2,4,6-Tribromophenol Disposition and Kinetics in Pregnant and Nursing Sprague Dawley Rats

2,4,6-Tribromophenol (TBP, CAS no. 118-79-6) is a brominated chemical used as a precursor, flame retardant, and wood antifungal agent. TBP is detected in environmental matrices and biota, including human breast milk, placenta, and serum. To address reports of TBP accumulation in human placenta and breast milk, studies were conducted to characterize TBP disposition and toxicokinetics in timed-pregnant or nursing Sprague Dawley rats following a single oral dose to the dam. Animals were administered [14C]-TBP (10 μmol/kg, 25 µCi/kg, 4 ml/kg) by gavage on gestation day 12 and 20, or postnatal day 12 and serially euthanized between 15 min and 24 h for collection of blood and tissues from the dam and fetuses/pups. Observed plasma TBP Cmax (3 and 7 nmol/ml) occurred at 15 min in both GD12 and GD20 dams while Cmax (3 nmol/ml) was observed at 30 min for PND12 dams. Concentrations in tissues followed plasma concentrations, with kidneys containing the highest concentrations at 30 min. GD12 litters contained a sustained 0.2%-0.3% of the dose (5-9 nmol/litter) between 15 min and 6 h while GD20 fetuses (2%-3%) and placentas (0.3%-0.5%) had sustained levels between 30 min and 12 h. The stomach contents (approx. 1 nmol-eq/g, 6-12 h), livers (0.04-0.1 nmol-eq/g) and kidneys (0.1-0.2 nmol-eq/g) of PND12 pups increased over time, indicating sustained exposure via milk. Systemic exposure to TBP and its metabolites occurs in both the directly exposed mother and the indirectly exposed offspring and is rapid and persistent after a single dose in pregnant and nursing rats.

Effect of exposure to 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) and polychlorinated biphenyls (PCBs) on mitochondrial DNA (mtDNA) copy number in rats

Mitochondria are intracellular organelles responsible for biological oxidation and energy production. These organelles are susceptible to damage from oxidative stress and compensate for damage by increasing the number of copies of their own genome, mitochondrial DNA (mtDNA). Cancer and environmental exposure to some pollutants have also been associated with altered mtDNA copy number. Since exposures to polychlorinated biphenyls (PCBs) and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) have been shown to increase oxidative stress, we hypothesize that mtDNA copy number will be altered with exposure to these compounds. mtDNA copy number was measured in DNA from archived frozen liver and lung specimens from the National Toxicology Program (NTP) study of female Harlan Sprague Dawley rats exposed to TCDD (3, 10, or 100 ng/kg/day), dioxin-like (DL) PCB 126 (10, 100, or 1000 ng/kg/day), non-DL PCB 153 (10, 100, or 1000 μg/kg/day), and PCB 126 + PCB 153 (10 ng/kg/day + 10 μg/kg/day, 100 ng/kg/day + 100 μg/kg/day, or 1000 ng/kg/day + 1000 μg/kg/day, respectively) for 13 and 52 weeks. An increase in mtDNA copy number was observed in the liver and lung of rats exposed to TCDD and the lung of rats exposed to the mixture of PCB 126 and PCB 153. A statistically significant positive dose-dependent trend was also observed in the lung of rats exposed to PCB 126 and a mixture of PCB 153 and PCB 126, although in neither case was the control copy number significantly exceeded at any dose level. These exposures produced a range of pathological responses in these organs in the two-year NTP studies. Conversely, there was a significant decrease or no change in mtDNA copy number in the liver and lung of rats exposed to non-DL PCB 153. This is consistent with a general lack of PCB 153 mediated liver or lung injury in the NTP study, with the exception of liver hypertrophy. Together, the results suggest that an increase in mtDNA copy number may serve as a sensitive, early biomarker of mitochondrial injury and oxidative stress that contributes to the development of the toxicity of dioxin-like compounds.

