Changes in Sensitivity to the Effects of Atrazine on the Luteinizing Hormone Surge in Female Sprague-Dawley Rats after Repeated Daily Doses: Correlation with Liver Enzyme Expression

BACKGROUND

Atrazine suppression of the LH surge slowly develops over time and peaks after 4 days; sensitivity to atrazine decreases after 8 or 14 days of dosing. Adaptation of the LH response was correlated with increased phase I and phase II liver enzyme activity/expression.

METHODS

The effect of atrazine on the LH surge was evaluated in female Sprague-Dawley rats administered 100 mg/kg/day atrazine by gavage for 1, 2, 3, or 4 consecutive days or 6.5, 50, or 100 mg/kg/day atrazine for 4, 8, or 14 days.

RESULTS

No statistically significant effects of atrazine were seen on peak plasma LH or LH area under the curve (AUC) after one, two, or three doses of 100 mg/kg/day. Four daily doses of 50 or 100 mg/kg atrazine significantly reduced peak LH and LH AUCs, whereas 6.5 mg/kg/day had no effect. After 8 or 14 days of treatment, statistically significantly reduced peak LH and LH AUC were observed in the 100 mg/kg/day dose group, but not in the 6.5 or 50 mg/kg/day dose groups, although significantly reduced LH was observed in one sample 9 hr after lights-on in the 50 mg/kg/day dose group on day 14. The number of days of treatment required to achieve a significant suppression of the LH surge is consistent with the repeat-dose pharmacokinetics of the chlorotriazines.

CONCLUSION

The apparent adaptation to the effect of atrazine on the LH surge after 8 or 14 days may be related to the induction of phase I or, more likely, phase II metabolism observed in this study after 8 days, or to a decreased sensitivity of the hypothalamic-pituitary-adrenal axis or an homeostatic adaption of the effect of atrazine on the LH surge mechanism. Birth Defects Research 110:246-258, 2018. © 2017 The Authors. Birth Defects Research Published by Wiley Periodicals, Inc.

PBPK-Based Probabilistic Risk Assessment for Total Chlorotriazines in Drinking Water

The risk of human exposure to total chlorotriazines (TCT) in drinking water was evaluated using a physiologically based pharmacokinetic (PBPK) model. Daily TCT (atrazine, deethylatrazine, deisopropylatrazine, and diaminochlorotriazine) chemographs were constructed for 17 frequently monitored community water systems (CWSs) using linear interpolation and Krieg estimates between observed TCT values. Synthetic chemographs were created using a conservative bias factor of 3 to generate intervening peaks between measured values. Drinking water consumption records from 24-h diaries were used to calculate daily exposure. Plasma TCT concentrations were updated every 30 minutes using the PBPK model output for each simulated calendar year from 2006 to 2010. Margins of exposure (MOEs) were calculated (MOE = [Human Plasma TCTPOD] ÷ [Human Plasma TCTEXP]) based on the toxicological point of departure (POD) and the drinking water-derived exposure to TCT. MOEs were determined based on 1, 2, 3, 4, 7, 14, 28, or 90 days of rolling average exposures and plasma TCT Cmax, or the area under the curve (AUC). Distributions of MOE were determined and the 99.9th percentile was used for risk assessment. MOEs for all 17 CWSs were >1000 at the 99.9(th)percentile. The 99.9(th)percentile of the MOE distribution was 2.8-fold less when the 3-fold synthetic chemograph bias factor was used. MOEs were insensitive to interpolation method, the consumer's age, the water consumption database used and the duration of time over which the rolling average plasma TCT was calculated, for up to 90 days. MOEs were sensitive to factors that modified the toxicological, or hyphenated appropriately no-observed-effects level (NOEL), including rat strain, endpoint used, method of calculating the NOEL, and the pharmacokinetics of elimination, as well as the magnitude of exposure (CWS, calendar year, and use of bias factors).

A comprehensive review of occupational and general population cancer risk: 1,3-Butadiene exposure-response modeling for all leukemia, acute myelogenous leukemia, chronic lymphocytic leukemia, chronic myelogenous leukemia, myeloid neoplasm and lymphoid neoplasm

Excess cancer risks associated with 1,3-butadiene (BD) inhalation exposures are calculated using an extensive data set developed by the University of Alabama at Birmingham (UAB) from an epidemiology study of North American workers in the styrene butadiene rubber (SBR) industry. While the UAB study followed SBR workers, risk calculations can be adapted to estimate both occupational and general population risks. The data from the UAB SBR study offer an opportunity to quantitatively evaluate the association between cumulative exposure to BD and different types of cancer, accounting for the number of tasks involving high-intensity exposures to BD as well as confounding associated with the exposures to the multiple other chemicals in the SBR industry. Quantitative associations of BD exposure and cancer, specifically leukemia, can be further characterized by leukemia type, including potential associations with acute myelogenous leukemia (AML), chronic lymphocytic leukemia (CLL), and chronic myelogenous leukemia (CML), and the groups of lymphoid and myeloid neoplasms. Collectively, these multiple evaluations lead to a comprehensive analysis that makes use of all of the available information and is consistent with the risk assessment goals of the USEPA and other regulatory agencies, and in line with the recommendations of the USEPA Science Advisory Board. While a range of cancer risk values can result from these multiple factors, a preferred case for occupational and general population risk is highlighted. Cox proportional hazards models are used to fit exposure-response models to the most recent UAB data. The slope of the model with cumulative BD ppm-years as the predictor variable is not statistically significantly greater than zero for CML, AML, or, when any one of eight exposure covariates is added to the model, for all leukemias combined. The slope for CLL is statistically significantly different from zero. The slope for myeloid neoplasms is not statistically significantly greater than zero while the slope for lymphoid neoplasms is statistically significantly greater than zero. The excess risk for the general population is largest for lymphoid neoplasms. The best estimates of the environmental concentrations (ECs) associated with an excess risk of 1/100,000 by age 70 years for lymphoid neoplasms, all leukemias, and CLL are EC(1/100,000)'s equal to 0.06, 0.16 and 0.38 ppm, respectively. The best estimates of the occupational BD exposure from 20 to 65 years of age associated with an excess risk of 1/10,000 by age 70 years for lymphoid neoplasms, all leukemias, and CLL are the EC(1/10,000)'s of 2.7, 7.3 and 15.1 ppm, respectively.

