Evaluation of thresholds for benomyl- and carbendazim-induced aneuploidy in cultured human lymphocytes using fluorescence in situ hybridization

Lack of in vivo mutagenicity of carbendazim in the liver and glandular stomach of MutaMice

BACKGROUND

Carbendazim (methyl 2-benzimidazolecarbamate, CASRN: 10605-21-7) exhibits spindle poisoning effects and is widely used as a fungicide. With respect to genotoxicity, carbendazim is deemed to be non-mutagenic in vitro, but it causes indicative DNA damage in vivo and chromosome aberrations in vitro and in vivo. In this study, we examined the mutagenicity of carbendazim in vivo.

RESULTS

MutaMice were treated with carbendazim orally at doses of 0 (corn oil), 250, 500, and 1,000 mg/kg/day once a day for 28 days. A lacZ assay was used to determine the mutant frequency (MF) in the liver and glandular stomach of mice. MutaMice were administered up to the maximum dose recommended by the Organization for Economic Co-operation and Development Test Guidelines for Chemicals No. 488 (OECD TG488). The lacZ MFs in the liver and glandular stomach of carbendazim-treated animals were not significantly different from those in the negative control animals. In contrast, positive control animals exhibited a significant increase in MFs in both the liver and glandular stomach.

CONCLUSIONS

Carbendazim is non-mutagenic in the liver and glandular stomach of MutaMice following oral treatment.

Guidance on aneugenicity assessment

The EFSA Scientific Committee was asked to provide guidance on the most appropriate in vivo tests to follow up on positive in vitro results for aneugenicity, and on the approach to risk assessment for substances that are aneugenic but not clastogenic nor causing gene mutations. The Scientific Committee confirmed that the preferred approach is to perform an in vivo mammalian erythrocyte micronucleus test with a relevant route of administration. If this is positive, it demonstrates that the substance is aneugenic in vivo. A negative result with evidence that the bone marrow is exposed to the test substance supports a conclusion that aneugenic activity is not expressed in vivo. If there is no evidence of exposure to the bone marrow, a negative result is viewed as inconclusive and further studies are required. The liver micronucleus assay, even though not yet fully validated, can provide supporting information for substances that are aneugenic following metabolic activation. The gastrointestinal micronucleus test, conversely, to be further developed, may help to assess aneugenic potential at the initial site of contact for substances that are aneugenic in vitro without metabolic activation. Based on the evidence in relation to mechanisms of aneugenicity, the Scientific Committee concluded that, in principle, health-based guidance values can be established for substances that are aneugenic but not clastogenic nor causing gene mutations, provided that a comprehensive toxicological database is available. For situations in which the toxicological database is not sufficient to establish health-based guidance values, some approaches to risk assessment are proposed. The Scientific Committee recommends further development of the gastrointestinal micronucleus test, and research to improve the understanding of aneugenicity to support risk assessment.

Experiments in the EpiDerm 3D Skin In Vitro Model and Minipigs In Vivo Indicate Comparatively Lower In Vivo Skin Sensitivity of Topically Applied Aneugenic Compounds

Risk management of in vitro aneugens for topically applied compounds is not clearly defined because there is no validated methodology to accurately measure compound concentration in proliferating stratum basale keratinocytes of the skin. Here, we experimentally tested several known aneugens in the EpiDerm reconstructed human skin in vitro micronucleus assay and compared the results to flow cytometric mechanistic biomarkers (phospho-H3; MPM2, DNA content). We then evaluated similar biomarkers (Ki-67, nuclear area) using immunohistochemistry in skin sections of minipigs following topical exposure the potent aneugens, colchicine, and hesperadin. Data from the EpiDerm model showed positive micronucleus responses for all aneugens tested following topical or direct media dosing with similar sensitivity when adjusted for applied dose. Quantitative benchmark dose-response analysis exhibited increases in the mitotic index biomarkers phospho-H3 and MPM2 for tubulin binders and polyploidy for aurora kinase inhibitors are at least as sensitive as the micronucleus endpoint. By comparison, the aneugens tested did not induce histopathological changes, increases in Ki-67 immunolabeling or nuclear area in skin sections from the in vivo minipig study at doses in significant excess of those eliciting a response in vitro. Results indicate the EpiDerm in vitro micronucleus assay is suitable for the hazard identification of aneugens. The lack of response in the minipig studies indicates that the barrier function of the minipig skin, which is comparable to human skin, protects from the effects of aneugens in vivo. These results provide a basis for conducting additional studies in the future to further refine this understanding.

Mutagenicity evaluation of pesticide analogs using standard and 6-well miniaturized bacterial reverse mutation tests

The Ames test is widely used in the mutagenicity evaluation of new and existing chemicals as a part of a compound selection strategy, regulatory control, the equivalence assessment, carcinogenic potential measurement etc. Intensification of the chemical industry and synthesis of plenty of new molecules has led to the necessity of tests with a higher throughput capacity. The 6-well miniaturized bacterial reverse mutation test and the standard Ames test were compared using 14 technical grade active ingredients (TGAIs) of pesticides. With some exceptions, the responses obtained in the miniscreen Ames are similar to those seen in the standard method: 4 overall test outcomes were negative and 9 were positive in both test versions, but 1 discordant result between the miniscreen and standard version. Comparison of the standard and the miniscreen Ames test resulted in 98% of concordance across five strains and conditions (±S9). The overall judgment is that the miniscreen Ames test can be used to assess the mutagenicity of pesticide analogs. It has the advantage of decreasing the number of materials and animals (for S9) and keeping a high-test performance.

