The use of Haber's law in standard setting and risk assessment.

Haber's Law simply states that the incidence and/or severity of a toxic effect depends on the total exposure, i.e. exposure concentration (c) rate times the duration time (t) of exposure (c x t). This rule, within constraints, is often used in setting exposure guidelines for toxic substances. Establishing reference doses (acceptable daily intakes) for long-term exposures when only the results of short-term studies are available requires the use of an uncertainty (safety) factor. The value of this uncertainty factor often approximates a value comparable to Haber's Law for extrapolation from short-term to long-term exposure durations. As a default procedure, cancer risk estimates are generally based on the average lifetime daily dose which is derived from the total cumulative exposure, i.e. Haber's (c x t). This has been shown both theoretically and empirically to be valid within a factor of 20 for carcinogenesis. This provides some credence for the use of an additional safety factor of 10, in some instances, for exposures of children to carcinogens. Finally, a generalization of Haber's Law, exposure concentration raised to a power times exposure duration, is discussed.

New issues in cancer risk assessment.

When a nonlinear dose-response at low doses can be justified, an acceptable daily intake for a carcinogen can be obtained by dividing a benchmark dose, associated with a low incidence of tumors in animals, by uncertainty factors to account for animal-to-human extrapolation, human variability, and risk reduction from a low observed adverse-effect level. This approach can utilize mechanistic information to justify smaller uncertainty factors than typical default values of 10. If a nonlinear dose-response cannot be justified, traditional linear extrapolation from the benchmark dose to zero sometimes gives similar results. This suggests a unified risk-assessment procedure based on uncertainty factors. The issue of cross-species extrapolation based on the risk relative to background risks, rather than excess risk, is examined. The relative risk approach reduces the estimates of cancer risk in humans based on common rodent tumors, such as the liver in some strains of mice.

A unified approach to risk assessment for cancer and noncancer endpoints based on benchmark doses and uncertainty/safety factors.

A fundamental goal of toxicology is to determine safe levels of human exposure to toxic substances. In the absence of information to establish dose-response relationships at low exposure levels generally experienced by humans, high-dose to low-dose linear extrapolation is generally used for estimating carcinogenic risks and the no-observed-adverse-effect-level divided by uncertainty (safety) factors is widely used for establishing human exposure guidelines for noncancer effects. The basis and impact of this dichotomy is examined and questioned. It is proposed that a unified approach be adopted for establishing human exposure guidelines for both cancer and noncancer endpoints. It is suggested that a lower confidence limit on the dose estimated to produce an excess incidence of adverse health effects in 10% of the individuals in a human study or 10% of the animals in laboratory experiments be used as a point-of-departure. This dose would be divided by appropriate uncertainty factors to establish human exposure guidelines. For severe irreversible adverse health effects we suggest a total default uncertainty factor (divisor) for animal data on the order of 10,000, which is comparable to current guidelines. For reversible biological effects a smaller default uncertainty factor on the order of 1000 may be employed. This is comparable to the divisor often used currently when the point-of-departure is the lowest-observed-adverse-effect-level. It is asserted that the toxicological information generally available does not warrant numerical estimates of risk at low levels of human exposure. Rather, we support a unified approach for all adverse health effects of dividing a benchmark dose by appropriate uncertainty factors to establish guidelines for human exposures to toxic substances.

Tumors and DNA adducts in mice exposed to benzo[a]pyrene and coal tars: implications for risk assessment.

Current methods to estimate the quantitative cancer risk of complex mixtures of polycyclic aromatic hydrocarbons (PAH) such as coal tar assume that overall potency can be derived from knowledge of the concentration of a few carcinogenic components such as benzo[a]pyrene (B[a]P). Genotoxic damage, such as DNA adducts, is thought to be an essential aspect of PAH-induced tumorigenesis and could be a biomarker for exposure useful for estimating risk. However, the role of B[a]P and the relationship of adduct formation in tumorigenesis have not been tested rigorously in models appropriate for human health risk assessment. Therefore, we directly compared tumor induction and adduct formation by B[a]P and coal tars in several experimental protocols, including one broadly accepted and used by regulators. We found that B[a]P content did not account for tumor incidences after exposure to coal tars. DNA adducts were found in both tumors and tumor-free tissue and tumor outcomes were not predicted by either quantitation of total DNA adducts or by the DNA adduct formed by B[a]P. These data suggest that risk assessments based on B[a]P content may not predict accurately risk to human health posed by environmental PAH.

U.S. Food and Drug Administration perspective of the inclusion of effects of low-level exposures in safety and risk assessment.

