A Framework for Human Relevance Analysis of Information on Carcinogenic Modes of Action

Using mode of action information to improve regulatory decision-making: an ECETOC/ILSI RF/HESI workshop overview

The European Centre for Ecotoxicology and Toxicology of Chemicals (ECETOC), the International Life Sciences Institute (ILSI) Research Foundation (RF), and the ILSI Health and Environmental Sciences Institute (HESI) hosted a workshop in November 2009 to review current practice in the application of mode of action (MOA) considerations in chemical risk assessment. The aim was to provide a rationale for a more general, but flexible approach and to propose steps to facilitate broader uptake and use of the MOA concept. There was consensus amongst the workshop participants that it will require substantial effort and cooperation from the multiple disciplines involved to embrace a common, consistent, and transparent approach. Setting up a repository of accepted MOAs and associated guidance concerning appropriate data to support specific MOAs for critical effects would facilitate categorization of chemicals and allow predictions of toxicity outcomes by read-across. This should in future contribute to the reduction of toxicity testing in animals. The workshop participants also acknowledged the value and importance of human data and the importance of integrating information from biological pathway analyses into current MOA/human relevance frameworks.

Tertiary-Butanol: a toxicological review

Tert-Butanol is an important intermediate in industrial chemical synthesis, particularly of fuel oxygenates. Human exposure to tert-butanol may occur following fuel oxygenate metabolism or biodegradation. It is poorly absorbed through skin, but is rapidly absorbed upon inhalation or ingestion and distributed to tissues throughout the body. Elimination from blood is slower and the half-life increases with dose. It is largely metabolised by oxidation via 2-methyl-1,2-propanediol to 2-hydroxyisobutyrate, the dominant urinary metabolites. Conjugations also occur and acetone may be found in urine at high doses. The single-dose systemic toxicity of tert-butanol is low, but it is irritant to skin and eyes; high oral doses produce ataxia and hypoactivity and repeated exposure can induce dependence. Tert-Butanol is not definable as a genotoxin and has no effects specific for reproduction or development; developmental delay occurred only with marked maternal toxicity. Target organs for toxicity clearly identified are kidney in male rats and urinary bladder, particularly in males, of both rats and mice. Increased tumour incidences observed were renal tubule cell adenomas in male rats and thyroid follicular cell adenomas in female mice and, non-significantly, at an intermediate dose in male mice. The renal adenomas were associated with alpha(2u)-globulin nephropathy and, to a lesser extent, exacerbation of chronic progressive nephropathy. Neither of these modes of action can function in humans. The thyroid tumour response could be strain-specific. No thyroid toxicity was observed and a study of hepatic gene expression and enzyme induction and thyroid hormone status has suggested a possible mode of action.

1,3-Butadiene: III. Assessing carcinogenic modes of action

1,3-Butadiene (BD) is a multisite carcinogen in laboratory rodents following lifetime exposure, with greater potency in the mouse than the rat, and is associated with an increase in leukemia mortality in highly exposed workers. Species differences in the formation of reactive metabolites underlie observed species differences in sensitivity to the carcinogenic effects of BD. The modes of action (MOAs) for human leukemia and rodent tumors are both likely related to mutagenic potencies of one or more of these metabolites. However, differences in the nature of genotoxic lesions associated with human leukemia and rodent tumors, along with their implications for risk assessment, require that they be discussed separately. The MOAs for BD are assessed in this review using the modified Hill criteria and human relevance framework. Key events in MOAs for human and rodent cancers are identified, along with important species differences and sources of nonlinearity for each event that can affect extrapolations made from high- to low-dose exposures. Because occupational exposures to BD have also included co-exposures to styrene and dimethyldithiocarbamide (DMDTC), potential interactions with BD carcinogenicity are also discussed. The MOAs for BD carcinogenesis will be used to guide key decisions made in the quantitative cancer dose-response assessment.

Chemical carcinogenesis

Understanding the relationship of chemicals to carcinogenesis has progressed significantly since the initial observations of Hill and Pott in the 1700's. Distinguishing between DNA-reactive chemicals and those which increase cancer risk by increasing cell proliferation has been a major breakthrough in delineating overall mechanisms. Competing processes for activation versus inactivation of chemicals occur at many levels, including metabolism, DNA repair, and cellular repair processes. These processes can be affected by other agents to decrease carcinogenesis (chemoprevention). Increasing knowledge of the multiple steps of carcinogenesis is leading to improved methods for screening chemicals for carcinogenic activity and for regulatory decision making. Improvements in assessment of modes of action involved in animal and in vitro models have led to more rational approaches to assessing relevance to humans. The advent of genomics and high-throughput technologies have contributed to investigations of mechanisms and is beginning to impact development of better methods for screening chemicals. Based on developments in basic research, epidemiology, and astute clinical observations, the major risk factors and etiologic agents have been identified for a majority of cancers, which is beginning to lead to methods to decrease cancer incidence overall and identify targets for early detection and treatment.

