Kirman CR, Hays SM, Aylward LL, Suh M, Proctor D. 2012. Physiologically-based pharmacokinetic model for mice, rats and humans orally-exposed to chromium. Presented at the Society of Toxicology’s 51st Annual Meeting, March 11-15, San Francisco, CA.
A multi-compartment physiologically based pharmacokinetic (PBPK) model was developed to describe the behavior of Cr(III) and Cr(VI) in mice, rats, and humans. Tissue compartments were included for gastrointestinal lumen, oral cavity, stomach, small intestines (duodenum, jejunum, ileum), blood, liver, kidney, bone, and a combined compartment for remaining tissues. Data from ex vivo reduction studies were used to characterize reduction of Cr(VI) in fed rodent gastric fluid as a second-order process. Ex vivo data for human stomach fluid were used to characterize reduction of Cr(VI) in humans as a pH-dependent process under fed and fasted conditions. Tissue time course data for total Cr were collected from rats and mice exposed to Cr(VI) in drinking water for 90 days at six concentrations ranging from 0.1 to 180 mg Cr(VI)/L. These data were used to supplement the tissue time course data collected by NTP for Cr(III) and Cr(VI). Clear species differences are identified for Cr delivery to the target tissue (small intestines), with higher concentrations achieved in mice, consistent with tumor formation upon chronic exposures, than rats, in which tumors were not observed. Plasma:RBC Cr ratios suggest that Cr(VI) entered portal circulation at 60 and 180 mg/L in rodents. Species differences were identified for distribution of Cr to the liver and kidney, with liver:kidney ratios higher in mice than in rats. Overall, the PBPK model provides a reasonable fit to the available data. Simulations in humans were conducted to evaluate diurnal variation in gastric pH, as a function of meal consumption, and its impact of Cr(VI) reduction. Additional simulations were conducted to address potentially sensitive subpopulations, including fasted individuals, individuals on proton pump inhibitor medication, and children. The predictions of internal dose using the PBPK model support dose-response assessments for cancer and noncancer endpoints and allow for interspecies extrapolations to humans at environmentally relevant exposure concentrations.