Corrigendum to "Toxicokinetics of perfluorobutane sulfonate (PFBS), perfluorohexane-1-sulphonic acid (PFHxS), and perfluorooctane sulfonic acid (PFOS) in male and female Hsd:Sprague Dawley SD rats after intravenous and gavage administration" [Toxicol. Rep. 6 (2019) 645-655].

[This corrects the article DOI: 10.1016/j.toxrep.2019.06.016.].

In vivo acute exposure to polychlorinated biphenyls: effects on free and total thyroxine in rats.

Hypothyroxinemia in rats has been well documented as a result of exposure to polychlorinated biphenyls (PCBs). Hypothetical mechanisms include induction of hepatic catabolic enzymes and cellular hormone transporters, and/or interference with plasma transport proteins. We hypothesized that if thyroxine displacement from transport proteins by PCBs occurs in vivo, it would result in increased free thyroxine (FT4). This study investigates the effects of a single oral dose of 2,2',4,4',5,5'-hexachlorobiphenyl (PCB 153 at 60 mg/kg) or 3,3',4,4',5,5'-hexachlorobiphenyl (PCB 169 at 1 mg/kg) on rats at 28 or 76 days of age. Total thyroxine (TT4) and FT4 were measured at 0.5, 1, 2, 4, 8, 24, or 48 hours post-dosing. Microsomal ethoxy- and pentoxy-resorufin-O-deethylase (EROD and PROD) activity and uridine diphosphoglucuronosyl transferase (UGT) activity were determined. No significant increase in TT4 or FT4 concentrations was seen at any time point. PCB 153 significantly decreased TT4 and FT4 in young and adult rats, with young rats showing a time-by-treatment interaction from 2 to 48 hours post-dosing in serum FT4. With PCB 169 exposure, young rats showed a decrease in FT4 only, whereas adult rats showed decreases in TT4 only. Hepatic EROD and PROD activities were both dramatically increased following PCB 169 and 153, respectively. Uridine diphosphoglucuronosyl transferase activity was increased only after PCB 169 exposure. These data demonstrate that neither PCB 153 nor PCB169 increased FT4, which supports the conclusion that these PCBs do not displace thyroxine from serum TTR, or if it does occur, there is no subsequent increase in serum FT4 in vivo.

Predictive modeling of a mixture of thyroid hormone disrupting chemicals that affect production and clearance of thyroxine.

Thyroid hormone (TH) disrupting compounds interfere with both thyroidal and extrathyroidal mechanisms to decrease circulating thyroxine (T(4)). This research tested the hypothesis that serum T(4) concentrations of rodents exposed to a mixture of both TH synthesis inhibitors (pesticides) and stimulators of T(4) clearance in the liver (polyhalogenated aromatic hydrocarbons, PHAHs) could be best predicted by an integrated addition model. Female Long-Evans rats, 23 days of age, were dosed with dilutions of a mixture of 18 PHAHs (2 dioxins, 4 dibenzofurans, and 12 PCBs, including dioxin-like and non-dioxin like PCBs) and a mixture of 3 pesticides (thiram, pronamide, and mancozeb) for four consecutive days. Serum was collected 24 hours after the last exposure and T(4) concentrations were measured by radioimmunoassay. Animals exposed to the highest dose of the mixture experienced a 45% decrease in serum T(4). Three additivity model predictions (dose addition, effect addition, and integrated addition) were generated based on single chemical data, and the results were compared. Effect addition overestimated the effect produced by the combination of all 21 chemicals. The results of the dose- and integrated-addition models were similar, and both provided better predictions than the effect-addition model. These results support the use of dose- and integrated additivity models in predicting the effects of complex mixtures.

Toxicokinetics of perfluorobutane sulfonate (PFBS), perfluorohexane-1-sulphonic acid (PFHxS), and perfluorooctane sulfonic acid (PFOS) in male and female Hsd:Sprague Dawley SD rats after intravenous and gavage administration.

