Ethylene oxide (EtO) is a highly reactive industrial chemical and known human carcinogen with a mutagenic mode of action (MOA). Its genotoxicity is primarily mediated through alkylation of DNA, forming the mutagenic adduct O6-(2-hydroxyethyl)-2'-deoxyguanosine (O6-HE-dG), albeit in small quantities, and the more abundant but less- or nonmutagenic N7-(2-hydroxyethyl)guanine (N7-HE-G) adduct. However, dose-response relationships of these DNA adducts, particularly at low inhalation exposure levels (< 3 ppm), remain unknown. These data are necessary to inform the biological plausibility of different statistical dose-response models that have been applied to human or animal data used for cancer risk assessment. In this study, B6C3F1 mice were exposed to EtO (0 to 200 ppm) for 6 h/d over 28 consecutive days. DNA adducts in lung, liver, bone marrow, and mammary gland were quantified using highly sensitive mass spectrometry platforms. N7-HE-G was detected in all tissues and exposure groups, showing linear dose-response relationships in the low-dose range (≤ 1 ppm) and increased sharply and exposure-disproportionately in the high-dose range (≥ 50 ppm). Despite high sensitivity, O6-HE-dG was undetectable in any tissue at exposure < 50 ppm, reflecting adduct levels that are below the current quantifiable limit. At higher exposures (≥ 50 ppm), O6-HE-dG exhibited a dose-response pattern of N7-HE-G. Notably, the mammary gland, despite being anatomically distant from the site of inhalation, exhibited the second-highest levels of both adducts at higher doses. This study provides the first reliable quantitative dose-response evidence of DNA adducts in tumor target and nontarget (liver) tissues across a wide range of EtO exposures. The two DNA adducts differ markedly in their abundance, repairability and mutagenic potential and together provide a molecular MOA dose-response framework to provide the biological foundation for informing quantitative cancer risk assessment and genotoxic hazard characterization.

Ethylene oxide (EtO) assessment relies on detecting the hemoglobin adduct N-(2-hydroxyethyl)-l-valine (HE-V). However, current methods require large blood volumes, limiting small-animal exposure studies and reliable detection of endogenous background levels of HE-V. We optimized a highly sensitive analytical workflow including hemoglobin extraction from microliter-scale blood samples, HE-V purification, and targeted LC-MS/MS quantitative analysis. This workflow reliably quantifies endogenous HE-V levels of unexposed mice from only 50 μg of purified hemoglobin (10 μL of blood) and is demonstrated to detect significant HE-V increases at EtO exposures as low as 0.05 ppm. Our platform enables practical EtO biomonitoring in volume-constrained experimental and environmental settings.

Ethylene oxide (EtO) is a widely used industrial compound with known carcinogenic potential in humans. Due to its high reactivity and short biological half-life, occupational exposure assessments (> 1 ppm) rely on the detection of stable biomarkers, such as N-(2-hydroxyethyl)-L-valine (HE-V), formed as hemoglobin adducts in blood. Existing analytical methods for HE-V detection often require large volumes of blood and purified hemoglobin due to limited sensitivity, restricting their application in characterizing the dose–response relationship between EtO exposure and HE-V accumulation—particularly at low environmental exposure levels relevant to assessing potential general population health risks. In this study, we aimed to characterize the molecular dosimetry of HE-V formation in B6C3F1 mice exposed by whole body inhalation to a broad range of concentrations of EtO: 0, 0.05, 0.1, 0.5, 1, 50, 100, and 200 ppm, 7 days/week for 4 weeks. To achieve this, we developed a sensitive LC–MS-based workflow for HE-V quantification, incorporating hemoglobin purification, HE-V release plus enrichment, and targeted mass spectrometric detection from as little as 10 μL of blood and 50 μg of extracted hemoglobin. A clear, dose-dependent increase in HE-V levels was observed following EtO exposure, with statistically significant elevations detected even at 0.05 ppm compared to endogenous background levels. At lower concentrations (0.5 to 1 ppm), HE-V levels increased linearly with dose, while higher concentrations (50 to 200 ppm) exhibited an upward-bending (increasing slope) dose response. No sex-specific differences were observed. Taken together, these findings indicate EtO exhibits linear systemic toxicokinetics at lower exposures that transition to nonlinear toxicokinetics in the range of higher exposures (likely due to saturation of glutathione-mediated detoxification), thus providing new quantitative insights to support improved risk assessments and toxicological evaluations of EtO exposure.

