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

Final Report on the Safety Assessment of Acetamide MEA

Acetamide MEA is used in cosmetics as a skin conditioning agent-humectant and hair conditioning agent. Oral LD50s of 27 g/kg were reported for Acetamide MEA in rats. No rabbits died following an acute dermal exposure of 20 ml/kg Acetamide MEA. In ocular irritation studies, 70% Acetamide MEA and cosmetic formulations containing 1.3% Acetamide MEA were classified as nonocular irritants in rabbits. Only mild skin irritation occurred following a 24-h skin exposure to undiluted Acetamide MEA. In the maximization test, Acetamide MEA was classified as a nonsensitizer in guinea pigs when tested at a concentration of 5.0%. Neither primary irritation nor sensitization reactions to 7.5% Acetamide MEA were observed in a human repeated insult patch test. Acetamide MEA was not nonmutagenic in the Ames assay. In the presence of nitrosating agents, Acetamide MEA may form N-nitroso compounds; acetamide may be a minor impurity in Acetamide MEA. On the basis of the data presented in this report, it is concluded that Acetamide MEA is safe as a cosmetic ingredient at concentrations not to exceed 7.5% in leave-on products and is safe in the present practice of use in rinse-off products. Cosmetic formulations containing Acetamide MEA should not contain nitrosating agents or significant amounts of free acetamide.

Toxicologic Evaluation of Cellulon™ Fiber; Genotoxicity, Pyrogenicity, Acute and Subchronic Toxicity

CellulonTM fiber, a cellulose produced by a bacterial fermentation process employing a strain of Acetobacter aceti subsp. xylinum, was tested for genotoxicity in four assays: (1) microbial reverse mutation assay in Salmonella typhimurium (Ames assay), (2) an assay for chromosomal aberrations in Chinese hamster ovary (CHO) cells, (3) an assay for induction of unscheduled DNA synthesis (UDS) in rat primary hepatocytes, and (4) the CHO/HGPRT forward mutation assay. Each assay was conducted at a wide range of dose levels, both with and without metabolic activation (assay 1, 2, and 4). Test results gave no indication that Cellulon fiber possessed any genotoxic potential. The pyrogenicity of five batches of Cellulon fiber was tested in the Limulus Amebocyte Lysate assay, gel-clot method. Test results were negative for the presence of gram-negative bacterial endotoxin. The primary eye and dermal irritation potential of Cellulon fiber were examined in New Zealand White rabbits. The Draize method was employed to evaluate and grade ocular and dermal irritation as a result of test material administration. Test results indicated that Cellulon fiber is a minor ocular irritant up to 1 hour postadministration. However, the resultant irritation was considered to be mechanical and related to the dry, granular form of the test material. In addition, test results indicated that Cellulon fiber is not a dermal irritant in the rabbit. The acute oral toxicity of Cellulon fiber was determined in Sprague-Dawley rats, and the LD50 was found to be greater than 2000 mg/kg of body weight. The subchronic toxicity of Cellulon fiber was examined in Sprague-Dawley rats fed diets containing 0, 5, and 10% Cellulon fiber or microcrystalline cellulose for 13 weeks. No dose-related effects on survival, growth, hematology, blood chemistry, organ weights, or pathologic lesions were observed. The results of these studies indicate that Cellulon fiber and microcrystalline cellulose are toxicologically equivalent and that Cellulon fiber does not possess genotoxic potential, is nonpyrogenic, and that animals are not adversely affected by acute or subchronic exposure to Cellulon fiber.

Preclinical safety evaluation of Gd-EOB-DTPA (Primovist)

OBJECTIVES

Gadoxetic acid [gadolinium-ethoxybenzyl-diethylenetriamine penta-acetic acid (Gd-EOB-DTPA); Primovist] is a liver specific contrast agent for magnetic resonance imaging. For risk assessment of the single diagnostic use the toxicity of this compound was assessed.

MATERIALS AND METHODS

Studies into acute, repeated-dose, reproductive and developmental toxicity, and local tolerance, contact sensitizing, and genotoxic potential were performed.

RESULTS

Lethality was observed after a single intravenous administration at doses 2 orders of magnitude higher than the clinical dose. The no observed adverse effect levels after repeated administration markedly exceeds the single diagnostic dose in humans and no unexpected organ toxicity was observed. No indications of reproductive and developmental toxicity, potential contact allergenic, and genotoxic effects were observed. Gd-EOB-DTPA was well tolerated after intravenous administration.

CONCLUSIONS

Gd-EOB-DTPA was well tolerated with high safety margins between the single diagnostic dose and the doses showing adverse effects in animal studies.

Effects of mRg2, a mixture of ginsenosides containing 60% Rg2, on the ultraviolet B-induced DNA repair synthesis and apoptosis in NIH3T3 cells

Ginseng has been used worldwidely as a traditional medicine of Asian countries for treatment of various diseases including cancer. The purpose of this study was to determine the effect of ginseng saponin mRg2, a mixture of ginsenosides containing 60% Rg2, on the repair and apoptosis of ultraviolet B (UVB)-exposed NIH3T3 cells. When cells were exposed to UVB and then incubated with normal growth medium for 48 h, cell viability, as determined by trypan blue exclusion assay decreased to about 25%. However, when mRg2 was included in the postincubation medium, the UVB-induced loss of cell viability was significantly reduced as compared with that postincubated in normal growth medium. 4,6-diamidino-2-phenylindole (DAPI) staining showed that postincubation of the UVB-exposed cells in medium containing mRg2 significantly reduced the apoptotic nuclear fragmentation. Interestingly, when cells were preincubated with mRg2 for 24 h and then exposed to various doses of UV, the amount of repair synthesis significantly increased as compared with those in cells exposed to UVB alone. Western blot analysis indicated that the mRg2 postincubation after UVB exposure potentiated the level of p53 and p21. The level of Triton nonextractable proliferating cell nuclear antigen (PCNA) also remained elevated by mRg2 postincubation. All these results suggest that mRg2 protects cells against UVB-induced genotoxicity by increasing DNA repair and decreasing apoptosis, in possible association with the modulation of protein levels involved in cell cycle arrest or progression.

