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

Mutagenic properties of dimethylaniline isomers in mice as evaluated by comet, micronucleus and transgenic mutation assays

Background

The carcinogenic potential of dimethylaniline (DMA) isomers in rodents and humans has been previously reported, and there is sufficient evidence for the carcinogenicity of 2,6-DMA in experimental animals. The target organ of carcinogenesis of 2,6-DMA is the nasal cavity. In the current study, six DMA isomers, 2,3-, 2,4-, 2,5-, 2,6-, 3,4- and 3,5-DMA, were evaluated for mutagenic properties.

Results

Male ddY mice (3/group) were treated intragastrically (i.g.) with 200 mg/kg of one of the six DMAs, and a comet assay was performed on samples of bone marrow, kidney, liver and lung at 3 and 24 h after the treatment. Positive responses were observed in the kidney, liver and lungs of mice from all of the DMA treatment groups after 3 h and in the bone marrow of mice treated with either 3,4- or 3,5-DMA after 3 h; however, these effects were diminished at the 24 h time point. The micronucleus induction in the bone marrow was analysed in the same mouse at 24 h after the treatment. No induction of micronucleated polychromatic erythrocytes was observed after treatment with any of the DMAs.Male transgenic Muta™ mice (five/group) were treated i.g. with 2,5-, 2,6- or 3,5-DMA at 100 mg/kg bw weekly for 4 weeks, and the lacZ and the cII mutation frequencies were examined in the nasal cavity, liver and bone marrow at 7 days after the last treatment. Statistically significant increases in the mutation frequencies of the lacZ and/or cII genes were observed in the nasal cavity of 2,5-DMA or 2,6-DMA treated mice. Sequence analysis showed increased incidences of AT to GC and GC to TA mutations in the nasal tissues.

Conclusions

These findings suggest that the carcinogenic activities of DMAs are associated with mutagenic events.

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.

Final report of the safety assessment of Acacia catechu gum, Acacia concinna fruit extract, Acacia dealbata leaf extract, Acacia dealbata leaf wax, Acacia decurrens extract, Acacia farnesiana extract, Acacia farnesiana flower wax, Acacia farnesiana gum, Acacia senegal extract, Acacia senegal gum, and Acacia senegal gum extract

These ingredients are derived from various species of the acacia plant. Only material derived from Acacia senegal are in current use according to industry data. The concentration at which these ingredients are reported to be used ranges from 9% in mascara to 0.0001% in tonics, dressings, and other hair-grooming aids. Gum arabic is a technical name for Acacia Senegal Gum. Gum arabic is comprised of various sugars and glucuronic acid residues in a long chain of galactosyl units with branched oligosaccharides. Gum arabic is generally recognized as safe as a direct food additive. Little information is available to characterize the extracts of other Acacia plant parts or material from other species. Acacia Concinna Fruit Extract was generally described as containing saponins, alkaloids, and malic acid with parabens and potassium sorbate added as preservatives. Cosmetic ingredient functions have been reported for Acacia Decurrens Extract (astringent; skin-conditioning agent--occlusive) and Acacia Farnesiana Extract (astringent), but not for the other Acacias included in this review. Toxicity data on gum arabic indicates little or no acute, short-term, or subchronic toxicity. Gum arabic is negative in several genotoxicity assays, is not a reproductive or developmental toxin, and is not carcinogenic when given intraperitoneally or orally. Clinical testing indicated some evidence of skin sensitization with gum arabic. The extensive safety test data on gum arabic supports the safety of Acacia Senegal Gum and Acacia Senegal Gum Extract, and it was concluded that these two ingredients are safe as used in cosmetic formulations. It was not possible, however, to relate the data on gum arabic to the crude Acacias and their extracts from species other than Acacia senegal. Therefore, the available data were considered insufficient to support the safety of Acacia Catechu Gum, Acacia Concinna Fruit Extract, Acacia Dealbata Leaf Extract, Acacia Dealbata Leaf Wax, Acacia Decurrens Extract, Acacia Farnesiana Extract, Acacia Farnesiana Flower Wax, Acacia Farnesiana Gum, and Acacia Senegal Extract in cosmetic products. The additional data needed to complete the safety assessment for these ingredients include (1) concentration of use; (2) identify the specific chemical constituents, and clarify the relationship between crude Acacias and their extracts and the Acacias and their extracts that are used as cosmetic ingredients; (3) data on contaminants, particularly relating to the presence of pesticide residues, and a determination of whether Acacia melanoxylon is used in cosmetics and whether acamelin (a quinone) and melacacidin (a flavin) are present in the Acacias that are being used; (4) skin sensitization study (i.e., dose response to be determined); (5) contact urticaria study at use concentration; and (6) ultraviolet (UV) absorption spectrum; if there is significant absorbance in the UVA or UVB range, then a photosensitization study may be needed. It was also noted that other data may be needed after clarification of the chemical constituents of the Acacia-derived ingredients.

