The impact of toxicity on carcinogenicity studies: implications for risk assessment

Occurrence and prioritization of non-volatile substances in recycled PET flakes produced in China

Recycled PET (rPET) is gaining popularity for use in the production of new food contact materials (FCMs) under the context of circular economy. However, the limited information on contaminants in rPET from China and concerns about their potential risk are major obstacles to their use in FCM in China. Fifty-five non-volatile compounds were tentatively identified in 126 batches of hot-washed rPET flakes aimed for food packaging applications in China. Although the 55 substances are not necessarily migratable and may not end up in the contacting media, their presence indicates a need for proper management and control across the value chain. For this reason, the 55 substances prioritized on the basis of level of concerns and in-silico genotoxicity profiler. Among them, dimethoxyethyl phthalate, dibutyl phthalate, bis(2-ethylhexyl) phthalate were classified as level V substances, and Michler's ketone and 4-nitrophenol were both categorized as level V substances and had the genotoxic structure alert, while 2,4,5-trimethylaniline was specified with genotoxic structure alert. The above substances have high priority and may pose a potential risk to human health, therefore special attention should be paid to their migration from rPET. Aside from providing valuable information on non-volatile contaminants present in hot-washed rPET flakes coming from China, this article proposed a prioritization workflow that can be of great help to identify priority substances deserving special attention across the value chain.

Persistent organic and inorganic pollutants in the effluents from the textile dyeing industries: Ecotoxicology appraisal via a battery of biotests

Today, the textile industry is considered as a leading economic sector in Tunisia. However, this sector demands a huge volume of water and a wide spectrum of chemicals, which is converted into potentially toxic wastewater leading to environmental perturbation and human health toxicity. Assessment of the environmental risks associated with textile wastewater becomes a necessity. In this study, textile dyeing wastewater samples were collected before and after the physico-chemical treatment carried out by textile companies located in Monastir-city-Tunisia and subjected to chemical analyzes in order to determine their physicochemical characteristics and the content of metals and textile dyes. The ecotoxicological assessment was performed using four organisms, namely Selenastrum capricornutum, Vibrio fischeri, Daphnia magna and Lepidium sativum, to represent different trophic levels. Based on chemical data, some physicochemical parameters (e.g. TSS, COD and TSS levels) and metals (e.g. Cr, Hg and Sb) in the textile dyeing effluents were revealed not in compliance with the Tunisian standard. Moreover, high quantities of three disperse dyes have been detected even in the textile dyeing wastewater samples before and after treatments. The ecotoxicological data confirmed that the textile dyeing influents displayed toxic effects to all the test organisms, with Selenastrum capricornutum being the most sensitive organism. While, the above toxic effects were decreased slightly when evaluating the treated effluents. Metals and textile disperse dyes could be associated with the observed toxic effects of the textile influents and effluents. In fact, the treatment process applied by the evaluated companies was only partially efficient at removing metals, disperse dyes and effluent ecotoxicity, suggesting potential risks to aquatic biota. These findings emphasize the importance of applying integrated chemical and biological approaches for continuous evaluation of the toxicity of the treated effluents to predict hazards on the environment.

Ecotoxicity profile of heavily contaminated surface water of two rivers in Tunisia

Persistent organic and inorganic contaminants generated by industrial effluent wastes poses a threat to the maintenance of aquatic ecosystems and public health. The Khniss and Hamdoun rivers, located in the central-east of Tunisia, receive regularly domestic and textile wastewater load. The present study aimed to survey the water quality of these rivers using physicochemical, analytical and toxicological approaches. In the physicochemical analysis, the recorded levels of COD and TSS in both samples exceed the Tunisian standards. Using the analytical approach, several metals and some textile dyes were detected. Indeed, 17 metals were detected in both samples in varying concentrations, which do not exceed the Tunisian standards. The sources of metals pollution can be of natural and anthropogenic origin. Three textile disperse dyes were detected with high levels compared to other studies: the disperse orange 37 was detected in the Khniss river with a concentration of 6.438 μg/L and the disperse red 1 and the disperse yellow 3 were detected in the Hamdoun river with concentrations of 3.873 μg/L and 1895 μg/L, respectively. Textile activities were the major sources of disperse dyes. For both samples, acute and chronic ecotoxicity was observed in all the studied organisms, however, no genotoxic activity was detected. The presence of metals and textile disperse dyes could be associated with the ecotoxicological effects observed in the river waters, in particular due to the industrial activity, a fact that could deteriorate the ecosystem and therefore threaten the human health of the population living in the study areas. Combining chemical and biological approaches, allowed the detection of water ecotoxicity in testing organisms and the identification of possible contributors to the toxicity observed in these multi-stressed water reservoirs.

A case example of a radiation-relevant adverse outcome pathway to lung cancer

BACKGROUND

Adverse outcome pathways (AOPs) describe how a measurable sequence of key events, beginning from a molecular initiator, can lead to an adverse outcome of relevance to risk assessment. An AOP is modular by design, comprised of four main components: (1) a Molecular Initiating Event (MIE), (2) Key Events (KEs), (3) Key Event Relationships (KERs) and (4) an Adverse Outcome (AO).

PURPOSE

Here, we illustrate the utility of the AOP concept through a case example in the field of ionizing radiation, using the Organisation for Economic Cooperation and Development (OECD) Users' Handbook. This AOP defines a classic targeted response to a radiation insult with an AO of lung cancer that is relevant to radon gas exposure.

MATERIALS AND METHODS

To build this AOP, over 500 papers were reviewed and categorized based on the modified Bradford-Hill Criteria. Data-rich key events from the MIE, to several measurable KEs and KERs related to DNA damage response/repair were identified.

RESULTS

The components for this AOP begin with direct deposition of energy as the MIE. Energy deposited into a cell can lead to multiple ionization events to targets such as DNA. This energy can damage DNA leading to double-strand breaks (DSBs) (KE1), this will initiate repair activation (KE2) in higher eukaryotes through mechanisms that are quick and efficient, but error-prone. If DSBs occur in regions of the DNA transcribing critical genes, then mutations (KE3) generated through faulty repair may alter the function of these genes or may cause chromosomal aberrations (KE4). This can impact cellular pathways such as cell growth, cell cycling and then cellular proliferation (KE5). This will form hyperplasia in lung cells, leading eventually to lung cancer (AO) induction and metastasis. The weight of evidence for the KERs was built using biological plausibility, incidence concordance, dose-response, time-response and essentiality studies. The uncertainties and inconsistencies surrounding this AOP are centered on dose-response relationships associated with dose, dose-rates and radiation quality.

