Bacterial mutagenicity and mammalian cell DNA damage by several substituted anilines

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

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

Green rust (GR) and glucose oxidase (GOX) based Fenton-like reaction: Capacity of sustainable release, promoted conversion of glucose through GOX-iron and pH self-adjustment

The Fenton reaction is regarded as highly efficient for the degradation of organic contaminants. However, the traditional Fenton reaction is still flawed in a narrow pH working range and low utilization efficiency of the reagents. Based on two striking features, a sustained release of H₂O₂ in-situ under the catalysis of glucose oxidase (GOX) and the rapid electron donation & transferability from green rust (GR), an adaptable biological Fenton-like system (GGGMFs) was established. The coupling roles of glucose, GOX and GR in the degradation of 3,4-dimethylaniline (3,4-DMA) and the types of reactive species were deduced by electron spin resonance (ESR), etc.. Results demonstrated that the suitable pH range of the system was optimized from acidic to circumneutral, which was favorable for practical application, owing to the heterogeneous formation of GR and the pH self-adjustable capacity of GOX-Glucose. Meanwhile, hydroxyl radical (·OH), superoxide radical (·O₂^-) and Fe (IV) were identified to be the main oxidizing reactive species. Taking different selectivity of the reactive species to certain pollutant functional groups into consideration, the degradation pathways of 3,4-DMA were proposed. Moreover, it was shown that GR not only acted as the activating substance of the Fenton-like reaction, but also enhanced the activity of GOX, resulting in the promotion of glucose conversion in GGGMFs. This study shed light on the enhancement mechanism consisting of two aspects: (i) the elimination of product inhibition (ii) the formation of a 2Fe(III)-FAD complex with FAD, the active center of GOX, which prompted the electronic transfer in the enzyme catalytic reaction.

Evaluation of toxicological aspects of three new benzoxazole compounds with sunscreen photophysical properties using in silico and in vitro methods

Sunscreening chemicals protect against damage caused by sunlight most absorbing UVA or UVB radiations. In this sense, 2-(2'-hydroxyphenyl)benzoxazole derivatives with amino substituents in the 4' and 5' positions have an outstandingly high Sun Protection Factor and adequate photostability, but their toxicity is not yet known. This study aimed to evaluate the toxicity of three synthetic 2-(2'-hydroxyphenyl)benzoxazole derivatives for their possible application as sunscreens. In silico tools were used in order to assess potential risks regarding mutagenic, carcinogenic, and skin sensitizing potential. Bioassays were performed in L929 cells to assess cytotoxicity in MTT assay and genotoxic activities in the Comet assay and micronucleus test. Also, the Salmonella/microsome assay was performed to evaluate gene mutations. The in silico predictions indicate a low risk of mutagenicity and carcinogenicity of the compounds while the skin sensitizing potential was low or inconclusive. The 2-(4'-amino-2'-hydroxyphenyl)benzoxazol compound was the most cytotoxic and genotoxic among the compounds evaluated in L929 cells, but none induced mutations in the Salmonella/microsome assay. The amino substituted at the 4' position of the phenyl ring appears to have greater toxicological risks than substituents at the 5' position of 2-(phenyl)benzoxazole. The findings warrant further studies of these compounds in cosmetic formulations.

DNA-damaging activities of twenty-four structurally diverse unsubstituted and substituted cyclic compounds in embryo-fetal chicken livers

DNA-damaging activities of twenty-four structurally diverse unsubstituted and substituted cyclic compounds were assessed in embryo-fetal chicken livers. Formation of DNA adducts and strand breaks were measured using the nucleotide ³²P-postlabelling (NPL) and comet assays, respectively. Unsubstituted monocyclic benzene, polycyclic fused ring compound naphthalene, covalently connected polycyclic ring compound biphenyl, and heterocyclic ring compound fluorene did not produce DNA damage. Amino-substituted monocyclic compounds, aniline and p-phenylenediamine, as well as polycyclic 1-naphthylamine were also negative. In contrast, carcinogenic monocyclic methyl-substituted anilines: o-toluidine, 2,6-xylidine, 3,4-dimethylaniline, 4-chloro-o-toluidine; 2 methoxy-substituted methylaniline: p-cresidine; 2,4 and 2,6 diamino- or dinitro- substituted toluenes all produced DNA damage. Genotoxic polycyclic amino-substituted 2-naphthylamine, 4-aminobiphenyl, benzidine, methyl-substituted 3,2'-dimethyl-4-aminobiphenyl and 4-dimethylaminoazobenzene as well as amino- and nitro- fluorenes substituted at the 1 or 2 positions also were positive in at least one of the assays. Overall, the DNA damaging activity of cyclic compounds in embryo-fetal chicken livers reflected the type and position of the substitution on the aromatic ring. Additionally, substituted polycyclic compounds exhibited higher DNA-damaging potency compared to monocyclic chemicals. These results are congruent with in vivo findings in other species, establishing chicken eggs as a reliable system for structure-activity assessment of members of groups of related chemicals.

Mutagenicity in Surface Waters: Synergistic Effects of Carboline Alkaloids and Aromatic Amines

For decades, mutagenicity has been observed in many surface waters with a possible link to the presence of aromatic amines. River Rhine is a well-known example of this phenomenon but responsible compound(s) are still unknown. To identify the mutagenic compounds, we applied effect-directed analysis (EDA) utilizing novel analytical and biological approaches to a water sample extract from the lower Rhine. We could identify 21 environmental contaminants including two weakly mutagenic aromatic amines, and the known alkaloid comutagen norharman along with two related β-carboline alkaloids, carboline, and 5-carboline, which were reported the first time in surface waters. Results of mixture tests showed a strong synergism of the identified aromatic amines not only with norharman, but also with carboline and 5-carboline. Additionally, other nitrogen-containing compounds also contributed to the mutagenicity when aromatic amines were present. Thus, comutagenicity of β-carboline alkaloids with aromatic amines is shown to occur in surface waters. These results strongly suggest that surface water mutagenicity is highly complex and driven by synergistic mechanisms of a complex compound mixture (of which many are yet unidentified) rather than by single compounds. Therefore, mixture effects should be considered not only from mutagens alone, but also including possible comutagens and nonmutagenic compounds.

