Assessing interaction thresholds for trichloroethylene in combination with tetrachloroethylene and 1,1,1-trichloroethane using gas uptake studies and PBPK modeling.

The volatile organic solvents trichloroethylene (TCE), tetrachloroethylene (perchloroethylene, PERC), and 1,1,1-trichloroethane (methylchloroform, MC) are widely distributed environmental pollutants and common contaminants of many chemical waste sites. To investigate the mode of pharmacokinetic interactions among TCE, PERC, and MC and to calculate defined "interaction thresholds", gas-uptake experiments were performed using a closed-chamber exposure system. In each experiment, two rats (Fischer 344, male, 8-9 weeks old) were exposed to different initial concentrations of TCE, PERC, and MC, applied singly or as a mixture, and their concentration in the gas phase of the chamber was monitored over a period of 6 h. A physiologically based pharmacokinetic (PBPK) model was developed to test multiple mechanisms of inhibitory interactions, i.e., competitive, non-competitive, or uncompetitive. All mixture exposure data were accurately described by a system of equations in which a PBPK model was provided for each chemical and each was regarded as an inhibitor of the others' metabolism. Sensitivity-analysis techniques were used to investigate the impact of key parameters on model output and optimize experimental design. Model simulations indicated that, among these three chemicals, the inhibition was competitive. The PBPK model was extended to assess occupationally relevant exposures at or below the current threshold-limit values (TLVs). Based on 10% elevation in TCE blood levels as a criterion for significant interaction and assuming TCE exposure is set at TLV of 50 ppm, the calculated interaction thresholds for PERC and MC were 25 and 135 ppm, respectively. TLV exposures to binary TCE/PERC mixture were below the 10% significance level. The interaction threshold for TCE and MC co-exposure would be reached at 50 and 175 ppm, respectively. Such interactive PBPK models should be of value in risk assessment of occupational and environmental exposure to solvent mixtures.