CONCLUSION

Overall, the AOP framework provided an effective means to organize the scientific knowledge surrounding the KERs and identify those with strong dose-response relationships and those with inconsistencies. This case study is an example of how the AOP methodology can be applied to sources of radiation to help support areas of risk assessment.

The impact of dose rate on the linear no threshold hypothesis

The goal of this manuscript is to define the role of dose rate and dose protraction on the induction of biological changes at all levels of biological organization. Both total dose and the time frame over which it is delivered are important as the body has great capacity to repair all types of biological damage. The importance of dose rate has been recognized almost from the time that radiation was discovered and has been included in radiation standards as a Dose, Dose Rate, Effectiveness Factor (DDREF) and a Dose Rate Effectiveness Factor (DREF). This manuscript will evaluate the role of dose rate at the molecular, cellular, tissue, experimental animals and humans to demonstrate that dose rate is an important variable in estimating radiation cancer risk and other biological effects. The impact of low-dose rates on the Linear-No-Threshold Hypothesis (LNTH) will be reviewed since if the LNTH is not valid it is not possible to calculate a single value for a DDREF or DREF. Finally, extensive human experience is briefly reviewed to show that the radiation risks are not underestimated and that radiation at environmental levels has limited impact on total human cancer risk.

A review of the genotoxic, mutagenic, and carcinogenic potentials of several lower acrylates

Lower alkyl acrylate monomers include methyl-, ethyl-, n-butyl-, and 2-ethylhexyl acrylate. These acrylates are used in the manufacture of acrylic polymers and copolymers for plastics, food packaging, adhesives, and cosmetic formulations. Although there is limited potential for human environmental exposure, occupational exposure can occur via inhalation and dermal contact. Recently, new genotoxicity data have been generated, along with in silico and in vitro read-cross analyses, for these acrylates. The availability of high-throughput screening (HTS) data through the ToxCast™/Tox21 databases allows for consideration of computational toxicology and organization of these data according to the ten key characteristics of carcinogens. Therefore, we conducted a comprehensive review to evaluate the mechanistic, toxicokinetic, animal, and human data, including HTS data, for characterizing the potential carcinogenicity, mutagenicity, and genotoxicity of these acrylates. Toxicokinetic data demonstrate that these acrylates are metabolized rapidly by carboxylesterase hydrolysis and conjugation with glutathione. HTS data demonstrated an overall lack of bioactivity in cancer-related pathways. Overall, the genotoxicity and mutagenicity data support a cytotoxic, non-genotoxic mechanism for these acrylates. Cancer bioassay studies conducted by the oral, dermal, and inhalation routes in animal models with these acrylates did not show any increase in tumor incidence, with two exceptions. At high doses, and secondary to chronic site-of-contact irritation and corrosion, rodent forestomach tumors were induced by oral gavage dosing with ethyl acrylate, and skin tumors were observed following chronic dermal dosing with 2-ethylhexyl acrylate in C3H/HeJ inbred mice (a strain with deficiencies in wound healing), but not in the outbred NMRI strain. For both dermal and forestomach cancers, tumorigenesis is secondary to high doses and long-term tissue damage, shown to be reversible. With evidence that these chemicals are not genotoxic, and that they cause forestomach and dermal tumors through chronic irritation and regenerative proliferation mechanisms, these acrylates are unlikely to pose a human cancer hazard.

Two-Year Toxicity Study of Doxylamine Succinate in B6C3F1 Mice

Doxylamine succinate, a commonly used antihistamine, was administered as an admixture in the feed to groups of male and female B6C3F1 mice at dosage levels of 0,190,375, and 750 parts per million (ppm) (based on free amine) for 65 weeks (12 per group) or 2 years (48 per group). Survival to terminal sacrifice in the 2-year groups was 85–98% with no significant differences between groups of the same sex. Final body weights of the highest dose group were 3.4% and 8.7% less than controls in males and females, respectively. Doxylamine produced liver lesions in male mice including hepatocellular hypertrophy, atypical hepatocytes, clear cell and mixed cell foci, and necrosis. In females, doxylamine produced liver fatty change, hepatocellular hypertrophy, and necrosis. Doxylamine produced a significant increase in hepatocellular adenomas in the mid- and high-dosage groups of males and in the high-dosage group of females. Thyroid follicular cell hyperplasia and thyroid follicular cell adenomas also were increased in treated mice of both sexes. A treatment-related increase in cytoplasmic alteration of the parotid salivary gland in males and an increased incidence in hyperplasia of the pituitary gland in females were observed.

Chemically exacerbated chronic progressive nephropathy not associated with renal tubular tumor induction in rats: an evaluation based on 60 carcinogenicity studies by the national toxicology program

Chronic progressive nephropathy (CPN) is a common age-related degenerative-regenerative disease of the kidney that occurs in both sexes of most strains of rats. Recently, claims have been made that enhanced CPN is a mode of action for chemically induced kidney tumors in male rats and that renal tubular tumors (RTTs) induced by chemicals that concomitantly exacerbate CPN are not relevant for human cancer risk assessments. Although CPN is an observable histopathological lesion that may be modified by diet, the etiology of this disease and the mechanisms for its exacerbation by chemicals are unknown, and it fails to meet fundamental principles for defining carcinogenic modes of action and human relevance. Our comprehensive evaluation of possible relationships between exacerbated CPN and induction of RTTs in 58 carcinogenicity studies, conducted by the National Toxicology Program, in male and 11 studies in female F344 rats using 60 chemicals revealed widespread inconsistency in the claimed association. Because the proposed hypothesis lacks evidence of biological plausibility, and due to inconsistent relationships between exacerbated CPN and kidney tumor incidence in carcinogenicity studies in rats, dismissing the human relevance of kidney tumors induced by chemicals that also exacerbate CPN in rats would be wrong.