Using ovality to predict nonmutagenic, orally efficacious pyridazine amides as cell specific spleen tyrosine kinase inhibitors

Inhibition of spleen tyrosine kinase has attracted much attention as a mechanism for the treatment of cancers and autoimmune diseases such as asthma, rheumatoid arthritis, and systemic lupus erythematous. We report the structure-guided optimization of pyridazine amide spleen tyrosine kinase inhibitors. Early representatives of this scaffold were highly potent and selective but mutagenic in an Ames assay. An approach that led to the successful identification of nonmutagenic examples, as well as further optimization to compounds with reduced cardiovascular liabilities is described. Select pharmacokinetic and in vivo efficacy data are presented.

Using chemical reactivity to provide insights into environmental transformations of priority organic substances: the Fe⁰-mediated reduction of Acid Blue 129

Sulfonated anthracenedione dyes are medium priority organic compounds targeted for environmental assessment under the Government of Canada's Chemical Management Plan (CMP). Since organic compounds undergo transformations in environmental matrices, understanding these transformations is critical for a proper assessment of their environmental fate. In the current study, we used zero-valent iron (ZVI) to provide insight into reductive transformation processes available to the anthracenedione dye, Acid Blue 129 (AB 129), a dye which is used in the textile industry. At high temperatures, we found that AB 129 was rapidly reduced (within 3 h) after being adsorbed onto the ZVI-surface, whereupon decomposition took place via multiple competitive and consecutive reaction pathways. Reaction products were identified using state-of-the-art accurate mass Liquid Chromatography-Quadrupole Time of Flight-Mass Spectroscopy (LC-QToF-MS). Five transformation products were identified, including a genotoxic (and thus, potentially carcinogenic) end-product, 2,4,6-trimethylaniline. The same products were found at room temperature, demonstrating that the transformation pathways revealed here could plausibly arise from biological and/or environmental reductions of AB 129. Our results demonstrate the importance of identifying reaction product arising from priority substances as part of the environmental risk assessment process.

A core in vitro genotoxicity battery comprising the Ames test plus the in vitro micronucleus test is sufficient to detect rodent carcinogens and in vivo genotoxins

In vitro genotoxicity testing needs to include tests in both bacterial and mammalian cells, and be able to detect gene mutations, chromosomal damage and aneuploidy. This may be achieved by a combination of the Ames test (detects gene mutations) and the in vitro micronucleus test (MNvit), since the latter detects both chromosomal aberrations and aneuploidy. In this paper we therefore present an analysis of an existing database of rodent carcinogens and a new database of in vivo genotoxins in terms of the in vitro genotoxicity tests needed to detect their in vivo activity. Published in vitro data from at least one test system (most were from the Ames test) were available for 557 carcinogens and 405 in vivo genotoxins. Because there are fewer publications on the MNvit than for other mammalian cell tests, and because the concordance between the MNvit and the in vitro chromosomal aberration (CAvit) test is so high for clastogenic activity, positive results in the CAvit test were taken as indicative of a positive result in the MNvit where there were no, or only inadequate data for the latter. Also, because Hprt and Tk loci both detect gene-mutation activity, a positive Hprt test was taken as indicative of a mouse-lymphoma Tk assay (MLA)-positive, where there were no data for the latter. Almost all of the 962 rodent carcinogens and in vivo genotoxins were detected by an in vitro battery comprising Ames+MNvit. An additional 11 carcinogens and six in vivo genotoxins would apparently be detected by the MLA, but many of these had not been tested in the MNvit or CAvit tests. Only four chemicals emerge as potentially being more readily detected in MLA than in Ames+MNvit--benzyl acetate, toluene, morphine and thiabendazole--and none of these are convincing cases to argue for the inclusion of the MLA in addition to Ames+MNvit. Thus, there is no convincing evidence that any genotoxic rodent carcinogens or in vivo genotoxins would remain undetected in an in vitro test battery consisting of Ames+MNvit.

Monocyclic aromatic amines as potential human carcinogens: old is new again

Alkylanilines are a group of chemicals whose ubiquitous presence in the environment is a result of the multitude of sources from which they originate. Exposure assessments indicate that most individuals experience lifelong exposure to these compounds. Many alkylanilines have biological activity similar to that of the carcinogenic multi-ring aromatic amines. This review provides an overview of human exposure and biological effects. It also describes recent investigations into the biochemical mechanisms of action that lead to the assessment that they are most probably more complex than those of the more extensively investigated multi-ring aromatic amines. Not only is nitrenium ion chemistry implicated in DNA damage by alkylanilines but also reactions involving quinone imines and perhaps reactive oxygen species. Recent results described here indicate that alkylanilines can be potent genotoxins for cultured mammalian cells when activated by exogenous or endogenous phase I and phase II xenobiotic-metabolizing enzymes. The nature of specific DNA damage products responsible for mutagenicity remains to be identified but evidence to date supports mechanisms of activation through obligatory N-hydroxylation as well as subsequent conjugation by sulfation and/or acetylation. A fuller understanding of the mechanisms of alkylaniline genotoxicity is expected to provide important insights into the environmental and genetic origins of one or more human cancers and may reveal a substantial role for this group of compounds as potential human chemical carcinogens.

