Annette M. Shipp

Evaluation of the Uncertainty in an Oral Reference Dose for Methylmercury Due to Interindividual Variability in Pharmacokinetics

An analysis of the uncertainty in guidelines for the ingestion of methylmercury (MeHg) due to human pharmacokinetic variability was conducted using a physiologically based pharmacokinetic (PBPK) model that describes MeHg kinetics in the pregnant human and fetus. Two alternative derivations of an ingestion guideline for MeHg were considered: the U.S. Environmental Protection Agency reference dose (RfD) of 0.1 microgram/kg/day derived from studies of an Iraqi grain poisoning episode, and the Agency for Toxic Substances and Disease Registry chronic oral minimal risk level (MRL) of 0.5 microgram/kg/day based on studies of a fish-eating population in the Seychelles Islands. Calculation of an ingestion guideline for MeHg from either of these epidemiological studies requires calculation of a dose conversion factor (DCF) relating a hair mercury concentration to a chronic MeHg ingestion rate. To evaluate the uncertainty in this DCF across the population of U.S. women of child-bearing age, Monte Carlo analyses were performed in which distributions for each of the parameters in the PBPK model were randomly sampled 1000 times. The 1st and 5th percentiles of the resulting distribution of DCFs were a factor of 1.8 and 1.5 below the median, respectively. This estimate of variability is consistent with, but somewhat less than, previous analyses performed with empirical, one-compartment pharmacokinetic models. The use of a consistent factor in both guidelines of 1.5 for pharmacokinetic variability in the DCF, and keeping all other aspects of the derivations unchanged, would result in an RfD of 0.2 microgram/kg/day and an MRL of 0.3 microgram/kg/day.

Evaluation of Uncertainty in Input Parameters to Pharmacokinetic Models and the Resulting Uncertainty in Output

Physiologically-based pharmacokinetic (PBPK) models may be used to predict the concentrations of parent chemical or metabolites in tissues, resulting from specified chemical exposures. An important application of PBPK modeling is in assessment of carcinogenic risks to humans, based on animal data. The parameters of a PBPK model may include metabolic parameters, blood/air and tissue/blood partition coefficients, and physiological parameters, such as organ weights and blood flow rates. Uncertainty in estimates of these parameters results in uncertainty regarding tissue concentrations and resulting risks. Data are reviewed relevant to the quantification of these uncertainties, for a PBPK model-based risk assessment for tetrachloroethylene. Probability distributions are developed to express uncertainty in model parameters, and uncertainties are propagated by a sequence of operations that simulates processes recognized as contributing to estimates of human risk. Distributions of PBPK model output and human risk estimates are used to characterize uncertainty resulting from uncertainty in model parameters.

Determination of a site-specific reference dose for methylmercury for fish-eating populations