Successful drug development despite adverse preclinical findings part 1: processes to address issues and most important findings

Unexpected adverse preclinical findings (APFs) are not infrequently encountered during drug development. Such APFs can be functional disturbances such as QT prolongation, morphological toxicity or carcinogenicity. The latter is of particular concern in conjunction with equivocal genotoxicity results. The toxicologic pathologist plays an important role in recognizing these effects, in helping to characterize them, to evaluate their risk for man, and in proposing measures to mitigate the risk particularly in early clinical trials. A careful scientific evaluation is crucial while termination of the development of a potentially useful drug must be avoided. This first part of the review discusses processes to address unexpected APFs and provides an overview over typical APFs in particular classes of drugs. If the mode of action (MoA) by which a drug candidate produces an APF is known, this supports evaluation of its relevance for humans. Tailor-made mechanistic studies, when needed, must be planned carefully to test one or several hypotheses regarding the potential MoA and to provide further data for risk evaluation. Safety considerations are based on exposure at no-observed-adverse-effect levels (NOAEL) of the most sensitive and relevant animal species and guide dose escalation in clinical trials. The availability of early markers of toxicity for monitoring of humans adds further safety to clinical studies. Risk evaluation is concluded by a weight of evidence analysis (WoE) with an array of parameters including drug use, medical need and alternatives on the market. In the second part of this review relevant examples of APFs will be discussed in more detail.

Application of the U.S. EPA mode of action Framework for purposes of guiding future research: a case study involving the oral carcinogenicity of hexavalent chromium

Mode of action (MOA) analysis provides a systematic description of key events leading to adverse health effects in animal bioassays for the purpose of informing human health risk assessment. Uncertainties and data gaps identified in the MOA analysis may also be used to guide future research to improve understanding of the MOAs underlying a specific toxic response and foster development of toxicokinetic and toxicodynamic models. An MOA analysis, consistent with approaches outlined in the MOA Framework as described in the Guidelines for Carcinogen Risk Assessment, was conducted to evaluate small intestinal tumors observed in mice chronically exposed to relatively high concentrations of hexavalent chromium (Cr(VI)) in drinking water. Based on review of the literature, key events in the MOA are hypothesized to include saturation of the reductive capacity of the upper gastrointestinal tract, absorption of Cr(VI) into the intestinal epithelium, oxidative stress and inflammation, cell proliferation, direct and/or indirect DNA modification, and mutagenesis. Although available data generally support the plausibility of these key events, several unresolved questions and data gaps were identified, highlighting the need for obtaining critical toxicokinetic and toxicodynamic data in the target tissue and in the low-dose range. Experimental assays that can address these data gaps are discussed along with strategies for comparisons between responsive and nonresponsive tissues and species. This analysis provides a practical application of MOA Framework guidance and is instructive for the design of studies to improve upon the information available for quantitative risk assessment.

Genotoxicity testing: moving beyond qualitative "screen and bin" approach towards characterization of dose-response and thresholds

For more than 40+ years, genotoxicity data have been interpreted in a qualitative, binary mode; a chemical is considered either positive or negative for a response in the test system. Although dose-response information is sometimes used in this decision, it is not routine to obtain the amount of information needed to inform risk assessment, for example to determine no-observed-genotoxic-effect-levels, primarily due to the historical view of genotoxic responses as "linear, no-threshold." Only recently have researchers begun to address this issue through robust experimental designs and application of statistical models. A growing body-of-evidence supports the existence of response thresholds for a number of mutagenic agents, in vitro and in vivo. Clearly, simple observation of a "hockey-stick" dose-response curve is not sufficient to establish a threshold. Collection of robust empirical data must be supported with an analysis of biological plausibility for the observed threshold. In this context, a chemical-specific mode-of-action (MOA) approach, which identifies key events responsible for the observed mutagenic effect, is extremely valuable. Biomarkers of key events, providing qualitative and quantitative information, can be integrated in a weight-of-evidence-based assessment of genotoxicity data from multiple test systems and used to identify data gaps to resolve/reduce uncertainties during the risk assessment process. To this end, specific recommendations on study design and data analysis are proposed. As the Environmental Mutagen Society celebrates its 40th anniversary, the field of genetic toxicology is marking a milestone on the path to a new paradigm, using a MOA, data-driven approach to answer questions about thresholds for genotoxic agents.

Just who is at risk? The ethics of environmental regulation

The willingness to view risk as part of daily life has vanished. A risk-averse mindset among environmental regulators engenders confusion between the ethics of intention and the ethics of consequence, leading to the elevation of the precautionary principle with unintended and often unfortunate outcomes. Environmental risk assessment is conservative, but the actual level of conservatism cannot be determined. High-end exposure assumptions and current toxicity criteria from the USEPA, based on linear extrapolation for carcinogens and default uncertainty factors for systemic toxicants, obscure the degree of conservatism in risk assessments. Ideally, one could choose a percentile of the target population to include within environmental standards, but this choice is complicated by the food, pharmaceutical and advertising industries, whose activities, inadvertent or not, often promote maladaptive and unhealthy lifestyle choices. There has lately been much discussion about background exposures and disease processes and their potential to increase the risk from environmental chemicals. Should these background exposures or disease processes, especially those associated with maladaptive individual choices, be included as part of a regulatory risk evaluation? A significant ethical question is whether environmental regulation should protect those pursuing a self-destructive lifestyle that may add to or synergize with otherwise innocuous environmental exposures. Choosing a target percentile of protection would provide an increased level of transparency and the flexibility to choose a higher or lower percentile if such a choice is warranted. Transparency and flexibility will lead to more responsive environmental regulation that balances protection of public health and the stewardship of societal resources.