Ethylene oxide (EtO) is primarily used as an intermediate in the manufacture of chemicals, with a minor use as a sterilant for medical equipment and food products. It is a direct-acting alkylating agent that reacts with cellular macromolecules, including proteins and DNA. EtO has been shown to induce tumors in rodents and humans. DNA reactivity has been the postulated mode of action (MOA) for its carcinogenicity. The current study has investigated the dose response for EtO-induced genetic damage to inform the biological plausibility of a dose-response model for cancer risk assessment. Male and female B6C3F1 mice (≥ 10/sex/concentration) were exposed to 0, 0.05, 0.1, 0.5, 1, 50, 100, or 200 ppm EtO by whole-body inhalation (6 h/day for 28 days, 7 days/week). Mutagenicity was assessed by determining the frequency of mutant Pig-a phenotype in reticulocytes (RET) and mature red blood cells (RBC) on Day 28. Cytogenetic damage was evaluated by the erythrocyte micronucleus (MN) test in blood samples collected on Days 5 and 28. EtO is a relatively weak genotoxicant with treatment-related increases in Pig-a and MN frequencies being seen primarily at 200 ppm. The hockey-stick shaped dose response for genetic damage may be conservatively interpreted as being no more than a linear response with a single slope. Thus, a cancer risk assessment dose-response model consisting of a single shallow linear slope throughout the exposure range is biologically plausible and consistent if EtO were acting through a mutagenic MoA for its carcinogenicity.

The two most recent cancer risk assessments for ethylene oxide (EO) are based on the same epidemiologic study of sterilant workers conducted by the National Institute of Occupational Safety and Health (NIOSH) but result in cancer risk estimates with three orders of magnitude difference, despite relying on the same assumption of a default linear (non-threshold) extrapolation. A major reason for the difference is the use of different exposure-response models (i.e., the standard Cox proportional hazards [CPH] versus a two-piece linear spline model with a steep initial slope) to derive the inhalation unit risk. The purpose of this research is to utilize analysis of a 10-year update of the Union Carbide Corporation (UCC) EO 2053 chemical worker cohort to examine the epidemiological evidence for the shape of the exposure-response model for EO. This updated UCC study provides an external dataset that is informative given high average cumulative exposures (67 ppm-years), extensive average follow-up of over 40 years, and number of male lymphoid cancer deaths (25) comparable to that observed in the NIOSH cohort. This independent analysis of a different cohort using continuous dose response modeling with cumulative or log cumulative exposure metrics provides no empirical support for a steep curve at low exposures. Furthermore, analyses of the categorical odds ratio estimates across different updates of the UCC cohort and for each sex in the NIOSH cohort provide further epidemiological evidence that the standard CPH model more plausibly describes the relationship between EO exposures and lymphoid mortality for both cohorts.

Ethylene oxide (EO) is an industrial chemical and sterilant that is released into ambient air from natural and unregulated anthropogenic sources that contribute to background exogenous exposure and from regulated industrial sources that contribute to additional exogenous exposure for near-facility populations. Metabolic processes contribute to substantial background endogenous exposures to EO, complicating the interpretation of the relation between total background exposure and the health significance of added industrial exogenous exposure. In 2021, Kirman and colleagues characterized the total and endogenous equivalent background concentrations for U.S. populations, which are substantially greater than the USEPA 2016 EO cancer reassessment risk-specific concentrations (0.00011–0.011 ppb), suggesting that the consideration of background exposure could be used as a reality check for the utility of the reassessment in managing EO risk for industrially exposed populations. New exposure biomarker data and background ambient concentration data for EO have become available since the 2021 assessment and are used here to refine the estimates of U.S. population total and endogenous equivalent background EO concentrations. Refined equivalent background concentrations as well as total equivalent exposure estimates for U.S. smokers provide context as to the health significance of near-industry population added exposure and a reality check for the utility of USEPA and TCEQ risk-specific concentrations in managing and communicating EO risk.