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.

The amoebicidal aqueous extract from Castela texana possesses antigenotoxic and antimutagenic properties

Due to long-term treatment toxicity and clinical resistance to drugs commonly used against E. histolytica, new drugs against amoebiasis are urgently needed. Castela texana ("chaparro amargo") is a shrub taken traditionally in teas and capsules of dry plant to treat intestinal amoebic infections. An aqueous extract was prepared and its mutagenic, genotoxic and cytotoxicity properties were evaluated in prokaryotic and eukaryotic systems. This extract was neither mutagenic when evaluated with the Ames test in Salmonella typhimurium strains TA98, TA100 and TA102, nor genotoxic in unscheduled DNA synthesis in hepatocyte cultures, even at the highest concentrations tested. In fact, C. texana extract showed antimutagenic activity on S. typhimurium strains TA98 and TA100 in the Ames test. Furthermore, it was capable of protecting liver cell cultures against unscheduled DNA synthesis induced by 2-acetylaminofluorene at a concentration of 6.77 microg/ml. A free-radical scavenging test was used in order to explore the antioxidant capacity of C. texana extract with S. typhimurium strain TA102 pretreated with norfloxacin, a free radical producer. This extract showed a free radical withdrawal effect. The effective chemoprotective activity of this extract could be due to the antioxidant capacity of the C. texana extract components. In this paper it is shown that the antiamoebic natural product, C. texana, is also antimutagenic and protects against induction of preneoplastic lesions in rat liver. These results justify further studies to extend it use to human beings.

Differences in genotoxicity of H(2)O(2) and tetrachlorohydroquinone in human fibroblasts

During autoxidation of the pentachlorophenol (PCP) metabolite tetrachlorohydroquinone (TCHQ) the semiquinone is formed as well as reactive oxygen species (ROS). It was examined if *OH or the semiquinone are the cause of TCHQ-induced genotoxicity by direct comparison of TCHQ- and H(2)O(2)-induced DNA damage in human cells. All endpoints tested (DNA damage, DNA repair, and mutagenicity) revealed a greater genotoxic potential for TCHQ than for H(2)O(2). In the comet assay, TCHQ induced DNA damage at lower concentrations than H(2)O(2). The damaging rate by TCHQ (tail moment (tm)/concentration) was 10-fold greater than by H(2)O(2). DNA repair was lower for TCHQ than for H(2)O(2) treatment. This was shown by measuring DNA repair in the unscheduled DNA synthesis (UDS) assay and the persistence of the DNA damage in the comet assay. In contrast to H(2)O(2), TCHQ in non-toxic concentrations was mutagenic in the hypoxanthine-guanine phosphoribosyltransferase (HPRT) locus of V79 cells. Finally, there were also differences observed in cytotoxicity (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) assay) of TCHQ and H(2)O(2). Whereas the TCHQ cytotoxicity was enhanced during a 21h recovery phase, the H(2)O(2) cytotoxicity did not change. The results demonstrated that the pronounced genotoxic properties of TCHQ in human cells were not caused by *OH radicals but more likely by the tetrachlorosemiquinone (TCSQ) radical.

Ciprofloxacin: in vivo genotoxicity studies

The fluoroquinolone ciprofloxacin is widely used in antimicrobial therapy. It inhibits the bacterial gyrase and in high concentrations in vitro also the functionally related eukaryotic topoisomerase-II, which resulted in genotoxic effects in several in vitro tests. In order to evaluate the relevance of these findings, ciprofloxacin was tested in vivo for genotoxic activity using the following test systems: micronucleus test in bone marrow of mice, cytogenetic chromosome analysis in Chinese hamster, dominant lethal assay in male mice and UDS tests in primary rat and mouse hepatocytes in vivo. These results are compared with already published in vitro and in vivo studies with ciprofloxacin. All in vivo genotoxicity revealed no genotoxic effect for ciprofloxacin. In addition, ciprofloxacin was found to be non-carcinogenic in two rodent long-term bioassays. Therefore, ciprofloxacin is considered to be safe for therapeutic use.

Synergistic DNA damaging effects of 4-nitroquinoline-1-oxide and non-effective concentrations of methyl methanesulfonate in human fibroblasts

DNA damage and DNA repair in human fibroblasts induced by the combination mixture of the genotoxic agents methyl methanesulfonate (MMS) and 4-nitroquinoline-1-oxide (4-NQO) were studied using the comet assay and the unscheduled DNA synthesis (UDS), respectively. Cells were simultaneously treated for 1h with the no observed effect concentration (noec) of MMS and increasing concentrations of 4-NQO or vice versa. Different results were obtained with the two types of mixtures. When the noec of 4-NQO was combined with increasing concentrations of MMS, no combination effects were observed. However, in experiments with increasing concentrations of 4-NQO and the noec of MMS, an increase in DNA damage and repair (and an enhancement of cytotoxicity) was demonstrated. Quantitative analysis of the effects by the isobologram method confirmed synergistic responses in both tests. We are proposing interactive actions between 4-NQO and MMS, whereby 4-NQO facilitates the attack of MMS on the DNA bases.