FutureTox IV Workshop Summary: Predictive Toxicology for Healthy Children

FutureTox IV, a Society of Toxicology Contemporary Concepts in Toxicology workshop, was held in November 2018. Building upon FutureTox I, II, and III, this conference focused on the latest science and technology for in vitro profiling and in silico modeling as it relates to predictive developmental and reproductive toxicity (DART). Publicly available high-throughput screening data sets are now available for broad in vitro profiling of bioactivities across large inventories of chemicals. Coupling this vast amount of mechanistic data with a deeper understanding of molecular embryology and post-natal development lays the groundwork for using new approach methodologies (NAMs) to evaluate chemical toxicity, drug efficacy, and safety assessment for embryo-fetal development. NAM is a term recently adopted in reference to any technology, methodology, approach, or combination thereof that can be used to provide information on chemical hazard and risk assessment to avoid the use of intact animals (U.S. Environmental Protection Agency [EPA], Strategic plan to promote the development and implementation of alternative test methods within the tsca program, 2018, https://www.epa.gov/sites/production/files/2018-06/documents/epa_alt_strat_plan_6-20-18_clean_final.pdf). There are challenges to implementing NAMs to evaluate chemicals for developmental toxicity compared with adult toxicity. This forum article reviews the 2018 workshop activities, highlighting challenges and opportunities for applying NAMs for adverse pregnancy outcomes (eg, preterm labor, malformations, low birth weight) as well as disorders manifesting postnatally (eg, neurodevelopmental impairment, breast cancer, cardiovascular disease, fertility). DART is an important concern for different regulatory statutes and test guidelines. Leveraging advancements in such approaches and the accompanying efficiencies to detecting potential hazards to human development are the unifying concepts toward implementing NAMs in DART testing. Although use of NAMs for higher level regulatory decision making is still on the horizon, the conference highlighted novel testing platforms and computational models that cover multiple levels of biological organization, with the unique temporal dynamics of embryonic development, and novel approaches for estimating toxicokinetic parameters essential in supporting in vitro to in vivo extrapolation.

Differential Receptor Tyrosine Kinase Phosphorylation in the Uterus of Rats Following Developmental Exposure to Tetrabromobisphenol A

Tetrabromobisphenol A (TBBPA) is a brominated flame retardant that induces endometrial adenocarcinoma and other uterine tumors in Wistar Han rats; however, early molecular events or biomarkers of TBBPA exposure remain unknown. We investigated the effects of TBBPA on growth factor receptor activation (phospho-RTK) in uteri of rats following early-life exposures. Pregnant Wistar Han rats were exposed to TBBPA (0, 0.1, 25, 250 mg/kg/day) via oral gavage on gestation day 6 through weaning of pups (PND 21). Pups were exposed in utero, through lactation, and by daily gavage from PND 22 to PND 90. Uterine horns were collected (at PND 21, PND 33, PND 90) and formalin-fixed or frozen for histologic, immunohistochemical, phospho-RTK arrays, or western blot analysis. At PND 21, the phosphor-RTKs, FGFR2, FGFR3, TRKC and EPHA1 were significantly increased at different treatment concentrations. Several phospho-RTKs were also significantly overexpressed at PND 33 which included epithelial growth factor receptor (EGFR), Fibroblast Growth Factor Receptor 3-4 (FGFR2, FGFR3, FGFR4), insulin-like growth factor receptor 1 (IGF1R), INSR, AXL, MERTK, PDGFRa and b, RET, Tyrosine Kinase with Immunoglobulin Like and EGF Like Domains 1 and 2 (TIE1; TIE2), TRKA, VEGFR2 and 3, and EPHA1 at different dose treatments. EGFR, an RTK overexpressed in endometrial cancer in women, remained significantly increased for all treatment groups at PND 90. Erb-B2 Receptor Tyrosine Kinase 2 (ERBB2) and IGF1R were overexpressed at PND 33 and remained increased through PND 90, although ERBB2 was statistically significant at PND 90. The phospho-RTKs, FGFR3, AXL, DTK, HGFR, TRKC, VEGFR1 and EPHB2 and 4 were also statistically significant at PND 90 at different dose treatments. The downstream effector, phospho-MAPK44/42 was also increased in uteri of treated rats. Our findings show RTKs are dysregulated following early life TBBPA exposures and their sustained activation may contribute to TBBPA-induced uterine tumors observed in rats later in life.

