Mutagenic activity of epoxy embedding reagents employed in electron microscopy

In silico assessment of chemical mutagenesis in comparison with results of Salmonella microsome assay on 909 chemicals

Genotoxicity is one of the important endpoints for risk assessment of environmental chemicals. Many short-term assays to evaluate genotoxicity have been developed and some of them are being used routinely. Although these assays can generally be completed within a short period, their throughput is not sufficient to assess the huge number of chemicals, which exist in our living environment without information on their safety. We have evaluated three commercially available in silico systems, i.e., DEREK, MultiCASE, and ADMEWorks, to assess chemical genotoxicity. We applied these systems to the 703 chemicals that had been evaluated by the Salmonella/microsome assay from CGX database published by Kirkland et al. We also applied these systems to the 206 existing chemicals in Japan that were recently evaluated using the Salmonella/microsome assay under GLP compliance (ECJ database). Sensitivity (the proportion of the positive in Salmonella/microsome assay correctly identified by the in silico system), specificity (the proportion of the negative in Salmonella/microsome assay correctly identified) and concordance (the proportion of correct identifications of the positive and the negative in Salmonella/microsome assay) were increased when we combined the three in silico systems to make a final decision in mutagenicity, and accordingly we concluded that in silico evaluation could be optimized by combining the evaluations from different systems. We also investigated whether there was any correlation between the Salmonella/microsome assay result and the molecular weight of the chemicals: high molecular weight (>3000) chemicals tended to give negative results. We propose a decision tree to assess chemical genotoxicity using a combination of the three in silico systems after pre-selection according to their molecular weight.

Glycidyloxy compounds used in epoxy resin systems: a toxicology review

The glycidyloxy compounds constitute an important group of chemicals used extensively in the formulation of epoxy resin systems employed in coatings, electronics, structural composites, and adhesives. Although extensive toxicological data are available on glycidyloxy compounds, use and understanding of the data have been hampered by two major problems: (1) proper identification and complexity of the epoxy systems in question, and (2) absence of meaningful classification of epoxy materials. This paper provides a classification scheme with CAS numbers and reviews the mammalian toxicology of the most common glycidyloxy derivatives used in epoxy resin systems based on both published and proprietary information. Although the toxicity of many of the glycidyloxy compounds used in epoxy resin systems can be characterized as low, the diversity of compounds found within this group precludes broad generalizations for the class. This comprehensive account should facilitate a clearer understanding of the potential health effects and allow for easier comparison among compounds containing the glycidyloxy moiety.

The role of epoxidation in 4-vinylcyclohexene-induced ovarian toxicity

4-Vinylcyclohexene (VCH) is present in gases discharged during synthetic rubber production. Chronic treatment of B6C3F1 mice and F-344 rats with VCH by gavage has been shown to induce ovarian tumors in mice but not in rats. Our objective was to understand the mechanism of the species difference in VCH-induced ovarian tumors. Since a critical step in the induction of ovarian tumors is destruction of the small oocyte, small oocyte counts obtained from serially sectioned ovaries were used as an index of toxicity. VCH or its epoxide metabolites [VCH-diepoxide, VCH-1,2-epoxide, and VCH-7,8-epoxide (in mice only)] were given to 28-day-old female mice and rats in corn oil, ip, at doses ranging from 0.07 to 7.4 mmol/kg body wt/day for 30 days. The dose which reduced the small oocyte count to 50% that of control was defined as the ED50. In mice, the ED50 for the reduction in small oocytes by VCH was 2.7 mmol/kg, whereas, no detectable oocyte loss occurred in rats at the highest dose of VCH (7.4 mmol/kg). The potency of the epoxides of VCH was greater than that of VCH in both species. The ED50 for oocyte loss by VCH-1,2-epoxide in mice and rats was 0.5 and 1.4 mmol/kg, respectively. In mice, VCH-7,8-epoxide had comparable potency to VCH-1,2-epoxide (ED50 = 0.7). VCH diepoxide was even more potent with ED50 values of 0.2 and 0.4 mmol/kg, in mice and rats, respectively. The dose response of the blood concentration of VCH-1,2-epoxide in mice after VCH showed that doses of VCH which caused minimal toxicity had the lowest blood level of this ovotoxic epoxide. Pretreatment of mice with the cytochrome P450 inhibitor chloramphenicol (200 mg/kg, ip) inhibited VCH epoxidation in vivo and in vitro and partially protected mice from VCH toxicity. Thus it appears that metabolism of VCH to epoxides and their subsequent destruction of oocytes are critical steps in VCH-induced ovarian tumors. Rats may be resistant to ovarian tumor induction by VCH because the amount of VCH converted to epoxides is insufficient to produce oocyte destruction.

