Prediction of rodent nongenotoxic carcinogenesis: evaluation of biochemical and tissue changes in rodents following exposure to nine nongenotoxic NTP carcinogens

We studied nine presumed nongenotoxic rodent carcinogens, as defined by the U.S. National Toxicology Program (NTP), to determine their ability to induce acute or subacute biochemical and tissue changes that may act as useful predictors of nongenotoxic rodent carcinogenesis. The chemicals selected included six liver carcinogens (two of which are peroxisome proliferators), three thyroid gland carcinogens, and four kidney carcinogens. We administered the chemicals (diethylhexyl phthalate, cinnamyl anthranilate, chlorendic acid, 1,4-dichlorobenzene, monuron, ethylene thiourea, diethyl thiourea, trimethyl thiourea, and d-limonene to the same strains of mice and rats used in the original NTP bioassays (nine chemicals to rats and seven to mice). Selected tissues (liver, thyroid gland, and kidney) were collected from groups of animals at 7, 28, and 90 days for evaluation. Tissue changes selected for study were monitored for all of the test groups, irrespective of the specificity of the carcinogenic responses observed in those tissues. This allowed us to assess both the carcinogen specificity and the carcinogen sensitivity of the events being monitored. We studied relative weight, cell labeling indices, and pathologic changes such as hypertrophy in all tissues; a range of cytochrome P450 enzymes and palmitoyl coenzyme A oxidase in the liver; changes in the levels of plasma total triiodothyronine, total thyroxine, and thyroid-stimulating hormone (TSH) as markers of thyroid gland function; and hyaline droplet formation, tubular basophilia, and the formation of granular casts in the kidney. There were no single measurements that alerted specifically to the carcinogenicity of the agents to the rodent liver, thyroid gland, or kidney. However, in the majority of cases, the chemical induction of cancer in a tissue was preceded by a range of biochemical/morphologic changes, most of which were moderately specific for a carcinogenic outcome, and some of which were highly specific for it (e.g., increases in TSH in the thyroid gland and increases in relative liver weight in the mouse). The only measurements that failed to correlate usefully with carcinogenicity were the induction of liver enzymes (with the exception of the enzymes associated with peroxisome proliferation). Most of the useful markers were evident at the early times studied (7 days and 28 days), but no overall best time for the measurement of all markers was identified. The judicious choice of markers and evaluation times can aid the detection of potential nongenotoxic rodent carcinogens.     

Genotoxic agents detected by plant bioassays

Seven higher plant species (Allium cepa, Arabidopsis thaliana, Glycine max, Hordeum vulgaris. Tradescantia paludosa, Vicia faba, and Zea mays) were reviewed for their ability to detect genotoxicity of chemical agents under the U.S. Environmental Protection Agency (U.S. EPA) Gene-Tox program in the late 1970s. Six bioassays-Allium and Vicia root tip chromosome breaks, Tradescantia chromosome break, Tradescantia micronucleus, Tradescantia-stamen-hair mutation, and Arabidopsis-mutation bioassays- were established from four plant systems that are currently in use for detecting the genotoxicity of environmental agents. Under the Gene-Tox program, the Crepis capillaris-chromosome-aberration test was added to the existing six bioassays. The current review is limited to chemical agents that exhibit a positive response to any of these seven plant bioassays. From 158 articles reviewed, 84 chemicals were compiled in three categories: carcinogens, clastogens, and mutagens. As none of these plant bioassays can detect tumor initiation or cancerous growth, the chemicals were categorized as carcinogens based on their characteristics defined by the U.S. EPA's Superfund Priority 1 List and/or by the chemical listings of the Sigma and Aldrich Chemical Companies. Certain mutagens were categorized in the same manner in addition to the agents detected as mutagens by these plant bioassays.     

