JaCVAM-organized international validation study of the in vivo rodent alkaline comet assay for detection of genotoxic carcinogens: II. Summary of definitive validation study results

Genotoxicity and safety pharmacology of the rVSVInd(GML)-mspSGtc vaccine against SARS-CoV-2 in Sprague-Dawley rats and Beagle dogs

The emergence of coronavirus disease (COVID-19) caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) led to a pandemic, prompting rapid vaccine development. Although vaccines are effective, the occurrence of rare adverse events following vaccination highlights the necessity of determining whether the benefits outweigh the risks posed by the infection itself. The recombinant Vesicular Stomatitis Virus (rVSV) platform is a promising vector for vaccines against emerging viruses. However, limited studies have evaluated the genotoxicity and safety pharmacology of this viral vector vaccine, which is crucial to ensure the safety of vaccines developed using this platform. Hence, the present study aimed to assess the genotoxicity and safety pharmacology of the rVSVInd(GML)-mspSGtc COVID-19 vaccine using micronucleus and comet assays, as well as neurobehavioral, body temperature, respiratory, and cardiovascular assessments in Sprague-Dawley rats and beagle dogs. The intramuscular administration of rVSVInd(GML)-mspSGtc at doses up to 1.5 × 109 PFU/animal did not increase the number of bone marrow micronucleated polychromatic erythrocytes or cause liver DNA damage. Additionally, it had no significant impact on neurobehavioral functions in rats and showed marginal temporary changes in body temperature, respiratory rate, heart rate, and electrocardiogram parameters in rats and dogs, all of which resolved within 24 h. Overall, following genotoxicity and pharmacological safety assessments, rVSVInd(GML)-mspSGtc displayed no notable systemic adverse effects in rats and dogs, suggesting its potential as a vaccine candidate for human clinical trials.

Genotoxicity evaluation of orally administered styrene monomer in mice using comet, micronucleus, and Pig-a endpoints

Male B6C3F1 mice were administered styrene monomer by oral gavage for 29 consecutive days at dose levels of 0, 75, 150, or 300 mg/kg/day. The highest dose level represented the maximum tolerated dose based on findings in a 28-day dose range-finding study, in which the bioavailability of orally administered styrene was also confirmed. The positive control group received ethyl nitrosourea (ENU; 51.7 mg/kg/day) on Study Days 1-3 and ethyl methanesulfonate (EMS; 150 mg/kg/day) on Study Days 27-29 by oral gavage. Approximately 3 h following the final dose, blood was collected to assess erythrocyte Pig-a mutant and micronucleus frequencies. DNA strand breakage was assessed in glandular stomach, duodenum, kidney, liver, and lung tissues using the alkaline comet assay. The %tail DNA for stomach, liver, lung, and kidney in the comet assay among the styrene-treated groups was neither significantly different from the respective vehicle controls nor was there any dose-related increasing trend in any of the tissues; results for duodenum were interpreted to be inconclusive because of technical issues. The Pig-a and micronucleus frequencies among styrene-treated groups also did not show significant increases relative to the vehicle controls and there was also no evidence for a dose-related increasing trend. Thus, orally administered styrene did not induce DNA damage, mutagenesis, or clastogenesis/aneugenesis in these Organization of Economic Co-operation and Development test guideline-compliant genotoxicity studies. Data from these studies can contribute to the overall assessment of genotoxic hazard and risk posed to humans potentially exposed to styrene.

Measuring DNA modifications with the comet assay: a compendium of protocols

The comet assay is a versatile method to detect nuclear DNA damage in individual eukaryotic cells, from yeast to human. The types of damage detected encompass DNA strand breaks and alkali-labile sites (e.g., apurinic/apyrimidinic sites), alkylated and oxidized nucleobases, DNA-DNA crosslinks, UV-induced cyclobutane pyrimidine dimers and some chemically induced DNA adducts. Depending on the specimen type, there are important modifications to the comet assay protocol to avoid the formation of additional DNA damage during the processing of samples and to ensure sufficient sensitivity to detect differences in damage levels between sample groups. Various applications of the comet assay have been validated by research groups in academia, industry and regulatory agencies, and its strengths are highlighted by the adoption of the comet assay as an in vivo test for genotoxicity in animal organs by the Organisation for Economic Co-operation and Development. The present document includes a series of consensus protocols that describe the application of the comet assay to a wide variety of cell types, species and types of DNA damage, thereby demonstrating its versatility.

Search for the optimal genotoxicity assay for routine testing of chemicals: Sensitivity and specificity of conventional and new test systems

Many conventional in vitro tests that are currently widely used for routine screening of chemicals have a sensitivity/specificity in the range between 60 % and 80 % for the detection of carcinogens. Most procedures were developed 30-40 years ago. In the last decades several assays became available which are based on the use of metabolically competent cell lines, improvement of the cultivation conditions and development of new endpoints. Validation studies indicate that some of these models may be more reliable for the detection of genotoxicants (i.e. many of them have sensitivity and specificity values between 80 % and 95 %). Therefore, they could replace conventional tests in the future. The bone marrow micronucleus (MN) assay with rodents is at present the most widely used in vivo test. The majority of studies indicate that it detects only 5-6 out of 10 carcinogens while experiments with transgenic rodents and comet assays seem to have a higher predictive value and detect genotoxic carcinogens that are negative in MN experiments. Alternatives to rodent experiments could be MN experiments with hen eggs or their replacement by combinations of new in vitro tests. Examples for promising candidates are ToxTracker, TGx-DDI, multiplex flow cytometry, γH2AX experiments, measurement of p53 activation and MN experiments with metabolically competent human derived liver cells. However, the realization of multicentric collaborative validation studies is mandatory to identify the most reliable tests.

Evaluation of Genetic Damage and Antigenotoxic Effect of Ascorbic Acid in Erythrocytes of Orochromis niloticus and Ambystoma mexicanum Using Migration Groups as a Parameter

The comet assay system is an efficient method used to assess DNA damage and repair; however, it currently provides the average result and, unfortunately, the heterogeneity of DNA damage loses relevance. To take advantage of this heterogeneity, migration groups (MGs) of cell comets can be formed. In this study, genetic damage was quantified in erythrocytes of Oreochromis niloticus and Ambystoma mexicanum exposed to ethyl methanesulfonate (ethyl methanesulfonate (EMS) 2.5, 5, and 10 mM over two hours) and ultraviolet C radiation (UV-C) for 5, 10, and 15 min using the tail length, tail moment, and migration group parameters. Additionally, blood cells were exposed to UV-C radiation for 5 min and treated post-treatment at 5, 10, and 15 mM ascorbic acid (AA) for two hours. With the MG parameter, it was possible to observe variations in the magnitude of genetic damage. Our data indicate that MGs help to detect basal and induced genetic damage or damage reduction with approximately the same efficiency of the tail length and tail moment parameters. MGs can be a complementary parameter used to assess DNA integrity in species exposed to mutagens.

