In Vitro, Ex Vivo, and In Vivo Determination of Thyroid Hormone Modulating Activity of Benzothiazoles

Metabolic signatures of prenatal exposure to 'Cocktails' of benzotriazoles and benzothiazoles and its health implications

Prenatal exposure to benzotriazoles and benzothiazoles (collectively as BTs) was associated with pregnancy complications. Identifying the metabolites associated with prenatal BTs exposure may help elucidate the mechanism and characterize the exposure risk. In this prospective study of 158 pregnant women from Wuhan, China, urinary BTs were repeatedly measured across three trimesters to provide an accurate estimation of exposure during pregnancy. We conducted high-throughput targeted metabolomics with great coverage and high accuracy to characterize the urinary metabolic profile in late pregnancy. We first identified the perturbed metabolites of cocktail BTs exposure and then pinned down to the pairwise associations between individual BTs and the identified metabolites. A total of 44 metabolites were identified as perturbed biomarkers of cocktail BTs exposure based on the variable influence on projection (VIP > 1.2) score. Further pairwise associations analysis showed positive association of BTs with oxidative stress related biomarkers and negative association of BTs with neuronal function metabolites. The shared metabolic signatures among BTs in the co-occurrence network of pairwise association analysis may partially be attributed to the correlation among cocktail BTs exposure. The findings provide the potential mechanisms of BTs-associated pregnancy complications and offer insight into the health implications for prenatal BTs exposure. Furthermore, the framework we employed, which integrates both cocktail exposure and individual exposure, may illuminate future epidemiological research that seeks to incorporate exposure to mixtures and omics scale data.

Comparing the Developmental Toxicity Delay and Neurotoxicity of Benzothiazole and Its Derivatives (BTHs) in Juvenile Zebrafish

In this study, a semi-static water exposure method was employed to investigate the early developmental and neurotoxic effects of four benzothiazole substances (BTHs), namely benzothiazole (BTH), 2-mercaptobenzothiazole (MBT), 2-hydroxybenzothiazole (BTON), and 2-aminobenzothiazole (2-ABTH), on zebrafish at an equimolar concentration of 10 μM. The findings revealed that all four BTHs exerted certain impacts on early development in zebrafish. MBT stimulated spontaneous movement in juvenile zebrafish, whereas BTON inhibited such movements. Moreover, all four BTHs hindered the hatching process of zebrafish larvae, with MBT exhibiting the strongest inhibition at 24 h post-fertilization (hpf). Notably, MBT acted as a melanin inhibitor by suppressing melanin production in juvenile zebrafish eyes and weakening phototaxis. Additionally, both BTH and BTON exhibited significantly lower speeds than the control group and other test groups under conditions without bright field stimulation; however, their speeds increased to average levels after percussion stimulation, indicating no significant alteration in motor ability among experimental zebrafish groups. Short-term exposure to these four types of BTHs induced oxidative damage in zebrafish larvae; specifically, BTH-, MBT-, and BTON-exposed groups displayed abnormal expression patterns of genes related to oxidative damage. Exposure to both BTH and MBT led to reduced fluorescence intensity in transgenic zebrafish labeled with central nervous system markers, suggesting inhibition of central nervous system development. Furthermore, real-time quantitative PCR results demonstrated abnormal gene expression associated with neural development. However, no significant changes were observed in 2-ABTH gene expression at this concentration. Overall findings indicate that short-term exposure to BTHs stimulates neurodevelopmental gene expression accompanied by oxidative damage.

Technical evaluation and standardization of the human thyroid microtissue assay

The success and sustainability of U.S. EPA efforts to reduce, refine, and replace in vivo animal testing depends on the ability to translate toxicokinetic and toxicodynamic data from in vitro and in silico new approach methods (NAMs) to human-relevant exposures and health outcomes. Organotypic culture models employing primary human cells enable consideration of human health effects and inter-individual variability but present significant challenges for test method standardization, transferability, and validation. Increasing confidence in the information provided by these in vitro NAMs requires setting appropriate performance standards and benchmarks, defined by the context of use, to consider human biology and mechanistic relevance without animal data. The human thyroid microtissue (hTMT) assay utilizes primary human thyrocytes to reproduce structural and functional features of the thyroid gland that enable testing for potential thyroid-disrupting chemicals. As a variable-donor assay platform, conventional principles for assay performance standardization need to be balanced with the ability to predict a range of human responses. The objectives of this study were to (1) define the technical parameters for optimal donor procurement, primary thyrocyte qualification, and performance in the hTMT assay, and (2) set benchmark ranges for reference chemical responses. Thyrocytes derived from a cohort of 32 demographically diverse euthyroid donors were characterized across a battery of endpoints to evaluate morphological and functional variability. Reference chemical responses were profiled to evaluate the range and chemical-specific variability of donor-dependent effects within the cohort. The data-informed minimum acceptance criteria for donor qualification and set benchmark parameters for method transfer proficiency testing and validation of assay performance.

A ubiquitous tire rubber additive induced serious eye injury in zebrafish (Danio rerio)

Previous studies have indicated that tire wear particles (TWPs) leachate exposure induced serious eye injury in fish through inhibiting the thyroid peroxidase (TPO) enzyme activity. However, the main TPO inhibitors in the leachate were still unknown. In this study, we identified 2-Mercaptobenzothiazole (MBT) as the potential TPO inhibitor in the TWPs leachate through references search, model prediction based on Danish QSAR and ToxCast database, molecular docking, and in vivo assay. We further explored the toxic mechanism of MBT under environmentally relevant concentrations. The decreased eye size of zebrafish larvae was mainly caused by the decreased lens diameter and cell density in the inner nuclear layer (INL) and outer nuclear layer (ONL) of the retina. Transcriptomics analysis demonstrated that the eye phototransduction function was significantly suppressed by inhibiting the photoreceptor cell proliferation process after MBT exposure. The altered opsin gene expression and decreased opsin protein levels were induced by weakening thyroid hormone signaling after MBT treatment. These results were comparable to those obtained from a known TPO inhibitor, methimazole. This study has identified MBT as the primary TPO inhibitor responsible for inducing eye impairment in zebrafish larvae exposed to TWPs leachate. It is crucial for reducing the toxicity of TWPs leachate in fish.

Associations between urinary concentrations of benzothiazole, benzotriazole, and their derivatives and lung cancer: A nested case-control study

Benzothiazole (BTH), benzotriazole (BTR), and their respective derivatives (BTHs and BTRs) are emerging environmental pollutants with widespread human exposure and oncogenic potential. Studies have demonstrated adverse effects of exposure to certain BTHs and BTRs on the respiratory system. However, no study has examined the associations between exposure to BTHs and BTRs and lung cancer risk. We aimed to examine the associations between urinary concentrations of BTHs and BTRs and the risk of lung cancer in the general population from Quzhou, China. We conducted a nested case-control study in an ongoing prospective Quzhou Environmental Exposure and Human Health (QEEHH) cohort, involving 20, 694 participants who provided urine samples during April 2019-July 2020. With monthly follow-up until November 2022, 212 lung cancer cases were recruited and 1:1 matched with healthy controls based on age and sex. We estimated odds ratios (ORs) and 95% confidence intervals (CIs) of lung cancer risk associated with urinary BTHs and BTRs concentrations using conditional logistic regression models after controlling for potential covariates. We also examined effect modification by several covariates, including sex, socioeconomic status, smoking status, alcohol consumption, and dietary habit. Creatinine-corrected urinary BTH and 2-hydroxy-benzothiazole (2-OH-BTH) levels were significantly associated with the risk of lung cancer, after adjusting for a variety of covariates. Participants in the highest quartile of BTH had a 95% higher risk of lung cancer, compared with those in the lowest quartile (adjusted OR = 1.95, 95% CI: 1.08-3.49; p for trend = 0.01). Participants with higher levels of urinary 2-OH-BTH had an 83% higher risk of lung cancer than those with lower levels (adjusted OR = 1.83, 95% CI: 1.16-2.88; p for trend = 0.01). Exposure to elevated levels of BTH and 2-OH-BTH may be associated with an increased risk of lung cancer. These associations were not modified by socio-demographic characteristics.

Neurobehavioral toxic effects and mechanisms of 2-aminobenzothiazole exposure on zebrafish

2-Aminobenzothiazole (NTH), a benzothiazole derivative, exhibits potent biochemical activities and plays a significant role in modern industry. Widespread and intensive utilization of NTH has led to its detection in aquatic environments, encompassing both groundwater and surface water. Despite its wide usage, the effect of NTH on developmental neurotoxicity in aquatic organisms remains uncharted. Therefore, the aim of this investigation was to create exposure models for short- and long-term studies in order to analyze the neurobehavioral toxic impact of NTH (0, 50, 500, and 5000 μg/L) on zebrafish, which includes motor function, anxiety, and memory performance, as well as to examine the mechanism of neurotoxicity. The results revealed a significant suppression of initial embryonic mobility by NTH. However, during short-term exposure experiments, it did not significantly impact the developmental neurobehavioral functions of zebrafish. In addition, significant effects on zebrafish were observed after long-term exposure to 50 and 500 μg/L NTH, mainly impacting locomotion, social behavior, anxiety, and cognitive functions. Moreover, NTH caused oxidative damage in adult zebrafish brain tissue, which was accompanied by abnormal expression of oxidative damage-related genes. Furthermore, the Real-Time PCR results indicated a significant suppression of genes related to exposure to NTH, specifically those in the GABA synthesis pathway (gabrg2, gad2, gad1b, and abat) and the 5-HT synthesis pathway (tph2, tph1b, pet1, and htr1aa). Taken together, this study demonstrates for the first time that chronic exposure to NTH decreases the expression of genes associated with the zebrafish GABA synthesis pathway and the 5-HT synthesis pathway. This suppression is accompanied by oxidative damage, ultimately resulting in neurobehavioral changes related to motor ability, anxiety, and memory performance.

