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

Contribution of toxicologic pathologists for the safety of human health in biomedical research-past, present, and future of the JSTP

The research field of “Toxicologic Pathology” evaluates potentially toxic chemical exposures and chemically mediated illnesses in humans and experimental animals. Comparative studies of chemical exposures between model organisms and humans are essential for the risk assessment of chemicals and human health. Here we review the development and activities of the Japanese Society of Toxicologic Pathology (JSTP) during its 37-year history. Toxicological pathology studies provide many interesting and valuable findings. Rodent cancer bioassay data demonstrate the importance of dose levels, times, and duration of exposures to chemicals that possibly cause human cancers. Studies of toxic injuries in the nasal cavity demonstrate that specific chemical compounds affect different target cells and tissues. These observations are relevant for current air pollution studies in the preventive medicine field. Future toxicological pathology studies will be enhanced by applying molecular pathology with advanced observation techniques. In addition to the nasal cavity, another sense organ such as the tongue should be a potential next program of our mission for risk assessment of inhaled and ingested chemicals. As a message to the younger members of the JSTP, interdisciplinary and global cooperation should be emphasized. Elucidating the mechanisms of toxicologic pathology with a combination of advanced expertise in genetics and molecular biology offers promise for future advances by JSTP members.

Dose-response and threshold-mediated mechanisms in mutagenesis: statistical models and study design

The objective of this paper is to review the use, in mutagenesis, of various mathematical models to describe the dose-response relationship and to try to identify thresholds. It is often taken as axiomatic that genotoxic carcinogens could damage DNA at any level of exposure, leading to a mutation, and that this could ultimately result in tumour development. This has led to the assumption that for genotoxic chemicals, there is no discernible threshold. This assumption is increasingly being challenged in the case of aneugens. The distinction between 'absolute' and 'pragmatic' thresholds is made and the difficulties in determining 'absolute' thresholds using hypothesis testing approaches are described. The potential of approaches, based upon estimation rather than statistical significance for the characterization of dose-response relationships, is stressed. The achievement of a good fit of a mathematical model to experimental data is not proof that the mechanism supposedly underlying this model is operating. It has been argued, in the case of genotoxic chemicals, that any effects produced by a genotoxic chemical which augments that producing a background incidence in unexposed individuals will lead to a dose-response relationship that is non-thresholded and is linear at low doses. The assumptions underlying this presumption are explored in the context of the increasing knowledge of the mechanistic basis of mutagenicity and carcinogenicity. The possibility that exposure to low levels of genotoxic chemicals may induce and enhance defence and repair mechanisms is not easily incorporated into many of the existing mathematical models and should be an objective in the development of the next generation of biologically based dose-response (BB-DR) models. Studies aimed at detecting or characterizing non-linearities in the dose-response relationship need appropriate experimental designs with careful attention to the choice of biomarker, number and selection of dose levels, optimum allocation of experimental units and appropriate levels of replication within and repetition of experiments. The characterization of dose-response relationships with appropriate measures of uncertainty can help to identify 'pragmatic' thresholds based upon biologically relevant criteria which can help in the regulatory process.

Long-term exposure to fluoride in drinking water and sister chromatid exchange frequency in human blood lymphocytes

The genetic toxicity of fluoride has been investigated extensively by various test systems. However, results obtained have been inconsistent. Fluoride has been reported to be non-genotoxic, genotoxic, and synergistic or antagonistic with certain mutagens. To date, there are no published human studies on the genotoxicity of fluoride. The purpose of this investigation was to determine genotoxic risks of long-term exposure to various concentrations of fluoride in drinking water in humans with normal or inadequate nutrition. Six groups of subjects with either normal or inadequate nutritional intakes were selected from areas of approximately 0.2, 1.0, or 4.8 ppm (10.5, 52.6, or 252.6 mumol/L) fluoride in water. The subjects had been continuous residents in the area for at least 35 years. Samples of drinking water, plasma, and urine were analyzed for fluoride content. Blood lymphocytes were examined to determine the frequency of sister chromatid exchange (SCE). Blood chemistry and electrolytes were also analyzed. The results showed that average daily fluoride intake as well as urine and plasma fluoride levels increased with increase in the fluoride content of the drinking water. The blood chemistry and electrolyte values were within the normal range for all populations, but several parameters were significantly different. While the numerical differences were small, the subjects with low fluoride in the water (0.11 and 0.23 ppm or 5.8 and 12.1 mumol/L) had significantly higher SCE frequencies than those with higher fluoride exposures.(ABSTRACT TRUNCATED AT 250 WORDS)

An appreciation of the maximum tolerated dose: an inadequately precise decision point in designing a carcinogenesis bioassay?

