International Workshop on Standardisation of Genotoxicity Test Procedures. Report of in vivo subgroup

Apoptosis and oxidative stress as relevant mechanisms of antitumor activity and genotoxicity of ZnO-NPs alone and in combination with N-acetyl cysteine in tumor-bearing mice

Background: Several in vitro studies have revealed that zinc oxide nanoparticles (ZnO-NPs) were able to target cancerous cells selectively with minimal damage to healthy cells.

Purpose: In the current study, we aimed to evaluate the antitumor activity of ZnO-NPs in Ehrlich solid carcinoma (ESC) bearing mice by measuring their effect on the expression levels of P53, Bax and Bcl2 genes as indicators of apoptotic induction in tumor tissues. Also, we assessed the potential ameliorative or potentiation effect of 100 mg/kg N-acetyl cysteine (NAC) in combination with ZnO-NPs.

Materials and methods: ESC bearing mice were gavaged with three different doses of ZnO-NPs (50, 300 and 500 mg/kg body weight) alone or in combination with NAC for seven consecutive days. In addition to measuring the tumor size, pathological changes, zinc content, oxidative stress biomarkers and DNA damage in ESC, normal muscle, liver and kidney tissues were assessed.

Results: Data revealed a significant reduction in tumor size with a significant increase in p53 and Bax and decrease in Bcl2 expression levels in the tissues of ZnO-NPs treated ESC bearing mice. Moreover, a significant elevation of MDA accompanied with a significant reduction of CAT and GST. Also, a marked increase in all comet assay parameters was detected in ZnO-NPs treated groups. On the other hand, the combined treatment with ZnO-NPs and NAC significantly reduced reactive oxygen species production and DNA damage in liver and kidney tissues in all ZnO-NPs treated groups.

Conclusion: ZnO-NPs exhibited a promising anticancer efficacy in ESC, this could serve as a foundation for developing new cancer therapeutics. Meanwhile, the combined treatment with ZnO-NPs and NAC could act as a protective method for the healthy normal tissue against ZnO-NPs toxicity, without affecting its antitumor activity.

Genotoxicity assessment of ethylenediamine dinitrate (EDDN) and diethylenetriamine trinitrate (DETN)

Ethylenediamine dinitrate (EDDN) and diethylenetriamine trinitrate (DETN) are relatively insensitive explosive compounds that are being explored as safe alternatives to other more sensitive compounds. When used in combination with other high explosives they are an improvement and may provide additional safety during storage and use. The genetic toxicity of these compounds was evaluated to predict the potential adverse human health effects from exposure by using a standard genetic toxicity test battery which included: a gene mutation test in bacteria (Ames), an in vitro Chinese Hamster Ovary (CHO) cell chromosome aberration test and an in vivo mouse micronucleus test. The results of the Ames test showed that EDDN increased the mean number of revertants per plate with strain TA100, without activation, at 5000μg/plate compared to the solvent control, which indicated a positive result. No positive results were observed with the other tester strains with or without activation in Salmonella typhimurium strains TA98, TA1535, TA1537, and Escherichia coli strain WP2 uvrA. DETN was negative for all Salmonella tester strains and E. coli up to 5000μg/plate both with and without metabolic activation. The CHO cell chromosome aberration assay was performed using EDDN and DETN at concentrations up to 5000μg/mL. The results indicate that these compounds did not induce structural chromosomal aberrations at all tested concentrations in CHO cells, with or without metabolic activation. EDDN and DETN, when tested in vivo in the CD-1 mouse at doses up to 2000mg/kg, did not induce any significant increase in the number of micronuclei in bone marrow erythrocytes. These studies demonstrate that EDDN is mutagenic in one strain of Salmonella (TA100) but was negative in other strains, for in vitro induction of chromosomal aberrations in CHO cells, and for micronuclei in the in vivo mouse micronucleus assay. DETN was not genotoxic in all in vitro and in vivo tests. These results show the in vitro and in vivo genotoxicity potential of these chemicals.

