The Salmonella typhimurium/mammalian microsomal assay. A report of the U.S. Environmental Protection Agency Gene-Tox Program

Genotoxicity studies on professional hair colorists exposed to oxidation hair dyes

The cytogenic repercussions of occupational exposure to oxidation hair dyes were assessed by using three assays in professional hair colorists. The assays were sister chromatid exchanges (SCE) in circulating lymphocytes to evaluate the interchange of DNA replication products at apparently homologous chromosomal loci, single cell gel electrophoretic (SCGE) assay to detect the presence of DNA strand breaks/alkali-labile damage, and the Ames assay using Salmonella typhimurium strain TA98 to detect the urine mutagenicity. The ability of these assays to detect genetic damage caused by oxidation hair dyes in man compared with closely matched controls produced the following findings. (i) The SCE assay could not detect the mutagenic effect in lymphocytes of exposed subjects from whom complete data were obtained. However, subjects (controls and exposed) with a history of smoking had slightly increased SCEs than the non-smokers in both groups. (ii) The extent of DNA migration (SCGE assay) did not distinguish between the samples in either the exposed or control subjects. Like the SCE results, the exposed and control smoker subjects showed a greater proportion of damaged lymphocytes with apparent migration of DNA. (iii) No clear differences in the mutagenic activity of the urine samples were observed between the exposed and control subjects. But, pooling exposed and controls together, a positive and significant variation in the urinary mutagenic effect was observed with the number of cigarettes smoked per day.

Comparative evaluation of the in vitro micronucleus test and the in vitro chromosome aberration test: industrial experience

Because of its rapidness, simplicity and potential for automation, the measurement of micronucleated cells in vivo is not only equivalent to the analysis of chromosome aberrations, but often even preferred within routine genotoxicity testing. In order to evaluate the correlation between the in vitro micronucleus assay (MNT) and the in vitro chromosome aberration test (CA), we collected data from four pharmaceutical companies obtained either in Chinese hamster cell lines (CHO-K5, CHO-K1, V79) or in human peripheral blood lymphocytes. Among the 57 compounds included in this comparison, 45 compounds gave rise to concordant results in both assays (26 compounds negative in both assays; 19 compounds positive in both assays). The high percentage of concordance, i.e. about 79% is very promising and can be even increased to about 88% by omitting the 3 aneugenic compounds and 2 compounds inducing endoreduplicated chromosomes which were found positive only in the in vitro MNT. The results are remarkable in particular considering that most of the compounds evaluated are 'standard' pharmaceutical compounds and thus are at most weak inducers of chromosome damage. Our comparison strongly supports that the in vitro micronucleus test is a suitable alternative to the in vitro chromosome aberration assay. Moreover, the MNT has the potential of not only detecting clastogens but additionally aneuploidy inducing chemicals.

Genotoxic potential of estrogens

A genotoxicity evaluation of three commonly used estrogens-ethinyl estradiol, cyclotriol and cyclodiol was undertaken using short-term in vitro and in vivo assays. None of the drugs caused significantly increased or decreased number of His+ mutants to appear in the Ames Salmonella assay, either with or without S9 mix or in a modified host-mediated version of this assay. However, the clastogenic potential of these drugs became evident from the increased number or chromosome aberrations and sister chromatid exchanges (SCEs) induced by these drugs in human lymphocyte cultures both in the presence and absence of S9 mix. Increased frequencies of micronuclei and of sister chromatid exchanges in mice confirmed their clastogenic potential.

Use of semi-permeable membrane devices to monitor pollutants in water and assess their effects: a laboratory test and field verification

Uptake of eight pesticides of different classes (organochlorines, synthetic pyrethroids, dinitroanilines, amides) by semi-permeable membrane devices (SPMDs) was studied in a laboratory continuous-flow system. After 20 days of exposure, membrane concentration factors were in the range of 50 000-120 000 for all the analytes except for the amide herbicides propachlor and alachlor, which were not detected in any of the SPMDs. The kinetic data and mathematical models were used to calculate analyte uptake rate constants, SPMD lipid/water partition coefficients, average sampling rates and concentrations of the pesticides in water. To assess the effects of concentrated pesticides, standard bioassays (Salmonella/histidine reversion assay and bioluminescence inhibition in Vibrio fischeri test) were incorporated in the SPMD technique. To test the method in an environmental situation, SPMDs were deployed in polluted sites in Lithuania. Polynuclear aromatic hydrocarbons were the major pollutants detected in the SPMDs deployed in the field. All the SPMD dialysates were highly toxic in the bioluminescence inhibition test but no mutagenic activity was observed in the Salmonella/histidine reversion assay.

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.

Biological properties of peroxide-containing tooth whiteners

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

In vitro studies of the genotoxic effects of bitumen and coal-tar fume condensates: comparison of data obtained by mutagenicity testing and DNA adduct analysis by 32P-postlabelling

Bitumens contain traces of polycyclic aromatic compounds (PACs), a part of which will end up in the fumes emitted during hot handling of bitumen-containing products, e.g. during roadpaving. Although exposure of workers to these fumes is low, it might lead to health problems. Studies on bitumen fume condensates (BFCs) showed weak to moderate mutagenic activities, but studies on DNA adduct formation have not been reported. Therefore, a study was initiated in which fumes were generated from two road grade bitumens, in such a way that they were representative of the fumes produced in the field. The combined vapour/particulates were tested in vitro for their ability to produce DNA adducts and in modified Ames mutation assays, using a number of different strains. An attempt was made to relate the results to chemical data, such as the content of a number of individual polycyclic aromatic hydrocarbons (PAHs) and with a measure for the total PAC content. As a reference material fume condensate from coal-tar (coal-tar pitch volatiles; CTPV) were subjected to the same tests. All fume condensates tested were mutagenic to all strains and induced the formation of DNA adducts. The patterns of DNA adducts, obtained by 32P-postlabelling, arising from the BFCs were qualitatively different from the patterns of adducts obtained from the CTPVs, implying qualitative differences in the nature of the compounds responsible for the formation of these adducts. This is corroborated by the observation that for BFCs quantitative adduct levels are higher than would be expected based on the PAH content. These data thus indicate that the PAHs analysed are not the sole components responsible for adduct formation from BFCs, but that an important contribution comes from other (hetero- and/or substituted-) PACs.

Computer prediction of possible toxic action from chemical structure: an update on the DEREK system

Computer-based assessment of potential toxicity has become increasingly popular in recent years. The knowledge-base system DEREK is developed under the guidance of a multinational Collaborative Group of expert toxicologists and provides a qualitative approach to toxicity prediction. Major developments of the DEREK program and knowledge-base have taken place in the last 3 years. Program developments include improvements in both the user interface and data processing. Work on the knowledge-base has concentrated on the areas of genotoxicity and skin sensitisation. DEREK's predictive capabilities for these toxicological end-points has been demonstrated. In addition to the continued expansion of the knowledge-base, a number of enhancements are planned in the DEREK program. In particular, work is in progress to develop further DEREK's ability to report the reasoning behind its predictions.