Disruption of thyroid hormone homeostasis in Ugt1a-deficient Gunn rats by microsomal enzyme inducers is not due to enhanced thyroxine glucuronidation

Microsomal enzyme inducers (MEI) that increase UDP-glucuronosyltransferases (UGTs) are thought to increase glucuronidation of thyroxine (T(4)), thus reducing serum T(4), and subsequently increasing thyroid stimulating hormone (TSH). Ugt1a1 and Ugt1a6 mediate T(4) glucuronidation. Therefore, this experiment determined the involvement of Ugt1a enzymes in increased T(4) glucuronidation, decreased serum T(4), and increased TSH after MEI treatment. Male Wistar and Ugt1a-deficient Wistar (Gunn) rats were fed a control diet or diet containing pregnenolone-16α-carbonitrile (PCN; 800 ppm), 3-methylcholanthrene (3-MC; 200 ppm), or Aroclor 1254 (PCB; 100 ppm) for 7 days. Serum T(4), triiodothyronine (T(3)), and TSH concentrations, hepatic T(4)/T(3) glucuronidation, and thyroid histology and follicular cell proliferation were investigated. PCN, 3-MC, and PCB treatments decreased serum T(4), whereas serum T(3) was maintained in both Gunn and Wistar rats (except for PCB treatment). TSH was increased in Wistar and Gunn rats after PCN (130 and 277%) or PCB treatment (72 and 60%). T(4) glucuronidation in Wistar rats was increased after PCN (298%), 3-MC (85%), and PCB (450%), but was extremely low in Gunn rats, and unchanged after MEI. T(3) glucuronidation was increased after PCN (121%) or PCB (58%) in Wistar rats, but only PCN increased T(3) glucuronidation in Gunn rats (43%). PCN treatment induced thyroid morphological changes and increased follicular cell proliferation in both strains. These data demonstrate that T(4) glucuronidation cannot be increased in Ugt1a-deficient Gunn rats. Thus, the decrease in serum T(4), increase in TSH, and increase in thyroid cell proliferation after MEI are not dependent on increased T(4) glucuronidation, and cannot be attributed to Ugt1a enzymes.

Triclosan: a critical review of the experimental data and development of margins of safety for consumer products

Triclosan (2,4,4'-trichloro-2'-hydroxy-diphenyl ether) is an antibacterial compound that has been used in consumer products for about 40 years. The tolerability and safety of triclosan has been evaluated in human volunteers with little indication of toxicity or sensitization. Although information in humans from chronic usage of personal care products is not available, triclosan has been extensively studied in laboratory animals. When evaluated in chronic oncogenicity studies in mice, rats, and hamsters, treatment-related tumors were found only in the liver of male and female mice. Application of the Human Relevance Framework suggested that these tumors arose by way of peroxisome proliferator-activated receptor alpha (PPARalpha) activation, a mode of action not considered to be relevant to humans. Consequently, a Benchmark Dose (BMDL(10)) of 47 mg/kg/day was developed based on kidney toxicity in the hamster. Estimates of the amount of intake from in the use of representative personal care products for men, women, and children were derived in two ways: (1) using known or assumed triclosan levels in various consumer products and assumed usage patterns (product-based estimates); and (2) using upper bound measured urinary triclosan levels from human volunteers (biomonitoring-based estimates) using data from the Centers for Disease Control and Prevention. For the product-based estimates, the margin of safety (MOS) for the combined exposure estimates of intake from the use of all triclosan-containing products considered were approximately 1000, 730, and 630 for men, women, and children, respectively. The MOS calculated from the biomonitoring-based estimated intakes were 5200, 6700, and 11,750 for men, women, and children, respectively. Based on these results, exposure to triclosan in consumer products is not expected to cause adverse health effects in children or adults who use these products as intended.

Effects of Hepatic Drug-metabolizing Enzyme Induction on Clinical Pathology Parameters in Animals and Man

Hepatic drug-metabolizing enzyme (DME) induction is an adaptive response associated with changes in preclinical species; this response can include increases in liver weight, hepatocellular hyperplasia and hypertrophy, and upregulated tissue expression of DMEs. Effects of DME induction on clinical pathology markers of hepatobiliary injury and function in animals as well as humans are not well established. This component of a multipart review of the comparative pathology of xenobiotically mediated induction of hepatic metabolizing enzymes reviews pertinent data from retrospective and prospective preclinical and clinical studies. Particular attention is given to studies with confirmation of DME induction and concurrent evaluation of liver and/or serum hepatobiliary marker enzyme activities and histopathology. These results collectively indicate that in the rat, when histologic findings are limited to hepatocellular hypertrophy, DME induction is not expected to be associated with consistent or substantive changes in serum or plasma activity of hepatobiliary marker enzymes such as alanine aminotransferase, alkaline phosphatase, and gamma glutamyltransferase. In the dog and the monkey, published studies also do not demonstrate a consistent relationship across DME-inducing agents and changes in these clinical pathology parameters. However, increased liver alkaline phosphatase or gamma glutamyltransferase activity in dogs treated with phenobarbital or corticosteroids suggests that direct or indirect induction of select hepatobiliary injury markers can occur both in the absence of liver injury and independently of induction of DME activity. Although correlations between tissue and serum levels of these hepatobiliary markers are limited and inconsistent, increases in serum/plasma activities that are substantial or involve changes in other markers generally reflect hepatobiliary insult rather than DME induction. Extrahepatic effects, including disruption of the hypothalamic-pituitary-thyroid axis, can also occur as a direct outcome of hepatic DME induction in humans and animals. Importantly, hepatic DME induction and associated changes in preclinical species are not necessarily predictive of the occurrence, magnitude, or enzyme induction profile in humans.