Antigenotoxic Effect of Ascorbic Acid and Resveratrol in Erythrocytes of Ambystoma mexicanum, Oreochromis niloticus and Human Lymphocytes Exposed to Glyphosate

Glyphosate is a controversial herbicide. Its genotoxicity and presence in various ecosystems have been reported. The use of ascorbic acid and resveratrol could protect different organisms from glyphosate-induced genetic damage. In the present study, specific genetic damage induced by glyphosate was evaluated in erythrocytes of Oreochromis niloticus, Ambystoma mexicanum and human lymphocytes. Simultaneously, the antigenotoxic capacity of various concentrations of ascorbic acid and resveratrol was evaluated by means of pretreatment and simultaneous treatment protocols. The 0.03, 0.05 and 0.07 mM concentrations of glyphosate induced significant genotoxic activity (p < 0.05) in human lymphocytes and in erythrocytes of the species studied, and could cause genomic instability in these populations. The reduction in genetic damage observed in human lymphocytes exposed to high concentrations of glyphosate is only apparent: excessive genetic damage was associated with undetectable excessive tail migration length. A significant (p < 0.05) antigenotoxic effect of ascorbic acid and resveratrol was observed in all concentrations, organisms and protocols used. Both ascorbic acid and resveratrol play an important role in maintaining the integrity of DNA. Ascorbic acid in Oreochromis niloticus, Ambystoma mexicanum reduced glyphosate-induced genetic damage to a basal level. Therefore, our data indicate that these antioxidants could help preserve the integrity of the DNA of organisms exposed to glyphosate. The consumption of antioxidants is a useful tool against the genotoxicity of glyphosate.

Assessment of cellular and molecular metrics for dose selection in an in vivo comet assay: A case study with MDI

The in vivo comet assay can evaluate the genotoxic potential of a chemical in theoretically any tissue that can be processed to a single cell suspension. This flexibility enables evaluation of point-of-contact tissues using a relevant route of test material administration; however, assessing cytotoxicity is essential for the interpretation of comet results. Histopathological evaluation is routinely utilized to assess cytotoxicity, but temporal- and cell-specific considerations may compromise applicability to the comet assay. In the present study, 1,1'-methylenebis(4-isocyanatobenzene) (4,4'-MDI) was administered to rats for 6 h by nose-only inhalation, and the comet assay was conducted to evaluate genotoxicity in the site-of-contact tissue (bronchoalveolar lavage cells) and distal tissues (liver and glandular stomach). Given the reactive nature of MDI, cellular and molecular metrics at the site-of-contact- including inflammation, macrophage activation, apoptosis/necrosis, and oxidative stress- were used to set appropriate exposure concentrations, in addition to the standard systemic measures of toxicity. In the range-finding study, a concentration of 4 mg/m³ was considered the maximum noninflammatory concentration; hence target concentrations of 2, 5, and 11 mg/m³ were selected for the comet study. In the lung lavage, MDI exposure substantially increased total protein and β-glucuronidase, along with cellular apoptosis. Although MDI did not increase the comet assay response (% tail DNA) in any of the tissues examined, the positive control (ethyl methanesulfonate, EMS) significantly increased % tail DNA in all tissues. In total, these data indicate that appropriate cellular and molecular measurements may facilitate dose selection to discern cellular status in the comet assay.

Overall lack of genotoxic activity among five common low- and no-calorie sweeteners: A contemporary review of the collective evidence

Low- and no-calorie sweeteners (LNCS) are food additives that have been widely consumed for many decades. Their safety has been well established by authoritative bodies globally and is re-evaluated periodically. The objective herein was to survey and summarize the genotoxicity potential of five commonly utilized LNCS: acesulfame potassium (Ace-K), aspartame, saccharin, steviol glycosides and sucralose. Data from peer-reviewed literature and the ToxCast/Tox21 database were evaluated and integrated with the most recent weight-of-evidence evaluations from authoritative sources. Emphasis was placed on assays most frequently considered for hazard identification and risk assessment: mutation, clastogenicity and/or aneugenicity, and indirect DNA damage, such as changes in DNA repair mechanisms or gene expression data. These five sweeteners have been collectively evaluated in hundreds of in vivo or in vitro studies that employ numerous testing models, many of which have been conducted according to specific testing guidelines. The weight-of-evidence demonstrates overall negative findings across assay types for each sweetener when considering the totality of study design, reliability and reporting quality, as well as the lack of carcinogenic responses (or lack of responses relevant to humans) in animal cancer bioassays as well as observational studies in humans. This conclusion is consistent with the opinions of authoritative sources that have consistently determined that these sweeteners lack mutagenic and genotoxic potential.

Validation of the 3D reconstructed human skin micronucleus (RSMN) assay: an animal-free alternative for following-up positive results from standard in vitro genotoxicity assays

In vitro test batteries have become the standard approach to determine the genotoxic potential of substances of interest across industry sectors. While useful for hazard identification, standard in vitro genotoxicity assays in 2D cell cultures have limited capability to predict in vivo outcomes and may trigger unnecessary follow-up animal studies or the loss of promising substances where animal tests are prohibited or not desired. To address this problem, a team of regulatory, academia and industry scientists was established to develop and validate 3D in vitro human skin-based genotoxicity assays for use in testing substances with primarily topical exposure. Validation of the reconstructed human skin micronucleus (RSMN) assay in MatTek Epi-200™ skin models involved testing 43 coded chemicals selected by independent experts, in four US/European laboratories. The results were analysed by an independent statistician according to predefined criteria. The RSMN assay showed a reproducibly low background micronucleus frequency and exhibited sufficient capacity to metabolise pro-mutagens. The overall RSMN accuracy when compared to in vivo genotoxicity outcomes was 80%, with a sensitivity of 75% and a specificity of 84%, and the between- and within-laboratory reproducibility was 77 and 84%, respectively. A protocol involving a 72-h exposure showed increased sensitivity in detecting true positive chemicals compared to a 48-h exposure. An analysis of a test strategy using the RSMN assay as a follow-up test for substances positive in standard in vitro clastogenicity/aneugenicity assays and a reconstructed skin Comet assay for substances with positive results in standard gene mutation assays results in a sensitivity of 89%. Based on these results, the RSMN assay is considered sufficiently validated to establish it as a ‘tier 2’ assay for dermally exposed compounds and was recently accepted into the OECD’s test guideline development program.

