PBPK modeling of intestinal and liver enzymes and transporters in drug absorption and sequential metabolism.

Experimental strategies have long been applied for in vitro or in vivo evaluation of the effect of transporters and/or enzymes on the bioavailability. However, the lack of specific inhibitors or inducers of transporters and enzymes and the multiplicity of nuclear receptors in gene regulation and cross talk have led to compromised assessment of these effects in vivo. These and other causes have resulted in confusion and controversy in transporter-enzyme interplay. In this review, physiologically-based pharmacokinetic (PBPK) intestinal and liver models are utilized to predict the contributions of enzymes and transporters on intestinal availability (F(I)) and hepatic availability (F(H)), with the aim to fully understand the impact of these variables on bioavailability (F(sys)) in vivo. We emphasize the often overlooked impact of influx and efflux clearances, and apply the PBPK models and their solutions to examine individual organ clearances of the intestine and the liver. In order to accurately predict oral bioavailability, these organ models are incorporated into the whole body PBPK model, and additional complicated scenarios such as segmental differences and zonal heterogeneity of transporters and enzymes in the intestine and liver and segregated blood flow patterns of the intestine are further discussed. The sequential metabolism of a drug to form primary and secondary metabolites in the first-pass organs is considered in PBPK modeling, revealing that the segregated flow model (SFM) of the intestine is more appropriate than the traditional PBPK intestinal model (TM). Examples are included to highlight the potential application of these PBPK models on the quantitative prediction of bioavailability.