Estimating the risk of liver cancer associated with human exposures to chloroform using physiologically based pharmacokinetic modeling

Monte-Carlo and multi-exposure assessment for the derivation of criteria for disinfection byproducts and volatile organic compounds in drinking water: Allocation factors and liter-equivalents per day

The probability distributions of total potential doses of disinfection byproducts and volatile organic compounds via ingestion, inhalation, and dermal exposure were estimated with Monte Carlo simulations, after conducting physiologically based pharmacokinetic model simulations to takes into account the differences in availability between the three exposures. If the criterion that the 95th percentile estimate equals the TDI (tolerable daily intake) is regarded as protecting the majority of a population, the drinking water criteria would be 140 (trichloromethane), 66 (bromodichloromethane), 157 (dibromochloromethane), 203 (tribromomethane), 140 (dichloroacetic acid), 78 (trichloroacetic acid), 6.55 (trichloroethylene, TCE), and 22 μg/L (perchloroethylene). The TCE criterion was lower than the Japanese Drinking Water Quality Standard (10 μg/L). The latter would allow the intake of 20% of the population to exceed the TDI. Indirect inhalation via evaporation from water, especially in bathrooms, was the major route of exposure to compounds other than haloacetic acids (HAAs) and accounted for 1.2-9 liter-equivalents/day for the median-exposure subpopulation. The ingestion of food was a major indirect route of exposure to HAAs. Contributions of direct water intake were not very different for trihalomethanes (30-45% of TDIs) and HAAs (45-52% of TDIs).

Determination of glyphosate and its metabolite in emergency room in Korea

The number of glyphosate intoxication cases has been increased after the regulation of paraquat. Unfortunately, there are no reports on the potential concentration of glyphosate for those acute intoxicated patients admitted to emergency rooms and the correlation between the concentration of glyphosate and clinical symptoms in Korea up to our knowledge. As a nonselective herbicide, analysis of glyphosate requires derivatization because of its amphoteric and strongly polar nature. In order to develop a method to determine the concentration of glyphosate and its metabolite, aminomethylphosphonic acid (AMPA) in blood samples without derivatization, liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS) was utilized with a hydrophilic interaction chromatography (HILIC) column. The validation of this method showed that the limits of detection (LODs) and limits of quantitation (LOQs) for glyphosate and AMPA were 50 and 100ng/mL, respectively. In addition, matrix effect, recovery rate, and accuracy and precision in intra and inter-day were evaluated during the validation study of this method. Blood samples acquired from five glyphosate intoxicated patients were analyzed to investigate the correlation between the concentration of glyphosate and clinical symptoms. These patients were previously admitted to the emergency room at a University Hospital in Korea after glyphosate was self-administered in suicide attempts or by accident. As results of blood sample study, the concentration of glyphosate and AMPA were found in the range of 1.0-171.1 and 0.2-2.6μg/mL, respectively. The concentration ratio of glyphosate to AMPA was 55-71. According to the clinical reports for those patients, they were in the age between 47 and 82 years old and administered about 50-400mL. The blood samples were collected within 2-5h after administration of glyphosate. Among the intoxicated patients, the most common clinical symptom was metabolic acidosis, identified in four patients. The comparison between the concentration of glyphosate and administered dosage did not show the correlation, which suggests further investigation on the effects of surfactants in glyphosate from different vendors.

Mathematical and statistical modeling in cancer systems biology

Cancer is a major health problem with high mortality rates. In the post-genome era, investigators have access to massive amounts of rapidly accumulating high-throughput data in publicly available databases, some of which are exclusively devoted to housing Cancer data. However, data interpretation efforts have not kept pace with data collection, and gained knowledge is not necessarily translating into better diagnoses and treatments. A fundamental problem is to integrate and interpret data to further our understanding in Cancer Systems Biology. Viewing cancer as a network provides insights into the complex mechanisms underlying the disease. Mathematical and statistical models provide an avenue for cancer network modeling. In this article, we review two widely used modeling paradigms: deterministic metabolic models and statistical graphical models. The strength of these approaches lies in their flexibility and predictive power. Once a model has been validated, it can be used to make predictions and generate hypotheses. We describe a number of diverse applications to Cancer Biology, including, the system-wide effects of drug-treatments, disease prognosis, tumor classification, forecasting treatment outcomes, and survival predictions.

Physiologically based pharmacokinetic/toxicokinetic modeling

Physiologically based pharmacokinetic (PBPK) models differ from conventional compartmental pharmacokinetic models in that they are based to a large extent on the actual physiology of the organism. The application of pharmacokinetics to toxicology or risk assessment requires that the toxic effects in a particular tissue are related in some way to the concentration time course of an active form of the substance in that tissue. The motivation for applying pharmacokinetics is the expectation that the observed effects of a chemical will be more simply and directly related to a measure of target tissue exposure than to a measure of administered dose. The goal of this work is to provide the reader with an understanding of PBPK modeling and its utility as well as the procedures used in the development and implementation of a model to chemical safety assessment using the styrene PBPK model as an example.

Toxicity testing in the 21st century: a vision and a strategy

With the release of the landmark report Toxicity Testing in the 21st Century: A Vision and a Strategy, the U.S. National Academy of Sciences, in 2007, precipitated a major change in the way toxicity testing is conducted. It envisions increased efficiency in toxicity testing and decreased animal usage by transitioning from current expensive and lengthy in vivo testing with qualitative endpoints to in vitro toxicity pathway assays on human cells or cell lines using robotic high-throughput screening with mechanistic quantitative parameters. Risk assessment in the exposed human population would focus on avoiding significant perturbations in these toxicity pathways. Computational systems biology models would be implemented to determine the dose-response models of perturbations of pathway function. Extrapolation of in vitro results to in vivo human blood and tissue concentrations would be based on pharmacokinetic models for the given exposure condition. This practice would enhance human relevance of test results, and would cover several test agents, compared to traditional toxicological testing strategies. As all the tools that are necessary to implement the vision are currently available or in an advanced stage of development, the key prerequisites to achieving this paradigm shift are a commitment to change in the scientific community, which could be facilitated by a broad discussion of the vision, and obtaining necessary resources to enhance current knowledge of pathway perturbations and pathway assays in humans and to implement computational systems biology models. Implementation of these strategies would result in a new toxicity testing paradigm firmly based on human biology.

Mode of action considerations in the quantitative assessment of tumour responses in the liver

Chemical carcinogenesis is a complex, multi-stage process and the relationship between dose and tumour formation is an important consideration in the risk assessment of chemicals. Extrapolation from empirical dose-response relationships obtained in experimental studies has been criticized, as it fails to take into account information on mode of action. Strategies for incorporating mode of action information into the risk assessment of chemical carcinogens are described, with a focus on hepatic cancer. Either toxicokinetic or toxicodynamic processes can be addressed. Whilst the former have been the focus of more attention to date, for example by using physiologically based modelling, there is increasing interest in the development of mode of action-based toxicodynamic models. These have the advantage that they do not require extreme assumptions, and may be amenable to paramaterization using human data. This is rarely if ever possible when using conventional dose-tumour response relationships. The approaches discussed are illustrated using chloroform as a case study. This compound is converted to a cytotoxic metabolite, phosgene, by CYP2E1 in liver and/or kidney. Cytotoxicity results in proliferative regeneration, with increased probability of tumour formation. Both physiologically based toxicokinetic and toxicodynamic models have been developed, and it is possible to use probabilistic approaches incorporating, for example, data on the distribution of hepatic CYP2E1 levels. Mode of action can provide an invaluable link between observable, experimental data, on both toxicokinetics and toxicodynamics, and chemical-specific risk assessment, based on physiological approaches.