Mouse liver tumor data: assessment of carcinogenic activity

Significant numbers of chemicals have been shown to be carcinogenic in mouse liver although they do not exhibit carcinogenic activity in other organs or tissues of mice or rats. This review focuses on the reasons for the unique susceptibility of the mouse liver to these carcinogens and the extent to which the carcinogenic activity of a chemical in mouse liver can be used to predict carcinogenicity in humans. Many of these mouse liver carcinogens lack genotoxic activity and, as such, have been proposed to be tumor promoters. Two mechanisms that may explain the action of nongenotoxic carcinogens in mouse liver are reviewed. These are: direct action on precursor cancer cells, either to accelerate their growth or to prevent their death and the selective growth advantage, resulting from regenerative hyperplasia of precursor cancer cells in response to the necrosis of normal cells produced by hepatotoxins. Estimating human health risks on the basis of mouse liver tumor data is believed to differ for nongenotoxic and genotoxic carcinogens in two fundamental ways. The first involves intraspecies extrapolation and the second involves low-dose extrapolation. In conclusion, although mouse liver tumor data are seen to be of value in estimating human health hazard, it is important to distinguish between genotoxic and nongenotoxic mechanisms in applying such data. Further study of the biochemical and molecular mechanisms of chemical carcinogens is necessary to determine the relationship between their activity in mouse liver and their activity in humans.     

Mechanism of skin tumorigenesis by contact sensitizers: the effect of the corticosteroid fluocinolone acetonide on inflammation and tumor induction by 2,4 dinitro-1-fluorobenzene in the skin of the TG.AC (v-Ha-ras) mouse.

The effect of the corticosteroid fluocinolone acetonide (FA) on skin tumor induction and inflammation by the contact sensitizer dinitrofluorobenzene (DNFB) was examined. This study broadly relates to the question of whether contact sensitizers, as electrophilic chemicals that produce protein adduction, may constitute an environmental cancer hazard. The specific aim of this study was to evaluate the extent to which the immunogenic inflammatory response to DNFB, in contrast to DNFB cytotoxicity, might be responsible for tumor induction. Experiments were conducted on a transgenic (TG.AC) mouse, incorporating a mutated ras oncogene (v-Ha-ras) that responds rapidly and profusely with skin papillomas to tumor promoters as if it were genetically initiated. Various doses and patterns of DNFB and FA were applied to the skin in a 2-week period; DNFB was given four times and FA was given either with the DNFB or daily. The tumor response to DNFB was completed by 8 weeks from the first dose and was consistent with a dose-squared relationship. FA was not tumorigenic alone; when given with DNFB, it caused only a small reduction in inflammation and tumor yield. When given daily, FA increased ulcerative skin damage, inflammation, and the yield tumors. The results suggest that tumorigenesis by DNFB, in the high-dose short-term regimen used here, is mainly due to its cytotoxicity and not contact sensitization.     

The potential role of chemically induced hyperplasia in the carcinogenic activity of the hypolipidemic carcinogens

Di(2-ethylhexyl)phthalate (DEHP) is a widely used plasticizing agent resulting in substantial human exposure and environmental contamination. In a chronic bioassay, high doses of DEHP induced hepatocellular carcinomas in female Fischer-344 rats and male and female B6C3F1 mice. Thus, there is considerable concern as to the species specificity, mechanism of action, and human risk assessment of DEHP. DEHP belongs to a class of agents described as hypolipidemic hepatocarcinogens. These chemicals share the ability to induce hepatic peroxisomal proliferation and range from very weak to very potent hepatocarcinogens. Unlike most identified carcinogens, the hypolipidemic carcinogens lack DNA reactivity in sensitive cell culture systems such as the Ames test. It has been proposed that active oxygen radicals, produced as a result of peroxisomal proliferation, induce DNA damage. While this is an attractive hypothesis, no genotoxic activity has been observed in hepatocytes with peroxisomal proliferation in treated animals. Another biological activity shared by this class of compounds is their ability to stimulate liver growth or hyperplasia. This additive hyperplasia results from direct mitogenic stimulation rather than regenerative growth following liver toxicity. This hyperplasia can be dramatic, with liver to body weight ratios from treated animals reaching two to three times normal. The degree of induced hyperplasia correlates well with the carcinogenic potency of these agents, whereas genotoxicity does not correlate at all. Increased cellular growth may result in spontaneous mutational events or promotional effects. While some feedback mechanism eventually inhibits liver growth, it is possible that key genes related to the regulation of cellular growth and cancer remain stimulated during continued administration of the chemical. Thus, determination of hyperplastic activity represents an attractive first-step approach to the short-term detection and study of the mode of action of nongenotoxic carcinogens.     

