Selective induction of micronuclei in the rat/mouse colon and liver by 1,2-dimethylhydrazine: a seven-tissue comparative study

Evaluation of a 4-day repeated-dose micronucleus test in rat glandular stomach and colon using aneugens and non-genotoxic non-carcinogens

Background

We previously developed a rodent gastrointestinal (GI) tract micronucleus (MN) test using the glandular stomach and/or colon, and evaluated this test method using several genotoxic carcinogens (clastogens) and genotoxic non-carcinogens; we demonstrated that this test method could detect genotoxic stomach and/or colon carcinogens with target organ specificity. In the present study, we further evaluated the sensitivity and specificity of the MN test for the rat glandular stomach and colon using three aneugens (colchicine, vinblastine sulfate, and docetaxel hydrate) and two non-genotoxic non-carcinogens (sodium chloride and sucrose).

Results

Male Crl:CD (SD) rats were administered test compounds through clinical administration route (orally or intravenously) for four consecutive days and then examined for the micronucleated cell frequencies in the glandular stomach and colon. We observed that all three aneugens significantly and dose-dependently increased the micronucleated cell frequencies in the stomach and colon. In contrast, neither of the two non-genotoxic non-carcinogens increased the micronucleated cell frequency in these tissues. Notably, an increase in cell proliferation was observed in the glandular stomach of rats administered a stomach toxicant, sodium chloride, but this increase did not affect the induction of micronuclei in the gastric cells.

Conclusions

In the present study, it was demonstrated that the glandular stomach and colon MN tests could detect aneugens as positive and could adequately evaluate non-genotoxic non-carcinogens as negative, including a chemical that enhances cell proliferation. These results provide important evidence supporting good performance of the rat glandular stomach and colon MN tests with a 4-day treatment regimen.

Titanium dioxide nanoparticle genotoxicity: A review of recent in vivo and in vitro studies

Titanium dioxide nanoparticles (TiO2 NPs, size <100 nm) find applications in a wide range of products including food and cosmetics. Studies have found that exposure to TiO2 NPs can cause inflammation, cytotoxicity, genotoxicity and cell apoptosis. In this article, we have reviewed the recent literature on the potential of TiO2 NPs to cause genotoxicity and summarized the results of two standard genotoxicity assays, the comet and micronucleus (MN) assays. Analysis of these peer-reviewed publications shows that the comet assay is the most common genotoxicity test, followed by MN, Ames, and chromosome aberration tests. These assays have reported positive as well as negative results, although there is inconsistency in some results that need to be confirmed further by well-designed experiments. We also discuss the possible mechanisms of TiO2 NP genotoxicity and point out areas that warrant further research.

Micronucleus test in rodent tissues other than liver or erythrocytes: Report of the IWGT working group

At the 6th International Workshop on Genotoxicity Testing, the liver micronucleus test (MNT) working group briefly discussed the MNT using tissues other than liver/erythrocytes. Many tissues other than liver/erythrocytes have been studied, primarily for research purposes. They have included the colon and intestinal epithelium, skin, spleen, lung, stomach, bladder, buccal mucosa, vagina, and fetal/neonatal tissues. These tissues were chosen because they were target sites of carcinogens, and/or relevant to a specific route of exposure. Recently, there has been particular focus on the gastrointestinal (GI) tract as it is a contact site associated with high exposure following oral gavage. Furthermore GI tumors are observed with high frequency in human populations. A collaborative study of the rat glandular stomach and colon MNT was conducted in conjunction with a collaborative study of the repeated-dose liver MNT. Based on limited data currently available, the rodent MNT using the glandular stomach and/or colon seems to detect genotoxic carcinogens with GI tract target-organ specificity. The working group concluded that the GI tract MNT would be a promising method to examine clastogenicity or aneugenicity of test chemicals in the stomach and/or colon. Further data will be needed to fully establish the methods, and to identify the sensitivity and specificity of the GI tract MNT.

