# Adverse outcome pathway: a path toward better data consolidation and global co-ordination of radiation research

Vinita Chauhan, Danielle Beaton, Nobuyuki Hamada, Ruth Wilkins, Julie Burtt, Julie Leblanc, Donald Cool, Jacqueline Garnier-Laplace, Dominique Laurier, Yevgenia Le, et al.

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Vinita Chauhan, Danielle Beaton, Nobuyuki Hamada, Ruth Wilkins, Julie Burtt, et al. Adverse outcome pathway: a path toward better data consolidation and global co-ordination of radiation research. International Journal of Radiation Biology, 2022, 10.1080/09553002.2021.2020363. hal-03736875

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# Adverse outcome pathway: a path toward better data consolidation and global co-ordination of radiation research

Vinita Chauhan, Danielle Beaton, Nobuyuki Hamada, Ruth Wilkins, Julie Burtt, Julie Leblanc, Donald Cool, Jacqueline Garnier-Laplace, Dominique Laurier, Yevgeniya Le, Yukata Yamada & Knut Erik Tollefsen

**To cite this article:** Vinita Chauhan, Danielle Beaton, Nobuyuki Hamada, Ruth Wilkins, Julie Burtt, Julie Leblanc, Donald Cool, Jacqueline Garnier-Laplace, Dominique Laurier, Yevgeniya Le, Yukata Yamada & Knut Erik Tollefsen (2022): Adverse outcome pathway: a path toward better data consolidation and global co-ordination of radiation research, *International Journal of Radiation Biology*, DOI: 10.1080/09553002.2021.2020363

**To link to this article:** https://doi.org/10.1080/09553002.2021.2020363

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# Adverse outcome pathway: a path toward better data consolidation and global co-ordination of radiation research

Vinita Chauhanª ID, Danielle Beatonb ID, Nobuyuki Hamadac ID, Ruth Wilkinsª ID, Julie Burttd ID, Julie Leblancd, Donald Coole ID, Jacqueline Garnier-Laplacef ID, Dominique Laurierg ID, Yevgeniya Leh, Yukata Yamadai, and Knut Erik Tollefsenj,k,l ID

ªEnvironmental Health Science Research Bureau, Health Canada, Ottawa, Canada; bCanadian Nuclear Laboratories, Chalk River, Canada; cRadiation Safety Unit, Biology and Environmental Chemistry Division, Sustainable System Research Laboratory, Central Research Institute of Electric Power Industry (CRIEPI), Komae, Japan; dDirectorate of Environmental and Radiation Protection and Assessment, Canadian Nuclear Safety Commission, Canada; eElectric Power Research Institute, Charlotte, NC, USA; fOrganisation for Economic Co-operation and Development Nuclear Energy Agency (OECD NEA), Paris, France; gHealth and Environment Division, Institute for Radiological Protection and Nuclear Safety (IRSN), Fontenay-aux-Roses, France; hCANDU Owners Group Inc, Toronto, Canada; iDepartment of Radiation Effects Research, National Institute of Radiological Sciences, National Institutes for Quantum Science and Technology, Chiba, Japan; jNorwegian Institute for Water Research (NIVA), Oslo, Norway; kNorwegian University of Life Sciences (NMBU), Ås, Norway; lCentre for Environmental Radioactivity (CERAD), Norwegian University of Life Sciences (NMBU), Ås, Norway

## ABSTRACT

**Background:** The purpose of toxicology is to protect human health and the environment. To support this, the Organisation for Economic Co-operation and Development (OECD), operating via its Extended Advisory Group for Molecular Screening and Toxicogenomics (EAGMST), has been developing the Adverse Outcome Pathway (AOP) approach to consolidate evidence for chemical toxicity spanning multiple levels of biological organization. The knowledge transcribed into AOPs provides a structured framework to transparently organize data, examine the weight of evidence of the AOP, and identify causal relationships between exposure to stressors and adverse effects of regulatory relevance. The AOP framework has undergone substantial maturation in the field of hazard characterization of chemicals over the last decade, and has also recently gained attention from the radiation community as a means to advance the mechanistic understanding of human and ecological health effects from exposure to ionizing radiation at low dose and low dose-rates. To fully exploit the value of such approaches for facilitating risk assessment and management in the field of radiation protection, solicitation of experiences and active cooperation between chemical and radiation communities are needed. As a result, the Radiation and Chemical (Rad/Chem) AOP joint topical group was formed on June 1, 2021 as part of the initiative from the High Level Group on Low Dose Research (HLG-LDR). HLG-LDR is overseen by the OECD Nuclear Energy Agency (NEA) Committee on Radiation Protection and Public Health (CRPPH). The main aims of the joint AOP topical group are to advance the use of AOPs in radiation research and foster broader implementation of AOPs into hazard and risk assessment. With global representation, it serves as a forum to discuss, identify and develop joint initiatives that support research and take on regulatory challenges.

