Development of a framework for risk assessment of dietary carcinogens

Although there are a number of guidance documents and frameworks for evaluation of carcinogenicity, none of the current methods fully reflects the state of the science. Common limitations include the absence of dose-response assessment and not considering the impact of differing exposure patterns (e.g., intermittent, high peaks vs. lower, continuous exposures). To address these issues, we have developed a framework for risk assessment of dietary carcinogens. This framework includes an enhanced approach for weight of evidence (WOE) evaluation for genetic toxicology data, with a focus on evaluating studies based on the most recent testing guidance to determine whether a chemical is a mutagen. Included alongside our framework is a discussion of resources for evaluating tissue dose and the temporal pattern of internal dose, taking into account the chemical's toxicokinetics. The framework then integrates the mode of action (MOA) and associated dose metric category with the exposure data to identify the appropriate approach(es) to low-dose extrapolation and level of concern associated with the exposure scenario. This framework provides risk managers with additional flexibility in risk management and risk communication options, beyond the binary choice of linear low-dose extrapolation vs. application of uncertainty factors.

A Review of Stable Isotope Labeling and Mass Spectrometry Methods to Distinguish Exogenous from Endogenous DNA Adducts and Improve Dose-Response Assessments

Cancer remains the second most frequent cause of death in human populations worldwide, which has been reflected in the emphasis placed on management of risk from environmental chemicals considered to be potential human carcinogens. The formation of DNA adducts has been considered as one of the key events of cancer, and persistence and/or failure of repair of these adducts may lead to mutation, thus initiating cancer. Some chemical carcinogens can produce DNA adducts, and DNA adducts have been used as biomarkers of exposure. However, DNA adducts of various types are also produced endogenously in the course of normal metabolism. Since both endogenous physiological processes and exogenous exposure to xenobiotics can cause DNA adducts, the differentiation of the sources of DNA adducts can be highly informative for cancer risk assessment. This review summarizes a highly applicable methodology, termed stable isotope labeling and mass spectrometry (SILMS), that is superior to previous methods, as it not only provides absolute quantitation of DNA adducts but also differentiates the exogenous and endogenous origins of DNA adducts. SILMS uses stable isotope-labeled substances for exposure, followed by DNA adduct measurement with highly sensitive mass spectrometry. Herein, the utilities and advantage of SILMS have been demonstrated by the rich data sets generated over the last two decades in improving the risk assessment of chemicals with DNA adducts being induced by both endogenous and exogenous sources, such as formaldehyde, vinyl acetate, vinyl chloride, and ethylene oxide.

Revisiting the bacterial mutagenicity assays: Report by a workgroup of the International Workshops on Genotoxicity Testing (IWGT)

The International Workshop on Genotoxicity Testing (IWGT) meets every four years to obtain consensus on unresolved issues associated with genotoxicity testing. At the 2017 IWGT meeting in Tokyo, four sub-groups addressed issues associated with the Organization for Economic Cooperation and Development (OECD) Test Guideline TG471, which describes the use of bacterial reverse-mutation tests. The strains sub-group analyzed test data from >10,000 chemicals, tested additional chemicals, and concluded that some strains listed in TG471 are unnecessary because they detected fewer mutagens than other strains that the guideline describes as equivalent. Thus, they concluded that a smaller panel of strains would suffice to detect most mutagens. The laboratory proficiency sub-group recommended (a) establishing strain cell banks, (b) developing bacterial growth protocols that optimize assay sensitivity, and (c) testing "proficiency compounds" to gain assay experience and establish historical positive and control databases. The sub-group on criteria for assay evaluation recommended that laboratories (a) track positive and negative control data; (b) develop acceptability criteria for positive and negative controls; (c) optimize dose-spacing and the number of analyzable doses when there is evidence of toxicity; (d) use a combination of three criteria to evaluate results: a dose-related increase in revertants, a clear increase in revertants in at least one dose relative to the concurrent negative control, and at least one dose that produced an increase in revertants above control limits established by the laboratory from historical negative controls; and (e) establish experimental designs to resolve unclear results. The in silico sub-group summarized in silico utility as a tool in genotoxicity assessment but made no specific recommendations for TG471. Thus, the workgroup identified issues that could be addressed if TG471 is revised. The companion papers (a) provide evidence-based approaches, (b) recommend priorities, and (c) give examples of clearly defined terms to support revision of TG471.

Main issues addressed in the 2014-2015 revisions to the OECD Genetic Toxicology Test Guidelines

The Organization for Economic Cooperation and Development (OECD) recently revised the test guidelines (TGs) for genetic toxicology. This article describes the main issues addressed during the revision process, and the new and consistent recommendations made in the revised TGs for: (1) demonstration of laboratory proficiency; (2) generation and use of robust historical control data; (3) improvement of the statistical power of the tests; (4) selection of top concentration for in vitro assays; (5) consistent data interpretation and determination of whether the result is clearly positive, clearly negative or needs closer consideration; and, (6) consideration of 3R's for in vivo assay design. The revision process resulted in improved consistency among OECD TGs (including the newly developed ones) and more comprehensive recommendations for the conduct and the interpretation of the assays. Altogether, the recommendations made during the revision process should improve the efficiency, by which the data are generated, and the quality and reliability of test results. Environ. Mol. Mutagen. 58:284-295, 2017. © 2017 Wiley Periodicals, Inc.

