A protocol and guide for the in vivo rat hepatocyte DNA-repair assay

Investigation of in vivo unscheduled DNA synthesis in rabbit corneas following instillation of genotoxic agents

PURPOSE

An unscheduled DNA synthesis (UDS) test is used for in vitro or in vivo genotoxicity evaluation. The UDS test with hepatocytes is well established; however, drug exposure levels at the application site for topically administered drugs (e.g. ophthalmic drugs) often exceed the exposure levels for systemic administration. To establish in vivo genotoxicity on the ocular surface, we performed the UDS test using rabbit corneas from eyes subjected to instillation of genotoxic agents.

MATERIALS AND METHODS

Five genotoxic agents - 1,1'-dimethyl-4,4'-bipyridinium dichloride (paraquat); acridine orange; ethidium bromide; acrylamide; and 4-nitroquinoline 1-oxide (4-NQO) - were instilled once onto both eyes of male Japanese white rabbits. Physiological saline or a general vehicle for ophthalmic solution were instilled as the negative controls. Dimethyl sulfoxide was instilled as the vehicle control. Isolated corneas were incubated with tritium-labelled thymidine and the number of sparsely labelled cells (SLCs, cells undergoing UDS) was counted by autoradiography.

RESULTS

Statistically significant increases in the mean appearance rates of SLCs in the corneal epithelium were noted in paraquat-, acridine orange-, ethidium bromide-, and 4-NQO-treated eyes compared with those of the controls. These increases generally appeared in a dose-dependent manner. Acrylamide did not induce an increase in the mean appearance rates of SLCs, presumably because it caused the generation of fewer metabolites in the cornea.

CONCLUSIONS

UDS tests revealed DNA damage in the cornea epitheliums treated with well-known genotoxic agents. These results suggest that the UDS test is one of the useful tools for the assessment of in vivo genotoxicity on the ocular surface in the development of ophthalmic drugs.

A comparison of transgenic rodent mutation and in vivo comet assay responses for 91 chemicals

A database of 91 chemicals with published data from both transgenic rodent mutation (TGR) and rodent comet assays has been compiled. The objective was to compare the sensitivity of the two assays for detecting genotoxicity. Critical aspects of study design and results were tabulated for each dataset. There were fewer datasets from rats than mice, particularly for the TGR assay, and therefore, results from both species were combined for further analysis. TGR and comet responses were compared in liver and bone marrow (the most commonly studied tissues), and in stomach and colon evaluated either separately or in combination with other GI tract segments. Overall positive, negative, or equivocal test results were assessed for each chemical across the tissues examined in the TGR and comet assays using two approaches: 1) overall calls based on weight of evidence (WoE) and expert judgement, and 2) curation of the data based on a priori acceptability criteria prior to deriving final tissue specific calls. Since the database contains a high prevalence of positive results, overall agreement between the assays was determined using statistics adjusted for prevalence (using AC1 and PABAK). These coefficients showed fair or moderate to good agreement for liver and the GI tract (predominantly stomach and colon data) using WoE, reduced agreement for stomach and colon evaluated separately using data curation, and poor or no agreement for bone marrow using both the WoE and data curation approaches. Confidence in these results is higher for liver than for the other tissues, for which there were less data. Our analysis finds that comet and TGR generally identify the same compounds (mainly potent mutagens) as genotoxic in liver, stomach and colon, but not in bone marrow. However, the current database content precluded drawing assay concordance conclusions for weak mutagens and non-DNA reactive chemicals.

Clarification of some aspects related to genotoxicity assessment

The European Commission requested EFSA to provide advice on the following: (1) the suitability of the unscheduled DNA synthesis (UDS) in vivo assay to follow‐up positive results in in vitro gene mutation tests; (2) the adequacy to demonstrate target tissue exposure in in vivo studies, particularly in the mammalian erythrocyte micronucleus test; (3) the use of data in a weight‐of‐evidence approach to conclude on the genotoxic potential of substances and the consequent setting of health‐based guidance values. The Scientific Committee concluded that the first question should be addressed in both a retrospective and a prospective way: for future assessments, it is recommended no longer performing the UDS test. For re‐assessments, if the outcome of the UDS is negative, the reliability and significance of results should be carefully evaluated in a weight‐of‐evidence approach, before deciding whether more sensitive tests such as transgenic assay or in vivo comet assay would be needed to complete the assessment. Regarding the second question, the Scientific Committee concluded that it should be addressed in lines of evidence of bone marrow exposure: toxicity to the bone marrow in itself provides sufficient evidence to allow concluding on the validity of a negative outcome of a study. All other lines of evidence of target tissue exposure should be assessed within a weight‐of‐evidence approach. Regarding the third question, the Scientific Committee concluded that any available data that may assist in reducing the uncertainty in the assessment of the genotoxic potential of a substance should be taken into consideration. If the overall evaluation leaves no concerns for genotoxicity, health‐based guidance values may be established. However, if concerns for genotoxicity remain, establishing health‐based guidance values is not considered appropriate.

Chromosomal damage and micronucleus induction by MP-124, a novel poly(ADP-ribose) polymerase-1 (PARP-1) inhibitor: Evidence for a non-DNA-reactive mode of action

MP-124, a novel poly(ADP-ribose) polymerase-1 (PARP-1) inhibitor that competes with the binding of the PARP substrate nicotinamide adenine dinucleotide (NAD), is being developed as a neuroprotective agent against acute ischemic stroke. MP-124 increased structural chromosomal aberration in CHL/IU cells, but showed negative results in the bacterial reverse mutation test, and the rat bone marrow micronucleus (MN) and the rat liver unscheduled DNA synthesis tests after the intravenous bolus injection. Thus, MP-124 did not appear to be direct-acting mutagen. Since, PARP-1 is a key enzyme in DNA repair, the effect of continuous PARP-1 inhibition by MP-124 was further examined in the rat MN test under 24-h intravenous infusion, and an increase in micronucleated immature erythrocytes (MNIE) was observed. The increase was clearly reduced by co-treatment with nicotinic acid, which resulted in increased intracellular NAD levels. This is consistent with the established activity of MP-124 as a competitive inhibitor of PARP and provides strong evidence that the DNA-damaging effect that leads to the increase in MNIE is a secondary effect of PARP-1 inhibition. This mechanism is expected to result in a threshold for the induction of MNIE by MP-124, and allows for the establishment of a safe margin of exposure for the therapeutic use of MP-124.

Unscheduled DNA synthesis (UDS) test with mammalian liver cells in vivo

The unscheduled DNA synthesis test is a short-term genotoxicity assay that allows identification of substances that induce DNA repair in liver cells of treated animals. This endpoint can be assessed by determining the uptake of radiolabelled nucleotide after excision-repair in cells that are not undergoing scheduled (S-phase) DNA synthesis. Here we describe the protocol for performing this well-known primary DNA damage test following the OECD guideline No. 486, EC, and published international recommendations.

Safety evaluation of Evesse EPC, an apple polyphenol extract rich in flavan-3-ols

The safety of the apple polyphenol extract EvesseEPC, which is rich in flavan-3-ols, particularly epicatechin, was evaluated. Both in a bacterial reverse mutation test and a mouse lymphoma assay, EvesseEPC showed a positive response in vitro. In vivo studies (UDS test in hepatocytes, bone marrow micronucleus test and comet assay in intestinal cells) were all negative and hence Evesse EPC is considered not to have genotoxic properties in vivo. In a 90-day study in rats, EvesseEPC was administered at dietary levels of 0%, 1.25%, 2% and 3.25%. Body weights were decreased in the high-dose group in both sexes without effects on feed or water intake. In the high-dose group, thrombocytes (males) and creatinine (both sexes) were decreased, prothrombin time (males) was increased, and liver, kidneys and spleen weights were increased (males), without histological correlates. Diffuse acinar cell hypertrophy, observed in the parotid salivary glands in all treatment groups, was not considered as adverse and presumably reflected a local, reversible and adaptive response to direct contact with EvesseEPC. The NOAEL for EvesseEPC in rats was 2% in the diet, equivalent to an overall average intake of 1.3 and 1.5 g/kg body weight/day for males and females, respectively.

