Quantitative factors in chemical carcinogenesis: variation in carcinogenic potency

Uncertainty of Estimates of Cancer Risks Derived by Extrapolation from High to Low Doses and from Animals to Humans

The uncertainties associated with extrapolating model-based cancer risks from high to low doses and animal-based cancer risks to humans are examined. It is argued that low-dose linear extrapolation based on statistical confidence limits calculated from animal data is designed to account for data uncertainty, model-selection uncertainty, and model-fitting instability. The intent is to err on the side of safety, that is, overstating rather than understating the true risk. The tendency toward conservatism in predicting human cancer risks from animal data based on linear extrapolation is confirmed by a real-data analysis of the various sources of uncertainty involved in extrapolating from animals to humans. Along with the tendency toward conservatism, a high degree of overall uncertainty in the interspecies extrapolation process is demonstrated. It is concluded that human cancer risk estimates based on animal data may underestimate the true risk by a factor of 10 or may overestimate that risk by a factor of 1,000.

Extending the threshold of regulation concept: de minimis limits for carcinogens and mutagens

Risk assessment processes for carcinogens are highly developed but risk assessment processes for mutagens are not well established. In the pharmaceutical industry, risk associated with exposure to carcinogens is tightly controlled. It is desirable to control risk associated with exposure to mutagens also, in spite of the greater uncertainty associated with the risk. In this paper, a published cancer potency database is used to frame the risk and to support risk management decisions. A de minimis exposure for mutagens is proposed and a decision matrix is presented to align available data with risk assessment approaches for carcinogens and mutagens.

Priority assessment of toxic substances in life cycle assessment. Part I: calculation of toxicity potentials for 181 substances with the nested multi-media fate, exposure and effects model USES-LCA

Toxicity potentials are standard values used in life cycle assessment (LCA) to enable a comparison of toxic impacts between substances. In most cases, toxicity potentials are calculated with multi-media fate models. Until now, unrealistic system settings were used for these calculations. The present paper outlines an improved model to calculate toxicity potentials: the global nested multi-media fate, exposure and effects model USES-LCA. It is based on the Uniform System for the Evaluation of Substances 2.0 (USES 2.0). USES-LCA was used to calculate for 181 substances toxicity potentials for the six impact categories freshwater aquatic ecotoxicity, marine aquatic ecotoxicity, freshwater sediment ecotoxicity, marine sediment ecotoxicity, terrestrial ecotoxicity and human toxicity, after initial emission to the compartments air, freshwater, seawater, industrial soil and agricultural soil, respectively. Differences of several orders of magnitude were found between the new toxicity potentials and those calculated previously.

Threshold of toxicological concern for chemical substances present in the diet: a practical tool for assessing the need for toxicity testing

The de minimis concept acknowledges a human exposure threshold value for chemicals below which there is no significant risk to human health. It is the underlying principle for the US Food and Drug Administration (FDA) regulation on substances used in food-contact articles. Further to this, the principle of Threshold of Toxicological Concern (TTC) has been developed and is now used by the Joint FAO/WHO Expert Committee on Food Additives (JECFA) in their evaluations. Establishing an accepted TTC would benefit consumers, industry and regulators, since it would preclude extensive toxicity evaluations when human intakes are below such threshold, and direct considerable time and cost resources towards testing substances with the highest potential risk to human health. It was questioned, however, whether specific endpoints that may potentially give rise to low-dose effects would be covered by such threshold. In this review, the possibility of defining a TTC for chemical substances present in the diet was examined for general toxicity endpoints (including carcinogenicity), as well as for specific endpoints, namely neurotoxicity and developmental neurotoxicity, immunotoxicity and developmental toxicity. For each of these endpoints, a database of specific no-observed-effect levels (NOELs) was compiled by screening oral toxicity studies. The substances recorded in each specific database were selected on the basis of their demonstrated adverse effects. For the neurotoxicity and developmental neurotoxicity databases, it was intended to cover all classes of compounds reported to have either a demonstrated neurotoxic or developmentally neurotoxic effect, or at least, on a biochemical or pharmacological basis were considered to have a potential for displaying such effects. For the immunotoxicity endpoint, it was ensured that only immunotoxicants were included in the database by selecting most of the substances from the Luster et al. database, provided that they satisfied the criteria for immunotoxicity defined by Luster. For the developmental toxicity database, substances were selected from the Munro et al. database that contained the lowest NOELs retrieved from the literature for more than 600 compounds. After screening these, substances showing any effect which could point to developmental toxicity as broadly defined by the US were recorded in the database. Additionally, endocrine toxicity and allergenicity were addressed as two separate cases, using different approaches and methodology. The distributions of NOELs for the neurotoxicity, developmental neurotoxicity and developmental toxicity endpoints were compared with the distribution of NOELs for non-specific carcinogenic endpoints. As the immunotoxicity database was too limited to draw such a distribution of immune NOELs, the immunotoxicity endpoint was evaluated by comparing immune NOELs (or LOELs-lowest-observed-effect levels-when NOELs were not available) with non-immune NOELs (or LOELs), in order to compare the sensitivity of this endpoint with non-specific endpoints. A different methodology was adopted for the evaluation of the endocrine toxicity endpoint since data currently available do not permit the establishment of a clear causal link between endocrine active chemicals and adverse effects in humans. Therefore, this endpoint was analysed by estimating the human exposure to oestrogenic environmental chemicals and evaluating their potential impact on human health, based on their contribution to the overall exposure, and their estrogenic potency relative to endogenous hormones. The allergenicity endpoint was not analysed as such. It was addressed in a separate section because this issue is not relevant to the overall population but rather to subsets of susceptible individuals, and allergic risks are usually controlled by other means (i.e. labelling) than the Threshold of Toxicological Concern approach. (ABSTRACT TRUNCATED)