Illustrative case using the RISK21 roadmap and matrix: prioritization for evaluation of chemicals found in drinking water

The HESI-led RISK21 effort has developed a framework supporting the use of twenty-first century technology in obtaining and using information for chemical risk assessment. This framework represents a problem formulation-based, exposure-driven, tiered data acquisition approach that leads to an informed decision on human health safety to be made when sufficient evidence is available. It provides a transparent and consistent approach to evaluate information in order to maximize the ability of assessments to inform decisions and to optimize the use of resources. To demonstrate the application of the framework's roadmap and matrix, this case study evaluates a large number of chemicals that could be present in drinking water. The focus is to prioritize which of these should be considered for human health risk as individual contaminants. The example evaluates 20 potential drinking water contaminants, using the tiered RISK21 approach in combination with graphical representation of information at each step, using the RISK21 matrix. Utilizing the framework, 11 of the 20 chemicals were assigned low priority based on available exposure data alone, which demonstrated that exposure was extremely low. The remaining nine chemicals were further evaluated, using refined estimates of toxicity based on readily available data, with three deemed high priority for further evaluation. In the present case study, it was determined that the greatest value of additional information would be from improved exposure models and not from additional hazard characterization.

Evaluation of urinary speciated arsenic in NHANES: issues in interpretation in the context of potential inorganic arsenic exposure

Urinary dimethylarsinic acid (DMA) and monomethylarsonic acid (MMA) are among the commonly used biomarkers for inorganic arsenic (iAs) exposure, but may also arise from seafood consumption and organoarsenical pesticide applications. We examined speciated urinary arsenic data from National Health and Nutrition Examination Survey (NHANES) 2009-2010 cycle to assess potential correlations among urinary DMA, MMA, and the organic arsenic species arsenobetaine. Urinary DMA and MMA were positively associated with urinary arsenobetaine, suggesting direct exposure to these species in seafood or metabolism of organic arsenicals to these species, although the biomonitoring data do not directly identify the sources of exposure. The magnitude of association was much larger for DMA than for MMA. The secondary methylation index (SMI, ratio of urinary DMA to MMA) observed in the NHANES program likewise is much higher in persons with detected arsenobetaine than in those without, again suggesting that direct DMA exposure is co-occurring with exposure to arsenobetaine. Urinary MMA was less correlated with organic arsenic exposures than DMA and, therefore, may be a more reliable biomarker for iAs exposure in the general US population. However, given the associations between both MMA and DMA and organic arsenic species in urine, interpretations of the urinary arsenic concentrations observed in the NHANES in the context of potential arsenic exposure should be made cautiously.

Mode of action and human relevance analysis for nuclear receptor-mediated liver toxicity: A case study with phenobarbital as a model constitutive androstane receptor (CAR) activator

The constitutive androstane receptor (CAR) and pregnane X receptor (PXR) are important nuclear receptors involved in the regulation of cellular responses from exposure to many xenobiotics and various physiological processes. Phenobarbital (PB) is a non-genotoxic indirect CAR activator, which induces cytochrome P450 (CYP) and other xenobiotic metabolizing enzymes and is known to produce liver foci/tumors in mice and rats. From literature data, a mode of action (MOA) for PB-induced rodent liver tumor formation was developed. A MOA for PXR activators was not established owing to a lack of suitable data. The key events in the PB-induced liver tumor MOA comprise activation of CAR followed by altered gene expression specific to CAR activation, increased cell proliferation, formation of altered hepatic foci and ultimately the development of liver tumors. Associative events in the MOA include altered epigenetic changes, induction of hepatic CYP2B enzymes, liver hypertrophy and decreased apoptosis; with inhibition of gap junctional intercellular communication being an associative event or modulating factor. The MOA was evaluated using the modified Bradford Hill criteria for causality and other possible MOAs were excluded. While PB produces liver tumors in rodents, important species differences were identified including a lack of cell proliferation in cultured human hepatocytes. The MOA for PB-induced rodent liver tumor formation was considered to be qualitatively not plausible for humans. This conclusion is supported by data from a number of epidemiological studies conducted in human populations chronically exposed to PB in which there is no clear evidence for increased liver tumor risk.

Evaluation of biomonitoring data from the CDC National Exposure Report in a risk assessment context: perspectives across chemicals

BACKGROUND

Biomonitoring data reported in the National Report on Human Exposure to Environmental Chemicals [NER; Centers for Disease Control and Prevention (2012)] provide information on the presence and concentrations of > 400 chemicals in human blood and urine. Biomonitoring Equivalents (BEs) and other risk assessment-based values now allow interpretation of these biomonitoring data in a public health risk context.

