Influence of physiological status on residues of lipophilic xenobiotics in livestock.

Data on the transfer of lipophilic xenobiotics from livestock feed and the environment to meat and milk are required for risk assessment purposes. Often, data are only available for lactating dairy cattle. Physiologically-based pharmacokinetic (PBPK) modeling has been used to explore differences between classes of food-producing animals. Blood and tissue levels of lipophilic xenobiotics under conditions approximating steady-state were simulated. Simulations of constant exposure were performed for lactating cows, non-lactating cattle, sheep, goats and swine. The tissue : blood partition coefficient, fat volume and fat blood flow were identified as critical determinants of predicted tissue concentrations. There may be significant breed differences in residues in milk and fat following exposure. Modeling was used to derive scaling factors that can be used to assist the extrapolation of transfer studies, carried out on lactating dairy cows, to other classes of cattle and different species.