Evaluation of the L5178Y mouse lymphoma cell mutagenesis assay: interlaboratory reproducibility and assessment

Statistics of Interlaboratory In Vitro Toxicological Studies

Interlaboratory studies are common in toxicology, particularly for the introduction of alternative assays. Numerous papers are available on the statistical analysis of interlaboratory studies, but these deal primarily with the case of a replicated single sample studied in several laboratories. This approach can be used for some assays, but for the majority, the results will be unsatisfactory, i.e. involving great variability between both the dose groups and the laboratories. However, the primary objective of toxicological assays is to achieve similarity between the sizes of effects, rather than to determine absolute values. In the parametric model, the sizes of effects are the studentised differences from the negative control or, for the commonly used dose-response designs, the similarity of the slopes of the dose-response curves. Standard approaches for the estimation of intralaboratory and interlaboratory variability, including Mandel plots, are introduced, and new approaches are presented for demonstrating similarity of effect sizes, with or without assuming a dose-response model. One approach is based on a modification of the parallel-line assay, the other is based on a modification of the interaction contrasts of the analysis of variance. SAS programs are given for all approaches, and real data from an interlaboratory immunotoxicological study are analysed as a demonstration.

The mouse lymphoma assay

The mouse lymphoma TK assay (MLA) is part of an in vitro battery of tests designed to predict risk assessment prior to in vivo testing. The test has the potential to detect mutagenic and clastogenic events at the thymidine kinase (tk) locus of L5178Y mouse lymphoma tk ( +/- ) cells by measuring resistance to the lethal nucleoside analogue triflurothymidine (TFT). Cells may be plated for viability and mutation in semi-solid agar (agar assay) or in 96-well microtitre plates (microwell assay). When added to selective medium containing TFT, wild-type tk ( +/- ) cells die, but TFT cannot be incorporated into the DNA of mutant tk ( -/- ) cells, which survive to form colonies that may be large (indicative of gene mutation) or small (indicative of chromosomal mutation) in nature. Mutant frequency is expressed as the number of mutants per 10(6) viable cells.

A core in vitro genotoxicity battery comprising the Ames test plus the in vitro micronucleus test is sufficient to detect rodent carcinogens and in vivo genotoxins

In vitro genotoxicity testing needs to include tests in both bacterial and mammalian cells, and be able to detect gene mutations, chromosomal damage and aneuploidy. This may be achieved by a combination of the Ames test (detects gene mutations) and the in vitro micronucleus test (MNvit), since the latter detects both chromosomal aberrations and aneuploidy. In this paper we therefore present an analysis of an existing database of rodent carcinogens and a new database of in vivo genotoxins in terms of the in vitro genotoxicity tests needed to detect their in vivo activity. Published in vitro data from at least one test system (most were from the Ames test) were available for 557 carcinogens and 405 in vivo genotoxins. Because there are fewer publications on the MNvit than for other mammalian cell tests, and because the concordance between the MNvit and the in vitro chromosomal aberration (CAvit) test is so high for clastogenic activity, positive results in the CAvit test were taken as indicative of a positive result in the MNvit where there were no, or only inadequate data for the latter. Also, because Hprt and Tk loci both detect gene-mutation activity, a positive Hprt test was taken as indicative of a mouse-lymphoma Tk assay (MLA)-positive, where there were no data for the latter. Almost all of the 962 rodent carcinogens and in vivo genotoxins were detected by an in vitro battery comprising Ames+MNvit. An additional 11 carcinogens and six in vivo genotoxins would apparently be detected by the MLA, but many of these had not been tested in the MNvit or CAvit tests. Only four chemicals emerge as potentially being more readily detected in MLA than in Ames+MNvit--benzyl acetate, toluene, morphine and thiabendazole--and none of these are convincing cases to argue for the inclusion of the MLA in addition to Ames+MNvit. Thus, there is no convincing evidence that any genotoxic rodent carcinogens or in vivo genotoxins would remain undetected in an in vitro test battery consisting of Ames+MNvit.

Mutant spectra of irradiated CHO AL cells determined with multiple markers analyzed by flow cytometry

We have previously developed a sensitive and rapid mammalian cell mutation assay which is based on a Chinese hamster ovary cell line that stably incorporates human chromosome 11 (CHO A(L)) and uses flow cytometry to measure mutations in CD59. We now show that multiparameter flow cytometry may be used to simultaneously analyze irradiated CHO A(L) cells for mutations in five CD genes along chromosome 11 (CD59, CD44, CD90, CD98, CD151) and also a GPI-anchor gene. Using this approach, 19 different mutant clones derived from individual sorted mutant cells were analyzed to determine the mutant spectrum induced by ionizing radiation. All clones analyzed were negative for CD59 expression and PCR confirmed that at least CD59 exon 4 was also absent. As expected, ionizing radiation frequently caused large deletions along chromosome 11. This technology can readily be used to rapidly analyze the mutant yield as well as the spectrum of mutations caused by a variety of genotoxic agents and provide greater insight into the mechanisms of mutagenesis.