Identification of adducts formed by reaction of N-acetoxy-3,5-dimethylaniline with DNA

Aromatic amines constitute one of the most extensively studied classes of chemical carcinogens. Although monocyclic aromatic amines are generally regarded as weak carcinogens, a recent epidemiologic study of bladder cancer found that the arylamine 3,5-dimethylaniline (3,5-DMA) may play a significant role in the etiology of this disease in man. Investigations using experimental animals also strongly suggested that DNA adducts-of indeterminate structure-formed by 3,5-DMA might account for its presumptive activity. The present study was undertaken to determine the structures of the major DNA adducts formed in vitro by the known, and possibly carcinogenic, N-hydroxylated metabolite. Calf thymus DNA (ct-DNA) was modified by reaction with N-acetoxy-3,5-dimethylaniline (N-AcO-3,5-DMA). After enzymatic hydrolysis of DNA to individual 2'-deoxyribonucleosides, adduct profiles were determined using HPLC/MS. 3,5-DMA formed four major DNA adducts, one to 2'-deoxyguanosine (dG), two to 2'-deoxyadenosine (dA), and one to 2'-deoxycytidine (dC). Reactions of N-AcO-3,5-DMA with dG, dA, and dC produced the same adducts as reaction with ct-DNA with very similar profiles. Adducts were isolated chromatographically and unambiguously characterized as N-(deoxyguanosin-8-yl)-3,5-dimethylaniline (dG-C8-3,5-DMA), 4-(deoxyadenosin- N(6)-yl)-3,5-dimethylaniline (dA- N(6)-3,5-DMA), N-(deoxyadenosin-8-yl)-3,5-dimethylaniline (dA-C8-3,5-DMA), and N-(deoxycytidin-5-yl)-3,5-dimethylaniline (dC-C5-3,5-DMA) by high-resolution mass spectra (HR-MS) and NMR spectroscopy including (1)H NMR, (13)C NMR, and two-dimensional NMR. This report includes the first detailed description of a dC adduct of an aromatic amine. The present results provide chemical support for a carcinogenic mechanism of action by 3,5-DMA based on N-hydroxylation and the intermediacy of a nitrenium ion in the formation of DNA adducts.

Dose-threshold for thyroid tumor-promoting effects of xylazine in rats

To clarify the threshold dose of thyroid tumor-promoting effects of xylazine hydrochloride (XZ), male F344 rats received pulverized basal diet containing 0, 250, 500, or 1000 ppm XZ for 26 weeks with or without initiation of 2400 mg/kg N-bis(2-hydroxypropyl)nitrosamine (DHPN). Thyroid weights significantly increased in the groups with or without DHPN initiation that were given 500 ppm XZ or more. The serum thyroxine (T4) and triiodothyronine (T3) levels decreased significantly in the XZ 250 and XZ 1000 ppm groups, respectively, although there were no remarkable changes in the serum thyroid-stimulating hormone (TSH) levels. Histopathologically, follicular cell hyperplasias and adenomas were induced in the DHPN-alone and DHPN+XZ groups, and the incidences and multiplicities of these lesions in the DHPN groups treated with 500 ppm XZ or more were significantly higher than those in the DHPN alone group. These results suggest that the threshold dose of rat thyroid tumor-promoting effects of XZ is between 250 and 500 ppm under the present experimental condition.

The effect of deuterium and fluorine substitution upon the mutagenicity of N-hydroxy-2,6-dimethylaniline

2,6-Dimethylaniline (2,6-DMA) is an intermediate in the manufacture of several products, including pesticides, dyestuffs, and synthetic resins. It is also present in nanogram amounts in tobacco smoke, and is a major metabolite of the potent anesthetic and antiarrhythmic drug lidocaine, as well as a nasal carcinogen in rats. As with other aromatic amines, 2,6-DMA can undergo metabolic activation through cytochrome p450-mediated N-hydroxylation, followed by O-esterification to a reactive derivative capable of forming DNA adducts. We have recently characterized four DNA adducts resulting from this metabolic pathway. Three of the adducts arose from reaction of the exocyclic heteroatoms of deoxyadenosine and deoxyguanosine with the carbon para to the arylamine nitrogen. The fourth adduct resulted from reaction of the 2,6-DMA nitrogen with the C8 atom of deoxyguanosine. In order to investigate the relative contribution of the exocyclic heteroatom adducts as compared to the C8-deoxyguanosine adduct to the toxicities elicited by 2,6-DMA, we synthesized and compared the mutagenicity of N-hydroxy-2,6-DMA, N-hydroxy-4-deutero-2,6-DMA, 2,6-dimethylnitrosobenzene, 4-deutero-2,6-dimethylnitrosobenzene, and N-hydroxy-4-fluoro-2,6-DMA. In Salmonella typhimurium TA100, the two deuterated compounds and their non-deuterated analogues gave similar mutagenic responses ( approximately 25 revertants/nmol). Likewise in S. typhimurium TA98, a similar mutant frequency ( approximately 0.7 revertants/nmol) was obtained with the four compounds. With N-hydroxy-4-fluoro-2,6-DMA, the mutant frequency was reduced by approximately 90% in S. typhimurium TA100 and approximately 50% in S. typhimurium TA98. The results suggest that multiple adducts contribute to base substitution mutations detected by S. typhimurium TA100 while the C8-deoxyguanosine adduct is primarily responsible for the frameshift mutations detected by S. typhimurium TA98.

Immunohistochemical and ultrastructural studies of 2,6-dimethylaniline-induced nasal proliferative lesions in a rat two-stage nasal carcinogenesis model initiated with N-bis(2-hydroxypropyl)nitrosamine

Proliferative lesions induced by 2,6-dimethylaniline (DMA) in a two-stage rat nasal carcinogenesis model were immunohistochemically and ultrastructurally investigated. Male F344 rats received diet containing 3,000 ppm DMA for 52 weeks after initiation with a single subcutaneous injection of 2400 mg/kg of N-bis(2-hydroxypropyl)nitrosamine (DHPN). Histopathologically, proliferation of Bowman's glands, glandular hyperplasias, dysplastic foci, adenomas, and carcinomas were observed in treated rats. These nasal lesions mostly arose in the olfactory mucosa of the nasal cavity. Immunohistochemically, they were positive for cytokeratin and/or collagen type IV antibodies. Ultrastructurally, intracytoplasmic dense secretory granules (200-850 nm in diameter), identical to those in normal Bowman's glands, were observed in all the lesions, providing further support from an origin from these glands. Based on their cellular characterization, growth pattern and/or proliferative activity, two morphological continua were evident, one from dysplastic foci to carcinomas and the other from proliferation of Bowman's glands to glandular hyperplasias and adenomas. These results suggest that dysplastic foci arise from Bowman's glands and progress to carcinomas, while proliferation of Bowman's glands result in glandular hyperplasias and adenomas.