Selective determination of 2-aminobenzothiazole in environmental water and organic extracts from fish and dust samples

In the present study, a homemade mixed-mode ion-exchange sorbent based on silica with embedded graphene microparticles is applied for the selective extraction of 2-aminobenzothiazole (NH2BT) followed by determination through liquid chromatography coupled to high-resolution mass spectrometry. The sorbent was evaluated for the solid-phase extraction of NH2BT from environmental water samples (river, effluent wastewater, and influent wastewater), and NH2BT was strongly retained through the selective cation-exchange interactions. Therefore, the inclusion of a clean-up step of 7 mL of methanol provided good selectivity for the extraction of NH2BT. The apparent recoveries obtained for environmental water samples ranged from 62 to 69% and the matrix effect from -1 to -14%. The sorbent was also evaluated in the clean-up step of the organic extract for the extraction of NH2BT from organic extracts of indoor dust samples (10 mL of ethyl acetate from pressurized liquid extraction) and fish (10 mL of acetonitrile from QuEChERS extraction). The organic extracts were acidified (adding a 0.1% of formic acid) to promote the cation-exchange interactions between the sorbent and the analyte. The apparent recoveries for fish samples ranged from 22 to 36% depending on the species. In the case of indoor dust samples, the recovery was 41%. It should be highlighted the low matrix effect encountered in such complex samples, with values ranging from -7 to 5% for fish and dust samples. Finally, various samples were analyzed. The concentration in river samples ranged from 31 to 136 ng/L; in effluent wastewater samples, from 55 to 191 ng/L; in influent wastewater samples, from 131 to 549 ng/L; in fish samples, from 14 to 57 ng/g dried weight; and in indoor dust samples, from  Collapse

Key Words

• Clean-up
• Complex water samples
• Dust
• Fish
• Homemade mixed-mode ion-exchange sorbent
• Selective solid-phase extraction

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MESH Headings

• Animals
• Tandem Mass Spectrometry/methods
• Wastewater
• Water/analysis
• Dust/analysis
• Water Pollutants, Chemical/analysis
• Solid Phase Extraction/methods
• Fishes
• Cations/analysis

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Grants

• 2020PMF-PIPF-33 Universitat Rovira i Virgili
• PID2020-114587GB-I00 Ministerio de Ciencia e Innovación

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Affiliation(s)

Alberto Moral Department of Analytical Chemistry and Organic Chemistry, Universitat Rovira i Virgili, Sescelades Campus, Marcel·lí Domingo 1, 43007, Tarragona, Spain
Francesc Borrull Department of Analytical Chemistry and Organic Chemistry, Universitat Rovira i Virgili, Sescelades Campus, Marcel·lí Domingo 1, 43007, Tarragona, Spain
Kenneth G Furton Department of Chemistry and Biochemistry, Florida International University, International Forensic Research Institute, Miami, FL, 33199, USA
Abuzar Kabir Department of Chemistry and Biochemistry, Florida International University, International Forensic Research Institute, Miami, FL, 33199, USA
Núria Fontanals Department of Analytical Chemistry and Organic Chemistry, Universitat Rovira i Virgili, Sescelades Campus, Marcel·lí Domingo 1, 43007, Tarragona, Spain.
Rosa Maria Marcé Department of Analytical Chemistry and Organic Chemistry, Universitat Rovira i Virgili, Sescelades Campus, Marcel·lí Domingo 1, 43007, Tarragona, Spain

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Evaluation of the diphenyl herbicide, oxyfluorfen, for effects on thyroid hormones in the juvenile rat

Recently, oxyfluorfen, a pre- and post-emergent diphenyl ether herbicide, was identified in our laboratory as an inhibitor of iodide uptake by the sodium iodide symporter (NIS), the first key step in the synthesis of thyroid hormones (THs). This inhibition was observed in vitro, using both a human NIS engineered cell line (hNIS-HEK293T-EPA) and a rat thyroid follicular cell line (FRTL-5). Oxyfluorfen was found to be a potent inhibitor of NIS activity with an EC50 of approximately 2 µM in both cell lines with no observed cytotoxicity at any concentration tested up to 100 μM. The current research tested the hypothesis that oxyfluorfen alters circulating concentrations of THs. This hypothesis was first tested in a pilot study with both juvenile male and female rats exposed to oxyfluorfen for 4 days at 0, 125, 250 and 500 mg/kg/day. Once we identified that this short-term 4-day oxyfluorfen exposure suppressed both total serum thyroxine (T4) and triiodothyronine (T3) at all doses, we tested seven lower concentrations of oxyfluorfen (0.8125 to 62.5 mg/kg day) in an 8-day exposure paradigm to more closely evaluate the dose-response. We found that oxyfluorfen suppressed serum T4 with a LOEL of 3.25 mg/kg/day and T3 with a LOEL 62.5 mg/kg/day. Analytical chemistry of the serum showed an accumulation over time following oral exposure to oxyfluorfen in both the 4- and 8-day groups. Analytical chemistry of the thyroid glands in the 8-day study revealed higher accumulation in the thyroid as compared to the serum (2 to 3- fold at 62.5 mg/kg). No changes in thyroid weight or serum TSH were observed following the 8-day exposure. This study is the first to demonstrate an effect of oxyfluorfen on serum thyroid hormones in the rat. Additional studies are needed to further evaluate the effects on thyroid homeostasis with extended exposures and the potential implications of the observed effects.

An optimized radioimmunoassay for quantification of total serum thyroxine (T4) in fetal, neonatal, and pregnant rats

Identifying xenobiotics that interrupt the thyroid axis has significant public health implications, given that thyroid hormones are required for brain development. As such, some developmental and reproductive toxicology (DART) studies now require or recommend serum total thyroxine (T4) measurements in pregnant, lactating, and developing rats. However, serum T4 concentrations are normally low in the fetus and pup which makes quantification difficult. These challenges can be circumvented by technologies like mass spectrometry, but these approaches are expensive and not always widely available. To demonstrate the feasibility of measuring T4 using a commercially available assay, we examine technical replicates of rat serum samples measured both by liquid chromatography mass spectrometry (LC/MS/MS) and radioimmunoassay (RIA). These samples were obtained from rats on gestational day 20 (dams and fetuses) or postnatal day 5 (pups), following maternal exposure to the goitrogen propylthiouracil (0-3 ppm) to incrementally decrease T4. We show that with assay modification, it is possible to measure serum T4 using low sample volumes (25-50 μL) by an RIA, including in the GD20 fetus exposed to propylthiouracil. This proof-of-concept study demonstrates the technical feasibility of measuring serum T4 in DART studies.

Occurrence and phase distribution of benzothiazoles in untreated highway stormwater runoff and road dust

The study about how tyre-derived particles can potentially worsen the water quality and how traffic pollution markers can affect the environment is crucial for environmental management. Road emissions are known to contribute to pollution in various environments, and benzothiazoles and their derivates can be used to trace pollutant inputs related to surface runoff in the aquatic system. A total of eight benzothiazoles were determined in highway stormwater runoff and road dust collected from February to August 2022 near Venice (Casale sul Sile, Veneto Region, Italy). A new analytical method was validated, by using an UHPLC system coupled to a mass spectrometer (triple quadrupole). The target compounds were determined in both dissolved phase and suspended particulate matter of runoff, and the road dust samples were divided into seven fractions depending on particle diameters to understand the fraction partitioning. The results indicate that 2-SO3H-BTH was the most concentrated benzothiazole in all the analysed substrates, suggesting tyre debris as the main source because it is usually used in the vulcanization process. 2-SO3H-BTH reached a mean concentration of 115 ± 59 µg L-1, 4 ± 3 µg L-1, and 411 ± 441 µg Kg-1 for dissolved phase, suspended particulate matter, and road dust, respectively, while 2-OH-BTH and BTH showed values about an order of magnitude lower. The size distribution of most BTHs suggests that they are distributed in the finest fraction of road dust. An exception was given by 2-SCNMeS-BTH being present only in particles with a diameter > 1 mm.

Cross-species applicability of an adverse outcome pathway network for thyroid hormone system disruption

Thyroid hormone system disrupting compounds are considered potential threats for human and environmental health. Multiple adverse outcome pathways (AOPs) for thyroid hormone system disruption (THSD) are being developed in different taxa. Combining these AOPs results in a cross-species AOP network for THSD which may provide an evidence-based foundation for extrapolating THSD data across vertebrate species and bridging the gap between human and environmental health. This review aimed to advance the description of the taxonomic domain of applicability (tDOA) in the network to improve its utility for cross-species extrapolation. We focused on the molecular initiating events (MIEs) and adverse outcomes (AOs) and evaluated both their plausible domain of applicability (taxa they are likely applicable to) and empirical domain of applicability (where evidence for applicability to various taxa exists) in a THSD context. The evaluation showed that all MIEs in the AOP network are applicable to mammals. With some exceptions, there was evidence of structural conservation across vertebrate taxa and especially for fish and amphibians, and to a lesser extent for birds, empirical evidence was found. Current evidence supports the applicability of impaired neurodevelopment, neurosensory development (eg, vision) and reproduction across vertebrate taxa. The results of this tDOA evaluation are summarized in a conceptual AOP network that helps prioritize (parts of) AOPs for a more detailed evaluation. In conclusion, this review advances the tDOA description of an existing THSD AOP network and serves as a catalog summarizing plausible and empirical evidence on which future cross-species AOP development and tDOA assessment could build.