Perfluorinated alkyl substances (PFAS) are persistent contaminants that have been detected in the environment and in humans. With the PFAS chemical class, there are perfluorinated alkyl acids, many of which have been associated with certain toxicities. Because toxicity testing cannot feasibly be conducted for each individual PFAS, the National Toxicology Program (NTP) designed studies to compare toxicities across different subclasses of PFAS and across PFAS of different chain lengths to better understand the structure-toxicity relationship. Pharmacokinetic studies were conducted in parallel to these toxicity studies to facilitate comparisons across PFAS and to provide context for human relevance. Here, the toxicokinetic parameters of perfluorobutane sulfonate (PFBS), perfluorohexane-1-sulphonic acid (PFHxS), or perfluorooctane sulfonate (PFOS) after a single intravenous or gavage administration in male and female Hsd:Sprague-Dawley rats are reported. Concentrations of these PFAS were measured in the liver, kidney, and brain. Plasma half-life increased with longer chain length after gavage administration: PFBS- males averaged 3.3 h, females 1.3 h; PFHxS- males averaged 16.3 days, females 2.1 days; PFOS- males and females averaged ˜ 20 days. There were dose-dependent changes in clearance and systemic exposure for all administered chemicals and the direction of change was different in PFOS compared to the others. Liver:plasma ratios of PFOS were the highest followed by PFHxS and PFBS, while brain:plasma ratios were low in all three sulfonates. Sex differences in plasma half-life and tissue distribution were observed for PFBS and PFHxS, but not PFOS. These data provide a direct comparison of the kinetics of three different perfluoroalkyl sulfonic acids and allow for the contextualization of toxicity data in rats for human risk assessment of this chemical class.

Toxicokinetics of 8:2 fluorotelomer alcohol (8:2-FTOH) in male and female Hsd:Sprague Dawley SD rats after intravenous and gavage administration.

Fluorotelomer alcohols (FTOHs) are used in the production of persistent per- and polyfluorinated alkyl substances (PFAS). Rodents and humans metabolize FTOHs to perfluoralkyl carboxylic acids which have several associated toxicities. Thus, understanding the toxicokinetics of these FTOHs and their metabolites will be useful for interpreting their toxicity for humans. Here, male and female Hsd:Sprague-Dawley SD rats were administered a single dose of 8:2-FTOH via gavage (males: 12, 24, 48 mg/kg; females: 40, 80, 160 mg/kg) or IV (males: 12 mg/kg; females: 40 mg/kg). Toxicokinetics of 8:2-FTOH and two primary metabolites, perfluorooctanoic acid (PFOA) and 7:3-fluorotelomer acid (7:3-FTA) were determined in plasma. Concentrations (total) of these chemicals were determined in the liver, kidney, and brain. There was rapid absorption and distribution of 8:2-FTOH after gavage administration in male rats. The plasma elimination half-life ranged from 1.1 to 1.7 hours. Kinetic parameters of 8:2-FTOH in females were similar to that in males. Bioavailability of 8:2-FTOH ranged from 22 to 41% for both sexes with no dose-dependent trends. 8:2-FTOH metabolites, PFOA and 7:3-FTA were detected in plasma following administration of the parent FTOH. Consistent with existing literature, the plasma half-life of PFOA was longer in males than in females (198-353 hours and 4.47-6.9 hours, respectively). The plasma half-life of 7:3-FTA was around 2-3 days in both sexes. 8:2-FTOH and 7:3-FTA were detected in all tissues; PFOA was found in the liver and kidney but not the brain. Detectable concentrations of metabolites persisted longer than the parent FTOH. These data demonstrate that in rats given a single gavage dose, 8:2-FTOH is rapidly absorbed, metabolized to form PFOA and 7:3-FTA, distributed to tissues, and eliminated faster than its metabolites. Sex differences were observed in the tissue distribution and elimination of PFOA, but not 8:2-FTOH and 7:3-FTA.

Carcinogenic activity of pentabrominated diphenyl ether mixture (DE-71) in rats and mice.

