Evaluation of the micronucleus test in vitro using Chinese hamster cells: results of four chemicals weakly positive in the in vivo micronucleus test

Age-related changes in radiation-induced micronuclei among healthy adults

The aim of the present study was to establish the extent of in vitro radioresponse of lymphocytes among 62 healthy adults of both genders and to estimate the distribution of baseline micronuclei and radiosensitivity among individuals of the study population using the cytochalasin block micronucleus test. A younger study group consisted of 10 males (mean age, 22.4 years; range, 21-27) and 12 females (mean age, 24.8 years; range, 20-29), whereas an older study group consisted of 18 males (mean age, 35.1 years; range, 30-44) and 22 females (mean age, 38.5 years; range, 30-48). For evaluation of radiosensitivity blood samples were irradiated in vitro using 60Co gamma-ray source. The radiation dose employed was 2 Gy, the dose rate 0.45 Gy/min. The study revealed a significant gender effect on baseline micronuclei favoring females (Z = 3.25, P < 0.001), while yields of radiation-induced micronuclei did not differ significantly (Z = 0.56, P < 0.56) between genders. The distribution of baseline micronuclei among the individuals tested followed Poisson distribution in both study groups and in both genders, whereas the distribution of radiosensitivity among individuals of the older study group did not fulfill Poisson expectations (Kolmogorov-Smirnof test, P < 0.01). In contrast to a nonsignificant difference in radiosensitivity between males and females of the same age group (Z = 1.97, P < 0.56), a statistically significant difference in radiosensitivity between younger and older group for both genders was found (Z = 3.03, P < 0.03). Since the individuals tested were healthy, the observed variability in radiation response is considered to be an early effect of ageing.

Genotoxicity of benzene and its metabolites

The potential role of genotoxicity in human leukemias associated with benzene (BZ) exposures was investigated by a systematic review of over 1400 genotoxicity test results for BZ and its metabolites. Studies of rodents exposed to radiolabeled BZ found a low level of radiolabel in isolated DNA with no preferential binding in target tissues of neoplasia. Adducts were not identified by 32P-postlabeling (equivalent to a covalent binding index <0.002) under the dosage conditions producing neoplasia in the rodent bioassays, and this method would have detected adducts at 1/10,000th the levels reported in the DNA-binding studies. Adducts were detected by 32P-postlabeling in vitro and following high acute BZ doses in vivo, but levels were about 100-fold less than those found by DNA binding. These findings suggest that DNA-adduct formation may not be a significant mechanism for BZ-induced neoplasia in rodents. The evaluation of other genotoxicity test results revealed that BZ and its metabolites did not produce reverse mutations in Salmonella typhimurium but were clastogenic and aneugenic, producing micronuclei, chromosomal aberrations, sister chromatid exchanges and DNA strand breaks. Rodent and human data were compared, and BZ genotoxicity results in both were similar for the available tests. Also, the biotransformation of BZ was qualitatively similar in rodents, humans and non-human primates, further indicating that rodent and human genotoxicity data were compatible. The genotoxicity test results for BZ and its metabolites were the most similar to those of topoisomerase II inhibitors and provided less support for proposed mechanisms involving DNA reactivity, mitotic spindle poisoning or oxidative DNA damage as genotoxic mechanisms; all of which have been demonstrated experimentally for BZ or its metabolites. Studies of the chromosomal translocations found in BZ-exposed persons and secondary human leukemias produced by topoisomerase II inhibitors provide some additional support for this mechanism being potentially operative in BZ-induced leukemia.

Apoptosis may contribute to false-positive results in the in vitro micronucleus test performed in extreme osmolality, ionic strength and pH conditions

Conditions in which clastogens produce positive responses have been increasingly challenged, and several situations have been described in which clastogenic responses would be considered not to be relevant. For example, extreme culture conditions lead to high variations of pH, osmolality or ionic strength. Apoptosis is induced in extreme culture conditions and contributes to false-positive results in the in vitro micronucleus test performed with CTLL-2 cells. These cells can enter apoptosis when exposed to apoptosis stimuli or after IL-2 deprivation, whereas the CTLL-2 Bcl2 cell line is protected from apoptosis due to the over-expression of the apoptosis inhibitor Bcl2 in bcl2-transfected CTLL-2 cells. The two cell lines were treated in extreme culture conditions of either pH or osmolality or were submitted to high ionic strength. The apoptosis level was measured in parallel with the in vitro micronucleus test using the annexin V-FITC method. Data obtained in the two cell lines suggested that apoptosis caused by extreme culture condition induces the formation of micronucleated cells, which leads to false-positive results in the in vitro micronucleus test.