The acute, genetic, developmental and inhalation toxicology of trans-1,3,3,3-tetrafluoropropene (HFO-1234ze)

HFO-1234ze is being developed as a refrigerant, propellant, and foam-blowing agent because it has a very low global warming potential (less than 10), as contrasted to the hydrofluorocarbons with values of over 500. Several toxicology studies were conducted to develop a toxicology profile for this material. There was no lethality in mice and rats receiving single 4-hour exposures up to 103,300 or 207,000 ppm, respectively. Exposures up to 120,000 ppm did not induce cardiac sensitization to adrenalin. Rats were exposed to HFO-1234ze at levels of 5,000, 20,000, and 50,000 ppm 6 hours/day 5 days/week for 2 weeks. Predominate findings of increased liver and kidney weights and histopathological changes in the liver and heart suggested that these organs were the targets for HFO-1234ze toxicity. In a 4-week study at 1000, 5000, 10,000, and 15,000 ppm, the only organ showing treatment-related effects was the heart. In a 90-day study with exposures of 1500, 5000, and 15,000 ppm 6 hours/day 5 days/week, again, the heart was the only target organ. The findings consisted of focal and multifocal mononuclear cell infiltrates in the heart. There was no evidence of fibrosis, and, when compared to the 2- and 4-week studies, there did not appear to be an increase in severity with length of exposure. HFO-1234ze was inactive in a mouse and rat micronucleus assay, an Ames assay, and an unscheduled DNA synthesis assay and was not clastogenic in human lymphocytes. It was also not a developmental toxin in either the rat or rabbit, even at exposure levels up to15,000 ppm.

1,3-Butadiene: II. Genotoxicity profile

1,3-Butadiene’s (BD’s) major electrophilic metabolites 1,2-epoxy-3-butene (EB), 1,2-dihydroxy-3,4-epoxybutane (EBD), and 1,2,3,4-diepoxybutane (DEB) are responsible for both its mutagenicity and carcinogenicity. EB, EBD, and DEB are DNA reactive, forming a variety of adducts. All three metabolites are genotoxic in vitro and in vivo, with relative mutagenic potencies of DEB >> EB > EBD. DEB also effectively produces gene deletions and chromosome aberrations. BD’s greater mutagenicity and carcinogenicity in mice over rats as well as its failure to induce chromosome-level mutations in vivo in rats appear to be due to greater production of DEB in mice. Concentrations of EB and DEB in vivo in humans are even lower than in rats. Although most studies of BD-exposed humans have failed to find increases in gene mutations, one group has reported positive findings. Reasons for these discordant results are examined. BD-related chromosome aberrations have never been demonstrated in humans except for the possible production of micronuclei in lymphocytes of workers exposed to extremely high levels of BD in the workplace. The relative potencies of the BD metabolites, their relative abundance in the different species, and the kinds of mutations they can induce are major considerations in BD’s overall genotoxicity profile.

A classification framework and practical guidance for establishing a mode of action for chemical carcinogens

The recently released U.S. Environmental Protection Agency (U.S. EPA) Supplemental Guidance for Assessing Risk from Early Life Exposure to Carcinogens (SGAC) provides guidance to account for potential increased early life susceptibility to carcinogens that are acting via a mutagenic mode of action. While determination of the mode of carcinogenic action is central to the SGAC procedures and other regulatory risk assessments, little guidance is given as to the approaches, criteria, and nature of the evidence required to define a mutagenic mode of action. The purpose of this paper is to provide a framework along with practical guidance for the process of assigning a mode of action. Strengths, weaknesses, reliability, and choice of a test battery are discussed for select bacterial, cell culture, whole animal and human cell assays. Common confounding factors of induced pathology, cytolethality, and regenerative cell proliferation in rodent cancer bioassays are discussed along with approaches to account for these effects in assigning a mode of action and in risk assessments. Specific examples are given to illustrate the complexity in generating a data set sufficient to move from the default regulatory position of assuming a genotoxic mode of action to actually assigning a nongenotoxic mode of action. A two-part framework is proposed for assigning a mode of action. First, a weight of evidence approach is used to assess mutagenic potential based on results of genetic toxicology test systems. Second, a descriptor is assigned to classify the degree to which mutagenic activity likely played a role in the mode of action of tumor formation. This option provides a more realistic way of describing the mode of action instead of being bound by the strict genotoxic vs. nongenotoxic choices.

Genetic toxicology of remifentanil, an opiate analgesic

Compounds that interact with opioid receptors are commonly used as analgesics. Opioid agonists vary in their potency and pharmacokinetic properties as well as in their affinity for distinct opioid receptors. The fentanyl opiate analogues are an important group of analgesics that interact with the mu opioid receptor. Remifentanil (GI87084) is a particularly interesting member of this group of opioids because its action is especially short in duration. This report examines the genetic toxicology of remifentanil. Remifentanil was not genotoxic in an Ames test, an in vitro chromosome aberration assay in Chinese hamster ovary cells, an in vivo micronucleus assay in rat erythrocytes, or an in vivo/in vitro unscheduled DNA synthesis assay in rat hepatocytes. In the in vitro L5178Y tk(+/-) mouse lymphoma assay, remifentanil produced a genotoxic response at dose levels >or=308 microg/mL only in the presence of rat liver S9 metabolic activation; primarily tiny and small mutant colonies were produced. This pattern of activity in a battery of genetic toxicology assays is not unique to remifentanil, but has also been observed for other pharmaceuticals, including the opioid fentanyl. A weight-of-evidence analysis, taking into consideration genotoxic mechanisms, in vivo results, and the conditions of clinical use, suggests remifentanil does not pose a genotoxic risk to patients.

A review of the genotoxicity of triallate

Triallate is a selective herbicidal chemical used for control of wild oats in wheat. It has an extensive genotoxicity database that includes a variety of in vitro and in vivo studies. The chemical has produced mixed results in in vitro assay systems. It was genotoxic in bacterial mutation Ames assays, predominantly in Salmonella typhimurium strains TA100 and TA1535 in the presence of S9. Weaker responses have been observed in TA100 and TA1535 in the absence of S9. Mixed results have been observed in strain TA98, whereas no genotoxicity has been observed in strains TA1537 and TA1538. The presence and absence of S9 and its source seem to play a role in the bacterial response to the chemical. There have also been conflicting results in other test systems using other bacterial genera, yeast, and mammalian cells. Chromosome effects assays (sister-chromatid exchange and cytogenetics assays) have produced mixed results with S9 but no genotoxicity without S9. Triallate has not produced any genotoxicity in in vitro DNA damage or unscheduled DNA synthesis assays using EUE cells, human lymphocytes, and rat and mouse hepatocytes. In a series of in vivo genotoxicity assays (cytogenetics, micronucleus, dominant lethal, and unscheduled DNA synthesis), there has been no indication of any adverse genotoxic effect. Metabolism data indicate that the probable explanation for the differences observed between the in vitro studies with S9 and without S9 and between the in vitro and the in vivo studies is the production of a mutagenic intermediate in vitro at high doses of triallate is expected to be at most only transiently present in in vivo studies. The weight of evidence strongly suggests that triallate is not likely to exert mutagenic activity in vivo due to toxicokinetics and metabolic processes leading to detoxification.

