Mathematical model of L5178Y mouse lymphoma forward mutation assay

The L5178Y/tk+/- mouse lymphoma specific gene and chromosomal mutation assay a phase III report of the U.S. Environmental Protection Agency Gene-Tox Program

The L5178Y/tk+/- (-)3.7.2C mouse lymphoma assay (MLA) which detects mutations affecting the heterozygous thymidine kinase (tk) locus is capable of responding to chemicals acting as clastogens as well as point mutagens. Improvements in the assay to enhance detection of this spectrum of genetic events are summarized, and criteria for evaluating the data are defined. Using these criteria, the Phase III Work Group reviewed and evaluated literature containing MLA results published from 1976 through 1993. The data base included 602 chemicals of which 343 were evaluated as positive, 44 negative, 18 equivocal, 54 apparently inappropriate for evaluation in this test system with the published protocols, and 142 that were inadequately tested, and thus a definitive call could not be made. The overall performance of the assay is summarized by chemical class, and the outcome of testing 260 chemicals in the MLA is compared with Gene-Tox and National Toxicology Program evaluations of rodent carcinogenesis bioassay results for the same chemicals. Based on the Work Group's evaluation of published MLA data for chemicals that were considered adequately tested, it is concluded that for most chemicals the L5178Y/tk+/- mouse lymphoma assay is eminently well suited for genotoxicity testing and for predicting the potential for carcinogenicity.

The mutagenic activity of selected compounds at the TK locus: rodent vs. human cells

The mutagenic (TFT resistance) and toxic responses of mouse lymphoma (MOLY) L5178Y cells and human lymphoblast (HULY) TK6 cells were compared for 13 chemicals. The mutagenic activities of 8 of the 13 chemicals (62%) examined in the HULY and MOLY assays are in agreement - the results being judged positive in both assays. However, a dramatic difference is observed when the two conditions of metabolic activation are considered separately; the overall concordance of 8/13 has been achieved by combining a 13/13 (100%) agreement in the absence of S9 with a 1/6 (17%) agreement in the presence of S9. In the absence of S9, the concentration ranges, lowest significant doses, and shapes of the concentration-response curves for both toxicity and mutagenicity were similar in spite of the differences in exposure times (4 h for MOLY, 20 for HULY) and expression times (2 days for MOLY, 3 days for HULY). The general agreement observed in the absence of S9 contrasted with the differences manifested in its presence. 6 compounds which were negative in the absence of S9 were tested in both the MOLY and HULY assays in the presence of S9. Of the 6 chemicals, only 1 was positive in both MOLY and HULY under the latter condition; 4 others were positive in MOLY and negative in HULY whereas 1 was positive in HULY and negative in MOLY.

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.

Responses of the L5178Y tk+/tk- mouse lymphoma cell forward mutation assay: III. 72 coded chemicals

Seventy-two chemicals were tested for their mutagenic potential in the L5178Y tk+/- mouse lymphoma cell forward mutation assay, using procedures based upon those described by Clive and Spector (Mutat Res 44:269-278, 1975) and Clive et al. (Mutat Res 59:61-108, 1979). Cultures were exposed to the chemicals for 4 hr, then cultured for 2 days before plating in soft agar with or without trifluorothymidine (TFT), 3 micrograms/ml. The chemicals were tested at least twice. Significant responses were obtained with allyl isothiocyanate, p-benzoquinone dioxime, benzyl acetate, 2-biphenylamine HCl, bis(2-chloro-1-methylethyl)ether, cadmium chloride, chlordane, chlorobenzene, chlorobenzilate, 2-chloroethanol, chlorothalonil, cytarabine.HCl, p,p'-DDE, diazinon, 2,6-dichloro-p-phenylenediamine, N,N-diethylthiourea, diglycidylresorcinol ether, 2,4-dimethoxy aniline.HCl, disperse yellow 3, endosulfan, 1,2-epoxyhexadecane, ethyl acrylate, ethyl benzene, ethylene thiourea, F D and C yellow Number 6, furan, heptachlor, isophorone, mercuric chloride, 4,4'-methylenedianiline.2 HCl, methyl viologen, nickel sulfate.6H2O, 4,4'-oxydianiline, pentachloroethane, piperonyl butoxide, propyl gallate, quinoline, rotenone, 2,4,5,6-tetrachloro-4-nitro-anisole, 1,1,1,2-tetrachloroethane, trichlorfon, 2,4,6-trichlorophenol, 2,4,5-trimethoxybenzaldehyde, 1,1,3-trimethyl-2-thiourea, 1-vinyl-3-cyclopetene dioxide, vinyl toluene, and ziram. Apart from 2-biphenylamine.HCl, 2-chloroethanol, disperse yellow 3, ethylene thiourea, FD and C yellow number 6, phenol, and 1,1,2-tetrachloroethane, rat liver S9 mix was not a requirement for these compounds. Chemicals not identified as mutagens were acid red, 11-aminoudecanoic acid, boric acid, 5-chloro-o-toluidine, coumaphos, cyclohexanone, decabromodiphenyl oxide, di(2-ethylhexyl)adipate, ferric chloride, fluometuron, melamine, monuron, phenesterin, phthalimide, reserpine, sodium dodecyl sulfate, 4,4-sulfonyldianiline, tetrachloroethylene, and zearalenone. The assay was incapable of providing a clear indication of whether some chemicals were mutagens; these were benzyl alcohol, 1,4-dichlorobenzene, phenol, succinic acid-2,2-dimethyl hydrazide, and toluene.

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 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.

Responses of the L5178Y tk+/tk- mouse lymphoma cell forward mutation assay to coded chemicals. I: Results for nine compounds

Nine substances were tested for their mutagenic potential in the L5178Y tk+/tk- mouse lymphoma cell forward mutation assay, by means of procedures based upon those described by Clive and Spector (Mutat Res 44:269-278, 1975) and Clive et al (Mutat Res 59:61-108, 1979). Cultures were exposed to the chemicals for 4 hr, then cultured for 2 days before plating in soft agar with or without trifluorothymidine (TFT), 3 micrograms/ml. The coded chemicals were tested at least twice. Significant responses were obtained with calcium chromate, 3-(chloromethyl)pyridine, 1,2-epoxybutane, monochloroacetic acid, dicyclohexylthiourea, 2,4-diaminophenol hydrochloride, and pentachloroanisole. Apart from pentachloroanisole, rat liver S9 mix was not a requirement for the clearly mutagenic activity of any of these compounds. Compounds not identified as mutagens were 3-amino-1,2,4-triazole and sucrose.

Mutagenicity of methylisocyanate and its reaction products to cultured mammalian cells

Methylisocyanate (MIC) induced mutagenic responses in the absence of exogenous activation in the mouse lymphoma cell forward mutation assay at concentrations as low as 8-24 microM. MIC produced predominantly small mutant colonies, suggesting the possibility of clastogenic activity. The intermediate hydrolysis product, methylamine, was also mutagenic without exogenous activation but required several hundred-fold higher concentrations (ca. 3 mM). N,N'-Dimethylurea, the final product in the reaction of methylisocyanate and water, was totally refractory in either the presence or absence of S9 for concentrations up to 57 mM (5 mg/ml). The ethyl ester of N-methylcarbamic acid was also tested since it was the only available analogue to the highly reactive N-methylcarbamic acid intermediate. This compound was mutagenic only in the presence of S9 at doses exceeding 5-40 microM, which suggested the possibility that the free acid, produced by enzymatic hydrolysis, is also mutagenic. The mutagenic activity of the ester resulted solely in the production of small mutant colonies.