A physiological pharmacokinetic description of the tissue distribution and enzyme-inducing properties of 2,3,7,8-tetrachlorodibenzo-p-dioxin in the rat

Transcriptomic analysis of AHR wildtype and Knock-out rat livers supports TCDD's role in AHR/ARNT-mediated circadian disruption and hepatotoxicity

Single, high doses of TCDD in rats are known to cause wasting, a progressive loss of 30 to 50% body weight and death within several weeks. To identify pathway perturbations at or near doses causing wasting, we examined differentially gene expression (DGE) and pathway enrichment in centrilobular (CL) and periportal (PP) regions of female rat livers following 6 dose levels of TCDD - 0, 3, 22, 100, 300, and 1000 ng/kg/day, 5 days/week for 4 weeks. At the higher doses, rats lost weight, had increased liver/body weight ratios and nearly complete cessation of liver cell proliferation, signs consistent with wasting. DGE curves were left shifted for the CL versus the PP regions. Canonical Phase I and Phase II genes were maximally increased at lower doses and remained elevated at all doses. At lower doses, ≤ 22 ng/kg/day in the CL and ≤ 100 ng/kg/day, upregulated genes showed transcription factor (TF) enrichment for AHR and ARNT. At the mid- and high-dose doses, there was a large number of downregulated genes and pathway enrichment for DEGs which showed downregulation of many cellular metabolism processes including those for steroids, fatty acid metabolism, pyruvate metabolism and citric acid cycle. There was significant TF enrichment of the hi-dose downregulated genes for RXR, ESR1, LXR, PPARalpha. At the highest dose, there was also pathway enrichment with upregulated genes for extracellular matrix organization, collagen formation, hemostasis and innate immune system. TCDD demonstrates most of its effects through binding the aryl hydrocarbon receptor (AHR) while the downregulation of metabolism genes at higher TCDD doses is known to be independent of AHR binding to DREs. Based on our results with DEG, we provide a hypothesis for wasting in which high doses of TCDD shift circadian processes away from the resting state, leading to greatly reduced synthesis of steroids and complex lipids needed for cell growth, and producing gene expression signals consistent with an epithelial-to-mesenchymal transition in hepatocytes.

Transfer of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) and polychlorinated biphenyls (PCBs) from oral exposure into cow's milk - part II: toxicokinetic predictive models for risk assessment

Understanding the transfer of polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs) as well as polychlorinated biphenyls (PCBs) from oral exposure into cow's milk is not purely an experimental endeavour, as it has produced a large corpus of theoretical work. This work consists of a variety of predictive toxicokinetic models in the realms of health and environmental risk assessment and risk management. Their purpose is to provide mathematical predictive tools to organise and integrate knowledge on the absorption, distribution, metabolism and excretion processes. Toxicokinetic models are based on more than 50 years of transfer studies summarised in part I of this review series. Here in part II, several of these models are described and systematically classified with a focus on their applicability to risk analysis as well as their limitations. This part of the review highlights the opportunities and challenges along the way towards accurate, congener-specific predictive models applicable to changing animal breeds and husbandry conditions.

Release and toxicity of adipose tissue-stored TCDD: Direct evidence from a xenografted fat model

BACKGROUND

Persistent organic pollutants (POPs) are known to accumulate in adipose tissues (AT). This storage may be beneficial by diverting POPs from other sensitive tissues or detrimental because of chronic release of pollutants as indirectly suggested during weight loss. The aim is to study the biological and/or toxic effects that chronic POP release from previously contaminated grafted AT could exert in a naïve mouse.

METHODS

C57BL/6J male mice were exposed intraperitoneally to 2,3,7,8-tetrachlorodibenzo-p-doxin (TCDD); their epididymal fat pads were collected and grafted on the back skin of uncontaminated recipient mice whose brain, liver, and epididymal ATs were analyzed (TCDD concentration, relevant gene expression). Kinetics of release and redistribution were modeled using Physiologically Based PharmacoKinetics (PBPK).

RESULTS

The grafts released TCDD over a period of 10 weeks with different kinetics of distribution in the three organs studied. A PBPK model was used to simulate the AT releasing process and the incorporation of TCDD into the major organs. At three weeks post-graft, we observed significant changes in gene expression in the liver and the host AT with signatures reminiscent of inflammation, gluconeogenesis and fibrosis as compared to the control.

CONCLUSIONS

This study confirms that AT-stored TCDD can be released and distributed to the organs of the recipient hence leading to distinct changes in gene expression. This original model provides direct evidence of the potential toxic-relevant effects when endogenous sources of contamination are present.

Risk for animal and human health related to the presence of dioxins and dioxin-like PCBs in feed and food

The European Commission asked EFSA for a scientific opinion on the risks for animal and human health related to the presence of dioxins (PCDD/Fs) and DL-PCBs in feed and food. The data from experimental animal and epidemiological studies were reviewed and it was decided to base the human risk assessment on effects observed in humans and to use animal data as supportive evidence. The critical effect was on semen quality, following pre- and postnatal exposure. The critical study showed a NOAEL of 7.0 pg WHO2005-TEQ/g fat in blood sampled at age 9 years based on PCDD/F-TEQs. No association was observed when including DL-PCB-TEQs. Using toxicokinetic modelling and taking into account the exposure from breastfeeding and a twofold higher intake during childhood, it was estimated that daily exposure in adolescents and adults should be below 0.25 pg TEQ/kg bw/day. The CONTAM Panel established a TWI of 2 pg TEQ/kg bw/week. With occurrence and consumption data from European countries, the mean and P95 intake of total TEQ by Adolescents, Adults, Elderly and Very Elderly varied between, respectively, 2.1 to 10.5, and 5.3 to 30.4 pg TEQ/kg bw/week, implying a considerable exceedance of the TWI. Toddlers and Other Children showed a higher exposure than older age groups, but this was accounted for when deriving the TWI. Exposure to PCDD/F-TEQ only was on average 2.4- and 2.7-fold lower for mean and P95 exposure than for total TEQ. PCDD/Fs and DL-PCBs are transferred to milk and eggs, and accumulate in fatty tissues and liver. Transfer rates and bioconcentration factors were identified for various species. The CONTAM Panel was not able to identify reference values in most farm and companion animals with the exception of NOAELs for mink, chicken and some fish species. The estimated exposure from feed for these species does not imply a risk.

Toxicokinetic Modeling as a Tool for Risk Estimation: 2,3,7,8-Tetrachlorodibenzo-P-Dioxin

Concepts of toxicokinetic modeling and the relevance of toxicokinetics for understanding dose-response relationships, species scaling, and risk estimation are broached. A physiological one-compartment model for 2,3,7,8-tetra-chlorodibenzo-p-dioxin (TCDD) is presented in detail. It describes the TCDD burden of the human body, which results from TCDD-contaminated food, in dependence of age. The model was validated using a series of measured values obtained by other authors and this group. They represent lipid-based concentrations of TCDD in liver, blood, adipose tissue, feces, and mother's milk in dependence of age. Special attention was paid to the TCDD burden in infants resulting from feeding with mother's milk or formula. Model simulations demonstrate that TCDD burden can amount to 10 mg/kg of lipids after nursing for 6 months with mother's milk exclusively This is still within the range of the concentrations found in adults. After the nursing period, TCDD burden declines. From the age of 7 years on, there is no longer a difference in the TCDD burden, independently of the food they had received as infants. According to the model, elimination half-life of TCDD from the body is not constant but increases during life starting from a few months in newborns to several years in adults.