Development of an inhalation unit risk factor for hexavalent chromium

A unit risk factor (URF) was developed for hexavalent chromium (CrVI). The URF is based on excess lung cancer mortality in two key epidemiological studies of chromate production workers. The Crump et al. (2003) study concerns the Painesville, OH worker cohort, while Gibb et al. (2000) regards the Baltimore, MD cohort. A supporting assessment was also performed for a cohort from four low-dose chromate plants (Leverkusen and Uerdingen, Germany, Corpus Christi, TX, Castle Hayne, NC). For the Crump et al. (2003) study, grouped observed and expected number of lung cancer mortalities along with cumulative CrVI exposures were used to obtain the maximum likelihood estimate and asymptotic variance of the slope (β) for the linear multiplicative relative risk model using Poisson regression modeling. For the Gibb et al. (2000) study, Cox proportional hazards modeling was performed with optimal exposure lag and adjusting for the effect of covariates (e.g., smoking) to estimate β values. Life-table analyses were used to develop URFs for each of the two key studies, as well as for supporting and related studies. The two key study URFs were combined using weighting factors relevant to confidence to derive the final URF for CrVI of 2.3E-03 per μgCrVI/m(3).

Misinterpretation of categorical rate ratios and inappropriate exposure-response model fitting can lead to biased estimates of risk: ethylene oxide case study

There are pitfalls associated with exposure-response modeling of human epidemiological data based on rate ratios (RRs). Exposure-response modeling is best based on individual data, when available, rather than being based on summary results of that data such as categorical RRs. Because the data for the controls (or the lowest exposure interval if there are not enough controls) are random and not known with certainty a priori, any exposure-response model fit to RRs should estimate the intercept rather than fixing it equal to one. Evaluation of a model's goodness-of-fit to the individual data should not be based on the assumption that summary RRs describe the true underlying exposure-response relationship. These pitfalls are illustrated by Monte Carlo simulation examples with known underlying models. That these pitfalls are a practical concern is illustrated by the need for U.S. EPA to reconsider its most recent evaluation of ethylene oxide. If they had avoided these pitfalls, their exposure-response modeling would have been in better agreement with the log-linear model fit to the individual data.

Consideration of rat chronic progressive nephropathy in regulatory evaluations for carcinogenicity

Chronic progressive nephropathy (CPN) is a spontaneous renal disease of rats which can be a serious confounder in toxicology studies. It is a progressive disease with known physiological factors that modify disease progression, such as high dietary protein. The weight of evidence supports an absence of a renal counterpart in humans. There is extensive evidence that advanced CPN, particularly end-stage kidney, is a risk factor for development of a background incidence of atypical tubule hyperplasia and renal tubule tumors (RTT). The likely cause underlying this association with tubule neoplasia is the long-term increased tubule cell proliferation that occurs throughout CPN progression. As a variety of chemicals are able to exacerbate CPN, there is a potential for those exacerbating the severity up to and including end-stage kidney to cause a marginal increase in RTT and their precursor lesions. Extensive statistical analysis of National Toxicology Program studies shows a strong correlation between high-grade CPN, especially end-stage CPN, and renal tumor development. CPN as a mode of action (MOA) for rat RTT has received attention from regulatory authorities only recently. In the absence of toxic effects elsewhere, this does not constitute a carcinogenic effect of the chemical but can be addressed through a proposed MOA approach for regulatory purposes to reach a decision that RTT, developing as a result of CPN exacerbation in rats, have no relevance for human risk assessment. Guidelines are proposed for evaluation of exacerbation of CPN and RTT as a valid MOA for a given chemical.

Statistical Comparison of Carcinogenic Effects and Dose-Response Relationships in Rats and Mice for 2,4-Toluene Diamine to those Ascribed to Toluene Diisocyanate

The U.S. National Toxicology Program (NTP) conducted 2-year bioassays of commercial grade toluene diisocyanate (TDI) (80% 2,4-TDI and 20% 2,6-TDI) and 2,4-toluene diamine (TDA) and concluded that both were carcinogenic in rodents. In the TDI study, there was an unproven but likely formation of TDA either because of flawed test-substance handling and storage conditions and/or the atypical exposure conditions employed. Although the carcinogenic responses in both studies were qualitatively similar, several statistical analyses were performed to substantiate this possibility more rigorously. Seven different statistical approaches combine to yield a robust and consistent conclusion that, if only a small fraction (approximately 5%) of the dose of TDI were hydrolyzed to TDA in the TDI study, then that would be sufficient to explain the observed carcinogenic responses in the TDI study.

An updated inhalation unit risk factor for arsenic and inorganic arsenic compounds based on a combined analysis of epidemiology studies

The United States Environmental Protection Agency (USEPA) developed an inhalation unit risk factor (URF) of 4.3E-03 per μg/m(3) for arsenic in 1984 for excess lung cancer mortality based on epidemiological studies of workers at two smelters: the Asarco smelter in Tacoma, Washington and the Anaconda smelter in Montana. Since the USEPA assessment, new studies have been published and exposure estimates were updated at the Asarco and Anaconda smelters and additional years of follow-up evaluated. The Texas Commission on Environmental Quality (TCEQ) has developed an inhalation URF for lung cancer mortality from exposures to arsenic and inorganic arsenic compounds based on a newer epidemiology study of Swedish workers and the updates of the Asarco and Anaconda epidemiology studies. Using a combined analysis approach, the TCEQ weighted the individual URFs from these three epidemiology cohort studies, to calculate a final inhalation URF of 1.5E-04 per μg/m(3). In addition, the TCEQ also conducted a sensitivity analysis, in which they calculated a URF based on a type of meta-analysis, and these results compared well with the results of the combined analysis. The no significant concentration level (i.e., air concentration at 1 in 100,000 excess lung cancer mortality) is 0.067μg/m(3). This value will be used to evaluate ambient air monitoring data so the general public in Texas is protected against adverse health effects from chronic exposure to arsenic.