Evaluation of the ability of a battery of three in vitro genotoxicity tests to discriminate rodent carcinogens and non-carcinogens I. Sensitivity, specificity and relative predictivity

The performance of a battery of three of the most commonly used in vitro genotoxicity tests--Ames+mouse lymphoma assay (MLA)+in vitro micronucleus (MN) or chromosomal aberrations (CA) test--has been evaluated for its ability to discriminate rodent carcinogens and non-carcinogens, from a large database of over 700 chemicals compiled from the CPDB ("Gold"), NTP, IARC and other publications. We re-evaluated many (113 MLA and 30 CA) previously published genotoxicity results in order to categorise the performance of these assays using the response categories we established. The sensitivity of the three-test battery was high. Of the 553 carcinogens for which there were valid genotoxicity data, 93% of the rodent carcinogens evaluated in at least one assay gave positive results in at least one of the three tests. Combinations of two and three test systems had greater sensitivity than individual tests resulting in sensitivities of around 90% or more, depending on test combination. Only 19 carcinogens (out of 206 tested in all three tests, considering CA and MN as alternatives) gave consistently negative results in a full three-test battery. Most were either carcinogenic via a non-genotoxic mechanism (liver enzyme inducers, peroxisome proliferators, hormonal carcinogens) considered not necessarily relevant for humans, or were extremely weak (presumed) genotoxic carcinogens (e.g. N-nitrosodiphenylamine). Two carcinogens (5-chloro-o-toluidine, 1,1,2,2-tetrachloroethane) may have a genotoxic element to their carcinogenicity and may have been expected to produce positive results somewhere in the battery. We identified 183 chemicals that were non-carcinogenic after testing in both male and female rats and mice. There were genotoxicity data on 177 of these. The specificity of the Ames test was reasonable (73.9%), but all mammalian cell tests had very low specificity (i.e. below 45%), and this declined to extremely low levels in combinations of two and three test systems. When all three tests were performed, 75-95% of non-carcinogens gave positive (i.e. false positive) results in at least one test in the battery. The extremely low specificity highlights the importance of understanding the mechanism by which genotoxicity may be induced (whether it is relevant for the whole animal or human) and using weight of evidence approaches to assess the carcinogenic risk from a positive genotoxicity signal. It also highlights deficiencies in the current prediction from and understanding of such in vitro results for the in vivo situation. It may even signal the need for either a reassessment of the conditions and criteria for positive results (cytotoxicity, solubility, etc.) or the development and use of a completely new set of in vitro tests (e.g. mutation in transgenic cell lines, systems with inherent metabolic activity avoiding the use of S9, measurement of genetic changes in more cancer-relevant genes or hotspots of genes, etc.). It was very difficult to assess the performance of the in vitro MN test, particularly in combination with other assays, because the published database for this assay is relatively small at this time. The specificity values for the in vitro MN assay may improve if data from a larger proportion of the known non-carcinogens becomes available, and a larger published database of results with the MN assay is urgently needed if this test is to be appreciated for regulatory use. However, specificity levels of <50% will still be unacceptable. Despite these issues, by adopting a relative predictivity (RP) measure (ratio of real:false results), it was possible to establish that positive results in all three tests indicate the chemical is greater than three times more likely to be a rodent carcinogen than a non-carcinogen. Likewise, negative results in all three tests indicate the chemical is greater than two times more likely to be a rodent non-carcinogen than a carcinogen. This RP measure is considered a useful tool for industry to assess the likelihood of a chemical possessing carcinogenic potential from batteries of positive or negative results.

