Advancing Chemical Safety: An In Silico Protocol for Endocrine Activity Assessment 

In the evolving landscape of chemical safety and regulatory science, the assessment of endocrine-disrupting chemicals (EDCs) has become a critical priority.  Endocrine disruption (ED) has been introduced as a classification and labelling category in Regulation (EC) No 1272/2008¹ and continues to be an important endpoint for assessment under the Canadian Environmental Protection Act (CEPA) ², and the U.S. EPA ³. A comprehensive evaluation of ED hazard includes an assessment of endocrine activity, which supports a mode of action hypothesis for adverse endocrine effects.  In a recently published in silico protocol ⁴, the use of computational tools to streamline endocrine activity assessments is described.  

A structured approach to assess endocrine activity 

The in silico protocol  presents a workflow for integrating computational predictions with experimental data and expert review to assess endocrine activity across the estrogen, androgen, thyroid, steroidogenesis (EATS) modalities.  

The evaluation is organized based on a  Hazard Assessment Framework (HAF) for endocrine disruption that maps molecular initiating events (MIEs) and key events (KE) to endpoints relevant to endocrine activity. For each MIE and KE, the application and interpretation of existing data (for example, in vitro data generated from guideline studies or high-throughput screening (HTS) data), (Quantitative) Structure–Activity Relationships (Q)SAR models, expert review elements, and read-across methodology are considered within the context of application. 

The approach is structured such that it allows for transparent, reproducible, and evidence-based assessments. This protocol supports a non-testing, weight-of-evidence approach that can enable chemical screening efforts and regulatory decision-making. It aligns with global efforts, including the OECD’s conceptual framework⁵ and the ECHA/EFSA framework⁶ for the assessment of endocrine disruptors. 

Case Studies⁴ 

Two chemicals were evaluated to demonstrate applications of the protocol: 

1. 4-Chloro-1-[2,2-dichloro-1-(4-chlorophenyl)ethenyl]-2-(methylsulfonyl)benzene 

Given the absence of experimental data, in silico models combined with expert reviews were used to derive EATS activity assessments. QSAR models flagged concern for estrogen and androgen activity. The QSAR model similarly predicted the activity of a structural analog with experimental data indicating endocrine activity and highlighted features of concern for developmental and reproductive toxicity due to estrogen receptor binding. In this example, structural analogs supported medium-confidence assessments, and the importance of expert review was highlighted. 

2. Chloroprene 

Chloroprene may undergo cytochrome oxidation to form metabolites that potentially contain reactive features. These metabolites are therefore of potential interest to the assessment of chloroprene’s endocrine activity. The metabolic transformation of chloroprene could lead to uncertainty in negative in vitro as well as in silico predictions in which metabolism is not considered. An assessment of both chloroprene and its metabolites suggested negative predictions for estrogen, androgen, and steroidogenesis activity, although the reliability of the assessments varied. The positive prediction of a metabolite for NIS inhibition introduces uncertainty in the thyroid activity assessment. An analysis of additional thyroid MIEs is therefore suggested, particularly given findings of increased thyroid carcinoma in a two-year cancer study. 

Looking Ahead 

The integration of in silico models with experimental data and expert review is aligned with best practice on the use of computational models in chemical safety assessment. This work supports efficient and transparent assessments of endocrine activity using computational tools. As regulatory bodies continue to adopt non-animal testing strategies, defining how computational tools can be integrated into safety assessments continues to be a valuable effort.