The adverse outcome pathway (AOP) concept provides a conceptual framework to collect, organize, and evaluate the knowledge about the progression of toxicity from molecular changes to adverse manifestations on the whole-organism or even ecosystem level. Diverse chemicals are known to trigger multiple AOPs and, while the AOPs themselves are chemical-agnostic, elucidation of AOPs associated with a particular chemical can contribute to a better understanding of that chemical’s hazards. Several new AOPs relevant for food contact chemicals have been proposed in publications appearing in the last months.
For example, Jean-Charles Carvaillo and colleagues from the University of Paris, France, used text mining and systems biology approaches to explore the AOPs associated with bisphenol S (BPS, CAS 80-09-1), a controversial alternative for bisphenol A (BPA, CAS 80-05-7). The main adverse outcomes identified by the authors included “reproductive health, endocrine disruption, impairments of metabolism, and obesity.” The authors highlighted “the relevance of the[ir] approach to decision makers assessing substituents to toxic chemicals.”
In a follow-up publication by the same group, Marylene Rugard and colleagues used similar approaches to decipher an AOP network linked to bisphenol F (BPF, CAS 620-92-8), where “various types of cancers such as breast and thyroid malignancies” were included as adverse outcomes. The authors suggested that their compilation can “support regulatory assessment” of BPF as well as “trigger new epidemiological and experimental studies.”
With regard to BPA itself, Elena Lo Piparo and colleagues from Food Safety Research, Nestlé Research, Lausanne, Switzerland, published the results of the “whole proteome virtual screening” that predicted BPA “to bind to many more proteins than previously described.” Most of these proteins also appeared to bind to the endogenous estrogen 17β-estradiol. The authors stated that their “findings provide a new and unprecedented insight into the complexity of chemical-protein interactions, highlighting the binding promiscuity of BPA and its broad binding similarity to the female sex hormone.”
Pamela Noyes and colleagues from the U.S. Environmental Protection Agency (EPA) reported on the development of an AOP network for thyroid disruption. Several food contact-relevant chemicals associated with thyroid disruption AOPs included perchlorate (CAS 14797-73-0), resorcinol (CAS 108-46-3), triclosan (CAS 3380-34-5), tetrabromobisphenol A (TBBPA, CAS 79-94-7), BPA, phthalates, and per- and polyfluoroalkyl substances (PFAS).
Kirsten Baken and colleagues from the Flemish Institute for Technological Research, Belgium, proposed “a strategy to validate a selection of human effect biomarkers” using AOPs, and used a case study for phthalates and reproductive effects as a proof of concept. In particular, they presented literature evidence that “the activation of several receptors, such as PPARα [(peroxisome-proliferator activated receptor alpha)], PPARγ, and GR [(glucocorticoid receptor)], may initiate events leading to impaired male and female fertility as well as other adverse effects of phthalate exposure.” The authors emphasized that “the proposed strategy connects the fields of epidemiology and toxicology and may strengthen the weight of evidence in observational studies that link chemical exposures to health outcomes.”
Andreas Kortenkamp from Brunel University London, UK, constructed an AOP network for “malformations of the male reproductive system” and used it “to identify pathways that converge at critical nodal points to produce downstream adverse effects.” This analysis concluded “that cumulative assessment groups for male reproductive health risks should not only include phthalates but also comprise androgen receptor (AR) antagonists, chemicals capable of disrupting steroid synthesis, InsL3 production, prostaglandin signaling and co-planar polychlorinated dibenzo-dioxins together with other dioxin-like compounds. This list goes far beyond what has been suggested previously.” Kortenkamp further compiled “a minimum set of chemicals to be assessed together with phthalates,” which includes multiple pharmaceuticals and dioxin-like pollutants, but also “phenolics such as bisphenol A and butylparaben.” He concluded that “AOP network analyses are essential to overcome difficulties in establishing groupings of chemicals for mixture risk assessments that derive from a narrow focus on mechanisms and modes of action.”
In a publication focused on the same topic, a group of international scientists led by Xabier Arzuaga from the U.S. Environmental Protection Agency (EPA) proposed eight “key characteristics of male reproductive toxicants as an approach for organizing and evaluating mechanistic evidence in human health hazard assessments.” Specifically, a male reproductive toxicant “1. Alters germ cell development, function, or death; 2. Alters somatic cell development, functions, or death; 3. Alters production and levels of reproductive hormones; 4. Alters hormone receptor levels/functions; 5. Is genotoxic; 6. Induces epigenetic alterations; 7. Induces oxidative stress; 8. Induces inflammation.”
Another group of scientists, led by Michele La Merrill from the University of California Davis, U.S., proposed ten “key characteristics of endocrine-disrupting chemicals as a basis for hazard identification” (FPF reported). These include the ability to “interact with or activate hormone receptors,” “antagonize hormone receptors,” “alter hormone receptor expression,” “alter signal transduction (including changes in protein or RNA expression, post-translational modifications and/or ion flux) in hormone-responsive cells,” “induce epigenetic modifications in hormone-producing or hormone-responsive cells,” “alter hormone synthesis,” “alter hormone transport across cell membranes,” “alter hormone distribution or circulating hormone levels,” “alter hormone metabolism or clearance,” and/or “alter the fate of hormone-producing or hormone-responsive cells.”
