The Genotoxicity of DNA Intercalating Drug 8-Methoxy pyrimido [4′,5′:4,5]thieno(2,3-b)quinoline-4(3H)-one

8-Methoxypyrimido[4′,5′:4,5]thieno(2,3-b)quinoline-4(3H)-one (MPTQ) is known to have antitumor and cytotoxic activities on various types of tumors. This compound showed a strong clastogenic effect on bone marrow cells of Swiss albino mice treated in vivo (17.5–35 mg/kg body weight). MPTQ induced micronuclei formation (MN) at doses of 17.5, 23.3, and 35 mg/kg. Dose and time-yield effect of MPTQ was studied in the case of chromosome aberration assay. MPTQ induced a statistically significant increase in the frequency of chromosome aberrations and micronuclei induction. The drug induced significant abnormal sperms even in the sperm shape abnormality assay. Based on the data reported in the literature, we have tried to establish the relationship between the clastogenic effect observed and process of MPTQ intercalation into DNA and the formation of protein-associated DNA-strand breaks probably promoted by topoisomerase enzymes.     

N-methyl-N'-nitro-N-nitrosoguanidine (MNNG)-induced in vivo clastogenicity: protective effects of aqueous neem leaf extract

We evaluated the modifying effects of aqueous neem leaf extract on the in vivo clastogenicity of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), a potent gastric carcinogen by quantitation of micronuclei and chromosomal aberrations in metaphase cells from the bone marrow of male Wistar rats. Intraperitoneal injection of MNNG (40 mg/kg body weight) induced a significant increase in the frequency of micronuclei and chromosomal aberrations. Pretreatment with aqueous neem leaf extract (100 mg/kg body weight) significantly reduced MNNG-induced increase in micronuclei and chromosomal aberrations. These results reveal the chemoprotective potential of aqueous neem leaf extract against the clastogenic effects of MNNG.     

Chromosomal composition of micronuclei in mouse bone marrow treated with rifampicin and nicotine, analyzed by multicolor fluorescence in situ hybridization with pancentromeric DNA probe

The mechanism underlying the induction of micronuclei induced by rifampicin and nicotine in mouse bone marrow was investigated by fluorescence in situ hybridization assay using mouse minor satellite DNA probe. Colchicine and mitomycin, known to be predominantly clastogenic and aneugenic, respectively were used as positive controls and these compounds produced the expected responses. In animals treated with different doses of rifampicin (10-320 mg/kg), the frequencies of micronucleated polychromatic erythrocytes (MNPCE) increased significantly after treatment with 160 and 320 mg/kg. Furthermore, rifampicin caused a significant depression of erythroblast proliferation at the high dose. At the two highest doses of 160 and 320 mg/kg rifampicin, a total of 0.96% and 1.44% MNPCE, respectively were found. Of the rifampicin-induced signal-positive MNPCE, an average of 58.1% of them was centromere-negative, reflecting the clastogenic activity of rifampicin. Correspondingly, about 41.9% of induced MNPCE were centromere-positive, representing the aneugenic activity of rifampicin. Eight and 16 mg/kg of nicotine induced 0.84% and 1.2% MNPCE, respectively, and of these an average of 29.5% showed one or more fluorescent signals, reflecting the predominant clastogenic activity of nicotine. The results obtained demonstrate that rifampicin induced both chromosome breakage and numerical chromosomal abnormalities, whereas nicotine induced one type of MNPCE and it could be considered a clastogenic agent.     

Evaluation of genetic toxicity of 6-diazo-5-oxo-l-norleucine (DON)

DON (6-diazo-5-oxo-l-norleucine), a glutamine antagonist, was demonstrated to exhibit analgesic, antibacterial, antiviral and anticancer properties. The study was performed to characterize its in vitro and in vivo genetic toxicity potential. DON was tested in the bacterial reverse mutation assay (Ames test) using Salmonella typhimurium tester strains (TA98, TA100, TA1535 and TA1537) and Escherichia coli tester strain (WP2 uvrA) with and without S9 and also with reductive S9. In addition, DON was tested for the chromosome aberrations in Chinese hamster ovary (CHO) cells with or without S9 to evaluate the clastogenic potential. Furthermore, DON was also evaluated for its in vivo clastogenic activity by detecting micronuclei in polychromatic erythrocyte (PCE) cells in bone marrow collected from the male mice dosed intravenously with 500, 100, 10, 1 and 0.1?mg/kg at 24 and 48-h post-dose. The Ames mutagenicity assay showed no positive mutagenic responses. However, the in vitro chromosome aberration assay demonstrated dose dependent statistically positive increase in structural aberrations at 4 and 20-h exposure without S9 and also at 4-h exposure with S9. The in vivo micronucleus assay also revealed a statistically positive response for micronucleus formation at 500, 100 and 10?mg/kg at 24 and 48-h post-dose. Thus, DON appears to be negative in the Ames test but positive in the in vitro chromosome aberration assay and in the in vivo micronucleus assay. In conclusion, the results indicate DON is a genotoxic compound with a plausible epigenetic mechanism.     