Modeling drug- and chemical-induced hepatotoxicity with systems biology approaches

We provide an overview of computational systems biology approaches as applied to the study of chemical- and drug-induced toxicity. The concept of “toxicity pathways” is described in the context of the 2007 US National Academies of Science report, “Toxicity testing in the 21st Century: A Vision and A Strategy.” Pathway mapping and modeling based on network biology concepts are a key component of the vision laid out in this report for a more biologically based analysis of dose-response behavior and the safety of chemicals and drugs. We focus on toxicity of the liver (hepatotoxicity) – a complex phenotypic response with contributions from a number of different cell types and biological processes. We describe three case studies of complementary multi-scale computational modeling approaches to understand perturbation of toxicity pathways in the human liver as a result of exposure to environmental contaminants and specific drugs. One approach involves development of a spatial, multicellular “virtual tissue” model of the liver lobule that combines molecular circuits in individual hepatocytes with cell–cell interactions and blood-mediated transport of toxicants through hepatic sinusoids, to enable quantitative, mechanistic prediction of hepatic dose-response for activation of the aryl hydrocarbon receptor toxicity pathway. Simultaneously, methods are being developing to extract quantitative maps of intracellular signaling and transcriptional regulatory networks perturbed by environmental contaminants, using a combination of gene expression and genome-wide protein-DNA interaction data. A predictive physiological model (DILIsym™) to understand drug-induced liver injury (DILI), the most common adverse event leading to termination of clinical development programs and regulatory actions on drugs, is also described. The model initially focuses on reactive metabolite-induced DILI in response to administration of acetaminophen, and spans multiple biological scales.

A mass balance of tri-hexabrominated diphenyl ethers in lactating cows

Beef and dairy products can be important vectors of human exposure to polybrominated diphenylethers (BDEs), and hence an understanding of BDE transfer from feed to cows' milk and tissue is important for BDE exposure assessment The fate of tri- to hexaBDEs in lactating cows exposed to a naturally contaminated diet was studied by analyzing feed, feces, and milk samples from a mass balance study. Tissue distribution was studied in one cowslaughtered afterthe experiment The carryover rates from feed to milk ranged from 0.15 to 0.35 for the major congeners. Lower values were observed for several of the tetrabrominated congeners, and this was attributed to metabolism. The dietary absorption efficiency decreased with increasing octanol-water partition coefficient of the BDE congener. The absorption behavior was consistent with a model based on chemical lipophilicity, but agreed less well with a model based on effective molecular diameter, and it violated Lipinski's "rule of 5". The lipid normalized concentrations were similar in all tissues analyzed including liver and milk, suggesting that tissue distribution is governed by partitioning into lipids. Overall, the behavior of the tri- to hexaBDEs was consistent with that observed for other classes of halogenated aromatic contaminants such as PCBs and PCDD/Fs, but it differed markedly from the behavior of the hepta- decaBDEs.

A model for energetics and bioaccumulation in marine mammals with applications to the right whale

We present a dynamic energy budget (DEB) model for marine mammals, coupled with a pharmacokinetic model of a lipophilic persistent toxicant. Inputs to the model are energy availability and lipid-normalized toxicant concentration in the environment. The model predicts individual growth, reproduction, bioaccumulation, and transfer of energy and toxicant from mothers to their young. We estimated all model parameters for the right whale; with these parameters, reduction in energy availability increases the age at first parturition, increases intervals between reproductive events, reduces the organisms' ability to buffer seasonal fluctuations, and increases its susceptibility to temporal shifts in the seasonal peak of energy availability. Reduction in energy intake increases bioaccumulation and the amount of toxicant transferred from mother to each offspring. With high energy availability, the toxicant load of offspring decreases with birth order. Contrary to expectations, this ordering may be reversed with lower energy availability. Although demonstrated with parameters for the right whale, these relationships between energy intake and energetics and pharmacokinetics of organisms are likely to be much more general. Results specific to right whales include energy assimilation estimates for the North Atlantic and southern right whale, influences of history of energy availability on reproduction, and a relationship between ages at first parturition and calving intervals. Our model provides a platform for further analyses of both individual and population responses of marine mammals to pollution, and to changes in energy availability, including those likely to arise through climate change.

Characterizing Uncertainty and Variability in Physiologically Based Pharmacokinetic Models: State of the Science and Needs for Research and Implementation

Physiologically based pharmacokinetic (PBPK) models are used in mode-of-action based risk and safety assessments to estimate internal dosimetry in animals and humans. When used in risk assessment, these models can provide a basis for extrapolating between species, doses, and exposure routes or for justifying nondefault values for uncertainty factors. Characterization of uncertainty and variability is increasingly recognized as important for risk assessment; this represents a continuing challenge for both PBPK modelers and users. Current practices show significant progress in specifying deterministic biological models and nondeterministic (often statistical) models, estimating parameters using diverse data sets from multiple sources, using them to make predictions, and characterizing uncertainty and variability of model parameters and predictions. The International Workshop on Uncertainty and Variability in PBPK Models, held 31 Oct-2 Nov 2006, identified the state-of-the-science, needed changes in practice and implementation, and research priorities. For the short term, these include (1) multidisciplinary teams to integrate deterministic and nondeterministic/statistical models; (2) broader use of sensitivity analyses, including for structural and global (rather than local) parameter changes; and (3) enhanced transparency and reproducibility through improved documentation of model structure(s), parameter values, sensitivity and other analyses, and supporting, discrepant, or excluded data. Longer-term needs include (1) theoretical and practical methodological improvements for nondeterministic/statistical modeling; (2) better methods for evaluating alternative model structures; (3) peer-reviewed databases of parameters and covariates, and their distributions; (4) expanded coverage of PBPK models across chemicals with different properties; and (5) training and reference materials, such as cases studies, bibliographies/glossaries, model repositories, and enhanced software. The multidisciplinary dialogue initiated by this Workshop will foster the collaboration, research, data collection, and training necessary to make characterizing uncertainty and variability a standard practice in PBPK modeling and risk assessment.

A Physiologically Based Pharmacokinetic Model of Docetaxel Disposition: from Mouse to Man

PURPOSE

Docetaxel (Taxotere), an important chemotherapeutic agent with shown activity in a broad range of cancers, is being investigated for use in combination therapies and as an antiangiogenic agent. Docetaxel exhibits a complex pharmacologic profile with high interpatient variability. Pharmacokinetic models capable of predicting exposure under various dosing regimens would aid the rational development of clinical protocols.

EXPERIMENTAL DESIGN

A pharmacokinetic study of docetaxel at 5 and 20 mg/kg was carried out in female BALB/c mice. Tissues were collected at various time points and analyzed by liquid chromatography-tandem mass spectrometry. Time course tissue distribution and pharmacokinetic data were used to build and validate a physiologically based pharmacokinetic (PBPK) model in mice. Specific and nonspecific tissue partitioning, metabolism, and elimination data were coupled with mouse physiologic variables to develop a PBPK model that describes docetaxel plasma and tissue pharmacokinetic. The PBPK model was then modified with human model variables to predict the plasma distribution of docetaxel.

RESULTS

Resulting simulation data were compared with actual measured data obtained from our pharmacokinetic study (mouse), or from published data (human), using pharmacokinetic variables calculated using compartmental or noncompartmental analysis to assess model predictability.

CONCLUSIONS

The murine PBPK model developed can accurately predict plasma and tissue levels at the 5 and 20 mg/kg doses. The human PBPK model is capable of estimating plasma levels at 30, 36, and 100 mg/m(2). This will enable us to develop and test various dosing regimens (e.g., metronomic schedules and combination therapies) to achieve specific tissue and plasma concentrations to maximize therapeutic benefit while minimizing toxicity.