Promoting effects of xylazine on development of thyroid tumors in rats initiated with N-bis(2-hydroxypropyl)nitrosamine and the mechanism of action

To cast light on whether xylazine hydrochloride (XZ), a veterinary medicine commonly used as a sedative agent for food-producing animals, has any promoting potential for thyroid carcinogenesis, the following studies were performed. In Experiment I, male F344 rats received a diet containing 1000 or 0 p.p.m. XZ for 52 weeks with or without initiation with 2400 mg/kg N:-bis(2-hydroxypropyl)nitrosamine (DHPN). Focal follicular cell hyperplasias, adenomas and/or carcinomas were induced in the DHPN alone, XZ alone and DHPN+XZ groups, and the incidences and multiplicities of these lesions in the DHPN+XZ group were significantly increased as compared with the DHPN alone case. In Experiment II, male F344 rats received a diet containing 1000 or 0 p.p.m. XZ and were examined for serum levels of triiodothyronine (T(3)), thyroxine (T(4)) and thyroid-stimulating hormone (TSH) at weeks 1, 2 and 4. In the XZ group, significant increase in thyroid weight and decrease in serum T(4) levels were observed at all time points. Serum T(3) and TSH levels were significantly decreased and increased, respectively, at week 1, but returned to within the control range thereafter. In Experiment III, male F344 rats received a diet containing 1000 or 0 p.p.m. XZ, they were examined for thyroid iodine uptake and organification of XZ after 1 and 2 weeks. The thyroidal iodine uptake per milligram of thyroid and the amount of iodine bound to 1 mg protein showed a tendency for decrease at week 1 and significant decrease at week 2. These results indicate that XZ has tumor-promoting effects on thyroid follicular cells, and suggest an involvement of alterations in thyroid-related hormone levels due to inhibition of thyroid iodine uptake and organification, resulting, provably, in serum TSH stimulation depending on continuous reduction of serum T(4) level through the feedback system in the pituitary-thyroid axis.

Sequential observation of 2,6-dimethylaniline-induced nasal lesions in a rat two-stage nasal carcinogenesis model after initiation with N-bis(2-hydroxypropyl) nitrosamine

Male F344 rats received diet containing 3,000 ppm 2,6-dimethylaniline (DMA) after initiation with a single subcutaneous injection of 2,400 mg/kg of N-bis(2-hydroxypropyl)nitrosamine (DHPN), and histological and electron microscopic examinations of the nasal cavity were performed at 4, 13, 26 and 52 weeks to examine sequential changes induced by DMA. Severe atrophy of Bowman's glands and epithelial disarrangement were apparent from week 4, followed by dilatation and/or proliferation of Bowman's glands, degeneration of epithelial cells, and proliferation of undifferentiated epithelial cells from week 13. Focal glandular hyperplasias, dysplastic foci, and adenomas were observed from week 26, and carcinomas at 52 week. These nasal lesions were mostly evident in the olfactory mucosa in the nasal cavity, and their severity and/or incidences, other than atrophy of Bowman's glands, increased with the treatment period. Electron microscopically, carcinoma cells demonstrated desmosomes, dense secretory granules identical to those in normal Bowman's glands, a basement membrane, and microvilli. These results suggest that Bowman's glands are the target of DMA, giving rise to nasal carcinomas after DHPN-initiation.

Tumor-promoting activity of 2,6-dimethylaniline in a two-stage nasal carcinogenesis model in N-bis(2-hydroxypropyl)nitrosamine-treated rats

The potential promotion activity on nasal carcinogenesis of 2,6-dimethylaniline (DMA), an alpha2-adrenergic agonist metabolite of xylazine which is used for food-producing animals as a sedative agent, was examined. Male F344 rats received diet containing 0 or 3000 ppm DMA for 52 weeks after initiation with N-bis(2-hydroxypropyl)nitrosamine (DHPN). Histopathological assessment showed the incidence of carcinomas in the DHPN+DMA group (33%) to be significantly elevated as compared with that for the DHPN-alone group (5%). Incidences and/or multiplicity of epithelial hyperplasias and dysplastic foci were also increased in the DHPN+DMA group. These lesions were exclusively observed in the olfactory mucosa. The lowest plasma levels of DMA in tumor- and dysplastic foci-bearing rats were 0.05 and 0.20 microg/ml, respectively. These results indicate that DHPN acts as an appropriate initiator for nasal carcinogenesis and that DMA exerts a tumor-promoting effect on the olfactory mucosa in the rat nasal cavity.

Characterization of methemoglobinemia induced by 3,5-xylidine in rats

The objective of this study was to characterize the dose effect and kinetics of methemoglobinemia in rats following oral or intravenous administration of 3,5-xylidine (XYL). The first set of experiments involved the intravenous administration of 0.06, 0.12, 0.24, 0.48, or 0.60 mmol XYL/kg to groups of 3 rats each and the serial sampling of blood from the tail vein of individual animals for the determination of methemoglobin levels. An additional series of experiments involved the oral administration of 0.24, 0.48, 0.72, 0.96, 1.2, 1.8, 2.4, or 4.8 mmol XYL/kg and the serial sampling of blood for the determination of methemoglobin levels. The results showed a dose-dependent induction of methemoglobinemia by XYL in the rat, for both routes of administration. The maximal percent methemoglobin observed in the treated animals was 28.90 +/- 0.34% and 32.67 +/- 2.14% for the intravenous (0.6 mmol/kg) and oral (4.8 mmol/kg) routes, respectively. The dose levels of 0.06 mmol/kg (iv) and 0.96 mmol/kg (po) were the no-observable-adverse-effect levels with respect to XYL-induced methemoglobinemia in the rat. The dose-effect information on XYL-induced methemoglobinemia obtained in this study may be useful for the characterization of noncarcinogenic risks of acute human exposure to this chemical.

Genotoxicity of 4-chloro-o-toluidine in Salmonella typhimurium, human lymphocytes and V79 cells

In the absence of a metabolizing system (S9 mix) 4-chloro-o-toluidine (4-COT) was found to be ineffective in a combination of assays for gene mutations in Salmonella typhimurium, for chromosome aberrations and sister chromatide exchanges in human lymphocytes, and for the induction of spindle disturbances in V79 Chinese hamster cells. In the presence of S9, 4-COT was also ineffective in producing structural or numerical changes in mammalian cells, but the yields of 4-COT induced revertants in S. typhimurium strains TA 100 and TA 98 were about 2-fold higher than those in controls.