Applying Mode-of-Action and Pharmacokinetic Considerations in Contemporary Cancer Risk Assessments: An Example with Trichloroethylene

The guidelines for carcinogen risk assessment recently proposed by the U.S. Environmental Protection Agency (U.S. EPA) provide an increased opportunity for the consideration of pharmacokinetic and mechanistic data in the risk assessment process. However, the greater flexibility of the new guidelines can also make their actual implementation for a particular chemical highly problematic. To illuminate the process of performing a cancer risk assessment under the new guidelines, the rationale for a state-of-the-science risk assessment for trichloroethylene (TCE) is presented. For TCE, there is evidence of increased cell proliferation due to receptor interaction or cytotoxicity in every instance in which tumors are observed, and most tumors represent an increase in the incidence of a commonly observed, species-specific lesion. A physiologically based pharmacokinetic (PBPK) model was applied to estimate target tissue doses for the three principal animal tumors associated with TCE exposure: liver, lung, and kidney. The lowest points of departure (lower bound estimates of the exposure associated with 10% tumor incidence) for lifetime human exposure to TCE were obtained for mouse liver tumors, assuming a mode of action primarily involving the mitogenicity of the metabolite trichloroacetic acid (TCA). The associated linear unit risk estimates for mouse liver tumors are 1.5 x 10(-6) for lifetime exposure to 1 microg TCE per cubic meter in air and 0.4 x 10(-6) for lifetime exposure to 1 microg TCE per liter in drinking water. However, these risk estimates ignore the evidence that the human is likely to be much less responsive than the mouse to the carcinogenic effects of TCA in the liver and that the carcinogenic effects of TCE are unlikely to occur at low environmental exposures. Based on consideration of the most plausible carcinogenic modes of action of TCE, a margin-of-exposure (MOE) approach would appear to be more appropriate. Applying an MOE of 1000, environmental exposures below 66 microg TCE per cubic meter in air and 265 microg TCE per liter in drinking water are considered unlikely to present a carcinogenic hazard to human health.

Approaches for applications of physiologically based pharmacokinetic models in risk assessment

Physiologically based pharmacokinetic (PBPK) models are particularly useful for simulating exposures to environmental toxicants for which, unlike pharmaceuticals, there is often little or no human data available to estimate the internal dose of a putative toxic moiety in a target tissue or an appropriate surrogate. This article reviews the current state of knowledge and approaches for application of PBPK models in the process of deriving reference dose, reference concentration, and cancer risk estimates. Examples drawn from previous U.S. Environmental Protection Agency (EPA) risk assessments and human health risk assessments in peer-reviewed literature illustrate the ways and means of using PBPK models to quantify the pharmacokinetic component of the interspecies and intraspecies uncertainty factors as well as to conduct route to route, high dose to low dose and duration extrapolations. The choice of the appropriate dose metric is key to the use of the PBPK models for the various applications in risk assessment. Issues related to whether uncertainty factors are most appropriately applied before or after derivation of human equivalent dose (or concentration) continue to be explored. Scientific progress in the understanding of life stage and genetic differences in dosimetry and their impacts on variability in susceptibility, as well as ongoing development of analytical methods to characterize uncertainty in PBPK models, will make their use in risk assessment increasingly likely. As such, it is anticipated that when PBPK models are used to express adverse tissue responses in terms of the internal target tissue dose of the toxic moiety rather than the external concentration, the scientific basis of, and confidence in, risk assessments will be enhanced.

On the incorporation of chemical-specific information in risk assessment

This paper describes the evolution of chemical risk assessment from its early dependence on generic default approaches to the current situation in which mechanistic and biokinetic data are routinely incorporated to support a more chemical-specific approach. Two methodologies that have played an important role in this evolution are described: mode-of-action evaluation and physiologically based biokinetic (PBBK) modelling. When used together, these techniques greatly increase the opportunity for the incorporation of biokinetic and mechanistic data in risk assessment. The resulting risk assessment approaches are more appropriately tailored to the specific chemical and are more likely to provide an accurate assessment of the potential hazards associated with human exposures. The appropriate application of PBBK models in risk assessment demands well-formulated statements about the chemical mode of action. It is this requirement for an explicit, mechanistic hypothesis that gives biologically motivated models their power, but at the same time serves as the greatest impediment to the acceptance of a chemical-specific risk assessment approach by regulators. The chief impediment to the regulatory acceptance and application of PBBK models in risk assessment is concern about uncertainties associated with their use. To some extent such concerns can be addressed by the development of generally accepted approaches for model evaluation and quantitative uncertainty analysis. In order to assure the protection of public health while limiting the economic and social consequences of over-regulation, greater dialogue between researchers and regulators is crucially needed to foster an increased use of emerging scientific information and innovative methods in chemical risk assessments.

Reverse dosimetry: interpreting trihalomethanes biomonitoring data using physiologically based pharmacokinetic modeling

Biomonitoring data provide evidence of exposure of environmental chemicals but are not, by themselves, direct measures of exposure. To use biomonitoring data in understanding exposure, physiologically based pharmacokinetic (PBPK) modeling can be used in a reverse dosimetry approach to assess a distribution of exposures possibly associated with specific blood or urine levels of compounds. Reverse dosimetry integrates PBPK modeling with exposure pattern characterization, Monte Carlo analysis, and statistical tools to estimate a distribution of exposures that are consistent with biomonitoring data in a population. The present study used an existing PBPK model for chloroform as a generic framework to develop PBPK models for other trihalomethanes (THMs). Using Monte Carlo sampling techniques, probabilistic information about pharmacokinetics and exposure patterns was included to estimate distributions of THMs concentrations in blood in relation to various exposure patterns in a diverse population. In addition, the possibility of inhibition of hepatic metabolism among THMs was evaluated under the scenarios of household exposure. These studies demonstrated how PBPK modeling can be used as a tool to estimate a population distribution of exposures that could have resulted in particular biomonitoring results. When toxicity level is known, this tool can also be used to estimate proportion of population above levels associated with health risk.

Use of a physiologically based pharmacokinetic model to identify exposures consistent with human biomonitoring data for chloroform

Biomonitoring data provide evidence of human exposure to environmental chemicals by quantifying the chemical or its metabolite in a biological matrix. To better understand the correlation between biomonitoring data and environmental exposure, physiologically based pharmacokinetic (PBPK) modeling can be of use. The objective of this study was to use a combined PBPK model with an exposure model for showering to estimate the intake concentrations of chloroform based on measured blood and exhaled breath concentrations of chloroform. First, the predictive ability of the combined model was evaluated with three published studies describing exhaled breath and blood concentrations in people exposed to chloroform under controlled showering events. Following that, a plausible exposure regimen was defined combining inhalation, ingestion, and dermal exposures associated with residential use of water containing typical concentrations of chloroform to simulate blood and exhaled breath concentrations of chloroform. Simulation results showed that inhalation and dermal exposure could contribute substantially to total chloroform exposure. Next, sensitivity analysis and Monte Carlo analysis were performed to investigate the sources of variability in model output. The variability in exposure conditions (e.g., shower duration) was shown to contribute more than the variability in pharmacokinetics (e.g., body weight) to the predicted variability in blood and exhaled breath concentrations of chloroform. Lastly, the model was used in a reverse dosimetry approach to estimate distributions of exposure consistent with concentrations of chloroform measured in human blood and exhaled breath.

Dose-Incidence Modeling: Consequences of Linking Quantal Measures of Response to Depletion of Critical Tissue Targets

In developing mechanistic PK-PD models, incidence of toxic responses in a population has to be described in relation to measures of biologically effective dose (BED). We have developed a simple dose-incidence model that links incidence with BED for compounds that cause toxicity by depleting critical cellular target molecules. The BED in this model was the proportion of target molecule adducted by the dose of toxic compound. Our modeling approach first estimated the proportion depleted for each dose and then calculated the tolerance distribution for toxicity in relation to either administered dose or log of administered dose. We first examined cases where the mean of the tolerance distribution for toxicity occurred when a significant proportion of target had been adducted (i.e., more than half). When a normal distribution was assumed to exist for the relationship of incidence and BED, the tolerance distribution based on administered dose for these cases becomes asymmetrical and logarithmic transformations of the administered dose axis lead to a more symmetrical distribution. These linked PK-PD models for tissue reactivity, consistent with conclusions from other work for receptor binding models (Lutz et al., 2005), indicate that log normal distributions with administered dose may arise from normal distributions for BED and nonlinear kinetics between BED and administered dose. These conclusions are important for developing biologically based dose response (BBDR) models that link incidences of toxicity or other biological responses to measures of BED.

Risk assessment of chemicals and pharmaceuticals in the pediatric population: a workshop report

ATSDR and RIVM organized an Expert Panel Workshop on the Differences Between Children and Adults and Their Relevance to Risk Assessment. The workshop was held in June 2003, in Brussels, Belgium. The purpose of the workshop was to identify data gaps in current scientific knowledge related to children's health and to recognize areas of mutual interest that would serve as the basis for upcoming ATSDR/RIVM cooperative projects. The aim for both agencies is a better understanding of the issues related to children's health, and the improvement of scientifically based (chemical) risk assessment in children. Topics discussed included clinical trials/toxicity studies, testing in juvenile animals, PBPK modeling in children, and children's risk assessment.