Age- and concentration-dependent elimination half-life of 2,3,7,8-tetrachlorodibenzo-p-dioxin in Seveso children

OBJECTIVE

Pharmacokinetic and statistical analyses are reported to elucidate key variables affecting 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) elimination in children and adolescents.

DESIGN

We used blood concentrations to calculate TCDD elimination half-life. Variables examined by statistical analysis include age, latency from exposure, sex, TCDD concentration and quantity in the body, severity of chloracne response, body mass index, and body fat mass.

PARTICIPANTS

Blood was collected from 1976 to 1993 from residents of Seveso, Italy, who were < 18 years of age at the time of a nearby trichlorophenol reactor explosion in July 1976.

RESULTS

TCDD half-life in persons < 18 years of age averaged 1.6 years while those > or =18 years of age averaged 3.2 years. Half-life is strongly associated with age, showing a cohort average increase of 0.12 year half-life per year of age or time since exposure. A significant concentration-dependency is also identified, showing shorter half-lives for TCDD concentrations > 400 ppt for children < 12 years of age and 700 ppt when including adults. Moderate correlations are also observed between half-life and body mass index, body fat mass, TCDD mass, and chloracne response.

CONCLUSIONS

Children and adolescents have shorter TCDD half-lives and a slower rate of increase in half-life than adults, and this effect is augmented at higher body burdens.

RELEVANCE

Modeling of TCDD blood concentrations or body burden in humans should take into account the markedly shorter elimination half-life observed in children and adolescents and concentration-dependent effects observed in persons > 400-700 ppt.

Estimated cancer risk of dioxins to humans using a bioassay and physiologically based pharmacokinetic model

The health risk of dioxins and dioxin-like compounds to humans was analyzed quantitatively using experimental data and mathematical models. To quantify the toxicity of a mixture of three dioxin congeners, we calculated the new relative potencies (REPs) for 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), 1,2,3,7,8-pentachlorodibenzo-p-dioxin (PeCDD), and 2,3,4,7,8- pentachlorodibenzofuran (PeCDF), focusing on their tumor promotion activity. We applied a liver foci formation assay to female SD rats after repeated oral administration of dioxins. The REP of dioxin for a rat was determined using dioxin concentration and the number of the foci in rat liver. A physiologically based pharmacokinetic model (PBPK model) was used for interspecies extrapolation targeting on dioxin concentration in liver. Toxic dose for human was determined by back-estimation with a human PBPK model, assuming that the same concentration in the target tissue may cause the same level of effect in rats and humans, and the REP for human was determined by the toxic dose obtained. The calculated REPs for TCDD, PeCDD, and PeCDF were 1.0, 0.34, and 0.05 for rats, respectively, and the REPs for humans were almost the same as those for rats. These values were different from the toxic equivalency factors (TEFs) presented previously (Van den Berg, M., Birnbaum, L., Bosveld, A.T.C., Brunstrom, B., Cook, P., Feeley, M., Giesy, J.P., Hanberg, A., Hasegawa, R., Kennedy, S.W., Kubiak, T., Larsen, J.C., Rolaf van Leeuwen, F.X., Liem, A.K.D., Nolt, C., Peterson, R.E., Poellinger. L., Safe, S., Schrenk, D., Tillitt, D, Tysklind, M., Younes, M., Waern, F., Zacharewski, T., 1998. Toxic equivalency factors (TEFs) for PCBs, PCDDs, PCDFs for humans and wildlife. Environ. Health Perspect. 106, 775-792). The relative risk of excess liver cancer for Japanese people in general was 1.7-6.5 x 10(-7) by TCDD only, and 2.9-11 x 10(-7) by the three dioxins at the present level of contamination.

T-screen to quantify functional potentiating, antagonistic and thyroid hormone-like activities of poly halogenated aromatic hydrocarbons (PHAHs)

The present study investigates chemical thyroid hormone disruption at the level of thyroid hormone receptor (TR) functioning. To this end the (ant)agonistic action of a series of xenobiotics was tested in the newly developed T-screen. This assay makes use of a GH3 rat pituitary cell line, that specifically proliferates when exposed to 3,3',5-triiodo-L-thyronine (T3). The growth stimulatory effect is mediated via T3-receptors. (Ant)agonistic and potentiating action of compounds was studied in absence and presence of T3 at its EC50 level (0.25 nM). The compounds tested included the specific TR-antagonist amiodarone, as well as a series of brominated diphenyl ethers (BDEs), including specifically synthesized BDEs with a structural resemblance to 3,5-diiodo-L-thyronine (T2), T3 and T4 (3,3',5,5'-tetraiodo-L-thyronine). The results obtained reveal that only BDE206 and amiodarone are specific antagonists. Interestingly some compounds which did not respond in the T-screen in absence of T3, potentiated effects when tested in combination with T3. This points at possibilities for disruption at the TR in vivo, where exposure generally occurs in presence of T3. Altogether the results of the present study show that the newly developed T-screen can be used as a valuable tool for identification and quantification of compounds active in disturbing thyroid hormone homeostasis at the level of TR-functioning.

CYP Induction-Mediated Drug Interactions: in Vitro Assessment and Clinical Implications

Cytochrome P450 (CYP) induction-mediated interaction is one of the major concerns in clinical practice and for the pharmaceutical industry. There are two major issues associated with CYP induction: a reduction in therapeutic efficacy of comedications and an induction in reactive metabolite-induced toxicity. Because CYP induction is a metabolic liability in drug therapy, it is highly desirable to develop new drug candidates that are not potent CYP inducer to avoid the potential of CYP induction-mediated drug interactions. For this reason, today, many drug companies routinely include the assessment of CYP induction at the stage of drug discovery as part of the selection processes of new drug candidates for further clinical development. The purpose of this article is to review the molecular mechanisms of CYP induction and the clinical implications, including pharmacokinetic and pharmacodynamic consequences. In addition, factors that affect the degree of CYP induction and extrapolation of in vitro CYP induction data to in vivo situations will also be discussed. Finally, assessment of the potential of CYP induction at the drug discovery and development stage will be discussed.

Physiologically based modeling of the accumulation in plasma and tissue lipids of a mixture of PCB congeners in female Sprague-Dawley rats

This study aimed to develop a physiologically based model for simulating the concentrations of polychlorinated biphenyls (PCBs) in tissue and plasma lipids of rats exposed to PCB mixtures. The model was based on the assumption that the neutral lipid fraction is the only critical determinant of the tissue distribution of PCBs, and that the solubility/retention in other tissue components is negligible. The volumes of the model compartments reflected the volumes of neutral lipids, whereas the flow rates corresponded to those of the neutral lipids in blood. Since the equilibrium ratio of PCB concentrations in neutral lipids of tissues and plasma equals 1, the present modeling approach does not require the use of tissue:blood partition coefficients. Metabolism rates were derived from the best visual fit of the model to the PCB concentrations in hepatic lipids determined on d 41 and 90 in rats exposed to a mixture containing 5, 50, or 500 microg PCBs (118, 138, 153, 170, 180 and 187) per kilogram body weight according to various protocols: (a) every-day dosing, (b) once-a-week dosing, (c) consecutive dosing for 13 d with no further treatment, and (d) 13 irregularly spaced doses. The resulting model consistently simulated the concentrations of PCBs in adipose tissue and plasma lipids of rats exposed according to the four described protocols. The physiologically based pharmacokinetic (PBPK) model developed in this study should be useful as a basis for interpretating blood or plasma lipid concentration data on PCBs collected during biomonitoring studies.

