Use of mode of action in risk assessment: past, present, and future

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.

Exposure limits for indoor volatile substances concerning the general population: The role of population-based differences in sensory irritation of the eyes and airways for assessment factors

Assessment factors (AFs) are essential in the derivation of occupational exposure limits (OELs) and indoor air quality guidelines. The factors shall accommodate differences in sensitivity between subgroups, i.e., workers, healthy and sick people, and occupational exposure versus life-long exposure for the general population. Derivation of AFs itself is based on empirical knowledge from human and animal exposure studies with immanent uncertainty in the empirical evidence due to knowledge gaps and experimental reliability. Sensory irritation in the eyes and airways constitute about 30-40% of OELs and is an abundant symptom in non-industrial buildings characterizing the indoor air quality and general health. Intraspecies differences between subgroups of the general population should be quantified for the proposal of more 'empirical' based AFs. In this review, we focus on sensitivity differences in sensory irritation about gender, age, health status, and vulnerability in people, based solely on human exposure studies. Females are more sensitive to sensory irritation than males for few volatile substances. Older people appear less sensitive than younger ones. However, impaired defense mechanisms may increase vulnerability in the long term. Empirical evidence of sensory irritation in children is rare and limited to children down to the age of six years. Studies of the nervous system in children compared to adults suggest a higher sensitivity in children; however, some defense mechanisms are more efficient in children than in adults. Usually, exposure studies are performed with healthy subjects. Exposure studies with sick people are not representative due to the deselection of subjects with moderate or severe eye or airway diseases, which likely underestimates the sensitivity of the group of people with diseases. Psychological characterization like personality factors shows that concentrations of volatile substances far below their sensory irritation thresholds may influence the sensitivity, in part biased by odor perception. Thus, the protection of people with extreme personality traits is not feasible by an AF and other mitigation strategies are required. The available empirical evidence comprising age, lifestyle, and health supports an AF of not greater than up to 2 for sensory irritation. Further, general AFs are discouraged for derivation, rather substance-specific derivation of AFs is recommended based on the risk assessment of empirical data, deposition in the airways depending on the substance's water solubility and compensating for knowledge and experimental gaps. Modeling of sensory irritation would be a better 'empirical' starting point for derivation of AFs for children, older, and sick people, as human exposure studies are not possible (due to ethical reasons) or not generalizable (due to self-selection). Dedicated AFs may be derived for environments where dry air, high room temperature, and visually demanding tasks aggravate the eyes or airways than for places in which the workload is balanced, while indoor playgrounds might need other AFs due to physical workload and affected groups of the general population.

In Silico Acute Aquatic Hazard Assessment and Prioritization Using a Grouped Target Site Model: A Case Study of Organic Substances Reported in Permian Basin Hydraulic Fracturing Operations

Hydraulic fracturing (HF) is commonly used to enhance onshore recovery of oil and gas during production. This process involves the use of a variety of chemicals to support the physical extraction of oil and gas, maintain appropriate conditions downhole (e.g., redox conditions, pH), and limit microbial growth. The diversity of chemicals used in HF presents a significant challenge for risk assessment. The objective of the present study is to establish a transparent, reproducible procedure for estimating 5th percentile acute aquatic hazard concentrations (e.g., acute hazard concentration 5th percentiles [HC5s]) for these substances and validating against existing toxicity data. A simplified, grouped target site model (gTSM) was developed using a database (n = 1696) of diverse compounds with known mode of action (MoA) information. Statistical significance testing was employed to reduce model complexity by combining 11 discrete MoAs into three general hazard groups. The new model was trained and validated using an 80:20 allocation of the experimental database. The gTSM predicts toxicity using a combination of target site water partition coefficients and hazard group-based critical target site concentrations. Model performance was comparable to the original TSM using 40% fewer parameters. Model predictions were judged to be sufficiently reliable and the gTSM was further used to prioritize a subset of reported Permian Basin HF substances for risk evaluation. The gTSM was applied to predict hazard groups, species acute toxicity, and acute HC5s for 186 organic compounds (neutral and ionic). Toxicity predictions and acute HC5 estimates were validated against measured acute toxicity data compiled for HF substances. This case study supports the gTSM as an efficient, cost-effective computational tool for rapid aquatic hazard assessment of diverse organic chemicals. Environ Toxicol Chem 2024;00:1-12. © 2024 ExxonMobil Petroleum and Chemical BV. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

