IntroductionThe Organisation for Economic Co-operation and Development (OECD) Adverse Outcome Pathway (AOP) framework is used to organize scientific knowledge in toxicology into linear sequences of causally related events that lead to adverse toxicological endpoints (Ankley et al., 2010; OECD, 2018). AOPs describe the critical interactions of a chemical or non-chemical stressor within a biological system. AOPs begins with a molecular initiating event (MIE) that leads to intermediate key events (KEs) and culminate in an adverse outcome (AO). KEs are connected by key event relationships (KERs) for which causality is evaluated using the modified Bradford-Hill criteria (Becker et al., 2015). These criteria include biological plausibility, evidence for the essentiality of KEs, and empirical evidence in the form of dose-, temporal- and incidence-concordance. The strength of these directional and causal relationships is evaluated through a weight of evidence analysis for each KER (Villeneuve et al., 2014). AOPs are purposefully simplified, describing KEs that can be routinely measured and are essential to pathway progression, to facilitate regulatory utility and test strategy development (Ankley et al., 2010). AOPs are developed in a linear manner; however, shared KEs lead to networks of AOPs. Through these networks, multiple MIEs can converge to lead to the same AOs. In addition, multiple different types of stressors may interact with the same MIE to progress AOPs. Thus, although simple in concept, the AOP network is the fundamental unit of application for risk assessment and can broadly reflect complex, multi-stressor interactions and outcomes. AOPs have primarily been used to describe the impacts of chemicals on human and ecological outcomes. However, there is growing interest in applying AOPs in the radiation field (NCRP 2020, Chauhan et al., 2019; Chauhan et al., 2024). A case example AOP to lung cancer that is relevant to stressors such as radon inhalation has been endorsed by the Nuclear Energy Agency (NEA) and the Working Group of the National Coordinators of the Test Guidelines Programme (WNT) and Working Party on Hazard Assessment (WPHA) of the OECD (https://aopwiki.org/aops/272). Furthermore, since June 2021, a Radiation/Chemical AOP Joint Topical Group under the auspices of the NEA High Level Group on Low Dose Research has been working to promote and integrate AOPs into radiation research and risk assessments (Chauhan, Beaton et al., 2022a; Chauhan, Hamada et al., 2022b; Chauhan, Beaton et al., 2024). As part of these efforts, AOPs are being developed to multiple AOs including those relevant to space exploration (Kozbenko et al., 2024; Carrothers et al., 2024; Sleiman et al., 2024; Sandhu et al., 2024). Herein we describe four AOPs (AOP#478; AOP#483; AOP#470; AOP#482) that form a network leading to non-cancer AOs of relevance to space travel.

Bone loss, commonly seen in osteoporosis, is a condition that entails a progressive decline of bone mineral density and microarchitecture, often seen in post-menopausal women. Bone loss has been widely reported in astronauts exposed to a plethora of stressors and in patients with osteoporosis following radiotherapy for cancer. Studies on mechanisms are well documented but the causal connectivity of events to bone loss development remains incompletely understood. Herein, the adverse outcome pathway (AOP) framework was used to organize data and develop a qualitative AOP beginning from deposition of energy (the molecular initiating event) to bone loss (the adverse outcome). A literature review was conducted to compile and evaluate the state of knowledge based on the modified Bradford Hill criteria. Following review of 1865 studies, an empirically supported AOP was developed, showing the progression to bone loss through many factors affecting the activities of bone-forming osteoblasts and bone-resorbing osteoclasts. The structural, functional, and quantitative basis of each proposed relationship was defined, for inference of causal changes between key events. Current knowledge and its gaps relating to dose-, time- and incidence-concordance across the key events were identified, as well as modulating factors that influence linkages. The new priorities for research informed by the AOP highlight areas for improvement to enable development of a quantitative AOP used to support risk assessment strategies for space travel or cancer radiotherapy.