A tiered approach to threshold of regulation

This paper presents methods for extending the principle of a single "threshold of regulation" to a range of dietary concentrations between 0.5 and 15 parts per billion by using structure-activity relationships, genotoxicity, and short-term toxicity data. The database used to develop the FDA's threshold of regulation was examined to determine whether structural parameters or the result of certain short-term toxicity tests could be used to define a subset of less potent substances that supports higher threshold levels. In addition, results of reproductive toxicity tests for 3306 compounds and other multidose toxicity tests for 2542 compounds were compared with the database of carcinogenic potencies to establish that carcinogenic endpoints are the most conservative toxicity endpoint for establishing thresholds of regulation.

A procedure for the safety evaluation of flavouring substances. Joint FAO/WHO Expert Committee on Food Additives

This review describes a procedure for the safety evaluation of flavouring substances. Over 2500 flavouring substances are currently in use in food. While toxicity data do not exist on all flavouring substances currently in use, within structurally related groups of flavouring substances many do have toxicity data and this information along with knowledge of structure-activity relationships and data on the daily intake provides a framework for safety evaluation. The safety evaluation procedure provides a scientifically based practical method of integrating data on intake, structure-activity relationships, metabolism and toxicity to evaluate flavouring substances in a timely manner. The procedure has been used recently by the Joint FAO/WHO Expert Committee on Food Additives (JECFA) to evaluate a total of 263 flavouring substances.

Potency grading in carcinogen classification

In 1992 the United Nations Conference on Environment and Development decided to harmonize carcinogen classification systems. A proposal for a harmonized classification system is currently being considered by the Organization for Economic Cooperation and Development (OECD). In many countries, classification of a chemical as carcinogenic triggers labeling requirements. Implicit in the labeling requirements are often restrictions on the sale of consumer products and workplace regulations. Many of the current classification systems for carcinogens use a single concentration limit for the minimum concentration of a carcinogen in a preparation (mixture) that requires labeling. For high-potency carcinogens, one concentration limit may not adequately express the hazard, whereas for low-potency carcinogens, one limit may overestimate the hazard caused by the carcinogen in the preparation (mixture). The potency grading system discussed consists of three potency groups: high-, medium-, and low-potency carcinogens. It is envisioned that the different classes will trigger different labeling requirements. In the process of potency grading, a preliminary conclusion as to whether a substance shows high, medium, or low potency is initially based on a tumorigenic dose descriptor. The preliminary potency evaluation may then be modified after due consideration of a number of additional elements. These may include evaluation of the dose-response curve; site-, species-, strain-, and sex-specific activity; mechanisms including genotoxicity; mechanistic relevance to humans; toxicokinetics; and other factors. The potency grading system discussed is applicable to most carcinogen classification systems, including that currently being considered by the OECD.