Toxicokinetic modeling and its applications in chemical risk assessment

In recent years physiologically based pharmacokinetic (PBPK) modeling has found frequent application in risk assessments where PBPK models serve as important adjuncts to studies on modes of action of xenobiotics. In this regard, studies on mode of action provide insight into both the sites/mechanisms of action and the form of the xenobiotic associated with toxic responses. Validated PBPK models permit calculation of tissue doses of xenobiotics and metabolites for a variety of conditions, i.e. at low-doses, in different animal species, and in different members of a human population. In this manner, these PBPK models support the low-dose and interspecies extrapolations that are important components of current risk assessment methodologies. PBPK models are sometimes referred to as physiological toxicokinetic (PT) models to emphasize their application with compounds causing toxic responses. Pharmacokinetic (PK) modeling in general has a rich history. Data-based PK compartmental models were developed in the 1930's when only primitive tools were available for solving sets of differential equations. These models were expanded in the 1960's and 1970's to accommodate new observations on dose-dependent elimination and flow-limited metabolism. The application of clearance concepts brought many new insights about the disposition of drugs in the body. In the 1970's PBPK/PT models were developed to evaluate metabolism of volatile compounds of occupational importance, and, for the first time, dose-dependent processes in toxicology were included in PBPK models in order to assess the conditions under which saturation of metabolic and elimination processes lead to non-linear dose response relationships. In the 1980's insights from chemical engineers and occupational toxicology were combined to develop PBPK/PT models to support risk assessment with methylene chloride and other solvents. The 1990's witnessed explosive growth in risk assessment applications of PBPK/PT models and in applying sensitivity and variability methods to evaluate model performance. Some of the compounds examined in detail include butadiene, styrene, glycol ethers, dioxins and organic esters/aids. This paper outlines the history of PBPK/PT modeling, emphasizes more recent applications of PBPK/TK models in health risk assessment, and discusses the risk assessment perspective provided by modern uses of these modeling approaches.

Development and application of a generalized physiologically based pharmacokinetic model for multiple environmental contaminants

The pharmacological disposition of four environmental contaminants resulting from acute and chronic exposure regimes is simulated using a general physiologically based pharmacological (PBPK) model. The model, which is detailed in supporting materials, is mechanistic in structure and relies on available physical-chemical partitioning and reactivity data, but experimental partitioning and absorption efficiency data can be used to refine the parameters. It is designed to complement environmental fate models, thus linking chemical emission rates with environmental and physiological behavior as part of the larger environmental risk assessment process. The model is illustratively applied to inhaled styrene and trichloroethene as well as ingested dibutyl phthalate and di(2-ethylhexyl) phthalate. The phthalate simulations include the corresponding monoester and conjugated monoester as metabolites. Tissue concentrations for each of the chemicals and metabolites are simulated for acute, occupational, and environmental exposure regimes. The same model is used for all chemicals and exposure regimes with only the physical-chemical properties, reaction rates, and exposure estimates being changed.

Dietary accumulation and biochemical responses of juvenile rainbow trout (Oncorhynchus mykiss) to 3,3',4,4',5-pentachlorobiphenyl (PCB 126)

Juvenile rainbow trout (Oncorhynchus mykiss) (initial weights 2-5 g) were exposed to three dietary concentrations (0, 12.4 and 126 ng g(-1), wet weight) of a 14C-labelled 3,3',4,4',5-pentachlorobiphenyl (PCB 126) for 30 days followed by 160 days of clean food. We assessed bioaccumulation, histology (liver and thyroid) and biochemical responses (liver ethoxyresorufin-O-deethylase (EROD), liver vitamins (retinoids and tocopherol) and muscle thyroid hormone levels) along with growth and survival. The half-life of PCB 126 in the rainbow trout ranged from 82 to 180 days while biomagnification factors (BMF) ranged from 2.5 to 4.1 providing further evidence that PCB 126 is among the most bioaccumulative PCB congeners. Toluene extractable 14C declined with time in the trout suggesting the possibility of some biotransformation and/or covalent bonding with biological macromolecules. The threshold for liver EROD induction by PCB 126 was approximately 0.1 ng g(-1) (wet weight). EROD activities in the low- and high treatments were 9 and 44 times greater than control, respectively, and remained elevated throughout the experiment. EROD activity was correlated with whole body concentrations of PCB 126 although there was evidence of EROD activity suppression in the highly exposed fish. Liver didehydroretinoids and tocopherol concentrations were depressed by the high PCB 126 dose after 30 days exposure. Initially, muscle concentrations of thyroxine (T4) and triiodo-L-thyronine (T3) declined as the fish grew during the experiment, and exposure to PCB 126 accelerated the growth related decline. More information is needed to assess the functional significance of the reduced muscular stores of thyroid hormones. Despite the changes in liver EROD, liver vitamins and muscle thyroid hormones, liver and thyroid histology in trout examined after 30 days exposure and growth parameters were unaffected by PCB 126. This indicates that the functional competences of the physiological factors associated with growth were maintained under the experimental conditions.

Doxorubicin pharmacokinetics: Macromolecule binding, metabolism, and excretion in the context of a physiologic model

The studies described herein were designed to determine whether doxorubicin (DOX) pharmacokinetics (PKs) could be described by a physiologically based PK model that incorporated macromolecule-specific binding and organ-specific metabolism and excretion. Model parameters were determined experimentally, or were gathered from the literature, in a species-specific manner, and were incorporated into a physiologically based description of DOX blood and tissue distribution for mice, dogs, and humans. The resulting model simulation data were compared with experimentally determined data using PK parameters calculated using compartmental or noncompartmental analysis to assess the predictability of the models. The resulting physiologically based PK model that was developed could accurately predict blood and tissue PKs of DOX in mice. When this model was interspecies extrapolated to predict DOX levels in dogs and humans undergoing treatment for cancer, predictions in dog plasma or human serum were also consistent with the actual clinical data. This model has potential utility for predicting the magnitude of PK interactions of DOX with other drugs, and for predicting changes in DOX PKs in any number of clinical situations.

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

The tissue-blood partition coefficients for a physiologically based pharmacokinetic (PBPK) model were determined, and the concentrations of 17 congeners of polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/Fs) in tissues in Japanese people were estimated using the model. According to the PBPK model established by Lawrence and Gobas [Chemosphere 35 (1997) 427-452], we assumed a steady-state fugacity model for Japanese people in general, and set the route of PCDD/Fs exposure only from food intake. The required partition coefficients for liver, kidney, adipose, muscle, skin, bile, gut and viscera (richly perfused tissue) were calculated using available autopsy data from eight Japanese men and women who were not accidentally exposed to PCDD/Fs. For validation of the partition coefficients, estimated PCDD/F concentrations in liver, kidney, fat, blood and muscle using the model were compared to other two sets of measured concentration data in Japanese tissues. Good agreement was obtained between estimated data and measured data, and most of the measured data were within the simulated concentration range in liver, kidney, blood and muscle. From these results, our model and calculated partition coefficients seem applicable for the estimation of congener-specific concentrations in human tissues.