Quantitative cancer risk assessment for ethylene oxide inhalation in occupational settings

The estimated occupational ethylene oxide (EO) exposure concentrations corresponding to specified extra risks are calculated for lymphoid mortality as the most appropriate endpoint, despite the lack of a statistically significant exposure-response relationship. These estimated concentrations are for occupational exposures--40 years of occupational inhalation exposure to EO from age 20 to age 60 years. The estimated occupational inhalation exposure concentrations (ppm) corresponding to specified extra risks of lymphoid mortality to age 70 years in a population of male and female EO workers are based on Cox proportional hazards models of the most recent updated epidemiology cohort mortality studies of EO workers and a standard life-table calculation. An occupational exposure at an inhalation concentration of 2.77 ppm EO is estimated to result in an extra risk of lymphoid mortality of 4 in 10,000 (0.0004) in the combined worker population of men and women from the two studies. The corresponding estimated concentration decreases slightly to 2.27 ppm when based on only the men in the updated cohorts combined. The difference in these estimates reflects the difference between combining all of the available data or focusing on only the men and excluding the women who did not show an increase in lymphoid mortality with EO inhalation exposure. The results of sensitivity analyses using other mortality endpoints (all lymphohematopoietic tissue cancers, leukemia) support the choice of lymphoid tumor mortality for estimation of extra risk.

Cancer risk assessment for 1,3-butadiene: Dose–response modeling from an epidemiological perspective

The dose-response assessment of the association between 1,3-butadiene (BD) and leukemia mortality among workers in the North American synthetic rubber industry is explored. Analyses are based on the most recent University of Alabama at Birmingham epidemiological study and exposure estimation. The U.S. EPA Science Advisory Board recommendations of using the most recent data and giving consideration to peak exposures to BD have been followed. If cumulative BD ppm-years is to be used as the predictor of the leukemia rate ratio, then the performance of that predictor is statistically significantly improved if the slope in the predictor is estimated with age and the cumulative number of BD peaks (where a BD peak is any exposure, regardless of duration, to a BD concentration above 100 ppm) added as categorical covariates. After age and the cumulative number of BD peaks are incorporated as categorical covariates in the Poisson regression model, the estimated concentration (EC(001)) corresponding to an excess risk of 0.001 as a result of continuous environmental exposure is 11.2 ppm; however, the estimated slope for BD cumulative ppm-years in the linear rate ratio for leukemia used to derive this EC(001) is not statistically significantly different from zero. Sensitivity analyses using alternative models indicate either essentially no risk or estimated EC(001) values of 9 and 77 ppm. Analyses suggesting the absence of a statistically significant low-dose risk versus cumulative BD ppm-years are presented. Sensitivity analyses of other malignant neoplasms of lymphatic and hematopoietic tissue (specifically, lymphoid and myeloid neoplasms) resulted in conclusions about the dose-response modeling methodology that were supportive of the methodology used for leukemia.

Statistical inferences about the mechanism of action in carcinogenicity studies

An innovative approach to dose-response modeling provides statistical insight into the relative likelihood of different mechanisms of action in cancer dose-response studies. Two illustrative examples are given based on time-to-tumor data on mammary fibroadenoma and adenocarcinoma in female Sprague-Dawley rats using 34 different dose metrics. The likelihood for the study outcome was calculated for each dose metric and compared with the background likelihood using a likelihood-ratio test. In the first example, fibroadenomas were strongly related to the presence or absence of mammary secretory activity, galactoceles, pituitary tumors, and abnormal diestrous days in weeks 1 to 26. Adenocarcinomas were the most strongly related to the number and percentage of abnormal estrous days. In these examples, the usual dose metric based on the dietary concentration of the pesticide had some explanatory ability but not nearly as much as the dose metrics more directly related to hormonal mechanisms of action.

Addressing nonlinearity in the exposure-response relationship for a genotoxic carcinogen: cancer potency estimates for ethylene oxide

Ethylene oxide (EO) has been identified as a carcinogen in laboratory animals. Although the precise mechanism of action is not known, tumors in animals exposed to EO are presumed to result from its genotoxicity. The overall weight of evidence for carcinogenicity from a large body of epidemiological data in the published literature remains limited. There is some evidence for an association between EO exposure and lympho/hematopoietic cancer mortality. Of these cancers, the evidence provided by two large cohorts with the longest follow-up is most consistent for leukemia. Together with what is known about human leukemia and EO at the molecular level, there is a body of evidence that supports a plausible mode of action for EO as a potential leukemogen. Based on a consideration of the mode of action, the events leading from EO exposure to the development of leukemia (and therefore risk) are expected to be proportional to the square of the dose. In support of this hypothesis, a quadratic dose-response model provided the best overall fit to the epidemiology data in the range of observation. Cancer dose-response assessments based on human and animal data are presented using three different assumptions for extrapolating to low doses: (1) risk is linearly proportionate to dose; (2) there is no appreciable risk at low doses (margin-of-exposure or reference dose approach); and (3) risk below the point of departure continues to be proportionate to the square of the dose. The weight of evidence for EO supports the use of a nonlinear assessment. Therefore, exposures to concentrations below 37 microg/m3 are not likely to pose an appreciable risk of leukemia in human populations. However, if quantitative estimates of risk at low doses are desired and the mode of action for EO is considered, these risks are best quantified using the quadratic estimates of cancer potency, which are approximately 3.2- to 32-fold lower, using alternative points of departure, than the linear estimates of cancer potency for EO. An approach is described for linking the selection of an appropriate point of departure to the confidence in the proposed mode of action. Despite high confidence in the proposed mode of action, a small linear component for the dose-response relationship at low concentrations cannot be ruled out conclusively. Accordingly, a unit risk value of 4.5 x 10(-8) (microg/m3)(-1) was derived for EO, with a range of unit risk values of 1.4 x 10(-8) to 1.4 x 10(-7) (microg/m3)(-1) reflecting the uncertainty associated with a theoretical linear term at low concentrations.

