Understanding the influence of time in biology for NGRA

Brussels, 9 October 2024 – In the transition from in vivo to in vitro toxicity testing, the influence of time has so far not received much attention. Therefore, the Centre for chemical safety assessment (ECETOC) gathered toxicologists, biologists, bioinformaticians, modelers and risk assessors for a workshop in Brussels to explore the influence of time in toxicity testing.

The scientists discussed how to consider the influence of when, how long, and how often a test organism, tissue etc. is exposed to a substance, as well as how quickly damage builds up, when creating and understanding toxicity testing used in next-generation risk assessments (e.g. in vitro assays).

Time is a key factor in human toxicology. It affects how biological systems (organism, tissue etc.) are exposed to potentially harmful substances. It takes time for substances to reach the area where they cause harm and then for the body to remove them through distribution, excretion, or metabolism. These processes all control how the concentration of a harmful substance changes over time at the site where it causes damage. Additionally, how exposure happens – whether a single dose or repeated doses over time – has an influence. Long-term or repeated exposure might cause the compounds to build up in the body at the site where it causes damage. Time is also crucial in how events progress from the initial harmful action to the first key biological event, and then to the final adverse outcome.

These factors equally affect studies carried out in vitro (e.g. using isolated cells or tissues), as well as tests on living organisms (in vivo).

The first day of this ECETOC workshop featured a series of presentations to set the scene and provide inspiration for the Day 2 breakout group discussions.

The breakout groups discussed the following potential harmful effects in terms of how to account for the role of time: skin sensitisation, neurodegenerative diseases, cancer risks, liver toxicity (cholestasis), and endocrine disruption affecting development and reproduction.

The workshop participants concluded that consideration of the potential impact of time on adverse outcomes is important in toxicology and recommendations were developed for how to incorporate time considerations in toxicity testing used in next-generation risk assessments (e.g. in vitro assays). Recommendations included:

• Evaluate the timing, frequency, and duration of real-life exposures to estimate the worst-case external exposure.
• Consider the impact of duration and timing of exposure, such as during different development stages, and make sure in vitro studies can replicate this sensitivity.
• Use models to determine the internal exposure at the target site and replicate in vivo exposures as far as possible.
• Consider the potential for cumulative exposure and how to replicate this in vitro.
• Use adverse outcome pathways (AOPs) to better understand time-sensitivities from first key biological event to the final adverse outcome.

Prof. Blaauboer, Utrecht University, a workshop participant, commented: “Time - both as duration of exposure (chronos), but also as timing (kairos) - is an important factor in the understanding of the toxicity of chemicals, and this must also be taken into consideration when interpreting the data from new approach methods. Given the recent vast developments in the field of toxicology towards the implementation of new methods in risk assessment, the recommendations of this highly relevant and timely workshop may be able to accelerate the modernisation of the field.”

The workshop report is available here.