Antigenotoxic effects of cimetidine against benzene induced micronuclei in mouse bone marrow erythrocytes

An in vivo micronucleus assay using Balb/C male mice was used to examine antigenotoxic effects of cimetidine (CM) on benzene (BZN) induced genotoxic effects. CM not only has therapeutic and immunomudolatory role, but it has also been shown to protect bone marrow stem cells from radiation induced clastogenic effects. Therefore, in the present study we attempt to investigate the protective effects and possible mechanisms involved in the effects of CM. An 8-week-old male Balb/C mice (22±4 g weight) were treated with different doses of BZN (400, 600 and 800 mg/kg body weight), i.p. and sampled at 24, 48 and 72 h after treatment by cervical dislocation. Various doses of CM (10, 15, 30 mg/kg) were used in association with BZN and 1–2 h prior to BZN treatment. Results show that BZN effectively induced micronuclei in polychromatic erythrocytes (PCEs). Application of CM led to a significant reduction of micronuclei in PCEs, i.e. 2-fold after 10 mg/kg and 3-fold after 30 mg/kg CM treatment. Results also indicate CM was more effective when used in combination with BZN. Therefore, results indicate that CM could reduce clastogenic effects of BZN. Although further investigations are needed to reveal the mechanistical background behind the effect, the most probable mechanism involved might be free radical scavenging. This mechanism might be associated with amplification of glutathione system and cytochrome P-450 inhibition.     

Effect of plant melanin pigment on the clastogenic effects of chemical mutagens in mice

The chromosome aberration assay in the bone marrow cells of C57BL/6 mice showed that melanin pigment (MP) in a dose range from 0.01 to 10 mg/kg does not influence the clastogenic effect of dioxidine (200 mg/kg, i.p.), while reducing the clastogenic effect of cyclophosphamide (20 mg/kg, i.p.) by a factor of 1.5-4 in various treatment regimes depending on the mutagen injection time.     

Genotoxicity evaluation of Isaria sinclairii (ISE) extract

The mutagenic potential Isaria sinclairii, a traditional Chinese medicine composed of the fruiting bodies of I. sinclairii and its parasitic host larva, was evaluated using short-term genotoxicity tests, namely, the Ames test, chromosome aberration (CA), and micronuclei (MN) tests. In a Salmonella typhimurium assay, I. sinclairii extract (ISE) did not produce any mutagenic response in the absence or presence of 59 mix with TA98, TA100, TA1535, and TA1537. In the chromosome aberration (CA) test, ISE induced no significant effect on Chinese hamster ovary (CHO) cells compared with control. In the MN test, no significant change in the occurrence of micronucleated polychromatic erythrocytes was observed in male ICR mice intraperitoneally administered ISE at doses of 15, 150, or 1500 mg/kg. These results indicate that ISE has no mutagenic potential in these in vitro and in vivo systems.     

Developmental Exposure to 4-hydroxy-2,3,3',4',5-pentachlorobiphenyl (4-OH-CB107): Long-Term Effects on Brain Development, Behavior, and Brain Stem Auditory Evoked Potentials in Rats