Observed versus predicted carboxyhemoglobin levels in cellulose triacetate workers exposed to methylene chloride

BACKGROUND

Occupational exposure to methylene chloride, together with carboxyhemoglobin concentrations, has not been studied previously.

METHODS

Carboxyhemoglobin levels were measured in non-smoking employees exposed to varying concentrations of methylene chloride during the manufacture of cellulose triacetate fibers. The observed carboxyhemoglobin levels were compared to predicted concentrations using a pharmacokinetic model.

RESULTS

The presence of carboxyhemoglobin in non-smokers exposed to methylene chloride results primarily from the metabolism of methylene chloride in the liver and exhibits a linear dose-response relationship. The observed levels of carboxyhemoglobin in non-smokers at the end of an 8-hour shift depend upon exposures to methylene chloride that day but are independent of occupational exposures on previous days. The observed daily concentrations of carboxyhemoglobin are consistent with predicted concentrations using a pharmacokinetic model. While varying exposure patterns were shown to change the rate of metabolite formation at the end of shift, these same exposure patterns had almost no effect on the total amount of carbon monoxide in the blood.

CONCLUSION

While the present study addresses the relationship between methylene chloride, carbon monoxide, carboxyhemoglobin and ischemic heart disease, it does not address the issue of tumorigenicity, which is also the basis for the current U.S. Occupational Health and Safety workplace exposure limit of 25 ppm. This study provides support for the conclusion that the current American Conference of Governmental Industrial Hygienists 8-hour Threshold Limit Value of 50 ppm adequately protects human health with regard to ischemic heart disease and carboxyhemoglobin formation among non-smokers.

Dose-response implications of the University of Alabama study of lymphohematopoietic cancer among workers exposed to 1,3-butadiene and styrene in the synthetic rubber industry

New quantitative cancer risk estimates for exposure to 1,3-butadiene are presented. These estimates are based on the most recent human epidemiologic data developed by Drs Delzell and Macaluso and their colleagues at the University of Alabama at Birmingham. The implications of Poisson regression analyses of the relative rate for leukemia are explored using their updated dose estimates and lymphohematopoietic cancer data. The Poisson regression model in these analyses has the same form as in the U.S. Environmental Protection Agency (EPA)'s draft risk assessment of 1,3-butadiene [U.S. Environmental Protection Agency, Health Risk Assessment of 1,3-Butadiene - External Review Draft, National Center for Environmental Assessment, Office of Research and Development, 63 Fed. Reg. 7167 (February 12, 1998) Publication NCEA-W-0267, Washington, 1998]. Consistent with the proposed cancer risk assessment guidelines of the EPA and the EPA's draft risk assessment, the exploration includes the maximum likelihood estimate of the 'effective concentration' (EC(01)) corresponding to an extra risk of leukemia of 0.01 (1%) from a lifetime continuous exposure to 1,3-butadiene based on a linear dose-response model and the cumulative 1,3-butadiene dose metric (ppm-years). The incorporation of the most recent exposure estimates results in a 2.5-fold decrease in the estimates of leukemia risks computed by EPA. In addition, three changes proposed by the American Chemistry Council (formerly the Chemical Manufacturers Association) to the EPA's Science Advisory Board (SAB) for EPA's draft risk assessment of 1,3-butadiene are incorporated into the calculation. This results in approximately an additional fivefold decrease in the risk estimates of leukemia. The leukemia cancer risk estimates in the EPA's draft risk assessment of 1,3-butadiene decrease by approximately a factor of 13-fold when the updated epidemiologic data and the alternative numbers proposed by industry to the SAB are both incorporated. Specifically, the maximum likelihood estimate of the EC(01) increases from EPA's 1.2 ppm to 2.8 ppm on the basis of the updated epidemiologic data and increases further to 15.1 ppm when the CMA's proposed changes are also incorporated.

Risk metrics and cumulative risk assessment methodology for the FQPA

The Food Quality Protection Act (FQPA) of 1996 mandates that the U.S. Environmental Protection Agency consider both aggregate and cumulative risks. Aggregate assessments account for multiple sources and routes of exposure for a single chemical. Cumulative assessments combine exposures to two or more chemicals that share a common mechanism of toxicity. Probabilistic risk assessment methods are described for determining a population's distribution of the dose from exposure and the combination of that exposure characterization with appropriate toxicological information to form a risk assessment. An individual's dose from exposure is characterized as a set of chemical- and route-specific dose profiles over time. For each individual and each chemical and route, a margin of exposure (MOE) is calculated by dividing a toxicologically relevant benchmark dose (e.g., an ED(10)) by the individual's dose from exposure. The set of these MOEs for an individual is combined into the individual's Total MOE. The distribution of the Total MOEs in a population is compared to an Acceptable MOE. Advantages of the Total MOE approach over approaches based on reference doses are discussed. Some general comments on risk metrics are made, and some general guidance for cumulative risk assessments is provided.

Ethylene oxide cancer risk assessment based on epidemiological data: application of revised regulatory guidelines

Ethylene oxide (EO) research has significantly increased since the 1980s, when regulatory risk assessments were last completed on the basis of the animal cancer chronic bioassays. In tandem with the new scientific understanding, there have been evolutionary changes in regulatory risk assessment guidelines, that encourage flexibility and greater use of scientific information. The results of an updated meta-analysis of the findings from 10 unique EO study cohorts from five countries, including nearly 33,000 workers, and over 800 cancers are presented, indicating that EO does not cause increased risk of cancers overall or of brain, stomach or pancreatic cancers. The findings for leukemia and non-Hodgkin's lymphoma (NHL) are inconclusive. Two studies with the requisite attributes of size, individual exposure estimates and follow up are the basis for dose-response modeling and added lifetime risk predictions under environmental and occupational exposure scenarios and a variety of plausible alternative assumptions. A point of departure analysis, with various margins of exposure, is also illustrated using human data. The two datasets produce remarkably similar leukemia added risk predictions, orders of magnitude lower than prior animal-based predictions under conservative, default assumptions, with risks on the order of 1 x 10(-6) or lower for exposures in the low ppb range. Inconsistent results for "lymphoid" tumors, a non-standard grouping using histologic information from death certificates, are discussed. This assessment demonstrates the applicability of the current risk assessment paradigm to epidemiological data.

