Investigation of sodium arsenite, thioacetamide, and diethanolamine in the alkaline comet assay: Part of the JaCVAM comet validation exercise.

As part of the Japanese Center for the Validation of Alternative Methods (JaCVAM)-initiative international validation study of the in vivo rat alkaline comet assay (comet assay), we examined sodium arsenite, thioacetamide, and diethanolamine. Using the JaCVAM approved study protocol version 14.2, each chemical was tested in male rats up to maximum tolerated dose levels and DNA damage in the liver and stomach was assessed approximately 3h after the final administration by gavage. Histopathology assessments of liver and stomach sections from the same animals were also examined for evidence of cytotoxicity or necrosis. No evidence of DNA damage was observed in the stomach of animals treated with sodium arsenite at 7.5, 15, or 30 mg/kg/day. However, equivocal findings were found in the liver, where increases in DNA migration were observed in two independent experiments, but not in all treated animals and not at the same dose levels. Thioacetamide caused an increase in DNA migration in the stomach of rats treated at 19, 38, and 75 mg/kg/day, but not in the liver, despite evidence of marked hepatotoxicity following histopathology assessments. No evidence of DNA damage was observed in the stomach or liver of animals treated with diethanolamine at 175, 350, or 700 mg/kg/day.

In vivo comet assay of acrylonitrile, 9-aminoacridine hydrochloride monohydrate and ethanol in rats.

As part of the Japanese Center for the Validation of Alternative Methods (JaCVAM)-initiative international validation study of the in vivo rat alkaline comet assay, we examined the ability of acrylonitrile, 9-aminoacridine hydrochloride monohydrate (9-AA), and ethanol to induce DNA damage in the liver and glandular stomach of male rats. Acrylonitrile is a genotoxic carcinogen, 9-AA is a genotoxic non-carcinogen, and ethanol is a non-genotoxic carcinogen. Positive results were obtained in the liver cells of male rats treated with known genotoxic compounds, acrylonitrile and 9-AA.

Evaluation of 4,4'-diaminodiphenyl ether in the rat comet assay: Part of the Japanese Center for the Validation of Alternative Methods (JaCVAM)-initiative international validation study of in vivo rat alkaline comet assay.

As a part of the Japanese Center for the Validation of Alternative Methods (JaCVAM)-initiative international validation study of the in vivo rat alkaline comet assay, 4,4'-diaminodiphenyl ether (DPE), a known rodent genotoxic carcinogen, was tested in this laboratory. Sprague Dawley rats (7-9 weeks of age) were given three oral doses of DPE, 24 and 21 h apart and liver or stomach sampled 3h after the final dose. Under the conditions of the test, no increases in DNA damage in liver and stomach were observed with DPE (up to 200 mg/kg/day). A dose-dependent decrease in DNA migration, compared to vehicle controls, was noted for DPE in rat stomach. Further analysis is required to elucidate fully whether this decrease is a consequence of the mode of action or due to the toxicity of DPE. What is perhaps surprising is the inability of the comet assay to detect a known rat genotoxic carcinogen in liver. Further investigation is needed to clarify whether this apparent lack of response results from limited tissue exposure or metabolic differences between species. This finding highlights a need for careful consideration of study design when evaluating assay performance as a measure of in vivo genotoxicity.

Optimal dose selection of N-methyl-N-nitrosourea for the rat comet assay to evaluate DNA damage in organs with different susceptibility to cytotoxicity.

The in vivo rodent alkaline comet assay (comet assay) is a promising technique to evaluate DNA damage in vivo. However, there is no agreement on a method to evaluate DNA damage in organs where cytotoxicity is observed. As a part of the Japanese Center for the Validation of Alternative Methods (JaCVAM)-initiative international validation study of the comet assay, we examined DNA damage in the liver, stomach, and bone marrow of rats given three oral doses of N-methyl-N-nitrosourea (MNU) up to the maximum tolerated dose based on systemic toxicity. MNU significantly increased the % tail DNA in all the organs. Histopathological analysis showed no cytotoxic effect on the liver, indicating clearly that MNU has a genotoxic potential in the liver. In the stomach, however, the cytotoxic effects were very severe at systemically non-toxic doses. Low-dose MNU significantly increased the % tail DNA even at a non-cytotoxic dose, indicating that MNU has a genotoxic potential also in the stomach. Part of the DNA damage at cytotoxic doses was considered to be a secondary effect of severe cell damage. In the bone marrow, both the % tail DNA and incidence of micronucleated polychromatic erythrocytes significantly increased at non-hematotoxic doses, which were different from the non-cytotoxic doses for liver and stomach. These findings indicate that an optimal dose for detecting DNA damage may vary among organs and that careful attention is required to select an optimum dose for the comet assay based on systemic toxicity such as mortality and clinical observations. The present study shows that when serious cytotoxicity is suggested by increased % hedgehogs in the comet assay, histopathological examination should be included for the evaluation of a positive response.

Evaluation of p-phenylenediamine, o-phenylphenol sodium salt, and 2,4-diaminotoluene in the rat comet assay as part of the Japanese Center for the Validation of Alternative Methods (JaCVAM)-initiated international validation study of in vivo rat alkaline comet assay.