Genotoxicity evaluation of norethisterone acetate

Genotoxic evaluation of a commonly used progestogen, norethisterone acetate, was undertaken using a combination of short-term in vitro and in vivo assays. The clastogenic potentiality of norethisterone acetate was evident from the chromosome aberrations and sister chromatid exchanges induced both with and without S9 mix in cultured human lymphocytes and also from the increased frequency of micronuclei formation and sister chromatid exchanges in mice. However, in the Ames Salmonella assay, both with and without S9 mix and in host-mediated assay, norethisterone acetate was unable to cause any significant increase/decrease in the His+ revertants/plate.

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.

Unexpected genetic toxicity to rodents of the N',N'-dimethyl analogues of MNU and ENU

Lijinsky and his colleagues have reported that the N',N'-dimethyl analogues of ENU and MNU [N',N'-dimethyl-N-ethyl-N-nitrosourea (DMENU) and trimethylnitrosourea (TMNU), respectively] are carcinogenic to rats despite their extreme hydrolytic stability which would reduce or preclude generation of alkylating species analogous to those formed upon hydrolysis of ENU and MNU. Lijinsky and his colleagues were unable to rationalize those activities of DMENU and TMNU despite extensive experimentation. We therefore decided to study this problem further. Whichever mode is accepted for the generation of electrophilic/mutagenic/carcinogenic reactive species from ENU and MNU, blocking of the free-NH2 group with methyl groups (-NMe2) should ablate or abolish activity. Consistent with this DMENU and TMNU gave negative results in the NBP alkylation test while the parent compounds gave an instantaneous deep blue coloration. Studies of the rate of hydrolysis of these four compounds revealed ENU and MNU to have half-lives of 8 min, while the alkylated analogues (DMENU and TMNU) had half-lives of 25 and 41 days, respectively. Hydrolysis of ENU and MNU, to yield the alkylating species, proceeds either via proton abstraction from the -NH2 group or by attack by water on the carbon of the carbonyl group. Methylation will inhibit both of these pathways, the first absolutely (no -NH2 protons) and the second partially, via steric inhibition. The slow hydrolysis observed for DMENU and TMNU suggests that the latter route of hydrolysis is applicable. Studies with strain TA1535 of Salmonella typhimurium (without S9 mix) confirmed the potent mutagenic activity for ENU and MNU (approximately 300-fold increase in revertants at 2,000 micrograms/plate and approximately 180-fold increase in revertants at 150 micrograms/plate respectively). In contrast, the methylated analogues showed only weak mutagenic activity (approximately 3-fold) at approximately 100-fold higher dose-levels. Addition of S9 mix did not affect the mutagenicity of DMENU or TMNU. To this point, hypothesis and data coincide. ENU and MNU are potent micronucleus-inducing agents to the mouse bone marrow, and given the above data, it was expected that DMENU and TMNU would show weak or no activity in that assay. In fact, the methylated analogues were as effective as ENU and MNU as clastogens to the mouse bone marrow. Four possible reasons for this conflict of theory and data are explored. The speculative explanation we favour for these effects is that the net alkylation of bone marrow DNA is the same for all four chemicals. With ENU and MNU, most of the alkylating activity is dissipated by rapid hydrolysis. Thus, only a small fraction of the administered dose survives to alkylate the bone marrow. Due to the enhanced stability of the methyl analogues most of the delivered dose will reach the bone marrow. However, because of their lower intrinsic reactivity, only a small fraction of the target dose will alkylate the bone marrow DNA during the time window of the experiment. If these opposing influences happen to balance out, the essentially identical bone marrow genetic toxicity for the four chemicals could be explained.

SOS chromotest results in a broader context: empirical relationships between genotoxic potency, mutagenic potency, and carcinogenic potency

Environmental monitoring requires that large numbers of samples be processed in a relatively short period of time. While microbioassays facilitate rapid testing, the results are often difficult to interpret in the broader context of human or animal health. Determining the consequences of exposure to genotoxic substances will ultimately require in situ monitoring of exposed organisms. However, it is immediately possible to construct a broad empirical framework within which available microbioassay results can be interpreted. To do this for SOS Chromotest results, we investigated the empirical relationships between SOS genotoxic potency and mutagenic potency (as measured with the Salmonella/microsome assay), as well as between genotoxic potency and carcinogenic potency (as measured using standard, chronic animal bioassays). Strong relationships were identified between; 1) genotoxic potency and mutagenic potency for 268 direct-acting substances (r2=0.76) and 2) genotoxic potency and mutagenic potency for 126 S9-activated substances (r2=0.65). Ordinary least squares regression analyses of the SOS genotoxicity-Salmonella mutagenicity relationship revealed a significant effect of SOS genotoxicity as well as differences in mutagenic potency that can be attributed to the Salmonella strain used to measure mutagenic potency. Analyses of S9-activated substances revealed a significant interaction between the SOS genotoxic potency (SOSIP) effect and the Salmonella strain effect. Two regression models relating SOS genotoxicity and Salmonella mutagenicity were used to predict the mutagenic potency of several industrial effluent extracts previously analyzed for SOS genotoxicity by White et al. [(1996): Environ Mol Mutagen 27:116-139]. Predictions are consistent with published mutagenic potency values for similar industrial waste materials. A consistent relationship was also identified between genotoxic potency and carcinogenic potency for 51 substances. Linear regression analyses revealed an effect of SOS genotoxic potency as well as differences in carcinogenic potency that may be attributable to experimental animal and route of exposure. The correlation between genotoxicity and carcinogenicity was fairly weak (maximum r value = 0.51). Previous studies revealed similar strength of association between Ames mutagenicity and carcinogenicity. Predicted carcinogenic potencies of previously examined genotoxic, industrial effluent extracts are generally low compared to the pure substances included in the data set.

Toxicological investigations in the semiconductor industry: IV. Studies on the subchronic oral toxicity and genotoxicity of vacuum pump oils contaminated by waste products from aluminum plasma etching processes

Dry etching processes in semiconductor manufacturing use ionized gases in closed reactors at pressures below 1 torr. Vacuum pump systems that service the reaction chambers are potential sources of exposure to complex mixtures of inorganic and organic compounds. These mixtures consist of unused process gases and process by-products that condense and accumulate in the vacuum pump oils. To evaluate potential hazards of dry etch vacuum equipment, a contaminated vacuum pump oil sample from a BCl3/Cl2 etching process was analyzed. The waste oil was administered by gavage for 14 or 28 days to male and female Wistar rats. Neither death nor behavioral changes occurred after subchronic treatment or during a 14-day posttreatment period. Only slight effects on body weights, clinical chemistry, and hematology data were seen in the exposed animals, although the livers of all waste oil-exposed rats of both sexes showed remarkable hypertrophic degenerations. Genotoxicological investigations were performed through the Ames assay (Salmonella assay) and the Micronucleus assay. The contaminated oil sample caused clear genotoxic effects in both test systems.