Borax counteracts genotoxicity of aluminum in rat liver

This study was carried out to evaluate the protective role of borax (BX) on genotoxicity induced by aluminum (Al) in rat liver, using liver micronucleus assay as an indicator of genotoxicity. Sprague-Dawley rats were randomly separated into six groups and each group had four animals. Aluminum chloride (AlCl3; 5 mg/kg b.w.) and BX (3.25 and 13 mg/kg b.w.) were injected intraperitoneally to rats. Besides, animals were also treated with Al for 4 consecutive days followed by BX for 10 days. Rats were anesthetized after Al and BX injections and the hepatocytes were isolated for counting the number of micronucleated hepatocytes (MNHEPs). AlCl3 was found to significantly ( p < 0.05) increase the number of MNHEPs. Rats treated with BX, however, showed no increase in MNHEPs. Moreover, simultaneous treatments with BX significantly modulated the genotoxic effects of AlCl3 in rats. It can be concluded that BX has beneficial influences and has the ability to antagonize Al toxicity.

The genotoxic, hepatotoxic, nephrotoxic, haematotoxic and histopathological effects in rats after aluminium chronic intoxication

Aluminium (Al) is used in water purification and is also present in several manufactured foods and medicines. Al is known to induce a broad range of physiological, biochemical and behavioural dysfunctions in laboratory animals and humans. This investigation was carried out to investigate the effects of subchronic exposure to Al (as AlCl3) in rats. Sprague-Dawley rats were randomly separated into two groups. Group 1 rats treated with sodium chloride served as the control, group 2 rats were treated with Al (as AlCl3, 5 mg/kg body weight) intraperitonally for 10 weeks. Animals were killed and blood samples were analyzed for blood serum alkaline phosphatase (ALP), aspartate aminotransferase (AST), alanine aminotransferase (ALT) and lactate dehydrogenase (LDH) enzyme activities and creatinine, urea (U) and uric acid (UA) levels for evaluating hepatotoxicity and nephrotoxicity. Blood parameters including red blood cells (RBCs), haemoglobin (Hb) concentration, haematocrit (Ht), platelets (PLTs) and white blood cells (WBCs) were compared between control and experimental group to assess haematoxicity. In order to determine the genotoxicity, the number of micronucleated hepatocytes (MNHEPs) was counted in isolated hepatocytes. In addition, histological alterations in liver and kidney samples were investigated. After exposure with Al, the enzymatic activities of ALP, AST, ALT and LDH, and the levels of U and UA significantly increased. RBC, WBC, PLT, Hb and Ht revealed significant decreases in experimental group compared to the control. AlCl3 caused a significant increase in MNHEPs. Furthermore, severe pathological damages were established in both liver and kidney samples. Subchronic exposure to low doses of Al can produce serious dysfunctions in rat blood, liver and kidney, and exposure to this metal can result in greater damages.

The rat gut micronucleus assay: a good choice for alternative in vivo genetic toxicology testing strategies

The in vivo bone marrow (BM) micronucleus assay is one of the three tests in the standard test battery to assess the genotoxic potential of a pharmaceutical candidate. In some cases, depending on results of in vitro studies, the route of administration or the degree of systemic exposure, in vivo assessment of genotoxicity in the BM alone may not be sufficient. Based on the potential for high gut exposures to orally administered compounds with low systemic exposures as well as the potential susceptibility of rapidly dividing cells of the intestinal tissues, we have developed a modified technique for evaluating micronuclei formation in both the duodenum and colon of rats based on earlier publications. Adult male Sprague Dawley rats were treated once daily for 2 days with either vehicle control or with the test articles acetyl salicylic acid (ASA), carbendazim (CAR), cyclophosphamide (CP), dimethylhydrazine (DMH), mitomycin C (MMC) or vinblastine sulfate (VIN). The duodenum, colon, and BM were harvested, processed, and analyzed for micronucleus induction. Results from these studies demonstrated differences in the susceptibility for different test compounds in the three tissues tested. When MMC and VIN were dosed by different routes at the same dose levels both compounds produced positive results in all three tissues by intraperitoneal injection but not oral administration. These studies suggest that overall the GI micronucleus assay might be a useful tool for clastogenic and aneugenic compounds that are expected to produce high sustained concentrations in the gastrointestinal tract with little systemic exposure.