PFAS use in electronic products and exposure risks during handling and processing of e-waste: A review

Poly- and perfluorinated alkyl substances (PFAS) have been and are used in electronic products due to their unique properties that improve product quality and performance. Ubiquities and persistence of some PFAS detected in environmental samples (water, soil, air) have attracted much attention and regulatory actions in recent years. This review provides an overview of PFAS use in electronic components; trends in quantities of e-waste generation; PFAS exposure pathways during e-waste handling and processing; reported PFAS in environmental samples and samples of serum, blood, and hair collected from people living near and working at e-waste processing sites. Processes used for manufacturing electronic components (e.g., embedded processes, additive manufacturing) make recycling or materials recovery from discarded electronic units and components very difficult and unfeasible. Exposure during numerous processing steps for materials recovery and scavenging at disposal sites can result in PFAS intake through inhalation, ingestion, and dermal routes. Chemical risk assessment approaches have been continuously evolving to consider chemical-specific dosimetric and mechanistic information. While the metabolic fate of PFAS is not well understood, some PFAS bioaccumulate and bind to proteins (but not to lipids) in biota and humans due to their surface-active characteristics and very low solubility in water and fat. It is difficult to associate the adverse health effects due to exposure to e-waste directly to PFAS as there are other factors that could contribute to the observed adverse effects. However, PFAS have been detected in the samples collected from different environmental compartments (e.g., water, soil, leachate, blood sera, rainwater) at and near e-waste processing sites, landfills, and near electronics and optoelectronics industries indicating that e-waste collection, processing, and disposal sites are potential PFAS exposure locations. Better monitoring of e-waste handling sites and detailed epidemiological studies for at risk populations are needed for assessing potential health risks due to PFAS exposure at these sites.

Standards of evidence and causality in regulatory science: Risk and benefit assessment

In this paper we analyze the Russo-Williamson Thesis (RWT) as a standard of evidence in regulatory science, in risk as well as benefit assessment. In our analysis we take account of the recent controversies that have taken place in regulation with respect to the evidentiary requirements necessary for regulatory decision making. RWT's main point is that not only probabilistic but also mechanistic evidence is necessary for being able to infer the existence of causal links. We ask in which way RWT could have an impact upon current decision making about subjecting to regulation (or, to the contrary, leaving them unregulated) certain chemical substances, food stuffs, health claims, and other typical objects of regulation. We show that the application of RWT resolves some of the problems posed by current standards of evidence. RWT makes it possible to determine with higher accuracy if a particular substance should be subjected to regulation or not, even though under certain circumstances RWT itself may turn into a source of regulatory error. The adequacy of RWT as a standard of evidence depends on the precise manner of its application to regulation (particularly the consideration of mechanistic evidence as a complementary or necessary requirement), as well as the assessment of its non-epistemic consequences.

Adverse outcome pathways for chemical toxicity and their applications to workers' health: a literature review

OBJECTIVE AND METHODS

Various papers related to the application of adverse outcome pathways (AOPs) for the prevention of occupational disease were reviewed. The Internet was used as the primary tool to search for the necessary research data and information, using such online resources as Google Scholar, ScienceDirect, Scopus, NDSL, and PubMed. The key search terms were "adverse outcome pathway," "toxicology," "risk assessment," "human," "worker," "occupational safety and health," and so on.

RESULTS AND CONCLUSION

The aim of this paper is to explain the use of AOP for the understanding of chemical toxicity as a conceptual means and to predict the toxic mechanism. The tools of AOP have emerged as a forward-looking alternative to the existing chemical risk assessment paradigm. AOP is being applied to the assessment of acute toxicity and to chronic toxic chemicals in the workplace. Not only can it lead to breakthroughs in occupational and environmental cancer prevention, it is also widely used in chemical risk assessment and has led to breakthroughs in the prevention of occupational disease in the workplace.