UDS induction by an array of standard carcinogens in human and rodent hepatocytes: effect of cryopreservation

The UDS induction assay with primary hepatocytes as the target cells is a determinative assay for chemical carcinogens. This assay is, however, limited to the availability of freshly prepared liver cells. A cryopreservation technique for liver cells has recently been described. Frozen cells have been shown to retain a variety of enzyme activities essential for xenobiotic metabolism after being thawed. In the present investigation, 19 direct or indirect-acting carcinogens were tested with respect to their capacity to induce DNA repair in primary as well as cryopreserved human and rat hepatocytes. Cryopreserved cells yielded results that were essentially indistinguishable from fresh cells. Only marginal differences were observed between hepatocytes of rat or human origin. These results demonstrate the suitability of cryopreserved hepatocytes as indicator cells for the study of UDS induction to discover possible carcinogenicity in chemicals.

Cell-cycle delay is induced in cells of a U937 promonocytic cell line by low-intensity light irradiation at 660 nm

Visible-light irradiation (VLI) at 660 nm and 11.5 J/cm2 inhibits proliferation of cells of the U937 promonocytic cell line, as monitored by autoradiographical analysis. The S-phase cell population is reduced at 6 h post-radiation treatment. Flow cytometric analysis confirms this, and also shows that light irradiation of cells induces a statistically significant increase in G2/M cells at 6 h post-radiation treatment. It has been postulated that VLI at 660 nm can alter cell-cycle progression by affecting intracellular concentrations of ions, in particular pH and calcium. However, no significant effects of light irradiation on these intracellular ions have been observed. These effects of VLI are not a consequence of radiation-induced DNA strand breaks, therefore events other than direct DNA damage are involved. These findings demonstrate a direct photobiological effect of VLI at 660 nm on the cell cycle, and indicate a previously unsuspected mechanism for the induction of cell-cycle delay that is neither a result of changes in the concentration of intracellular ions nor initiated by DNA strand breaks.

Inhibition of xenobiotic-induced genotoxicity in cultured precision-cut human and rat liver slices

In this study precision-cut liver slices have been used to evaluate the effects of the flavone tangeretin, the flavonoid glycoside naringin and the flavanone naringenin (the aglycone derived from naringin) on xenobiotic-induced genotoxicity. Liver slices were cultured for 24 h in medium containing [3H]thymidine and the test compounds and then processed for autoradiographic determination of unscheduled DNA synthesis (UDS). The cooked food mutagen 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) markedly induced UDS in cultured human liver slices and both 2-acetylaminofluorene (2-AAF) and aflatoxin B1 (AFB1) induced UDS in cultured rat liver slices. Tangeretin (20 and 50 microM) was found to be a potent inhibitor of 5 and 50 microM PhIP-induced UDS in human liver slices, whereas 20 and 50 microM naringenin was ineffective and naringin only inhibited genotoxicity at a concentration of 1000 microM. In rat liver slices 50 microM tangeretin inhibited 10 and 50 microM 2-AAF-induced UDS, whereas 50 microM naringenin and 100 and 1000 microM naringin were ineffective. None of the three flavonoids examined inhibited 5 microM AFB1-induced UDS in rat liver slices. The inhibition of PhIP- and 2-AAF-induced UDS by tangeretin is probably attributable to the inhibition of the human and rat cytochrome P-450 isoforms which are responsible for the bioactivation of these two genotoxins. Although flavonoids can modulate xenobiotic-induced genotoxicity in human and rat liver slices, any protective effect is dependent on the particular combination of genotoxin and flavonoid examined. These results demonstrate that cultured precision-cut liver slices may be utilised as an in vitro model system to examine the modulation of xenobiotic-induced genotoxicity by flavonoids and other dietary components.

Lack of effect of coumarin on unscheduled DNA synthesis in precision-cut human liver slices

In this study the effect of coumarin on unscheduled DNA synthesis (UDS) in precision-cut human liver slices has been examined. Liver slices from tissue samples from four donors were cultured for 24 hr in medium containing [3H]thymidine and 0-5.0 mM coumarin using a dynamic organ culture system and processed for autoradiographic evaluation of UDS. As positive controls liver slices were also cultured with three known genotoxic agents, namely 0.02 and 0.05 mM 2-acetylaminofluorene (2-AAF), 0.002 and 0.02 mM aflatoxin B1 (AFB1) and 0.005 and 0.05 mM 2-amino-1-methyl-6-phenylimidazo [4,5-b]pyridine (PhIP). UDS was quantified as the net grain count in centrilobular hepatocytes and as the percentage of centrilobular hepatocyte nuclei with more than five net grains. Compared with control liver slice cultures, treatment with 0.05-5.0 mM coumarin had no effect on UDS. In contrast, treatment with 0.02 and 0.05 mM 2-AAF, 0.002 and 0.02 mM AFB1 and 0.005 and 0.05 mM PhIP produced significant increases in the net grain counts of centrilobular hepatocytes. The greatest induction of UDS was observed in liver slices treated with 0.05 mM PhIP. Treatment with 2-AAF, AFB1 and PhIP also produced significant increases in the number of centrilobular hepatocyte nuclei with more than five net grains. At the concentrations examined neither coumarin. 2-AAF, AFB1 nor PhIP had any significant effect on replicative DNA synthesis in 24 hr cultured human liver slices. These results demonstrate that coumarin does not induce UDS in cultured human liver slices. However, all three positive control compounds produced marked significant increases in UDS, thus confirming the functional viability of the human liver slice preparations used in this study. The results of this study suggest that coumarin is not a genotoxic agent in human liver.