Dioxin-like compound exposures and DNA methylation in the Anniston Community Health Survey Phase II

The Anniston Community Health Survey (ACHS-I) was initially conducted from 2005 to 2007 to assess polychlorinated biphenyl (PCB) exposures in Anniston, Alabama residents. In 2014, a follow-up study (ACHS-II) was conducted to measure the same PCBs as in ACHS-I and additional compounds e.g., polychlorinated dibenzo-p-dioxins (PCDDs), polychlorinated dibenzofurans (PCDFs), and dioxin-like non-ortho (cPCBs) substituted PCBs. In this epigenome-wide association study (EWAS), we examined the associations between PCDD, PCDF, and PCB exposures and DNA methylation. Whole blood DNA methylation was measured using Illumina EPIC arrays (n=292). We modeled lipid-adjusted toxic equivalencies (TEQs) for: ΣDioxins (sum of 28 PCDDs, PCDFs, cPCBs, and mPCBs), PCDDs, PCDFs, cPCBs, and mPCBs using robust multivariable linear regression adjusting for age, race, sex, smoking, bisulfite conversion batch, and estimated percentages of six blood cell types. Among all exposures we identified 10 genome-wide (Bonferroni p≤6.74E-08) and 116 FDR (p≤5.00E-02) significant associations representing 10 and 113 unique CpGs, respectively. Of the 10 genome-wide associations, seven (70%) occurred in the PCDDs and four (40%) of these associations had an absolute differential methylation ≥1.00%, based on the methylation difference between the highest and lowest exposure quartiles. Most of the associations (six, 60%) represented hypomethylation changes. Of the 10 unique CpGs, eight (80%) were in genes shown to be associated with dioxins and/or PCBs based on data from the 2019 Comparative Toxicogenomics Database. In this study, we have identified a set of CpGs in blood DNA that may be particularly susceptible to dioxin, furan, and dioxin-like PCB exposures.

Pharmacokinetics of bisphenol A in humans following dermal administration

BACKGROUND

Human exposures to bisphenol A (BPA) are widespread. The current study addresses uncertainties regarding human pharmacokinetics of BPA following dermal exposure.

OBJECTIVE

To examine the absorption, distribution, metabolism and excretion of BPA in humans following dermal administration.

METHODS

We dermally administered deuterated BPA (d6-BPA) to 10 subjects (6 men and 4 women) at a dose of 100 µg/kg over a 12-hour period and conducted blood and urine analysis from the beginning of dosing through a three- or six-day period. We present time-course serum and urine concentrations of total and unconjugated ("free") d6-BPA and used this information to calculate terminal half-life and area under the curve.

RESULTS AND CONCLUSIONS

Detectable serum levels of total d6-BPA were observed at 1.4 h after the start of dosing, and a maximum serum concentration (C_max) of 3.26 nM was observed. Free d6-BPA was detectable in serum 2.8 h after start of dermal administration, with C_max of 0.272 nM. Beginning at approximately seven hours and continuing to 12 h (which corresponds to cessation of exposure), the concentration of free and total serum d6-BPA plateaued. The terminal half-lives of total d6-BPA and free d6-BPA in the body were 21.4 ± 9.81 h and 17.6 ± 7.69 h, respectively. Elimination from the body was rate-limited by kinetics in the dermal compartment. Free d6-BPA was a greater percentage of the area under the curve of total serum BPA (8.81%) compared to the 0.56% observed in our previously published oral study. Recovery of total d6-BPA in urine was <2% of the applied dose after six days. Analysis of the area under the curve for dermal and oral administration revealed that 2.2% of the dermal dose became systemically available. These data are in line with prior studies indicating how pharmacokinetics of BPA differ following oral and dermal exposures. Dermal exposure resulted in a longer apparent half-life and higher free:total d6-BPA ratio compared to oral.

Telomeres as targets for the toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and polychlorinated biphenyls (PCBs) in rats

