Consideration of both genotoxic and nongenotoxic mechanisms in predicting carcinogenic potential

Cell proliferation in the livers of male mice and rats exposed to the carcinogen P-dichlorobenzene: evidence for thresholds

In a previous study, p-dichlorobenzene (pDCB), which is associated with tumorigenicity in male rat kidney and livers of mice of both genders, was found to produce acute increases in cell proliferation in those tissues. To determine whether sustained cell proliferation in the liver in susceptible species correlated with reported carcinogenic effects, we examined the effect of pDCB on cell proliferation in the livers and toxicity to the glutamine synthetase-expressing hepatocyte (GS+) subpopulation of male B6C3F1C3F1 mice and F344 rats. Mice were exposed for up to 4 weeks to 600, the maximally tolerated dose which increased liver tumors, 300 or 150 mg/kg. Rats were exposed to 300, 150 or 75 mg/kg for up to 4 weeks. In mice, the cumulative replicating fraction (CRF) in the livers of the high dose animals was significantly increased 16-fold at 1 week and 4-fold at 4 weeks. The CRF was also increased at 300 mg/kg at 1 week, but this subsided at 4 weeks. No increase was seen in the low dose group. In rats, the CRFs of the livers at 1 week were increased at 300 and 150 mg/kg, but returned to normal at 4 weeks. The size of the hepatic GS+ area was not affected in mice or in rats after 1 week of exposure, but comparable decreases were observed at all exposures at 4 weeks in mice. The data therefore suggest that sustained increases of cell division in the mouse liver may contribute to the increases in liver tumors. The transient increase in rat liver suggests that this is not sufficient to enhance tumor development. The absence of sustained increases of the CRFs at the low dose in mice, which was one-fourth of the hepatocarcinogenic dose, implies the existence of a threshold in pDCB hepatocarcinogenesis.

Chemically-induced nasal carcinogenesis and epithelial cell proliferation: a brief review

An increased rate of cell proliferation has long been recognized as an important factor in both human and experimental carcinogenesis, and may be a major risk factor for cancer development in a number of tissues. Limited information exists, however, regarding the relevance of increased cell proliferation and nasal cancer. Examples of toxicological studies utilizing nasal cell proliferation data as an important endpoint are briefly reviewed. Data for one of the most extensively studied chemicals, the weakly genotoxic carcinogen formaldehyde, support the contention that the concentration-response relationship for tumor incidence is a function of formaldehyde-induced target cell proliferation, in addition to other factors including target cell population size. The increasing importance of utilizing cell proliferation data in determining dose-response relationships and in biologically-based risk assessment models is discussed.

Neonatal mouse assay for tumorigenicity: alternative to the chronic rodent bioassay

The chronic rodent bioassay for tumors has been utilized systematically for 25 years to identify chemicals with carcinogenic potential in man. In general, those chemicals exhibiting tumorigenicity at multiple sites in both mice and rats have been regarded as possessing strong carcinogenic potential in humans. In comparison, the value of data collected for those test chemicals exhibiting more sporadic tumorigenicity results (e.g., single species/single sex or dose-independent) has been questioned. As knowledge of the carcinogenic process has increased, several alternative test systems, usually faster and less expensive than the 2-year bioassay, have been suggested for identification of the strongly acting, transspecies carcinogens. The International Conference on Harmonization for Technical Requirements for the Registration of Pharmaceuticals for Human Use has proposed an international standard that allows for the use of one long-term rodent carcinogenicity study, plus one supplementary study to identify potential human pharmaceutical carcinogens. The neonatal mouse assay for tumorigenicity has been used since 1959; however, relative to other alternate tests, little has been written about this system. It is clear that this assay system successfully identifies transspecies carcinogens from numerous chemical classes, thus recommending itself as a strong candidate for a supplementary study to identify potential human carcinogens. In contrast, there are decidedly less data available from this assay in response to pharmaceuticals shown to exhibit weak and/or conflicting results in the 2-year bioassay, knowledge invaluable to the regulatory process. This paper reviews the historical development and our experience with the neonatal mouse assay and includes suggestions for a standardized protocol and strategies to document its response to "weak" and/or "nongenotoxic" carcinogens.

Lobule-dependent zonal measurement (LZM) method for the determination of cell proliferation in the liver

Using the conventional method of measurement (randomly distributed fields) there is a risk of overlooking a biologically relevant effect due to the different zonal expression of cell proliferation. Therefore a lobule-dependent zonal measurement (LZM) method was developed that guaranteed the independent evaluation of the hepatic zones in lobules of a comparable size. With this method, applied in a 3-month-study, a statistically significant increase in cell proliferation after one week (71%) and a distinct increase after six weeks (49%) and thirteen weeks (32%) of exposure could be proved, whereas the conventional method revealed after one week only a slight increase (22%), after six weeks a distinct but not statistically significant increase (56%) and after thirteen weeks even a decrease of 24%. In comparison, the LZM-method revealed a panlobular increase of cell proliferation after one week of administration, that shifted to the periportal region after six and thirteen weeks representing the target zone as it was shown by electronmicroscopy later on. Since the test substance evoked an increased tumor incidence in both sexes in a lif-span study, the enhanced cell proliferation, in our case especially in the periportal area, seems to be a key mechanism in tumor development. These data confirm that the, LZM-method represents a very sensitive and stable method in the evaluation of cell proliferation in the liver.

Increased nuclear ploidy, not cell proliferation, is sustained in the peroxisome proliferator-treated rat liver

Peroxisome proliferators are believed to induce liver tumors in rodents due to sustained increase in cell proliferation and oxidative stress resulting from the induction of peroxisomal enzymes. The objective of this study was to conduct a sequential analysis of the early changes in cell-cycle kinetics and the dynamics of rat liver DNA synthesis after treatment with a peroxisome proliferator. Immunofluorescent detection of proliferating cell nuclear antigen (PCNA) and bromodeoxyuridine (BrdU) incorporation into DNA during S phase we used to assess rat hepatocyte proliferation in vivo during dietary administration of Wy-14,643, a known peroxisome proliferator and hepatocarcinogen in rodents. Rats were placed on diet containing 0.1% WY-14,643 and implanted subcutaneously with 5-bromo-2'deoxyuridine containing osmotic pumps 4 days prior to being sacrificed on days 4, 11, and 25 of treatment. Isolated liver nuclei labeled with fluorscein isothiocyanate (FITC)-anti-BrdU/PI and FITC-anti-PCNA/PI were analyzed for S-phase kinetics using flow cytometry. Morphometric analysis was performed to evaluate nuclear and cell size and enumeration of BrdU labeled cells, binucleated hepatocytes, and mitotic index. The BrdU labeling index increased 2-fold in livers of Wy-14,643-treated rats at day 4, but distribution of cells in G1, S phase, and G2-M did not differ significantly from controls. PCNA-positive cells decreased from 36% on day 4 to 17% on day 25, whereas the percentage of PCNA-positive cells in controls increased 2-fold from day 4 to day 11 and remained unchanged up to day 25. The differences in the number of PCNA-positive nuclei between control and Wy-14,643-treated groups were statistically significant only on day 4. Binucleated hepatocytes, determined by morphometric analysis, increased slightly on day 25 in treated rats parallel to an increase in the percentage of cells in G2-M phase. Significant shifts were noted in nuclear diameter and nuclear area after 11 and 25 days of treatment with Wy-14,643. Hepatic cell populations with nuclei > 9 microns diameter and nuclear area > 64 microns2 increased in Wy-14,643-fed rats during the treatment period compared with the control, indicating hepatic karyomegaly and hyperploidy, whereas percentage of distribution of nuclei based on diameter and area remained consistently unchanged in control animals from 4 through 25 days of sham treatment. The flow cytometric and morphometric analysis indicated an initial wave of DNA synthesis in response to Wy-14,643. The hepatomegaly was sustained over the treatment period accompanied by increase in ploidy with a significant shift toward hyperploidic hepatocytes. The increase in DNA content was almost entirely accounted for by the overall polypoidy increase rather than by an absolute increase in cells.