Cancer dose-response modeling of epidemiological data on worker exposures to aldrin and dieldrin

The paper applies classical statistical principles to yield new tools for risk assessment and makes new use of epidemiological data for human risk assessment. An extensive clinical and epidemiological study of workers engaged in the manufacturing and formulation of aldrin and dieldrin provides occupational hygiene and biological monitoring data on individual exposures over the years of employment and provides unusually accurate measures of individual lifetime average daily doses. In the cancer dose-response modeling, each worker is treated as a separate experimental unit with his own unique dose. Maximum likelihood estimates of added cancer risk are calculated for multistage, multistage-Weibull, and proportional hazards models. Distributional characterizations of added cancer risk are based on bootstrap and relative likelihood techniques. The cancer mortality data on these male workers suggest that low-dose exposures to aldrin and dieldrin do not significantly increase human cancer risk and may even decrease the human hazard rate for all types of cancer combined at low doses (e.g., 1 microgram/kg/day). The apparent hormetic effect in the best fitting dose-response models for this data set is statistically significant. The decrease in cancer risk at low doses of aldrin and dieldrin is in sharp contrast to the U.S. Environmental Protection Agency's upper bound on cancer potency based on mouse liver tumors. The EPA's upper bound implies that lifetime average daily doses of 0.0000625 and 0.00625 microgram/kg body weight/day would correspond to increased cancer risks of 0.000001 and 0.0001, respectively. However, the best estimate from the Pernis epidemiological data is that there is no increase in cancer risk in these workers at these doses or even at doses as large as 2 micrograms/kg/day.

Monograph: reassessment of human cancer risk of aldrin/dieldrin

In 1987, the US Environmental Protection Agency (EPA) classified aldrin and dieldrin as category B2 carcinogens, i.e. probable human carcinogens, based largely on the increase in liver tumors in mice fed either organochlorine insecticide. At that date, the relevant epidemiology was deemed inadequate to influence the cancer risk assessment. More time has now elapsed since early exposures of manufacturing workers to aldrin/dieldrin; therefore, updated epidemiological data possess more power to detect exposure-related differences in cancer risk and mortality. Also, recent experimental studies provide a plausible mode of action to explain the mouse specificity of dieldrin-induced hepatocarcinogenesis and call into question the relevance of this activity to human cancer risk. This monograph places this new information within the historic and current perspectives of human cancer risk assessment, including EPA's 1996 Proposed Guidelines for Carcinogen Risk Assessment. Updated epidemiological studies of manufacturing workers in which lifetime exposures to aldrin/dieldrin have been quantified do not indicate increased mortality or cancer risk. In fact, at the middle range of exposures, there is evidence of a decrease in both mortality from all causes and cancer. Recent experimental studies indicate that dieldrin-induced hepatocarcinogenesis in mice occurs through a nongenotoxic mode of action, in which the slow oxidative metabolism of dieldrin is accompanied by an increased production of reactive oxygen species, depletion of hepatic antioxidant defenses (particularly alpha-tocopherol), and peroxidation of liver lipids. Dieldrin-induced oxidative stress or its sequelae apparently result in modulation of gene expression that favors expansion of initiated mouse, but not rat, liver cells; thus, dieldrin acts as a nongenotoxic promoter/accelerator of background liver tumorigenesis in the mouse. Within the framework of EPA's Proposed Guidelines for Carcinogen Risk Assessment, it is proposed that the most appropriate cancer risk descriptor for aldrin/dieldrin, relating to the mouse liver tumor response, is 'not likely a human carcinogen', a descriptor consistent with the example of phenobarbital cited by EPA.

Some implications for quantitative risk assessment if hormesis exists

The existence of hormesis should impact quantitative risk assessment in at least seven fundamental ways. (1) The dose-response models for bioassay and epidemiological data should have greater flexibility to fit the observed shape of the dose-response data and no longer be forced to always be linearly increasing at low doses. (2) Experimental designs should be altered to provide greater opportunity to identify the hormetic component of a dose-response relationship. (3) Rather than a lifetime average daily dose or its analog for shorter time periods, dose scales or metrics should be used that reflect the age or time dependence of the dose level. (4) Low-dose risk characterization should include the likelihood of beneficial effects and the likelihood that a dose level has reasonable certainty of no appreciable adverse health effects. (5) Exposure assessments should make greater efforts to characterize the distribution of actual doses from exposure rather than just upper bounds. (6) Uncertainty characterizations should be expanded to include both upper and lower bounds, and there should be an increased explicit use of expert judgement and weight-of-evidence based distributional analyses reflecting more of the available relevant dose-response information and alternative risk characterizations. (7) Risk should be characterized in terms of the net effect of a dose on health rather than a dose's effect on a single factor affecting health - for example, risk would be better expressed in terms of mortality from all causes combined rather than a specific type of fatal disease.

Modeling to incorporate defense mechanisms into the estimation of dose responses

Several adverse health effects (including cancer and noncancer effects) may be the result of an imbalance between exogenous and endogenous invading substances and defense mechanisms. In these cases the probability of an adverse effect depends on how much the exposure to a substance increases or decreases the number of defenders or their efficiency as well as increasing or decreasing the number of invaders. Rather than using a dose scale such as parts per million or milligram/kilogram/day in these cases, dose-response models can directly incorporate the impact of defense mechanisms by using a dose scale that corresponds to the number of invaders that break through the defenders and become free to do their damage. The number of breakthroughs at a specific age, the cumulative number of breakthroughs by a specific age, or the cumulative number of breakthroughs in a window of time would usually be the appropriate age-dependent dose. Although a lifetime average daily dose level can be used as a surrogate for an age-dependent dose in simplistic dose-response models, the age-dependent dose itself can be used in more biologically based models that include time, reflect the key role of feedback mechanisms, and treat the human body as an age-dependent dynamic system responding to internal and external stimuli and not as a system at equilibrium. Some illustrative biologic examples of defense mechanisms and invader-defender interactions are presented. Several numerical examples are given in which the dose incorporates the age-dependent effects of a substance on the number of invaders, the number of defenders, and/or the defenders' efficiencies.