Guidance for the classification of carcinogens under the Globally Harmonised System of Classification and Labelling of Chemicals (GHS)

The United Nations Conference on Environment and Development (UNCED) has developed criteria for a globally harmonised system of classification and labelling of chemicals (GHS). With regard to carcinogenicity, GHS distinguishes between Category 1 ('known or presumed human carcinogens') and Category 2 ('suspected human carcinogens'). Category 1 carcinogens are divided into Category 1A ('known to have carcinogenic potential for humans'), based largely on human evidence, and 1B ('presumed to have carcinogenic potential for humans'), based largely on experimental animal data. Concerns have been raised that the criteria for applying these carcinogenicity classifications are not sufficiently well defined and potentially allow different conclusions to be drawn. The current document describes an attempt to reduce the potential for diverse conclusions resulting from the GHS classification system through the application of a series of questions during the evaluation of data from experiments with rodents; epidemiological data, which could lead to Category 1A, have not been considered. Answers to each question can lead either to a classification decision or to the next question, but this process should only be implemented in an environment of informed scientific opinion. The scheme is illustrated with five case studies. These questions are: (1) Has a relevant form of the substance been tested? (2) Is the study design relevant to human exposure? (3) Is there a substance-related response? (4) Is the target tissue exposure relevant to humans? (5) Can a mode of action be established? (6) Is the mode of action relevant to humans? (7) What is the potency?

Application of the Margin of Exposure (MOE) approach to substances in food that are genotoxic and carcinogenic

This paper presents the work of an expert group established by the International Life Sciences Institute - European branch (ILSI Europe) to follow up the recommendations of an international conference on "Risk Assessment of Compounds that are both Genotoxic and Carcinogenic: New Approaches". Twelve genotoxic and carcinogenic chemicals that can be present in food were selected for calculation of a Margin of Exposure (MOE) between a point of departure on the dose-response for oral carcinogenicity in animal studies and estimates of human dietary exposure. The MOE can be used to support prioritisation of risk management action and, if the MOE is very large, on communication of a low level of human health concern. Depending on the approaches taken in determining the point of departure and the estimation of exposure, it is possible to derive very different values for the MOE. It is therefore essential that the selection of the cancer endpoint and mathematical treatment of the data are clearly described and justified if the results of the MOE approach are to be trusted and of value to risk managers. An outline framework for calculating an MOE is proposed in order to help to ensure transparency in the results.

Mode-of-action analysis for induction of rat liver tumors by pyrethrins: relevance to human cancer risk

High doses of pyrethrins have been shown to produce liver tumors in female rats. Pyrethrins are not genotoxic agents. Pyrethrins produce liver tumors in rats by a mode of action (MOA) involving induction of hepatic xenobiotic metabolising enzymes, hypertrophy, increased cell proliferation, and the development of altered hepatic foci. The relevance of pyrethrins-induced rat liver tumors to human health was assessed by using the 2006 International Programme on Chemical Safety Human Relevance Framework. The postulated rodent tumor MOA was tested against the Bradford Hill criteria and was found to satisfy the conditions of dose and temporal concordance, biological plausibility, coherence, strength, consistency, and specificity that fit with an established mode of action for rodent liver tumor formation by phenobarbital and related compounds, which are activators of the constitutive androstane receptor. Other possible MOAs including mutagenicity, cytotoxicity, hepatic peroxisome proliferation, porphyria, and hormonal pertubation were excluded. The proposed MOA is considered not to be plausible in humans because pyrethrins, like phenobarbital, do not induce cell proliferation in human hepatocytes. Moreover, epidemiological studies with phenobarbital demonstrate that such compounds do not increase the risk of liver tumors in humans. It is concluded that pyrethrins do not pose a hepatocarcinogenic hazard for humans.

Derivation of a reference dose and drinking water equivalent level for 1,2,3-trichloropropane

In some US potable water supplies, 1,2,3-trichloropropane (TCP) has been present at ranges of non-detect to less than 100 ppb, resulting from past uses. In subchronic oral studies, TCP produced toxicity in kidneys, liver, and other tissues. TCP administered by corn oil gavage in chronic studies produced tumors at multiple sites in rats and mice; however, interpretation of these studies was impeded by substantial premature mortality. Drinking water equivalent levels (DWELs) were estimated for a lifetime of consumption by applying biologically-based safety/risk assessment approaches, including Monte Carlo techniques, and with consideration of kinetics and modes of action, to possibly replace default assumptions. Internationally recognized Frameworks for human relevance of animal data were employed to interpret the findings. Calculated were a reference dose (=39 microg/kg d) for non-cancer and Cancer Values (CV) (=10-14 microg/kg d) based on non-linear dose-response relationships for mutagenicity as a precursor of cancer. Lifetime Average Daily Intakes (LADI) are 3130 and 790-1120 microg/person-d for non-cancer and cancer, respectively. DWELs, estimated by applying a relative source contribution (RSC) of 50% to the LADIs, are 780 and 200-280 microg/L for non-cancer and cancer, respectively. These DWELs may inform establishment of formal/informal guidelines and standards to protect public health.