The utility of two rodent species in carcinogenic risk assessment of pharmaceuticals in Europe

For the past 20-30 years, lifespan carcinogenicity studies for pharmaceuticals have been required to be carried out in two rodent species. Due to scientific progress, the necessity/justification of lifespan studies in two species for the assessment of carcinogenic risk of pharmaceuticals is currently under discussion. A study in one species (either rat or mouse) might suffice. To appraise the need for a study in a second species, a database was compiled of all pharmaceuticals tested for carcinogenicity for which a marketing authorization was applied for in Germany and The Netherlands since 1980. The incidence of treatment-related tumor findings was determined in either rat or mouse or in both. Tumor findings occurred for nearly 50% of all compounds, with the rat being more sensitive than the mouse. Specific attention was given to the question whether tumor findings in mice ever caused the regulatory authorities to refuse registration, to restrict the proposed therapeutic indication of a pharmaceutical, or to apply a cautionary label. It was found that no tumor findings in mice alone ever led to such a regulatory action. In addition, whether mouse studies had been important in interpreting the results of rat studies was determined. A negative mouse study (no tumors found) was rarely used to declare the rat findings irrelevant to humans. A mechanistic explanation was used as a much more important argument in the assessment of tumor findings in rats. In case of transspecies findings, the target organs were the usual ones, such as lung and liver, or the tumors occurred as a result of an exaggerated pharmacodynamic action expected from the pharmacology of the compound. The results of the database thus question the need of maintaining the requirement of rodent carcinogenicity studies in two species.

Measurement of replicative DNA synthesis (RDS) by a 5-bromo-2'-deoxyuridine (BrdU) labeling technique for detection of hepatocyte proliferation

The hypothesis has been proposed that cell proliferation, or replicative DNA synthesis (RDS) in S-phase cells, is a nongenotoxic (Ames-negative) mechanism involved in tumorigenesis, providing a very useful conceptual basis for carcinogen testing. In this present study, hepatocyte RDS experiments were conducted using 5-bromo-2'-deoxyuridine (BrdU) labeling in combination with histopathological observation, comparing our results with earlier findings for in situ [3H]thymidine (TdR) labeling. The present BrdU data proved to be consistent with the previous TdR data in all but one case. Hepatocyte RDS induction was observed for some chemicals without hepatotoxicity. BrdU labeling in combination with histopathological observation is therefore a reliable approach to assessment of test compound effects in vivo.

The pH 6.7 Syrian hamster embryo cell transformation assay for assessing the carcinogenic potential of chemicals

Cell transformation models have been established for studying the cellular and molecular basis of the neoplastic process. Transformation models have also been utilized extensively for studying mechanisms of chemical carcinogenesis and, to a lesser degree, screening chemicals for their carcinogenic potential. Complexities associated with the conduct of cell transformation assays have been a significant factor in discouraging broad use of this approach despite their reported good predictivity for carcinogenicity. We previously reported that many of the experimental difficulties with the Syrian hamster embryo (SHE) cell transformation assay could be reduced or eliminated by culturing these cells at pH 6.7 culture conditions compared to the historically used pH 7.1-7.3. We and others have shown that morphological transformation (MT), the earliest recognizable phenotype in the multi-step transformation process and the endpoint used in the standard assay to indicate a chemical's transforming activity, represents a pre-neoplastic stage in this model system. In the collaborative study reported here, in which approx. 50% of the chemicals were tested under code in one laboratory (Hazelton) and the other 50% evaluated by several investigators in the second laboratory (P & G), we have evaluated 56 chemicals (30 carcinogens, 18 non-carcinogens, 8 of inconclusive carcinogenic activity) in the SHE cell transformation assay conducted at pH 6.7 culture conditions with a standardized, Good Laboratory Practices-quality protocol. An overall concordance of 85% (41/48) between SHE cell transformation and rodent bioassay results was observed with assay sensitivity of 87% (26/30) and specificity of 83% (15/18), respectively. The assay exhibited a sensitivity of 78% (14/18) for Salmonella assay negative carcinogens, supporting its value for detecting non-mutagenic carcinogens. For maximum assay sensitivity, two exposure durations were required, namely a 24-h exposure and a 7-day exposure assay. Depending on the duration of chemical treatment required to induce transformation, insight into the mechanism of transformation induction may also be gained. Based on the data reported here, as well as the larger historical dataset reviewed by Isfort et al. (1996), we conclude that the SHE cell transformation assay provides an improved method for screening chemicals for carcinogenicity relative to current standard genotoxicity assays.

Application of in vitro cell transformation assays to predict the carcinogenic potential of chemicals

Genotoxicity test batteries have become a standard fool for identifying chemicals that may have potential carcinogenic risk to humans. It is now apparent, however, that the use of genotoxicity batteries for assessing carcinogenic potential has limitations including an overall low specificity and a limited ability to detect carcinogens acting via 'nongenotoxic' mechanisms. In vitro cell transformation models, because they measure a chemical's ability to induce preneoplastic or neoplastic endpoints regardless of mechanism, may fulfil the current need for an in vitro biologically relevant model with increased predictiveness for determining carcinogenic potential. This review will focus on data demonstrating the similarities of chemically induced cell transformation in vitro to carcinogenesis in vivo. Furthermore, a growing database demonstrating a high overall correlation between cell transformation results with those of the rodent bioassay will also be discussed. Finally, the inclusion of cell transformation approaches for assessing the carcinogenic potential of chemicals relative to currently used genotoxicity batteries will be presented.

Evaluation of the potential carcinogenicity and genetic toxicity to humans of the herbicide acetochlor

Comprehensive toxicological studies of the herbicide acetochlor are presented and discussed. Although it gave a negative profile of responses in the many toxicity tests conducted there were some findings that prompted further investigation. First, although non-mutagenic in the Salmonella assay, acetochlor was clastogenic to mammalian cells treated in vitro. This clastogenic potential was not expressed in vivo in four rodent cytogenetic assays (bone marrow and germ cells). Second, although acetochlor gave a negative response in rat liver UDS assays when tested at the acute MTD, gavage administration of a single, supra-MTD dose (2000 mg/kg) gave a weak positive assay response. This dose-level (2000 mg/kg) was necrotic to the liver, depressed hepatic glutathione levels by up to approximately 80%, altered the metabolism of acetochlor, and was associated with up to 33% lethality. In contrast, reference liver genotoxins such as DMN, DMH and 2AAF were shown to elicit UDS in the absence of such effects, and at approximately 400 x lower dose-levels. Finally, microscopic nasal polypoid adenomas were induced in the rat when acetochlor was administered for two years at the maximum tolerated dose (MTD). The tumours were not life-threatening, they did not metastasize, and no DNA damage was induced in the nasal cells of rats maintained on a diet containing the MTD of acetochlor for either 1 or 18 weeks (comet assay). In order to probe the mechanism of action of these high dose toxicities a series of chemical and genetic toxicity studies was conducted on acetochlor and a range of structural analogues. These revealed the chloroacetyl substructure to be the clastogenic species in vitro. Although relatively inert, this substituent is preferentially reactive to sulphydryl groupings, most evidently, to glutathione (GSH). Similar chemical reactivity and clastogenicity in vitro was observed for two related chemicals bearing a chloroacetyl group, both of which have been defined as non-carcinogens in studies reported by the US.NTP. These collective observations indicate that the source of the clastogenicity of acetochlor in vitro is also the source of its rapid detoxification in the rat in vivo, via reaction with GSH. Metabolic studies of acetochlor are described which reveal the formation of a series of GSH-associated biliary metabolites in the rat that were not produced in the mouse. The metabolism of acetochlor in the rat changes with increasing dose-levels, probably because of depletion of hepatic GSH. It is most likely that a rat-specific metabolite is responsible for the rat nasal tumours observed uniquely at elevated dose-levels. The absence of genetic toxicity to the nasal epithelium of rats exposed acutely or subchronically to acetochlor favours a non-genotoxic mechanism for the induction of these adenomas. The observation of a time- and dose-related increase in S-phase cells in the nasal epithelium is consistent with this conclusion. Despite some confusion caused by the early use of perilethal gavage administrations of acetochlor to rodents, and supra-MTD dietary concentrations in some of the chronic studies, the available MTD data are consistent with acetochlor not posing a genetic or carcinogenic hazard to humans.