Effect of some cooked food mutagens on unscheduled DNA synthesis in cultured precision-cut rat, mouse and human liver slices

Precision-cut liver slices were prepared from male Fischer 344 rats, female CDF1 mice and humans (both male and female subjects). Liver slices were cultured for 24 hr in medium containing [3H]thymidine and either PhIP, IQ, MeIQ, MeIQx, Glu-P-1 or Trp-P-1, and then processed for auto-radiographic evaluation of unscheduled DNA synthesis (UDS). All six cooked food mutagens examined produced concentration-dependent increases in UDS in human liver slices. PhIP was the most potent compound examined, followed by MeIQx, IQ and then MeIQ, Glu-P-1 and Trp-P-1. Significant increases in UDS were observed with PhIP, IQ and MeIQx at concentrations as low as 5 microM in the culture medium. The same rank order of potency was not apparent in either rat or mouse liver slices. In rat liver slices only MeIQ significantly induced UDS, although positive results were obtained with two other genotoxins, namely 2-acetylaminofluorene and aflatoxin B1. Apart from MeIQx, all the cooked food mutagens produced significant increases in UDS in mouse liver slices. This study demonstrates the usefulness of precision-cut liver slices to evaluate species differences in xenobiotic-induced genotoxicity. Both marked compound and species differences in induction of UDS were observed. The data provide further evidence that dietary cooked food mutagens are potential human carcinogens.

In vivo mutagenicity studies with iodinated glycerol azeo

Previous studies have shown that iodinated glycerol azeo is positive in a number of in vitro mutagenicity assays including the Ames assay (TA100; TA1535), mouse lymphoma assay, Chinese hamster ovary (cytogenetic) assay and in one in vivo study, the sex-linked-recessive-lethal assay in Drosophila. Prior studies have also shown that the drug is negative in the mouse micronucleus assay. We now report that the drug is also negative for mutagenic activity in a number of other in vivo tests. Single intraperitoneal doses of 25, 125 and 250 mg/kg were without effect in the rat bone marrow chromosomal aberration assay. Single oral doses of 30, 75, 150 and 300 were negative in the rat hepatocyte DNA-repair assay. Single intraperitoneal doses of 30 and 100 mg/kg were without effect in the sister chromatid exchange (SCE) assay in the mouse. Statistically significant effects were seen at 200 and 300 mg/kg in the initial SCE assay and at 300 and 350 mg/kg in the confirmatory SCE assay. The rationale for considering the SCE results to be anomalous and thus not relative to the overall safety evaluation of the drug is presented.

Evaluation of the biocompatibility of a nonceramic hydroxyapatite

In this study, the biocompatibility of calcium phosphate cement (CPC) was systematically investigated, which included systemic injection acute toxicity assay, cell culture cytotoxicity assay, gene mutation assay (Ames test), chromosome aberration assay (micronucleus test), DNA damage assay (unscheduled DNA synthesis test) and implant histological evaluation. The results showed that the CPC had no toxicity and all tests for mutagenicity and potential carcinogenicity of CPC extracts are negative. CPC was implanted into the femurs of rabbits under no-load condition and the histology of specimens revealed that the implant tightly joined with the surrounding bone, only a very slight histological inflammatory reaction occurred. Thus, CPC is a highly biocompatible material and seems to be safe for application in humans.

Evaluation of positive controls for the in vitro unscheduled DNA synthesis assay using hepatocytes from induced (Aroclor 1254) and uninduced male cynomolgus monkey

We have evaluated the use of four different positive control compounds for assessing UDS in monkey hepatocytes and have found three of these, methylmethanesulfonate, benzo[a]pyrene, and dimethylbenz[a]anthracene, to produce strong positive responses in vitro. Dimethylnitrosamine induced only weak responses. We also report that the strength of the response induced by procarcinogens was not enhanced in hepatocytes taken from Aroclor 1254-pretreated monkeys, even though substantial induction of cytochrome P450 enzymes was demonstrated in these cells. These studies raise the question of the utility of employing an in vivo induction system to enhance the monkey UDS assay.

Lack of genotoxicity of piperonyl butoxide

The genotoxicity of piperonyl butoxide has been investigated in bacterial mutation assays using tester strains TA98, TA100, TA1535, TA1537 and TA1538. The assays were conducted both with and without metabolic activation. Piperonyl butoxide was tested for mutation with and without metabolic activation in the CHO/HGPRT assay. Chromosomal aberrations were investigated also using Chinese hamster ovary (CHO) cells and effects on DNA were evaluated by in vitro unscheduled DNA synthesis (UDS) test using rat liver primary cell cultures. Piperonyl butoxide was not shown to be genotoxic in any assay system. The data presented supports the view that the liver tumors observed in rodents at dose levels above the maximally tolerated dose (MTD) result from a secondary non-genotoxic mechanism.

