An approach for incorporating tissue composition data into physiologically based pharmacokinetic models

The objective of this study was to develop an approach for incorporating tissue composition data into physiologically based pharmacokinetic (PBPK) models in order to facilitate ''built-in'' calculation of tissue:air partition coefficients (PCs) of volatile organic chemicals. The approach involved characterizing tissue compartments within PBPK models as a mixture of neutral lipids, phospholipids, and water (instead of using the conventional description of them as ''empty'' boxes). This approach enabled automated calculation of the tissue solubility of chemicals from n-octanol and water solubility data, since these data approximate those of solubility in tissue lipids and water. Tissue solubility was divided by the saturable vapor concentration at 37 degrees C to estimate the tissue:air PCs within PBPK models, according to the method of Poulin and Krishnan (1995c). The highest and lowest lipid and water levels for human muscle, liver, and adipose tissues were obtained from the literature and incorporated within the tissue composition-based PBPK model to calculate the tissue:air PCs of dichloromethane (DCM) and simulate the pharmacokinetics of DCM in humans. The PC values predicted for human tissues were comparable to those estimated using rat tissues in cases where the relative levels of lipids and water were comparable in both species. These results suggest that the default assumption of using rat tissue:air PCs in human PBPK models may be acceptable for certain tissues (liver, adipose tissues), but questionable for others (e.g., muscle). The PBPK modeling exercise indicated that the interindividual differences in tissue dose arising from variations of tissue:air PCs may not be reflected sufficiently by venous blood concentrations. Overall, the present approach of incorporating tissue composition data into PBPK models would not only enhance the biological basis of these models but also provide a means of evaluating the impact of interindividual and interspecies differences in tissue composition on the tissue dose surrogates used in PBPK-based risk assessments.