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.

Cardiovascular diseases (CVDs) are complex, encompassing many types of heart pathophysiologies and associated etiologies. It has been shown that fractionated radiation exposure at high doses (3-17 Gy) to the heart increases the incidence of CVD, as evident from radiotherapy studies. However, the effects of low doses of radiation on the cardiovascular system or the effects from space travel, where radiation and microgravity are important contributors to damage, are not clearly understood. Herein, the adverse outcome pathway (AOP) framework was applied to develop an AOP to vascular remodeling from the deposition of energy. Following the creation of a preliminary pathway with the guidance of field experts and authoritative reviews, a scoping review was conducted which informed final key event (KE) selection and facilitated evaluation Bradford Hill criteria of the key event relationships (KERs). The AOP begins with a molecular initiating event of deposition of energy; ionization events increase oxidative stress, which concurrently causes the release of pro-inflammatory mediators and alters signaling pathways. These KEs alter nitric oxide levels leading to endothelial dysfunction and subsequent vascular remodeling (the adverse outcome). The work identifies evidence needed to strengthen understanding of the causal associations for the KERs, emphasizing where there are knowledge gaps and uncertainties in both qualitative and quantitative understanding. The AOP is anticipated to direct future research to better understand the effects of space on the human body and potentially develop countermeasures to better protect future space travelers.

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.

The understanding of radiation-induced non-cancer effects on the central nervous system (CNS) is essential for the medical setting (e.g., radiotherapy), and occupational exposures, such as nuclear workers or astronauts. Herein, the adverse outcome pathway (AOP) approach was used to consolidate relevant studies in the area of cognitive decline for identification of research gaps, countermeasure development, and for eventual use in risk assessments. AOPs are an analytical construct describing critical events to an adverse outcome (AO) in a simplified form beginning with a molecular initiating event (MIE). An AOP was constructed utilizing mechanistic information to build empirical support for the key event relationships (KERs) between the MIE of deposition of energy to the AO of learning and memory impairment through multiple key events (KEs). The evidence for the AOP was developed through a scoping review of the literature. In this AOP, the MIE is connected to the AO via six KEs of increased oxidative stress, increased deoxyribonucleic acid (DNA) strand breaks, altered signaling pathways, tissue resident cell activation, increased pro-inflammatory mediators and neural remodeling. Deposition of energy directly leads to oxidative stress, increased DNA strand breaks, an increase of pro-inflammatory mediators and tissue resident cell activation. These KEs, which are themselves interconnected, converge through increased DNA strand breaks, altered signaling pathways and pro-inflammatory routes and directly lead to neural remodeling. Broadly, it is envisioned that the outcome of these efforts could be applied to other cognitive disorders and support ongoing work by international authorities to review the system of radiological protection.

Cataracts are one of the leading causes of blindness, with estimated 95 million people affected worldwide. A hallmark of cataract development is lens opacification, typically associated not only with aging, but also radiation exposure as encountered by interventional radiologists and astronauts during the long-term space mission. To better understand radiation-induced cataracts, the adverse outcome pathway (AOP) framework was used to structure and evaluate knowledge across biological levels of organization. AOPs identify a sequence of key events (KEs) connected by key event relationships (KERs) beginning with a molecular initiating event (MIE) to an adverse outcome (AO) of relevance to regulatory decision-making. To construct the cataract AOP, a scoping review methodology was used to filter, screen, and review studies based on the modified Bradford Hill criteria. Eight KEs were moderately supported by empirical evidence across the adjacent (directly-linked) relationships using well-established endpoints. Over half of the evidence collected was informed by biological plausibility. Early KEs of oxidative stress and protein modifications could be the focus of countermeasures. Several identified knowledge gaps and inconsistencies in the AOP can be the basis of future research, most notably directed to experiments at low or moderate doses and dose-rates, relevant to radiation workers and other occupational exposures.