups since 2015. Key Characteristics are 10 features of agents known to cause cancer in humans. From 2015 to 2022, a total of 19 Monographs (73 agents combined) used Key Characteristics for cancer hazard classification. We hypothesized that a retrospective analysis of applications of the Key Characteristics approach to cancer hazard classification using heterogenous mechanistic data on diverse agents would be informative for systematic reviews in decision-making. We extracted information on the conclusions, data types, and the role mechanistic data played in the cancer hazard classification from each Monograph. Statistical analyses identified patterns in the use of Key Characteristics, as well as trends and correlations among Key Characteristics, data types, and ultimate decisions. Despite gaps in data for many agents and Key Characteristics, several significant results emerged. Mechanistic data from in vivo animal, in vitro animal, and in vitro human studies were most impactful in concluding that an agent could cause cancer via a Key Characteristic. To exclude the involvement of a Key Characteristic, data from large-scale systematic in vitro testing programs such as ToxCast, were most informative. Overall, increased availability of systemized data streams, such as human in vitro data, would provide the basis for more confident and informed conclusions about both positive and negative associations and inform expert judgments on cancer hazard.

Procedural application of mode-of-action and human relevance analysis: styrene-induced lung tumors in mice

Risk assessment of human health hazards has traditionally relied on experiments that use animal models. Although exposure studies in rats and mice are a major basis for determining risk in many cases, observations made in animals do not always reflect health hazards in humans due to differences in biology. In this critical review, we use the mode-of-action (MOA) human relevance framework to assess the likelihood that bronchiolar lung tumors observed in mice chronically exposed to styrene represent a plausible tumor risk in humans. Using available datasets, we analyze the weight-of-evidence 1) that styrene-induced tumors in mice occur through a MOA based on metabolism of styrene by Cyp2F2; and 2) whether the hypothesized key event relationships are likely to occur in humans. This assessment describes how the five modified Hill causality considerations support that a Cyp2F2-dependent MOA causing lung tumors is active in mice, but only results in tumorigenicity in susceptible strains. Comparison of the key event relationships assessed in the mouse was compared to an analogous MOA hypothesis staged in the human lung. While some biological concordance was recognized between key events in mice and humans, the MOA as hypothesized in the mouse appears unlikely in humans due to quantitative differences in the metabolic capacity of the airways and qualitative uncertainties in the toxicological and prognostic concordance of pre-neoplastic and neoplastic lesions arising in either species. This analysis serves as a rigorous demonstration of the framework's utility in increasing transparency and consistency in evidence-based assessment of MOA hypotheses in toxicological models and determining relevance to human health.

SDHi fungicides: An example of mitotoxic pesticides targeting the succinate dehydrogenase complex

Succinate dehydrogenase inhibitors (SDHi) are fungicides used to control the proliferation of pathogenic fungi in crops. Their mode of action is based on blocking the activity of succinate dehydrogenase (SDH), a universal enzyme expressed by all species harboring mitochondria. The SDH is involved in two interconnected metabolic processes for energy production: the transfer of electrons in the mitochondrial respiratory chain and the oxidation of succinate to fumarate in the Krebs cycle. In humans, inherited SDH deficiencies may cause major pathologies including encephalopathies and cancers. The cellular and molecular mechanisms related to such genetic inactivation have been well described in neuroendocrine tumors, in which it induces an oxidative stress, a pseudohypoxic phenotype, a metabolic, epigenetic and transcriptomic remodeling, and alterations in the migration and invasion capacities of cancer cells, in connection with the accumulation of succinate, an oncometabolite, substrate of the SDH. We will discuss recent studies reporting toxic effects of SDHi in non-target organisms and their implications for risk assessment of pesticides. Recent data show that the SDH structure is highly conserved during evolution and that SDHi can inhibit SDH activity in mitochondria of non-target species, including humans. These observations suggest that SDHi are not specific inhibitors of fungal SDH. We hypothesize that SDHi could have toxic effects in other species, including humans. Moreover, the analysis of regulatory assessment reports shows that most SDHi induce tumors in animals without evidence of genotoxicity. Thus, these substances could have a non-genotoxic mechanism of carcinogenicity that still needs to be fully characterized and that could be related to SDH inhibition. The use of pesticides targeting mitochondrial enzymes encoded by tumor suppressor genes raises questions on the risk assessment framework of mitotoxic pesticides. The issue of SDHi fungicides is therefore a textbook case that highlights the urgent need for changes in regulatory assessment.

Ten Years of Using Key Characteristics of Human Carcinogens to Organize and Evaluate Mechanistic Evidence in IARC Monographs on the Identification of Carcinogenic Hazards to Humans: Patterns and Associations

Systematic review and evaluation of the mechanistic evidence only recently been instituted in cancer hazard identification step of decision-making. One example of organizing and evaluating mechanistic evidence is the Key Characteristics approach of the International Agency for Research on Cancer (IARC) Monographs on the Identification of Carcinogenic Hazards to Humans. The Key Characteristics of Human Carcinogens were proposed almost 10 years ago and have been used in every IARC Monograph since 2015. We investigated the patterns and associations in the use of Key Characteristics by the independent expert Working Groups. We examined 19 Monographs (2015-2022) that evaluated 73 agents. We extracted information on the conclusions by each Working Group on the strength of evidence for agent-Key Characteristic combinations, data types that were available for decisions, and the role mechanistic data played in the final cancer hazard classification. We conducted both descriptive and association analyses within and across data types. We found that IARC Working Groups were cautious when evaluating mechanistic evidence: for only ∼13% of the agents was strong evidence assigned for any Key Characteristic. Genotoxicity and cell proliferation were most data-rich, while little evidence was available for DNA repair and immortalization Key Characteristics. Analysis of the associations among Key Characteristics revealed that only chemical's metabolic activation was significantly co-occurring with genotoxicity and cell proliferation/death. Evidence from exposed humans was limited, while mechanistic evidence from rodent studies in vivo was often available. Only genotoxicity and cell proliferation/death were strongly associated with decisions on whether mechanistic data was impactful on the final cancer hazard classification. The practice of using the Key Characteristics approach is now well-established at IARC Monographs and other government agencies and the analyses presented herein will inform the future use of mechanistic evidence in regulatory decision-making.