Clarifying carcinogenicity of ethylbenzene

Ethylbenzene has been evaluated for carcinogenic activity in Fischer rats and B6C3F1 mice exposed by inhalation (Chan et al., 1998; Chan, 1999) and in Sprague-Dawley rats after oral exposure (Maltoni et al., 1985,1997). Bioassay findings are summarized below to expand on those not stated clearly or completely in Saghir et al. (2010). Overall in these three studies animals exposed to ethylbenzene had increased tumors in rats for kidneys, testes, head (including rare neuroesthesioepitheliomas), and total malignant tumors, whilst in mice tumor incidences were increased in the lung and liver (Huff, 2002). Thus ethylbenzene was carcinogenic by two exposure routes to both sexes of two species of rodents, two strains of rats, and one strain of mice, causing collectively tumors in five different target organs and a composite of "total malignant" tumors.

Hepatic Enzyme Induction

Hepatic enzyme induction is generally an adaptive response associated with increases in liver weight, induction of gene expression, and morphological changes in hepatocytes. The additive growth and functional demands that initiated the response to hepatic enzyme induction cover a wide range of stimuli including pregnancy and lactation, hormonal fluctuations, dietary constituents, infections associated with acute-phase proteins, as well as responses to exposure to xenobiotics. Common xenobiotic enzyme inducers trigger pathways involving the constitutive androstane receptor (CAR), the peroxisome proliferator-activated receptor (PPAR), the aryl hydrocarbon receptor (AhR), and the pregnane-X-receptor (PXR). Liver enlargement in response to hepatic enzyme induction is typically associated with hepatocellular hypertrophy and often, transient hepatocyte hyperplasia. The hypertrophy may show a lobular distribution, with the pattern of lobular zonation and severity reflecting species, strain, and sex differences in addition to effects from specific xenobiotics. Toxicity and hepatocarcinogenicity may occur when liver responses exceed adaptive changes or induced enzymes generate toxic metabolites. These undesirable consequences are influenced by the type and dose of xenobiotic and show considerable species differences in susceptibility and severity that need to be understood for assessing the potential effects on human health from similar exposures to specific xenobiotics.

Aromatic Amines in Experimental Cancer Research: Tissue-Specific Effects, an Old Problem and New Solutions

Carcinogenic aromatic amines usually produce tumors in specific target tissue, such as 2-acetylaminofluorene (AAF) producing liver tumors in rats, in contrast to some other structurally related arylamines. A hypothesis is presented that explains the mode of action in this rat liver model. Genotoxic and nongenotoxic effects work together and make AAF a complete rat liver carcinogen. The cytotoxic, promoting effects are particularly important. N-Hydroxy-2-aminofluorene and 2-nitrosofluorene, two metabolites of AAF, are able to uncouple the mitochondrial respiratory chain. They entertain a redox cycle that removes electrons from the respiratory chain and impairs ATP production. The dose-dependent opening of the mitochondrial permeability transition pore signals the viability of the cell. If the pore is opened to a certain extent, the cell is eliminated by apoptosis. As a consequence, oval cells proliferate, and as this process is overloaded, the liver transforms into a cirrhosis-like situation and thus provides the conditions under which initiated liver cells develop tumors. Such an interpretation is based on assumptions that have been debated for a long time. Some of these often forgotten developments are reviewed in support of the hypothesis, which allows a more comprehensive view of the complex in vivo situation at a time when in vitro models prevail.

Comparative QSTR studies for predicting mutagenicity of nitro compounds

Mutagenicity and carcinogenicity are toxicological endpoints which pose a great concern being the major determinants of cancers and tumours. Nitroarenes possess genotoxic properties as they can form various electrophilic intermediates and adducts with biological systems. Different QSTR techniques were employed to develop models for the prediction of mutagenicity of nitroarenes using a diverse set of 197 nitro aromatic and hetero aromatic molecules. The 2D and 3D QSTR methods used for model development gave statistically significant results. The alignment for 3D methods was obtained by maximum common substructures (MCS) approach, by taking the most mutagenic molecule of the dataset as the template. All the QSTR models were developed with the same set of training and test set molecules. The 3D contours and 2D contribution maps along with molecular fingerprints provide useful information about the mutagenic potentials of the molecules. The GFA based model shows thermodynamic and topological descriptors play an important role in characterizing mutagenicity of nitroarenes. Atomic-level thermodynamic descriptor namely AlogP throws light on hydrophobic features and helps to understand the bilinear model. Topological aspects of these classes of compounds were depicted by the fragment fingerprints and Balaban indices obtained from HQSAR and GFA models, respectively. The predictive abilities of 2D and 3D QSTR models may be useful as a vibrant predictive tool to screen out mutagenic nitroarenes and design safer non-mutagenic nitro compounds.

Benzene-induced cancers: abridged history and occupational health impact

Benzene-induced cancer in humans was first reported in the late 1920s. Carcinogenesis findings in animals were not reported conclusively until 1979. Industry exploited this "discrepancy" to discredit the use of animal bioassays as surrogates for human exposure experience. The cardinal reason for the delay between first recognizing leukemia in humans and sought-after neoplasia in animals centers on poor design and conduct of experimental studies. The first evidence of carcinogenicity in animals manifested as malignant tumors of the zymbal glands (sebaceous glands in the ear canal) of rats, and industry attempted to discount this as being irrelevant to humans, as this organ is vestigial and not present per se in humans. Nonetheless, shortly thereafter benzene was shown to be carcinogenic to multiple organ sites in both sexes of multiple strains and multiple species of laboratory animals exposed via various routes. This paper presents a condensed history of the benzene bioassay story with mention of benzene-associated human cancers.

The biological sense of cancer: a hypothesis

BACKGROUND

Most theories about cancer proposed during the last century share a common denominator: cancer is believed to be a biological nonsense for the organism in which it originates, since cancer cells are believed to be ones evading the rules that control normal cell proliferation and differentiation. In this essay, we have challenged this interpretation on the basis that, throughout the animal kingdom, cancer seems to arise only in injured organs and tissues that display lost or diminished regenerative ability.