Target site model: Predicting mode of action and aquatic organism acute toxicity using Abraham parameters and feature-weighted k-nearest neighbors classification

A database of 1480 chemicals with 47 associated modes of action compiled from the literature encompasses a wide range of chemical classes (alkanes, polycyclic aromatic hydrocarbons, pesticides, and polar compounds) and includes toxicity data for 79 different aquatic genera. The data were split into a calibration group and a validation group (80/20) to apply k-nearest neighbors (k-NN) methodology to predict the toxic mode of action for the compound. Other approaches were tested (support vector machines and linear discriminant analysis) as well as variations in the k-NN technique (distance weighting, feature weighting). Best-prediction results were found with k = 3, in a voting platform with optimized feature weighting. Using the predicted mode of action, the appropriate polyparameter target site model for that mode of action is applied to calculate the 50% lethal concentration (LC50). Predicted LC50s for the validation database resulted in a root-mean squared error (RMSE) of 0.752. This can be compared to an RMSE of 0.655 for the same validation set using the reference mode of action labels. The complete database resulted in an RMSE of 0.793 for reference mode of action labels. This confirms that the classification model has sufficient accuracy for predicting the mode of action and for determining toxicity using the target site model. Environ Toxicol Chem 2019;38:375-386. © 2018 SETAC.

Target site model: Application of the polyparameter target lipid model to predict aquatic organism acute toxicity for various modes of action

A database of 2049 chemicals with 47 associated modes of action (MoA) was compiled from the literature. The database includes alkanes, polycyclic aromatic hydrocarbons, pesticides, inorganic, and polar compounds. Brief descriptions of some critical MoA classification groups are provided. The MoA from the 14 sources were assigned using a variety of reliable experimental and modeling techniques. Toxicity information, chemical parameters, and solubility limits were combined with the MoA label information to create the data set used for model development. The model database was used to generate linear free energy relationships for each specific MoA using multilinear regression analysis. The model uses chemical-specific Abraham solute parameters estimated from AbSolv to determine MoA-specific solvent parameters. With this procedure, critical target site concentrations are determined for each genus. Statistical analysis showed a wide range in values of the solvent parameters for the significant MoA. Environ Toxicol Chem 2019;38:222-239. © 2018 SETAC.

Application of the Adverse Outcome Pathway Framework to Risk Assessment for Predicting Carcinogenicity of Chemicals

As industry develops in modern society, many chemicals are being used. The safety of chemicals is an important issue because humans are constantly exposed to chemicals throughout their daily life. Through a risk assessment, the hazardous human effects of chemicals can be identified. Recently, the adverse outcome pathway (AOP) framework has been used to predict the adverse effects of chemicals. As a conceptual framework for organizing existing biological knowledge, the AOP consists of a molecular initiating event, key events, and an adverse outcome. These independent elements represent biological responses and are connected by key event relationships. This AOP framework provides intuitive hazard identification that can be helpful for carcinogenic risk assessment of chemicals. In this review, we introduce the application of the AOP framework to risk assessment for predicting carcinogenicity of chemicals and illustrate the utility of this approach for cancer prevention.

Glyphosate toxicity and carcinogenicity: a review of the scientific basis of the European Union assessment and its differences with IARC

Glyphosate is the most widely used herbicide worldwide. It is a broad spectrum herbicide and its agricultural uses increased considerably after the development of glyphosate-resistant genetically modified (GM) varieties. Since glyphosate was introduced in 1974, all regulatory assessments have established that glyphosate has low hazard potential to mammals, however, the International Agency for Research on Cancer (IARC) concluded in March 2015 that it is probably carcinogenic. The IARC conclusion was not confirmed by the EU assessment or the recent joint WHO/FAO evaluation, both using additional evidence. Glyphosate is not the first topic of disagreement between IARC and regulatory evaluations, but has received greater attention. This review presents the scientific basis of the glyphosate health assessment conducted within the European Union (EU) renewal process, and explains the differences in the carcinogenicity assessment with IARC. Use of different data sets, particularly on long-term toxicity/carcinogenicity in rodents, could partially explain the divergent views; but methodological differences in the evaluation of the available evidence have been identified. The EU assessment did not identify a carcinogenicity hazard, revised the toxicological profile proposing new toxicological reference values, and conducted a risk assessment for some representatives uses. Two complementary exposure assessments, human-biomonitoring and food-residues-monitoring, suggests that actual exposure levels are below these reference values and do not represent a public concern.