Studies of nervous system effects of glyphosate, a widely used herbicide, have not been critically examined. The aim of this paper was to systematically review glyphosate-induced neurotoxicity literature to determine its usefulness in regulatory decision-making. The review was restricted to mammalian studies of behavior, neuropathology, and neuropharmacology; in vitro and other biochemical studies were considered supplementary information. Glyphosate formulation studies were also considered, despite uncertainties regarding toxicities of the formulated products; no studies used a formulation vehicle as the control. Inclusion criteria were developed a priori to ensure consistent evaluation of studies, and in vivo investigations were also ranked using ToxRTool software to determine reliability. There were 27 in vivo studies (open literature and available regulatory reports), but 11 studies were considered unreliable (mostly due to critical methodological deficiencies). There were only seven acceptable investigations on glyphosate alone. Studies differed in terms of dosing scenarios, experimental designs, test species, and commercial product. Limitations included using only one dose and/or one test time, small sample sizes, limited data presentation, and/or overtly toxic doses. While motor activity was the most consistently affected endpoint (10 of 12 studies), there were considerable differences in outcomes. In six investigations, there were no marked neuropathological changes in the central or peripheral nervous system. Other neurological effects were less consistent, and some outcomes were less convincing due to influences including high variability and small effect sizes. Taken together, these studies do not demonstrate a consistent impact of glyphosate on the structure or function of the mammalian nervous system.

The purpose of the present investigation is to analyze the in vivo genotoxicity dose–response data of ethylene oxide (EO) and the applicability of the derived point-of-departure (PoD) values when estimating permitted daily exposure (PDE) values. A total of 40 data sets were identified from the literature, and benchmark dose analyses were conducted using PROAST software to identify a PoD value. Studies employing the inhalation route of exposure and assessing gene or chromosomal mutations and chromosomal damage in various tissues were considered the most relevant for assessing risk from EO, since these effects are likely to contribute to adverse health consequences in exposed individuals. The PoD estimates were screened for precision and the values were divided by data-derived adjustment factors. For gene mutations, the lowest PDE was 285 parts per trillion (ppt) based on the induction of lacI mutations in the testes of mice following 48 weeks of exposure to EO. The corresponding lowest PDE value for chromosomal mutations was 1,175 ppt for heritable translocations in mice following 8.5 weeks of EO exposure. The lowest PDE for chromosomal aberrations was 238 ppt in the mouse peripheral blood lymphocytes following 48 weeks of inhalation exposure. The diverse dose–response data for EO-induced genotoxicity enabled the derivation of PoDs for various endpoints, tissues, and species and identified 238 ppt as the lowest PDE in this retrospective analysis.

The purpose of the present investigation is to analyze the in vivo genotoxicity dose-response data of ethylene oxide (EO) and the applicability of the derived point-of-departure (PoD) values when estimating permitted daily exposure (PDE) values. A total of 40 data sets were identified from the literature, and benchmark dose analyses were conducted using PROAST software to identify a PoD value. Studies employing the inhalation route of exposure and assessing gene or chromosomal mutations and chromosomal damage in various tissues were considered the most relevant for assessing risk from EO, since these effects are likely to contribute to adverse health consequences in exposed individuals. The PoD estimates were screened for precision and the values were divided by data-derived adjustment factors. For gene mutations, the lowest PDE was 285 parts per trillion (ppt) based on the induction oflacImutations in the testes of mice following 48 weeks of exposure to EO. The corresponding lowest PDE value for chromosomal mutations was 1,175 ppt for heritable translocations in mice following 8.5 weeks of EO exposure. The lowest PDE for chromosomal aberrations was 238 ppt in the mouse peripheral blood lymphocytes following 48 weeks of inhalation exposure. The diverse dose-response data for EO-induced genotoxicity enabled the derivation of PoDs for various endpoints, tissues, and species and identified 238 ppt as the lowest PDE in this retrospective analysis.