Mechanistic insight into the adverse outcome of tire wear and road particle leachate exposure in zebrafish (Danio rerio) larvae

Tire wear particles (TWP) have become the major microplastic pollution in China. Road runoff containing TWP leachate can decrease the eye size and even induced mortality in the aquatic organisms. However, the toxic mechanism of TWP and road particles (RP) leachate on aquatic organisms is still unclear. In this study, the zebrafish embryos were exposed to TWP or RP leachate for 5 days at both environmental relevant and high concentrations. The adverse outcome pathways (AOPs) were screened from individual to molecular levels. The morphological and behavioral analysis demonstrated that the leachate exposure mainly impaired the eye development of zebrafish larvae and inhibited the larval swim behavior and phototactic response, which are the adverse outcomes. The phototransduction modulated by zebrafish retina was significantly down-regulated through transcriptomics and metabolomics analysis. The eye histopathological analysis showed that the decreased thickness of the retinal outer nuclear layer (ONL) and retinal pigmented epithelium (RPE) after leachate exposure were caused by the decreased photoreceptor cells. Moreover, the expression of NR2E3 and TPO genes showed concentration-dependent down-regulation after leachate exposure. The inhibition of photoreceptor cell proliferation was identified as the main reason for photoreceptor cell decrease in zebrafish larval eye. This study, for the first time, uncovered the underlying toxic mechanism of TWP and RP on zebrafish larval eyes.

Reducing uncertainties in quantitative adverse outcome pathways by analysis of thyroid hormone in the neonatal rat brain

A number of xenobiotics interfere with thyroid hormone (TH) signaling. Although adequate supplies of TH are necessary for normal brain development, regulatory reliance on serum TH as proxies for brain TH insufficiency is fraught with significant uncertainties. A more direct causal linkage to neurodevelopmental toxicity induced by TH-system disrupting chemicals is to measure TH in the target organ of most concern, the brain. However, the phospholipid-rich matrix of brain tissue presents challenges for TH extraction and measurement. We report optimized analytical procedures to extract TH in brain tissue of rats with recoveries >80% and low detection limits for T3, rT3, and T4 (0.013, 0.033, and 0.028 ng/g, respectively). Recovery of TH is augmented by enhancing phospholipid separation from TH using an anion exchange column coupled with a stringent column wash. Quality control measures incorporating a matrix-matched calibration procedure revealed excellent recovery and consistency across a large number of samples. Application of optimized procedures revealed age-dependent increases in neonatal brain T4, T3, and rT3 on the day of birth (postnatal day, PN0), PN2, PN6, and PN14. No sex-dependent differences in brain TH were observed at these ages, and similar TH levels were evident in perfused versus non-perfused brains. Implementation of a robust and reliable method to quantify TH in the fetal and neonatal rat brain will aid in the characterization of the thyroid-dependent chemical interference on neurodevelopment. A brain- in addition to a serum-based metric will reduce uncertainties in assessment of hazard and risk on the developing brain posed by thyroid system-disrupting chemicals.

Prenatal exposure to benzotriazoles and benzothiazoles and child neurodevelopment: A longitudinal study

Benzotriazoles (BTRs) and benzothiazoles (BTHs) are emerging benzo-heterocyclic compounds that may induce neurotoxicity. However, the effect of prenatal exposure to BTs (BTRs and BTHs) on child neurodevelopment has not been elucidated. We aimed to explore the associations between maternal urinary concentrations of BTs in single or in mixture with child neurodevelopment at the age of two. This study recruited 513 mother-child pairs based on a birth cohort from 2014 to 2015 in Wuhan. Maternal urinary concentrations of eight BTs (four BTRs and four BTHs) in the first, second, and third trimesters were measured. The mental development index (MDI) and psychomotor development index (PDI) of children, as two indexes of neurodevelopment, were assessed at two years old by the Bayley Scales. In the analyses of single BTs, prenatal average tolyltriazole (TTR) exposure level was associated with decreased boys' MDI scores (β = -2.84, 95 % CI: -5.11, -0.57) and prenatal average 1-H-benzotriazole (1-H-BTR) exposure level was associated with decreased boys' PDI scores (β = -1.44, 95 % CI: -2.70, -0.17), respectively. Maternal urinary concentrations of benzothiazole (BTH) in the 1^st trimester (β = -1.79, 95 % CI: -2.78, -0.80), 2^nd trimester (β = -1.14, 95 % CI: -2.19, -0.09), and the prenatal average exposure (β = -2.15, 95 % CI: -3.69, -0.61) were also negatively associated with boys' PDI scores. However, no significantly negative association was observed among girls. In the further mixture analysis, the quantile g-computation model found a significant negative association between prenatal average concentrations of BTs in mixture and boys' PDI scores [β = -4.80 (95 % CI: -9.08, -0.52)], and BTH weighted the highest in the negative association. As far as we know, this is the first research to estimate the effect of prenatal exposure to BTs on child neurodevelopment. The findings showed that prenatal exposure to BTs was negatively associated with neurodevelopment among boys, suggesting that the associations may be modified by infant sex.

Cumulative maternal and neonatal effects of combined exposure to a mixture of perfluorooctanoic acid (PFOA) and perfluorooctane sulfonic acid (PFOS) during pregnancy in the Sprague-Dawley rat

Globally, biomonitoring data demonstrate virtually all humans carry residues of multiple per- and polyfluoroalkyl substances (PFAS). Despite pervasive co-exposure, limited mixtures-based in vivo PFAS toxicity research has been conducted. Perfluorooctanoic acid (PFOA) and perfluorooctane sulfonic acid (PFOS) are commonly detected PFAS in human and environmental samples and both produce adverse effects in laboratory animal studies, including maternal and offspring effects when orally administered during pregnancy and lactation. To evaluate the effects of combined exposure to PFOA and PFOS, we orally exposed pregnant Sprague-Dawley rats from gestation day 8 (GD8) to postnatal day 2 (PND2) to PFOA (10-250 mg/kg/d) or PFOS (0.1-5 mg/kg/d) individually to characterize effects and dose response curve parameters, followed by a variable-ratio mixture experiment with a constant dose of PFOS (2 mg/kg/d) mixed with increasing doses of PFOA (3-80 mg/kg/d). The mixture study design was intended to: 1) shift the PFOA dose response curves for endpoints shared with PFOS, 2) allow comparison of dose addition (DA) and response addition (RA) model predictions, 3) conduct relative potency factor (RPF) analysis for multiple endpoints, and 4) avoid overt maternal toxicity. Maternal serum and liver concentrations of PFOA and PFOS were consistent between the individual chemical and mixture experiments. Combined exposure with PFOS significantly shifted the PFOA dose response curves towards effects at lower doses compared to PFOA-only exposure for multiple endpoints and these effects were well predicted by dose addition. For endpoints amenable to mixture model analyses, DA produced equivalent or better estimates of observed data than RA. All endpoints evaluated were accurately predicted by RPF and DA approaches except for maternal gestational weight gain, which produced less-than-additive results in the mixture. Data support the hypothesis of cumulative effects on shared endpoints from PFOA and PFOS co-exposure and dose additive approaches for predictive estimates of mixture effects.

The performance and pathway of benzothiazole degradation by electron beam irradiation

Benzothiazole (BTH) is a typical refractory heterocyclic compound that can be used as a photosensitive material in organic synthesis and conditional plant resource research. The extensive use of BTH has led to high BTH concentrations in natural environment, such as in tap water and urine, which tend to inhibit animal hormone synthesis and induce genotoxicity. Traditional wastewater treatment processes cannot effectively remove BTH. Therefore, we aimed to use the electron beam method, an emerging method for pollutant degradation, to degrade BTH in water. Experiments showed that BTH can be effectively degraded (up to 90%) when the electron beam reaches 5 kGy and irradiation conformed perfectly to the pseudo first-order kinetics model. Experimental results showed that acidic conditions are more favorable for electron beam degradation of BTH, while the degradation of most other inorganic ions is inhibited (except SO₄^2-). Hydroxyl radicals (•OH) was confirmed to play a major role in degradation by the experiment, and the mineralization rate was greatly increased by the addition of H₂O₂ and K₂S₂O₈. In addition, our experimental and theoretical calculations showed that the degradation of BTH occurred mainly through the opening of the benzene ring. Theoretical calculations showed that the toxicity of BTH decreased significantly after electron beam degradation, making it an effective way to degrade BTH.

Gestational Exposure to Perchlorate in the Rat: Thyroid Hormones in Fetal Thyroid Gland, Serum, and Brain

Iodine is essential for the production of thyroid hormones. Perchlorate is an environmental contaminant that interferes with iodine uptake into the thyroid gland to reduce thyroid hormone synthesis. As thyroid hormones are critical for brain development, exposure to perchlorate during pregnancy is of concern for the developing fetal brain. In this study, we (1) define profiles of thyroid hormone in the maternal and fetal compartments of pregnant rats in response to inhibition of the sodium-iodide symporter (NIS) by perchlorate and (2) expand inquiry previously limited to serum to include fetal thyroid gland and brain. Perchlorate was added to the drinking water (0, 1, 30, 300, and 1000 ppm) of pregnant rat dams from gestational days (GD) 6-20. On GD20, blood, thyroid gland, and brain were collected from the fetus and dam for thyroid hormone and molecular analyses. Thyroid gland and serum thyroid hormones were dose-dependently reduced, with steeper declines evident in the fetus than in the dam. The thyroid gland revealed perturbations of thyroid hormone-action with greater sensitivity in the fetus than the dam. Thyroid hormones and thyroid hormone-responsive gene expression were reduced in the fetal cortex portending effects on brain development. These findings are the first quantitative assessments of perchlorate-induced deficits in the fetal thyroid gland and fetal brain. We provide a conceptual framework to develop a quantitative NIS adverse outcome pathway for serum thyroid hormone deficits and the potential to impact the fetal brain. Such a framework may also serve to facilitate the translation of in vitro bioactivity to the downstream in vivo consequences of NIS inhibition in the developing fetus.

