Hypothesis-driven weight of evidence analysis to determine potential endocrine activity of MTBE

Current testing programs for pesticides adequately capture endocrine activity and adversity for protection of vertebrate wildlife

The toxicity and ecotoxicity of pesticide active ingredients are evaluated by a number of standardized test methods using vertebrate animals. These standard test methods are required under various regulatory programs for the registration of pesticides. Over the past two decades, additional test methods have been developed with endpoints that are responsive to endocrine activity and subsequent adverse effects. This article examines the available test methods and their endpoints that are relevant to an assessment of endocrine-disrupting properties of pesticides. Furthermore, the article highlights how weight-of-evidence approaches should be applied to determine whether an adverse response in (eco)toxicity tests is caused by an endocrine mechanism of action. The large number of endpoints in the current testing paradigms for pesticides make it unlikely that endocrine activity and adversity is being overlooked. Integr Environ Assess Manag 2023;00:1-21. © 2023 Bayer CropScience and The Authors. Integrated Environmental Assessment and Management published by Wiley Periodicals LLC on behalf of Society of Environmental Toxicology & Chemistry (SETAC).

Methyl-tert-butyl ether (MTBE): integration of rat and mouse carcinogenicity data with mode of action and human and rodent bioassay dosimetry and toxicokinetics indicates MTBE is not a plausible human carcinogen

Methyl-tert-butyl ether (MTBE) is a fuel oxygenate used in non-United States geographies. Multiple health reviews conclude that MTBE is not a human-relevant carcinogen, and this review provides updated mode of action (MOA), exposure, dosimetry and risk perspectives supporting those conclusions. MTBE is non-genotoxic and has large margins of exposure between blood concentrations at the overall rat 400 ppm inhalation NOAEL and blood concentrations in typical workplace or general population exposures. Non-cancer and threshold cancer hazard quotients range from a high of 0.046 for fuel-pump gasoline station attendants and are 100-1,000-fold lower for general population exposures. Cancer risks conservatively assuming genotoxicity for these same scenarios are all less than 1 × 10^-6. The onset of MTBE nonlinear toxicokinetics (TK) in rats at inhalation exposures less than 3,000 ppm, a dose that is also not practically achievable in fuel-use scenarios, indicates that high-dose specific male rat kidney and testes (3,000 and 8,000 ppm) and female mouse liver tumors (8000 ppm) are not quantitatively relevant to humans. Mode of action analyses also indicate MTBE male rat kidney tumors, and lesser so female mouse liver tumors, are not qualitatively relevant to humans. Thus, an integrated analysis of the toxicology, exposure/dosimetry, TK, and MOA data indicates that MTBE presents minimal human cancer and non-cancer risks.

A mechanistic evaluation of the potential for octamethylcyclotetrasiloxane to produce effects via endocrine modes of action

This review is a hypothesis driven, mechanistic evaluation of the potential for octamethylcyclotetrasiloxane (D4) to produce any effects via endocrine modes of action. D4 is a volatile, lipophilic liquid used in the production of high molecular weight dimethylsiloxane polymers. These are used in a variety of industrial, medical, cleaning, and personal care products, and they may contain low levels of residual D4. Low concentrations of D4 are found in the environment and there is potential for low level human exposure. All of the measured environmental and workplace levels of D4 fall below no observed effect levels (NOEL). Most of the effects of high dose D4 involve the female reproductive system. In the mature intact female rat following chronic high dose exposure, D4 may cause inhibition of mating and ovulation, decreased live litter sizes, small increases in the estrogen to progesterone ratio primarily through decreases in progesterone, and increases in uterine hyperplasia. When endogenous estrogens are very low, high dose D4 causes increases in some uterine parameters. To assess whether these high dose effects can be attributed to an endocrine mode of action, endpoints are ranked for relevance and strength, consistent with published concepts. When sufficient information is available the level of activity of D4 for producing the observed effect is compared with that of potent endocrines. The conclusions reached are that all of the effects of D4 fall well short of any established criteria for D4 to be capable of producing any adverse effect via an endocrine mode of action.