A brief overview is provided of some of the general safety and risk assessment procedures used by the different centers of the U.S. Food and Drug Administration (U.S. FDA) to evaluate low-level exposures. The U.S. FDA protects public health by regulating a wide variety of consumer products including foods, human and animal drugs, biologics, and medical devices under the federal Food, Drug, and Cosmetic Act. The diverse legal and regulatory standards in the act allow for the consideration of benefits for some products (e.g., drugs) but preclude them from others (e.g., food additives). When not precluded by statutory mandates (e.g., Delaney prohibition), the U.S. FDA considers both physiologic adaptive responses and beneficial effects. For the basic safety assessment paradigm as presently used, for example in the premarket approval of food additives, the emphasis is on the identification of adverse effects and no observed adverse effect level(s) (NOAEL). Generally, the NOAEL is divided by safety factors to establish an acceptable exposure level. This safety assessment paradigm does not preclude the consideration of effects whether they are biologically adaptive or beneficial at lower dose levels. The flexibility to consider issues such as mechanisms of action and adaptive and beneficial responses depends on the product under consideration. For carcinogenic contaminants and radiation from medical devices, the U.S. FDA considers the potential cancer risk at low exposure levels. This generally involves downward extrapolation from the observed dose-response range. The consideration of adverse effects of other toxicologic end points (e.g., reproductive, immunologic, neurologic, developmental) associated with low exposure levels is also becoming more of a reality (e.g., endocrine disrupters). The evaluation of the biologic effects of low-level exposures to toxic substances must include whether the effect is adverse or a normal physiologic adaptive response and also determine the resiliency of a physiologic system. The public health mandate of the U.S. FDA includes an active research program at the National Center for Toxicological Research and the other U.S. FDA centers to support the regulatory mission of the U.S. FDA. This includes the development of knowledge bases, predictive strategies, and toxicologic studies to investigate effects at the lower end of the dose-response range. Because of the wide diversity of legal and regulatory standards for various products regulated by the U.S. FDA agency-wide safety and risk assessment procedures and policies generally do not exist.

Biologically-based dose-response model for neurotoxicity risk assessment.

Domoic acid is a tricarboxylic amino acid that is structurally-related to kainic acid and glutamic acid. It is produced by phytoplankton that may contaminate seafood. To determine domoate's toxicological effects and their pathogenesis, cynomolgus monkeys were dosed intravenously at one of a range of bolus doses from 0.25 to 4.0 mg/kg. Histochemical staining, using silver methods, revealed degenerating axons and cell bodies. Doses in the range of 0.5-1.0 mg/kg produced a small area of silver grains restricted to axons of the hippocampal CA2 stratum lucidum, the most sensitive brain area identified. Quantitation of the abundance of these silver grains yielded continuous dose-response data. A four step quantitative risk estimation approach was used: (1) determination of a dose-response model; (2) determination of the distribution of measurements (variability) about the model; (3) determination of an adverse or abnormal level with the use of the control data; and (4) estimation of the probability that a measure is beyond the abnormal level as a function of dose. The currently used safety-factor (S-F) approach, the benchmark (BM) approach and this quantitative (Q) approach was used to assess the same data set. Assuming a 5% oral absorption of domoic acid, acceptable doses would be achieved if subjects ate 200 g of seafood containing 12, 6 and 10 ppm domoic acid for the S-F, BM and Q approaches, respectively. This quantitative approach uses all the available data, takes into account the variability of the data and provides an actual risk at a given dose of domoic acid.

Regulatory cancer risk assessment based on a quick estimate of a benchmark dose derived from the maximum tolerated dose.