Mapping Mechanistic Pathways of Acute Oral Systemic Toxicity Using Chemical Structure and Bioactivity Measurements

Regulatory agencies around the world have committed to reducing or eliminating animal testing for establishing chemical safety. Adverse outcome pathways can facilitate replacement by providing a mechanistic framework for identifying the appropriate non-animal methods and connecting them to apical adverse outcomes. This study separated 11,992 chemicals with curated rat oral acute toxicity information into clusters of structurally similar compounds. Each cluster was then assigned one or more ToxCast/Tox21 assays by looking for the minimum number of assays required to record at least one positive hit call below cytotoxicity for all acutely toxic chemicals in the cluster. When structural information is used to select assays for testing, none of the chemicals required more than four assays and 98% required two assays or less. Both the structure-based clusters and activity from the associated assays were significantly associated with the GHS toxicity classification of the chemicals, which suggests that a combination of bioactivity and structural information could be as reproducible as traditional in vivo studies. Predictivity is improved when the in vitro assay directly corresponds to the mechanism of toxicity, but many indirect assays showed promise as well. Given the lower cost of in vitro testing, a small assay battery including both general cytotoxicity assays and two or more orthogonal assays targeting the toxicological mechanism could be used to improve performance further. This approach illustrates the promise of combining existing in silico approaches, such as the Collaborative Acute Toxicity Modeling Suite (CATMoS), with structure-based bioactivity information as part of an efficient tiered testing strategy that can reduce or eliminate animal testing for acute oral toxicity.

Characterization of the mechanistic linkages between iodothyronine deiodinase inhibition and impaired thyroid-mediated growth and development in Xenopus laevis using iopanoic acid

Iodothyronine deiodinases (DIO) are key enzymes that influence tissue-specific thyroid hormone levels during thyroid-mediated amphibian metamorphosis. Within the larger context of evaluating chemicals for thyroid system disrupting potential, chemical activity toward DIOs is being evaluated using high-throughput in vitro screening assays as part of U.S. EPA's ToxCast program. However, existing data gaps preclude any inferences between in vitro chemical inhibition of DIOs and in vivo outcomes relevant to ecological risk assessment. This study aimed to generate targeted data in a laboratory model species (Xenopus laevis) using a model DIO inhibitor, iopanoic acid (IOP), to characterize linkages between in vitro potency, in vivo biochemical responses, and adverse organismal outcomes. In vitro potency of IOP toward DIOs was evaluated using previously developed in vitro screening assays, which showed concentration-dependent inhibition of human DIO1 (IC50: 97 µM) and DIO2 (IC50: 231 µM) but did not inhibit human or X. laevis DIO3 under the assay conditions. In vivo exposure of larval X. laevis to 0, 2.6, 5.3 and 10.5 µM IOP caused thyroid-related biochemical profiles in the thyroid gland and plasma consistent with hyperthyroxinemia but resulted in delayed metamorphosis and significantly reduced growth in the highest two exposure concentrations. Independent evaluations of dio gene expression ontogeny, together with existing literature, supported interpretation of IOP-mediated effects resulting in a proposed adverse outcome pathway for DIO2 inhibition leading to altered amphibian metamorphosis. This study highlights the types of mechanistic data needed to move toward predicting in vivo outcomes of regulatory concern from in vitro bioactivity data.

Catalyst-dependent chemoselective insertion of diazoalkanes into N-H\C-H\O-H\C-O bonds of 2-hydroxybenzothiazoles

The control of chemoselective insertions of diazoalkanes with 2-hydroxybenzothiazoles is challenging. Herein, the chemoselective N-H, O-H, C-O or C-H bond insertions of diazoalkanes with 2-hydroxybenzothiazoles are achieved using B(C6F5)3, Rh2(OAc)4...

Targeted Pathway-based In Vivo Testing Using Thyroperoxidase Inhibition to Evaluate Plasma Thyroxine as a Surrogate Metric of Metamorphic Success in Model Amphibian Xenopus laevis

Chemical safety evaluation is in the midst of a transition from traditional whole-animal toxicity testing to molecular pathway-based in vitro assays and in silico modeling. However, to facilitate the shift in reliance on apical effects for risk assessment to predictive surrogate metrics having characterized linkages to chemical mechanisms of action, targeted in vivo testing is necessary to establish these predictive relationships. In this study, we demonstrate a means to predict thyroid-related metamorphic success in the model amphibian Xenopus laevis using relevant biochemical measurements during early prometamorphosis. The adverse outcome pathway for thyroperoxidase inhibition leading to altered amphibian metamorphosis was used to inform a pathway-based in vivo study design that generated response-response relationships. These causal relationships were used to develop Bayesian probabilistic network models that mathematically determine conditional dependencies between biochemical nodes and support the predictive capability of the biochemical profiles. Plasma thyroxine concentrations were the most predictive of metamorphic success with improved predictivity when thyroid gland sodium-iodide symporter gene expression levels (a compensatory response) were used in conjunction with plasma thyroxine as an additional regressor. Although thyroid-mediated amphibian metamorphosis has been studied for decades, this is the first time a predictive relationship has been characterized between plasma thyroxine and metamorphic success. Linking these types of biochemical surrogate metrics to apical outcomes is vital to facilitate the transition to the new paradigm of chemical safety assessments.

Contamination profiles and health impact of benzothiazole and its derivatives in PM2.5 in typical Chinese cities

Although benzothiazole and its derivatives (BTHs) are considered emerging contaminants in diverse environments and organisms, little information is available about their contamination profiles and health impact in ambient particles. In this study, an optimized method of ultrasound-assisted extraction coupled with the selected reaction monitoring (SRM) mode of GC-EI-MS/MS was applied to characterize and analyze PM_2.5-bound BTHs from three cities of China (Guangzhou, Shanghai, and Taiyuan) during the winter of 2018. The total BTH concentration (ΣBTHs) in PM_2.5 samples from the three cities decreased in the order of Guangzhou > Shanghai > Taiyuan, independently of the PM_2.5 concentration. Despite the large variation in concentration of ΣBTHs in PM_2.5, 2-hydroxybenzothiazole (OTH) was always the predominant compound among the PM_2.5-bound BTHs and accounted for 50-80% of total BTHs in the three regions. Results from human exposure assessment and toxicity screening indicated that the outdoor exposure risk of PM_2.5-bound BTHs in toddlers was much higher than in adults, especially for OTH. The developmental and reproduction toxicity of OTH was further explored in vivo and in vitro. Exposure of mouse embryonic stem cells (mESCs) to OTH for 48 h significantly increased the intracellular reactive oxygen species (ROS) and induced DNA damage and apoptosis via the functionally activating p53 expression. In addition, the growth and development of zebrafish embryos were found to be severely affected after OTH treatment. An overall metabolomics study was conducted on the exposed zebrafish larvae. The results indicated that exposure to OTH inhibited the phenylalanine hydroxylation reaction, which further increased the accumulation of toxic phenylpyruvate and acetylphenylalanine in zebrafish. These findings provide important insights into the contamination profiles of PM_2.5-bound BTHs and emphasize the health risk of OTH.

In vitro assays for characterization of distinct multiple catalytic activities of thyroid peroxidase using LC-MS/MS

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Multiple reactions catalyzed by thyroid peroxidase (TPO) were monitored by a battery of unique in vitro assays.

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Monoiodination and diiodination of L-Tyr to MIT and DIT was examined in a single assay.

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MIT to DIT and T3 to T4 monoiodination reactions were monitored separately.

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DIT to T4 conversion assay was used to study coupling of iodotyrosine phenolic rings.

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Distinct Km, Vmax, Kcat and Kcat/ Km values for each of the TPO catalysed reaction are presented.

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Differential response of 5 known inhibitors with specific TPO reactions was studied.

A diverse set of environmental contaminants have raised a concern about their potential adverse effects on endocrine signaling. Robust and widely accepted battery of in vitro assays is available to assess the disruption of androgenic and estrogenic pathways. However, such definitive systems to investigate effects on the disruption of thyroid pathways by the xenobiotics are not yet well established. One of the major “Molecular Initiating Events” (MIEs) in thyroid disruption involves targeting of thyroid peroxidase (TPO), a key enzyme involved in thyroid hormone synthesis. TPO catalyzes mono- and diiodination of L-Tyrosine (L-Tyr) to generate 3-Iodo-l-tyrosine (MIT) and 3,5-Diiodo-l-tyrosine (DIT), respectively, followed by the coupling of iodinated tyrosine rings to generate thyroid hormones, 3,3’5-Triiodo-l-thyronine (T3) and Levothyroxine (T4).

We sought to develop a robust, sensitive, and rapid in vitro assay systems to evaluate the effects of test chemicals on the multiple catalytic activities of thyroid peroxidase. Simple in vitro assays were designed to study TPO mediated distinct reactions using a single LC-MS/MS method.