Evaluation of a modified CD71 MicroFlow method for the flow cytometric analysis of micronuclei in rat bone marrow erythrocytes

The aim of this study was to evaluate a modified flow cytometric method for the quantification of micronuclei in rat bone marrow reticulocytes. The method identified uses the erythrocyte pure fraction from cellulose filtered bone marrow with slight modifications to the widely published MicroFlow(®) method developed by Litron Laboratories, Rochester, NY for the detection of micronuclei in peripheral blood. A number of experiments were conducted to compare the micronucleus induction measured by flow cytometry with traditional microscopic analysis in male rats treated daily for 2 days with appropriate vehicle controls or various doses of cyclophosphamide (CP), mitomycin C (MMC), vinblastine sulfate (VBS), 1,2-dimethylhydrazine (DMH), etoposide (ETO), colchicine (COL), or 4-nitroquinoline-1-oxide (4NQO). In addition, for a subset of chemical we compared the induction of micronuclei in bone marrow and peripheral blood. The results from this study showed a very good correlation of micronucleus frequencies in bone marrow between microscopic analysis and the flow cytometry as well as between blood and bone marrow. In general, micronucleus frequencies of test compound treated animals and inter-animal variability were slightly lower by flow cytometric analysis compared to manual slide analysis. The data presented in this study support the use of the CD71 flow method for the analysis of micronuclei in rat bone marrow and also suggest that peripheral blood may be equally as sensitive as bone marrow in detecting a micronucleus response in short term studies.

Propolis prevents aluminium-induced genetic and hepatic damages in rat liver

Aluminium is present in several manufactured foods and medicines and is also used in water purification. Therefore, the present experiment was undertaken to determine the effectiveness of propolis in modulating the aluminium chloride (AlCl(3)) induced genotoxicity and hepatotoxicity in liver of rats. Animals were assigned to 1 of 4 groups: control; 34 mg AlCl(3)/kg bw; 50mg propolis/kg bw; AlCl(3) (34 mg/kg bw) plus propolis (50mg/kg bw), respectively. Rats were orally administered their respective doses daily for 30 days. At the end of the experiment, rats were anesthetized and hepatocytes (HEP) were isolated for counting the number of micronucleated hepatocytes (MNHEPs). In addition, the levels of serum enzymes and histological alterations in liver were investigated. AlCl(3) caused a significant increase in MNHEPs, alkaline phosphatase, transaminases (AST and ALT) and lactate dehydrogenase (LDH). Furthermore, severe pathological damages such as: sinusoidal dilatation, congestion of central vein, lipid accumulation and lymphocyte infiltration were established in liver. On the contrary, treatment with propolis alone did not cause any adverse effect on above parameters. Moreover, simultaneous treatments with propolis significantly modulated the toxic effects of AlCl(3). It can be concluded that propolis has beneficial influences and could be able to antagonize AlCl(3) toxicity.

In vivo erythrocyte micronucleus assay III. Validation and regulatory acceptance of automated scoring and the use of rat peripheral blood reticulocytes, with discussion of non-hematopoietic target cells and a single dose-level limit test

The in vivo micronucleus assay working group of the International Workshop on Genotoxicity Testing (IWGT) discussed new aspects in the in vivo micronucleus (MN) test, including the regulatory acceptance of data derived from automated scoring, especially with regard to the use of flow cytometry, the suitability of rat peripheral blood reticulocytes to serve as the principal cell population for analysis, the establishment of in vivo MN assays in tissues other than bone marrow and blood (for example liver, skin, colon, germ cells), and the biological relevance of the single-dose-level test. Our group members agreed that flow cytometric systems to detect induction of micronucleated immature erythrocytes have advantages based on the presented data, e.g., they give good reproducibility compared to manual scoring, are rapid, and require only small quantities of peripheral blood. Flow cytometric analysis of peripheral blood reticulocytes has the potential to allow monitoring of chromosome damage in rodents and also other species as part of routine toxicology studies. It appears that it will be applicable to humans as well, although in this case the possible confounding effects of splenic activity will need to be considered closely. Also, the consensus of the group was that any system that meets the validation criteria recommended by the IWGT (2000) should be acceptable. A number of different flow cytometric-based micronucleus assays have been developed, but at the present time the validation data are most extensive for the flow cytometric method using anti-CD71 fluorescent staining especially in terms of inter-laboratory collaborative data. Whichever method is chosen, it is desirable that each laboratory should determine the minimum sample size required to ensure that scoring error is maintained below the level of animal-to-animal variation. In the second IWGT, the potential to use rat peripheral blood reticulocytes as target cells for the micronucleus assay was discussed, but a consensus regarding acceptability for regulatory purposes could not be reached at that time. Subsequent validation efforts, combined with accumulated published data, demonstrate that blood-derived reticulocytes from rats as well as mice are acceptable when young reticulocytes are analyzed under proper assay protocol and sample size. The working group reviewed the results of micronucleus assays using target cells/tissues other than hematopoietic cells. We also discussed the relevance of the liver micronucleus assay using young rats, and the importance of understanding the maturation of enzyme systems involved in the processes of metabolic activation in the liver of young rats. Although the consensus of the group was that the more information with regard to the metabolic capabilities of young rats would be useful, the published literature shows that young rats have sufficient metabolic capacity for the purposes of this assay. The use of young rats as a model for detecting MN induction in the liver offers a good alternative methodology to the use of partial hepatectomy or mitogenic stimulation. Additional data obtained from colon and skin MN models have been integrated into the data bases, enhancing confidence in the utility of these models. A fourth topic discussed by the working group was the regulatory acceptance of the single-dose-level assay. There was no consensus regarding the acceptability of a single dose level protocol when dose-limiting toxicity occurs. The use of a single dose level can lead to problems in data interpretation or to the loss of animals due to unexpected toxicity, making it necessary to repeat the study with additional doses. A limit test at a single dose level is currently accepted when toxicity is not dose-limiting.