Alternatives to the 2-species bioassay for the identification of potential human carcinogens

It is proposed that the standard 2-species rodent cancer bioassay protocol, as perfected by the US National Toxicology Program (NTP), has already fulfilled its most useful role by providing an unequalled carcinogenicity database by which to re-assess the type of carcinogen worthy of definition. Continued use of this resource and time consuming protocol can no longer be justified, except in rare circumstances of high and protracted human exposure to a chemical of unknown carcinogenicity. In those rare instances an enlarged bioassay of three or four test species should perhaps be considered, there being nothing fundamental about the rat/mouse combination. In the large majority of cases, however, a practical estimation of the carcinogenic potential of a chemical can be formed in the absence of lifetime carcinogenicity bioassay data. This can be achieved by its sequential study, starting with an appreciation of its chemical structure and anticipated reactivity and mammalian metabolism. After the shortterm evaluation of a range of additional properties of the agent, including its genetic toxicity, rodent toxicity and tissue-specific toxicity, confident predictions of the genotoxic and/or non-genotoxic carcinogenic potential of the agent can be made. In most situations these predictions will be suitable for framing hazard reduction measures among exposed humans. In some situations it may be necessary to evaluate these predicted activities using limited bioassays, a range of which are considered. Extensions of these limited carcinogenicity bioassays to a standard 2-year/2-species bioassay can only be supported in cases where the non-carcinogenicity of the agent becomes the important thing to define. The US NTP have evaluated the carcinogenicity of approximately 400 chemicals over the past 20 years, at a cost of hundreds of millions of US dollars. The experience gained by that and related initiatives, worldwide, can now be harnessed to classify thousands of priority chemicals as being either probable carcinogens or probable noncarcinogens. That can now be achieved using a fraction of the earlier resources and in a fraction of the time that would be required for the conduct of 2-species bioassays. The comfort factor for one group of people of the order of the present system, coupled to the comfort factor for another group of the delay in carcinogenicity assessment enforced by the present council of perfection, are the two main factors delaying transfer to a streamlined system for assessing the carcinogenic potential of chemicals to humans. A third delaying factor in the need for new and focused test data. Coordinated acquisition of such data could rapidly remove the first two obstacles.

Application of structural concepts to evaluate the potential carcinogenicity of natural products

The expert structure-activity relational system CASE/MULTICASE was used to obtain an assessment of the possible carcinogenicity of selligueain A, a plant-derived sweetener. Based upon a series of authoritative data bases it was predicted that this chemical had some marginal potential for being a 'non-genotoxic' rodent carcinogen. The relevance of this potential to possible human health risks is problematic. Still, given the fact that successful sweetener may be widely consumed, should this chemical be developed further, experimental determinations of its potential carcinogenicity appear in order.

A computerized connectivity approach for analyzing the structural basis of mutagenicity in Salmonella and its relationship with rodent carcinogenicity

We have applied a new software program, based on graph theory and developed by our group, to predict mutagenicity in Salmonella. The software analyzes, as information in input, the structural formula and the biological activities of a relatively large database of chemicals to generate any possible molecular fragment with size ranging from two to ten nonhydrogen atoms, and detects (as predictors of biological activity) those fragments statistically associated with the biological property investigated. Our previous work used the program to predict carcinogenicity in small rodents. In the current work we applied a modified version of the program, which bases its predictions solely on the most important fragment present in a given molecule, considering as practically negligible the effects of additional less important fragments. For Salmonella mutagenicity we used a database of 551 compounds, and the program achieved a level of predictivity (73.9%) comparable to that obtained by other authors using the Computer Automated Structure Evaluation (CASE) program. We evaluated the relative contributions of biophores and biophobes to overall predictivity: biophores tended to be more important than biophobes, and chemicals containing both biophores and biophobes were more difficult to predict. Many of the molecular fragments identified by the program as being strongly associated with mutagenic activity were similar to the structural alerts identified by the human experts Ashby and Tennant. Our results tend to confirm that structural alerts useful to predict Salmonella mutagenicity are generally not very strong predictors of rodent carcinogenicity. Although the predictivity level achieved for oncogenic activity improved when the program was directly trained with carcinogenicity data, carcinogenicity as a biological endpoint was still more difficult to predict than Salmonella mutagenicity.

Renal activation of trichloroethene and S-(1,2-dichlorovinyl)-L-cysteine and cell proliferative responses in the kidneys of F344 rats and B6C3F1 mice

Covalent binding of reactive intermediates formed by renal beta-lyase activation of S-(1,2-dichlorovinyl)-L-cysteine (DCVC) has been suggested to be responsible for the greater renal sensitivity of rats than mice to the carcinogenic effects of chronic treatment with trichloroethene (TRI). Previous work demonstrated that the activation of DCVC results in acid-labile adducts to protein that can be distinguished from adducts formed by other pathways of TRI metabolism. By analyzing acid-labile adduct formation, the relationship between DCVC formation and activation from TRI and increases in rates of cell division in the kidneys of male F344 rats and B6C3F1 mice could be investigated. The delivered dose of DCVC from an oral dose of 1000 mg/kg TRI was approximately six times greater in rats than mice. However, renal activation of DCVC in mice was approximately 12 times greater than in rats. Therefore, the overall activation of TRI was about two times greater in mice than rats. Induction of cell replication in liver and kidney following doses of 1, 5, or 25 mg/kg DCVC or 1000 mg/kg TRI was also measured through the use of miniosmotic pumps that delivered BrdU subcutaneously for 3 d. Acid-labile adduct formation from DCVC and TRI displayed a consistent relationship with increased cell replication in mice and between mice and rats. Both cell replication and acid-labile adduct formation in rats given 25 mg/kg DCVC were approximately equal to that observed in mice given 1 mg/kg. Increased cell replication was not observed in rats receiving 1 or 5 mg/kg DCVC or 1000 mg/kg TRI, nor were there histological signs of nephrotoxicity. Thus, net activation of TRI by the cysteine S-conjugate pathway was found to be greater in mice than rats and these findings appeared related to differences in cell proliferative responses of the kidneys of the two species. Based on these data, it would appear that other factors must contribute to the greater sensitivity of the rat to the induction of renal carcinogenesis by TRI.

