A model-free approach to low-dose extrapolation

Estimates of risk associated with exposure to low levels of carcinogenic substances present in the environment are generally obtained by linear extrapolation from higher exposure levels at which risks can be estimated directly. In this paper, we examine the scientific basis for the assumption of low-dose linearity in carcinogenic risk assessment and the different statistical methods that have been proposed for linear extrapolation. A model-free approach to linear extrapolation is described and illustrated using epidemiological data on radiation carcinogenesis. The statistical properties of this method are empirically assessed using 572 selected sets of bioassay data.

Quantitative factors in chemical carcinogenesis: variation in carcinogenic potency

The quantitative assessment of toxicological data on the carcinogenic potential of chemicals requires consideration of a number of factors, including mathematical models of the mechanism of carcinogenic action and pharmacokinetic models for the metabolic activation of the parent compound to its reactive metabolite. In this article, the use of such models in estimating carcinogenic potency and in predicting risks at low levels of exposure is discussed, along with other factors involved in the evaluation of carcinogen bioassay data. The Carcinogenic Potency Database (CPDB) established by Gold et al. (1984, Environ. Health Perspect. 58, 9-322) is used to illustrate the application of quantitative approaches to carcinogenic risk assessment and to examine the variation in the potency of chemical carcinogens. Based on an analysis of 585 experiments selected from the CPDB, the risk-specific (10(-6) doses (RSDs) obtained by linear extrapolation from the TD50 were generally within a factor of 5-10 of those derived from the linearized multistage model. The RSDs obtained by linear extrapolation from the TD50 are roughly log-normally distributed with a median of about 20-90 ng/kg/day, depending on the subset of the CPDB considered. This distribution has been used by Rulis (1986, in Food Protection Technology (C. W. Felix, Ed.), pp. 29-37, Lewis, Chelsea, MI) to explore the concept of a threshold of regulation for chemical carcinogens present in the environment at low levels.

Role of epidemiology in health risk assessment

Human health risk assessment has been the object of systematic study in recent years, with formal models of risk assessment and risk management having been proposed by several national and international health agencies. The particular model developed by the Environmental Health Directorate of Health and Welfare Canada was examined in some detail and used to focus on the role of epidemiology in the overall process of risk assessment. In addition to providing information fundamental to the identification of environmental carcinogens and the estimation of carcinogenic risks, epidemiology may also play a role in shaping risk perception and in improving risk communication practices. Taken collectively, epidemiologic data on health risks provide a basis for improved disease surveillance and prioritization of public health concerns. Both descriptive and analytic epidemiologic protocols may be used to gather information on disease etiology. Because of the potential for bias and confounding in observational studies of human populations, epidemiological data should be subjected to careful evaluation in accordance with established criteria before a causal relationship between exposure and disease is inferred. Toxicological studies using nonhuman test systems may be used to avoid these problems, but at the expense of obtaining indirect information on human health risks. Nonetheless, toxicological data provide an important complement to epidemiological data, providing information on potential health risks in advance of human exposure and offering a means of indirectly assessing risks in situations where human studies fail to provide informative results. The complementary roles of epidemiology and toxicology in health risk assessment were examined using four case studies. While the epidemiological evidence linking tobacco consumption to lung cancer is now unequivocal, the corresponding data on involuntary smoking, although strongly suggestive of increasing the relative risk of lung cancer, requires further confirmation before providing the same degree of evidence as now exists for active smoking. At present, the best estimates suggest that overall mortality attributable to active smoking may exceed that due to passive smoking by roughly 100-fold. Despite this large difference in health impact, passive smoking continues to be the focus of much public concern, in part because of the involuntary nature of the risk involved. Because of the abundance of good epidemiological data on tobacco, toxicology has assumed a secondary role in defining the health risks associated with smoking. In contrast, while epidemiological studies with saccharin and formaldehyde have provided unequivocal evidence of carcinogenic effects in animals exposed to high doses, thereby raising concerns over potential human carcinogenicity.(ABSTRACT TRUNCATED AT 400 WORDS)