Towards a science-based testing strategy to identify maternal thyroid hormone imbalance and neurodevelopmental effects in the progeny - Part IV: the ECETOC and CLE Proposal for a Thyroid Function-Related Neurodevelopmental Toxicity Testing and Assessment Scheme (Thyroid-NDT-TAS)

Following the European Commission Endocrine Disruptor Criteria, substances shall be considered as having endocrine disrupting properties if they (a) elicit adverse effects, (b) have endocrine activity, and (c) the two are linked by an endocrine mode-of-action (MoA) unless the MoA is not relevant for humans. A comprehensive, structured approach to assess whether substances meet the Endocrine Disruptor Criteria for the thyroid modality (EDC-T) is currently unavailable. Here, the European Centre for Ecotoxicology and Toxicology of Chemicals Thyroxine Task Force and CropLife Europe propose a Thyroid Function-Related Neurodevelopmental Toxicity Testing and Assessment Scheme (Thyroid-NDT-TAS). In Tier 0, before entering the Thyroid-NDT-TAS, all available in vivo, in vitro and in silico data are submitted to weight-of-evidence (WoE) evaluations to determine whether the substance of interest poses a concern for thyroid disruption. If so, Tier 1 of the Thyroid-NDT-TAS includes an initial MoA and human relevance assessment (structured by the key events of possibly relevant adverse outcome pathways) and the generation of supportive in vitro/in silico data, if relevant. Only if Tier 1 is inconclusive, Tier 2 involves higher-tier testing to generate further thyroid- and/or neurodevelopment-related data. Tier 3 includes the final MoA and human relevance assessment and an overarching WoE evaluation to draw a conclusion on whether, or not, the substance meets the EDC-T. The Thyroid-NDT-TAS is based on the state-of-the-science, and it has been developed to minimise animal testing. To make human safety assessments more accurate, it is recommended to apply the Thyroid-NDT-TAS during future regulatory assessments.

An Adverse Outcome Pathway Network for Chemically Induced Oxidative Stress Leading to (Non)genotoxic Carcinogenesis

Nongenotoxic (NGTX) carcinogens induce cancer via other mechanisms than direct DNA damage. A recognized mode of action for NGTX carcinogens is induction of oxidative stress, a state in which the amount of oxidants in a cell exceeds its antioxidant capacity, leading to regenerative proliferation. Currently, carcinogenicity assessment of environmental chemicals primarily relies on genetic toxicity end points. Since NGTX carcinogens lack genotoxic potential, these chemicals may remain undetected in such evaluations. To enhance the predictivity of test strategies for carcinogenicity assessment, a shift toward mechanism-based approaches is required. Here, we present an adverse outcome pathway (AOP) network for chemically induced oxidative stress leading to (NGTX) carcinogenesis. To develop this AOP network, we first investigated the role of oxidative stress in the various cancer hallmarks. Next, possible mechanisms for chemical induction of oxidative stress and the biological effects of oxidative damage to macromolecules were considered. This resulted in an AOP network, of which associated uncertainties were explored. Ultimately, development of AOP networks relevant for carcinogenesis in humans will aid the transition to a mechanism-based, human relevant carcinogenicity assessment that involves a substantially lower number of laboratory animals.

Evidential requirements for the regulatory hazard and risk assessment of respiratory sensitisers: methyl methacrylate as an example

This review addresses the need for a framework to increase the consistency, objectivity and transparency in the regulatory assessment of respiratory sensitisers and associated uncertainties. Principal issues are considered and illustrated through a case study (with methyl methacrylate). In the absence of test methods validated for regulatory use, formal documentation of the weight-of-evidence for hazard classification both at the level of integration of individual studies within lines of evidence and across a broad range of data streams was agreed to be critical for such a framework. An integrated approach is proposed to include not only occupational studies and clinical evidence for the regulatory assessment of respiratory sensitisers, but also information on structure and physical and chemical factors, predictive approaches such as structure activity analysis and in vitro and in vivo mechanistic and toxicokinetic findings. A weight-of-evidence protocol, incorporating integration of these sources of data based on predefined considerations, would contribute to transparency and consistency in the outcome of the assessment. In those cases where a decision may need to be taken on the basis of occupational findings alone, conclusions should be based on transparent weighting of relevant data on the observed prevalence of occupational asthma in various studies taking into account all relevant information including the range and nature of workplace exposures to the substance of interest, co-exposure to other chemicals and study quality.

A Pragmatic Framework for the Application of New Approach Methodologies in One Health Toxicological Risk Assessment

Globally, industries and regulatory authorities are faced with an urgent need to assess the potential adverse effects of chemicals more efficiently by embracing new approach methodologies (NAMs). NAMs include cell and tissue methods (in vitro), structure-based/toxicokinetic models (in silico), methods that assess toxicant interactions with biological macromolecules (in chemico), and alternative models. Increasing knowledge on chemical toxicokinetics (what the body does with chemicals) and toxicodynamics (what the chemicals do with the body) obtained from in silico and in vitro systems continues to provide opportunities for modernizing chemical risk assessments. However, directly leveraging in vitro and in silico data for derivation of human health-based reference values has not received regulatory acceptance due to uncertainties in extrapolating NAM results to human populations, including metabolism, complex biological pathways, multiple exposures, interindividual susceptibility and vulnerable populations. The objective of this article is to provide a standardized pragmatic framework that applies integrated approaches with a focus on quantitative in vitro to in vivo extrapolation (QIVIVE) to extrapolate in vitro cellular exposures to human equivalent doses from which human reference values can be derived. The proposed framework intends to systematically account for the complexities in extrapolation and data interpretation to support sound human health safety decisions in diverse industrial sectors (food systems, cosmetics, industrial chemicals, pharmaceuticals etc.). Case studies of chemical entities, using new and existing data, are presented to demonstrate the utility of the proposed framework while highlighting potential sources of human population bias and uncertainty, and the importance of Good Method and Reporting Practices.

Updated systematic assessment of human, animal and mechanistic evidence demonstrates lack of human carcinogenicity with consumption of aspartame

Aspartame has been studied extensively and evaluated for its safety in foods and beverages yet concerns for its potential carcinogenicity have persisted, driven primarily by animal studies conducted at the Ramazzini Institute (RI). To address this controversy, an updated systematic review of available human, animal, and mechanistic data was conducted leveraging critical assessment tools to consider the quality and reliability of data. The evidence base includes 12 animal studies and >40 epidemiological studies reviewed by the World Health Organization which collectively demonstrate a lack of carcinogenic effect. Assessment of >1360 mechanistic endpoints, including many guideline-based genotoxicity studies, demonstrate a lack of activity associated with endpoints grouped to key characteristics of carcinogens. Other non-specific mechanistic data (e.g., mixed findings of oxidative stress across study models, tissues, and species) do not provide evidence of a biologically plausible carcinogenic pathway associated with aspartame. Taken together, available evidence supports that aspartame consumption is not carcinogenic in humans and that the inconsistent findings of the RI studies may be explained by flaws in study design and conduct (despite additional analyses to address study limitations), as acknowledged by authoritative bodies.