Evaluation of Possible Carcinogenic Risk to Humans Based on Liver Tumors in Rodent Assays

The two-year rodent bioassay remains the mainstay for carcinogenicity testing, although numerous difficulties have been identified. Fundamentally, a chemical can increase the risk of cancer (1) by damaging DNA directly (DNA reactive) or (2) indirectly by increasing the number of DNA replications (non–DNA reactive). Mechanistic research has identified numerous precursor lesions in the sequence of key events necessary for neoplasia development. Based on these concepts, the author has proposed a short-term (thirteen-week) assay for screening for carcinogenic potential based on a mode of action analysis and on readily available, identifiable preneoplastic changes. A screening assay that detects all potential rodent hepatocarcinogens has been previously identified ( Toxicol Pathol32 [2004], 393–401) including increased liver weight, hepatocellular necrosis, hypertrophy, and cytomegaly. Labeling index for DNA replication might supply additional support. These markers have high sensitivity but low specificity. However, most chemicals can be appropriately classified as to their mode(s) of action for hepatocarcinogenesis with follow-up mechanistic studies, and a rational evaluation of their relevance to humans can be made. A similar process can be envisioned for other tissues for evaluation for carcinogenic potential.

An evaluation of the mode of action framework for mutagenic carcinogens case study II: chromium (VI)

In response to the 2005 revised U.S Environmental Protection Agency's (EPA) Cancer Guidelines, a strategy is being developed to include all mutagenicity and other genotoxicity data with additional information to determine whether the initiating step in carcinogenesis is through a mutagenic mode of action (MOA). This information is necessary to decide if age-dependent adjustment factors (ADAFs) should be applied to the risk assessment. Chromium (VI) [Cr (VI)], a carcinogen in animals and humans via inhalation, was reassessed by the National Toxicology Program (NTP) in 2-year drinking water studies in rodents. From these data, NTP concluded that the results with Cr (VI) showed clear evidence of carcinogenicity in male and female mice and rats. Cr (VI) is also mutagenic, in numerous in vitro assays, in animals (mice and rats) and in humans. Accordingly, Cr (VI) was processed through the MOA framework; postulated key steps in tumor formation were interaction of DNA with Cr (VI) and reduction to Cr (III), mutagenesis, cell proliferation, and tumor formation. Within the timeframe and tumorigenic dose range for early events, genetic changes in mice (single/double-stranded DNA breaks) commence within 24 hr. Mechanistic evidence was also found for oxidative damage and DNA adduct formation contributing to the tumor response. The weight of evidence supports the plausibility that Cr (VI) may act through a mutagenic MOA. Therefore, the Cancer Guidelines recommend a linear extrapolation for the oral risk assessment. Cr (VI) also induces germ cell mutagenicity and causes DNA deletions in developing embryos; thus, it is recommended that the ADAFs be applied.

Proposed mode of action of benzene-induced leukemia: Interpreting available data and identifying critical data gaps for risk assessment

Mode of action is defined as a series of key biological events leading to an observed toxicological effect (for example, metabolism to a toxic entity, cell death, regenerative repair and tumors). It contrasts with mechanism of action, which generally involves a detailed understanding of the molecular basis for an effect. A framework to consider the weight of evidence for hypothesized modes of action in animals and their relevance to humans, has been widely adopted and used by government agencies and international organizations. The framework, developed and refined through its application in case studies for principally non-DNA-reactive carcinogens, has more recently been extended to DNA-reactive carcinogens, non-cancer endpoints and different life stages. In addition to increasing transparency, use of the framework promotes consistency in decision-making concerning adequacy of weight of evidence, facilitates peer input and review and identifies critical research needs. The framework provides an effective tool to facilitate discussion between the research and risk assessment communities on critical data gaps, which if filled, would permit more refined estimates of risk. As a basis for additionally coordinating and focusing research on critical data gaps in a risk assessment context, five key events in the mode of action for benzene-induced leukemia are proposed: (1) benzene metabolism via Cytochrome P450, (2) the interaction of benzene metabolites with target cells in the bone marrow, (3) formation of initiated, mutated target cells, (4) selective proliferation of the mutated cells and (5) production of leukemia. These key events are considered in a framework analysis of human relevance as a basis to consider appropriate next steps in developing research strategies.

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.

Meeting the common needs of a more effective and efficient testing and assessment paradigm for chemical risk management

Significant advances have been made in human health and ecological risk assessment over the last decade. Substantial challenges, however, remain in providing credible scientific information in a timely and efficient manner to support chemical risk assessment and management decisions. A major challenge confronting risk managers is the need for critical information to address risk uncertainties in large chemical inventories such as high- and medium-production-volume industrial chemicals or pesticide inert ingredients. From a strategic and tactical viewpoint, an integrated approach that relies on all existing knowledge and uses a range of methods, including those from emerging and novel technologies, is needed to advance progressive and focused testing strategies, as well as to advance the utility and predictability of the risk assessment by providing more relevant information. A hypothesis-based approach that draws on all relevant information is consistent with the vision articulated in the 2007 report by the National Research Council, Toxicity Testing in the 21st Century: A Vision and a Strategy. This article describes the current practices in evaluating chemical risks and ongoing efforts to enhance the quality and efficiency of risk assessment and risk management decisions within the Office of Prevention, Pesticides, and Toxic Substances at the U.S. Environmental Protection Agency.