Mutagenic effects of carbosulfan, a carbamate pesticide

The genotoxic effects of carbosulfan were evaluated using chromosome aberration (CA), bone marrow micronucleus (MN) and sperm abnormality assays in mice. All the three acute doses (1.25, 2.5 and 5mg/kg) of carbosulfan induced significant dose-dependent increase in the frequency of CA (P<0.02), micronucleated polychromatic erythrocytes (PCEs) (P<0.05) and sperm head abnormalities (P<0.05) but did not affect the total sperm count. The highest acute dose of carbosulfan induced >7-fold increase in the frequency of CA, >3.5-fold increase in the frequency of micronucleated PCEs and >4.6-fold increase in the frequency of sperms with abnormal head morphology following intraperitoneal exposure as compared to the untreated controls. The present findings suggest that carbosulfan is a potent genotoxic agent and may be regarded as a potential germ cell mutagen also.

Analysis of micronuclei and DNA single-strand breaks in mouse splenocytes and peripheral lymphocytes after oral administration of tetramethylthiuram disulfide (thiram)

The fungicide thiram (tetramethylthiuram disulfide, TMTD) was administered by repeated oral intubations to groups of male B6C3F1 mice at 100, 300 and 900 mg/kg body weight for 4 consecutive days, or at 300 mg/kg for 8 and 12 days. 24 hr after the last treatment animals were killed, and splenocyte cultures were set up for the analysis of micronuclei by the cytokinesis-block method. DNA single strand breaks (ssb) and alkali labile sites were also analysed by the single cell gel electrophoresis (Comet) assay in splenocytes and lymphocytes of animals receiving the 8- and 12-day treatments. Parallel experiments with human peripheral lymphocytes were carried out to assess the ability of thiram to induce micronuclei and DNA ssb and alkaline labile sites under in vitro conditions. No significant increase of micronucleated splenocytes was observed in treated animals, despite some evidence of treatment-related cellular toxicity. A borderline excess of DNA damage was suggested by the Comet assay on circulating lymphocytes, whereas negative results were obtained with splenocytes. In vitro, positive results with both genetic end points were obtained in assays with human lymphocytes in the dose ranges 0.5-24 microg/ml and 0.1-8 microg/ml for micronucleus and Comet assays, respectively. These results suggest that thiram, despite its established genotoxicity in vitro, is devoid of appreciable clastogenic and/or aneugenic activity in vivo after oral administration to mice at the maximum tolerated dose.

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.

A strategy for the application of transgenic rodent mutagenesis assays

The past several years have seen an enormous increase in the development and use of transgenic animal models to measure mutations in specific inserted reporter genes. These systems provide gene mutation data in vivo in a wide range of relevant tissues. Numerous laboratories are now using these systems with consistent results. This paper describes the unique niche that transgenic mutagenesis systems can fill in product development and registration strategies. In addition to tissue-specific mechanistic studies, transgenic assays are available to follow up mutagenic effects demonstrated in Salmonella, Escherichia coli, mouse lymphoma (L5178Y) cells, or other in vitro systems.

Use of cyclophosphamide as a positive control in dominant lethal and micronucleus assays

Analyses of dominant lethal (DL) mutations and micronuclei (MN) are 2 important and widely used genotoxicity assays to measure drug-induced chromosome damage in germ cells and somatic cells, respectively. Cyclophosphamide (CP) has been widely used as a positive control in the single-dose mouse MN assay; however, its utility as a positive control for the DL assay has not been fully studied. In the present study, CP was tested in both assays under similar experimental conditions and MN seen in somatic tissue (bone marrow) were correlated with DL mutations seen in germinal tissue. In a dose-range finding study, groups of 5 male mice were dosed i.p. daily for 5 days at 0, 30 or 40 mg/kg CP and bone marrow was harvested 24 h later for MN assay. CP induced a dose-related increase (7- and 11-fold over control at 30 and 40 mg/kg) in micronucleated polychromatic erythrocytes (MNPCEs) and decreased %PCEs (to 60% and 54% of controls at 30 and 40 mg/kg, respectively). Based on this, a definitive DL and MN study was conducted using separate groups of 30 male mice at 0 and 40 mg/kg CP with a daily times 5 dosing regimen. For the MN assay, bone marrow was collected 24 h after the last dose from 5 animals and evaluated for MNPCEs and %PCEs. For the DL assay, each male was caged with 2 untreated females per week for 8 weeks to cover the postmeiotic germ cell stages. On day 17 after the initiation of breeding, the females were evaluated for the number of implantation sites and live, dead and resorbed implants. The results indicated that CP induced about a 17-fold increase in MNPCEs and a 46% decrease in PCEs in relation to controls. In the DL assay, CP produced a slight (13%) but statistically significant reduction in fertility index at week 7 of mating. Also, the total number of implants was significantly lower during weeks 1, 2, 3, 6 and 7 and the numbers of dead implants and postimplantation loss (PIL) were increased for weeks 1, 2 and 3 (55%, 71% and 34% PIL, respectively) over controls. These data clearly show that CP produced clastogenicity and some toxicity in both somatic tissue and germinal tissue. It was concluded that a dose of 40 mg/kg CP can be used as a positive control compound in the DL assay and in the multiple-dose marrow MN assay.