HYPOTHESIS

According to our hypothesis, a tumor cell would be the only one able to respond to the demand to proliferate in the organ of origin. It would be surrounded by "normal" aged cells that cannot respond to that signal. According to this interpretation, cancer would have a profound biological sense: it would be the ultimate way to attempt to restore organ functions and structures that have been lost or altered by aging or noxious environmental agents. In this way, the features commonly associated with tumor cells could be reinterpreted as progressively acquired adaptations for responding to a permanent regenerative signal in the context of tissue injury. Analogously, several embryo developmental stages could be dependent on cellular damage and death, which together disrupt the field topography. However, unlike normal structures, cancer would have no physiological value, because the usually poor or non-functional nature of its cells would make their reparative task unattainable.

CONCLUSION

The hypothesis advanced in this essay might have significant practical implications. All conventional therapies against cancer attempt to kill all cancer cells. However, according to our hypothesis, the problem might not be solved even if all the tumor cells were eradicated. In effect, if the organ failure remained, new tumor cells would emerge and the tumor would reinitiate its progressive growth in response to the permanent regenerative signal of the non-restored organ. Therefore, efficient anti-cancer therapy should combine an attack against the tumor cells themselves with the correction of the organ failure, which, according to this hypothesis, is fundamental to the origin of the cancer.

Age-dependent sensitivity of Big Blue transgenic mice to the mutagenicity of N-ethyl-N-nitrosourea (ENU) in liver

The incidence of childhood cancer is increasing and recent evidence suggests an association between childhood cancer and environmental exposure to genotoxins. In the present study, the Big Blue transgenic mouse model was used to determine whether specific periods in early life represent windows of vulnerability to mutation induction by genotoxins in mouse liver. Groups of mice were treated with single doses of 120 mg N-ethyl-N-nitrosourea (ENU)/kg body weight or the vehicle either transplacentally to the 18-day-old fetus or at postnatal days (PNDs) 1, 8, 15, 42 or 126; the animals were sacrificed 6 weeks after their treatment. The cII mutation assay was performed to determine the mutant frequencies (MFs) in the livers of the mice. Liver cII MFs for both sexes were dependent on the age at which the animals were treated. Perinatal treatment with ENU (either transplacental treatment to the 18-day-old fetus or i.p. injection at PND 1) induced relatively high MFs. However, ENU treatment at PNDs 8 and 15 resulted in the highest mutation induction. The lowest mutation induction occurred in those animals treated as adults (PND 126). For instance, the cII MF for the PND 8 female group was 646 x 10(-6) while the MF for female adults was only 145 x 10(-6), a more than 4-fold difference. Molecular analysis of the mutants found that A:T-->T:A transversions and A:T-->G:C transitions characterized the pattern of mutations induced by ENU in both the neonate and adult mice, while the predominate type of mutation in the controls was G:C-->A:T. The results indicate that mouse liver is most sensitive to ENU-induced mutation during infancy. This period correlates well with the age-dependent sensitivity to carcinogenicity in mouse liver, suggesting that mutation is an important rate-limiting factor for age-related carcinogenesis.

Relevance of animal carcinogenesis findings to human cancer predictions and prevention

Use of laboratory animals to identify carcinogenic potential of chemicals, mixtures, and other agents has a modern history of greater than 40 years from which much useful scientific and public health information can be derived. While laboratory animals differ from humans in some respects that may affect responses to hazardous exposures, use of such models is based on experimental evidence indicating that there are more genetic, genomic, physiological, biochemical, and metabolic similarities than differences among mammalian species. Issues of concordance of responses between rodent species and between rodents and humans as well as repeatability and site-specificity are important considerations in evaluating laboratory animal carcinogenicity results. Variables in experimental design such as animal strain, diet, route of exposure, and study, duration as well as single-site versus multisite carcinogenic responses all influence interpretation and intelligent use of study data. Similarities and differences in site-specific laboratory animal and corresponding human cancers should also be considered in study evaluation. Recent attempts to explore genetically engineered mice and to humanize the mouse for more relevant identification of carcinogen hazard identification have yielded mixed results. In the end we are confronted by the realization that virtually all animal cancer models are useful but imperfect surrogates for humans. Assuming the percentage of chemicals currently in commerce that are estimated to be potent animal or human carcinogens is quite low, the task of identifying agents with significant carcinogenic potential is daunting and important. The biological conundrum of scientific debate regarding the relevance of carcinogenicity studies in laboratory animals is likely to continue. Nonetheless public health considerations must take precedence when deciding human safety issues.

Toxicity characterization of environmental chemicals by the US National Toxicology Program: an overview

The US National Toxicology Program (NTP) is an interagency program whose mission is to evaluate agents of public health concern by developing and applying the tools of modern toxicology and molecular biology. Chemicals substances or physical agents selected for toxicology and carcinogenesis evaluations by the NTP are usually studied in a series of subacute (14-day exposure), subchronic (90-day exposure) and chronic (2-year exposure) studies in rodents. The NTP has published more than 500 reports of the findings and conclusions from its toxicology and carcinogenesis studies. In more specialized studies, the NTP also evaluates adverse effects on the structure and function of the immune, reproductive, nervous, and respiratory systems. The program attempts to evaluate and appropriately incorporate new technologies to improve the way we study the toxicity of chemicals. For example, the program has extensively evaluated several transgenic mouse models for their potential use as short-term cancer screens and has been a full participant in an international effort to examine their usefulness in pharmaceutical registration. Toxicogenomics, an emerging scientific field that examines the expression of thousands of genes simultaneously in response to chemical exposure, holds promise for future application to better understand the underlying mechanisms of chemical toxicity. A number of public health issues being addressed by the NTP are not only of national importance but also have global impact, such as the potential for endocrine disruptors to influence development and carcinogenesis and the safety of herbal medicines and dietary supplements. The program participates in the preparation of national and international toxicity testing guidelines and the findings from NTP studies are widely used for risk assessments by international organizations and federal agencies. The NTP maintains databases that contain toxicity, and health and safety information on a large number of chemicals. These databases are available from the NTP web site (http://ntp-server.niehs.nih.gov) and are accessed over 100000 times a month from around the world.