Pesticide residues in food: investigation of disparities in cancer risk estimates

Much of the public perceives that exposure to synthetic pesticide residues in the diet is a major cause of cancer. The National Research Council (NRC), in a 1987 report, Regulating Pesticides in Food: The Delaney Paradox, evaluated cancer risks for 29 pesticides that are rodent carcinogens and estimated that the risks for 23 were greater than one-in-a-million. In contrast, our group has ranked possible carcinogenic hazards from a variety of human exposures to rodent carcinogens using the HERP (Human Exposure/Rodent Potency) index, and found that dietary residues of synthetic pesticides ranked low. This paper evaluates the disparities in these analyses by examining the two components of risk assessment: carcinogenic potency in rodents and human exposure. Potency estimates based on rodent bioassay data are shown to be similar whether calculated, as in the NRC report, as the regulatory q1* or as TD50. In contrast, estimates of dietary exposure to residues of synthetic pesticides vary enormously, depending on whether they are based on the Theoretical Maximum Residue Contribution (TMRC) calculated by the Environmental Protection Agency vs. the average dietary residues measured by the Food and Drug Administration in the Total Diet Study (TDS). The TMRC is the theoretical maximum human exposure anticipated under the most severe field application conditions, which are far greater than dietary residues measured in the TDS. Several independent exposure studies suggest that the FDA dietary residues are reasonable estimates of average human exposures, whereas TMRC values are large overestimates. Using standard methodology and measured dietary residues in the TDS, the estimate of excess cancer risk from average lifetime exposure to synthetic pesticide residues in the diet appears to be less than one-in-a-million for each of the ten pesticides for which adequate data were available.

Quantitative risk assessment and the limitations of the linearized multistage model

1. Quantifying carcinogenic risk is an important objective for assisting in the assessment and management of risks from chemical exposure. The most widely used of the many mathematical models proposed for extrapolation of carcinogenicity data from animal studies to low dose human exposures is the linearized multistage (LMS) model. This has, in effect, become the default approach for much of Quantitative Risk Assessment (QRA). The practical properties of this model have been investigated. 2. Analysis of stimulated data using the LMS model showed (i) that the Maximum Likelihood Estimate (MLE) of the low dose slope, q1, was unstable and extremely sensitive to small changes in the data; (ii) the 95% Upper Confidence Limit (UCL) estimate, q1*, preferred by the US Environmental Protection Agency (EPA) was insensitive with only small changes in values being obtained for large changes in the data; (iii) data sets where there was no statistical significance could give risk estimates similar to those obtained from data sets with clear dose-related effects; (iv) the size of the values of the Virtually Safe Dose (VSD) obtained did not necessarily relate to the biological interpretation of the data sets; (v) the value of q1* obtained was closely related to the top dose used in the study. 3. Limitations of the LMS model were illustrated by examples of its use in assessing the carcinogenicity of 2, 3, 7, 8-TCDD leading to the conclusion that the existing models are not suitable for routine use in the estimation of the risk from chemical carcinogens. The use of the LMS model has been justified in part by its original derivation from a mathematical model based upon a multistage model of carcinogenesis. However, estimates of the parameters of the model used to provide estimates of low dose risk to humans have no direct relationship to specific biological event in carcinogenesis. Further developments in mathematical models and increased understanding of the biological events underlying the carcinogenesis will lead to more biologically plausible QRA methods which would then justify serious consideration of QRA by regulatory authorities throughout the world.

Threshold of estimated toxicity for regulation of indirect food additives

In response to the objectives of this ATSDR workshop, 2 new procedures are described for assessing the safe use of indirect food additives. First, this workshop provided a timely forum in which to describe a newly proposed Threshold of Regulation (T/R) Policy under which the Food and Drug Administration (FDA) would exempt certain indirect food additives from the formal premarket petition process. Second, this workshop offered an opportunity to discuss 2 circumstances in which Quantitative Structure Activity Relationship (QSAR) methodologies might be utilized in the future to provide decision-support information for substances used in food-contact articles.

Improving the regulation of carcinogens by expediting cancer potency estimation

The statutory language of the Safe Drinking Water and Toxic Enforcement Act of 1986 (Proposition 65; California Health and Safety Code 25249.5 et seq.) encourages rapid adoption of "no significant risk levels" (NSRLs), intakes associated with estimated cancer risks of no more than 1 in 100,000. Derivation of an NSRL for a carcinogen listed under Proposition 65 requires the development of a cancer potency value. This paper discusses the methodology for the derivation of cancer potencies using an expedited procedure, and provides potency estimates for a number of agents listed as carcinogens under Proposition 65. To derive expedited potency values, default risk assessment methods are applied to data sets selected from an extensive tabulation of animal cancer bioassays according to criteria used by regulatory agencies. A subset of these expedited values is compared to values previously developed by regulatory agencies using conventional quantitative risk assessment and found to be in good agreement. Specific regulatory activities which could be facilitated by adopting similar expedited procedures are identified.

