Characterizing Apixaban Pharmacokinetics Through Physiologically‐Based Pharmacokinetic Modeling: Critical Role of Biliary Secretion and Enterohepatic Circulation in Humans

ABSTRACT Apixaban, a factor Xa inhibitor, is a direct oral anticoagulant with a well‐balanced elimination; it is eliminated evenly via feces, urine (with no active secretion), and as metabolites after oral administration. The common understanding is that biliary secretion and enterohepatic circulation (EHC) of apixaban are limited in humans, and that fecal excretion may be attributable to intestinal secretion. However, a decrease in apixaban blood concentration with activated charcoal coadministration in humans suggests possible involvement of EHC. This study aimed to evaluate the contribution of biliary excretion, EHC, and intestinal secretion to apixaban pharmacokinetics (PK) using a physiologically‐based pharmacokinetic (PBPK) model. A top‐down analysis was performed using blood concentration and mass balance data from healthy volunteers. Model parameters were optimized using the Cluster‐Gauss Newton method (CGNM), followed by the bootstrap method. The model accurately described observed data and indicated moderate to high biliary secretion relative to metabolic clearance. Simulated biliary secretion into the duodenum well predicted the biliary secretion data in humans (< 1% of dose at 8 h post‐dose). Virtual knockout of EHC resulted in a shortened half‐life from 8.7 to 2.9 h, and 17% and 55% decrease in area under the concentration curve (AUC) and fecal excretion after intravenous dosing, respectively, confirming the significant contribution of biliary excretion and EHC. The model also accurately described apixaban PK with activated charcoal coadministration at 2 or 6 h post‐dose. Although further experimental validation (e.g., sandwich‐cultured hepatocytes) would strengthen these findings, our study demonstrates that biliary secretion and EHC play a substantial role in apixaban elimination and disposition in humans.