Modifying role of trace elements on the mutagenicity of benzo[a]pyrene

Benzo[a]pyrene (BaP) is a polycyclic aromatic hydrocarbon that is found in tobacco smoke and various environmental contaminants and has been shown to be carcinogenic and mutagenic in animal and cell culture studies, respectively. Research studies suggest that various nutritional factors such as the antioxidant vitamins and selenium are very promising as potential anticarcinogenic agents. Moreover, some evidence exists showing that both iron and germanium, at specific dosage levels, may possess antimutagenic potential. This study examined the influence of ferrous sulfate and germanium oxide, independently, upon the mutagenic potential of BaP in the Ames test. Four test strains of Salmonella typhimurium were exposed to BaP (15 micrograms/plate) in the presence of different dosage levels of iron (0-1000 micrograms/plate) and germanium (0-600 micrograms/plate). In the case of iron, it was observed that, depending upon the strain tested, iron reduced BaP's mutagenicity. In strain TA98, this was a significant effect at 100 micrograms/plate and higher. In strains TA97a and TA100, iron concentrations had to reach 250 micrograms/plate or higher to produce significant effects. Iron was much less effective in reducing BaP mutagenicity in strain TA102. In general, germanium was not as effective in reducing the mutagenic potential of BaP. Only in the case of the highest concentrations tested (400 and 600 micrograms/plate) was any effect noted, and this in only three of the four strains evaluated.

In vitro and in vivo genotoxicity of hormonal drugs. VI. Fluoxymesterone

Genotoxic evaluation of a commonly used synthetic steroidal androgen, fluoxymesterone, was undertaken using a combination of in vitro and in vivo assays. The clastogenic potential of fluoxymesterone was evident from the chromosome aberrations and sister chromatid exchanges induced by it in the cultured human lymphocytes and also from the increased frequencies of micronuclei and sister chromatid exchanges in bone marrow cells of mice. However, in Ames Salmonella assay both with and without S9 mix and in host-mediated assay using bacterial strains of S. typhimurium as indicator organism, fluoxymesterone did not cause any significant increase/decrease in His+ revertants.

Mutagenicity and mutation spectra of 2-acetylaminofluorene at frameshift and base-substitution alleles in four DNA repair backgrounds of Salmonella

We used colony probe hybridization procedures to determine the mutations in approximately 600 revertants of the -1 frameshift allele hisD3052 and approximately 200 revertants of the base-substitution allele hisG46 of Salmonella typhimurium induced by 2-acetylaminofluorene (2-AAF) in the presence of Aroclor-induced rat liver S9. 2-AAF was primarily a frameshift mutagen, exhibiting 5 times more frameshift than base-substitution activity. The only frameshift mutation 2-AAF induced at the hisD3052 allele was a hotspot (-2) deletion within the sequence CGCGCGCG. The addition of the pKM101 plasmid had a small effect on the mutagenic potency of 2-AAF at this allele in a uvr+ background and no effect on the mutation spectra in either a uvr+ or uvr- background. The small amount of base-substitution activity exhibited by 2-AAF at the hisG46 allele required the presence of both the pKM101 plasmid and the uvrB mutation. The base substitutions were G.C-->T.A transversions (86%) and G.C-->A.T transitions (14%), and 85% of the substitutions were at the second position of the CCC target of the hisG46 allele; the remainder were at the first position. We propose that the hotspot frameshift may be initiated by N-acetyl-2-aminofluorene adducts located at the C(8) position of any of the guanines except the first one in the CGCGCGCG hotspot sequence. The mutation might then result from correct incorporation of cytosine opposite the adducted guanine, followed by a 2-base slippage according to our recently proposed correct-incorporation/slippage model. The hotspot mutation may also result from a 2-AAF-induced B- to Z-DNA transition at the repeating GpC site as well as by the action of enzymes involved in DNA metabolism, such as DNA resolvases or topoisomerases, on DNA structures that have been distorted by 2-AAF adducts. The small amount of 2-AAF-induced base-substitution activity may be due to mispairing of adenine opposite the minor aminofluorene adduct at the C(8) position of guanine.

The gradient plate assay: a modified Ames assay used as a prescreen for the identification of bacterial mutagens

Bacterial test systems have been used extensively to identify the mutagenic potential of new compounds. In particular, the Ames test has gained worldwide acceptance and is required by many regulatory agencies to support product registration. The gradient plate assay (GPA) is a modification of the Ames test. It is used as a high capacity prescreen to detect the mutagenic potential of synthetic intermediates, impurities, and research compounds over a concentration gradient. Since the development of the GPA, over 4000 compounds have been tested in the assay. Selection and use of the GPA in our laboratory is due to many factors: reliability; sensitivity; capacity; timeliness of reporting results; and establishment of safety standard in the laboratory. In this manuscript, results of the GPA method are compared with results from the traditional Ames assay. To date, 113 compounds of identical lots have been evaluated in both tests, and in all but 3 instances the results are the same. Thus, the GPA is an ideal assay for use as a prescreen in determining the ability of a compound to induce bacterial mutation.

Anchorage independent colony growth of fetal hamster lung epithelial cells after treatment with diepoxybutane

To test the reliability of a new cell transformation assay, a cloned fetal Syrian hamster lung epithelial cell line (M3E3/C3) was used. The target cells originating from the respiratory tract were treated in vitro over a concentration range of 0-10(-5) M/l with diepoxybutane, cultured during the expression period of 28 or 35 days and then transferred into soft agar. Anchorage independent colony growth in soft agar occurs only if cells are transformed. Growth and number of colonies were taken as a score of the carcinogenic potential of the test substance. Under the conditions of this cell transformation assay it was possible to detect the carcinogenic potential of diepoxybutane unequivocally.

Modified suspension Ames test for testing proteinaceous substances: an initial step

The basic Salmonella/microsome assay (Ames test) is a valuable primary tool by which to discriminate mutagens from non-mutagens. For a variety of chemical test substances this test is easily conducted according to international guidelines for genotoxicity testing. However, the testing of proteinaceous substances in the basic Ames test may generate false positives owing to the presence of growth-promoting constituents in the test sample, such as histidine or its precursors. It was hypothesized that the growth-promoting capacities of biological test samples might be overcome by testing according to the 'suspension variant' of the Ames test, which uses very rich growth conditions thereby overwhelming any growth-enhancing constituents present in a biological test sample. This hypothesis appeared to be correct, although several important modifications had to be made to the suspension assay. The most important aspect of this 'new suspension Ames test' appeared to be the plating of overnight regrown bacteria in the poorest way possible (by omitting histidine and nutrient broth from the overlay agar). This study may comprise an initial step in the development of a modified suspension Ames test for testing proteinaceous substances.