DNA adduct formation by 7H-dibenzo[c,g]carbazole and its tissue- and organ-specific derivatives in Chinese hamster V79 cell lines stably expressing cytochrome P450 enzymes

The cytochrome P4501A subfamily (CYP1A) is involved in the metabolic activation of 7H-dibenzo[c,g]carbazole (DBC) and its tissue- and organ-specific derivatives, N-methyldibenzo[c,g]carbazole (MeDBC)and 5,9-dimethyldibenzo[c,g]carbazole (diMeDBC). In this study, we have evaluated the relationship between the tissue specificity and (32)P-postlabeled adduct patterns produced by these compounds by using a panel of Chinese hamster V79 cell lines stably expressing human CYP1A1 and CYP1A2 and/or N-acetyltransferase. Treatment of the parental cell lines V79MZ and V79NH, which are devoid of any CYP activity, with DBC and its derivatives did not result in detectable adducts. The highest DNA adduct levels were found in CYP1A1-expressing V79MZh1A1 cells after DBC and MeDBC treatment (24.5 +/- 7.2 and 16.2 +/- 3.6 adducts/10(8) nucleotides, respectively). Exposure of this cell line to DBC resulted in five distinct spots, while six spots with different chromatographic mobilities were detected in MeDBC-treated cells. DiMeDBC produced only very low levels of DNA adducts in V79MZh1A1 cells. DBC and MeDBC formed relatively low levels of DNA adducts in CYP1A2-expressing V79MZh1A2 cells (0.7 +/- 0.2 and 2.1 +/- 1.2 adducts/10(8) nucleotides, respectively). DBC formed three weak spots and MeDBC five spots in V79MZh1A2 cells, and all the spots had different chromatographic mobilities. In contrast, diMeDBC did not induce any DNA adducts in these cells, although diMeDBC induced a significant dose-dependent increase in micronucleus frequency under similar treatment conditions (r = 0.76; P < 0.001). The significant increase in DNA damage in the Comet assay following incubation of exposed cells with a repair-specific endonuclease (Fpg protein) suggests that base modifications such as 8-oxodG or Fapy-adducts might be responsible for the genotoxicity of diMeDBC in V79MZh1A2 cells. The similarities between the DNA adduct patterns produced by DBC and MeDBC in V79MZh1A1 and V79MZh1A2 cells suggest that biotransformation mediated via CYP1A1 and CYP1A2 might depend on a PAH-type pathway involving the aromatic ring system.

Using CTLL-2 and CTLL-2 bcl2 cells to avoid interference by apoptosis in the in vitro micronucleus test

In vitro assays for chromosome aberrations (i.e., in vitro micronucleus and in vitro metaphase analysis tests) frequently produce false-positive or exaggerated-positive results. Our previous work suggested that apoptosis interferes with these tests, producing misleading results. These previous studies were conducted by performing the in vitro micronucleus test in CTLL-2 cells and a CTLL-2 cell derivative stably transfected with the apoptosis inhibitor gene bcl2. In the present study, these previous observations were extended by examining micronucleus induction with a larger number of compounds in both CTLL-2 and CTLL-2 bcl2 cells and measuring apoptosis with annexin V-FITC. Both cell lines were treated with different classes of compounds that were anticipated to be exclusively apoptosis inducers, or compounds known to be clastogens or aneugens, some of which were anticipated to be both genotoxic and apoptotic. We were able to confirm that compounds that are only apoptogenic induced micronuclei in CTLL-2 but not CTLL-2 bcl2 cells, indicating that the positive responses are due to apoptosis in CTLL-2 cells. Some genotoxins (clastogens and aneugens) did not produce apoptosis by the annexin V assay and gave similar responses in CTLL-2 and CTLL-2 bcl2 cells. Finally, higher responses were induced in CTLL-2 cells compared to CTLL-2 bcl2 cells that were treated with aneugens or clastogens that were also apoptosis inducers, suggesting that the greater response in CTLL-2 cells is a consequence of both genotoxicity and apoptosis. Finally, it was demonstrated that just eliminating CTLL-2 cells having three or more micronuclei from scoring was not adequate for correctly evaluating agents that only produce apoptosis. The results indicate that coupling the in vitro micronucleus test in both CTLL-2 and CTLL-2 bcl2 cells with the measurement of apoptosis is able to distinguish the genotoxic effects of a test compound from its apoptotic potential and is able to avoid interference from apoptosis in the in vitro micronucleus test. These observations may provide the basis for a useful genotoxicity assay.