Application of AOPs to assist regulatory assessment of chemical risks - Case studies, needs and recommendations

While human regulatory risk assessment (RA) still largely relies on animal studies, new approach methodologies (NAMs) based on in vitro, in silico or non-mammalian alternative models are increasingly used to evaluate chemical hazards. Moreover, human epidemiological studies with biomarkers of effect (BoE) also play an invaluable role in identifying health effects associated with chemical exposures. To move towards the next generation risk assessment (NGRA), it is therefore crucial to establish bridges between NAMs and standard approaches, and to establish processes for increasing mechanistically-based biological plausibility in human studies. The Adverse Outcome Pathway (AOP) framework constitutes an important tool to address these needs but, despite a significant increase in knowledge and awareness, the use of AOPs in chemical RA remains limited. The objective of this paper is to address issues related to using AOPs in a regulatory context from various perspectives as it was discussed in a workshop organized within the European Union partnerships HBM4EU and PARC in spring 2022. The paper presents examples where the AOP framework has been proven useful for the human RA process, particularly in hazard prioritization and characterization, in integrated approaches to testing and assessment (IATA), and in the identification and validation of BoE in epidemiological studies. Nevertheless, several limitations were identified that hinder the optimal usability and acceptance of AOPs by the regulatory community including the lack of quantitative information on response-response relationships and of efficient ways to map chemical data (exposure and toxicity) onto AOPs. The paper summarizes suggestions, ongoing initiatives and third-party tools that may help to overcome these obstacles and thus assure better implementation of AOPs in the NGRA.

Data-driven learning of narcosis mode of action identifies a CNS transcriptional signature shared between whole organism Caenorhabditis elegans and a fish gill cell line

With the large numbers of man-made chemicals produced and released in the environment, there is a need to provide assessments on their potential effects on environmental safety and human health. Current regulatory frameworks rely on a mix of both hazard and risk-based approaches to make safety decisions, but the large number of chemicals in commerce combined with an increased need to conduct assessments in the absence of animal testing makes this increasingly challenging. This challenge is catalysing the use of more mechanistic knowledge in safety assessment from both in silico and in vitro approaches in the hope that this will increase confidence in being able to identify modes of action (MoA) for the chemicals in question. Here we approach this challenge by testing whether a functional genomics approach in C. elegans and in a fish cell line can identify molecular mechanisms underlying the effects of narcotics, and the effects of more specific acting toxicants. We show that narcosis affects the expression of neuronal genes associated with CNS function in C. elegans and in a fish cell line. Overall, we believe that our study provides an important step in developing mechanistically relevant biomarkers which can be used to screen for hazards, and which prevent the need for repeated animal or cross-species comparisons for each new chemical.

Towards achieving a modern science-based paradigm for agrochemical carcinogenicity assessment

The rodent cancer bioassay has been the standard approach to fulfill regulatory requirements for assessing human carcinogenic potential of agrochemicals, food additives, industrial chemicals, and pharmaceuticals. Decades of research have described the limitations of the rodent cancer bioassay leading to international initiatives to seek alternatives and establish approaches that modernize carcinogenicity assessment. Biologically relevant approaches can provide mechanistic information and increased efficiency for evaluating hazard and risk of chemical carcinogenicity to humans. The application of human-relevant mechanistic understanding to support new approaches to carcinogenicity assessment will be invaluable for regulatory decision-making. The present work outlines the challenges and opportunities that authorities should consider as they come together to build a roadmap that leads to global acceptance and incorporation of fit-for-purpose, scientifically defensible new approaches for human-relevant carcinogenicity assessment of agrochemicals.

Commentary: cumulative risk assessment of perfluoroalkyl carboxylic acids and perfluoralkyl sulfonic acids: what is the scientific support for deriving tolerable exposures by assembling 27 PFAS into 1 common assessment group?

This commentary proposes an approach to risk assessment of mixtures of per- and polyfluorinated alkyl substances (PFAS) as EFSA was tasked to derive a tolerable intake for a group of 27 PFAS. The 27 PFAS to be considered contain different functional groups and have widely variable physicochemical (PC) properties and toxicokinetics and thus should not treated as one group based on regulatory guidance for risk assessment of mixtures. The proposed approach to grouping is to split the 27 PFAS into two groups, perfluoroalkyl carboxylates and perfluoroalkyl sulfonates, and apply a relative potency factor approach (as proposed by RIVM) to obtain two separate group TDIs based on liver toxicity in rodents since liver toxicity is a sensitive response of rodents to PFAS. Short chain PFAS and other PFAS structures should not be included in the groups due to their low potency and rapid elimination. This approach is in better agreement with scientific and regulatory guidance for mixture risk assessment.

A Collaborative Initiative to Establish Genomic Biomarkers for Assessing Tumorigenic Potential to Reduce Reliance on Conventional Rodent Carcinogenicity Studies

There is growing recognition across broad sectors of the scientific community that use of genomic biomarkers has the potential to reduce the need for conventional rodent carcinogenicity studies of industrial chemicals, agrochemicals, and pharmaceuticals through a weight-of-evidence approach. These biomarkers fall into 2 major categories: (1) sets of gene transcripts that can identify distinct tumorigenic mechanisms of action; and (2) cancer driver gene mutations indicative of rapidly expanding growth-advantaged clonal cell populations. This call-to-action article describes a collaborative approach launched to develop and qualify biomarker gene expression panels that measure widely accepted molecular pathways linked to tumorigenesis and their activation levels to predict tumorigenic doses of chemicals from short-term exposures. Growing evidence suggests that application of such biomarker panels in short-term exposure rodent studies can identify both tumorigenic hazard and tumorigenic activation levels for chemical-induced carcinogenicity. In the future, this approach will be expanded to include methodologies examining mutations in key cancer driver gene mutation hotspots as biomarkers of both genotoxic and nongenotoxic chemical tumor risk. Analytical, technical, and biological validation studies of these complementary genomic tools are being undertaken by multisector and multidisciplinary collaborative teams within the Health and Environmental Sciences Institute. Success from these efforts will facilitate the transition from current heavy reliance on conventional 2-year rodent carcinogenicity studies to more rapid animal- and resource-sparing approaches for mechanism-based carcinogenicity evaluation supporting internal and regulatory decision-making.