OBJECTIVES

We compared the measured biomarker concentrations in the NER with BEs and similar risk assessment values to provide an across-chemical risk assessment perspective on the measured levels for approximately 130 analytes in the NER.

METHODS

We identified available risk assessment-based biomarker screening values, including BEs and Human Biomonitoring-I (HBM-I) values from the German Human Biomonitoring Commission. Geometric mean and 95th percentile population biomarker concentrations from the NER were compared to the available screening values to generate chemical-specific hazard quotients (HQs) or cancer risk estimates.

CONCLUSIONS

Most analytes in the NER show HQ values of < 1; however, some (including acrylamide, dioxin-like chemicals, benzene, xylene, several metals, di-2(ethylhexyl)phthalate, and some legacy organochlorine pesticides) approach or exceed HQ values of 1 or cancer risks of > 1 × 10-4 at the geometric mean or 95th percentile, suggesting exposure levels may exceed published human health benchmarks. This analysis provides for the first time a means for examining population biomonitoring data for multiple environmental chemicals in the context of the risk assessments for those chemicals. The results of these comparisons can be used to focus more detailed chemical-specific examination of the data and inform priorities for chemical risk management and research.

Which fish should I eat? Perspectives influencing fish consumption choices

BACKGROUND

Diverse perspectives have influenced fish consumption choices.

OBJECTIVES

We summarized the issue of fish consumption choice from toxicological, nutritional, ecological, and economic points of view; identified areas of overlap and disagreement among these viewpoints; and reviewed effects of previous fish consumption advisories.

METHODS

We reviewed published scientific literature, public health guidelines, and advisories related to fish consumption, focusing on advisories targeted at U.S. populations. However, our conclusions apply to groups having similar fish consumption patterns.

DISCUSSION

There are many possible combinations of matters related to fish consumption, but few, if any, fish consumption patterns optimize all domains. Fish provides a rich source of protein and other nutrients, but because of contamination by methylmercury and other toxicants, higher fish intake often leads to greater toxicant exposure. Furthermore, stocks of wild fish are not adequate to meet the nutrient demands of the growing world population, and fish consumption choices also have a broad economic impact on the fishing industry. Most guidance does not account for ecological and economic impacts of different fish consumption choices.

CONCLUSION

Despite the relative lack of information integrating the health, ecological, and economic impacts of different fish choices, clear and simple guidance is necessary to effect desired changes. Thus, more comprehensive advice can be developed to describe the multiple impacts of fish consumption. In addition, policy and fishery management interventions will be necessary to ensure long-term availability of fish as an important source of human nutrition.

Evidence on the human health effects of low-level methylmercury exposure

BACKGROUND

Methylmercury (MeHg) is a known neuro-toxicant. Emerging evidence indicates it may have adverse effects on the neuro-logic and other body systems at common low levels of exposure. Impacts of MeHg exposure could vary by individual susceptibility or be confounded by beneficial nutrients in fish containing MeHg. Despite its global relevance, synthesis of the available literature on low-level MeHg exposure has been limited.

OBJECTIVES

We undertook a synthesis of the current knowledge on the human health effects of low-level MeHg exposure to provide a basis for future research efforts, risk assessment, and exposure remediation policies worldwide.

DATA SOURCES AND EXTRACTION

We reviewed the published literature for original human epidemiologic research articles that reported a direct biomarker of mercury exposure. To focus on high-quality studies and those specifically on low mercury exposure, we excluded case series, as well as studies of populations with unusually high fish consumption (e.g., the Seychelles), marine mammal consumption (e.g., the Faroe Islands, circumpolar, and other indigenous populations), or consumption of highly contaminated fish (e.g., gold-mining regions in the Amazon).

DATA SYNTHESIS

Recent evidence raises the possibility of effects of low-level MeHg exposure on fetal growth among susceptible subgroups and on infant growth in the first 2 years of life. Low-level effects of MeHg on neuro-logic outcomes may differ by age, sex, and timing of exposure. No clear pattern has been observed for cardio-vascular disease (CVD) risk across populations or for specific CVD end points. For the few studies evaluating immunologic effects associated with MeHg, results have been inconsistent.

CONCLUSIONS

Studies targeted at identifying potential mechanisms of low-level MeHg effects and characterizing individual susceptibility, sexual dimorphism, and non-linearity in dose response would help guide future prevention, policy, and regulatory efforts surrounding MeHg exposure.

Balancing the benefits of n-3 polyunsaturated fatty acids and the risks of methylmercury exposure from fish consumption

Fish and shellfish are widely available foods that provide important nutrients, particularly n-3 polyunsaturated fatty acids (n-3 PUFAs), to many populations globally. These nutrients, especially docosahexaenoic acid, confer benefits to brain and visual system development in infants and reduce risks of certain forms of heart disease in adults. However, fish and shellfish can also be a major source of methylmercury (MeHg), a known neurotoxicant that is particularly harmful to fetal brain development. This review documents the latest knowledge on the risks and benefits of seafood consumption for perinatal development of infants. It is possible to choose fish species that are both high in n-3 PUFAs and low in MeHg. A framework for providing dietary advice for women of childbearing age on how to maximize the dietary intake of n-3 PUFAs while minimizing MeHg exposures is suggested.