Environmentally relevant exposures of male mice to carbendazim and thiram cause persistent genotoxicity in male mice

Carbendazim and thiram are fungicides used in combination to prevent mold destruction of crops. Studies have demonstrated genotoxicity by these agents, but have not used concentrations below their water solubility limits in drinking water to test for persistence of genotoxicity due to chronic exposure. Ten 8-week old male Swiss-Webster mice were exposed to tap water, or nominal concentrations of 20 μM carbendazim, 20 μM thiram or 20 μM of both fungicides for 90 days (total of 40 mice). Five mice from tap water controls, carbendazim, thiram and combination-treated groups (20 mice total) had genotoxicity detected by comet assay of lymphocytes at the termination of the exposure period. The other 20 mice (4 treatment groups) were all switched to tap water and allowed a 45-day recovery period to check for persistence of DNA damage. The damage was compared with commercial control cells exposed to increasingly harsh treatment by etopside. Comet assay (mean % tail DNA + SE) of control mice (9.8 + 0.9) was similar to commercial control (CC0) cells (8.5 + 0.9). Carbendazim, thiram or the combination treatment caused similar mean % tail DNA with 33.0 + 2.9, 30.1 + 3.3 and 29.1 + 1.8, respectively, comparable with commercial cells slightly damaged by etopside (CC1 with 31.4 + 2.9) with no statistical change in water or food intake, body weight or liver or kidney weights. The key result was that a 45-day recovery period had no observable difference in the DNA damage as assessed by DNA % in comet tail with tap water controls and CCO control cells at 7.0 + 0.7 and 9.7 + 1.2 versus 27.5 + 1.9, 29.3 + 2.2 and 32.0 + 1.8, respectively, for carbendazim, thiram and combination treatments. It is of concern that the use of these agents in developing countries with little training or regulation results in water pollution that may cause significant persistent DNA damage in animal or human populations that may not be subject to repair.

Role of aneuploidy in the carcinogenic process: Part 3 of the report of the 2017 IWGT workgroup on assessing the risk of aneugens for carcinogenesis and hereditary diseases

Aneuploidy is regarded as a hallmark of cancer, however, its role is complex with both pro- and anti-carcinogenic effects evident. In this IWGT review, we consider the role of aneuploidy in cancer biology; cancer risk associated with constitutive aneuploidy; rodent carcinogenesis with known chemical aneugens; and chemotherapy-related malignant neoplasms. Aneuploidy is seen at various stages in carcinogenesis. However, the relationship between induced aneuploidy occurring after exposure and clonal aneuploidy present in tumours is not clear. Recent evidence indicates that the induction of chromosomal instability (CIN), may be more important than aneuploidy per se, in the carcinogenic process. Down Syndrome, trisomy 21, is associated with altered hematopoiesis in utero which, in combination with subsequent mutations, results in an increased risk for acute megakaryoblastic and lymphoblastic leukemias. In contrast, there is reduced cancer risk for most solid tumours in Down Syndrome. Mouse models with high levels of aneuploidy are also associated with increased cancer risk for particular tumours with long latencies, but paradoxically other types of tumour often show decreased incidence. The aneugens reviewed that induce cancer in humans and animals all possess other carcinogenic properties, such as mutagenicity, clastogenicity, cytotoxicity, organ toxicities, hormonal and epigenetic changes which likely account for, or interact with aneuploidy, to cause carcinogenesis. Although the role that aneuploidy plays in carcinogenesis has not been fully established, in many cases, it may not play a primary causative role. Tubulin-disrupting aneugens that do not possess other properties linked to carcinogenesis, were not carcinogenic in rodents. Similarly, in humans, for the tubulin-disrupting aneugens colchicine and albendazole, there is no reported association with increased cancer risk. There is a need for further mechanistic studies on agents that induce aneuploidy, particularly by mechanisms other than tubulin disruption and to determine the role of aneuploidy in pre-neoplastic events and in early and late stage neoplasia.

Targets and mechanisms of chemically induced aneuploidy. Part 1 of the report of the 2017 IWGT workgroup on assessing the risk of aneugens for carcinogenesis and hereditary diseases

An aneuploidy workgroup was established as part of the 7th International Workshops on Genotoxicity Testing. The workgroup conducted a review of the scientific literature on the biological mechanisms of aneuploidy in mammalian cells and methods used to detect chemical aneugens. In addition, the current regulatory framework was discussed, with the objective to arrive at consensus statements on the ramifications of exposure to chemical aneugens for human health risk assessment. As part of these efforts, the workgroup explored the use of adverse outcome pathways (AOPs) to document mechanisms of chemically induced aneuploidy in mammalian somatic cells. The group worked on two molecular initiating events (MIEs), tubulin binding and binding to the catalytic domain of aurora kinase B, which result in several adverse outcomes, including aneuploidy. The workgroup agreed that the AOP framework provides a useful approach to link evidence for MIEs with aneuploidy on a cellular level. The evidence linking chemically induced aneuploidy with carcinogenicity and hereditary disease was also reviewed and is presented in two companion papers. In addition, the group came to the consensus that the current regulatory test batteries, while not ideal, are sufficient for the identification of aneugens and human risk assessment. While it is obvious that there are many different MIEs that could lead to the induction of aneuploidy, the most commonly observed mechanisms involving chemical aneugens are related to tubulin binding and, to a lesser extent, inhibition of mitotic kinases. The comprehensive review presented here should help with the identification and risk management of aneugenic agents.