The regulatory status of xylazine for use in food-producing animals in the United States

Xylazine is commonly used in veterinary medicine as a tranquillizer or adjunct to surgical anaesthesia. Although its use is approved in companion animals and certain species of deer, xylazine remains unapproved for use in food-producing animals in the United States. This paper reviews existing toxicological and residue chemistry information on xylazine in food animals, particularly cattle, and discusses the regulatory status of the drug in the US, as well as the conclusions reached by the Joint FAO/WHO Expert Committee on Food Additives in its recent evaluation of xylazine.

An evaluation of micronucleus induction in bone marrow and in hepatocytes isolated from collagenase perfused liver or from formalin-fixed liver using four-week-old rats treated with known clastogens

The bone marrow (BM) micronucleus (MN) test is a sensitive assay for identifying clastogens. However, some clastogenic compounds and metabolites may never reach the BM. The liver has been suggested as an alternative tissue to BM but adult rat liver has a low mitotic index that increases the difficulty of evaluating hepatocytes (HEP) for MN induction. Chemical mitogens and partial hepatectomy have been used to increase HEP proliferation to improve the sensitivity for detection of clastogenic compounds, but these practices raise concerns for the evaluation of drug candidates. The use of 4-wk-old rats provides an alternative to mitogenic stimulation because livers from these animals have approximately 5.4% of their HEP in S-phase. HEP were isolated by collagenase perfusion, or from formalin-fixed tissue, from 4-wk-old treated rats. Six compounds were evaluated for the incidence of MN in HEP that were isolated by both methods. The results for MN induction by these compounds were similar for the two methods and confirmed that formalin-fixed tissue is an acceptable source of cells for evaluating MN induction in HEP. BM polychromatic erythrocytes (PCE) also were harvested at the end of the live phase for each study and then evaluated for the incidence of MN. Diethylnitrosamine and 2-nitrofluorene induced MN in HEP but had no effect in PCE. 2-Acetylaminofluorene, cyclophosphamide and 7,12-dimethylbenz[a]anthracene did not induce MN in HEP but were positive in PCE. The direct-acting clastogen, mitomycin C, was positive in both HEP and PCE. These results indicate that this modified liver micronucleus test, using 4-wk-old rats, offers an alternative to existing methods that use mitogens or partial hepatectomy to stimulate cell replication. Analysis of MN from formalin-fixed tissue provides additional flexibility by allowing the investigator to assess MN induction at a later time.

Validation of an automated image analysis micronucleus scoring system

The Loats Automated Micronucleus Scoring System was developed to assist with the evaluation of compounds for the ability to induce micronuclei in mouse bone marrow polychromatic erythrocytes (PCE). This image analysis system can identify PCE as well as normochromatic erythrocytes (NCE) and calculate the PCE/NCE ratio as an index for bone marrow toxicity. Two studies were conducted to provide slides for a comparison of micronucleated PCE values collected manually to those collected by the automated system. Mitomycin C was used as a micronucleus-inducing agent to elicit a positive response and Lilly compound 303497 was used as an example of a compound negative for the induction of micronuclei. No statistically significant differences were observed between micronucleus counts obtained manually and those obtained by the automated system. The PCE/NCE ratios calculated by the automated system were also similar to those determined from the manually collected PCE and NCE counts for the vehicle and positive controls, however, differences in the ratios were observed in compound treatment groups. These differences were attributed to a larger population of transitional cells in the treated groups. These results confirm that the Loats Automated Micronucleus Scoring System is an acceptable alternative to manual evaluation of mouse bone marrow slides for the incidence of micronucleated PCE.