Interaction between the substrate and the high-valent-iron-oxo porphyrin cofactor as a possible factor influencing the regioselectivity of cytochrome P450 catalysed aromatic ring hydroxylation of 3-fluoro(methyl)anilines

In the present study the in vitro and in vivo aromatic ring hydroxylation of a series of amino and/or methyl containing fluorobenzenes, i.e. 3-fluoro(methyl)anilines, was investigated and compared to the calculated density distribution of the reactive frontier pi-electrons of the aromatic substrate. This was done (1) to study to what extent the regioselectivity of the aromatic ring hydroxylation of the 3-fluoro(methyl)anilines could be predicted on the basis of the calculated chemical reactivity, as was previously observed for a series of fluorinated benzenes and monofluoroanilines, and (2) to investigate which factors contribute to possible deviations from the predictions on the basis of the calculated chemical reactivity. Results obtained show that the in vitro and in vivo aromatic ring hydroxylation of the series of 3-fluoro(methyl)anilines correlates qualitatively with the calculated frontier orbital density distribution for electrophilic attack by the cytochrome P450(FeO)3+ species. These results indicate that the HOMO/HOMO-1 frontier orbital densities, i.e. the chemical reactivity of the carbon centres for an electrophilic attack, predict the preferential as well as the non-reactive sites for cytochrome P450 catalysed aromatic ring hydroxylation of the tested model compounds. The absolute values, however, deviated in a systematic way; C4 para hydroxylation being observed to a higher extent than expected on the basis of chemical reactivity and C2/C6 ortho hydroxylation being observed to a lower extent than expected. Additional experiments were performed using different microsomal preparations and microperoxidase-8. The latter is a mini-heme protein of eight amino acids without a substrate binding site. In incubations of the model compounds with different types of microsomal preparations, as well as with MP-8 and purified reconstructed cytochrome P4502B1, similar systematic deviations between the predicted and observed regioselectivity of aromatic hydroxylation were observed. These results show that the regioselectivity of aromatic ring hydroxylation of the 3-fluoro(methyl)anilines cannot be predominantly ascribed to an interaction between the substrate and the substrate binding site of the cytochromes P450 dictating a specific stereoselective positioning of the substrate in the active site. More likely, the systematic deviations between the observed and predicted regioselectivity of hydroxylation of the tested model substrates should be ascribed to an (orienting) interaction between the substrate and the activated cytochrome P450(FeO)3+ cofactor.

1H NMR T1 relaxation rate study on substrate orientation of fluoromethylanilines in the active sites of microsomal and purified cytochromes P450 1A1 and 2B1

The present study describes 1H NMR T1 relaxation rate studies on fluoromethylanilines bound to the active sites of microsomal and purified cytochromes P450 1A1 and 2B1. From the data obtained, insights into the average orientation of the substrates with respect to the paramagnetic Fe3+ centre in the cytochromes P450 could be derived. Particular attention was paid to a possible extra relaxation pathway for methyl protons compared to the aromatic protons, due to the rotational motion of the CH3 around the sigma-C-CH3 bond. However, this effect appeared to be minimal and to result in at most a few percent underestimation of the actual distance of the methyl protons to the Fe3+ centre. Furthermore, the data obtained demonstrate that all aromatic protons are at about the same average distance from the paramagnetic centre. The results also demonstrate that the fluromethylanilines are bound to the active sites of cytochromes P450 1A1 and 2B1 in a similar way. A time-averaged orientation of the substrate with the Fe3+ above the aromatic ring, with the pi-orbitals of the aromatic ring and those of the porphyrin rings in a parallel position, providing possibilities for energetically favourable pi-pi interaction defines the orientation which best fits the results of the present study. Possibilities for a flip-flop rotation around an axis in the plane of the aromatic ring can be included in this picture, as such rotations would still result in a similar average distance of all aromatic protons to the Fe3+ paramagnetic centre. The results obtained also indicate that possible differences in metabolite patterns resulting from conversion of the fluoromethylanilines by different cytochromes P450, especially P450 1A1 and 2B1, are unlikely to be caused by a specific orientation of the substrate imposed by the substrate binding site of the enzyme.

The genetic toxicology of putative nongenotoxic carcinogens

This report examines a group of putative nongenotoxic carcinogens that have been cited in the published literature. Using short-term test data from the U.S. Environmental Protection Agency/International Agency for Research on Cancer genetic activity profile (EPA/IARC GAP) database we have classified these agents on the basis of their mutagenicity emphasizing three genetic endpoints: gene mutation, chromosomal aberration and aneuploidy. On the basis of results of short-term tests for these effects, we have defined criteria for evidence of mutagenicity (and nonmutagenicity) and have applied these criteria in classifying the group of putative nongenotoxic carcinogens. The results from this evaluation based on the EPA/IARC GAP database are presented along with a summary of the short-term test data for each chemical and the relevant carcinogenicity results from the NTP, Gene-Tox and IARC databases. The data clearly demonstrate that many of the putative nongenotoxic carcinogens that have been adequately tested in short-term bioassays induce gene or chromosomal mutations or aneuploidy.

Rat liver microsomal metabolism of 2-halogenated 4-methylanilines

1. Rat liver microsomal metabolism of 2-fluoro-, 2-chloro- and 2-bromo-4-methylaniline was investigated using h.p.l.c. Metabolites identified include products from side-chain C-hydroxylation (benzyl alcohols and benzaldehydes) and N-hydroxylation (hydroxylamines and nitroso derivatives). Aromatic ring hydroxylation was not a major reaction pathway. 2. A new type of microsomal metabolite was detected which was identified as a secondary amine, i.e. a halogenated N-(4'-aminobenzyl)-4-methylaniline. 3. In addition to these products azoxy, azo and hydrazo derivatives were formed. 4. Benzyl alcohols and halogenated N-(4'-aminobenzyl)-4-methylanilines were the major microsomal metabolites for all three 2-halogenated 4-methylanilines. 5. Quantification of the metabolite patterns demonstrated an influence of the type of halogen substituent on the rate of microsomal metabolism. The rate of side-chain C-hydroxylation increases in the order 2-fluoro-4-methylaniline < 2-chloro-4-methylaniline < 2-bromo-4-methylaniline. 6. The rate of N-hydroxylation increases from 2-bromo-4-methylaniline < 2-fluoro-4-methylaniline < 2-chloro-4-methylaniline. That 2-chloro-4-methylaniline is N-hydroxylated to a larger extent is in accordance with its greater mutagenicity, twice that of 2-bromo-4-methylaniline.