Correlation of individual papilloma latency time with DNA adducts, 8-hydroxy-2'-deoxyguanosine, and the rate of DNA synthesis in the epidermis of mice treated with 7,12-dimethylbenz[alpha]anthracene

The question was addressed whether the risk of cancer of an individual in a heterogeneous population can be predicted on the basis of measurable biochemical and biological variables postulated to be associated with the process of chemical carcinogenesis. Using the skin tumor model with outbred male NMRI mice, the latency time for the appearance of a papilloma was used as an indicator of the individual cancer risk. Starting at 8 weeks of age, a group of 29 mice was treated twice weekly with 20 nmol of 7,12-dimethylbenz[alpha]anthracene (DMBA) applied to back skin. The individual papilloma latency time ranged from 13.5 to 25 weeks of treatment. Two weeks after the appearance of the first papilloma in each mouse, an osmotic minipump delivering 5-bromo-2'-deoxyuridine was s.c. implanted and the mouse was killed 24 hr later. Levels of DMBA-DNA adducts, of 8-hydroxy-2'-deoxyguanosine, and various measures of the kinetics of cell division were determined in the epidermis of the treated skin area. The levels of 8-hydroxy-2'-deoxyguanosine and the fraction of cells in DNA replication (labeling index for the incorporation of 5-bromo-2'-deoxyuridine) were significantly higher in those mice that showed short latency times. On the other hand, the levels of DMBA-DNA adducts were lowest in animals with short latency times. The latter finding was rather unexpected but can be explained as a consequence of the inverse correlation seen for the labeling index: with each round of cell division, the adduct concentration is reduced to 50% because the new DNA strand is free of DMBA adducts until the next treatment. Under the conditions of this bioassay, therefore, oxygen radical-related genotoxicity and the rate of cell division, rather than levels of carcinogen-DNA adducts, were found to be of predictive value as indicators of an individual cancer risk.

The in vivo-in vitro replicative DNA synthesis (RDS) test with hepatocytes prepared from male B6C3F1 mice as an early prediction assay for putative nongenotoxic (Ames-negative) mouse hepatocarcinogens

To assess the efficacy of the in vivo-in vitro hepatocyte replicative DNA synthesis (RDS) test as a short-term assay, 41 putative nongenotoxic (Ames-negative) mouse hepatocarcinogens, as well as 31 noncarcinogens, were examined using male 8-week-old B6C3F1 mice and an in vitro [methyl-3]thymidine-incorporation technique. Animals were exposed to the maximum tolerated dose (MTD) and 1/2 MTD of each chemical by gavage and after 24, 39 or 48 h, hepatocytes were prepared with a collagenase-perfusion technique. Assessment of the distribution of spontaneous RDS in a total of 337 control mice gave an average incidence of 0.15 +/- 0.08% within the range of 0 to 0.39% (mean +/- 3 x SE) with a 99.7% probability. Values of 0.4% or more for RDS incidences induced by test samples were therefore judged as indicating a positive response in our RDS test. Under the experimental conditions applied, 32 of 41 putative nongenotoxic hepatocarcinogens gave clear positive responses (positive sensitivity: 78%), and of 31 noncarcinogens 25 samples gave negative responses (negative specificity: 81%), thus giving an overall concordance for the RDS test with long-term findings of 79%.

Expression of exposure in negative carcinogenicity studies: dose/body weight, dose/body surface area, or plasma concentrations?

Evaluation of positive findings in a rodent carcinogenicity study and the subsequent extrapolation to humans is based on chemical structure, mutagenicity, pharmacology, hormone changes, chronic toxicity, and the nature of the tumors induced. For negative studies, adequacy of exposure may become an issue. The use of plasma concentrations as a metric for exposure assumes that each species responds in a similar manner to a given concentration; data are now available that demonstrate that this is not generally true for carcinogenicity. The use of the body surface area metric (i.e., mg/m2) is a special case of interspecies allometric scaling (i.e., W0.67). For a chemical to be amenable to such scaling in toxicology, it must satisfy 3 criteria: (a) the concentration-time profile of the putative toxicant at the site of action must be governed by a scalable pharmacokinetic process (e.g., glomerular filtration); (b) the mechanism of action and the susceptibility of each species to a given systemic exposure must be the same and, for example, be independent of lifespan, cellular repair mechanism/rate, and so forth; and (c) the biological response must depend only on size (e.g., not on race, strain, gender, age, or parity). Carcinogens rarely, if ever, meet these criteria. An empirical analysis of carcinogenic potency data in rodents and in humans shows that, in general, exposure is best expressed in terms of mg/kg body weight.

Induced regenerative cell proliferation in livers and kidneys of male F-344 rats given chloroform in corn oil by gavage or ad libitum in drinking water

These studies were designed to establish the dose response relationships for the induction of cytolethality and regenerative cell proliferation in the liver and kidneys of male F-344 rats given chloroform by gavage or in drinking water. Rats were administered oral doses of 0, 10, 34, 90 or 180 mg/kg/day chloroform dissolved in corn oil by gavage for 4 days or for 5 days/week for 3 weeks. A second group of rats was given chloroform ad libitum in the drinking water at concentrations of 0, 60, 200, 400, 900 or 1800 ppm for 4 days or 3 weeks. Bromodeoxyuridine (BrdU) was administered via an implanted osmotic pump 3.5 days prior to necropsy to label cells in S-phase. Cells having incorporated BrdU were visualized in tissue sections immunohistochemically and the labelling index (LI) evaluated as the percentage of S-phase cells. Rats treated with 90 or 180 mg/kg/day by gavage for 4 days had mild to moderate degeneration of renal proximal tubules and centrilobular hepatocytes. These alterations were absent or slight after 3 weeks of treatment. LI were increased in the kidney cortex only in the rats treated with 180 mg/kg/day for 4 days. A dose-dependent increase in LI was seen in rat liver after 4 days of treatment with 90 and 180 mg/kg/day by gavage, but the LI remained elevated after 3 weeks of treatment only at the 180 mg/kg/day dose. When chloroform was administered in the drinking water, no microscopic alterations were seen in the kidneys after 4 days of treatment. As a general observation, rats treated for 3 weeks with 200 ppm chloroform and greater had slightly increased numbers of focal areas of regenerating renal proximal tubular epithelium and cell proliferation than were noted in the controls, but no clear dose response relationship was evident. However, the overall renal LI was not increased at any dose or time point. Similarly, only mild hepatocyte vacuolation was observed in rats given 1800 ppm chloroform in the water for 3 weeks with no increase in the hepatic LI at any time point, even though the rats were consuming chloroform at a rate of 106 mg/kg/day at the 1800 ppm drinking water concentration. These data indicate more severe hepatic and renal toxicity when chloroform is administered by gavage than in the drinking water and a different pattern of regenerative proliferation in the kidney.(ABSTRACT TRUNCATED AT 400 WORDS)

An examination of the potential "genotoxic" carcinogenicity of a biopesticide derived from the neem tree

Structural analyses of azadirachtin, a promising biopesticide recently introduced into the United States, indicates that this natural product has the potential for acting as a "genotoxic" carcinogen. In view of the fact that genotoxic carcinogens are regarded as presenting a potential carcinogenic risk to humans, the present finding suggests that the possible metabolism of azadirachtin to DNA-reactive products be evaluated experimentally.

