Weight-of-the-evidence evaluation of 2,4-D potential for interactions with the estrogen, androgen and thyroid pathways and steroidogenesis

Abstract

A comprehensive weight-of-the-evidence evaluation of 2,4-dichlorophenoxyacetic acid (2,4-D) was conducted for potential interactions with the estrogen, androgen and thyroid pathways and with steroidogenesis. This assessment was based on an extensive database of high quality in vitro, in vivo ecotoxicological and in vivo mammalian toxicological studies. Epidemiological studies were also considered. Toxicokinetic data provided the basis for determining rational cutoffs above which exposures were considered irrelevant to humans based on exceeding thresholds for saturation of renal clearance (TSRC); extensive human exposure and biomonitoring data support that these boundaries far exceed human exposures and provide ample margins of exposure. 2,4-D showed no evidence of interacting with the estrogen or androgen pathways. 2,4-D interacts with the thyroid axis in rats through displacement of thyroxine from plasma binding sites only at high doses exceeding the TSRC in mammals. 2,4-D effects on steroidogenesis parameters are likely related to high-dose specific systemic toxicity at doses exceeding the TSRC and are not likely to be endocrine mediated. No studies, including high quality studies in the published literature, predict significant endocrine-related toxicity or functional decrements in any species at environmentally relevant concentrations, or, in mammals, at doses below the TSRC that are relevant for human hazard and risk assessment. Overall, there is no basis for concern regarding potential interactions of 2,4-D with endocrine pathways or axes (estrogen, androgen, steroidogenesis or thyroid), and thus 2,4-D is unlikely to pose a threat from endocrine disruption to wildlife or humans under conditions of real-world exposures.ARTICLE HISTORY

Human-relevant potency threshold (HRPT) for ERα agonism

“…The HRPT derived here for the ERα agonist MoA can be used to evaluate chemicals according to the criteria for identification of EDCs proposed by the European Commission, which requires establishing the biological plausibility of a causal link between the adverse effects of a chemical and an endocrine MoA. For the ERα agonist MoA, plausibility would require that the adverse effects of the chemical are among those known to be produced by ERα agonists in chronic studies using animals with intact endocrine systems (Neal et al 2017 ) or in humans, and that their mechanistic potency via the ERα agonist MoA exceeds the HRPT of 1E-04 relative to the supra-threshold reference ERα agonists, e.g., 17β-estradiol or 17α-ethinyl estradiol.…”

“…If the level of mechanistic potency exhibited by D4 were sufficient to produce physiologically relevant ERα-mediated effects in intact animals, alterations in a variety of estrogen-sensitive endpoints could be observed (Neal et al 2017 ). As predicted by its sub-threshold mechanistic potency, however, D4 fails to produce an estrogenic pattern of effects in either chronic repeated-dose (Dekant et al 2017 ) or multigenerational reproduction (Siddiqui et al 2007 ) toxicity tests in rats.…”

Issue analysis: key characteristics approach for identifying endocrine disruptors

“…For more than a decade, weight of evidence (WoE) evaluations have been the standard method for determining whether a chemical meets that definition (World Health Organization [WHO] 2002 ; U.S. EPA 2011 ; OECD 2012 ). WoE methods consider all data pertinent to satisfying the EDC definition and evaluating those data with respect to relevance, reliability, strength, and coherence with established endocrine physiology and pharmacology (Rhomberg et al 2010 ; Borgert et al 2011 , 2014 ; Rhomberg 2014 ; Lutter et al 2015 ; Bridges and Solomon 2016 ; Mihaich et al 2017 ; Neal et al 2017 ; Mihaich and Borgert 2018 ; Borgert 2022 ).…”

“…The steps in a WoE evaluate whether a chemical can operate through an endocrine mechanism and whether that endocrine mechanism is then responsible for an adverse effect(s) of the chemical. Of critical importance is that WoE methods provide a transparent means to resolve conflicts in the data through comparisons to known positive and negative controls for established endocrine MoAs (Rhomberg et al 2010 ; Borgert et al 2011 , 2014 ; Rhomberg 2014 ; Lutter et al 2015 ; Bridges and Solomon 2016 ; Mihaich et al 2017 ; Neal et al 2017 ; Mihaich and Borgert 2018 ; Borgert 2022 ).…”

“…Recently, a new approach for identifying EDC hazards has been proposed that organizes and evaluates data according to ten so-called “Key Characteristics (KCs) of EDCs” (La Merrill et al 2020 ). The approach claims to address the lack of a widely accepted, systematic approach for identifying “EDC hazards,” but completely ignores the WoE literature cited here (Rhomberg et al 2010 ; Borgert et al 2011 , 2014 ; Rhomberg 2014 ; Lutter et al 2015 ; Bridges and Solomon 2016 ; Mihaich et al 2017 ; Neal et al 2017 ; Mihaich and Borgert 2018 ; Borgert 2022 ). Proponents of the KC approach define EDCs as “exogenous chemicals that interfere with hormone action, thereby increasing the risk of adverse health outcomes, including cancer, reproductive impairment, cognitive deficits and obesity” and use chemicals alleged to be EDCs to illustrate the “KC-based approach.” In fact, the KC approach searches for and organizes data according to broad, mechanistically indistinct categories such as “alters hormone metabolism or clearance,” and “alters signal transduction in hormone-responsive cells,” which are phenomena that occur by endocrine as well as non-endocrine mechanisms (Marty et al 2018 ).…”