Dimethylhydrazine: a reliable positive control for the short sampling time in the UDS assay in vivo

In the first international guideline addressing the unscheduled DNA synthesis (UDS) assay in vivo (OECD guideline no. 486, adopted July 1997) only the genotoxic liver carcinogen N-nitrosodimethylamine (NDMA) is proposed as positive control for the short sampling time. Since NDMA is extremely volatile, alternative positive controls should be identified to facilitate handling and reduce exposure risk during routine testing. At Bayer AG and at RCC-CCR GmbH, the genotoxic but non-volatile dimethylhydrazine (DMH; as dihydrochloride) was used instead as positive control in livers of Wistar rats and to a limited extent of NRMI mice after 2-4h exposure. As shown by the data presented in this paper DMH induced a positive result in a total of 21 UDS in vivo studies over a period of 7 years. A negative result was never seen for DMH. Due to these results DMH was proven to be a suitable and reliable positive control in the UDS assay in vivo. Consequently, DMH should be considered as positive control for the short sampling time in the next issue of OECD guideline no. 486.

Lack of effect of furfural on unscheduled DNA synthesis in the in vivo rat and mouse hepatocyte DNA repair assays and in precision-cut human liver slices

The ability of furfural to induce unscheduled DNA synthesis (UDS) in hepatocytes of male and female B6C3F(1) mice and male F344 rats after in vivo administration and in vitro in precision-cut human liver slices has been studied. Preliminary toxicity studies established the maximum tolerated dose (MTD) of furfural to be 320 and 50 mg/kg in the mouse and rat, respectively. Furfural was dosed by gavage at levels of 0 (control), 50, 175 and 320 mg/kg to male and female mice and 0, 5, 16.7 and 50 mg/kg to male rats. Hepatocytes were isolated by liver perfusion either 2-4 h or 12-16 h after treatment, cultured in medium containing [3H]thymidine for 4 h and assessed for UDS by grain counting of autoradiographs. Furfural treatment did not produce any statistically significant increase or any dose-related effects on UDS in mouse and rat hepatocytes either 2-4 h or 12-16 h after dosing. In contrast, UDS was markedly induced in mice and rats 2-4 h after treatment with 20 mg/kg dimethylnitrosamine and 12-16 h after treatment of mice and rats with 200 mg/kg o-aminoazotoluene and 50 mg/kg 2-acetylaminofluorene (2-AAF), respectively. Precision-cut human liver slices from four donors were cultured for 24 h in medium containing [3H]thymidine and 0-10 mM furfural. Small increases in the net grain count (i.e. nuclear grain count less mean cytoplasmic grain count) observed with 2-10 mM furfural were not due to any increase in the nuclear grain count. Rather, it was the result of concentration-dependent decreases in the mean cytoplasmic grain counts and to a lesser extent in nuclear grain counts, due to furfural-induced cytotoxicity. In contrast, marked increases in UDS (both net grain and nuclear grain counts) were observed in human liver slices treated with 0.02 and 0.05 mM 2-AAF, 0.002 and 0.02 mM aflatoxin B(1) and 0.005 and 0.05 mM 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine. This study demonstrates that furfural does not induce UDS in the hepatocytes of male and female B6C3F(1) mice and male F344 rats after oral treatment at doses up to the MTDs. Moreover, human liver slice studies suggest that furfural is also not a genotoxic agent in human liver.

Lack of effect of coumarin on unscheduled DNA synthesis in the in vivo rat hepatocyte DNA repair assay

The ability of coumarin to induce UDS in male Sprague-Dawley CD rat hepatocytes in vivo was assessed using the unscheduled DNA synthesis (UDS) assay. From a preliminary toxicity study the oral maximum tolerated dose (MTD) of coumarin was determined to be 320 mg/kg body weight. For the UDS studies, rats were treated with 0 (corn oil control), 32 (one-tenth the MTD), 107 (one-third the MTD) and 320 (MTD) mg/kg coumarin via oral gavage. Rats were also treated with 20mg/kg body weight dimethylnitrosamine (DMN) or 50mg/kg body weight 2-acetylaminofluorene (2-AAF) as positive controls for the 2-4 hr and 12-16 hr expression of UDS, respectively. Hepatocytes were isolated by liver perfusion either 2-4 hr or 12-16 hr after treatment and cultured in medium containing [methyl-(3)H]thymidine for 4 hr and assessed for UDS by grain counting of autoradiographs. Coumarin treatment at doses of 32-320 mg/kg body weight had no statistically significant or dose-related effect on UDS in rat hepatocytes either 2-4 hr or 12-16 hr after dosing. In contrast, both DMN 2-4 hr after dosing and 2-AAF 12-16 hr after dosing produced significant increases in UDS assessed as the net nuclear grain count. Both genotoxins also increased the percentage of hepatocyte nuclei with greater than 5 net grains. Treatment with coumarin, DMN and 2-AAF had no statistically significant effect on the proportion of rat hepatocytes undergoing replicative DNA synthesis. In summary, this study demonstrates that coumarin does not induce UDS in hepatocytes of male Sprague-Dawley CD rats after oral administration at doses up to the MTD of 320 mg/kg. The responsiveness of the animals used in this study to genotoxic agents was demonstrated by the clear induction of DNA repair after treatment with DMN and 2-AAF.

Inhalation toxicity and genotoxicity of hydrochlorofluorocarbon (HCFC)-225ca and HCFC-225cb

The acute, subchronic and genetic toxicity of the hydrochlorofluorocarbons HCFC-225ca and HCFC-225cb were evaluated to assist in establishing proper handling guides. In acute inhalation studies, rats were exposed for 4 h to various concentrations of each isomer. Based on the mortality incidence, the LC50 value for HCFC-225cb for males and females (combined) was 36800 ppm. For HCFC-225ca, the LC50 for males and females (combined) was 37300 ppm. Narcotic-like effects, e.g. prostration, incoordination and reduced motor activity, were observed during exposure to either isomer, but these signs were not evident 15 min after termination of exposure. Histopathological examination of the liver revealed an increase in mitotic figures with vacuolation of hepatocytes and fluid-filled, congested hepatic sinusoids. In cardiac sensitization studies, HCFC-225cb induced a cardiac sensitization response at 20000 ppm, with one fatal response, whereas a blend of the two isomers (45% HCFC-225ca/55% HCFC-225cb) produced a cardiac sensitization response at 15000 ppm. In 4-week subchronic inhalation studies, male and female rats were whole-body exposed to HCFC-225cb at concentrations of 0, 1000, 5000 or 15000 ppm for 6 h a day, 5 days per week. Similarly, male and female rats were whole-body exposed to HCFC-225ca concentrations of 0, 50, 500 or 5000 ppm for 6 h a day, 5 days per week. During exposure, narcotic-like and irritant effects were observed. A dose-related decrease in cholesterol and triglycerides was observed in the treated rats, with males being affected more than females. Increases in liver weight were observed in most male and female rats exposed to either isomer. The increase in liver weight was consistent in male rats with microscopic evidence of hepatocyte hypertrophy. Although liver weight was increased in female rats, no hepatocyte enlargement was observed in treated female rats. Increases in cytochrome P-450 and beta-oxidation activities were also observed in male and female rats exposed to either isomer. Neither of the HCFC-225 isomers was mutagenic in the Ames reverse mutation assay, or clastogenic in the chromosomal aberration assay with Chinese hamster lung cells. Also, neither isomer induced unscheduled DNA synthesis in liver cells. However, both isomers were clastogenic in the chromosomal aberration assay with human lymphocytes in the absence of S-9. No increases in aberrant cells were observed in activated cells exposed to either isomer.