An exposure-response curve for copper excess and deficiency

There is a need to define exposure-response curves for both Cu excess and deficiency to assist in determining the acceptable range of oral intake. A comprehensive database has been developed where different health outcomes from elevated and deficient Cu intakes were assigned ordinal severity scores to create common measures of response. A generalized linear model for ordinal data was used to estimate the probability of response associated with dose, duration and severity. The model can account for differences in animal species, the exposure medium (drinking water and feed), age, sex, and solubility. Using this model, an optimal intake level of 2.6 mg Cu/d was determined. This value is higher than the current U.S. recommended dietary intake (RDI; 0.9 mg/d) that protects against toxicity from Cu deficiency. It is also lower than the current tolerable upper intake level (UL; 10 mg/d) that protects against toxicity from Cu excess. Compared to traditional risk assessment approaches, categorical regression can provide risk managers with more information, including a range of intake levels associated with different levels of severity and probability of response. To weigh the relative harms of deficiency and excess, it is important that the results be interpreted along with the available information on the nature of the responses that were assigned to each severity score.

Creating context for the use of DNA adduct data in cancer risk assessment: I. Data organization

The assessment of human cancer risk from chemical exposure requires the integration of diverse types of data. Such data involve effects at the cell and tissue levels. This report focuses on the specific utility of one type of data, namely DNA adducts. Emphasis is placed on the appreciation that such DNA adduct data cannot be used in isolation in the risk assessment process but must be used in an integrated fashion with other information. As emerging technologies provide even more sensitive quantitative measurements of DNA adducts, integration that establishes links between DNA adducts and accepted outcome measures becomes critical for risk assessment. The present report proposes an organizational approach for the assessment of DNA adduct data (e.g., type of adduct, frequency, persistence, type of repair process) in concert with other relevant data, such as dosimetry, toxicity, mutagenicity, genotoxicity, and tumor incidence, to inform characterization of the mode of action. DNA adducts are considered biomarkers of exposure, whereas gene mutations and chromosomal alterations are often biomarkers of early biological effects and also can be bioindicators of the carcinogenic process.

An evaluation of the mode of action framework for mutagenic carcinogens case study: Cyclophosphamide

In response to the 2005 revised US Environmental Protection Agency (EPA) Cancer Guidelines, a Risk Assessment Forum's Technical Panel has devised a strategy in which genetic toxicology data combined with other information are assessed to determine whether a carcinogen operates through a mutagenic mode of action (MOA). This information is necessary for EPA to decide whether age-dependent adjustment factors (ADAFs) should be applied to the cancer risk assessment. A decision tree has been developed as a part of this approach and outlines the critical steps for analyzing a compound for carcinogenicity through a mutagenic MOA (e.g., data analysis, determination of mutagenicity in animals and in humans). Agents, showing mutagenicity in animals and humans, proceed through the Agency's framework analysis for MOAs. Cyclophosphamide (CP), an antineoplastic agent, which is carcinogenic in animals and humans and mutagenic in vitro and in vivo, was selected as a case study to illustrate how the framework analysis would be applied to prove that a carcinogen operates through a mutagenic MOA. Consistent positive results have been seen for mutagenic activity in numerous in vitro assays, in animals (mice, rats, and hamsters) and in humans. Accordingly, CP was processed through the framework analysis and key steps leading to tumor formation were identified as follows: metabolism of the parent compound to alkylating metabolites, DNA damage followed by induction of multiple adverse genetic events, cell proliferation, and bladder tumors. Genetic changes in rats (sister chromatid exchanges at 0.62 mg/kg) can commence within 30 min and in cancer patients, chromosome aberrations at 35 mg/kg are seen by 1 hr, well within the timeframe and tumorigenic dose range for early events. Supporting evidence is also found for cell proliferation, indicating that mutagenicity, associated with cytotoxicity, leads to a proliferative response, which occurs early (48 hr) in the process of tumor induction. Overall, the weight of evidence evaluation supports CP acting through a mutagenic MOA. In addition, no data were found that an alternative MOA might be operative. Therefore, the cancer guidelines recommend a linear extrapolation for the risk assessment. Additionally, data exist showing that CP induces mutagenicity in fetal blood and in the peripheral blood of pediatric patients; thus, the ADAFs would be applied.

