Comparative results of 327 chemical carcinogenicity studies

Simultaneous preconcentration and pre-column derivatization for rapid analysis of nitrilotriacetic acid in environmental waters by high performance liquid chromatography

A simplified sample pretreatment procedure was developed for quantitative measurement of nitrilotriacetic acid (NTA) in environmental water. On the basis of coordination capacity between NTA and metal ions, aluminum-based metal organic framework (MOF, MIL-53(Al)) was adopted for the adsorption of NTA, followed by stripping with copper sulfate as the eluent. The adsorbed NTA was converted into Cu-NTA during the desorption process, which facilitated the ensuing measurement by high performance liquid chromatography (HPLC). A linear range within 0.10 - 10 mg L^-1 was achieved, along with a limit of detection (LOD, S/N=3, n=7) of 0.03 mg L^-1 and an enrichment factor of 10.4. The developed method was validated by the analysis of sea water, influent of wastewater treatment plant and industrial wastewater, with satisfactory recoveries (90.2 - 91.1%) obtained.

Uterine Changes in Female Mice Following Lifetime Inhalation of Wholly Vaporized Unleaded Gasoline: A Possible Relationship to Observed Liver Tumors?

An increased incidence of primary hepatocellular tumors was reported in female B6C3F1 mice following lifetime exposure to high levels (2056 ppm) of wholly vaporized unleaded gasoline. This effect was not observed in male mice, nor in females exposed to either 67 or 292 ppm gasoline. No explanation for the sex and dose-specific effect was discussed in the initial report or has been subsequently published. At necropsy, a decreased incidence of enlarged/cystic uteri was also noted among high dose females; however, no correlating histopathologic changes were reported. Because the liver neoplastic response was limited to the females, and because spontaneous liver tumors are known to be influenced by the hormonal environment, we reexamined the uterine tissues microscopically. We observed a dramatic difference among treatment groups in the severity of cystic endometrial hyperplasia. The incidence of moderate cystic endometrial hyperplasia at the terminal sacrifice was 76% in the control animals whereas none of the animals at the highest exposure level had lesions that were graded moderate or severe. In addition, uterine atrophy was present in 35% of mice examined at the high exposure level and was absent in other groups. The changes noted are consistent with an altered hormonal influence on the uterus. The finding of a decrease in cystic endometrial hyperplasia and an increase in uterine atrophy in the same dose group in which liver tumors were found suggests that these effects may be interrelated.

Establishment of a Stable Cell Line Expressing Green Fluorescence Protein-fused Hypoxia Inducible Factor-1α for Assessment of Carcinogenicity of Chemical Toxicants

Hypoxia inducible factor 1α (HIF-1α) is a potential marker of carcicnogenesis since it is overexpresssed in many human cancers such as brain, breast, and uterus, and its role has implicated in tumor cell growth and metastasis. In this study, we established a stable cell line that express green fluorescence protein (GFP)-fused hypoxia inducible factor 1α (HIF-1α) and evaluated the potential use of this cell line for assessment of carcinogenicity of chemical toxicants. Western blot analysis as well as fluorescence measurements showed that protein-level of GFP-HIF-1α was significantly enhanced in a dose-dependent manner upon treatment of hypoxia mimicking agents such as dexferrioxamine and CoCl₂. Well-Known tumor promoters such as mitomycin and methyl methane-sulfonate. significantly induced the fluorescence intensity of GFP-HIF-1α, whereas the known negative controls such as o-anthranilic acid and benzethonium chloride, did not. These results indicate that HIF-1α could be a biological parameter for detection of tumor initiators/promoters and suggest that the GFP-HIF-1α cell line is a useful system for screening of carcinogenic toxicants.

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.

Toward an evidence-based toxicology

The increasing demands on toxicology of large-scale risk assessment programmes for chemicals and emerging or expanding areas of chemical use suggest it is timely to review the toxicological toolbox. Like in clinical medicine, where an evidence-based medicine (EBM) is critically reviewing traditional approaches, toxicology has the opportunity to reshape and enlarge its methodology and approaches on the basis of compounded scientific knowledge. Such revision would have to be based on structured reviews of current practice, ie, assessment of test performance characteristics, mechanistic understanding, extended quality assurance, formal validation and the use of integrated testing strategies. This form of revision could optimize the balance between safety, costs and animal welfare, explicitly stating and, where possible, quantifying uncertainties. After a self-critical reassessment of current practices and evaluation of the thus generated information, such an evidence-based toxicology (EBT) promises to make better use of resources and to increase the quality of results, facilitating their interpretation. It shall open up hazard and also risk assessments to new technologies, flexibly accommodating current and future mechanistic understanding. An EBT will be better prepared to answer the continuously growing safety demands of modern societies.