In vivo comet assay in rabbit corneal epithelial cells following ocular instillation with genotoxic compounds

Background

The in vivo comet assay is used to evaluate the genotoxic potential of compounds by detecting DNA strand breaks in cells isolated from animal tissue. The comet assay of hepatocytes is well established; however, the levels of systemic drug exposure following systemic administration are often insufficient to evaluate the genotoxic potential of compounds on the ocular surface following ocular instillation. To investigate the possibility of using the comet assay as a genotoxic evaluation tool for the ocular surface, we performed this assay on the corneal epithelial cells of rabbit eyes 2 h after the single ocular instillation of five genotoxic compounds, namely ethidium bromide, 1,1′-dimethyl-4,4′-bipyridinium dichloride (paraquat), methyl methanesulfonate (MMS), acrylamide, and 4-nitroquinoline 1-oxide (4-NQO).

Results

The mean % tail DNA, as an indicator of DNA damage, in the corneal epithelial cells treated with ethidium bromide, MMS, and 4-NQO exhibited statistically significant increases compared with those in the negative controls (saline or 5 % dimethyl sulfoxide in saline). However, paraquat and acrylamide did not increase the mean % tail DNA, presumably because of the high antioxidant levels and low cytochrome P450 levels present in the corneal epithelium, respectively.

Conclusions

The comet assay was able to detect genotoxic potential on the ocular surface following ocular instillation with genotoxic compounds. The study findings indicate that the in vivo comet assay may provide a useful tool for assessing the genotoxicity of compounds topically administrated on the ocular surface under mimicking clinical condition.

Recombinogenic, genotoxic, and cytotoxic effects of azathioprine using in vivo assays

Azathioprine (Aza) is a purine antimetabolite immunosuppressant that is widely employed for immunosuppressive therapy in post-transplant recipients or patients with autoimmune diseases. Chronic use of immunosuppressants might produce several side effects, including a high rate of neoplasms in these patients. Considering that genotoxic effects are associated with an increased risk of developing cancer, the aim of this study was to examine the recombinogenic, genotoxic, and cytotoxic effects of Aza using Somatic Mutation and Recombination Test (SMART) in Drosophila melanogaster, as well as comet and micronucleus assays in mouse bone marrow cells. Further, the adverse effects of Aza were determined in mouse hepatic and renal tissues using histopathological analysis. Data demonstrated that Aza induced significant increased genotoxicity in D. melanogaster and mouse bone marrow cells at all concentrations tested. Homologous recombination was the predominant genotoxic event noted for the first time to be initiated by Aza in SMART. In histopathological analysis, Aza did not show any marked toxic activity in mouse hepatic and renal tissues. Therefore, the high rate of neoplasms reported in patients with long-term use of Aza may be attributed, at least partially, to the genotoxic action of this drug.

Antioxidative defense against omeprazole-induced toxicogenetical effects in Swiss mice

BACKGROUND

Omeprazole (OME), a most frequently used proton pump inhibitor in gastric acidosis, is evident to show many adverse effects, including genetic instability. This study evaluated toxicogenic effects of OME in Mus musculus.

METHODS

For this study, 40 male Swiss mice were divided into 8 groups (n = 5) and treated with OME at doses of 10, 20, and 40 mg/kg and/or treated with the antioxidants retinol palmitate (100 IU/kg) and ascorbic acid (2.0 μM/kg). Cyclophosphamide 50 mg/kg, (cytotoxic agent) and the vehicle were served as positive and negative control group, respectively. After 14 days of treatment, the stomach cells along with the bone marrow and peripheral blood lymphocytes were collected and submitted to the comet assay (alkaline version) and micronucleus test. Additionally, hematological and biochemical parameters of the animals were also determined inspect of vehicle group.

RESULTS

The results suggest that OME at all doses induced genotoxicity and mutagenicity in the treated cells. However, in association with the antioxidants, these effects were modulated and/or inhibited along with a DNA repair capacity.

CONCLUSIONS

Taken together, antioxidants (such as retinol palmitate and ascorbic acid) may be one of the best options to counteract OME-induced cytogenetic instability.

Comet assay: a versatile but complex tool in genotoxicity testing

The comet assay is a versatile method for measuring DNA strand breaks in individual cells. It can also be applied to cells isolated from treated animals. In this review, we highlight advantages and limitations of this in vivo comet assay in a regulatory context. Modified versions of the standard protocol detect oxidized DNA bases and may be used to reveal sites of DNA base loss, DNA interstrand crosslinks, and the extent of DNA damage induced indirectly by reactive oxygen species elicited by chemical-induced oxidative stress. The assay is, however, at best semi-quantitative, and we discuss possible approaches to improving DNA damage quantitation and highlight the necessity of optimizing protocol standardization to enhance the comparability of results between laboratories. As a genotoxicity test in vivo, the in vivo comet assay has the advantage over the better established micronucleus erythrocyte test that it can be applied to any organ, including those that are specific targets of chemical carcinogens or those that are the first sites of contact of ingested or inhaled mutagens. We illustrate this by examples of its use in risk assessment for the food contaminants ochratoxin and furan. We suggest that improved quantitation is required to reveal the full potential of the comet assay and enhance its role in the battery of in vivo approaches to characterize the mechanisms of toxicity and carcinogenicity of chemicals and to aid the determination of safe human exposure limits.

Performance of high-throughput CometChip assay using primary human hepatocytes: a comparison of DNA damage responses with in vitro human hepatoma cell lines

Primary human hepatocytes (PHHs) are considered the "gold standard" for evaluating hepatic metabolism and toxicity of xenobiotics. In the present study, we evaluated the genotoxic potential of four indirect-acting (requiring metabolic activation) and six direct-acting genotoxic carcinogens, one aneugen, and five non-carcinogens that are negative or equivocal for genotoxicity in vivo in cryopreserved PHHs derived from three individual donors. DNA damage was determined over a wide range of concentrations using the CometChip technology and the resulting dose-responses were quantified using benchmark dose (BMD) modeling. Following a 24-h treatment, nine out of ten genotoxic carcinogens produced positive responses in PHHs, while negative responses were found for hydroquinone, aneugen colchicine and five non-carcinogens. Overall, PHHs demonstrated a higher sensitivity (90%) for detecting DNA damage from genotoxic carcinogens than the sensitivities previously reported for HepG2 (60%) and HepaRG (70%) cells. Quantitative analysis revealed that most of the compounds produced comparable BMD₁₀ values among the three types of hepatocytes, while PHHs and HepaRG cells produced similar BMD_1SD values. Evidence of sex- and ethnicity-related interindividual variation in DNA damage responses was also observed in the PHHs. A literature search for in vivo Comet assay data conducted in rodent liver tissues demonstrated consistent positive/negative calls for the compounds tested between in vitro PHHs and in vivo animal models. These results demonstrate that CometChip technology can be applied using PHHs for human risk assessment and that PHHs had higher sensitivity than HepaRG cells for detecting genotoxic carcinogens in the CometChip assay.

