Evaluation of a liver micronucleus assay in young rats (III): A study using nine hepatotoxicants by the Collaborative Study Group for the Micronucleus Test (CSGMT)/Japanese Environmental Mutagen Society (JEMS)–Mammalian Mutagenicity Study Group (MMS)

Hepatocyte proliferation activity in untreated rats, measured by immunohistochemical detection of Ki-67: The effect of age on the repeated-dose liver micronucleus assay

The repeated-dose liver micronucleus (RDLMN) assay is a widely accepted method for detecting genotoxic substances. We investigated the effect of animal age on this assay. Proliferation activity in the liver tissue of untreated rats at age = 3.5, 6, 8, 10, or 12 weeks was measured via immunohistochemical expression of Ki-67 protein. The percentage of Ki-67-positive hepatocytes decreased markedly with age, reaching very low levels after 10 weeks, indicating decline with age of proliferative capacities in the liver. We calculated the area under the curve (AUC) of the approximate curve generated from the percentage of Ki-67-positive cells, to estimate the hepatocyte proliferation activity over the dosing period in the two regimens of the 4-week RDLMN assay: dosing initiated at age = 6 or 8 weeks. Hepatocyte proliferation activity of the former regimen was approximately double that of the latter. We also calculated the AUC for the juvenile-rat method, in which rats are treated for two days at age = 3.5 weeks. The AUC calculated for that method was approximately half of that for the 4-week repeated-dosing regimen initiated at 6 weeks of age. These findings suggest that the 4-week RDLMN assay with dosing initiated at age = 6 weeks could be approximately twice as sensitive as the other two methods.

The genotoxic potential of mixed nitrosamines in drinking water involves oxidative stress and Nrf2 activation

Nitrosamine by-products in drinking water are designated as probable human carcinogens by the IARC, but the health effects of simultaneous exposure to multiple nitrosamines in drinking water remain unknown. Genotoxicity assays were used to assess the effects of both individual and mixed nitrosamines in finished drinking water produced by a large water treatment plant in Shanghai, China. Cytotoxicity and genotoxicity were measured at 1, 10-, 100- and 1000-fold actual concentrations by the Ames test, Comet assay, γ-H2AX assay, and the cytokinesis-block micronuclei assay; oxidative stress and the Nrf2 pathway were also assessed. Nitrosamines detected in drinking water included NDMA (36.45 ng/L), NDPA (44.68 ng/L), and NEMA (37.27 ng/L). Treatment with a mixture of the three nitrosamines at 1000-fold actual drinking-water concentration induced a doubling of revertants in Salmonella typhimurium strain TA100, DNA and chromosome damage in HepG2 cells, while 1-1000-fold concentrations of compounds applied singly lacked these effects. Treatment with 100- and 1000-fold concentrations increased ROS, GSH, and MDA and decreased SOD activity. Thus, nitrosamine mixtures showed greater genotoxic potential than that of the individual compounds. N-Acetylcysteine protected against the nitrosamine-induced chromosome damage, and Nrf2 pathway activation suggested that oxidative stress played pivotal roles in the genotoxic property of the nitrosamine mixtures.

Detection of hepatocarcinogens by combination of liver micronucleus assay and histopathological examination in 2-week or 4-week repeated dose studies

Background

Currently, revisions to the ICH S1 guidance on rodent carcinogenicity testing are being proposed. Application of this approach would reduce the use of animals in accordance with the 3Rs principles (reduce/refine/replace). The method would also shift resources to focus on more scientific mechanism-based carcinogenicity assessments and promote safe and ethical development of new small molecule pharmaceuticals. In the revised draft, findings such as cellular hypertrophy, diffuse and/or focal cellular hyperplasia, persistent tissue injury and/or chronic inflammation, preneoplastic changes, and tumors are listed as histopathology findings of particular interest for identifying carcinogenic potential. In order to predict hepatocarcinogenicity of test chemicals based on the results from 2- or 4-week repeated dose studies, we retrospectively reanalyzed the results of a previous collaborative study on the liver micronucleus assay. We focused on liver micronucleus induction in combination with histopathological changes including hypertrophy, proliferation of oval cells or bile duct epithelial cells, tissue injuries, regenerative changes, and inflammatory changes as the early responses of hepatocarcinogenesis. For these early responses, A total of 20 carcinogens, including 14 genotoxic hepatocarcinogens (Group A) and 6 non-liver-targeted genotoxic carcinogens (Group B) were evaluated.