Absence of carcinogenic activity in Fischer rats and B6C3F1 mice following 103-week inhalation exposures to toluene

Toluene, methylbenzene, is used to back-blend gasoline, as a chemical intermediate, and as a solvent; more than 7 million tonnes are produced each year in the United States. Following 14-15-week toxicity studies to estimate appropriate exposure concentrations for the carcinogenesis bioassays, toxicology and carcinogenesis studies of toluene (>99% pure) were conducted by whole-body inhalation exposures of F344/N rats and B6C3F1 mice of each sex for 15 months or two years. Toluene levels were 0 (chamber controls), 600, and 1,200 ppm for rats and 0, 120, 600, and 1200 ppm for mice. Exposures were 6.5 hr/day 5 days/wk. Genetic toxicology studies using Salmonella typhimurium, mouse L5178Y lymphoma cells, and Chinese hamster ovary cells were negative. No chemically related neoplasm was found in male rats, and one nasal, two kidney, and two forestomach neoplasms observed in female rats were considered not to be associated with the toluene exposure. For mice, no biologically important increase was observed for any nonneoplastic or neoplastic lesion. Studies by others had reported carcinogenicity of toluene, especially for total malignant tumors.

Chemicals studied and evaluated in long-term carcinogenesis bioassays by both the Ramazzini Foundation and the National Toxicology Program: in tribute to Cesare Maltoni and David Rall

The Ramazzini Foundation (RF) in Bentivoglio, Italy and the National Toxicology Program (NTP) in Research Triangle Park, North Carolina have carried out several hundred chemical carcinogenesis bioassays: 200 by RF and 500 by NTP. Of these, 21 have been evaluated by both laboratories. The 14 chemicals for which both laboratories have designed, conducted, and reported bioassay results are: acrylonitrile, benzene, chlorine, diesel fuel, ethylbenzene, methylene chloride (dichloromethane), propylene, styrene, styrene oxide, toluene, trichloroethylene, trichlorofluoromethane, vinylidene chloride, and xylenes. The other seven chemicals (two are fibers) were evaluated by both laboratories, but results have not yet been published. Results of these 14 interlaboratory studies were compared both to explore consistency of carcinogenic responses and to identify possible factors that may reveal reasons for any differences observed. Individual carcinogenesis results from each laboratory were duplicated and complementary. Of the 14 chemicals compared, 11 (80%) were either carcinogenic (9 chemicals) or noncarcinogenic (2 chemicals) in both studies. Eight of the paired chemicals had at least one carcinogenic target site in common. The other three were carcinogenic in one laboratory but not in the other. Possible explanations for these differences include dose, method of administration, duration of follow-up, and whether or not total tumors are counted. The collaboration between these two pioneering bioassay laboratory programs contributes greatly to our understanding of chemical carcinogenesis and results in better protection of workers and the general population from chemical diseases, especially cancers.

Development of quantitative structure-activity relationship (QSAR) models for predicting risk of exposure from carcinogens in animals

Quantitative structure-activity relationship (QSAR) models capable of predicting acute toxicity and carcinogen potency of polychlorinated dibenzo-p-dioxin (PCDD), polychlorinated hydrocarbons, and chlorinated insecticides have been formulated. Median lethal dose (LD50) for PCDD-exposed mice correlated negatively with polarity and positively with (H acceptor x 2 chi), whereas LD50 for PCDD-exposed guinea pigs correlated with (H acceptor x density). Both (H acceptor x 2 chi) and (H acceptor x density) exhibited parabolic relationship with log P (partition coefficient). Carcinogenic potency, determined from order of magnitude (OM) values, correlated negatively with log P and positively with (length x width). Thus, a hydrophobic mechanism plays a key role in the lethal effects of PCDD in mice, whereas both hydrophobic and electronic mechanisms are involved in the lethal effects of PCDD in guinea pigs. However, the molecule's lipophilicity, length, and width may play important roles in the carcinogenic effects of chlorinated compounds.

Long-term chemical carcinogenesis bioassays predict human cancer hazards. Issues, controversies, and uncertainties

Long-term carcinogenesis bioassays are the most valued and predictive means for identifying potential carcinogenic hazards of various agents to humans. Agents may be chemicals, chemical mixtures, multiple chemicals, combinations of chemicals, residues and contaminants, commercial products and formulations, and various exposure circumstances. Life-styles, dietary factors, and occupational exposure circumstances are very difficult, but not totally impossible, to evaluate experimentally. Historically, the first chemical bioassay took place in the early part of this century: Yamagiwa and Ichikawa in 1915, showed that coal tar applied experimentally to rabbit ears caused skin carcinomas. Since then, nearly 1500-2000 bioassays of one sort or another have been carried out. Importantly, however, some of these bioassays must be considered inadequate for judging the absence of carcinogenicity, since there were various limitations on the way they were performed: too few animals, too short a duration, too low exposure concentrations, too limited pathology, as examples. Thus, each bioassay must be critically evaluated, especially those reported to be negative, because "false negatives" are certainly more hazardous to human health than are "false positives". Likewise, one must be careful not to discount bioassay results simply because a target organ in rodents may not have a direct counterpart in humans (e.g., Zymbal glands), or because an organ site in rodents may not be a major site of cancers in humans (e.g., mouse liver). The design and conduct of a bioassay is not simple, however, and one must be fully aware of possible pitfalls as well as viable and often necessary alternatives. Similarly, evaluating results and interpreting findings must be approached with the utmost objectivity and consistency. These and other select issues, controversies, and uncertainties possibly encountered in long-term bioassays are covered in this paper. One fact remains abundantly clear: for every known human carcinogen that has been tested adequately in laboratory animals, the findings of carcinogenicity are concordant.

The High Production Volume Chemical Challenge Program : The Rodent LD50 and its Possible Replacement

The High Production Volume Chemical Challenge Program provides an opportunity to re-examine the usefulness and informational value of tests currently used to obtain preliminary hazard identification data. With a view to assessing the mechanistic information provided by the rodent LD50 test and to ascertain the possibility of replacing it with other "more acceptable" assays, we used a recently developed approach to determine the relationship of the LD50 assay to other toxicological protocols. Our analyses indicate that, of the assays examined, the LD50 was significantly related to toxicity in cultured cells and to binding at the Ah receptor.