Regulatory Genotoxicity Testing: A Critical Appraisal

This review considers current approaches to regulatory genotoxicity testing, focusing on how the use of animals can be further replaced, reduced and refined. The complementary roles of in vitro and in vivo testing, and the justification for using animals, are discussed in detail. Recommendations are made for improvements and further work, in the light of the considerable current controversy surrounding the composition and deployment of testing strategies, and the interpretation of the data generated, particularly for carcinogenicity prediction. The major problems are the oversensitivity of in vitro tests and the insensitivity of in vivo assays. On the basis of an analysis of some published databases, it is concluded that there is insufficient support for using in vivo genotoxicity assays for screening. Also, it is questionable whether the scientific benefits of using such assays always outweigh the costs to the animals involved. The considerable efforts being made to harmonise in vivo protocols and to develop improved methods for detecting genotoxicity are discussed. It is recommended that more emphasis be placed on characterising genotoxins in vitro, especially for mechanisms of activity, to optimise the benefits of any confirmatory animal tests.. Also, regulatory agencies are urged to require better-designed and more-scientifically sound protocols, in which animal numbers are minimised and data interpretation, particularly that of negative results, is facilitated. Lastly, in the development and validation of transgenic rodent systems, emphasis should be placed on developing protocols in which other acute toxicity and metabolism endpoints can be measured simultaneously with in vivo mutagenesis, while minimising animal numbers.

Recommendations for the performance of UDS tests in vitro and in vivo

The Working Group (WG) dealt with the harmonization of routine methodologies of tests for unscheduled DNA synthesis (UDS) both in vitro and in vivo. In contrast to the existing guidelines from OECD, EPA and EC on in vitro UDS tests (there is no Japanese UDS guideline), the Working Group recommends that in general in vitro UDS tests should be performed with primary hepatocytes. For routine applications any other cell types would need special justification. Hepatocytes from male rats are preferable, unless there are contra-indications on the basis of e.g. toxicokinetic data. According to the OECD, EPA and EC guidelines, UDS may be analysed by means of autoradiography (AR) or liquid scintillation counting (LSC). The WG recommends use of AR. LSC is less suitable due to the problem of differentiation between UDS activity and replicative DNA synthesis, and the disadvantage that cells cannot be analysed individually. Since a specific cell type was recommended by the WG, methodological aspects could be described in more detail than in the present guidelines. For in vitro tests, it was agreed that the initial viability of freshly isolated hepatocytes should be at least 70%. With regard to the need for confirmatory experiments in the event of a clear-cut negative result, the majority view was that confirmation by a second (normally not identical) experiment is still needed; this is in line with the present OECD and EC guidelines. Evaluation of results from UDS tests should be based primarily on net nuclear grain (NNG) values, although it is recognised that nuclear and cytoplasmic grains result from different biological processes. Since grain counts are influenced by a number of methodological parameters, no global threshold NNG value can be recommended for discrimination of positive and negative UDS results. For in vitro assays, the criteria for positive findings go beyond those of the present guidelines and two alternative approaches are given which are based on (1) dose-dependent increases in NNG values and (2) reproducibility, dose-effect relationship and cytotoxicity. At present there is no official guideline on the performance of in vivo UDS tests. Some fundamental recommendations given for in vitro methodology also apply to the in vivo assay. For routine testing with the in vivo UDS test, again the general use of hepatocytes from male rats is recommended. However, concerning the requirement to use one or two sexes, consistency with other in vivo genotoxicity assays (e.g. the micronucleus assay) would be preferable. As for the in vitro methodology, AR is preferred rather than LSC.(ABSTRACT TRUNCATED AT 400 WORDS)