Mouse-specific carcinogens: an assessment of hazard and significance for validation of short-term carcinogenicity bioassays in transgenic mice

1. The International Conference on the Harmonisation of Technical Requirements for the Registration of Pharmaceuticals for human use (ICH) has agreed that bioassay data from only one species, the rat, supported by appropriate mutagenicity and pharmacokinetic data and also information from new (unvalidated) short term in vivo screening tests for potential carcinogenicity, could be used for the licensing of human medicines. This proposal has been supported by reviews of the utility of testing pharmaceuticals in the mouse which have concluded that the mouse bioassay contributes little to regulatory decisions. The current review was undertaken to identify 'genuine' mouse-specific carcinogens using the Gold Carcinogenicity Potency Database (CPD) for the initial identification of potential mouse-specific carcinogens from published literature. Hazard assessments were completed for these chemicals with particular attention focused on the 'genuine' mouse-specific carcinogens. The significance of such chemicals has been discussed together with consideration of on-going work on the validation of short-term carcinogenicity bioassays using transgenic mice. 2. Seventy-six potential mouse specific carcinogens were identified through the Gold Carcinogenicity Potency Database. Following more detailed consideration a total of ten chemicals were excluded from further consideration (three were multispecies carcinogens, five were considered to be non-carcinogenic in the mouse, and the data for two were uninterpretable). The review focused on the remaining 66 chemicals. There was equivocal evidence of carcinogenicity to the rat for 28 chemicals and inadequate data for a further 23 chemicals. Fifteen 'genuine' mouse-specific carcinogens were identified. These 15 chemicals comprise two genotoxic mouse-specific carcinogens (N-methylolacrylamide (924-42-5), 2,6-Dichloro-p-phenylenediamine (609-20-1); five non-genotoxic mouse-specific carcinogens 2-Aminobiphenyl.HCl (2185-92-4), Captan (133-06-2), Dieldrin (60-57-7), Diethylhexyladipate (103-23-1), and Probenicid (57-66-9); five mouse-specific carcinogens with equivocal evidence of mutagenicity were identified; (2,4-diaminophenol.2HCl (137-09-7), Dipyrone (68-89-3), Ozone (10028-15-6), Vinylidene chloride (75-35-4), and Zearalenone (17924-92-4)), and three mouse-specific carcinogens with inadequate mutagenicity data (Benzaldehyde (100-52-7), Piperonyl sulphoxide (120-62-7), Ripazepam (26308-28-1)). 3. It is suggested that the two genotoxic mouse carcinogens would have been considered as potential carcinogens in the absence of a mouse bioassay. Of the five non-genotoxic mouse-specific carcinogens; three induced tumours in mouse liver only and are considered as being of low potential hazard to human health. The remaining two chemicals would have been missed in the absence of a mouse bioassay (2-aminobiphenyl (2185-92-4) and captan (133-06-2)) and thus are good candidates for evaluation in the short term bioassays in transgenic mice currently being validated. 4. The hardest group of mouse-specific carcinogens to evaluate are those for which there is equivocal or inadequate mutagenicity data. The difficulty in evaluating these particular chemicals emphasises the need for adequate mutagenicity data in addition to adequate carcinogenicity data in order to assess potential hazards to human health. Hazard assessments and a consideration of the potential role for short-term bioassays in transgenic mice for the eight chemicals in this subgroup are presented. 5. A number of general conclusions have been derived from this review. Firstly, there are insufficient published genotoxicity data to allow a full assessment fo mutagenic potential for 57/76 of the potential mouse-specific carcinogens identified from the CPD. This is surprising given the clear value of such data in interpreting bioassay results and the much greater resources required for carcinogenicity bioassays. (ABSTRACT TRUNCATED)

Evaluation of the genotoxicity of aniline.HCl in Drosophila melanogaster

The induction of X-chromosome malsegregation, sex-linked recessive lethals and II-III autosomal translocations by aniline.HCl was investigated in Drosophila melanogaster. Nondisjunction was tested in 2 and 4 d old virgin females fed on aniline.HCl solutions (3, 5, 10 and 15%) using a system where exceptional females (XXY) and only 1/4 of the expected regular progeny are viable. After mating, the females were subcultured daily. Similarly treated 7-day-old wild-type males were used to run classical II-III translocation and recessive lethal tests; for the latter, the solutions were also injected intraabdominally. In all cases, five broods were obtained. A direct correlation was observed between concentration and toxicity. Furthermore, males were more sensitive than females, and the latter's sensitivity was higher at 4-day-old than at 2-day-old. This could be attributed to a decrease with age in the efficiency of a detoxifying mechanism, or to the generation of a toxic metabolite in older flies. Significant increases in nondisjunction were observed with 5, 10 and 15% solutions suggesting the existence of a threshold. No dose effect was detected within the range of the effective concentrations used. The increases were observed in the first subculture (representing mostly stage 14 oocyte, i.e., cells in metaphase I) and in the third subculture, representing cells in which the spindle has not yet formed, thereby pointing to a direct effect of the chemical on the chromosomes and not on the spindle. It is proposed that the second sensitivity peak detected might be the outcome of the transient loss of a protective configuration provided by the karyosome, due to its expansion in stages 9 and 10 of the developing oocytes. No sex-linked lethals or translocations were induced.

