Rodent carcinogens: setting priorities

Biological properties of peroxide-containing tooth whiteners

Peroxides have been used in tooth whitening for more than 100 years. Current peroxide-containing whiteners can be classified into three categories: (1) those containing high concentrations of peroxides for professional use only; (2) materials dispensed by dentists and used by patients at home; and (3) over-the-counter products available directly to consumers for home use. Hydrogen peroxide (H2O2) and carbamide peroxide are the most commonly used active ingredients in these whiteners. Both peroxides have long been used safely in oral health products and are accepted by the US Food and Drug Administration. However, questions have been raised regarding the safety of at-home whiteners because the peroxides appear to constitute a new use. Substantial differences exist in the manner of application between at-home whiteners and oral health products. In addition, tooth whiteners are a mixture of various ingredients; possible interactions may occur because of the active nature of peroxides. Therefore, the safety evidence for peroxide-containing whiteners is considered inadequate. This paper will review the history of using peroxides for tooth whitening, the toxicology of H2O2 and carbamide peroxide, and available information on the safety of whiteners. The rationale and approaches for evaluating biological properties of peroxide containing whiteners are also discussed.

Simultaneous measurement of unscheduled and replicating DNA synthesis by means of a new cell culture insert DNA retention method: rapid induction of replicating DNA synthesis in response to genotoxic carcinogens

In order to measure simultaneously replicating DNA synthesis (RDS) and unscheduled DNA synthesis (UDS) in rat hepatocytes responding to exposure to carcinogens, a new method, namely the "cell culture insert DNA retention (CDR)" method, was developed. All CDR procedures for cell culture, digestion of cytoplasm and retention of DNA were performed on membranes attached to cell culture containers. Four subgroups of primary cultures of hepatocytes prepared from rats were exposed to a genotoxic or non-genotoxic carcinogen with or without 10 mM hydroxyurea and incubated for 4 h with 10 microCi/ml [3H]thymidine. The membranes were then processed for both liquid scintillation and autoradiography. Among seven tested chemicals, three genotoxic agents, 3,2'-dimethyl-4-aminobiphenyl, 2-acetylaminofluorene and diethylnitrosamine, and two non-genotoxic carcinogens, nafenopin and phenobarbital, induced RDS within 4 h after the exposure, indicating that these carcinogenic agents induce cell proliferation is non-proliferating rat hepatocytes prior to the emergence of genotoxic changes. Several indices were devised to characterize the genotoxicity of the tested chemicals. The induction patterns obtained showed a wide variation in the individual characteristics of carcinogen-induced genotoxicity and mitogenicity in the early phase of initiation. This is the first report of simultaneous measurement, by using a combination of autoradiography and liquid scintillation, of UDS and RDS induced in rat hepatocytes. The described CDR approach will be useful for risk assessment and characterization of carcinogenic and tumor-promoting agents.

The effects of exposure to "synthetic" chemicals on human health: a review

This article examines how scientists use human, animal, and bacterial evidence to develop policy recommendations about the health consequences of human exposure to modern chemicals. Human evidence is limited because many epidemiological studies are contaminated with selection effects or unobserved heterogeneity. Changes in the aggregate incidence of morbidity (such as cancer) in the population over time are not a substitute for the lack of good individual-level data because incidence data are contaminated by the medicalization of cancer. Animal tests are also problematic because the expense of conducting experiments leads researchers to use only enough animals to allow detection of large differences in cancer incidence between controls and experimental animals that can only arise if the exposure doses are large. Predictions about the cancer incidence that would result in humans at much lower exposure levels, thus, require statistical inferences that implicitly make choices between false positive and false negative inference errors. Policy recommendations about carcinogens, therefore, are as much the product of value choices as "scientific" knowledge.

Environmental contaminants in breast milk from the central Asian republics

Concern has been widespread about possible high levels of environmental contamination in areas of the former Soviet Union. Some of this concern has focussed on toxicants in human milk and their potential adverse effects on the breastfeeding child, but published data have been virtually unavailable. This study collected previously unpublished analyses of breast milk, water, cow's milk, and dairy product samples from several of the Republics during the last decade, providing an opportunity for comparing the level of contamination to similar data from other countries. The general levels of contamination are similar to those observed in other countries. Social and economic conditions as well as the contamination of water and substitute foods in the Republics make alternative methods of infant feeding demonstrably less desirable for the child than the measured levels of breast milk contamination.