The application of global sensitivity analysis in the development of a physiologically based pharmacokinetic model for m-xylene and ethanol co-exposure in humans

Global sensitivity analysis (SA) was used during the development phase of a binary chemical physiologically based pharmacokinetic (PBPK) model used for the analysis of m-xylene and ethanol co-exposure in humans. SA was used to identify those parameters which had the most significant impact on variability of venous blood and exhaled m-xylene and urinary excretion of the major metabolite of m-xylene metabolism, 3-methyl hippuric acid. This analysis informed the selection of parameters for estimation/calibration by fitting to measured biological monitoring (BM) data in a Bayesian framework using Markov chain Monte Carlo (MCMC) simulation. Data generated in controlled human studies were shown to be useful for investigating the structure and quantitative outputs of PBPK models as well as the biological plausibility and variability of parameters for which measured values were not available. This approach ensured that a priori knowledge in the form of prior distributions was ascribed only to those parameters that were identified as having the greatest impact on variability. This is an efficient approach which helps reduce computational cost.

Cytotoxic effect of linear alkylbenzene sulfonate on human intestinal Caco-2 cells: associated biomarkers for risk assessment

Linear alkylbenzene sulfonate (LAS) is a synthetic anionic surfactant widely present in the environment due to its intensive production and use in the detergency field. Admitting that current procedure of risk assessment has limits in providing realistic risk assessment data and predicting the cumulative effect of the toxicant mixtures, the incorporation of information regarding the mode of action and cell response mechanism seems to be a potential solution to overcome these limits. In this regard, we investigated in this study the LAS cytotoxicity on human intestinal Caco-2 cells, trying to unveil the protein actors implicated in the cell response using proteomics approach in order to give a better understanding of the toxicological effect and allow the identification of appropriate biomarkers reflecting the mode of action associated with LAS. As results, we demonstrated that LAS induces a time- and dose-dependent cytotoxicity in Caco-2 cells accompanied by an induction of oxidative stress followed by an excessive increase of intracellular calcium level. Proteomics approach helped in discovering three informative biomarkers of effect associated with LAS cytotoxic effect, reported for the first time: calreticulin, thioredoxin, and heat shock cognate 71 (HSP7C), confirmed by real-time PCR and western blot analysis. These biomarkers could serve for more reliable future risk assessment studies that consider the toxicants mode of action in order to help in the prediction of potential cumulative effects of environmentally coexisting contaminants.

Incorporating epigenetic data into the risk assessment process for the toxic metals arsenic, cadmium, chromium, lead, and mercury: strategies and challenges

Exposure to toxic metals poses a serious human health hazard based on ubiquitous environmental presence, the extent of exposure, and the toxicity and disease states associated with exposure. This global health issue warrants accurate and reliable models derived from the risk assessment process to predict disease risk in populations. There has been considerable interest recently in the impact of environmental toxicants such as toxic metals on the epigenome. Epigenetic modifications are alterations to an individual's genome without a change in the DNA sequence, and include, but are not limited to, three commonly studied alterations: DNA methylation, histone modification, and non-coding RNA expression. Given the role of epigenetic alterations in regulating gene and thus protein expression, there is the potential for the integration of toxic metal-induced epigenetic alterations as informative factors in the risk assessment process. In the present review, epigenetic alterations induced by five high priority toxic metals/metalloids are prioritized for analysis and their possible inclusion into the risk assessment process is discussed.

Metal oxide nanoparticles induce unique inflammatory footprints in the lung: important implications for nanoparticle testing

BACKGROUND

Metal oxide nanoparticles (NPs) have been widely used in industry, cosmetics, and biomedicine.