Chromosome Missegregation and Aneuploidy Induction in Human Peripheral Blood Lymphocytes In vitro by Low Concentrations of Chlorpyrifos, Imidacloprid and α-Cypermethrin

Chlorpyrifos, imidacloprid, and α-cypermethrin are some of the most widely used insecticides in contemporary agriculture. However, their low-dose, nontarget genotoxic effects have not been extensively assayed. As one of the most relevant cancer biomarkers, we aimed to assess the aneuploidy due to chromosome missegregation during mitosis. To aim it we treated human lymphocytes in vitro with three concentrations of insecticides equivalents relevant for real scenario exposure assessed by regulatory agencies. We focused on chlorpyrifos as conventional and imidacloprid and α-cypermethrin as sustainable use insecticides. Cytokinesis-blocked micronucleus assay was performed coupled with fluorescence in situ hybridization (FISH) with directly labeled pancentromeric probes for chromosomes 9, 18, X and Y. None of the insecticides induced significant secondary DNA damage in terms of micronuclei (MN), nuclear buds (NB), or nucleoplasmic bridges (NPB). However, significant disbalances in chromosomes 9, 18, X and Y, and in insecticide-treated cells has been observed. According to recent studies, these disbalances in chromosome numbers may be atributted to defect sister chromatid cohesion which contribute to the increase of chromosome missegregation but not to micronuclei incidence. We conclude that tested insecticidal active substances exert chromosome missegregation effects at low concentrations, possibly by mechanism of sister chromatid cohesion. These findings may contribute to future risk assesments and understanding of insecticide mode of action on human genome. Environ. Mol. Mutagen. 60:72-84, 2019. © 2018 Wiley Periodicals, Inc.

Development of a data-processing method based on Bayesian k-means clustering to discriminate aneugens and clastogens in a high-content micronucleus assay

Genotoxicants can be identified as aneugens and clastogens through a micronucleus (MN) assay. The current high-content screening-based MN assays usually discriminate an aneugen from a clastogen based on only one parameter, such as the MN size, intensity, or morphology, which yields low accuracies (70-84%) because each of these parameters may contribute to the results. Therefore, the development of an algorithm that can synthesize high-dimensionality data to attain comparative results is important. To improve the automation and accuracy of detection using the current parameter-based mode of action (MoA), the MN MoA signatures of 20 chemicals were systematically recruited in this study to develop an algorithm. The results of the algorithm showed very good agreement (93.58%) between the prediction and reality, indicating that the proposed algorithm is a validated analytical platform for the rapid and objective acquisition of genotoxic MoA messages.

Genotoxicity of flubendazole and its metabolites in vitro and the impact of a new formulation on in vivo aneugenicity

The anti-parasitic benzimidazole flubendazole has been used for many years to treat intestinal infections in humans and animals. Previous genotoxicity studies have shown that the compound is not a bacterial mutagen and a bone marrow micronucleus test, using a formulation that limited systemic absorption, was negative. The purpose of this study is to explore the genotoxicity of flubendazole and its main metabolites in in vitro micronucleus studies and to test a new oral formulation that improves systemic absorption in an in vivo micronucleus test. The isolated metabolites were also screened using the Ames test for bacterial mutagenicity. It was found that flubendazole, like other chemically related benzimidazoles used in anti-parasitic therapies, is a potent aneugen in vitro The hydrolysed metabolite of flubendazole is negative in these tests, but the reduced metabolite (R- and S-forms) shows both aneugenic and clastogenic activity. However, in vitro micronucleus tests of flubendazole in the presence of rat liver S9 gave almost identical signals for aneugenicity as they did in the absence of S9, suggesting that any clastogenicity from the reduced metabolite is not sufficient to change the overall profile. Like flubendazole itself, both metabolites are negative in the Ames test. Analysis of dose-response curves from the in vitro tests, using recently developed point of departure approaches, demonstrate that the aneugenic potency of flubendazole is very similar to related anti-parasitic benzimidazoles, including albendazole, which is used in mass drug administration programmes to combat endemic filarial diseases. The in vivo micronucleus test of the new formulation of flubendazole also showed evidence of induced aneugenicity. Analysis of the in vivo data allowed a reference dose for aneugenicity to be established which can be compared with therapeutic exposures of flubendazole when this has been established. Analysis of the plasma from the animals used in the in vivo micronucleus test showed that there is increased exposure to flubendazole compared with previously tested formulations, as well as significant formation of the non-genotoxic hydrolysed metabolite of flubendazole and small levels of the reduced metabolite. In conclusion, this study shows that flubendazole is a potent aneugen in vitro with similar potency to chemically related benzimidazoles currently used as anti-parasitic therapies. The reduced metabolite also has aneugenic properties as well as clastogenic properties. Treatment with a new formulation of flubendazole that allows increased systemic exposure, compared with previously used formulations, also results in detectable aneugenicity in vivo. Based on the lack of carcinogenicity of this class of benzimidazoles and the intended short-term dosing, it is unlikely that flubendazole treatment will pose a carcinogenic risk to patients.