Biomimetic approach to biomembrane models studies: medium influence on the interaction kinetics of some phenylurea derivatives herbicides

The ability of herbicides to interact with cell membranes outer lipid layer and subsequently to penetrate inside cells can be a prerequisite for exhibiting a toxic activity for both the directly exposed workers and the end consumers as the herbicides are present in the soil and water. The effect exerted by fenuron, chlorotoluron, metobromuron, monolinuron, and chlorbromuron, five structurally similar phenylurea herbicides, on the thermotropic behavior of model membranes, represented by dimyristoylphosphatidylcholine (DMPC) vesicles, was investigated by differential scanning calorimetry. The examined compounds, when dispersed in liposomes during their preparation, exerted a different action on the gel-to-liquid crystal phase transition of DMPC multilamellar vesicles. The ability of phenylurea herbicides, as a finely powdered solid, to migrate through an aqueous medium and interact with biomembrane models was also studied. This transfer process was compared with these compounds intermembrane transfer from herbicide-loaded liposomes to empty ones. These processes can mimic absorption kinetics mediated by hydrophilic or lipophilic media. Different rate and entity of interaction occurred between model membranes and solid phenylurea herbicides. Different behavior was observed by considering the time-dependent studies carried out by contacting, for increasing times, equivalent amounts of empty DMPC vesicles with phenylurea herbicide-loaded ones; all compounds were able to migrate from loaded to empty DMPC vesicles. Thus, phenylurea herbicides are able to reach and penetrate biological membranes when dispersed in a lipophilic or hydrophilic medium; these processes are related to the substituents present on the compounds backbone. The obtained experimental results seem to validate the employed strategy to study the ability of bioactive compounds to both interact with biological membranes and be adsorbed inside a membrane mimicking a biological cell when dispersed in a lipophilic or hydrophilic medium.

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.

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.

Compendium of chemical carcinogens by target organ: results of chronic bioassays in rats, mice, hamsters, dogs, and monkeys

A compendium of carcinogenesis bioassay results organized by target organ is presented for 738 chemicals that are carcinogenic in chronic-exposure, long-term bioassays in at least 1 species. This compendium is based primarily on experiments in rats or mice; results in hamsters, monkeys, and dogs are also reported. The compendium can be used to identify chemicals that induce tumors at particular sites and to determine whether target sites are the same for chemicals positive in more than 1 species. The source of information is the Carcinogenic Potency Database (CPDB). which includes results of 6073 experiments on 1458 chemicals (positive or negative for carcinogenicity) that have been reported in Technical Reports of the National Cancer Institute/National Toxicology Program or in papers in the general published literature. The published CPDB includes detailed analyses of each test and citations. The CPDB is publicly available in several formats (http://potency.berkeley.edu). Chemical carcinogens are reported for 35 different target organs in rats or mice. Target organs in humans are also summarized for 82 agents that have been evaluated as human carcinogens at a particular target site by the International Agency for Research on Cancer (IARC). Comparisons are provided of target organs for mutagens versus nonmutagens and rats versus mice.

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.

Importance of species selection in drug toxicity testing

The development of efficacious and safe new human pharmaceuticals continues to be highly dependent on well-designed and carefully executed animal toxicology studies. Although the protocols of most toxicology studies now conform to a standard format, careful selection of the most appropriate and relevant laboratory species is essential. This selection is usually based on pilot toxicity and general pharmacology studies in rodent and non-rodent animals, together with supporting comparative drug disposition studies to determine bioavailability and metabolic profile. A knowledge of the human metabolic profile from early in vitro studies with liver preparations can be particularly valuable.

An update of the National Toxicology Program database on nasal carcinogens

Nearly 500 long-term rodent carcinogenicity studies carried out by the National Cancer Institute and the National Toxicology Program were examined, and 12 chemicals were identified that produced nasal tumors: allyl glycidol ether, p-cresidine, 1,2-dibromo-3-chloropropane, 1,2-dibromoethane, 2,3-dibromo-1-propanol, dimethylvinyl chloride, 1,4-dioxane, 1,2-epoxybutane, iodinated glycerol, procarbazine, propylene oxide, and 2,6-xylidine. All 12 of these chemicals produced nasal tumors in rats, and 5 also produced nasal tumors in mice. Most of the nasal carcinogens (1) produced tumor increases in both sexes, (2) produced tumors at other sites as well, (3) had significantly reduced survival at doses that were carcinogenic, and (4) were genotoxic. Only 5 of the 12 nasal carcinogens were administered by inhalation. A variety of different types of nasal cavity tumors were produced, and specific tumor rates are given for those chemicals causing multiple tumor types. Increased incidences of nasal neoplasms were often accompanied by suppurative/acute inflammation, epithelial/focal hyperplasia and squamous metaplasia. However, high incidences of these nonneoplastic nasal lesions were also frequently seen in inhalation studies showing no evidence of nasal carcinogenicity, suggesting that in general nasal carcinogenesis is not associated with the magnitude of chronic toxicity observed at this site.