The use of quantitative histological and molecular data for risk assessment and biologically based model development

In organs with diverse cell populations, it is not uncommon for one type of cell to respond while others are spared. Even in an organ with common cell types, such as hepatocytes within the liver, the population of cells may respond with different sensitivities for injury or for biochemical responses to toxicants. In the liver, many tumor promoters induce cytochrome P450 enzymes and other proteins in centrilobular cells at much lower doses than required to cause induction in periportal cells. In addition, these induction responses appear to occur at the level of individual cells--a 50% response of the liver for induction does not represent 50% induction in all cells. Instead, half of the cells are fully induced and half are unaffected. Cells "switch" from one phenotypic state to another. Over the past 10 years, several attempts have been made to model these cellular switches and to understand their relevance for hepatic tumor promotion and risk assessment. The data used for analyzing these switches include responses of the entire liver (total induction), responses of individual cells in the liver (regional induction), and cellular responses such as proliferation and apoptosis. This brief overview describes the development of biologically based, dose-response (BBDR) models for protein induction and tumor promotion in liver by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) with emphasis on the role of specific types of histological and molecular data in providing insights about mechanisms for cellular switches and their implications for tumor promotion. As the biological basis of these switches become unraveled and incorporated into the models, these BBDR models should eventually serve to improve risk assessments with a variety of liver tumor promoters with receptor-based modes of action.

Physiological modeling of a proposed mechanism of enzyme induction by TCDD

A physiological model was previously constructed to facilitate extrapolation of surrogates for the effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in rat liver to doses comparable to human environmental exposures. The model included induction of P450 isozymes and suggested the presence of multiple binding sites with different affinities for the TCDD-liganded Ah receptor at CYP1A1 dioxin responsive elements. The model also indicated that protein synthesis on the mRNA template exhibited saturation kinetics with respect to message levels. In the present work the earlier model was revised to include the increased proteolysis of the Ah receptor on binding TCDD, more realistic representations of gene transcription and mRNA translation, and different stability for each mRNA. The revised model includes multiple TCDD-liganded Ah receptor binding sites for CYP1A1 and CYP1B1 genes, a lag of 0.2 day for production of mRNA and induced proteins, and stabilization of mRNA by a poly(A) tail. The model reproduced the transient depletion of the Ah receptor subsequent to binding ligand and the dose-response of the receptor in rats treated with biweekly oral doses of TCDD in corn oil. The model reproduced tissue TCDD concentrations observed for several dosing scenarios. Such robustness indicates the utility of the model in estimating internal dose. The model also reproduced the observed dose-response patterns for mRNA and protein for CYP1A1, CYP1A2, and CYP1B1 after repeated dosing. Neither of the two dissociation constants for the Ah receptor bound to the CYP1B1 gene is negligible, supporting the assumption of multiple response elements for this gene. The poorer induction of CYP1B1 was predicted to be due to lower affinity of the dioxin responsive elements for binding the liganded Ah receptor, suggesting the involvement of other regulatory factors, and a shorter poly(A) tail on CYP1B1 mRNA, leading to a shorter lifetime. Saturation in the kinetics of protein synthesis was linked to the limited number of ribosomes that could bind to each message molecule, resulting in fewer ribosomes bound per message at higher doses. Predicted induction at low doses was found to vary widely with the assumptions used in the construction of a model. More detailed descriptions of biological processes might provide more reliable predictions of enzyme induction.

Toxicokinetics

The toxicokinetic determinants of dioxin and related chemicals depend on three major properties: lipophilicity, metabolism, and binding to CYP1A2 in the liver. The induction of CYP1A2 is partially under the control of the aryl hydrocarbon receptor (AhR). Lipophilicity increases with more chlorination and controls absorption and tissue partitioning. Metabolism is the rate-limiting step for elimination. Induction of CYP1A2 leads to hepatic sequestration of TCDD. Binding to this inducible hepatic protein results in non-linear dose-dependent tissue distribution: with increasing doses, the relative concentration in extra-hepatic tissues decreases while that in liver increases. The induction of this protein occurs in both animals and humans and results in an increase in the liver to fat ratio of these compounds. Humans have similar sensitivities to rodents for dioxin-like compounds when using tissue concentration (from in vitro studies), body burden, average lifetime serum lipid concentration, or lifetime area-under-the-curve concentration based on both low dose (biochemical) and high dose (cancer) driven endpoints. To reach the same tissue concentration in humans as rodents however, humans need a lower daily intake than rodents based on differences in pharmacokinetic behaviour. This clearly indicates that physiologically based pharmacokinetic models should be explored for the estimation of the daily intake of dioxin-like compounds in humans based on tissue dose levels or derivatives of those.

Risk ranges for various endpoints following exposure to 2,3,7,8-TCDD

Conducting a dose-response analysis for an environmental contaminant requires a careful evaluation of most of the available data focusing on both the magnitude of the effect and the possible ranges of dose-response shapes which fit the data. This paper calculates potency values (1% response exposures) for human and animal data on cancer, non-cancer and biological effect endpoints for TCDD and finds that a reasonable estimate for 1% excess cancer would be between 1 and 50 pg/kg/day. The paper also evaluates the adequacy of linear and non-linear models for fitting these data and concludes that the assumption of a threshold dose-response is not fully supported by these data.

Uncertainty in biomonitoring and kinetic modeling

Uncertainty in exposure assessment and uncertainty in kinetic models of early effects after exposure to a toxin are addressed in this paper. Sources of uncertainty in the determination of exposure of workers in chemical industry exposed to dioxins are exhibited and a simple kinetic model for biomonitor measurements of the concentrations from occupational exposure is derived. Model uncertainty, and uncertainty in the model parameters of physiologically-based pharmacokinetic models (PBPK models) are addressed when these models are used to estimate the effective dose in risk assessment. Uncertainty in the model parameters originating from the use of different statistical analysis methods is exhibited for Hill type nonlinear kinetics of enzyme induction mediated by a toxin.

A pharmacodynamic analysis of TCDD-induced cytochrome P450 gene expression in multiple tissues: dose- and time-dependent effects

The ability of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD, dioxin) to alter gene expression and the demonstration that the induction of CYP1A2 is responsible for hepatic TCDD sequestration suggest that both pharmacokinetic and pharmacodynamic events must be incorporated for a quantitative description of TCDD disposition. In this paper, a biologically based pharmacodynamic (BBPD) model for TCDD-induced biochemical responses in multiple tissues was developed. The parameters responsible for tissue response were estimated simultaneously with a refined physiologically based pharmacokinetic (PBPK) model developed by Wang et al. (1997a), by using the time-dependent effects of TCDD on induced CYP1A1/CYP1A2 gene expression in multiple target tissues (liver, lungs, kidneys, and skin) of female Sprague-Dawley rats treated with 10 microgram TCDD/kg for 30 min, 1, 3, 8, or 24 h, or 7, 14, or 35 days. This refined BBPD model developed based on the time-course of TCDD-induced CYP1A1/CYP1A2 protein expression, and associated enzymatic activities well described the dose-dependent effects of TCDD on cytochrome P450 protein expression and associated enzyme activities in the multiple tissues of female Sprague-Dawley rats at 3 days following a single exposure to TCDD (0.01-30.0 micromgram TCDD/kg). This is the first BBPD model to quantitatively describe the time- and dose-dependent effects of TCDD on induced CYP1A1/CYP1A2 protein expression and associated enzyme activities in multiple target tissues for TCDD-induced biochemical responses.

