Paracelsus to parascience: the environmental cancer distraction

Entering a new millennium seems a good time to challenge some old ideas, which in our view are implausible, have little supportive evidence, and might best be left behind. In this essay, we summarize a decade of work, raising four issues that involve toxicology, nutrition, public health, and government regulatory policy. (a) Paracelsus or parascience: the dose (trace) makes the poison. Half of all chemicals, whether natural or synthetic, are positive in high-dose rodent cancer tests. These results are unlikely to be relevant at the low doses of human exposure. (b) Even Rachel Carson was made of chemicals: natural vs. synthetic chemicals. Human exposure to naturally occurring rodent carcinogens is ubiquitous, and dwarfs the general public's exposure to synthetic rodent carcinogens. (c) Errors of omission: micronutrient inadequacy is genotoxic. The major causes of cancer (other than smoking) do not involve exogenous carcinogenic chemicals: dietary imbalances, hormonal factors, infection and inflammation, and genetic factors. Insufficiency of many micronutrients, which appears to mimic radiation, is a preventable source of DNA damage. (d) Damage by distraction: regulating low hypothetical risks. Putting huge amounts of money into minuscule hypothetical risks damages public health by diverting resources and distracting the public from major risks.     

Alleged misconceptions' distort perceptions of environmental cancer risks.

In a series of papers, Ames and colleagues allege that the scientific and public health communities have perpetuated a series of 'misconceptions' that resulted in inaccurate identification of chemicals that pose potential human cancer risks, and misguided cancer prevention strategies and regulatory policies. They conclude that exposures to industrial and synthetic chemicals represent negligible cancer risks and that animal studies have little or no scientific value for assessing human risks. Their conclusions are based on flawed and untested assumptions. For instance, they claim that synthetic residues on food can be ignored because 99.99% of pesticides humans eat are natural, chemicals in plants are pesticides, and their potential to cause cancer equals that of synthetic pesticides. Similarly, Ames does not offer any convincing scientific evidence to justify discrediting bioassays for identifying human carcinogens. Ironically, their arguments center on a ranking procedure that relies on the same experimental data and extrapolation methods they criticize as being unreliable for evaluating cancer risks. We address their inconsistencies and flaws, and present scientific facts and our perspectives surrounding Ames' nine alleged misconceptions. Our conclusions agree with the International Agency for Research on Cancer, the National Toxicology Program, and other respected scientific organizations: in the absence of human data, animal studies are the most definitive for assessing human cancer risks. Animal data should not be ignored, and precautions should be taken to lessen human exposures. Dismissing animal carcinogenicity findings would lead to human cancer cases as the only means of demonstrating carcinogenicity of environmental agents. This is unacceptable public health policy.     

Endogenous mutagens and the causes of aging and cancer

A very large oxidative damage rate to DNA occurs as part of normal metabolism. In each rat cell the steady-state level is estimated to be about 10⁶ oxidative adducts and about 10⁵ new adducts are formed daily. It is argued that this endogenous DNA damage is a major contributor to aging and the degenerative diseases of aging, such as cancer. The oxidative damage rate in mammalian species with a high metabolic rate, short life span, and high age-specific cancer rate is much higher than the rate in humans, a long-lived creature with a lower metabolic rate and a lower age-specific cancer rate. It is argured that deficiency of micronutrients, such as dietary antioxidants or folate, is a major contributor to human cancer and degenerative diseases.

Understanding the role of mitogenesis in mutagenesis is critical for clarifying the mechanisms of carcinogenesis and interpreting high-dose animal cancer tests. High-dose animal cancer tests have been done mainly on synthetic industrial chemicals, yet almost all of the chemicals humans are exposed to are natural. About half of natural chemicals tested in high-dose animal cancer tests are rodent carcinogens, a finding that is consistent with the view that high-dose tests frequently increase mitogenesis rates. Animals have numerous defenses against toxins that make them very well buffered against low doses of almost all toxins, whether synthetic or natural.     

Synergy between systemic toxicity and genotoxicity: relevance to human cancer risk

The health risk manager and policy analyst must frequently make recommendations based upon incomplete toxicity data. This is a situation which is encountered in the evaluation of human carcinogenic risks as animal cancer bioassay results are often not available. In this study, in order to assess the relevance of other possible indicators of carcinogenic risks, we used the "chemical diversity approach" to estimate the magnitude of the human carcinogenic risk based upon Salmonella mutagenicity and systemic toxicity data of the "universe of chemicals" to which humans have the potential to be exposed. Analyses of the properties of 10,000 agents representative of the "universe of chemicals" suggest that chemicals that have genotoxic potentials as well as exhibiting greater systemic toxicity are more likely to be carcinogens than non-genotoxicants or agents that exhibit lesser toxicity. Since "genotoxic" carcinogenicity is a hallmark of recognized human carcinogens, these findings are relevant to human cancer risk assessment.     