**Conclusion:** The Rad/Chem AOP joint topical group will specifically engage, promote, and implement the use of the AOP framework to: (a) organize and evaluate mechanistic knowledge relevant to the protection of human and ecosystem health from radiation; (b) identify data gaps and research needs pertinent to expanding knowledge of low dose and low dose-rate radiation effects; and (c) demonstrate utility to support risk assessment by developing radiation-relevant case studies. It is envisioned that the Rad/Chem AOP joint topical group will actively liaise with the OECD EAGMST AOP developmental program to collectively advance areas of common interest and, specifically, provide recommendations for harmonization of the AOP framework to accommodate non-chemical stressors, such as radiation.

## ARTICLE HISTORY

Received 23 September 2021
Revised 20 November 2021
Accepted 13 December 2021

## KEYWORDS

OECD; NEA; HLG-LDR;
CRPPH; radiation and
chemical adverse outcome
pathway joint topical
group; adverse outcome
pathway; molecular
initiating event; key event;
adverse outcome; hazard
assessment; risk assessment;
low dose radiation;
radiation protection

# Radiation and chemical perspective

Historically, the international radiation protection system has been built on the basis of analyses of epidemiological data, in particular from Japanese atomic bomb survivors in

Hiroshima and Nagasaki. These data support the assumption made for radiation protection purposes that, for low dose or low dose-rate exposures (i.e. below 100 mSv or below 0.1 mSv/minute), stochastic effects (e.g. cancer risk) are

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assumed to follow a dose response with no threshold. Due to uncertainties in the area of low dose and low dose-rate health risks, the adoption of the linear-non-threshold (LNT) extrapolation model remains controversial (ICRP 2007, 2012). Current radiobiological knowledge on effects at low doses or low dose-rates also shows that mechanisms involved in carcinogenesis are much more complex than considered a few decades ago (ICRP 2007, 2012). Uncertainties also apply for some non-cancer effects where dose thresholds might be lower than previously considered. Integration of epidemiological and mechanistic research results appears today as a way to improve knowledge in low dose and low dose-rate health risks (ICRP 2007, 2012; Hamada and Fujimichi 2014; NCRP 2015, 2018).

With the development and larger-scale implementation of new approach methodologies (NAMs), such as in silico (computational) efforts, in vitro and targeted in vivo studies under controlled radiation exposure conditions, an unprecedented amount of mechanistically-informed data has been generated in an increasingly large number of species (Parish et al. 2020). These data, which help identify and characterize effects as bioindicators (i.e. a cellular alteration that is on a critical pathway to the adverse outcome itself) or as biomarkers (i.e. a biological phenotype that can be used to indicate a response to an exposure at a molecular, cell or tissue level) may facilitate a more thorough mechanistic understanding of radiation effects (Preston et al. 2021). Although the biomarker (e.g. chromosome aberration, DNA adduct, gene expression change, specific metabolite) may not be directly involved in the development of the adverse outcome (e.g. initiation and/or progression of a key apical endpoint of medical or regulatory concern), the bioindicator (e.g. a specific mutation in a target cell that is associated with tumor formation) is considered key to the adverse outcome. It would be of value to collectively consolidate the knowledge of cancer and non-cancer effects of radiation in mammalian and non-mammalian models at low/intermediate/high radiation doses and dose-rates (NCRP 2015). However, in the context of low dose and low dose-rate radiation exposures, substantial uncertainties arise as to how these data can be systematically used to enhance hazard characterization and inform human health and ecological risk assessment (Boice 2017). This is partly due to the central attributes of radiation, namely the energy deposited, radiation quality, dose, and dose-rate including the exposure

route (internal, external or mixed). These often define the type of molecular perturbations initiated and the target tissues/organs affected. However, when combined with an individual difference in radiation response, the ability to characterize hazard and identify risk from radiation exposures may be complicated. Since the vast majority of radiation exposures occur in the low dose/low dose-rate range, our current radiation risk paradigm remains mechanistically uninformative until we take advantage of the new data.