Determination of parameters responsible for pharmacokinetic behavior of TCDD in female Sprague-Dawley rats

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is the most toxic member of a class of planar and halogenated chemicals. Improvements in exposure assessment of TCDD require scientific information on the distribution of TCDD in target tissues and cellular responses induced by TCDD. Since 1980, several physiologically based pharmacokinetic (PBPK) models for TCDD and related compounds have been reported. Some of these models incorporated the induction of a hepatic binding protein in response to interactions of TCDD, the Ah receptor, and DNA binding sites and described the TCDD disposition in a biological system for certain data sets. Due to the limitations of the available experimental data, different values for the same physical parameters of these models were obtained from the different studies. The inconsistencies of the parameter values limit the application of PBPK models to risk assessment. Therefore, further refinement of previous models is necessary. This paper develops an improved PBPK model to describe TCDD disposition in eight target tissues. The interaction of TCDD with the Ah receptor and with hepatic inducible CYP1A2 were also incorporated into the model. This model accurately described the time course distribution of TCDD following a single oral dose of 10 microg/kg, as well as the TCDD concentration on Day 3 after six different doses, 0.01, 0.1, 0.3, 1, 10, and 30 microg TCDD/kg, in target tissues. This study extends previous TCDD models by illustrating the validity and the limitation of the model and providing further confirmation of the potential PBPK model for us in optimal experimental design and extrapolation across doses and routes of exposure. In addition, this study demonstrated some critical issues in PBPK modeling.

A pharmacokinetic analysis of interspecies extrapolation in dioxin risk assessment

This study entails a pharmacokinetic analysis of the relationship between the external dose of 2,3,7,8-tetrachlorodibenzo-p-dioxin (dioxin, TCDD) and resulting concentrations of TCDD in internal tissues and organs of humans and rodent species. The methodology is based on the development and testing of physiologically based pharmacokinetic models for several rodent species and humans. The results indicate that the relationship between the external dose of TCDD and resulting TCDD concentrations in liver and adipose tissue of humans and various species of rats and mice can vary by as much as 725 fold, illustrating that humans and experimental animals differ considerably in their ability to convert external dosages of dioxin to tissue concentrations. Interspecies scaling factors are reported to express the differences in tissue concentrations of dioxin between mice, rats and humans in response to an equivalent external dose. The significance of these findings for conducting human cancer and ecological risk assessments is discussed. It is recommended that pharmacokinetic differences be considered explicity in risk estimation, while separately recognizing interspecies differences in pharmacodynamics (sensitivity).

Regional hepatic CYP1A1 and CYP1A2 induction with 2,3,7,8-tetrachlorodibenzo-p-dioxin evaluated with a multicompartment geometric model of hepatic zonation

A physiologically based pharmacokinetic (PBPK) model for 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) was combined with a five-compartment geometric model of hepatic zonation to predict both total and regional induction of CYP450 proteins within the liver. Three literature studies on TCDD pharmacokinetics and protein induction in female rats were analyzed. In simulating low-dose behavior for mRNA in whole liver and, particularly, in representing immunohistochemical observations, the five-compartment model was more successful than conventional homogeneous one-compartment liver models. The five-compartment liver model was used with the affinity of TCDD for the Ah receptor (AhR) held constant across all the liver (Kb = 0.2 nM). The presumed affinities of the AhR-TCDD complex for TCDD responsive elements in the CYP1A1 (Kd1) and CYP1A2 (Kd2) genes varied between adjacent compartments by a factor of 3. This parameterization leads to predicted 81-fold differences in affinities between the centrilobular and the periportal regions. The affinities used for AhR-TCDD complex binding to TCDD response elements for CYP1A2 in compartment 3 (the midzonal area) ranged from 0.08 to 1.0 nM in the three studies modeled. For CYP1A1 the corresponding dissociation constant in compartment 3 varied from 0.6 to 2.0 nM. In each compartment, the Hill coefficient for induction had to be 4 or greater to match the immunohistochemical results. This multi-compartment liver model is consistent with data on protein and mRNA induction throughout the liver and on the regional distribution of these proteins. No previous model has incorporated regional variations in induction. The PBPK analysis based on the multicompartment liver model suggests that the low-dose behavior for hepatic CYP1A1/CYP1A2 induction by TCDD is highly non-linear.

2,3,7,8-Tetrachlorodibenzo-p-dioxin mechanistic data and risk assessment: gene regulation, cytotoxicity, enhanced cell proliferation, and tumor promotion

2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) has been found to cause several tumor types in rodents, but TCDD has not been proven to cause cancer in humans, although there have been reported associations. TCDD does not bind to DNA, and indirect tests for DNA damage have been mostly negative. Tumorigenicity by TCDD in rodents has been linked to cellular necrosis, enhanced cell proliferation and tumor promotion. TCDD binds to the Ah receptor, which induces CYP1A1. This binding may be involved in tumorigenicity in rodents; however, additional TCDD-induced toxic changes appear to be required. Biopersistence and organ distribution may play an important role in TCDD dosage extrapolation to humans, but these have not been adequately determined.

The future of mechanistic research in risk assessment: where are we going and can we get there from here?

Quantitative estimates of human health risk are often based on mathematical models fit to experimental or epidemiological data. Recent years have witnessed a trend towards the use of mechanistic models in risk assessment applications. Such models afford a more biologically based interpretation of the data and a firmer scientific basis for extrapolation beyond the conditions under which the original data were obtained. In this article, we review some recent advances in the development of biologically based models for mutagenesis, carcinogenesis and developmental toxicity. Pharmacokinetic and receptor-binding models and their roles in mechanistic risk assessment are also discussed. The future of mechanistic research in risk assessment is contemplated, including the need for more elaborate experiments to obtain the data necessary for mechanistic modeling.

Biologically-based models of dioxin pharmacokinetics

Significant advances have been made in the development of physiologically-based models of dioxin pharmacokinetics (PBPK) in the last 5-6 years. These models incorporate explicit descriptions of biological factors which determine tissue dosimetry of dioxin and include some description of dioxin-mediated pharmacodynamic events. Biological determinants of dioxin disposition include fat solubility, specific and inducible binding in the liver, diffusion-limited tissue distribution and metabolic elimination. PBPK models have been successfully used to predict the dose and time-dependent chemical disposition and protein induction properties of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) over a wide variety of experimental data sets with rodents. The models have also been extended to describe the disposition of a brominated dioxin, 2,3,7,8-tetrabromodibenzo-p-dioxin. As these quantitative descriptions of disposition are more fully refined, particularly with regard to pharmacodynamic descriptions of dioxin-mediated alterations in gene expression, more accurate predictions of tissue dosimetry and tissue responses will be performed across dose, species and related polyhalogenated aromatic hydrocarbons. Accurate, mechanistic dosimetry models will facilitate biologically-based approaches to the human risk assessment of these important and ubiquitous environmental contaminants.

Biochemical mechanisms and cancer risk assessment models for dioxin

Biologically realistic mechanistic models of carcinogenesis by TCDD are composed of equations representing biochemical events leading to altered expression of proteins involved in the response or equations representing the kinetics of proliferation of clones of mutant cells. A biochemically augmented physiological dosimetry model reproduces the observed altered expression of liver proteins in female rats exposed to dioxin. The model suggests that oxidation of estradiol to DNA reactive quinones or semiquinones by CYP1A2 protein induced by TCDD may contribute to an increased mutational rate. It suggests that TCDD-stimulated production of a peptide ligand of the epidermal growth factor (EGF) receptor and subsequent activation of the receptor's tyrosine kinase activity may increase the rate of proliferation of susceptible cells. These calculated quantities can serve as indices of toxicity and can be used to predict tumor incidence as a function of exposure.