Cancers arise in specific tissues. One difficulty with the present definitions of the Maximum Tolerated Dose (MTD), as they pertain to the rodent cancer bioassay, is that they base MTD on relatively crude parameters associated with the well-being of the entire animal rather than with the lack of specific tissue toxicity. Additional factors that could be included in the MTD definition, or could be separately determined, are addressed. Many of these factors refer to toxic behavior in one or a few tissues and, if used in setting the MTD, may mask more relevant events occurring at higher dose levels in other tissues. Reducing the MTD to a level that fails to take into account pesticide or drug-related toxicity may lead to the loss of relevant information in the bioassay. It is concluded, therefore, that there are two possible approaches to a more appropriate use of the MTD. The highest dose of the test agent (MTD) may be chosen (i) to lie below the thresholds of carcinogenicity-related non-genotoxic toxicity or (ii) the present high level MTD may continue to be used and tumors that arise may be classified as being irrelvant to humans at some or all exposure levels. The latter approach is to be preferred. It has the potential to avoid missing high level effects of the test agent that may be relevant to the human population.

International Commission for Protection Against Environmental Mutagens and Carcinogens. ICPEMC publication No. 19. The classification of carcinogens identified in the rodent bioassay as potential risks to humans: what type of substance should be tested next?

The relevance of rodent cancer bioassay data to humans is discussed in relation to the needs of regulatory agencies. The usefulness of in vivo and in vitro genotoxicity testing in this connection is also discussed. In the case of rodent carcinogens that do not elicit genotoxicity, it is suggested that homeostatic imbalance, cell proliferation, and other processes may play a major role in tumor development and its importance to the possible ability of the test agent to induce human cancer. These possibilities need to be evaluated on a case by case basis. The methods by which chemicals are selected for the rodent cancer bioassay are also discussed and it is pointed out that naturally-occurring constituents of human foods should in future receive greater priority as a consequence of anticipated changes resulting from biotechnology.

Structural basis of carcinogenicity in rodents of genotoxicants and non-genotoxicants

A set of 189 chemicals tested in the National Toxicology Program Cancer Bioassay was subjected to analysis by CASE, the Computer-Automated Structure Evaluation system. In the data set, 63% of the chemicals were carcinogens, approx. 40% of the carcinogens were non-genotoxic, i.e., they possessed neither "structural alerts" for DNA reactivity as defined by Ashby and Tennant, 1988, nor were they mutagenic for Salmonella. The data base can be characterized as a "combined rodent" compilation as chemicals were characterized as "carcinogenic" if they were carcinogenic in either rats or mice or both. CASE identified 23 fragments which accounted for the carcinogenicity, or lack thereof, of most of the chemicals. The sensitivity and specificity were unexpectedly high: 1.00 and 0.86, respectively. Based upon the identified biophores and biophobes, CASE performed exceedingly well in predicting the activity of chemicals not included among the 189 in the original set. CASE predicted correctly the carcinogenicity of non-genotoxic carcinogens thereby suggesting a structural commonality in the action of this group of carcinogens. As a matter of fact biophores restricted to non-genotoxic carcinogens were identified as were "non-electrophilic" biophores shared by genotoxic and non-genotoxic carcinogens. The findings suggest that the CASE program may help in the elucidation of the basis of the action of non-genotoxic carcinogens.

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)

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.

Absence of effect in stochastic processes: its influence on test validation and test use

Negativity in stochastic processes presents problems in interpretation because it is never possible to attain an absolutely adequate assurance of safety with such processes. Thus, it is extremely difficult to have complete confidence in the utility of validation studies of new methods in comparison to those that are accepted to be well established, especially when these processes are stochastic in the statistical sense of this term. The regulatory scientist who must make decisions on the basis of available evidence, therefore, has to make a number of assumptions in dealing with negativity. It is important to review the validity and usefulness of these assumptions from time to time to ensure that they cannot be replaced by improved methodology in the light of new scientific knowledge.