Estimates of the proportion of chemicals that were carcinogenic or anticarcinogenic in bioassays conducted by the National Toxicology Program.

Estimates were made of the proportion of chemicals that were carcinogenic, anticarcinogenic, or either in 397 long-term bioassays conducted by the National Toxicology Program (NTP). The estimates were obtained from the global pattern of p-values obtained from statistical tests applied to individual experiments. These tests accounted for multiple comparisons using a randomization procedure and were found to operate at the correct level of significance. Representative estimates of the proportion of carcinogens [with 90% confidence intervals (CI)] compared to the NTP estimates were as follows: male mice, 0.32 (CI, 0.19-0.44), NTP = 0.29; female mice, 0. 28 (CI, 0.15-0.41), NTP = 0.34; male rats, 0.35 (CI, 0.23-0.47), NTP = 0.36; female rats, 0.34 (CI, 0.21-0.46), NTP = 0.28; all sexes and species, 0.59 (CI, 0.49-0.69), NTP = 0.51. Representative estimates of the proportion of anticarcinogens were as follows: male mice, 0. 34; female mice, 0.27; male rats, 0.40; female rats, 0.44; all sexes and species, 0.66. Thus, there was as much or more evidence in this study for anticarcinogenesis as carcinogenesis. Even though the estimators used were negatively biased, it was estimated that 85% of the chemicals were either carcinogenic or anticarcinogenic at some site in some sex-species group. This suggests that most chemicals given at high enough doses will cause some sort of perturbation in tumor rates.     

某自来水厂各处理单元出水中有机物的致突变性研究

目的检测和评价某市某自来水厂各处理单元出水中有机物的遗传毒性。方法通过鼠伤寒沙门菌致突变试验（Ames test）检测该自来水厂各处理单元出水的致突变性及微量波动试验（“microtitre”fluctuation test）检测自来水厂某些单元出水的致突变性,并对两种生物检测方法进行了比较。结果自来水厂各处理单元出水中有机物的Ames试验结果为阳性;微量波动试验比Ames试验对TA98,TA100出现阳性结果时提取物的剂量更低。结论某市自来水各处理单元出水中的有机物具有致突变作用;微量波动试验可提高对水中有机物致突变性检测的灵敏度。     

Anomalous nonidentity between Salmonella genotoxicants and rodent carcinogens: nongenotoxic carcinogens and genotoxic noncarcinogens.

According to current data, the capacity to cause nonprogrammed or unscheduled cell proliferation in target tissues, a common characteristic of chemical carcinogens, may play a more important role in the development of tumors than does genotoxicity. This paper provides strong support for the validity of this conclusion. Ames-negative nongenotoxicants may be considered to be carcinogenic primarily because of their ability to induce cell proliferation in animal tissues and organs. In addition, such nongenotoxic carcinogens may also provide latent and modest DNA (equivocal) modifications that never lead to Ames-positive events. Conversely, noncarcinogenesis by Ames-positive agents is likely to be linked to a lack of stimulation of cell division. Nongenotoxic and genotoxic carcinogens rely on both cell proliferation and equivocal DNA modification for their full carcinogenicity. Such equivocal DNA modifications do not appear to be formed by tumor promoters. The role of cell proliferation may provide a favorable milieu for the occurrence of genetic instability, give rise to selective "apoptosis-resistant abnormal cells," and then affect clonal expansion of these cells. Therefore, understanding the influence of nongenotoxic and genotoxic carcinogens on cell proliferation capability is a key point in determining the mechanisms of chemical carcinogenesis. Considering the contradictory and common features of genotoxicants and carcinogens, early detection of nonprogrammed cell proliferation is the most effective approach to predict human and rodent carcinogenicity.     