In the present study the developmental neurotoxic effects ofthe PCB metabolite 4-OH-2,3,3',4',5-pentachlorobiphenyl (4-OH-CB107)were compared with effects caused by a mixture of parent polychlorinatedbiphenyl (PCB) congeners (Aroclor 1254). Pregnant female Wistarrats were exposed to 0.5 or 5 mg 4-OH-CB107, or 25 mg Aroclor1254 per kg body weight from gestation days 10 to 16. Plasmathyroid hormone levels were significantly decreased in the offspringof all treatment groups at postnatal day 4 (PND 4). Behavioralexperiments using an open field paradigm revealed an impairedhabituation in male offspring of all treatment groups at PND130. Passive avoidance experiments indicated significant influenceson the time course of step-down latencies across trials in exposedmale rats. Catalepsy induced by haloperidol showed increasesin latencies to movement onset in female offspring exposed to0.5 mg 4-OH-CB107 compared to Aroclor 1254 treated offspringat PND 168–175. Male offspring exposed to 4-OH-CB107 orAroclor 1254 showed decreases in latencies compared to controlanimals. Brain stem auditory evoked potentials (BAEPs) measuredat PND 300–310 showed significant increases in auditorythresholds in the low frequency range between Aroclor 1254 and4-OH-CB107 (5 mg/kg bw) treated animals. Measurements of neurotransmitterlevels revealed effects of Aroclor 154 exposure on both thedopaminergic and the serotonergic systems, whereas 4-OH-CB107exposure affected dopaminergic and noradrenergic systems, withslight but not significant effects on the serotonergic system.These results indicate that 4-OH-CB107 is able to induce long-termeffects on behavior and neurodevelopment. The observed effectsfor 4-OH-CB107 are similar to, but in some aspects differentfrom, the effects observed after Aroclor 1254 exposure.     

Baicalein: an in vitro antigenotoxic compound from Scutellaria baicalensis

Bioassay-guided fractionation of an H2O extract of the root of Scutellaria baicalensis has furnished an in vitro antigenotoxic flavonoid, baicalein (1) and 2',5,5',7-tetrahydroxy-6',8-dimethoxyflavone (2). Compound 1 exhibited a dose-dependent inhibition of aflatoxin B1 (AFB1) and N-methyl-N'-nitro-N-nitrosoguanidine mutagenicity in the Salmonella typhimurium bacterial mutation assay. In the chromosome aberration assay, compound 1, at a concentration of 5 microM, reduced the frequency of chromosome aberration induced by AFB1 but increased the clastogenic effect of AFB1 at a concentration of 50 microM.     

Pharmacological profile of 4-(2',4'-difluorobiphenyl-4-yl)-2-methylbutyric acid (deoxoflobufen)

4-(2',4'- Difluorobiphenyl-4-yl)-2-methylbutyric acid (deoxoflobufen, VUFB 19053, CAS 847475-35-8) has been developed as a new omega-biphenyl-alkanoic acid and studied in comparison with the racemic form of 4-(2',4'-difluorobiphenyl-4-yl)-2-methyl-4-oxobutanoic acid (flobufen, CAS 112344-52-2). The compounds were tested in a series of models including acute inflammation induced by carrageenan, adjuvant arthritis, in vitro inhibition of the leuktotriene B4 (LTB4) production, reaction of the graft versus the host (GVHR), production of specific antibodies against ovalbumin, peritoneal exudate formation induced by thioglycollate and phagocytosis of thioglycollate-stimulated mouse peritoneal macrophages. Deoxoflobufen exhibited strong anti-inflammatory, antiarthritic and immunomodulatory effects in most of the performed tests. Anti-inflammatory and antiarthritic effects are fully comparable with those of flobufen, however, the compound is less toxic and has apparently stronger immunomodulating effects.     

Modification by lacidipine of the clastogenic effect of dioxidine in vivo

The influence of lacidipine (0.1-10 mg/kg, intragastric) on the clastogenic effect of dioxidine (100 and 200 mg/kg, i.p.) under conditions of their single and repeated (five-fold, 24-h interval) administration was studied by the chromosome aberration assay in the metaphase bone marrow cells of BALB/c and C57BL/6 male mice. It was found that single (5 or 10 mg/kg) and repeated (10 mg/kg) introduction of lacidipine enhances the clastogenic effect of dioxidine in both genotypes. At the same time, a single treatment of C57BL/6 mice with 0.1 and 1 mg/kg of lacidipine sometimes significantly reduced the clastogenic effect of dioxidine (200 mg/kg). Thus, lacidipine exhibits a comutagen effect in vivo when administered at large doses (5 and 10 mg/kg) but not at small doses, where the drug sometimes acted as antimutagen in C57BL/6 mice.     

