Charles D. Hébert

Investigation of the Mode of Action Underlying the Tumorigenic Response Induced in B6C3F1 Mice Exposed Orally to Hexavalent Chromium

Chronic ingestion of high concentrations of hexavalent chromium [Cr(VI)] in drinking water induces intestinal tumors in mice. To investigate the mode of action (MOA) underlying these tumors, a 90-day drinking water study was conducted using similar exposure conditions as in a previous cancer bioassay, as well as lower (heretofore unexamined) drinking water concentrations. Tissue samples were collected in mice exposed for 7 or 90 days and subjected to histopathological, biochemical, toxicogenomic, and toxicokinetic analyses. Described herein are the results of toxicokinetic, biochemical, and pathological findings. Following 90 days of exposure to 0.3–520 mg/l of sodium dichromate dihydrate (SDD), total chromium concentrations in the duodenum were significantly elevated at ≥ 14 mg/l. At these concentrations, significant decreases in the reduced-to-oxidized glutathione ratio (GSH/GSSG) were observed. Beginning at 60 mg/l, intestinal lesions were observed including villous cytoplasmic vacuolization. Atrophy, apoptosis, and crypt hyperplasia were evident at ≥ 170 mg/l. Protein carbonyls were elevated at concentrations ≥ 4 mg/l SDD, whereas oxidative DNA damage, as assessed by 8-hydroxydeoxyguanosine, was not increased in any treatment group. Significant decreases in the GSH/GSSG ratio and similar histopathological lesions as observed in the duodenum were also observed in the jejunum following 90 days of exposure. Cytokine levels (e.g., interleukin-1β) were generally depressed or unaltered at the termination of the study. Overall, the data suggest that Cr(VI) in drinking water can induce oxidative stress, villous cytotoxicity, and crypt hyperplasia in the mouse intestine and may underlie the MOA of intestinal carcinogenesis in mice.

Comparison of the Effects of Hexavalent Chromium in the Alimentary Canal of F344 Rats and B6C3F1 Mice Following Exposure in Drinking Water: Implications for Carcinogenic Modes of Action

Exposure to high concentrations of hexavalent chromium (Cr[VI]) in drinking water is reported to induce oral mucosa tumors in F344 rats and intestinal tumors in B6C3F1 mice. To investigate the modes of action underlying these tumors, 90-day drinking water studies (with interim necropsy at day 8) were conducted with concentrations of 0.1–182 mg/l Cr(VI), administered as 0.3–520 mg/l sodium dichromate dihydrate. Blood and tissue samples were analyzed for chromium content, oxidative stress, iron levels, and gross and microscopic lesions. Results for the F344 rats are described herein and compared with results from B6C3F1 mice published previously. After 90 days of exposure, total chromium concentrations in the rat and mouse oral mucosae were comparable, yet significant dose-dependent decreases in the reduced-to-oxidized glutathione ratio (GSH/GSSG) were observed only in rats. In the duodenum, changes in GSH/GSSG were only observed in mice. Levels of 8-hydroxydeoxyguanosine were not increased in the oral or duodenal mucosae of either species. Glutathione levels were increased in the duodenum but decreased in the jejunum of both species, indicating potential differential responses in the intestinal segments. Histiocytic infiltration was observed in the duodenum of both species, yet duodenal cytokines were repressed in mice but increased in rats. Serum and bone marrow iron levels were more decreased in rats than mice. Collectively, these data suggest that Cr(VI)-induced carcinogenesis in the rodent alimentary canal involves oxidative stress; however, differences in histopathology, cytokines, and iron status suggest potential contributions from other factors as well.

Toxicity and carcinogenicity studies of methylene blue trihydrate in F344N rats and B6C3F1 mice.