Physiologically based pharmacokinetic and pharmacodynamic modeling in health risk assessment and characterization of hazardous substances

Recent advances in physiologically based pharmacokinetic and pharmacodynamic (PBPK/PD) modeling have introduced novel approaches for evaluating toxicological problems. Because PBPK models are amenable to extrapolation of tissue dosimetry, they are increasingly being applied to chemical risk assessment. A comprehensive listing of PBPK/PD models for environmental chemicals developed to date is referenced. Salient applications of PBPK/PD modeling to health risk assessments and characterization of hazardous substances are illustrated with examples.

An in vitro method for determination of tissue partition coefficients of non-volatile chemicals such as 2,3,7,8-tetrachlorodibenzo-p-dioxin and estradiol

The development of an in vitro vial equilibration technique for determining tissue and liquid partition coefficients for non-volatile chemicals is described. Radiolabeled chemical dissolved in propylene carbonate is equilibrated with tissues or liquid at 37 degrees C in a vial system. The solvent must be essentially immiscible with the test material. The amount of chemical movement to the tissue or liquid is compared to an appropriate reference vial, and tissue or liquid:solvent partition coefficients are calculated. Tissue:solvent values divided by blood:solvent values provide tissue:blood partition coefficients required for developing physiologically based pharmacokinetic models for chemicals. These models are useful for estimating internal tissue doses to assess human risk from exposure to chemicals. Partition coefficients for various rat tissues, 0.9% saline solution and olive oil were determined in this study for radiolabeled 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and for the less fat-soluble compound, estradiol. The TCDD tissue:propylene carbonate partition coefficients were found to be: blood, 0.091; fat, 17.02; liver, 0.419; brain, 0.632; kidney, 0.305; muscle, 0.408. For estradiol, the tissue:propylene carbonate partition coefficients were: blood, 0.286; fat, 0.169; liver, 1.032; brain, 0.554. The TCDD results compared well with values reported and estimated from a more protracted in vivo approach. Thus, this current technique offers a simpler and time-saving alternative to in vivo approaches for determining the partition coefficients of non-volatile compounds.

Significance of TCDD-induced changes in protein phosphorylation in the adipocyte of male guinea pigs

The chemical TCDD (2,3,7,8-tetrachlorodibenzo-p-dioxin) at very low concentrations has been found to cause a rapid rise in protein phosphorylation activities in the extranuclear fraction (i.e., cytosol and cellular membranes) of the adipose tissue from male guinea pigs. The effect occurred both in vivo and when TCDD was added to adipose in tissue culture (in situ). To study the cause for such a TCDD-induced biochemical change, isolated adipose tissues were treated with TCDD in conjunction with various diagnostic agents known to affect protein synthesis (actinomycin D and cycloheximide) and protein phosphorylation activities (genistein). The stimulatory effect of TCDD on protein phosphorylation was not totally abolished by actinomycin D; however, cycloheximide and genistein partially suppressed the effect of TCDD. Such an action of TCDD was accompanied by a quick rise in ras GTP binding activity as well as a rise in phosphorylation of nuclear c-myc protein product within 10 minutes after TCDD addition, indicating that TCDD is likely to activate the signal transduction pathway for growth factors. In view of the absence of an inhibitory action of actinomycin D and the short time required for TCDD to induce these changes, we have formulated a working hypothesis that stimulation of protein phosphorylation activities by TCDD may not be mediated by a transcriptional process. To test this possibility, an extranuclear fraction was obtained from the homogenate of adipose tissue, and the effect of TCDD was studied in vitro cell- and nucleus-free conditions. It was found that TCDD was capable of activating protein phosphorylation activity under cell free conditions even in the absence of nuclei. Such an action of TCDD is dependent on the action of the Ah receptor.

The toxicokinetics and metabolism of polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs) and their relevance for toxicity

This article reviews the present state of the art regarding the toxicokinetics and metabolism of polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs). The absorption, body distribution, and metabolism can vary greatly between species and also may depend on the congener and dose. In biota, the 2,3,7,8-substituted PCDDs and PCDFs are almost exclusively retained in all tissue types, preferably liver and fat. This selective tissue retention and bioaccumulation are caused by a reduced rate of biotransformation and subsequent elimination of congeners with chlorine substitution at the 2,3,7, and 8 positions. 2,3,7,8-Substituted PCDDs and PCDFs also have the greatest toxic and biological activity and affinity for the cytosolic arylhydrocarbon (Ah)-receptor protein. The parent compound is the causal agent for Ah-receptor-mediated toxic and biological effects, with metabolism and subsequent elimination of 2,3,7,8- substituted congeners representing a detoxification process. Congener-specific affinity of PCDDs and PCDFs for the Ah-receptor, the genetic events following receptor binding, and toxicokinetics are factors that contribute to the relative in vivo potency of an individual PCDD or PCDF in a given species. Limited human data indicate that marked species differences exist in the toxicokinetics of these compounds. Thus, human risk assessment for PCDDs and PCDFs needs to consider species-, congener-, and dose-specific toxicokinetic data. In addition, exposure to complex mixtures, including PCBs, has the potential to alter the toxicokinetics of individual compounds. These alterations in toxicokinetics may be involved in some of the nonadditive toxic or biological effects that are observed after exposure to mixtures of PCDDs or PCDFs with PCBs.

Toxicokinetic mixture interactions between chlorinated aromatic hydrocarbons in the liver of the C57BL/6J mouse: 2. Polychlorinated dibenzo-p-dioxins (PCDDs), dibenzofurans (PCDFs) and biphenyls (PCBs)

Six groups of C57BL/6J mice received single oral doses of 1.5-10.6 nmol/kg 1,2,3,7,8-pentachlorodibenzo-p-dioxin (PnCDD), 1,2,3,6,7,8-hexachlorodibenzo-p-dioxin (HxCDD) or 2,3,4,7,8-pentachlorodibenzofuran (PnCDF) as single compounds or in combination with 300 mumol/kg 2,2',4,4',5,5'-hexachlorobiphenyl (HxCB). Two other groups of mice received a mixture of the first three compounds, either with or without HxCB. The hepatic deposition and elimination of the compounds and their CYP1a dependent 7-ethoxyresorufin-O-deethylation (EROD) activity were studied until day 175. Interactive effects on the hepatic deposition of PnCDD were observed in most of the mixed dose groups. For HxCDD and PnCDF interactive effects were either very small or absent. No interactive effects were observed on hepatic elimination rates of PnCDD, HxCDD or PnCDF. No evidence was found for the influence of HxCB cotreatment on the hepatic concentration-response curves of the three compounds or their mixture. Based on the results from the present study it is concluded that PCDDs, PCDFs and PCBs may influence each other's, toxicokinetics when administered in mixtures.

Dioxin hepatic carcinogenesis: biologically motivated modeling and risk assessment

There are several key portions of the exposure-dose-response continuum with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD, dioxin) that have to be described quantitatively in developing a comprehensive mechanistically based dose-response model. These include: (i) the accumulation of TCDD in the target tissue, (ii) formation of a complex between dioxin and the Ah receptor, (iii) activation of transcription of growth regulatory genes by the TCDD-Ah receptor complex, (iv) cellular events on tumor initiation, promotion, and progression. Physiologically based pharmacokinetic (PBPK) models have been used as tools to integrate knowledge of the determinants of dioxin disposition, including specific binding to dioxin-inducible hepatic cytochromes, and to link TCDD tissue dosimetry with gene activation by pharmacodynamic (PD) models crafted to examine dioxin-regulated gene expression. Biological studies on growth factor regulation suggest hypotheses for the role of these gene products in transient cell proliferation, prolonged growth suppression, and hepatic tumor promotion. We have used these hypotheses as the basis for stochastic cell growth models of the promotional events with TCDD and to suggest experimental strategies for future research. The combination of PBPK, PBPD and stochastic cell growth models provides a seamless exposure-dose-response model for TCDD induction of liver tumors in rodents. This comprehensive exposure-dose-response model should prove useful for risk assessment, experimental design, and analysis of noncancer endpoints with this potent, ubiquitous environmental contaminant. This paper outlines progress in formulating and evaluating these models for TCDD.