Copper and human health: biochemistry, genetics, and strategies for modeling dose-response relationships

Copper (Cu) and its alloys are used extensively in domestic and industrial applications. Cu is also an essential element in mammalian nutrition. Since both copper deficiency and copper excess produce adverse health effects, the dose-response curve is U-shaped, although the precise form has not yet been well characterized. Many animal and human studies were conducted on copper to provide a rich database from which data suitable for modeling the dose-response relationship for copper may be extracted. Possible dose-response modeling strategies are considered in this review, including those based on the benchmark dose and categorical regression. The usefulness of biologically based dose-response modeling techniques in understanding copper toxicity was difficult to assess at this time since the mechanisms underlying copper-induced toxicity have yet to be fully elucidated. A dose-response modeling strategy for copper toxicity was proposed associated with both deficiency and excess. This modeling strategy was applied to multiple studies of copper-induced toxicity, standardized with respect to severity of adverse health outcomes and selected on the basis of criteria reflecting the quality and relevance of individual studies. The use of a comprehensive database on copper-induced toxicity is essential for dose-response modeling since there is insufficient information in any single study to adequately characterize copper dose-response relationships. The dose-response modeling strategy envisioned here is designed to determine whether the existing toxicity data for copper excess or deficiency may be effectively utilized in defining the limits of the homeostatic range in humans and other species. By considering alternative techniques for determining a point of departure and low-dose extrapolation (including categorical regression, the benchmark dose, and identification of observed no-effect levels) this strategy will identify which techniques are most suitable for this purpose. This analysis also serves to identify areas in which additional data are needed to better define the characteristics of dose-response relationships for copper-induced toxicity in relation to excess or deficiency.

Data considerations for regulation of water contaminants

There are several pieces of legislation based on human health assessment that set the framework for U.S. EPA's regulation of water contaminants, such as bromate. The Safe Drinking Water Act, for example, specifies that the best available science be used in support of regulation of drinking water contaminants, and highlights that regulations must provide protection to sensitive human populations. Recent EPA guidance, including the 2005 Cancer Guidelines, emphasize analyzing data, and using defaults only in the absence of adequate data. This represents a major shift from the former practice of invoking default methodologies or values unless it was judged that there were sufficient data to depart from them. The Guidelines further present a framework for assessing data in order to determine if a mode of action (MOA) can be established, based on a modification of the Bradford-Hill criteria for causality. A similar approach is used by the International Programme on Chemical Safety (IPCS). To illustrate the application of the framework for evaluating animal tumors, three case studies are considered here. In the first example (chloroform carcinogenicity), sufficient data exist to identify the MOA in animals, and the data are used to illustrate the evaluation of the plausibility of the animal MOA in humans, taking into account toxicokinetics and toxicodynamics. In this case, the MOA was judged to be relevant to humans, and was used to determine the approach for the cancer quantitation. In the second example (naphthalene inhalation carcinogenicity), the key question is whether the weight of evidence (WOE) is sufficient to establish the MOA in animals. Atrazine-induced mammary tumors form the final example, illustrating the reasoning used to determine that the tumor MOA in animals was not considered relevant to humans; atrazine is therefore considered not likely to be a human carcinogen.

The interaction effects of binary mixtures of benzene and toluene on the developing heart of medaka (Oryzias latipes)

The United States Environmental Protection Agency (USEPA) has pursued the estimation of risk of adverse health effects from exposure to chemical mixtures since the early 1980s. Methods used to calculate risk estimates of mixtures were often based on single chemical information that required assumptions of dose-addition or response-addition and did not consider possible changes in response due to interaction effects among chemicals. Full factorial designs for laboratory studies can produce interactions information, but these are expensive to perform and may not provide the information needed to evaluate specific environmentally relevant mixtures. In this research, groups of Japanese medaka (Oryzias latipes) embryos were exposed to binary mixtures of benzene and toluene as well as to each of these chemicals alone. Endpoint specific dose-response models were built for the hydrocarbon mixture under an assumption of dose-additivity, using the single chemical dose-response information on benzene and toluene. The endpoints included heart rate, heart rate progression, and lethality. Results included a synergistic response for heart rate at 72 h of development, and either additivity or antagonism for all other endpoints at 96 h of development. This work uses an established statistical method to evaluate the toxicity of an environmentally relevant mixture to ascertain whether interaction effects are occurring, thus providing additional information on toxicity.

Methods and rationale for derivation of a reference dose for methylmercury by the U.S. EPA

In 2001, the U.S. Environmental Protection Agency derived a reference dose (RfD) for methylmercury, which is a daily intake that is likely to be without appreciable risk of deleterious effects during a lifetime. This derivation used a series of benchmark dose (BMD) analyses provided by a National Research Council (NRC) panel convened to assess the health effects of methylmercury. Analyses were performed for a number of endpoints from three large longitudinal cohort studies of the neuropsychological consequences of in utero exposure to methylmercury: the Faroe Islands, Seychelles Islands, and New Zealand studies. Adverse effects were identified in the Faroe Islands and New Zealand studies, but not in the Seychelles Islands. The NRC also performed an integrative analysis of all three studies. The EPA applied a total uncertainty factor (UF) of 10 for intrahuman toxicokinetic and toxicodynamic variability and uncertainty. Dose conversion from cord blood mercury concentrations to maternal methylmercury intake was performed using a one-compartment model. Derivation of potential RfDs from a number of endpoints from the Faroe Islands study converged on 0.1 microg/kg/day, as did the integrative analysis of all three studies. EPA identified several areas for which further information or analyses is needed. Perhaps the most immediately relevant is the ratio of cord:maternal blood mercury concentration, as well as the variability around this ratio. EPA assumed in its dose conversion that the ratio was 1.0; however, available data suggest it is perhaps 1.5-2.0. Verification of a deviation from unity presumably would be translated directly into comparable reduction in the RfD. Other areas that EPA identified as significant areas requiring further attention are cardiovascular consequences of methylmercury exposure and delayed neurotoxicity during aging as a result of previous developmental or adult exposure.