Lack of effect of piperonyl butoxide on unscheduled DNA synthesis in precision-cut human liver slices

In this study the effect of piperonyl butoxide (PBO) on unscheduled DNA synthesis in precision-cut human liver slices has been examined. Liver slices prepared from tissue samples from five human donors were cultured in medium containing [3H]thymidine and 0-2.5 mM PBO using a dynamic organ culture system. After 24 h the liver slices were processed for autoradiographic examination of UDS. As positive controls, liver slices were also cultured with three known genotoxic agents, namely 2-acetylaminofluorene (2-AAF), aflatoxin B1 (AFB1) and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP). UDS was quantified as the net grain count in centrilobular hepatocytes and as the percentage of centrilobular hepatocyte nuclei with > 5 and > 10 net grains. Compared to control liver slice cultures PBO had no effect on UDS. In contrast, treatment with 0.02 and 0.05 mM 2-AAF, 0.002 and 0.02 mM AFB1 and 0.005 and 0.05 mM PhIP produced significant increases in net grain counts of centrilobular hepatocytes. The greatest induction of UDS was observed in liver slices treated with 0.05 mM PhIP. Treatment with 2-AAF, AFB1 and PhIP also produced increases in the number of centrilobular hepatocyte nuclei with > 5 and > 10 net grains. At the concentrations examined neither PBO, 2-AAF nor PhIP had any significant effect on replicative DNA synthesis in 24 h cultured human liver slices. In cultured liver slices treated with 0.02, but not 0.002, mM AFB1 a significant reduction in the rate of replicative DNA synthesis was observed. These results demonstrate that PBO does not induce UDS in cultured human liver slices. However, all three positive control compounds produced marked significant increases in UDS, thus confirming the functional viability of the human liver slice preparations used in this study. In conclusion, these results provide further evidence that PBO is a non-genotoxic agent which does not damage DNA in human liver.

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.

Enhanced cyto- and genotoxicity of tetracycline in Wilson disease fibroblasts

Tetracycline (TC) exerts DNA damaging properties which are accelerated in the presence of copper(II). Thereby, reactive oxygen species are generated. We investigated, if copper-accumulating cells show a higher sensitivity to TC compared to normal cells. Fibroblasts with an increased copper content were derived from patients of two genetic disorders, Wilson disease (WD) and Menkes disease (MD). Cytotoxic and genotoxic effects of TC were investigated in different human fibroblasts. The inhibition of cell growth by TC was measured in two normal fibroblast lines, fibroblast lines of two patients with WD and one patient with MD. While TC inhibited cell growth at similar concentrations in normal fibroblasts and the MD fibroblasts, the WD cells were much more sensitive. Furthermore, an increased inhibition of DNA synthesis and an enhanced induction of unscheduled DNA synthesis (UDS) was found in WD cells after a TC-treatment compared to normal cells.

Validation of a flow cytometric in vitro DNA repair (UDS) assay in rat hepatocytes

An in vitro flow cytometric (FCM) DNA repair assay has been developed and validated by comparison to conventional autoradiography (ARG). Both assays measure unscheduled DNA synthesis (UDS). Cultures of hepatocytes from young male Sprague-Dawley rats were exposed to a battery of 26 chemicals plus bromodeoxyuridine (BrdUrd) or 3H-thymidine (3H-dT) for 18-20 h before harvest. Selection of test chemicals was based upon both their genotoxicity classifications and carcinogenicity bioassay results in male rats. DNA repair in chemically treated cultures was detected flow cytometrically by measuring the uptake of BrdUrd in non-replicating (G1, G2, mitotic and 4C) cells. Intracellular levels of incorporated BrdUrd were visualized by immunochemical labeling with fluorescein isothiocyanate (FITC), and total cellular DNA content was simultaneously estimated by counterstaining samples with the nucleic acid intercalator, propidium iodide (PI). Information was obtained from 10(4) cells/sample. Since repairing cells incorporate significantly less BrdUrd per unit of time than replicating cells, low intensity BrdUrd-FITC fluorescent signals from repairing cells are readily discriminated from high intensity signals from replicating cells when displayed on linear univariate histograms. Further distinction between repairing and replicating cells was achieved by displaying the DNA contents of all cells on linear bivariate histograms. Thus, repairing cells were resolved without subjecting these cultures to agents which suppress replicative synthesis (e.g., hydroxyurea). Results from these concurrent FCM and ARG investigations include the following: (1) conclusions (DNA repair positive or negative) were in agreement, with one exception, cinnamyl anthranilate, for which cytotoxic doses produced a positive FCM response, but lack of intact hepatocytes in parallel ARG preparations prevented analysis; (2) similar sensitivities for most of the positive chemicals were reported; (3) a high correlation (85%) exists between the reported genotoxicity classification and these DNA repair results in the absence of overt cytotoxicity; (4) a poor correlation exists between these DNA repair results and hepatocarcinogenesis (only 4/11 liver carcinogens tested positive) or overall carcinogenesis in the male rat (only 9/21 carcinogens tested positive). This FCM assay provides a rapid, sensitive, safe and reliable means of identifying agents which induce DNA repair in mammalian cells.

A comparison of the DNA-damaging, the cytotoxic and genotoxic properties of tetracycline in human fibroblasts in the presence and absence of light

The reduction of the colony-forming ability, the induction of DNA strand breaks and DNA repair were determined in human fibroblasts after treatment with tetracycline (TC) in the presence and absence of light. In all experiments human fibroblasts were more sensitive to incubations of TC in the light than in the dark. Induction of DNA single-strand breaks and DNA repair were detected in the cells after a 1-h incubation with TC under light but not in the dark. In contrast to these results, TC induced single-strand breaks in isolated PM2 DNA in the dark, however, to a lower extent than in the presence of light. In both cases strand break formation was totally suppressed by adding catalase. The formation of a TC-derived radical by ESR and a decomposition product by UV-vis spectroscopy was observed in the presence and absence of light; their rate of formation in the dark was much smaller than in the light.

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.