Dimethyl fumarate protects thioacetamide-induced liver damage in rats: Studies on Nrf2, NLRP3, and NF-κB

The present study was designed to investigate the hepatoprotective potential of dimethyl fumarate (DMF) against thioacetamide (TAA)-induced liver damage. Wistar rats were treated with DMF (12.5, 25, and 50 mg/kg/day, orally) and TAA (200 mg/kg intraperitoneally, every third day) for 6 consecutive weeks. TAA exposure significantly reduced body weight, increased liver weight and index, and intervention with DMF did not ameliorate these parameters. DMF treatment significantly restored TAA-induced increase in the levels of aspartate aminotransferase, alanine aminotransferase, γ-glutamyl transferase, total bilirubin, uric acid, malondialdehyde, reduced glutathione, and histopathological findings such as inflammatory cell infiltration, deposition of collagen, necrosis, and bridging fibrosis. DMF treatment significantly ameliorated TAA-induced hepatic stellate cell activation, increase in inflammatory cascade markers (NACHT, LRR, and PYD domains-containing protein 3; NLRP3, apoptosis-associated speck like protein containing a caspase recruitment domain; ASC, caspase-1, nuclear factor-kappa B; NF-κB, interleukin-6), fibrogenic makers (α-smooth muscle actin; ɑ-SMA, transforming growth factor; TGF-β1, fibronectin, collagen 1) and antioxidant markers (nuclear factor (erythroid-derived 2)-like factor 2; Nrf2, superoxide dismutase-1; SOD-1, catalase). The present findings concluded that DMF protects against TAA-induced hepatic damage mediated through the downregulation of inflammatory cascades and upregulation of antioxidant status.

Multiple-endpoint genotoxicity assay for colon carcinogen 1,2-dimethylhydrazine

Human risk assessment of the toxic potency of chemicals typically includes genotoxicity assays for predicting carcinogenicity. Gene mutation frequency and chromosomal aberration are two major genotoxicity endpoints in standardized in vitro and in vivo assays. The weight-of-evidence approach in risk assessment is more focused on in vivo assay results; however, animal welfare considerations are aimed at the reduction, replacement, and refinement (3R's) of animal experiments, including a reduction in the number of experimental animals. Proposals to reduce experimental animals in genotoxicity testing include the incorporation of genotoxicity endpoint(s) into other toxicological studies and the combination of two or more assays detecting different genotoxicity endpoints in the same animals. In this study, we used 1,2-dimethylhydrazine as a model chemical of colon carcinogen to assess gene mutation frequency and chromosomal aberration in vivo simultaneously. Specifically, a gene mutation frequency assay was combined with a multiple-organ micronucleus test (peripheral blood, bone marrow, liver, and colon) in F344 gpt delta transgenic rats. Both gpt mutant frequency and micronucleated cell frequency significantly increased in colon and liver but not in bone marrow. Interestingly, we found that the colon carcinogen induced both gene mutations and micronuclei in the targeted colon tissue. Thus, we demonstrated that the mechanism of a carcinogen could be derived from an animal experiment using a lower number of experimental animals as currently recommended. Moreover, a significant increase in mutant frequency in colon and liver was already observed on the first day after treatment completion, as well as on the third day, which is the guideline-recommended period. Thus, this endpoint is compatible with other genotoxicity assays. We confirmed that performing the micronucleus assay in combination with a gene mutation assay in F344 gpt delta transgenic rats is useful to evaluate different genotoxic endpoints simultaneously in the same animals, which reduces the number of experimental animals.

FEMA GRAS assessment of natural flavor complexes: Mint, buchu, dill and caraway derived flavoring ingredients

In 2015, the Expert Panel of the Flavor and Extract Manufacturers Association (FEMA) initiated a re-evaluation of the safety of over 250 natural flavor complexes (NFCs) used as flavor ingredients. NFC flavor materials include a variety of essential oils and botanical extracts. The re-evaluation of NFCs is conducted based on a constituent-based procedure outlined in 2005 and updated in 2018 that evaluates the safety of NFCs for their intended use as flavor ingredients. This procedure is applied in the re-evaluation of the generally recognized as safe (GRAS) status of NFCs with constituent profiles that are dominated by alicyclic ketones such as menthone and carvone, secondary alcohols such as menthol and carveol, and related compounds. The FEMA Expert Panel affirmed the GRAS status of Peppermint Oil (FEMA 2848), Spearmint Oil (FEMA 3032), Spearmint Extract (FEMA 3031), Cornmint Oil (FEMA 4219), Erospicata Oil (FEMA 4777), Curly Mint Oil (FEMA 4778), Pennyroyal Oil (FEMA 2839), Buchu Leaves Oil (FEMA 2169), Caraway Oil (FEMA 2238) and Dill Oil (FEMA 2383) and determined FEMA GRAS status for Buchu Leaves Extract (FEMA 4923), Peppermint Oil, Terpeneless (FEMA 4924) and Spearmint Oil, Terpeneless (FEMA 4925).

In vivo genotoxicity testing strategies: Report from the 7th International workshop on genotoxicity testing (IWGT)

The working group reached complete or majority agreement on many issues. Results from TGR and in vivo comet assays for 91 chemicals showed they have similar ability to detect in vivo genotoxicity per se with bacterial mutagens and Ames-positive carcinogens. TGR and comet assay results were not significantly different when compared with IARC Group 1, 2 A, and unclassified carcinogens. There were significantly more comet assay positive responses for Group 2B chemicals, and for IARC classified and unclassified carcinogens combined, which may be expected since mutation is a sub-set of genotoxicity. A liver comet assay combined with the bone marrow/blood micronucleus (MNviv) test would detect in vivo genotoxins that do not exhibit tissue-specific or site-of-contact effects, and is appropriate for routine in vivo genotoxicity testing. Generally for orally administered substances, a comet assay at only one site-of-contact GI tract tissue (stomach or duodenum/jejunum) is required. In MNviv tests, evidence of target tissue exposure can be obtained in a number of different ways, as recommended by ICH S2(R1) and EFSA (Hardy et al., 2017). Except for special cases the i.p. route is inappropriate for in vivo testing; for risk evaluations more weight should be given to data from a physiologically relevant administration route. The liver MN test is sufficiently validated for the development of an OECD guideline. However, the impact of dosing animals >6 weeks of age needs to be evaluated. The GI tract MN test shows promise but needs more validation for an OECD guideline. The Pig-a assay detects systemically available mutagens and is a valuable follow-up to in vitro positive results. A new freeze-thaw protocol provides more flexibility. Mutant reticulocyte and erythrocyte frequencies should both be determined. Preliminary data are available for the Pig-a assay in male rat germ cells which require validation including germ cell DNA mutation origin.