In vivo studies on genotoxicity of pure and commercial linuron

The ureic herbicide linuron [3-(3, 4-dichlorophenyl)-1-methoxy-1-methylurea] (CAS 330-55-2) was investigated for genotoxicity in a series of in vivo experiments. Since human exposure to herbicides is not only to the active principles, but also to all the chemicals present in the commercial formulation, we tested both pure and commercial linuron. Groups of rats were treated with gavage containing different doses of the herbicide (pure compound or commercial formulation) for 14 days. The doses were 150, 300 and 450 mg/kg b.wt. for the pure compound and 315.8, 631.6 and 947.4 mg/kg b.wt. for the commercial formulation (47.5% of linuron). Faeces and urine were collected at regular intervals. Urine specimens were analysed for their mutagenic metabolites, thioethers and D-glucaric acid content. Faeces extracts were tested for mutagenicity. Linuron's ability to cause DNA damage and cytogenetic effects was also investigated after treating groups of rats once with different doses of pure or commercial linuron. DNA single-strand breaks were assessed in rat liver using the alkaline elution technique and the single-cell microgel electrophoresis assay (SCGE: 'comet' assay), and in rat testes cells with the SCGE assay. Micronuclei induction was analysed in rat bone marrow erythrocytes. Results obtained were mainly negative when the excretion of mutagenic metabolites in urine and faeces of animals treated with the pure compound or with the linuron-based commercial formulation were monitored, whereas an increase in the urinary excretion of thioethers and D-glucaric acid was observed in rats treated with the commercial formulation. No increase in the frequency of micronucleated polychromatic erythrocytes was observed in the treated animals. However, linuron affected the viability of hepatocytes isolated from animals treated with higher doses. This cytotoxicity was accompanied by the induction of DNA single-strand breaks in the liver, as seen by the alkaline elution assay. The potential of pure linuron to induce in vivo DNA damage was confirmed with the microgel-electrophoresis technique ('comet' assay). Cytotoxicity was also seen in rat testes cells. However, no indication of DNA damage was visible.

Evaluation of false positive and false negative outcomes in NTP long-term rodent carcinogenicity studies

The decision-making process used by the National Toxicology Program (NTP) in its evaluation of long-term rodent carcinogenicity studies was investigated to determine whether or not this procedure resulted in an excessive number of false positive or false negative outcomes. All site-specific tumor incidences that were found to be significantly (p < 0.05) increased either by a trend test or by pairwise comparisons of each dosed group against the controls in 218 NTP 2-year studies with Fischer 344 rats and/or B6C3F1 mice were tabulated and compared to the number of statistically significant tumor increases expected to occur by chance. Our evaluation suggests that false positive rates are fairly low in NTP long-term studies. Assessing false negative rates is more difficult because of the limited sensitivity of the bioassay for detecting subtle carcinogenic effects. Moreover, reduced body weights frequently occur in dosed animals, and the positive correlation between the incidences of certain site-specific tumors and body weight may mask the detection of carcinogenic effects. Despite these difficulties, our analysis did identify one tumor showing evidence of false negative outcomes: interstitial cell tumors of the testis in male Fischer 344 (F344) rats. This tumor showed considerably more significant (p < 0.05) increased incidences than expected by chance, yet none were considered to be chemically-related. However, the biological significance of interstitial cell tumor increases in F344 rats is uncertain because of the high background rate of neoplasia (> 90%) for this target site.

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.

The value of animal test information in environmental control decisions

Value of information (VOI) analytic techniques are used to evaluate the benefit of performing animal bioassays to provide information about the cancer potency of specific chemical compounds. These tools allow the identification of the conditions in which the cost of reducing uncertainty about potency, by performing a subchronic or chronic bioassay, is justified by the benefit of having improved information for making control decisions. The decision analytic results are readily scaled to apply to a range of human contact rates (exposures) and a variety of control strategies. The sensitivity of results to uncertainty about animal to human extrapolation and the design of the bioassay is explored. An evaluation of the possible gains in general understanding about the mechanisms of carcinogenicity resulting from chronic bioassays is beyond the scope of this approach.

Correlations between chemically related site-specific carcinogenic effects in long-term studies in rats and mice

We examined a database of 379 long-term carcinogenicity studies in rats and mice to evaluate sex and species correlations in site-specific carcinogenic responses. Within a species, most target sites showed a strong correlation between males and females. For example, chemicals producing forestomach or liver tumors in males were likely to produce these same types of tumors in females. There was also a significant correlation between species for certain site-specific carcinogenic effects, most notably tumors of the forestomach, liver, and thyroid gland. In contrast, adrenal pheochromocytoma, preputial/clitoral gland neoplasms, and lung tumors showed no significant interspecies correlation. Many chemicals produced a syndrome of carcinogenic effects involving tumors of the skin, Zymbal gland, preputial/clitoral gland, mammary gland, and/or oral cavity. Regarding different target sites, there appeared to be a correlation between thyroid and liver tumors both within and between species. Further, all chemicals producing mesotheliomas in male rats also produced mammary gland neoplasms in female rats. In contrast, kidney and urinary bladder tumors showed no significant association with any other tumor type in rats or mice. If a chemical produced a site-specific carcinogenic effect in female rats or mice, there was approximately a 65% probability that the chemical would also be carcinogenic at that same site in males. The interspecies correlation was somewhat lower: approximately 36% of the site-specific carcinogenic effects observed in one species (rats or mice) were also observed in the other species.(ABSTRACT TRUNCATED AT 250 WORDS)

RITA--Registry of Industrial Toxicology Animal-data--progress of the working group

A data base for the collection of histopathological diagnoses of neoplastic and pre-neoplastic lesions and related data from control animals involved in long-term carcinogenicity or toxicity studies has been operated since 1988. Factors which may have an influence on the spontaneous tumor rate are integrated in the data base. Systematized nomenclature and standardized diagnostic criteria have been established as well as the applied data acquisition and data validation procedure.