Induction of a whole chromosome loss by colcemid in human cells elucidated by discrimination between FISH signal overlap and chromosome loss

Aneuploidy is a change in the number of chromosomes and an essential component in tumorigenesis. Therefore, accurate and sensitive detection of aneuploidy is important in screening for carcinogens. In vitro micronucleus (MN) assay has been adopted in the recently revised International Conference on Harmonization of Technical Requirements for Registration of Pharmaceuticals for Human Use (ICH) S2 guideline and can be employed to predict both clastogenic and aneugenic chromosomal aberrations in interphase cells. However, distinguishing clastogens and aneugens is not possible using this assay. The Organization for Economic Co-operation and Development (OECD) guideline TG487 therefore recommends the use of centromere/kinetochore staining in micronuclei to differentiate clastogens from aneugens. Here, we analyzed numerical changes of a specific chromosome in cytokinesis-blocked binucleated cells by fluorescence in situ hybridization (FISH) using the specific centromere probe in human lymphoblastoid TK6 cells treated with aneugens (colcemid and vincristine) or clastogens (methyl methanesulfonate [MMS] and 4-nitroquinoline-1-oxide [4-NQO]). Colcemid and vincristine significantly increased the frequencies of nondisjunction and loss of FISH signals, while MMS and 4-NQO slightly increased only the frequency of loss of FISH signals. The loss of FISH signals of a specific chromosome from two to one per nucleus implies either a loss of a whole chromosome or an overlap of two signals. To distinguish a chromosome loss from signal overlap, we investigated the number of FISH signals and the fluorescent intensity of each signal per nucleus using a probe specific for whole chromosome 2 in binucleated TK6 cells and primary human lymphocytes treated with colcemid and MMS. By discriminating between chromosome loss and FISH signal overlap, we revealed that colcemid, but not MMS, induced a loss of a whole chromosome in primary lymphocytes and TK6 cells.

Seven benzimidazole pesticides combined at sub-threshold levels induce micronuclei in vitro

Benzimidazoles act by disrupting microtubule polymerisation and are capable of inducing the formation of micronuclei. Considering the similarities in their mechanisms of action (inhibition of microtubule assembly by binding to the colchicine-binding site on tubulin monomers), combination effects according to the principles of concentration addition might occur. If so, it is to be expected that several benzimidazoles contribute to micronucleus formation even when each single one is present at or below threshold levels. This would have profound implications for risk assessment, but the idea has never been tested rigorously. To fill this gap, we analysed micronucleus frequencies for seven benzimidazoles, including the fungicide benomyl, its metabolite carbendazim, the anthelmintics albendazole, albendazole oxide, flubendazole, mebendazole and oxibendazole. Thiabendazole was also tested but was inactive. We used the cytochalasin-blocked micronucleus assay with CHO-K1 cells according to OECD guidelines, and employed an automated micronucleus scoring system based on image analysis to establish quantitative concentration–response relationships for the seven active benzimidazoles. Based on this information, we predicted additive combination effects for a mixture of the seven benzimidazoles by using the concepts of concentration addition and independent action. The observed effects of the mixture agreed very well with those predicted by concentration addition. Independent action underestimated the observed combined effects by a large margin. With a mixture that combined all benzimidazoles at their estimated threshold concentrations for micronucleus induction, micronucleus frequencies of ~15.5% were observed, correctly anticipated by concentration addition. On the basis of independent action, this mixture was expected to produce no effects. Our data provide convincing evidence that concentration addition is applicable to combinations of benzimidazoles that form micronuclei by disrupting microtubule polymerisation. They present a rationale for grouping these chemicals together for the purpose of cumulative risk assessment.

A study of proteotoxicity and genotoxicity induced by the pesticide and fungicide on marine invertebrate (Donax faba)

Chlorpyrifos (CPF) and carbendazim (CBZ) are widely used pesticide and fungicide in India. The 96 h LC(50) values of chlorpyrifos and carbendazim for the marine invertebrate Donax faba, were 247.72 μg L(-1) and 200.82 μg L(-1) respectively. CBZ was found to accumulate less than CPF. Gill, body and foot tissues were used as target organs in biomarker and genotoxic studies. The results showed that treatment with chlorpyrifos and carbendazim increased the MDA levels, decreased the GSH levels and changed the total protein, SOD, CAT, GPx and AChE activities in all the tissues compared to those of the control (p<0.05). These results suggest that lipid peroxidation can be a possible pathway for the toxicity of chlorpyrifos and carbendazim. The antioxidant enzyme levels decrease as a result of the consumption of enzymes to neutralize free radicals generated by CPF and CBZ. Comet tail was observed in the first three doses, in all the tissues, of which the mean tail length differed significantly (p<0.01) from those of the unexposed ones. D. faba can be used as an indicator organism to assess the genotoxic risks of chemical contamination in the marine environment using comet assay. Theses biomarkers can be used further to know these agrochemicals impact on coastal marine bivalves.

Follow-up actions from positive results of in vitro genetic toxicity testing

Appropriate follow-up actions and decisions are needed when evaluating and interpreting clear positive results obtained in the in vitro assays used in the initial genotoxicity screening battery (i.e., the battery of tests generally required by regulatory authorities) to assist in overall risk-based decision making concerning the potential effects of human exposure to the agent under test. Over the past few years, the International Life Sciences Institute (ILSI) Health and Environmental Sciences Institute (HESI) Project Committee on the Relevance and Follow-up of Positive Results in In Vitro Genetic Toxicity (IVGT) Testing developed a decision process flow chart to be applied in case of clear positive results in vitro. It provides for a variety of different possibilities and allows flexibility in choosing follow-up action(s), depending on the results obtained in the initial battery of assays and available information. The intent of the Review Subgroup was not to provide a prescriptive testing strategy, but rather to reinforce the concept of weighing the totality of the evidence. The Review Subgroup of the IVGT committee highlighted the importance of properly analyzing the existing data, and considering potential confounding factors (e.g., possible interactions with the test systems, presence of impurities, irrelevant metabolism), and chemical modes of action when analyzing and interpreting positive results in the in vitro genotoxicity assays and determining appropriate follow-up testing. The Review Subgroup also examined the characteristics, strengths, and limitations of each of the existing in vitro and in vivo genotoxicity assays to determine their usefulness in any follow-up testing.