The comet assay in animal models: From bugs to whales - (Part 2 Vertebrates)

The comet assay has become one of the methods of choice for the evaluation and measurement of DNA damage. It is sensitive, quick to perform and relatively affordable for the evaluation of DNA damage and repair at the level of individual cells. The comet assay can be applied to virtually any cell type derived from different organs and tissues. Even though the comet assay is predominantly used on human cells, the application of the assay for the evaluation of DNA damage in yeast, plant and animal cells is also quite high, especially in terms of biomonitoring. The present extensive overview on the usage of the comet assay in animal models will cover both terrestrial and water environments. The first part of the review was focused on studies describing the comet assay applied in invertebrates. The second part of the review, (Part 2) will discuss the application of the comet assay in vertebrates covering cyclostomata, fishes, amphibians, reptiles, birds and mammals, in addition to chordates that are regarded as a transitional form towards vertebrates. Besides numerous vertebrate species, the assay is also performed on a range of cells, which includes blood, liver, kidney, brain, gill, bone marrow and sperm cells. These cells are readily used for the evaluation of a wide spectrum of genotoxic agents both in vitro and in vivo. Moreover, the use of vertebrate models and their role in environmental biomonitoring will also be discussed as well as the comparison of the use of the comet assay in vertebrate and human models in line with ethical principles. Although the comet assay in vertebrates is most commonly used in laboratory animals such as mice, rats and lately zebrafish, this paper will only briefly review its use regarding laboratory animal models and rather give special emphasis to the increasing usage of the assay in domestic and wildlife animals as well as in various ecotoxicological studies.

Dosage time affects alkylating agents induced micronuclei in mouse peripheral blood reticulocytes through the function of erythropoietin

Previously, we reported that the frequency of micronucleated reticulocytes (MNRETs) in the peripheral blood of male C3H/He mice intraperitoneally administered ethylnitrosourea (ENU) (25 mg/kg body weight) in the dark period (zeitgeber time, ZT15) was higher than in the light period (ZT3). In this study, to clarify the mechanism underlying this phenomenon, we investigated the differences in micronucleus (MN) induction observed between ZT3 and ZT15 using five chemicals, methylnitrosourea (MNU), ethylmethane sulfonate (EMS), mitomycin C, cyclophosphamide and vincristin. MNU and EMS, monofunctional alkylating agents, showed higher frequencies of MNRETs in the ZT15 than the ZT3 treatment similar to ENU. However, no differences were observed for the other chemicals. In the comet assay, more DNA damage was induced by ENU in the ZT15 than the ZT3 treatment. Furthermore, the plasma erythropoietin (EPO) level, a known effector of MN induction with anti-apoptotic activity mediated by Bcl-xL expression, was higher in the dark than in the light period. EPO did not increase the frequency of MNRETs. However, in the ENU treatment group at ZT3 following EPO injection a significant increase of MNRETs was observed similar to the ZT15 treatment. Higher expression of apoptosis-related genes such as Bcl-xL was induced in bone marrow cells from mice treated with ENU at ZT15 compared with ZT3. From these results, it was speculated that the differences in MN induction in the peripheral blood of mice exposed to monofunctional alkylating agents such as ENU depend on apoptotic or anti-apoptotic conditions related to the circadian rhythms of EPO in bone marrow.

A comparison of transgenic rodent mutation and in vivo comet assay responses for 91 chemicals

A database of 91 chemicals with published data from both transgenic rodent mutation (TGR) and rodent comet assays has been compiled. The objective was to compare the sensitivity of the two assays for detecting genotoxicity. Critical aspects of study design and results were tabulated for each dataset. There were fewer datasets from rats than mice, particularly for the TGR assay, and therefore, results from both species were combined for further analysis. TGR and comet responses were compared in liver and bone marrow (the most commonly studied tissues), and in stomach and colon evaluated either separately or in combination with other GI tract segments. Overall positive, negative, or equivocal test results were assessed for each chemical across the tissues examined in the TGR and comet assays using two approaches: 1) overall calls based on weight of evidence (WoE) and expert judgement, and 2) curation of the data based on a priori acceptability criteria prior to deriving final tissue specific calls. Since the database contains a high prevalence of positive results, overall agreement between the assays was determined using statistics adjusted for prevalence (using AC1 and PABAK). These coefficients showed fair or moderate to good agreement for liver and the GI tract (predominantly stomach and colon data) using WoE, reduced agreement for stomach and colon evaluated separately using data curation, and poor or no agreement for bone marrow using both the WoE and data curation approaches. Confidence in these results is higher for liver than for the other tissues, for which there were less data. Our analysis finds that comet and TGR generally identify the same compounds (mainly potent mutagens) as genotoxic in liver, stomach and colon, but not in bone marrow. However, the current database content precluded drawing assay concordance conclusions for weak mutagens and non-DNA reactive chemicals.

Gamma-H2AX immunofluorescence for the detection of tissue-specific genotoxicity in vivo

The phosphorylation of histone H2AX in Serine 139 (gamma-H2AX) marks regions of DNA double strand breaks and contributes to the recruitment of DNA repair factors to the site of DNA damage. Gamma-H2AX is used widely as DNA damage marker in vitro, but its use for genotoxicity assessment in vivo has not been extensively investigated. Here, we developed an image analysis system for the precise quantification of the gamma-H2AX signal, which we used to monitor DNA damage in animals treated with known genotoxicants (EMS, ENU and doxorubicin). To compare this new assay to a validated standard procedure for DNA damage quantification, tissues from the same animals were also analyzed in the comet assay. An increase in the levels of gamma-H2AX was observed in most of the tissues from animals treated with doxorubicin and ENU. Interestingly, the lesions induced by doxorubicin were not easily detected by the standard comet assay, while they were clearly identified by gamma-H2AX staining. Conversely, EMS appeared strongly positive in the comet assay but only mildly in the gamma-H2AX immunofluorescence. These observations suggest that the two methods could complement each other for DNA damage analysis, where gamma-H2AX staining allows the detection of tissue-specific effects in situ. Moreover, since gamma-H2AX staining can be performed on formalin-fixed and paraffin-embedded tissue sections generated during repeated-dose toxicity studies, it does not require any further treatments or extra procedures during dissection, thus optimizing the use of resources and animals. Environ. Mol. Mutagen. 60:4-16, 2019. © 2018 Wiley Periodicals, Inc.

