Trichloroethylene cancer risk: simplified calculation of PBPK-based MCLs for cytotoxic end points

Assessing the impact of child/adult differences in hepatic first-pass effect on the human kinetic adjustment factor for ingested toxicants

The objective of this study was to evaluate the impact of interindividual differences in hepatic first-pass effect (FPE) on the magnitude of the human kinetic adjustment factor (HKAF) for ingested toxicants. This factor aims at replacing a default value of 3.2 used in non-cancer risk assessment. Coupled with Monte Carlo simulations, steady-state equations that account for FPE were used to obtain distributions of arterial blood concentrations (CAss) and rates of metabolism in adults, neonates, infants and toddlers continuously exposed to an oral dose of 1 μg/kg/d of theoretical CYP2E1 and CYP1A2 substrates. For such substrates exhibiting a range of blood:air partition coefficients (Pb: 1-10,000) and hepatic extraction ratios in an average adult (E(ad): 0.01-0.99), HKAFs were computed as the ratio of the 95th percentile of dose metrics for each subpopulation over the 50th percentile value in adults. The reduced hepatic enzyme content in neonates as compared to adults resulted in correspondingly diminished FPE. Consequently, HKAFs greater than 3.2 could be observed, based on CAss only, in the following cases: for some CYP2E1 substrates with E(ad) ≤ 0.3, in neonates (max.: 6.3); and for some CYP1A2 substrates with E(ad) ≤ 0.1 and 0.7, in, respectively, neonates and infants (max.: 28.3). Overall, this study pointed out the importance of accounting for child/adult differences in FPE when determining the HKAF for oral exposure.

Trichloroethylene risk assessment: a review and commentary

Trichloroethylene (TCE) is a widespread environmental contaminant that is carcinogenic when given in high, chronic doses to certain strains of mice and rats. The capacity of TCE to cause cancer in humans is less clear. The current maximum contaminant level (MCL) of 5 ppb (microg/L) is based on an US Environment Protection Agency (USEPA) policy decision rather than the underlying science. In view of major advances in understanding the etiology and mechanisms of chemically induced cancer, USEPA began in the late 1990s to revise its guidelines for cancer risk assessment. TCE was chosen as the pilot chemical. The USEPA (2005) final guidelines emphasized a "weight-of-evidence" approach with consideration of dose-response relationships, modes of action, and metabolic/toxicokinetic processes. Where adequate data are available to support reversible binding of the carcinogenic moiety to biological receptors as the initiating event (i.e., a threshold exists), a nonlinear approach is to be used. Otherwise, the default assumption of a linear (i.e., nonthreshold) dose-response is utilized. When validated physiologically based pharmacokinetic (PBPK) models are available, they are to be used to predict internal dosimetry as the basis for species and dose extrapolations. The present article reviews pertinent literature and discusses areas where research may resolve some outstanding issues and facilitate the reassessment process. Key research needs are proposed, including role of dichloroacetic acid (DCA) in TCE-induced liver tumorigenesis in humans; extension of current PBPK models to predict target organ deposition of trichloroacetic acid (TCA) and DCA in humans ingesting TCE in drinking water; use of human hepatocytes to ascertain metabolic rate constants for use in PBPK models that incorporate variability in metabolism of TCE by potentially sensitive subpopulations; measurement of the efficiency of first-pass elimination of trace levels of TCE in drinking water; and assessment of exogenous factors' (e.g., alcohol, drugs) ability to alter metabolic activation and risks at such low-level exposure.

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.

Organic contamination in dialysis water: trichloroethylene as a model compound

BACKGROUND

Routine water monitoring in a haemodialysis centre revealed high trichloroethylene (TCE) concentrations. The aim of this study is to describe the measures adopted after organic contamination of dialysis water in order to avoid the possibility of patient exposure. We also carried out in vitro experiments to evaluate the accumulation of TCE in various devices normally used in a dialysis water treatment system (DWTS).

METHODS

In vivo and in vitro blood and water TCE levels were determined by means of solid phase microextraction-gas chromatography/mass spectrometer.