Gemfibrozil-induced peroxisome proliferation and hepatomegaly in male F344 rats

Gemfibrozil is a widely used hypolipidemic drug in humans that causes peroxisome proliferation and hepatocarcinogenesis in rodents. The induction of hepatomegaly and hepatic peroxisome proliferation (measured as peroxisomal acyl CoA oxidase activity), was determined and compared to another peroxisome proliferator, WY-14,643 (0.1% in the diet) in male F344 rats. In a 21-day study, dietary no-observable-effect and lowest-observable-effect levels of gemfibrozil for both hepatomegaly and peroxisome proliferation were 0.002% and 0.005%, respectively. In a 42-day study, dietary concentrations of 0.9-2.0% gemfibrozil induced a similar magnitude of hepatomegaly to WY-14,643 (2.3-fold) but a higher level of peroxisome proliferation (16-18-fold) than the maximum induction for WY-14,643 (13-fold). The plateau in magnitude of gemfibrozil-induced peroxisome proliferation across the 0.9-2.0% dietary concentrations was associated with a plateau in serum concentration of gemfibrozil (approximately 20 micrograms/ml), similar to concentrations reported in human subjects receiving oral gemfibrozil. These results indicate that maximal induction of peroxisome proliferation by gemfibrozil can exceed that of a more potent compound such as WY-14,643, and further suggest that maximal induction of peroxisome proliferation can be limited by steady-state serum concentrations. Moreover, the reported lack of hepatic responses to gemfibrozil in humans is unlikely to be the result of inefficacy or unavailability of this drug, compared to other peroxisome proliferators, in rodents.

Organochlorine compounds and estrogen-related cancers in women

The organochlorines, a diverse group of some 15,000 compounds, have been implicated increasingly as being harmful to humans. Some congeners of DDT and PCB elicit very weak estrogenic responses in animals, while the dioxin TCDD and related compounds have antiestrogenic properties. This review summarizes the evidence regarding whether certain organochlorine compounds, usually as persistent food-chain contaminants, increase the risk of breast and endometrial cancers through their estrogenic potential. In humans, neither ecologic data nor occupational studies provide clear support for an association between organochlorine exposure and the occurrence of these cancers. In our summary analysis of occupational exposure, the rate ratio of breast cancer for exposed cf unexposed women was 0.84 (95 percent confidence interval [CI] = 0.50-1.33) for PCBs and 1.08 (CI = 0.68-1.58) for TCDD. Similarly, effect estimates close to unity were found in summary analysis of breast cancer case-control studies regarding levels of DDE and PCB in adipose tissue or serum. In two recent nested case-control studies using stored specimens, the odds ratio per standard deviation increase in serum p,p'-DDE was 1.27 (CI = 0.95-1.69). Although estrogenic effects of certain organochlorine compounds should be easier to detect on the endometrium, we know of no analytic epidemiologic studies of endometrial cancer published to data. We conclude that available data do not indicate that organochlorines will affect the risk of these two cancers in any but the most unusual situation.

Mitochondrial medicine

In the mitochondrion, inherited defects have been identified in the electron transport system by which ATP is formed, as well as in the transport and metabolism of fuels. Clinical findings in diseases due to these defects can be related to abnormal accumulations of metabolic intermediates and inadequate or inefficient ATP generation. In the oxidative process within the mitochondrion, chemical oxidants are generated, which can cause cellular damage. As the body's defences against the oxidants decline, oxidative damage appears to contribute to the ageing process itself as well as to age-related degenerative diseases. Understanding in this area has accelerated with knowledge of the synthesis, structure and function of the mitochondrion and its specific DNA. The frontier is expected to advance rapidly as causal relationships between these diseases and mitochondrial dysfunction, and the potential role of antioxidants in therapy, are better defined.

