Determination of tissue-blood partition coefficients for a physiological model for humans, and estimation of dioxin concentration in tissues

A Physiologically Based Pharmacokinetic (PBPK) Modeling Framework for Mixtures of Dioxin-like Compounds

Humans are exposed to persistent organic pollutants, such as dioxin-like compounds (DLCs), as mixtures. Understanding and predicting the toxicokinetics and thus internal burden of major constituents of a DLC mixture is important for assessing their contributions to health risks. PBPK models, including dioxin models, traditionally focus on one or a small number of compounds; developing new or extending existing models for mixtures often requires tedious, error-prone coding work. This lack of efficiency to scale up for multi-compound exposures is a major technical barrier toward large-scale mixture PBPK simulations. Congeners in the DLC family, including 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), share similar albeit quantitatively different toxicokinetic and toxicodynamic properties. Taking advantage of these similarities, here we reported the development of a human PBPK modeling framework for DLC mixtures that can flexibly accommodate an arbitrary number of congeners. Adapted from existing TCDD models, our mixture model contains the blood and three diffusion-limited compartments-liver, fat, and rest of the body. Depending on the number of congeners in a mixture, varying-length vectors of ordinary differential equations (ODEs) are automatically generated to track the tissue concentrations of the congeners. Shared ODEs are used to account for common variables, including the aryl hydrocarbon receptor (AHR) and CYP1A2, to which the congeners compete for binding. Binary and multi-congener mixture simulations showed that the AHR-mediated cross-induction of CYP1A2 accelerates the sequestration and metabolism of DLC congeners, resulting in consistently lower tissue burdens than in single exposure, except for the liver. Using dietary intake data to simulate lifetime exposures to DLC mixtures, the model demonstrated that the relative contributions of individual congeners to blood or tissue toxic equivalency (TEQ) values are markedly different than those to intake TEQ. In summary, we developed a mixture PBPK modeling framework for DLCs that may be utilized upon further improvement as a quantitative tool to estimate tissue dosimetry and health risks of DLC mixtures.

Nonlinear Method to Predict the Distribution of Structurally Diverse Compounds between Blood and Tissue

In this work, methods for predicting the distribution of compounds between blood and tissue were investigated using nonlinear regression analysis. For the tissue/blood partition coefficient, 282 compounds and 810 activity data for seven tissues were selected. Twenty-four parameters were studied for each state of the compound. A study set with the most compounds and activity data in similar studies was established, and a model with good prediction ability was obtained. A total of 773 data points were randomly divided into two data sets: the training set contained 623 data points (n = 623, r = 0.822, s = 0.438, F = 142.2, and Q = 0.814) and the test set contained 150 data points (n = 150, r = 0.814, and s = 0.334). Furthermore, individual tissue/blood distribution coefficients were also studied in depth to obtain separate models for predicting the distribution coefficient of a drug between blood and a tissue. By applying these models, not only can the tissue/blood distribution coefficient or single tissue/blood distribution coefficient of the seven tissues and organs of the human body be predicted, but the distribution of the drug molecules in different states (neutral, cation, and anion) in the three tissue components can also be predicted.

Physiologically based toxicokinetic models and in silico predicted partition coefficients to estimate tetrachlorodibenzo-p-dioxin transfer from feed into growing pigs

Tetrachlorodibenzo-p-dioxin (TCDD) is a ubiquitous, toxic, persistent and bioaccumulative organic pollutant. TCDD can potentially enter the food chain through contaminated food of animal origin as a consequence of feed contamination. Prediction of the TCDD transfer from feed into animal products is thus important for human health risk assessment. Here, we develop several physiologically based toxicokinetic (PBTK) models of TCDD transfer from contaminated feed into growing pigs (Sus scrofa) exposed to doses ranging from 24.52 to 3269.25 ng of TCDD. We test the consequences of explicit dose-dependent absorption (DDA) versus the indirect effects of a self-induced liver metabolism (SIM). The DDA and SIM models showed similar fit to experimental data, although currently it is not possible to unequivocally make statement on a mechanistic preference. The performance of both toxicokinetic models was successfully evaluated using the 1999 Belgian case of contaminated fats for feeding. In combination with toxicokinetic models of other dioxin congeners, they can be used to formulate maximum allowance levels of dioxins in feedstuffs for pigs. Additionally, the implementation of in silico-predicted partition coefficients was explored as a useful alternative to predict TCDD tissue distribution in low-dose scenarios without recurring to animal experiments.