Glibenclamide protects against thioacetamide-induced hepatic damage in Wistar rat: investigation on NLRP3, MMP-2, and stellate cell activation

Glibenclamide (GLB), most widely used in the treatment of type II diabetes mellitus, inhibits K⁺_ATP channel in pancreatic-β cells and releases insulin, while thioacetamide (TAA) is a well-known hepatotoxicant and most recommended for the induction of acute and chronic liver disease. The purpose of this study was to evaluate the hepatoprotective potential of GLB against TAA-induced hepatic damage in Wistar rats. TAA (200 mg/kg, ip, twice weekly) and GLB (1.25, 2.5, and 5 mg/kg/day, po) were administered for 6 consecutive weeks. Different biochemical, DNA damage, histopathological, TEM, immunohistochemical, and western blotting parameters were evaluated. GLB treatment has no effects on the TAA-induced significant decrease in body and liver weights. TAA treatment significantly increased liver index and treatment with GLB has no effect the same. TAA treatment altered the liver morphology, whereas treatment with GLB normalized the alteration in morphology. Further, significant increase in oxidative stress, apoptosis, and DNA damage was found in TAA-treated animals and GLB treatment significantly reduced these effects. TAA-induced plasma transaminases and serum ALP levels were significantly restored by GLB. Furthermore, histopathological findings showed the presence of lymphocyte infiltration, collagen deposition, bridging fibrosis, degeneration of portal triad, and necrosis in TAA-treated animals and GLB intervention significantly reduced the same. TEM images revealed that GLB significantly normalized the hepatic stellate cell morphology as well as restored the number of lipid droplets. GLB treatment significantly downregulated the expressions of TGF-β1, α-SMA, NLRP3, ASC, caspase-1, and IL-1β, and upregulated MMP-2 and catalase against TAA-induced liver damage. The outcomes of the present study confirmed that GLB ameliorated the liver damage induced by TAA.

Role of pathology peer review in interpretation of the comet assay

When a comet assay, an increasingly popular in vivo genotoxicity test, shows a positive test result, interpretation of that response requires ruling out any confounding tissue site toxicity. Since the comet assay typically uses only two or three daily doses of test agent, precursor tissue changes indicative of toxicity may be easily overlooked. Using case examples for two flavoring agents, perillaldehyde and 4,5-epoxydec-2(trans)-enal, we highlight the role of pathology peer review in verifying precursor tissue changes indicative of tissue site toxicity, thereby increasing confidence in final interpretation of comet assay results. Given global deliberation regarding safety assessment of compounds entering the marketplace, we recommend consideration of pathology peer review for equivocal and positive comet assays so that interpretations are universally consistent.

Advantages of evaluating γH2AX induction in non-clinical drug development

γH2AX, the phosphorylated form of a histone variant H2AX at Ser 139, is already widely used as a biomarker to research the fundamental biology of DNA damage and repair and to assess the risk of environmental chemicals, pollutants, radiation, and so on. It is also beginning to be used in the early non-clinical stage of pharmaceutical drug development as an in vitro tool for screening and for mechanistic studies on genotoxicity. Here, we review the available information on γH2AX-based test systems that can be used to develop drugs and present our own experience of practically applying these systems during the non-clinical phase of drug development. Furthermore, the potential application of γH2AX as a tool for in vivo non-clinical safety studies is also discussed.

DNA damage in a liver tissue of metal exposed Clethrionomys glareolus

It is widely known that some toxic agents may act on DNA strand resulting in its damages. One of the possible impairments is formation of abasic sites in DNA. The aim of this study was to indicate a presence of these DNA sites in the liver tissue of bank voles inhabiting the vicinity of zinc/lead smelters. Samples that were used originated from animals collected from unpolluted (Niepołomice, Teleśnica Oszwarowa, Mikołajki) and polluted (Miasteczko Śląskie, Katowice, Olkusz) populations. They significantly differed in terms of tissue lead concentrations in the kidney and liver. The means of detected AP sites per 10⁵ bp ranged between 3.39 (Teleśnica Oszwarowa) to 5.13 (Miasteczko Śląskie). Statistical analysis (ANOVA) showed no difference in terms of number of the AP sites between single populations. However, t-test showed significant difference between the unpolluted and polluted populations. Factorial ANOVA indicated that sex is not a factor influencing the number of AP sites. The analyses revealed statistically significant relationships between the number of AP sites and Cu concentrations in the liver, and also Pb and Cd concentrations in the kidney.

Sevoflurane and isoflurane genotoxicity in kidney cells of mice

The aim of this study was to evaluate the DNA damage and repair in kidney cells of Swiss albino mice after repeated exposure to sevoflurane and isoflurane and compare their detrimental effects. We used the alkaline comet assay to establish the genetic damage and measured three parameters: tail length, tail moment, and tail intensity of comets. These parameters were measured immediately after exposure to the above mentioned inhalation anaesthetics, two hours, six hours, and 24 hours later and were compared with the control group. Mean values of all three parameters were significantly higher in experimental groups compared to the control group. DNA damage in kidney cells of mice exposed to sevoflurane increased continuously before it reached its peak 24 hours after exposure. Isoflurane induced the highest DNA damage two hours after exposure. Levels of DNA damage recorded 24 h after cessation of exposure to both tested compounds suggest that sevoflurane was slightly more genotoxic than isoflurane to kidney cells of mice. According to these results, the currently used volatile anaesthetics sevoflurane and isoflurane are able to damage DNA in kidney cells of mice. Such findings suggest a possibility for similar outcomes in humans and that fact must be taken into account in everyday clinical practice.