Heterocyclic amines formed by cooking food: comparison of bioassay results with other chemicals in the Carcinogenic Potency Database

Results in the Carcinogenic Potency Database (CPDB) on 11 mutagenic heterocyclic amines (HA) tested for carcinogenicity in rats, mice and cynomolgus monkeys are compared to results for other chemicals. An analysis of strength of evidence of carcinogenicity for HA vs. other mutagenic carcinogens and vs. all rodent carcinogens, indicates strong carcinogenicity of HA in terms of positivity rates and multiplicity of target sites. The liver is the most frequent target site in each species. Despite several target sites in each species, concordance in target sites between rats and mice is restricted to the liver for each HA except one. In cynomolgus monkeys, liver tumors have been induced rapidly by 2-amino-3-methylimidazo[4,5-f]quinoline (IQ). Human exposures to HA in cooked animal foods are small, in the low ppb range. A comparison of possible carcinogenic hazards from a variety of exposures to rodent carcinogens in the American diet is presented, using an index (Human Exposure/Rodent Potency, HERP) that relates human exposure to carcinogenic potency in rodents. Results indicate that there is a large background of exposures to naturally-occurring rodent carcinogens in typical portions of common foods, and that possible hazards from HA rank below those of most natural pesticides and products of cooking or food preparation; synthetic pesticide residues also rank low.

An overview of the report: correlation between carcinogenic potency and the maximum tolerated dose: implications for risk assessment

Current practice in carcinogen bioassay calls for exposure of experimental animals at doses up to and including the maximum tolerated dose (MTD). Such studies have been used to compute measures of carcinogenic potency such as the TD50 as well as unit risk factors such as q1 * for predicting low-dose risks. Recent studies have indicated that these measures of carcinogenic potency are highly correlated with the MTD. Carcinogenic potency has also been shown to be correlated with indicators of mutagenicity and toxicity. Correlation of the MTDs for rats and mice implies a corresponding correlation in TD50 values for these two species. The implications of these results for cancer risk assessment are examined in light of the large variation in potency among chemicals known to induce tumors in rodents.

Uncertainty in cancer risk estimates

Several existing databases compiled by Gold et al. for carcinogenesis bioassays are examined to obtain estimates of the reproducibility of cancer rates across experiments, strains, and rodent species. A measure of carcinogenic potency is given by the TD50 (daily dose that causes a tumor type in 50% of the exposed animals that otherwise would not develop the tumor in a standard lifetime). The lognormal distribution can be used to model the uncertainty of the estimates of potency (TD50) and the ratio of TD50's between two species. For near-replicate bioassays, approximately 95% of the TD50's are estimated to be within a factor of 4 of the mean. Between strains, about 95% of the TD50's are estimated to be within a factor of 11 of their mean, and the pure genetic component of variability is accounted for by a factor of 6.8. Between rats and mice, about 95% of the TD50's are estimated to be within a factor of 32 of the mean, while between humans and experimental animals the factor is 110 for 20 chemicals reported by Allen et al. The common practice of basing cancer risk estimates on the most sensitive rodent species-strain-sex and using interspecies dose scaling based on body surface area appears to overestimate cancer rates for these 20 human carcinogens by about one order of magnitude on the average.(ABSTRACT TRUNCATED AT 250 WORDS)

Rodent carcinogens: setting priorities

The human diet contains an enormous background of natural chemicals, such as plant pesticides and the products of cooking, that have not been a focus of carcinogenicity testing. A broadened perspective that includes these natural chemicals is necessary. A comparison of possible hazards for 80 daily exposures to rodent carcinogens from a variety of sources is presented, using an index (HERP) that relates human exposure to carcinogenic potency in rodents. A similar ordering would be expected with the use of standard risk assessment methodology for the same human exposure values. Results indicate that, when viewed against the large background of naturally occurring carcinogens in typical portions of common foods, the residues of synthetic pesticides or environmental pollutants rank low. A similar result is obtained in a separate comparison of 32 average daily exposures to natural pesticides and synthetic pesticide residues in the diet. Although the findings do not indicate that these natural dietary carcinogens are important in human cancer, they cast doubt on the relative importance for human cancer of low-dose exposures to synthetic chemicals.