Application of pharmacokinetic modelling for 2,3,7,8-tetrachlorodibenzo-p-dioxin exposure assessment

Polychlorinated dibenzo-p-dioxins, polychlorinated dibenzofurans, and mono- and non-ortho polychlorinated biphenyls (dioxin-like PCBs) are identified as a family or group of organic compounds known as 'dioxins' or dioxin-like chemicals (DLCs). The most toxic member of this group is 2,3,7,8-tetrachlorodibenzo-(p)-dioxin (TCDD). Historically, DLCs have caused a variety of negative human health effects, but a disfiguring skin condition known as chloracne is the only health effect reported consistently. As part of translational research to make computerized models accessible to health risk assessors, the Concentration- and Age-Dependent Model (CADM) for TCDD was recoded in the Berkeley Madonna simulation language. The US Agency for Toxic Substances and Disease Registry's computational toxicology laboratory used the recoded model to predict TCDD tissue concentrations at different exposure levels. The model simulations successfully reproduced the National Health and Nutrition Examination Survey (NHANES) 2001-2002 TCDD data in age groups from 6 to 60 years and older, as well as in other human datasets. The model also enabled the estimation of lipid-normalized serum TCDD concentrations in breastfed infants. The model performed best for low background exposures over time compared with a high acute poisoning case that could due to the large dose and associated liver toxicity. Hence, this model may be useful for interpreting human biomonitoring data as a part of an overall DLC risk assessment.

Physiologically based pharmacokinetic (PBPK) models for lifetime exposure to PCB 153 in male and female harbor porpoises (Phocoena phocoena): model development and evaluation

Physiologically based pharmacokinetic (PBPK) models were developed for the most persistent polychlorinated biphenyl (PCB 153) in male and female harbor porpoises (Phocoena phocoena) to elucidate processes such as uptake, distribution, and elimination. Due to its limited metabolic capacities, long life span, and top position in marine food chains, this species is highly sensitive to pollution. The models consist of 5 compartments, liver, blubber, kidney, brain, and a compartment which accounts for the rest of the body, all connected through blood. All physiological and biochemical parameters were extracted from the literature, except for the brain/blood partition coefficient and rate of excretion, which were both fitted to data sets used for validation of the models. These data sets were compiled from our own analyses performed with GC-MS on tissue samples of harbor porpoises. The intake of PCB 153 was from milk from birth to 4 months, and after weaning fish was the main food source. Overall, these models reveal that concentrations of PCB 153 in males increase with age but suggest that, as the animals grow older, metabolic transformation can be a possible pathway for elimination as well. In contrast, the model for females confirms that gestation and lactation are key processes for eliminating PCB 153 as body burdens decrease with age. These PBPK models are capable of simulating the bioaccumulation of PCB 153 during the entire life span of approximately 20 years of the harbor porpoises.

Predicting PCDD/F and dioxin-like PCB contamination levels in bovine edible tissues from in vivo sampling

European Union regulation has defined maximum levels for polychlorinated dibenzo-p-dioxins, polychlorinated dibenzofurans (PCDD/F) and dioxin-like polychlorinated biphenyls (PCB) in food from animal origin. In case of particular event where livestock is potentially exposed to a specific source of contamination, the possibility of accurate estimation of the PCDD/F and dioxin-like PCB levels in edible tissues of the animals from in vivo and weakly invasive biological matrices (fast animal recovery, no major side effects) may be of clear valuable interest. This study investigated the correlations between contamination levels determined in subcutaneous adipose tissue and blood samples obtained from living animals and those measured after slaughtering in muscle and liver. The obtained results demonstrated very good correlations between these in vivo and ex vivo samples in terms of PCDD/F and PCB contamination levels. Finally, it seems to be demonstrated that a weakly invasive biopsy of subcutaneous adipose tissue performed on living animal (potentially completed by a blood sample) can be used in order to predict contamination levels in muscle and liver destined to human consumption.

Polychlorinated dibenzo-p-dioxins/furans and polychlorinated biphenyls in human adipose tissue from Zhejiang Province, China

This is the first report on polychlorinated dibenzo-p-dioxin/furan (PCDD/F) and polychlorinated biphenyl (PCB) contamination of human adipose tissue from China. A total of 24 human adipose tissue samples from a general population in Zhejiang Province were analyzed for PCDD/F and PCB by high-resolution gas chromatography-high-resolution mass spectrometry. Total PCDD/F concentrations in human adipose tissue ranged from 33.9 to 504 pg g(-1)lipid (mean 108 pg g(-1)lipid). Corresponding values for dioxin-like PCBs ranged from 4.1 to 125 ng g(-1)lipid (mean 32.8 ng g(-1)lipid). Mean total WHO toxicity equivalent (TEQ) values for PCDD/Fs and PCBs in human adipose tissue were 9.22 and 16.2 pg g(-1)lipid, respectively. OCDD was the dominant PCDD/F congener, and 2,3,4,7,8-PeCDF and 1,2,3,7,8-PeCDD accounted for more than 70% of the WHO PCDD/F TEQ. In all samples, PCB-118, PCB-156 and PCB-105 were the main PCB congeners. PCB-153 concentrations were the highest of all indicator PCBs (mean 52.5 ng g(-1)lipid). The contamination levels and profiles are compared with those reported for European and Asian countries.