OBJECTIVES

We examined hazards of several well-characterized high production volume NPs because of increasing concern about occupational exposure via inhalation.

METHODS

A panel of well-characterized NPs [cerium oxide (CeO₂NP), titanium dioxide (TiO₂NP), carbon black (CBNP), silicon dioxide (SiO₂NP), nickel oxide (NiONP), zinc oxide (ZnONP), copper oxide (CuONP), and amine-modified polystyrene beads] was instilled into lungs of rats. We evaluated the inflammation potencies of these NPs 24 hr and 4 weeks postinstillation. For NPs that caused significant inflammation at 24 hr, we then investigated the characteristics of the inflammation. All exposures were carried out at equal-surface-area doses.

RESULTS

Only CeO₂NP, NiONP, ZnONP, and CuONP were inflammogenic to the lungs of rats at the high doses used. Strikingly, each of these induced a unique inflammatory footprint both acutely (24 hr) and chronically (4 weeks). Acutely, patterns of neutrophil and eosinophil infiltrates differed after CeO₂NP, NiONP, ZnONP, and CuONP treatment. Chronic inflammatory responses also differed after 4 weeks, with neutrophilic, neutrophilic/lymphocytic, eosinophilic/fibrotic/granulomatous, and fibrotic/granulomatous inflammation being caused respectively by CeO₂NP, NiONP, ZnONP, and CuONP.

CONCLUSION

Different types of inflammation imply different hazards in terms of pathology, risks, and risk severity. In vitro testing could not have differentiated these complex hazard outcomes, and this has important implications for the global strategy for NP hazard assessment. Our results demonstrate that NPs cannot be viewed as a single hazard entity and that risk assessment should be performed separately and with caution for different NPs.

The combined effect of abandoned mines and agriculture on groundwater chemistry

Although it is well known that both mining and agriculture disturb groundwater quality, their mutual interactions are much less well documented, though agricultural activities may prevail once mining operations have ended. To study these potential interactions and their impacts on water chemistry, we monitored the chemical composition of groundwater at the outlet of a gold exploration gallery in an area of intensive agricultural activity along with an isotopic study of the groundwater, a reactive artificial tracer test that involved injecting H2O2 into the gallery, and geochemical modelling. The isotopic study revealed denitrification of the NO3-bearing groundwater that takes place through oxidation of the sulphide minerals associated with the gold deposit and leads to anomalous concentrations of some metals such as Zn, Co and Ni. It also contributes to liberating As into the groundwater, where the tracer test confirmed that As is sensitive to the redox conditions. The currently observed high arsenic concentrations in the groundwater are interpreted as resulting mainly from the former mining activities through a remobilization of As sorbed on or co-precipitated with the iron oxides that formed when the gallery was excavated. The geochemical modelling enabled us to calculate the respective role of each process involved in the As accumulation in the groundwater. It is also inferred that NO3 contamination from agricultural activities disturbs arsenic remobilization--by consuming available electron donors (e.g. organic matter), NO3 limits the reduction of iron oxides and consequently the release of arsenic.

Mechanistic approach to understanding the toxicity of the azole fungicide triadimefon to a nontarget aquatic insect and implications for exposure assessment

Mechanistic and stereoselective based in vitro metabolism assays were utlilized to gain insight into the toxic mode of action of the 1,2,4-triazole fungicide, triadimefon, with black fly (Diptera: Simuliidae) larvae. Based on results from enzyme inhibitor studies, the metabolism of triadimefon in black fly larvae microsomes was found to occur predominantly via an oxidative P450-mediated pathway; triadimenol was formed via the stereoselective reduction of the prochiral carbonyl group of triadimefon. The relatively minor contribution of carbonyl reduction suggests that triadimefon may inhibit ecdysone 20-monooxygenase and disrupt insect molting hormone biosynthesis. 48-h LC50 tests for triadimefon and triadimenol with black fly larvae yielded median values (with 95% confidence intervals) of 6.1 (5.8-6.4) and 22.3 (20.3-24.1) mg/L respectively. The exposure of black fly larvae to sublethal concentrations of triadimefon resulted in increased microsomal P450 activity and affected the microsomal rates of both triadimefon depletion and triadimenol formation. In contrast to trout, black fly larvae produced a higher fraction of the more toxic triadimenol stereoisomers, which may explain in part why triadimefon exhibited a significantly greater toxicity with black fly larvae than trout. These results illustrate that while LC50 tests conducted with commercial triadimenol would presumably expose each organism to the same relative abundance of the four triadimenol stereoisomers, LC50 tests with triadimefon ultimately expose each organism to a unique set of triadimenol stereoisomers depending upon the organism's stereoselective metabolism.