Adverse Outcome Pathway for Antimicrobial Quaternary Ammonium Compounds

Quaternary ammonium compounds (QACs) or quats are a large class of antimicrobial chemicals used in households and institutions as sanitizers and disinfectants. These chemicals are utilized as food processing sanitizers, algicides, in the process of water treatment, and preservatives in cosmetics. The aim of this study was to determine an Adverse Outcome Pathway (AOP) whereby two widely used QACs, alkyl dimethyl benzyl ammonium chloride (ADBAC) and didecyl dimethyl ammonium chloride (DDAC), may result in respiratory tract and gastrointestinal tract effects. When inhaled or ingested, these QACs are incorporated into the epithelial cell membrane at the point of contact. With sufficient dosage, the epithelial membrane is disrupted, reducing its fluidity, and releasing cellular contents. Further, ADBAC and DDAC might disrupt mitochondrial functions leading to decreased ATP production. Both events might lead to cell death, either attributed to direct lysis, necrosis, or apoptosis. Pro-inflammatory mediators are recruited to the tissue, inducing inflammation, edema, and excess mucus production. The primary tissue-level adverse outcome is epithelial degeneration and dysplasia. Most important, no apparent metabolism or distribution is involved in QAC action. Based upon this knowledge, it is suggested to replace default Uncertainty Factors for risk assessments with a set of Data Derived Extrapolation Factors.

Concept for the Evaluation of Carcinogenic Substances in Population-Based Human Biomonitoring

The Human Biomonitoring (HBM) Commission at the German Environment Agency holds the opinion that for environmental carcinogens for which no exposure levels can be assumed and are harmless to health, health-based guidance values corresponding to the classical definition of the HBM-I or HBM-II value cannot be established. Therefore, only reference values have been derived so far for genotoxic carcinogens from exposure data of the general population or subpopulations. The concept presented here opens up the possibility of performing health risk assessments of carcinogenic substances in human biomonitoring, and thus goes decisively beyond the purely descriptive statistical reference value concept. Using the presented method, quantitative dose descriptors of internal exposure can be derived from those of external exposure, provided that sufficient toxicokinetic information is available. Dose descriptors of internal exposure then allow the simple estimate of additional lifetime cancer risks for measured biomarker concentrations or, conversely, of equivalent concentrations for selected risks, such as those considered as tolerable for the general population. HBM data of chronic exposures to genotoxic carcinogens can thus be used to assess the additional lifetime cancer risk referring to the general population and to justify and prioritize risk management measures.

Beyond pharmaceuticals: Fit-for-purpose new approach methodologies for environmental cardiotoxicity testing

Environmental factors play a substantial role in determining cardiovascular health, but data informing the risks presented by environmental toxicants is insufficient. In vitro new approach methodologies (NAMs) offer a promising approach with which to address the limitations of traditional in vivo and in vitro assays for assessing cardiotoxicity. Driven largely by the needs of pharmaceutical toxicity testing, considerable progress in developing NAMs for cardiotoxicity analysis has already been made. As the scientific and regulatory interest in NAMs for environmental chemicals continues to grow, a thorough understanding of the unique features of environmental cardiotoxicants and their associated cardiotoxicities is needed. Here, we review the key characteristics of as well as important regulatory and biological considerations for fit-for-purpose NAMs for environmental cardiotoxicity. By emphasizing the challenges and opportunities presented by NAMs for environmental cardiotoxicity we hope to accelerate their development, acceptance, and application.

Ecotoxicological impacts caused by high demand surfactants in Latin America and a technological and innovative perspective for their substitution

Nowadays, water pollution represents a great concern due to population growth, industrialization, and urbanization. Every day hazardous chemical products for humans and aquatic organisms are disposed of arbitrarily from homes and industries. Even though detergents are considered an essential market, there is evidence of environmental impacts caused by surfactants like nonylphenol ethoxylate (NPE) and linear alkylbenzene sulfonates (LAS). Regulations about maximum allowable concentrations in sewage, surface water, and drinking water are scarce or null, mostly in developing countries like Latin American countries. Therefore, this review explores these two common toxic surfactants (NPE and LAS) and proposes a technological, innovative, and ecological perspective on detergents. Also, it establishes a starting point for industries to minimize adverse effects on humans and environmental health caused by these compounds.

Quantification of Hepatocellular Mitoses in a Toxicological Study in Rats Using a Convolutional Neural Network

Convolutional neural networks (CNNs) have been recognized as valuable tools for rapid quantitative analysis of morphological changes in toxicologic histopathology. We have assessed the performance of CNN-based (Halo-AI) mitotic figure detection in hepatocytes in comparison with detection by pathologists. In addition, we compared with Ki-67 and 5-bromodesoxyuridin (BrdU) immunohistochemistry labeling indices (LIs) obtained by image analysis. Tissues were from an exploratory toxicity study with a glycogen synthase kinase-3 (GSK-3) inhibitor. Our investigations revealed that (1) the CNN achieved similarly accurate but faster results than pathologists, (2) results of mitotic figure detection were comparable to Ki-67 and BrdU LIs, and (3) data from different methods were only moderately correlated. The latter is likely related to differences in the cell cycle component captured by each method. This highlights the importance of considering the differences of the available methods upon selection. Also, the pharmacology of our test item acting as a GSK-3 inhibitor potentially reduced the correlation. We conclude that hepatocyte cell proliferation assessment by CNNs can have several advantages when compared with the current gold standard: it relieves the pathologist of tedious routine tasks and contributes to standardization of results; the CNN algorithm can be shared and iteratively improved; it can be performed on routine histological slides; it does not require an additional animal experiment and in this way can contribute to animal welfare according to the 3R principles.

Rethinking chronic toxicity and carcinogenicity assessment for agrochemicals project (ReCAAP): A reporting framework to support a weight of evidence safety assessment without long-term rodent bioassays

Rodent cancer bioassays have been long-required studies for regulatory assessment of human cancer hazard and risk. These studies use hundreds of animals, are resource intensive, and certain aspects of these studies have limited human relevance. The past 10 years have seen an exponential growth of new technologies with the potential to effectively evaluate human cancer hazard and risk while reducing, refining, or replacing animal use. To streamline and facilitate uptake of new technologies, a workgroup comprised of scientists from government, academia, non-governmental organizations, and industry stakeholders developed a framework for waiver rationales of rodent cancer bioassays for consideration in agrochemical safety assessment. The workgroup used an iterative approach, incorporating regulatory agency feedback, and identifying critical information to be considered in a risk assessment-based weight of evidence determination of the need for rodent cancer bioassays. The reporting framework described herein was developed to support a chronic toxicity and carcinogenicity study waiver rationale, which includes information on use pattern(s), exposure scenario(s), pesticidal mode-of-action, physicochemical properties, metabolism, toxicokinetics, toxicological data including mechanistic data, and chemical read-across from similar registered pesticides. The framework could also be applied to endpoints other than chronic toxicity and carcinogenicity, and for chemicals other than agrochemicals.