Soil contamination detected using bacterial and plant mutagenicity tests and chemical analyses

Soil contaminants are common in industrialized countries, causing widespread contamination directly of soil and indirectly of ground water and food. Among these pollutants particular attention should be paid to soil mutagens and carcinogens due to their potentially hazardous effects on animal populations and human health. The aim of this research was to evaluate the genotoxicity of contaminated soils by means of an integrated chemical/biological approach, using a short-term bacterial mutagenicity test (Ames test), a plant genotoxicity test (Tradescantia/micronucleus test), and chemical analyses. Soil samples were collected in a highly industrialized area in the Lombardy region, in Northern Italy. Soil samples were extracted with water or with organic solvents. Water extracts of soil samples were tested using the Tradescantia genotoxicity test and organic solvent extracts were analyzed for their polycyclic aromatic hydrocarbon (PAH) concentrations and for their mutagenicity with the Ames test. Heavy metal concentrations were also studied. Some soil samples showed mutagenic activity with the Ames test and clastogenicity with the Tradescantia/micronucleus test. The same soils showed high concentrations of genotoxic PAH and heavy metals.     

The significance of mouse liver tumor formation for carcinogenic risk assessment: results and conclusions from a survey of ten years of testing by the agrochemical industry.

A survey was performed on the results of 138 carcinogenicity studies conducted in various mouse strains by the agrochemical industry over the period 1983-1993. Data for liver tumor incidence, liver weight, and histopathology were collected along with data on genotoxicity. Studies were judged positive or negative for liver tumor formation on the basis of apparent dose response, malignancy, and difference from historical control values using a weight of evidence approach. Thirty-seven studies were judged to be positive for liver tumorigenicity in one or both sexes. There was no evidence showing an influence of the mouse strain and the duration of the study on the proportion of positive studies. Although 8 of the chemicals tested in the 138 studies were positive in the Ames test, only one of these was judged positive for carcinogenicity. Only 6 of the 37 positive chemicals had any other reported positive genotoxicity findings. A clear relationship between hepatomegaly at 1 year after exposure and a positive tumorigenic outcome at 18 months or 2 years after exposure was demonstrated. Whereas the average relative liver weight of top dose animals was 110% of control in negative studies, it was 150% in positive studies. Likewise, very few negative studies demonstrated significant pathological findings after 1 year, whereas the majority of positive studies had significant liver pathology. The implications of these findings for extrapolation to humans are discussed.     

Identifying chemical carcinogens and assessing potential risk in short-term bioassays using transgenic mouse models.

Cancer is a worldwide public health concern. Identifying carcinogens and limiting their exposure is one approach to the problem of reducing risk. Currently, epidemiology and rodent bioassays are the means by which putative human carcinogens are identified. Both methods have intrinsic limitations: they are slow and expensive processes with many uncertainties. The development of methods to modify specific genes in the mammalian genome has provided promising new tools for identifying carcinogens and characterizing risk. Transgenic mice may provide advantages in shortening the time required for bioassays and improving the accuracy of carcinogen identification; transgenic mice might now be included in the testing armamentarium without abandoning the two-year bioassay, the current standard. We show that mutagenic carcinogens can be identified with increased sensitivity and specificity using hemizygous p53 mice in which one allele of the p53 gene has been inactivated. Furthermore, the TG.AC transgenic model, carrying a v-Ha-ras construct, has developed papillomas and malignant tumors in response to a number of mutagenic and nonmutagenic carcinogens and tumor promoters, but not to noncarcinogens. We present a decision-tree approach that permits, at modest extra cost, the testing of more chemicals with improved ability to extrapolate from rodents to humans.     