Carcinogenic risk assessment of complex mixtures

Although procedures for assessing the carcinogenic risks associated with exposure to individual chemicals are relatively well developed, risk assessment methods for mixtures of chemicals are still in the developmental stage. In this paper, we examine the difficulties in assessing the risks of exposure to complex mixtures, with special reference to the potential for synergistic effects among the components of the mixture. Statistical models for describing the joint action of multiple exposures are reviewed, and their implications for low-dose risk assessment are examined. The potential use of pharmacokinetic models to describe the metabolism of mixtures is also considered. Application of these results in regulating mixtures of carcinogenic substances is illustrated using examples involving multiple contaminants in drinking water and polycyclic aromatic hydrocarbons produced from combustion sources.

Cancer risks due to occupational exposure to formaldehyde: results of a multi-site case-control study in Montreal

A case-control study was undertaken in Montreal to investigate the possible associations between occupational exposures and cancers of the following sites: oesophagus, stomach, colo-rectum, liver, pancreas, lung, prostate, bladder, kidney, melanoma and lymphoid tissue. In total, 3,726 cancer patients and 533 population controls were interviewed to obtain detailed lifetime job histories and information on potential confounders. Each job history was translated into a history of occupational exposures. Because of current concerns about formaldehyde carcinogenicity, we carried out a special analysis of the association between exposure to formaldehyde and each type of cancer covered by this study. Separate statistical analyses were carried out for each type of cancer using population controls as well as a control series drawn from among the other cancer sites in the study. Although nearly a quarter of all subjects had undergone occupational exposure to formaldehyde, the levels of exposure were in general quite low. There was no persuasive evidence of an increased risk of any type of cancer among men exposed to these levels of formaldehyde. However, the possibility of a small increase in risk could not be ruled out.

Statistical considerations in the interpretation of negative carcinogenicity data

The regulation of toxic substances present in the environment requires that carcinogens be distinguished from noncarcinogens on the strength of the available toxicological and epidemiological evidence for carcinogenicity. In this article, we consider the difficulties associated with establishing strong evidence against carcinogenicity. In particular, the ability of both animal and human studies to detect small increases in tumor occurrence rates is evaluated in statistical terms. Consideration is also given to resolving apparent conflicts between the toxicological and the epidemiological sources of data.

Statistical issues in the analysis of the long-term carcinogenicity bioassay in small rodents: an empirical evaluation of statistical decision rules

Data on 49 randomly selected studies from the NCI/NTP Carcinogenesis Bioassay Program were reanalyzed using four statistical decision rules to classify substances as either being negative or falling into one of three categories indicating increasing evidence of oncogenicity. The data available for analysis were the crude marginal counts of numbers of animals with specified lesions, as well as the number of animals surviving at the time the studies were terminated. Statistical analysis was based primarily on the Cochran-Armitage test for linear trend in proportions, with and without the use of historical control information. If only concurrent controls were used, classifications of carcinogenicity obtained in between 34 and 57% of the studies, depending on the decision rule used. The incorporation of historical control information into the Cochran-Armitage test statistic led to almost universal findings of carcinogenicity. The data base assembled here was used to estimate false negative and false positive rates for each of the four decision rules.

Pharmacokinetics of methylmercury in the blood of monkeys (Macaca fascicularis)

Statistical analysis of the blood mercury profiles of groups of two and four adult female cynomolgus monkeys (Macaca fascicularis) given single oral doses of 500 micrograms and 50 microCi (25.3 micrograms) methylmercury/kg body wt, respectively, indicates that a two-compartment model best describes the absorption and elimination of methylmercury in blood. Absorption was largely complete within 6 hr, and the half-time of methylmercury during the terminal elimination phase ranged from 10 to 15 days. In addition, three groups of five adult female cynomolgus monkeys were dosed with methylmercury every Monday, Wednesday, and Friday for periods up to 2 years at effective doses of 10, 25, or 50 micrograms methylmercury/kg body wt/day. The average blood levels at steady state were estimated to be 0.27 +/- 0.02, 0.69 +/- 0.03, and 1.51 +/- 0.08 ppm, respectively, with average time taken to achieve 95% of the steady-state blood level being about 92 days. The steady-state blood levels obtained via extrapolation of the results from the two single-dose experiments were significantly different from those actually achieved, indicating that the average steady-state blood levels under chronic dosing conditions may not be accurately estimated on the basis of short-term experiments. The data were also used to examine the impact of different dosing intervals on variation in blood mercury levels.