In vitro and in vivo genotoxicity evaluation of hormonal drugs. II. Dexamethasone

Genotoxicity evaluation of a widely used glucocorticoid medicine, dexamethasone, was undertaken using in vitro and in vivo assays. Analyses of chromosomal aberrations, sister-chromatid exchanges (SCEs) in human lymphocytes and micronuclei and SCEs in mouse bone marrow showed the drug to be capable of attacking the genetic material. However, the Ames/Salmonella assay, both with and without S9 mix, did not show any increase in His+ revertants.

Ethylene thiourea (ETU). A review of the genetic toxicity studies

Ethylene thiourea (ETU) is a common contaminant, metabolite and degradation product of the fungicide class of ethylene bisdithiocarbamates (EBDCs); as such, they present possible exposure and toxicological concerns to exposed individuals. ETU has been assayed in many different tests to assess genotoxicity activity. While a great number of negative results are found in the data base, there is evidence that demonstrates ETU is capable of inducing genotoxic endpoints. These include responses for gene mutations (e.g. Salmonella), structural chromosomal alterations (e.g. aberrations in cultured mammalian cells as well as a dominant lethal assay) and other genotoxic effects (e.g. bacterial rec assay and several yeast assays). It is important to consider the magnitude of the positive responses as well as the concentrations/doses used when assessing the genotoxicity of ETU. While ETU induces a variety of genotoxic endpoints, it does not appear to be a potent genotoxic agent. For example, it is a weak bacterial mutagen in the Salmonella assay without activation in strain TA1535 at concentrations generally above 1000 micrograms/plate. Weak genotoxic activity of this sort is usually observed in most of the assays with positive results. Since ETU does not appear very potent and is not extremely toxic to test cells and organisms, it is not surprising to find that ETU does not produce consistent effects in many of the assays reviewed. Consequently, in many instances, mixed results for the same assay type are reported by different investigators, but as reviewed herein, these results may be dependent upon the test conditions in each individual laboratory. A primary shortcoming with many of the reported negative results is that the concentrations or doses used are not high enough for an adequate test for ETU activity. There are also problems with many of the negative assays generally in protocol or reporting, particularly with the in vivo studies (e.g. inappropriate sample number and/or sampling times; inadequate top dose employed). Overall, while ETU does not appear to be a potent genotoxic agent, it is capable of producing genotoxic effects (e.g. gene mutations, structural chromosomal aberrations). This provides a basis for weak genotoxic activity by ETU. Furthermore, based on a suggestive dominant lethal positive result, there may be a concern for heritable effects. Due to the many problems with the conduct and assessment of the in vivo assays, it is worth repeating in vivo cytogenetic assays and a dominant lethal assay (with acceptable test procedures and data generation) to determine if these results would continue to support a heritable mutagenicity concern.

Isolation and prevalidation of an Escherichia coli tester strain for the use in mechanistic and metabolic studies of genotoxins

We have isolated an Escherichia coli tester strain for the use in mechanistic and metabolic studies of genotoxins. We started with one of the more used and better characterized E. coli K-12 laboratory strains, AB1157. We isolated a lipopolysaccharide defective mutant of strain AB1886 which is an excision repair deficient derivative of AB1157 and introduced a newly constructed plasmid pKR11, encoding mucAB, resulting in strain MR2101/pKR11. A genotoxicity assay was designed, monitoring the reversion to arginine prototrophy and a preliminary validation was carried out against Ames tester strain TA100 with a set of diagnostic compounds. The results seem to indicate that strain MR2101/pKR11 is an adequate tester strain which can be a useful tool in mechanistic studies. Moreover, this strain can serve as mother strain to isolate improved and more specialized tester strains.

Series: 'Current issues in mutagenesis and carcinogenesis.' No. 42. Strategies and philosophies of genotoxicity testing: what is the question?

A number of statements concerning the uses and effectiveness of in vitro and in vivo genetic toxicity tests have recently been made. Certain of these statements are examined using genetic toxicity and carcinogenicity data available in the literature.

Medium-term rat liver bioassay for rapid detection of carcinogens and modifiers of hepatocarcinogenesis

For rapid detection of carcinogenic agents, a medium-term liver bioassay has been established in our laboratory using preneoplastic glutathione S-transferase placental form (GST-P) positive foci in the rat liver as endpoint marker lesions. A total of 237 compounds have so far been tested in this system and the results compared with reported Salmonella/microsome and long-term carcinogenicity test findings. The positive rate was found to be extremely high, 97% (28 of 29 compounds) for genotoxic hepatocarcinogens; and satisfactory, 86% (23 of 27 chemicals) for nongenotoxic ones. The positive rate for carcinogens targeting organs other than the liver, however, is relatively low (24%). Malathion and vinclozolin proved positive, although both have been reported to be noncarcinogenic in rats and mice. Those chemicals which exerted positive results in this system might be hepatopromoting agents even if hepatocarcinogenicity has not been established. Five of the six false-negative hepatocarcinogens could be categorized as peroxisome proliferators. In addition, a number of inhibitory agents for GST-P-positive foci development have been detected and many are categorized as antioxidants. The validity of this system as a tool for rapid detection of carcinogenic and chemopreventive agents is discussed.

Evaluation of the genotoxic potential of alkyleneamines

A series of 6 alkyleneamines [ethylenediamine (EDA), diethylenetriamine (DETA), triethylenetetramine (TETA), tetraethylenepentamine (TEPA), aminoethylethanolamine (AEEA), and aminoethylpiperazine (AEP)] were evaluated for potential genotoxic activity using a battery of in vitro and in vivo assays. Only TETA was considered mutagenic in the Salmonella typhimurium mutation assay. All 6 alkyleneamines tested except AEP were considered inactive in the Chinese hamster ovary (CHO) gene mutation assay. AEP was considered inconclusive in this assay. In 2 the sister-chromatid exchange (SCE) assay, EDA, DETA and AEEA were inactive with or without metabolic activation. TETA, TEPA and AEP were considered active in the induction of SCE in CHO cells. With hepatocytes, no positive effects of EDA, DETA AEEA and AEP upon unscheduled DNA syntheses (UDS) were noted. However, TETA and TEPA produced significant increases in the amount of UDS activity, and thus were considered positive in inducing primary DNA damage in this assay. In a micronucleus study with Swiss-Webster mice, no clastogenic activity was observed with TETA, TEPA and AEP. The overall weight of evidence from the in vitro and in vivo tests suggested that EDA, DETA and AEEA were not mutagenic, while TETA was mutagenic, and TEPA and AEP had a weak mutagenic potential.