Comparison of four different treatment conditions of extended exposure in the in vitro micronucleus assay using TK6 lymphoblastoid cells

In the OECD Guideline 487, a total of four extended exposure treatment conditions are proposed for the in vitro micronucleus (MNvit) assay in the presence and absence of a cytokinesis block and with or without a recovery period. This guideline also states that rodent cell lines and human lymphocytes can be used as shown by many validated studies but that human cell lines such as TK6 and HepG2 are not yet validated. In this present study each extended exposure condition was characterized by investigation using TK6 cells and nine chemicals known to be able to induce micronucleus (MN) in rodent cell lines. The results revealed two concerns: six chemicals did not show significant MN induction in the 'cytokinesis block without recovery period'; two aneugens showed no dose-dependent cytotoxicity in the 'cytokinesis block with recovery period'. Further investigation revealed that 3-4 times higher spontaneous MN frequency than that in the other conditions is a possible reason for the low sensitivity, and this high spontaneous MN frequency was not observed in Chinese hamster lung cells under the identical treatment condition. With regard to the two conditions without cytokinesis block, two negative substances were evaluated and found to be negative, suggesting the validity of the TK6 test system for these conditions. Although our findings showed a few concerns for the treatment with cytokinesis block, the TK6 cells were considered to be a reliable cell line to be used for detecting potential inducers of MN in the in vitro micronucleus assay based on the overall results.

Determination of DNA damage in floriculturists exposed to mixtures of pesticides

The aim of the study was to determine possible DNA damage in floriculturists chronically exposed to pesticides. Leukocytes from 52 workers, 46 environmentally exposed, and 38 control individuals were evaluated with the comet assay. Serum from all individuals was also analyzed for pesticides using gas chromatography coupled to mass spectrometry. A statistically significant difference in DNA fragmentation in the pesticide exposed group compared to the other two groups (P < .001) was found. No differences between environmentally exposed and control individuals were detected. The statistical analysis showed no significant correlation between DNA damage and sex, age, drinking or smoking habits, as well as years of exposure. One or more pesticides were detected in 50% of the floriculturists, while in the rest of the individuals, a chemical related with the preparation of pesticides, such as additives, plasticizers, or solvents, was found. Our study shows that chronic exposure to pesticides produces DNA damage in floriculturists. It also suggests that this type of monitoring could be valuable in recommending preventive measures.

Evaluation of high-throughput genotoxicity assays used in profiling the US EPA ToxCast chemicals

Three high-throughput screening (HTS) genotoxicity assays-GreenScreen HC GADD45a-GFP (Gentronix Ltd.), CellCiphr p53 (Cellumen Inc.) and CellSensor p53RE-bla (Invitrogen Corp.)-were used to analyze the collection of 320 predominantly pesticide active compounds being tested in Phase I of US. Environmental Protection Agency's ToxCast research project. Between 9% and 12% of compounds were positive for genotoxicity in the assays. However, results of the varied tests only partially overlapped, suggesting a strategy of combining data from a battery of assays. The HTS results were compared to mutagenicity (Ames) and animal tumorigenicity data. Overall, the HTS assays demonstrated low sensitivity for rodent tumorigens, likely due to: screening at a low concentration, coverage of selected genotoxic mechanisms, lack of metabolic activation and difficulty detecting non-genotoxic carcinogens. Conversely, HTS results demonstrated high specificity, >88%. Overall concordance of the HTS assays with tumorigenicity data was low, around 50% for all tumorigens, but increased to 74-78% (vs. 60% for Ames) for those compounds producing tumors in rodents at multiple sites and, thus, more likely genotoxic carcinogens. The aim of the present study was to evaluate the utility of HTS assays to identify potential genotoxicity hazard in the larger context of the ToxCast project, to aid prioritization of environmentally relevant chemicals for further testing and assessment of carcinogenicity risk to humans.

In vitro genotoxic assessment of xenobiotic diacylglycerols in an in vitro micronucleus assay

Xenobiotic diacylglycerols (DG) may induce pathological disorders by causing abnormal chromosomal segregation, which could be aneuploid. In this study, seven xenobiotic-diacylglycerols (four of drug origin and three of pesticide origin) were evaluated for their ability to induce aneuploidy in mammalian cultures using in vitro cytokinesis blocked micronucleus (CBMN) assay coupled with kinetochore labeling and interphase fluorescent in situ hybridization. Out of seven xeno-DGs, two (ibuprofen-DG and fenbufen-DG) induced statistically significant (P < 0.001) and dose-dependent increase in micronucleus induction, but this apparent micronucleus induction was very weak in case of fenbufen-DG. These MN were produced predominantly by aneugenic and clastogenic mechanisms, respectively, confirmed by immunofluorescent labeling of kinetochores. Fluorescent in situ hybridization analysis revealed that ibuprofen-DG induced significantly higher nondisjunction for chromosomes 10, 17, and 18. Other xenobiotic diacylglycerols (indomethacin-DG, salicylic acid-DG, 4-(2-methyl-4-chlorophenoxy) butanoic acid-DG (MCPB-DG), 2-(2-methyl-4-chlorophenoxy) propanoic acid-DG (MCPP-DG) and 2-(4-dichlorophenoxy)-butanoic acid-DG (2,4 DB-DG) did not induce micronuclei, but the concentrations tested did not reach levels that caused the marked growth suppression typically required for testing for regulatory testing purposes. However, the levels of growth suppression achieved were similar to that seen with ibuprofen-DG, which was positive. This study shows that xeno-DGs, which have been neglected in the past for their possible link to any pathological disorders, need serious assessment of their mutagenic potential.