Early biochemical markers of effects: enzyme induction, oncogene activation and markers of oxidative damage

Experimental carcinogenicity studies focus on identification of single carcinogens. Humans, however, appear exposed to a variety of low doses of carcinogens. Furthermore, few chemical entities are carcinogenic or toxic per se, but require metabolic activation to form ultimate carcinogens or toxins. In contrast to experimental animals, humans show considerable difference in genetic properties. In that situation it is particularly important to estimate individual capability for metabolic activation. To an increasing extent, activation includes formation of toxic oxygen metabolites. Particular targets for activated species are DNA and lipids; in particular low-density lipoproteins (LDL). Modifications of DNA are important for initiating the multistep process of carcinogenesis, in particular if oncogenes are activated or if tumor supressor genes are inactivated. Such DNA modification can be identical regardless of the reactive specimens being a xenobiotic or an oxygen species. Modification of LDL can start the process of atherosclerosis by transforming macrophages into foam cells, deposited as fatty streaks in the arterial wall. Biomarkers for activation capacity of xenobiotics include the use of prototype substrates and molecular techniques to determine genetic polymorphisms. Oxidative DNA modification can be measured from urinary excretion of oxidatively modified deoxynucleosides, particularly guanosine. Future efforts have to include individual measurements in order to improve the 'resolution' of molecular epidemiological approaches.

The causes and prevention of cancer

Epidemiological evidence indicates that avoidance of smoking, increased consumption of fruits and vegetables, and control of infections will have a major effect on reducing rates of cancer. Other factors include avoidance of intense sun exposure, increases in physical activity, and reduction of alcohol consumption and possibly red meat. A substantial reduction in breast cancer is likely to require modification of sex hormone levels, and development of practical methods for doing so is a high research priority. Resolution of the potential protective roles of specific antioxidants and other constituents of fruits and vegetables deserves major attention. Mechanistic studies of carcinogenesis indicate an important role of endogenous oxidative damage to DNA that is balanced by elaborate defense and repair processes. Also key is the rate of cell division, which is influenced by hormones, growth, cytotoxicity, and inflammation, as this determines the probability of converting DNA lesions to mutations. These mechanisms may underlie many epidemiologic observations.

The development of mitochondrial medicine

I consider mitochondrial medicine a tentative designation for an area within clinical medicine still to be delineated. Its development extends over a period of 35 years, from its discovery in 1959 [1]. Progress had been gradual until recent years when it has become explosive in nature with extensions in many different directions. My presentation is an effort to illustrate this evolution with emphasis on especially important observations which by leaps advanced the area. We are fortunate to have here several of the distinguished investigators, who have contributed so much to those advances. They will share with us their deep knowledge in different aspects of mitochondrial medicine, what is known, what remains to be elucidated, and what the problems are to be encountered in that elucidation.

Analysis of the potential carcinogenicity of coffee and its related compounds in a medium-term liver bioassay of rats

The potential carcinogenicity of coffee and related compounds was examined using a medium-term liver bioassay based on the induction of glutathione S-transferase placental form (GST-P)-positive foci in F344 rats. A total of 230 males were initially injected with diethylnitrosamine (200 mg/kg body weight, ip) or saline as controls and 2 wk later were fed on diet or drinking water supplemented as follows for 6 wk: 5% regular instant coffee; 5% decaffeinated instant coffee; freshly brewed coffee, 8 g in 140 ml water; 0.1% caffeine, 0.2% methylglyoxal, 0.2% glyoxal; or 0.3% theophylline in the drinking water (w/v); and 0.4% theobromine in the diet (w/w). All rats were subjected to two-thirds partial hepatectomy at wk 3 and killed at wk 8. The resultant values for GST-P-positive hepatic focus induction were slightly increased with methylglyoxal and decreased with glyoxal and theobromine compared with the corresponding controls. Although the increase in number of foci for methylglyoxal was statistically significant at P < 0.05, the value was within the historical control levels. Regular and decaffeinated instant coffee as well as fresh-brewed coffee, caffeine and theophylline exerted no effects on focus development. Thus, the coffee-related compounds examined demonstrated no obvious enhancing potential, and it is therefore concluded that coffee and its main constituents are not carcinogenic for the rat liver.