Profiles of chemically-induced tumors in rodents: quantitative relationships

The rodent carcinogenicity bioassay has been used for several decades for evaluating hundreds of chemicals, with the two aims of better understanding the etiologies of cancer, and of assessing the hazard posed by environmental and industrial chemicals. This has generated an enormous wealth of data and information on the phenomenon of chemical carcinogenicity. However, this information cannot be appreciated easily, since too many details may obscure the general trends present in the data; on the contrary, the use of computerized data analysis techniques suitable for the exploration of large databases makes its investigation much more fruitful, and its results more reliable. For this work, we collected a database of 536 rodent carcinogens, and we investigated the profiles of tumors (target organs) induced in the four experimental systems which are usually employed (rat and mouse, male and female). The analysis was performed with an Artificial Neural Network called Kohonen Self-Organizing Map, which is a computer-intensive method aimed at making the relevant information emerge automatically from the data itself. The analysis generated a global view, as well as a quantitative measure of the associations among the individual tumor types, and among the tumor profiles induced by the chemicals. In the complex interplay between the organ and species specificity of tumor induction, the species specificity generally overcame organ specificity, except for a few tumors (namely Lymphatic System, Brain, Forestomach, Stomach and Thyroid Gland). Moreover, the species specificity was remarkably stronger than the trans-species sex specificity. For three chemical classes (Aromatic Amines, Electrophilic/Alkylating Agents, Nitroarenes) most represented in the database, we investigated the hypothesis that a single mechanism of interaction with DNA would produce one, or a few very similar tumor profiles. Our analysis pointed out that no obvious association exists between chemical/mode of action class, and tumor profile. On the contrary, none of these classes induces a single tumor or pattern of tumors, but rather it appears that each class produces tumors at a wide range of sites. This suggests that an important determinant of the differences in tumor profile are the events that surround the ultimate mechanism of interaction with DNA.

The potential of organ specific toxicity for predicting rodent carcinogenicity

Relationships between organ specific toxicity (specifications of the presence or absence of 43 morphological effects in 32 organs) observed from 13-week subchronic studies and rodent carcinogenicity were investigated by manually measuring the concordance of each feature and also automatically using the RL (Rule Learner) induction program. Of the 32 organs, the presence or absence of any effect in liver or kidney was found very relevant to rodent carcinogenicity. While the concordance of Salmonella genotoxicity with rodent carcinogenicity was only 60%, the battery of liver and kidney was 74% accurate with 75% sensitivity and 71% specificity. Further, using the RL program, rule sets based on organ specific toxicity together with the default predictions based on Salmonella mutagenicity were on average 80% accurate with 83% sensitivity and 82% specificity.

Comparison of contact site cancer potency across dose routes: case study with epichlorohydrin

Risk assessment for airborne carcinogens is often limited by a lack of inhalation bioassay data. While extrapolation from oral-based cancer potency factors may be possible for some agents, this is not considered feasible for contact site carcinogens. The change in contact sites (oral: g.i. tract; inhalation: respiratory tract) when switching dose routes leads to possible differences in tissue sensitivity as well as chemical delivery. This research evaluates the feasibility to extrapolate across dose routes for a contact site carcinogen through a case study with epichlorohydrin (EPI). EPI cancer potency at contact sites is compared across three bioassays involving different dose routes (gavage, drinking water, inhalation) through the use of dosimetry models to adjust for EPI delivery to contact sites. Results indicate a large disparity (two orders of magnitude) in potency across the three routes of administration when expressed as the externally applied dose. However, when expressed as peak delivered dose, inhalation and oral potency estimates are similar and overall, the three potency estimates are within a factor of seven. The results suggest that contact site response to EPI is more dependent upon the rate than the route of delivery, with peak concentration the best way to extrapolate across dose routes. These results cannot be projected to other carcinogens without further study.

Risk assessment and the use of information on underlying biologic mechanisms: a perspective

Recent years have seen the rapid expansion of scientific understanding of the underlying biologic bases of toxic reactions to chemicals. Use of this information in health risk assessment is expanding, but it has yet to reach its full potential. This article considers what has successfully been done, what approaches are now being developed, and what impediments and difficulties have been encountered in attempts to bring case-specific, mechanistic toxicological information to bear on risk estimation. In hazard identification, mechanistic information can help explain the bearing of various empirical experimental results for inferring human hazard, can increase the sensitivity of detection, and can be considered in attempts to replace 2-year animal bioassays with hazard identification methods that rest on identifying key biological properties underlying carcinogenicity rather than relying only on the experimental observation of tumors. In carcinogen potency estimation, mechanistic information can potentially extend relevant observation to lower dose levels, provide the basis for choosing among empirically based dose-response models, lead to potency estimates through relationships with quantitative measures of short-term test outcomes, and can be considered as a basis for providing direct observation of the biological parameters in biologically based dose-response modeling.

Emerging issues in mouse liver carcinogenesis

The mouse liver is the primary target site for carcinogenesis of more than 200 chemicals (including pesticides, food additives, pharmaceuticals, and industrial intermediates) tested in long-term toxicity safety assessment assays. Mouse liver tumors develop through defined morphological stages (similar to those found in other species) whether their origin is of undetermined etiology (spontaneous) or induced by chemicals. The morphologic type of hepatocytes in the various stages of hepatocarcinogenesis is sometimes associated with the specific inducing agent. Liver tumors developing in toxic livers often have more benign appearances and may progress to carcinomas at a slower rate than tumors developing in histologically normal livers. Specific tumors, dependent on the inducing chemical, may regress under defined protocols. Genotoxic and nongenotoxic mouse hepatocarcinogens each may induce tumors of either high malignant or low malignant potential. Liver tumors with specific H-ras oncogene mutations may appear morphologically and biologically similar to those without proven ras mutations. Thus, distinguishing mechanism of carcinogenesis by liver tumor morphology and mutation spectra may be difficult. Additionally, the presence of liver tumors with a morphology and a ras oncogene mutation spectrum characteristic of spontaneous tumors in histologically normal livers of mice exposed to a nongenotoxic test chemical may indicate promotion of spontaneous hepatocarcinogenesis by one of several potential mechanisms. Further research into the mechanisms responsible for the increased incidences of liver tumors in mice exposed to test chemicals could enhance human cancer risk assessments.