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

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

Virtual tissues in toxicology

New approaches are vital for efficiently evaluating human health risk of thousands of chemicals in commerce. In vitro models offer a high-throughput approach for assaying chemical-induced molecular and cellular changes; however, bridging these perturbations to in vivo effects across chemicals, dose, time, and species remains challenging. Technological advances in multiresolution imaging and multiscale simulation are making it feasible to reconstruct tissues in silico. In toxicology, these "virtual" tissues (VT) aim to predict histopathological outcomes from alterations of cellular phenotypes that are controlled by chemical-induced perturbations in molecular pathways. The behaviors of thousands of heterogeneous cells in tissues are simulated discretely using agent-based modeling (ABM), in which computational "agents" mimic cell interactions and cellular responses to the microenvironment. The behavior of agents is constrained by physical laws and biological rules derived from experimental evidence. VT extend compartmental physiologic models to simulate both acute insults as well as the chronic effects of low-dose exposure. Furthermore, agent behavior can encode the logic of signaling and genetic regulatory networks to evaluate the role of different pathways in chemical-induced injury. To extrapolate toxicity across species, chemicals, and doses, VT require four main components: (a) organization of prior knowledge on physiologic events to define the mechanistic rules for agent behavior, (b) knowledge on key chemical-induced molecular effects, including activation of stress sensors and changes in molecular pathways that alter the cellular phenotype, (c) multiresolution quantitative and qualitative analysis of histologic data to characterize and measure chemical-, dose-, and time-dependent physiologic events, and (d) multiscale, spatiotemporal simulation frameworks to effectively calibrate and evaluate VT using experimental data. This investigation presents the motivation, implementation, and application of VT with examples from hepatotoxicity and carcinogenesis.

Development of a quantitative model incorporating key events in a hepatotoxic mode of action to predict tumor incidence

Biologically based dose-response (BBDR) modeling of environmental pollutants can be utilized to inform the mode of action (MOA) by which compounds elicit adverse health effects. Chemicals that produce tumors are typically labeled as either genotoxic or nongenotoxic. Though both the genotoxic and the nongenotoxic MOA may be operative as a function of dose, it is important to note that the label informs but does not define a MOA. One commonly proposed MOA for nongenotoxic carcinogens is characterized by the key events cytotoxicity and regenerative proliferation. The increased division rate associated with such proliferation can cause an increase in the probability of mutations, which may result in tumor formation. We included these steps in a generalized computational pharmacodynamic (PD) model incorporating cytotoxicity as a MOA for three carcinogens (chloroform, CHCl(3); carbon tetrachloride, CCL(4); and N,N-dimethylformamide, DMF). For each compound, the BBDR model is composed of a chemical-specific physiologically based pharmacokinetic model linked to a PD model of cytotoxicity and cellular proliferation. The rate of proliferation is then linked to a clonal growth model to predict tumor incidences. Comparisons of the BBDR simulations and parameterizations across chemicals suggested that significant variation among the models for the three chemicals arises in a few parameters expected to be chemical specific (such as metabolism and cellular injury rate constants). Optimization of model parameters to tumor data for CCL(4) and DMF resulted in similar estimates for all parameters related to cytotoxicity and tumor incidences. However, optimization of the CHCl(3) data resulted in a higher estimate for one parameter (BD) related to death of initiated cells. This implies that additional steps beyond cytotoxicity leading to induced cellular proliferation can be quantitatively different among chemicals that share cytotoxicity as a hypothesized carcinogenic MOA.

Risk assessment practice for essential metals

This article addresses the content of the workshop, including a panel discussion relevant to delineation of a path forward in relation to risk assessment of essential metals. The state of the art of risk assessment and associated issues for essential metals are outlined initially, followed by brief illustration by the case studies considered at the workshop (i.e., copper, zinc, and manganese). Approaches for the future testing strategies of essential metals are discussed in terms of options to increase efficiency and accuracy of assessments. Subsequently, recommendations for pragmatic next steps to advance progress and facilitate uptake by the regulatory risk assessment community are presented.

Derivation of a melamine oral reference dose (RfD) and drinking-water total allowable concentration