Cytogenetic effects of quinalphos in mice

The cytogenetic effect of quinalphos was studied in Swiss albino mice using the micronucleus test, bone marrow and germ cell chromosome assays and sperm morphology assay. Quinalphos at 5, 10 and 15 mg/kg body weight was administered orally to mice. Quinalphos induced micronuclei in the bone-marrow cells of mice and also caused a significant increase in chromosomal aberrations in bone-marrow cells (at 10 and 15 mg/kg body weight dose levels) and in germ cells (at all tested doses). A high incidence of abnormal sperms was also observed in mice treated with quinalphos.     

Modulatory effects of gentisic acid against genotoxicity and hepatotoxicity induced by cyclophosphamide in Swiss albino mice

Objectives This study evaluated the protective effects of gentisic acid (GA) against genotoxicity and hepatotoxicity induced by cyclophosphamide (CP) in Swiss albino mice. Methods Mice were pretreated with GA orally at doses of 50 and 100 mg/kg for 14 consecutive days before the administration of a single intraperitoneal dose of 50 mg/kg CP. The ameliorative effect of GA on genotoxicity was studied using the in‐vivo bone marrow micronuclei induction test, DNA integrity and alkaline unwinding assay. The activity of various oxidative stress enzymes were estimated in hepatic tissue. Key findings A single intraperitoneal administration of CP in mice increased the malondialdehyde level, depleted the glutathione content and antioxidant enzyme activity (glutathione peroxidase, glutathione reductase, catalase and quinone reductase), and induced DNA strand breaks and micronuclei induction. Oral pretreatment with GA at both doses caused a significant reduction in malondialdehyde and glutathione levels, restoration of antioxidant enzyme activity, reduction in micronuclei formation and DNA fragmentation. Serum toxicity marker enzymes such as aspartate aminotransferase, alanine aminotransferase and lactate dehydrogenase were increased after CP treatment but restored in GA pretreated groups. Conclusion The results support the protective effect of GA against CP induced genotoxicity and hepatotoxicity.     

Evaluation of the genotoxicity of zerumbone in cultured human peripheral blood lymphocytes

The chromosomal aberrations (CA) assay and micronucleus (MN) test were employed to investigate the effect in vitro of zerumbone (ZER) on human chromosomes. ZER is a sesquiterpene compound isolated from the rhizomes of wild ginger, Zingiber zerumbet Smith. The rhizomes of the plant are employed as a traditional medicine for some ailments and as condiments. ZER has been shown to have anti-cancer and apoptosis-inducing properties against various human tumour cells. It has also been shown to be active in vivo against a number of induced malignancies. Studies on ZER genotoxicity in cultured human peripheral blood lymphocytes (PBL) have not been reported so far. Therefore, the present study was undertaken to investigate the ability of ZER to induce chromosomal aberrations and micronuclei formation in human lymphocytes in vitro. Human blood samples were obtained from four healthy, non-smoking males aged 25–35 years. Cultures were exposed to the drug for 48 h at four final concentrations: 10, 20, 40 and 80 μM. Mitomycin C (MMC) was used as a positive control. The results of chromosomal aberrations assay showed that ZER was not clastogenic, when compared to untreated control, meanwhile MN test results showed a dose-dependent increase in MN formation. The overall clastogenic effect of ZER on human PBL was statistically not significant. In conclusion, ZER is a cytotoxic but not a clastogenic substance in human PBL.     

Inhibitory effects of histone deacetylase inhibitor depsipeptide on benzo(a)pyrene- and cyclophosphamide-induced genotoxicity in Swiss albino mice

Depsipeptide (FK228 or FR901228) was evaluated in the mouse bone marrow micronucleus test for its possible protective effect against chromosomal damage induced by benzo(a)pyrene and cyclophosphamide. Three doses of depsipeptide (0.5, 1, and 1.5 mg/kg body weight) were given intravenously to mice for 7 consecutive days prior to administration of genotoxins under investigation. All the three doses of depsipeptide were effective in exerting a protective effect against both benzo(a)pyrene and cyclophosphamide. A significant suppression (34.9% to 67.5%) in the micronuclei formation induced by benzo(a)pyrene and (25.7% to 71.5%) cyclophosphamide was observed following intravenous administration of depsipeptide at doses of 0.5, 1, and 1.5 mg/kg in Swiss albino mice.     

Safety studies on epigallocatechin gallate (EGCG) preparations. Part 1: genotoxicity.