A protocol for the in vitro micronucleus test. I. Contributions to the development of a protocol suitable for regulatory submissions from an examination of 16 chemicals with different mechanisms of action and different levels of activity

The in vitro micronucleus (IVM) test is currently used as a screen during the early stages of pharmaceutical development to identify chemicals likely to produce positive outcomes in the in vitro chromosome aberration assay. For several reasons, the assay is being considered as an alternative to the aberration assay, but the current screening protocols are not rigorous enough to fully satisfy concerns about genotoxic safety. This manuscript describes the investigation of several protocol parameters to assist with the development of a regulatory guideline for the IVM test. The parameters investigated are: the effect of cytochalasin B on the outcome of the assay when conducted with continually growing cell lines; the need for an extended exposure in the absence of metabolic activation; and the number of cells to be counted for a valid assay. In addition, two statistical procedures for the analysis of data from the test are described. The results of the investigation indicate that cytochalasin B does not effect the outcome of the test, that the extended exposure treatment is not necessary, that counting 2000 cells is preferable to counting 1000, and that the data can be appropriately analyzed using a trend test.

Effects of the formaldehyde releasing preservatives dimethylol urea and diazolidinyl urea in several short-term genotoxicity tests

The two formaldehyde (FA)-releasers dimethylol urea (DMU) and diazolidinyl urea (DZU) are widely used as preservatives or additives. They were tested for genotoxicity in three short-term test systems, i.e. in the Salmonella typhimurium mutagenicity assay, in the in vitro micronucleus test with V79 Chinese hamster cells and in the in vitro tubulin assembly assay using isolated tubulin from pig brains. The polymerization products obtained in the tubulin assembly assay were examined additionally by electron microscopy. In the S. typhimurium mutagenicity assay with the pre-incubation assay both FA-releasers tested show a clear and concentration-dependent increase in the number of revertants in strains TA98, TA100 and TA102 with and without metabolic activation (rat liver S9 mix). In all cases, a biologically relevant increase in the number of revertants was achieved within the concentration range tested (DZU: 0.04-1.8 micromol per plate, DMU: 0.21-8.33 micromol per plate). FA was tested at 0.06-2.5 micromol per plate and lead to similar effects.Both compounds induce the formation of micronuclei (concentration range tested: DZU: 2.5-50 micromol/l, DMU: 3.3-333 micromol/l). However, DMU shows a comparatively weaker effect exclusively in the absence of the metabolizing enzymes. By contrast, DZU yields a distinct increase of the micronucleus rate in the absence and in the presence of S9. In addition, DZU predominantly causes an increase of large micronuclei, which suggests that this compound has a marked aneugenic potential. Cytotoxic effects accompany the clastogenic effects of both DMU and DZU. The examination of DMU and DZU in view of a possible aneugenic potential in the tubulin assembly assay yielded the following results: DMU at concentrations up to 10 mmol/l did not influence the formation of microtubuli, whereas DZU inhibited this process completely at 3 mmol/l. FA at 6 mmol/l completely inhibited the tubulin assembly. These results could clearly be confirmed by electron microscopy examination. The different potential of the two compounds with respect to the inhibition of tubulin formation is apparently due to a significant difference in the degree of FA release. According to these results, both compounds have to be considered as genotoxic in vitro. On account of these data and because of the widespread use of these two compounds in various products used in daily life, a reevaluation of the risk associated with these compounds seems to be necessary.

Evaluation of the genotoxic potential of some microbial volatile organic compounds (MVOC) with the comet assay, the micronucleus assay and the HPRT gene mutation assay

Microbial volatile organic compounds (MVOC), metabolites of fungi detected in indoor moulds and in working places in compost facilities are considered as a potential health hazard. Their toxicological relevance, however, is largely unknown and data are rare. The aim of this study was to evaluate in vitro the genotoxic, clastogenic and mutagenic potential of same typical MVOC. For the study of DNA damage human lung carcinoma epithelial A549 cells, V79 Chinese hamster fibroblasts and human peripheral blood cells were exposed and subjected to the alkaline comet assay (single cell gel test). Taking the Chinese hamster V79 cell line as a target clastogenic effects were studied by the micronucleus test and mutagenic effects by the hypoxanthine-guanine-phosphoribosyl transferase gene mutation test (HPRT test). The cytogenic effects of MVOC were assessed by a clonogenic assay using the A549 cell line. The alkylating agent methyl methanesulfonate (MMS) was taken as a positive control. The results indicate that MVOC induced DNA damage is only seen in conditions in which also cytotoxic effects are observed. Clastogenic and mutagenic effects could not be detected.