Pentabrominated diphenyl ether (PBDE) flame retardants have been phased out in Europe and in the United States, but these lipid soluble chemicals persist in the environment and are found human and animal tissues. PBDEs have limited genotoxic activity. However, in a 2-year cancer study of a PBDE mixture (DE-71) (0, 3, 15, or 50 mg/kg (rats); 0, 3, 30, or 100 mg/kg (mice)) there were treatment-related liver tumors in male and female Wistar Han rats [Crl:WI(Han) after in utero/postnatal/adult exposure, and in male and female B6C3F1 mice, after adult exposure. In addition, there was evidence for a treatment-related carcinogenic effect in the thyroid and pituitary gland tumor in male rats, and in the uterus (stromal polyps/stromal sarcomas) in female rats. The treatment-related liver tumors in female rats were unrelated to the AhR genotype status, and occurred in animals with wild, mutant, or heterozygous Ah receptor. The liver tumors in rats and mice had treatment-related Hras and Ctnnb mutations, respectively. The PBDE carcinogenic activity could be related to oxidative damage, disruption of hormone homeostasis, and molecular and epigenetic changes in target tissue. Further work is needed to compare the PBDE toxic effects in rodents and humans.

Inhibition of human and rat CYP1A2 by TCDD and dioxin-like chemicals.

Dioxins have been shown to bind and induce rodent CYP1A2, producing a dose-dependent hepatic sequestration in vivo. The induction of CYP1A2 activity has been used as a noninvasive biomarker for human exposure to dioxins; while there is a consistent relationship between exposure and hepatic CYP1A2 induction in rodents, this relationship has only been observed in some of the highest exposed human populations. This may be explained by inhibition of CYP1A2 activity by dioxins as some rodent studies demonstrate that rodent CYP1A2 activity can in fact be inhibited by dioxins in vitro. CYP1A2 activity was examined using a series of dioxins to inhibit human and rat CYP1A2 activity in species-specific CYP1A2 SUPERSOMES using three common CYP1A2 substrates. Methoxyresorufin was a more efficient substrate than acetanalide or caffeine in this in vitro system. Rat and human CYP1A2 enzymatic activity is inhibited by TCDD, PCDD, TCDF, 4-PeCDF, and PCBs 126, 169, 105, 118, and 156 in a concentration-dependent manner. These data demonstrate that the in vitro metabolism of prototype substrates is similar between the rat and human CYP1A2 SUPERSOME preparations and that dioxins inhibit CYP1A2 activity in both species. Because of the potential for inhibition of CYP1A2 activity by TCDD and other dioxins, studies examining CYP1A2 induction in dioxin-exposed populations using these substrates should be viewed cautiously.

Toxicokinetics of BDE 47 in female mice: effect of dose, route of exposure, and time.

2,2',4,4'-Tetrabromodiphenyl ether (BDE 47) is present in commercial mixtures of polybrominated diphenyl ethers (PBDEs), which are used as flame retardants in a wide variety of consumer products. Despite its small contribution to PBDE global production and usage, BDE 47 is the major congener found in environmental samples and human tissue. No human data are currently available regarding the toxicokinetics of BDE 47 either as an individual congener or in the commercial mixture. Because previous studies have suggested potential toxicokinetic differences between rodent species, this study was conducted in an effort to fully characterize absorption, distribution, and excretion parameters following a single dose with respect to dose, time, and route of exposure in female C57BL/6 mice. Over 80% of the administered dose was absorbed after oral or intratracheal administration, whereas approximately 62% was absorbed when the dose was applied dermally. Disposition was dictated by lipophilicity as adipose and skin were major depot tissues. BDE 47 was rapidly excreted in the urine and feces. Of particular interest was the amount of parent compound found in the urine, which was a major factor in determining an initial whole-body half life of 1.5 days after a single oral exposure. Elimination, both whole-body and from individual tissues, was biphasic. Initial half-lives were 1-3 days, whereas terminal half-lives were much longer, suggesting the potential for bioaccumulation. This toxicokinetic behavior has important implications for extrapolation of toxicological studies to the assessment of health risk in humans.