Effects of corn oil, time-related changes, and inter-laboratory variability on tumor occurrence in control Fischer 344 (F344/N) rats

Survival, body weight, and site-specific tumor rates in untreated, corn oil gavage, and water gavage control Fischer 344 (F344/N) rats from 88 National Toxicology Program (NTP) long term carcinogenicity studies were evaluated to determine which factors were primarily responsible for inter-study variability. For male rats, previously-reported decreases in leukemia and increases in body weight, survival, and pancreatic acinar cell tumors attributable to corn oil gavage were confirmed. Corn oil did not appear to affect tumor rates in female rats. The gavage technique per se did not appear to influence tumor rates in rats of either sex. Previously reported time-related increases in certain site-specific neoplasia in control rats appeared to have stabilized in recent years, but control tumor rates are still much greater than those seen a decade ago. More recent studies continue to show increasing rates of leukemia and mammary gland tumors and decreasing survival. Female rats also continue to show time-related increases in maximum mean body weight. Inter-laboratory variability in body weight and in the rates of a number of site-specific neoplasms were also significant. High mean body weights in control groups were found to be associated with increased rates of mammary and pituitary tumors. Our evaluation supports the view that if historical control data are to be utilized in the interpretation of experimental results, primary emphasis should be given to lab and route of administration-specific tumor rates for studies that are contemporary to the study under evaluation. It also suggests that certain experimental design changes (e.g., dietary modifications) may be needed to reduce tumor rates and to increase survival.

Long-term chemical carcinogenesis experiments for identifying potential human cancer hazards: collective database of the National Cancer Institute and National Toxicology Program (1976-1991)

The carcinogenicity database used for this paper originated in the late 1960s by the National Cancer Institute (NCI) and since 1978 has been continued and made more comprehensive by the National Toxicology Program (NTP). The extensive files contain, among other sets of information, detailed pathology data on more than 400 long-term (most often 24-month) chemical carcinogenesis studies, comprising nearly 1600 individual experiments having at least 10 million tissue sections that have been evaluated for toxicity and carcinogenicity.(ABSTRACT TRUNCATED AT 250 WORDS)

Predicting the carcinogenicity of chemicals in humans from rodent bioassay data

Regulatory agencies currently rely on rodent carcinogenicity bioassay data to predict whether or not a given chemical poses a carcinogenic threat to humans. We argue that it is always more useful to know a chemical's carcinogenic potency (with confidence limits) than to be able to say only qualitatively that it has been found to be a carcinogen. In a typical bioassay, a chemical is administered to groups of 50 to 100 rodents at the highest feasible level (the maximum tolerated dose) and rarely at less than 1/10 this dose in order to maximize the statistical significance of any increase in tumors that might result. Recently, much experimental work has focused on the mechanisms by which site-specific toxicity arising from chronic administration at the maximum tolerated dose may lead to carcinogenicity. Extrapolation of high-dose results to low doses does not take into consideration the possibility of a threshold dose, below which the carcinogenic potency is much lower or even zero. Threshold dose-response phenomena may be much more relevant to the etiology of cancer in the rodent bioassays than was earlier realized; if so, there is an even greater need for establishing dose-dependent potency estimates. The emphasis of this review is on the interspecies comparison of high-dose potencies. The qualitative and quantitative comparison of carcinogenicities between mice and rats and between rodents and humans is reviewed and discussed. We conclude that there is a good qualitative (yes/no) correlation for both the rat/mouse and the rodent/human comparison. There is also a good correlation of the carcinogenic potencies between rats and mice, and the upper limits on potencies in humans are consistent with rodent potencies for those chemicals for which human exposure data are available. For the rodent/human comparison, the best estimate of the interspecies potency factor is lognormally distributed around 1 when the potencies in both species are measured in units of (mg/kg-day)-1.

Target organs in chronic bioassays of 533 chemical carcinogens

A compendium of carcinogenesis bioassay results organized by target organ is presented for 533 chemicals that are carcinogenic in at least one species. This compendium is based primarily on experiments in rats or mice; results in hamsters, nonhuman primates, and dogs are also reported. The compendium can be used to identify chemicals that induce tumors at particular sites, and to determine whether target sites are the same for chemicals positive in more than one species. The Carcinogenic Potency Database (CPDB), which includes results of 3969 experiments, is used in the analysis. The published CPDB includes details on each test, and literature references. Chemical carcinogens are reported for 35 different target organs in rats or mice. More than 80% of the carcinogens in each of these species are positive in at least one of the 8 most frequent target sites: liver, lung, mammary gland, stomach, vascular system, kidney, hematopoietic system, and urinary bladder. An analysis is presented of how well one can predict the carcinogenic response in mice from results in rats, or vice versa. Among chemicals tested in both species, 76% of rat carcinogens are positive in mice, and 71% of mouse carcinogens are positive in rats. Prediction is less accurate to the same target site: 52% of rat carcinogens are positive in the same site in mice, and 48% of mouse carcinogens are positive in the same site in rats. The liver is the most frequent site in common between rats and mice.

NTP carcinogens--interpretational problems

The National Toxicology Program (NTP) was established in 1978 with the broad goal of strengthening the science base of chemical toxicity, thus providing better information to regulatory and research agencies. Since that time the NTP has conducted in-depth toxicity/carcinogenesis studies on over 200 chemicals of importance to industry, the public at large and the general environment; clearly the largest such database in the world. This database is unique in that it represents an objective fairly standard accumulation of peer-reviewed information on a myriad of chemicals composed of various chemical classes, non-carcinogens as well as carcinogens. The results of these studies are reported as "no evidence, equivocal evidence, some evidence or clear evidence of carcinogenic activity" in a single sex/species. There is also an "inadequate" category for studies that have major limitations. Although noted, no attempt is made to give added weight to chemicals which cause neoplasms at multiple sites, at rare versus common sites, in both species/sexes, which occur early in the study, at low as well as high doses, or those observed in the presence or absence of toxicity (necrosis, degeneration, etc.) in the same organ. Such observational data may serve as "markers" or "alerts" for whether a chemical's in vivo carcinogenic activity is the result of mutagenic or non-mutagenic activity.