Application of integrated genetic monitoring: the optimal approach for detecting environmental carcinogens

Short-term in vitro genetic toxicity assays have not fulfilled their anticipated role in predicting the carcinogenicity of environmental agents reliably and economically. A reduction in emphasis from nonanimal systems to relevant animal assays and population monitoring will help to reestablish the credibility of this field. An analysis of the various steps in the carcinogenic process indicates the biological responses occurring during these stages can be utilized for early detection of environmental carcinogens. Emphasis should be placed on using the earliest significant response that indicates genetic damage (e.g., gene mutations and chromosome alterations). Assays that detect pregenomic damage (e.g., adduct formation), without evidence of subsequent heritable genetic alterations, may produce misleading results for risk assessment and should not be considered as stand-alone monitoring procedures. Late biological responses may occur in tissues or organs where genetic damage may be difficult to measure, and the opportunity for intervention diminishes as we approach the clinical outcome. For example, analyzing localized cells that contain activated protooncogenes and inactivated tumor suppressor genes, although they further document adverse response from exposure to carcinogens, may be of greater value for indicating clinical outcome than for genetic monitoring. With few notable exceptions, the window of opportunity for genetic monitoring is the period after exposure where genetic damage is evident and where circulating lymphocytes can faithfully record this damage. An ongoing study of butadiene-exposed workers illustrates an optimum protocol, where multiple assays can be carried out and correlated with both external and internal measurements of exposure.(ABSTRACT TRUNCATED AT 250 WORDS)

Cell proliferation and formaldehyde-induced respiratory carcinogenesis

Formaldehyde is a nasal carcinogen in the rat but the cancer risk this chemical poses for humans remains to be determined. Formaldehyde induces nonlinear, concentration-dependent increases in nasal epithelial cell proliferation and DNA-protein cross-link formation following short-term exposure. Presented in this review are results from a mechanistically based formaldehyde inhalation study in which an important endpoint was the measurement of cell proliferation indices in target sites for nasal tumor induction. Male Fischer 344 rats were exposed to 0, 0.7, 2, 6, 10, or 15 ppm formaldehyde for up to 2 years (6 hr/day, 5 day/week). Statistically significant increases in cell proliferation were confined to the 10 and 15 ppm groups, which remained elevated throughout the study. The concentration-dependent increases in cell proliferation correlated strongly with the tumor response curve, supporting the proposal that sustained increases in cell proliferation are an important component of formaldehyde carcinogenesis. The nonlinearity observed in formaldehyde-induced rodent nasal cancer is consistent with a high-concentration effect of regenerative cell proliferation of the target organ coupled with the genotoxic effects of formaldehyde. Cell kinetic data from these studies provide important information that may be utilized in the assessment of risk for humans exposed to formaldehyde.

Dose-response relationship for rat liver DNA damage caused by 49 rodent carcinogens

An experimental approach was taken to the question of dose-response curves for chemical carcinogenesis. DNA damage in female rat liver was chosen as the experimental parameter because all chemicals found to damage hepatic DNA were rodent carcinogens. The lowest dose causing DNA damage was determined for the 12 active chemicals (1,2-dibromoethane, 1,2-dibromo-3-chloropropane, 1,2-dichloroethane, 1,4-dioxane, methylene chloride, auramine O, Michler's ketone, selenium sulfide, 1,3-dichloropropene, 1,2-dimethylhydrazine, N-nitroso-piperidine and butylated hydroxytoluene). The resulting dose-response curves for rat hepatic DNA damage were plotted versus log of the molar dose (all activity was in five orders of magnitude) and versus percent of chemicals' oral rat LD50 (most of the activity was in only two orders of magnitude). Dose-response studies of the active chemicals were analyzed by regression methods. With the exception of butylated hydroxytoluene, the dose-response curves fit a linear model well (r2 = 0.886) and a quadratic model even better (r2 = 0.947). Based on experimental data from 11 DNA-damaging carcinogens (a dose range of 6 orders of magnitude), an equation and graph of the dose-response relationship of an 'average DNA-damaging carcinogen' is presented over the x-axis dose range of eight orders of magnitude.

Mechanisms of non-genotoxic carcinogenesis

Until recently, the mechanism of carcinogenesis has been regarded as a two-stage phenomenon involving damage to the genetic material, which initiates the process, followed by a cell-division stimulus, which promotes the development of the tumour. However, exposure to some chemicals has been shown to result in carcinogenesis without involvement of the initiation step. The mechanism of non-genotoxic carcinogenesis is not fully understood, but is believed to involve stimulation of cell division with a consequent increased probability of a mutation occurring spontaneously. In this article, Ian Shaw and Huw Jones review the theories of non-genotoxic carcinogenesis with reference to specific examples of known non-genotoxic carcinogens.

Evaluation of the yeast DEL assay with 10 compounds selected by the International Program on Chemical Safety for the evaluation of short-term tests for carcinogens

The Saccharomyces cerevisiae DEL assay detects a wide variety of nonmutagenic carcinogens (Schiestl et al. (1989) Carcinogenesis, 10, 1445-1455). This study shows the effect on DEL recombination of 8 carcinogenic compounds (o-toluidine, hexamethylphosphoramide, safrole, acrylonitrile, benzene, diethylhexylphthalate, phenobarbital and diethylstilbestrol) and 2 noncarcinogenic compounds (caprolactam and benzoin). These chemicals have been selected by the Program on Chemical Safety for the evaluation of short-term tests for carcinogens, because sufficient carcinogenicity data for these compounds exist, and because they are difficult to detect with the Salmonella assay. 5 of 8 carcinogens reproducibly gave a strong positive response and the noncarcinogen benzoin was negative. Thus, 60% of the chemicals tested in this study have been correctly identified with the DEL assay compared to only 20% with the Salmonella assay.

An in vivo-in vitro replicative DNA synthesis (RDS) test using rat hepatocytes as an early prediction assay for nongenotoxic hepatocarcinogens screening of 22 known positives and 25 noncarcinogens

To evaluate the applicability of an in vivo-in vitro replicative DNA synthesis (RDS) test using rat hepatocytes, we conducted the RDS test with 22 nongenotoxic (Ames-negative) hepatocarcinogens and 25 noncarcinogens under our standardized conditions and judgement criteria. Compared to controls (RDS incidence of under 1.0%), the RDS test gave positive results for 18 hepatocarcinogens (positive sensitivity: 82%), and negative results for 20 noncarcinogens (negative specificity: 80%), and thus the overall concordance was 81%. These findings strongly suggest that the RDS test is an extremely useful method for early detection of nongenotoxic hepatocarcinogens.

Complementarity of genotoxic and nongenotoxic predictors of rodent carcinogenicity

Twenty-one chemicals carcinogenic in rodent bioassays were selected for study. The chemicals were administered by gavage in two dose levels to female Sprague-Dawley rats. The effects of these 21 chemicals on four biochemical assays [hepatic DNA damage by alkaline elution (DD), hepatic ornithine decarboxylase activity (ODC), serum alanine aminotransferase activity (ALT), and hepatic cytochrome P-450 content (P450)] were determined. Available data from seven cancer predictors published by others [the Ames test (AMES), mutation in Salmonella typhimurium TA 1537 (TA 1537), structural alerts (SA), mutation in mouse lymphoma cells (MOLY), chromosomal aberrations in Chinese hamster ovary cells (ABS), sister chromatid exchange in hamster ovary cells (SCE), and the ke test (ke)] were also compiled for these 21 chemical carcinogens plus 28 carcinogens and 62 noncarcinogens already published by our laboratory. From the resulting 111 (chemicals) by 11 (individual cancer predictors) data matrix, the five operational characteristics (sensitivity, specificity, positive predictivity, negative predictivity, and concordance) of each of the 11 individual cancer predictors (four biochemical parameters of this study and seven cancer predictors of others) are presented. Two examples of complementarity or synergy of composite cancer predictors were found. To obtain maximum concordance it was necessary to combine both genotoxic and nongenotoxic cancer predictors. The composite cancer predictor (DD or [ODC and P450] or [ODC and ALT]) had higher concordance than did any of the four individual cancer predictors from which it was constructed. Similarly, the composite cancer predictor (TA 1537 or DD or [ODC and P450] or [ODC and ALT]) had higher concordance than any of its five individual constituent cancer predictors. Complementarity or synergy has been demonstrated both 1) among genotoxic cancer predictors (DD and TA 1537) and 2) between nongenotoxic (ODC, P450, and ALT) and genotoxic cancer predictors (TA 1537 and DD).