Historical perspective on risk assessment in the federal government

This article traces the evolution of risk assessment as an essential analytical tool in the federal government. In many programs and agencies, decisions cannot be made without the benefit of information from risk assessment. Although this analytical tool influences important public health and economic decisions, there is widespread dissatisfaction with the day-to-day practice of risk assessment. The article describes the sources of dissatisfaction that have been voiced by scientists, regulators, interest groups and ordinary citizens. Problems include the use of arbitrary exposure scenarios, the misuse of the 'carcinogen' label, the excessive reliance on animal cancer tests, the lack of formal uncertainty analysis the low priority assigned to noncancer endpoints, the poor communication of risk estimates and the neglect of inequities in the distribution of risk. Despite these limitations, the article argues that more danger rests in efforts to make decisions without any risk assessment. Recent Congressional and Administration interest in risk assessment is encouraging because it offers promise to learn from past mistakes and set in motion steps to enhance the risk assessment process.

Mechanisms, chemical carcinogenesis, and risk assessment: cell proliferation and cancer

Mechanisms of carcinogenesis--and in particular chemically associated carcinogenicity--have attracted considerable scientific and public attention in the last decade. Much insight has been gained that will lead to more reasoned and better prevention, intervention, and treatment for the reduction of environmentally caused cancers. However, there seems to be an exaggerated tendency to embrace "mechanisms" not yet fully characterized, completely tested, unequivocally proven, and consensus accepted. More than 100 agents and exposure circumstances have been identified as causally or strongly associated with human cancers; for many the evidence was discovered first in experimental animals. More chemicals have been uncovered as carcinogenic in experimental animals, with as yet no or little available information in exposed human populations. Additional and expanded mechanistic and epidemiological studies should further elucidate the relevance of these agents to adverse human health effects, including cancers. Claims are being posed that certain chemical-specific "mechanisms" in experimental systems are irrelevant to humans, and thus chemicals thought to be aberrantly carcinogenic in animals would present no cancer hazard to exposed humans. Nonetheless before undeniable proof becomes available, we must continue to proceed with sensitive and responsible caution. This commentary offers a central and personal view of one such mechanism: cell proliferation and cancer.

Sequential functional and morphological alterations during hepatocarcinogenesis induced in rats by feeding of a low dose of 2-acetylaminofluorene

The early cellular events in liver carcinogenesis were studied in Fischer-344 male rats that either were fed 200 ppm 2-acetylaminofluorene (AAF) for up to 10 wk or were fed the carcinogen for 8 wk followed by maintenance for an additional 24 wk. By 1 wk of exposure, AAF caused a reduction in the number of glutamine synthetase (GS)-positive centrilobular hepatocytes, an increase in DNA synthesizing hepatocytes in the central areas of the hepatic lobules, and a shift from multinucleated to mononucleated hepatocytes, although overt hepatocellular necrosis was not evident. By 3 wk, altered hepatocellular foci characterized by deficiencies in iron storage (IS-) and collagen production and by expression of gamma-glutamyl transferase (GGT+) and placental-type glutathione transferase (PGT+) activity appeared. Single PGT+ cells were also found. During continued exposure, foci increased in number, size, and total area with the increases escalating between 8 and 10 wk of exposure. Cessation of AAF exposure at 8 wk resulted in a slight decrease in the number of foci after a further 6 wk of maintenance, but with continued maintenance for another 6 and 12 wk, the number again increased. IS- characterized the majority of foci during carcinogen administration, whereas after cessation of exposure, GGT+ and PGT+ foci predominated. None of the foci were positive for GS. After AAF exposure for 10 wk, a few neoplasms developed and greater numbers occurred after maintenance for a further 24 wk of rats exposed for 8 wk. We conclude the following: (a) the low dose of AAF caused subtle alterations in function and proliferation of normal hepatocytes and converted hepatocytes into focus cells; (b) reduction of the GS+ area is a sensitive indicator of cytotoxicity of AAF; (c) the development of some foci at an early stage depends on a promoting action of AAF, which ceased when the carcinogen was withdrawn, allowing some foci to undergo reversion; (d) a strong linkage exists in expression of IS-, GGT+, and PGT+ in foci; (e) the carcinogenic process accelerates in the absence of any indication of increased cytotoxicity by AAF; and (f) under the conditions of this study, no GS+ foci, adenomas, and carcinomas were found, indicating that no carcinogen-induced expression of GS occurred in these lesions and that GS expression is not linked to other phenotypic abnormalities.

Rodent carcinogenicity and toxicity, in vitro mutagenicity, and their physical chemical determinants

In this paper, we considered rodent carcinogenicity and toxicity, and four in vitro mutagenicity systems, and we made a global comparison between their different response profiles to a common set of 297 chemicals. This analysis is complemented with a study of the physical chemical properties of active and inactive compounds in the different systems. A clearcut separation between the different classes of toxicological end-points (carcinogenicity, in vivo toxicity, in vitro carcinogenicity) was evident. The observed lack of association between carcinogenicity and toxicity supports the validity of the rodent bioassays; this is contrary to the position that the positive results obtained are due mainly to the use of excessive doses that exert cytotoxic effects. We found substantial consistency in the responses of the in vivo toxicity systems (maximum tolerated dose and LD50), but we also found that remarkable differences exist between the in vitro mutagenicity assay systems. The study of the structure-activity relationships showed that: (a) the hydrophobic-electronic properties of the chemicals influence rodent carcinogenicity, with the tendency of carcinogens to be more electrophilic and more hydrophobic than non-carcinogens; (b) steric effects are implied in in vitro mutagenicity, bulkier molecules being less mutagenic than smaller molecules; (c) no clear association between in vivo toxicity and physical chemical properties was apparent. The differences between carcinogenicity and in vitro mutagenicity may hypothetically be related to their different experimental procedures. The relatively short treatment of in vitro mutagenicity requires that chemicals penetrate easily into the cells, and are well dissolved into the aqueous medium, size and hydrophilicity thus being critical for the action of the chemicals. The size of the molecules is not critical in the long-term rodent carcinogenicity experiments, where other factors, like bioaccumulation (hydrophobicity) and electronic reactivity, become essential.

