Influence of cytochrome P-450 inhibitors on the inhalative uptake of methyl chloride and methylene chloride in male B6C3F1 mice

A Bayesian analysis of the influence of GSTT1 polymorphism on the cancer risk estimate for dichloromethane

The carcinogenicity of dichloromethane (DCM) is related to metabolic activation mediated by glutathione transferase theta 1 (GSTT1), whereas oxidation serves as a detoxification pathway. The aim of this study was to calculate the excess cancer risk from DCM, using Bayesian statistics. In a first step, a previously developed population physiologically based pharmacokinetic (PBPK) model for DCM was simultaneously fitted to extensive human toxicokinetic data from 27 male volunteers exposed to 250-1000 ppm DCM (Astrand et al. Scand. J. Work Environ. Health 1, 78-94, 1975; Engström and Bjurström, Scand. J. Work Environ. Health 7, 215-224, 1977) using Markov chain Monte Carlo simulation. Improved population estimates were obtained for the PBPK model parameters. In a second step, excess cancer risk was calculated for lifelong exposure to 1-1000 ppm DCM by Monte Carlo simulation. Data on GSTT1 gene frequencies in the Swedish population were used, including all three genotypes. Estimated mean and median excess risks were in general agreement with those previously derived (El-Masri et al. Toxicol. Appl. Pharmacol. 158, 221-230, 1999). However, we estimate higher excess risks at the upper confidence limits. Furthermore, our simulations suggest that 1% of the Swedish population is not covered by a factor 4.2-7.1 away from the mean target dose. The majority of the fraction of the population not covered was classified as GSTT1 homozygote. This indicates that a higher uncertainty factor than the commonly used 3.16 should be considered in noncancer risk assessment for substances with polymorphic bioactivation.

Dichloromethane (methylene chloride): metabolism to formaldehyde and formation of DNA-protein cross-links in B6C3F1 mice and Syrian golden hamsters

Dichloromethane (DCM) is metabolized via a glutathione transferase (GST)-dependent pathway to formaldehyde (HCHO), a mutagenic compound that could play an important role in the carcinogenic effects of DCM observed in the liver and lungs of B6C3F1 mice at 2000 and 4000 ppm. Syrian hamsters metabolize DCM more slowly than mice via this pathway, and hamsters exposed to 3500 ppm showed no apparent carcinogenic response. The possible formation of DNA-protein cross-links (DPX) from DCM in both species was examined. Male mice and hamsters were pre-exposed for 2 days (6 hr/day) to 4000 ppm of DCM and on the third day were exposed (6 hr) to a decaying concentration (4500 to 2500 ppm) of [14C]DCM. DPX were detected in mouse liver, but not in mouse lung, hamster liver, or hamster lung. The failure to detect DPX in mouse lung does not exclude their possible formation in a subpopulation of lung cells. Metabolic incorporation of 14C derived from [14C]DCM into DNA suggested a higher rate of turnover of some mouse lung cells than of hamster lung cells, but no large difference in the turnover rates of liver cells in the two species under these conditions. These results demonstrate that HCHO derived from DCM can form DNA-protein cross-links in the liver of the B6C3F1 mouse. The formation of DPX is dependent on the activity of the GST pathway, and species such as hamsters and humans having much lower rates of DCM metabolism via this pathway may not generate toxicologically significant concentrations of HCHO and DPX.