Environmental risk-management decisions in the U.S. involving potential exposures to methylmercury currently use a reference dose (RfD) developed by the U.S. Environmental Protection Agency (USEPA). This RfD is based on retrospective studies of an acute poisoning incident in Iraq in which grain contaminated with a methylmercury fungicide was inadvertently used in the baking of bread.1 The exposures, which were relatively high but lasted only a few months, were associated with neurological effects in both adults (primarily paresthesia) and infants (late walking, late talking, etc.). It is generally believed that the developing fetus represents a particularly sensitive subpopulation for the neurological effects of methylmercury. The USEPA derived an RfD of 0.1 g/kg/day based on benchmark dose (BMD) modeling of the combined neurological endpoints reported for children exposed in utero. This RfD included an uncertainty factor of 10 to consider human pharmacokinetic variability and database limitations (lack of data on multigeneration effects or possible long-term sequelae of perinatal exposure). Alcoa signed an Administrative Order of Consent for the conduct of a remedial investigation/feasibility study (RI/FS) at their Point Comfort Operations and the adjacent Lavaca Bay in Texas to address the effects of historical discharges of mercury-containing wastewater. In cooperation with the Texas Natural Resource Conservation Commission and USEPA Region VI, Alcoa conducted a baseline risk assessment to assess potential risk to human health and the environment. As a part of this assessment, Alcoa pursued the development of a site-specific RfD for methylmercury to specifically address the potential human health effects associated with the ingestion of contaminated finfish and shellfish from Lavaca Bay. Application of the published USEPA RfD to this site is problematic; while the study underlying the RfD represented acute exposure to relatively high concentrations of methylmercury, the exposures of concern for the Point Comfort site are from the chronic consumption of relatively low concentrations of methylmercury in fish. Since the publication of the USEPA RfD, several analyses of chronic exposure to methylmercury in fish-eating populations have been reported. The purpose of the analysis reported here was to evaluate the possibility of deriving an RfD for methylmercury, specifically for the case of fish ingestion, on the basis of these new studies. In order to better support the risk-management decisions associated with developing a remediation approach for the site in question, the analysis was designed to provide information on the distribution of acceptable ingestion rates across a population, which could reasonably be expected to be consistent with the results of the epidemiological studies of other fish-eating populations. Based on a review of the available literature on the effects of methylmercury, a study conducted with a population in the Seychelles Islands was selected as the critical study for this analysis. The exposures to methylmercury in this population result from chronic, multigenerational ingestion of contaminated fish. This prospective study was carefully conducted and analyzed, included a large cohort of mother-infant pairs, and was relatively free of confounding factors. The results of this study are essentially negative, and a no-observed-adverse-effect level (NOAEL) derived from the estimated exposures has recently been used by the Agency for Toxic Substances and Disease Registry (ATSDR) as the basis for a chronic oral minimal risk level (MRL) for methylmercury. In spite of the fact that no statistically significant effects were observed in this study, the data as reported are suitable for dose-response analysis using the BMD method. Evaluation of the BMD method used in this analysis, as well as in the current USEPA RfD, has demonstrated that the resulting 95% lower bound on the 10% benchmark dose (BMDL) represents a conservative estimate of the traditional NOAEL, and that it is superior to the use of “average” or “grouped” exposure estimates when dose-response information is available, as is the case for the Seychelles study. A more recent study in the Faroe Islands, which did report statistically significant associations between methylmercury exposure and neurological effects, could not be used for dose-response modeling due to inadequate reporting of the data and confounding from co-exposure to polychlorinated biphenyls (PCBs). BMD modeling over the wide range of neurological endpoints reported in the Seychelles study yielded a lowest BMDL for methylmercury in maternal hair of 21 ppm. This BMDL was then converted to an expected distribution of daily ingestion rates across a population using Monte Carlo analysis with a physiologically based pharmacokinetic (PBPK) model to evaluate the impact of interindividual variability. The resulting distribution of ingestion rates at the BMDL had a geometric mean of 1.60 g/kg/day with a geometric standard deviation of 1.33; the 1st, 5th, and 10th percentiles of the distribution were 0.86, 1.04, and 1.15 g/kg/day. In place of the use of an uncertainty factor of 3 for pharmacokinetic variability, as is done in the current RfD, one of these lower percentiles of the daily ingestion rate distribution provides a scientifically based, conservative basis for taking into consideration the impact of pharmacokinetic variability across the population. On the other hand, it was felt that an uncertainty factor of 3 for database limitations should be used in the current analysis. Although there can be high confidence in the benchmark-estimated NOAEL of 21 ppm in the Seychelles study, some results in the New Zealand and Faroe Islands studies could be construed to suggest the possibility of effects at maternal hair concentrations below 10 ppm. In addition, while concerns regarding the possibility of chronic sequelae are not supported by the available data, neither can they be absolutely ruled out. The use of an uncertainty factor of 3 is equivalent to using a NOAEL of 7 ppm in maternal hair, which provides additional protection against the possibility that effects could occur at lower concentrations in some populations. Based on the analysis described above, the distribution of acceptable daily ingestion rates (RfDs) recommended to serve as the basis for site-specific risk-management decisions at Alcoa’s Point Comfort Operations ranges from approximately 0.3 to 1.1 g/kg/day, with a population median (50th percentile) of 0.5 g/kg/day. By analogy with USEPA guidelines for the use of percentiles in applications of distributions in exposure assessments, the 10th percentile provides a reasonably conservative measure. On this basis, a site-specific RfD of 0.4 g/kg/day is recommended.

Requirements for a biologically realistic cancer risk assessment for inorganic arsenic

A remarkable feature of the carcinogenicity of inorganic arsenic (As,) is the observation that human exposures to Asi have been strongly associated with increases in skin, lung, and internal cancers, but As, does not typically cause tumors in standard laboratory animal test protocols. Considerable controversy has centered on whether there is epidemiological evidence of a “threshold” for the carcinogenic effects of Asi, or at least of a highly nonlinear dose–response. Saturation of metabolism in the dose-range associated with tumors does not appear to be adequate to produce a major impact on the dose-response for carcinogenicity. If there is a strong nonlinearity, it results from the nature of the carcinogenic mechanism(s) of Asi. However, no single hypothesis for the mechanism of Asi carcinogenicity has widespread support. A biologically realistic cancer risk assessment for Asi would requirea quantitative description of the dose of active arsenic species in target tissues, the interactions between active arsenic and tissue constituents, and the manner in which these interactions result in tumor formation in multiple organs in humans, but not in experimental animals. Although Asi has only infrequently been associated with tumors in animal studies, it has repeatedly been shown to act as a comutagen in vitro and as a cocarcinogen in vivo. Asi is clastogenic, producing chromatid aberrations, but does not produce point mutations at single gene loci. Of particular interest, Asi has been shown to inhibit repair of DNA single-strand breaks, a possible mechanism for its observed comutagenicity and cocarcinogenicity. We propose a cocarcinogenic mode of action in which Asi acts primarily on intermediate cells deficient in cell cycle control at a late stage in a preexisting carcinogenic process. This interaction enhances ge-nomic fragility and accelerates conversion of premalignant lesions to more aggressive, clinically observable tumors. An indirect effect of As, on DNA repair is consistent with the expectation of a nonlinear dose-response rather than the linear dose-response traditionally assumed for mutagenic carcinogens. However, defining the exact nature of this tumor dose-response will require further experimental data on the dose-response for the cellular effects of Asi. Because Asi carcinogenicity is unlikely to be observed in normal experimental animals not exposed to other carcinogens, studies in animals and cell lines deficient in cell cycle control should also be considered. Experimental studies specifically designed to address the key mechanistic and dose-response issues for Asi carcinogenicity are critically needed to support public health policy decisions regarding current environmental exposures to Asi.