Derivation of U.S. EPA's oral Reference Dose (RfD) for methylmercury

Mercury (Hg) cycles in the environment through a series of complex chemical and physical transformations that occur in air, soils, and water bodies. One component of the environmental mercury cycle is the formation of methylmercury (MHg) primarily by aquatic and marine microorganisms and the accumulation of MHg in foodwebs, particularly in piscivorous species. Human consumption of piscivorous fish and other piscivorus animals is the most common pathway of exposure to MHg. For non-carcinogenic toxic endpoints, the U.S. EPA typically develops a Reference Dose (RfD). This is generally interpreted to be a concentration of a chemical which can be consumed on a daily basis over a lifetime without expectation of adverse effect. There is substantial evidence in both animal and humans that MHg is a neurotoxicant in the adult and the child as well as a developmental neurotoxicant for the fetus. Epidemics of MHg poisoning in Japan and Iraq have resulted from high-dose exposures to MHg. In these epidemics adults, children, nursing infants and fetuses were affected by MHg. The epidemics demonstrate that neurotoxicity is the health effect of greatest concern and that effects on the developing human nervous system apparently occur at lower exposures than those affecting the adult nervous system. We describe how the data from the Iraqi MHg epidemic were used to derive the current RfD of 0.1 microgram/Kgbw/day (U.S. EPA, 1995;).

Nurses' and pharmacists' exposure to antineoplastic drugs: findings from industrial hygiene scans and urine mutagenicity tests

Data from 83 nurses and pharmacists handling antineoplastic drugs and 35 nurse/pharmacist controls who participated in a national study of antineoplastic drug-handling risks were examined to investigate antineoplastic drug exposure. Measures of external exposure included self-completion drug logs and industrial hygiene scans conducted in clinical settings. Internal exposure was measured by urine mutagenicity tests on end-of-week 24-hour urine specimens. To control for potential confounders, the staff was asked to complete food and hobby diaries and to avoid identified mutagenic substances for 1 week before collection of 24-hour urine samples. On the scans of the drug handlers, 13% showed one or more spots of drug contamination on gloved and ungloved hands, gowns, or shoes. Of the 24-hour urine samples, 15% were mutagenic for Salmonella typhimurium: Rates did not differ significantly for drug handlers and controls. Among nurses who both prepared and administered antineoplastics, those with positive mutagenicity tests handled more doses of the drugs, used less skin protection, and had more skin contact with the drugs than those with negative tests. Nurses who only administered the drugs and had positive mutagenicity tests handled fewer doses of drugs than those with negative tests, but they also reported less use of protection and more skin contact. For both groups of nurses, skin contact with antineoplastics was associated with positive mutagenicity test results (p < 0.01).

Use of genetic toxicology data in U.S. EPA risk assessment: the mercury study report as an example

Assessment of human health risks of environmental agents has often been limited to consideration of the potential for the agent to cause cancer or general systemic toxicity after long-term exposure. The U.S. Environmental Protection Agency (U.S. EPA) is increasingly moving toward the development of integrated assessments, which consider all potential health end points including developmental toxicity, neurotoxicity, immunotoxicity, reproductive effects, and germ cell mutagenicity. The U.S. EPA has a responsibility to assess risks to nonhuman species or ecosystems when appropriate data are available. An example of a recent integrated human health and ecological risk assessment can be found in the U.S. EPA Mercury Study Report to Congress. This report covers the following topics in separate volumes: an inventory of anthropogenic mercury emissions in the United States; an exposure assessment using measured and predicted values and including indirect dietary exposure; an evaluation of human health risks; an assessment of ecologic risk wherein water criteria are presented for several wildlife species; an overall integrated characterization of human and nonhuman risk; and a discussion of risk management considerations. In the evaluation of human health risk, genetic toxicology data were considered for three forms of mercury: elemental, inorganic (divalent), and methylmercury. These data were used in judgments of two types of potential health effects (carcinogenicity and germ cell mutagenicity). In assessment of potential carcinogenicity of inorganic and methylmercury, genetic toxicity data were key. Data for clastogenicity in the absence of mutagenicity supported the characterization of inorganic and methylmercury as materials that produce carcinogenic effects only at high, toxic doses. The evidence for clastogenicity, coupled with information on metabolism and distribution, resulted in a judgment of a moderate degree of concern (or weight of evidence) that inorganic mercury can act as a human germ cell mutagen. For methylmercury, the degree of concern for germ cell mutagenicity is high.