Comparative activity of human carcinogens and NTP rodent carcinogens in the mouse bone marrow micronucleus assay: an integrative approach to genetic toxicity data assessment.

The mouse bone marrow micronucleus (MN) assay holds a key position in all schemes for detecting potential human carcinogens and mutagens. It was therefore of concern when Shelby et al. reported that only 5 of 25 rodent carcinogens defined by the U.S. NTP were positive in the assay. Further, each of these positive responses was weak and indistinguishable from the 4 positive responses observed among the 24 NTP noncarcinogens tested. To focus these findings, the activity in the MN assay of 26 human carcinogens, 6 reference rodent genotoxins, and the 9 NTP chemicals positive in the MN assay have been displayed in a common format. This involved plotting the minimum positive dose level (expressed as mumole/kilogram) and the maximum fold-increase in micronucleated polychromatic erythrocytes frequency observed at any dose level. By displaying the high sensitivity of the micronucleus assay to the reference human and rodent genotoxins, this analysis emphasizes the weakness in the MN assay responses given by the NTP carcinogens reported by Shelby et al. This, in turn, poses questions about the intrinsic hazard of this selection of NTP rodent carcinogens. Using fotemustine and vitamin C as models of a toxic and a nontoxic chemical known to be active in the MN assay, this analysis describes a method by which their relative potential human hazard can be distinguished (a synthetic, as opposed to an analytical approach to data assessment). The possibility that some weak responses observed in the MN assay at elevated dose levels may be stress induced is considered.     

An assessment of the genotoxic effects of landfill leachates using bacterial and plant tests

Two bacterial tests (the Ames test and the umu-test) and the Allium test were used to assess the genotoxicity of aqueous leachates from municipal solid waste landfill sites in Southern Poland. A comparison of the sensitivity of the applied tests was performed. None of the tested samples revealed genotoxic activity in the umu-test and thus did not appear to be sensitive enough for evaluations of leachates. Two out of 22 leachates were described as genotoxic in the Ames test and 3 out of 6 leachates in the Allium test. All of the analyzed leachates samples affected cell divisions. Two of the tests applied, the Allium and Ames test, revealed the high genotoxicity of leachate 4. Among the bioassays used in these studies, the Allium test proved to be more sensitive than bacterial tests for the investigation of leachate toxicity. The results suggest that the Allium cytogenetic bioassay is efficient and simple for genotoxicity studies of leachates. The potential correlations between the chemical characteristics and genotoxic effects is discussed. The biological effects of selected appeared to be related to chemical parameters. Leachates have a genotoxic potential and pose a risk to human health and the environment. A combination of biotests and chemical analyses is the best approach for the assessment of the risk or impact of leachates.     

自来水痕量有机污染物检测及致突变性研究

目的:在对自来水中痕量有机污染物检测的基础上研究其对人体健康的潜在危害。方法:采集某市3个自来水厂出厂水(A、B、C)和对照水样(D),XAD-2树脂富集自来水样中的痕量有机物,气相色谱/质谱(GC/MS)联用法检测分析;鼠伤寒沙门氏菌回复突变试验(Am es试验)检测其致突变性。结果:在选定的条件下,本次试验累计检出痕量有机物335种,其中EPA(美国环保署)优先污染物10余种,致癌、致畸、致突变20余种。A水厂出厂水在第一次Ames试验中TA98和TA100在不加S9时呈阳性(MR>2);A、B、C水厂出厂水在第二次试验中呈可疑阳性(±),存在剂量反应关系。对照水样Ames试验结果为阴性。结论:GC/MS联用分析、Ames试验联合运用是研究水中有机污染物致突变性的有效方法。     

Mammary gland neoplasia in long-term rodent studies.

