Hypothesis-driven weight of evidence evaluation indicates ethylbenzene lacks endocrine disruption potential by EATS pathways

Ethylbenzene (EB) was placed on List 2 for Tier 1 endocrine screening in the U.S. EPA’s two-tiered Endocrine Disruptor Screening Program (EDSP) and was scheduled for evaluation under TSCA. Results of toxicology studies on EB were used to evaluate estrogen, androgen, thyroid, and steroidogenic (EATS) endpoints by a Weight of Evidence (WoE) methodology, as required by U.S. EPA and OECD guidelines for evaluating a chemical’s endocrine disruptive potential. The WoE method involved problem formulation, systematic literature search and selection, data quality evaluation, relevance weighting of endpoint data, and application of specific interpretive criteria. Data on EB were sufficient to assess its effects on endpoints that would be expected to respond to chemicals that operate via EATS modes of action (MoAs) in various screening assays (Tier 1) and toxicity tests (Tier 2) that evaluate reproduction, development, and sub-chronic and chronic toxicity. In those studies, EB produced a pattern of responses inconsistent with the responses that would be expected for hormones and chemicals known to operate via EATS MoAs. Endocrine-sensitive endpoints that respond to EB administration generally do so only at dose levels above its kinetic maximum dose, indicating a lack of relevance to potential effects at lower dose levels in either the test species or humans. This comprehensive WoE evaluation demonstrates that EB lacks the potential to exhibit endocrine disruptive properties and cannot be deemed an endocrine disruptor or potential endocrine disruptor. Because this WoE evaluation was based largely on Tier 2-level studies of the type considered by the U.S. EPA and OECD to be more definitive than results of Tier 1 EDSP screening results, no additional useful information would be obtained by subjecting EB to further endocrine screening. As such, further endocrine screening of EB would be unjustified from animal welfare perspectives. This analysis supports a regulatory decision to halt further testing of EB for endocrine disruption unless unique and compelling data to the contrary arise.