A spreadsheet program for modeling quantitative structure-pharmacokinetic relationships for inhaled volatile organics in humans

The extent and profile of target tissue exposure to toxicants depend upon the pharmacokinetic processes, namely, absorption, distribution, metabolism and excretion. The present study developed a spreadsheet program to simulate the pharmacokinetics of inhaled volatile organic chemicals (VOCs) in humans based on information from molecular structure. The approach involved the construction of a human physiologically-based pharmacokinetic (PBPK) model, and the estimation of its parameters based on quantitative structure-property relationships (QSPRs) in an Excel spreadsheet. The compartments of the PBPK model consisted of liver, adipose tissue, poorly perfused tissues and richly perfused tissues connected by circulating blood. The parameters required were: human physiological parameters such as cardiac output, breathing rate, tissue volumes and tissue blood flow rates (obtained from the biomedical literature), tissue/air partition coefficients (obtained using QSPRs developed with rat data), blood/air partition coefficients (Pb) and hepatic clearance (CL). Using literature data on human Pb and CL for several VOCs (alkanes, alkenes, haloalkanes and aromatic hydrocarbons), multi-linear additive QSPR models were developed. The numerical contributions to human Pb and CL were obtained for eleven structural fragments (CH3, CH2, CH, C, C [double bond] C, H, Cl, Br, F, benzene ring, and H in the benzene ring structure). Using these data as input, the PBPK model written in an Excel spreadsheet simulated the inhalation pharmacokinetics of ethylbenzene (33 ppm, 7 h) and dichloromethane (100 ppm, 6 h) in humans exposed to these chemicals. The QSPRs developed in this study should be useful for predicting the inhalation pharmacokinetics of VOCs in humans, prior to testing and experimentation.

Physiologically-based pharmacokinetic and toxicokinetic models in cancer risk assessment

Physiologically-based pharmacokinetic (PBPK) and toxicokinetic models are increasingly being used for the conduct of high dose to low dose and interspecies extrapolations required in cancer risk assessment. These models, by simulating tissue dose of toxic chemicals, help address the uncertainty associated with the default approaches for interspecies and high dose to low dose extrapolations. The applicability of PBPK models in cancer risk assessment has been demonstrated with a number of chemicals (e.g., acrylonitrile, 2-butoxyethanol, chloroform, 1,4-dioxane, methyl chloroform, methylene chloride, styrene, trichloroethylene, tetrachloroethylene, vinyl chloride, vinyl acetate). Recent advances in PBPK modeling facilitate the consideration of population distribution of parameter values, age-dependent changes in physiology and metabolism, multi-route exposures as well as multichemical interactions for application in cancer risk assessment. Whereas the average values for various input parameters have been used to evaluate the age-dependency of tissue dose, the Markov Chain Monte Carlo technique can be applied to address variability and uncertainty in parameter estimates, thus facilitating a more accurate estimation of cancer risk in the population. The PBPK models also uniquely facilitate the simulation of tissue dose, and thereby cancer risks, associated with multi-route and multichemical exposure situations. Overall, the recent advances reviewed in this article point to the continued enhancement of the scientific basis and applicability of PBPK models in cancer risk assessment.

Assessing the dose-dependency of allometric scaling performance using physiologically based pharmacokinetic modeling

The performance of allometric scaling of dose as a power of body weight under a variety of extrapolation conditions with respect to species, route, exposure intensity, and mechanism/mode of action, remains untested in many cases. In this paper, animal-human internal dose ratio comparisons have been developed for 12 chemicals (benzene, carbon tetrachloride, chloroform, diisopropylfluorophosphate, ethanol, ethylene oxide, methylene chloride, methylmercury, styrene, tetrachloroethene, trichloroethene, and vinyl chloride). This group of predominantly volatile and lipophilic chemicals was selected on the basis that their kinetics have been well-studied and can be predicted in mice, rats, and humans using physiologically based pharmacokinetic (PBPK) models. PBPK model predictions were compared to the allometric scaling predictions for interspecies extrapolation. Recommendations for the application of the allometric scaling are made with reference to internal dose measure (mode of action) and concentration level. The results of this assessment generally support the use of scaling factors recommended in the published literature, which includes scaling factors of 1.0 for risk assessments in which toxicity is attributed to the parent chemical or stable metabolite, and -0.75 for dose-response assessments in which toxicity is attributed to the formation of a reactive metabolite from an inhaled compound. A scaling factor of 0.75 is recommended for dose-response assessments of orally administered compounds in which toxicity is attributed to the parent chemical or stable metabolite and 1.0 for risk assessments in which toxicity is attributed to the formation of a reactive metabolite from a compound administered by the oral route. A dose-dependency in the results suggests that the scaling factors appropriate at high exposures may differ from those at low exposures, primarily due to the impact of saturable metabolism.

Chloroform: exposure estimation, hazard characterization, and exposure-response analysis

Chloroform has been assessed as a Priority Substance under the Canadian Environmental Protection Act. The general population in Canada is exposed to chloroform principally through inhalation of indoor air, particularly during showering, and through ingestion of tap water. Data on concentrations of chloroform in various media were sufficient to serve as the basis for development of deterministic and probabilistic estimates of exposure for the general population in Canada. On the basis of data acquired principally in studies in experimental animals, chloroform causes hepatic and renal tumors in mice and renal tumors in rats. The weight of evidence indicates that chloroform is likely carcinogenic only at concentrations that induce the obligatory precursor lesions of cytotoxicity and proliferative regenerative response. Since this cytotoxicity is primarily related to rates of formation of reactive, oxidative metabolites, dose response has been characterized in the context of rates of formation of reactive metabolites in the target tissue. Results presented here are from a "hybrid" physiologically based pharmacokinetic (PBPK) animal model that was revised to permit its extension to humans. The relevant measure of exposure response, namely, the mean rate of metabolism in humans associated with a 5% increase in tumor risk (TC05), was estimated on the basis of this PBPK model and compared with tissue dose measures resulting from 24-h multimedia exposure scenarios for Canadians based on midpoint and 95th percentiles for concentrations in outdoor air, indoor air, air in the shower compartment, air in the bathroom after showering, tap water, and food. Nonneoplastic effects observed most consistently at lowest concentrations or doses following repeated exposures of rats and mice to chloroform are cytotoxicity and regenerative proliferation. As for cancer, target organs are the liver and kidney. In addition, chloroform has induced nasal lesions in rats and mice exposed by both inhalation and ingestion at lowest concentrations or doses. The mean rate of metabolism associated with a 5% increase in fatty cysts estimated on the basis of the PBPK model was compared with tissue dose measures resulting from the scenarios already described, and lowest concentrations reported to induce cellular proliferation in the nasal cavities of rats and mice were compared directly with midpoint and 95th percentile estimates of concentrations of chloroform in indoor air in Canada. The degree of confidence in the underlying database and uncertainties in estimates of exposure and in characterization of hazard and dose response are delineated.

Cancer risk associated with household exposure to chloroform

Chloroform (CHCl3) the trihalomethane most prevalent in drinking water, is a proven animal carcinogen and a suspected human carcinogen. Consequently, standards have been issued by health authorities to limit its concentration in drinking water. These limits are based solely on ingestion, without taking into account inhalation and skin contact. Exposure to CHCl3 was assessed for 18 men (age: mean 38 years; range 23-51) following a 10-min shower in their respective residences located in the Quebec City region (Canada). CHCl3 concentration was measured in alveolar air samples collected before, immediately after, and 15 min and 30 min following the shower. Indoor air and water concentrations were determined concomitantly. Mean CHCl3 concentrations in the air of the shower stall and in water were respectively 147 microg/m3 (SD = 56.2 microg/m3) and 20.1 microg/L (SD = 9.0 microg/L). Water concentrations were comparable to those documented in a large proportion of distribution networks in Canada. The mean increase in alveolar air CHCl3 concentration (deltaCHCIALV) at the end of the shower was 33 microg/m3 (SD = 14.7 microg/m3). A multiple-regression analysis revealed that deltaCHCl3ALV values were only associated with chloroform concentration in air of the shower stall. DeltaCHCl3ALV were described using a physiologically based pharmacokinetic (PBPK) model. This model was then used to estimate concentrations of CHCl3 metabolites bound to liver and kidney macromolecules following a shower, and also according to exposure scenarios that integrate drinking-water ingestion and air inhalation. The concentration predicted in the liver following a worst-case exposure scenario was 0.41 microg CHCl3 equivalents/kg of tissue, some 6,000 times lower than the lowest concentration that did not increase the incidence of hepatic tumors in laboratory animals. Data indicate that for this range of exposure the safety margin appears therefore considerable with respect to the potential carcinogenic effect of household exposure to CHCl3.