Organochlorine compounds in relation to breast cancer, endometrial cancer, and endometriosis: an assessment of the biological and epidemiological evidence

There is an increasing public and scientific concern that certain chlorinated compounds, recognized as environmental pollutants, may cause estrogen-related neoplastic disease in humans. The main hypothesis has been that certain organochlorines, through their estrogenic actions, might cause breast cancer. From experimental studies, both in vitro and in vivo, there is evidence that certain organochlorine compounds may cause estrogenic effects, whereas others may cause antiestrogenic effects. In limited studies, some of these compounds in high doses have also been shown to increase and reduce the frequency of estrogen-related tumors in animals. The epidemiological findings regarding the association between organochlorines and breast cancer are inconclusive. However, the largest and best designed study has been interpreted as negative with respect to DDT and polychlorinated biphenyls (PCB) in relation to breast cancer. Associations between organochlorine exposure and endometrial cancer or endometriosis have even more limited empirical basis. The hypothesis that human exposure to environmental levels or organochlorines would favor an estrogenic overactivity leading to an increase in estrogen-dependent formation of mammary or endometrial tumors is not supported by the existing in vitro, animal and epidemiological evidence. It can, however, not be conclusively rejected on the basis of available data.

Descriptive study of lipid-modulating drug use in a French professional population

The objectives of this cross-sectional study presented as the first stage of a cohort follow-up pharmacoepidemiological study of lipid modulators, are to determine the extent and pattern of use of antihyperlipidemics and concomitant drugs and to compare antihyperlipidemic subgroups. The study population are the 17,244 respondents to the 1991 questionnaire in which questions on drug utilization were asked for the first time. It comprises 40.5% of the men and 32% of the women of the population originally targeted in 1989, i.e. the entire workforce of a national company within an age range of 35-45 years for men and 35-50 years for women. Self-reported health events and drug use over the previous year were measured. Overall 1 year prevalence of use of lipid lowering drugs was 7.7% (9.5% in men and 2.7% in women). Most antihyperlipidemics were fibrates (n = 878), with fenofibrate ranking above ciprofibrate, as opposed to HMG-CoA reductase inhibitors (n = 436), principally simvastatin. Only 60% of antihyperlipidemic drug users were following a prescribed lipid-lowering diet; over half were taking other drugs concomitantly, most often b-blockers. The simvastatin group differs from the fibrate group by an excess of prevalent thyroid disease. The link between simvastatin and thyroid disease may be indication-related. In conclusion, the relevant features of this study are the extensive use of antihyperlipidemics, frequently, without concomitant diet, and the marked preference for antihyperlipidemics for which long term safety is unknown. Extent of use and choices of antihyperlipidemics are corroborated by estimations in the general middle-aged population.

The development of mitochondrial medicine

Primary defects in mitochondrial function are implicated in over 100 diseases, and the list continues to grow. Yet the first mitochondrial defect--a myopathy--was demonstrated only 35 years ago. The field's dramatic expansion reflects growth of knowledge in three areas: (i) characterization of mitochondrial structure and function, (ii) elucidation of the steps involved in mitochondrial biosynthesis, and (iii) discovery of specific mitochondrial DNA. Many mitochondrial diseases are accompanied by mutations in this DNA. Inheritance is by maternal transmission. The metabolic defects encompass the electron transport complexes, intermediates of the tricarboxylic acid cycle, and substrate transport. The clinical manifestations are protean, most often involving skeletal muscle and the central nervous system. In addition to being a primary cause of disease, mitochondrial DNA mutations and impaired oxidation have now been found to occur as secondary phenomena in aging as well as in age-related degenerative diseases such as Parkinson, Alzheimer, and Huntington diseases, amyotrophic lateral sclerosis and cardiomyopathies, atherosclerosis, and diabetes mellitus. Manifestations of both the primary and secondary mitochondrial diseases are thought to result from the production of oxygen free radicals. With increased understanding of the mechanisms underlying the mitochondrial dysfunctions has come the beginnings of therapeutic strategies, based mostly on the administration of antioxidants, replacement of cofactors, and provision of nutrients. At the present accelerating pace of development of what may be called mitochondrial medicine, much more is likely to be achieved within the next few years.