Evaluation of possible genotoxic mechanisms for acrylonitrile tumorigenicity

Acrylonitrile (ACN) exposure is associated with tumors in rat brain, Zymbal gland, and mammary gland. Adducts affecting base pairing were formed in isolated DNA exposed in vitro to the ACN metabolite cyanoethylene oxide (CNEO). DNA from liver, which is not a cancer target organ in ACN-exposed rats, contained low levels of 7-(2-oxoethyl)guanine, and adduct believed not to interfere with base pairing. No adducts have been detected in brain DNA from ACN-exposed rats, suggesting that brain tumors may have arisen by mechanisms other than ACN-DNA reactivity. Genotoxicity assays of ACN have indicated no particular carcinogenic mechanism. Positive reverse mutagenesis in Salmonella typhimurium HisG46 base substitution tester strains by ACN is attributable to CNEO. Other in vitro genotoxicity test assays of ACN have yielded mixed results, without consistent effect of metabolic activation. Some positive genotoxicity data for ACN appear to result from artifacts or from non-DNA-reactive mechanisms. In vivo micronucleus, chromosome aberration, and autoradiographic unscheduled DNA synthesis assays were negative for ACN. The comparative genotoxicity of vinyl chloride and ACN indicates that despite other similarities, they cause rodent tumors by different mechanisms. Also, they absence of ACN-DNA adduct formation in the rat brain suggests the operation of epigenetic mechanisms.

Genotoxicity of aloeemodin in vitro and in vivo

The present in vitro and in vivo experiments were undertaken to clarify the genotoxic potential of the hydroxyanthrachinone aloeemodin which can be found in different plant derived products for therapy of constipation. The results demonstrate that aloeemodin is able to induce mutagenic effects in vitro. Positive results were obtained in the chromosomal aberration assay with CHO cells, as well as in the Salmonella reverse mutation assay (frameshift mutations in strains TA 1537, TA 1538 and TA 98). No mutagenic potential of aloeemodin, however, was observed in the gene mutation assay with mammalian cells in vitro (HPRT assay in V79 cells). Each assay was performed in the presence and absence of an extrinsic metabolic activation system (S9-mix). In in vivo studies (micronucleus assay in bone marrow cells of NMRI mice; chromosome aberration assay in bone marrow cells of Wistar rats; mouse spot text [DBA/2JxNMRI]) no indication of a mutagenic activity of aloeemodin was found. Information about a possible reaction of aloeemodin with DNA was derived from an in vivo UDS assay. Hepatocytes of aloeemodin-treated male Wistar rats did not show DNA damage via repair synthesis. All these data suggest that aloeemodin is able to interact with DNA under certain in vitro conditions. However, in vivo the results that were negative did not indicate a genotoxic potential. Therefore, it may be assumed that a genotoxic risk for man might be unlikely.

An in vitro and an in vivo unscheduled DNA synthesis assay with a zinc oxide/hexachloroethane (Zn/HCE) smoke

1. Since Zn/HCE smoke has been shown previously to be weakly positive in the Ames test, and negative in the bone marrow micronucleus assay, other assays including a second in vivo assay examining unscheduled DNA synthesis (UDS) in rat hepatocytes has been carried out, as recommended by the UK Department of Health guidelines. 2. Zn/HCE smoke was assessed for its ability to induce DNA repair in an UDS assay both in vitro in cultured rat hepatocytes and in rat hepatocytes after in vivo treatment by inhalation. 3. For the in vitro investigation, two studies were carried out assessing media exposed to Zn/HCE smoke using at least seven concentrations up to a toxic level. At the highest concentration of Zn/HCE smoke, where some viable cells were seen, an increase in UDS was observed in both experiments. However this was not statistically significant, was only seen at a level where toxicity was observed and was therefore considered not to be biologically significant. 4. In the in vivo investigation, one study was carried out in three separate parts, assessing two doses of Zn/HCE smoke characterised by their zinc content as approximately 20 and 56 micrograms l-1 air. A dose-related increase in UDS was observed which was not statistically significant. The positive control behaved as anticipated, showing a highly statistically significant response. 5. It was concluded that Zn/HCE smoke did not induce unscheduled DNA repair in the in vitro or in vivo UDS assays under the conditions used in the studies. The overall lack of genotoxic effect of this smoke in this and previous studies in this laboratory would not suggest a major health hazard.

Comparison of the microgel electrophoresis assay and other assays for genotoxicity in the detection of DNA damage

Genotoxic agents can be detected by measuring DNA damage which result in the migration of DNA from single cells in agarose, using an electrophoretic field under alkaline conditions. The alkaline microgel electrophoresis technique was compared with in vitro structural chromosomal aberration (SCA) and mutation assays using V79 Chinese hamster lung cells and in vivo assays such as the bone marrow micronucleus assay in mice and a hepatocyte DNA repair assay in rats. Genotoxicants tested were those routinely used as positive control compounds in the various assays. In vitro assays included liver S9 for metabolic activation of cyclophosphamide (CP) for the SCA assay and benzo[a]pyrene (BP) for the mutation assay. A highly significant increase in DNA migration was induced by these agents under circumstances where a significant increase in DNA damage was detected using other endpoints. The alkaline microgel electrophoresis assay thus demonstrated the ability to detect DNA damage coinciding with the induction of DNA damage detected in these other assays for genotoxicity.

Comparison of autoradiography, liquid scintillation counting and immunoenzymatic staining of 5-bromo-2'-deoxyuridine for measurement of unscheduled DNA synthesis and replicative DNA synthesis in rat liver

Different methods for evaluating unscheduled DNA synthesis (UDS) and replicative DNA synthesis (RDS) were studied in hepatocytes of F344 rats exposed in vivo to dimethylnitrosamine (DMN) or CCl4. Hepatocytes were isolated and incubated in Williams' medium E supplemented with either [3H]thymidine for autoradiography or 5-bromo-2'-deoxyuridine for immunoenzymatic staining. In the method of liquid scintillation counting, the cells were incubated with [3H]thymidine with or without hydroxyurea. The nuclear fraction was isolated and the incorporation of [3H]thymidine into nuclear DNA was determined by a liquid scintillation counter. DMN at doses of 0.625-5 mg/kg body weight induced UDS of 1.6-37.9 (0 dose; -6.9) net grains/nucleus measured by autoradiography and 337-1377 (0 dose; 177) dpm/microgram DNA in the presence of hydroxyurea measured by a liquid scintillation counter. CCl4 at doses of 50-400 mg/kg body weight induced RDS in 1.5-12.1% (0 dose; 0.12%) and 1.8-14.6% (0 dose; 0.16%) of cells with the methods of autoradiography and immunoenzymatic staining, respectively, and of 2991-24256 (0 dose; 324) dpm/microgram DNA in the absence of hydroxyurea with the method of liquid scintillation counting. Similar dose-dependent induction of UDS and RDS was observed with these methods. These results suggest that the methods of liquid scintillation counting and immunoenzymatic staining have almost the same sensitivity for measuring UDS and RDS as that of autoradiography.