A publication by Patience Browne and colleagues from the Organisation for Economic Cooperation and Development (OECD), Paris, France, provided an overview of the “approaches and considerations [used by the OECD] for regulatory evaluation of endocrine disruptors.” The authors stressed that “from the outset, endocrine testing has always required integration of multiple methods that provide data on different levels of biological organization,” and therefore this area is “particularly adaptable to . . . [AOP] frameworks and integrated test methods built around AOPs.” They then reviewed “the status of endocrine disruptors in the OECD context,” discussed “examples where innovation from research is needed to improve or bridge gaps in endocrine testing,” and provided “suggestions for regulators and researchers to facilitate uptake of innovative methods for endocrine disruptor regulatory testing.”
Nynke Kramer from the Institute for Risk Assessment Sciences, Utrecht University, together with several industry representatives affiliated with the International Life Sciences Institute (ILSI), screened “safety evaluation reports of food additives listed in Annex II of Regulation (EC) No 1333/2008 of the European Union” in order to “qualitatively and quantitatively characterize toxicity induced in laboratory animals.” These data were then used “to identify the critical adverse effects used for risk assessment and to investigate whether food additives share common AOPs.” The common adverse effects identified by this analysis included “effects on the liver, kidney, cardiovascular system, lymphatic system, central nervous system and reproductive system.” The authors noted that “AOPs are available for many of these apical endpoints, albeit to different degrees of maturity.” On the contrary, for several “other adverse outcomes pertinent to food additives, including gastrointestinal irritation and corrosion, AOPs are lacking.” The authors suggested that “efforts should focus on developing AOPs for these particular endpoints.”
An AOP-focused review of studies on the toxicity of microplastics found that “microplastic toxicology research thus far has focused on ecotoxicity using apical endpoints and only a few studies deal with toxicity mechanisms.” Therefore, the authors called for “more studies on toxicity mechanisms . . . to fill these gaps in data.” Based on the few studies currently available, reactive oxygen species (ROS) formation was highlighted as one potential mechanism.
Two reviews on quantitative AOPs (qAOPs), led by Edward Perkins from the U.S. Army Engineer Research and Development Center discussed how qAOPs can be built and applied in chemical hazard prediction, hypothesis testing, and risk assessment. Lastly, a review on “regulatory assessment and risk management of chemical mixtures,” by Stephanie Bopp and colleagues from the Joint Research Centre (JRC) of the European Commission, Ispra, Italy, highlighted the AOP framework as a useful tool for “integrating different types of toxicity data and grouping” of chemicals.
Carvaillo, J.-C., et al. (2019). “Linking bisphenol S to adverse outcome pathways using a combined text mining and systems biology approach.” Environmental Health Perspectives 127: 047005.
Rugard, M., et al. (2020). “Deciphering adverse outcome pathway network linked to bisphenol F using text mining and systems toxicology approaches.” Toxicological Sciences 173:32-40.
Lo Piparo, E., et al. (2019). “Bisphenol A binding promiscuity: A virtual journey through the universe of proteins.” ALTEX 37: 085-094.
Noyes, P. E., et al. (2019). “Evaluating chemicals for thyroid disruption: Opportunities and challenges with in vitro testing and adverse outcome pathway approaches.” Environmental Health Perspectives 127:095001.
Baken, K. A., et al. (2019). “A strategy to validate a selection of human effect biomarkers using adverse outcome pathways: Proof of concept for phthalates and reproductive effects.” Environmental Research 175: 235-256.
Kortenkamp, A. (2020). “Which chemicals should be grouped together for mixture risk assessments of male reproductive disorders?” Molecular and Cellular Endocrinology 499: 110581.
Arzuaga, X., et al. (2019). “Proposed key characteristics of male reproductive toxicants as an approach for organizing and evaluating mechanistic evidence in human health hazard assessments.” Environmental Health Perspectives 127: 065001.
La Merrill, M. A., et al. (2019). “Consensus on the key characteristics of endocrine-disrupting chemicals as a basis for hazard identification.” Nature Reviews Endocrinology 16: 45-57.
Browne, P., et al. (2020). “OECD approaches and considerations for regulatory evaluation of endocrine disruptors.” Molecular and Cellular Endocrinology 504: 110675.
Kramer, N. I., et al. (2019). “Characterizing the coverage of critical effects relevant in the safety evaluation of food additives by AOPs.” Regulatory Toxicology 93: 2115-2125.
Jeong, J., and Choi, J. (2019) “Adverse outcome pathways potentially related to hazard identification of microplastics based on toxicity mechanisms.” Chemosphere 231: 249-255.
Perkins, E. J., et al. (2019). “Building and applying quantitative adverse outcome pathway models for chemical hazard and risk assessment.” Environmental Toxicology and Chemistry 38: 1850-1865.
Perkins, E. J., et al. (2019). “Chemical hazard prediction and hypothesis testing using quantitative adverse outcome pathways.” ALTEX 36: 91-102.
Bopp, S. K., et al. (2019). “Regulatory assessment and risk management of chemical mixtures: Challenges and ways forward.” Critical Reviews in Toxicology 49: 174-189.