As part of the Japanese Center for the Validation of Alternative Methods (JaCVAM)-initiated international validation study of in vivo rat alkaline comet assay (comet assay), p-phenylenediamine dihydrochloride (PPD), o-phenylphenol sodium salt (OPP), and 2,4-diaminotoluene (2,4-DAT), were analyzed in this laboratory as coded test chemicals. Male Sprague-Dawley rats (7-9 weeks of age) were given three oral doses of the test compounds, 24 and 21 h apart and liver and stomach were sampled 3h after the final dose administration. Under the conditions of the test, no increases in DNA damage were observed in liver and stomach with PPD and OPP up to 100 and 1000 mg/kg/day, respectively. 2,4-DAT, a known genotoxic carcinogen, induced a weak but reproducible, dose-related and statistically significant increase in DNA damage in liver cells while no increases were observed in stomach cells.

Use of a standardized JaCVAM in vivo rat comet assay protocol to assess the genotoxicity of three coded test compounds; ampicillin trihydrate, 1,2-dimethylhydrazine dihydrochloride, and N-nitrosodimethylamine.

As part of the Japanese Center for the Validation of Alternative Methods (JaCVAM)-initiative international validation study of the in vivo rat alkaline comet assay (comet assay), our laboratory examined ampicillin trihydrate (AMP), 1,2-dimethylhydrazine dihydrochloride (DMH), and N-nitrosodimethylamine (NDA) using a standard comet assay validation protocol (v14.2) developed by the JaCVAM validation management team (VMT). Coded samples were received by our laboratory along with basic MSDS information. Solubility analysis and range-finding experiments of the coded test compounds were conducted for dose selection. Animal dosing schedules, the comet assay processing and analysis, and statistical analysis were conducted in accordance with the standard protocol. Based upon our blinded evaluation, AMP was not found to exhibit evidence of genotoxicity in either the rat liver or stomach. However, both NDA and DMH were observed to cause a significant increase in % tail DNA in the rat liver at all dose levels tested. While acute hepatoxicity was observed for these compounds in the high dose group, in the investigators opinion there were a sufficient number of consistently damaged/measurable cells at the medium and low dose groups to judge these compounds as genotoxic. There was no evidence of genotoxicity from either NDA or DMH in the rat stomach. In conclusion, our laboratory observed increased DNA damage from two blinded test compounds in rat liver (later identified as genotoxic carcinogens), while no evidence of genotoxicity was observed for the third blinded test compound (later identified as a non-genotoxic, non-carcinogen). This data supports the use of a standardized protocol of the in vivo comet assay as a cost-effective alternative genotoxicity assay for regulatory testing purposes.

Combination comet/micronucleus assay validation performed by BioReliance under the JaCVAM initiative.

In the international validation study of the in vivo rat alkaline comet assay (comet assay), the Japanese Center for the Validation of Alternative Methods (JaCVAM) provided three coded chemicals to BioReliance, 1,3-dichloropropene, ethionamide and busulfan, to be tested in a combined in vivo comet/micronucleus assay. Induction of DNA damage (comet) in liver, stomach and jejunum (1,3-dichloropropene only) cells, and induction of MNPCEs in bone marrow, were examined in male Sprague-Dawley (Hsd:SD) rats following oral administration of the test chemical for three consecutive days. A dose range finding (DRF) test was performed with each chemical to determine the maximum tolerated dose (MTD). Based on the results of the DRF test; 1,3-dichloropropene was tested at 50, 100 and 200 mg/kg/day; ethionamide was tested at 125, 250 and 500 mg/kg/day, and busulfan was tested at 10, 20 and 40 mg/kg/day. The results indicated that 1,3-dichloropropene induced DNA damage only in liver cells at all three test article doses, while no effects were observed in the stomach and jejunum cells. Additionally, it did not increase MNPCEs in the bone marrow. 1,3-Dichloropropene was concluded to be negative in the MN assay but positive in the comet assay. Ethionamide did not induce DNA damage in liver. However, in stomach, statistically significant decreases (although still within historical range) in % tail DNA at all test article doses compared to the vehicle control were observed. There was no increase in MNPCEs in the bone marrow. Thus, ethionamide was concluded to be negative in the comet/MN combined assay. Busulfan did not induce DNA damage in any of the organs tested (liver and stomach) but it did induce a significant increase in MNPCEs in the bone marrow. Busulfan was concluded to be negative in the comet assay but positive in the MN assay.

Evaluation of methyl methanesulfonate, 2,6-diaminotoluene and 5-fluorouracil: Part of the Japanese center for the validation of alternative methods (JaCVAM) international validation study of the in vivo rat alkaline comet assay.

As a part of the Japanese Center for the Validation of Alternative Methods (JaCVAM)-initiative international validation study of the in vivo rat alkaline comet assay (comet assay), we examined methyl methanesulfonate, 2,6-diaminotoluene, and 5-fluorouracil under coded test conditions. Rats were treated orally with the maximum tolerated dose (MTD) and two additional descending doses of the respective compounds. In the MMS treated groups liver and stomach showed significantly elevated DNA damage at each dose level and a significant dose-response relationship. 2,6-diaminotoluene induced significantly elevated DNA damage in the liver at each dose and a statistically significant dose-response relationship whereas no DNA damage was obtained in the stomach. 5-fluorouracil did not induce DNA damage in either liver or stomach.