Influence of dietary antioxidants on the mutagenicity of 7,12-dimethylbenz[a]anthracene and bleomycin in female rats

Studies on agents that modulate carcinogen-induced genotoxic effects in experimental animals provide end points that can be used for assessing the antimutagenic or anticarcinogenic properties of putative chemopreventive compounds and for predicting their protective efficacy in humans. In this study, we investigated the ability of the dietary antioxidant Vitamins C, E, beta-carotene and the mineral selenium to inhibit the mutant frequency (MF) induced by treatment of rats with 7,12-dimethylbenz[a]anthracene (DMBA), a mammary carcinogen and bleomycin (BLM), an anti-tumor agent that can damage DNA by free radical mechanisms. Both chemicals have been previously shown to be mutagenic in the rat lymphocyte Hprt assay. Adult female Fischer 344 rats were given the antioxidants singly or in a combination 2 weeks prior to mutagen treatment. Antioxidant intake continued for an additional 4 weeks post-mutagen treatment. At sacrifice, spleens were aseptically removed for the isolation of lymphocytes to conduct the mutagenesis assay at the Hprt locus. The DMBA and BLM treatment induced a marked increase in MF, 52.8 x 10(-6) and 19.2 x 10(-6), respectively, over the controls. The MFs seen in the individual antioxidants alone (single or mixture) were relatively similar to the controls, with the exception of Vitamins C and E, that had 1.7- and 1.5-fold increase, respectively. The degree of inhibitory response was dependent on the type of mutagen and the particular antioxidant. BLM/antioxidant combination had inhibitions ranging from 44 to 80%, while DMBA/antioxidant system ranged from 60 to 93%, with Vitamins C and E achieving the highest inhibition in both systems. The mixture displayed low inhibitory responses, 44.6% for BLM/mix and 47% DMBA/mix. On the whole, the results indicate that the dietary constituents tested are antimutagenic; however, because of the gradations seen with the responses, the protective efficacy of these antioxidants may depend on the type of mutagen/carcinogen they encounter. Pending molecular analysis of mitochondrial DNA mutations will also indicate whether there is a shift in the mutational spectra produced by the carcinogens in the presence of antioxidants.

Induction of micronuclei by 7H-dibenzo[c,g]carbazole and its tissue specific derivatives in Chinese hamster V79MZh1A1 cells

The clastogenicity/aneugenicity of N-heterocyclic polycyclic aromatic pollutant 7H-dibenzo[c,g]carbazole (DBC) and its two synthetic derivatives N-methyl DBC (MeDBC) and 5,9-dimethyl DBC (diMeDBC) was evaluated in the genetically engineered Chinese hamster V79 cell line V79MZh1A1 with stable expression of human cytochrome P4501A1 and in the parental V79MZ cell line without any cytochrome P450 activity. While none of the three carbazoles changed significantly the level of micronuclei in the parental V79MZ cells, a variable, but statistically significant rise of micronucleus frequencies was assessed in V79MZh1A1 cells. DBC induced dose-dependent increase in the number of micronuclei at harvest times of 24 and 48h and MeDBC at sampling time of 48h in V79MZh1A1 cells in comparison to untreated cells, however, no significant time-dependent increase in micronucleus frequencies was found. The use of the antikinetochore immunostaining revealed that DBC and MeDBC induced approximately equal levels of both kinetochore positive (C+) and kinetochore negative (C-) micronuclei. DiMeDBC, a strict hepatocarcinogen, did not manifest any effect on micronucleus induction in V79MZh1A1 cells. These studies suggest that genetically engineered Chinese hamster V79 cell lines expressing individual CYP cDNAs are a useful in vitro model for evaluation the role of particular cytochromes P450 in biotransformation of DBC and its tissue and organ specific derivatives.