UDS-test with freon 11 (R-11)

Trichlorofluoromethane, in concentrations of 80, 400, 2 000, 10 000 and 50 000 ppm, was administered to rats of both sexes (Sprague Dawley) by inhalation exposure.2 000 ppm in air (= 11 200 mg/m(3)) amount to twice the MAK value of 1 000 ppm. At exposure times of 4 hours this corresponds to the MAK value defined for an 8 hour workday.Unscheduled DNA synthesis (UDS) was measured in single-cell suspensions of hepatocytes, pulmonary epithelial cells and lymphocytes of the spleen, respectively.In the pulmonary cells concentrations of 2 000, 10 000 and 50 000 ppm of freon 11 lead to a significantly increased mean silver grain count compared to a negative control group.In spleen and liver cells increasing concentrations of R 11 tend to increase the incorporation of thymidine into the DNA of the cells. These changes of the extent of unscheduled DNA synthesis can, however, not be statistically verified.

The effect of reducing the number of cells scored on the performance of the in vivo rat liver UDS assay

The most labour-intensive feature of the in vivo rat liver UDS assay is the scoring of hepatocyte autoradiograms by microscope. Even with image analyser and computer equipment the scoring phase of a full study might require half of the technical effort applied. Practice recommended by guidelines has been to score 50 cells/slide and two slides per animal. Now sufficient data have been accumulated, an evaluation was made to observe whether this procedure was necessary. An analysis of the accumulated UDS database in our laboratory was made to determine the sources of variability of mean net nuclear grain count, [N-C]. It was observed that the two largest components of variation in negative control animal mean [N-C] were between-day and interanimal variability. The contribution from sampling error during slide scoring was relatively small. Theoretical calculations showed that the greater sampling error derived from scoring 30 rather than 50 cells/slide would result in only a marginal increase in total assay variation. To test this, 30 cells/slide were randomly selected from the 50 cells scored originally in negative control animals in each of 18 studies over an 18-month period. It was confirmed that reducing the number of cells had a negligible effect on the variation of negative control animal mean [N-C] values. Furthermore, analysis of data from 10 more studies confirmed that within-study variation would be essentially unaffected by scoring 30 cells/slide. The use of 30 rather than 50 cells per slide (a total of 60 cells per animal) has therefore been adopted for all current studies and scoring procedures modified to avoid operator bias during the selection of a smaller number of cells.

Detection of polycyclic aromatic hydrocarbon exposure damage using different methods in laboratory animals

Benzo[a]pyrene, benzo[b]fluoroanthene and dibenzo[a,h]anthracene dissolved in a 1:2 mixture of dimethylsulphoxide (DMSO) and water were administered to two groups of female mice, each group containing 15 mice. The doses were administered orally (via gavage) at the respective rates of 1 and 100 micrograms kg-1 body weight five times per week for a period of 9 weeks. The influence of the polycyclic aromatic hydrocarbons (PAHs) was determined using the following methods: determination of DNA-PAH adducts, of chromosome injuries (micronucleus test), of induction of repair using the unscheduled DNA synthesis (UDS) test, and by examination of the DNA structure after nucleoid sedimentation. All the methods investigated provided evidence of a significant effect resulting from exposure to PAHs on the parameters examined. Following chronic exposure to PAHs, the formation of DNA-PAH adducts and injury to the genetic material, as well as the appearance of micronuclei (micronucleus test), the induction of unscheduled DNA synthesis (UDS test) and mutation of the DNA structure (nucleoid sedimentation), were demonstrated. The described methods therefore provide a means for the detection of genetic damage caused by PAH exposure in humans.

Effects of dietary restriction on induction of unscheduled DNA synthesis (UDS) and replicative DNA synthesis (RDS) in rat liver

The effects of dietary restriction on the induction of unscheduled DNA synthesis (UDS) and replicative DNA synthesis (RDS) were studied in the hepatocytes of F344 rats exposed in vivo to dimethylnitrosamine (DMN) or CCl4. The animals were given food ad libitum, a restricted amount of food (4 g/rat/overnight) or no food. Hepatocytes were isolated 2 h after oral administration of DMN at a dose of 5 mg/kg body weight and 48 h after oral administration of CCl4 at a dose of 400 mg/kg body weight, and incubated for 4 h in Williams' medium E supplemented with either [3H]thymidine for UDS or 5-bromodeoxyuridine for RDS. UDS was determined by autoradiography and RDS was determined by the immunoenzymatic staining method. The background levels of UDS (net grains/nucleus) and RDS (cells in S phase) in control were -12.4 and 0.64% for ad libitum feeding, -6.8 and 0.04% for restricted feeding, and -8.1 and 0% for fasting. UDS induced by DMN and RDS induced by CCl4 were 19.4 and 3.3% for ad libitum feeding, 34.5 and 10.4% for restricted feeding, and 47.8 and 15.1% for fasting. DMN demethylase activity in rat liver was also found to increase with dietary restriction. These results indicate that dietary restriction modulates the responses of UDS and RDS in the liver of rats.

Investigations of the genotoxicity and cell proliferative activity of dichlorvos in mouse forestomach

This study investigated the possible mechanism by which dichlorvos may have caused forestomach tumours in mice in a chronic corn oil gavage cancer bioassay [NTP (1989) Toxicology and carcinogenesis studies of dichlorvos in F344/N rats and B6C3F1 mice (gavage studies). National Toxicology Program Technical Report 342, NIH Publ. No 89-2598]. For this purpose, a method has been developed to assess the genotoxicity of irritant substances on mouse forestomach epithelium. Groups of five B6C3F1 mice were given a single oral dose of dichlorvos, the genotoxic forestomach carcinogen 1-methyl-3-nitro-1-nitrosoguanidine (MNNG) or the irritant, non-genotoxic forestomach carcinogen butylated hydroxyanisole (BHA). After periods of 2-48 h, three parameters were assessed: unscheduled DNA synthesis (UDS) by autoradiography of tissue sections, replicative DNA synthesis (RDS) also by autoradiography of incorporated [3H]thymidine, and histopathological changes, including hyperplasia. MNNG induced UDS but not RDS or hyperplasia in forestomach epithelium, consistent with its genotoxic mode of action. BHA and dichlorvos did not induce UDS, consistent with absence of genotoxic activity in the forestomach after in vivo exposure. In contrast, BHA and dichlorvos induced RDS and subsequent hyperplasia, which is likely to result from irritant damage. These data suggest that the chronic effects of dichlorvos on mouse forestomach epithelium in the oral gavage bioassay were mediated via enforced cell proliferation, rather than by a genotoxic mechanism.

Lack of chloroform-induced DNA repair in vitro and in vivo in hepatocytes of female B6C3F1 mice

Chloroform has been shown to induce hepatocellular carcinomas in female B6C3F1 mice when administered by gavage, but not when given in drinking water. When administered in corn oil at the carcinogenic doses of 238 and 477 mg/kg, chloroform induced necrosis and sustained regenerative cell proliferation in the liver. To investigate the mode of action of tumor induction in the target cells, the ability of chloroform to induce unscheduled DNA synthesis (UDS) was examined in the in vitro and in vivo hepatocyte DNA repair assays. In the in vitro assay, primary hepatocyte cultures from female B6C3F1 mice were incubated with concentrations from 0.01 to 10 mM chloroform in the presence of 3H-thymidine. UDS was assessed by quantitative autoradiography. No induction of DNA repair was observed at any concentration. In the in vivo assay, animals were treated by gavage with 238 and 477 mg/kg chloroform in corn oil. Primary hepatocyte cultures were prepared 2 and 12 hr later, incubated with 3H-thymidine, and assessed for induction of UDS as above. No DNA repair activity was seen at either dose or at either timepoint. These negative results in the target organ are consistent with the concept that neither chloroform nor its metabolites are directly DNA reactive and that the carcinogenicity of chloroform is secondary to induced cytolethality and regenerative cell proliferation.

