Evaluating hexabromocyclododecane (HBCD) toxicokinetics in humans and rodents by physiologically based pharmacokinetic modeling
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Hexabromocyclododecane
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Toxicodynamics
Inhalation exposure
Pharmacodynamics
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Hexabromocyclododecane
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A multi-compartment physiologically based pharmacokinetic (PBPK) model was developed to describe the behavior of Cr(III) and Cr(VI) in humans. Compartments were included for gastrointestinal lumen, oral mucosa, stomach, small intestinal tissue, blood, liver, kidney, bone, and a combined compartment for remaining tissues. As chronic exposure to high concentrations of Cr(VI) in drinking water cause small intestinal cancer in mice, the toxicokinetics of Cr(VI) in the upper gastrointestinal tract of rodents and humans are important for assessing internal tissue dose in risk assessment. Fasted human stomach fluid was collected and ex vivo Cr(VI) reduction studies were conducted and used to characterize reduction of Cr(VI) in human stomach fluid as a mixed second-order, pH-dependent process. For model development, toxicokinetic data for total chromium in human tissues and excreta were identified from the published literature. Overall, the PBPK model provides a good description of chromium toxicokinetics and is consistent with the available total chromium data from Cr(III) and Cr(VI) exposures in typical humans (i.e., model predictions are within a factor of three for approximately 86% of available data). By accounting for key species differences, sources of saturable toxicokinetics, and sources of uncertainty and variation, the rodent and human PBPK models can provide a robust characterization of toxicokinetics in the target tissue of the small intestine allowing for improved health risk assessment of human populations exposed to environmentally-relevant concentrations.
Ex vivo
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Alpha-hexabromocyclododecane (α-HBCD) is an emerging persistent organic pollutant present in the hexabromocyclododecane (HBCD) commercial mixture. HBCD is used as an additive flame retardant in a wide variety of household consumer products. Three main stereoisomers, alpha (α), beta (β), and gamma (γ), comprise roughly 10, 10, and 80% of the mixture, respectively. Despite its small contribution to HBCD global production and usage, α-HBCD is the major stereoisomer found in wildlife and human tissues including breast milk and blood in North America, European Union, and Asia. No mammalian or human data are currently available regarding the toxicokinetics of α-HBCD. This study was conducted in an effort to fully characterize the absorption, distribution, metabolism, and elimination of α-HBCD following a single and repeated exposure with respect to dose, time, and route of administration in female C57BL/6 mice. Results indicate that ∼90% of the administered dose (3 mg/kg) was absorbed after oral exposure. Disposition was (1) dictated by lipophilicity, as adipose, liver, muscle, and skin were major depots and (2) was dose dependent with nonlinear accumulation at higher doses. Elimination, both whole-body and from individual tissues, was biphasic. α-HBCD-derived radioactivity was excreted in the feces as parent and metabolites, whereas urine only contained metabolites. Presence of polar metabolites in the blood and urine were a major factor in determining the rapid initial whole-body half-life after a single oral exposure. Initial half-lives were ∼1–3 days and much longer terminal half-lives of 17 days were observed, suggesting the potential for α-HBCD bioaccumulation. A 10-day repeated study supports α-HBCD bioaccumulation potential. Stereoisomerization previously observed after exposure to γ-HBCD was not seen after exposure of α-HBCD. The toxicokinetic behavior reported here has important implications for the extrapolation of toxicological studies of the commercial HBCD mixture to the assessment of risk of α-HBCD which is the major stereoisomer found in wildlife and people.
Hexabromocyclododecane
Brominated flame retardant
Chlorinated paraffins
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Hexabromocyclododecane
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Physiologically based pharmacokinetic (PBPK) modeling is a powerful in silico tool that can be used to simulate the toxicokinetics and tissue distribution of xenobiotic substances, such as perfluorooctanoic acid (PFOA), in organisms. However, most existing PBPK models have been based on the flow-limited assumption and largely rely on in vivo data for parametrization. In this study, we propose a permeability-limited PBPK model to estimate the toxicokinetics and tissue distribution of PFOA in male rats. Our model considers the cellular uptake and efflux of PFOA via both passive diffusion and transport facilitated by various membrane transporters, association with serum albumin in circulatory and extracellular spaces, and association with intracellular proteins in liver and kidney. Model performance is assessed using seven experimental data sets extracted from three different studies. Comparing model predictions with these experimental data, our model successfully predicts the toxicokinetics and tissue distribution of PFOA in rats following exposure via both IV and oral routes. More importantly, rather than requiring in vivo data fitting, all PFOA-related parameters were obtained from in vitro assays. Our model thus provides an effective framework to test in vitro-in vivo extrapolation and holds great promise for predicting toxicokinetics of per- and polyfluorinated alkyl substances in humans.
Perfluorooctanoic acid
Xenobiotic
Membrane permeability
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In this report a rodent Physiologically Based PharmacoKinetic (PBPK) model for 2,3,7,8-tetrachlorodibenzodioxin is described. Validation studies, in which model simulations of TCDD disposition were compared with in vivo TCDD disposition in rodents exposed to TCDD, showed that the model adequately predicted the in vivo toxicokinetics of TCDD across species (mouse, rat), dose-schedule (acute semi-chronic chronic) and route of administration (oral subcutaneous). It was concluded that PBPK models are suited to establish quantitative relationships between the external dose of TCDD (amount of TCDD administered per kg body weight) and the internal dose (concentration of TCDD at the organ level). The relationship between internal dose and the resulting organ toxicity will be elaborated in subsequent reports. The results will be used in a quantitative risk assessment of the human exposure situation, in particular the exposure of suckling infants to dioxins from mother's milk.
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Internal dose
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Abstract : As a follow-up on the development of the PBPK model for soman in the guinea pig and man a new model is being developed for VX. This report describes in short the work and results which has led to this model. Based on the Physiology (Physiologically-Based) of the investigated species the 'fate' or kinetics (Pharmaco Kinetics) of a compound can be predicted after an intoxication in several organs or compartments by using a computer simulation model. The toxicokinetics of the nerve agent VX are fundamentally different from those of the nerve agents sarin or soman. Especially the persistent character of VX in blood at equitoxic doses is apparent. Animal experiments have shown the presence of toxicologically relevant concentrations of VX in blood a long time after intoxication. A number of input parameters need to be obtained either through literature searches or by research. The 6-compartment model for soman was adapted and expanded with an additional compartment, the skin. Also the reactions with the enzymes, AChE, BuChE and CaE were described individually. With the implementation of a skin compartment it becomes possible to simulate dermal exposure scenarios. Using this model and the available set of input parameters it is not yet possible to simulate the intravenous kinetics of VX in the `normal' guinea pig. After adapting the numbers of slow binding sites (CaE) the toxicokinetics in the hairless guinea pig are simulated very well. Some preliminary simulations of percutaneuos exposure also show a reasonable fit of the VX toxicokinetics in the blood of the hairless guinea pig.
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