Telomere length (TL) can be affected by various factors, including age and oxidative stress. Changes in TL have been associated with chronic disease, including a higher risk for several types of cancer. Environmental exposure of humans to PCBs and dioxins has been associated with longer or shorter leukocyte TL. Relative telomere length (RTL) may serve as a biomarker associated with neoplastic and/or non-neoplastic responses observed with chronic exposures to TCDD and PCBs. RTL was measured in DNA isolated from archived frozen liver and lung tissues from the National Toxicology Program (NTP) studies conducted in female Harlan Sprague Dawley rats exposed for 13, 30, and 52 weeks to TCDD, dioxin-like (DL) PCB 126, non-DL PCB 153, and a mixture of PCB 126 and PCB 153. RTL was assessed by quantitative polymerase chain reaction (qPCR). Consistent with literature, decreased liver and lung RTL was seen with aging. Relative to time-matched vehicle controls, RTL was increased in both the liver and lung tissues of rats exposed to TCDD, PCB 126, PCB 153, and the mixture of PCB 126 and PCB 153, which is consistent with most epidemiological studies that found PCB exposures were associated with increased leukocyte RTL. Increased RTL was observed at doses and/or time points where little to no pathology was observed. In addition to serving as a biomarker of exposure to these compounds in rats and humans, increases in RTL may be an early indicator of neoplastic and non-neoplastic responses that occur following chronic exposure to TCDD and PCBs.

Scientific Basis for Managing PFAS as a Chemical Class

This commentary presents a scientific basis for managing as one chemical class the thousands of chemicals known as PFAS (per- and polyfluoroalkyl substances). The class includes perfluoroalkyl acids, perfluoroalkylether acids, and their precursors; fluoropolymers and perfluoropolyethers; and other PFAS. The basis for the class approach is presented in relation to their physicochemical, environmental, and toxicological properties. Specifically, the high persistence, accumulation potential, and/or hazards (known and potential) of PFAS studied to date warrant treating all PFAS as a single class. Examples are provided of how some PFAS are being regulated and how some businesses are avoiding all PFAS in their products and purchasing decisions. We conclude with options for how governments and industry can apply the class-based approach, emphasizing the importance of eliminating non-essential uses of PFAS, and further developing safer alternatives and methods to remove existing PFAS from the environment.

Polychlorinated biphenyl exposure and DNA methylation in the Anniston Community Health Survey

Anniston, Alabama was home to a major polychlorinated biphenyl (PCB) production facility from 1929 until 1971. The Anniston Community Health Survey I and II (ACHS-I 2005-2007, ACHS-II 2013-2014) were conducted to explore the effects of PCB exposures. In this report we examined associations between PCB exposure and DNA methylation in whole blood using EPIC arrays (ACHS-I, n = 518; ACHS-II, n = 299). For both cohorts, 35 PCBs were measured in serum. We modelled methylation versus PCB wet-weight concentrations for: the sum of 35 PCBs, mono-ortho substituted PCBs, di-ortho substituted PCBs, tri/tetra-ortho substituted PCBs, oestrogenic PCBs, and antiestrogenic PCBs. Using robust multivariable linear regression, we adjusted for age, race, sex, smoking, total lipids, and six blood cell-type percentages. We carried out a two-stage analysis; discovery in ACHS-I followed by replication in ACHS-II. In ACHS-I, we identified 28 associations (17 unique CpGs) at p ≤ 6.70E-08 and 369 associations (286 unique CpGs) at FDR p ≤ 5.00E-02. A large proportion of the genes have been observed to interact with PCBs or dioxins in model studies. Among the 28 genome-wide significant CpG/PCB associations, 14 displayed replicated directional effects in ACHS-II; however, only one in ACHS-II was statistically significant at p ≤ 1.70E-04. While we identified many novel CpGs significantly associated with PCB exposures in ACHS-I, the differential methylation was modest and the effect was attenuated seven years later in ACHS-II, suggesting a lack of persistence of the associations between PCB exposures and altered DNA methylation in blood cells.

Synthetic Chemicals and Cardiometabolic Health Across the Life Course Among Vulnerable Populations: a Review of the Literature from 2018 to 2019

PURPOSE OF REVIEW

Although vulnerable populations are disproportionately exposed to synthetic chemicals with endocrine disrupting properties, few recent reviews have summarized the impact of synthetic chemicals on cardiometabolic health among these groups.