Rapid DNA degradation in primary rat hepatocytes treated with diverse cytotoxic chemicals: analysis by pulsed field gel electrophoresis and implications for alkaline elution assays

The use of genetic toxicology tests for hazard identification is complicated by the fact that some in vitro tests using cultured mammalian cells are subject to potential artifacts that can make it difficult to distinguish between direct, chemically-induced genotoxicity, and DNA damage that occurs secondary to chemically-induced cytotoxicity (e.g., mediated by endogenous nucleases). Recently, we demonstrated that cytotoxicity-induced DNA double strand breaks (dsb) can produce artifacts in the in vitro alkaline elution/rat hepatocyte assay [Elia et al., 1993]. To explore this further, we used pulsed field gel/DNA dsb assays to characterize the relationship between chemically-induced cytotoxicity and the degradation of genomic DNA to high molecular weight fragments. Two sets of compounds were tested: 17 cytotoxic agents judged to be neither genotoxic nor carcinogenic, and 10 known genotoxic carcinogens. We found a close correlation between chemically-induced cytotoxicity and the rapid degradation of DNA to high molecular weight, double-stranded fragments. In contrast, the classic genotoxic chemicals tested generally did not trigger DNA dsb fragmentation at doses that were genotoxic but not immediately cytotoxic. These data indicate that pulsed field gel/DNA dsb assays can be used with in vitro genetic toxicology assays to help distinguish between genotoxic and cytotoxic mechanisms of DNA damage.

Assessment of the relation between the initial viability and the attachment of freshly isolated rat hepatocytes used for the in vivo/in vitro DNA repair assay (UDS)

Crucial steps of the in vivo/in vitro DNA repair assay (UDS) are the hepatocyte isolation procedure and the establishment of the hepatocyte cultures. Since the attachment of the isolated hepatocytes on the surface of the culture vessel is an essential prerequisite for the in vitro part of this assay to yield scorable autoradiograms, we assessed the relation between the initial viabilities of hepatocyte preparations and the resulting attachment efficiencies from 286 rats. The initial viability was determined by means of the trypan blue dye exclusion assay. The actual cell number was corrected for the viability and a constant number of 2.5 x 10(5) viable cells were seeded into each well of gelatinized six-well dishes. The amount of adherent cells was determined after a 1.5-h attachment period using a recently described modification (Fautz et al., 1991) of the neutral red dye absorption assay. The attachment is described by the optical density at 540 nm obtained after the elution of neutral red from the adherent cells (OD540 value). To facilitate a comparison of the data we divided the 286 animals into eight arbitrary viability groups. The mean values of the viability groups were 53.1, 62.2, 66.3, 68.4, 70.9, 73.6, 76.9, and 84.0% living cells. Although there was a great interindividual variation, the resulting mean OD540 values were nearly uniform, about 0.5, in all eight groups, regardless of the initial viability of the hepatocytes. UDS data obtained from 46 animals treated with the positive control chemical 2-acetylaminofluorene demonstrated that there was no correlation between the in vitro DNA repair capacity and the initial viability or the attachment efficiency of the hepatocytes. Our results suggest that (i) great interindividual differences exist between the attachment of particular cell preparations with no regard to the initial viability, (ii) the correction of the cell number for viability leads to relatively uniform OD540 mean values and (iii) for an in vivo/in vitro UDS assay even cell suspensions with relatively low viabilities can be used since they will yield adherent cultures which are capable of DNA repair synthesis. The latter item often allows a reduction in the number of animals required for this in vivo assay because it is not necessary to perform repeated experiments because of low viability preparations.

In vivo-in vitro replicative DNA synthesis (RDS) test using perfused rat livers as an early prediction assay for nongenotoxic hepatocarcinogens: I. Establishment of a standard protocol

To establish a standard protocol for an in vivo-in vitro hepatocyte replicative DNA synthesis (RDS) test using male F344 rats for screening nongenotoxic (the Ames-negative) hepatocarcinogens, experimental conditions were examined. After treatment with three model hepatocarcinogens, isolated hepatocytes showed highest RDS incidences when plated at a density of 5 x 10(4) cells/ml. Spontaneous RDS incidences in hepatocytes from rats aged 9 weeks or older showed a constant value. The use of hepatocytes from 9-week-old rats at the 5 x 10(4) cells/ml plating density was therefore determined as the standard. Based on the distribution of mean spontaneous RDS incidences over 105 additional experiments (0.4 +/- 0.18%, with SEM), an RDS incidence of over 1% was adopted as the criterion for a positive response in our rat liver RDS test.

Inhibition of sulfotransferase affecting unscheduled DNA synthesis induced by 2-acetylaminofluorene: an in vivo and in vitro comparison

The unscheduled DNA repair (UDS) assay was conducted using the in vivo and in vitro procedures to investigate the role of arylsulfotransferases (AST) in the genotoxicity of 2-acetylaminofluorene (AAF). The in vivo assay had 4 groups of rats that consisted of those treated with pentachlorophenol (PCP), PCP and AAF, or AAF and an untreated control. The in vitro assay used hepatocytes from 3-methylcholanthrene or corn oil (control) treated rats. In both the in vivo and in vitro UDS assays AAF induced DNA damage. PCP, an inhibitor of arylsulfotransferase, significantly decreased AAF induced DNA damage. In the in vivo assay, PCP induced a significant increase in UDS and confounded an investigation of the role of sulfotransferase. The in vitro UDS assay more clearly defined the effect of PCP on AAF genotoxicity.

D and C Red No. 9: Genotoxic or non-genotoxic carcinogen?

The azo-compound, D and C Red No. 9 was assayed for genotoxicity in vivo using the rat micronucleus test and the rat ex vivo liver UDS assay. Uniformly negative results were obtained in both assays, even though large oral doses were used (2 g/kg). These results suggest that the tumorigenic effects of this compound in rats are mediated through non-genotoxic rather than a genotoxic mechanism. Further experiments using additional end-points such as 32P-post-labelling would further substantiate this conclusion.

Chromosome aberrations, micronuclei and sister-chromatid exchanges (SCEs) in rat liver induced in vivo by hepatocarcinogens including heterocyclic amines

The induction of chromosome aberrations, micronuclei and SCEs was studied in hepatocytes of F344 rats exposed in vivo to hepatocarcinogens. Hepatocytes were isolated and allowed to proliferate in Williams' medium E supplemented with epidermal growth factor. Cells were fixed after a culture period of 48 h. Oral administration of dimethylnitrosamine at doses of 2.5-20 mg/kg body weight (bw) induced (1) chromosome aberrations in up to 27% of the metaphase cells 2-48 h after its administration, (2) SCEs with a frequency of up to 0.9 per chromosome 2-48 h after its administration, and (3) micronuclei in up to 2.9% of the cells 16-48 h after its administration. Oral administration of 2-acetylaminofluorene at doses of 6.25-200 mg/kg bw induced (1) chromosome aberrations in up to 35% of the metaphase cells after 2-48 h, (2) SCEs at up to 0.9 per chromosome and (3) micronuclei in up to 2.5% of the cells with a maximum after 4 h. Oral administration of CCl4, a non-genotoxic hepatocarcinogen, at a dose of 1600 mg/kg bw did not induce chromosome aberrations, SCEs or micronuclei within 4-72 h. Intraperitoneal injections of Trp-P-1, Glu-P-1, MeIQx, IQ and nitro-IQ resulted in chromosome aberrations in up to 16% of the metaphase cells and SCEs at up to 0.9 per chromosome, while injections of Trp-P-2 and Glu-P-2 produced SCEs at up to 0.7 and 1.1 per chromosome, respectively. The present method of in vivo cytogenetic assay using rats without partial hepatectomy or mitogen treatment in vivo should be useful for evaluating the tumor-initiating activities of hepatocarcinogens.