Biological monitoring screening of patients provided antineoplastic drugs including adriamycin, cyclophosphamide, 5-fluorouracil, methotrexate, and vincristine

The aim of the present study was to establish screening biomarkers of exposure to antineoplastic drugs administered to 11 patients undergoing cancer chemotherapy. Among the anticancer drugs administered were cyclophosphamide (all), Adriamycin (5 of 11), methotrexate (3 of 11), 5-fluorouracil (4 of 11), vincristine (3 of 11), megestrol acetate (1 of 11), and procarbazine (1 of 11). The noninvasive urinary parameters investigated were thioethers, D-glucaric acid, elements, and forward and reverse mutagenesis using bacterial bioassays. The data were analyzed in terms of the observed concentrations and those corrected for personal baseline. Personal baseline correction for parameters with significant nonexposure baseline levels was essential. While glucaric acid and thioethers were increased by the drug treatments, the correlations with baseline-uncorrected data showing an inverse relationship proved spurious, because saturation of the detoxification systems occurred at the high doses administered. Glucaric acid was also influenced by methotrexate and vincristine. Thioether content was affected by cyclophosphamide only. The forward mutagenesis assay was directly correlated to cyclophosphamide dose but the reverse assay was not, in the presence or absence of rat S9 fraction. The forward assay was not sensitive to the effects of smoking. Relative to controls, the elements changed by cyclophosphamide were K, S, and P. Those affected by Adriamycin were Ca, Mg, and Na; 5-fluorouracil affected Ca, Mg, Na, and C; methotrexate changed P and S. The forward mutagenesis assay and D-glucaric acid concentrations were the screening biomarkers best suited to monitoring for extent of exposure to these antineoplastic drugs.

Metabolism of mutagenic polycyclic aromatic hydrocarbons by photosynthetic algal species

Polycyclic aromatic hydrocarbons (PAH) known to produce carcinogenic and mutagenic effects have been shown to contaminate waters, sediments and soils. While it is accepted that metabolites of these compounds are responsible for most of their biological effects in mammals, their metabolism, and to a large extent their bioactivity, in aquatic plants have not been explored. Cultures of photosynthetic algal species were assayed for their ability to metabolize benzo[a]pyrene (BaP), a carcinogenic PAH under conditions which either permitted (white light) or disallowed (gold light) photooxidation of the compound. Growth of Selenastrum capricornutum, a fresh-water green alga, was completely inhibited when incubated in white light with 160 micrograms BaP/l medium. By contrast concentrations at the upper limit of BaP solubility in aqueous medium had no effect on algal growth when gold light was used. BaP quinones and phenol derivatives were found to inhibit growth of Selenastrum under white light incubation. BaP phototoxicity and metabolism were observed to be species-specific. All 3 tested species of the order Chlorococcales were growth-inhibited by BaP in white light whereas neither the green alga Chlamydomonas reinhardtii nor a blue-green, a yellow-green or an euglenoid alga responded in this fashion. Assays of radiolabeled BaP metabolism in Selenastrum showed that the majority of radioactivity associated with BaP was found in media as opposed to algal cell pellets, that the extent of metabolism was BaP concentration dependent, and that the proportion of various metabolites detected was a function of the light source. After gold light incubation, BaP diols predominated while after white light treatment at equal BaP concentrations, the 3,6-quinone was found in the highest concentration. Extracted material from algal cell pellets and from media was tested for mutagenicity in a forward mutation suspension assay in Salmonella typhimurium using resistance to 8-azaguanine for selection. Direct-acting mutagens were detected in extracted media from incubation of Selenastrum with 400 micrograms BaP/l for 1 day in gold light. Extracts of media from algae incubated in gold light from 1 to 4 days with 1200 micrograms BaP/l were found to have direct-acting mutagens as well as those requiring further metabolism. Media extracts from white light incubations of BaP were mutagenic upon addition of rat liver homogenates. Activity of these materials from white light treatment are largely attributable to unmetabolized BaP.

Mutagenicity of 7H-dibenzo[c,g]carbazole and metabolites in Salmonella typhimurium

7H-Dibenzo[c,g]carbazole (DBC) is a potent carcinogen of environmental import. Reverse-mutation plate-incorporation assays for mutagenicity were undertaken in Salmonella typhimurium strains TA98 and TA100. Results were negative when no exogenous activation system was used, as well as when assays incorporated liver homogenates (S9) from rats, mice and rabbits. By contrast DBC was mutagenic in a forward mutation assay in Salmonella strain TM677 using resistance to 8-azaguanine for selection. Metabolites of DBC were generated by incubation with rat-liver microsomes and separated by HPLC. Two of these metabolites were directly mutagenic for Salmonella strain TM 677 while two others were mutagenic upon addition of S9. Synthetic phenolic derivatives of DBC were also mutagenic in this assay when further metabolized. It is likely that metabolites of DBC phenols constitute the biologically active forms.