Grain counting in the in vitro hepatocyte DNA-repair assay

The in vitro hepatocyte DNA-repair assay is a widely used useful method in assessing the genotoxic activity of both directly and indirectly acting chemical agents. This article discusses the criteria presently employed in the autoradiographic evaluation of unscheduled DNA synthesis, and suggests that the subtraction of either the average or the highest cytoplasmic grain count, usually carried out to obtain the net nuclear grain count, may represent a potential source of errors when the test compound is a weakly genotoxic or a non-genotoxic agent. As a matter of fact, a response can be classified as positive or negative depending on the procedure used to quantitate the cytoplasmic background, and the subtraction of this background from the nuclear count is not founded on a sound theoretical basis because of the following reasons: the different nature of the processes responsible for the generation of nuclear and cytoplasmic grains; and the quantitatively different effect that the test compounds may have on the nuclear and the cytosolic labelling.

Effects of selected secondary metabolites of Fusarium moniliforme on unscheduled synthesis of DNA by rat primary hepatocytes

The Fusarium moniliforme mycotoxins--fusarin C, fumonisin B1, moniliformin and bikaverin--were evaluated for genotoxicity by their ability to induce unscheduled DNA synthesis (UDS) in primary rat hepatocytes. Isolated hepatocytes were exposed to several concentrations of moniliformin (5.0-500 microM), bikaverin (1.0-500 microM), fumonisin B1 (0.5-250 microM), or fusarin C (1.0-100 microM). Aflatoxin B1, a known inducer of UDS, was included as a positive control. UDS was determined by autoradiography of cells after their exposure to [3H]thymidine. The highest doses of fusarin C and bikaverin caused cell death, but no cytotoxicity was observed in cells exposed to moniliformin or fumonisin B1. Fumonisin B1, moniliformin and bikaverin were not genotoxic in the UDS assay. The results of the UDS assay with fusarin C were inconclusive since a marginal effect on UDS was obtained.

Synergistic effects of U46 D fluid (dimethylammonium salt of 2,4-D) and CuCl2 on cytotoxicity and DNA repair in human fibroblasts

The cytotoxicity of U46 D Fluid was tested in human fibroblasts after pretreatment with non-toxic or slightly toxic concentrations of CuCl2. While cell survival, colony-forming ability and protein synthesis were not affected by pretreatment with CuCl2, the inhibition of cell growth was enhanced as was inhibition of DNA synthesis. Synergistic effects of CuCl2 and U46 D Fluid were also detected on the induction of DNA repair measured by unscheduled DNA synthesis. While neither U46 D Fluid nor CuCl2 alone induced DNA repair, preincubation with CuCl2 followed by treatment with U46 D Fluid strongly provoked DNA repair.

Application of the neutral red assay (NR assay) to monolayer cultures of primary hepatocytes: rapid colorimetric viability determination for the unscheduled DNA synthesis test (UDS)

The neutral red (NR) absorption method was adapted for the determination of cell viability in the UDS assay with primary hepatocyte cultures of the rat. The NR method is rapid, easy to perform, and suitable for handling of large numbers of cultures simultaneously. It can be used for concentration range-finding pre-experiments. In addition, it can easily be integrated into a UDS test protocol for documentation of toxic effects if supplementary cultures for each concentration are established. The time schedule required for the NR assay makes it possible for one person to process the hepatocytes for autoradiography and at the same time determine the toxicity.

Differential stability of drug-metabolizing enzyme activities in primary rat hepatocytes, cultured in the absence or presence of dexamethasone

The effects of primary hepatocyte culture on the rat cytochrome P450-dependent monooxygenase system and several conjugating enzyme activities were examined using a culture system similar to those used for evaluation of chemicals as potential genotoxins. Cytochrome P450 and cytochrome b5 contents progressively decreased throughout the 72-h culture period to less than 25% of initial values, whereas cytochrome P450 reductase rapidly decreased by 50% during attachment, but then remained stable. Cytochrome P450-dependent testosterone hydroxylase activities decreased more rapidly in culture than did cytochrome P450 content reaching less than 50% of attachment levels by 24 h. Cytochrome P450IIIA immunoreactive protein decreased at a similar rate to testosterone-6 beta-hydroxylase. Activated UDP-glucuronyltransferase activities towards 1-naphthol and testosterone declined more slowly over the 72 h than cytochrome P450 and remained at 50-60% of initial values at 72 h. UDP-glucuronyltransferase activity towards digitoxigenin monodigitoxoside (DIG) did not decrease during culture. Glutathione-S-transferase and sulfotransferase activities also declined during the 72 h at rates which appeared to be isozyme-dependent. Addition of 1 microM dexamethasone (DEX) to the culture medium increased UDP-glucuronyltransferase activity towards DIG, cytochrome P450 reductase and testosterone-6 beta-hydroxylase activities up to 2.5-, 2.0- and 7-fold, respectively and induced cytochrome P450IIIA immunoreactive protein(s) in the hepatocytes after 24 and 48 h of culture; DEX was less effective at the 72 h time-point. DEX treatment also significantly accelerated the decreases in glutathione-S-transferase activities and in sulfotransferase activities towards 1-naphthol and estrone. Thus, it appears that primary rat hepatocytes cultured under standard conditions, not only rapidly lose their monooxygenase capabilities, but also some of their capacity for conjugation. Furthermore, the use of DEX in cell culture medium to enhance cell survival does not maintain total drug-metabolizing enzyme capability, but appears to transiently and selectively increase expression of certain isozymes at the expense of others.