Carcinogenic potential of sanguinarine, a phytochemical used in 'therapeutic' black salve and mouthwash

Black salves are escharotic skin cancer therapies in clinical use since the mid 19th century. Sanguinaria canadensis, a major ingredient of black salve formulations, contains a number of bioactive phytochemicals including the alkaloid sanguinarine. Despite its prolonged history of clinical use, conflicting experimental results have prevented the carcinogenic potential of sanguinarine from being definitively determined. Sanguinarine has a molecular structure similar to known polyaromatic hydrocarbon carcinogens and is a DNA intercalator. Sanguinarine also generates oxidative and endoplasmic reticulum stress resulting in the unfolded protein response and the formation of 8-hydroxyguanine genetic lesions. Sanguinarine has been the subject of contradictory in vitro and in vivo genotoxicity and murine carcinogenesis test results that have delayed its carcinogenic classification. Despite this, epidemiological studies have linked mouthwash that contains sanguinarine with the development of oral leukoplakia. Sanguinarine is also proposed as an aetiological agent in gallbladder carcinoma. This literature review investigates the carcinogenic potential of sanguinarine. Reasons for contradictory genotoxicity and carcinogenesis results are explored, knowledge gaps identified and a strategy for determining the carcinogenic potential of sanguinarine especialy relating to black salve are discussed. As patients continue to apply black salve, especially to skin regions suffering from field cancerization and skin malignancies, an understanding of the genotoxic and carcinogenic potential of sanguinarine is of urgent clinical relevance.

Cytogenotoxicological defense of retinyl palmitate in the front damage of antineoplastics

Cancer, the multifactorial pathology and to date is the most lethal causes of death in the world. Cyclophosphamide (CPA) and doxorubicin (DOX) are the individually or combindly used two anticancer drugs. The antineoplastic drugs-mediated genetic instability can be overcome by using antioxidants. The study evaluated the cytogenotoxic modulatory potentials of retinyl palmitate (RP) caused by CPA and DOX in Swiss mice. For this, adult Mus musculus of either sex were divided equally regarding to the gender. Toxicogenetic effects were induced by the intraperitoneal (i.p.) administration of the CPA (20mg/kg) and/or DOX (2mg/kg), following to test for comet assay and micronucleus test in bone marrow cells after 48h (DOX) and 7h (CPA) of the administration of RP (100 IU/kg). Both CPA and DOX significantly (p<0.05) increased with the index and frequency of damages, clastogenic and/or aneugenic effects with the augmenting of micronuclei, demonstrating the cytotoxicity interference on the ratio of normochromatic to polychromatic erythrocytes and bone marrow cells of mice, that were found to reduce in RP treatment groups. In conclusion, RP has a modulatory effect on CPA and DOX-mediated cytogenotoxic events. The findings may be a good indication to manage the antioneoplastic drug-induced stress mediated detrimental effects by using RP, especially as a side effect minimizer.

2-Nitroanisole-induced oxidative DNA damage in Salmonella typhimurium and in rat urinary bladder cells

2-Nitroanisole (2-NA) is used in the manufacturing of azo dyes and causes cancer, mainly in the urinary bladder. Previous in vivo genotoxic data seems to be insufficient to explain the mechanism through which 2-NA induces carcinogenesis, and several bladder carcinogens were reported to induce oxidative DNA damage. Thus, we examined the potential induction of oxidative DNA damage by 2-NA using bacterial strain YG3008, a mutM_ST-deficient derivative of strain TA100. Consequently, strain YG3008, when compared with strain TA100, was found to be more sensitive to 2-NA, indicating oxidative DNA damage in bacterial cells. For further investigation, we performed the comet assay using the urinary bladder and liver of rats, with and without human 8-oxoguanine DNA-glycosylase 1 (hOGG1), to confirm the potential of 2-NA for inducing oxidative DNA damage. Simultaneously, we conducted a micronucleus test using bone marrow from rats to assess the genotoxicity of 2-NA in vivo. 2-NA was administered orally to male Fischer 344 rats for 3 consecutive days. The rats were divided into 6 treatment groups: 3 groups treated with 2-NA at doses of 125, 250, and 500mg/kg; a group treated with the combination of 2-NA and glutathione-SH (GSH); a negative control group; and a positive control group. The comet assay without hOGG1 detected no DNA damage in the liver or urinary bladder, and the micronucleus test did not show clastogenic effects in bone marrow cells. However, the comet assay with hOGG1 was positive in the urinary bladder samples, indicating the induction of oxidative DNA damage in the urinary bladder for the group treated with 2-NA at 500mg/kg. Moreover, an antioxidant of GSH significantly reduced oxidative DNA damage caused by 2-NA. These results indicate that oxidative DNA damage is a possible mode of action for carcinogenesis in the urinary bladder of rats treated with 2-NA.

Genotoxicity assessment of the flavouring agent, perillaldehyde

Perillaldehyde, a natural monocyclic terpenoid found most abundantly in the herb perilla, has a long history of use as a flavouring ingredient to add spiciness and citrus taste to foods. Previously, it was judged to be safe by several international expert panels. To confirm the safety of flavourings placed on the European Union list of flavourings, perillaldehyde was selected by the European Food Safety Authority as a representative of a subgroup of alicyclic aldehyde flavouring substances to be evaluated for genotoxic potential. Perillaldehyde was tested in a bacterial reverse mutation assay, an in vitro micronucleus assay in human lymphocytes, an HPRT assay in mouse lymphoma cells, and a micronucleus/comet assay in Han Wistar rats. In contrast to previously published results, perillaldehyde induced mutation in Salmonella typhimurium strain TA98 in the absence of metabolic activation. The comet assay was negative for duodenum and weakly positive for liver but only at a hepatotoxic dose of perillaldehyde. All other genotoxicity assays were negative. These data do not provide an indication of any genotoxic potential for perillaldehyde, and they provide the primary basis for recent scientific opinions regarding perillaldehyde genotoxicity announced by several international organizations responsible for safety assessment of food additives and flavourings.

The application of the comet assay to assess the genotoxicity of environmental pollutants in the nematode Caenorhabditis elegans

This study aimed to establish a protocol for cell dissociation from the nematode Caenorhabditis elegans (C. elegans) to assess the genotoxicity of the environmental pollutant benzo[a]pyrene (BaP) using the alkaline version of the single cell electrophoresis assay (comet assay). BaP genotoxicity was assessed in C. elegans (wild-type [WT]; N2, Bristol) after 48h exposure (0-40μM). Induction of comets by BaP was concentration-dependent up to 20μM; comet% tail DNA was ∼30% at 20μM BaP and ∼10% in controls. Similarly, BaP-induced DNA damage was evaluated in C. elegans mutant strains deficient in DNA repair. In xpa-1 and apn-1 mutants BaP-induced comet formation was diminished to WT background levels suggesting that the damage formed by BaP that is detected in the comet assay is not recognised in cells deficient in nucleotide and base excision repair, respectively. In summary, our study provides a protocol to evaluate DNA damage of environmental pollutants in whole nematodes using the comet assay.