The discovery of RPR 200765A, a p38 MAP kinase inhibitor displaying a good oral anti-arthritic efficacy

RPR132331, a 2-(2-dioxanyl)imidazole, was identified as an inhibitor of tumour necrosis factor (TNF)alpha release from lipopolysaccharide (LPS)-stimulated human monocytes. An intensive programme of work exploring the biology, toxicity and physical chemistry of a novel series of inhibitors, derived from RPR132331, has led to the identification of RPR200765A, a development candidate for the treatment of rheumatoid arthritis (RA). RPR200765A is a potent and selective inhibitor of p38 MAP kinase (IC50 = 50 nM). It inhibits LPS-stimulated TNFalpha release both in vitro, from human monocytes (EC50 = 110 nM), and in vivo in Balb/c mice (ED50 = 6 mg/kg). At oral doses between 10 and 30 mg/kg/day it reduces the incidence and progression in the rat streptococcal cell wall (SCW) arthritis model when administered in either prophylactic or therapeutic dosing regimens. The compound, which is a mesylate salt and exists as a stable monohydrate, shows good oral bioavailabiltiy (F = 50% in the rat) and excellent chemical stability. The data from the SCW disease model suggests that RPR200765A could exhibit a profile of disease modifying activity in rheumatoid arthritis (RA) patients which is not observed with current drug therapies.

Biomarkers of carcinogenesis in humans exposed to polycyclic aromatic hydrocarbons

The frequencies of micronuclei (MN) in cytokinesis-blocked lymphocytes of 91 steel foundry workers were analysed. On the basis of ambient PAH levels at the work stands and 1-hydroxypyrene concentrations in the urine, the coke-oven workers were the most exposed as compared to the rollers reference group. The difference in results for the two groups studied was not statistically significant, although MN were slightly higher for coke-oven workers. The frequency of MN did not increase with exposure: after some increase in 1-10 years, a decreasing tendency was observed.

The application of the micronucleus test in Chinese hamster V79 cells to detect drug-induced photogenotoxicity

Recent reports on the photochemical carcinogenicity and photochemical genotoxicity of fluoroquinolone antibacterials led to an increasing awareness for the need of a standard approach to test for photochemical genotoxicity. In this study the micronucleus test using V79 cells was adapted to photogenotoxicity testing. Results of using different UVA/UVB relationships enabled us to identify a suitable irradiation regimen for the activation of different kinds of photosensitizers. Using this regimen, 8-methoxypsoralen and the fluoroquinolones lomefloxacin, grepafloxacin and Bay Y 3118 were identified to cause micronuclei and toxicity upon photochemical activation. Among the phenothiazines tested, chlorpromazine and 2-chlorophenothiazine, were positive for both endpoints, whereas triflupromazine was only slightly photoclastogenic in the presence of strong phototoxicity. Among the other potential human photosensitizers tested (oxytetracycline, doxycycline, metronidazole, emodin, hypericin, griseofulvin), only hypericin was slightly photogenotoxic. Photochemical toxicity in the absence of photochemical genotoxicity was noted for doxycycline and emodin. With the assay system described, it is possible to determine photochemical toxicity and photochemical genotoxicity concomitantly with sufficient reliability.

Laser pyrolysis products-genotoxic, clastogenic and mutagenic effects of the particulate aerosol fractions

Laser therapy has gained wide acceptance and application in many medical disciplines. Nevertheless, during surgical procedures, the thermal destruction of tissue creates a smoke plume. Recent research data indicate that pyrolysates liberated during vaporisation of tissue induce DNA damage. However, assessing potential health hazards during medical laser treatment requires comprehensive insight into the cytotoxic, genotoxic, clastogenic and mutagenic capacity of laser pyrolysis products (LPP). Therefore, the aim of this study was to evaluate the cytotoxic, genotoxic, clastogenic and mutagenic potential of substances resulting from laser irradiation. Four different types of porcine tissues were irradiated with a surgical CO2 laser, the aerosols were sampled under defined conditions and subjected to the SCE test, micronucleus test and the HPRT test. The results showed that the pyrolysis products are strong inducers of cytotoxic effects. The pyrolysis products induced positive effects in the SCE test, micronucleus test and the HPRT test. The ability and extent to induce genotoxic and mutagenic effects turned out to be dependent on the type of tissue that had been irradiated. In general, the effects were most pronounced with liver pyrolysate. In all test systems, a clear dose relationship could be established. In conclusion, we were able to prove that the particulate fraction of laser pyrolysis aerosols originating from biological tissues undoubtedly have to be classified as cytotoxic, genotoxic, clastogenic and mutagenic. Therefore, they could be potential health hazards for humans.