Report of a comparative study of DNA damage and repair assays in primary rat hepatocytes with five coded chemicals

We report the results of a collaborative study for the detection of chemical-induced DNA damage in primary cultures of rat hepatocytes. The methods include the detection of unscheduled DNA synthesis (UDS) with either autoradiography (5 laboratories) or liquid scintillation counting (2 laboratories) and the assessment of DNA single-strand breaks with the alkaline elution assay (1 laboratory). Interlaboratory standardization was omitted in order to prove the agreement of the assays under routine conditions. Five coded chemicals were tested. For 4 chemicals (2-acetylaminofluorene, thiourea, glycerine and potassium chloride) the UDS data were consistent in all laboratories, thus indicating a high consensus of the test systems applied in the different laboratories. Those 3 chemicals that were not expected to elicit genotoxic activity (thiourea, glycerine, and potassium chloride) yielded negative results in all laboratories. 2-Acetylaminofluorene, a known DNA-damaging agent in hepatocytes, gave strongly positive responses in all laboratories. In contrast, N-nitrosodiphenylamine led to equivocal responses.

Evaluation of four methods for scoring cytoplasmic grains in the in vitro unscheduled DNA synthesis (UDS) assay

The in vitro unscheduled DNA synthesis (UDS) assay measures DNA repair (incorporation of [3H]thymidine) following in vitro treatment of rat primary hepatocytes. The autoradiographic method was used to detect UDS by counting developed silver grains in the photographic emulsion overlaying nuclei and cytoplasmic areas of the hepatocytes. In this communication we report results using 4 scoring methods: (1) the 2 most heavily labeled cytoplasmic areas adjacent to the nucleus (our standard method), (2) the cytoplasmic area left of the nucleus, (3) the cytoplasmic areas left and right of the nucleus, and (4) 2 cytoplasmic areas whose positions were selected at random. Rat primary hepatocyte cultures treated with a medium control, a solvent control (dimethyl sulfoxide) and 5 known genotoxic chemicals (2-acetylaminofluorene, dimethylnitrosamine, diethylnitrosamine, methyl methanesulfonate and ethyl methanesulfonate) were scored using these 4 methods. The average or maximum cytoplasmic grain count was subtracted from the nuclear grain count to yield net grains/nucleus (NG). In general, NG counts for Methods 2, 3 and 4 were similar, although shifted about 3-10 grains higher than Method 1 for controls and most treated groups. Methods 2, 3 and 4 showed more experiment-to-experiment variability in sensitivity for detecting statistically significant increases in treated groups than did our standard method. Thus, the alternative methods afforded no consistent improvements in sensitivity or reduction of variability for this assay. Subtraction of the average or the highest cytoplasmic count had virtually no effect on the sensitivity of the assay, but simply requires an appropriate adjustment of the criteria for a positive response.

Strain differences in in vitro rat hepatocyte unscheduled DNA synthesis (UDS): effect of UV is independent of strain while increased sensitivity is apparent using Fischer-344 instead of Sprague-Dawley rats

The unscheduled DNA synthesis (UDS) assay measures DNA repair following in vitro treatment of rat primary hepatocytes. This report compares the UDS response of primary hepatocytes from 2 widely used rat strains, the Fischer-344 (F344) and Sprague-Dawley (SD) strains. Ultraviolet (UV) light and 5 known genotoxic chemicals were evaluated in each strain in parallel experiments. The chemicals tested were 2-acetylaminofluorene (2-AAF), 4-aminobiphenyl (4-AB), benzidine, dimethylnitrosamine (DMN) and N-propyl-N'-nitro-N-nitrosoguanidine (PNNG). Four of these compounds (2-AAF, 4-AB, benzidine and DMN) require metabolic activation. Benzidine and PNNG were both negative using SD rat hepatocytes, but were weakly positive using F344 rat hepatocytes. In the first of 2 experiments, 4-AB was inconclusive in SD hepatocytes, but strongly positive in F344 cells. In the second experiment, 4-AB was positive in hepatocytes from both strains. 2-AAF was more strongly positive in F344 cells than in SD cells. DMN and UV light induced positive dose responses with little or no differences between strains. It is concluded that hepatocytes from F344 rats may be more sensitive, qualitatively and quantitatively, than hepatocytes from SD rats as indicators of UDS. This difference is not due to intrinsic differences in DNA repair mechanisms but is probably due to differences in drug-metabolizing enzymes between these strains. Thus, for routine screening, F344 rats are preferable for measurement of the in vitro UDS-inducing potential of compounds.

Recommended protocols based on a survey of current practice in genotoxicity testing laboratories: I. Unscheduled DNA synthesis assay in rat hepatocyte cultures

A protocol based primarily on current laboratory practices in the performance of the unscheduled DNA synthesis (UDS) assay with primary rat hepatocyte cultures has been developed. These guidelines were developed using tabulated responses to a detailed questionnaire completed by North American and European governmental, university and contract laboratories involved with the UDS test. This report identifies those modifications to previously described methodologies which are used on a regular basis and also serves to clarify confusing or inconsistent practices. Although this protocol pertains specifically to the use of primary rat hepatocyte cultures, it can be modified to incorporate other types of cells in which certain aspects remain the same.

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)

Induction of DNA-repair synthesis in primary rat hepatocytes by epoxides

The genotoxicity of 10 epoxides was investigated in the UDS test with primary rat hepatocytes. The sensitivity of the assay was demonstrated using 2-acetylaminofluorence. The epoxides 1,2-epoxyoctane, 1,2-epoxydecane, epoxycyclooctane, epoxycyclododecane, (+)-limoneoxide, alpha-pinaneoxide, transstilbeneoxide, and cis-2,3-epoxysuccinic acid, which are known to be non-mutagenic in the Ames test, as well as the bacterial mutagen, 1,2-epoxyphenoxypropane did not induce UDS in primary hepatocytes of the rat. However, a positive UDS response obtained with glycidyltrimethylammonium chloride showed that metabolic inactivation of the oxirane ring in hepatocytes is influenced by further structural substituents.