Breast cancer, the most frequent spontaneous malignancy diagnosed in women in the western world, is continuously increasing in incidence in industrialized nations. Although breast cancer develops in women as the result of a combination of external and endogenous factors such as exposure to ionizing radiation, diet, socioeconomic status, and endocrinologic, familial, or genetic factors, no specific etiologic agent(s) or the mechanisms responsible of the disease has been identified as yet. Thus, experimental models that exhibit the same complex interactions are needed for testing various mechanisms and for assessing the carcinogenic potential of given chemicals. Rodent mammary carcinomas represent such a model to a great extent because, in these species, mammary cancer is a multistep complex process that can be induced by either chemicals, radiation, viruses, or genetic factors. Long-term studies in rodent models have been particularly useful for dissecting the initiation, promotion, and progression steps of carcinogenesis. The susceptibility of the rodent mammary gland to develop neoplasms has made this organ a unique target for testing the carcinogenic potential of specific genotoxic chemicals and environmental agents. Mammary tumors induced by indirect- or direct-acting carcinogens such as 7, 12-dimethlbenz(a)anthracene or N-methyl-N-nitrosourea are, in general, hormone dependent adenocarcinomas whose incidence, number of tumors per animal, tumor latency, and tumor type are influenced by the age, reproductive history, and endocarinologic milieu of the host at the time of carcinogen exposure. Rodent models are informative in the absence of human data. They have provided valuable information on the dose and route of administration to be used and optimal host conditions for eliciting maximal tumorigenic response. Studies of the influence of normal gland development on the pathogenesis of chemically induced mammary carcinomas have clarified the role of differentiation in cancer initiation. Comparative studies with the development of the human breast and the pathogenesis of breast cancer have contributed to validate rodent-to-human extrapolations. However, it has not been definitively established what type of information is necessary for human risk assessment, whether currently toxicity testing methodologies are sufficient for fulfilling those needs, or whether treatment-induced tumorigenic responses in rodents are predictive of potential human risk. An alternative to the traditional bioassays are mechanism-based toxicology and molecular and cellular approaches, combined with comparative in vitro systems. These approaches might allow the rapid screen of chemicals for setting priorities for further studies to determine the dose-response relationship for chemical effects at low doses, to assess effects other than mutagenesis and/or tumorigenesis, or to establish qualitative and quantitative relationships of biomarkers to toxic effects. Until there is enough information on the predictive value of mechanism-based toxicology for risk assessment, this approach should be used in conjunction with and validated by the traditional in vivo long-term bioassays.     

Evaluation of genotoxicity of sodium hypochlorite, chlorine dioxide and peracetic acid using plant tests

The aim of this research was to evaluate the potential genotoxic effects of three drinking water disinfectants by means of in vivo short-term mutagenicity tests using plants. The study was carried out in laboratory using distilled water disinfected with sodium hypochlorite (NaClO), chlorine dioxide (ClO2) and peracetic acid (PAA) at different concentrations both in neutral and acid conditions. Untreated distilled water was used as a negative control. Micronuclei test in Tradescantia pollen cells and chromosomal aberration test in root cells of Allium cepa were the bioassays performed by exposing directly plant bioindicators to treated and untreated distilled water. The Tradescantia/micronuclei test gave positive results in most of the ClO2-treated water samples but only at acid pH. The Allium cepa test showed genotoxicity in NaClO-treated samples at acid pH and in a ClO2-treated sample at pH 7. PAA-treated samples were always nongenotoxic. Since the concentrations tested of free disinfectants are usually present in drinking water for biocidal purposes, genotoxicity of these compounds could be a public health problem.     