RECENT FINDINGS

Of 37 eligible epidemiological studies among vulnerable populations published between January 2018 and April 2019 in which over half were prospective, the most investigated populations were pregnant women and children. Racial/ethnic minorities, individuals of low socioeconomic status (SES), and those occupationally exposed were studied the least. The most studied persistent organic pollutants (POPs) were per-/poly-fluoroalkyl substances (PFAS), and the most studied non-POPs were phenols. Across chemical classes, studies found certain POPs (e.g., PFAS) and non-POPs (i.e., phenols, phthalates, and parabens) to be associated with gestational diabetes and dysregulated glucose metabolism. Results for other cardiometabolic health outcomes were inconsistent but suggested certain chemicals may negatively affect cardiometabolic health. Synthetic chemicals likely adversely affect cardiometabolic health, but current findings were inconclusive. Few recent studies focused on racial/ethnic minorities, low SES, and occupationally exposed populations. To address poor cardiometabolic health and related disparities, more studies across vulnerable populations are warranted.

Effect of GenX on P-Glycoprotein, Breast Cancer Resistance Protein, and Multidrug Resistance-Associated Protein 2 at the Blood-Brain Barrier

BACKGROUND

Ammonium 2,3,3,3-tetrafluoro-2-(heptafluoropropoxy)propanoic acid (GenX) is a replacement for perfluorooctanoic acid in the production of fluoropolymers used in a variety of consumer products. GenX alters fetal development and antibody production and elicits toxic responses in the livers and kidneys of rodents. The GenX effect on the blood-brain barrier (BBB) is unknown. The BBB protects the brain from xenobiotic neurotoxicants and harmful endogenous metabolites.

OBJECTIVES

We aimed to investigate the effects of GenX on the transport activity and expression of P-glycoprotein (P-gp), breast cancer resistance protein (BCRP), and multidrug resistance-associated protein 2 (MRP2) at the BBB.

METHODS

Transporter activities were measured in isolated rat brain capillaries by a confocal microscopy-based method. ATPase (enzymatic hydrolysis of adenosine triphosphate to inorganic phosphate) levels were measured in vitro. Western blotting determined P-gp and BCRP protein levels. Cell survival after GenX exposure was determined for two human cell lines.

RESULTS

Nanomolar levels of GenX inhibited P-gp and BCRP but not MRP2 transport activities in male and female rat brain capillaries. P-gp transport activity returned to control levels after GenX removal. GenX did not reduce P-gp- or BCRP-associated ATPase activity in an in vitro transport assay system. Reductions of P-gp but not BCRP transport activity were blocked by a peroxisome proliferator-activated receptor γ (PPAR γ ) antagonist. GenX reduced P-gp and BCRP transport activity in human cells.

CONCLUSION

In rats, GenX at 0.1 - 100   nM rapidly (in 1-2 h) inhibited P-gp and BCRP transport activities at the BBB through different mechanisms. PPAR γ was required for the GenX effects on P-gp but not BCRP transport activity. https://doi.org/10.1289/EHP5884.

Screening for Developmental Neurotoxicity at the National Toxicology Program: The Future Is Here

The National Toxicology Program (NTP) receives requests to evaluate chemicals with potential to cause adverse health effects, including developmental neurotoxicity (DNT). Some recent requests have included classes of chemicals such as flame retardants, polycyclic aromatic compounds, perfluoroalkyl substances, and bisphenol A analogs with approximately 20-50 compounds per class, many of which include commercial mixtures. However, all the compounds within a class cannot be tested using traditional DNT animal testing guideline studies due to resource and time limitations. Hence, a rapid and biologically relevant screening approach is required to prioritize compounds for further in vivo testing. Because neurodevelopment is a complex process involving multiple distinct cellular processes, one assay will unlikely address the complexity. Hence, the NTP sought to characterize a battery of in vitro and alternative animal assays to quantify chemical effects on a variety of neurodevelopmental processes. A culmination of this effort resulted in a NTP-hosted collaborative project with approximately 40 participants spanning across domains of academia, industry, government, and regulatory agencies; collaborators presented data on cell-based assays and alternative animal models that was generated using a targeted set of compounds provided by the NTP. The NTP analyzed the assay results using benchmark concentration (BMC) modeling to be able to compare results across the divergent assays. The results were shared with the contributing researchers on a private web application during the workshop, and are now publicly available. This article highlights the overview and goals of the project, and describes the NTP's approach in creating the chemical library, development of NTPs data analysis strategy, and the structure of the web application. Finally, we discuss key issues with emphasis on the utility of this approach, and knowledge gaps that need to be addressed for its use in regulatory decision making.