Relative sensitivity of 32P-postlabelling of DNA and the autoradiographic UDS assay in the liver of mice exposed to 2-acetylaminofluorene (2AAF)

In contrast to earlier studies conducted at lower dose levels, 2AAF is shown to induce a positive UDS response in the liver of mice dosed orally at dose levels between 500 and 1000 mg/kg. Similarly exposed mice had low levels of 2AAF-related hepatic DNA adducts at dose levels in the range 10-1000 mg/kg 2AAF, as determined by 32P-postlabelling analysis. It is concluded that the attenuated UDS response observed in the mouse liver, as compared to the rat liver, is due primarily to metabolic differences between these two species, coupled to a reduced capacity for UDS in the mouse liver for a given level of total 2AAF-related adducts per unit of DNA. These observations are compared and contrasted with identical studies conducted in the rat and reported in the preceding paper (Gallagher et al., 1991).

Relative sensitivity of 32P-postlabelling of DNA and the autoradiographic UDS assay in the liver of rats exposed to 2-acetylaminofluorene (2AAF)

Groups of male Alderley Park rats were dosed concomitantly with 2-acetylaminofluorene (2AAF) by gavage at doses between 0.01 mg/kg and 40 mg/kg, and livers sampled 2-72 h later. The liver of one group of animals was perfused to yield hepatocytes which were assayed in vitro for unscheduled DNA synthesis (UDS) via incorporation of tritiated thymidine and autoradiography. DNA was extracted from the livers of the other group and DNA adduct levels determined using the 32P-postlabelling technique. The major C-8 2-aminofluorene/guanosine adduct and 3 minor adducts were quantitated, enabling the relative sensitivity of the 2 techniques to be compared. A dose- and time-related UDS response was observed, which, at the most sensitive time-point (12 h) enabled DNA repair to be discerned at a dose level of 0.1-1 mg/kg of 2AAF, a response classified as formally positive at 5 mg/kg 2AAF. Only the C-8 adduct, as determined by 32P-postlabelling, was discernible at 0.01 mg/kg of 2AAF, although other adducts were visible on autoradiograms at higher dose levels. It is concluded that as part of a well-defined dose response, UDS can be discerned with confidence for doses of 2AAF between approximately 0.1 and 5 mg/kg, and DNA adducts for doses of 2AAF between approximately 0.01 and 1 mg/kg. Discernible UDS for 2AAF in the rat liver is apparent at approximately 13 DNA (total) adducts/10(8) nucleotides, or approximately 8 DNA (C-8) adducts/10(8) nucleotides. The presumed C-8 2-acetylaminofluorene/guanosine adduct, prepared by reaction of 2-acetoxy-2-acetylaminofluorene (2AAAF) with DNA, was a significant but unreliable marker of 2AAF/DNA adducts in the rat liver in vivo. DNA repair did not appear to remove DNA adducts selectively, and adducts remained in DNA when discernible DNA repair had ceased.

The application of a wedge perfusion technique to the in vivo-in vitro rat hepatocyte DNA-repair assay

The in vivo-in vitro rat hepatocyte DNA-repair assay is regarded as labour-intensive and time-consuming to perform. This has tended to impose limitations on its use as a routine procedure for assessing the potential genotoxicity of chemicals. We have developed a simple wedge-perfusion technique which enables hepatocytes to be isolated from several different rats simultaneously. Hepatocyte yield and metabolic capacity are comparable to those isolated by conventional whole-liver perfusion. Hepatocyte viability was generally superior to that obtained when performing multiple in situ perfusions for the rat hepatocyte UDS assay. The median lobe is routinely used but no difference was observed in the UDS response to the positive control genotoxic agents, methyl methanesulphonate (MMS, CAS No. 66-27-3) and 2-acetylaminofluorene (AAF, CAS No. 53-96-3), in hepatocytes isolated from the median or either lateral lobe. The use of Williams medium E or Leibovitz L15 culture medium did not influence the response. This perfusion technique greatly reduces the time, equipment and personnel required and therefore the cost for hepatocyte isolation. It also facilitates the inclusion of concurrent control groups at each time point of assay.

Evaluation of the mutagenic potential of the antichagasic drug Rochagan in healthy and chagasic rodents

Benznidazole (bz) is the active component of the antichagasic drug Rochagan. Tests were carried out to detect the induction of chromosomal aberrations and micronuclei in rodent bone marrow cells and peripheral blood cells, respectively. Rats were exposed to acute treatment with Rochagan by gavage at total doses of 150, 300, 1500, 2000 and 3000 mg bz/kg body weight and killed at different times. In the chronic treatments, healthy and chagasic Balb/c mice were treated with Rochagan by gavage at a dose of 100 mg bz/kg/day for 10 and 25 days. No significant increase in frequency of chromosomal aberrations in bone marrow cells or of micronuclei in peripheral blood cells was detected in the animals acutely or chronically exposed to Rochagan in vivo.

Examination of potential mechanisms of carcinogenicity of 1,4-dioxane in rat nasal epithelial cells and hepatocytes

Several long-term studies with 1,4-dioxane (dioxane) have shown it to induce liver tumors in mice, and nasal and liver tumors in rats when administered in amounts from 0.5 to 1.8% in the drinking water (Argus et al. 1965; Kociba et al. 1974; National Cancer Institute, 1978). In order to examine potential mechanisms of action, chemically-induced DNA repair (as an indicator of DNA reactivity) and cell proliferation (as an indicator of promotional activity) were examined in nasal turbinate epithelial cells and hepatocytes of male Fischer-344 rats treated with dioxane. Neither dioxane nor 1,4-dioxane-2-one, one of the proposed metabolites, exhibited activity in the in vitro primary rat hepatocyte DNA repair assay, even from cells that had been isolated from animals given either 1 or 2% dioxane in the drinking water for 1 week to induce enzymes that might be responsible for producing genotoxic metabolites. No activity was seen in the in vivo hepatocyte DNA repair assay in animals given a single dose of up to 1000 mg/kg dioxane or up to 2% dioxane in the drinking water for 1 week. Treatment of rats with 1.0% dioxane in the drinking water for 5 days yielded no increase in liver/body weight nor induction of palmitoyl CoA oxidase, indicating that dioxane does not fit into the class of peroxisomal proliferating carcinogens. The percentage of cells in DNA synthesis phase (S-phase) was determined by administration of 3H-thymidine and subsequent quantitative histoautoradiography. The hepatic labeling index (LI) did not increase at either 24 or 48 h following a single dose of 1000 mg/kg dioxane.(ABSTRACT TRUNCATED AT 250 WORDS)

Cell isolation and genotoxicity assessment in gastric mucosa

It has been claimed that in vitro digestion of in vivo DNA-labeled gastric mucosa is suitable for evaluation of genotoxic effects of drugs or chemicals. This method was then used to show that omeprazole (a novel antiulcer drug) was potentially genotoxic. In this study we have examined the method used and the interaction of omeprazole and its derivatives with purified DNA. The method was shown to enrich for dividing cells (6.92 +/- 0.693%, N = 43, 2-hr labeling) in the digest from the intact tissue and was therefore unsuitable for estimating unscheduled DNA synthesis in the gastric mucosa induced by chemicals or drugs including omeprazole. It was further shown that neither omeprazole or its acid-activated product, a cationic sulfenamide, were able to react with isolated purified DNA from either a prokaryote (E. coli) or a eukaryote (salmon sperm). Hence any conclusions using this method attributing acute genotoxic effects to any chemical are based on unrecognized artifacts of the technique and are unsound. In addition, these results negate the suggestion that omeprazole or its gastric metabolites are genotoxic.