The effects of inhalation exposure to bromo-dichloromethane on specific rat CYP isoenzymes

Several cytochrome P450 (CYP) isoenzymes may be involved in the metabolism of bromo-dichloromethane (BDCM), a drinking water disinfection byproduct. After 4-h inhalation exposures of male F344 rats to BDCM between 100 and 3200 p.p.m., hepatic microsomal methoxyresorufin demethylase (MROD), ethoxyresorufin de-ethylease (EROD) and pentoxyresorufin dealkylase (PROD) activities showed modest increases at low exposure levels and larger decreases at high exposure levels, compared with controls. Western blots for CYP1A2 and CYP2B1 showed similar trends. In addition, p-nitrophenol hydroxylase (PNP) activity was measured and Western blots for CYP2E1 were performed. CYP2E1 and CYP2B1 isoenzymes are known to metabolize BDCM (Thornton-Manning, J.R., Gao, P., Lilly, P.D., Pegram, R.A., 1993. Acute bromodichloromethane toxicity in rats pretreated with cytochrome P450 inducers and inhibitors. The Toxicologist 13: 361). When compared with a multiple gavage study of BDCM in female F344 rats (Thornton-Manning, J.R., et al., 1994. Toxicology 94, 3-18), the results of the two studies for EROD, PROD, and PNP activities were qualitatively the same; PNP activity did not change, while both PROD and EROD activities decreased at high exposures. In the current work, Western blots for CYP2E1, CYP2B1 and CYP1A2 supported the results from the PNP, PROD and MROD activities, respectively. The decreases in MROD and PROD activities and in Western blots for CYP1A2 and CYP2B1 at high exposures suggest that BDCM may be a suicide substrate for these CYP isoenzymes. Other important conclusions that can be drawn from the comparison between the current and prior work are that the liver response is similar for both sexes, and it is also similar for inhalation and gavage exposures under these conditions. Finally, the decrease in EROD activity at high doses, found in both studies, may be a further reflection of CYP1A2 activity, since little or no CYP1A1 activity is normally found in uninduced rat liver and CYP1A2 is known to metabolize ethoxyresorufin, although much more slowly than CYP1A1.

Physiological-model-based derivation of the adult and child pharmacokinetic intraspecies uncertainty factors for volatile organic compounds

The intraspecies uncertainty factor (UF(HH)=10x) is used in the determination of the reference dose or reference concentration and accounts for the pharmacokinetic and pharmacodynamic heterogeneity within the human population. The Food Quality Protection Act of 1996 mandated the use of an additional uncertainty factor (UF(HC)=10x) to take into account potential pre- and postnatal toxicity and lack of completeness of the data with respect to exposure and toxicity to children. There is no conclusive experimental or theoretical justification to support or refute the magnitude of the UF(HH) and UF(HC) nor any conclusive evidence to suggest that a factor of 100 is needed to account for intrahuman variability. This study presents a new chemical-specific method for estimating the pharmacokinetic (PK) component of the interspecies uncertainty factor (UF(HH-PK) and UF(HC-PK)) for volatile organic compounds (VOCs). The approach utilizes validated physiological-based pharmacokinetic (PBPK) models and simplified physiological-model-based algebraic equations to translate ambient exposure concentration to tissue dose in adults and children the ratio of which is the UF(HH-PK) and UF(HC-PK). The results suggest that: (i) the UF(HH-PK) and UF(HC-PK) are chemical specific; (ii) for the chemicals used in this study there is no significant difference between UF(HH-PK) and UF(HC-PK); (iii) the magnitude of UF(HH-PK) and UF(HC-PK) varies between 0.033 and 2.85 with respect to tissue and blood concentrations; (iv) the body weight, the rate of ventilation, the fraction of cardiac output flowing to the liver, the blood : air partition coefficient, and the hepatic extraction ratio are the only parameters that play a critical role in the variability of tissue and blood doses within species; and (v) the magnitude of the UF(HH-PK) and UF(HC-PK) obtained with the simplified steady-state equations is essentially the same with that obtained with PBPK models. Overall, this study suggests that no adult-children differences in the parent chemical concentrations of the VOCs are likely to be observed during inhalation exposures. The physiological-model-based approaches used in the present study to estimate the UF(HH-PK) and UF(HC-PK) provide a scientific basis for their magnitude. They can replace the currently used empirical default approaches to provide chemical-specific UF(HH-PK) in future risk assessments.

The utility of PBPK in the safety assessment of chloroform and carbon tetrachloride

Occupational exposure limits (OELs) for individual substances are established on the basis of the available toxicological information at the time of their promulgation, expert interpretation of these data in light of industrial use, and the framework in which they sit. In the United Kingdom, the establishment of specific OELs includes the application of uncertainty factors to a defined starting point, usually the NOAEL from a suitable animal study. The magnitude of the uncertainty factors is generally determined through expert judgment including a knowledge of workplace conditions and management of exposure. PBPK modeling may help in this process by informing on issues relating to extrapolation between and within species. This study was therefore designed to consider how PBPK modeling could contribute to the establishment of OELs. PBPK models were developed for chloroform (mouse and human) and carbon tetrachloride (rat and human). These substances were chosen for examination because of the extent of their toxicological databases and availability of existing PBPK models. The models were exercised to predict the rate (chloroform) or extent (carbon tetrachloride) of metabolism of these substances, in both rodents and humans. Monte Carlo analysis was used to investigate the influence of variability within the human and animal model populations. The ratio of the rates/extent of metabolism predicted for humans compared to animals was compared to the uncertainty factors involved in setting the OES. Predictions obtained from the PBPK models indicated that average rat and mouse metabolism of carbon tetrachloride and chloroform, respectively, are much greater than that of the average human. Application of Monte Carlo analysis indicated that even those people who have the fastest rates or most extensive amounts of metabolism in the population are unlikely to generate the levels of metabolite of these substances necessary to produce overt toxicity in rodents. This study highlights the value that the use of PBPK modeling may add to help inform and improve toxicological aspects of a regulatory process.

A physiological toxicokinetic model for exogenous and endogenous ethylene and ethylene oxide in rat, mouse, and human: formation of 2-hydroxyethyl adducts with hemoglobin and DNA

Ethylene (ET) is a gaseous olefin of considerable industrial importance. It is also ubiquitous in the environment and is produced in plants, mammals, and humans. Uptake of exogenous ET occurs via inhalation. ET is biotransformed to ethylene oxide (EO), which is also an important volatile industrial chemical. This epoxide forms hydroxyethyl adducts with macromolecules such as hemoglobin and DNA and is mutagenic in vivo and in vitro and carcinogenic in experimental animals. It is metabolically eliminated by epoxide hydrolase and glutathione S-transferase and a small fraction is exhaled unchanged. To estimate the body burden of EO in rodents and human resulting from exposures to EO and ET, we developed a physiological toxicokinetic model. It describes uptake of ET and EO following inhalation and intraperitoneal administration, endogenous production of ET, enzyme-mediated oxidation of ET to EO, bioavailability of EO, EO metabolism, and formation of 2-hydroxyethyl adducts of hemoglobin and DNA. The model includes compartments representing arterial, venous, and pulmonary blood, liver, muscle, fat, and richly perfused tissues. Partition coefficients and metabolic parameters were derived from experimental data or published values. Model simulations were compared with a series of data collected in rodents or humans. The model describes well the uptake, elimination, and endogenous production of ET in all three species. Simulations of EO concentrations in blood and exhaled air of rodents and humans exposed to EO or ET were in good agreement with measured data. Using published rate constants for the formation of 2-hydroxyethyl adducts with hemoglobin and DNA, adduct levels were predicted and compared with values reported. In humans, predicted hemoglobin adducts resulting from exposure to EO or ET are in agreement with measured values. In rodents, simulated and measured DNA adduct levels agreed generally well, but hemoglobin adducts were underpredicted by a factor of 2 to 3. Obviously, there are inconsistencies between measured DNA and hemoglobin adduct levels.