Unscheduled DNA synthesis in rat pleural mesothelial cells treated with mineral fibres

Unscheduled DNA synthesis (UDS) was studied in confluent rat pleural mesothelial cells (RPMCs) arrested in G0/G1 with hydroxyurea (HU) and treated with various fibre types, i.e., chrysotile, crocidolite or attapulgite. In addition, the effects of UV light and of benzo[a]pyrene were determined as references. Using autoradiography after [3H]thymidine incorporation ([3H]dThd), RPMCs treated with 4 micrograms/cm2 of chrysotile fibres exhibited a low but significant enhancement of net grains compared to untreated cells. Treatment with higher doses of chrysotile was not possible because of the impairment of microscopic observation due to the presence of the fibres. Using liquid scintillation counting, RPMCs treated with chrysotile or crocidolite showed a significant dose-dependent increase in [3H]dThd incorporation compared to untreated cells. In contrast, attapulgite did not enhance [3H]dThd incorporation compared to untreated cells. Treatment of RPMCs with 1, 2 or 4 micrograms/ml of benzo[a]pyrene resulted in a significant increase in [3H]dThd incorporation. In order to discount a possible role of S cells in the augmentation of [3H]dThd incorporation, despite the presence of 5 mM HU, S cells were counted by autoradiography. Results indicated that the percentage of S cells was similar in asbestos-treated and untreated cultures. Stimulation of the S phase also seems unlikely because treatment of RPMCs with asbestos fibres in the absence of HU resulted in a reduction of [3H]dThd incorporation attributed to an impairment of the S phase by the fibres. 1-4 micrograms/ml benzo[a]pyrene or 10-50 J/m2 UV light resulted in an approximate doubling of [3H]dThd incorporation. The effects of inhibitors of DNA repair were determined in chrysotile-treated RPMCs. [3H]dThd incorporation was inhibited by cytosine arabinoside and nalidixic acid. These results show that asbestos produces UDS in RPMCs.

Activity of 2-acetylaminofluorene as a UDS inducing agent in B6C3F1 mouse hepatocytes in vitro

2-Acetylaminofluorene (2AAF) is shown to be reproducibly active in inducing UDS in cultured hepatocytes from a B6C3F1 mouse liver.

Quantification of unscheduled DNA synthesis by a whole cell counting method

A procedure was developed for the quantification of the autoradiographic assay for unscheduled DNA synthesis. Relative to commonly used practices for grain counting, this procedure provides a more accurate net nuclear grain count by eliminating the subjectivity currently associated with selection of the areas to be counted for the cytoplasmic background count. Briefly, the object area and aperture area modes of an ARTEK 880 colony counter are used to collect values for the total number of silver grains over a particular cell (nuclear and cytoplasmic counts), as well as for the nuclear and cytoplasmic areas. These values are then employed in a short algorithm to determine the net nuclear grain count. This new method provides greater sensitivity for defining weak UDS responses and the data collected readily lends itself to statistical analysis.

The in vitro unscheduled DNA synthesis (UDS) assay in rat primary hepatocytes: evaluation of 2-furoic acid and 7 drug candidates

The in vitro unscheduled DNA synthesis assay (UDS) is part of the routine genetic toxicology screening at The Upjohn Company. The purpose of this paper is to report results for 8 compounds which were tested in the in-house genetic toxicology program. These compounds represent diverse chemical structure and most of them entered the screening program because they are biologically active in efficacy screens. All tests were carried out under Good Laboratory Practices Regulations of the U.S. Food and Drug Administration. None of the materials reported here produced an increase in UDS and therefore the UDS results with these compounds do not suggest potential for genotoxicity.

The in vitro unscheduled DNA synthesis (UDS) assay in rat primary hepatocytes: evaluation of 24 drug candidates

The in vitro unscheduled DNA synthesis assay (UDS) is part of the routine genetic toxicology screening at The Upjohn Company. The purpose of this paper is to report results for 24 drug candidates which were tested as coded compounds. These compounds are very diverse in chemical structure and represent classes of compounds selected because of biological activity in a variety of preliminary drug efficacy screens. None of the compounds reported here produced an increase in UDS, and therefore, the UDS results with these materials do not suggest potential for mutagenesis or carcinogenesis.

The in vitro unscheduled DNA synthesis (UDS) assay in rat primary hepatocytes. Validation of improved methods for primary culture including data on the lack of effect of ionizing radiation

The in vitro unscheduled DNA synthesis (UDS) assay was evaluated for inclusion in a battery of assays used at The Upjohn Company for evaluation of lead compounds in the development of new and existing drug entities. This evaluation process encompassed aspects of the isolation of hepatocytes and tests of reference mutagens and genotoxins. The flow rate of perfusion solutions and their temperatures were critical in the isolation of high viability hepatocytes in good yield. The attachment of freshly isolated hepatocytes to coverslips was greatly enhanced by coating the coverslips with type III collagen. Results of testing 12 known genotoxic agents (UV light, cyclophosphamide, 7,12-dimethylbenzanthracene, dimethylnitrosamine, diethylnitrosamine, 2-acetylaminofluorene, benzo[a]pyrene, methyl methanesulfonate, ethyl methanesulfonate, N-propyl-N'-nitro-N-nitrosoguanidine, benzidine and 4-aminobiphenyl) were in agreement with the literature. The use of X-ray did not induce unscheduled DNA synthesis in hepatocytes. This latter finding draws attention to the inability of this assay to detect agents which result in 'short-patch' repair of damage.