Comparisons of estimated human body burdens of dioxinlike chemicals and TCDD body burdens in experimentally exposed animals.

Humans are exposed to mixtures of polyhalogenated aromatic hydrocarbons, and the potential health effects of these exposures are uncertain. A subset of this class of compounds produce similar spectra of toxicity in experimental animals as does 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), and these chemicals have been classified as "dioxins." In this study, we compared the body burdens of dioxins that produce effects in experimental animals to body burdens associated with these effects in humans. Human body burdens were estimated from lipid-adjusted serum concentrations of dioxins, assuming dioxins are equally distributed in body fat and an adult has 22% body fat. The toxic equivalency factor (TEF) method was used to calculate body burdens of dioxins in humans. These calculations included dibenzo-p-dioxins, dibenzofurans, and polychlorinated biphenyls. In the general population, average background concentrations were estimated at 58 ng TCDD equivalents (TEQ)/kg serum lipid, corresponding to a body burden of 13 ng TEQ/kg body weight. Populations with known exposure to dioxins have body burdens of 96-7,000 ng TEQ/kg body weight. For effects that have been clearly associated with dioxins, such as chloracne and induction of CYP1A1, humans and animals respond at similar body burdens. Induction of cancer in animals occurs at body burdens of 944-137,000 ng TCDD/kg body weight, while noncancer effects in animals occur at body burdens of 10-12,500 ng/kg. Available human data suggest that some individuals may respond to dioxin exposures with cancer and noncancer effects at body burdens within one to two orders of magnitude of those in the general population.

Tissue disposition of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in maternal and developing long-evans rats following subchronic exposure.

Prenatal exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) produces alterations in the reproductive system of the developing pups. The objective of this study was to determine the disposition of TCDD in maternal and fetal Long-Evans (LE) rats following subchronic exposure, since the adverse reproductive and developmental effects have been extensively characterized in this strain of rat. LE rats were dosed by gavage with 1, 10, or 30 ng [(3)H]TCDD/kg in corn oil, 5 days/week for 13 weeks. At the end of 13 weeks, females were mated and dosing continued every day throughout gestation. Dams were sacrificed on gestation day (GD) 9, GD16, GD21, and post-natal day 4 and analyzed for [(3)H]TCDD-derived activity in maternal and fetal tissues. Maternal body burdens were equivalent at different time points, indicating that the dams were at steady state. Maternal body burdens were approximately 19, 120, and 300 ng TCDD/kg following doses of 1, 10, and 30 ng TCDD/kg, respectively. Individual embryo concentrations on GD9 were 1.6, 7, and 16 pg TCDD/g after maternal exposure of 1, 10, and 30 ng/kg/d, respectively. On GD 16, fetal liver, urogenital tract, head, and body concentrations were similar and averaged 1.4, 7.8, and 16.4 pg TCDD/g after administration of 1, 10, or 30 ng TCDD/kg/d, respectively, indicating no preferential sequestration within the different fetal tissues. These concentrations of TCDD within fetal tissues after subchronic exposure are comparable to those seen after a single dose of 50, 200, or 1000 ng TCDD/kg administered on GD15, a critical period of gestation.

Effects of short-term in vivo exposure to polybrominated diphenyl ethers on thyroid hormones and hepatic enzyme activities in weanling rats.