RESULTS

High TCE concentrations were found throughout the DWTS; acceptably low levels were obtained only by replacing the activated charcoal, ionic-exchange resins, microfilters and PVC pipes. The adsorption and realising capacities of these devices were tested in vitro, and the elimination curves made it possible to calculate the total percentage of the previously absorbed TCE mass released into the water. Evidence of exposure was confirmed by the relatively high TCE levels in the patient blood samples taken 30 days after the last exposure even if the subjects were asymptomatic. In vivo experiments showed that the blood gain of TCE through the low flux membrane during the course of dialysis was about 77+/-10.4% of the amount carried by dialysis fluid as calculated on the basis of its partition coefficient value (K(b/w) 3.75).

CONCLUSIONS

This study shows that, when present in dialysis water, the lipophilic TCE contaminant can accumulate in various devices, thus transforming them into possible sources of exposure. This highlights the importance of periodically monitoring dialysis water for organic substances that have a great affinity to the blood compartment, in order to prevent occasional or chronic patient exposure.

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.

Trichloroethylene effects on gene expression during cardiac development

BACKGROUND

Halogenated hydrocarbon exposure is associated with changes in gene expression in adult and embryonic tissue. Our study was undertaken to identify differentially expressed mRNA transcripts in embryonic hearts from Sprague-Dawley rats exposed to trichloroethylene (TCE) or potential bio-transformation products dichloroethylene (DCE) and trichloroacetic acid (TCAA).

METHODS

cDNA subtractive hybridization was used to selectively amplify expressed mRNA obtained from control or halogenated hydrocarbon exposed rat embryos. The doses used were 1100 and 110 ppm (8300 and 830 microM) TCE, 110 and 11 ppm (1100 and 110 microM) DCE, and 27.3 and 2.75 mg/ml (100 and 10 mM) TCAA. Control animals were given distilled drinking water throughout the period of experiments.

RESULTS

Sequencing of over 100 clones derived from halogenated hydrocarbon exposed groups resulted in identification of numerous differentially regulated gene sequences. Up-regulated transcripts identified include genes associated with stress response (Hsp 70) and homeostasis (several ribosomal proteins). Down-regulated transcripts include extracellular matrix components (GPI-p137 and vimentin) and Ca(2+) responsive proteins (Serca-2 Ca(2+)-ATPase and beta-catenin). Two possible markers for fetal TCE exposure were identified: Serca-2 Ca(2+)-ATPase and GPI-p137, a GPI-linked protein of unknown function. Differential regulation of expression of both markers by TCE was confirmed by dot blot analysis and semi-quantitative RT-PCR with levels of TCE exposure between 100 and 250 ppb (0.76 and 1.9 microM) sufficient to decrease expression.

CONCLUSIONS

Sequences down-regulated with TCE exposure appear to be those associated with cellular housekeeping, cell adhesion, and developmental processes, while TCE exposure up-regulates expression of numerous stress response and homeostatic genes.

Health risk assessment on residents exposed to chlorinated hydrocarbons contaminated in groundwater of a hazardous waste site

We conducted this study to estimate residents' chronic hazard and carcinogenic risk in a groundwater-contaminated community after on-site remediation in Taiwan during 1999-2000. We followed guidelines for assessing hazardous waste sites of the U.S. Environmental Protection Agency (EPA) and used empirically measured contaminant levels and exposure parameters to perform health risk assessment on seven chlorinated hydrocarbons. We measured groundwater concentrations of vinyl chloride, tetrachloroethylene, trichloroethylene, 1,1-dichloroethylene, 1,1,1-trichloroethane, cis-1,2-dichloroethylene, and 1,1-dichloroethane in 49 off-site residential wells by gas chromatography/mass spectrometry. Exposure parameters were mainly derived from our field survey of 382 residents, and partially from U.S. EPA default values. Total exposure dose estimation included routes of inhalation during showering and dermal absorption of showers and other activities involved with hand-water contacts. The ingestion route of water was not included because most residents drank boiled water with negligible contaminants. We calculated a hazard index (HI) for all seven chlorinated hydrocarbons and carcinogenic risks for known human carcinogen of vinyl chloride and probable human carcinogens of tetrachloroethylene and trichloroethylene, which had the same target organ, the liver. The HI values for reasonable maximal exposure (RME) and average exposure were 14.3 and 0.2, respectively. The cancer risks based on RME and average exposure (in parentheses) for vinyl chloride, tetrachloroethylene, and trichloroethylene were 8.4 x 10(-6) (7.3 x 10(-9)), 1.9 x 10(-4) (1.3 x 10(-7)), and 1.4 x 10(-4) (1.2 x 10(-6)), respectively. We applied Monte Carlo simulations to the sensitivity analysis, which showed that the contaminant levels, exposure duration, and time for showers were major determinants of health risks. We concluded that the contaminated groundwater was still unsafe for use even after the contaminated site underwent remediation by extraction and treatment in 1997.