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.

Enhanced mutagenicity of anisidine isomers in bacterial strains containing elevated N-acetyltransferase activity

In previous studies on the mutagenicity of anisidine isomers, the ortho isomer was considered to be mutagenic towards standard Ames tester strains, while the para isomer gave equivocal results. In the present study we show that both para- and ortho-anisidine isomers are mutagenic in a Salmonella typhimurium tester strain containing elevated levels of N-acetyltransferase (YG1029). p-Anisidine gave a positive mutagenic response using either hamster S9 or ram seminal vesicle microsomes (RSVM) as an activating system, while o-anisidine gave a positive response only with the hamster S9 fraction. The mutagenic response from p-anisidine was greater than with o-anisidine in each case. In tests with p-anisidine and RSVM, the addition of arachidonic acid was not necessary to observe a mutagenic response. Catalase produced a dose-dependent decrease in the mutagenic response with p-anisidine and RSVM; this indicates that endogenous hydrogen peroxide from the bacteria acts as a substrate for the peroxidase activity of RSVM prostaglandin H synthase. These results demonstrate that both anisidine isomers are mutagenic and that N-acetyltransferase enzymes play an important role in their metabolism to mutagenic species.

The genetic toxicology of ortho-toluidine

ortho-Toluidine, a monosubstituted aniline and an intermediate in the dyeing industry, with a number of uses in other fields such as rubber processing and pharmaceutical production, has been in production for over 100 years. It is metabolised in vivo into a number of compounds, some of which are active genotoxins. It has been demonstrated to be a carcinogen in mice and rats and is a suspected human carcinogen. o-Toluidine has a wide range of genetic effects. It is a weak bacterial, fungal and mammalian mutagen, although the conditions required are stringent. The metabolising system used is of particular importance. o-Toluidine is also a clastogen, generally on prolonged exposure. It induces aneuploidy in both fungi and mammalian cultured cells. It also produces DNA damage (single-strand breaks and unscheduled DNA synthesis, UDS) and causes cell transformation. o-Toluidine can be considered a general genotoxin demonstrable under special conditions, particularly with regard to metabolism.

Structure-activity relationships of aromatic amines in the Ames Salmonella typhimurium assay

The author tried in a somewhat limited work to quantitatively correlate the electronic and steric intramolecular interactions of substituents on the amino group (influencing the enzymatic reactions of aromatic amines) and the mutagenic event. It was assumed that there is a correlation between these biotransformations and the electronic state of aromatic amines at the ionic dissociation equilibrium. The approach is rather empirical and arbitrary but the overall agreement between experimental mutagenic potencies and the values calculated was encouraging and led the author to further developments. It is hoped that the concepts used in this work may be applied to other aromatic molecules bearing an amino group.

Genotoxicity of aniline derivatives in various short-term tests

Various substituted aniline derivatives were tested for genotoxicity in several short-term tests in order to examine the hypothesis that a substitution at both ortho positions (2,6-disubstitution) could prevent genotoxicity due to steric hindrance of an enzymatic activation to electrophilic intermediates. In the Salmonella/microsome assay, 2,6-dialkylsubstituted anilines and 2,4,6-trimethylaniline (2,4,6-TMA) were weakly mutagenic in strain TA100 when 20% S9 mix was used, although effects were small compared to those of 2,4-dimethylaniline and 2,4,5-trimethylaniline (2,4,5-TMA). In Drosophila melanogaster, however, 2,4,6-TMA and 2,4,6-trichloroaniline (TCA) were mutagenic in the wing spot test at 2-3 times lower doses than 2,4,5-TMA. In the 6-thioguanine resistance test in cultured fibroblasts, 2,4,6-TMA was again mutagenic at lower doses than 2,4,5-TMA. Two methylene-bis-aniline derivatives were also tested with the above methods: 4,4'-methylene-bis-(2-chloroaniline) (MOCA) was moderately genotoxic in all 3 test systems whereas 4,4'-methylene-bis-(2-ethyl-6-methylaniline) (MMEA) showed no genotoxicity at all. DNA binding studies in rats, however, revealed that both MOCA and MMEA produced DNA adducts in the liver at levels typically found for moderately strong genotoxic carcinogens. These results indicate that the predictive value of the in vitro test systems and particularly the Salmonella/microsome assay is inadequate to detect genotoxicity in aromatic amines. Genotoxicity seems to be a general property of aniline derivatives and does not seem to be greatly influenced by substitution at both ortho positions.

The genotoxicity of a variety of aniline derivatives in a DNA repair test with primary cultured rat hepatocytes

The genotoxicity of a variety of aniline derivatives was examined by a DNA repair test with rat hepatocytes. Out of 37 aniline derivatives, 6 chemicals, i.e., 2,4,6-trimethylaniline (mesidine), 2,4-xylidine, 3,5-diaminobenzoic acid, 3,4-diaminochlorobenzene, 2-chloro-4-methylaniline and 4-chloro-N-methylaniline, elicited positive DNA repair responses. The results are in agreement with the bacterial mutagenicities with or without norharman of these compounds. Positive compounds of unknown carcinogenicity in the present assay, i.e., 3,5-diaminobenzoic acid, 2-chloro-4-methylaniline and 4-chloro-N-methylaniline are suspected of being potentially carcinogenic.