Polybrominated diphenyl ethers (PBDEs), used as flame retardants, are ubiquitous environmental contaminants. PBDEs act as endocrine disruptors via alterations in thyroid hormone homeostasis. We examined thyroid hormone concentrations and hepatic enzyme activity in weanling rats exposed to three commercial PBDE mixtures: DE-71, DE-79, and DE-83R. Female Long-Evans rats, 28 days old, were orally administered various doses of DE-71, DE-79, or DE-83R for 4 days. Serum and liver samples were collected 24 h after the last dose and analyzed for serum total thyroxine (T(4)), triiodothyronine (T(3)), thyroid-stimulating hormone (TSH), hepatic microsomal ethoxy- and pentoxy-resorufin-O-deethylase (EROD and PROD), and uridinediphosphate-glucuronosyltransferase (UDPGT) activities. The PBDE-treated groups did not exhibit significant changes in body weight; however, increased liver weights, as well as 10- to 20-fold induction in EROD and 30- to 40-fold induction in PROD were found in the DE-71-- and DE-79--treated animals. DE-71 and DE-79 caused dose-dependent depletion of T(4), accompanied by up to 3- to 4-fold induction in UDPGT activities. Serum total T(4) was decreased a maximum of 80% for DE-71 and 70% for DE-79 in the highest dose, with benchmark doses (BMDs) of approximately 12.74 mg/kg/day for DE-71 and 9.25 mg/kg/day for DE-79. Dose-related effects in serum T(3) levels were less apparent, with maximal reductions of 25-30% at the highest dose for both DE-71 and DE-79. The two mixtures showed no effect on serum TSH levels. Benchmark dose analysis revealed that the two mixtures were comparable in altering thyroid hormone levels and hepatic enzyme activity. DE-83R was not effective in altering any of the measured parameters. The present study suggests that short-term exposure to some commercial PBDE mixtures interferes with the thyroid hormone system via upregulation of UDPGTS:

Extrapolation of a PBPK model for dioxins across dosage regimen, gender, strain, and species.

A physiologically based pharmacodynamic (PBPK) model for 2,3,7, 8-tetrachlorodibenzo-p-dioxin (TCDD) was developed based on pharmacokinetic data from acute oral exposures of TCDD to female Sprague-Dawley rats (Wang et al., 1997, Toxicol Appl. Pharmacol 147, 151-168). In the present study, the utility of this model to predict the disposition of TCDD in male and female Sprague-Dawley and female Wistar rats exposed to TCDD through different dosage regimens was examined. The ability of the model to predict the disposition of 2-iodo-3,7,8-trichlorodibenzo-p-dioxin (ITrCDD) in mice (Leung, et al., 1990, Toxicol. Appl. Pharmacol. 103, 399-410) was also examined. The ability of the model to predict across routes of exposure was assessed with intravenous injection data (5.6 microg/kg bw) (Li et al., 1995, Fundam. Appl. Toxicol. 27, 70-76) in female rats. Analysis across gender extrapolations used data for male Sprague-Dawley rats exposed intravenously to 9.25 microg TCDD/kg bw (Weber et al., 1993, Fundam. Appl. Toxicol. 21, 523-534). The analysis of across-dosage regimen and stains of rats extrapolations were assessed using data from rats exposed to TCDD through a loading/maintenance dosage regimen (Krowke et al., 1989, Arch. Toxicol. 63, 356-360). The physiological differences between gender, strain, and species were taken into account when fitting the PBPK model to these data sets. The results demonstrate that the PBPK model for TCDD developed for female Sprague-Dawley rats exposed by acute oral dosing accurately predicts the disposition of TCDD, for different gender and strain of rats across varying dosage regimens, as well as in a strain of mice. Minimal changes in fitted parameters were required to provide accurate predictions of these data sets. This study provides further confirmation of the potential use of physiological modeling in understanding pharmacokinetics and pharmacodynamics.

Dose-response relationships for disposition and hepatic sequestration of polyhalogenated dibenzo-p-dioxins, dibenzofurans, and biphenyls following subchronic treatment in mice.