Influence of alkylphenols and trace elements in toxic, genotoxic, and endocrine disruption activity of wastewater treatment plants

Toxicity and endocrine interference of influent and effluent waters from domestic and industrial wastewater treatment plants were determined. In addition, chemical analyses were performed to detect the presence of 17beta-estradiol, 17alpha-ethinyl estradiol, nonylphenol, 4-octylphenol, and p-t-octylphenol as well as lead, copper, and cadmium in these matrices. The results showed that despite low acute toxic potential, most of the samples tested showed both genotoxicity and endocrine interference. Furthermore, to establish whether the observed effects were caused by the alkylphenols and the heavy metals detected, toxic, genotoxic, and endocrine interference tests also were performed on pure chemicals. The acute toxicity was measured on the crustacean Daphnia magna. The estrogenic activity was determined by using the yeast estrogen screen with Saccharomyces cerevisiae RMY326, whereas the SOS Chromotest and Ames test detected the genotoxicity on Escherichia coli PQ37 and Salmonella typhimurium TA98 and TA100, respectively. The results showed that the toxicity found in the matrices did not match the values found for pure chemicals, but a clear correlation was found between alkylphenols and genotoxicity. Both heavy metals and alkylphenols took part in the endocrine interference activity.     

Regulation of priority carcinogens and reproductive or developmental toxicants.

In California, 370 carcinogens and 112 reproductive/developmental toxicants have been identified as a result of the State's Safe Drinking Water and Toxic Enforcement Act of 1986. They include pesticides, solvents, metals, industrial intermediates, environmental mixtures, and reactive agents. Occupational, environmental, and consumer product exposures that involve these agents are regulated under the Act. At levels of concern, businesses must provide warnings for and limit discharges of those chemicals. The lists of chemicals were compiled following systematic review of published data, including technical reports from the U.S. Public Health Service--National Toxicology Program (NTP), and evaluation of recommendations from authoritative bodies such as the International Agency for Research on Cancer (IARC) and the U.S. Environmental Protection Agency (USEPA). Given the large number of chemicals that are carcinogens or reproductive/developmental toxicants, regulatory concerns should focus on those that have high potential for human exposure, e.g., widely distributed or easily absorbed solvents, metals, environmental mixtures, or reactive agents. In this paper, we present a list of 33 potential priority carcinogens and reproductive/developmental toxicants, including alcoholic beverages, asbestos, benzene, chlorinated solvents, formaldehyde, glycol ethers, lead, tobacco smoke, and toluene.     

Relationship between allergic contact dermatitis and electrophilicity.

To evaluate the role of electrophilicity in the induction of allergic contact dermatitis (ACD) in humans, we compared the structure-activity relationship (SAR) model of ACD with those of electrophilic and nonelectrophilic subsets of chemicals in the ACD database. For these analyses, electrophilicity was defined as the potential of a chemical to induce mutations in Salmonella. It was found that electrophilicity accounted for approximately 30-40% of ACD-inducing ability, and the remainder was associated with nonelectrophilic structures. The identification of these moieties opens the possibility for studying their role in ACD.     

Changes in the classification of carcinogenic chemicals in the work area

Carcinogenic chemicals in the work area are currently classified into three categories in section III of the German List of MAK and BAT Values (list of values on maximum workplace concentrations and biological tolerance for occupational exposures). This classification is based on qualitative criteria and reflects essentially the weight of evidence available for judging the carcinogenic potential of the chemicals. It is proposed that these categories – IIIA1, IIIA2, IIIB – be retained as Categories 1, 2, and 3, to correspond with European Union regulations. On the basis of our advancing knowledge of reaction mechanisms and the potency of carcinogens, these three categories are supplemented with two additional categories. The essential feature of substances classified in the new categories is that exposure to these chemicals does not contribute significantly to risk of cancer to man, provided that an appropriate exposure limit (MAK value) is observed. Chemicals known to act typically by nongenotoxic mechanisms and for which information is available that allows evaluation of the effects of low-dose exposures, are classified in Category 4. Genotoxic chemicals for which low carcinogenic potency can be expected on the basis of dose-response relationships and toxicokinetics, and for which risk at low doses can be assessed are classified in Category 5. The basis for a better differentiation of carcinogens is discussed, the new categories are defined, and possible criteria for classification are described. Examples for Category 4 (1,4-dioxane) and Category 5 (styrene) are presented.