The in vivo effect of 2,6-xylidine on induction of micronuclei in mouse bone marrow cells

The ability of 2,6-xylidine to produce chromosome breakage and/or spindle malformation in vivo was evaluated by an assessment of the capacity of the compound to induce micronuclei in bone marrow polychromatic erythrocytes. Male ICR mice were administered a single oral dose of 350, 175 or 87.5 mg/kg of 2,6-xylidine by oral gavage and bone marrow was extracted from the femurs 24, 48 and 72 h thereafter. The frequency of micronuclei in animals treated with 2,6-xylidine was not different from that observed for the corresponding solvent treated controls.

Carcinomas of the urinary bladder in a 4-chloro-o-toluidine cohort

In an historical mortality study, conducted on a cohort of 335 male employees in 4-chloro-o-toluidine (4-COT) production and processing plants, no deaths due to cancer of the urinary bladder had been identified. However, after completion of this study, urothelial carcinomas were recorded in eight of the employees, two of whom have died in the meantime (as of December 1986). All eight persons had been employed in the 4-COT production plant before improvements in industrial hygiene were introduced in 1970 (a subcohort of 116 men). This presumably higher level of exposure to monocyclic arylamines lasted for 14.0 years (median), and the total exposure time (before and after 1970) in the 4-COT plant was 25.5 years (median). The standardized incidence rate for urothelial carcinomas in the 4-COT subcohort was 73 times higher than expected and was comparable with the results obtained for polycyclic arylamines, which have been identified as human carcinogenic agents. On the basis of our results as association may be postulated between occupational exposure to 4-COT and carcinomas of the urinary bladder observed among production workers.

Species differences in the toxicity of p-chloro-o-toluidine to rats and mice. Covalent binding to hepatic macromolecules and hepatic non-parenchymal cell DNA and an investigation of effects upon the incorporation of [3H] thymidine into capillary endothelial cells

The interaction of p-[14C] chloro-o-toluidine with hepatic macromolecules of rats and mice has been investigated. At all time points after single administration the extent of binding decreased in the order protein greater than RNA greater than DNA in both species. The level of binding to mouse liver DNA was greater than that to rat liver DNA after both single and repeated administration. In vitro studies showed that mouse liver fractions catalysed the binding of p-chloro-o-toluidine to calf thymus DNA more readily than rat liver fractions. Conversely, binding to protein and RNA was more marked in the rat than in the mouse. Species differences in DNA repair rates were not observed. The results failed to demonstrate a preferential persistence of binding to mouse liver nonparenchymal cell DNA. Autoradiographic determinations did not demonstrate any effect of p-chloro-o-toluidine upon the incorporation of [3H] thymidine into subcutaneous capillary endothelial cells. The results suggest that different reactive metabolites are responsible for binding to DNA and protein, and that the pattern of reactive metabolites formed from p-chloro-o-toluidine in the mouse differs from that formed in rats.

Species differences in the covalent binding of p-chloro-o-toluidine to DNA

The covalent binding of p-chloro-o-toluidine to hepatic macromolecules was assessed in rats and mice. At all timepoints investigated covalent binding to DNA was most marked in mice, whilst binding to proteins was more pronounced in rats. Two major hydrophobic DNA-adducts were formed in both species. One of these was formed to a much greater extent (6-30 fold) in mice. Thus, species differences in the metabolism of p-chloro-o-toluidine could account for the fact that mice are more susceptible to the carcinogenic effects of this compound.

Mutagenicity of N-hydroxylamines and N-hydroxycarbamates towards strains of Escherichia coli and Salmonella typhimurium

The mutagenic activity of several arylamines, alkyl- and arylcarbamates and their corresponding N-hydroxylated derivatives towards Escherichia coli WP2uvrA was investigated using the fluctuation test without a metabolic activation system. None of the parent amines or carbamates were mutagenic while several arylhydroxylamines and N-hydroxycarbamates were direct-acting base-pair substitution mutagens. With the exception of n-hexyl-N-hydroxycarbamate, the mutagenic activity of the N-hydroxycarbamates increased with increase in the length of alkyl substituent. Some arylamines and arylhydroxylamines were further examined, again without a metabolic activation system, using a plate test in conjunction with bacterial strains which detect either base-pair or frameshift mutagens. The arylhydroxylamines were found to cause both base-pair and frameshift mutations but were more active as frameshift mutagens. Possible reasons for the observed mutagenic activity are considered.

Testing of 24 food, drug, cosmetic, and fabric dyes in the in vitro and the in vivo/in vitro rat hepatocyte primary culture/DNA repair assays

Twenty-four dyes currently or previously used in the food, drug, cosmetic, and textile industries were tested in the in vitro rat hepatocyte primary culture/DNA repair (HPC/DR) assay and, to a limited extent, in the in vivo/in vitro HPC/DR assay. The positive control, Solvent Yellow 3 (o-aminoazotoluene), and five other dyes (4-dimethylaminobenzeneazo-1-naphthalene, 4-dimethylaminobenzeneazo-2-naphthalene, Direct Blue 53, Acid Blue 9, and 4-dimethylaminostilbene) induced DNA repair in rat hepatocytes both in vitro and in vivo, while 13 of the dyes (Food Red 1, Food Red 5, Food Orange 4, Food Red 7, Acid Red 14, Acid Red 27, Pigment Red 53, Acid Yellow 23, Food Black 1, Food Green 3, Acid Red 51, Acid Blue 74, and Natural Red 4) did not produce any detectable DNA repair in either the in vitro or in vivo/in vitro assays. Direct Blue 14 had weak activity in vitro but none was detected in vivo. In contrast, Solvent Yellow 5 was not active in vitro, but produced a weak positive response in vivo. Negative responses were also obtained for Solvent Yellow 14 and Acid Green 5 in the in vitro assay, whereas the responses produced by these dyes in the in vivo/in vitro assay were judged to be equivocal. An equivocal response was also obtained for Direct Red 28 in the in vivo/in vitro assay as well as in the in vitro assay. These findings provide information about the potential genotoxicity of a number of dyes for which previous genotoxicity data has been inconsistent or inadequate. For some dyes (eg, Solvent Yellow 5), discrepancies between the results obtained in the in vitro and in vivo/in vitro assays may implicate a role for intestinal microflora in their metabolic activation.