Although progress is being made in using epidemiological studies and field data relevant to low doses and low dose-rates, hurdles arise from the need for large populations and/or extensive field data sets to parametrize models and to perform robust statistical analysis. In recent years, several analyses of large studies or international pooled analyses made it possible to estimate the dose-risk relationship at low doses for solid cancers or specific cancer sites (Richardson et al. 2015; Grant et al. 2017; Little et al., 2017; Lubin et al. 2017), even if associated with large uncertainties. A recent meta-analysis, restricted to low dose epidemiological studies (mean dose of less than 100 mSv), concluded that results directly support the existence of excess risks associated with low doses for solid cancers and leukemia (Hauptmann et al. 2020). Large datasets are being constructed based on low dose-exposed individuals among patients (Bernier et al., 2019) or workers (Laurier et al. 2017; Boice et al. 2018). This should lead to an improvement in the quantification of the dose-risk relationship at low doses in the near future, including refinement of the shape of the dose-risk relationship and the impact of modifying factors, such as sex, age and time since exposure. Such datasets also provide the opportunity to use approaches, such as biologically-based mathematical models, to estimate quantitative relationships between radiation exposure and cancer risk (Kaiser et al. 2020a, 2020b).

In the last decade, chemical regulators have faced a similar challenge to increase the inclusion of relevant mechanistically-based data into chemical hazard and risk assessment. New chemicals are being manufactured at a high pace as exemplified by the over 180 million chemicals and substances registered in the Chemical Abstracts Service (CAS) registry in 2021 (https://www.cas.org/cas-data/cas-registry) and over 350,000 unique chemicals and substances used on the global markets (Wang et al. 2020). For this reason, there is a need for large scale toxicity testing using

<figure><img src="image_18.png" /><figcaption>**Figure 1.** Schematic of an adverse outcome pathway (AOP) beginning with a molecular initiating event (MIE) and linked to key events (KE) by key event relationships (KER) to an adverse outcome (AO) in the most simplistic and unidirectional manner. Also highlighted are the non-adjacent (dotted arrows) and adjacent relationships (solid arrows).</figcaption></figure>
<figure><img src="image_19.png" /></figure>

rapid, cost-effective, ethically acceptable, and mechanistic-
ally informative methods (NRC 2007). There is also the
need for substantial scientific efforts and resources to lever-
age these new technologies and systematically organize data
within a common and standardized knowledge framework.
The AOP and mode of action (MOA) framework have
been proposed and identified as being instrumental for the
successful implementation of such data into a research and
regulatory framework (Ankley et al. 2010). Although AOPs
were originally formulated to support ecological risk assess-
ment, the MOA framework was conceptualized for regula-
tors to support human hazard and risk assessment. These
approaches have now been synchronized and both requir-
ing causal linkages to be established, with AOPs intended
to be stressor agnostic and beginning with a molecular ini-
tiating event and MOAs being more chemical-specific, and
focused on the toxicity pathway of the specific stressor
(Ankley et al. 2010).

AOP framework

The AOP framework was conceived by the US Environmental Protection Agency (EPA) to bring regulatory chemical toxicity testing into the 21st century by using the data generated by, mechanistically informative biologically based and high throughput techniques and by having a rational basis to meet the challenge of the 3Rs of animal testing – Replacement, Reduction and Refinement – in a move away from animal testing for endpoints that were not mechanistically informative (Villeneuve et al. 2014).

AOP development is based on five general principles:

1. AOPs are not stressor or chemical specific (i.e. stressor agnostic);

2. AOPs are modular, consisting of key events (KEs) and key event relationships (KERs) that can be shared between two or more pathways and/or AOPs;

3. An individual AOP is a pragmatic unit of development and evaluation;

4. For most real-world applications, AOP networks are the functional unit of prediction;

5. AOPs are living documents and iterative in their development.

The OECD AOP program currently operates under the
Extended Advisory Group for Molecular Screening and
Toxicogenomics (EAGMST) and is co-managed by the
European Commission Joint Research Centre (JRC) and the
EPA (https://www.oecd.org/chemicalsafety/testing/adverse-
outcome-pathways-molecular-screening-and-toxicogenomics.
htm). The AOP begins with a molecular initiating event
(MIE) and is followed by KEs that map out a hypothetical
toxicity pathway to an adverse outcome (AO) via KERs
(Figure 1) (OECD 2016a, 2016b). An AOP is a purposefully
simplified toxicity pathway encompassing KEs (analogous to
bioindicators as used by the radiation community) that are
both measurable and essential to the manifestation of an
AO. These essential critical events are causally linked using

all relevant data across multiple levels of biological organiza-
tion and the weight of evidence (WoE) for biological plausi-
bility, empirical support, and uncertainties/inconsistencies
are evaluated using modified Bradford-Hill considerations
(Becker et al. 2015). The AOP framework is designed to be
transparent and crowd-sourced with stringent evaluation cri-
teria and following an international review processes prior
to endorsement in the AOP development program (https://
www.oecd.org/chemicalsafety/testing/projects-adverse-out-
come-pathways.htm).