Expression of myc, fos, and Ha-ras in the livers of furan-treated F344 rats and B6C3F1 mice

Furan administered by gavage for 2 yr has been reported to induce hepatocellular carcinomas in male and female B6C3F1 mice and in male but not female F344 rats. Chronic exposure studies in our laboratory using bioassay conditions showed extensive hepatocellular toxicity and sustained increases in regenerative cell proliferation after 1, 3, and 6 wk of treatment in male and female rats and male mice. Altered expression of growth-control genes associated with this hyperproliferative state may enhance the susceptibility of these genes to mutation or may provide a selective growth advantage to preneoplastic cells. Quantitative northern blot analysis of mRNA was used to examine the expression of the oncogenes myc, fos, and Ha-ras in the livers of animals treated with furan. In male rats, a single administration of 30 mg/kg furan produced necrosis and a subsequent wave of cell proliferation 48 h after treatment and induced transient peaks in the expression of myc, fos, and Ha-ras 6-24 h after treatment. In male rat liver from our cell proliferation studies, only a slight increase in myc expression was seen at the end of week 1 of treatment. However, beginning at week 3 and increasing at week 6, up to a 15-fold increase over control values was observed in the expression of myc in the treated animals. The only other notable increase in expression observed in any animals from the cell proliferation study was a threefold increase in myc at week 6 in treated female rats. The absence of an increase in Ha-ras expression in the male mouse liver suggests that the unique pattern of Ha-ras mutations previously reported in furan-induced mouse liver tumors is not due to increased mutational susceptibility related to overexpression of this gene. The lack of sustained expression of myc, fos, and Ha-ras in rapidly proliferating liver suggests that continuous expression of these genes is not necessary to maintain increased rates of cell replication. The large increase in myc expression in male but not female rats suggests an adaptive change that may be related to the sex-specific incidence of furan-induced hepatocellular carcinomas in rats.

Lack of chloroform-induced DNA repair in vitro and in vivo in hepatocytes of female B6C3F1 mice

Chloroform has been shown to induce hepatocellular carcinomas in female B6C3F1 mice when administered by gavage, but not when given in drinking water. When administered in corn oil at the carcinogenic doses of 238 and 477 mg/kg, chloroform induced necrosis and sustained regenerative cell proliferation in the liver. To investigate the mode of action of tumor induction in the target cells, the ability of chloroform to induce unscheduled DNA synthesis (UDS) was examined in the in vitro and in vivo hepatocyte DNA repair assays. In the in vitro assay, primary hepatocyte cultures from female B6C3F1 mice were incubated with concentrations from 0.01 to 10 mM chloroform in the presence of 3H-thymidine. UDS was assessed by quantitative autoradiography. No induction of DNA repair was observed at any concentration. In the in vivo assay, animals were treated by gavage with 238 and 477 mg/kg chloroform in corn oil. Primary hepatocyte cultures were prepared 2 and 12 hr later, incubated with 3H-thymidine, and assessed for induction of UDS as above. No DNA repair activity was seen at either dose or at either timepoint. These negative results in the target organ are consistent with the concept that neither chloroform nor its metabolites are directly DNA reactive and that the carcinogenicity of chloroform is secondary to induced cytolethality and regenerative cell proliferation.

Predicting rodent carcinogenicity of Ames test false positives by in vivo biochemical parameters

28 chemicals known to be mutagenic in the Ames test but not carcinogenic in rodent bioassays were selected for study. The chemicals were administered by gavage in 2 dose levels to female Sprague-Dawley rats. The effects of these 28 chemicals on 4 biochemical assays (hepatic DNA damage by alkaline elution (DD), hepatic ornithine decarboxylase activity (ODC), serum alanine aminotransferase activity (ALT), and hepatic cytochrome P-450 content (P450)) were determined. The scientific approach taken was to either experimentally find individual cancer predictors of high specificity or to mathematically create composite predictors of high specificity. Composite predictive parameters are defined as follows: CP = [ODC and P450], CT = [ALT and ODC], and TS = [DD or CP or CT]. The specificity (percent of rodent noncarcinogens which test negative) of DD, ODC, ALT, P450, CP, CT and TS was 100%, 46%, 89%, 86%, 93%, 93% and 86%, respectively. For these 28 mutagenic noncarcinogens, the specificity of structural alerts (SA) 13%, mutation in mouse lymphoma cells (MOLY) 0%, chromosomal aberrations in Chinese hamster ovary cells (ABS) 13%, and sister-chromatid exchange in Chinese hamster ovary cells (SCE) 0% were much lower. The ke test, an experimental measure of electron attachment, had a specificity of 33%. DD was the only DNA related parameter to predict well the noncarcinogenic rodent bioassay result of Ames false-positive chemicals. 5 nongenotoxic parameters (ALT, P450, CP, CT and [CP or CT]) predicted the rodent bioassay result well. Depending on the prevalence of chemicals carcinogenic to humans, the problem of Ames test false positives for predicting human cancer may be either small or large.

The SOS chromotest: a review

The SOS chromotest is reviewed through over 100 publications corresponding to the testing of 751 chemicals. 404 (54%) of these chemicals present a genotoxic activity detectable in the SOS chromotest. Their SOS inducing potencies span more than 8 orders of magnitude. For 452 compounds, the results obtained in the SOS chromotest could be compared to those obtained in the Ames test. It was found that 373 (82%) of these compounds give similar responses in both tests (236 positive and 137 negative responses). Thus the discrepancies between both tests concern 79 compounds (18%). A case by case analysis shows that many of these compounds are at the same time very weak SOS inducers and very weak mutagens. Thus we think that, most of the time, the discrepancies between the two tests may be accounted for by differences in the interpretation of the results rather than by the experimental results themselves. However, there are some compounds which are clearly SOS inducers but devoid of mutagenic activity in the Ames test (such as quinoline-1-oxide) and to a larger extent, clearly mutagenic compounds which do not induce the SOS response in the SOS chromotest (such as benzidine, cyclophosphamide, acridines, ethidium bromide). We also analyzed the correlation between SOS induction, mutagenesis and carcinogenesis according to the classification of Lewis. For 65 confirmed carcinogens (class 1), the sensitivity, i.e., the capacity to identify carcinogens, was 62% with the SOS chromotest and 77% with the Ames test. For 44 suspected carcinogens (class 2), the sensitivity was 66% with the SOS chromotest and 68% with the Ames test. Thus, we confirmed previous observations made on 83 compounds that there is a close correlation between the results given by both bacterial tests. The capacity of the Ames test to identify carcinogens is higher than that of the SOS chromotest. However, because the number of false positive compounds was lower in the SOS chromotest, the specificity, i.e., the capacity to discriminate between carcinogens and non-carcinogens of the SOS chromotest, appeared higher than that of the Ames test. Thus, the results of the SOS chromotest and of the Ames test can complement each other. The SOS chromotest is one of the most rapid and simple short-term test for genotoxins and is easily adaptable to various conditions, so that it could be used as an early--perhaps the earliest--test in a battery.

Revisiting the role of mutagenesis in the induction of lung cancers in rats by diesel emissions

Recent studies have reported the induction of lung tumors in rats chronically exposed to levels of diesel-engine emissions which interfered with lung clearance. The present analysis suggests that the mutagenic substances (e.g. dinitropyrenes, polycyclic aromatic hydrocarbons) present in diesel particulates are not major contributors to rat-lung carcinogenesis. The cancers presumably result from lung "overloading" and the ensuing inflammatory response. In the absence of additional data, this putative mechanism of diesel-caused cancer cannot be extrapolated to other species at risk from exposure to diesel emissions.