A pharmacokinetic modeling of inorganic arsenic: a short-term oral exposure model for humans

This study presents a pharmacokinetic modeling of inorganic arsenic disposition in human body for short-term oral exposures. Effort on the development of the model is directed toward the prediction of the kinetic behavior of inorganic arsenic and its metabolites in the body. The current model considers the 4 circulating species; AsIII, AsV, and two metabolites such as monomethylarsenic (MMA) and dimethylarsenic (DMA) in the blood and tissue groups. While it is difficult to estimate some parameters used in the models at this time, the current model assumptions and predictions seem to be consistent with the experimental observations found in the literature. Hence, the current model, when more fully developed, is expected to provide insight into the behavior of inorganic arsenic and its methylated metabolites within the body, and may help increase the understanding of risk assessment issues associated with inorganic arsenic in drinking water.

Metabolism of chloroform by cytochrome P450 2E1 is required for induction of toxicity in the liver, kidney, and nose of male mice

Chloroform is a nongenotoxic-cytotoxic liver and kidney carcinogen and nasal toxicant in some strains and sexes of rodents. Substantial evidence indicates that tumor induction is secondary to events associated with cytolethality and regenerative cell proliferation. Therefore, pathways leading to toxicity, such as metabolic activation, become critical information in mechanism-based risk assessments. The purpose of this study was to determine the degree to which chloroform-induced cytotoxicity is dependent on the cytochromes P450 in general and P450 2E1 in particular. Male B6C3F(1), Sv/129 wild-type (Cyp2e1+/+), and Sv/129 CYP2E1 knockout (Cyp2e1-/- or Cyp2e1-null) mice were exposed 6 h/day for 4 consecutive days to 90 ppm chloroform by inhalation. Parallel control and treated groups, excluding Cyp2e1-null mice, also received an i.p. injection (150 mg/kg) of the irreversible cytochrome P450 inhibitor 1-aminobenzotriazole (ABT) twice on the day before exposures began and 1 h before every exposure. Cells in S-phase were labeled by infusion of BrdU via an implanted osmotic pump for 3.5 days prior to necropsy, and the labeling index was quantified immunohistochemically. B6C3F(1) and Sv/129 wild-type mice exposed to chloroform alone had extensive hepatic and renal necrosis with significant regenerative cell proliferation. These animals had minimal toxicity in the nasal turbinates with focal periosteal cell proliferation. Administration of ABT completely protected against the hepatic, renal, and nasal toxic effects of chloroform. Induced pathological changes and regenerative cell proliferation were absent in these target sites in Cyp2e1-/- mice exposed to 90 ppm chloroform. These findings indicate that metabolism is obligatory for the development of chloroform-induced hepatic, renal, and nasal toxicity and that cytochrome P450 2E1 appears to be the only enzyme responsible for this cytotoxic-related metabolic conversion under these exposure conditions.

A comprehensive approach for integration of toxicity and cancer risk assessments

Experimental observations and theoretical considerations indicate a dose threshold for most chemically induced noncancer toxic effects below which the increased risk of toxicity is zero. Thus, the historical approach for minimizing risk from toxic chemicals has been to experimentally determine a no-observed-adverse-effect-level (NOAEL) and then to apply safety or uncertainty factors to estimate a dose not expected to produce that toxic effect in humans. In contrast, for radiation and chemically induced cancer, it has been assumed that all agents operate by a genotoxic mode of action and that some risk can be assigned to even vanishingly small doses. Accordingly, risk assessments for carcinogens have commonly been based on the assumption that the tumor dose-response curve at low doses is linear and passes through the origin. Mode of action is defined as a fundamental obligatory step in the induction of toxicity or cancer. It is now clear that tumor induction can arise in a variety of ways including not only a DNA-reactive genotoxic mode of action, but also non-DNA-reactive nongenotoxic-cytotoxic and nongenotoxic-mitogenic modes of action. Initial risk assessment approaches that recognized this distinction identified a chemical carcinogen as either genotoxic or nongenotoxic, with no middle ground. The realization that there is a continuum whereby different chemicals can act by a combination of modes of action and the recent explosion of research into molecular mechanisms of carcinogenesis indicate that all relevant information should be integrated into the risk assessment process on a case by case basis. A comprehensive approach to risk assessment demands that default assumptions be replaced with an integrated understanding of the rate-limiting steps in the induction of toxicity or cancer along with quantitative measures of the shapes of those dose-response curves. The examples of more contemporary risk assessments are presented for chloroform and vinyl acetate.

Application of an automated HS-GC method in partition coefficient determination for xylenes and ethylbenzene in rat tissues

An automated static head space-gas chromatography method was used in the determination of partition coefficients (Kd) for the xylene isomers and ethylbenzene in blood, brain, muscle, kidney, liver and fat of Sprague Dawley rats. Since homogenization resulted in the potential loss of analytes from tissue samples, unhomogenized samples were used. With a few exceptions, tissue:air Kd values were independent of the concentrations of the analytes, singly or as a mixture. The tissue:blood Kd values were determined. For each tissue and analyte, the value obtained for each analyte concentration was within +/- 10% of the mean value calculated for the entire concentration range.

Chloroform-induced cytolethality in freshly isolated male B6C3F1 mouse and F-344 rat hepatocytes

Chloroform is carcinogenic in rodents but is not mutagenic or DNA reactive. Chloroform-induced hepatocarcinogenesis in rodents is believed to be secondary to events associated with cytotoxicity and cell proliferation. Understanding the mechanisms of chloroform toxicity may provide insights into the mechanisms of carcinogenicity. The goal of these studies was to characterize the cytotoxicity of chloroform in male B6C3F1 mouse and F-344 rat hepatocytes in vitro. We used an in vitro suspension-culture system that reproduced the exposure of the liver to chloroform and the expression of toxicity in vivo. Simulations of a physiologically based dosimetry model for chloroform indicated that the livers of mice and rats were exposed to chloroform concentrations up to 5 mM for 3 h after hepatotoxic doses of chloroform. Freshly isolated male mouse and rat hepatocytes were exposed to chloroform in sealed flasks and then cultured for 24 h as monolayers. Following a 2- or 3-h exposure in suspension, chloroform induced concentration-dependent cytotoxicity (lactate dehydrogenase release) in culture at concentrations higher than 1 mM. Cytolethality was not increased under reduced oxygen tension, indicating that reductive metabolism does not contribute to chloroform-induced toxicity. A threshold of 1 mM chloroform was also found for glutathione (GSH) depletion, with a 50% depletion at 3.8 mM after 2 h. Addition of dithiothreitol, a reducing agent, did not prevent chloroform-induced toxicity, indicating that oxidation of sulfhydryl groups is not critical for toxicity. The lack of protein sulfhydryl group depletion is consistent with this conclusion. Cotreatment with the cytochrome P450 inhibitor 1-phenylimidazole prevented both cytolethality and GSH depletion, indicating that metabolism is necessary for chloroform-induced toxicity. Both species exhibited similar sensitivity toward chloroform toxicity, indicating that toxicity is not sufficient to explain different susceptibility in heptocarcinogenicity. As chloroform metabolism is saturated in the micromolar range, our results indicate that both metabolism and exposure of the liver cells to high concentrations of chloroform are required for toxicity.

Investigation of the potential impact of benchmark dose and pharmacokinetic modeling in noncancer risk assessment

There has been relatively little attention given to incorporating knowledge of mode of action or of dosimetry of active toxic chemical to target tissue sites in the calculation of noncancer exposure guidelines. One exception is the focus in the revised reference concentration (RfC) process on delivered dose adjustments for inhaled materials. The studies reported here attempt to continue in the spirit of the new RfC guidelines by incorporating both mechanistic and delivered dose information using a physiologically based pharmacokinetic (PBPK) model, along with quantitative dose-response information using the benchmark dose (BMD) method, into the noncancer risk assessment paradigm. Two examples of the use of PBPK and BMD techniques in noncancer risk assessment are described: methylene chloride, and trichloroethylene. Minimal risk levels (MRLs) based on PBPK analysis of these chemicals were generally similar to those based on the traditional process, but individual MRLs ranged from roughly 10-fold higher to more than 10-fold lower than existing MRLs that were not based on PBPK modeling. Only two MRLs were based on critical studies that presented adequate data for BMD modeling, and in these two cases the BMD models were unable to provide an acceptable fit to the overall dose-response of the data, even using pharmacokinetic dose metrics. A review of 10 additional chemicals indicated that data reporting in the toxicological literature is often inadequate to support BMD modeling. Three general observations regarding the use of PBPK and BMD modeling in noncancer risk assessment were noted. First, a full PBPK model may not be necessary to support a more accurate risk assessment; often only a simple pharmacokinetic description, or an understanding of basic pharmacokinetic principles, is needed. Second, pharmacokinetic and mode of action considerations are a crucial factor in conducting noncancer risk assessments that involve animal-to-human extrapolation. Third, to support the application of BMD modeling in noncancer risk assessment, reporting of toxicity results in the toxicological literature should include both means and standard deviations for each dose group in the case of quantitative endpoints, such as relative organ weights or testing scores, and should report the number of animals affected in the case of qualitative endpoints.