Humans are exposed to mixtures of polyhalogenated dibenzo-p-dioxins, dibenzofurans, and biphenyls mainly through the diet. Many of these chemicals are dioxin-like and their relative toxicity is related to their ability to bind and activate the Ah receptor. The present study examines the structure-activity relationship for disposition of these chemicals in female B6C3F1 mice following subchronic exposures. Mice were treated 5 days/week for 13 weeks by oral gavage with different doses of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), 1,2,3,7,8-pentachlorodibenzo-p-dioxin (PeCDD),2,3,7,8-tetrabromodibenzo-p-dioxin (TBDD), 2,3,7,8-tetrachlorodibenzofuran (TCDF), 1,2,3,7,8-pentachlorodibenzofuran (1-PeCDF), 2,3,4,7,8-pentachlorodibenzofuran (4-PeCDF), octachlorodibenzofuran (OCDF), 3,3',4,4',5-pentachlorobiphenyl (126), 3,3',4,4',5,5'-hexachlorobiphenyl (169), 2,3,3',4,4'-pentachlorobiphenyl (105), 2,3',4,4',5-pentachlorobiphenyl (118), and 2,3,3',4,4',5-hexachlorobiphenyl (156). All of the chemicals examined exhibited dose-dependent increases in the liver/fat concentrations except PCBs 105, 118, and 156. While TCDD is the most potent toxicant in this class of chemicals, 4-PeCDF, PeCDD, OCDF, TCDF, and PCB126 were sequestered in hepatic tissue to a greater extent than was TCDD. The high affinity for hepatic tissue supports the presence of an inducible hepatic binding protein for some dixin-like chemicals. The differences in disposition between these chemicals suggests that pharmacokinetic differences between congeners is important in the relative potency of these chemicals.

2,3,7,8-Tetrachlorodibenzo-p-dioxin in pregnant Long Evans rats: disposition to maternal and embryo/fetal tissues.

Prenatal exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) interferes with fetal development at doses lower than those causing overt toxicity in adult animals. In a multigeneration study (Murray et al., 1979), female rats that were administered 0.01 microgram TCDD/kg/day in their diet did not experience reduced fertility; however, reduced fertility was seen in the F1 and F2 generations. Exposure to TCDD during development produces alterations in the reproductive system of the developing pups, such as delayed puberty and reduced sperm counts in males (Mably et al., 1992a; Gray et al., 1995) and malformations in the external genitalia of females (Gray and Ostby, 1995). Therefore, the objectives of this study were to determine maternal and fetal tissue concentrations of TCDD that are associated with the adverse reproductive effects seen by Gray and co-workers. Pregnant Long Evans rats received a single oral dose of 1.15 micrograms [3H]TCDD/kg on Gestation Day (GD) 8 and maternal as well as fetal tissue concentrations of TCDD were measured on GD9, GD16, and GD21. On GD9, the highest level of TCDD localized in the maternal liver (25.1% dose). In addition, the amount reaching all the embryos on GD9 was 0.01% of the administered dose, which resulted in a concentration of 0.02% dose/g. The amount of TCDD reaching the fetal compartment (fetuses + placentas) increased to 0.12% dose/tissue on GD16 and 0.71% by GD21. The concentration of TCDD within the fetal compartment (0.01% dose/g) on GD16 was comparable to that found in the maternal blood and spleen. Concentrations of TCDD in a single embryo/fetus were 39.6, 18.1, and 22.1 pg/g on GD9, GD16, and GD21, respectively. Estimates of hepatic half-life of elimination in pregnant rats suggested that TCDD may be eliminated faster in pregnant LE rats. Therefore, measurements of biliary elimination were made in pregnant and nonpregnant LE rats to compare rates of metabolism; however, biliary elimination of TCDD is not affected by pregnancy. In conclusion, this dose administered during a critical period of organogenesis causes adverse effects on the developing reproductive system of rodents. This dose produced a body burden of 22.1 pg TCDD/g within a single fetus on GD21. This indicates that low-level TCDD exposure during the perinatal stage of life can produce adverse effects within the developing pups.

Acute administration of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in pregnant Long Evans rats: association of measured tissue concentrations with developmental effects.