Response variability across the lifespan is an important consideration in toxicology and risk assessment, and the toxic effects of drugs and chemicals during adolescence need more research. This paper summarizes a workshop presented in March 2019, at the Society of Toxicology Annual Meeting in Baltimore, Maryland, that brought together experts in research on drug dependence and toxicity related to nicotine, cannabis, cocaine, and other illicit drugs during adolescence. The goal of the workshop was to address the following issues: (1) Do the effects of adolescent exposure differ from the same exposure in adults? (2) Are there unique biological markers of adolescent brain development? If so, what are they and how reliable are they? (3) Since multiple factors influence substance use disorder, can we disentangle risk factors for abuse and/or toxicity? What are the underlying biological susceptibilities that lead to dependence and neurotoxicity? What are the social, psychosocial and environmental factors that contribute to abuse susceptibilities? This paper reviews drug policy and national trends in adolescent substance use; the public health consequences of e-cigarettes; rat models of adolescent-onset nicotine self-administration and persisting effects of gestational nicotine; sex-dependent effects of delta-9-tetrahydrocannabinol on adolescent brain-behavior relationships; and translational approaches for identifying adolescent risk factors for transition to drug dependence. There is strong evidence that drug exposure prior to adulthood has longer lasting effects on behavior and the underlying neural circuitry. These effects, which are sex-dependent and influenced by stress, may be candidates as predictors of adolescent vulnerability. A major challenge to determining if adolescents have a unique susceptibility to dependence is whether and to what extent the human data allow distinction between the increased risk due to biological immaturity, an underlying biological susceptibility to dependence, or psychosocial and environmental factors for substance dependence. Factors important to consider for development of animal models include the timing and pattern of exposure as it relates to adolescence; age of assessment, and direct comparison with similar effects following exposures to adults to demonstrate that these effects are unique to adolescence. Here we provide a roadmap for further research into what makes adolescent brain development unique.

Thyroid hormones (THs; T3 and T4) play a role in development of cardiovascular, reproductive, immune and nervous systems. Thus, interpretation of TH changes from rodent studies (during pregnancy, in fetuses, neonates, and adults) is critical in hazard characterization and risk assessment. A roundtable session at the 2017 Society of Toxicology (SOT) meeting brought together academic, industry and government scientists to share knowledge and different perspectives on technical and data interpretation issues. Data from a limited group of laboratories were compiled for technical discussions on TH measurements, including good practices for reliable serum TH data. Inter-laboratory historical control data, derived from immunoassays or mass spectrometry methods, revealed: 1) assay sensitivities vary within and across methodologies; 2) TH variability is similar across animal ages; 3) laboratories generally achieve sufficiently sensitive TH quantitation levels, although issues remain for lower levels of serum TH and TSH in fetuses and postnatal day 4 pups; thus, assay sensitivity is critical at these life stages. Best practices require detailed validation of rat serum TH measurements across ages to establish assay sensitivity and precision, and identify potential matrix effects. Finally, issues related to data interpretation for biological understanding and risk assessment were discussed, but their resolution remains elusive.

The 2016 U.S. Environmental Protection Agency (EPA) Integrated Risk Information System (IRIS) assessment for ethylene oxide (EO) estimated a 10−6 increased inhalation cancer risk of 0.1 parts per trillion, based on National Institute of Occupational Safety and Health (NIOSH) epidemiology studies of sterilization facility workers exposed to EO between 1938 and 1986. The worker exposure estimates were based on a NIOSH statistical regression (NSR) model “validated” with EO levels measured after 1978. Between 1938 and 1978, when EO data was unavailable, the NSR model predicts exposures lowest in 1938 increasing to peak levels in 1978. That increasing EO concentration trend arose, in part, because engineering/industrial-hygiene (E/IH) factors associated with evolving EO-sterilization equipment and operations before 1978 were not properly considered in the NSR model. To test the NSR model trend prediction, a new E/IH-based model was developed using historical data on EO kill concentrations, EO residue levels in sterilized materials, post-wash EO concentrations in a sterilization chamber, and information on facility characteristics and sterilizer operator practices from operators familiar with pre-1978 industry conditions. The E/IH 90th percentile of 8 h time-weighted average EO exposures (C90) for highly exposed sterilizer operators was calibrated to match 1978 C90 values from the NSR model. E/IH model C90 exposures were estimated to decrease over time from levels 16 and were four-fold greater than NSR-estimated exposures for workers during 1938–1954 and 1955–1964. This E/IH modeled trend is opposite to that of NSR model predictions of exposures before 1978, suggesting that EPA’s exclusive reliance on the NIOSH cohort to estimate EO cancer risk should be re-examined.

International and U.S. guidance for developmental neurotoxicity (DNT) testing of chemicals are provided by OECD DNT test guideline 426, OECD extended one-generation reproductive toxicity study (EOGRTS) test guideline 443, and U.S. EPA DNT test guideline 870.6300. These guidelines with their multiple behavioral and neuropathology endpoints are among the most logistically complex, labor-intensive bioassays conducted for regulatory purposes. This article identifies key issues in the practical implementation and analysis of neurobehavioral and neuropathology endpoints, with emphasis on quantitative brain morphometry testing requirements. This article also reviews the development of DNT guidelines, strengths and limitations of validation efforts, regulatory triggers for DNT testing, and recent guidance from government and workgroup efforts to enhance conduct and interpretation of studies for regulatory risk assessment purposes.