Evaluation of liver and peripheral blood micronucleus assays with 9 chemicals using young rats

We conducted simultaneous liver and peripheral blood micronucleus assays in young rats with seven rodent hepatocarcinogens-4,4'-methylenedianiline (MDA), quinoline, o-toluidine, 4-chloro-o-phenylenediamine (CPDA), dimethylnitrosamine (DMN), p-dimethylaminoazobenzene (DAB), and di(2-ethylhexyl)phthalate (DEHP)-and two mutagenic chemicals-kojic acid and methylmethanesulfonate (MMS). Quinoline, DMN, and DAB were positive in the liver assay, while o-toluidine, kojic acid, DAB, and MMS were positive in the peripheral blood assay. o-Toluidine, kojic acid, and DAB are reportedly negative in mouse bone marrow micronucleus assays, indicating a species difference. Our results revealed a correlation between micronucleus induction in hepatocytes and hepatocarcinogenicity. This technique can be useful for the detection of micronucleus-inducing chemicals that require metabolic activation, and it enables simultaneous comparison of the micronucleus-inducing potential of chemicals in the liver and peripheral blood in the same individual.

Inhibition of tumor progression and neoangiogenesis using cyclic RGD-peptides in a chemically induced colon carcinoma in rats

Integrins are cell surface molecules that mediate cell adhesion, but are also important regulators of tumor cell interactions with their microenvironment, tumor cell survival and growth. In addition, the alpha(v)beta3-integrins appear to be critical for microvessel formation in tumor-induced neoangiogenesis. The present study is the first to investigate the effects of therapeutic alpha(v)beta3-integrin inhibition in a chemically induced tumor model that largely resembles human colon carcinomas. Tumor induction was performed in 47 male Sprague-Dawley rats using 1,2 dimethylhydrazin (21 mg/kg) twice a week. After 20 weeks of tumor induction, 100% of the animals developed adenocarcinomas with a median of 13.5 macroscopic tumor nodules (range 12-17), but no distant metastases. During further tumor induction for an additional 10 weeks, rats were treated three times/week with (a) 15 mg/kg RGDfV-peptide that can block vitronectin and fibronectin receptors; (b) an equimolar amount of an ineffective cyclic control peptide; or (c) with equimolar amounts of a linear RGDS-peptide. At the end of this treatment period, rats were sacrificed, and tumor load was quantified macroscopically and confirmed by histological examination. For investigation of the involvement of tumor-induced neoangiogenesis microvessel, density was determined using CD31-immunostaining. After 30 weeks, control animals (group B) had 5-18 tumors (median 14.5). If rats were treated with RGDfV-peptide (group A), the number of tumor nodules was significantly reduced (P < 0.005) to a median of seven macroscopic tumors (range 2-10 tumors), which also represented a significant reduction (P < 0.005) compared with prior to treatment. Application of noncylic RGDS-peptides (group C) did not affect the number of tumor nodules (median 18; range 10-30 tumors). The diameters of tumor nodules were comparable (3.2-6.1 mm) in animals of all groups. In addition, microvessel density was significantly (P < 0.05) reduced in tumors in group A compared to control rats. The major side effect in the treatment group was increased susceptibility to respiratory infections. Our results demonstrate that alpha(v)beta3-integrin-receptor inhibition appears to be a therapeutic strategy for colorectal cancer. In our therapeutic model, late onset of treatment with integrin-blocking peptides resulted in an inhibition of tumor growth and a reduced tumor load which appeared to be mediated, at least in part, by inhibition of neoangiogenesis.