Evaluation of the human hazard of the liver and lung tumors in mice treated with permethrin based on mode of action

The non-genotoxic synthetic pyrethroid insecticide permethrin produced hepatocellular adenomas and bronchiolo-alveolar adenomas in female CD-1 mice, but not in male CD-1 mice or in female or male Wistar rats. Studies were performed to evaluate possible modes of action (MOAs) for permethrin-induced female CD-1 mouse liver and lung tumor formation. The MOA for liver tumor formation by permethrin involves activation of the peroxisome proliferator-activated receptor alpha (PPARα), increased hepatocellular proliferation, development of altered hepatic foci, and ultimately liver tumors. This MOA is similar to that established for other PPARα activators and is considered to be qualitatively not plausible for humans. The MOA for lung tumor formation by permethrin involves interaction with Club cells, followed by a mitogenic effect resulting in Club cell proliferation, with prolonged administration producing Club cell hyperplasia and subsequently formation of bronchiolo-alveolar adenomas. Although the possibility that permethrin exposure may potentially result in enhancement of Club cell proliferation in humans cannot be completely excluded, there is sufficient information on differences in basic lung anatomy, physiology, metabolism, and biologic behavior of tumors in the general literature to conclude that humans are quantitatively less sensitive to agents that increase Club cell proliferation and lead to tumor formation in mice. The evidence strongly indicates that Club cell mitogens are not likely to lead to increased susceptibility to lung tumor development in humans. Overall, based on MOA evaluation it is concluded that permethrin does not pose a tumorigenic hazard for humans, this conclusion being supported by negative data from permethrin epidemiological studies.

Beyond AOPs: A Mechanistic Evaluation of NAMs in DART Testing

New Approach Methodologies (NAMs) promise to offer a unique opportunity to enable human-relevant safety decisions to be made without the need for animal testing in the context of exposure-driven Next Generation Risk Assessment (NGRA). Protecting human health against the potential effects a chemical may have on embryo-foetal development and/or aspects of reproductive biology using NGRA is particularly challenging. These are not single endpoint or health effects and risk assessments have traditionally relied on data from Developmental and Reproductive Toxicity (DART) tests in animals. There are numerous Adverse Outcome Pathways (AOPs) that can lead to DART, which means defining and developing strict testing strategies for every AOP, to predict apical outcomes, is neither a tenable goal nor a necessity to ensure NAM-based safety assessments are fit-for-purpose. Instead, a pragmatic approach is needed that uses the available knowledge and data to ensure NAM-based exposure-led safety assessments are sufficiently protective. To this end, the mechanistic and biological coverage of existing NAMs for DART were assessed and gaps to be addressed were identified, allowing the development of an approach that relies on generating data relevant to the overall mechanisms involved in human reproduction and embryo-foetal development. Using the knowledge of cellular processes and signalling pathways underlying the key stages in reproduction and development, we have developed a broad outline of endpoints informative of DART. When the existing NAMs were compared against this outline to determine whether they provide comprehensive coverage when integrated in a framework, we found them to generally cover the reproductive and developmental processes underlying the traditionally evaluated apical endpoint studies. The application of this safety assessment framework is illustrated using an exposure-led case study.

Towards a qAOP framework for predictive toxicology - Linking data to decisions

The adverse outcome pathway (AOP) is a conceptual construct that facilitates organisation and interpretation of mechanistic data representing multiple biological levels and deriving from a range of methodological approaches including in silico, in vitro and in vivo assays. AOPs are playing an increasingly important role in the chemical safety assessment paradigm and quantification of AOPs is an important step towards a more reliable prediction of chemically induced adverse effects. Modelling methodologies require the identification, extraction and use of reliable data and information to support the inclusion of quantitative considerations in AOP development. An extensive and growing range of digital resources are available to support the modelling of quantitative AOPs, providing a wide range of information, but also requiring guidance for their practical application. A framework for qAOP development is proposed based on feedback from a group of experts and three qAOP case studies. The proposed framework provides a harmonised approach for both regulators and scientists working in this area.

Causal Criteria in Medical and Biological Disciplines: History, Essence, and Radiation Aspects. Report 4, Part 1: The Post-Hill Criteria and Ecolgoical Criteria

Part 1 of Report 4 is focused on the development and modifications of causal criteria after A.B. Hill (1965). Criteria from B. MacMahon et al. (1970-1996), regarded as the first textbook for modern epidemiology, were considered, and it was found that the named researchers did not offer anything new despite the frequent mention of this source in relation to the theme. A similar situation emerged with the criteria of M. Susser: the three obligatory points of this author, "Association" (or "Probability" of causality), "Time order," and "Direction of effect," are trivial, and two more special criteria, which are the development of "Popperian Epidemiology," i.e., "Surviability" of the hypothesis when it is tested by different methods (included in the refinement in Hill's criterion "Consistency of association") and "Predictive performance" of the hypothesis are more theoretical and hardly applicable for the practice of epidemiology and public health. The same restrictions apply to the similar "Popperian" criteria of D.L. Weed, "Predictability" and "Testability" of the causal hypothesis. Although the universal postulates of A.S. Evans for infectious and noninfectious pathologies can be considered exhaustive, they are not used either in epidemiology or in any other discipline practice, except for the field of infectious pathologies, which is probably explained by the complication of the ten-point complex. The little-known criteria of P. Cole (1997) for medical and forensic practice are the most important. The three parts of Hill's criterion-based approaches are important in that they go from a single epidemiological study through a cycle of studies (coupled with the integration of data from other biomedical disciplines) to re-base Hill's criteria for assessing the individual causality of an effect. These constructs complement the earlier guidance from R.E. Gots (1986) on establishing probabilistic personal causation. The collection of causal criteria and the guidelines for environmental disciplines (ecology of biota, human ecoepidemiology, and human ecotoxicology) were considered. The total dominance of inductive causal criteria, both initial and in modifications and with additions, was revealed for an apparently complete base of sources (1979-2020). Adaptations of all known causal schemes based on guidelines have been found, from Henle-Koch postulates to Hill and Susser, including in the international programs and practice of the U.S. Environmental Protection Agency. The Hill Criteria are used by the WHO and other organizations on chemical safety (IPCS) to assess causality in animal experiments for subsequent extrapolation to humans. Data on the assessment of the causality of effects in ecology, ecoepidemiology, and ecotoxicology, together with the use of Hill's criteria for animal experiments, are of significant relevance not only for radiation ecology, but also for radiobiology.