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

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

Toward the development of an equitable cancer prevention

The most reasonable and socially acceptable development of cancer prevention should be the blending of the population approach, that is the shifting of the distribution of risk factors across an entire population in a favourable direction, with the high risk approach aimed at the identification, surveillance and possibly early interventions on individuals with particularly high values of predisposition to cancer. Interventions aimed at reducing or eliminating genetically determined weaknesses with regard to interactions with the environment, will not make, therefore, in any way obsolete or redundant, interventions aimed at eliminating or reducing exposure to environmental carcinogens.

Molecular cloning of three sulfotransferase cDNAs from mouse liver

Sulfate conjugation plays an important role in the biotransformation of not only xenobiotics but also many endogenous substances. Sulfotransferases, the enzymes that are responsible for this process, exist as a superfamily of genes. It has long been recognized that significant species differences exist among drug and carcinogen metabolizing enzymes such as cytochrome P450. Species differences in both regulation and catalytic activities of sulfotransferases may also exist. To investigate this, we conducted cDNA cloning and cDNA expression studies of sulfotransferase in the mouse. Three sulfotransferase cDNA clones were isolated from a female B6CBA mouse liver. Two of the clones, mSTa1 and mSTa2, were highly homologous to each other. Alignment of mSTa1 and mSTa2 cDNAs' nucleotide sequences with those of other sulfotransferase cDNAs revealed the greatest sequence identity with the rat STsmp cDNA. This analysis suggests that mSTa1, mSTa2 and rSTsmp cDNAs are derived from orthologous genes belonging to the alcohol/hydroxysteroid sulfotransferase gene family. The third clone, mSTp1 showed high identity to rSTp, hSTp1, hSTp3, and rSTp1C1, suggesting that mSTp1 belongs to the phenol family.

Animal studies on the role of 50/60-Hertz magnetic fields in carcinogenesis

A number of epidemiological studies have suggested that exposure to 50/60-Hz magnetic fields (MF) from power lines and electrical equipment may be associated with a modestly increased incidence of various types of cancer. Laboratory studies have indicated that nonionizing radiation has no mutagenic effect, i.e. does not initiate cancer. Thus, if 50/60-Hz MF are truly associated with an increased risk of cancer, then these fields must act as a promoter or co-promoter of cancer in cells that have already been initiated. This paper reviews the evidence produced by animal studies. As shown in this review, the available animal data on 50/60-Hz MF exposures seem to indicate that intermediate MF exposure exerts co-promoting effects in different tumor models, particularly cocarcinogenesis models of breast cancer, while chronic (up to life-time) exposure may exert promoting effects on "spontaneous" development of certain tumors. The tumor promoting or co-promoting effects of 50/60-Hz MF exposure found in several animal studies could relate to actions of MF on gene expression, immune surveillance, and Ca2+ homeostasis as demonstrated by in vitro experiments in cell cultures. However, the most plausible evidence for an in vivo effect of MF exposure which could be related to tumor promotion is reduction of circulating levels of melatonin, i.e. a hormone which is inhibitory to the growth of a wide range of cancers, particularly breast cancer. Animal studies have shown that 50-Hz MF exposure at flux densities as low as 0.3-1 mu Tesla significantly reduces nocturnal melatonin levels in plasma. While decrease of melatonin levels alone could explain tumor promoting or co-promoting effects of MF exposure, recent data indicate that MF exposure also impairs the effects of melatonin at the cellular level. Thus, the oncostatic effect of melatonin on cell proliferation of a human breast cancer cell line was antagonized by 60-Hz MF exposure at a flux density of about 1 mu Tesla. All these data indicate that interactions between 50/60-HZ MF exposure and melatonin may be the key mechanism of any carcinogenic effects. Although the existing experimental evidence is still insufficient for discerning a cause-effect relationship for MF exposure and human disease or injury, it does suggest the need for further laboratory research under well-defined laboratory exposure conditions to allow for a realistic assessment of the possible health risks and their magnitude.

