Improved extrapolation of hepatobiliary clearance from in vitro sandwich cultured rat hepatocytes through absolute quantification of hepatobiliary transporters.

Previously we have reported that hepatobiliary transporter expressions in sandwich cultured hepatocytes (SCH) are altered 2- to 5-fold. This change could limit the model's predictive power for in vivo biliary clearance. The present study was designed to better establish in vitro to in vivo correlation (IVIVC) of biliary clearance. Eleven compounds representing the substrates of Mrp2/Abcc2, Bcrp/Abcg2 and Bsep/Abcb11 were tested in the sandwich cultured rat hepatocyte (SCRH) model. Simultaneously, the absolute difference of hepatobiliary transporters between rat livers and SCRH at day 5 post culture was determined by LC-MS/MS. This difference was integrated into the well-stirred hepatic prediction model. A correction factor named "g_factor" was mathematically defined to reflect the difference in hepatobiliary transporter expressions between the SCRH model and in vivo models, as well as the contribution of multiple transporters. When the g_factor correction was applied, the in vivo biliary clearance prediction was significantly improved. In addition, for those compounds which are poorly permeable and/or undergo transporter-dependent active uptake, the known intracellular concentrations of substrates were used to estimate intrinsic bile clearance. This led to further improvement in the prediction of in vivo bile secretion. While the rate-limiting processes of uptake transporters in the SCRH model remain to be further determined, we showed that integration of the absolute difference of hepatobiliary transporter proteins and transport contributions could improve the predictability of SCRH model. This integration is fundamental for increased confidence in the IVIVC of human biliary clearance.