Results of the International Validation of the in vivo rodent alkaline comet assay for the detection of genotoxic carcinogens: Individual data for 1,2-dibromoethane, p-anisidine, and o-anthranilic acid in the 2nd step of the 4th phase Validation Study under the JaCVAM initiative.

As part of the Japanese Center for the Validation of Alternative Methods (JaCVAM)-initiative International Validation Study of an in vivo rat alkaline comet assay, we examined 1,2-dibromoethane (DBE), p-anisidine (ASD), and o-anthranilic acid (ANT) to investigate the effectiveness of the comet assay in detecting genotoxic carcinogens. Each of the three test chemicals was administered to 5 male Sprague-Dawley rats per group by oral gavage at 48, 24, and 3h before specimen preparation. Single cells were collected from the liver and glandular stomach at 3h after the final dosing, and the specimens prepared from these two organs were subjected to electrophoresis under alkaline conditions (pH>13). The percentage of DNA intensity in the comet tail was then assessed using an image analysis system. A micronucleus (MN) assay was also conducted using these three test chemicals with the bone marrow (BM) cells collected from the same animals simultaneously used in the comet assay, i.e., combination study of the comet assay and BM MN assay. A genotoxic (Ames positive) rodent carcinogen, DBE gave a positive result in the comet assay in the present study, while a genotoxic (Ames positive) non-carcinogen, ASD and a non-genotoxic (Ames negative) non-carcinogen, ANT showed negative results in the comet assay. All three chemicals produced negative results in the BM MN assay. While the comet assay findings in the present study were consistent with those obtained from the rodent carcinogenicity studies for the three test chemicals, we consider the positive result in the comet assay for DBE to be particularly meaningful, given that this chemical produced a negative result in the BM MN assay. Therefore, the combination study of the comet assay and BM MN assay is a useful method to detect genotoxic carcinogens that are undetectable with the BM MN assay alone.

Genotoxicity evaluation of benzene, di(2-ethylhexyl) phthalate, and trisodium ethylenediamine tetraacetic acid monohydrate using a combined rat comet/micronucleus assays.

As a part of the Japanese Center for the Validation of Alternative Methods (JaCVAM)-initiative international validation study of the in vivo alkaline comet assay (comet assay), we examined DNA damage in the liver, stomach, and bone marrow of rats dosed orally three times with up to 2000 mg/kg of benzene, di(2-ethylhexyl) phthalate, and trisodium ethylenediamine tetraacetic acid monohydrate. All three compounds gave negative results in the liver and stomach. In addition, a bone marrow comet and micronucleus analysis revealed that benzene, but not di(2-ethylhexyl) phthalate or trisodium ethylenediamine tetraacetic acid monohydrate induced a significant increase in the median % tail DNA and micronucleated polychromatic erythrocytes, compared with the respective concurrent vehicle control. These results were in good agreement with the previously reported genotoxicity findings for each compound. The present study has shown that combining the micronucleus test with the comet assay and carrying out these analyses simultaneously is effective in clarifying the mechanism of action of genotoxic compounds such as benzene.

Alkaline comet assay in liver and stomach, and micronucleus assay in bone marrow, from rats treated with 2-acetylaminofluorene, azidothymidine, cisplatin, or isobutyraldehyde.

As part of the Japanese Center for the Validation of Alternative Methods (JaCVAM) initiative international validation study of the in vivo rat alkaline comet assay (comet assay), we examined the ability of the assay to determine the genotoxicity of 2-acetylaminofluorene (AAF), azidothymidine (AZT), cisplatin (CPN), and isobutyraldehyde (IBA) in liver and glandular stomach of male Sprague-Dawley rats. Rats were given oral doses of test compound or control once daily for three days. High dose levels were approximately maximum tolerated doses and were based on preliminary range-finding studies. Tissues were harvested 3h after the final dose (48h after the initial dose). A bone marrow micronucleus assay (MN) was also conducted on the rats treated with AZT, CPN, and IBA. Acute toxic effects of treatment were determined primarily through histomorphologic analysis of liver and stomach but also by body weight and serum liver enzyme changes. The comet assay was conducted on fresh tissue preparations but frozen samples from two studies were also assayed. Statistically significant dose-related differences in comet % DNA in tail were found in liver and stomach for the genotoxin AZT and in liver for the genotoxin CPN, but not in liver or stomach for the non-genotoxin IBA. Statistically significant differences in % DNA in tail were measured in liver for the low and mid dose of the genotoxin AAF, but not the high dose. The comet assays of frozen liver suspensions from CPN- and AAF-treated rats yielded comparable results to the assays of fresh preparations. There were no indications of significant toxicity induced by any treatment. The micronucleus assay was positive for CPN and AZT and negative for IBA. In conclusion, the in vivo comet assay is capable of detecting genotoxic effects of a variety of chemicals and may fill an important role in the genotoxicity test battery.