Herein, we describe a battery of assays to investigate the iodination of L-Tyr to MIT and DIT, MIT to DIT as well as, T3 to T4 catalyzed by rat thyroid TPO. Importantly, two sequential reactions involving mono- and diiodination of L-Tyr could be analyzed in a single assay. The assay that monitors in vitro conversion of DIT to T4 was developed to study the coupling of tyrosine rings. Enzyme kinetics studies revealed distinct characteristics of multiple reactions catalyzed by TPO. Further, the known TPO inhibitors were used to assess their potency towards individual TPO substrates and reactions. The resultant half maximum inhibitory concentration (IC₅₀) values highlighted differential targeting of TPO catalyzed reactions by the same inhibitor. Overall results underscore the need to develop more nuanced approaches that account for distinct multiple catalytic activities of TPO.

Rapid, high-throughput method for the quantification of thyroid hormones in rat blood serum using isotope-dilution LC-MS/MS

Aim: Numerous guideline studies required for regulatory toxicity testing now include the measurement of the thyroid hormones 3,3',5-triiodo-L-thyronine (T3) and L-thyroxine (T4) in blood serum from rodents. A rapid, high-throughput method for the determination of the thyroid hormones T4 and T3 is reported. Materials & methods: Sample preparation is done using a 96-well microtiter plate format. Stable isotope analogs of both hormones are used as internal standards for study and quality control samples. Results & conclusion: The validated quantification levels are T3: 10 pg/ml and T4: 1 ng/ml, with CVs of <10% at the limit of quantification and up to 50*limit of quantification. The use of isotope analog internal standards eliminates the need for quantitative transfers and complete evaporations.

Prenatal exposure to benzotraizoles and benzothiazoles in relation to fetal and birth size: A longitudinal study

The presence of benzotriazoles and benzothiazoles (BTs) in maternal urine and amniotic fluid indicated the widespread fetal exposure to the contaminants. We investigated the associations of prenatal BTs exposure with fetal and birth size and explored the window of susceptibility. Participants consisted of 856 mother-infant pairs drawn from a prospective birth cohort between 2014 and 2015 in Wuhan, China. Prenatal BTs exposure were measured in multiple urine samples collected across three trimesters. We observed positive associations between prenatal exposure to specific BTs (e.g., 1-H-benzotriazole, 1-hydroxy-benzotriazole and 2-amino-benzothiazole) and femur length (FL) and birth length z-scores among girls. In boys, a 2-fold increase of averaged concentration of urinary benzothiazole (BTH) was associated with decrement in FL (β = -0.068, p < 0.001) and birth length (β = -0.055, p = 0.005) z-scores. Further analysis indicated that the negative associations between urinary concentrations of BTH and birth length z-score among boys were observed at exposure measurement in 25-35 gestational weeks. This study reported the associations between prenatal exposure to BTs and fetal and birth size, suggests the associations maybe in a sex-specific manner and the window of exposure may influence susceptibility. These findings require replication in future research.

Prenatal exposure to benzotriazoles and benzothiazoles and cord blood mitochondrial DNA copy number: A prospective investigation

BACKGROUND

Mitochondria are sensitive to environmental toxicants due to the limited repair capacity. Exposure to benzotriazoles (BTRs) and benzothiazoles (BTHs) may contribute to adverse health outcomes through oxidative stress, which may interfere with mitochondrial function. However, the mitochondrial effects of exposure to BTs (BTRs and BTHs) have not yet been elucidated, particularly in human investigations.

OBJECTIVES

We examined the associations between trimester-specific urinary BTRs and BTHs concentrations and cord blood mitochondrial DNA copy number (mtDNAcn) in a prospective birth cohort.

METHODS

The present study included 742 mother-infant pairs who participated in a birth cohort between 2014 and 2015 in Wuhan and had data on urinary concentrations of BTRs and BTHs and mtDNAcn in cord blood. Concentrations of BTs were repeatedly measured in maternal urine samples at different trimesters using high performance liquid chromatography-tandem mass spectrometry. Relative mtDNAcn in umbilical cord blood was analyzed by quantitative real-time polymerase chain reaction. Generalized estimating equations were used to evaluate the associations between BTs exposure across gestation and mtDNAcn in cord blood.

RESULTS

In the present study, we observed a positive association between urinary 2-methylthio-benzothiazole (2-MeS-BTH) concentrations in the first trimester and cord blood mtDNAcn, with marginal significance [percent changes (%Δ) = 3.97, 95% confidence interval (CI): -0.05, 8.16, p = 0.05], while urinary 2-amino-benzothiazole concentrations in the third trimester were significantly negatively associated with cord blood mtDNAcn (%Δ = -5.89, 95% CI: -10.32, -1.24). Similar patterns of associations were demonstrated between urinary 1-H-benzotriazole (1-H-BTR) and xylyltriazole concentrations in the third trimester and cord blood mtDNAcn (%Δ = -4.18 to -3.23). In sex-specific analysis, we identified that maternal urinary 1-H-BTR in the first trimester and 2-MeS-BTH in the third trimester were positively associated with cord blood mtDNAcn among male infants but not female (P for interaction = 0.05 for 1-H-BTR, P for interaction = 0.05 for 2-MeS-BTH, respectively).

CONCLUSIONS

We found evidence that prenatal exposure to BTRs and BTHs were associated with cord blood mtDNAcn alternation, and these associations were modified by infant gender. Further investigations are needed to corroborate these findings.

Agrochemicals disrupt multiple endocrine axes in amphibians

Concern over global amphibian declines and possible links to agrochemical use has led to research on the endocrine disrupting actions of agrochemicals, such as fertilizers, fungicides, insecticides, acaricides, herbicides, metals, and mixtures. Amphibians, like other species, have to partition resources for body maintenance, growth, and reproduction. Recent studies suggest that metabolic impairments induced by endocrine disrupting chemicals, and more particularly agrichemicals, may disrupt physiological constraints associated with these limited resources and could cause deleterious effects on growth and reproduction. Metabolic disruption has hardly been considered for amphibian species following agrichemical exposure. As for metamorphosis, the key thyroid hormone-dependent developmental phase for amphibians, it can either be advanced or delayed by agrichemicals with consequences for juvenile and adult health and survival. While numerous agrichemicals affect anuran sexual development, including sex reversal and intersex in several species, little is known about the mechanisms involved in dysregulation of the sex differentiation processes. Adult anurans display stereotypical male mating calls and female phonotaxis responses leading to successful amplexus and spawning. These are hormone-dependent behaviours at the foundation of reproductive success. Therefore, male vocalizations are highly ecologically-relevant and may be a non-invasive low-cost method for the assessment of endocrine disruption at the population level. While it is clear that agrochemicals disrupt multiple endocrine systems in frogs, very little has been uncovered regarding the molecular and cellular mechanisms at the basis of these actions. This is surprising, given the importance of the frog models to our deep understanding of developmental biology and thyroid hormone action to understand human health. Several agrochemicals were found to have multiple endocrine effects at once (e.g., targeting both the thyroid and gonadal axes); therefore, the assessment of agrochemicals that alter cross-talk between hormonal systems must be further addressed. Given the diversity of life-history traits in Anura, Caudata, and the Gymnophiona, it is essential that studies on endocrine disruption expand to include the lesser known taxa. Research under ecologically-relevant conditions will also be paramount. Closer collaboration between molecular and cellular endocrinologists and ecotoxicologists and ecologists is thus recommended.

Development of an In Vitro Human Thyroid Microtissue Model for Chemical Screening

Thyroid hormones (TH) are essential for regulating a number of diverse physiological processes required for normal growth, development, and metabolism. The US EPA Endocrine Disruptor Screening Program (EDSP) has identified several molecular thyroid targets relevant to hormone synthesis dynamics that have been adapted to high-throughput screening (HTS) assays to rapidly evaluate the ToxCast/Tox21 chemical inventories for potential thyroid disrupting chemicals (TDCs). The uncertainty surrounding the specificity of active chemicals identified in these screens and the relevance to phenotypic effects on in vivo human TH synthesis are notable data gaps for hazard identification of TDCs. The objective of this study was to develop a medium-throughput organotypic screening assay comprised of reconstructed human thyroid microtissues to quantitatively evaluate the disruptive effects of chemicals on TH production and secretion. Primary human thyroid cells procured from qualified euthyroid donors were analyzed for retention of NK2 homeobox 1 (NKX2-1), Keratin 7 (KRT7), and Thyroglobulin (TG) protein expression by high-content image analysis to verify enrichment of follicular epithelial cells. A direct comparison of 2-dimensional (2D) and 3-dimensional (3D) 96-well culture formats was employed to characterize the morphology, differential gene expression, TG production, and TH synthesis over the course of 20 days. The results indicate that modeling human thyroid cells in the 3D format was sufficient to restore TH synthesis not observed in the 2D culture format. Inhibition of TH synthesis in an optimized 3D culture format was demonstrated with reference chemicals for key molecular targets within the thyroid gland. Implementation of the assay may prove useful for interpreting phenotypic effects of candidate TDCs identified by HTS efforts currently underway in the EDSP.