Results

In the Group A chemicals, 5 chemicals (NPYR, MDA, NDPA, 2,6-DNT, and NMOR) showed all of the 6 early responses in hepatocarcinogenesis. Five chemicals (DMN, 2,4-DNT, QUN, 2-AAF, and TAA) showed 4 responses, and 4 chemicals (DAB, 2-NP, MCT, and Sudan I) showed 3 responses. All chemicals exhibited at least 3 early responses.

Contrarily, in the Group B chemicals (6 chemicals), 3 of the 6 early responses were observed in 1 chemical (MNNG). No more than two responses were observed in 3 chemicals (MMC, MMS, and KA), and no responses were observed in 2 chemicals (CP and KBrO3).

Conclusion

Evaluation of liver micronucleus induction in combination with histopathological examination is useful for detecting hepatocarcinogens. This assay takes much less time than routine long-term carcinogenicity studies.

[Genotoxicity of psychotropic drugs in experimental and clinical studies]

The analysis of experimental data on the study of the genotoxic activity of psychotropic drugs published over the past 25 years has been carried out. It has been shown that the information describing the genotoxicity of psychotropic drugs is characterized by fragmentation, contradictions, and the conditions for their experimental production often do not meet modern requirements. Conclusions about the presence or absence of genotoxic properties can be made only for 9.6% 94 examined drugs. The need for a large-scale systematic reassessment of the genotoxicity of psychotropic drugs, especially drugs of the first generation, on the basis of modern methodology, including studies of mutagen-modifying activity, has been proven. The expediency of monitoring the genotoxic status of patients receiving psychotropic drugs is emphasized, which should contribute to an adequate assessment of the genotoxic risk of their use and objectification of approaches when choosing a drug for the safe therapy. The urgency of conducting research to determine the role of primary DNA damage in the pathogenesis of mental illnesses has been substantiated.

The effect of aging on the repeated-dose liver micronucleus assay

BACKGROUND

The liver micronucleus (MN) assay is an effective and important in vivo test for detecting genotoxic compounds. In particular, the repeated-dose liver MN (RDLMN) assay which greatly facilitates incorporation of the liver MN assay into the general toxicity study has been developed. Usefulness of the RDLMN assay was appraised highly in the 7th International Workshops on Genotoxicity Testing (2017 in Tokyo) in that sufficient numbers and types of chemicals were studied and easy integration into the general toxicity study is preferred from the 3R's point of view. However, it was pointed out that it is necessary to evaluate the effect of age at the start of 4-week repeated administration, since there are limited data, where only those of rats of 6 week of age at the start of administration are available. In this study, we conducted the 4-week RDLMN assay using rats of 6 and 8 weeks of age (at the start of administration) to investigate the effect of age on the liver MN inducibility. Clofibrate, a weak inducer of liver MN, was used in this study to detect the slight difference in the liver MN induction.

RESULTS

The liver MN induced by clofibrate was detected in both rats of 6 and 8 weeks of age at the start of administration. However, the liver MN induction was lower in rats of 8 weeks of age compared to rats of 6 weeks of age at the start of administration.

CONCLUSION

These results suggest that the liver MN inducibility decreases with age. Therefore, we recommend the use of rats of 6 weeks of age at start of administration to reliably detect the liver MN induction in the RDLMN assay.

Evaluation of the novel liver micronucleus assay using formalin-fixed tissues

Background

The repeated-dose liver micronucleus (RDLMN) assay is an effective and important in vivo test for detecting genotoxic compounds, particularly for those that require metabolic activation to show genotoxicity. In a collaborative study by the Collaborative Study Group for the Micronucleus Test (CSGMT)/The Japanese Environmental Mutagen Society (JEMS) – Mammalian Mutagenicity Study Group (MMS), micronucleus induction of 22 chemicals with the RDLMN assay employing the collagenase digestion method was examined and reported on. Recently, we have developed a method which enables retrospective evaluation of micronucleus induction in formalin-fixed liver tissues (the formalin-fixed method) obtained in general toxicity studies completed in the past. Using this method, we were able to easily evaluate clastogenic potential of chemicals from the formalin-fixed tissues obtained in the general toxicity studies.