Toxicological evaluation of 4-vinylcyclohexene. II. Induction of ovarian tumors in female B6C3F1 mice by chronic oral administration of 4-vinylcyclohexene

4-Vinylcyclohexene (VCH), a dimer of 1,3-butadiene present in the gases discharged during tire curing, was examined for its toxic and carcinogenic effects in Fischer 344 (F344) rats and B6C3F1 mice by 2-yr chronic testing. VCH was administered orally by gavage in corn oil at doses of 0 (vehicle control), 200, or 400 mg/kg body weight to groups of 50 F344 rats and B6C3F1 mice of each sex for 103 wk (5 d/wk). Because the studies of VCH in male and female rats and in male mice were considered to be inadequate studies of carcinogenicity due to the extensive and early mortality at the high dose or both doses tested, as well as the lack of conclusive evidence of a carcinogenic effect, the present article focuses on the results of the 2-yr study of VCH in female B6C3F1 mice. Survival of high-dose female mice was lower (p less than 0.001) than that of the vehicle controls, whereas survival of low-dose and survival of vehicle control female mice were comparable. Mean body weights of high-dose female mice were generally slightly lower than those of the vehicle controls, whereas the mean body weights of low-dose female mice were generally greater than or comparable to those of the vehicle controls. Oral administration of VCH by gavage to female B6C3F1 mice was associated with an increased incidence of a number of nonneoplastic lesions, including mild acute inflammatory lesions and epithelial hyperplasia of the forestomach, congestion of the lungs and adrenal glands at the high dose, and cytologic alteration of the adrenal cortex at both doses. However, the most striking finding was the markedly increased (p less than 0.01) incidences of uncommon ovarian neoplasms, including mixed benign tumors, granulosa-cell tumors, and granulosa-cell tumors or carcinomas (combined), in both groups of dosed female mice. In addition, the increased incidence of adrenal-gland capsular adenomas in high-dose female mice may have been compound-related.

Toxicological evaluation of 4-vinylcyclohexene. I. Prechronic (14-day) and subchronic (13-week) gavage studies in Fischer 344 rats and B6C3F1 mice

4-Vinylcyclohexene (VCH), a dimer of 1,3-butadiene present in the gases discharged during tire curing, was examined for its toxic effects in Fischer 344 (F344) rats and B6C3F1 mice by 14-d prechronic and 13-wk subchronic testing. In the 14-d studies, VCH was administered orally by gavage in corn oil at doses of 0 (vehicle control), 300, 600, 1250, 2500, or 5000 mg/kg body weight to groups of five F344 rats and B6C3F1 mice of each sex, while the doses for the 13-wk studies (10 animals/group; 5 d/wk) were 0 (vehicle control), 50, 100, 200, 400, or 800 mg/kg body weight for rats and 0 (vehicle control), 75, 150, 300, 600, or 1200 mg/kg body weight for mice. All rats and most mice in the 14-d studies died when administered doses greater than or equal to 1250 mg/kg, although no compound-related gross or histopathologic effects were observed. In the 13-wk studies, extensive mortality was observed only in mice dosed at 1200 mg/kg. Final body weights were reduced in the 13-wk studies in male rats receiving doses greater than or equal to 400 mg VCH/kg, in female rats receiving 800 mg/kg, and in female mice receiving 600 mg/kg. Compound-related histopathologic effects in the 13-wk studies included hyaline droplet degeneration of the proximal convoluted tubules of the kidney in dosed male rats, the severity of which was dose-related, and a reduction in the number of primary follicles and mature graafian follicles in the ovaries of female mice receiving 1200 mg VCH/kg. No compound-related gross or histopathologic effects were evident in dosed female rats or male mice in the 13-wk studies.

The Reproductive Effects Assessment Group's report on the mutagenicity of 1,3-butadiene and its reactive metabolites

A major data gap for assessing heritable risk from exposure to 1,3-butadiene is the lack of mammalian mutagenicity data. The data base on the mutagenic potential of 1,3-butadiene is limited to three bacterial studies from the same laboratory. Two of these studies were positive only in the presence of liver S9 mix from chemically pretreated animals. In vitro data suggest that 1,3-butadiene is metabolized to two epoxide intermediates. 3,4-Epoxybutene, one potential reactive metabolite of 1,3-butadiene, is a monofunctional alkylating agent and is a direct-acting mutagen in bacteria. In addition, unpublished data suggest that 3,4-epoxy-butene induces DNA damage and chromosomal aberrations in mice. Another potential reactive metabolite, 1,2:3,4-diepoxybutane, is a bifunctional alkylating agent and is mutagenic in a wide variety of organisms (bacteria, fungi, and the germ cells of Drosophila). This metabolite also induces DNA damage in mice and in cultured hamster cells, is clastogenic in fungi and cultured rat cells, and produces chromosome damage/breakage in Drosophila germ cells. These data, when combined with evidence that 1,3-butadiene is carcinogenic in rodent gonadal tissues and is associated with gonadal atrophy in mice, constitute suggestive evidence that 1,3-butadiene may be a human germ cell mutagen. However, because the mutagenicity of 1,3-butadiene has been studied only in bacteria, studies in mammalian test systems are needed to further characterize the mutagenic potential of 1,3-butadiene.