Evaluation of human relevance of Nicofluprole-induced rat thyroid disruption

Nicofluprole is a novel insecticide of the phenylpyrazole class conferring selective antagonistic activity on insect GABA receptors. After repeated daily dietary administration to Wistar rats for 28/90 days, Nicofluprole induced increases in thyroid (and liver) weight, associated with histopathology changes. Nicofluprole did not inhibit thyroid peroxydase nor sodium/iodide symporter, two key players in the biosynthesis of thyroid hormones, indicating the absence of a direct thyroid effect. The results seen in rats suggested a mode of action of Nicofluprole driven by the molecular initiating event of CAR/PXR nuclear receptor activation in livers, with key events of increases in liver weight and hypertrophy, decreasing circulatory thyroid hormones, a compensatory increase in TSH release and follicular cell hypertrophy. To explore the relevance of these changes to humans, well established in vitro rat and human sandwich-cultured hepatocytes were exposed to Nicofluprole up to 7 days. A concentration-dependent CYP3A induction (PXR-activation), an increase in T4-glucuronoconjugation accompanied by UGT1A/2B inductions was observed in rat but not in human hepatocytes. The inductions seen with Nicofluprole in rat (in vivo and in vitro in hepatocytes) that were absent in human hepatocytes represent another example of species-selectivity of nuclear CAR/PXR receptor activators. Importantly, the different pattern observed in rat and human models demonstrate that Nicofluprole-related thyroid effects observed in the rat are with no human relevance.

Guidance Document on Scientific criteria for grouping chemicals into assessment groups for human risk assessment of combined exposure to multiple chemicals

This guidance document provides harmonised and flexible methodologies to apply scientific criteria and prioritisation methods for grouping chemicals into assessment groups for human risk assessment of combined exposure to multiple chemicals. In the context of EFSA's risk assessments, the problem formulation step defines the chemicals to be assessed in the terms of reference usually through regulatory criteria often set by risk managers based on legislative requirements. Scientific criteria such as hazard-driven criteria can be used to group these chemicals into assessment groups. In this guidance document, a framework is proposed to apply hazard-driven criteria for grouping of chemicals into assessment groups using mechanistic information on toxicity as the gold standard where available (i.e. common mode of action or adverse outcome pathway) through a structured weight of evidence approach. However, when such mechanistic data are not available, grouping may be performed using a common adverse outcome. Toxicokinetic data can also be useful for grouping, particularly when metabolism information is available for a class of compounds and common toxicologically relevant metabolites are shared. In addition, prioritisation methods provide means to identify low-priority chemicals and reduce the number of chemicals in an assessment group. Prioritisation methods include combined risk-based approaches, risk-based approaches for single chemicals and exposure-driven approaches. Case studies have been provided to illustrate the practical application of hazard-driven criteria and the use of prioritisation methods for grouping of chemicals in assessment groups. Recommendations for future work are discussed.

Club Cells Are the Primary Target for Permethrin-Induced Mouse Lung Tumor Formation

Permethrin has been shown to increase lung adenomas in female CD-1 mice, but not in male mice or Wistar rats. The proposed mode of action (MOA) for permethrin-induced female mouse lung tumor formation involves morphological changes in Club cells; increased Club cell proliferation; increased Club cell hyperplasia, and lung tumor formation. In this study, the treatment of female CD-1 mice with tumorigenic doses (2500 and 5000 ppm) of permethrin, but not with a nontumorigenic dose (20 ppm), for 14 and/or 28 days increased Club cell replicative DNA synthesis. Global gene expression analysis of female mouse lung samples demonstrated that permethrin treatment up-regulated 3 genes associated with cell proliferation, namely aldehyde dehydrogenase 3a1 (Aldh3a1), oxidative stress-induced growth inhibitor 1, and thioredoxin reductase 1. Treatment with 2500 and 5000 ppm, but not 20 ppm, permethrin for 7 days produced significant increases in mRNA levels of these 3 genes. Immunohistochemical analysis demonstrated that Club cell secretory protein, CYP2F2, and ALDH3A1 colocalized in Club cells; confirmed by flow cytometry analysis of lung cells employing KI67 as a cell proliferation marker. Overall, the present data extend the proposed MOA by demonstrating that Club cells are the primary initial target of permethrin administration in female mouse lungs. As humans are quantitatively much less sensitive to agents that increase Club cell proliferation and lung tumor formation in mice, it is most likely that permethrin could not produce lung tumors in humans. This conclusion is supported by available negative epidemiological data from several studies.

Application of humanized mice to toxicology studies: Evaluation of the human relevance of the mode of action for rodent liver tumor formation by activators of the constitutive androstane receptor (CAR)

The constitutive androstane receptor (CAR)-mediated mode of action (MOA) for phenobarbital (PB)-induced rodent liver tumor formation has been established, with increased hepatocyte proliferation, which is a key event in tumor formation. Previous studies have demonstrated that PB and other CAR-activators stimulate proliferation in cultured rodent hepatocytes, but not in cultured human hepatocytes. However, in the genetically humanized CAR and pregnane X receptor (PXR) mouse (hCAR/hPXR mouse, downstream genes are still mouse), PB increased hepatocyte proliferation and tumor production in vivo. In contrast to the hCAR/hPXR mouse, studies with chimeric mice with human hepatocytes (PXB-mouse, both receptor and downstream genes are human) demonstrated that PB did not increase human hepatocyte proliferation in vivo. PB increased hepatocyte proliferation in a chimeric mouse model with rat hepatocytes, indicating that the lack of human hepatocyte proliferation is not due to any functional defect in the chimeric mouse liver environment. Gene expression analysis demonstrated that the downstream genes of CAR/PXR activation were similar in hCAR/hPXR and CD-1 mice, but differed from those observed in chimeric mice with human hepatocytes. These findings strongly support the conclusion that the MOA for CAR-mediated rodent liver tumor formation is qualitatively implausible for humans. Indeed, epidemiological studies have found no causal link between PB and human liver tumors. There are many similarities with respect to hepatic effects and species differences between rodent CAR and peroxisome proliferator-activated receptor α activators. Based on our research, the chimeric mouse with human hepatocytes (PXB-mouse) is reliable for human cancer risk assessment of test chemicals.

Guidance on risk assessment of nanomaterials to be applied in the food and feed chain: human and animal health

The EFSA has updated the Guidance on risk assessment of the application of nanoscience and nanotechnologies in the food and feed chain, human and animal health. It covers the application areas within EFSA's remit, including novel foods, food contact materials, food/feed additives and pesticides. The updated guidance, now Scientific Committee Guidance on nano risk assessment (SC Guidance on Nano-RA), has taken account of relevant scientific studies that provide insights to physico-chemical properties, exposure assessment and hazard characterisation of nanomaterials and areas of applicability. Together with the accompanying Guidance on Technical requirements for regulated food and feed product applications to establish the presence of small particles including nanoparticles (Guidance on Particle-TR), the SC Guidance on Nano-RA specifically elaborates on physico-chemical characterisation, key parameters that should be measured, methods and techniques that can be used for characterisation of nanomaterials and their determination in complex matrices. The SC Guidance on Nano-RA also details aspects relating to exposure assessment and hazard identification and characterisation. In particular, nanospecific considerations relating to in vitro/in vivo toxicological studies are discussed and a tiered framework for toxicological testing is outlined. Furthermore, in vitro degradation, toxicokinetics, genotoxicity, local and systemic toxicity as well as general issues relating to testing of nanomaterials are described. Depending on the initial tier results, additional studies may be needed to investigate reproductive and developmental toxicity, chronic toxicity and carcinogenicity, immunotoxicity and allergenicity, neurotoxicity, effects on gut microbiome and endocrine activity. The possible use of read-across to fill data gaps as well as the potential use of integrated testing strategies and the knowledge of modes or mechanisms of action are also discussed. The Guidance proposes approaches to risk characterisation and uncertainty analysis.