Cancer incidence among Danish workers exposed to trichloroethylene

Human evidence regarding the carcinogenicity of the animal carcinogen trichloroethylene (TCE) is limited. We evaluated cancer occurrence among 803 Danish workers exposed to TCE, using historical files of individual air and urinary measurements of TCE-exposure. The standardized incidence ratio (SIR) for cancer overall was close to unity for both men and women who were exposed to TCE. Men had significantly elevated SIRs for non-Hodgkin's lymphoma (SIR = 3.5; n = 8) and cancer of the esophagus (SIR = 4.2; n = 6). Among women, the SIR for cervical cancer was significantly increased (SIR = 3.8; n = 4). No clear dose-response relationship appeared for any of these cancers. We found no increased risk for kidney cancer. In summary, we found no overall increase in cancer risk among TCE-exposed workers in Denmark. For those cancer sites where excesses were noted, the small numbers of observed cases and the lack of dose-related effects hinder etiological conclusions.

Dermal benzene and trichloroethylene induce aneuploidy in immature hematopoietic subpopulations in vivo

Accumulation of genetic damage in long-lived cell populations with proliferative capacity is implicated in tumorigenesis. Hematopoietic stem cells (hsc) maintain lifetime hematopoiesis, and recent studies demonstrate that hsc in leukemic patients are cytogenetically aberrant. We postulated that exposure to agents associated with increased leukemia risk would induce genomic changes in cells in the hsc compartment. Aneusomy involving chromosomes 2 and 11 in sorted hsc (Lin(-)c-kit(+)Sca-1(+)) and maturing lymphoid and myeloid cells from mice that received topical doses of benzene (bz) or trichloroethylene (TCE) was quantified using fluorescence in situ hybridization. Six days after bz or TCE exposure, aneuploid cells in the hsc compartment increase four- to eightfold in a dose- and schedule-independent manner. Aneuploid lymphoid and myeloid cells from bz- and TCE-treated mice approximate controls, except after repeated benzene exposures. Aneuploid cells are more frequent in the hsc compartment than in mature hematopoietic subpopulations. Hematotoxicity was also quantified in bz- and TCE-exposed hematopoietic subpopulations using two colony-forming assays: CFU-GM (colony-forming units/granulocyte-macrophage progenitors) and CAFC (cobblestone area-forming cells). Data indicate that bz is transiently cytotoxic (< or =1 week) to hsc subpopulations, and induces more persistent toxicity (>2 weeks) in maturing, committed progenitor subpopulations. TCE is not hematotoxic at the doses applied. In conclusion, we provide direct evidence for induction of aneuploidy in cells in the hsc compartment by topical exposure to bz and TCE. Disruption of genomic integrity and/or toxicity in hsc subpopulations may be one step in leukemic progression.

A civil action and statistical assessments of the spatial pattern of disease: do we have a cluster?

A reported "cluster" of excess childhood leukemia cases and possible environmental causes in Woburn, Massachusetts, formed a key motivation for the events described in the popular book and motion picture A Civil Action. Although statistical methods to assess spatial clustering existed prior to the events in Woburn, increasing interest in environmental risk factors and recent developments in geographical information systems and data availability prompt increased attention to such methods and their application to public health data. In this article, we review statistical and epidemiological concepts involved in the analysis of disease clusters. We discuss data issues, outline some methodological approaches, and illustrate ideas using data regarding leukemia incidence in upstate New York for the years 1978-1982.