Due to its high nitrogen content, melamine has been used to adulterate food to increase apparent protein content. In 2008, thousands of Chinese infants consumed reconstituted formula derived from melamine-adulterated milk. Urinary-tract stones (comprised of melamine and uric acid) accumulated in some victims and lead to acute renal failure or death. Premature infants and children (<2 yr) have an increased susceptibility to ingested melamine. Due to incomplete reporting, the human data were inadequate to identify a no-observed-adverse-effect level (NOAEL) for melamine-induced pediatric urolithiasis. Urolithiasis, urinary bladder cystitis, and ulcerations were observed in F344 rats after subchronic or chronic ingestion of melamine at > or =72 mg/kg-d. Bladder epithelial damage was followed by epithelial hyperplasia that progressed to bladder papillomas and carcinomas in male but not female F344 rats or male or female B6C3F1 mice. Short-term assays suggest, at best, weak genotoxic activity, and kinetic data show that melamine is not metabolized. Since reliable exposure information was lacking from the clinical reports, an oral reference dose (RfD) based on urolithiasis in male rats after 13 wk of continuous melamine ingestion was calculated as a 10% benchmark dose (38 mg/kg-d). Incorporation of 10-fold interspecies and intraspecies (for the increased susceptibility of infants) uncertainty factors and a threefold database uncertainty factor (for the lack of immunological, neurological and reproduction toxicity data) yields an oral RfD of 0.13 mg/kg-d. Assuming the 70-kg adult consumes 2 L of drinking water daily, a total allowable concentration of 0.9 mg/L (900 microg/L) was calculated for melamine in drinking water.

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

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

The Key Events Dose-Response Framework: a cross-disciplinary mode-of-action based approach to examining dose-response and thresholds

The ILSI Research Foundation convened a cross-disciplinary working group to examine current approaches for assessing dose-response and identifying safe levels of intake or exposure for four categories of bioactive agents—food allergens, nutrients, pathogenic microorganisms, and environmental chemicals. This effort generated a common analytical framework—the Key Events Dose-Response Framework (KEDRF)—for systematically examining key events that occur between the initial dose of a bioactive agent and the effect of concern. Individual key events are considered with regard to factors that influence the dose-response relationship and factors that underlie variability in that relationship. This approach illuminates the connection between the processes occurring at the level of fundamental biology and the outcomes observed at the individual and population levels. Thus, it promotes an evidence-based approach for using mechanistic data to reduce reliance on default assumptions, to quantify variability, and to better characterize biological thresholds. This paper provides an overview of the KEDRF and introduces a series of four companion papers that illustrate initial application of the approach to a range of bioactive agents.

The use of Mechanisms and Modes of Toxic Action in Integrated Testing Strategies: The Report and Recommendations of a Workshop held as part of the European Union OSIRIS Integrated Project

This report on The Potential of Mode of Action (MoA) Information Derived from Non-testing and Screening Methodologies to Support Informed Hazard Assessment, resulted from a workshop organised within OSIRIS (Optimised Strategies for Risk Assessment of Industrial Chemicals through Integration of Non-test and Test Information), a project partly funded by the EU Commission within the Sixth Framework Programme. The workshop was held in Liverpool, UK, on 30 October 2008, with 35 attendees. The goal of the OSIRIS project is to develop integrated testing strategies (ITS) fit for use in the REACH system, that would enable a significant increase in the use of non-testing information for regulatory decision making, and thus minimise the need for animal testing. One way to improve the evaluation of chemicals may be through categorisation by way of mechanisms or modes of toxic action. Defining such groups can enhance read-across possibilities and priority settings for certain toxic modes or chemical structures responsible for these toxic modes. Overall, this may result in a reduction of in vivo testing on organisms, through combining available data on mode of action and a focus on the potentially most-toxic groups. In this report, the possibilities of a mechanistic approach to assist in and guide ITS are explored, and the differences between human health and environmental areas are summarised.

Application of key events analysis to chemical carcinogens and noncarcinogens

The existence of thresholds for toxicants is a matter of debate in chemical risk assessment and regulation. Current risk assessment methods are based on the assumption that, in the absence of sufficient data, carcinogenesis does not have a threshold, while noncarcinogenic endpoints are assumed to be thresholded. Advances in our fundamental understanding of the events that underlie toxicity are providing opportunities to address these assumptions about thresholds. A key events dose-response analytic framework was used to evaluate three aspects of toxicity. The first section illustrates how a fundamental understanding of the mode of action for the hepatic toxicity and the hepatocarcinogenicity of chloroform in rodents can replace the assumption of low-dose linearity. The second section describes how advances in our understanding of the molecular aspects of carcinogenesis allow us to consider the critical steps in genotoxic carcinogenesis in a key events framework. The third section deals with the case of endocrine disrupters, where the most significant question regarding thresholds is the possible additivity to an endogenous background of hormonal activity. Each of the examples suggests that current assumptions about thresholds can be refined. Understanding inter-individual variability in the events involved in toxicological effects may enable a true population threshold(s) to be identified.

A Data-Based Assessment of Alternative Strategies for Identification of Potential Human Cancer Hazards

The two-year cancer bioassay in rodents remains the primary testing strategy for in-life screening of compounds that might pose a potential cancer hazard. Yet experimental evidence shows that cancer is often secondary to a biological precursor effect, the mode of action is sometimes not relevant to humans, and key events leading to cancer in rodents from nongenotoxic agents usually occur well before tumorigenesis and at the same or lower doses than those producing tumors. The International Life Sciences Institute (ILSI) Health and Environmental Sciences Institute (HESI) hypothesized that the signals of importance for human cancer hazard identification can be detected in shorter-term studies. Using the National Toxicology Program (NTP) database, a retrospective analysis was conducted on sixteen chemicals with liver, lung, or kidney tumors in two-year rodent cancer bioassays, and for which short-term data were also available. For nongenotoxic compounds, results showed that cellular changes indicative of a tumorigenic endpoint can be identified for many, but not all, of the chemicals producing tumors in two-year studies after thirteen weeks utilizing conventional endpoints. Additional endpoints are needed to identify some signals not detected with routine evaluation. This effort defined critical questions that should be explored to improve the predictivity of human carcinogenic risk.