Simultaneous Confidence Bands for Abbott-Adjusted Quantal Response Models

We study use of a Scheffé-style simultaneous confidence band as applied to low-dose risk estimation with quantal response data. We consider two formulations for the dose-response risk function, an Abbott-adjusted Weibull model and an Abbott-adjusted log-logistic model. Using the simultaneous construction, we derive methods for estimating upper confidence limits on predicted extra risk and, by inverting the upper bands on risk, lower bounds on the benchmark dose, or BMD, at which a specific level of 'benchmark risk' is attained. Monte Carlo evaluations explore the operating characteristics of the simultaneous limits.

In vitro micronucleus assay scored by flow cytometry provides a comprehensive evaluation of cytogenetic damage and cytotoxicity

This laboratory has previously reported on the development of a flow cytometry-based method for scoring in vitro micronuclei in mouse lymphoma (L5178Y) cells [S.L. Avlasevich, S.M. Bryce, S.E. Cairns, S.D. Dertinger, In vitro micronucleus scoring by flow cytometry: differential staining of micronuclei versus apoptotic and necrotic chromatin enhances assay reliability, Environ. Molec. Mutagen. 47 (2006) 56-66]. With this method, necrotic and mid/late stage apoptotic cells are labeled with the fluorescent dye ethidium monoazide. Cells are then washed, stripped of their cytoplasmic membranes, and incubated with RNase plus a pan-nucleic acid dye (SYTOX Green). This process provides a suspension of free nuclei and micronuclei that are differentially stained relative to chromatin associated with dead/dying cells. The current report extends this line of investigation to include the human cell line TK6. Additionally, methods are described that facilitate simultaneous quantitative analysis of cytotoxicity, perturbations to the cell cycle, and what we hypothesize is aneuploidization. This comprehensive cytogenetic damage assay was evaluated with the following diverse agents: etoposide, ionizing radiation, methyl methanesulfonate, vinblastine, ethanol, and staurosporine. Cells were harvested after 30h of continuous treatment (in the case of chemicals), or following graded doses of radiation up to 1Gy. Key findings include the following: (1) Significant discrepancies in top dose selection were found for five of the six agents studied when relative survival measurements were based on Coulter counting versus flow cytometry. (2) Both microscopy- and flow cytometry-based scoring methods detected dose-dependent micronucleus formation for the four genotoxic agents studied, whereas no significant increases were observed for the presumed non-genotoxicants ethanol and staurosporine when top dose selection was based on flow cytometric indices of cytotoxicity. (3) SYTOX and ethidium monoazide fluorescence signals conveyed cell cycle and cell death information, respectively, and appear to represent valuable aids for interpreting micronucleus data. (4) The frequency of hypodiploid nuclei increased in response to each of the genotoxic agents studied, but not following exposure to ethanol or staurosporine. Collectively, these results indicate that a comprehensive assessment of genotoxicity and other test article-induced toxicities can be acquired simultaneously using a simple two-color flow cytometry-based technique.

In vitro approaches to develop weight of evidence (WoE) and mode of action (MoA) discussions with positive in vitro genotoxicity results

A recent analysis by Kirkland et al. [Kirkland, D., Aardema, M., Henderson, L. and Müller, L. (2005) Evaluation of the ability of a battery of 3 in vitro genotoxicity tests to discriminate rodent carcinogens and non-carcinogens. I. Sensitivity, specificity and relative predictivity. Mutat. Res. 584, 1-256] demonstrated an extremely high false positive rate for in vitro genotoxicity tests when compared with carcinogenicity in rodents. In many industries, decisions have to be made on the safety of new substances, and health risk to humans, without rodent carcinogenicity data being available. In such cases, the usual way to determine whether a positive in vitro genotoxicity result is relevant (i.e. indicates a hazard) for humans is to develop weight of evidence (WoE) or mode of action (MoA) arguments. These are based partly on further in vitro investigations, but usually rely heavily on tests for genotoxicity in one or more in vivo assays. However, for certain product types in the European Union, the use of animals for genotoxicity testing (as well as for other endpoints) will be prohibited within the next few years. Many different examples have been described that indicate DNA damage and genotoxic responses in vitro can arise through non-relevant in vitro events that are a result of the test systems and conditions used. The majority of these non-relevant in vitro events can be grouped under a category of 'overload of normal physiology' that would not be expected to occur in exposed humans. However, obtaining evidence in support of such MoAs is not easy, particularly for those industries prohibited from carrying out in vivo testing. It will become necessary to focus on in vitro studies to provide evidence of non-DNA, threshold or in vitro-specific processes and to discuss the potential for such genotoxic effects to occur in exposed humans. Toward this end, we surveyed the published literature for in vitro approaches that may be followed to determine whether a genotoxic effect observed in vitro will occur in humans. Unfortunately, many of the approaches we found are based on only a few published examples and validated approaches with consensus recommendations often do not exist. This analysis highlights the urgent need for developing consensus approaches that do not rely on animal studies for dealing with in vitro genotoxins.