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.     

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.     

Mutagenic activity of carcinogens detected in transgenic rodent mutagenicity assays at dose levels used in chronic rodent cancer bioassays

Data on transgenic rodent mutagenicity of five human carcinogens were summarised and compared with the results from rodent carcinogenicity studies. Four out of five carcinogens showed mutagenic activity already at daily dose levels which induced cancer in long-term rodent bioassays in at least one target tissue of carcinogenesis. In several of these studies, even single dose applications were sufficient to significantly increase the mutation frequency in vivo. Other genotoxic carcinogens required application of multiple dosing at dose-levels used in rodent cancer bioassays to show their in vivo mutagenicity. A rodent respiratory tract carcinogen, 1,2-dibromoethane (DBE), following inhalation exposure, displayed no mutagenic activity, neither in lung nor in nasal mucosa, at a single 2-h exposure to 30 ppm, which is below the highest concentration used in a NTP cancer bioassay. In contrast, after multiple treatment for 10 days at the same daily doses, a significant increase of the mutation frequency in nasal mucosa was apparent. We conclude, that especially when studying new chemicals in these transgenic rodent mutation assays, a multiple dosing protocol should be preferred. For dose selection, the same criteria could be applied as for chronic rodent bioassays.     

How many high production chemicals are rodent carcinogens? Why should we care? What do we need to do about it?

High production volume (HPV) chemicals are produced in or imported to the US in amounts greater than 1 million pounds per chemical per year. The EPA has identified thousands of HPVs. Due to their abundance, such chemicals bring a substantial risk for human exposure, and as a result some level of adverse consequences to health are to be expected. In order to examine the potential for cancer risk associated with HPVs, this paper examines HPVs that have been tested in the National Toxicology Program's rodent cancer bioassay. The chemicals tested in the bioassay represent a small sample of the universe of environmental chemicals to which people are exposed. Unexpectedly, 60% of the 128 HPVs evaluated in the bioassay proved to be rodent carcinogens. This value compares to a predicted prevalence of only 16.5% carcinogens in a previous study. The previous study concluded that HPVs were less likely to be toxic by a variety of other test criteria as well. However, the approach involved identifying putative carcinogens using quantitative chemical structure-activity relationships (QSAR) in contrast to the direct tabulation of bioassay test results performed here. Detailed examination of bioassay results reveals that test outcomes depend heavily on route of administration as well as on dose level, sex, strain, and species used. Since most of these factors have little or no apparent relationship to chemical structure, results of this study suggest that QSAR, as well as virtually all other alternative methods, may not generally provide accurate predictions of carcinogenic potential. As we wait for efficient and effective methods for predicting carcinogens to be developed, people continue to be exposed to environmental carcinogens. Progress on regulating environmental carcinogens as well as on developing more effective test methods has been minimal since "war on cancer" began approximately 30 years ago. The present study questions whether the current strategy for dealing with environmental carcinogens will ever be successful. Close examination of rodent cancer test results seems to suggest that almost all chemicals may have carcinogenic potential in some genotypes under some exposure circumstances. If this hypothesis is correct, it would explain the general lack of progress in developing methods to differentiate carcinogens from noncarcinogens. A completely new strategy for dealing with cancer caused by exposures to environmental chemicals seems to be needed.     

Critical factors in assessing risk from exposure to nasal carcinogens

Anatomical, physiological, biochemical and molecular factors that contribute to chemical-induced nasal carcinogenesis are either largely divergent between test species and humans, or we know very little of them. These factors, let alone the uncertainty associated with our knowledge gap, present a risk assessor with the formidable task of making judgments about risks to human health from exposure to chemicals that have been identified in rodent studies to be nasal carcinogens. This paper summarizes some of the critical attributes of the hazard identification and dose–response aspects of risk assessments for nasal carcinogens that must be accounted for by risk assessors in order to make informed decisions. Data on two example compounds, dimethyl sulfate and hexamethylphosphoramide, are discussed to illustrate the diversity of information that can be used to develop informed hypotheses about mode of action and decisions on appropriate dosimeters for interspecies extrapolation. Default approaches to interspecies dosimetry extrapolation are described briefly and are followed by a discussion of a generalized physiologically based pharmacokinetic model that, unlike default approaches, is flexible and capable of incorporating many of the critical species-specific factors. Recent advancements in interspecies nasal dosimetry modeling are remarkable. However, it is concluded that without the development of research programs aimed at understanding carcinogenic susceptibility factors in human and rodent nasal tissues, development of plausible modes of action will lag behind the advancements made in dosimetry modeling.     