Hypertension in Relation to Dioxins and Polychlorinated Biphenyls from the Anniston Community Health Survey Follow-Up

BACKGROUND

In 2014, we conducted a longitudinal study [Anniston Community Health Survey (ACHS II)] 8 y after the baseline (ACHS I).

OBJECTIVES

We investigated the relationship between persistent chlorinated compounds and hypertension in residents living around the former polychlorinated biphenyl (PCB) production plant in Anniston, Alabama. We also examined the potential role of inflammatory cytokines in those with hypertension.

METHODS

A total of 338 participants had their blood pressure measured and medications recorded, gave a blood sample, and completed a questionnaire. Prevalent hypertension was defined as taking antihypertensive medication or having systolic blood pressure >140 mmHg and/or diastolic pressure >90 mmHg; incident hypertension used similar criteria in those who developed hypertension since the baseline in 2005-2007. PCB congeners were categorized into structure-activity groups, and toxic equivalencies (TEQs) were calculated for dioxin-like compounds. Descriptive statistics, logistic and linear regressions, as well as Cox proportional hazard models, were used to analyze the associations between exposures and hypertension.

RESULTS

Prevalent hypertension (78%) in ACHS II showed statistically significant adjusted odds ratios (ORs) for PCBs 74, 99, 138, 153, 167, 177, 183, and 187, ranging from 2.18 [95% confidence interval (CI): 1.10, 4.33] to 2.76 (95% CI: 1.14, 6.73), as well as for two estrogenic-like PCB groups, and the thyroid-like group [ORs ranging from 2.25 (95% CI: 1.07, 4.75) to 2.54 (95% CI: 1.13, 5.74)]. Furthermore, analysis of quartiles demonstrated a monotonic relationship for dioxin-like non-ortho (non-o)-PCB TEQs [fourth vs. first quartile: 3.66 (95% CI: 1.40, 9.56)]. Longitudinal analyses of incident hypertension supported those positive associations. The results were strongest for the di-o-PCBs [hazard ratio (HR)=1.93 (95% CI: 0.93, 4.00)] and estrogenic II PCB group [HR=1.90 (95% CI: 0.96, 3.78)] but were weaker for the dioxin TEQs.

DISCUSSION

Findings supportive of positive associations were reported for dioxin-like mono-o- and non-o-PCBs as well as for nondioxin-like estrogenic and thyroid-like congeners with prevalent and incident hypertension, suggesting that multiple pathways may be involved in hypertension development. https://doi.org/10.1289/EHP5272.

Sex-specific behavioral effects following developmental exposure to tetrabromobisphenol A (TBBPA) in Wistar rats

Tetrabromobisphenol A (TBBPA) has become a ubiquitous indoor contaminant due to its widespread use as an additive flame retardant in consumer products. Reported evidence of endocrine disruption and accumulation of TBBPA in brain tissue has raised concerns regarding its potential effects on neurodevelopment and behavior. The goal of the present study was to examine the impact of developmental TBBPA exposure, across a wide range of doses, on sexually dimorphic non-reproductive behaviors in male and female Wistar rats. We first ran a pilot study using a single TBBPA dose hypothesized to produce behavioral effects. Wistar rat dams were orally exposed using cookie treats to 0 or 0.1 mg TBBPA/kg bw daily from gestational day (GD) 9 to postnatal day (PND) 21 to assess offspring (both sexes) activity and anxiety-related behaviors. Significant effects were evident in females, with exposure increasing activity levels. Thus, this dose was used as the lowest TBBPA dose in a subsequent, larger study conducted as part of a comprehensive assessment of TBBPA toxicity. Animals were exposed to 0, 0.1, 25, or 250 mg TBBPA/kg bw daily by oral gavage starting on GD 6 through PND 90 (dosed dams GD 6 - PND 21, dosed offspring PND 22 - PND 90). Significant behavioral findings were observed for male offspring, with increased anxiety-like behavior as the primary phenotype. These findings demonstrate that exposure to environmental contaminants, like TBBPA, can have sex-specific effects on behavior highlighting the vulnerability of the developing brain.