In situ and suspension protocols for chemically-induced mutation at the tk locus in L5178Y MOLY cells: dose response and colony size distribution

We used EMS up to concentrations of 0.25 microliters/ml (292 micrograms/ml) to induce mutations at the tk locus in L5178Y MOLY cells, measured the cellular response by the in situ mutation assay protocol and compared these results to those obtained in a concomitant suspension assay. EMS induced mutagenic responses with both protocols. The mutant fraction for the solvent control was 89 mutants per million viable colonies for the suspension protocol and 426 mutations per million viable cells plated for the in situ protocol. These numbers increase to 447 and 2073 respectively, with 0.25 microliter/ml EMS treatment. Sizing curves indicated that the in situ protocol detected a greater proportion of smaller colonies than did the suspension protocol. Not only were the number of small colonies greater than large colonies in the in situ protocol, but their rate of increase was also slightly higher than that of the large colonies. The in situ protocol also reduces the time and cost of experimentally performing the assay compared to the suspension protocol. In this paper we compare the use of the suspension and in situ protocols to measure chemically-induced mutations and demonstrate that the latter method detects a larger fraction of induced mutations at the tk locus in L5178Y MOLY cells.

Comparison of common gene mutation tests in mammalian cells in culture: a position paper of the GUM Commission for the Development of Guidelines for Genotoxicity Testing

In gene mutation tests a decision concerning mutations is made on the basis of hereditary functional changes. In terms of the large amount of data available, the most suitable tests for routine testing in mammalian cells in culture are the tests for acquisition of 6-thioguanine resistance in Chinese hamster cells (V79 and CHO) and for acquisition of alpha,alpha,alpha-trifluorothymidine resistance in the mouse lymphoma line L5178Y TK+/- 3.7.2C. The molecular bases, peculiarities, advantages and disadvantages of these systems will be presented. Which system is to be preferred in any particular case depends among other things on the purpose of the study and the extent to which a technically competent performance of these comparatively exacting tests can be guaranteed.

L5178Y mouse lymphoma cell mutation assay results with 41 compounds

Forty-one chemicals were tested for their abilities to induce trifluorothymidine resistance in L5178Y mouse lymphoma (MOLY) cells. These chemicals were included in the National Toxicology Program's evaluation of four in vitro short-term toxicity assays for predicting carcinogenicity in the rodent bioassay. Of the 41 chemicals examined for this report, 8 were equivocal in the rodent bioassay, and 7 were questionable in- the MOLY assay. If these chemicals are eliminated from an analysis of concordance, the remaining 26 chemicals lead to a concordance of 69% with a sensitivity of 71%. The specificity could not be determined because only two non-carcinogens were detected.

Evaluation of the L5178Y mouse lymphoma cell mutagenesis assay: quality-control guidelines and response categories

A data-based approach to formulating quality-control criteria for the mouse lymphoma cell forward mutation assay is described. Quality-control guidelines for solvent controls, positive controls, and compound-treated cultures were developed based on analysis of over 800 experiments. Frequency distributions of experimental parameters of control cultures, such as mutant frequencies, cloning efficiencies, and suspension growths, were examined. Cloning efficiency and relative total growth affected the variability only when the test chemical was highly toxic. This information was used to generate the quality-control criteria, which were applied to an experiment before it was evaluated for a response. The response categories for classifying the effect of test chemicals on the assay system are defined in terms of (1) the statistically significant differences in average mutant frequency between solvent control cultures and cultures exposed to a chemical and (2) the trend of the dose-related responses.

Evaluation of the L5178Y mouse lymphoma cell mutagenesis assay: methods used and chemicals evaluated

A general protocol, modified from the one described by Clive and Spector (Mutat Res 31:17-29, 1975), was followed by two laboratories, Litton Bionetics, Inc., and SRI International, to evaluate 63 coded chemicals from 16 chemical classes for mutagenic activity at the thymidine kinase locus in L5178Y TK+/- 3.7.2C mouse lymphoma cells. The general protocol is discussed. Some procedural variations introduced by both laboratories are described and discussed in terms of their potential effect on the comparative results of the assay. Also included are the chemical structures, molecular weights, and functional classifications of the 63 chemicals. The assay appeared to tolerate the specific procedural variations in each laboratory without changing its reliability.