Chemically induced micronucleus formation in V79 cells--comparison of three different test approaches

The in vitro micronucleus test (MNT) is a useful assay for the detection of mutagenic events on both the chromosomal and the genomic level. The main disadvantage for introducing the in vitro MNT into official test guidelines seems to be the disparity of existing protocols. To contribute to the aim of standardisation, three different methodological approaches of the in vitro MNT with V79 cells were compared: the standard assay using an asynchronically growing mixed cell population, the cytokinesis block (CB) assay and a modified MNT, the so-called mitotic shake-off (MSO) method. V79 cells were thus treated with two known aneugens (colcemide and griseofulvin) and two clastogens (mitomycin C and cyclophosphamide) over various time periods. The cultures of the CB assay were additionally exposed to cytochalasin B (Cyt-B), an inhibitor of cell, but not of nucleus division. After treatment, the cells were harvested and analysed for the appearance of micronuclei (MN). All three assays yielded positive results for all test substances. These results support the suitability of the MNT with V79 cells with regard to the ability to detect the genotoxic potential of both clastogens and aneugens independent of the test protocol applied. Thus, all three methods are appropriate for MN detection, but due to the fact that the application of Cyt-B has no advantages for a cell line like V79 in which nearly all cells undergo a normal cell cycle, its use is not recommended.

Effects of uranium ore dust on cultured human lung cells

Effects of uranium ore dust on cell proliferation, lipid peroxidation and micronuclei formation were compared with silica (DQ12) and titanium oxide in normal human distal airway epithelial cells (NHDE), human lung cancer cells (A549) and human lung fibroblast cells. Cell proliferation was significantly inhibited with uranium ore dust and silica but not with titanium oxide. Lipid peroxidation was significantly enhanced only with uranium ore dust. Micronuclei formation was significantly stimulated with uranium ore dust in A549 and NHDE cells, but not in fibroblast cells. Silica stimulated micronuclei formation only in A549 cells. The results showed the outstanding effect of uranium ore dust on lipid peroxidation and micronuclei formation in human lung cells compared to silica and titanium dioxide.

Induction of sister chromatid exchanges and micronuclei by titanium dioxide in Chinese hamster ovary-K1 cells

Titanium dioxide (TiO2) has color properties of extreme whiteness and brightness, is relatively inexpensive, and is extensively used as a white pigment in a variety of materials. TiO2, an effective blocker of ultraviolet light, is frequently added to sunscreens and cosmetic creams. However, the genotoxicity of TiO2 remains to be controversial. In this report, we have demonstrated that TiO2 can be transported into Chinese hamster ovary-K1 (CHO-K1) cells. The effects of TiO2 on induction of sister chromatid exchanges (SCE) and micronuclei (MN) were then studied in these cells. The SCE frequency in CHO-K1 cells treated with TiO2 at a nonlethal dose range (0 to 5 microM) for 24 h was significantly and dose-dependently increased. By the conventional MN assay, TiO2 at the dose ranged from 0 to 20 microM slightly increased the MN frequency in CHO-K1 cells. However, in the cytokinesis-block MN assay, the number of MN per 1000 binucleated cells was significantly and dose-dependently enhanced in CHO-K1 cells treated TiO2 at the same dose range for 24 h. These results suggest that TiO2 is a potential genotoxic agent.

1,4-Dioxane is not mutagenic in five in vitro assays and mouse peripheral blood micronucleus assay, but is in mouse liver micronucleus assay

1,4-Dioxane, an animal carcinogen, was not previously genotoxic in in vitro assays. We reevaluated the compound's genotoxic potential in five in vitro genotoxicity tests in the presence and absence of S9 mix using recommended new protocols. We used the bacterial reverse mutation assay with Salmonella TA and E. coli WP2 strains, including the plate and preincubation methods, the CHO chromosomal aberration assay, including examination of polyploid induction and extended sampling time, the CHO sister-chromatid exchange assay with short and long treatment time, the mouse lymphoma tk assay (microtiter method), including longer treatment time (24 hr), and the CHO micronucleus assay with short and long treatment times. The highest concentration we used was five mg/ml or plate. We also evaluated the genotoxic effect of 1,4-dioxane in vivo by conducting peripheral blood and liver micronucleus assays in the same mice after single oral administration of up to 3,000 mg/kg. All in vitro assays and the peripheral blood micronucleus assay were negative. The mouse liver micronucleus assay, on the other hand, was positive, indicating that 1,4-dioxane might be genotoxic. It is also conceivable that the positive result in mouse liver micronucleus assay was due to a nongenotoxic mechanism, i.e., errors in genetic repair following enhancement of hepatocyte proliferation.