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.

Liver carcinogenesis is not a predicted outcome of chemically induced hepatocyte proliferation

Cell proliferation has long been recognized as a basic component of multistage carcinogenesis. Based largely on the finding that certain nongenotoxic chemical carcinogens induce cell proliferation in the same organ that develops tumors after long-term exposure, some suggest that the increased rates of cell division account for the carcinogenicity of these chemicals. This paper examines relationships between chemically induced liver toxicity, cell proliferation, and liver carcinogenesis; major factors include consistency, transient vs. sustained dose-response correspondence, and scientific plausibility. For a presumed mechanism to be valid, a sustained proliferative response is critical, largely because transient increases in hepatocyte proliferation are not sufficient to induce cancer or promote liver tumor development. A consistent association between liver toxicity and carcinogenicity has not been established. Our evaluation of studies on purported relationships between chemically induced cell proliferation and liver carcinogenesis shows: 1) that inconsistencies in sex and species specificity exist, 2) that a large percentage of proliferative responses are transient, 3) that inconsistencies in response to various hepatic peroxisome proliferators are common, and 4) that dose-response and duration relationships have not been sufficiently examined. Studies of proliferative responses of putative preneoplastic cells in the liver indicate that these cells divide faster than normal hepatocytes and also have higher death rates. Chemicals that induce cell proliferation in preneoplastic foci do not always provide a persistent increase in replication rates, even with continuous exposure. A selective growth advantage to preneoplastic cells in the liver may be provided either by an enhancement of the replication rates of these cells compared to the surrounding normal hepatocytes, by inhibition of cell loss, or by inhibition of the growth rate of normal cells. More work is needed to understand how chemical carcinogens and noncarcinogens affect cell division and cell loss of normal hepatocytes and of preneoplastic cells; measurements of hepatocyte proliferation alone are not sufficient to elucidate mechanisms of liver tumor development or to predict liver carcinogenesis. Because of our limited knowledge of the complex molecular changes occurring during liver cancer, it would be inappropriate and far too premature to amend scientific risk assessment procedures for nongenotoxic chemical carcinogens based on oversimplified or incompletely tested speculations.

Limiting the uncertainty in risk assessment by the development of physiologically based pharmacokinetic and pharmacodynamic models

Analysis of the default cancer risk assessment methodology suggests that the confidence interval usually associated with the prediction of an upper bound on risk underestimates the uncertainty in the risk estimate. This underestimate of uncertainty is based on the use of a large number of policy decisions or professional judgements that are incorporated into the methodology as exact values with no estimate of error. An alternative approach is to develop a comprehensive biologically based risk assessment that provides scientific data to substitute for many of the policy decisions of the default methodology.

Cancer etiology: agents causally associated with human cancer

Cancer is a multifactorial and multistage process, the exact mechanisms of which are still only partially known. However, even in the absence of a complete understanding of the process of carcinogenesis, we have been able to identify several factors which modify the risk of tumour development in humans. These include both endogenous and environmental factors, ranging from exposure to a single identified chemical to the occupations we follow in order to make our living. Cancer prevention strategies may differ in different parts of the world. In Europe, lung cancer is responsible for about one fourth of all cancer deaths and most of it could be prevented by eliminating tobacco smoking. Other exposures that can be controlled include occupational exposures to agents known to cause cancer at sites such as lung, bladder, paranasal sinuses, leukaemia, lymphoma and liver, as well as exposure to sunlight, known to be associated with both non-melanocytic and melanocytic skin cancer. Liver cancer is a common cancer in other regions of the world where hepatitis B virus (HBV) infection is endemic; in these areas, fungal contamination of food is also common. While immunization against HBV may be the method of choice in the long run, reduction of exposure to aflatoxins might be a more useful intermediate goal in primary prevention because of the strong interaction between hepatitis B and aflatoxin exposure on liver cancer risk. To date, few chemical agents have been proved to be of etiological relevance to cancer in humans at sites such as the breast (with the exception of oestrogenic hormones), ovary, colon-rectum and prostate.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Structural relationships between mutagenicity, maximum tolerated dose, and carcinogenicity in rodents

The CASE structure-activity relational system was applied to a study of the structural bases of toxicity as expressed in the maximum tolerated dose (MTD) of a group of chemicals for which rodent carcinogenicity and mutagenicity data were also available. All of the results were obtained under the aegis of the U.S. National Toxicology Program. The analyses revealed that there was a structural basis for the MTD in mice and in rats and that these overlapped considerably. There was also some overlap between structural determinants of the MTD and of carcinogenicity in rodents but there was also a significant "antagonism" between such fragments; i.e., fragments associated with high toxicity (low MTD) were associated with lack of carcinogenicity and vice versa. The highest overlaps observed were between the structural determinant for a low MTD (i.e., high toxicity) and mutagenicity in Salmonella.

Thyroid follicular cell carcinogenesis: results from 343 2-year carcinogenicity studies conducted by the NCI/NTP

The National Toxicology Program data base on 343 mouse and rat carcinogenesis studies was reviewed to determine the frequency of and relationship between hyperplastic and neoplastic follicular lesions of the thyroid gland. The frequency of chemically related lesions in the thyroid was also compared to neoplastic lesions in the liver to investigate a possible correlation. The percentage of studies observed to have positive or equivocal chemically related thyroid proliferative lesions was rats: male, 14%, female, 11%; mice; male, 8%; female, 9%. When positive in one sex for a given chemical, there was a 60-80% chance of it being positive in the other sex of the same species, although interspecies correlation was not as strong. Thyroid follicular cell neoplasia without hyperplasia was uncommon in mice but was common in rats. Chemicals that caused thyroid proliferative changes were more likely (P less than 0.05) to produce liver neoplasms (both within and between species) than were chemicals causing no thyroid changes. However, this correlation was far from perfect, with many chemicals producing thyroid proliferative lesions, but not liver neoplasms and vice versa. This suggests that universal correlations are not supportable by the data and that individual chemicals should be evaluated on a case-by-case basis.