Chemicals associated with site-specific neoplasia in 1394 long-term carcinogenesis experiments in laboratory rodents

The carcinogenicity data base used for this paper originated in the late 1960s by the National Cancer Institute and since 1978 has been continued and made more comprehensive by the National Toxicology Program. The extensive files contain among other sets of information detailed pathology data on more than 400 long-term (most often 24 month) chemical carcinogenesis studies, comprised of nearly 1600 individual experiments having at least 10 million tissue sections that have been evaluated for toxicity and carcinogenicity. Using the current data set of 379 studies made up of 1394 experiments, we have compiled listings of chemicals having like carcinogenic target sites for each of the 34 organs or systems for which histopathology diagnoses have been recorded routinely. The most common tumor site is the liver (15% of all experiments), followed in rank order by: lung, hematopoietic system and kidneys, mammary glands, forestomach, thyroid glands, Zymbal glands, urinary bladder, skin and uterus/cervix, and circulatory system and adrenal glands. These compilations are most useful for maintaining a historic perspective when evaluating the carcinogenicity of contemporary experiments. Equally important, the chemical-tumor-organ connection permits an evaluation of how well chemically induced cancers in a particular organ in one sex or species will predict or correlate with the other sex or species. Using liver cancers as an example, the overall interspecies concordance is 80%. Likewise target site predictions can be made for chemicals selected for study that may be similar to those already evaluated; thereby experimental protocols could be adjusted to allow, for example, more extensive pathology on preselected target organs (i.e., serial sections of the kidney). Further from these observations, one could decide to use two strains of mice to evaluate a short-chain chlorinated aliphatic compound or to study a human carcinogen in a sex-species known to develop chemically induced tumors in the same site observed in humans. Structural classes of chemicals having a propensity for certain organs can be easily identified from these data. Sex-species responders to particular induced cancers become clearly evident, such as in the ovary of female mice or in the kidney of male rats.

Inhalation of tetranitromethane causes nasal passage irritation and pulmonary carcinogenesis in rodents

Fischer 344 rats and B6C3F1 mice were exposed for 2 years to vapors of tetranitromethane at concentrations below (0.5 ppm) and slightly above (2 or 5 ppm) the current U.S. recommended occupational exposure limit. Under the conditions of exposure of 6 h/day, 5 days/week, tetranitromethane was found to cause mild irritation and hyperplastic lesions in the nasal passages, but not nasal cavity neoplasms were observed. In contrast, nearly all animals exposed to the higher TNM concentrations, and the majority of animals exposed to the lower concentrations developed alveolar/bronchiolar adenoma or carcinoma; squamous cell neoplasms of the lung also occurred in exposed rats. The extent of the lung tumor response, and the low concentrations of tetranitromethane required for this response, are unprecedented in National Toxicology Program (NTP) studies.

Carcinogenesis studies in rodents for evaluating risks associated with chemical carcinogens in aquatic food animals

Fish and shellfish caught in polluted waters contain potentially dangerous amounts of toxic and carcinogenic chemicals. Public concern was heightened when a large percentage of winter flounder taken from Boston Harbor was found to have visible cancer of the liver; winter flounder outside the estuary area had no liver lesions. Long-term chemical carcinogenesis studies could be easily and feasibly designed using laboratory rodents offered diets containing fish caught in polluted waters. Induced cancers in rodents would corroborate field observations in fish; positive results from these studies would provide further evidence about potential human health hazards from eating substantial amounts of chemically contaminated fish. Nonetheless, fish and aquatic organisms should be viewed as environmental biological monitors of pollution or of potential human health hazards, and authorities responsible for assuring clean and safe rivers, bodies of water, and biota should give more attention to these valid biological indicators or sentinels of environmental pollution. Consequently, fish and other sea creatures alone should serve as alarms regarding whether water areas constitute public health hazards.

The chemotherapeutic potential of glycol alkyl ethers: structure-activity studies of nine compounds in a Fischer-rat leukemia transplant model

Structure-activity studies with nine glycol alkyl ethers were conducted with a cellular leukemia transplant model in male Fischer rats. This in vivo assay measures the effects of chemical treatment on neoplastic progression in transplant recipients. Chemicals were given ad libitum in the drinking water simultaneously with the transplants and continued throughout the study. In all, 20 million leukemic cells were injected s.c. into syngeneic rats, which after 60 days resulted in a 10-fold increase in relative spleen weights, a 100-fold increase in white blood cell counts, and a 50% reduction in red blood cell (RBC) indices and platelet counts. At this interval, ethylene glycol monomethyl ether (2-ME) given at a dose of 2.5 mg/ml in the drinking water completely eliminated all clinical, morphological, and histopathological evidence of leukemia, whereas the same dose of ethylene glycol monoethyl ether (2-EE) reduced these responses by about 50%. Seven of the glycol ethers were ineffective as anti-leukemic agents, including ethylene glycol, the monopropyl, monobutyl, and monophenyl ethylene glycol ethers, diethylene glycol, and the monomethyl and monoethyl diethylene glycol ethers. 2-ME more than doubled the latency period of leukemia expression and extended survival for at least 210 days. A minimal effective dose for a 50% reduction in the leukemic responses was 0.25 mg/ml 2-ME in the drinking water (15 mg/kg body weight), whereas a 10-fold higher dose of 2-EE was required for equivalent antileukemic activity. In addition, the in vitro exposure of a leukemic spleen mononuclear cell culture to 2-ME caused a dose- and time-dependent reduction in the number of leukemia cells after a single exposure to 1-100 microM concentrations, whereas the 2-ME metabolite, 2-methoxyacetic acid, was only half as effective. The two glycol alkyl ethers with demonstrable anti-leukemic activity, 2-ME and 2-EE, also exhibited a favorable efficacy-to-toxicity ratio and should be considered for further development as chemotherapeutic agents.