Organizing information in this manner effectively allows for the identification of knowledge gaps that can be used to guide future research (Conolly et al. 2017; Knapen et al. 2018; Pittman et al. 2018; Pollesch et al. 2019). It allows studies across various domains of research, using different study types to be organized into AOPs to identify priority research areas that will better support the regulatory or research questions of relevance (Figure 2). Since the framework has been predominantly supported by the chemical research and regulatory communities, participation from scientific professionals in the radiation research field has been limited. However, the increase in the number of workshops and publications in this area in the past few years is clearly demonstrating an increasing interest and growing momentum in the radiation protection field toward AOP advancement (NCRP 2015; Chauhan et al. 2019, 2021a, 2021b; Helm and Rudel 2020, 2021c; Stainforth et al. 2021).

Benefits of AOPs

Equipped with such a systematic organization of knowledge,
researchers and regulators gain the capability to evaluate the
scientific basis of causation from early biological indicators
at the cellular level, to intermediate tissue and organ levels
and to latent individual and population biomarkers, of an
adverse outcome of regulatory relevance (NCRP 2020).
Thus, any risk of chemicals, radiation, or mixture of mul-
tiple stressors could then be evaluated within the relevant
context of individual and population effects (Beyer et al.
2014; Salbu et al. 2019; Chauhan et al. 2021a, 2021c, 2021e).
In this way, relevant mechanistic data that have been gener-
ated from exposures involving medical, environmental, and
occupational scenarios from different data sources (in silico,
in vitro, in vivo, epidemiological and field studies) could
support the OECD's AOP framework (Figure 2). The bene-
fits of using this approach have been identified through case
examples in both human and ecological hazard characteriza-
tions (Gomes et al. 2018; Xie et al. 2020, 2019; Song et al.
2020a, 2020b; Chauhan et al. 2021b, 2021c). In addition, the
existence of more than 10 AOPs for ionizing radiation and
over 5 AOPs for non-ionizing radiation (https://aopwiki.org;
last accessed November 1st, 2021) is a testament to the level
of interest of the radiation community in developing AOPs.
Radiation AOPs would help enhance our understanding of
crucial molecular, cellular, tissue and organismal-level events
that are detectable and relevant to adverse effects progres-
sion. Bringing data together in formalized frameworks
would also support: (1) increasing understanding of tissue-
<figure><img src="image_20.png" /><figcaption>**Figure 2.** A hypothetical schematic showing how (A) accumulated data (e.g. in silico, in vitro and in vivo), that supports a similar key event across various radiation fields (i.e. medical, occupational and environmental) can be (B) organized into key events (squares, circles, and pentagons). Not all the key events will form relationships. Those that do form key event relationships (C) can be structured into an adverse outcome pathway. In certain cases, where evidence is lacking in the radiation field, it can be supplemented with other scientific disciplines (e.g. studies with other stressors than radiation) (grey box).</figcaption></figure>

<figure><img src="image_21.png" /><figcaption>**Figure 3.** Some research areas that the adverse outcome pathway approach could support.</figcaption></figure>

level sensitivities; (2) identifying bioindicators and bio-
markers to increase understanding of disease progression or
its detection; (3) linking low dose and low dose-rate effects
to health outcomes to facilitate risk characterization using

disease-based guidance; and (4) refining hazard and risk
assessment for co-exposure scenarios (Figure 3).
Additionally, the framework could broadly guide future col-
laborative research by identifying knowledge gaps and
<figure><img src="image_22.png" /></figure>

improving understanding of relevance and role of underlying compensatory or adaptive responses on individual events along the AOP, where knowledge during the past decade has expanded. Further, the framework can facilitate the use of advanced data mining and machine learning techniques to search for relevant information from the vast body of research that is available. With a growing need to integrate systems biology into hazard- and risk-related assessments, the chemical and radiation fields would benefit from cooperation and sharing of knowledge, including the advancement of AOPs.

