An Interlaboratory Validation Study of the Improved Transformation Assay Employing Balb/c 3T3 Cells: Results of a Collaborative Study on the Two-stage Cell Transformation Assay by the Non-genotoxic Carcinogen Study Group.

The Non-genotoxic Carcinogen Study Group of the Environmental Mutagen Society of Japan organised the first step of an interlaboratory validation study on an improved cell transformation assay employing Balb/c 3T3 A31-1-1 cells. Nineteen laboratories participated in this study. The modified transformation assay was evaluated for its responsiveness, its interlaboratory reproducibility and its transferability. In this study, a mixture of Dulbecco's modified Eagle's medium and nutrient mixture F12, supplemented with insulin-transferrin-ethanolamine-sodium selenite and 2% fetal bovine serum (FBS) was used during the period of expression of transformed foci, intead of the usual minimum essential medium with 10% FBS. 20-Methylcholanthrene (MCA) and 12-O-tetradecanoylphorbol-13-acetate (TPA) were selected as a prototype initiator and a tumour promoter, respectively. Two series of experiments were conducted. In the first series, the transformation activity of MCA was examined at various concentrations. In the absence of the promoting treatment with TPA, exposure to MCA only weakly induced transformed foci. In the presence of 0.1µg/ml TPA, all laboratories observed significant dose-dependent increases in the number of transformed foci with increasing MCA concentrations. In the second series of experiments, various concentrations of TPA were tested. In the absence of initiating treatment with MCA, exposure to TPA weakly induced transformed foci in about half of the laboratories. In the presence of 0.2µg/ml MCA, all the laboratories observed significant dose-dependent increases in the number of transformed foci with increasing TPA concentrations. The results from this study support the usefulness of this modified two-stage transformation assay with Balb/c 3T3 cells.

Improvement of carcinogen detection in the BALB/3T3 cell transformation assay by using a rich basal medium supplemented with low concentration of serum and some growth factors.

To improve the detection sensitivity and reproducibility of the transformation assay using BALB/3T3 cells, we scrutinized a new assay method in which the cells were replated in a medium containing a low concentration of serum after carcinogen treatment. Dulbecco's modified Eagle's medium plus Ham F12 (DME.F12) supplemented with a mixture of insulin, transferrin, ethanolamine and sodium selenite (ITES) and a low concentration of fetal calf serum (FCS) caused transformation at a high frequency with a high reproducibility. The transformation frequency in the culture treated with N-methyl-N'-nitro-N-nitrosoguanidine was the highest in DME.F12 medium containing ITES and 2% FCS, and it decreased at either a lower or higher FCS concentration. Moreover, the transformation frequency was not markedly influenced by the difference in lot of FCS, probably due to reduction in FCS concentration. According to the present method, the transformation frequency was 2-times higher and transformed foci appeared much earlier than by the method using the original medium. Next, we examined some geno- and non-genotoxic carcinogens, and some genotoxic non-carcinogens to confirm the availability of this method for predicting potential carcinogens. This method could precisely identify not only genotoxic carcinogens but also non-genotoxic carcinogens and genotoxic non-carcinogens. In conclusion, these findings suggest that this method is useful for detection of chemical carcinogens because it provides high sensitivity, high reproducibility and accurate predictivity, without the requirement of 12-O-tetradecanoylphorbol 13-acetate as a promoter, making it harmless to examiner.

[Mutagenicity studies of (+/-)-4-diethylamino-1,1,-dimethylbut-2-yn-1-yl 2-cyclohexyl-2-hydroxy-2-phenylacetate monohydrochloride monohydrate (NS-21), a novel drug for urinary frequency and incontinence].

The mutagenicity of (+/-)-4-diethylamino-1,1-dimethylbut-2-yn-1-yl 2-cyclohexyl-2-hydroxy-2-phenylacetate monohydrochloride monohydrate (NS-21), a new drug for the treatment of urinary frequency and incontinence, was investigated by the reverse mutation test in bacteria, the chromosome aberration test in vitro, and the micronucleus test in mice. The reverse mutation test was performed at a dose from 31.3 to 4000 micrograms/plate, at which dose cell killing was observed, using Salmonella typhimurium TA100, TA1535, TA98, and TA1537, and Escherichia coli WP2uvrA. NS-21 did not increase revertant colonies significantly in any of the test strains with or without metabolic activation system (S9 mix). The chromosome aberration test was carried out at a dose from 3.75 to 140 micrograms/ml, at which dose more than 50% cell proliferation was inhibited, using cultured Chinese hamster lung cells (CHL/IU). No significant increases of the frequencies of cells with chromosome aberrations were observed with or without S9 mix. The micronucleus test was conducted in the bone marrow cells of Slc : ddY male mice. Mice were given NS-21 by a single oral administration at doses of 0, 43.8, 87.5, 175, and 350 mg/kg, the geometric mean dose between the maximum tolerated dose and the minimum lethal dose. There were no significant increases in the frequencies of micronucleated polychromatic erythrocytes at any dose levels. These results show that NS-21 has no mutagenic activity in vitro or in vivo.

[Mutagenicity studies of prednisolone farnesylate (PNF)].

Prednisolone farnesylate (PNF) was tested for mutagenicity by Ames test using Salmonella typhimurium (TA100, TA1535, TA98, TA1537) and Escherichia coli (WP2 uvrA), for clastogenic activity in vitro by the chromosomal aberration test in a Chinese hamster fibroblast cell line (CHL), and for induction of micronuclei by the micronucleus test in male ICR mice. 1) In Ames test, PNF with and without metabolic activation showed no mutagenicity in any strains at any dose levels (312-5,000 micrograms/plate). 2) In the chromosomal aberration test, PNF with metabolic activation produced a slight increase in the incidence of structural chromosomal aberrations in CHL cells at 1,500 micrograms/ml. 3) In the micronucleus test, a single administration of PNF caused no significant increase of micronucleated polychromatic erythrocytes at any doses (250-2,000 mg/kg).

The micronucleus assay with peripheral blood reticulocytes by acridine orange supravital staining with 1-beta-D-arabinofuranosylcytosine.

Dose-dependent induction of micronuclei with 1-beta-D-arabinofuranosylcytosine (ara-C) was clearly shown in CD-1 mouse peripheral blood reticulocytes (RETs) using an acridine orange (AO) supravital staining method, as well as in the conventional bone marrow assay. The maximum frequencies of micronucleated RETs (MNRETs) in peripheral blood and of micronucleated polychromatic erythrocytes (MNPCEs) in bone marrow were comparable, as shown in two laboratories independently. The maximum frequencies of MNRETs in peripheral blood lagged about 24 and 12 h behind those of MNPCEs in bone marrow in experiments with 24- and 12-h sampling intervals, respectively. The proportion of each type of RET was examined periodically after treatment with ara-C at doses ranging from 6.25 to 50.0 mg/kg. The proportion of type I RETs among total RETs decreased 24 or 48 h after treatment according to the dose level. This suggest that this ratio could be a good indicator of the bone marrow cell toxicity of test chemicals.