A hypothesis-driven weight-of-evidence analysis to evaluate potential endocrine activity of perfluorohexanoic acid

Perfluorohexanoic acid (PFHxA) is a potential impurity and environmental degradation product of C6-based fluorotelomer products. Considering the potential endocrine activity of perfluoroalkyl acids, a hypothesis-driven weight-of-evidence (WoE) analysis was conducted to evaluate the potential endocrine disruptor activity of PFHxA, as defined by World Health Organization (WHO), across estrogen (E), androgen (A), thyroid (T), and steroidogenesis (S) pathways. A comprehensive literature search identified primary and secondary studies across species for review. The ToxCast/Tox21 database provided in vitro data. Studies identified were reviewed for reliability, and relevance, with endocrine endpoints ranked, and lines of evidence evaluated across pathways. Overall, PFHxA showed no endocrine effects in Japanese medaka, juvenile rainbow trout, chickens or reproductive parameters in northern bobwhite with no significant activity in rodent repeated-dose toxicity, lifetime cancer, or reproductive and developmental studies. In vitro, there was weak or negative activity for T transport protein or activation of E, A or T receptors. PFHxA was also negative in vitro and in vivo for disrupting steroidogenesis. Based on this WoE endocrine analysis, PFHxA exposure did not cause adverse effects associated with alterations in endocrine activity in these models, as such would not be characterized as an endocrine disruptor according to the WHO definition.

Distinguishing between endocrine disruption and non-specific effects on endocrine systems

The endocrine system is responsible for growth, development, maintaining homeostasis and for the control of many physiological processes. Due to the integral nature of its signaling pathways, it can be difficult to distinguish endocrine-mediated adverse effects from transient fluctuations, adaptive/compensatory responses, or adverse effects on the endocrine system that are caused by mechanisms outside the endocrine system. This is particularly true in toxicological studies that require generation of effects through the use of Maximum Tolerated Doses (or Concentrations). Endocrine-mediated adverse effects are those that occur as a consequence of the interaction of a chemical with a specific molecular component of the endocrine system, for example, a hormone receptor. Non-endocrine-mediated adverse effects on the endocrine system are those that occur by other mechanisms. For example, systemic toxicity, which perturbs homeostasis and affects the general well-being of an organism, can affect endocrine signaling. Some organs/tissues can be affected by both endocrine and non-endocrine signals, which must be distinguished. This paper examines in vitro and in vivo endocrine endpoints that can be altered by non-endocrine processes. It recommends an evaluation of these issues in the assessment of effects for the determination of endocrine disrupting properties of chemicals. This underscores the importance of using a formal weight of evidence (WoE) process to evaluate potential endocrine activity.

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

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.

A critical review finds styrene lacks direct endocrine disruptor activity

The European Commission lists styrene (S) as an endocrine disruptor based primarily on reports of increased prolactin (PRL) levels in S-exposed workers. The US Environmental Protection Agency included S in its list of chemicals to be tested for endocrine activity. Therefore, the database of S for potential endocrine activity is assessed. In vitro and in vivo screening studies, as well as non-guideline and guideline investigations in experimental animals indicate that S is not associated with (anti)estrogenic, (anti)androgenic, or thyroid-modulating activity or with an endocrine activity that may be relevant for the environment. Studies in exposed workers have suggested elevated PRL levels that have been further examined in a series of human and animal investigations. While there is only one definitively known physiological function of PRL, namely stimulation of milk production, many normal stress situations may lead to elevations without any chemical exposure. Animal studies on various aspects of dopamine (DA), the PRL-regulating neurotransmitter, in the central nervous system did not give mechanistic explanations on how S may affect PRL levels. Overall, a neuroendocrine disruption of PRL regulation cannot be deduced from a large experimental database. The effects in workers could not consistently be reproduced in experimental animals and the findings in humans represented acute reversible effects clearly below clinical and pathological levels. Therefore, unspecific acute workplace-related stress is proposed as an alternative mode of action for elevated PRL levels in workers.

Short-term fish reproduction assays with methyl tertiary butyl ether with zebrafish and fathead minnow: Implications for evaluation of potential for endocrine activity

The authors report on short-term fish reproduction assays in zebrafish and fathead minnow conducted to examine the potential for methyl tertiary butyl ether (MTBE) to cause effects on the endocrine system. Both studies were performed under good laboratory practice and in accordance with Organisation for Economic Co-operation and Development and US Environmental Protection Agency test guidelines. The results of the first study demonstrated that exposure to a high test concentration (147 mg/L) of MTBE impaired reproductive output of female zebrafish, evident by a reduction in fecundity. Based on the endpoints evaluated in the present study however, there was no supporting evidence to indicate that this effect was caused by disruption of or interaction with the endocrine system. In the second study, fathead minnows exposed to a wider but lower range of test concentrations showed no effects on any reproductive parameter of male or female fish, at the maximum recommended testing concentration of 100 mg/L (62 mg/L measured). The results of these 2 guideline studies indicate that MTBE does not interact with the hypothalamic-pituitary-gonadal axis of zebrafish or fathead minnow.