## Path forward

To facilitate co-ordination and collaboration between the chemical toxicologists and radiation biologists, a Rad/Chem AOP joint topical group was formed on June 1, 2021 through the High-Level Group on Low-Dose Research (HLG-LDR), a subsidiary body of the OECD Nuclear Energy Agency (NEA) Committee on Radiological Protection and Public Health. This Rad/Chem AOP joint topical group aims to bring together organizations pursuing, or considering the use of AOPs in radiation research and work to demonstrate the value of this approach to support the radiation field with guidance from the OECD AOP program experts. It also aims to identify work in progress by the OECD EAGMST AOP development program that would benefit the radiation field and consider opportunities for new initiatives. This could include strategies for effective data aggregation and evaluation, promotion of AOP development, harmonization of AOP-relevant information/content and support of the review process of AOPs submitted to the AOP development program. Current members of the Rad/Chem AOP joint topical group represent a number of organizations in Asia, Europe, and North America and span a broad range of research and regulatory initiatives, as well as human and environmental/ecological topical issues related to radiation protection. Members have expressed an interest in using the framework for the purposes of: (1) assembling knowledge around co-exposure scenarios involving radiation (and chemical) insults; (2) developing qualitative and quantitative AOPs for better understanding the dose/dose-rate effects; (3) defining bioindicators (and biomarkers) of ecological impact or human disease; and (4) guiding experimental studies in areas where there is a lack of data. Some relevant adverse outcomes of interest, besides the obvious interest in cancer, include but not limited to: (1) developmental deficiencies (e.g. cognitive failure, bone loss, catalysts, circulatory diseases); (2) growth and/or reproductive abnormalities (e.g. fertility, fecundity, population recruitment); and (3) kidney toxicity from exposures involving uranium. One main challenge is the ability of AOPs to improve the consolidation of the quantitative relationships between radiation exposure and health risks at low dose/low-dose rate levels. Also of interest, is to better define the limitations of this framework, particularly related to individual susceptibilities, adaptive mechanisms, compensatory effects and confounders, such as nutritional status, life stage

and gender susceptibility that may influence individual events along the AOP continuum. This remains a significant challenge in successful implementation of AOPs for hazard and risk assessment from both chemical and radiation exposures. The Rad/Chem AOP joint topical group, therefore, provides a forum through which such considerations can be discussed, documented and progressed.

## Vision

The vision of the Rad/Chem AOP joint topical group is to generate impactful deliverables in three main areas: (1) promotion of AOPs; (2) engagement of radiation societies and journals; and (3) AOP developmental projects. By building on these elements, uptake and demonstration of the value of the AOP tools in the field of radiation research will be accelerated. A balance will be struck between addressing human and environmentally/ecologically relevant issues to ensure holistic handling and provide synergy between efforts. Additionally, the group will help develop a strategy for effectively curating the vast amount of mechanistic, epidemiological, and monitoring data that currently represents radiation-induced injury in the context of different exposure scenarios (external and internal), exposure duration (chronic and acute), radiation types (low- and high-linear energy transfer ionizing radiation), and dose-rates (low and high). Considerations on species/sex/life stage sensitivity and how this can be interpreted within an AOP framework will also be discussed in the context of linking experimental data to human and wildlife data. The group will broadly address questions relevant to radiation risk, laboratory to epidemiology (or field) extrapolations and multiple stressor assessments. In these areas, the topical group will identify and promote priority case studies that could be put forth toward AOP development by the broader community and points of experimental collaboration. The Rad/Chem AOP joint topical group aims to work toward better collaboration and co-ordination between the chemical and radiation fields, which have thus far been working independently.

The Rad/Chem AOP joint topical group's mission is to:

* Demonstrate the efficiency of the approach to identify knowledge gaps through better organization of data.

* Advance the understanding of adverse effects/health outcomes for human and non-human species.

* Demonstrate the value of collaborative expansive studies.

* Address questions on low dose effects in the context of human and ecosystem health.

* Contribute to and advance the goals of the OECD AOP program toward non-chemical stressors.

## Workplan

Concrete tasks to help achieve the mission have been established in the form of a workplan (Table 1). The Rad/Chem AOP joint topical group will build on the three pillars of work described below.
<figure><img src="image_23.png" /><figcaption>**Table 1.** List of proposed initiatives within the workplan of the Rad/Chem AOP joint topical group.</figcaption></figure>

<table><thead><tr><td>Sub-group</td><td>Description</td></tr></thead><tbody><tr><td>Horizon Style Exercise</td><td>Development of a survey that can be distributed to the radiation community to help identify the priority areas that will guide AOP development, identify the Subject Matter Experts, resources, areas for collaborative AOP development. The survey can also be designed to promote AOPs.</td></tr><tr><td>Strategy for focused AOP workshops</td><td>Develop a strategy and organize focused meetings and workshops to support addressing specific AOP challenges.</td></tr><tr><td>Strategy for engaging societies and journals</td><td>Development of an actionable workplan for engaging journals and societies in promoting and reviewing AOPs.</td></tr><tr><td>Building case examples of radiation-relevant AOPs</td><td>Case examples of AOP development that cover regulatory questions of interest, how best to capture the data using systematic review and data aggregation tools and any specific considerations for developing radiation AOPs.</td></tr><tr><td>Case examples of radiation OMICS-informed AOPs</td><td>Development of a strategy to apply omics data to an AOP relevant to the field of ionizing radiation. What tools and methods are needed; considerations and drawbacks.</td></tr><tr><td>Review on AOPs applied to radioecological challenges</td><td>Providing a review of current knowledge of ionizing-radiation effects, availability of AOPs and challenges associated with application to radioecology.</td></tr></tbody></table>