Evaluation of the L5178Y mouse lymphoma cell mutagenesis assay: intralaboratory results for sixty-three coded chemicals tested at Litton Bionetics, Inc

The reliability of the L5178Y TK+/- forward mutation assay as a rapid screen for genotoxicity was evaluated by testing 63 coded chemicals. Replicate treatments were used, and at least two independent experiments were performed for each test condition. The test conditions consisted of no exogenous activation, activation by Aroclor 1254-induced Fischer 344 rat liver S9 homogenate, and in some cases activation by noninduced Fischer 344 rat liver S9. The results were organized into tables that show the mutant colony counts, mutant frequency, and toxicity for each test chemical treatment, positive control treatment, and solvent negative control cultures. The repeat experiments were highly consistent and yielded contradictory evaluations for only a few of the chemicals studied. Fifty-one of the chemicals (81%) were evaluated as mutagenic under one or both of the test conditions. A range in minimum effective concentrations of almost 10(6)-fold (0.008 to 5,000 micrograms/ml) was observed among the mutagenic chemicals. Nine chemicals (14%) were considered to be nonmutagenic. Three chemicals (progesterone, p-rosaniline HCl, and 1,1,1-trichloroethane) gave responses that were not easily evaluated under any test condition: evidence for mutagenesis was obtained in some experiments but not for all repeat studies. Under nonactivation conditions, specifically, the mutagenic activities of 4,4'-bis(dimethylamino)benzophenone, progesterone, and p-rosaniline HCl remained uncertain. With S9 activation, uncertain evidence for mutagenesis was obtained for 2-naphthylamine, progesterone, and 1,1,1-trichloroethane. In some cases, changes in the treatment conditions could lead to different evaluations of the mutagenic activity, and these possibilities are discussed in the descriptive evaluations of each chemical. Comparisons of the observed responses with published results were possible for 29 of the compounds and yielded highly confirmatory evaluations.

Evaluation of the L5178Y mouse lymphoma cell mutagenesis assay: intralaboratory results for sixty-three coded chemicals tested at SRI International

SRI used the L5178Y mouse lymphoma cell forward mutation assay to determine the mutagenic activity of 63 coded chemicals from 16 chemical classes. Replicate experiments were performed to assess the reproducibility of the assay within the laboratory. The evaluations (positive or negative) of the first two repeat experiments with the chemicals were the same for 116 (87%) of 134 tests. Evaluational differences between the first two experiments were fewer in the presence of induced S9 (6 tests) than in the absence of S9 (12 tests). The most commonly observed variability was the magnitude of positive mutagenic responses; this may be attributed to factors such as compound solubilities, S9 activation conditions, and differential recovery of mutant cells. Some consistency was observed in the responses of compounds of various chemical classes. Generally, antibiotics (ABO) and the azo dyes, azoxy and hydrazo compounds, diazoalkanes, nitriles and azides (AZO), were mutagenic with S9; alkyl, acyl, and aryl halides, halogenated ethers, and halohydrins (HAL) were more strongly mutagenic with than without S9; and monofunctional polycyclic aromatic hydrocarbons and fluorenones (PAH) were mutagenic only with S9. Amine-1-oxides (AMO), alkyl and aryl epoxides (EPO), and nitroalkanes, nitroaromatics, nitroquinolines, nitrofurans, and nitroimidazoles (NIT) were mutagenic with and without S9; amides, sulfonamides, aromatic amines, aliphatic amines, hydroxylamines, and benzidine and its derivatives (AMI) were mutagenic without S9; and methyl carbamate (the only monofunctional carbamate) and thioureas (CBM) induced a negative response under both conditions.

Prediction of chemical carcinogenicity in rodents from in vitro genetic toxicity assays

Four widely used in vitro assays for genetic toxicity were evaluated for their ability to predict the carcinogenicity of selected chemicals in rodents. These assays were mutagenesis in Salmonella and mouse lymphoma cells and chromosome aberrations and sister chromatid exchanges in Chinese hamster ovary cells. Seventy-three chemicals recently tested in 2-year carcinogenicity studies conducted by the National Cancer Institute and the National Toxicology Program were used in this evaluation. Test results from the four in vitro assays did not show significant differences in individual concordance with the rodent carcinogenicity results; the concordance of each assay was approximately 60 percent. Within the limits of this study there was no evidence of complementarity among the four assays, and no battery of tests constructed from these assays improved substantially on the overall performance of the Salmonella assay. The in vitro assays which represented a range of three cell types and four end points did show substantial agreement among themselves, indicating that chemicals positive in one in vitro assay tended to be positive in the other in vitro assays.