Rodent tumor profiles, Salmonella mutagenicity and risk assessment

The tumorigenesis profiles of 116 chemicals, which proved to induce cancer in the NCI/NTP experimentation, were studied by multivariate data analysis methods. Three main patterns of tumor induction were evident. One chemical (benzene) was not classifiable in any of the 3 clusters of chemicals. The carcinogen classes based on patterns of tumor induction did not reflect a repartition between Ames-positive and Ames-negative chemicals. Therefore any classification of carcinogens as either 'primary' (genotoxic, hence assumed to pose a greater risk) or 'secondary' (presumably carcinogenic via non-genotoxic mechanisms) would seem to be a subject for research and speculation, and, for the present, an unsuitable basis for risk assessment.

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

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

Long-term assays for carcinogenicity

During the past two decades, the rodent bioassay for detection of chemical carcinogens has reached a high standard of performance with both an increased number of animals and dose levels and a more detailed assessment of findings. However, the basic principles of testing and evaluation of results have remained essentially unchanged. Problems such as the length of the testing, use of maximum tolerated dose (MTD), selection of strains, variability of spontaneous tumors, discordant results between mouse and rat, and the classification of chemical carcinogens according to their mechanism of action have all remained unsolved. By contrast, the results of short-term tests and of other biological analyses do not always show a direct correlation with those of the long-term bioassays; this can be interpreted as an indication of different mechanisms of carcinogenicity. Currently available medium-term tests may detect carcinogenic activity of chemicals at particular organs in a period of time (weeks to months) relatively shorter than that of the 2-year carcinogenesis bioassay, and they may also provide additional information on mechanisms of carcinogenicity.

Structural basis of the mutagenicity of phenylazoaniline dyes

The CASE (Computer-Automated Structure Evaluation) methodology was used to gain an understanding of the basis of mutagenicity of phenylazoanilines. It was found that the activity of these molecules is dependent upon an intact moiety that spans the azo linkage, i.e., the azo bond must remain intact for mutagenicity. The study also addressed the effect of sulfonation on the activity of these azo dyes. It was revealed that sulfonation at only certain sites resulted in loss of mutagenicity. Sulfonation of the structural moiety responsible for the activity of phenylazoaniline dyes did not necessarily result in complete elimination of activity as this substitution could generate new structural moieties which contribute to the activity of the molecules.

Multiple-site carcinogenicity of benzene in Fischer 344 rats and B6C3F1 mice

Toxicology and carcinogenesis studies of benzene (CAS No. 71-43-2; greater than 99.7% pure) were conducted in groups of 60 F344/N rats and 60 B6C3F1 mice of each sex for each of three exposure doses and vehicle controls. These composite studies on benzene were designed and conducted because of large production volume and widespread human exposure, because of the epidemiologic association with leukemia, and because previous experiments were considered inadequate or inconclusive for determining carcinogenicity in laboratory animals. Using the results from 17-week studies, doses for the 2-year studies were selected based on clinical observations (tremors in higher dosed mice), on clinical pathologic findings (lymphoid depletion in rats and leukopenia in mice), and on body weight effects. Doses of 0, 50, 100, or 200 mg/kg body weight benzene in corn oil were administered by gavage to male rats, 5 days per week, for 103 weeks. Doses of 0, 25, 50, or 100 mg/kg benzene in corn oil were administered by gavage to female rats and to male and female mice for 103 weeks. Ten animals in each of the 16 groups were killed at 12 months, and necropsies were performed. Hematologic profiles were performed at 3-month intervals. For the 2-year studies, mean body weights of the top dose groups of male rats and of both sexes of mice were lower than those of the controls. Survivals of the top dose group of rats and mice of each sex were reduced; however, at week 92 for rats and week 91 for mice, survival was greater than 60% in all groups; most of the dosed animals that died before week 103 had neoplasia. Compound-related nonneoplastic or neoplastic effects on the hematopoietic system, Zymbal gland, forestomach, and adrenal gland were found both for rats and mice. Further, the oral cavity was affected in rats, and the lung, liver, Harderian gland, preputial gland, ovary, and mammary gland were affected in mice. Under the conditions of these 2-year gavage studies, there was clear evidence of carcinogenicity of benzene in male F344/N rats, female F344/N rats, male B6C3F1 mice, and female B6C3F1 mice. In male rats, benzene caused increased incidences of Zymbal gland carcinomas, squamous cell papillomas and squamous cell carcinomas of the oral cavity, and squamous cell papillomas and squamous cell carcinomas of the skin. In female rats, benzene caused increased incidences of Zymbal gland carcinomas and squamous cell papillomas and squamous cell carcinomas of the oral cavity.(ABSTRACT TRUNCATED AT 400 WORDS)