The proposed U.S. Environmental Protection Agency carcinogen risk assessment guidelines employ a benchmark dose as a point of departure (POD) for low-dose risk assessment. If information on the carcinogenic mode of action for a chemical supports a nonlinear dose-response curve below the POD, a margin-of-exposure ratio between the POD and anticipated human exposure would be considered. The POD would be divided by uncertainty (safety) factors to arrive at a reference dose that is likely to produce no, or at most negligible, cancer risk for humans. If nonlinearity below the POD is not supported by sufficient evidence, then linear extrapolation from the incidence at the POD to zero would be used for low-dose cancer risk estimation. The carcinogen guidelines suggest that the lower 95% confidence limit on the dose estimated to produce an excess of tumors in 10% of the animals (LTD10) be used for the POD. Due to the relatively narrow range of doses in 2-year rodent bioassays and the limited range of statistically significant tumor incidence rates, the estimate of the LTD10 obtained from 2-year bioassays is constrained to a relatively narrow range of values. Because of this constraint, a simple, quick, and relatively precise determination of the LTD10 can be obtained by the maximum tolerated dose (MTD) divided by 7. All that is needed is a 90-day study to establish the MTD. It is shown that the LTD10 determined by this relatively easy procedure is generally within a factor of 10 of the LTD10 that would be estimated using tumor incidence rates from 2-year bioassays. Estimates of cancer potency from replicated 2-year bioassays, and hence estimates of cancer risk, have been show to vary by a factor of 4 around a median value. Thus, there may be little gain in precision of cancer risk estimates derived from a 2-year bioassay, compared to the estimate based on the MTD from a 90-day study. If the anticipated human exposure were estimated to be small relative to the MTD/7 = LTD10, there may be little value in conducting a chronic 2-year study in rodents because the estimate of cancer risk would be low regardless of the results of a 2-year bioassay. Linear extrapolation to a risk of less than 1 in 100,000 and use of an uncertainty factor, e.g., of 10,000, would give the same regulatory "safe dose." Linear extrapolation to a virtually safe dose associated with a cancer risk estimate of less than one in a million would be 10 times lower than the reference dose based on the LTD10/10,000.

Health risk assessment practices in the U.S. Food and Drug Administration.

The U.S. Food and Drug Administration (FDA) regulates a wide variety of consumer products. Safety issues involve chemical and microbial contaminants in food, biologies, and medical devices; side effects from prescription and nonprescription drugs; residues of animal drugs in food; and radiation from electronic devices. Because of this wide diversity, the legal standards, rules, and policies governing the regulation of these products differ considerably. Hence, risk assessment and risk management practices within the FDA are of necessity quite diverse. This paper presents a summary of risk assessment practices at each of the product centers of the FDA (Center for Food Safety and Applied Nutrition, Center for Drug Evaluation and Research, Center for Biologics Evaluation and Research, Center for Devices and Radiological Health, and Center for Veterinary Medicine) and of the development of risk assessment procedures at the National Center for Toxicological Research.

On interspecies correlations of carcinogenic potencies.

It has been established in the literature that constraints on the designs of experiments used to estimate carcinogenic potencies cause overestimation of true biological interspecies correlations of such potencies. This article explores the potential for appreciable underestimation of interspecies correlations, due to the experimental error that occurs in the estimation of carcinogenic potencies.

Risk assessment of nongenotoxic carcinogens based upon cell proliferation/death rates in rodents.

Increased cell proliferation increases the opportunity for transformations of normal cells to malignant cells via intermediate cells. Nongenotoxic cytotoxic carcinogens that increase cell proliferation rates to replace necrotic cells are likely to have a threshold dose for cytotoxicity below which necrosis and hence, carcinogenesis do not occur. Thus, low dose cancer risk estimates based upon nonthreshold, linear extrapolation are inappropriate for this situation. However, a threshold dose is questionable if a nongenotoxic carcinogen acts via a cell receptor. Also, a nongenotoxic carcinogen that increases the cell proliferation rate, via the cell division rate and/or cell removal rate by apoptosis, by augmenting an existing endogenous mechanism is not likely to have a threshold dose. Whether or not a threshold dose exists for nongenotoxic carcinogens, it is of interest to study the relationship between lifetime tumor incidence and the cell proliferation rate. The Moolgavkar-Venzon-Knudson biologically based stochastic two-stage clonal expansion model is used to describe a carcinogenic process. Because the variability in cell proliferation rates among animals often makes it impossible to detect changes of less than 20% in the rate, it is shown that small changes in the cell proliferation rate, that may be obscured by the background noise in rates, can produce large changes in the lifetime tumor incidence as calculated from the Moolgavkar-Venzon-Knudson model. That is, dose response curves for cell proliferation and tumor incidence do not necessarily mimic each other. This makes the use of no observed effect levels (NOELs) for cell proliferation rates often inadmissible for establishing acceptable daily intakes (ADIs) of nongenotoxic carcinogens. In those cases where low dose linearity is not likely, a potential alternative to a NOEL is a benchmark dose corresponding to a small increase in the cell proliferation rate, e.g., 1%, to which appropriate safety (uncertainty) factors can be applied to arrive at an ADI.

Risk assessment strategies for neuroprotective agents.