In this study, to evaluate the usefulness of the formalin-fixed method, we have conducted a liver micronucleus assay using the formalin-fixed liver samples obtained from the above collaborative study (18 of 22 test chemicals) and carried out a comparison with the results obtained by the collagenase digestion method.

Results

Comparison of the collagenase digestion and formalin-fixed methods was conducted using the results of the micronucleus assays with a total of 18 test chemicals which included 12 genotoxic hepatocarcinogens (Group A), 4 genotoxic carcinogens but not liver targeted (Group B), and 2 nongenotoxic hepatocarcinogens (Group C). The formalin-fixed method obtained the similar results as the collagenase digestion method in 10 out of the 12 chemicals of Group A, and all chemicals of Group B and Group C. Although the results were statistically contradictive due to different levels of concurrent negative control, the 2 other chemicals of Group A showed comparable responses between the two methods.

Conclusion

The present study shows that the formalin-fixed method is capable of detecting liver carcinogens with sensitivity equal to or higher than that of the collagenase digestion method. We recommend use of the formalin-fixed method because of its capability of enabling retrospective evaluation of micronucleus induction in the formalin-fixed liver tissues obtained in general toxicity studies completed in the past.

Effectiveness of the liver micronucleus assay using juvenile mice

This study investigated the effectiveness of the liver micronucleus (MN) assay using juvenile mice. Therefore, we analyzed various hepatic cytochrome P450 (CYP)- mediated activities of ethoxyresorufin O-deethylation, pentoxyresorufin O-dealkylation, tolbutamide hydroxylation, bufuralol 1’-hydroxylation, aniline hydroxylation and midazolam 4-hydroxylation by CYP1A, CYP2B, CYP2C, CYP2D, CYP2E and CYP3A, respectively, in non-treated male ICR mice aged between 3 and 8 weeks. The enzyme efficiency levels in 3- and 4-week-old mice were approximately similar to or higher than those in 8-week-old mice, except for CYP1A and CYP2E in 3- and 4-week-old mice, respectively. Since these results suggest that juvenile mice have sufficient activities for most CYP enzymes, we also conducted a liver MN assay using diethylnitrosamine (DEN), a rodent hepatocarcinogen, on male ICR mice aged between 3 and 6 weeks. A peripheral blood (PB) MN assay was performed simultaneously in 4-week-old mice. Assays incorporating DEN produced positive results in 3- and 4-week-old mice and showed a dose-dependent increase in the micronucleated hepatocyte frequencies at 4 weeks. Both the liver MN assay in 5- and 6-week-old mice and the PB MN assay had negative results when using DEN. These results suggest that 3- and 4-week-old mice have micronuclei-inducing potential in the liver to detect genotoxic compounds using the liver MN assay.

Tissue-specific in vivo genetic toxicity of nine polycyclic aromatic hydrocarbons assessed using the Muta™Mouse transgenic rodent assay

Test batteries to screen chemicals for mutagenic hazard include several endpoints regarded as effective for detecting genotoxic carcinogens. Traditional in vivo methods primarily examine clastogenic endpoints in haematopoietic tissues. Although this approach is effective for identifying systemically distributed clastogens, some mutagens may not induce clastogenic effects; moreover, genotoxic effects may be restricted to the site of contact and/or related tissues. An OECD test guideline for transgenic rodent (TGR) gene mutation assays was released in 2011, and the TGR assays permit assessment of mutagenicity in any tissue. This study assessed the responses of two genotoxicity endpoints following sub-chronic oral exposures of male Muta™Mouse to 9 carcinogenic polycyclic aromatic hydrocarbons (PAHs). Clastogenicity was assessed via induction of micronuclei in peripheral blood, and mutagenicity via induction of lacZ transgene mutations in bone marrow, glandular stomach, small intestine, liver, and lung. Additionally, the presence of bulky PAH-DNA adducts was examined. Five of the 9 PAHs elicited positive results across all endpoints in at least one tissue, and no PAHs were negative or equivocal across all endpoints. All PAHs were positive for lacZ mutations in at least one tissue (sensitivity=100%), and for 8 PAHs, one or more initial sites of chemical contact (i.e., glandular stomach, liver, small intestine) yielded a greater response than bone marrow. Five PAHs were positive in the micronucleus assay (sensitivity=56%). Furthermore, all PAHs produced DNA adducts in at least one tissue. The results demonstrate the utility of the TGR assay for mutagenicity assessment, especially for compounds that may not be systemically distributed.

