Pathway Based Toxicology and Fit-for-Purpose Assays

Abstract

The field of toxicity testing for non-pharmaceutical chemicals is in flux with multiple initiatives in North America and the EU to move away from animal testing to mode-of-action based in vitro assays. In this arena, there are still obstacles to overcome, such as developing appropriate cellular assays, creating pathway-based dose-response models and refining in vitro-in vivo extrapolation (IVIVE) tools. Overall, it is necessary to provide assurances that these new approaches are adequately protective of human and ecological health. Another major challenge for individual scientists and regulatory agencies is developing a cultural willingness to shed old biases developed around animal tests and become more comfortable with mode-of-action based assays in human cells. At present, most initiatives focus on developing in vitro alternatives and assessing how well these alternative methods reproduce past results related to predicting organism level toxicity in intact animals. The path forward requires looking beyond benchmarking against high dose animal studies. We need to develop targeted cellular assays, new cell biology-based extrapolation models for assessing regions of safety for chemical exposures in human populations, and mode-of-action-based approaches which are constructed on an understanding of human biology. Furthermore, it is essential that assay developers have the flexibility to 'validate' against the most appropriate mode-of-action data rather than against apical endpoints in high dose animal studies. This chapter demonstrates the principles of fit-for-purpose assay development using pathway-targeted case studies. The projects include p53-mdm2-mediated DNA-repair, estrogen receptor-mediated cell proliferation and PPARα receptor-mediated liver responses.

A Modern Genotoxicity Testing Paradigm: Integration of the High-Throughput CometChip® and the TGx-DDI Transcriptomic Biomarker in Human HepaRG™ Cell Cultures

“…New tools and approaches are urgently needed to allow regulatory agencies worldwide to evaluate a backlog of chemicals for potential adverse human health effects (1)(2)(3)(4)(5)(6)(7). Twenty-first century toxicology requires more affordable tests that are higherthroughput, higher-content, human-relevant, and mechanistic in nature for effective chemical evaluation (8)(9)(10)(11)(12)(13). Applying in vitro toxicogenomic (TGx) biomarkers in metabolically competent human cells in culture is a new approach methodology (NAM) that can help to accomplish these goals.…”

The State of the Art and Challenges of In Vitro Methods for Human Hazard Assessment of Nanomaterials in the Context of Safe-by-Design

“…The applicability domain of each assay should be well-understood (which assay can predict what kind of in vivo (human) toxicity) to be able to use assays that are fit-for-purpose. When looking at the transition to animal free testing in general, this is one of the issues that needs addressing for soluble chemicals as well [ 263 ]. In that respect, in vitro toxicity testing of NMs should be mechanism-based by looking for specific effects or MOAs [ 264 ].…”

Considerations for Improving Metabolism Predictions for In Vitro to In Vivo Extrapolation

“…HT-IVIVE estimates of human equivalent dose using only parent chemical clearance have been successfully used to support prioritization or screening-level risk evaluations from HT in vitro bioactivity data (Rotroff et al, 2010;Yoon et al, 2012;Wetmore et al, 2013;Wetmore et al, 2015;Sipes et al, 2017;Casey et al, 2018;Wambaugh et al, 2018). However, when used with data from biologically relevant fit-for-purpose assays (Clewell et al, 2016;Hartman et al, 2018;Beames et al, 2020) to derive quantitative estimates of risk, more sophisticated IVIVE models (quantitative-IVIVE; Q-IVIVE) that account for issues such as slow clearance, active transport, extra-hepatic metabolism, and metabolic bioactivation are likely to be necessary (Yoon et al, 2012;2016).…”