A toxicity pathway-based approach for modeling the mode of action framework of lead-induced neurotoxicity

BACKGROUND

The underlying mechanisms of lead (Pb) toxicity are not fully understood, which makes challenges to the traditional risk assessment. There is growing use of the mode of action (MOA) for risk assessment by integration of experimental data and system biology. The current study aims to develop a new pathway-based MOA for assessing Pb-induced neurotoxicity.

METHODS

The available Comparative Toxicogenomic Database (CTD) was used to search genes associated with Pb-induced neurotoxicity followed by developing toxicity pathways using Ingenuity Pathway Analysis (IPA). The spatiotemporal sequence of disturbing toxicity pathways and key events (KEs) were identified by upstream regulator analysis. The MOA framework was constructed by KEs in biological and chronological order.

RESULTS

There were a total of 71 references showing the relationship between lead exposure and neurotoxicity, which contained 2331 genes. IPA analysis showed that the neuroinflammation signaling pathway was the core toxicity pathway in the enriched pathways relevant to Pb-induced neurotoxicity. The upstream regulator analysis demonstrated that the aryl hydrocarbon receptor (AHR) signaling pathway was the upstream regulator of the neuroinflammation signaling pathway (11.76% overlap with upstream regulators, |Z-score|=1.451). Therefore, AHR activation was recognized as the first key event (KE1) in the MOA framework. The following downstream molecular and cellular key events were also identified. The pathway-based MOA framework of Pb-induced neurotoxicity was built starting with AHR activation, followed by an inflammatory response and neuron apoptosis.

CONCLUSION

Our toxicity pathway-based approach not only advances the development of risk assessment for Pb-induced neurotoxicity but also brings new insights into constructing MOA frameworks of risk assessment for new chemicals.

Construction of Mode of Action for Cadmium-Induced Renal Tubular Dysfunction Based on a Toxicity Pathway-Oriented Approach

Although it is recognized that cadmium (Cd) causes renal tubular dysfunction, the mechanism of Cd-induced nephrotoxicity is not yet fully understood. Mode of action (MOA) is a developing tool for chemical risk assessment. To establish the mechanistic MOA of Cd-induced renal tubular dysfunction, the Comparative Toxicogenomics Database (CTD) was used to obtain genomics data of Cd-induced nephrotoxicity, and Ingenuity^® Pathway Analysis (IPA) software was applied for bioinformatics analysis. Based on the perturbed toxicity pathways during the process of Cd-induced nephrotoxicity, we established the MOA of Cd-induced renal tubular dysfunction and assessed its confidence with the tailored Bradford Hill criteria. Bioinformatics analysis showed that oxidative stress, DNA damage, cell cycle arrest, and cell death were the probable key events (KEs). Assessment of the overall MOA of Cd-induced renal tubular dysfunction indicated a moderate confidence, and there are still some evidence gaps to be filled by rational experimental designs.

An adverse outcome pathway on the disruption of retinoic acid metabolism leading to developmental craniofacial defects

Adverse outcome pathway (AOP) is a conceptual framework that links a molecular initiating event (MIE) via intermediate key events (KEs) with adverse effects (adverse outcomes, AO) relevant for risk assessment, through defined KE relationships (KERs). The aim of the present work is to describe a linear AOP, supported by experimental data, for skeletal craniofacial defects as the AO. This AO was selected in view of its relative high incidence in humans and the suspected relation to chemical exposure. We focused on inhibition of CYP26, a retinoic acid (RA) metabolizing enzyme, as MIE, based on robust previously published data. Conazoles were selected as representative stressors. Intermediate KEs are RA disbalance, aberrant HOX gene expression, disrupted specification, migration, and differentiation of neural crest cells, and branchial arch dysmorphology. We described the biological basis of the postulated events and conducted weight of evidence (WoE) assessments. The biological plausibility and the overall empirical evidence were assessed as high and moderate, respectively, the latter taking into consideration the moderate evidence for concordance of dose-response and temporal relationships. Finally, the essentiality assessment of the KEs, considered as high, supported the robustness of the presented AOP. This AOP, which appears of relevance to humans, thus contributes to mechanistic underpinning of selected test methods, thereby supporting their application in integrated new approach test methodologies and strategies and application in a regulatory context.

Towards a science-based testing strategy to identify maternal thyroid hormone imbalance and neurodevelopmental effects in the progeny - part II: how can key events of relevant adverse outcome pathways be addressed in toxicological assessments?

The current understanding of thyroid-related adverse outcome pathways (AOPs) with adverse neurodevelopmental outcomes in mammals has been reviewed. This served to establish if standard rodent toxicity test methods and in vitro assays allow identifying thyroid-related modes-of-action potentially leading to adverse neurodevelopmental outcomes, and the human relevance of effects - in line with the European Commission's Endocrine Disruptor Criteria. The underlying hypothesis is that an understanding of the key events of relevant AOPs provides insight into differences in incidence, magnitude, or species sensitivity of adverse outcomes. The rodent studies include measurements of serum thyroid hormones, thyroid gland pathology and neurodevelopmental assessments, but do not directly inform on specific modes-of-action. Opportunities to address additional non-routine parameters reflecting critical events of AOPs in toxicological assessments are presented. These parameters appear relevant to support the identification of specific thyroid-related modes-of-action, provided that prevailing technical limitations are overcome. Current understanding of quantitative key event relationships is often weak, but would be needed to determine if the triggering of a molecular initiating event will ultimately result in an adverse outcome. Also, significant species differences in all processes related to thyroid hormone signalling are evident, but the biological implications thereof (including human relevance) are often unknown. In conclusion, careful consideration of the measurement (e.g. timing, method) and interpretation of additional non-routine parameters is warranted. These findings will be used in a subsequent paper to propose a testing strategy to identify if a substance may elicit maternal thyroid hormone imbalance and potentially also neurodevelopmental effects in the progeny.