2,4,6-Tribromophenol Exposure Decreases P-Glycoprotein Transport at the Blood-Brain Barrier

2,4,6-Tribromophenol (TBP, CAS No. 118-79-6) is a brominated chemical used in the production of flame-retardant epoxy resins and as a wood preservative. In marine environments, TBP is incorporated into shellfish and consumed by predatory fish. Food processing and water treatment facilities produce TBP as a byproduct. 2,4,6-Tribromophenol has been detected in human blood and breast milk. Biologically, TBP interferes with estrogen and thyroid hormone signaling, which regulate important transporters of the blood-brain barrier (BBB). The BBB is a selectively permeable barrier characterized by brain microvessels which are composed of endothelial cells mortared by tight-junction proteins. ATP-binding cassette (ABC) efflux transporters on the luminal membrane facilitate the removal of unwanted endobiotics and xenobiotics from the brain. In this study, we examined the in vivo and ex vivo effects of TBP on two important transporters of the BBB: P-glycoprotein (P-gp, ABCB1) and Multidrug Resistance-associated Protein 2 (MRP2, ABCC2), using male and female rats and mice. 2,4,6-Tribromophenol exposure ex vivo resulted in a time- (1-3 h) and dose- (1-100 nM) dependent decrease in P-gp transport activity. MRP2 transport activity was unchanged under identical conditions. Immunofluorescence and western blotting measured decreases in P-gp expression after TBP treatment. ATPase assays indicate that TBP is not a substrate and does not directly interact with P-gp. In vivo dosing with TBP (0.4 µmol/kg) produced decreases in P-gp transport. Co-treatment with selective protein kinase C (PKC) inhibitors prevented the TBP-mediated decreases in P-gp transport activity.

Tetrabromobisphenol A (TBBPA) Alters ABC Transport at the Blood-Brain Barrier

Tetrabromobisphenol A (TBBPA, CAS No. 79-94-7) is a brominated flame retardant used in 90% of epoxy coated circuit boards. Exposures to TBBPA can induce neurotoxicity and disrupt MAPK, estrogen, thyroid, and PPAR-associated signaling pathways. Because these pathways also regulate transporters of the central nervous system barriers, we sought to determine the effect of TBBPA on the expression and activity of 3 ATP binding cassette (ABC) transporters of the blood-brain barrier (BBB). Using a confocal based assay, we measured the ex vivo and in vivo effects of TBBPA on P-glycoprotein (P-gp), breast cancer resistant protein (BCRP), and multidrug resistance-associated protein 2 (MRP2) transport activity in rat brain capillaries. Our rationale for using a rat model was based on tissue availability, ease of handling, and availability of historical TBBPA toxicokinetic data. We found that TBBPA (1-1000 nM) exposure had no significant effect on multidrug resistance-associated protein 2 transport activity in either sex, suggesting TBBPA does not compromise the physical integrity of the BBB. However, low concentrations of TBBPA (1-100 nM) significantly decreased breast cancer resistant protein transport activity in both sexes. Additionally, TBBPA exposures (1-100 nM), elicited a sex-dependent response in P-gp transport: increasing transport activity in males and decreasing transport activity in females. All TBBPA dependent changes in transport activity were dose- and time-dependent. Inhibitors of either transcription or translation abolished the TBBPA dependent increases in male P-gp transport activity. Western blot and immunofluorescent assays confirmed the TBBPA dependent P-gp increases expression in males and decreases in females. Antagonizing PPAR-γ abolished the TBBPA dependent increases in males but not the decreases in females. However, the decreases in female P-gp transport were blocked by an ER-α antagonist. This work indicates that environmentally relevant concentrations of TBBPA (1-100 nM) alter ABC transporter function at the BBB. Moreover, permeability changes in the BBB can alter brain homeostasis, hinder central nervous system drug delivery, and increase the brain's exposure to harmful xenobiotic toxicants.

The Influence of Obesity on the Pharmacokinetics of Dioxin in Mice: An Assessment Using Classical and PBPK Modeling

