Theoretical considerations on two equations for estimating the extent of absorption after oral administration of drugs.

Purpose. The amount of drug absorbed into portal blood after oral dosing (Dp.o,g) has been estimated using Fick’s principle (Q-method), i.e., Dp.o,g = Qh · (AUCp.o,g − AUCp.o,c), where Qh is the portal blood flow rate, and AUCp.o,g and AUCp.o,c are the areas under the concentration-time curves of portal vein and systemic blood after oral dosing, respectively. However, this method may underestimate Dp.o,g, when the drug is subject to systemic intestinal elimination. An alternate equation (CL-method; Dp.o,g = CLS · AUCp.o,g) is described using a simple pharmacokinetic model, to estimate Dp.o,g in the presence of systemic intestinal elimination, where CLS is systemic clearance.