Proctor D, Haws L,Thompson C, Harris M. 2011. Use of mode of action and pharmacokinetics to inform the cancer risk assessment of ingested Cr(VI): A case study. Presented at the Society of Toxicology’s 50th Annual Meeting, March 6-10, 2011. Washington, D.C.
We conducted a Mode of Action (MOA) analysis for intestinal tumors observed in mice following chronic exposure to Cr(VI) in drinking water. Data gaps in the MOA were used to design a 90-day drinking water study including the collection of histopathological, biochemical, toxicogenomic, and pharmacokinetic data, as well as measures of DNA modification in rats and mice at concentrations of sodium dichromate dihydrate (SDD) from 0 to 520 mg/L. These data were used to evaluate whether the MOA for intestinal tumors is initiated through mutagenic or nonmutagenic mechanisms, interspecies differences, and dose-response at environmentally-relevant exposures. After 90 days of exposure in mice, crypt hyperplasia and villous atrophy were observed in the duodenum at exposures ≥ 170 mg/L SDD. Cytoplasmic signs of toxicity and stress were observed at ≥ 60 mg/L SDD. Clear signs of cellular oxidative stress (8-isoprostane) were apparent at ≥ 60 mg/L SDD, whereas changes in redox (GSH/GSSG) were apparent starting at 4 mg/L SDD. Together, these data suggest graded, dose-dependent changes in cell physiology beginning with mild oxidation, followed by oxidative stress, cytoplasmic toxicity, villous blunting and regenerative cell proliferation. These findings indicate a point-ofcontact response for Cr(VI) in the lumen with the epithelium of the small intestine, and target tissue response in the mouse at concentrations ≥ 4 mg/L SDD. Oxidative DNA damage was not significantly elevated in the duodenum, but preliminary data on chromium content in genomic DNA of intestinal epithelium suggests a similar dose-dependent increase in the duodenum and jejunum of mice. These data, along with an evaluation of in vivo mutations and gene expression in the duodenum and oral mucosa of mice, and corresponding information in the rat, were used to inform the MOA for rodent tumors. Toxicokinetic models were used to relate tissue responses to relevant internal dose metrics, model dose-response, and for interspecies extrapolation.