Uptake, metabolism and mutagenic activity of aromatic glycidyl compounds

Aromatic diglycidyl compounds are very active mutagens when assayed in in vitro tests. In vivo, however, resorcinol diglycidyl ether provided no evidence for the clastogenic activity, while diglycidylaniline exhibited definite mutagenic activity in the micronucleus test. Since the only difference between these two compounds lies in the binding mode of the glycidyl groups to the aromatic nucleus (i.e. ether oxygen vs. aminic nitrogen), this apparent discrepancy in mutagenic activity led to the question of the mechanisms involved in such an activity difference. Although no clear signs of differential uptake or excretion could be detected in mice, differences could be seen in the spectrum of urinary metabolites; while resorcinol diglycidyl ether seemed to become fully converted to the genetically inactive bis-diol compound, a sizeable proportion of diglycidylaniline was converted only to the diol-epoxide. In vitro investigations and enzyme kinetic measurements with postmitochondrial supernatant of rat or mouse liver homogenate (S-9) finally yielded the biochemical explanation for this behaviour, as they showed a very low affinity of the diol-epoxide metabolite of diglycidylaniline for the epoxide hydrolase, normally involved in the degradation of such compounds. The diol-epoxide obtained from resorcinol diglycidyl ether, on the other hand, has an affinity to the degradation enzyme similar to, or even higher than, the one measured with the parent substance.

The mutagenicity of mono- and di-functional aromatic glycidyl compounds

Although epoxides are very important compounds in today's technology, they are not as well investigated for their carcinogenic and mutagenic properties as their economical importance would suggest. The present study tried, on one hand, to bridge the gap between microbial testing of such compounds and the in vivo mammalian test system. On the other hand, the comparative testing of structurally related substances was expected to yield some clues as to the structural determinants of possible in vivo mutagenic activity. Our investigations with 4 compounds of similar structure, namely glycidylmethylaniline (GMA), diglycidylaniline (DGA), phenylglycidylether (PGE), and resorcinol diglycidyl ether (RDGE), first demonstrated that, in bacterial assays, all 4 epoxides acted as potent mutagens. Only the 2 difunctional epoxides, however, were active in an in vitro chromosomal aberration assay with CHO cells. That such in vitro results cannot be supposed to represent the in vivo situation is again demonstrated by discovery that only one of the two in vitro active substances (and in fact it is the less active one) also has a damaging effect in vivo. It, therefore, seems quite justifiable to conclude that two structural features must be combined in an aromatic glycidyl compound if it is to exert chromosome damaging activity in vivo: The substance must possess 2 epoxy functions, and they must be bound to the aromatic nucleus by an aminic nitrogen. The industrial use of aromatic glycidyl ethers might, therefore, be considered not to pose a great systemic risk for mutagenicity or carcinogenicity, without completely excluding the possibility of local effects.

Alkylating properties and genetic activity of 4-vinylcyclohexene metabolites and structurally related epoxides

The mutagenicity of the epoxides 4-vinyl-1,2-epoxycyclohexane, 4-epoxyethyl-1,2-epoxycyclohexane, 4-epoxyethyl-1,2-dihydroxycyclohexane, 1,2-epoxycyclohexane and styrene oxide was assayed on the TA100 strain of S. typhimurium and V79 Chinese hamster cells. In the latter cell system, both point mutation (6-thioguanine resistance) and chromosomal damage (anaphase bridges and micronuclei) were scored. Genetic effects were related to the alkylating properties of the epoxides. For this purpose, alkylation of 4-(p-nitrobenzyl)pyridine (NBP) and sodium-p-nitrothiophenolate (NTP) was measured and values for the substrate constant (s) were calculated. 4-Epoxyethyl-1,2-epoxycyclohexane, 1,2-epoxycyclohexane and styrene oxide, characterized by the highest reactivity toward NBP and by an s value in the vicinity of 1, were mutagenic in all test systems. 4-Vinyl-1,2-epoxycyclohexane and 4-epoxyethyl-1,2-dihydroxycyclohexane, characterized by lower NBP reactivity and higher s value (1.30-1.38), did not induce reversion in S. typhimurium or 6-thioguanine-resistant mutants in V79 cells, but were as effective as the 3 other compounds in the induction of chromosomal damage.