Comparison of integrated genotoxicity endpoints in rats after acute and subchronic oral doses of 4-nitroquinoline-1-oxide

During interlaboratory validation trials for the Pig-a gene mutation assay we assessed the genotoxicity of 4-nitroquinoline-1-oxide (4NQO) across endpoints in multiple tissues: induction of Pig-a mutant red blood cells (RBCs) and reticulocytes (RETs); micronucleated RETs (MN RETs); and DNA damage in blood and liver via the alkaline Comet assay (%tail intensity [TI]). In a previous subchronic toxicity study with 28 daily doses, biologically meaningful increases were observed only for Pig-a mutant RBCs/RETs while marginal increases in the frequency of MN RET were observed, and other clastogenic endpoints were negative. Follow up acute studies were performed using the same cumulative doses (0, 35, 70, 105, and 140 mg/kg) administered in a bolus, or split over three equal daily doses, with samples collected up to 1 month after the last dose. Both of the acute dosing regimens produced similar results, in that endpoints were either positive or negative, regardless of 1 or 3 daily doses, but the three consecutive daily dose regimen yielded more potent responses in TI (in liver and blood) and Pig-a mutant frequencies. In these acute studies the same cumulative doses of 4NQO induced positive responses in clastogenic endpoints that were negative or inconclusive using a subchronic study design. Additionally, a positive control group using combination doses of cyclophosphamide and ethyl methanesulfonate was employed to assess assay validity and potentially identify a future positive control treatment for integrated genetic toxicity studies.

Inter-laboratory comparison of the in vivo comet assay including three image analysis systems

To compare the extent of potential inter-laboratory variability and the influence of different comet image analysis systems, in vivo comet experiments were conducted using the genotoxicants ethyl methanesulfonate and methyl methanesulfonate. Tissue samples from the same animals were processed and analyzed-including independent slide evaluation by image analysis-in two laboratories with extensive experience in performing the comet assay. The analysis revealed low inter-laboratory experimental variability. Neither the use of different image analysis systems, nor the staining procedure of DNA (propidium iodide vs. SYBR® Gold), considerably impacted the results or sensitivity of the assay. In addition, relatively high stability of the staining intensity of propidium iodide-stained slides was found in slides that were refrigerated for over 3 months. In conclusion, following a thoroughly defined protocol and standardized routine procedures ensures that the comet assay is robust and generates comparable results between different laboratories.

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.

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.

Comet assay evaluation of six chemicals of known genotoxic potential in rats

As a part of an international validation of the in vivo rat alkaline comet assay (comet assay) initiated by the Japanese Center for the Validation of Alternative Methods (JaCVAM) we examined six chemicals for potential to induce DNA damage: 2-acetylaminofluorene (2-AAF), N-nitrosodimethylamine (DMN), o-anisidine, 1,2-dimethylhydrazine dihydrochloride (1,2-DMH), sodium chloride, and sodium arsenite. DNA damage was evaluated in the liver and stomach of 7- to 9-week-old male Sprague Dawley rats. Of the five genotoxic carcinogens tested in our laboratory, DMN and 1,2-DMH were positive in the liver and negative in the stomach, 2-AAF and o-anisidine produced an equivocal result in liver and negative results in stomach, and sodium arsenite was negative in both liver and stomach. 1,2-DMH and DMN induced dose-related increases in hedgehogs in the same tissue (liver) that exhibited increased DNA migration. However, no cytotoxicity was indicated by the neutral diffusion assay (assessment of highly fragmented DNA) or histopathology in response to treatment with any of the tested chemicals. Therefore, the increased DNA damage resulting from exposure to DMN and 1,2-DMH was considered to represent a genotoxic response. Sodium chloride, a non-genotoxic non-carcinogen, was negative in both tissues as would be predicted. Although only two (1,2-DMH and DMN) out of five genotoxic carcinogens produced clearly positive results in the comet assay, the results obtained for o-anisidine and sodium arsenite in liver and stomach cells are consistent with the known mode of genotoxicity and tissue specificity exhibited by these carcinogens. In contrast, given the known genotoxic mode-of-action and target organ carcinogenicity of 2-AAF, it is unclear why this chemical failed to convincingly increase DNA migration in the liver. Thus, the results of the comet assay validation studies conducted in our laboratory were considered appropriate for five out of the six test chemicals.

p-Chloroaniline, t-butylhydroquinone, and methyl carbamate: Rat in vivo comet test, JaCVAM trial phase 4.2

As part of the Japanese Center for the Validation of Alternative Methods (JaCVAM)-initiative international validation study of in vivo rat alkaline comet assay, we examined p-Chloroaniline, t-butylhydroquinone, and methyl carbamate. All test materials and controls were dosed orally by gavage. p-Chloroaniline produced a statistically significant increase in the mean and median % tail intensity which was also outside of the historical control range in the liver and stomach of Sprague-Dawley rats. t-Butylhydroquinone caused a statistically significant increase in the mean % tail intensity in the liver and stomach and a statistically significant increase in the median % tail intensity in the liver; however, these results are not considered to be biologically significant as all values obtained fell within the current vehicle historical control range and within the negative control range for mean % tail intensity set by the Validation Management Team (VMT) as a requirement for an acceptable assay. Methyl carbamate did not induce a statistically significant change in the mean or median % tail intensity in either liver or stomach.

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.

Assessment of the in vivo genotoxicity of cadmium chloride, chloroform, and D,L-menthol as coded test chemicals using the alkaline comet assay

As part of the Japanese Center for the Validation of Alternative Methods (JaCVAM) international validation study of in vivo rat alkaline comet assays, we examined cadmium chloride, chloroform, and D,L-menthol under blind conditions as coded chemicals in the liver and stomach of Sprague-Dawley rats after 3 days of administration. Cadmium chloride showed equivocal responses in the liver and stomach, supporting previous reports of its poor mutagenic potential and non-carcinogenic effects in these organs. Treatment with chloroform, which is a non-genotoxic carcinogen, did not induce DNA damage in the liver or stomach. Some histopathological changes, such as necrosis and degeneration, were observed in the liver; however, they did not affect the comet assay results. D,L-Menthol, a non-genotoxic non-carcinogen, did not induce liver or stomach DNA damage. These results indicate that the comet assay can reflect genotoxic properties under blind conditions.