Oxidants, antioxidants, and the degenerative diseases of aging

Metabolism, like other aspects of life, involves tradeoffs. Oxidant by-products of normal metabolism cause extensive damage to DNA, protein, and lipid. We argue that this damage (the same as that produced by radiation) is a major contributor to aging and to degenerative diseases of aging such as cancer, cardiovascular disease, immune-system decline, brain dysfunction, and cataracts. Antioxidant defenses against this damage include ascorbate, tocopherol, and carotenoids. Dietary fruits and vegetables are the principal source of ascorbate and carotenoids and are one source of tocopherol. Low dietary intake of fruits and vegetables doubles the risk of most types of cancer as compared to high intake and also markedly increases the risk of heart disease and cataracts. Since only 9% of Americans eat the recommended five servings of fruits and vegetables per day, the opportunity for improving health by improving diet is great.

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.

Tumor promotion in a breast cancer model by exposure to a weak alternating magnetic field

In view of the methodological problems of epidemiological studies on associations between exposures to 50/60 Hz magnetic fields (MF) and increased incidence of cancers, laboratory studies are necessary to determine if 50/60 Hz MF are cancer promoters or can progress cancers. The objective of the present study was to determine if an alternating MF of low flux density exerts tumor-promoting or co-promoting effects in a model of breast cancer in female rats. Mammary tumors were induced by the chemical carcinogen 7,12-dimethylbenz(a)anthracene (DMBA). A group of 99 rats was exposed to a homogeneous MF of 50 Hz, 100 microT (microtesla), for 24 h/day 7 day/week for a period of 91 days; another group of 99 rats was sham-exposed under the same environmental conditions as the MF-exposed rats. The exposure chambers were identical for MF-exposed and sham-exposed animals. DMBA was administered orally at a dose of 5 mg/kg at the first day of exposure and at weekly intervals thereafter up to a total dose of 20 mg per rat. The animals were palpated once weekly to assess the development of mammary tumors. In controls, DMBA induced tumors in about 40% of the animals within three months of first application. Eight weeks after DMBA application the MF-exposed rats exhibited significantly more tumors than sham-exposed animals. This difference in the rate of tumor development was observed throughout the period of exposure. At the end of the three-month period of MF exposure the tumor incidence in MF-exposed rats was 50% higher than in sham-exposed rats, the difference being statistically significant. Furthermore, the size of tumors as estimated by palpation was significantly larger in the MF-exposed compared to sham-exposed rats. The data demonstrates that long-term exposure of DMBA-treated female rats to an alternating MF of low flux density promotes the growth and increases the incidence of mammary tumors, thus strongly indicating that MF exposure exerts tumor-promoting and/or copromoting effects.

Issues and controversies surrounding qualitative strategies for identifying and forecasting cancer causing agents in the human environment

Certain chemicals, mixtures of chemicals, exposure circumstances, life-styles and personal or cultural habits, occupations, viruses, living conditions, and physical agents have been causally associated with cancers in humans. Most however are not considered potentially carcinogenic, and the proportion of 'agents' eventually identified to cause cancer is projected to be relatively low. Current methods to identify carcinogenic potential of chemicals rely largely on short-term in vitro and in vivo tests, mid- & long-term in vivo assays, molecular mechanisms, epidemiological investigations, and structural-activity-effect-relationships. Thus, the scientific and public health communities must continue to utilize available means and concomitantly strive to develop newer methods and tools to more easily, quickly, cheaply, and reliably identify carcinogens in the human milieu. Since adequate human studies are typically absent, the most useful method for identifying potential human carcinogens continues to be long-term carcinogenesis experiments. Agents identified as causing cancers in humans have been shown to cause cancer in animals, and this knowledge, together with similarities in mechanisms of carcinogenesis across species, led to the scientific logic and public health strategy that chemicals shown clearly to be carcinogenic in animals should be considered as being likely and anticipated to present cancer risks to humans. The quest of hazard identification efforts is cancer prevention, largely by reducing or eliminating exposures to chemicals that cause cancer and other diseases.