Bone marrow micronucleus assay: a review of the mouse stocks used and their published mean spontaneous micronucleus frequencies

We have examined published negative control data from 581 papers on micronucleated bone marrow polychromatic erythrocytes (mnPCE) for differences in mean frequency and the frequency distribution profile among the mouse stocks used with the bone marrow micronucleus assay. For the 55 mouse stocks with published micronucleus assay data, the overall mean frequency is 1.95 mnPCE/1,000 PCE (1.95 mnPCE/1,000); for the 13 stocks most commonly used in the assay, it is 1.88 mnPCE/1,000. During the last 5 years, the mnPCE rate for these 13 major stocks has been 1.74 mnPCE/1,000. This current mean frequency is a substantial decrease from the mean of 3.07 mnPCE/1,000 observed for these 13 stocks for data published prior to 1981. Of the major stocks, the highest mean mnPCE negative control frequencies were observed for MS/Ae > BALB/c > C57Bl/6, and the lowest for CD-1 < Swiss Webster. We note that hybrid mouse stocks appear to have lower and less variable negative control frequencies than either of their parent strains and that the negative control frequency for some progeny stocks have diverged significantly from that of the parent stocks. Overall mean negative control frequencies appear to be correlated with breadth of the frequency distribution profile of published mean negative control values. Furthermore, a possible correlation between negative control frequency in the micronucleus assay and sensitivity to clastogens of different mouse strains may be indicated. The databases generated here allow us to define a range of norms for both the historical mean frequency and individual experimental mean frequencies for most stocks, but in particular, for the more commonly used mouse stocks. Our analysis, for the most part, bears out the recommendation of the first Gene-Tox Report on the micronucleus assay that the historical negative control frequency for a mouse stock should fall between 1 and 3 mnPCE/1,000. Eighty-six percent of the most commonly used mouse stocks have historical mean frequencies within this range. Though individual experimental mean values would not necessarily be expected to fall within the 1-3.00 mnPCE/1,000 range, 65.3% of the 2,327 published negative control values do, and 83.5% are < 3 mnPCE/1,000. The frequency with which an individual experimental mean value lies outside the 1.00 to 3.00 mnPCE/1,000 range differs among stocks and appears related to the mouse mean frequency. We suggest that the recommended range for historical mean frequency be extended slightly, to approximately 3.4 mnPCE/1,000, to accommodate some commonly used strains with overall mean negative control frequencies just above 3.00 mnPCE/1,000.(ABSTRACT TRUNCATED AT 400 WORDS)

Diphenylhydantoin is not genotoxic in a battery of short-term cytogenetic assays

5,5-Diphenylhydantoin (DPH) is an antiepileptic drug associated with an increase in malformations in infants born to women taking DPH during pregnancy. Positive and negative results have been reported by various investigators for in vivo and in vitro chromosome aberration (CAB) assays, in vivo and in vitro sister chromatid exchange (SCE) assays, and in vivo micronucleus tests (MNT). In this laboratory, DPH was tested in an in vitro CAB assay using Chinese hamster ovary cells with and without an S-9 activation system, an in vivo SCE assay in female CD-1 mice, an in vivo MNT, using both male and female CD-1 mice, and a transplacental micronucleus test. The results from this comprehensive battery of cytogenetic tests were uniformly negative and support a conclusion that the known teratogen, DPH, is not clastogenic.

The evaluation of a multiple dosing protocol for the mouse bone-marrow micronucleus assay using benzidine and 2,6-xylidine

Male ICR mice were treated with 1, 2 or 3 daily doses of either benzidine or 2,6-xylidine. Groups of 5 animals were sacrificed 24 h after the last dose and the bone marrow examined for micronuclei. Benzidine was given at dose levels of 40 and 200 mg/kg and 2,6-xylidine was given at dose levels of 75 and 375 mg/kg. These doses represent 10 and 50% of the respective median lethal doses. Benzidine produced a significant (p less than 0.001) dose related increase in the incidence of micronucleated polychromatic erythrocytes (MPE), while 2,6-xylidine had no effect on the frequency of micronucleated cells. Statistical analyses of the data indicated that the incidence of MPE was independent of the number of doses administered prior to bone marrow harvest.

Pooled inference across sexes for the in vivo micronucleus assay

The Japanese Environmental Mutagen Society has investigated the extent of sex differences in the in vivo micronucleus assay (Sutou et al., 1986). In light of their findings, this manuscript reexamines the statistical analysis of the assay. A test statistic which pools the inference over sexes is introduced. The sensitivity of this statistic is examined in comparison with the more traditional procedure of analysis within each sex. The impact of extra-Poisson variation among animals on the validity and sensitivity of the test procedures is also examined.