Mutation spectra of Glu-P-1 in Salmonella: induction of hotspot frameshifts and site-specific base substitutions

We used colony probe hybridization and PCR/DNA sequence analysis to determine the mutations in approximately 1,640 revertants of the -1 frameshift allele hisD3052 and approximately 260 revertants of the base substitution allele hisG46 of Salmonella typhimurium induced by the heterocyclic amine cooked food mutagen 2-amino-6-methyldipyrido[1,2-a:3',2'-d]imidazole (Glu-P-1). All of the mutations were at sites containing guanine, which is the base at which Glu-P-1 forms DNA adducts. A hotspot mutation involving the deletion of a CG or GC within the sequence CGCGCGCG accounted for 100% of the Glu-P-1-induced mutations at the frameshift allele in strains TA1978 (uvr+) and TA1538 (delta uvrB) and 99% in TA98 (delta uvrB, pKM101). To explain the induction of these hotspot mutations by Glu-P-1, we describe here a more detailed version of our recently proposed correct incorporation/slippage model [Genetics:136:731, 1994]. We propose that after cytosine is incorporated correctly opposite a Glu-P-1-adducted guanine, various slipped intermediates may form (a total of 18), depending on which guanine is adducted and whether it remains within the helix or becomes extrahelical. This variety of mutational pathways may account for the high mutability of the hotspot sequence by Glu-P-1. Although the pKM101 plasmid does not influence the mutagenic potency or mutational spectrum of Glu-P-1 at the frameshift allele, it is required by Glu-P-1 to revert the base substitution allele, where Glu-P-1 induces G-C --> T-A transversions (75%) and G-C --> tA-T transitions (25%) exclusively at a single site (the second position of the CCC codon of the hisG46 allele). The limited (20-30 times less) base substitution mutagenic potency of Glu-P-1 relative to its frameshift mutagenic potency as well as the extreme site specificity exhibited by Glu-P-1 for base substitutions may have bearing on the lack of base substitutions identified in ras genes in Glu-P-1-induced rat colon tumors.

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.

Amphibian micronucleus test(s): a simple and reliable method for evaluating in vivo genotoxic effects of freshwater pollutants and radiations. Initial assessment

A micronucleus test was developed using larvae from two urodele amphibians (Pleurodeles waltl and Ambystoma mexicanum) and an anuran (Xenopus laevis). The methods for maintenance of adults, egg laying, and rearing the larvae are described, and the conditions required for optimal response are given for each of these species. The tests are carried out during a period of intense erythropoiesis when red blood cells are actively dividing in circulating blood. The micronuclei are observed on blood smears. The genotoxic effects of X-rays were evaluated at 12 different doses over a range of 6-1200 rad. All doses, even the very low dose of 6 rad, gave positive results. The test substances were added to the water in which the larvae were reared, and the results obtained after treatment for 12 days and/or 8 days with 47 different chemical compounds are listed. Detailed results are given as the lowest concentration producing a positive response or the highest concentration producing a negative response. The reliability of the test system using the newt is now well established, while the tests using the other two amphibian species are still under evaluation. Integration of this test in a test battery for quality control of water would aid the evaluation of risks to human health, as well as the protection of aquatic ecosystems.

Problem of the harmonization of philosophies for genotoxicity testing

Recently, Ashby (1993) published a paper on 'Precedents or possibilities: which should guide the harmonization of mutagenicity test protocols and carcinogen prediction strategies?'. Having been involved in two activities referred to by Ashby (Berlin Workshop "Current issues in genetic toxicology from the regulatory viewpoint" in conjunction with the 22nd EEMS meeting; development of an EC strategy for genotoxicity testing of new chemicals), I would like to give some further information and add my personal opinion on two aspects of Ashby's presentation. More transparency as to the background of different approaches might help to achieve harmonization.

Prediction of carcinogenicity from two versus four sex-species groups in the carcinogenic potency database

Prediction of a positive result in rodent carcinogenesis bioassays using two instead of four sex-species groups is examined for the subset of chemicals in the Carcinogenic Potency Database that have been tested in four sex-species groups and are positive in at least one (n = 212). Under the conditions of these bioassays, a very high proportion of rodent carcinogens that are identified as positive by tests in four groups is also identified by results from one sex of each species (86-92%). Additionally, chemicals that are classified as "two-species carcinogens" or "multiple-site carcinogens" on the basis of results from four sex-species groups are also identified as two-species or multiple-site carcinogens on the basis of two sex-species groups. Carcinogenic potency (TD50) values for the most potent target site are similar when based on results from two compared to four sex-species groups. Eighty-five percent of the potency values are within a factor of 2 of those obtained from tests in 4 sex-species groups, 94% are within a factor of 4, and 98% are within a factor of 10. This result is expected because carcinogenic potency values are constrained to a narrow range about the maximum dose tested in a bioassay, and the maximum doses administered to rats and mice are highly correlated and similar in dose level. Information that can be known in advance of a 2-yr bioassay (mutagenicity, class, route, and maximum dose to test) does not identify groups of rodent carcinogens for which four sex-species groups are required to identify carcinogenicity. The range of accurate prediction of carcinogenicity using only male rats and female mice is 93% among mutagens and 88% among nonmutagens; for various routes of administration, 88-100%; for various chemical classes, 75-100%; and for various levels of the maximum dose tested, 81-100%. Results are similar for the pair male rats and male mice. Using a strength of evidence approach, weaker carcinogens are somewhat less likely than stronger carcinogens to be identified by two sex-species groups. Strength of evidence is measured using the proportion of experiments on a chemical that are positive, the extent to which tumors occur in animals that die before terminal sacrifice, and whether the chemical induces tumors at more than one site and in more than one species.

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.

Mutagenicity to Salmonella, Drosophila and the mouse bone marrow of the human antineoplastic agent fotemustine: prediction of carcinogenic potency

The antineoplastic agent fotemustine is shown to be a base-pair mutagen to Salmonella. Activity is more marked in the uvrB-proficient strain G46 than in the repair-deficient strain TA1535. This is consistent with its ability to cross-link DNA. Potent activity as a somatic and germ-cell mutagen to Drosophila was also observed. A potent clastogenic response was given by fotemustine in the mouse bone marrow following either oral gavage or intraperitoneal injection of a single dose of 5 mg/kg. In each of these respects it is shown to be indistinguishable from the structurally related antineoplastic agent and human carcinogen MeCCNU. It is concluded that fotemustine should be regarded as having clear potential to induce cancer in humans. Based on these data, including the preponderance of chromosome breakages over recessive lethal mutations in Drosophila, an estimated rodent carcinogenic potency (TD50) of between 15-150 mg/kg is suggested for fotemustine.