Structural evidence for a dichotomy in rodent carcinogenesis: involvement of genetic and cellular toxicity

The structural determinants identified as associated with carcinogenicity in rodents were compared to the structural determinants associated with mutagenicity in Salmonella and toxicity to cultured cells. The analysis revealed that the determinants of carcinogenicity could be separated into two almost mutually exclusive groups: Those associated with mutagenicity (and electrophilicity) and those associated with cell toxicity. These findings provide evidence for two mechanisms of cancer induction: A genotoxic one and a non-genotoxic one associated with cell toxicity.

Correlation of carcinogenic potency with mouse-skin 32P-postlabeling and muta-Rmouse lac Z- mutation data for DMBA and its K-region sulphur isostere: comparison with activities observed in standard genotoxicity assays

The genotoxicities in vitro and in vivo of the mouse-skin carcinogen 7,12-dimethylbenz[a]anthracene (DMBA) have been compared with those of its weakly carcinogenic 4,5-sulphur analogue, 6,11-dimethylbenzo[b]naphtho-[2,3-d]thiophene (S-DMBA). The only datasets that correlated with the relative carcinogenicity of these agents to the skin were those conducted using topically exposed mouse skin. Thus, both chemicals induced lacZ- mutations in the skin of lacZ+ transgenic mice, and both produced DNA adducts on mouse-skin DNA as assessed using the 32P-postlabeling technique. In each case, DMBA gave a stronger response than did S-DMBA. In contrast to these responses, only DMBA was active in the mouse bone-marrow micronucleus assay and in the C3H10T1/2 in vitro cell transformation assay. Both chemicals were mutagenic to Salmonella and of approximately equal potency. The molecular geometry of DMBA and S-DMBA are compared, and divergent CASE predictions of activity in the Salmonella assay and skin-painting bioassay are discussed. The importance of conducting predictive genotoxicity assays in systems close to those in which carcinogenicity is to be assessed is emphasized by these data.

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.

A possible role for cell proliferation in potassium bromate (KBrO3) carcinogenesis

Accumulation of alpha 2u-globulin and induction of cell proliferation were examined in kidneys of rats exposed to KBrO3, KBr or NaBrO3 in their drinking water. Hyaline droplets observed after KBrO3 or NaBrO3 administration to male rats were specifically immunostained for alpha 2u-globulin. Increases in cell proliferation were found in the proximal tubules of male rats given KBrO3 or NaBrO3 but not KBr for 2, 4, and 8 weeks. No such change was evident in KBrO3-treated female rats or the distal tubules of any treated animal. The concordance between hyaline droplet accumulation and increased cell turnover suggests that KBrO3- and NaBrO3-induced cell replication in kidneys of male rats may result from alpha 2u-globulin nephropathy. Considering the fact that KBrO3 has genotoxic potential involving oxidative stress, we hypothesize that the induced cell proliferation might predominantly play an additive role in its carcinogenesis. Furthermore, the present data, showing similar effects of NaBrO3 on the rat kidney, are of direct significance to its risk assessment.

An unusual profile of musk xylene-induced drug-metabolizing enzymes in rat liver

We have demonstrated previously that musk xylene, a non-mutagenic carcinogen, is a novel and specific inducer of CYP1A2 in rats (Iwata et al., Biochem Biophys Res Commun 184: 149-153, 1992). In the present study, the effects of musk xylene (50, 100 or 200 mg/kg body weight, i.p., for 5 consecutive days) on both Phase I and Phase II metabolizing enzymes in rat liver were investigated further and more completely. Among the mixed-function oxidases monitored, 7-ethoxycoumarin deethylase and 7-pentoxyresorufin depentylase activities were increased at all dose levels from 1.6- to 1.7-fold and 2.6- to 3.1-fold, respectively. Benzo[a]pyrene hydroxylase activity was increased significantly at only the 200 mg/kg dose level of musk xylene (1.5-fold). Regarding Phase II enzymes, activities of both cytosolic DT-diaphorase and glutathione S-transferase (GST) were increased up to 2.0- to 2.4-fold by musk xylene in a dose-dependent manner. Western blot analysis revealed that the changes in these activities were caused by increases in the amounts of DT-diaphorase and GST Ya subunit. Microsomal UDP-glucoronyltransferase (UDPGT) activity assayed with p-nitrophenol as substrate was increased 1.6- to 2.0-fold. These results show that musk xylene induces both Phase I cytochrome P450 mixed-function oxidase (CYP1A2 specific) and Phase II metabolizing enzyme systems (DT-diaphorase, GST Ya subunit and UDPGT) in rat liver.

Physiological pharmacokinetics and cancer risk assessment

There has been considerable progress in recent years in developing physiological models for the pharmacokinetics of toxic chemicals and in the application of these models in cancer risk assessment. Physiological pharmacokinetic models consist of a number of individual compartments, based on the anatomy and physiology of the mammalian organism of interest, and include specific parameters for metabolism, tissue binding, and tissue reactivity. Because of the correspondence between these compartments and specific tissues or groups of tissues, these models are particularly useful for predicting the doses of biologically active forms of toxic chemicals at target tissues under a wide variety of exposure conditions and in different animal species, including humans. Due to their explicit characterization of the biological processes governing pharmacokinetic behaviour, these models permit more accurate predictions of the dose of active metabolites reaching target tissues in exposed humans and hence of potential cancer risk. In addition, physiological models also permit a more direct evaluation of the impact of parameter uncertainty and inter-individual variability in cancer risk assessment. In this article, we review recent developments in physiologic pharmacokinetic modeling for selected chemicals and the application of these models in carcinogenic risk assessment. We examine the use of these models in integrating diverse information on pharmacokinetics and pharmacodynamics and discuss challenges in extending these pharmacokinetic models to reflect more accurately the biological events involved in the induction of cancer by different chemicals.

The influence of chemical structure on the extent and sites of carcinogenesis for 522 rodent carcinogens and 55 different human carcinogen exposures

Gold and her colleagues have tabulated the results of rodent bioassays on 522 chemicals and have analysed the data. The present study complements those analyses by providing a perspective from the viewpoint of the chemical structure of the carcinogens. The chemical structure of each of the carcinogens is displayed and the Gold database is represented with the test agents as the primary variable. The carcinogens are gathered into six chemical classes and each chemical is assessed for structural alerts to DNA reactivity. The database is then analysed using an integration of the following parameters: bioassay in rat, mouse or both; structural alert status; chemical class; sites and multiplicity of carcinogenesis, and trans-species carcinogenicity. A series of Figures is presented that enables rapid acquaintance with what represents the core database of rodent carcinogenicity. The several analyses presented combine in endorsing the reality of two broad classes of rodent carcinogen--presumed DNA-reactive and others (putative genotoxic and non-genotoxic carcinogens, but semantics have been largely avoided). Vainio and his colleagues have tabulated 55 situations in which humans have succumbed to chemically induced cancer, and have listed the tissues affected. This database of human carcinogens has been analysed in the present study as done for the rodent carcinogen database, and comparisons made between the two. The predominance of putative genotoxic carcinogens in the human database was confirmed, as was the reality of putative non-genotoxic carcinogenicity in humans. It is concluded that putative genotoxic rodent carcinogenesis can be correlated both with chemical structure and the extent and nature of the induced effect, and that it is of clear relevance to humans. In contrast, it is concluded that putative non-genotoxic rodent carcinogenesis is more closely related to the test species than to the test chemical, and that it is essentially unpredictable in the absence of mechanistic models. In the absence of such models nongenotoxic carcinogenic effects should be extrapolated to humans with caution. Progress in the accurate prediction and extrapolation of rodent carcinogenicity will be helped by a common, if only temporary, enabling acceptance that not all carcinogens are intrinsically genotoxic.