The photogenotoxicity of titanium dioxide particles

We employed a series of in vitro genotoxicity assays--a single cell gel (SCG) assay with mouse lymphoma L5178Y cells, a microbial mutation assay with Salmonella typhimurium, a mammalian cell mutation assay with L5178Y cells, and a chromosomal aberration assay with Chinese hamster CHL/IU cells--to evaluate the photogenotoxicity of titanium dioxide (TiO2) particles. Without UV/visible light irradiation, TiO2 particles exhibited no or weak genotoxicity. With irradiation, however, TiO2 particles exhibited significant genotoxicity in the SCG and chromosomal aberration assays. Therefore, we concluded that TiO2 particles are photogenotoxic.

Application of the in vitro rat hepatocyte micronucleus assay in genetic toxicology testing

The investigation of micronuclei in mitogenic stimulated hepatocytes in vitro is a quite new area of research. Nevertheless, a relatively large database comprising more than 40 tested compounds of various classes has been generated up to now. This paper reviews the available data for the in vitro rat hepatocyte micronucleus assay, showing a sensitivity of this assay in identifying mutagens and genotoxic liver carcinogens of about 85%. Additionally, all of the tested non-carcinogens gave negative results. The use of primary hepatocytes instead of permanently dividing mammalian cell lines for the investigation of micronucleus induction has several advantages. (1) The broad spectrum of metabolizing enzymes expressed in primary hepatocytes ensures an adequate activation of most xenobiotics. (2) No transfer of activated metabolites via the culture medium is necessary in this system, since the metabolizing cells are the target cells themselves. (3) Whilst in experiments with permanently dividing cells the use of S9-mix restricts the treatment period with the test compounds to 2-6 h in the hepatocyte micronucleus assay continuous treatment of up to 48 h is possible. Investigations with the pyrrolizidine alkaloids retrorsine, monocrotaline and isatidine, strong mutagens and liver carcinogens, clearly showed that at least for isatidine a prolonged exposure period is essential to detect its mutagenic potential. This compound gave positive results in rat hepatocytes but not in V79-cells/S9-mix cultures. (4) The results obtained with the hepatocyte micronucleus assay are in good agreement with the genotoxic profiles of most of the compounds tested. Only three polycyclic aromatic hydrocarbons led to 'false-negative' results, since they strongly inhibited hepatocyte proliferation and thereby prevented micronucleus formation. (5) Hepatocytes are target cells of special interest when compounds are investigated which act specifically in the liver. Especially for hepatocarcinogens classified as non-genotoxins in standard genotoxicity tests or for chemicals showing DNA-repair induction in hepatocytes but no mutagenicity in standard tests, the hepatocyte micronucleus assay can contribute to clarify the situation. (6) The rat hepatocyte micronucleus assay can be performed easily and without great efforts in parallel to the in vitro hepatocyte DNA repair test (UDS-test), using the same hepatocyte batches. (7) Similar to the two versions of the UDS-test, the hepatocyte micronucleus assay can be performed following an in vivo-in vitro protocol. In order to further validate the hepatocyte micronucleus assay, as a next step controlled interlaboratory studies should be initiated.

The position of the in vitro micronucleus test within the battery of screening for genotoxic potential determination and the regulatory guidelines

The in vitro micronucleus test is available to demonstrate the clastogenic and aneugenic potencies of chemicals. This test is easier to evaluate than metaphase analysis and, although used routinely by some laboratories as a prescreening test for the determination of chromosomal mutation induction potency, it has not been recognised by regulatory authorities as a test to be included in the battery of assays for human risk assessment. This limitation is due to the lack of a fixed protocol, a limited data bank, particularly in the case of clastogens, and to the lack of sufficient robust interlaboratory validation studies. If we aim to recognize this test in the form of an OECD guideline and to introduce it as an alternative to in vitro metaphase analysis in cell culture, it is necessary to begin a collaborative study in order to determine the best protocol which is valid for the detection of all clastogens as well as to determine clearly the limits of the test. The use of this in vitro test as a prescreening assay could be very useful as it takes structural and numerical chromosomal abnormalities into account and is simpler to perform than in vitro metaphase analysis.