Physiological pharmacokinetics and cancer risk assessment

There has been considerable progress in recent years in developing physiological models for the pharmacokinetics of toxic chemicals and in the application of these models in cancer risk assessment. Physiological pharmacokinetic models consist of a number of individual compartments, based on the anatomy and physiology of the mammalian organism of interest, and include specific parameters for metabolism, tissue binding, and tissue reactivity. Because of the correspondence between these compartments and specific tissues or groups of tissues, these models are particularly useful for predicting the doses of biologically active forms of toxic chemicals at target tissues under a wide variety of exposure conditions and in different animal species, including humans. Due to their explicit characterization of the biological processes governing pharmacokinetic behaviour, these models permit more accurate predictions of the dose of active metabolites reaching target tissues in exposed humans and hence of potential cancer risk. In addition, physiological models also permit a more direct evaluation of the impact of parameter uncertainty and inter-individual variability in cancer risk assessment. In this article, we review recent developments in physiologic pharmacokinetic modeling for selected chemicals and the application of these models in carcinogenic risk assessment. We examine the use of these models in integrating diverse information on pharmacokinetics and pharmacodynamics and discuss challenges in extending these pharmacokinetic models to reflect more accurately the biological events involved in the induction of cancer by different chemicals.

Modeling receptor-mediated processes with dioxin: implications for pharmacokinetics and risk assessment

Dioxin (2,3,7,8-tetrachlorodibenzo-p-dioxin; TCDD), a widespread polychlorinated aromatic hydrocarbon, caused tumors in the liver and other sites when administered chronically to rats at doses as low as 0.01 microgram/kg/day. It functions in combination with a cellular protein, the Ah receptor, to alter gene regulation, and this resulting modulation of gene expression is believed to be obligatory for both dioxin toxicity and carcinogenicity. The U.S. EPA is reevaluating its dioxin risk assessment and, as part of this process, will be developing risk assessment approaches for chemicals, such as dioxin, whose toxicity is receptor-mediated. This paper describes a receptor-mediated physiologically based pharmacokinetic (PB-PK) model for the tissue distribution and enzyme-inducing properties of dioxin and discusses the potential role of these models in a biologically motivated risk assessment. In this model, ternary interactions among the Ah receptor, dioxin, and DNA binding sites lead to enhanced production of specific hepatic proteins. The model was used to examine the tissue disposition of dioxin and the induction of both a dioxin-binding protein (presumably, cytochrome P4501A2), and cytochrome P4501A1. Tumor promotion correlated more closely with predicted induction of P4501A1 than with induction of hepatic binding proteins. Although increased induction of these proteins is not expected to be causally related to tumor formation, these physiological dosimetry and gene-induction response models will be important for biologically motivated dioxin risk assessments in determining both target tissue dose of dioxin and gene products and in examining the relationship between these gene products and the cellular events more directly involved in tumor promotion.

A physiologically based pharmacokinetic model for nicotine disposition in the Sprague-Dawley rat

A physiologically based pharmacokinetic (PBPK) model was developed to describe the disposition of nicotine in the Sprague-Dawley (SD) rat. Parameters for the model were either obtained from the literature (blood flows, organ volumes) or determined experimentally (partition coefficients). Nicotine metabolism was defined in the liver compartment by the first-order rate constants KNC and KNP which control the rate of nicotine metabolism to cotinine and "polar metabolites" (PM), respectively. These rate constants were estimated by optimizing the model fit to pharmacokinetic data obtained by administering an intraarterial (S)-[5-3H]nicotine bolus of 0.1 mg/kg to 6 rats. Model simulations that optimized for the appearance of cotinine in plasma estimated KNC and KNP to be 75.8 and 24.3 hr-1, respectively. Use of these constants in the model allowed us to accurately predict nicotine plasma kinetics and the fraction of the dose eliminated by renal (8.5%) and metabolic (91.5%) clearance. To validate the model's ability to predict tissue kinetics of nicotine, 21 male SD rats were administered 0.1 mg/kg (S)-[5-3H]nicotine intraarterially. At seven time points following treatment, 3 rats were euthanized and tissues were removed and analyzed for nicotine. Model-predicted nicotine tissue kinetics were in agreement with those determined experimentally in muscle, liver, skin, fat, and kidney. The brain, heart, and lung exhibited nonlinear nicotine elimination, suggesting that saturable nicotinic binding sites may be important in nicotine disposition in these organs. Inclusion of saturable receptor binding expressions in the mathematical description of these compartments resulted in better agreement with the experimental data. The Bmax and KD estimated by model simulations for these tissues were brain, 0.009 and 0.12; lung, 0.039 and 2.0; and heart, 0.039 nmol/tissue and 0.12 nM, respectively. This PBPK model can successfully describe the tissue and plasma kinetics of nicotine in the SD rat and will be a useful tool for pharmacologic studies in humans and experimental animals that require insight into the plasma or tissue concentration-effect relationship.

Pathology reevaluation of the Kociba et al. (1978) bioassay of 2,3,7,8-TCDD: implications for risk assessment

The chronic bioassay of 2,3,7,8-tetrachlorodibenzo-p-dioxin (2,3,7,8-TCDD) reported in 1978 by Kociba et al. has been considered to be the primary evidence supporting its carcinogenicity, and is the basis for most dioxin regulations in North America and Western Europe. Because the histopathological criteria for proliferative lesions in the rat liver have changed significantly since 1978, a reevaluation of the liver slides was conducted recently by an independent panel of pathologists. Using current National Toxicology Program criteria, their study showed, in contrast to the original findings, that about two-thirds fewer tumors were present in the livers of female Sprague-Dawley rats. The no-observed-adverse-effect level (NOAEL) for hepatocellular carcinomas was 0.01 micrograms/kg/d rather than 0.001 micrograms/kg/d, which had been reported in 1978. In light of these significant findings, a quantitative dose-response assessment of 2,3,7,8-TCDD was undertaken to predict the potential carcinogenic risks to humans. Risk-specific doses (RsDs) and cancer potency factors (CPFs) were calculated by applying the linearized multistage (LMS) model to the combined incidences of hepatocellular carcinomas and adenomas, classified in accordance with the 1990 histopathological criteria. Based on the weight of evidence regarding the mechanism of action of 2,3,7,8-TCDD, body weight rather than surface area was selected as the appropriate means for scaling rodent data to predict the human response. Using the survival-adjusted data, the RsD for a 1 in 1,000,000 (10(-6)) plausible upper bound (95%) lifetime incremental cancer risk was 370 fg/kg/d based only on the incidence of hepatocellular carcinomas, and 100 fg/kg/d when hepatocellular carcinomas and adenomas were combined. The corresponding upper-bound (95%) CPFs were 2700 and 9700 (mg/kg/d)-1, respectively. These results indicate that the carcinogenic risk to humans from exposure to 2,3,7,8-TCDD is at least 16-fold lower than previous estimates derived from the Kociba et al. (1978) bioassay.