Collaborative validation study of the in vivo micronucleus test using mouse colonic epithelial cells

The in vivo micronucleus test using mouse colonic epithelial cells was evaluated as the 11th collaborative study organized by the Collaborative Study Group on the micronucleus test (CSGMT) with three model chemicals that were known to induce chromosome damage in mouse colonic cells. Five laboratories participated in this validation study. All three model chemicals, i.e. 1,2-dimethylhydrazine dihydrochloride (1,2-DMH), N-methyl-N-nitrosourea (MNU), and mitomycin C (MMC), induced micronucleated colonic epithelial cells in a 4-day exposure protocol in all participating laboratories. We confirmed that the present single cell suspension method could be used to detect the model chemicals as micronucleus inducers in mouse colonic epithelial cells. Advantages of this method are that experiments are easy to perform and that intact cells can be analyzed. The present study suggested that the colon micronucleus assay proposed here is useful for mechanistic studies of colon carcinogenesis.

The comet assay with multiple mouse organs: comparison of comet assay results and carcinogenicity with 208 chemicals selected from the IARC monographs and U.S. NTP Carcinogenicity Database

The comet assay is a microgel electrophoresis technique for detecting DNA damage at the level of the single cell. When this technique is applied to detect genotoxicity in experimental animals, the most important advantage is that DNA lesions can be measured in any organ, regardless of the extent of mitotic activity. The purpose of this article is to summarize the in vivo genotoxicity in eight organs of the mouse of 208 chemicals selected from International Agency for Research on Cancer (IARC) Groups 1, 2A, 2B, 3, and 4, and from the U.S. National Toxicology Program (NTP) Carcinogenicity Database, and to discuss the utility of the comet assay in genetic toxicology. Alkylating agents, amides, aromatic amines, azo compounds, cyclic nitro compounds, hydrazines, halides having reactive halogens, and polycyclic aromatic hydrocarbons were chemicals showing high positive effects in this assay. The responses detected reflected the ability of this assay to detect the fragmentation of DNA molecules produced by DNA single strand breaks induced chemically and those derived from alkali-labile sites developed from alkylated bases and bulky base adducts. The mouse or rat organs exhibiting increased levels of DNA damage were not necessarily the target organs for carcinogenicity. It was rare, in contrast, for the target organs not to show DNA damage. Therefore, organ-specific genotoxicity was necessary but not sufficient for the prediction of organ-specific carcinogenicity. It would be expected that DNA crosslinkers would be difficult to detect by this assay, because of the resulting inhibition of DNA unwinding. The proportion of 10 DNA crosslinkers that was positive, however, was high in the gastrointestinal mucosa, stomach, and colon, but less than 50% in the liver and lung. It was interesting that the genotoxicity of DNA crosslinkers could be detected in the gastrointestinal organs even though the agents were administered intraperitoneally. Chemical carcinogens can be classified as genotoxic (Ames test-positive) and putative nongenotoxic (Ames test-negative) carcinogens. The Ames test is generally used as a first screening method to assess chemical genotoxicity and has provided extensive information on DNA reactivity. Out of 208 chemicals studied, 117 are Ames test-positive rodent carcinogens, 43 are Ames test-negative rodent carcinogens, and 30 are rodent noncarcinogens (which include both Ames test-positive and negative noncarcinogens). High positive response ratio (110/117) for rodent genotoxic carcinogens and a high negative response ratio (6/30) for rodent noncarcinogens were shown in the comet assay. For Ames test-negative rodent carcinogens, less than 50% were positive in the comet assay, suggesting that the assay, which detects DNA lesions, is not suitable for identifying nongenotoxic carcinogens. In the safety evaluation of chemicals, it is important to demonstrate that Ames test-positive agents are not genotoxic in vivo. This assay had a high positive response ratio for rodent genotoxic carcinogens and a high negative response ratio for rodent genotoxic noncarcinogens, suggesting that the comet assay can be used to evaluate the in vivo genotoxicity of in vitro genotoxic chemicals. For chemicals whose in vivo genotoxicity has been tested in multiple organs by the comet assay, published data are summarized with unpublished data and compared with relevant genotoxicity and carcinogenicity data. Because it is clear that no single test is capable of detecting all relevant genotoxic agents, the usual approach should be to carry out a battery of in vitro and in vivo tests for genotoxicity. The conventional micronucleus test in the hematopoietic system is a simple method to assess in vivo clastogenicity of chemicals. Its performance is related to whether a chemical reaches the hematopoietic system. Among 208 chemicals studied (including 165 rodent carcinogens), 54 rodents carcinogens do not induce micronuclei in mouse hematopoietic system despite the positive finding with one or two in vitro tests. Forty-nine of 54 rodent carcinogens that do not induce micronuclei were positive in the comet assay, suggesting that the comet assay can be used as a further in vivo test apart from the cytogenetic assays in hematopoietic cells. In this review, we provide one recommendation for the in vivo comet assay protocol based on our own data.