Non-disjunction events induced by albendazole in human cells

Albendazole (ABZ), a benzimidazole carbamate used for the treatment of several human helminthiases has high affinity for tubulin, which results in an inhibition of microtubule polymerization, blocking several vital processes in the parasites, such as motility and nutrient uptake. The ability of ABZ to act as mitotic spindle poison leads to a potential risk for aneuploidy induction in exposed human beings. ABZ, as well as albendazole sulphoxide (ABZSO), its main metabolite, induce micronuclei in human cells in a dose-dependent manner. Despite recognition that ABZ and ABZSO increase micronucleus frequency, their potential as inducers of non-disjunction in human cells, an event considered more frequent than chromosome loss, and one of the main mechanisms involved in aneuploidy induction, has not been evaluated. In the present work, we investigated the ability of ABZ and ABZSO to induce non-disjunction in cultured human lymphocytes. Non-disjunction was scored by chromosome-specific FISH using a classical or alpha satellite probe for chromosomes 1 and 7, respectively. Significant increase in non-disjunction events that involved either chromosome were observed in cells treated with ABZ or ABZSO. Both ABZ and ABZSO induced non-disjunction at lower concentrations than those at which MN were observed.

The use of the in vitro micronucleus assay to detect and assess the aneugenic activity of chemicals

The successful validation of the in vitro micronucleus assay by the SFTG now provides the opportunity for this highly cost effective assay to be used to screen chemicals for their ability to induce both structural (clastogenic) and numerical (aneugenic) chromosome changes using interphase cells. The use of interphase cells and a relatively simple experimental protocol provides the opportunity to greatly increase the statistical power of cytogenetic studies on chemical interactions. The application of molecular probes capable of detecting kinetochores and centromeres provides the opportunity to classify mechanisms of micronucleus induction into those which are primarily due to chromosome loss or breakage. When a predominant mechanism of micronucleus induction has been shown to be based upon chromosome loss then further investigation can involve the determination of the role of non-disjunction in the induction of aneuploidy. The binucleate cell modification of the in vitro micronucleus assay can be combined with the use of chromosome specific centromere probes to determine the segregation of individual chromosomes into daughter nuclei. The combination of these methods provides us with powerful tools for the investigation of mechanisms of genotoxicity particularly in the low dose regions.

Do dose response thresholds exist for genotoxic alkylating agents?

The demonstration and acceptance of dose response thresholds for genotoxins may have substantial implications for the setting of safe exposure levels. Here we test the hypothesis that direct-acting DNA reactive agents may exhibit thresholded dose responses. We examine the potential mechanisms involved in such thresholded responses, particularly in relation to those of alkylating agents. As alkylating agents are representative model DNA reactive compounds with well characterized activities and DNA targets, they could help shed light on the general mechanisms involved in thresholded dose responses for genotoxins. Presently, thresholds have mainly been described for agents with non-DNA targets. We pay particular attention here to the contribution of DNA repair to genotoxic thresholds. A review of the literature shows that limited threshold data for alkylating agents are currently available, but the contribution of DNA repair in thresholded dose responses is suggested by several studies. The existence of genotoxic thresholds for alkylating agents methylmethanesulfonate is also supported here by data from our laboratory. Overall, it is clear that different endpoints induced by the same alkylator, can possess different dose response characteristics. This may have an impact on the setting of safe exposure levels for such agents. The limited information available concerning the dose response relationships of alkylators can nevertheless lead to the design of experiments to investigate the mechanisms that may be involved in threshold responses. Through using paired alkylators inducing different lesions, repaired by different pathways, insights into the processes involved in genotoxic thresholds may be elucidated. Furthermore, as alkyl-guanine-DNA transferase, base excision repair and mismatch repair appear to contribute to genotoxic thresholds for alkylators, cells deficient in these repair processes may possess altered dose responses compared with wild-type cells and this approach may help understand the contribution of these repair pathways to the production of thresholds for genotoxic effects in general. Finally, genotoxic thresholds are currently being described for acute exposures to single agents in vitro, however, dose response data for chronic exposures to complex mixtures are, as yet, a long way off.

Identification of a gene expression profile that discriminates indirect-acting genotoxins from direct-acting genotoxins

During the safety evaluation process of new drugs and chemicals, a battery of genotoxicity tests is conducted starting with in vitro genotoxicity assays. Obtaining positive results in in vitro genotoxicity tests is not uncommon. Follow-up studies to determine the biological relevance of positive genotoxicity results are costly, time consuming, and utilize animals. More efficient methods, especially for identifying a putative mode of action like an indirect mechanism of genotoxicity (where DNA molecules are not the initial primary targets), would greatly improve the risk assessment for genotoxins. To this end, we are participating in an International Life Sciences Institute (ILSI) project involving studies of gene expression changes caused by model genotoxins. The purpose of the work is to evaluate gene expression tools in general, and specifically for discriminating genotoxins that are direct-acting from indirect-acting. Our lab has evaluated gene expression changes as well as micronuclei (MN) in L5178Y TK(+/-) mouse lymphoma cells treated with six compounds. Direct-acting genotoxins (where DNA is the initial primary target) that were evaluated included the DNA crosslinking agents, mitomycin C (MMC) and cisplatin (CIS), and an alkylating agent, methyl methanesulfonate (MMS). Indirect-acting genotoxins included hydroxyurea (HU), a ribonucleotide reductase inhibitor, taxol (TXL), a microtubule inhibitor, and etoposide (ETOP), a DNA topoisomerase II inhibitor. Microarray gene expression analysis was conducted using Affymetrix mouse oligonucleotide arrays on RNA samples derived from cells which were harvested immediately after the 4 h chemical treatment, and 20 h after the 4 h chemical treatment. The evaluation of these experimental results yields evidence of differentially regulated genes at both 4 and 24 h time points that appear to have discriminating power for direct versus indirect genotoxins, and therefore may serve as a fingerprint for classifying chemicals when their mechanism of action is unknown.