A strategy for the risk assessment of human genotoxic metabolites

The role of metabolism in genotoxicity and carcinogenicity of many chemicals is well established. Accordingly, both in vitro metabolic activation systems and in vivo assays are routinely utilized for genotoxic hazard identification of drug candidates prior to clinical investigations. This should, in most cases provide a high degree of confidence that the genotoxic potential of the parent and associated metabolites have been characterized. However, it is well known that significant differences can exist between human metabolism and that which occurs with in vitro and in vivo genotoxicity tests. This poses challenges when considering the adequacy of hazard identification and cancer risk assessment if a human metabolite of genotoxic concern is identified during the course of drug development. Since such challenges are particularly problematic when recognized in the later stages of drug development, a framework for conducting a carcinogenic risk assessment for human genotoxic metabolites is desirable. Here, we propose a risk assessment method that is dependent upon the availability of quantitative human and rodent ADME (absorption, distribution, metabolism, excretion) data, such that exposures to a metabolite of genotoxic concern can be estimated at the intended human efficacious dose and the maximum dose used in the 2-year rodent bioassay(s). The exposures are then applied to the risk assessment framework, based on known cancer potencies, that allows one to understand the probability of a known or suspect genotoxic metabolite posing a carcinogenic risk in excess of 1 in 100,000. Practical case examples are presented to illustrate the application of the risk assessment method within the context of drug development and to highlight its utility and limitations.

Mode of action frameworks: a critical analysis

Mode of action (MOA) information is increasingly being applied in human health risk assessment. The MOA can inform issues such as the relevance of observed effects in laboratory animals to humans, and the variability of response within the human population. Several collaborative groups have developed frameworks for analyzing and utilizing MOA information in human health risk assessment of environmental carcinogens and toxins, including the International Programme on Chemical Safety, International Life Sciences Institute, and U.S. Environmental Protection Agency. With the goal of identifying gaps and opportunities for progress, we critically evaluate several of these MOA frameworks. Despite continued improvement in incorporating biological data in human health risk assessment, several notable challenges remain. These include articulation of the significant role of scientific judgment in establishing an MOA and its relevance to humans. In addition, binary (yes/no) decisions can inappropriately exclude consideration of data that may nonetheless be informative to the overall assessment of risk. Indeed, the frameworks lack a broad consideration of known causes of human disease and the potential for chemical effects to act additively with these as well as endogenous background processes. No integrated analysis of the impact of multiple MOAs over the same dose range, or of varying MOAs at different life stages, is included. Separate consideration of each MOA and outcome limits understanding of how multiple metabolites, modes, and toxicity pathways contribute to the toxicological profile of the chemical. An extension of the analyses across outcomes with common modes is also needed.

On use of the multistage dose-response model for assessing laboratory animal carcinogenicity

We explore how well a statistical multistage model describes dose-response patterns in laboratory animal carcinogenicity experiments from a large database of quantal response data. The data are collected from the US EPA's publicly available IRIS data warehouse and examined statistically to determine how often higher-order values in the multistage predictor yield significant improvements in explanatory power over lower-order values. Our results suggest that the addition of a second-order parameter to the model only improves the fit about 20% of the time, while adding even higher-order terms apparently does not contribute to the fit at all, at least with the study designs we captured in the IRIS database. Also included is an examination of statistical tests for assessing significance of higher-order terms in a multistage dose-response model. It is noted that bootstrap testing methodology appears to offer greater stability for performing the hypothesis tests than a more-common, but possibly unstable, "Wald" test.