Role of biomarkers in monitoring exposures to chemicals: present position,future prospects

Biomarkers are becoming increasingly important in toxicology and human health. Many research groups are carrying out studies to develop biomarkers of exposure to chemicals and apply these for human monitoring. There is considerable interest in the use and application of biomarkers to identify the nature and amounts of chemical exposures in occupational and environmental situations. Major research goals are to develop and validate biomarkers that reflect specific exposures and permit the prediction of the risk of disease in individuals and groups. One important objective is to prevent human cancer. This review presents a commentary and consensus views about the major developments on biomarkers for monitoring human exposure to chemicals. A particular emphasis is on monitoring exposures to carcinogens. Significant developments in the areas of new and existing biomarkers, analytical methodologies, validation studies and field trials together with auditing and quality assessment of data are discussed. New developments in the relatively young field of toxicogenomics possibly leading to the identification of individual susceptibility to both cancer and non-cancer endpoints are also considered. The construction and development of reliable databases that integrate information from genomic and proteomic research programmes should offer a promising future for the application of these technologies in the prediction of risks and prevention of diseases related to chemical exposures. Currently adducts of chemicals with macromolecules are important and useful biomarkers especially for certain individual chemicals where there are incidences of occupational exposure. For monitoring exposure to genotoxic compounds protein adducts, such as those formed with haemoglobin, are considered effective biomarkers for determining individual exposure doses of reactive chemicals. For other organic chemicals, the excreted urinary metabolites can also give a useful and complementary indication of exposure for acute exposures. These methods have revealed ‘backgrounds’ in people not knowingly exposed to chemicals and the sources and significance of these need to be determined, particularly in the context of their contribution to background health risks.     

Role of biomarkers in monitoring exposures to chemicals: present position, future prospects

Biomarkers are becoming increasingly important in toxicology and human health. Many research groups are carrying out studies to develop biomarkers of exposure to chemicals and apply these for human monitoring. There is considerable interest in the use and application of biomarkers to identify the nature and amounts of chemical exposures in occupational and environmental situations. Major research goals are to develop and validate biomarkers that reflect specific exposures and permit the prediction of the risk of disease in individuals and groups. One important objective is to prevent human cancer. This review presents a commentary and consensus views about the major developments on biomarkers for monitoring human exposure to chemicals. A particular emphasis is on monitoring exposures to carcinogens. Significant developments in the areas of new and existing biomarkers, analytical methodologies, validation studies and field trials together with auditing and quality assessment of data are discussed. New developments in the relatively young field of toxicogenomics possibly leading to the identification of individual susceptibility to both cancer and non-cancer endpoints are also considered. The construction and development of reliable databases that integrate information from genomic and proteomic research programmes should offer a promising future for the application of these technologies in the prediction of risks and prevention of diseases related to chemical exposures. Currently adducts of chemicals with macromolecules are important and useful biomarkers especially for certain individual chemicals where there are incidences of occupational exposure. For monitoring exposure to genotoxic compounds protein adducts, such as those formed with haemoglobin, are considered effective biomarkers for determining individual exposure doses of reactive chemicals. For other organic chemicals, the excreted urinary metabolites can also give a useful and complementary indication of exposure for acute exposures. These methods have revealed 'backgrounds' in people not knowingly exposed to chemicals and the sources and significance of these need to be determined, particularly in the context of their contribution to background health risks.     

The genetic toxicity of human carcinogens and its implications

23 chemicals and chemical combinations have been designated by the International Agency for Research on Cancer (IARC) as causally associated with cancer in humans. The literature was searched for reports of their activity in the Salmonella mutagenicity assay and for evidence of their ability to induce chromosome aberrations or micronuclei in the bone marrow of mice or rats. In addition, the chemical structures of these carcinogens were assessed for the presence of electrophilic substituents that might be associated with their mutagenicity and carcinogenicity. The purpose of this study was to determine which human carcinogens exhibit genetic toxicity in vitro and in vivo and to what extent they can be detected using these two widely employed short-term tests for genetic toxicity. The results of this study revealed 20 of the 23 carcinogens to be active in one or both short-term tests. Treosulphan, for which short-term test results are not available, is predicted to be active based on its structure. The remaining two agents, asbestos and conjugated estrogens, are not mutagenic to Salmonella; asbestos is not likely to induce cytogenetic effects in the bone marrow and the potential activity of conjugated estrogens in the bone marrow is difficult to anticipate. These findings show that genetic toxicity is characteristic of the majority of IARC Group 1 human carcinogens. If these chemicals are considered representative of human carcinogens, then two short-term tests may serve as an effective primary screen for chemicals that present a carcinogenic hazard to humans.     