Carcinogenicity of chemicals: the weight of evidence

1. The evidence discussed here is derived from epidemiology, long-term bioassays in laboratory animals, and predictive short-term tests. 2. Epidemiological data are obtained directly from human studies and are most compelling when they demonstrate a large relative risk and a clear dose-response in association with a distinctive tumour type. Exposure to a suspected carcinogen and the doses involved are, however, often difficult to determine, and the most sophisticated epidemiological methods are relatively insensitive. There are no epidemiological data for most occupational/environmental chemicals. 3. Long-term bioassays can present major problems in design, interpretation and extrapolation. Particular difficulties are associated with the planning of appropriate dose levels and the occurrence of certain tumours at high incidence in both control and test groups. Results from animal bioassays set priorities for concern and action but they cannot be reliably used for quantitative assessment of human risk. 4. Evidence of potential carcinogenicity derived from short-term predictive tests, involving a wide variety of systems with diverse end-points, is increasingly important. Emphasis is placed on the need for more in vivo procedures with a broadening of the scope of somatic cell targets.

Methodologies for interpretation of short-term test results which may allow reduction in the use of animals in carcinogenicity testing

Assessment of the risk to humans posed by chemical substances currently relies primarily on experimental exposure of animals in lifetime feeding studies. Short-term tests for genotoxicity are much less costly and use fewer or no animals, but have not replaced the long-term animal bioassay because their results do not coincide completely. We have developed methodologies for interpretation of short-term tests which improve the usefulness of their results, and may allow them to replace the long-term animal bioassay in some circumstances.

Application of a cellular test battery in the decision point approach to carcinogen identification

The assessment of the potential carcinogenicity of a chemical requires a systematic approach taking into account various types of data. Important information on the DNA reactivity and other genetic effects of chemicals can be obtained from a battery of cellular tests. A battery is described which includes DNA repair in hepatocytes, mutagenesis in Salmonella typhimurium, mutagenesis, chromosome alterations, and transformation in mammalian cells. The interpretation of findings in this battery for the identification of potential carcinogenicity of chemicals is discussed.

Lack of genotoxic effects of fluoride in the mouse bone-marrow micronucleus test

The purpose of this investigation was to examine the potential genotoxic influence of sodium fluoride (NaF) on mammalian cells by means of a mouse bone-marrow micronucleus test. Mice of genotype B6C3F1 were obtained at about eight weeks of age and maintained on a low-fluoride diet (< 0.2 ppm F) and distilled water ad libitum throughout the experiment. At approximately 12 weeks of age, the animals were randomly assigned to seven groups, with multiple sampling schedules. The animals were intubated with various doses of NaF, ranging from 0.1 mg/kg to the Maximum Tolerable Dose (MTD), with sampling at 30, 48, and 72 hours after treatment. Negative (distilled water) and positive (cyclophosphamide) controls were included. Coded slides of femur marrow cells were prepared and examined, without identification as to treatments, for the frequency of micronucleated polychromatic erythrocytes (MN-PCE). The humera were analyzed for fluoride for monitoring of the absorption of fluoride following oral intubation. The results indicated that doses of NaF up to the MTD did not significantly increase the frequencies of MN-PCE when compared with the negative controls, although the bone fluoride content increased as the dose of NaF was increased.

International Commission for Protection against Environmental Mutagens and Carcinogens. ICPEMC publication No. 13. The need for biological risk assessment in reaching decisions about carcinogens

The prudent assumption that carcinogen bioassays in rodents predict for human carcinogenicity is examined. It is suggested that in certain cases, as for example the induction of tumors against a high incidence in controls, or in situations in which high dose toxicity may be a critical factor in the induction of cancer, the probability that animal bioassays predict for humans may be low. The term 'biological risk assessment' is introduced to describe that part of risk assessment concerned with the relevance of specific animal results to the induction of human cancer. Biological risk assessment, which is almost entirely dependent on an understanding of carcinogenesis mechanisms, is an important addition to present mathematical modeling used to predict the effects of animal carcinogens that have been demonstrated after high dose exposure, to the effects of the much smaller doses to which humans are perceived to be exposed. Evidence for the conclusions reached by biological risk assessment may sometimes be supported by a careful review of human epidemiological data.

Invited contribution: an objective approach to the development of short-term tests predictive of carcinogenicity

The Carcinogenicity Prediction and Battery Selection procedure was developed to address two problems: (1) the identification of highly predictive, yet cost-effective, batteries of short-term tests and (2) the objective prediction of the potential carcinogenicity of chemicals based upon the results of short-term tests even when a mixture of positive and negative results is obtained. In the present report the usefulness of the Carcinogenicity Prediction and Battery Selection procedure is demonstrated using benzo[a]pyrene, benzoin and diethylstilbestrol as examples. In addition, its applicability in the analysis of all the possible outcomes of a battery is illustrated together with an analysis of the worth of additional testing.