Prenatal exposure to TCDD interferes with fetal development at doses lower than those causing overt toxicity in adult animals. Exposure to TCDD during development produces alterations in the reproductive system of the developing pups- delayed puberty and reduced sperm counts in males and malformations in the external genitalia of females. The objectives of this study were to determine maternal and fetal tissue concentrations of TCDD after acute exposure and whether these tissue concentrations can be used to estimate the intensity of the developmental abnormalities reported by other laboratories. Pregnant Long Evans rats received a single, oral dose of 0.05, 0.20, 0.80, or 1.0 microg [3H]-TCDD/kg on gestation day (GD) 15, and maternal and fetal tissue concentrations of TCDD were measured on GD16 and GD21. On GD16, maternal liver contained the greatest amount of TCDD (30-47% administered dose). One day after administration of 0.20 microg TCDD/kg on GD15, there were 13.2 pg TCDD/g present in an individual fetus. This concentration is associated with delayed puberty and decreased epididymal sperm counts in male pups as well as malformations in the external genitalia of females. For the responses studied, tissue concentration measured during a critical period of gestation adequately predicts the intensity of the response. In addition, there was a strong correlation between fetal body burden and maternal body burden on GD16. A dose of 0.05 microg TCDD/kg resulted in maternal body burdens of 30.6+/-3.1 and 26.6+/-3.1 ng TCDD/kg on GD16 and GD21, respectively. In conclusion, low-level TCDD exposure during the perinatal stage of life can produce adverse effects within the developing pups and that tissue concentration measured during a critical period is the appropriate dose metric to predict adverse reproductive and developmental effects.

Subchronic Exposure of [3H]- 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in female B6C3F1 mice: relationship of steady-state levels to disposition and metabolism.

The present study of subchronic low exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) at or near steady-state levels tries to emulate the most probable mode for human exposure, dietary consumption. This study is the first and most intensive pharmacokinetic study to be reported with repeated dosing, multiple times, and multiple doses examining disposition of TCDD-derived radioactivity and CYP1A activities in mice. For time-course relationships, animals were dosed (daily, Monday-Friday) with 0, 1.5, or 150 ng [3H]TCDD/kg for 4, 8, 13, or 17 weeks and also for 13 weeks followed by 4 weeks with no dosing. For dose-response relationships, animals were dosed for 13 weeks (daily, Monday-Friday) with 0, 0.15, 0.45, 1.5, 4.5, 15, 45, 150, or 450 ng [3H]TCDD/kg. Additional animals dosed for 13 weeks (daily, Monday-Friday) with 1.5 or 150 ng [(3)H]TCDD/kg were housed in metabolism cages. Time- and dose-dependencies of TCDD were confirmed in all measured tissues. Liver/fat (L/F) concentration ratios ranged from 0.2-3.4 (low to high dose). Hepatic CYP1A1 enzymatic activity increased (p < 0.05) starting at 0.15 ng/kg/day with L/F of 0.2 and body burden of 2.8 ng TCDD/kg body weight. By examining TCDD exposures at or near steady state, this study reports for the first time and provides direct evidence of low-dose effects on a measured reversible response at body burdens that are within background levels of the general human population. In addition, this study emphasizes cumulative effects of daily dosing and suggests the importance of tissue dosimetry or body burden for a persistent chemical such as TCDD.

Oxidative stress in female B6C3F1 mice following acute and subchronic exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD).

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a highly persistent trace environmental contaminant and is one of the most potent toxicants known to man. Hassoun et al. (1998, Toxicol. Sci. 42, 23-27) reported an increase in the production of reactive oxygen species (ROS) in the brain of female B6C3F1 mice following subchronic exposure to TCDD at doses as low as 0.45 ng/kg/day. In the present study, oxidative stress was characterized in liver, spleen, lung, and kidney following subchronic (0.15-150 ng/kg; 5 days/week for 13 weeks, po) or acute exposure (0.001-100 microg/kg, po) to TCDD in order to investigate the interaction between tissue concentration and time for production of ROS. Seven days following acute administration of TCDD, mice were sacrificed; they demonstrated increases in liver superoxide anion production (SOAP) and thiobarbituric acid reactive substances (TBARS) at doses of 10 and 100 microg/kg, associated with hepatic TCDD concentrations of 55 and 321 ng/g, respectively. Liver obtained from mice following subchronic TCDD exposure demonstrated an increase in SOAP and TBARS above controls at doses of 150 ng/kg/day with liver TCDD concentration of only 12 ng/g. Interestingly, glutathione (GSH) levels in lung and kidney following sub-chronic TCDD exposure were decreased at the low dose of 0.15 ng/kg/day. This effect disappeared at higher TCDD doses. The data suggest that higher tissue TCDD concentrations are required to elicit oxidative stress following acute dosing than with subchronic TCDD exposure. Therefore, the mechanism of ROS production following TCDD exposure does not appear to be solely dependent upon the concentration of TCDD within the tissue. In addition, very low doses of TCDD that result in tissue concentrations similar to the background levels found in the human population produced an effect on an oxidative stress endogenous defense system. The role of this effect in TCDD-mediated toxicity is not known and warrants further investigation.