The uses of carcinogen-DNA adduct measurement in establishing mechanisms of mutagenesis and in chemoprevention

DNA adducts generated by carcinogenic chemicals reflects human exposure and DNA adducts are related to tumor formation. Most chemical carcinogens require activation to reactive intermediates that bind to nucleophilic centers in proteins and nucleic acids thereby forming covalent adducts. Also, many of the chemicals considered carcinogenic for humans form covalent DNA adducts. Therefore, such DNA damage is generally considered to be causative and linked to tumor formation. In this article we have summarized the work done for many years on the role of DNA adduct formation as an indicator of their carcinogenicity. We have also addressed the important role for measurement of DNA adducts in studies with potential chemopreventive agents for which it is central to have a marker that can be measured more rapidly than changes in cancer incidence.     

Pesticide-Induced Immunotoxicity: Are Humans at Risk?

One of the first immunotoxicology studies determined that exposure of ducks to DDT reduced their resistance to a virus infection. The immunotoxic potential of insecticides and herbicides has subsequently been studied extensively in laboratory animals, driven by the global distribution and use of these chemicals. (Ten of the twelve persistent organic pollutants, identified by the United Nations Environmental Program as posing the greatest threat to humans and wildlife, are pesticides; all have been reported to alter immune function under laboratory conditions.) Nevertheless, our knowledge of the human health risks associated with pesticide use and exposure is far from complete. This paper provides a brief overview of the potential effects of chemicals on the immune system, and host factors that mitigate or exacerbate immunotoxic effects. Examples of rodent studies that exemplify categories of pesticide-induced immune system effects are then provided as an introduction to a discussion of pesticide immunotoxicity in humans.     

The alkaline single cell gel electrophoresis assay with mouse multiple organs: results with 30 aromatic amines evaluated by the IARC and U.S. NTP

The genotoxicity of 30 aromatic amines selected from IARC (International Agency for Research on Cancer) groups 1, 2A, 2B and 3 and from the U.S. NTP (National Toxicology Program) carcinogenicity database were evaluated using the alkaline single cell gel electrophoresis (SCG) (Comet) assay in mouse organs. We treated groups of four mice once orally at the maximum tolerated dose (MTD) and sampled stomach, colon, liver, kidney, bladder, lung, brain, and bone marrow 3, 8 and 24 h after treatment. For the 20 aromatic amines that are rodent carcinogens, the assay was positive in at least one organ, suggesting a high predictive ability for the assay. For most of the SCG-positive aromatic amines, the organs exhibiting increased levels of DNA damage were not necessarily the target organs for carcinogenicity. It was rare, in contrast, for the target organs not to show DNA damage. Organ-specific genotoxicity, therefore, is necessary but not sufficient for the prediction of organ-specific carcinogenicity. For the 10 non-carcinogenic aromatic amines (eight were Ames test-positive and two were Ames test-negative), the assay was negative in all organs studied. In the safety evaluation of chemicals, it is important to demonstrate that Ames test-positive agents are not genotoxic in vivo. Chemical carcinogens can be classified as genotoxic (Ames test-positive) and putative non-genotoxic (Ames test-negative) carcinogens. The alkaline SCG assay, which detects DNA lesions, is not suitable for identifying non-genotoxic carcinogens. The present SCG study revealed a high positive response ratio for rodent genotoxic carcinogens and a high negative response ratio for rodent genotoxic non-carcinogens. These results suggest that the alkaline SCG assay can be usefully used to evaluate the in vivo genotoxicity of chemicals in multiple organs, providing for a good assessment of potential carcinogenicity.     

Prevalence of genotoxic chemicals among animal and human carcinogens evaluated in the IARC Monograph series.