Classification according to chemical structure, mutagenicity to Salmonella and level of carcinogenicity of a further 42 chemicals tested for carcinogenicity by the U.S. National Toxicology Program

This paper is an extension and update of an earlier review published in this journal (Ashby and Tennant, 1988). A summary of the rodent carcinogenicity bioassay data on a further 42 chemicals tested by the U.S. National Toxicology Program (NTP) is presented. An evaluation of each chemical for structural alerts to DNA-reactivity is also provided, together with a summary of its mutagenicity to Salmonella. The 42 chemicals were numbered and evaluated as an extension of the earlier analysis of 222 NTP chemicals. The activity patterns and conclusions derived from the earlier study remain unchanged for the larger group of 264 chemicals. Based on the extended database of 264 NTP chemicals, the sensitivity of the Salmonella assay for rodent carcinogens is 58% and the specificity for the non-carcinogens is 73%. A total of 32 chemicals were defined as equivocal for carcinogenicity and, of these, 11 (34%) are mutagenic to Salmonella. An evaluation is made of instances where predictions of carcinogenicity, based on structural alerts, disagree with the Salmonella mutagenicity result (12% of the database). The majority of the disagreements are for structural alerts on non-mutagens, and that places these alerts as a sensitive primary screen with a specificity lower than that of the Salmonella assay. That analysis indicates some need for assays complementary to the Salmonella test when screening for potential genotoxic carcinogens. It also reveals that the correlation between structural alerts and mutagenicity to Salmonella is probably greater than 90%. Chemicals predicted to show Michael-type alkylating activity (i.e., CH2 = CHX; where X = an electron-withdrawing group, e.g. acrylamide) have been confirmed as a structural alert, and the halomethanes (624 are possible) have been classified as structurally-alerting. To this end an extended carcinogen-alert model structure is presented. Among the 138 NTP carcinogens now reviewed, 45 (33%) are non-mutagenic to Salmonella and possess a chemical structure that does not alert to DNA-reactivity. These carcinogens therefore either illustrate the need for complementary genetic screening tests to the Salmonella assay, or they represent the group of non-genotoxic carcinogens referred to most specifically by Weisburger and Williams (1981); the latter concept is favoured.

Occurrence and relevance of chemically induced benign neoplasms in long-term carcinogenicity studies

Recent carcinogenicity studies conducted and evaluated by the National Toxicology Program/National Institute of Environmental Health Sciences were examined to determine the frequency of chemically increased incidences of neoplasia. Many of the chemicals originally selected for study were chosen because of an a priori suggestion that they might be carcinogens. Of the 143 chemical studies evaluated, usually involving male and female rats and mice, 42 (29%) did not induce any neoplasms, 20 (14%) gave marginal or equivocal neoplastic responses, and 81 (57%) showed positive neoplastic responses in one or more of the 524 species-gender experiments. Of these 81 positive studies, 60 (74%) were considered positive based on malignant neoplasia, 16 (20%) were positive due primarily to benign neoplasia, but had supporting evidence of malignant neoplasia in the same organ/tissue, and 5 (6%) were positive based only on benign neoplasia. These five chemicals are a) allyl isothiocyanate (transitional cell papillomas of the urinary bladder in male rats), b) 2-amino-4-nitrophenol (tubular cell adenomas of the kidney in male rats), c) asbestos intermediate range chrysotile (adenomatous polyps of the large intestine in male rats), d) decabromodiphenyl oxide (neoplastic nodules of the liver in male and female rats), and e) nitrofurazone (fibroadenomas of the mammary gland in female rats and benign mixed tumors and granulosa cell tumors of the ovary in female mice). For all but one of these lesions (mammary gland), the occurrence in historic controls is low. Thus, only 5 of the 143 chemicals studied (3.5%) induced benign neoplasia alone, and those observed benign neoplasms are known to progress to malignancy. Accordingly, we consider chemically induced benign neoplasia to be an important indicator of a chemical's carcinogenic potential in rodents, and believe it should continue to be made an integral part of the overall weight-of-the-evidence evaluation process for identifying potential human health hazards.

Interspecies extrapolation in carcinogenesis: prediction between rats and mice

Interspecies extrapolation in carcinogenesis is studied by evaluating prediction from rats to mice and from mice to rats. The Carcinogenic Potency Database, which includes 3500 cancer tests conducted in rats or mice on 955 compounds, is used for the analysis. About half of the chemicals tested for carcinogenicity are positive in at least one test, and this proportion is similar when rats and mice are considered separately. For 392 chemicals tested in both species, 76% of the rat carcinogens are positive in the mouse, and 70% of mouse carcinogens are positive in the rat. When compounds composed solely of chlorine, carbon, hydrogen, and, optionally, oxygen are excluded from the analysis, 75% of mouse carcinogens are positive in the rat. Overall concordance (the percentage positive in both species plus the percentage negative in both) is 76%. Three factors that affect prediction between rats and mice are discussed: chemical class, mutagenicity in the Salmonella assay, and the dose level at which a chemical is toxic. Prediction is more accurate for mutagens than non-mutagens and for substances that are toxic at low (versus only at high) doses. Species differences are not the result of failure in the bioassay to attain the maximum tolerated dose in the negative species or of more frequent testing in the positive species. An analysis of the predictive value of positivity for the 10 most common target sites indicates that most sites are good predictors of carcinogenicity at some site in the other species; the poorest predictors among these common sites are the rat urinary bladder and the mouse liver.