Comparative evaluation of the in vitro micronucleus test and the in vitro chromosome aberration test: industrial experience

Because of its rapidness, simplicity and potential for automation, the measurement of micronucleated cells in vivo is not only equivalent to the analysis of chromosome aberrations, but often even preferred within routine genotoxicity testing. In order to evaluate the correlation between the in vitro micronucleus assay (MNT) and the in vitro chromosome aberration test (CA), we collected data from four pharmaceutical companies obtained either in Chinese hamster cell lines (CHO-K5, CHO-K1, V79) or in human peripheral blood lymphocytes. Among the 57 compounds included in this comparison, 45 compounds gave rise to concordant results in both assays (26 compounds negative in both assays; 19 compounds positive in both assays). The high percentage of concordance, i.e. about 79% is very promising and can be even increased to about 88% by omitting the 3 aneugenic compounds and 2 compounds inducing endoreduplicated chromosomes which were found positive only in the in vitro MNT. The results are remarkable in particular considering that most of the compounds evaluated are 'standard' pharmaceutical compounds and thus are at most weak inducers of chromosome damage. Our comparison strongly supports that the in vitro micronucleus test is a suitable alternative to the in vitro chromosome aberration assay. Moreover, the MNT has the potential of not only detecting clastogens but additionally aneuploidy inducing chemicals.

Micronucleus induction by neocarzinostatin and methyl methanesulfonate in ionizing radiation--sensitive Chinese hamster V79 cell mutants

Two mutant V79 Chinese hamster cell lines (V-E5, XR-V15B) which show hypersensitivities to DNA damage and their two parental cell lines (V79-LE, V79-B) were used for micronucleus studies. The characteristics of V-E5 strongly resemble those of cells derived from patients suffering from the genomic instability syndrome ataxia telangiectasia, whereas XR-V15B has a decreased ability to rejoin double-strand breaks. The two cell lines V-E5 and XR-V15B showed increased spontaneous micronucleus frequencies and higher sensitivity for micronucleus induction by methyl methanesulfonate (MMS) and neocarzinostatin (NCS) both with and without the use of cytochalasin B in the micronucleus assay. Thus, defects in cellular responses to DNA damage are modulating factors in micronucleus formation.

Antimalarial activity of the bisquinoline trans-N1,N2-bis (7-chloroquinolin-4-yl)cyclohexane-1,2-diamine: comparison of two stereoisomers and detailed evaluation of the S,S enantiomer, Ro 47-7737

The S,S enantiomer of the bisquinoline trans-N1,N2-bis(7-chloroquinolin-4-yl)cyclohexane-1,2-diamine, Ro 47-7737, is significantly more potent against chloroquine-resistant Plasmodium falciparum than the R,R enantiomer and the previously described racemate. Both the enantiomers and the racemate are more potent inhibitors of heme polymerization than chloroquine, and their activities are probably mediated by inhibition of this parasite-specific process. The S,S enantiomer, Ro 47-7737, was studied in more detail and proved to be a potent antimalarial in the treatment of P. vivax ex vivo and P. berghei in vivo. Its suppression of P. berghei growth in a mouse model (50% effective dose, 2.3 mg/kg of body weight) was equal to that of chloroquine and mefloquine, and Ro 47-7737 was found to be more potent than these two drugs in the Rane test, in which the curative effect of a single dose is monitored. The dose at which 50% of animals were permanently cured (34 mg/kg) was markedly superior to those of chloroquine (285 mg/kg) and mefloquine (> 250 mg/kg). When administered orally at 50 mg/kg, Ro 47-7737 also showed a faster clearance of parasites than either chloroquine or mefloquine, and unlike the other two compounds, Ro 47-7737 showed no recrudescence. In a study to compare prophylactic efficacies of oral doses of 50 mg/kg, Ro 47-7737 provided protection for 14 days compared to 3 days for mefloquine and 1 day for chloroquine. The good curative and prophylactic properties of the compound can be explained in part by its long terminal half-life. The ability to generate parasite resistance to Ro 47-7737 was also assessed. With a rodent model, resistance could be generated over eight passages. This rate of resistance generation is comparable to that of mefloquine, which has proved to be an effective antimalarial for many years. Toxicity liabilities, however, ruled out this compound as a candidate for drug development.