Using PBPK modeling and comprehensive realism methodology for the quantitative cancer risk assessment of butadiene

The National Academy of Sciences and many others have noted the need for quantitative health risk assessment methodology that goes beyond a simple screening analysis based on upper bounds on risk. The Academy recommended adoption of methodologies which provide a higher-tier analysis based on realistic estimates of risk which reflect more of the available biological information. In recent years, scientists have challenged the assumption of low-dose linearity and other default assumptions in cancer risk assessment. These challenges have stimulated the continued evolution of quantitative risk assessment methodologies, because effective risk management requires accurate characterizations of uncertainty and greater utilization of cost-benefit analyses for decision making. "Comprehensive Realism" is an emerging quantitative weight-of-evidence based risk assessment methodology for both cancer and noncancer health effects which utilizes probability distributions and decision analysis techniques to reflect more of the available human and animal dose-response data. The current state of knowledge about the relative plausibility of alternative dose-response analyses is also addressed in this approach. The framework discussed here should lead to a higher-tier assessment of butadiene.

Oncogenicity testing of 2-ethylhexanol in Fischer 344 rats and B6C3F1 mice

2-Ethylhexanol (2EH) is a weak nongenotoxic hepatic peroxisome proliferator in the rat. It is a high-volume chemical intermediate in the preparation of the plasticizers bis-(2-ethylhexyl) adipate (DEHA), bis-(2-ethylhexyl) phthalate (DEHP), and tris-(2-ethylhexyl) phosphate (TEHP), which are weak hepatocellular tumorigens in female mice. In consequence, the oncogenic potential of 2EH was evaluated in male (M) and female (F) rats and mice (50 animals/sex/group). Oral gavage doses of 2EH in 0.005% aqueous Cremophor EL (polyoxyl-35 castor oil) were given five times a week to rats: 0 (water), 0 (vehicle), 50, 150, and 500 mg/kg for 24 months, and to mice: 0 (water), 0 (vehicle), 50, 200, and 750 mg/kg for 18 months. Statistical comparisons of data were made between vehicle controls and treatment groups. There were no differences of biological significance between data from vehicle and water control groups. In rats, there were no dose-related changes at 50 mg/kg. There was reduced body weight gain at 150 mg/kg (M, 16; F, 12%) and 500 mg/kg (M, 33; F, 31%) and an increased incidence of lethargy and unkemptness. There were dose-related increases in relative liver, stomach, brain, kidney, and testis weights at sacrifice. Female rat mortality was markedly increased at 500 mg/kg. There was marked aspiration-induced bronchopneumonia in rats at 500 mg/kg; hematologic, gross, and microscopic changes, including tumors, were otherwise comparable among all rat groups. In mice at 50 and 200 mg/kg there were no dose-related changes and essentially no time-dependent or time-independent adverse trends in liver tumor incidence at the 5% significance level. At 750 mg/kg mouse body weight gain was reduced (M, 26; F, 24%), and mortality increased (M and F, 30%) versus vehicle controls. At 750 mg/kg there was a slight increase in nonneoplastic focal hyperplasia in the forestomach of mice (M 5/50, F 4/50) versus vehicle controls (M 1/50, F 1/50). There were increases in mouse relative liver (F, 21%) and stomach (M, 13%; F, 19%) weights at 750 mg/kg. There was a 12% incidence of hepatic basophilic foci and an 18% incidence of hepatocellular carcinomas in male mice at 750 mg/kg, not statistically significant compared with either control by Fisher's exact test. There was a 12% incidence of hepatic basophilic foci and a 10% incidence of hepatocellular carcinomas in female mice at 750 mg/kg, statistically significant (p < 0.05) compared with vehicle but not with water controls by Fisher's exact test. There were no metastases. Time-dependent and -independent statistical analyses showed an adverse trend in the incidence of hepatocellular carcinomas in male and female mice, correlated with toxicity (expressed as mortality) at 750 mg/kg. The time-adjusted incidence of hepatocellular carcinomas in male mice (18.8%) was within the historical normal range at the testing facility (0-22%), but that in females (13.1%) lay outside the normal range (0-2%). Under the conditions of these studies 2EH was not oncogenic in rats, but there were weak adverse trends in hepatocellular carcinoma incidence in mice at high dose levels which may have been associated with toxicity. The major effects of chronic dosing were mortality in female rats at 500 mg/kg and in male and female mice at 750 mg/kg, accompanied by reductions in body weight gain in rats at 150 and 500 mg/kg and in mice at 750 mg/kg. Direct comparison of any tumorogenic effects of 2EH given alone to female mice with those due to 2EH formed in vivo from DEHA, DEHP, or TEHP is limited by the high mortality caused by 2ER in female mice at equivalent doses of 2EH. While 2EH may be a contributing factor in the hepatocellular carcinogenesis in female mice associated with the chronic administration of DEHA and DEHP, it is unlikely to be the entire proximate carcinogen.

Challenges to default assumptions stimulate comprehensive realism as a new tier in quantitative cancer risk assessment

The current practice in carcinogen risk assessment of using a linearized multistage model and assuming low-dose linearity is based on several false premises. In many cases linearity at low doses would not be expected based on the interaction between the multiple components in the carcinogenic process. The two-stage growth models, involving multiple mutations and cell birth and death rates, provide one means of exploring these interactions. In addition, if carcinogenesis is considered to be the imbalance between invading substances and defense mechanisms, then the cancer probability depends on how much the substance increases or decrease the number of defenders or their efficiency as well as increasing or decreasing the number of invaders. Challenges to low-dose linearity and other default assumptions have stimulated the development of new risk assessment methodologies as have the need for more realistic estimates of risk, better uncertainty characterization, and greater utilization of cost-benefit analyses, and other tools for risk management decision making. "Comprehensive realism" is an emerging quantitative weight-of-evidence risk assessment methodology which is designed to reflect all of the relevant and available information and the current state of knowledge about the health risks associated with a substance.