## Promotion of the approach

A horizon-style exercise is proposed as an effective means to promote AOPs. A similar approach has been successfully implemented in the chemical toxicology field (LaLone et al. 2017). It will be used to measure the radiation community's level of knowledge, interest and hesitation to use the approach and identify needs for future development and implementation of work plans. It is a method for systematically searching and identifying emerging trends, opportunities, and limitations in using the AOP framework. Ideas and input will be collected from individuals/groups across government, academia, industry and non-governmental organizations. Additionally, using a ranking exercise, key questions will be identified regarding the needs and limitations that should be considered in the advancement of the AOP framework and its regulatory applications. Based on the key question solicitation, an international survey will be undertaken to inform future workshops and AOP developmental projects within the OECD AOP program. The survey will help identify the subject matter experts that can advise on relevant AOPs and data to support defining critical events along the AOP continuum. The survey could also identify the research centers with the bioinformatics and biostatistical competence needed to identify, evaluate and select data to incorporate into AOPs. This exercise will ultimately address what regulatory/research questions can be supported by the framework, how to effectively use the framework to identify the knowledge base and gaps, and what the areas of controversy and/or misalignment are with the chemical research community.

## Engagement of institutions

Another important task of the Rad/Chem AOP joint topical group is to engage professional science societies and scientific journals in the advancement of AOP-relevant approaches. These professional bodies will be important in enabling uptake and the understanding of AOPs by the broader research and regulatory community. In particular, scientific journals will be valuable in helping to determine the best approaches for identifying studies that meet AOP criteria and to support publishing targeted studies related to the development, evaluation, demonstration, and use of AOPs. A piloted approach could be launched in a similar

way as currently adopted for more chemical-centric AOPs, by supporting dedicated 'AOP reports' in selected journals (e.g. exemplified by Schmid et al. 2021). Additionally, such accompanying papers describing the technical summary of AOPs submitted to the AOP-Wiki (https://aopwiki.org/) would help develop AOPs through the formalized endorsement process in the OECD EAGMST AOP development program. This early engagement of scientific societies and journals will be a powerful vehicle to demonstrate the immediate impact of AOPs in research and regulatory communities.

## Development of AOPs

Workshops will be developed annually to ensure continuous evolution of the AOP framework to support the needs of the radio-toxicology community. A number of workshops have already been held, bringing communities together for knowledge exchange (Chauhan et al. 2019, 2021d, 2021e). This will be critical to ensuring that there is active engagement and interest in the work and that challenges are discussed in a transparent manner, with consensus on the path forward and with clear indicators of success. Some of these challenges are reviewed in Chauhan et al. (2021b) and can form the basis of focused working groups. This will allow for better integration of AOPs and consideration by regulatory and advisory organizations. The workshops can also direct efforts to finding solutions to open-ended questions on how to pragmatically harmonize radiation and chemical-centric AOPs (and supporting events), what systematic approaches are most relevant for reviewing literature in the context of AOP development, and the WoE considerations to support radiation-specific AOP development. Case studies will be initiated to identify the best way to use NAMs to populate radiation-relevant AOPs with data. Specific considerations will be given to the possibilities and limitations of NAMs, which tools are needed, and what subject matter expertise (SME) is required to progress in the work. Lastly, several working groups have already been initiated through a recent Multidisciplinary European Low Dose Initiative (MELODI)/European Radioecology Alliance (ALLIANCE) workshop (https://melodiallianceaopworkshop.vimeet.events/en/) that complement existing initiatives worldwide and support further development of AOP case studies within the
<figure><img src="image_24.png" /></figure>

radiation field (Chauhan et al. 2021e). These case studies will be important to identify suitable approaches and methodologies for AOPs in the radiation field. Further, the case studies will help evaluate the suitability of existing templates and guidance documents and how to refine them to effectively and efficiently support AOP development in the radiation field.