Risk characterization of methyl tertiary butyl ether (MTBE) in tap water

Methyl tertiary butyl ether (MTBE) can enter surface water and groundwater through wet atmospheric deposition or as a result of fuel leaks and spills. About 30% of the U.S. population lives in areas where MTBE is in regular use. Ninety-five percent of this population is unlikely to be exposed to MTBE in tap water at concentrations exceeding 2 ppb, and most will be exposed to concentrations that are much lower and may be zero. About 5% of this population may be exposed to higher levels of MTBE in tap water, resulting from fuel tank leaks and spills into surface or groundwater used for potable water supplies. This paper describes the concentration ranges found and anticipated in surface and groundwater, and estimates the distribution of doses experienced by humans using water containing MTBE to drink, prepare food, and shower/bathe. The toxic properties (including potency) of MTBE when ingested, inhaled, and in contact with the skin are summarized. Using a range of human toxic potency values derived from animal studies, margins of exposure (MOE) associated with alternative chronic exposure scenarios are estimated to range from 1700 to 140,000. Maximum concentrations of MTBE in tap water anticipated not to cause adverse health effects are determined to range from 700 to 14,000 ppb. The results of this analysis demonstrate that no health risks are likely to be associated with chronic and subchronic human exposures to MTBE in tap water. Although some individuals may be exposed to very high concentrations of MTBE in tap water immediately following a localized spill, these exposures are likely to be brief in duration due to large-scale dilution and rapid volatilization of MTBE, the institution of emergency response and remediation measures to minimize human exposures, and the low taste and odor thresholds of MTBE which ensure that its presence in tap water is readily detected at concentrations well below the threshold for human injury.

Chloroform mode of action: implications for cancer risk assessment

Risk assessment methodology, particularly pertaining to potential human carcinogenic risks from exposures to environmental chemicals, is undergoing intense scrutiny from scientists, regulators, and legislators. The current practice of estimating human cancer risk is based almost exclusively on extrapolating the results of chronic, high-dose studies in rodents to estimate potential risk in humans. However, many scientists are questioning whether the logic used in this current risk assessment methodology is the best way to safeguard public health. A major tool of human cancer risk assessment is the linearized multistage (LMS) model. The LMS model has been identified as an aspect of risk assessment that could be improved. One way to facilitate this improvement is by developing a way to incorporate a carefully derived, more biologically relevant mechanism of action data on carcinogenesis. Recent data on chloroform indicate that the dose-response relationship for chloroform-induced tumors in rats and mice is nonlinear, based upon events secondary to cell necrosis and subsequent regeneration as the likely mode of action for the carcinogenic effects of chloroform. In light of these data, there is a sound scientific basis for removing some of the uncertainty that accompanies current cancer risk assessments of chloroform. The following points summarize the critical data: (1) a substantial body of data demonstrates a lack of direct in vivo or in vitro genotoxicity of chloroform; (2) chloroform induces liver and kidney tumors in long-term rodent cancer bioassays only at doses that induce frank toxicity at these target sites; (3) the chloroform doses required to produce tumors in susceptible species exceed the MTD, often by a considerable margin; (4) cytotoxicity and compensatory cell proliferation are associated with the chloroform doses required to induce liver or kidney tumors in susceptible rodent species; (5) there are no instances of chloroform-induced tumors that are not preceded by this pattern of dose-dependent toxic responses; (6) it is biologically plausible that cytolethality leads to chronically stimulated cell proliferation and related events such as inflammation and growth stimulation which act to initiate and promote the carcinogenic process; and (7) the consistently linked cellular events of cytolethality and subsequent cell proliferation, for which doses of no adverse effect have been clearly shown to exist, are one of the biological prerequisites for chloroform-induced tumors in animals. Based on these data, it is inappropriate to extrapolate cancer risk from high doses that produce necrosis and regenerative cell proliferation to low doses that do not with a model that presumes genotoxicity and a linear dose-response relationship. The weight of the scientific evidence concerning chloroform-induced tumors in rodents is consistent with and supports a cancer risk assessment methodology based on mode of action as the basis for establishing regulatory standards for this compound.

The FEMA GRAS assessment of furfural used as a flavour ingredient. Flavor and Extract Manufacturers' Association

The Expert Panel of the Flavor and Extract Manufacturers' Association (FEMA) has assessed the safety of furfural for its continued use as a flavour ingredient. The safety assessment takes into account the current scientific information on exposure, metabolism, pharmacokinetics, toxicology, carcinogenicity and genotoxicity. Furfural was reaffirmed as GRAS (GRASr) as a flavour ingredient under conditions of intended use based on: (1) its mode of metabolic detoxication in humans; (2) its low level of flavour use compared with higher intake levels as a naturally occurring component of food; (3) the safety factor calculated from results of subchronic and chronic studies, (4) the lack of reactivity with DNA; and (5) the conclusion that the only statistically significant finding in the 2-year NTP bioassays, an increased incidence of hepatocellular adenomas and carcinomas in the high-dose group of male mice, was secondary to pronounced hepatotoxicity. Taken together, these data do not indicate any risk to human health under conditions of use as a flavour ingredient. This evidence of safety is supported by the occurrence of furfural as a natural component of traditional foods, at concentrations in the diet resulting in a 'natural intake' that is at least 100 times higher than the intake of furfural from use as a flavour ingredient.

Glutathione S-transferase-mediated mutagenicity of trihalomethanes in Salmonella typhimurium: contrasting results with bromodichloromethane off chloroform

Trihalomethanes (THMs) are the most prevalent disinfection by-products identified in chlorinated drinking water. Among the THMs, chloroform (CHCl3) generally occurs at the highest concentration in finished water, but the concentrations of each of the brominated THMs (CHBrCl2, CHBr2Cl, and CHBr3) can exceed that of CHCl3. Each of these four THMs was carcinogenic in rodents in chronic oral dosing studies. This study assessed THM mutagenicity in a strain of Salmonella typhimurium TA1535 that was transfected with rat theta-class glutathione S-transferase T1-1 (+GST). The +GST strain and its nontransfected parent strain (-GST) were employed in a plate-incorporation assay and exposed for 24 hr to the vapor of individual THMs at concentrations up to 25,600 ppm in sealed Tedlar bags. Base-substitution revertants were produced in the +GST strain in a dose-dependent fashion by CHBrCl2 but not by CHCl3. At 4800 ppm CHBrCl2, which produced a calculated agar concentration of 0.67 mM, there were 419 +/- 75 revertants per plate compared to a spontaneous level of 23 +/- 5. CHCl3 produced a doubling of revertants only at the two highest concentrations tested (19,200 and 25,600 ppm). These results indicate that bromination of THMs confers the capability for theta-class GST-mediated transformation to mutagenic intermediates at low substrate concentrations, suggesting the possibility of a similar activation route in humans. Further, the very low affinity of the GSH-dependent pathway for CHCl3 demonstrates that different THMs can induce adverse effects via different mechanisms, indicating that risk evaluations of THMs should not treat members of this class as if they shared a common mode of action.

Risk assessment of inhaled chloroform based on its mode of action

The development of scientifically sound risk assessments based on mechanistic data will enable society to better allocate scarce resources. Inadequate risk assessments may result in potentially dangerous levels of hazardous chemicals, whereas overly conservative estimates can result in unnecessary loss of products or industries and waste limited resources. Risk models are used to extrapolate from high-dose rodent studies to estimate potential effects in humans at low environmental exposures and determine a virtually safe dose (VSD). When information to the contrary is not available, the linearized multistage (LMS) model, a conservative model that assumes some risk of cancer at any dose, is traditionally employed. In the case of airborne chloroform, the dose at which an increased lifetime cancer risk of 10(-6) could be calculated was chosen as the target VSD. Applying the LMS model to the mouse liver tumor data from a corn-oil gavage bioassay yields a VSD of 0.000008 ppm chloroform in the air. The weight of evidence indicates that chloroform is not directly mutagenic but, rather, acts through a nongenotoxic-cytotoxic mode of action. In this case, tumor formation results from events secondary to induced cytolethality and regenerative cell proliferation. Toxicity is not observed in rodents when chloroform is not converted to toxic metabolites at a rate sufficient to kill cells. Thus, tumors would not be anticipated at doses that do not induce cytolethality, contrary to the predictions of the LMS model. Inhalation studies in rodents show no cytolethality or regenerative cell proliferation in mouse liver at a chloroform concentration of 10 ppm as the no observed effect level (NOEL) or below. Using that NOEL and a safety factor approach, one can develop a VSD of 0.01 ppm. Integrating these data into the risk assessment process will yield risk estimates that are appropriate to the route of administration and consistent with the mode of action.