Analysis of in vivo mutation data can inform cancer risk assessment

Under the new U.S. Environmental Protection Agency (EPA) Cancer Risk Assessment Guidelines [U.S. EPA, 2005. Guidelines for Carcinogen Risk Assessment. EPA/630/P-03/001B, March 2005], the quantitative model chosen for cancer risk assessment is based on the mode-of-action (MOA) of the chemical under consideration. In particular, the risk assessment model depends on whether or not the chemical causes tumors through a direct DNA-reactive mechanism. It is assumed that direct DNA-reactive carcinogens initiate carcinogenesis by inducing mutations and have low-dose linear dose-response curves, whereas carcinogens that operate through a nonmutagenic MOA may have nonlinear dose-responses. We are currently evaluating whether the analysis of in vivo gene mutation data can inform the risk assessment process by better defining the MOA for cancer and thus influencing the choice of the low-dose extrapolation model. This assessment includes both a temporal analysis of mutation induction and a dose-response concordance analysis of mutation with tumor incidence. Our analysis of published data on riddelliine in rats and dichloroacetic acid in mice indicates that our approach has merit. We propose an experimental design and graphical analysis that allow for assessing time-to-mutation and dose-response concordance, thereby optimizing the potential for in vivo mutation data to inform the choice of the quantitative model used in cancer risk assessment.

Application of the Risk Assessment Approaches in the USEPA Proposed Cancer Guidelines to Inorganic Arsenic

Publisher Summary There is convincing evidence from a number of epidemiological studies that exposure to inorganic arsenic (Asi) in air or in drinking water has been associated with an increased incidence of cancer in exposed populations. However, there is less agreement on two critical risk assessment issues: quantification of the dose-response for these exposed populations, especially in the low-dose region and extrapolation of that dose-response relationship to current exposures of the public in air and drinking water. Recent evaluations of the biological basis for Asi carcinogenicity have suggested that the mode of action is nonlinear and may even have an effective threshold. Therefore, under the new USEPA proposed cancer guidelines, inorganic As might better be evaluated using the margin of exposure (MOE) approach rather than linear extrapolation as is now used. The purpose of this investigation was to explore the application of the MOE approach to Asi using epidemiological data for both oral and inhalation routes of exposure. Unfortunately, while qualitative data support a nonlinear mode of action for the carcinogenicity of Asi, quantitative data are inadequate to support a determination of the exposure levels at which nonlinearity might occur. On the basis of benchmark analyses of recently published epidemiological data, it would appear that the risk of cancer from lifetime consumption of Asi in drinking water at the current maximum contaminant level (MCL) might be significant. In contrast, current environmental As inhalation exposures appear unlikely to entail significant risks of cancer.

Nickel Carcinogenicity in Relation to the Health Risks from Residual Oil Fly Ash

ABSTRACT Epidemiological studies of workers in the nickel industry, animal exposure studies, and reports on the potential mechanisms of nickel-induced toxicity and carcinogenicity indicate that only crystalline sulfidic nickel compounds have been clearly established as carcinogenic or potentially carcinogenic in humans. This observation indicates the need to modify and update regulatory approaches for nickel to reflect noncancer toxicity values for some individual nickel species. Analysis of nickel compounds in residual oil fly ash (ROFA) indicates that sulfidic nickel compounds (e.g., nickel subsulfide, nickel sulfide) are not present. Thus, the potential for emission of carcinogenic nickel compounds from residual oil fly ash appears to be low. Preliminary reference concentrations (RfCs) for a number of nickel compounds, based on non-carcinogenic endpoints, are proposed on the basis of the benchmark dose approach in conjunction with NTP data for nickel species.