It will also be important to foster discussions on the established pathways of radiation-induced injury and common macromolecular events across exposure scenarios at the various levels of biological organization. It is well understood within the field of radiation sciences that influential evidence and the emerging hallmarks of adverse outcomes will help to continuously modernize the AOP framework. These initial discussions could be the starting point for building outcome- and exposure-specific working groups. These working groups could evolve to independently work outside of the Rad/Chem AOP joint topical group. They could consider the significance of co-exposure scenarios, which could be cooperatively developed with experts from other fields. Rad/Chem AOP joint topical group discussions could also be initiated on the value of using data from other scientific disciplines (i.e. chemistry, ecology, and social science and humanities) to support AOPs in the radiation field and on the best ways to identify appropriate studies and define the WoE for the AOPs. Given that limitations in radiation risk estimates arise partly from uncertainties in dosimetry, the AOP framework, as it currently stands, may need to be adapted to address questions of dose and dose rate for acute and chronic exposure scenarios, and, external and internal exposures to various types of low and high linear energy transfer radiation. This may require the integration of other methodologies into AOP development. Therefore, discussions should also be fostered on methodologies that can be combined with AOPs to help strengthen the evidence of causality. AOPs cannot be used as standalone tools, but used in conjunction with other approaches such as biologically-based mathematical models to quantify the stressor-response and key event response-response relationships for future implementation into risk assessments (Kaiser et al. 2020a, 2020b; Song et al. 2020a; Moe et al. 2021). These connections with other fields should be made early on to ensure an understanding of needs and to better translate knowledge from AOP development to risk models. AOPs will improve the regulatory system only if they provide this quantitative understanding of health risks by consolidating the modeling of the dose-response relationships.

## Conclusion

Although interest in AOPs is emerging in the radiation field, much work is needed to demonstrate their impact in organizing biological and toxicological information to assist in data interpretation and method development. Bringing diverse data together for predictive utility will be a challenging but much needed step to advance research in many areas given the importance of radiation quality, dose-rate and dose on eventual health outcomes. It will help show

that a coordinated research framework is needed to make up for the inconsistencies in study reporting and lack of data on causal linkages. Such an undertaking would require collaboration and guidance on how best to report data produced from different institutions investigating similar regulatory questions. While the chemical field have shown considerable progress in applying this approach, it has not yet been the case so far in the area of radiation. Building on and leveraging the work being done by the chemical community would be a good starting point and assist in identifying commonalities and differences to inform the broader development and inclusion of radiation AOPs in the OECD EAGMST AOP development program. Integrating AOPs into institutional research workplans will involve training, knowledge transfer, and outreach initiatives to bring awareness of the AOP program to radiation biologists, epidemiologists, policymakers and regulators. This is where collaboration with the OECD EAGMST AOP program will be beneficial as it will ensure alignment with any existing projects underway in the EAGMST and will strengthen relationships between the chemical and radiation fields. As the program is well-advanced, with representation from the chemical and ecological research and regulatory community, this expertise will be leveraged to help improve data consolidation to support radiation hazard and risk assessment. It is envisioned that selected AOP coaches could provide guidance on initiatives undertaken by the Rad/Chem AOP joint topical group, with the goal of developing impactful joint proposals that would better translate scientific evidence into societal benefits. Productive dialogue between research scientists and regulatory communities can be achieved through organized international activities. The work of the Rad/Chem AOP joint topical group will be a starting point that will allow for a continuous evolution of the AOP framework within the field of radiation protection.

## Acknowledgements

The authors are grateful to Annick Laporte for support on the graphics and Sami Qutob and Katya Feder for critical review of the manuscript. The opinions expressed and arguments employed herein are those of the authors and do not necessarily reflect the official views of the Organisation for Economic Co-operation and Development, the Nuclear Energy Agency or of the governments of their member countries.

## Disclosure statement

No potential conflict of interest was reported by the author(s).

## Funding

This work is partially funded by grants from the Research Council of Norway (RCN) through its Centre of Excellence (CoE) funding scheme [Project No. 223268/F50], the RCN project 268294 'Cumulative hazard and risk assessment of complex mixtures and multiple stressors (MixRisk)' and NIVAs Computational Toxicology Program, NCTP (www.niva.no/nctp).
<figure><img src="image_25.png" /></figure>

## Notes on contributors

**Vinita Chauhan**, Ph.D, is a Senior Research Scientist at the Consumer and Clinical Radiation Protection Bureau of Health Canada. She is a Canadian delegate of the HLG-LDR and Extended Advisory Group on Molecular Screening and Toxicogenomics (EAGMST) of the OECD. She chairs the HLG-LDR Rad/Chem AOP Joint Topical Group and is the co-founder of Canadian Organization of Health Effects from Radiation Exposure (COHERE) initiative.