International Commission for Protection Against Environmental Mutagens and Carcinogens. ICPEMC Working Paper No. 3. The concept of negativity in experimental carcinogenesis

The problems that arise in the interpretation of experimental data on chemical carcinogenesis are addressed. In particular, the difficulties in demonstrating negative results are shown to present problems in delineating carcinogens from noncarcinogens. The use of the virtually safe dose estimated under the assumption of low dose linearity is shown to lead to potentially anomalous results if used indiscriminately in bioassays in which no statistically significant increase in tumor occurrence is induced. It is suggested that there is a need to establish an operational definition of negativity in carcinogenesis, with the realization that this definition may be revised in light of new information. The establishment of negativity in aligning data from positive and negative experiments and in considering possible thresholds is also discussed.

Methylmercury-induced mitochondrial DNA synthesis in neural tissue of cats

The association between selected neuropathological lesions and effects on mitochondrial and nuclear DNA synthesis was explored in cats exposed in vivo to methylmercuric chloride. Two groups of eight adult female cats ingested 0 or 176 micrograms Hg/kg body wt/day as methylmercuric chloride added daily to their diet. Treated animals and concurrent controls were sacrificed following the onset of clinical signs of toxicity, with the mean termination time being about 12 (range 7-15) weeks. Terminal Hg levels for the control and treated groups respectively were 0.16 +/- 0.02 and 12 +/- 1 ppm in the cerebrum and 0.16 +/- 0.01 and 14 +/- 1 ppm in blood. Hydroxyurea-resistant [3H]thymidine incorporation into DNA in cultured explants of cerebrum and cerebellum, as measured by scintillation counting of extracted DNA, was elevated for treated animals. Autoradiographic analysis indicated that the excess DNA synthesis was cytoplasmic, and deoxyribonuclease resistant, suggesting a mitochondrial DNA origin. The excess DNA synthesis was pronounced in cell types prone to neurodegeneration, specifically the Purkinje cells and the granular cell layer in the cerebellum and the large neurons in the cerebrum. Mitochondrial DNA from neural tissues of an additional five pairs of cats treated for 8 weeks was isolated from cesium chloride/ethidium bromide density gradients. Thymidine incorporation into mitochondrial DNA was greater in methylmercury-treated than control animals. These observations indicate that methylmercury affects mitochondrial DNA synthesis in vivo with a tissue specificity parallel to that of neuropathological lesions.

Determining "safe" levels of exposure: safety factors or mathematical models?

The object of regulatory toxicology is to determine "safe" levels of human exposure to toxicants present in the environment. The traditional safety factor approach is compared to more recent mathematical modeling techniques, outlining the underlying assumptions and statistical properties of each procedure. Several linear extrapolation procedures are examined in detail using computer simulation, along with the impact of nonlinear kinetics on the extrapolation process.

Mutagenicity screening of foods. II. Results with fruits and vegetables

A survey of the mutagenic potential of a wide variety of food products has been initiated with results for 28 different beverages reported previously [Stoltz et al, 1982b]. Here, results for samples of 46 widely consumed fruits and vegetables from six general categories are given. Each sample was concentrated and fractionated by polarity and solubility to give five fractions, each of which was assayed for mutagenic potential with Salmonella typhimurium TA98 and TA100. Although statistical analysis of the data resulted in positive findings for 22 fruit and vegetable samples, only six products (grapes, onions, peaches, raisins, raspberries, strawberries) demonstrated potent mutagenic activity.