The effects of body fat mass on the elimination of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) was examined in mice. When male C57BL/6J mice are fed a high-fat, simple carbohydrate diet (HFD) for 13 weeks, they develop an obese phenotype. In contrast, A/J mice fed an HFD do not become obese. After 13 weeks on a normal diet (ND) or HFD, male C57BL/6J and A/J mice received a single dose by gavage of 0.1 or 5.0 µg of 2,3,7,8-tetrachloro[1,6-3H] dibenzo-p-dioxin per kg body weight. Using classical pharmacokinetics, the blood elimination half-life of TCDD was approximately 10 and 2 times longer in the C57BL/6J on the HFD compared with the mice on the ND at 0.1 and 5.0 μg/kg doses, respectively. The diet did not increase the blood half-life of TCDD in the A/J mice, which did not get obese. Using a physiologically based pharmacokinetic model for TCDD that incorporated experimentally derived percent body fat mass and tissue partition coefficients, as well as data on hepatic sequestration, did not provide accurate predictions to the data and could not explain the increase in half-life of TCDD in the HFD groups. This work demonstrates that obesity influences the half-life of TCDD, but other undetermined factors are involved in its elimination because the increase in body fat mass, decreases in cytochrome P4501A2, and altered partition coefficients could not completely explain the prolonged half-life.

2,4,6-Tribromophenol Disposition and Kinetics in Rodents: Effects of Dose, Route, Sex, and Species

2,4,6-tribromophenol (TBP, CAS No. 118-79-6) is widely used as a brominated flame retardant and wood antifungal agent. TBP is frequently detected in environmental matrices, biota, and humans. In female SD rats, systemically available TBP (10 µmol/kg, IV) was rapidly excreted primarily via urine, with approximately 61% of the dose recovered after 4 h, and 89%-94% in 24 h; 5% was recovered in feces; and 1%-2% in blood/tissues. TBP administered to female SD rats (0.1-1000 µmol/kg) by gavage was well absorbed, with approximately 25% eliminated via urine after 4 h and approximately 88% after 24 h. Approximately 11% of a single oral dose was recovered in bile. Male SD rats and B6C3F1/J mice of both sexes had similar disposition profiles when administered a single oral dose of TBP (10 µmol/kg). Following administration, fecal recoveries varied only slightly by dose, sex, or species. TBP readily passed unchanged through both human (ex vivo only) and rat skin with between 55% and 85% of a 100 nmol/cm2 passing into or through skin. Concentrations of TBP in blood fit a two-compartment model after IV-dosing and a one-compartment model after oral dosing. Urine contained a mixture of TBP, TBP-glucuronide, and TBP-sulfate. Fecal extracts contained only parent TBP whereas bile contained only TBP-glucuronide. TBP did not appear to bioaccumulate or alter its own metabolism after repeated administration. TBP was readily absorbed at all doses and routes tested with an oral bioavailability of 23%-27%; 49% of TBP is expected to be dermally bioavailable in humans. From these data, we conclude that humans are likely to have significant systemic exposure when TBP is ingested or dermal exposure occurs.

Dermal disposition of Tetrabromobisphenol A Bis(2,3-dibromopropyl) ether (TBBPA-BDBPE) using rat and human skin

Tetrabromobisphenol A Bis(2,3-dibromopropyl) ether (TBBPA-BDBPE) is a high production volume brominated flame retardant (BFR) used in consumer products, resulting in ubiquitous human exposure. Although the major route of exposure for this chemical is believed to be via ingestion, dermal contact is likely via contaminated dust. Independent trials of a single dose of 100 nmol/cm2 (∼1 μCi [14C]/cm2) of [14C]-radiolabeled TBBPA-BDBPE was applied to whole rat skin (in vivo) or split-thickness human and rat skin (ex vivo) to estimate in vivo human percutaneous uptake. [14C]-radioactivity was quantified to determine dermal absorption (dose retained in dosed skin) and penetrance (dose recovered in receptor fluid [ex vivo] or tissues/excreta [in vivo]) over 24 h. In vivo absorption and penetration for rat skin was 26% and 1%, with a maximum flux of 44 ± 9 pmol/cm2/h. In ex vivo rat skin, absorption and penetration and absorption values were 23% and 0.3% (flux = 26 ± 8 pmol/cm2/h). In ex vivo human skin, 53% was absorbed and penetration was 0.2% with a maximal flux of 16 ± 12 pmol/cm2/h. Computed maximal flux for in vivo human skin was 21 ± 9 pmol/cm2/h with expected total absorption of ∼80% and a penetration of <1%. HPLC-radiometric analyses of samples showed that TBBPA-BDBPE was not metabolized in ex vivo or in vivo studies. These studies indicate that TBBPA-BDBPE is likely to be dermally bioavailable even after washing and dermal contact with this chemical should be considered an important route of exposure.