The aim of this work was to extrapolate in vitroand preclinical animal data to simulate the pharmacokinetic parameters of UK-343,664, a P-glycoprotein (P-gp) and CYP3A4 substrate, in human. In addition, we aimed to develop a simulation model to demonstrate the involvement and the controversial complex interaction of intestinal P-gp and CYP3A4 in its nonlinear absorption, first-pass extraction, and pharmacokinetics using the advanced compartmental absorption and transit (ACAT) model. Finally, we aimed to compare the results predicted from the model to the reported findings in human clinical studies. In situperfusion, allometric scaling, PBPK Rodger mechanistic approach, in vitrometabolism, and fitting to in vivodata were used to mechanistically explain the absorption, distribution and metabolism, respectively. GastroPlus was used to build the integrated simulation model in human for UK-343,664 to mechanistically explain the observed clinical data at 30, 100, 200, 400, and 800 mg oral doses. The measured in vitrovalue for CYP3A4 Km(465 μM) in rCYPs was converted to units of μg/mL, corrected for assumed microsomal binding (17.8%) and applied to all metabolic processes. The measured in vitrovalues of Vmaxfor CYP3A4 (38.9 pmol/min/pmol), 2C8, 2C9, 2C19, and 2D6 were used along with the in vitroCYP3A4 Kmto simulate liver first pass extraction and systemic clearance. The measured in vitrovalues of Vmaxfor CYP3A4 and 2D6 were used along with the in vitroCYP3A4 Kmto simulate gut first pass extraction. Vmaxand Kmvalues for P-gp were obtained by fitting to in vivodata and used to simulate gut efflux transport activity. Investigation of the interaction mechanism of P-gp and CYP3A4 in the intestine was achieved by comparing the influence of a virtual knockout of P-gp or gut metabolism on the fraction absorbed, fraction reaching the portal vein, and fraction metabolized in the gut. Comparison between simulation and in vivoresults showed that the in silicosimulation provided a mechanistic explanation of the observed nonlinear absorption kinetics of UK-343,664 in human following its administration in the range of 30–800 mg as oral solutions. The simulation results of the pharmacokinetic parameters, AUC and Cmax, by GastroPlus were comparable with those observed in vivo. This simulation model is one possible mechanistic explanation of the observed in vivodata and suggests that the nonlinear dose dependence could be attributed to saturation of both the efflux transport by P-gp and the intestinal metabolism. However, the concentration ranges for either protein saturation did not overlap and resulted in much greater than dose proportional increases in AUC. At low doses, producing intraenterocyte concentrations below the fitted value of Kmfor P-gp, the influence of P-gp appears to be protective and results in a lower fraction of gut 3A4 metabolism. At higher doses, as P-gp becomes saturated the fraction of gut 3A4 extraction increases, and eventually at the highest doses, where 3A4 becomes saturated, the fraction of gut 3A4 extraction again decreases. Such a complex interpretation of this in vitro–in vivoextrapolation (IVIVE) is another example of the value and insight obtained by physiologically based absorption simulation.