Whole-body physiologically based pharmacokinetic models

This review summarizes the most recent developments in and applications of physiologically based pharmacokinetic (PBPK) modeling methodology originating from both the pharmaceutical and environmental toxicology areas. It focuses on works published in the last 5 years, although older seminal papers have also been referenced. After a brief introduction to the field and several essential definitions, the main body of the text is structured to follow the major steps of a typical PBPK modeling exercise. Various applications of the methodology are briefly described. The major future trends and perspectives are outlined. The main conclusion from the review of the available literature is that PBPK modeling, despite its obvious potential and recent incremental developments, has not taken the place it deserves, especially in pharmaceutical and drug development sciences.

Congener-specific tissue distribution and hepatic sequestration of PCDD/Fs in wild herring gulls from Bohai Bay, North China: comparison to coplanar PCBs

Tissue distribution is an important property of pharmacokinetic behaviors of dioxins to provide information for risk assessment to wild avian species. In this study, concentrations of polychlorinated dibenzo-p-dioxins (PCDDs), polychlorinated dibenzofurans (PCDFs), and coplanar polychlorinated biphenyls (coplanar PCBs) were determined in muscle, liver, spleen, kidney, brain, and adipose of wild herring gulls collected from Bohai Bay, North China. Tissue distribution results showed preferential accumulation of PCDD/Fs in liver and of co-PCBs in adipose. The congener patterns of coplanar PCBs were constant in different tissues, but the congener patterns for PCDD/F were tissue-specific. The liver/adipose concentration ratios for PCDD/ Fs were found to increase statistically significantly with log K(ow), providing the quantitative relationship of structure-activity for hepatic sequestration of PCDD/Fs for the first time. Furthermore, this relationship was compared with those developed on the basis of previous results reported in the literature showing that the wild herring gulls in Bohai Bay are still in the exposure period.

Tissue distribution and bioconcentration factors of PCDD/Fs in the liver and adipose tissue following chronic ingestion of contaminated milk in rats

The aim of this study was to determine the tissue distribution of 17 PCDD/Fs following the chronic ingestion of contaminated milk in rats and to assess the "target tissue/milk" BioConcentration Factors (BCFs) of these molecules. Contaminated milk, collected in a polluted area, has been incorporated into the diet of male rats at a low dose (31 pg I-TEQ/day/rat). For this exposure, the accumulation of PCDD/Fs in target tissues (liver and adipose tissue) was limited, the tissue concentrations stabilising between 90 and 120 days of daily intake to levels close to 3 pg/g of tissue (all tissues and molecules combined). The tissue distribution seemed to be governed by the congeners properties and by the tissue characteristics. An increase in the chlorination degree of dioxins caused a decrease in their incorporation in the adipose tissue, and consequently of the BCF values. Moreover, the distribution of dioxins between hepatocytes and adipocytes differed: unlike the liver, the quantities of dioxins in the adipose tissue were significantly (P<0.05) correlated to the quantity of tissue fat. Only in the liver, the incorporation of PCDDs seemed to be facilitated when the chlorination degree of these congeners increased, the reverse phenomenon having been observed for PCDFs. However, for the same level of chlorination, the BCFs of PCDFs were 2.4 times higher than those of PCDDs in this tissue. The absence of correlation between the quantity of dioxins and that of fat and the BCFs differences of theses congeners suggested that dioxins fixation process in the liver was selective.

Perinatal dioxin exposure and later effects--a review

Negative effects of perinatal exposure to background levels of dioxins and PCBs in Europe and the USA have been documented. Four facets of development are reviewed in this paper: 1. Brain development and thyroid hormone metabolism. 2. Hepatic effects. 3. Hematopoietic system effects. 4. Lung function. Effects on IQ and behaviour have been documented in children on both sides of the Atlantic Ocean. Non-dioxin-like PCBs, measured in maternal and cord blood and current plasma samples have been implicated. Interference with thyroid hormone metabolism in the mother, in the foetus and in the newborn baby could be responsible for these effects on brain development. During early gestation the foetus is completely dependent on maternal thyroxine (T4). Lower T4 levels in the mother, caused by dioxins and PCBs, might negatively influence (early) brain development. It is plausible that the intrauterine dependency on maternal T4 and the high T4 need shortly after birth makes both these periods vulnerable for environmental influences. Effects of dioxin exposure on thyroid hormone metabolism have been described in the period shortly after birth. These effects are no longer found after two years of age indicating a transient effect. In animal studies, in utero exposure has led to effects on brain development due to abnormal induction of liver enzymes. This induction resulted in lower testosterone and estrogen levels, interfering with brain development in the vulnerable period of language development and the development of visuo-spatial abilities. In humans this developmental period occurs around the thirtieth week of pregnancy. Follow-up studies in puberty and adolescence of the different cohorts studied is necessary to evaluate these negative influences. Damaging effects on the liver found shortly after birth have proven to be transient. Effects on the haematopoietic system are clear immediately after birth, for instance on white blood cells and thrombocytes. An increase in middle ear infections (otitis media) in relation to current levels of PCBs at the age of 4 years was described in the Rotterdam study. Negative effects on lung function in the sense of increased obstruction was found after 8 years in relation to perinatal exposure to dioxins in the Zaandam study. This rather new finding might explain the sharp increase in lung problems in children in the Western world.