**Danielle Beaton**, Ph.D, is a Research Scientist with Canadian Nuclear Laboratories. Her current research focuses on the effects of low dose radiation on biological systems.

**Nobuyuki Hamada**, RT, Ph.D, is a Senior Research Scientist at CRIEPI Radiation Safety Unit and a Visiting Professor at Hiroshima University Research Institute for Radiation Biology and Medicine. He serves on ICRP Task Groups 102, 111 and 119, NCRP PAC 1, OECD/NEA/ CRPPH/HLG-LDR/Rad/Chem AOP Joint Topical Group, and Consultation Committee on AOP development for space flight health outcomes (Canadian project).

**Ruth C. Wilkins**, Ph.D, is a Research Scientist at the Consumer and Clinical Radiation Protection Bureau of Health Canada and the Chief of the Ionizing Radiation Health Sciences Division. She graduated with a Ph.D in Medical Physics from Carleton University and has been employed at Health Canada for the past 25 years. She is an Adjunct Professor and lecturer in Radiobiology in the Department of Physics at Carleton University and the alternative representative of Canada to the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR).

**Julie Burtt** is a Radiation Biologist with the Canadian Nuclear Safety Commission. Her research is focused on the potential health effects from exposure to low doses of ionizing radiation. She is a technical expert for UNSCEAR. She is a member of the ICRP (Committee 4), and she also serves on several NEA committees.

**Julie Leblanc**, Ph.D, is a Radiation Biologist and Women in STEM Special Advisor with the Canadian Nuclear Safety Commission. Her research is focused on the potential health effects from exposure to low doses of ionizing radiation. She is a mentee in the ICRP Task Group 111.

**Donald Cool**, Ph.D, is a Technical Executive with Electrical Power Research Institute in Charlotte, North Carolina, United States. He is Vice-Chair of the Main Commission of the ICRP, and a Council Member of the US National Council on Radiation Protection and Measurements.

**Jacqueline Garnier-Laplace**, Ph.D, a Senior Radiation Protection Specialist, is Deputy Head of the Division of Radiological Protection and Human Aspects of Nuclear Safety at the OECD Nuclear Energy Agency. She is the Scientific Secretary of the Committee on Radiological Protection and Public Health, and of the NEA HLG-LDR. Previously, she headed Radiation Protection research at France's IRSN. She served as scientific secretary of ICRP Committee 1 from 2017 to 2021, and is currently serving on Committee 4 with the same function.

**Dominique Laurier**, Ph.D, is a Senior Epidemiologist and head of a research department on the biological and health effects of low-dose radiation exposure at the French Institute for Radiation Protection and Nuclear Safety (IRSN). He is Chair of Committee 1 of the International Commission on Radiological Protection (ICRP), French representative to UNSCEAR, and Chair of the NEA HLG-LDR.

**Yevgeniya Le**, Ph.D., PMP, is a Program Manager for Health, Safety and Environment research and development program at CANDU Owners Group Inc. She is also Adjunct Professor in the Department of Biochemistry, Microbiology and Immunology at the University of Ottawa. Her research and work focus on increasing the understanding

of the health and environmental impacts of low dose and low-dose rate ionizing radiation.

**Yutaka Yamada**, D.V.M., Ph.D., is a Research Scientist at National Institutes for Quantum Science and Technology. His main fields are radiation biology and radiotoxicology, and his studies have focused on radiation-induced pulmonary carcinogenesis and the biological effects from exposure to low doses of ionizing radiation.

**Knut Erik Tollefesen**, Ph.D, is a Chief Scientist and Program Manager at the Norwegian Institute for Water Research's (NIVA) Computational Toxicology Program, NCTP (www.niva.no/nctp) and an Adjunct professor at the Norwegian University of Life Sciences (NMBU). He is an ecotoxicologist with over 20 years of experience in experimental ecotoxicology, computational toxicology and risk assessment. He chairs the HLG-LDR Rad/Chem AOP Joint Topical Group and participates in the OECD EAGMST.

## ORCID

Vinita Chauhan http://orcid.org/0000-0002-4498-0915

Danielle Beaton http://orcid.org/0000-0002-5764-5139

Nobuyuki Hamada http://orcid.org/0000-0003-2518-6131

Ruth Wilkins http://orcid.org/0000-0002-9621-477X

Julie Burtt http://orcid.org/0000-0002-0795-7142

Donald Cool http://orcid.org/0000-0003-4105-0242

Jacqueline Garnier-Laplace http://orcid.org/0000-0002-3486-147X

Dominique Laurier http://orcid.org/0000-0003-1432-4738

Knut Erik Tollefesen http://orcid.org/0000-0002-7534-0937

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