Investigation of the impact of pharmacokinetic variability and uncertainty on risks predicted with a pharmacokinetic model for chloroform

A sensitivity and uncertainty analysis was performed on the Reitz et al. (Toxicol. Appl. Pharmacol., 1990: 105, 443) physiologically based pharmacokinetic (PBPK) risk assessment model for chloroform. The analytical approach attempted to separately consider the impacts of interindividual variability and parameter uncertainty on the predicted values of the dose metrics in the model, as well as on liver cancer risk estimates obtained with the model. An important feature of the analytical approach was that an attempt was made to incorporate information on correlation between important parameters, for example, the observed correlation between total blood flow and alveolar ventilation rate. Using the published PBPK model for chloroform, the best estimate of the average population risk based on the preferred pharmacodynamic dose metric (PTDEAD), representing cell death, is 9.2 x 10(-7); this estimate is more than 500-fold lower than the risk estimate of 5.3 x 10(-4) based on an alternative pharmacokinetic dose metric (AVEMMB), which represents tissue adduct formation. However, when interindividual variability was considered the range of individual risks (from the 5th to the 95th percentile of the population) predicted with PTDEAD was extremely broad (from 3.0 x 10(-13) to 3.2 x 10(-4)), while individual risks predicted with AVEMMB only varied over a factor of four (from 1.9 x 10(-4) to 7.4 x 10(-4)). As a result, the upper 95th percentile of the distribution of individual risk estimates based on the preferred cell death metric were within a factor of three of the 95th percentile for the pharmacokinetic alternative. The crucial factor with respect to the much greater variability of chloroform risk estimates based on cell death is that the dose metric, PTDEAD, is exquisitely sensitive to variation of the parameters in the model defining the response of cells to the cytotoxicity of chloroform. Unfortunately, these key parameters are also highly uncertain, as well as strongly correlated. As a result it proved impossible to accurately quantify the additional impact of parameter uncertainty on the dose metrics and risk estimates for chloroform. In general, however, the approach used in this study should be useful for differentiating the impact of interindividual variability and parameter uncertainty on PBPK-based risk assessments of other chemicals where the sensitivity, uncertainty, and correlation of the key parameters are more limited.

Use of physiologically based pharmacokinetic modeling to investigate individual versus population risk

Because of the heterogeneity of the human population, it is generally expected that there will be a broad range of observed susceptibilities to the biological effects of exposure to chemicals or drugs. Often it is possible to distinguish specific classes of individuals, such as infants or the elderly, who appear to be more susceptible to a specific effect. Non-cancer risk assessment often address this variability by dividing the experimentally determined acceptable exposure level by an uncertainty factor of 10 to protect sensitive individuals; cancer risk assessments typically do not address this issue in any quantitative fashion. Physiologically based pharmacokinetic (PBPK) modeling provides the capability to quantitatively describe the potential impact of pharmacokinetic factors on the variability of individual risk. In particular, PBPK models can be used to determine the impact of differences in key metabolism enzymes, whether due to multiple genotypic expression, such as cytochrome P450 polymorphisms, or just due to normal variation in enzyme activities within the general population. Other potential modulators of sensitivity which can be addressed quantitatively with a PBPK model include physical condition, level of activity, disease states, age, hormonal status, and interactions with other chemicals and drugs. In each case, the PBPK model provides a quantitative structure for determining the effect of these various factors on the relationship between the external (environmental) exposure and the internal (biologically effective) target tissue exposure. When coupled with Monte Carlo analysis, the PBPK model provides a method to assess the quantitative impact of these sources of variability on individual risk (as opposed to average population risk) by comparing model-predicted risks over the distribution of individual parameter values.

Comparison of chloroform-induced toxicity in the kidneys, liver, and nasal passages of male Osborne-Mendel and F-344 rats

Chloroform given by gavage in corn oil at 180 mg/kg per day induced kidney tumors in male Osborne-Mendel rats. Chloroform-induced cytotoxicity and regenerative cell proliferation have been observed in the kidneys of male F-344 rats. In order to compare the acute sensitivity of male Osborne-Mendel with F-344 rats, animals from both strains were administered a single gavage dose of 0, 10, 34, 90, 180, or 477 mg/kg chloroform and necropsied 48 h later. Known target tissues were examined for histological changes. Regenerative cell proliferation was assessed as a labeling index (LI, percent of cells in S-phase) as determined by nuclear incorporation of bromodeoxyuridine. The epithelial cells of the proximal tubules of the kidney cortex were the primary target cells for cytotoxicity and regenerative cell proliferation. A dose-dependent increase in the LI was present in the kidney of Osborne-Mendel rats given doses of 10 mg/kg chloroform and above and in F-344 rats given 90 mg/kg and above. The maximal increase in the LI was 4.5- or 3.7-fold over control in Osborne-Mendel or F-344 given 477 mg/kg, respectively. The only increase in the hepatocyte LI was in the F-344 rats given 477 mg/kg. Edema and periosteal hypercellularity were observed in the nasal passages of both strains at doses of 90 mg/kg and above. These data indicate that Osborne-Mendel and F-344 rats are about equally susceptible to chloroform-induced nephrotoxicity. These results provide a basis for linking the extensive data base on mechanisms of action of chloroform toxicity in F-344 rats to the Osborne-Mendel rat and support the hypothesis that events secondary to chloroform-induced cytolethality and regenerative cell proliferation played a role in the induction of renal tumors in the Osborne-Mendel rat.

Reducing uncertainty in risk assessment by using specific knowledge to replace default options

This paper has advocated the development of specific scientific information, especially information on the mechanisms of action of chemicals, to use in place of default options in assessing human cancer risks. Four examples have been discussed that build largely on information from the CIIT research program. These four examples are worthy of consideration as a group, with a view to developing insights for increasing the effectiveness and efficiency of obtaining such data in the future and, most of all, to increase their acceptance for use instead of default options. In my view, key features of all four examples are that the data are framed within an exposure-dose-response paradigm and that there is a clear linkage to the end point of concern-cancer. As the number of techniques available for making observations at the cellular and molecular levels continues to increase at a rapid pace, linking these observations to the health end points of concern such as cancer is going to be increasingly important, especially in enhancing the value of the observations for risk assessment purposes. Equally as important, the mechanistic observations must be linked to realistic exposures and associated tissue dose that can be related to realistic human exposure scenarios. In my opinion, the likelihood of obtaining information of value for risk assessment purposes using the most sophisticated of molecular and cellular techniques will be of limited value if the exposures or doses are not realistically linked to those likely to be encountered by humans. The mechanism of alpha 2u-globulin nephropathy and its association with kidney tumors in male rats and the conclusion that the male rat kidney tumor findings are not applicable to assessing human cancer risk is an example of a qualitative decision. I suspect this may be a somewhat unusual case. As one looks across the various mammalian species used for experimentation and makes comparisons with humans, a unifying theme is the relative abundance of similarities. Indeed, this is a major argument for the use of laboratory animals to obtain information relevant to humans. Nonetheless, vigilance to differences among species is important. When differences are observed, we must capitalize on them to better understand the underlying biological mechanisms that mediate the differences. If, as I have suggested, laboratory animal species are more like than different from humans in their basic biological characteristics, there is a rationale for continuing to use laboratory animals as sources of data to help assess human risks of exposure to chemicals. It follows from this that quantitative differences among species such as observed with both formaldehyde and 1,3-butadiene assume major importance for assessing human risks. In my opinion, quantitation of the likely human carcinogenic potency of chemicals is of major importance. It is not sufficient to simply classify chemicals with regard to the likelihood of their being human carcinogens, as done by IARC (1994) and U.S. EPA (1986). IARC has placed more than 60 chemicals or processes (such as coke production) in group 1, carcinogenic to humans; more than 50 in group 2a, probably carcinogenic to humans; and 250 in group 2b, possibly carcinogenic to humans. This rank order implies differing levels of concern for three categories. However, even this rough three-bin system does not convey a very clear picture as to the degree of concern that should be accorded a given chemical for producing cancer. For example, the chemicals categorized as group 1, human carcinogens, using potency estimates developed by the U.S. EPA differ in potency by roughly 4 orders of magnitude. For example, a lifetime cancer risk is 6.2 x 10(-2) per micrograms/m3 for bischloromethyl ether and 8.3 x 10(-6) for benzene (NRC, 1994). Differences such as this offer strong arguments for complementing simplistic hazard identification schemes such as the IARC and EPA carcinogen classification systems w