Methylene blue trihydrate has a variety of biomedical and biologically therapeutic applications. Groups of 50 male and 50 female rats and mice were administered methylene blue trihydrate in 0.5% aqueous methylcellulose solution by gavage at doses of 0, 5, 25, or 50mg/kg bw/day (rats) or 0, 2.5, 12.5, and 25mg/kg bw/day (mice), 5 days per week for 2 years. In rats survival of all dosed groups was similar to that of the vehicle controls, whereas mice exhibited a dose-dependent increase in survival. Rats receiving 25 and 50mg/kg bw/day and mice receiving 25mg/kg bw/day developed mild anemia. The incidences of pancreatic islet cell adenoma and adenoma or carcinoma (combined) were increased in all dosed groups of male rats, but increases were statistically significant in 25mg/kg bw/day males only and the dose-response was non-linear. There was a corresponding increase in the incidence of pancreatic islet cell hyperplasia but statistically significant only in the 50mg/kg bw/day male rats. There were no significant increases in neoplastic transformation observed in the mice; however, positive trends were noted for adenoma or carcinoma (combined) of the small intestine and malignant lymphoma.

In-vitro and in-vivo metabolic studies of the candidate chemopreventative pentamethylchromanol using liquid chromatography/tandem mass spectrometry.

Objectives This study focuses on the in‐vitro metabolic profiles of pentamethyl‐chromanol in human, rat, dog and non‐human primates, and characterizes the associated metabolic kinetics and specific human isozymes responsible for metabolism. Additional investigations compare in‐vitro data with in‐vivo metabolic data from rats and dogs.

Toxicity of Lactide in Dogs after 2 and 13 Weeks of Daily Oral Dosing

Two-week and 13-week studies were conducted to determine the toxicity of lactide when the compound is administered orally in gelatin capsules to beagle dogs. In the 2-week study, daily doses of 0, 10, 100, 400, 1000 and 2500 mg/kg body weight/day were administered to dogs of both sexes for 14 consecutive days. All dogs survived to the end of the study. Clinical signs consistent with irritation of the alimentary tract occurred in dogs in the 1000 and 2500 mg/kg dose groups. Reductions in body weight gain and in absolute and relative thymus weights were observed in the same two dose groups, and reductions in absolute and relative spleen weights were seen in the 2500 mg/kg dose group. These changes were considered to be secondary to the stress resulting from irritation of the gastrointestinal tract. Gross and microscopic lesions were indicative of irritation, and included dark foci and haemorrhage of the stomach lining, and erosion and ulceration of the stomach and the oesophagus. Also noted in all high-dose dogs was renal tubular regeneration, which may represent repair of previous necrosis of the tubular epithelium. In the 13-week study, no deaths occurred when dogs were given daily oral doses of 0, 4, 20 or 100 mg/kg body weight/day. No clinical signs of toxicity were observed, and the compound had no effect on body weights, food consumption, or any of the clinical chemistry, haematology or urinalysis parameters assessed. Gross and microscopic findings considered to be potentially related to lactide administration were minimal, and included a stomach focus in one dog of each sex in the 100 mg/kg group. The stomach focus in the 100 mg/kg female dog was manifested microscopically as a stomach ulcer. Based on these results, the primary toxic effect of lactide was considered to be irritation of the gastrointestinal tract, and the no-observed-adverse-effect level (NOAEL) after subchronic oral dosing in dogs was considered to be 100 mg/kg/day.

Species and gender differences in the carcinogenic activity of trimethylolpropane triacrylate in rats and mice

Trimethylolpropane triacrylate (TMPTA) is a multifunctional monomer with industrial applications. To determine the carcinogenic potential, male and female F344/N rats and B6C3F1/N mice were administered TMPTA (0, 0.3, 1.0, or 3.0mg/kg) in acetone dermally for 2 years. There were no differences in the body weights and survival in the treated animals compared to controls. Nonneoplastic skin lesions at the site of application included epidermal hyperplasia and hyperkeratosis in both rats and mice. There were no incidences of tumors at the site of application in rats and mice. Rare malignant liver neoplasms were observed in female mice that included hepatoblastoma in the 0.3 and 3.0mg/kg groups, and hepatocholangiocarcinoma in the 1.0 and 3.0mg/kg groups. The incidences of uterine stromal polyp and stromal polyp or stromal sarcoma (combined) in female mice occurred with positive trends and the incidences were significantly increased in the 3.0mg/kg group. A marginal increase in the incidences of malignant mesothelioma in male rats may have been related to TMPTA treatment. In conclusion, our studies show that TMPTA is a dermal irritant in both rats and mice of either sex. Increased incidences of tumor formation were observed in female mice and male rats.