The importance of measured end-points in demonstrating the occurrence of interactions: A case study with methylchloroform and m-xylene

Abstract

Mixed exposures may result in significant changes in one biomarker of exposure without altering another biomarker, and this may have unknown significance in terms of exposure assessment and overall toxicity of the mixture. Results from a previous investigation showed that human exposure to methylchloroform (MC, 400 ppm) and m-xylene (XYL, 200 ppm) during 4 h did not result in any significant effect on blood concentrations of these solvents, suggesting the absence of interaction between MC and XYL. Those results were adequately described by conducting a physiologically-based toxicokinetic (PBTK) modeling of the MC-XYL interaction in humans; however, the model suggested that urinary excretion of MC metabolites would be reduced as a result of combined exposure, whereas that of XYL metabolites would not be modified. An experimental verification of this model prediction was then undertaken with rats. In this study, Sprague-Dawley rats (n, 5) were exposed during 4 h to MC (400 ppm) or XYL (200 ppm), alone or as a mixture. Results showed that combined exposure did not affect the blood concentration of MC whereas that of XYL was increased throughout the 2-h blood collection period following exposure. The excretion of MC metabolites during a period of 48 h following the onset of exposure, i.e., trichloroethanol (TCE: -71%) and trichloroacetic acid (TCA: -73%), were significantly reduced. Methylhippuric acid (MHA) was not affected by co-exposure to MC as expected from the PBTK model forecasts. These results exemplify the use of a priori PBPK modeling for designing interaction studies and choosing appropriate/sensitive end- points for demonstrating the occurrence of potential interactions.