The Toxicology of 1,4-Dioxane - Environmental Investigation and Remediation - page 250

followed 24 h later by an i.p. injection of 30 mg/kg diethylnitrosamine * (initiation treatment).
1,4-Dioxane was then administered daily by saline gavage at doses of 0, 100, or 1000 mg/kg per
day, i ve days per week for seven weeks. Control rats were given saline following diethylnitrosamine
initiation. 1,4-Dioxane was also administered to groups of rats that were not given the diethyl-
nitrosamine initiation treatment (saline used instead of diethylnitrosamine). Ten days after the last
dose, animals were sacrii ced, and liver sections were stained for GGT (i.e., preneoplastic enzyme-
altered foci). The number and total volume of GGT-positive foci were determined. 1,4-Dioxane did
not increase the number or volume of GGT-positive foci in rats that were not given the diethyl-
nitrosamine initiation treatment. The high dose of 1,4-dioxane (1000 mg/kg per day) given as a
promoting treatment (i.e., following diethylnitrosamine injection) produced an increase in the
number of GGT-positive foci and the total foci volume. Histopathological changes were also noted
in the livers of high-dose rats. Enlarged, foamy hepatocytes were observed in the midzonal region
of the liver; the foamy appearance was due to the presence of numerous fat-containing cytoplasmic
vacuoles. These results suggest that cytotoxic doses of 1,4-dioxane may be associated with tumor
promotion of 1,4-dioxane in rat liver.
5.3.3.3 Mechanistic Studies Evaluating Mode of Action for Carcinogenicity
Several studies have been performed to evaluate potential mechanisms for the carcinogenicity of
1,4-dioxane (Stott et al., 1981; Kitchin and Brown, 1990; Goldsworthy et al., 1991). Stott et al.
(1981) evaluated 1,4-dioxane in several test systems, including Salmonella typhimurium mutagen-
icity in vitro (Ames test), rat hepatocyte DNA repair activity in vitro , DNA synthesis determination
in male Sprague Dawley rats following acute gavage dosing or an 11-week drinking-water expo-
sure and hepatocyte DNA alkylation and DNA repair following a single gavage dose. This study
used daily doses of 0, 10, 100, or 1000 mg/kg; the highest dose was considered to be a tumorigenic
dose level. Liver histopathology and liver-to-body-weight ratios were also evaluated in rats from
acute gavage or repeated-dose drinking-water experiments (only for groups receiving 10 and
1000 mg/kg per day).
The histopathology evaluation indicated that liver cytotoxicity (i.e., centrilobular hepatocyte
swelling) was present in rats from the 1000 mg/kg per day dose group that received 1,4-dioxane in
the drinking water for 11 weeks (Stott et al., 1981). An increase in the liver-to-body-weight ratio
accompanied by an increase in hepatic DNA synthesis was also seen in high-dose animals. No
effect on histopathology, liver weight, or DNA synthesis was observed in acutely exposed rats or rats
that were exposed to a lower daily dose of 10 mg/kg for 11 weeks. 1,4-Dioxane produced negative
i ndings in the remaining genotoxicity assays conducted as part of this study (i.e., Salmonella typh-
imurium mutagenicity, in vitro and in vivo rat hepatocyte DNA repair, and DNA alkylation in rat
liver). The lack of genotoxicity in this study suggests that a mutagenic mechanism for 1,4-dioxane
hepatocellular carcinoma in rats is unlikely.
Goldsworthy et al. (1991) evaluated potential mechanisms for the nasal and liver carcinogenicity
of 1,4-dioxane in the rat. DNA repair activity was evaluated as a measure of DNA reactivity, and
DNA synthesis was measured as an indicator of cell proliferation or promotional activity. In vitro
DNA repair was evaluated in primary hepatocyte cultures from control and 1,4-dioxane-treated rats
(1% or 2% in the drinking water for one week). DNA repair and DNA synthesis were also measured
in vivo following a single gavage dose of 1000 mg/kg, a drinking-water exposure of 1% (1500 mg/
kg per day) for one week, or a drinking-water exposure of 2% (3000 mg/kg per day) for two weeks.
Liver-to-body-weight ratios and palmitoyl CoA (coenzyme A) oxidase activity were measured in the
rat liver to determine whether peroxisome proliferation † played a role in the liver carcinogenesis due
to 1,4-dioxane. In vivo DNA repair was evaluated in rat nasal epithelial cells derived from either the
* Diethylnitrosamine (CASRN 55-18-5) is also called n-nitrosodiethylamine or NDEA.
† Peroxisome proliferation is a mechanism by which some drugs and chemicals induce liver cancer in rodents. Peroxisomes
participate in fatty acid metabolism in cells.