Public interest in green tea has grown recently due to the potential health benefits from its consumption. Epigallocatechin gallate (EGCG), a principal polyphenolic component of green tea, is considered key to these healthful qualities. Although numerous studies have evaluated the anti-cancer effects of green tea and EGCG, few have examined the safety of EGCG consumption. The genotoxic potential of a concentrated EGCG preparation was tested in Salmonella and L5178Y tk+/- mouse lymphoma cell assays to further define the safety of Teavigo, a high-concentration EGCG extract of Camellia sinensis leaves produced by the same novel method. No mutagenic activity was detected in the bacterial system; however, a clastogenic 'trend' from the formation of hydrogen peroxide was noted in the murine cells. The oral administration of 500, 1000, or 2000 mg EGCG/kg to mice did not induce micronuclei formation in bone marrow cells. Similarly, administering 400, 800, or 1200 mg EGCG/kg/day in their diet for 10 days did not induce bone marrow cell micronuclei and produced plasma EGCG concentrations comparable to those reported in human studies. The intravenous injection of 10, 25 and 50 mg EGCG/kg/day to rats resulted in much higher plasma concentrations and demonstrated an absence of genotoxic effects. From these studies, it is concluded that Teavigo (EGCG) is not genotoxic.     

Teratological and cytogenetical evaluation of two antihistamines (pipethiadene and pizotifen maleate) in mice

The teratogenic and cytogenetic effects of two drugs with antihistamine properties, Pipethiadene and Pizotifen maleate, were investigated. Three groups of pregnant mice were treated daily with oral doses (0.24, 0.6 and 1.2 mg/kg) of these drugs from day 4 to day 16 of gestation. The following parameters were investigated: reproductive health of the dams, external, skeletal and visceral malformations of fetuses and frequencies of micronuclei and chromosome aberrations in bone marrow cells of dams. Oral administration of Pipethiadene or Pizotifen maleate produced no teratogenic effects. No elevation was observed in the frequencies of micronuclei and chromosome aberrations. However, the significant reduction of fetal weight after all doses of Pipethiadene or Pizotifen maleate was found to correlate well with the decreased values of the mitotic indices of bone marrow cells of mice, suggesting a potential embryotoxic effect of the tested substances.     

Effects of flavonol derivatives on the micronuclei formation by N-methyl-N′-nitro-N-nitrosoguanidine and the enhancement of bleomycin-induced chromosome aberration by N-methyl-N′-nitro-N-nitrosoguanidine

Flavonol derivatives were tested for their anticlastogenic effect against induction of micronuclei by N-methyl-N′-nitro-N-nitrosoguanidine(MNNG), and against induction of chromosome aberration by bleomycin or MNNG/bleomycin. For micronucleus assay, each flavonol derivative (0, 0.001, 0.01, 0.1, 1, 10 and 100 mg/kg) was administered orally twice with 24 h interval, together with intraperitoneally administered MNNG (150 mg/kg). The results showed that most flavonol derivatives tested were effective in suppressing the frequencies of micronuclei induced by MNNG. For chromosome aberration assay, each flavonol derivative (0, 0.1, 1, 10, and 100 mg/kg) was administered to mice orallyin vivo, and then mice were sacrificed and spleen lymphocyte cultures were made. Bleomycin (3 μg/ml) was treated to the mouse spleen lymphocyte cultures at 24 h after con A initiation. There were no marked decrease tendencies in chromosome aberration unless all doses of galangin and some doses of several flavonol derivatives tested. In the another experiment, we have evaluated the effect of flavonol derivatives on the enhancement of bleomycin-induced chromosome aberration by MNNG. Most of flavonol derivatives reduced the incidence of chromosome aberration induced by in vitro treatment of bleomycin followed byin vivo treatment of MNNG. Galangin particulary showed a dose-dependent decrease tendency. Other flavonol derivatives showed slightly decrease tendencies although there were no dose-dependent relationships. These results suggest that most of flavonol derivatives may be capable of protecting the inibition of DNA-repair by MNNG. Our data indicate clearly that flavonol derivatives can suppress MNNG-induced genotoxicity such as an induction of MNPCEs. Therefore, our results could suggest that flavonol derivatives may be useful as a chemopreventive agent of MNNG.     