Induction of oxidative stress in brain tissues of mice after subchronic exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin.

The ability of single doses of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) to induce oxidative stress in hepatic and some extrahepatic tissues of animals is well documented. However, no previous study has examined the ability of TCDD to induce oxidative stress and tissue damage in brain in vivo. In this study the ability of TCDD to induce oxidative stress in brain tissues of mice was studied after subchronic exposures. Groups of female B6C3F1 mice were treated orally with TCDD (0, 0.45, 1.5, 15, and 150 ng/kg/day) for 13 weeks, 5 days/week. The animals were euthanized 3 days after the last treatment and brain tissues were collected. Biomarkers of oxidative stress including production of superoxide anion, lipid peroxidation, and DNA-single-strand breaks (SSB) were determined. TCDD treatment resulted in significant and dose-dependent increases in the production of superoxide anion as assessed by reduction of cytochrome c. Significant increases were also observed in lipid peroxidation and DNA-SSB in those tissues, as assessed by the presence of thiobarbituric acid-reactive substances and the alkaline elution technique, respectively. These results clearly indicate that subchronic exposure to low doses of TCDD can induce oxidative tissue damage in brain tissues which may at least in part play a role in the effects of TCDD on the central nervous system.

Pargyline and naltrexone fail to antagonize the gustatory avoidance response induced by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine.

The administration of the dopaminergic neurotoxin, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) to rats following the consumption of a novel saccharin solution induced a gustatory avoidance response evidenced by a significant decrease in saccharin intake following its representation. Pretreatment with pargyline, an agent which prevents the metabolism of MPTP to the 1-methyl-4-phenylpyridinium ion (MPP+), did not potentiate or antagonize the magnitude of this aversion. Likewise, pretreatment with naltrexone, an opiate antagonist, failed to alter the gustatory avoidance response induced by MPTP. These observations indicate that MPTP is capable of inducing a gustatory avoidance response and that the mechanism mediating this response is not dependent upon the conversion of MPTP to MPP+ and does not involve the opiate receptor.

Use of toxic equivalency factors for risk assessment for dioxins and related compounds.

TCDD is the most toxic member of a class of polyhalogenated aromatic hydrocarbons that are structurally related, have a similar mechanism of action, and cause the same spectrum of responses. Because of the need to assess the risk from complex mixtures of these chemicals, the international community has adopted an interim approach that assigns relative potency factors to this family of chemicals, based on a comparison with the potency of TCDD. Each chemical that fits the criteria for this class is assigned a toxic equivalency factor, TEF, which is some fraction of that of TCDD. The total toxic equivalency of a mixture, TEQ, is the sum of the weighted potency of each compound in the mixture. Although there may be some variability between different responses in the determination of a TEF value for a compound, endpoint-specific TEFs are usually very similar. There may also be some species differences in TEFs. Again, if pharmacokinetic factors are taken into account, they are usually relatively minor. TEFs based on intake values may also exhibit some differences when compared to those based on target tissue concentrations. Using scientific judgment and a broad data base, interim TEF values have been recommended for PCDDs, PCDFs, and dioxin-like PCBs. Using such values, the TEF approach has been successful at predicting the toxicity of real world mixtures. Ongoing studies from our laboratory have validated the approach for synthetic mixtures that approximate congener ratios found in food samples. Whether non-additive interactions occur with nondioxin-like compounds found in environmentally relevant concentrations remain to be determined.