Hepatocyte cell proliferation in mice after inhalation exposure to unleaded gasoline vapor

Chronic inhalation exposure to unleaded gasoline (UG) induced an increase in liver tumors in female but not male mice. Unleaded gasoline exhibits little, if any, genotoxic activity in vitro or in vivo in the female mouse liver, suggesting that other biological effects such as the induction of cell turnover or altered growth control may play a role in this carcinogenic process. To better understand the role of UG-induced hepatocyte proliferation with respect to the dose- and sex-specific tumor response, male and female B6C3F1 mice were housed in 1-m3 single-pass flow-through inhalation chambers and administered UG under exposure conditions that produced tumors in the chronic studies. Mice were exposed to targeted concentrations of 67, 292, or 2056 ppm PS-6 blend of UG vapor 6 h/d, 5 d/wk, for up to 13 wk. Liver weights were elevated significantly in male and female mice exposed to 2056 ppm UG at wk, 1, 3, 6, and 13. No elevation in liver-specific serum enzymes was noted in treated animals, nor were there any significant histopathological changes in the liver, indicating a lack of overt hepatotoxicity. Hepatocyte proliferation, expressed as nuclear labeling index (Ll), was measured immunohistochemically after 5-bromo-2'-deoxyuridine administration via an osmotic minipump implanted three days before the animals were killed. A 6-to 10-fold increase in Ll compared to controls was observed in male and female mice exposed to 2056 ppm UG at wk 1, with a return to control levels at wk 3, 6, and 13. Mice exposed to 67 or 292 ppm UG did not show any increase in Ll. The mode by which an agent induces cell proliferation is an important consideration in mechanistic studies and the risk assessment process. These data indicate an early transient mitogenic stimulation of cell proliferation, rather than regeneration secondary to cytotoxicity, in the livers of UG-treated mice. The observed proliferative response after UG exposure in the male mouse in the absence of a tumorigenic response suggests that effects in addition to the early transient hepatocyte proliferation response are critical in understanding the sex-specific hepatocarcinogenic response of this complex mixture.

Airborne carcinogens

Air pollution has been recognized as a cancer risk for many years. More than 2,800 different chemicals have been identified in the air or emission sources. Only about 10% of these chemicals have been evaluated in bioassays for genetic or carcinogenic effects. Hydrocarbons, nitrogen-containing organics, and halogenated organics account for nearly 60% of the airborne chemicals that have been studied in long term animal cancer bioassays or short-term genetic bioassays. The sources that emit the highest number of these potentially carcinogenic chemicals are sources involving combustion (e.g., tobacco smoke, automobile exhaust, and coal combustion). Quantitative estimates of the risk of airborne carcinogens in outdoor air consistently show that polycyclic organic matter (POM) from products of incomplete combustion (PICs) make the largest single contribution to human cancer risk. Although the POM emissions from various air pollution sources are chemically similar and induce cancer by a similar genotoxic mechanism, the cancer risk per unit of exposure of these emissions may vary by several orders of magnitude. Among these combustion sources motor vehicle emissions account for the greatest cancer risk in outdoor air. Environmental tobacco smoke (ETS) and radon are the major sources of cancer risk from indoor exposures. There are, however, many uncertainties in identifying the important airborne carcinogens and quantitating the human cancer risk of air pollution. One important uncertainty is the role of atmospheric transformation products in human cancer.

Persistent proliferation of normal hepatocytes and promotion of preneoplastic development by N-nitrosodibenzylamine in rats

In a traditional long-term study N-nitrosodibenzylamine (NDBzA) was proven to be noncarcinogenic, but recently the substance was found to produce genotoxic lesions in hepatocytes. Our own experiments have shown that relatively low single doses of NDBzA cause liver hypertrophy and additive proliferation of hepatocytes in rats. Both effects are known from well-documented promoters and non-genotoxic carcinogens, respectively, in rodents. Investigation of NDBzA in an initiation-promotion assay (IP assay) showed it to cause an increase in the number and size of preneoplastic liver cell foci. This occurred only after initiation with diethylnitrosamine, but not when 2-acetylaminofluorene was used. Another property of NDBzA is its sustained mitotic stimulation of extrafocal hepatocytes. This is inconsistent with their adaptive loss of susceptibility to mitogens in IP assays using other promoters of hepatocarcinogenesis. The following conclusions can be drawn. First, "differential inhibition" of the proliferation of extrafocal hepatocytes, in contrast to the selective mitostimulation of preneoplastic cells, is obviously no prerequisite for cancer development. Second, primary mitogenicity of a compound in short-term studies can be a useful indicator for tumorigenic potential. In the case of NDBzA the data available at present are still insufficient to classify it unequivocally in terms of genotoxic or nongenotoxic carcinogenicity.

Predicting rodent carcinogenicity of halogenated hydrocarbons by in vivo biochemical parameters

Forty halogenated hydrocarbons of known rodent carcinogenicity (24 carcinogens, 16 noncarcinogens), including many promoters of carcinogenesis, nongenotoxic carcinogens, and hepatocarcinogens, were selected for study. The chemicals were administered by gavage in two dose levels to female Sprague-Dawley rats. The effects of these 40 chemicals on four biochemical assays [hepatic DNA damage by alkaline elution (DD), hepatic ornithine decarboxylase activity (ODC), serum alanine aminotransferase activity (ALT), and hepatic cytochrome P-450 content (P450)] were determined. Composite predictive parameters are defined as follows: CP = [ODC and P450], CT = [ALT and ODC], and TS = [DD or CP or CT]. The operational characteristics of TS for predicting rodent cancer were sensitivity 58%, specificity 81%, positive predictivity 82%, negative predictivity 57%, and concordance 68%. The concordance for the Ames test (45%) and structural alerts (SA; 46%) was much lower. TS also outperformed the Ames test and SA in producing fewer false positives (the specificity of TS was 81% vs. only 63% for the Ames test and 57% for SA). For predicting the carcinogenicity of the most difficult halogenated hydrocarbons (Ames and SA negative chemicals), TS was capable of successfully predicting the carcinogenicity of 8 (carbon tetrachloride, chloroform, alpha-hexachlorocyclohexane, kepone, mirex, monuron, p,p'-DDE, and 2,4,6-trichlorophenol) out of 16 of these non-DNA-reactive halogenated hydrocarbon carcinogens. All 8 of these halogenated hydrocarbons were positive in either CP or CT. This evidence shows that nongenotoxic carcinogenesis is best predicted by nongenotoxic parameters such as CP or CT (components of the predictor TS).

Cytoskeleton modifications induced by phenobarbital, 2-acetylaminofluorene and 4-acetylaminofluorene in normal and initiated/selected hepatocytes: relation with the "resistant" phenotype

Initiated/selected (ISH) and normal (NH) rat hepatocytes were used to study cytoskeleton modifications induced by three liver acting chemicals: 2-AAF, a liver complete carcinogen; PB, a liver tumor promoter; and 4-AAF, a non-carcinogen analogue of 2-AAF. Cytoskeleton alterations were visualized by disappearance of F-actin fibers and tubulin depolymerization. The three drugs induced actin fragmentation in normal hepatocytes; a net loss of actin protein was observed with PB. They also induced varied tubulin depolymerization. The principal difference between chemicals is that 2-AAF led to non-reversible effects, in comparison with PB and 4-AAF which induced reversible damages on cytoskeleton. By contrast to normal hepatocytes, the cytoskeleton of ISH obtained from rats subjected to the "resistant" hepatocyte protocol was much less susceptible to the effect of the three chemicals. Moreover, we observed a lack of LDH release in the culture medium and a very rapid inducibility of GST activity after exposure of ISH to drugs. The moderate effect of the three chemicals on actin and tubulin in ISH could thus be explained by the "resistant" metabolic profile of these cells.