Estimation of dioxin risk to Japanese from the past to the future

The transport processes of seventeen 2,3,7,8-chlorinated congeners of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) from their major sources to humans were modeled to estimate the time course, from the past to the future, of the human health risk to the Japanese population. The comparison between measured and estimated values showed that the present modeling approach represented the background levels of congeners in the environment, daily intake, and body burden reasonably well, except for in the case of 2,3,7,8-TCDF. Although PCDD/Fs in herbicides have contributed greatly to the daily intake and body burden to the Japanese population in the past, the main sources of the present intake and burden of PCDD/Fs is estimated to be incinerators. The margin of exposure (MOE) for the risk of morphological reproductive alteration in female offspring exposed prenatally was calculated based on the estimated maternal body burden. The results indicated that the MOE values were in the single digits, implying that these values may not be sufficiently large to guarantee the safety of female offspring of mothers born in the 1950s, whereas the MOE values for female offspring born in and after the latter half of the 1990s may be sufficiently large to guarantee safety.

Simulation of dioxin accumulation in human tissues and analysis of reproductive risk

Polychlorinated dibenzo-p-dioxin, polychlorinated dibenzofuran and dioxin-like polychlorinated biphenyl concentrations in human liver, kidney, fat, blood, muscle, richly perfused tissue (brain, lung etc.) and skin were simulated to assess the health risk for Japanese fetuses. A 40-year time course of dioxin accumulation via food ingestion was simulated using a physiologically based pharmacokinetic (PBPK) model. In richly perfused tissue, the concentration estimated by the PBPK model showed better agreement with measured concentrations than that calculated by the one-compartment model. Fetal dioxin concentration was simulated based on the assumption that the fetal concentration was almost equal to the concentration in the mother's richly perfused tissue. To assess the reproductive risk, the estimated concentration in human fetus was compared with that in rat fetus in which reproductive function showed signs of alteration by 2,3,7,8-TCDD in previous reports [Toxicol. Appl. Pharmacol. 114 (1992) 118; 146 (1997) 11; Toxicol. Sci. 53 (2000) 411; 57 (2000) 275]. The present daily intake of 2,3,7,8-TCDD is approximately 1/50 of the amount that leads to possible reproductive toxicity in the next generation. However, when 29 kinds of dioxin congeners are considered, the present level is 1/5 of the hazardous levels. For species extrapolation of dioxin risk, further study on tissue concentration versus toxicity is required.

Possible range of dioxin concentration in human tissues: simulation with a physiologically based model

In risk evaluation of dioxins, monitoring chemical concentrations in human tissues is an important step, and these concentration data can be utilized along with animal toxicity data for extrapolation of human manifestation. However large differences in dioxin concentrations usually exist even among individuals who have never been accidentally exposed to high quantities of dioxin, and this may cause problems in risk analysis. Body size, age, and history of food consumption are factors responsible for these interindividual differences in addition to exposure levels. Using a physiologically based pharmacokinetic (PBPK) model, the influence of differences in body weight, gastrointestinal absorption, and half-life and intake of dioxin were examined on tissue chemical concentration. Dioxin concentrations over a 40-yr time course in human liver, kidneys, fat, blood, muscle and richly perfused tissue were simulated for polychlorinated dibenzo-p-dioxins (PCDDs), polychlorinated dibenzofurans (PCDFs), and coplanar polychlorinated biphenyls (CoPCBs). Model parameters such as tissue-blood partition coefficients for CoPCBs were prepared, and sensitivity analysis was also performed on these parameters. The range of tissue concentrations was approximately 0.17 to 4.1 times the standard concentration, which was calculated using standard model parameters. The simulated ranges included more than 80% of the individual anatomical data for 2,3,7,8-tetrachlorodibenzo-p-dioxin, 1,2,3,7,-pentachlorodibenzo-p-dioxin, and 3,3',4,4',5-pentachlorobiphenyl in liver, fat, and blood. These results suggest that differences in body weight, gastrointestinal absorption, and food intake behavior may partially explain variation in tissue concentrations among individuals, and the possible interindividual uncertainty, which is approximately 24 for the general Japanese population.