DNA—Protein Cross-Links (DPX) and Cell Proliferation in B6C3F1 Mice but Not Syrian Golden Hamsters Exposed to Dichloromethane: Pharmacokinetics and Risk Assessment with DPX as Dosimeter

Abstract

Dichloromethane (DCM) (methylene chloride; CH2Cl2) is metabolized via a glutathione S-transferase-mediated pathway to formaldehyde (HCHO), a mutagenic compound that could play a role in the carcinogenic effects of DCM observed in the liver and lungs of B6C3F1 mice at 2000 and 4000 ppm. Mice but not hamsters formed DNA-protein cross-links (DPX) in the liver at DCM concentrations ranging from approximately 500 to 4000 ppm. The formation of DPX was a nonlinear function of the airborne concentration of DCM. In addition, mice exposed to DCM (6 hr/day, 3 days) at concentrations ranging from approximately 1500 to 4000 ppm showed an increased rate of DNA synthesis in the lung indicating cell proliferation, but increased cell turnover was not detected in mouse lung at exposure concentrations of 150 or 500 ppm. Hamsters showed no evidence of cell proliferation in the lung at any concentration, and cell proliferation was not apparent in the livers of either mice or hamsters. An extended physiologically based pharmacokinetic (PBPK) model for DPX formation in mouse liver was developed, based on a published PBPK model for DCM (Andersen, M.E., Clewell, H.J., III, Gargas, M.L., Smith, F.A., and Reitz, R.H. (1987). Toxicol. Appl. Pharmacol. 87, 185-205). The extended PBPK model was fitted to the DPX data using the PBPK model-estimated area under the curve for DCM in mouse liver as the independent variable. Parameter estimates for HCHO disposition in the livers of mice exposed to dichloromethane were similar to previously published estimates for HCHO disposition in the nasal mucosa of rats exposed to formaldehyde. Using the extended PBPK model, estimates were made of the yields of DPX presumably formed in mouse liver at the DCM concentrations used in a bioassay (Mennear, J.H., McConnell, E.E., Huff, J.E., Renne, R.A., and Giddens, E. (1988). Ann. NY Acad. Sci. 534, 343-351). The tumor incidence data in mice were fitted to the DPX yields and to the airborne concentration of DCM as alternative measures of exposure using the linearized multistage (LMS) model. The two dose measures yielded similar maximum likelihood estimates for the cancer risk at concentrations from 10 to 100 ppm, but the upper 95% confidence limit on the risk was reduced by two orders of magnitude when DPX rather than the airborne concentration was used as the measure of exposure. The results demonstrate that an internal dosimeter such as DPX can markedly improve the precision of low-dose risk estimates, while having only a minor effect on the maximum likelihood estimates calculated with the LMS model.