An enhanced 13-week bioassay: an alternative to the 2-year bioassay to screen for human carcinogenesis

The 2-year rodent bioassay has become the standard carcinogenicity screen for chemicals, despite concerns of costs, time, and excessive doses. More importantly, there are increasing concerns regarding its relevance to human carcinogenic risk, especially for non-DNA reactive chemicals. Cancer risk can be increased either by direct damage to DNA (DNA reactivity) or by increased cell proliferation. Utilizing the scientific basis for mode of action analysis in the framework that has been developed for extrapolating to human relevance, a short-term screen is proposed as a replacement for the 2-year bioassay. Chemicals are evaluated for DNA reactivity, immunosuppression, and estrogenic activity, known mechanisms of human carcinogenesis, by in vitro and/or in vivo tests. The chemical can then be evaluated for toxicity and/or increased cell proliferation in target tissues. This relies primarily on evaluation of organ weights and histopathology, and also utilizes data from blood and urine chemistries and DNA-labeling indices. Significant concern is raised regarding the relevance of evaluation of tissues that are present in rats or mice but not humans, and the relevance of proliferative responses in rodent endocrine tissues. In developing alternative procedures to evaluate chemicals for possible carcinogenic activity in humans, it is important not to rely on the 2-year rodent bioassay for validation of the new procedure. It is time to discontinue the performance of the 2-year rodent bioassay.

Dose-response and operational thresholds/NOAELs for in vitro mutagenic effects from DNA-reactive mutagens, MMS and MNU

The dose-response relationships for in vitro mutagenicity induced by methylmethanesulfonate (MMS) or methylnitrosourea (MNU) in L5178Y mouse lymphoma (ML) cells were examined. DNA adducts (N7-methylguanine, N7MeG and O(6)-methylguanine, O(6)MeG) were quantified as biomarkers of exposure. Both endpoints were assessed using 5replicates/dose (4-h treatment) with MMS or MNU (0.0069-50muM), or vehicle (1% DMSO). Mutant frequency (MF) (thymidine kinase (TK) locus) was determined using the soft agar cloning methodology and a 2-day expression period; in addition, microwell and Sequester-Express-Select (SES) methods were used for MMS. Isolated DNA was acid-hydrolyzed, and adducts quantified by LC/ESI-MS/MS, using authentic and internal standards. MF dose-responses were analyzed using several statistical approaches, all of which confirmed that a threshold dose-response model provided the best fit. NOAELs for MF were 10muM MMS and 0.69muM MNU, based on ANOVA and Dunnett's test (p<0.05). N7MeG adducts were present in all cell samples, including solvent-control cells, and were increased over control levels in cells treated with >/=10muM MMS or 3.45muM MNU. O(6)MeG levels were only quantifiable at >/=10muM MNU; O(6)MeG was not quantifiable in control or MMS-treated cells at current detection limits. Thus, (1) cells treated with </=0.69muM MNU or </=10muM MMS did not demonstrate increases in TK(-) MF, but did demonstrate quantifiable levels of N7MeG adducts; and (2) the levels of N7MeG adducts did not correlate with induced MF, as MNU-treated cells had fewer N7MeG adducts but higher MF compared with MMS-treated cells, for quasi-equimolar doses. Taken together, these results demonstrate operational thresholds, defined as the highest dose for which the response is not significantly (statistically or biologically) distinguishable from the control/background values, for induction of mutations and N7MeG adducts in ML cells treated with MMS or MNU, and a lack of correlation between induced MF and levels of N7MeG adducts.

Hemangiosarcoma in rodents: mode-of-action evaluation and human relevance

Although rarely occurring in humans, hemangiosarcomas (HS) have become important in evaluating the potential human risk of several chemicals, including industrial, agricultural, and pharmaceutical agents. Spontaneous HS arise frequently in mice, less commonly in rats, and frequently in numerous breeds of dogs. This review explores knowledge gaps and uncertainties related to the mode of action (MOA) for the induction of HS in rodents, and evaluates the potential relevance for human risk. For genotoxic chemicals (vinyl chloride and thorotrast), significant information is available concerning the MOA. In contrast, numerous chemicals produce HS in rodents by nongenotoxic, proliferative mechanisms. An overall framework is presented, including direct and indirect actions on endothelial cells, paracrine effects in local tissues, activation of bone marrow endothelial precursor cells, and tissue hypoxia. Numerous obstacles are identified in investigations into the MOA for mouse HS and the relevance of the mouse tumors to humans, including lack of identifiable precursor lesions, usually late occurrence of the tumors, and complexities of endothelial biology. This review proposes a working MOA for HS induced by nongenotoxic compounds that can guide future research in this area. Importantly, a common MOA appears to exist for the nongenotoxic induction of HS, where there appears to be a convergence of multiple initiating events (e.g., hemolysis, decreased respiration, adipocyte growth) leading to either dysregulated angiogenesis and/or erythropoiesis that results from hypoxia and macrophage activation. These later events lead to the release of angiogenic growth factors and cytokines that stimulate endothelial cell proliferation, which, if sustained, provide the milieu that can lead to HS formation.