Evaluation of potential sodium-iodide symporter (NIS) inhibitors using a secondary Fischer rat thyroid follicular cell (FRTL-5) radioactive iodide uptake (RAIU) assay

The Fischer rat thyroid follicular cell line (FRTL-5) endogenously expresses the sodium-iodide symporter (NIS) and has been used to identify environmental chemicals that perturb thyroid hormone homeostasis by disruption of NIS-mediated iodide uptake. Previously, a high-throughput radioactive iodide uptake (RAIU) screening assay incorporating the hNIS-HEK293T-EPA cell line was used to identify potential human NIS (hNIS) inhibitors in 1028 ToxCast Phase I (ph1_v2) and Phase II chemicals. In this study, the FRTL-5 cell line was evaluated and applied as a secondary RAIU assay coupled with cell viability assays to further prioritize highly active NIS inhibitors from the earlier screening. Assay validation with ten reference chemicals and performance assessment by chemical controls suggest the FRTL-5 based assays are robust and highly reproducible. Top-ranked chemicals from the ToxCast screening were then evaluated in both FRTL-5 and hNIS RAIU assays using newly sourced chemicals to strengthen the testing paradigm and to enable a rat vs. human species comparison. Eighteen of 29 test chemicals showed less than 1 order of magnitude difference in IC₅₀ values between the two assays. Notably, two common perfluorinated compounds, perfluorooctanesulfonic acid (PFOS) and perfluorohexane sulfonate (PFHxS), demonstrated strong NIS inhibitory activity [IC₅₀ - 6.45 (PFOS) and - 5.70 (PFHxS) log M in FRTL-5 RAIU assay]. In addition, several chemicals including etoxazole, methoxyfenozide, oxyfluorfen, triclocarban, mepanipyrim, and niclosamide also exhibited NIS inhibition with minimal cytotoxicity in both assays and are proposed for additional testing using short-term in vivo assays to characterize effects on thyroid hormone synthesis.

Early pregnancy exposure to benzotriazoles and benzothiazoles in relation to gestational diabetes mellitus: A prospective cohort study

BACKGROUND

Benzotriazoles (BTRs) and benzothiazoles (BTHs) are emerging contaminants with potential insulin modulation activities. Pregnancy exposure to BTs (BTRs and BTHs) may be a risk factor for the development of gestational diabetes mellitus (GDM). However, epidemiological studies are limited.

OBJECTIVES

We prospectively investigated the associations of exposure to BTs at early pregnancy with the blood glucose levels and the risks of GDM.

METHODS

A prospective cohort of 1770 pregnant women who were free of diabetes at baseline was established between 2013 and 2015 in Wuhan, China. Urine samples collected at 13.1 ± 1.1 weeks of gestation were analyzed to estimate the exposure level of BTs. The diagnosis of GDM was based on a 75 g oral glucose tolerance test (OGTT) conducted at 26.4 ± 2.4 weeks of gestation. We examined the associations between urinary concentration of BTs and blood glucose levels by linear regression models. The associations of urinary BTs concentrations with the relative risk (RR) of GDM were evaluated by generalized estimating equations with Poisson regression. Effect modifications by fetus sex and pre-pregnancy body mass index (BMI) were further evaluated in the sensitivity analysis.

RESULTS

A total of 147 (8.31%) pregnant women were diagnosed with GDM. Median concentrations of urinary BTs did not differ significantly between pregnant women with and without GDM. It was found that urinary levels of benzothiazole and 2-hydroxy-benzothiazole (2-OH-BTH) were positively associated with 2-hour blood glucose (p for trend < 0.050). Comparing the high exposure group with the low exposure group of 2-OH-BTH, the adjusted RR of GDM was 1.79 (95% CI = 1.18 to 2.69, p for trend = 0.012). Sensitivity analysis indicated that the positive association of the urinary 2-OH-BTH level with the RR of GDM remained significant among pregnant women who had a male fetus (RR = 1.76, 95% CI = 1.02 to 3.03, p for trend = 0.041) and those with a normal pre-pregnancy BMI (RR = 1.85, 95% CI = 1.09 to 3.11, p for trend = 0.034).

CONCLUSIONS

These findings suggested that higher urinary level of 2-OH-BTH in early pregnancy was associated with impaired glucose homeostasis and the increased risk of GDM. The results underscore the need of follow-up studies to validate the findings and elucidate the underlying biological mechanism.

Evaluating Iodide Recycling Inhibition as a Novel Molecular Initiating Event for Thyroid Axis Disruption in Amphibians

The enzyme iodotyrosine deiodinase (dehalogenase, IYD) catalyzes iodide recycling and promotes iodide retention in thyroid follicular cells. Loss of function or chemical inhibition of IYD reduces available iodide for thyroid hormone synthesis, which leads to hormone insufficiency in tissues and subsequent negative developmental consequences. IYD activity is especially critical under conditions of lower dietary iodine and in low iodine environments. Our objective was to evaluate the toxicological relevance of IYD inhibition in a model amphibian (Xenopus laevis) used extensively for thyroid disruption research. First, we characterized IYD ontogeny through quantification of IYD mRNA expression. Under normal development, IYD was expressed in thyroid glands, kidneys, liver, and intestines, but minimally in the tail. Then, we evaluated how IYD inhibition affected developing larval X. laevis with an in vivo exposure to a known IYD inhibitor (3-nitro-l-tyrosine, MNT) under iodine-controlled conditions; MNT concentrations were 7.4-200 mg/L, with an additional 'rescue' treatment of 200 mg/L MNT supplemented with iodide. Chemical inhibition of IYD resulted in markedly delayed development, with larvae in the highest MNT concentrations arrested prior to metamorphic climax. This effect was linked to reduced glandular and circulating thyroid hormones, increased thyroidal sodium-iodide symporter gene expression, and follicular cell hypertrophy and hyperplasia. Iodide supplementation negated these effects, effectively rescuing exposed larvae. These results establish toxicological relevance of IYD inhibition in amphibians. Given the highly conserved nature of the IYD protein sequence and scarcity of environmental iodine, IYD should be further investigated as a target for thyroid axis disruption in freshwater organisms.

Evaluating Chemicals for Thyroid Disruption: Opportunities and Challenges with in Vitro Testing and Adverse Outcome Pathway Approaches

BACKGROUND

Extensive clinical and experimental research documents the potential for chemical disruption of thyroid hormone (TH) signaling through multiple molecular targets. Perturbation of TH signaling can lead to abnormal brain development, cognitive impairments, and other adverse outcomes in humans and wildlife. To increase chemical safety screening efficiency and reduce vertebrate animal testing, in vitro assays that identify chemical interactions with molecular targets of the thyroid system have been developed and implemented.

OBJECTIVES

We present an adverse outcome pathway (AOP) network to link data derived from in vitro assays that measure chemical interactions with thyroid molecular targets to downstream events and adverse outcomes traditionally derived from in vivo testing. We examine the role of new in vitro technologies, in the context of the AOP network, in facilitating consideration of several important regulatory and biological challenges in characterizing chemicals that exert effects through a thyroid mechanism.

DISCUSSION

There is a substantial body of knowledge describing chemical effects on molecular and physiological regulation of TH signaling and associated adverse outcomes. Until recently, few alternative nonanimal assays were available to interrogate chemical effects on TH signaling. With the development of these new tools, screening large libraries of chemicals for interactions with molecular targets of the thyroid is now possible. Measuring early chemical interactions with targets in the thyroid pathway provides a means of linking adverse outcomes, which may be influenced by many biological processes, to a thyroid mechanism. However, the use of in vitro assays beyond chemical screening is complicated by continuing limits in our knowledge of TH signaling in important life stages and tissues, such as during fetal brain development. Nonetheless, the thyroid AOP network provides an ideal tool for defining causal linkages of a chemical exerting thyroid-dependent effects and identifying research needs to quantify these effects in support of regulatory decision making. https://doi.org/10.1289/EHP5297.

High-throughput screening and chemotype-enrichment analysis of ToxCast phase II chemicals evaluated for human sodium-iodide symporter (NIS) inhibition

In support of the Endocrine Disruptor Screening Program (EDSP), the U.S.EPA's Office of Research and Development (ORD) is developing high-throughput screening (HTS) approaches to identify chemicals that alter target sites in the thyroid hormone (TH) pathway. The sodium iodide symporter (NIS) is a transmembrane glycoprotein that mediates iodide uptake into the thyroid as the initial step of TH biosynthesis. Previously, we screened 293 ToxCast chemicals (ph1v2) using a HEK293T cell line expressing human NIS in parallel radioactive iodide uptake (RAIU) and cell viability assays to identify potential environmental NIS inhibitors. Here, we expanded NIS inhibitor screening for a set of 768 ToxCast Phase II (ph2) chemicals, and applied a novel computational toxicology approach based on the ToxPrint chemotype to identify chemical substructures associated with NIS inhibition. Following single-concentration screening (at 1 × 10-4 M with a 20% inhibition cutoff), 235 samples (228 chemicals) were further tested in multiple-concentration (1 × 10-9 - 1 × 10-4 M) format in both RAIU and cell viability assays. The 167 chemicals that exhibited significant RAIU inhibition were then prioritized using combined RAIU and cell viability responses that were normalized relative to the known NIS inhibitor sodium perchlorate. Some of the highest ranked chemicals, such as PFOS, tributyltin chloride, and triclocarban, have been previously reported to be thyroid disruptors. In addition, several novel chemicals were identified as potent NIS inhibitors. The present results were combined with the previous ph1v2 screening results to produce two sets of binary hit-calls for 1028 unique chemicals, consisting of 273 positives exhibiting significant RAIU inhibition, and 63 positives following application of a cell viability filter. A ToxPrint chemotype-enrichment analysis identified >20 distinct chemical substructural features, represented in >60% of the active chemicals, as significantly enriched in each NIS inhibition hit-call space. A shared set of 9 chemotypes enriched in both hit-call sets indicates stable chemotype signals (insensitive to cytotoxicity filters) that can help guide structure-activity relationship (SAR) investigations and inform future research.