Comparison of target organs of carcinogenicity for mutagenic and non-mutagenic chemicals

A comparison of target organs for mutagens and non-mutagens is presented for 351 rodent carcinogens in the Carcinogenic Potency Database (CPDB) with mutagenicity evaluations in Salmonella. Results are consistent with the hypotheses that in high-dose rodent tests mitogenesis is important in the carcinogenic response for mutagens and non-mutagens alike, and that mutagens have a multiplicative interaction for carcinogenicity because they can both damage DNA directly and cause cell division at high doses. These hypotheses would lead one to expect several results that are found in the analysis: First, a high proportion of both mutagens and non-mutagens induce tumors in rodent bioassays at the MTD. Second, mutagens compared to non-mutagens are: (a) more likely to be carcinogenic; (b) more likely to induce tumors at multiple target sites; and (c) more likely to be carcinogenic in two species. Among carcinogens that induce tumors at multiple sites in both rats and mice, 81% are mutagens; in comparison, among carcinogens that are positive at only a single target site in one species and are negative in the other, 42% are mutagens. Since tissue distribution and pharmacokinetics would not be expected to differ systematically between mutagens and non-mutagens, one would not expect systematic differences in the particular organs in which tumors are induced. Results do not support the idea that mutagens and non-mutagens induce tumors in different target organs. Both mutagens and non-mutagens induce tumors in a wide variety of sites, and most organs are target sites for both. Moreover, the same sites tend to be the most common sites for both: 79% or more of both mutagenic and non-mutagenic carcinogens are positive in each species in at least one of the 8 most frequent target sites: liver, lung, mammary gland, stomach, vascular system, kidney, hematopoietic system and urinary bladder. Species differences are discussed as well as results for particular target organs: liver, Zymbal's gland and kidney.

Demonstration of initiation potential of carcinogens by induction of preneoplastic glutathione S-transferase P-form-positive liver cell foci: possible in vivo assay system for environmental carcinogens

In a development trial for an initiation bioassay system, 7 known carcinogens and 1 suspected carcinogen were examined. In experiment 1, group 1 animals were initially subjected to partial hepatectomy (PH) 12 h before administration of diethylnitrosamine, 2-amino-3-methylimidazo[4,5-f]-quinoline (IQ), captafol, alpha-hexachlorocyclohexane or diethylstilbestrol (DES), then 2 weeks later underwent a promotion procedure comprising administration of phenobarbital (0.05% in diet) for 8 weeks and D-galactosamine (300 mg/kg, i.g.) at week 3. Group 2 received the promotion protocol alone as in group 1. Initiating potential was assayed on the basis of significant increase in values of preneoplastic placental form glutathione S-transferase-positive (GST-P+) foci of more than 3 cells in cross section at week 10. Numbers and areas of GST-P+ foci in group 1 given IQ, captafol and DES were significantly increased as compared to group 2, confirming the validity of the protocol as an initiation assay. In Experiment 2, group 1 rats were subjected to PH and 12 h later received a suspected carcinogenic mixture of opium pyrolysate (OP) or carcinogenic pesticide p,p'-dichloro-diphenyltrichloroethane or hexachlorobenzene. Application of a modified promotion procedure comprising cholic acid (0.15%) and carbon tetrachloride (1 ml/kg, i.g.) revealed significant initiation potential for OP. Overall the results indicate that the current protocols may be useful for detection of the initiation potential of carcinogens irrespective of their mutagenicity.

Genetic toxicology studies with a tumor promoter

A diuretic antihypertensive agent, SC-33643 (8-[2-ethoxyethyl]-7-phenyl-[1,2,4]triazolo[4,3-c]pyrimidine-5- amine, also known as bemitradine), was tested in the Ames test, in the mouse lymphoma TK +/- mutation assay, in the Chinese hamster ovary cell hypoxanthine guanine phosphoribosyl transferase (CHO/HGPRT) mutation test and in the CHO chromosome aberration assay with and without metabolic activation. Additionally, the compound was tested in the rat primary hepatocyte unscheduled DNA synthesis (UDS) assay and in the mouse bone marrow micronucleus assay. The results were uniformly negative. Contrary to expectations based on the results of the battery of genetic toxicology tests, the compound produced liver, thyroid and mammary tumors in the rat (reported separately). Subsequently, SC-36741 (5-amino-7-phenyl-[1,2,4]triazolo-[1,5-c] pyrimidine-8-ethanol, also known as desethylbemitradine), a major metabolite of SC-33643, was tested in the Ames test, in the CHO/HGPRT mutation test and in the CHO chromosome aberration assay with and without metabolic activation, and was also tested in the rat primary hepatocyte/UDS assay and in the mouse bone marrow micronucleus assay. This metabolite also produced negative results in these tests. Therefore, SC-33643 is a non-genotoxic carcinogen producing tumors in rats without altering DNA or chromosomes.

Studies for a genotoxic potential of some endogenous and exogenous sex steroids. I. Communication: examination for the induction of gene mutations using the Ames Salmonella/microsome test and the HGPRT test in V79 cells

The mutagenicity results and data of nine progestins (cyproterone acetate, dehydrospirorenone, gestodene, gestonorone caproate, levonorgestrel, norethisterone, norethisterone acetate, norethisterone enanthate, norethynodrel), one hypothetical metabolite (6,7-epoxy-cyproterone acetate), four estrogens (estradiol, ethinylestradiol, cyclodiol, cyclotriol), and four other sex steroids (atamestane, lilopristone, onapristone, propylmesterolone) are reported. All 17 sex steroids were investigated using the Ames salmonella/microsome direct plate incorporation protocol, and seven were additionally tested using the preincubation modification. Seven sex steroids were also studied in the HG-PRT test with V79 cells for the induction of gene mutations in mammalian cells. The metabolite was examined in the Ames salmonella/microsome assay using the standard protocol and the preincubation modification. In all assays the test compounds were investigated up to concentration levels where cytotoxicity and/or visible precipitation occurred or at least the solubility limit of the test compound was reached. For all assays, evaluation of the data indicates that neither any of the sex steroids nor the hypothetical metabolite was able to induce gene mutations whether in the absence or the presence of an extrinsic metabolizing system (S9 mix).