There is a paucity of information concerning the developmental neurotoxicity (DNT) hazard posed by industrial and environmental chemicals. New testing approaches will most likely be based on batteries of alternative and complementary (non-animal) tests. As DNT is assumed to result from the modulation of fundamental neurodevelopmental processes (such as neuronal differentiation, precursor cell migration or neuronal network formation) by chemicals, the first generation of alternative DNT tests target these processes. The advantage of such types of assays is that they capture toxicants with multiple targets and modes-of-action. Moreover, the processes modelled by the assays can be linked to toxicity endophenotypes, i.e., alterations in neural connectivity that form the basis for neurofunctional deficits in man. The authors of this review convened in a workshop to define criteria for the selection of positive/negative controls, to prepare recommendations on their use, and to initiate the setup of a directory of reference chemicals. For initial technical optimization of tests, a set of > 50 endpoint-specific control compounds was identified. For further test development, an additional "test" set of 33 chemicals considered to act directly as bona fide DNT toxicants is proposed, and each chemical is annotated to the extent it fulfills these criteria. A tabular compilation of the original literature used to select the test set chemicals provides information on statistical procedures, and toxic/non-toxic doses (both for pups and dams). Suggestions are provided on how to use the > 100 compounds (including negative controls) compiled here to address specificity, adversity and use of alternative test systems.

Neuropathology methods in rodent developmental neurotoxicity (DNT) studies have evolved with experience and changing regulatory guidance. This article emphasizes principles and methods to promote more standardized DNT neuropathology evaluation, particularly procurement of highly homologous brain sections and collection of the most reproducible morphometric measurements. To minimize bias, brains from all animals at all dose levels should be processed from brain weighing through paraffin embedding at one time using a counterbalanced design. Morphometric measurements should be anchored by distinct neuroanatomic landmarks that can be identified reliably on the faced block or in unstained sections and which address the region-specific circuitry of the measured area. Common test article-related qualitative changes in the developing brain include abnormal cell numbers (yielding altered regional size), displaced cells (ectopia and heterotopia), and/or aberrant differentiation (indicated by defective myelination or synaptogenesis), but rarely glial or inflammatory reactions. Inclusion of digital images in the DNT pathology raw data provides confidence that the quantitative analysis was done on anatomically matched (i.e., highly homologous) sections. Interpreting DNT neuropathology data and their presumptive correlation with neurobehavioral data requires an integrative weight-of-evidence approach including consideration of maternal toxicity, body weight, brain weight, and the pattern of findings across brain regions, doses, sexes, and ages.

A comprehensive review of published and previously unpublished studies was performed to evaluate the neonicotinoid insecticides for evidence of developmental neurotoxicity (DNT). These insecticides have favorable safety profiles, due to their preferential affinity for nicotinic receptor (nAChR) subtypes in insects, poor penetration of the mammalian blood-brain barrier, and low application rates. Nevertheless, examination of this issue is warranted, due to their insecticidal mode of action and potential exposure with agricultural and residential uses. This review identified in vitro, in vivo, and epidemiology studies in the literature and studies performed in rats in accordance with GLP standards and EPA guidelines with imidacloprid, acetamiprid, thiacloprid, clothianidin, thiamethoxam, and dinotefuran, which are all the neonicotinoids currently registered in major markets. For the guideline-based studies, treatment was administered via the diet or gavage to primiparous female rats at three dose levels, plus a vehicle control (≥20/dose level), from gestation day 0 or 6 to lactation day 21. F1 males and females were evaluated using measures of motor activity, acoustic startle response, cognition, brain morphometry, and neuropathology. The principal effects in F1 animals were associated with decreased body weight (delayed sexual maturation, decreased brain weight, and morphometric measurements) and acute toxicity (decreased activity during exposure) at high doses, without neuropathology or impaired cognition. No common effects were identified among the neonicotinoids that were consistent with DNT or the neurodevelopmental effects associated with nicotine. Findings at high doses were associated with evidence of systemic toxicity, which indicates that these insecticides do not selectively affect the developing nervous system.