Developmental Thyroid Hormone Insufficiency Induces a Cortical Brain Malformation and Learning Impairments: A Cross-Fostering Study

Thyroid hormones (THs) are essential for brain development, but few rodent models exist that link TH inefficiency to apical neurodevelopmental endpoints. We have previously described a structural anomaly, a heterotopia, in the brains of rats treated in utero with propylthiouracil (PTU). However, how the timing of an exposure relates to this birth defect is unknown. This study seeks to understand how various temporal treatments of the mother relates to TH insufficiency and adverse neurodevelopment of the offspring. Pregnant rats were exposed to PTU (0 or 3 ppm) through the drinking water from gestational day 6 until postnatal day (PN) 14. On PN2 a subset of pups was cross-fostered to a dam of the opposite treatment, to create 4 conditions: pups exposed to PTU prenatally, postnatally, during both periods, or not at all (control). Both PTU and TH concentrations were characterized in the mother and offspring over time, to capture the dynamics of a developmental xenobiotic exposure. Brains of offspring were examined for heterotopia presence and severity, and adult littermates were assessed for memory impairments. Heterotopia were observed under conditions of prenatal exposure, and its severity increased in animals in the most prolonged exposure group. This malformation was also permanent, but not sex biased. In contrast, behavioral impairments were limited to males, and only in animals exposed to PTU during both the gestational and postnatal periods. This suggests a distinct TH-dependent etiology for both phenotypes, and illustrates how timing of hypothyroxinemia can induce abnormal brain structure and function.

Assessment of the cytotoxicity, antioxidant activity and chemical composition of extracts from the cyanobacterium Fischerella major Gomont

Cyanoprokaryotes (Cyanobacteria/Cyanophyta) are ancient photosynthetic prokaryotic organisms with cosmopolitan distribution. They are producers of a number of biologically active substances with antitumor and antifungal activity, vitamins, antibiotics, algaecides, insecticides, repellents, hormones, immunosuppressants and toxins. So far, the cyanobacterium Fischerella major Gomont has not been studied regarding its impact on the environment and human health. In this study, the cytotoxic, antioxidant and antitumor activities of four extracts prepared from Fischerella major were evaluated in vitro. In addition, the total phenolic content and the potential for production of cyanotoxins were also analyzed. The conducted GC/MS analysis identified 45 compounds with different chemical nature and biological activity. Presence of microcystins and saxitoxins was detected in all Fischerella major extracts. In vitro testing on cell cultures showed a significant concentration- and time-dependent cytotoxic effect on all cell lines (HeLa, SK-Hep-1 and FL) treated at three exposure times (24, 48 and 72 h) with four extracts. A selective antitumor effect was not observed. This is the first study demonstrating biological activity of extracts from Fischerella major, which makes it an interesting subject for further research, including environmental risk assessments (as producer of cyanotoxins) or as a potential source of pharmaceuticals.

Screening the ToxCast Phase 1 Chemical Library for Inhibition of Deiodinase Type 1 Activity

Thyroid hormone (TH) homeostasis is dependent upon coordination of multiple key events including iodide uptake, hormone synthesis, metabolism, and elimination, to maintain proper TH signaling. Deiodinase enzymes catalyze iodide release from THs to interconvert THs between active and inactive forms, and are integral to hormone metabolism. The activity of deiodinases has been identified as an important endpoint to include in the context of screening chemicals for TH disruption. To begin to address the potential for chemicals to inhibit these enzymes an adenovirus expression system was used to produce human deiodinase type 1 (DIO1) enzyme, established robust assay parameters for nonradioactive determination of iodide release by the Sandell-Kolthoff method, and employed a 96-well plate format for screening chemical libraries. An initial set of 18 chemicals was used to establish the assay, along with the known DIO1 inhibitor 6-propylthiouracil as a positive control. An additional 292 unique chemicals from the EPA's ToxCast phase 1_v2 chemical library were screened. Chemicals were initially screened at a single high concentration of 200 µM to identify potential DIO1 inhibitors. There were 50 chemicals, or 17% of the TCp1_v2 chemicals tested, that produced >20% inhibition of DIO1 activity. Eighteen of these inhibited DIO1 activity >50% and were further tested in concentration-response mode to determine IC50s. This work presents an initial effort toward identifying chemicals with potential for affecting THs via inhibition of deiodinases and sets the foundation for further testing of large chemical libraries against DIO1 and the other deiodinase enzymes involved in TH function.

Metal-free C-H mercaptalization of benzothiazoles and benzoxazoles using 1,3-propanedithiol as thiol source

A facile and effective C-H functionalization strategy for the synthesis of 2-mercaptobenzothiazoles and 2-mercaptobenzoxazoles is described. 1,3-Propanedithiol was employed to convert benzothiazoles and benzoxazoles to the corresponding heteroarylthiols in the presence of potassium hydroxide and DMSO. This novel protocol is featured by direct C-H mercaptalization of heteroarenes and a simple reaction system.

Profiles, variability, and predictors of urinary benzotriazoles and benzothiazoles in pregnant women from Wuhan, China

BACKGROUND

Benzotriazoles (BTRs) and benzothiazoles (BTHs) are emerging contaminants with high production volume worldwide, which exhibit potential health risk to human. To date, little is known about the exposure of BTRs and BTHs (BTs) on human, especially in the context of pregnancy.

OBJECTIVES

We aimed to characterize the exposure profiles, temporal variability, and potential predictors of urinary BTs during pregnancy.

METHODS

Between 2014 and 2015, we recruited 856 pregnant women in Wuhan who provided urine samples at three trimesters (13.1 ± 1.1, 23.7 ± 3.2, and 35.7 ± 3.4 gestational weeks). We measured the urinary concentrations of five BTRs (1‑H‑benzotriazole, 1‑hydroxy‑benzotriazole, xylyltriazole, tolyltriazole, 5‑chloro‑1‑H‑benzotriazole) and five BTHs (benzothiazole, 2‑hydroxy‑benzothiazole, 2‑methylthio‑benzothiazole, 2‑amino‑benzothiazole, 2‑thiocyanomethylthio‑benzothiazole) to characterize the exposure profiles of BTs. We calculated the intra-class correlation coefficients (ICCs) to assess the temporal variability and investigated potential predictors of urinary BTs by using the mixed models.

RESULTS

Most of the targeted BTs were detected in over 50% of urine samples, except for 5‑chloro‑1‑H‑benzotriazole (9.3%) and 2‑thiocyanomethylthio-benzothiazole (1.4%). The predominant BTRs in urine was 1‑hydroxy‑benzotriazole [Geometric Mean (GM): 0.77 ng/mL]. Benzothiazole was the major derivative in urine samples with a GM concentration of 1.6 ng/mL. Correlations among BTHs (r = 0.04-0.39) were higher than that among BTRs (r = 0.02-0.14). The exposure pattern was constant at low level and co-exposure to all the targeted compounds was infrequent during pregnancy. Urinary concentrations of BTRs exhibited considerable within-subject variation (ICCs: 0.12-0.56) during pregnancy. Relatively high temporal reliability was observed for urinary concentrations of BTHs with ICCs ranging from 0.42 to 0.85. It was found that parity, household income, pregnancy occupational status, sampling season and menstrual cycle were associated with urinary concentrations of BTs in pregnant women (P < 0.05).

CONCLUSIONS

To the best of our knowledge, this is the first study to report the exposure profiles, variability and predictors of urinary BTs among pregnant women. Exposure assessment using multiple samples is essential in reducing measurement errors and identifying susceptible window of exposure in etiological studies. The potential predictors of urinary BTs raised concerns on tracing exposure routes and eliminating confounding variables in future studies.

A Review of Environmental Occurrence, Fate, Exposure, and Toxicity of Benzothiazoles

Benzothiazole and its derivatives (BTs) are high production volume chemicals that have been used for several decades in a large number of industrial and consumer products, including vulcanization accelerators, corrosion inhibitors, fungicides, herbicides, algicides, and ultraviolet (UV) light stabilizers. Several benzothiazole derivatives are used commercially, and widespread use of these chemicals has led to ubiquitous occurrence in diverse environmental compartments. BTs have been reported to be dermal sensitizers, respiratory tract irritants, endocrine disruptors, carcinogens, and genotoxicants. This article reviews occurrence and fate of a select group of BTs in the environment, as well as human exposure and toxicity. BTs have frequently been found in various environmental matrices at concentrations ranging from sub-ng/L (surface water) to several tens of μg/g (indoor dust). The use of BTs in a number of consumer products, especially in rubber products, has resulted in widespread human exposure. BTs undergo chemical, biological, and photolytic degradation in the environment, creating several transformation products. Of these, 2-thiocyanomethylthio-benzothiazole (2-SCNMeS-BTH) has been shown to be the most toxic. Epidemiological studies have shown excess risks of cancers, including bladder cancer, lung cancer, and leukemia, among rubber factory workers, particularly those exposed to 2-mercapto-benzothiazole (2-SH-BTH). Human exposure to BTs continues to be a concern.