The power and interpretation of the carcinogenicity bioassay

Carcinogenicity is a major consideration in the assessment of risks due to environmental chemicals. The carcinogen bioassay therefore is a very important component of the battery of toxicological tests used in hazard evaluation. The strengths and limitations of this bioassay are discussed with emphasis upon the unresolved practical considerations, the interpretation of negative results, the significance of tumors induced in the presence of a high background incidence of naturally occurring tumors, and the difficulties in transspecies extrapolation. These factors, in combination with consideration of the biological mechanisms of chemical cancer induction, will be valuable in assessing the potential risk to man posed by individual chemicals.

Saccharin: a toxicological and historical perspective

Saccharin, first synthesized in 1879, eventually became popular as an inexpensive substitute for sugar, particularly as a non-caloric sweetner. The dispute concerning the safety of saccharin for human consumption is almost as old as saccharin itself. In this article, the history concerning the uses of saccharin and the accompanying controversy are reviewed. In addition, the spectrum of toxicological and epidemiological studies to which saccharin has been subjected are also examined. While the toxicological data indicate that saccharin is probably the agent solely responsible for the bladder tumors observed in second generation male rats, the epidemiological studies provide, at best, an equivocal relationship between the consumption of saccharin and bladder cancer. A benefit-risk evaluation for saccharin showed few, if any documentable benefits from the use of saccharin and much genuine uncertainty concerning the potential risks for ingestion by man. This element of genuine uncertainty as to the extent of human risk posed to man is the crux of saccharin's past and its foreseeable future.

A comparison of statistical methods for low dose extrapolation utilizing time-to-tumor data

The assessment of health risks due to low levels of exposure to potential environmental hazards based on the results of toxicological experiments necessarily involves extrapolation of results obtained at relatively high doses to the low dose region of interest. In this paper, different statistical extrapolation procedures which take into account both time-to-response and the presence of competing risks are compared using a large simulated data base. The study was designed to cover a range of plausible dose response models as well as to assess the effects of competing risks, background response, latency and experimental design on the performance of the different extrapolation procedures. It was found that point estimates of risk in the low dose region may differ from the actual risk by a factor of 1000 or more in certain situations, even when precise information on the time of occurrence of the particular lesion of interest is available. Although linearized upper confidence limits on risk can be highly conservative when the underlying dose response curve is sublinear in the low dose region, they were found not to exceed the actual risk in the low dose region by more than a factor of 10 in those cases where the underlying dose response curve was linear at low doses.

Toxicological procedures for assessing the carcinogenic potential of agricultural chemicals

Pesticides and other agricultural chemicals are now widely used throughout the world as a means of improving crop yields in order to meet the increasing demands being placed upon the global food supply. In Canada, the use of such chemicals is controlled through government regulations established jointly by the Department of Agriculture and the Department of National Health & Welfare. Such regulations require a detailed evaluation of the toxicological characteristics of the chemical prior to its being cleared for use. In this paper, procedures for assessing the carcinogenic potential of agricultural and other chemicals are discussed. Consideration is given to both the classical long-term in vivo carcinogen bioassay in rodent or other species and the more recently developed short-term in vitro tests based on genetic alterations in bacterial and other test systems.

Mutagenicity screening of foods I. Results with beverages

Following a number of recent reports on the presence of mutagens in certain foods, a general survey of the mutagenic potential of a wide variety of food products has been initiated. Here, results for samples of 28 widely consumed beverages from 13 general categories are reported. Each sample was concentrated and fractionated by polarity and solubility to give up to seven fractions, each of which was assayed for mutagenic potential with Salmonella typhimurium TA98 and TA100 +/- fortified liver homogenate. Fractions showing evidence of either mutagenicity or toxicity were retested at the same and lower concentrations. The utility of the fractionation procedure and the sensitivity of the screening strategy were established by assaying six beverages spiked with known mutagens prior to fractionation. Statistical analysis of the data resulted in positive findings for seven beverages, although confirmation of these results through analysis of a second sample was obtained only for red wine, grape juice, and instant coffee. The remaining 21 beverages showed no strong evidence of mutagenic activity. For those foods for which the variation among replicate plates was largest, the false-positive rate for the two-stage screening procedure employed was estimated to be less than 1% while the false-negative rate for a beverage inducing a threefold increase in the background mutation rate was conservatively estimated to be limited to 14%.