Evaluation of repeated dose micronucleus assays of the liver using N-nitrosopyrrolidine: a report of the collaborative study by CSGMT/JEMS.MMS

The repeated dose liver micronucleus (RDLMN) assay has the potential to detect liver carcinogens, and can be integrated into a general toxicological study. To assess the performance of the assay, N-nitrosopyrrolidine (NPYR), a genotoxic hepatocarcinogen, was tested in 14- or 28-day RDLMN assays. NPYR was orally administered to rats at a daily dose of 25, 50 or 100 mg/kg. One day after the last administration, a portion of the liver was removed and hepatocyte micronucleus (MN) specimens were prepared by the new method recently established by Narumi et al. In addition, a bone marrow MN assay and a histopathological examination of the liver were conducted. The detection of Phospho-Histone H3 was performed by immunohistochemistry to evaluate the proliferation rate of hepatocytes. The results showed significant increase in the number of micronucleated hepatocytes and Phospho-Histone H3-positive cells from the lowest dose in both 14- and 28-day RDLMN assays. On the other hand, the bone marrow MN assay yielded a negative result, which was in accordance with the existing report of the bone marrow MN assay using mice. Upon histopathological examination, inflammatory lesions and hypertrophy were noted, which may explain the increase in the hepatocyte proliferation and the enhancement of MN induction by NPYR. Our findings indicate that the RDLMN assay could be a useful tool for comprehensive risk assessment of carcinogenicity by providing information on both genotoxicity and histopathology when integrated into a general repeat dosing toxicity assay.

Identification of specific mRNA signatures as fingerprints for carcinogenesis in mice induced by genotoxic and nongenotoxic hepatocarcinogens

Long-term rodent carcinogenicity studies for evaluation of chemicals and pharmaceuticals concerning their carcinogenic potential to humans are currently receiving critical revision. Additional data from mechanistic studies can support cancer risk assessment by clarifying the underlying mode of action. In the course of the IMI MARCAR project, a European consortium of EFPIA partners and academics, which aims to identify biomarkers for nongenotoxic carcinogenesis, a toxicogenomic mouse liver database was generated. CD-1 mice were orally treated for 3 and 14 days with 3 known genotoxic hepatocarcinogens: C.I. Direct Black 38, Dimethylnitrosamine and 4,4'-Methylenedianiline; 3 nongenotoxic hepatocarcinogens: 1,4-Dichlorobenzene, Phenobarbital sodium and Piperonyl butoxide; 4 nonhepatocarcinogens: Cefuroxime sodium, Nifedipine, Prazosin hydrochloride and Propranolol hydrochloride; and 3 compounds that show ambiguous results in genotoxicity testing: Cyproterone acetate, Thioacetamide and Wy-14643. By liver mRNA expression analysis using individual animal data, we identified 64 specific biomarker candidates for genotoxic carcinogens and 69 for nongenotoxic carcinogens for male mice at day 15. The majority of genotoxic carcinogen biomarker candidates possess functions in DNA damage response (eg, apoptosis, cell cycle progression, DNA repair). Most of the identified nongenotoxic carcinogen biomarker candidates are involved in regulation of cell cycle progression and apoptosis. The derived biomarker lists were characterized with respect to their dependency on study duration and gender and were successfully used to characterize carcinogens with ambiguous genotoxicity test results, such as Wy-14643. The identified biomarker candidates improve the mechanistic understanding of drug-induced effects on the mouse liver that result in hepatocellular adenomas and/or carcinomas in 2-year mouse carcinogenicity studies.