High-Throughput Screening and Quantitative Chemical Ranking for Sodium-Iodide Symporter Inhibitors in ToxCast Phase I Chemical Library

Thyroid uptake of iodide via the sodium-iodide symporter (NIS) is the first step in the biosynthesis of thyroid hormones that are critical for health and development in humans and wildlife. Despite having long been a known target of endocrine disrupting chemicals such as perchlorate, information regarding NIS inhibition activity is still unavailable for the vast majority of environmental chemicals. This study applied a previously validated high-throughput approach to screen for NIS inhibitors in the ToxCast phase I library, representing 293 important environmental chemicals. Here 310 blinded samples were screened in a tiered-approach using an initial single-concentration (100 μM) radioactive-iodide uptake (RAIU) assay, followed by 169 samples further evaluated in multi-concentration (0.001 μM-100 μM) testing in parallel RAIU and cell viability assays. A novel chemical ranking system that incorporates multi-concentration RAIU and cytotoxicity responses was also developed as a standardized method for chemical prioritization in current and future screenings. Representative chemical responses and thyroid effects of high-ranking chemicals are further discussed. This study significantly expands current knowledge of NIS inhibition potential in environmental chemicals and provides critical support to U.S. EPA's Endocrine Disruptor Screening Program (EDSP) initiative to expand coverage of thyroid molecular targets, as well as the development of thyroid adverse outcome pathways (AOPs).

Impaired swim bladder inflation in early life stage fathead minnows exposed to a deiodinase inhibitor, iopanoic acid

Inflation of the posterior and/or anterior swim bladder is a process previously demonstrated to be regulated by thyroid hormones. We investigated whether inhibition of deiodinases, which convert thyroxine (T4) to the more biologically active form, 3,5,3'-triiodothyronine (T3), would impact swim bladder inflation. Two experiments were conducted using a model deiodinase inhibitor, iopanoic acid (IOP). First, fathead minnow embryos were exposed to 0.6, 1.9, or 6.0 mg/L or control water until 6 d postfertilization (dpf), at which time posterior swim bladder inflation was assessed. To examine anterior swim bladder inflation, a second study was conducted with 6-dpf larvae exposed to the same IOP concentrations until 21 dpf. Fish from both studies were sampled for T4/T3 measurements and gene transcription analyses. Incidence and length of inflated posterior swim bladders were significantly reduced in the 6.0 mg/L treatment at 6 dpf. Incidence of inflation and length of anterior swim bladder were significantly reduced in all IOP treatments at 14 dpf, but inflation recovered by 18 dpf. Throughout the larval study, whole-body T4 concentrations increased and T3 concentrations decreased in all IOP treatments. Consistent with hypothesized compensatory responses, deiodinase-2 messenger ribonucleic acid (mRNA) was up-regulated in the larval study, and thyroperoxidase mRNA was down-regulated in all IOP treatments in both studies. These results support the hypothesized adverse outcome pathways linking inhibition of deiodinase activity to impaired swim bladder inflation. Environ Toxicol Chem 2017;36:2942-2952. Published 2017 Wiley Periodicals Inc. on behalf of SETAC. This article is a US government work and, as such, is in the public domain in the United States of America.

Neurodevelopment and Thyroid Hormone Synthesis Inhibition in the Rat: Quantitative Understanding Within the Adverse Outcome Pathway Framework

Adequate levels of thyroid hormone (TH) are needed for proper brain development, deficiencies may lead to adverse neurologic outcomes in humans and animal models. Environmental chemicals have been linked to TH disruption, yet the relationship between developmental exposures and decline in serum TH resulting in neurodevelopmental impairment is poorly understood. The present study developed a quantitative adverse outcome pathway where serum thyroxin (T4) reduction following inhibition of thyroperoxidase in the thyroid gland are described and related to deficits in fetal brain TH and the development of a brain malformation, cortical heterotopia. Pregnant rats were exposed to 6-propylthiouracil (PTU 0, 0.1, 0.5, 1, 2, or 3 parts per million [ppm]) from gestational days 6-20, sequentially increasing PTU concentrations in maternal thyroid gland and serum as well as in fetal serum. Dams exposed to 0.5 ppm PTU and higher exhibited dose-dependent decreases in thyroidal T4. Serum T4 levels in the dam were significantly decreased with exposure to 2 and 3 ppm PTU. In the fetus, T4 decrements were first observed at a lower dose of 0.5 ppm PTU. Based on these data, fetal brain T4 levels were estimated from published literature sources, and quantitatively linked to increases in the size of the heterotopia present in the brains of offspring. These data show the potential of in vivo assessments and computational descriptions of biologic responses to predict the development of this structural brain malformation and use of quantitative adverse outcome pathway approach to evaluate brain deficits that may result from exposure to other TH disruptors.

Occurrence of and exposure to benzothiazoles and benzotriazoles from textiles and infant clothing

Benzothiazoles (BTHs) and benzotriazoles (BTRs) are used in a wide range of applications, including rubber vulcanization and corrosion inhibition. Limited studies have reported the occurrence of BTHs and BTRs in textiles, including children's clothing. In this study, 79 textile samples (raw as well as tailored) collected in Albany, New York, USA, were analyzed to determine the occurrence of BTH, BTR and their seven common derivatives. BTH, 2-methylthio-benzothiazole (2-Me-S-BTH) and 2-hydroxy-benzothiazole (2-OH-BTH) were found in textiles at a detection rate (DR) of 86%, 54% and 19%, respectively. The DRs of tolyltriazole (TTR), BTR and 5-chloro-benzotriazole (5-Cl-BTR) in textiles were below 20%. Although BTH was the most frequently detected compound, BTR levels were elevated in certain textiles and the overall mean concentrations of BTR in textiles were higher than those of BTH. The concentrations of BTH in textiles ranged from 6.1 to 1120ng/g. The highest concentration of BTR (14,000ng/g) was found in a printed graphic of infant's bodysuit. On the basis of the measured concentrations, we calculated dermal exposure doses to BTHs and BTRs by infants. The dermal exposure doses were high from the use of socks (244 to 395pg/kg·bw/d), and the exposure doses of BTHs and BTRs from textiles were as high as 3740pg/kg·bw/d. Printed graphics on clothes, as well as socks, accounted for a major proportion of the exposure doses to BTHs and BTRs.

An "EAR" on Environmental Surveillance and Monitoring: A Case Study on the Use of Exposure-Activity Ratios (EARs) to Prioritize Sites, Chemicals, and Bioactivities of Concern in Great Lakes Waters

Current environmental monitoring approaches focus primarily on chemical occurrence. However, based on concentration alone, it can be difficult to identify which compounds may be of toxicological concern and should be prioritized for further monitoring, in-depth testing, or management. This can be problematic because toxicological characterization is lacking for many emerging contaminants. New sources of high-throughput screening (HTS) data, such as the ToxCast database, which contains information for over 9000 compounds screened through up to 1100 bioassays, are now available. Integrated analysis of chemical occurrence data with HTS data offers new opportunities to prioritize chemicals, sites, or biological effects for further investigation based on concentrations detected in the environment linked to relative potencies in pathway-based bioassays. As a case study, chemical occurrence data from a 2012 study in the Great Lakes Basin along with the ToxCast effects database were used to calculate exposure-activity ratios (EARs) as a prioritization tool. Technical considerations of data processing and use of the ToxCast database are presented and discussed. EAR prioritization identified multiple sites, biological pathways, and chemicals that warrant further investigation. Prioritized bioactivities from the EAR analysis were linked to discrete adverse outcome pathways to identify potential adverse outcomes and biomarkers for use in subsequent monitoring efforts.

Mass loading and removal of benzotriazoles, benzothiazoles, benzophenones, and bisphenols in Indian sewage treatment plants

Little is known about the occurrence of emerging environmental contaminants, such as benzotriazoles (BTRs), benzothiazoles (BTHs), benzophenones (BzPs), and bisphenol analogues (BPs) in India. In this study, we determined the occurrence and removal of BTRs, BTHs, BzPs, and BPs in five Indian sewage treatment plants (STPs). The respective measured mean concentrations (N = 5) in influents and effluents were 370 and 57.4 ng L^-1 for BTRs, 50800 and 20200 ng L^-1 for BTHs, 351 and 163 ng L^-1 for BzPs, and 98.0 and 9.6 ng L^-1 for BPs. Among the target chemicals analyzed, BTHs were found at elevated levels, and the measured levels were some of the highest ever reported in the literature. The mean concentrations (N = 5) of BTRs, BTHs, BzPs, and BPs in sludge were 44.2, 51200, 124, and 200 ng g^-1 dry wt, respectively. The removal efficiencies for BTRs, BTHs, BzPs, and BPs ranged as follows: 54.2-85.6%, 23.4-85.0%, 51.8-71%, and 76.0-97.0%, respectively, and were comparable to those reported for other countries. Elevated concentrations of BTHs in Indian STPs can be related to consumption of these chemicals in a wide range of products including paper, textile and rubber materials.

Benzothiazoles in indoor air from Albany, New York, USA, and its implications for inhalation exposure

Benzothiazole and its derivatives (collectively referred to BTHs) are used widely in many consumer (e.g., textiles) and industrial (e.g., rubber) products. Very little is known about the occurrence of BTHs in indoor air and the inhalation exposure of humans to these substances. In this study, 81 indoor air samples collected from various locations in Albany, New York, USA, in 2014 were analyzed for BTHs by high-performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS). BTHs were found in all indoor air samples, and the overall concentrations in bulk air (vapor plus particulate phases) were in the range of 4.36-2229 ng/m(3) (geometric mean: 32.7 ng/m(3)). The highest concentrations (geometric mean: 148 ng/m(3)) were found in automobiles, followed by homes (49.5)>automobile garages (46.0)>public places, e.g., shopping malls (24.2)>barbershops (18.9) >offices (18.8)>laboratories (15.1). The estimated geometric mean daily intake (EDI) of BTHs for infants, toddlers, children, teenagers, and adults through indoor air inhalation from homes was 27.7, 26.3, 17.9, 10.5, and 7.77 ng/kg-bw/day, respectively. The estimated contribution of indoor air to total BTHs intake was approximately 10%. This is the first study on the occurrence of BTHs in indoor air.