Effects of modifying agents on conformity of enzyme phenotype and proliferative potential in focal preneoplastic and neoplastic liver cell lesions in rats

Development of preneoplastic lesions in the rat liver under the influence of various modifiers was investigated with particular attention to changes in simultaneous expression of altered enzyme phenotype within the lesions (conformity) and proliferation potential. Degree of conformity of marker enzymes such as glutathione S-transferase placental form (GST-P), glucose-6-phosphate dehydrogenase (G6PD), glucose-6-phosphatase, adenosine triphosphatase and gamma-glutamyltranspeptidase was compared with levels of 5-bromo-2-deoxyuridine labeling. After initiation with diethylnitrosamine, rats were administered the hepatopromoter sodium phenobarbital (PB, 0.05%), the antioxidant ethoxyquin (EQ, 0.5%), or a peroxisome proliferator, clofibrate (CF, 1.0%) or di(2-ethylhexyl)-phthalate (0.3%) and killed at week 16 or 32. The PB promoting regimen was clearly associated with increase in the numbers of high conformity class lesions simultaneously expressing three to five enzymes, and elevated proliferation potential. The inhibitor, EQ, in contrast, brought about a time-dependent decrease in conformity so that only 1 or 2 alterations were most commonly observed at week 32. Lesion populations in the peroxisome proliferator- and especially CF-treated cases were characterized by obvious dissociation between degree of conformity and proliferative status. Such treatment-dependent differences were not always correlated with the size of the lesion. The results thus suggested that the conformity and proliferation potential of preneoplastic lesions are dependent on modification treatment. Overall, GST-P was found to be the most reliable marker, although G6PD was less influenced in the peroxisome proliferator cases.

Influence of lighting conditions on toxicity and genotoxicity of various PAH in the newt in vivo

We evaluated the influence of near-ultraviolet light (UVA) on the cytotoxicity and genotoxicity of 7 polycyclic aromatic hydrocarbons (PAH) in larvae of the amphibian Pleurodeles waltl. Benz[a]anthracene (BA), 7,12-benz[a]anthraquinone (BAQ) and anthracene (Ac) proved to be lethal at low doses (some ppb), and the following order of genotoxicity was observed: BA approximately BAQ > DMBA > DMA (9,10-dimethylanthracene). Ac, AQ (9,10-anthraquinone) and DBA (dibenz[a,h]anthracene) were not found to be clastogenic. In the larvae reared in normal conditions (subdued natural daylight/darkness alternation) or in continuous darkness, the BA derivatives were shown to be more genotoxic than BA itself: DMBA > BAQ > BA; BA (> or = 187.5 ppb) slightly increased the level of micronuclei in circulating erythrocytes, while DMBA was strongly clastogenic, in line with their reported carcinogenicity. In other experiments, rearing media alone (i.e., water containing BA, BAQ or DMBA) were UVA-irradiated for 24 h, and then tested on larvae in the dark ('IR-UV/dark' conditions). Photodegradation of BA (50 and 100 ppb) gave rise to clastogenic products. By contrast, DMBA (12.5, 25 or 50 ppb) was destroyed by UVA, and we suggested that any potentially mutagenic photoproducts formed were not in sufficient amounts to yield a positive response in the newt micronucleus test.

1,4-Dioxane: prediction of in vivo clastogenicity

1,4-Dioxane was analyzed with the CASE program to determine the structural basis of its potential genotoxicity and carcinogenicity. These investigations led to the prediction that while 1,4-dioxane was not genotoxic in vitro, it was an inducer of micronuclei in the bone marrow of rats and a carcinogen for both rats and mice. If it is assumed that the induction of micronuclei is the result of DNA damage, then this potential and the previous report of the in vivo induction of DNA strand breaks in rat liver raise the possibility of a genotoxic action for 1,4-dioxane. However it is also conceivable that we have identified a structural feature which contributes to the induction of micronuclei by a non-genotoxic mechanism.

An evaluation of the E. coli K-12 uvrB/recA DNA repair host-mediated assay. I. In vitro sensitivity of the bacteria to 61 compounds

A differential DNA repair test was evaluated in vitro, using derivatives of E. coli K-12 343/113 with the genotype uvrB-/recA- and uvrB+/recA+. The aim of this study was to characterize the sensitivity of the assay to different compounds in vitro and thereby provide information on the usefulness of this end-point as an indicator of genotoxicity in a host-mediated assay. Sixty-one compounds from diverse chemical groups were tested and of these 32 gave a positive result. The results obtained were compared with results from the Ames test and were in agreement for 49 out of the 61 compounds tested. Chemicals that were detected in this test but negative in the Ames test were 4-aminophenol, catechol, diethylstilbestrol, thioacetamide and thiourea. Seven of the compounds tested gave a negative result in E. coli but were positive in Salmonella. These were 4-aminobiphenyl, benzo[a]pyrene, cyclophosphamide, 1-naphthylamine, N-nitrosobutylpropylamine, quinoline and 2-toluidine. The performance of the in vitro test and reasons for the discrepant results with the Ames test are discussed. The overall concordance between the two tests was about 80%. On the basis of these results we consider these bacterial strains, and differential DNA repair as an end-point, to be sufficiently accurate as an indicator of genotoxicity in vitro and thereby also in vivo.

An evaluation of the E. coli K-12 uvrB/recA DNA repair host-mediated assay. II. In vivo results for 36 compounds tested in the mouse

The aim of this study was to further evaluate the E. coli K-12 DNA repair host-mediated assay, as a short-term in vivo genotoxicity test, to be used as a complement to the micronucleus test in the routine testing of chemicals and drugs. The assay involves the administration of the test substance to mice by the route of choice, followed by the intravenous administration of a mixture of DNA repair deficient and proficient derivatives of E. coli K-12. After an incubation period the relative survival of the two strains was determined in blood, liver, lungs, kidneys and testes of the host. A significant preferential reduction of the DNA repair deficient strain in any organ indicates that the test substance possesses genotoxic properties. A total of 36 substances, 26 carcinogens, 4 weak or non-carcinogens and 6 unclassified substances, were tested in this assay. Positive results were obtained for 23 compounds. Of the carcinogens 18 were positive and of the non-carcinogens 3 were negative. The overall concordance between the assay and carcinogenicity was 72%. In general, alkylating agents and direct-acting nitroso compounds showed genotoxic activity in all organs tested, while the other substances were positive in a limited number of organs. With oral administration, which was the most commonly used administration route in the study, the organ showing a positive response most often was the blood. The results from the present study were compared with results from the micronucleus test, which were available for 26 of the substances. Results were in agreement for 15 of the substances, while 8 substances were positive in the present assay and negative in the micronucleus test: 4-aminobiphenyl, 2-anisidine, epichlorohydrin, formaldehyde, 1- and 2-naphthylamine, 2-nitrophenylenediamine and 4-nitroquinoline-N-oxide. The substances negative in the E. coli DNA repair host-mediated assay, but positive in the micronucleus test were: benzene, catechol and cyclophosphamide. It is concluded from this evaluation that the E. coli K-12 DNA repair host-mediated assay detects a number of carcinogens that are negative in the micronucleus test, while detecting most of the compounds that are positive in the latter. The advantages of this test are that differential DNA repair measures a broad spectrum of genetic damage, an in vitro/in vivo comparison is possible with the same test organisms, results can be obtained from various organs and the test is rapid.(ABSTRACT TRUNCATED AT 400 WORDS)

Testing by artificial intelligence: computational alternatives to the determination of mutagenicity

In order to develop methods for evaluating the predictive performance of computer-driven structure-activity methods (SAR) as well as to determine the limits of predictivity, we investigated the behavior of two Salmonella mutagenicity data bases: (a) a subset from the Genetox Program and (b) one from the U.S. National Toxicology Program (NTP). For molecules common to the two data bases, the experimental concordance was 76% when "marginals" were included and 81% when they were excluded. Three SAR methods were evaluated: CASE, MULTICASE and CASE/Graph Indices (CASE/GI). The programs "learned" the Genetox data base and used it to predict NTP molecules that were not present in the Genetox compilation. The concordances were 72, 80 and 47% respectively. Obviously, the MULTICASE version is superior and approaches the 85% interlaboratory variability observed for the Salmonella mutagenicity assays when the latter was carried out under carefully controlled conditions.