Recommended protocols for the liver micronucleus test: Report of the IWGT working group

At the 6th International Workshop on Genotoxicity Testing (IWGT), the liver micronucleus test working group discussed practical aspects of the in vivo rodent liver micronucleus test (LMNT). The group members focused on the three methodologies currently used, i.e., a partial hepatectomy (PH) method, a juvenile/young rat (JR) method, and a repeated-dose (RD) method in adult rodents. Since the liver is the main organ that metabolizes chemicals, the LMNT is expected to detect clastogens, especially those that need metabolic activation in the liver, and aneugens. Based on current data the three methods seem to have a high sensitivity and specificity, but more data, especially on non-genotoxic but toxic substances, would be needed to fully evaluate the test performance. The three methods can be combined with the micronucleus test (MNT) using bone marrow (BM) and/or peripheral blood (PB). The ability of the PH method to detect both clastogens and aneugens has already been established, but the methodology is technically challenging. The JR method is relatively straightforward, but animal metabolism might not be fully comparable to adult animals, and data on aneugens are limited. These two methods also have the advantage of a short testing period. The RD method is also straightforward and can be integrated into repeated-dose (e.g. 2 or 4 weeks) toxicity studies, but again data on aneugens are limited. The working group concluded that the LMNT could be used as a second in vivo test when a relevant positive result in in vitro mammalian cell genotoxicity tests is noted (especially under the condition of metabolic activation), and a negative result is observed in the in vivo BM/PB-MNT. The group members discussed LMNT protocols and reached consensus about many aspects of test procedures. However, data gaps as mentioned above remain, and further data are needed to fully establish the LMNT protocol.

Genotoxicity of phenacetin in the kidney and liver of Sprague-Dawley gpt delta transgenic rats in 26-week and 52-week repeated-dose studies

Transgenic rat mutation assays can be used to assess genotoxic properties of chemicals in target organs for carcinogenicity. Mutations in transgenes are genetically neutral and accumulate during a treatment period; thus, assays are suitable for assessing the genotoxic risk of chemicals using a repeated-dose treatment paradigm. However, only a limited number of such studies have been conducted. To examine the utility of transgenic rat assays in repeated-dose studies, we fed male and female Sprague-Dawley gpt delta rats with a 0.5% phenacetin-containing diet for 26 and 52 weeks. A long-term feeding of phenacetin is known to induce renal cancer in rats. Phenacetin administration for 52 weeks in males significantly increased gpt (point mutations) mutant frequency (MF) in the kidney, the target organ of carcinogenesis. In the liver, the nontarget organ of carcinogenesis, gpt MFs were significantly elevated in phenacetin treatment groups of both genders during 26- and 52-week treatments. Furthermore, sensitive to P2 interference (Spi(-)deletions) MF increased in the liver of both genders following 52-week treatment. MFs were higher after treatment for 52 weeks than after treatment for 26 weeks. Frequencies of phenacetin-induced mutations were higher in the liver than in the kidney, suggesting that the intensity of genotoxicity does not necessarily correlate with the induction of tumor formation. Results from gpt delta rat assays of repeated-dose treatments are extremely useful to elucidate the relationship between gene mutations and carcinogenesis in the target organ induced by cancer-causing agents.

Evaluation of the Pig-a, micronucleus, and comet assay endpoints in a 28-day study with ethyl methanesulfonate

Ethyl methanesulfonate (EMS) was evaluated as part of the validation effort for the rat Pig-a mutation assay and compared with other well-established in vivo genotoxicity endpoints. Male Sprague-Dawley (SD) rats were given a daily dose of 0, 6.25, 12.5, 25, 50, or 100 mg/kg/day EMS for 28 days, and evaluated for a variety of genotoxicity endpoints in peripheral blood, liver, and colon. Blood was sampled pre-dose (Day 1) and at various time points up to Day 105. Pig-a mutant frequencies were determined in total red blood cells (RBCs) and reticulocytes (RETs) as RBC(CD59-) and RET(CD59-) frequencies. The first statistically significant increases in mutant frequencies were seen in RETs on Day 15 and in RBCs on Day 29 with the maximum RET(CD59-) on Day 29 and of RBC(CD59-) on Day 55. The lowest dose producing a statistically significant increase of RET(CD59-) was 12.5 mg/kg on Day 55 and 25 mg/kg for RBC(CD59-) on Day 55. EMS also induced significant increases in % micronucleated RETs (MN-RETs) in peripheral blood on Days 3, 15, and 28. No statistically significant increases in micronuclei were seen in liver or colon. Results from the in vivo Comet assay on Day 29 showed generally weak increases in DNA damage in all tissues evaluated with little evidence for accumulation of damage seen over time. The results with EMS indicate that the assessment of RBC(CD59-) and/or RET(CD59-) in the Pig-a assay could be a useful and sensitive endpoint for a repeat dose protocol and complements other genotoxicity endpoints.