Cytogenetic biomonitoring of a floriculturist population in Italy: micronucleus analysis by fluorescence in situ hybridization (FISH) with an all-chromosome centromeric probe

Flower production in greenhouses associated with a heavy use of pesticides is very wide-spread in the western part of the Ligurian region (Italy). The formation of micronuclei in peripheral blood lymphocytes is a valuable cytogenetic biomarker in human populations occupationally exposed to genotoxic compounds. In the present study we investigated the micronucleus frequency in peripheral blood lymphocytes of 52 floriculturists and 24 control subjects by use of the cytokinesis-block methodology associated with fluorescence in situ hybridization with a pan-centromeric probe that allowed to distinguish centromere-positive (C+) and centromere-negative (C-) micronuclei. The comparison between floriculturists and controls did not reveal any statistically significant difference in micronucleus frequency, although an increase was observed with increasing pesticide use, number of genotoxic pesticides used and duration of exposure. An increase in C+ as well as in C- micronuclei and in the percentage of C+ micronuclei with respect to the total number of micronuclei was detected in floriculturists, suggesting a higher contribution of C+ micronuclei in the total number scored. The percentage C+ micronuclei was not related to the duration of exposure or to the number of genotoxic pesticides used, but a higher percentage (66.52% versus 63.78%) was observed in a subgroup of subjects using benzimidazolic compounds, compared with the floriculturist population exposed to a complex pesticide mixture not including benzimidazolics. These results suggest a potential human hazard associated with the exposure to this class of aneuploidy-inducing carcinogens.

Effect of the dithiocarbamate pesticide zineb and its commercial formulation, the azzurro. V. Abnormalities induced in the spindle apparatus of transformed and non-transformed mammalian cell lines

Abnormalities induced in the mitotic spindle by zineb and azzurro (1.0-25.0 micro g/ml, 24h) were evaluated in Chinese hamster ovary (CHO) and HeLa cells, and in non-transformed human fibroblasts (NTHF). Spindles were stained with FITC-conjugated anti-beta tubulin. Treatment with 10.0 micro g/ml of zineb induced complete inhibition of cell viability in NTHF cells while 10.0 micro g/ml of azzurro decreased cell growth down to 62%. Higher doses of both compounds induced cell death. In HeLa and CHO cells, 15.0 micro g/ml of zineb and 10.0-15.0 micro g/ml of azzurro decreased viability, whereas 25.0 micro g/ml of both compounds was cytotoxic. A significantly decreased mitotic index (MI) was observed in NTHF treated with 5.0 micro g/ml zineb or azzurro, whereas 10.0 micro g/ml of both chemicals were necessary to induce the same phenomenon in HeLa and CHO cells. Treatment with 1.0-5.0 micro g/ml of zineb or azzurro induced a dose-dependent increase of degenerated spindles in NTHF and the number of degenerated or multipolar spindles in HeLa and CHO cells increased in a dose-dependent manner with 1.0-10.0 micro g/ml zineb and azzurro. Although zineb and azzurro were able to induce mitotic spindle abnormalities in all cell types, non-transformed cells were less resistant than immortalized cells.

Regulatory recognition of indirect genotoxicity mechanisms in the European Union

The European Union (EU) system for the regulation of chemicals includes approval systems for pharmaceuticals, pesticides and biocides, requirements for hazard classification and for risk assessment of industrial chemicals. Regulators have traditionally used the commonly accepted categorisation of chemicals into genotoxic (DNA-reactive) or non-genotoxic agents in their decision-making processes, and have generally considered that there is no threshold level for the former group. The recognition that a number of genotoxic agents operate by indirect genotoxicity mechanisms such as induction of aneuploidy, oxidative stress, inhibition of DNA synthesis or cytotoxicity presents new problems for the regulator. The dose-response relationship for a number of such agents is generally accepted to show a threshold, however, the degree of acceptance of the threshold effect differs in different EU regulatory systems. The classification system for mutagens is based primarily on intrinsic hazard rather than risk, and the classification criteria do not allow for a less stringent classification for chemicals operating by a threshold mechanism. In contrast, regulatory approval systems for plant protection products and therapeutic agents are based on a risk assessment approach, in which a demonstrated threshold effect for a genotoxic agent is likely to be an important factor in reaching a decision concerning authorisation of the product.

Reflections on the implications of thresholds of mutagenic activity for the labelling of chemicals by the European Union

During the course of the safety evaluation and regulatory control of chemicals it is important to distinguish between "potential hazard" and "actual risk" of exposure to toxins. In the case of DNA reactive chemicals, it has been prudent to assume that hazard is expressed as risk at low exposure concentrations. However, analysis of the dose-response relationships of both DNA reactive and non-DNA reactive genotoxins (e.g., aneugens) indicate that there are exposure concentrations below which protective mechanisms such as DNA repair activity and the presence of multiple targets may lead to the prediction of no risk until threshold concentrations are achieved. Current European Union management procedures for mutagenic chemicals are based predominantly upon hazard assessment rather than assessment of actual risk under likely exposure scenarios. As our knowledge of protective mechanisms increases, the time is now appropriate to undertake a re-evaluation of European Union criteria and to base the clarification mutagenic chemical more firmly upon the basis of actual risks to the human population and to the environment.