Increased hepatic activity of inducible nitric oxide synthase in rats fed on a high-fat diet

The effects were examined of the dietary level of fat on the activity of inducible nitric oxide synthase (iNOS) in the liver of rats. In experiment 1, rats were fed on a diet containing 5% or 20% beef tallow or safflower oil for 32 d. The animals were given a subcutaneous injection of the carcinogen, 1,2-dimethylhydrazine (DMH), on d 4. The activity of hepatic iNOS was significantly elevated by the high-fat diet, but was unaffected by the dietary source of the fat examined. In experiment 2, rats were fed on a 5% or 20% beef tallow diet for 11 d or 32 d with or without the DMH treatment. Feeding the high-fat diet and DMH treatment caused higher activity of hepatic iNOS. In experiment 3, the high-fat diet elevated hepatic iNOS activity and the amount of its protein in the lipopolysaccharide-treated rats. The results suggest that hepatic NO production is enhanced by a high-fat diet.

Organ-specific genotoxicity of the potent rodent colon carcinogen 1,2-dimethylhydrazine and three hydrazine derivatives: difference between intraperitoneal and oral administration

We used a modification of the alkaline single cell gel electrophoresis (SCG) (Comet) assay to test the in vivo genotoxicity of four hydrazine derivatives--1,2-dimethylhydrazine (SDMH), 1,1-dimethylhydrazine (UDMH), hydrazine (HZ), and procarbazine (PCZ)--in mouse liver, lung, kidney, brain, and bone marrow, and in the mucosa of stomach, colon, and bladder. Mice were sacrificed 3 and 24 h after intra-peritoneal (i.p.) and oral (p.o.) administration. SDMH at 20 mg/kg i.p. yielded statistically significant DNA damage in all tested organs except for lung. In the gastrointestinal tract, SDMH was genotoxic in the stomach and the colon after i.p. treatment but only in the colon after 20 and 30 mg/kg p.o. treatment. UDMH at 50 mg/kg i.p. yielded DNA damage in the liver and lung at 3 h. PCZ at 200 mg/kg i.p. caused DNA damage in the liver, kidney, lung, brain, and bone marrow. UDMH and PCZ were positive in the stomach and colon p.o. but not by i.p. treatment. HZ at 100 mg/kg yielded DNA damage in the stomach, liver, and lung when given i.p. and in the brain when p.o. Thus, the administration route is important when evaluating organ-specific genotoxicity in multiple organs.

An in vivo micronucleus assay for detecting the clastogenic effect in rat kidney cells

A micronucleus assay in vivo has been developed that is based on the use of freshly isolated kidney cells from mononephrectomized rats. In this validation study, a statistically significant increase in the frequency of micronucleated cells was detected in rats given i.p. a single dose of four kidney carcinogens, N-nitrosodimethylamine, N-nitrosodiethylamine, N-ethyl-N-hydroxyethylnitrosamine and N-nitroso-N-methylurea. The clastogenic effect was more marked when the same dose was injected for 3 successive days. As compared to controls, treated rats displayed a reduction in the frequency of binucleated cells, presumably due to a toxicity-induced inhibition of cellular proliferation. The proposed method should be suitable for the detection of the clastogenic effect of procarcinogens biotransformed into reactive species in the kidney.