A compilation of genotoxicity and carcinogenicity data on aromatic aminosulphonic acids

A review is presented to evaluate existing information on genotoxicity and carcinogenicity testing of various aromatic aminosulphonic acids (AASAs). A great variety of water-soluble azo dyes can form aromatic phenyl- or naphthyl-aminosulphonic acids by chemical and enzymatic reduction. AASAs are also used as intermediates in the synthesis of azo dyes and azo pigments and can arise as contaminants in the final products. Comparisons have been made with the data available on the corresponding unsulphonated analogues, some of which are known to be genotoxic and/or carcinogenic. The vast majority of the AASAs were conclusively non-mutagenic in the Ames test. In most cases the absence of genotoxicity was also demonstrated with a variety of other test systems in vitro and in vivo. It is concluded that AASAs, in contrast with some of their unsulphonated analogues, generally have no or very low genotoxic and tumorigenic potential.

Melphalan, a second chemical for which specific-locus mutation induction in the mouse is maximum in early spermatids

Melphalan (MLP), a bifunctional alkylating agent structurally related to the highly mutagenic chemical chlorambucil (CHL), was found to induce high frequencies of specific-locus mutations in postspermatogonial germ cells of the mouse, and to be one of only a few chemicals that is also mutagenic in spermatogonial stem cells. Productivity patterns following MLP exposures resembled those that had been found for CHL. Mutation rates in successive male germ-cell stages were measured at three MLP-exposure levels in a total of 95,375 offspring. While the induced (experimental minus historical-control) mutation rate is relatively low in stem-cell spermatogonia (1.2 x 10(-5) per locus at a weighted-mean exposure of 7.3 mg/kg), it is about 5 times higher in poststem-cell stages overall, and peaks at 26.7 x 10(-5) per locus in early spermatids at a weighted-mean exposure of only 5.7 mg/kg. This "type-2 pattern" of mutation yield (Russell et al., 1990), i.e., peak sensitivity in early spermatids, has heretofore been found for only one other chemical, CHL. Mutation-rate data earlier reported for CHL (Russell et al., 1989) were augmented in the present study for comparison with MLP-induced rates. Because of the greater toxicity of MLP, average exposures used for this chemical were only about one-half of those for CHL. When MLP and CHL mutation rates are extrapolated to equimolar doses, they appear very similar for poststem-cell stages overall. However, in the case of CHL, a somewhat higher proportion of the mutations is induced in early spermatids than in the case of MLP.

Lack of modifying effects of 6-mercaptopurine in a medium term bioassay system for liver carcinogenesis using male F344 rats

Carcinogenic and modification potential of 6-mercaptopurine (6-MP) was studied in a medium-term bioassay system for rat liver carcinogenesis. F344 male rats were initiated with a single dose (200 mg/kg body wt.) of diethylnitrosamine (DEN) i.p. and fed diets containing either 0.005% or 0.02% 6-MP with or without 0.05% phenobarbital (PB) for 6 weeks. Quantitative data revealed that 6-MP did not enhance the appearance of enzyme-altered preneoplastic foci and nodules even when administered at the highest dose (0.02%) despite showing an immunosuppressive effect and slight liver cell damage. Neither of the doses of 6-MP exerted any significant influence on the enhancing effect of PB when administered simultaneously in the medium-term-bioassay.

Considerations in the U.S. Environmental Protection Agency's testing approach for mutagenicity

OPP: This paper provides the rationale and support for the decisions the OPP will make in requiring and reviewing mutagenicity information. The regulatory requirement for mutagenicity testing to support a pesticide registration is found in the 40 CFR Part 158. The guidance as to the specific mutagenicity testing to be performed is found in the OPP's Pesticide Assessment Guidelines, Subdivision F, Hazard Evaluation: Human and Domestic Animals (referred to as the Subdivision F guideline). A revised Subdivision F guideline has been presented that becomes the current guidance for submitters of mutagenicity data to the OPP. The decision to revise the guideline was the result of close examination of the version published in 1982 and the desire to update the guidance based on developments since then and current state-of-the-science. After undergoing Agency and public scrutiny, the revised guideline is to be published in 1991. The revised guideline consists of an initial battery of tests (the Salmonella assay, an in vitro mammalian gene mutation assay and an in vivo cytogenetics assay which may be either a bone marrow assay for chromosomal aberrations or for micronuclei formation) that should provide an adequate initial assessment of the potential mutagenicity of a chemical. Follow-up testing to clarify results from the initial testing may be necessary. After this information as well as all other relevant information is obtained, a weight-of-evidence decision will be made about the possible mutagenicity concern a chemical may present. Testing to pursue qualitative and/or quantitative evidence for assessing heritable risk in relation to human beings will then be considered if a mutagenicity concern exists. This testing may range from tests for evidence of gonadal exposure to dominant lethal testing to quantitative tests such as the specific locus and heritable translocation assays. The mutagenicity assessment will be performed in accordance with the Agency's Mutagenicity Risk Assessment Guidelines. The mutagenicity data would also be used in the weight-of-evidence consideration for the potential carcinogenicity of a chemical in accordance with the Agency's Carcinogen Risk Assessment Guidelines. In instances where there are triggers for carcinogenicity testing, mutagenicity data may be used as one of the triggers after a consideration of available information. It is felt that the revised Subdivision F guideline will provide appropriate, and more specific, guidance concerning the OPP approach to mutagenicity testing for the registration of a pesticide. It also provides a clearer understanding of how the OPP will proceed with its evaluation and decision making concerning the potential heritable effects of a test chemical.(ABSTRACT TRUNCATED AT 400 WORDS)

Genetic activity of the human carcinogen sulphur mustard towards Salmonella and the mouse bone marrow

Sulphur mustard is clearly mutagenic to the repair-proficient (uvrB+) strain G46 of S. typhimurium. It is also a micronucleus-inducing agent to the mouse bone marrow. These data strengthen the database indicating that most human carcinogens are genotoxic.