Glutathione-dependent bioactivation of haloalkenes

Several halogenated alkenes are nephrotoxic in rodents. A mechanism for the organ-specific toxicity of these compounds to the kidney has been elucidated. The mechanism involves hepatic glutathione conjugation to dihaloalkenyl or 1,1-difluoroalkyl glutathione S-conjugates, which are cleaved by gamma-glutamyltransferase and dipeptidases to cysteine S-conjugates. Haloalkene-derived cysteine S-conjugates may have four fates in the organism: (a) They may be substrates for renal cysteine conjugate beta-lyases, which cleave them to form reactive intermediates identified as thioketenes (chloroalkene-derived S-conjugates), thionoacyl halides (fluoroalkene-derived S-conjugates not containing bromide), thiiranes, and thiolactones (fluoroalkene-derived S-conjugates containing bromine); (b) cysteine S-conjugates may be N-acetylated to excretable mercapturic acids; (c) they may undergo transamination or oxidation to the corresponding 3-mercaptopyruvic acid S-conjugate; (d) finally, oxidation of the sulfur atom in halovinyl cysteine S-conjugates and corresponding mercapturic acids forms Michael acceptors and may also represent a bioactivation reaction. The formation of reactive intermediates by cysteine conjugate beta-lyase may play a role in the target-organ toxicity and in the possible renal tumorigenicity of several chlorinated olefins widely used in many chemical processes.

What do animal cancer tests tell us about human cancer risk?: Overview of analyses of the carcinogenic potency database

Many important issues in carcinogenesis can be addressed using our Carcinogenic Potency Database, which analyzes and standardizes the literature of chronic carcinogenicity tests in laboratory animals. This review is an update and overview of our analyses during the past 15 years, using the current database that includes results of 5152 experiments on 1298 chemicals. We address the following: 1. More than half the 1298 chemicals tested in long-term experiments have been evaluated as carcinogens. We describe this positivity rate for several subsets of the data (including naturally occurring and synthetic chemicals), and we hypothesize and important role in the interpretation of results for increased cell division due to administration of high doses. 2. Methodological issues in the interpretation of animal cancer tests: constraints on the estimation of carcinogenic potency and validity problems associated with using the limited data from bioassays to estimate human risk, reproducibility of results in carcinogenesis bioassays, comparison of lifetable and summary methods of analysis, and summarizing carcinogenic potency when multiple experiments on a chemical are positive. 3. Positivity is compared in bioassays for two closely related species, rats and mice, tested under similar experimental conditions. We assess what information such a comparison can provide about interspecies extrapolation. 4. Rodent carcinogens induce tumors in 35 different target organs. We describe the frequency of chemicals that induce tumors in rats or mice at each target site, and we compare target sites of mutagenic and nonmutagenic rodent carcinogens. 5. A broad perspective on evaluation of possible cancer hazards from rodent carcinogens is given, by ranking 74 human exposures (natural and synthetic) on the HERP indes.

Combining physiologically based pharmacokinetic modeling with Monte Carlo simulation to derive an acute inhalation guidance value for trichloroethylene

Using the Monte Carlo method and physiologically based pharmacokinetic modeling, an occupational inhalation exposure to trichloroethylene consisting of 7 h of exposure per day for 5 days was simulated in populations of men and women of 5000 individuals each. The endpoint of concern for occupational exposure was drowsiness. The toxicologic condition leading to drowsiness was assumed to be high levels of both trichloroethanol and trichloroethylene. Therefore, the output of the simulation or dose metric was the maximum value of the sum of the concentration of trichloroethylene in blood and the concentration of trichloroethanol within its volume of distribution occurring within 1 week of exposure. The distributions of the dose metric in the simulated populations were lognormal. To protect 99% of a worker population, a concentration of 30 ppm over a 7-h period of the work day should not be exceeded. Subjecting a susceptible individual (the 99th percentile of the dose metric) to 200 ppm (the ACGIH short-term exposure limit or STEL) for 15 min twice a day over a work week necessitates a 2.5-h rest in fresh air following the STEL exposure to allow the blood concentrations of trichloroethylene and trichloroethanol to drop to levels that would not cause drowsiness. Both the OSHA PEL and the ACGIH TLV are greater than the value of 30 ppm derived here. As well as suggesting a new occupational guidance value, this study provides an example of this method of guidance value derivation.