Summary of carcinogenic potency and positivity for 492 rodent carcinogens in the carcinogenic potency database

A tabulation of carcinogenic potency (TD50) by species for 492 chemicals that induce tumors in rats or mice is presented. With the use of the Carcinogenic Potency Database, experimental results are summarized by indicating in which sex-species groups the chemical was tested and the respective evaluations of carcinogenicity. A comparison of three summary measures of TD50 for chemicals with more than one positive experiment per species shows that the most potent TD50 value is similar to measures that average values or functions of values. This tabulation can be used to investigate associations between rodent potency and other factors such as mutagenicity, teratogenicity, chemical structure, and human exposure.

Development and validation of a cellular transplant model for leukemia in Fischer rats: a short-term assay for potential anti-leukemic chemicals

The efficacy of a leukemia cell transplant model to measure potential chemotherapeutic activity was tested with five different chemicals that had previously been evaluated in 2-year studies. Leukemic spleen cells from Fischer rats were injected subcutaneously into syngeneic recipients and the effects of chemical treatment on tumor progression were evaluated at 70 days post-transplant. The data from the short-term assay were in all cases correlated with the trends reported for mononuclear cell leukemia in 2-year studies, where two chemicals were reported to decrease the incidence and three chemicals were reported to increase the incidence of leukemia. Short-term treatment with the two chemicals which caused negative trends for leukemia (2-ethoxyethanol or ethylene glycol monoethyl ether; 4-hexylresorcinol) delayed and/or reduced tumor growth in the transplant model in a dose-related fashion, as exhibited by reduction or elimination of splenomegaly and leukoblastosis, and a reversal in the depression of red blood cell indices or platelet counts. By contrast, the rate of tumor progression was increased in the short-term assay of the three chemicals which previously caused increased trends for leukemia in 2-year studies (pyridine; 2,4,6-trichlorophenol, dichlorvos). The severity of the mononuclear cell leukemia in the transplant recipients, as measured by histopathological examination of spleen and liver, was correlated with the changes in tumor growth rates. The in vivo leukemia transplant model is a short-term assay that could be used to screen a variety of potential chemotherapeutic agents, or to study structure-activity relationships within one class of chemicals.

Effects of quinoline and 8-hydroxyquinoline on mouse bone marrow erythrocytes as measured by the micronucleus assay

Both quinoline and 8-hydroxyquinoline (HOQ) were tested for their genotoxicity in CD1 male mice by using a bone marrow micronucleus assay. Mice were intraperitoneally treated in single injections with three dose levels (25, 50, and 100 mg/kg) of each chemical with corn oil as solvent vehicle. Bone marrow was sampled at 24, 48, and 72 h postinjection. Quinoline resulted in a significant dose-related increase in the number of micronucleated polychromatic erythrocytes (MPCE) at the 24 h sampling time for all doses tested. The high dose (100 mg/kg) and the medium dose (50 mg/kg) also induced statistically significant increases (P less than .05) in the number of MPCEs at 48 h interval. The ratios of polychromatic to normochromatic erythrocytes at the 24 h sampling time were lower for the treated than the control animals. Although HOQ resulted in some increases in the number of MPCEs over the control, this compound induced a statistically significant increase in the number of micronucleated normochromatic erythrocytes (MNCEs) at all three doses following 24 h treatment. Both low and medium doses also induced a higher incidence of MNCEs at the 48 and 72 h sampling times. No data were available for the high dose at these times. The cytotoxic effect of this compound was expressed as low PCE/NCE ratios with all doses at 24 h after injection and as a high mortality rate in animals treated with the high dose (100 mg/kg).

Tetrachloroethane, pentachloroethane, and hexachloroethane: genetic and biochemical studies

Tetrachloroethane (TTCE), pentachloroethane (PCE), and hexachloroethane (HCE) were tested in diploid strain (D7) of the yeast Saccharomyces cerevisiae in suspension test with and without mammalian metabolic activation (S9). TTCE, PCE, and HCE gave positive results on cells harvested from logarithmic growth phase; only PCE induced a significant increase (P less than or equal to .01) of mitotic gene conversion and point reverse mutation on cells from stationary growth phase with metabolic activation (S9). The in vivo effects on cytochrome P450 content (cyt. P450), pentoxyresorufin O-dealkylase (P450-like, class IIB, PROD), and ethoxy-resorufin O-deethylase (P448-like, class IA, EROD) activities were examined in hepatic microsomes from mice 24 h after acute intoxication. All the halogenated hydrocarbons displayed a marked toxic effect as shown by the significant decrease in cyt. P450 levels (maximum of 76% decrease, with TTCE 753.2 mg/kg) and EROD (maximum of 69% decrease, with PCE 925.4 mg/kg), and to a lesser extent in PROD (maximum of 52.4% decrease, with HCE 3150 mg/kg). Although a general decrease of P450 functions was observed, the toxic effects of TTCE and PCE seem to be preferentially related to P448 forms.