A toxicogenomic approach for the prediction of murine hepatocarcinogenesis using ensemble feature selection

The current strategy for identifying the carcinogenicity of drugs involves the 2-year bioassay in male and female rats and mice. As this assay is cost-intensive and time-consuming there is a high interest in developing approaches for the screening and prioritization of drug candidates in preclinical safety evaluations. Predictive models based on toxicogenomics investigations after short-term exposure have shown their potential for assessing the carcinogenic risk. In this study, we investigated a novel method for the evaluation of toxicogenomics data based on ensemble feature selection in conjunction with bootstrapping for the purpose to derive reproducible and characteristic multi-gene signatures. This method was evaluated on a microarray dataset containing global gene expression data from liver samples of both male and female mice. The dataset was generated by the IMI MARCAR consortium and included gene expression profiles of genotoxic and nongenotoxic hepatocarcinogens obtained after treatment of CD-1 mice for 3 or 14 days. We developed predictive models based on gene expression data of both sexes and the models were employed for predicting the carcinogenic class of diverse compounds. Comparing the predictivity of our multi-gene signatures against signatures from literature, we demonstrated that by incorporating our gene sets as features slightly higher accuracy is on average achieved by a representative set of state-of-the art supervised learning methods. The constructed models were also used for the classification of Cyproterone acetate (CPA), Wy-14643 (WY) and Thioacetamid (TAA), whose primary mechanism of carcinogenicity is controversially discussed. Based on the extracted mouse liver gene expression patterns, CPA would be predicted as a nongenotoxic compound. In contrast, both WY and TAA would be classified as genotoxic mouse hepatocarcinogens.

Report on stage III Pig-a mutation assays using N-ethyl-N-nitrosourea-comparison with other in vivo genotoxicity endpoints

N-Ethyl-N-nitrosourea (ENU) was evaluated as part of the Stage III trial for the rat Pig-a gene mutation assay. Groups of six- to eight-week-old male Sprague Dawley (SD) or Fischer 344 (F344) rats were given 28 daily doses of the phosphate buffered saline vehicle, or 2.5, 5, or 10 mg/kg ENU, and evaluated for a variety of genotoxicity endpoints in peripheral blood, spleen, liver, and colon. Blood was sampled predose (Day-1) and at various time points up to Day 57. Pig-a mutant frequencies were determined in total red blood cells (RBCs) and reticulocytes (RETs) as RBC(CD592-) and RET(CD592-) frequencies. Consistent with the results from a reference laboratory, RBC(CD592-) and RET(CD592-) frequencies increased in a dose and time-dependent manner, producing significant increases at all doses by Day 15, with similar frequencies seen in both rat strains. ENU also induced small but significant increases in % micronucleated RETs on Days 4 and 29. No significant increases in micronuclei were seen in the liver or colon of the ENU-treated SD rats. Hprt and Pig-a lymphocyte mutation assays conducted on splenocytes from Day 56 F344 rats detected two- to fourfold stronger responses for Hprt than Pig-a mutations. Results from the in vivo Comet assay in SD rats at Day 29 showed generally weak increases in DNA damage in all tissues evaluated. The results with ENU indicate that the Pig-a RET and RBC assays are reproducible, transferable, and complement other genotoxicity endpoints that could potentially be integrated into 28-day repeat dose rat studies.

Micronucleus assays in rodent tissues other than bone marrow

This report updates previous reviews that were conducted as part of the third and fourth International Workshops on Genetic Toxicology Testing of micronucleus (MN) assays in rodent tissues other than bone marrow. Tissues discussed here are liver, lung, skin, colon, spleen, testes and foetal/neonatal tissues with transplacental exposure. Previous reviews have been updated to include literature published after 2000. In addition to the previously described tissues, MN assays in bladder, buccal mucosal cells, stomach and vagina are also included. MN assays using tissues other than bone marrow are critical for risk assessments, for in situ evaluation and for studies of systemic genotoxic effects and modes of action. Protocols for the majority of assays in tissues other than bone marrow have not yet been well standardised and validated for regulatory application, and further development is needed to support regulatory studies.