The role of regenerative cell proliferation in chloroform-induced cancer

Chloroform produces cancer by a nongenotoxic-cytotoxic mode of action, with no increased cancer risk expected at noncytotoxic doses. The default risk assessment for inhaled chloroform relies on liver tumor incidence from a gavage study with female B6C3F1 mice and estimates a virtually safe dose (VSD) at an airborne concentration of 0.000008 ppm of chloroform. In contrast, a 1000-fold safety factor applied to the NOAEL for liver cytotoxicity from inhalation studies yields a VSD of 0.01 ppm. This estimate relies on inhalation data and is more consistent with the mode of action of chloroform.

Physiologically based pharmacokinetic (PB-PK) models in the study of the disposition and biological effects of xenobiotics and drugs

Physiologically based pharmacokinetic (PB-PK) models have been used to study the mechanisms of disposition of drugs and xenobiotics for almost 70 years. Their widespread application in toxicology began 15 years ago with models for polychlorinated biphenyls and other persistent lipophilic compounds. Quantitative applications of PB-PK moels in carcinogen risk assessment date to the development of a number of PB-PK models for dichloromethane in the mid 1980s. The expanding use of these models is primarily related to their ability to make more accurate predictions of target tissue dose for different exposure situations in different animal species, including humans, and to evaluate quantitatively the mechanisms of disposition of chemicals within the body. This paper discusses contemporary uses of PB-PK modeling in the context of risk assessment with xenobiotics and of safety assessment with drugs.

Biologically based dose response model for hepatic toxicity: a mechanistically based replacement for traditional estimates of noncancer risk

Uncertainty in risk assessment can be reduced by increasing the use of relevant data specific to the particular xenobiotic and exposed organism. We describe the development of a preliminary, mechanism-based exposure response model for chloroform hepatotoxicity consistig of toxicokinetic (TK) and toxicodynamic (TD) submodels. The TK submodel is based on an existing physiologically based toxicokinetic (PBTK) model for chloroform. The TD submodel consists of an empirical function linking tissue dose, defined by the PBTK model, with hepatocyte killing and subsequent regenerative cellular replication. Chloroform-induced cell killing was inferred quantitatively from dose-response hepatic labelling index studies conducted in female B6C3F1 mice and male F344 rats. The overall model was scaled to humans by conventional scaling of the TK submodel and by using the TD submodel as is, i.e. as developed from the rodent data. The resulting human model was used to analyze a case of human poisoning which developed after repeated ingestion of large doses of cough syrup containing chloroform and alcohol. The model predicted the observed toxic response after the capacity for chloroform metabolism was increased by a factor of 3 from the value estimated using human liver microsomes. This is an acceptable adjustment of this parameter, given the uncertainty associated with the extrapolation from microsomes and the coexposure to alcohol. This preliminary result is encouraging, suggesting that the model, at its current stage of development, is able to approximate actual human risks of hepatotoxicity from chloroform exposure. The extensive use of data on chloroform TK and cytolethality-induced regenerative cellular replication for model development suggests that the model has reduced uncertainty relative to the current U.S. EPA oral reference dose (RfD) calculation for chloroform, which does not use any mechanistic or dose-response data.

Development of physiologically based pharmacokinetic and physiologically based pharmacodynamic models for applications in toxicology and risk assessment

Pharmacokinetics (PK) involves the study of the rates of absorption, distribution, excretion, and biotransformation of chemicals and their metabolites. PK models can be used to reconstruct extensive time-course data sets based on a small number of kinetic parameters. These models can be used to predict the results of new experiments and integrate studies on kinetics, disposition and metabolism in various animal species [1]. The 2 main approaches that have been pursued in developing PK models are: (1) data-based compartmental modeling; and (2) physiologically based compartmental modeling. Data-based models rely on the collection of time-course concentration data and fitting these data with mathematical models. Compartments in these models do not necessarily reflect the anatomy and physiology of the animal, and the kinetic constants derived from these models do not have obvious physiological or biochemical counterparts. In physiologically based pharmacokinetic (PBPK) models, compartments correspond more closely to actual anatomical structures, defined with respect to their volumes, blood flows, chemical binding (partitioning) characteristics, and ability to metabolize or excrete the compounds of interest. Because the kinetic parameters of these models reflect tissue blood flows, partitioning, and biochemical constants, these models are more readily scaled from one animal species to another [2]. PBPK models have been used to understand the disposition of chemicals in the body for almost 70 years. Their more widespread application in toxicology dates back only 15 years or so to models developed for polychlorinated biphenyls and other persistent lipophilic compounds. Quantitative applications of PBPK models in risk assessment date to the development of a number of PBPK models for methylene chloride in the mid 1980s. The burgeoning use of PBPK models in toxicology research and chemical risk assessment today is primarily related to their ability to make more accurate predictions of target tissue dose for different exposure situations in different animal species, including humans. This overview includes a discussion of the development of these PBPK models in toxicology and speculates about future applications of PBPK and physiologically based pharmacodynamic (PBPD) models in chemical risk assessment.

The application of physiologically based pharmacokinetic modeling in human health risk assessment of hazardous substances

Physiologically based pharmacokinetic (PBPK) modeling is an important tool for improving the accuracy of human health risk assessments for hazardous substances in the environment. The proper use of PBPK modeling can reduce uncertainties that currently exist in risk assessment procedures by providing more scientifically credible extrapolations across species and routes of exposure, and from high experimental doses to potential environmental exposures. Current applications of PBPK models range from relatively straightforward uses for the extrapolation of chemical kinetics across species, route, and duration of exposure to much more demanding chemical risk assessment applications requiring a description of complex pharmacodynamic phenomena such as mitogenicity and hyperplasia secondary to cytotoxicity. PBPK modeling helps to identify the factors that are most important in determining the health risks associated with exposure to a chemical, and provides a means for estimating the impact of those factors both on the average risk to a population and on the specific risk to an individual. The chief challenge in the application of PBPK modeling in human health risk assessment lies in the need to generate chemical-specific data to support the development and validation of the models. Extensive use of rapidly developing in vitro and structure-activity relationship techniques is needed to provide the data required for the large number of hazardous chemicals currently contaminating the environment.

Cancer and non-cancer risk assessment: not so different if you consider mechanisms

Default risk assessment procedures use threshold models for non-carcinogens and a non-threshold model for carcinogens. This a priori distinction reflects the fact that the default procedures do not consider mechanisms of action of specific chemicals. When mechanisms are considered, the distinction is not necessary. Starting with the premise that the goal of risk assessment is to identify actual risk for specific chemicals, three major, generic components of the overall mechanism translating exposure into a response of regulatory interest are identified. These are the specific mechanisms linking (1) exposure with dose to target tissue, (2) target tissue dose with short-term responses such as cytolethality or mutation, and (3) short-term responses with ensuing long-term responses such as cancer or cirrhosis. (Short-term responses may be regulatory end points of interest, or they may be intermediate steps on the way to longer-term sequelae). On-going research on formaldehyde and chloroform is described to illustrate how these three components of the overall mechanism can be examined experimentally and used in specific models. The impact of mechanism-based risk assessment on uncertainty is also considered. Uncertainty is a function of the extent to which the model used for risk assessment misspecifies the actual mechanism of action for the chemical in question. There is a trade-off between (a) mechanism-based models that may reduce uncertainty but are expensive and time-consuming to develop and (b) default models that are not chemical-specific but can be used with minimal data sets. Experience with mechanism-based risk assessment may allow modification of default procedures to minimize this trade-off. A future default procedure for carcinogen risk assessment might allow specification of mode of action. For example, while DNA reactive-carcinogens would still be assumed to have linear low-dose risk, carcinogens acting through purely cytotoxic mechanisms might be assumed to have sharply non-linear or even threshold dose-response curves.