Neurotoxicity may be defined as any adverse effect on the structure or function of the central and/or peripheral nervous system by a biological, chemical, or physical agent. Neurotoxic effects may be permanent or reversible, produced by neuropharmacological or neurodegenerative properties of a neurotoxicant, or the result of direct or indirect actions on the nervous system. A multidisciplinary approach is necessary to assess neurotoxicity because of the complexity and diverse functions of the nervous system. Many of the relevant effects can be measured directly by neurochemical, neurophysiological, and neuropathological techniques, whereas, others must be inferred from observed behavior. Some neurotoxicological data can be derived directly from humans. Neurotoxicity in humans is most commonly measured by relatively noninvasive neurophysiologic and neurobehavioral methods that assess cognitive, affective, sensory, and motor function. For most toxicological assessments, however, it is necessary to rely on information derived from animal models. There are many approaches that can be used to assess neurotoxicity, including whole animal (in vivo) and tissue/cell culture (in vitro) testing. Neurotoxicity can be described at multiple levels of organization, including neurochemical, anatomical, physiological, and behavioral. An important aspect of neurotoxic endpoint evaluation involves risk assessment procedures. Risk assessment may be defined as an empirically-based process used to determine the probability that adverse or abnormal effects are associated with exposure to a chemical, physical or biological agent. Risk management, on the other hand, is the process that applies information obtained through the risk assessment process to determine whether the assessed risk should be reduced and, if so, to what extent. For chemicals such as neuroprotective agents and other drugs designed to provide therapeutic benefits, information concerning these benefits is considered during the risk management phase. The risk assessment process usually involves four steps: hazard identification, dose-response assessment, exposure assessment, and risk characterization. Neurotoxicity risk assessment models of the future may well include biomarkers of both effect and exposure as well as biologically-based mechanistic and pharmacokinetic considerations derived from both epidemiologic and experimental data.

Neurotoxicity modeling for risk assessment.

The setting of acceptable exposure levels for neurotoxicants has followed the traditional approach of dividing experimental no-observed-adverse-effect-levels (NOAELs) by safety/uncertainty factors. NOAELs are believed by many toxicologists to represent levels having zero or negligible risk, while uncertainty factors are used to account for a number of sources of variation. Although the use of NOAELs in this manner has been criticized because of their imprecise quantitative definition, NOAELs for nonquantal neurotoxic effects have not been replaced by more precisely defined quantities (e.g., benchmark doses), partly due to the absence of a generally accepted methodology for attaching specific risk levels to low exposures. The present paper describes a quantitative approach to modeling nonquantal neurotoxic effects for risk assessment, which can be used to obtain results similar to the familiar results obtained in risk assessment for carcinogenicity and developmental toxicity. The steps involved in implementing the process are discussed, with particular attention being given to the critical step of defining an adverse neurologic effect. An experimental data set is used to illustrate the methodology.

Modeling for risk assessment of neurotoxic effects.

The regulation of noncancer toxicants, including neurotoxicants, has usually been based upon a reference dose (allowable daily intake). A reference dose is obtained by dividing a no-observed-effect level by uncertainty (safety) factors to account for intraspecies and interspecies sensitivities to a chemical. It is assumed that the risk at the reference dose is negligible, but no attempt generally is made to estimate the risk at the reference dose. A procedure is outlined that provides estimates of risk as a function of dose. The first step is to establish a mathematical relationship between a biological effect and the dose of a chemical. Knowledge of biological mechanisms and/or pharmacokinetics can assist in the choice of plausible mathematical models. The mathematical model provides estimates of average responses as a function of dose. Secondly, estimates of risk require selection of a distribution of individual responses about the average response given by the mathematical model. In the case of a normal or lognormal distribution, only an estimate of the standard deviation is needed. The third step is to define an adverse level for a response so that the probability (risk) of exceeding that level can be estimated as a function of dose. Because a firm response level often cannot be established at which adverse biological effects occur, it may be necessary to at least establish an abnormal response level that only a small proportion of individuals would exceed in an unexposed group. That is, if a normal range of responses can be established, then the probability (risk) of abnormal responses can be estimated.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Application of biomarkers to risk assessment.

Due to difficulties in conducting epidemiological studies, most estimates of cancer risk are based on data from animal bioassays. Extrapolation of cancer risk estimates in animals to humans requires an assumption of equal potency across species based on the average daily dose. The purpose of this paper is to examine the ability to predict tumor incidence across species from DNA adduct concentrations resulting from exposure to carcinogens. A 100-fold range of structurally diverse adduct concentrations corresponding to the same tumor incidence raises questions about quantitative predictability across chemical classes and across species. Differences in adduct structure, mutagenic efficiency, adduct repair rates, and cellular proliferation could account for some of the differences. For specific carcinogen-DNA adducts, the steady-state levels associated with a 50% tumor incidence appear to vary over a narrower range. An equal incidence of liver tumors was obtained at equal concentrations of aflatoxin B1-DNA adducts for rats and trout. A 2- to 3-fold range of 4-aminobiphenyl-DNA adduct concentrations between mice and dogs appears to be associated with nearly equal bladder tumor incidence, on the basis of limited data. In humans, a 5-fold higher concentration of a 4-aminobiphenyl-DNA adduct in bladders of smokers than of nonsmokers is compatible with the relative risk of bladder cancer due to smoking. DNA adduct concentrations certainly can be used to improve quantification of chemical exposures for epidemiological studies. Although promising, more data are needed to judge the usefulness of DNA adduct concentrations to predict cancer incidence across species.