Reconsideration of the genetic risk assessment for ethylene oxide exposures

The US Environmental Protection Agency (EPA) developed a genetic risk assessment model for exposures to ethylene oxide utilizing data on the induction of reciprocal translocations in male germ cells [Rhomberg et al. 1990]. This particular approach served as a reasonable initial attempt, albeit somewhat limited with regard to endpoint and only partially attentive to the mechanisms of induction of genetic alterations and the behavior of chromosomes during meiosis. The present paper discusses the scientific basis for a reassessment of the EPA model, providing data and hypotheses related to effective dose to the target cells and shape of the dose-response relationship at low doses, and dose rates. While the present genetic risk assessment approach is discussed in terms of ethylene oxide, it would be applicable to most mutagenic chemicals. The outcome of the discussion is that the genetic risk for exposed males from reciprocal translocation induction will be negligible at low doses since the dose-response curve is likely to be a function of the square of the dose. In addition, the proportion of genetically unbalanced live born offspring in humans arising from reciprocal translocation carriers is less than 10% of the frequency formed through meiotic segregation and fertilization for such carriers. Simply from a consideration of mechanism--namely, the very high probability of DNA repair prior to the next S-phase for a resting oocyte--it would be predicted that there would be a very low to negligible frequency of translocations in female germ cells from ethylene oxide exposure. It is further stressed that additional components of a genetic risk model require a consideration of all germ cell stages in the male, and the inclusion of calculations for point and deletion mutations. Some indications of likely response are presented with these points in mind.

Use of probabilistic expert judgment in uncertainty analysis of carcinogenic potency

A new approach to characterizing the state of knowledge about carcinogenic potency is described. In this approach, the carcinogenic risk posed by a specific dose is characterized by a probability distribution, indicating the relative likelihood of different risk estimates. The approach utilizes expert judgment and a probability tree and is illustrated in a case study of chloroform exposure. Experts in cancer biology/toxicology, pharmacokinetics, and dose-response modeling were identified by a panel of science-policy specialists. In a workshop, experts reviewed the chloroform data, received training in probability elicitation, and constructed a consensual probability tree based on biological theories of cancer causation. Distributions of carcinogenic risk were developed based on the probability tree, chloroform data, judgmental probabilities provided by the experts, and classical statistical techniques. Risk distributions varied considerably between experts, with some predicting essentially no risk from 100 ppb chloroform in drinking water while other have at least some probability on risks generally considered of regulatory significance. Estimated human risk was much lower when extrapolating from liver tumors in animals than from kidney tumors. Issues of scientific disagreement leading to different risk distributions between experts are discussed. The resulting risk distributions are compared to standard EPA risk calculations for the same exposure scenario as well as to the expert judgement of epidemiologists about cancer risks of chlorinated drinking water. Issues in combining expert judgments are discussed, and several alternative methods are presented. Strengths and weaknesses of the distributional approach are discussed.

Another flaw in the linearized multistage model upper bounds on human cancer potency

The statistical methodology used by the EPA and other federal agencies to characterize human cancer potencies based on animal experiments greatly exaggerates the estimates and bounds for human cancer risks in some instances. The current methodology incorporates simplified assumptions and approximations that fail to properly assess the impact of quantitative differences in human and experimental animal background transition rates from stage to stage in the multistage carcinogenic process. Because the majority of tumorigenic responses in rats and mice occur in organs with a high background tumor incidence and, hence, high background stage transition rates, the current simplified methodology often significantly overstates human risk. This newly recognized flaw in conjunction with the several previously recognized flaws in the current characterizations of human cancer potencies argues strongly for a change away from the current characterizations based solely on a default screening methodology to a more comprehensive, biologically based risk assessment methodology.

A distributional approach to characterizing low-dose cancer risk

Since cancer risk at very low doses cannot be directly measured in humans or animals, mathematical extrapolation models and scientific judgment are required. This article demonstrates a probabilistic approach to carcinogen risk assessment that employs probability trees, subjective probabilities, and standard bootstrapping procedures. The probabilistic approach is applied to the carcinogenic risk of formaldehyde in environmental and occupational settings. Sensitivity analyses illustrate conditional estimates of risk for each path in the probability tree. Fundamental mechanistic uncertainties are characterized. A strength of the analysis is the explicit treatment of alternative beliefs about pharmacokinetics and pharmacodynamics. The resulting probability distributions on cancer risk are compared with the point estimates reported by federal agencies. Limitations of the approach are discussed as well as future research directions.

Assessment of safety/risk of chemicals: inception and evolution of the ADI and dose-response modeling procedures

This article reviews the procedures for the assessment of safety/risk of chemicals to human health. Because the nature and severity of toxicity and the extent of the database vary from chemical to chemical, the assessment is done on a case by case basis. Essentially 5 steps are involved in the assessment: (a) identification of hazards based on appropriate human and animal data; (b) determination of the dose-response relationship of the adverse effects of the chemical; (c) extrapolation of the dose-response data from test subjects to human populations; (d) estimation of the exposure; and (e) assessment of the safety/risk of the chemical under a specified exposure. Emphasis in this article, however, is placed on the extrapolation of the dose-response data to the human situation. The extrapolation is done by the identification of a no-observed-adverse-effect level (NOAEL) and the application of a safety factor, thereby arriving at an acceptable daily intake (ADI). The safety factor is selected on the basis of, inter alia, the severity of the adverse effect and the adequacy of the database. On the other hand, with genotoxic carcinogens, mathematical modeling is used for extrapolation. This is because the effects of genotoxic carcinogens are generally believed to have no threshold. The ADI approach, which involves the identification of a NOAEL, is therefore not applicable. A number of mathematical models have been developed to assess, from the dose-response data, either the risks that may be associated with a specified dose, or the 'virtually safe dose' at a specified risk level. The evolution, application and shortcomings of these procedures and the potential improvements in the ADI approach and in the dose-response characterization based on these mathematical models are also discussed.