In vivo short-term assays of repair and replication of rat liver DNA

A short-term in vivo method for assay of repair and replication of rat liver DNA has been developed, by which possible hepatocarcinogens could be identified in a few days. F344 rats were treated orally with two genotoxic hepatocarcinogens, dimethylnitrosamine (DMN) and 2-acetylaminofluorene (2AAF), or a nongenotoxic hepatocarcinogen, carbon tetrachloride (CCl4). Then at suitable times after treatment, their hepatocytes were isolated by a two-step collagenase perfusion technique in situ and incubated with [3H]dThd with or without hydroxyurea, which inhibits DNA replication. Their nuclear DNA was then extracted and the incorporation of [3H]dThd into nuclear DNA was determined in a liquid scintillation counter. Unscheduled DNA synthesis (DNA repair), induced by DMN at doses of 2.5-10 mg/kg body weight and by 2AAF at doses of 12.5-50 mg/kg body weight, could be detected 2 h and 4 h after their administration as an increase of DNA synthesis of up to 5.8-fold and 6.0-fold, respectively, in the presence of hydroxyurea. Replicative DNA synthesis, induced by CCl4 at a dose of 200 mg/kg body weight, could be detected 48 h after its administration as a 23-fold increase of DNA synthesis in the absence of hydroxyurea and was inhibited approximately 97%-99% by hydroxyurea. Replicative DNA synthesis induced by 2AAF at a dose of 25 mg/kg body weight 16 h after its administration could be detected as a 6.8-fold increase of DNA synthesis in the absence of hydroxyurea. These results show that unscheduled and replicative DNA synthesis can be clearly distinguished by simultaneous measurements of the incorporation of [3H]dThd into nuclear DNA in the presence and absence of hydroxyurea.

Testing of hydralazine in in vivo-in vitro hepatocyte assays for UDS and stimulation of replicative DNA synthesis

The vasodilator hydralazine was tested for induction of DNA-repair synthesis and stimulation of replicative DNA synthesis in rat hepatocytes after administration in vivo, either once or repetitively. No increase in unscheduled or replicative DNA synthesis was observed. By contrast, positive controls clearly induced DNA-repair synthesis, either after a single treatment (4-aminobiphenyl, dimethylnitrosamine and methyl methanesulphonate) or after repetitive treatment (benzo[a]pyrene), or stimulated replicative DNA synthesis (carbon tetrachloride and dimethylnitrosamine). Thus, hydralazine displayed no genotoxic and no tumour-promoting activity in these in vivo-in vitro test systems.

The rat-liver carcinogen N-nitrosomorpholine initiates unscheduled DNA synthesis and induces micronuclei in the rat liver in vivo

Alkylation of DNA is generally accepted as the primary event in the carcinogenicity of nitrosamines. However, the cyclic nitrosamine N-nitrosomorpholine (NMOR), a potent rat hepatocarcinogen, has been reported as binding at very low levels to the liver DNA of treated rats. This led us to investigate the activity of NMOR in two in vivo rat-liver genotoxicity assays--for the induction of unscheduled DNA synthesis (UDS) and the production of micronucleated hepatocytes in the liver micronucleus assay (LMN). Rats treated with oral doses of NMOR (10-200 mg/kg) gave a positive liver UDS response either 2.5 h or 12 h after dosing. Similarly, treatment with oral doses of NMOR (10 or 100 mg/kg) followed by mitogenic stimulation with 4-acetylaminofluorene (4AAF) resulted in high incidences of micronucleated hepatocytes in the LMN assay. These data confirm that the genotoxicity reported for NMOR in vitro can be reproduced in vivo and that NMOR interacts with liver DNA of treated rats. Earlier reports of only very weak binding of radiolabelled NMOR to rat liver DNA in vivo are discussed within the context of these data.

An automated alkaline elution system: DNA damage induced by 1,2-dibromo-3-chloropropane in vivo and in vitro

An automated alkaline elution system for the detection of DNA damage has been developed. After manual application of samples, which is completed within 5 min, the subsequent supply of liquids, changes in flow rates, and temperature are controlled automatically. The system operates 16 filters and may easily be expanded. The sensitivity of the fluorometric DNA determinations with the Hoechst 33258 dye is increased by using an elution buffer (20 mM Na2EDTA, pH 12.50) with low background fluorescence. DNA is determined using an automated setup similar to the one recently presented by Sterzel et al. (1985, Anal. Biochem. 147, 462-467). The most significant modification is the use of a neutralization buffer which allows variations in the pH of eluted fractions. This change increases the sensitivity of the DNA measurements. The automated alkaline elution system was evaluated using the nematocide 1,2-dibromo-3-chloropropane (DBCP) in a study of its genotoxic effects in the testes and the kidneys. Significant DNA damage was induced in testicular cells by 2.5 microM DBCP (1 h) in vitro and 85 mumol/kg DBCP ip (3 h) in vivo. The damage appeared after short treatment times (10 min in vivo). Variations in the observed DBCP response in vivo were largely due to interanimal variations. The automated alkaline elution system proved to be a sensitive assay also for the detection of DNA damage in kidney nuclei prepared from rats exposed to DBCP. Provided that kidney nuclei from untreated rats, mice, or hamster were kept ice-cold until lysing, 85-100% of their DNA was retained after 16 h of elution, indicating highly intact DNA. Under the same conditions, guinea pig DNA was rapidly degraded unless the nuclei were prepared in a buffer with a higher concentration of Na2EDTA (20 mM).

The potent hepatocarcinogen methapyrilene does not form DNA adducts in livers of Fischer 344 rats

The antihistamine methapyrilene hydrochloride has been shown to be a potent hepatocarcinogen in Fischer 344 rats. It has also been evaluated in a number of short-term in vitro genotoxicity assays resulting in conflicting reports. Short-term in vivo assays suggest that it may act as a promoter. We studied its ability to form DNA adducts in the target organ using the highly sensitive 32P-postlabeling technique. Methapyrilene failed to induce formation of DNA adducts in hepatocellular DNA at doses which induced S-phase DNA synthesis. These data suggest that methapyrilene does not induce the carcinogenesis process through a direct genotoxic mechanism.

A non-invasive micronucleus assay in the rat liver

The potent rat-liver mitogen 4-acetylaminofluorene (4AAF) is shown here to provide an effective replacement for the surgical procedure of 2/3 partial hepatectomy (2/3PH) in the in vivo rat-liver micronucleus assay described by Tates and his colleagues. This protocol modification enables the assay to be conducted on a routine basis. Control observations for both 2/3PH and 4AAF-treated rats are presented, together with evidence indicating 4AAF itself to be without activity in the assay, irrespective of the mitogenic stimulus. The activities of the rat carcinogens DMN, 2AAF, DMH and 6BT, and of the non-carcinogens 4AAF and 4N are demonstrated. Recommendations for the conduct of the modified assay are made.