Hepatic effects of a highly purified 2,2',3,4,4',5,5'-heptachlorbiphenyl (PCB 180) in male and female rats

PCB 180 (2,2',3,4,4',5,5'-heptachlorobiphenyl) is a persistent and accumulating polychlorinated biphenyl abundantly present in food and the environment. In this study, we used highly purified PCB 180 (dioxinlike impurities: 2.7 ng TEQ(WHO)/g PCB 180) in a 28-day toxicity study in young adult Sprague-Dawley rats. Male and female rats were given total doses of 3, 10, 30, 100, 300, 1000 or 1700 mg/kg b.w. PCB 180 by gavage. Increased liver weights were observed at ≥ 300 mg/kg b.w. in males and females. No increases in serum ALT or ALP activities were found. A significant increase in liver pentoxyresorufin O-dealkylase (PROD) activity was found in males at ≥ 10 mg/kg b.w. and in females at ≥ 30 mg/kg b.w. In both genders, a significant induction of hepatic 7-ethoxyresorufin O-deethylase (EROD) activity was also observed in males at ≥ 10 mg/kg b.w. and in females at ≥ 300 mg/kg b.w. Western blotting showed that mainly cytochromes P450 (CYPs) 2B1/2 and 3A1 were induced while slight effects were seen on CYP1A1, CYP1A2 and CYP1B1. However, no induction of CYP1A1, 1A2 and 1B1 was found on the mRNA level, except for a slight effect in females at 1000 mg/kg b.w. Furthermore, hepatic UDP-glucuronosyltransferases (UGTs) 1A1 and 1A6 were markedly induced in males and slightly induced in females. The hepatic concentrations of apolar retinoids were decreased in males at ≥ 30 mg/kg b.w. and in females at ≥ 300 mg/kg b.w. Taken together our findings show that pure PCB 180 leads to hepatic changes in a dose range which did not cause CYP1A1 induction but causes centrilobular liver hypertrophy, affects drug-metabolizing enzymes involved in the metabolism of exogenous and endogenous substrates and leads to changes in liver retinoid levels. A benchmark dose (BMD) approach is presented in order to model lowest effective dose levels for these effects. Comparison of PCB 180 liver level related to BMDL? for hepatic hypertrophy in rats with human data on 'total' hepatic PCB levels in individuals without history of specific exposure suggests a relatively small margin of tissue burden in the range of 37-fold. Our results show that the highly pure non dioxin-like PCB 180 exerted strong effects different to dioxin-like compounds and that the low TEQ contamination allowed a characterization of the PCB as non-dioxinlike.     

Quinoid metabolites of 4-monochlorobiphenyl induce gene mutations in cultured Chinese hamster v79 cells.

4-Monochlorobiphenyl (PCB3) is a component of commercial polychlorinated biphenyl (PCB) products and is an airborne environmental pollutant. Our recent study with transgenic Fischer 344 rats revealed the mutagenic potential of PCB3 in the livers of male rats. PCB3 is converted in vitro to hydroxylated metabolites, to hydroquinones (HQs, e.g., 2',5'-HQ and 3',4'-HQ), and can be further oxidized to quinones (Qs, e.g., 2',5'-Q and 3',4'-Q). This raises the question whether the mutagenic potential of PCB3 is due to the mutagenicity of PCB3 itself or of one of the metabolites. In this study, we investigated the mutagenicity of PCB3, of the monohydroxylated metabolites 2'-hydroxy (HO)-, 3'-HO-, and 4'-HO, of the HQs 3',4'-HQ and 2',5'-HQ and of the Qs 3',4'-Q and 2',5'-Q in cultured Chinese hamster V79 cells. The induction of gene mutations was determined at the hypoxanthine-guanine phosphoribosyltransferase (hprt) gene locus by selection with 6-thioguanine. The induction of chromosome and genome mutations was assessed using the micronucleus assay and immunochemical differentiation of micronuclei containing whole chromosomes (kinetochore positive) and DNA fragments (kinetochore negative). The induction of chromosome and genome mutations, detected as micronuclei, was only observed at higher, cytotoxic concentrations of monohydroxylated, catecholic, and quinoid metabolites of PCB3. However, both PCB3-Qs induced a significant increase in the mutant frequency of the hprt gene and did so at submicromolar concentrations. Thus, the present study demonstrates for the first time the mutagenicity of PCB3 metabolites in mammalian cells and identifies quinoid metabolites of PCB3 as potential ultimate mutagens.