Quantitative cancer risk assessments for 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)

State-of-the-art quantitative risk assessment techniques, including consideration of new time-to-response data, have been applied to chronic animal bioassay data on the dietary intake of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). The non-linear shapes of the dose-response relationships for the hepatocellular carcinogenic responses have been estimated, and a review of the quantitative impacts of several of the choices involved in the quantitative risk assessment considers, particularly, the definition of the carcinogenic responses of concern, the experimental data set, the pathology evaluation, a biologically effective dose scale versus the administered dose, methods of making the fitted model responsive to the data at the lower experimental doses, consistency in dose-response shapes for different data sets, fitted model values versus bounds, the utilization of time-to-response information incorporating the lateness of the carcinogenic responses, and the method of characterizing the maximum acceptable dose. The estimated virtually safe dose for an increase of 0.000001 (one in a million) in the probability of hepatocellular neoplastic nodule and/or carcinoma in a female rat is approximately 0.1 ng/kg body weight/day in the diet. The estimated mean free dose, corresponding to a reduction in the expected amount of time without hepatocellular neoplastic nodule and/or carcinoma proportional to 1 wk in 70 yr, is in the range of 1-5 ng/kg body weight/day in the diet of a female rat. No species-to-species extrapolations nor human exposure assessments have been made. However, these estimated risks correspond to dietary intakes that are at least 150 times greater than the 0.0006365 ng/kg body weight/day intake described by the Centers for Disease Control as a reasonable level to begin consideration of action to limit human exposure.

Some issues in the quantitative modeling portion of cancer risk assessment

Several questions should be asked in order to determine the relevance and scientific merit of a quantitative cancer risk assessment. Twenty such questions are formulated here and briefly discussed. These questions are intended to identify important issues and serve as a checklist for risk managers and developers of quantitative risk assessments. Among the many factors involved in these questions are the carcinogenic response, quantal response models, time to response, competing risks, model shapes, goodness of fit, dose scale, high-to-low-dose extrapolation, consistency across different subjects, animal-to-human extrapolation, route-to-route extrapolation, exposure durations and patterns, short-term tests, consistency with human epidemiological data, human exposures, statistical variability, impacts of assumptions and policy decisions and value judgments, risk characterization, upper and lower bounds, and keeping pace with scientific advances.

A comparison of statistical methods for low dose extrapolation utilizing time-to-tumor data

The assessment of health risks due to low levels of exposure to potential environmental hazards based on the results of toxicological experiments necessarily involves extrapolation of results obtained at relatively high doses to the low dose region of interest. In this paper, different statistical extrapolation procedures which take into account both time-to-response and the presence of competing risks are compared using a large simulated data base. The study was designed to cover a range of plausible dose response models as well as to assess the effects of competing risks, background response, latency and experimental design on the performance of the different extrapolation procedures. It was found that point estimates of risk in the low dose region may differ from the actual risk by a factor of 1000 or more in certain situations, even when precise information on the time of occurrence of the particular lesion of interest is available. Although linearized upper confidence limits on risk can be highly conservative when the underlying dose response curve is sublinear in the low dose region, they were found not to exceed the actual risk in the low dose region by more than a factor of 10 in those cases where the underlying dose response curve was linear at low doses.

Estimation of "safe doses" in carcinogenic experiments

The statistical methodology for carcinogenic safety testing here developed has the following advantages: (1) Rather than making possibly unwarranted assumptions about a minimum slope in a dose response relationship, the present method represents an objective data based method of estimating "safe" doses. It is applicable to any specified permissible risk (of exceeding the spontaneous rate) and the latter must of course be specified by F.D.A. (2) Although the model which is used for the estimation of safe doses and their lower confidence points is parametric it comprises an adequately large number of parameters to allow for differences in the idiosyncracies of suspected carcinogens, host species and to a limited degree for variations in the experimental protocols. (3) The same computer program will cover the analysis of an experiments in which times to tumor have been recorded as well as experiments in which only tumor incidence rates have been recorded or mixtures of the two. "Better experimentation" is "rewarded" in that the lower confidence limits for the "safe doses" should be higher and more closely approach the true safe dose as the experimental effort increases. (4) The maximum likelihood estimation procedure is sophisticated and has asymptotic optimality properties. It utilizes the latest techniques of "convex programming" and the computer algorithm is straightforward and fast. The methodology proposed here also has the following shortcomings: (1) The model is (multi)parametric. However, it is of the form of the product model for age specific hazard rates which is now widely accepted. (2) Robustness studies on the effect of model breakdown on the estimated safe doses are as yet limited and should be followed up with more extensive studies. (3) Obviously no estimates of "safe doses" can be made if the spontaneous incidence rate is zero and the experimental dose levels have been chosen too small and no tumors have been observed. Similarly if the experimental doses are too small and the tumor incidence rates are all comparable with the spontaneous rate the estimation procedure is afflicted by extremely large errors. The situation improves slightly if the incidence for the highest dose level is higher than that of the lower dose levels which are all approximately equal. In such situations more satisfactory experimental data are needed. Some general recommendations are as follows: (1) Whenever possible it is preferable to record times to tumor and not just incidence rates. However, for experiments of sufficiently long duration necropsies following the varying times of death will provide adequate information on the time dependence of tumor incidence. It may also be advisable to deliberately vary the times of sacrifice to two or three different times. (2) Other considerations being equal it is preferable to have a large number of dose levels rather than more animals per dose level...

A "super-population viewpoint' for finite population sampling

Frequently it is reasonable for a sample surveyor to view the finite population of interest as an independent sample of size N from an infinite super-population. This super-population viewpoint is contrasted to the classical frequentist theory of finite population sampling and the classical theory of infinite population sampling. A new technique for making inferences about finite population "parameters' is developed and shown to be applicable for any survey design. Two example applications are given: the estimation of strata- and population means in stratified sampling and the use of the so-called regression estimators for the same purpose.