ABSTRACT:The routine assessment of xenobiotic in vivo kinetic behavior is currently dependent upon data obtained through animal experimentation, although in vitro surrogates for determining key absorption, distribution, metabolism, and elimination properties are available. Here we present a unique, generic, physiologically based pharmacokinetic (PBPK) model and demonstrate its application to the estimation of rat plasma pharmacokinetics, following intravenous dosing, from in vitro data alone. The model was parameterized through an optimization process, using a training set of in vivo data taken from the literature and validated using a separate test set of in vivo discovery compound data. On average, the vertical divergence of the predicted plasma concentrations from the observed data, on a semilog concentration-time plot, was approximately 0.5 log unit. Around 70% of all the predicted values of a standardized measure of area under the concentration-time curve (AUC) were within 3-fold of the observed values, as were over 90% of the training set t 1/2 predictions and 60% of those for the test set; however, there was a tendency to overpredict t 1/2 for the test set compounds. The capability of the model to rank compounds according to a given criterion was also assessed: of the 25% of the test set compounds ranked by the model as having the largest values for AUC, 61% were correctly identified. These validation results lead us to conclude that the generic PBPK model is potentially a powerful and cost-effective tool for predicting the mammalian pharmacokinetics of a wide range of organic compounds, from readily available in vitro inputs only.Physiologically based pharmacokinetic (PBPK) models are mathematical descriptions of the flow of blood throughout the body, developed for the simulation of xenobiotic absorption, distribution, and elimination. The essential concepts were outlined over 60 years ago in a farsighted paper (Teorell, 1937) that presented many of the mathematical relationships required to simulate blood flow and tissue distribution.Simulation modeling ideas were developed further by Mapleson (1973), to explain the effect of anesthetics, and early attempts to apply the approach to drugs were published in the 1960s by Bellman et al. (1961). Probably the most important contributions in that period were made by Bischoff and Dedrick (1968), who demonstrated that PBPK models could be used for the a priori prediction of the pharmacokinetics of thiopental. During the following decades, developments were made by academics such as Rowland (Rowland, 1986), Sugiyama (Sugiyama and Ito, 1998), and Amidon (Yu and Amidon, 1999), as well as scientists working in the environmental health field, in particular Anderson and Clewell (Andersen et al., 2002). Recent reviews (Grass and Sinko, 2002;Leahy, 2003) have discussed the application of these approaches to the prediction of pharmacokinetics in drug discovery.It is of interest to us to apply the PBPK approach to the estimation of plasma levels in animals from in vitro d...