An evaluation of Salmonella (Ames) test data in the published literature: application of statistical procedures and analysis of mutagenic potency

We searched the published literature for Salmonella test data on some 450 chemicals. Only 137 of more than 400 articles containing original data satisfied minimum criteria for a quantitative analysis [1751 experiments, comprising data on 152 chemicals (Table 1)]. Many of these papers did not report basic information about the test protocol (Table 2). We used previously described statistical procedures (Bernstein et al., 1982) to estimate the initial slopes of the dose-response curves and corresponding standard errors. We also applied tests for significance and linear goodness-of-fit. We then used the results of these analyses to examine several issues: (1) Linearity of the low dose region of the dose-response curve. We found that the overwhelming majority of curves were linear, though ability to detect non-linearity of dose-response curves in the standard plate test is only limited. 7% of all experiments to which the goodness-of-fit test was applied were curves of increasing slope, and with a few possible exceptions, these were not obviously associated with any particular mutagens, even those generally considered to produce non-linear effects such as MNNG and EMS (Table 3). (2) Performance of the statistical test for significance. Results of the statistical test for significance of the dose-response were compared with author's opinions as to positivity. In almost all cases (94%) results of the statistical test and authors opinions were the same. In the examples of conflicting opinions, the reasons were: (a) the statistical test places more weight than do most authors on the presence of a linear dose-response; (b) most authors tend to require at least a 2-fold increase over the spontaneous background for 'significance', and (c) when the number of spontaneous revertants is small (e.g., TA1537), authors tend to require a larger increase in induced revertants than when the spontaneous background is large, whereas the statistical procedure makes no such distinction. These factors result in the statistical test tending to identify more experiments as positive than do authors, provided there is a linear dose-response, and authors tending to judge more experiments as positive when the dose-response is not linear. (3) Reproducibility. Among the 1751 experiments there were 122 data-sets (a total of 333 experiments) in which the same chemical was tested by two or more different laboratories under the same protocol. 21 of the 122 data-sets had some disagreement between experiments as to whether results were positive or negative (Table 4).(ABSTRACT TRUNCATED AT 400 WORDS)

Testing natural indigo for genotoxicity

The genotoxicity of indigo has been assessed by two short-term tests. The mutagenicity of natural indigo was compared with that of synthetic indigo. Both chemicals were tested using the standard procedure of the Salmonella/microsome mutagenicity test as described by Ames. The substance exhibits mutagenicity towards strains TA1538 and TA98 when S9 preparations of rat liver induced with Aroclor 1254 were present in the medium. The clastogenic potential was evaluated by the micronucleus test in the bone marrow of male mice. The test compound was administered twice with an interval of 24 h, the animals were killed 30 h and 54 h after the first treatment. When the test compound was given by oral gavage as two equal dosages of 0.1, 1 and 1.2 g/kg body weight, no statistically significant increase in the percentage of polychromatic erythrocytes with micronuclei was observed for any group treated with natural indigo.

The induction of bacterial mutation and hepatocyte unscheduled DNA synthesis by monosubstituted anilines

A group of 45 monosubstituted aniline compounds was tested for the induction of point mutations in Salmonella typhimurium and Escherichia coli as well as for unscheduled DNA synthesis (UDS) in rat hepatocyte culture. Eleven compounds were bacterial mutagens, and five compounds induced UDS. Among these a correspondence between mutagenicity and UDS occurred for only two compounds (o-phenylenediamine and 4-aminobiphenyl), and these were also reported to be carcinogenic in rodents. Bacterial mutation was observed for one compound (p-phenylenediamine) not carcinogenic in rodents, and six suspect carcinogens were not detected in either test. In addition, eight compounds of unknown carcinogenic potential induced either bacterial mutation or UDS.

A review of the genotoxicity of food, drug and cosmetic colours and other azo, triphenylmethane and xanthene dyes

The genetic toxicology of the major dyestuffs used in foods, drugs and cosmetics has been reviewed. Published data for azo, triphenylmethane and xanthene dyes from short-term assays for muta-carcinogenicity have been summarized and discussed according to usage, current and previous worldwide legislative status. Certain other synthetic food dyes, commercial mixtures, natural and polymeric colourants as well as a section on aminoazobenzene and its derivatives have been included. Genotoxicity has been discussed with reference to structural chemistry, levels of exposure, absorption and metabolism and to epidemiological information. The extent of agreement between data from different tests and correlations with animal cancer assays have been considered. Synthetic dyes from the 3 major structural classes exhibit genotoxicity, whilst only 2 natural colours have proved active. Activity may be due to the presence of certain functional groups, notably nitro- and amino-substituents which are metabolized to ultimate electrophiles that may be stabilized by electronic interaction with aryl rings. Metabolic processes such as azo-reduction may be activating or detoxifying. the low but significant correlation between animal carcinogenicity and short-term test data may be increased with further screening, especially involving chromosome assays. It is suggested that a human cancer hazard may exist where significant quantities of finished benzidine dye samples are handled. Such risks from exposures to other colours and the possibility of human germ-line mutation induction by dyestuffs cannot be meaningfully assessed.

The relationship of the carcinogenic/mutagenic potential of arylamines to their singlet-triplet nitrenium ion energies

Utilizing intermediate neglect of differential overlap (INDO) and ab initio methodology, trends in the energy differences between the singlet and triplet states for mono- and polycyclic aryl nitrenium ions have been estimated. Calculations reveal an empirical correlation between the energy separation of the singlet and triplet states of the nitrenium ion and the ability of the parent amine to behave as a carcinogen or mutagen. Non-carcinogenic/non-mutagenic arylamines were characterized by nitrenium ions whose singlet states were much less stable than the triplet. Carcinogenic/mutagenic amines were characterized by nitrenium ions whose singlet states were of similar or greater stability than the triplet. By examination of the charge density at key ring atoms of the singlet and triplet species, a rational approach to the stabilization of one species relative to the other has merged and forms the basis for prediction of genotoxicity in closely related structures. The application of this empirical correlation to the prediction of the carcinogenic/ mutagenic potential of arylamines is discussed.