Mathematical modeling of reproductive and developmental toxic effects for quantitative risk assessment.

A new mathematical dose-response model for reproductive and developmental risk assessment is proposed. The model includes the possibility of an exposure threshold as well as a litter-size effect. Correlation of responses of offspring from the same litter is taken into account through the use of the beta-binomial distribution. Confidence limits for low-dose extrapolation are based on the asymptotic distribution of the likelihood ratio. An empirical comparison of the proposed procedure to that of Rai and Van Ryzin demonstrates the improvement that can be achieved with the new procedure.

Bladder and liver tumorigenesis induced by 2-acetylaminofluorene in different F1 mouse hybrids: variation within genotypes and effects of using more than one genotype on risk assessment.

Several F1 mouse hybrids were used in a chronic bioassay to determine whether such an experimental design would provide greater statistical power than using only the B6C3F1 hybrid. For this purpose, the dose response of formation of hepatocellular and bladder tumors after 30 mo of feeding 2-acetylaminofluorene (2-AAF) in the diet was assessed in 4 F1 mouse hybrids, including the B6C3F1 hybrid. No strain background-related differences in frequency of bladder neoplasms between any F1 hybrids were detected. Bladder tumors occurred only at the highest 2-AAF dose in female mice. In males the lowest dose was already sufficient to induce bladder neoplasms with incidences of 25-48% adjusted for different nontumor mortality patterns across doses. No marked strain-related differences in hepatocellular tumor rates were apparent in either sex. Higher frequencies of hepatocellular neoplasms were observed among the untreated control males of the B6C3, AY, and CVA F1 hybrids than among the comparable females. Among treated mice, the lowest 2-AAF dose increased liver tumor incidence, more so among the females than among the males. The different background genomes resulted in somewhat different risk assessments for liver tumor formation in males due to differences in the time-to-tumor curves. Except for the much higher background liver tumor rate in the CVY mice, the adjusted liver tumor incidences were similar across the four hybrids. Hence, the levels of statistical significance obtained for dose-response trends and comparisons of treated and control groups were similar using 48 animals per dose groups with B6C3 mice, or combinations of 24 animals per dose from 2 genotypes, or 12 animals per dose from the 4 hybrid genotypes. Estimates of carcinogenic potency for bladder tumors were similar, within a factor of two, across the four hybrids. However, estimates of liver tumor potency across genotypes varied by a factor of two and six for females and males, respectively. Thus, the mean of cancer potency estimates across these genotypes would be more representative for mice than results from any single genotype. As in chronic carcinogenesis studies with other test agents, neoplasms developed in only a certain proportion, rather than in all, of the genetically identical animals exposed to a given dose of the toxicant for the same length of time under the same controlled environmental conditions. This phenotypic variability in toxic responses may reflect differential regulation of gene expression among the genetically identical test animals.

Risk assessment for neurotoxic effects.

Regulation of neurotoxicants is generally based on setting allowable doses (exposures) by dividing a no observed adverse effect level (NOAEL) by uncertainty factors that hopefully account for interspecies and intraspecies differences for extrapolations of experimental results obtained in animals to humans. This procedure makes no use of estimates of risk as a function of dose or does it acknowledge any risk at the NOAEL. The purpose of this paper is to illustrate how bioassay data can be used to estimate the risk of neurotoxic effects as a function of dose. In the absence of direct measurements of neurotoxic effects, biomarkers associated with neurotoxic effects can be used as measures of toxicity. In the absence of a definition of an adverse effect, an abnormal level for a measure of toxicity can be established which occurs only in a small fraction of a population which is not exposed to the substance under investigation. Risk is defined as the proportion of a population whose levels of a measure of toxicity equal or exceeds the abnormal level of the measure under study. The procedure is illustrated using data for neurochemical, neurohistological, and behavioral effects of methylenedioxymethamphetamine (MDMA) administered to rats or monkeys. This procedure is more versatile than the NOAEL/uncertainty factor approach since it provides estimates of risk as a function of dose of a potential neurotoxic substance.