Mutagenicity of algal metabolites of benzo(a)pyrene for Salmonella typhimurium

The metabolism and growth effects of benzo(a)pyrene (BaP) were studied using a freshwater green alga, Selenastrum capricornutum. Algal cultures were incubated under gold light with BaP added at concentrations of 40, 160, 400, and 1,200 micrograms/liter for the periods of 1-4 days. The metabolites and BaP were identified and quantified from ethyl acetate extracts of both algal cells and incubation medium. The ethyl acetate extracts were evaluated for genotoxicity using a micro-volume Salmonella typhimurium forward mutation assay with resistance to 8-azaguanine for selection. This assay detected the presence of small quantities of BaP and was particularly sensitive to the mutagenicity of BaP diols. Of those extracts prepared from algae and medium from cultures exposed to 400 micrograms BaP/liter (10 micrograms/25 ml culture), only algal cell extracts from one day's growth were mutagenic. In cultures exposed to 1,200 micrograms BaP/liter (30 micrograms/25 ml culture), mutagenic materials were produced or persisted in both algae and media throughout the 4-day incubation. The observed mutagenic response can be attributed in part to the presence of unmetabolized BaP or to BaP diols.

Mutagenicity of benzo(a)pyrene metabolites generated on the isolated perfused lung following particulate exposure

The isolated perfused rabbit lung (IPL) is being used to study the effects of particulate exposure on the pulmonary metabolism of benzo(a)pyrene (BaP). Pasturealla-free New Zealand white rabbits were treated intraperitoneally with BaP prior to kill. The isolated lungs were then administered either 14C-labeled BaP alone or BaP plus Fe2O3 or fly ash by intratracheal injection. Rates of appearance of BaP metabolites in the perfusing blood were determined. The extent of metabolism, distribution of metabolites, and types of metabolites produced were quantified for various lung tissue types by high-performance liquid chromatography and liquid scintillation spectrometry. Procedures were developed to apply the Salmonella/microsome test in the assay of mutagenicity of lung tissue and blood extracts as an indicator of their biologic activity. With few exceptions, blood extracts from IPL receiving BaP only were not mutagenic. Lung, trachea-bronchi, and macrophage extracts, by contrast, were mutagenic. A part of this activity could be attributed to BaP metabolites rather than to parent compound remaining in extracts. When lungs were exposed to Fe2O3 or to fly ash, only macrophage extracts were consistently mutagenic. This activity was due to significant amounts of unmetabolized BaP.

Mutagenicity testing of chlorinated biphenyls and chlorinated dibenzofurans

4 chlorinated biphenyl and 5 chlorinated dibenzofuran compounds have been evaluated in the reversion assay developed by B.N Ames using Salmonella typhimurium histidine auxotrophs. All these compounds (2,4,2'-4'-tetrachlorobiphenyl, 3,4,3',4'-tetrachlorobiphenyl, 4-chlorobiphenyl, 2,4,,6,2',4',6'-hexachlorobiphenyl, dibenzofuran, 2,9-dichlorodibenzofuran, 3,6-dichlorodibenzofuran, 2,3,7,8-tetrachlorodibenzofuran and octachlorodibenzofuran) were nonmutagenic for strains TA98 and TA100 when tested over a 3-log dose range. They were also not mutagenic whether or not varying concentrations of microsomal extracts (S9) from uninduced rats or from rats induced by several methods were included in the experimental protocol.

Evaluation of coal liquefaction technologies by Salmonella mutagenesis

Coal liquefaction materials made by two processes were found to be mutagenic in the Salmonella/microsome assay. Data from this type of in vitro assay can be used in the toxicological assessment of these processes. Such evaluations of the health and environmental impacts of technologies would aid in the development of alternate energy sources.

Mutagenicity of products from coal gasification and liquefaction in the Salmonella/microsome assay

As a first step in the assessment of their possible bio-effects, coal-related materials were tested for mutagenicity in the Salmonella/microsome assay. Of three coal gasification by-products tested, only a tar was mutagenic for any of four Salmonella strains. The following liquefaction materials were mutagenic for strains TA1538, TA98, and/or TA100: A liquefaction vehicle oil and coal hydrogenation filtered liquid, separated bottoms, vacuum overhead, and vacuum bottoms. Neither powdered coal nor water produced as a by-product of the hydrogenation process was positive in the Salmonella test. No coal-related material was mutagenic for the missense mutant TA1535 or for any strain in the absence of metabolic activation provided by rat hepatic homogenates (S9). In all but one instance Aroclor 1254-induced S9 provided the maximum activation for mutagenesis. Fractionation of all samples was undertaken by serial extraction with organic solvents of increasing polarity (hexane, toluene, methylene chloride, acetonitrile). Highly mutagenic materials were found in fractions of the hydrogenation filtered liquid, vacuum overhead, and vacuum bottoms. Thus far non-mutagenic samples have not yielded mutagenic components upon fractionation.