To determine whether genotoxic and non-genotoxic carcinogens contribute similarly to the cancer burden in humans, an analysis was performed on agents that were evaluated in Supplements 6 and 7 to the IARC Monographs for their carcinogenic effects in humans and animals and for the activity in short-term genotoxicity tests. The prevalence of genotoxic carcinogens on four groups of agents, consisting of established human carcinogens (group 1, n = 30), probable human carcinogens (group 2A, n = 37), possible human carcinogens (group 2B, n = 113) and on agents with limited evidence of carcinogenicity in animals (a subset of group 3, n = 149) was determined. A high prevalence in the order of 80 to 90% of genotoxic carcinogens was found in each of the groups 1, 2A and 2B, which were also shown to be multi-species/multi-tissues carcinogens. The distribution of carcinogenic potency in rodents did not reveal any specific characteristic of the human carcinogens in group 1 that would differentiate them from agents in groups 2A, 2B and 3. The results of this analysis indicate that (a) an agent with unknown carcinogenic potential showing sufficient evidence of activity in in vitro/in vivo genotoxicity assays (involving as endpoints DNA damage and chromosomal/mutational damage) may represent a hazard to humans; and b) an agent showing lack of activity in this spectrum of genotoxicity assays should undergo evaluation for carcinogenicity by rodent bioassay, in view of the present lack of validated short-term tests for non-genotoxic carcinogens. Overall, this analysis implies that genotoxic carcinogens add more to the cancer burden in man than non-genotoxic carcinogens. Thus, identification of such genotoxic carcinogens and subsequent lowering of exposure will remain the main goal for primary cancer prevention in man.     

The structural basis of the carcinogenic and mutagenic potentials of phytoalexins

CASE, a structure-activity relational system, was used to predict the proportion of substances to be carcinogenic and mutagenic among plant pesticides (phytoalexins) and other natural products compared to that of randomly selected chemicals. There were no significant differences between phytoalexins and other natural products. On the other hand, the natural products, as a group, were predicted to be less mutagenic and carcinogenic than randomly selected chemicals. 37% of natural products are predicted to be rodent carcinogens.     

Prevention of cancer and other chronic diseases worldwide based on sound mechanisms

The transformation of normal cells by DNA reactive, genotoxic carcinogens and the growth promotion and development of mutated cells by enhancing factors is involved in the overall basic mechanism of cancer induction. Thus, discrimination between genotoxic carcinogens and nongenotoxic chemicals is essential. The dose-response curves, reversibility, and organ-and species specificity are distinct. Genotoxic carcinogens are mutagenic, form DNA adducts, induce DNA repair, and form hydroxy radicals and inappropriate peroxidation reactions that antioxidants such as those in vegetables, fruits, and tea can decrease. In contrast, promoters do not form DNA adducts, but raise cell duplication rates, among other attributes. In the USA, about 35% of known cancers are associated with tobacco use and about 55% with inappropriate nutritional habits. Cancer induction can be decreased by avoiding the formation of carcinogens, reducing their metabolic activation, or increasing their detoxification. Excessive dietary salt, and heterocyclic arylamines formed in cooking of meats or fish, and high intake of 40% of calories in fats are health risks, but vegetables, fruits, tea, soy products, and fibers are protective. We review nutritional factors involved in cancer and chronic disease causation and prevention.     

The non-genotoxicity to rodents of the potent rodent bladder carcinogens o-anisidine and p-cresidine.

The two potent rodent bladder carcinogens o-anisidine and p-cresidine, and the structurally related non-carcinogen 2,4-dimethoxyaniline, have been extensively evaluated for genotoxicity to rodents and found to be inactive. Most data were generated on o-anisidine, an agent that is also only marginally genotoxic in vitro. The two carcinogens induced methaemoglobinaemia in rodents indicating that the chemicals are absorbed and metabolically oxidized. Despite their total lack of genotoxicity in vivo, the two carcinogens have the hall-marks of being genotoxic carcinogens given that most test animals of both sexes of B6C3F1 mice and F344 rats are reported to have succumbed rapidly to malignant bladder cancer. No reasons for this dramatic conflict of test data are so far apparent. The experiments described involve, in one or other combination, 2 strains of mice (including B6C3F1) and 4 strains of rat (including F344), the use of oral and i.p routes of exposure and observations made after 1, 3 or 6 doses of test chemical. 6 tissues (including the rat bladder) were assayed using 3 genetic endpoints (unscheduled DNA synthesis, DNA single-strand breaks and micronuclei induction). Aroclor-induced rats were employed in one set of experiments with o-anisidine. In the case of one set of mouse bone-marrow micronucleus experiments the same batch of the 3 chemicals as used in the cancer bioassays, and the same strain of mouse, were used. Possible further experiments and the implications of these findings are discussed.