Purpose: Enterohepatic recirculation (EHR) is an important drug disposition process. Liver metabolism is generally considered as the driving force of the EHR. All drugs are metabolized in hepatocytes and then excreted into bile. However, our recent study indicates that non-hepatic metabolism (like intestinal metabolism) has the same impact in the EHR process. Some compounds could be metabolized before enter liver and uptake by hepatocyte. Without any hepatic metabolism, the compounds would go through EHR as well. The purpose of the study is to elucidate the mechanism of hepatic uptake of these compounds and the structure related activity.
Methods: An in situ perfusion model was established to study the mechanism. Portal vein and bile duct were cannulated with tubes. Different flavonoids and their glucuronides were perfused through portal vein. The perfusion lasted for 2.5 hours. Bile and plasma (from tail vein) were collected every 30 minutes. The recirculation ratio (the concentration of the compound in bile divided by that in portal vein perfusate) would be calculated as an indicator of the recirculation efficiency. For one of the flavonoid (wogonoside), 5 different concentrations were perfused through portal vein. An OATP transporter inhibitor (rifampicin) was perfused with wogonoside.
Results: Wogonoside was nearly 100% recirculated while baicalin has less than 10% of recirculation ratio. The perfused aglycones had lower recirculation ratio. Wogonin (wogonoside aglycone) was recirculated 4% in bile while apigenin was recirculated 8% and baicalein (baicalin aglycone) was not recirculated (below detection limit). When perfused with rifampicin, the wogonoside bile concentration was decreased by 40%. The recirculation of wogonoside fitted Michaelis-Menten kinetics (r2=0.96) and the Vmax and Km were 273.97 uM/hr and 22.38uM.
Conclusion: The results indicating that without hepatic metabolism, the EHR is still existing, which is against general recognition of EHR. The unknown mechanism of the hepatic uptake was partially clarified by our study. The recirculation of wogonoside indicates a concentration dependent and inhibitable pathway. Thus we concluded that the hepatic uptake of wogonoside is a transporter mediated pathway. The hepatic uptake of wogonoside is mainly mediated by OATPs. OATP 1B1/1B3 play a significant role in the uptake process. The recirculation system showed preference to some special chemical structure, which indicates a structure related relationship.
Song Gao– Assistant Professor, Texas Southern University, Houston, Texas
Min Zeng– Visiting Scholar, University of Houston, Houston, Texas
Jiong Liu– Visiting Scholar, University of Houston, Houston, Texas
Bijun Xia– Visiting Scholar, University of Houston, Houston, Texas
Ming Hu– Professor of Pharmaceutics, University of Houston, Houston, Texas