An updated mode of action and human relevance framework evaluation for Formaldehyde-Related nasal tumors

Formaldehyde is a reactive aldehyde naturally present in all plant and animal tissues and a critical component of the one-carbon metabolism pathway. It is also a high production volume chemical used in the manufacture of numerous products. Formaldehyde is also one of the most well-studied chemicals with respect to environmental fate, biology, and toxicology-including carcinogenic potential, and mode of action (MOA). In 2006, a published MOA for formaldehyde-induced nasal tumors in rats concluded that nasal tumors were most likely driven by cytotoxicity and regenerative cell proliferation, with possible contributions from direct genotoxicity. In the past 15 years, new research has better informed the MOA with the publication of in vivo genotoxicity assays, toxicogenomic analyses, and development of ultra-sensitive methods to measure endogenous and exogenous formaldehyde-induced DNA adducts. Herein, we review and update the MOA for nasal tumors, with particular emphasis on the numerous studies published since 2006. These new studies further underscore the involvement of cytotoxicity and regenerative cell proliferation, and further inform the genotoxic potential of inhaled formaldehyde. The data lend additional support for the use of mechanistic data for the derivation of toxicity criteria and/or scientifically supported approaches for low-dose extrapolation for the risk assessment of formaldehyde.

Critical evaluation of the human relevance of the mode of action for rodent liver tumor formation by activators of the constitutive androstane receptor (CAR)

Many nongenotoxic chemicals have been shown to produce liver tumors in mice and/or rats by a mode of action (MOA) involving activation of the constitutive androstane receptor (CAR). Studies with phenobarbital (PB) and other compounds have identified the key events for this MOA: CAR activation; increased hepatocellular proliferation; altered foci formation; and ultimately the development of adenomas/carcinomas. In terms of human relevance, the pivotal species difference is that CAR activators are mitogenic agents in mouse and rat hepatocytes, but they do not stimulate increased hepatocellular proliferation in humans. This conclusion is supported by substantial in vitro studies with cultured rodent and human hepatocytes and also by in vivo studies with chimeric mice with human hepatocytes. Examination of the literature reveals many similarities in the hepatic effects and species differences between activators of rodent CAR and the peroxisome proliferator-activated receptor alpha (PPARα), with PPARα activators also not being mitogenic agents in human hepatocytes. Overall, a critical analysis of the available data demonstrates that the established MOA for rodent liver tumor formation by PB and other CAR activators is qualitatively not plausible for humans. This conclusion is supported by data from several human epidemiology studies.

Application of adverse outcome pathway networks to integrate mechanistic data informing the choice of a point of departure for hydrogen sulfide exposure limits

Acute exposure to hydrogen sulfide initiates a series of hallmark biological effects that occur progressively at increasing exposure levels: odor perception, conjunctivitis, olfactory paralysis, "knockdown," pulmonary edema, and apnea. Although effects of exposure to high concentrations of hydrogen sulfide are clear, effects associated with chronic, low-level exposure in humans is under debate, leading to uncertainty in the critical effect used in regulatory risk assessments addressing low dose exposures. This study integrates experimental animal, observational epidemiology, and occupational exposure evidence by applying a pathway-based approach. A hypothesized adverse outcome pathway (AOP) network was developed from 34 studies, composed of 4 AOPs sharing 1 molecular initiating events (MIE) and culminating in 4 adverse outcomes. A comparative assessment of effect levels and weight of evidence identified an AOP leading to a biologically-plausible, low-dose outcome relative to the other outcomes (nasal lesions, 30 ppm versus olfactory paralysis, >100 ppm; neurological effects, >80 ppm; pulmonary edema, >80 ppm). This AOP (i.e. AOP1) consists of the following key events: cytochrome oxidase inhibition (>10 ppm), neuronal cell loss (>30 ppm), and olfactory nasal lesions (defined as both neuronal cell loss and basal cell hyperplasia; >30 ppm) in rodents. The key event relationships in this pathway were supported by moderate empirical evidence and have high biological plausibility due to known mechanistic understanding and consistency in observations for diverse chemicals.

Hazard characterization of carcinogenicity, mutagenicity, and reproductive toxicity for short chain primary nitroalkanes

Nitroalkanes are organic aliphatic hydrocarbon compounds with a nitro moiety that are commonly used as solvents or intermediates to synthesize a variety of organic compounds due to their inherent reactivity. In June 2020, a harmonized classification and labeling (CLH) proposal was submitted to the European Chemicals Agency (ECHA) for the following harmonized carcinogenicity, mutagenicity, and reproductive toxicity ("CMR") classifications for nitromethane (NM), nitroethane (NE), and 1-nitropropane (1-NP): NM Carc. 1B and Repr. 1B; NE Repr. 1B; and 1-NP Repr. 2. In this assessment, a weight of evidence (WoE) evaluation of studies on animal carcinogenicity and reproductive and developmental toxicity, genotoxicity, and mode of action for these three nitroalkanes was performed to critically assess the relevance of the proposed CMR classifications. Overall, the WoE indicates that NM, NE, and 1-NP are not carcinogenic, genotoxic, nor selective reproductive or developmental toxicants. Based on our analysis, classifying NM, NE, and 1-NP as Category 2 reproductive toxicants is most appropriate. Furthermore, not classifying NE and 1-NP with respect to their carcinogenicity is appropriate based on the available studies for this endpoint coupled with negative results in genotoxicity studies, metabolism data, and in silico predictions. We determined that the classification for NM of Carc. 1B is not appropriate, based on the fact that rat mammary and harderian tumors are likely not relevant to humans and lung and liver tumors reported in mice were equivocal in their dose-response and statistical significance.

Per- and Polyfluoroalkyl Substance Toxicity and Human Health Review: Current State of Knowledge and Strategies for Informing Future Research

Reports of environmental and human health impacts of per- and polyfluoroalkyl substances (PFAS) have greatly increased in the peer-reviewed literature. The goals of the present review are to assess the state of the science regarding toxicological effects of PFAS and to develop strategies for advancing knowledge on the health effects of this large family of chemicals. Currently, much of the toxicity data available for PFAS are for a handful of chemicals, primarily legacy PFAS such as perfluorooctanoic acid and perfluorooctane sulfonate. Epidemiological studies have revealed associations between exposure to specific PFAS and a variety of health effects, including altered immune and thyroid function, liver disease, lipid and insulin dysregulation, kidney disease, adverse reproductive and developmental outcomes, and cancer. Concordance with experimental animal data exists for many of these effects. However, information on modes of action and adverse outcome pathways must be expanded, and profound differences in PFAS toxicokinetic properties must be considered in understanding differences in responses between the sexes and among species and life stages. With many health effects noted for a relatively few example compounds and hundreds of other PFAS in commerce lacking toxicity data, more contemporary and high-throughput approaches such as read-across, molecular dynamics, and protein modeling are proposed to accelerate the development of toxicity information on emerging and legacy PFAS, individually and as mixtures. In addition, an appropriate degree of precaution, given what is already known from the PFAS examples noted, may be needed to protect human health. Environ Toxicol Chem 2021;40:606-630. © 2020 SETAC.