Assessment of drug interactions in hepatobiliary transport using rhodamine 123 in sandwich-cultured rat hepatocytes

The purpose of the present study was to explore the utility of sandwich-cultured rat hepatocytes as an in vitro tool to examine drug interactions at the hepatic transport level. Rhodamine 123 was used as a model substrate for P-glycoprotein-mediated biliary excretion. Effects of various types of P-glycoprotein modulation on the biliary excretion index (BEI; a relative measure of the extent of biliary excretion) and the in vitro biliary clearance (CLbile) were determined. Significant reductions in rhodamine 123 BEI and CLbile were noted in the presence of the P-glycoprotein inhibitors verapamil ( 30 - 100 muM) and progesterone ( 100 muM). The P-glycoprotein activator quercetin ( 10 - 100 muM) enhanced rhodamine 123 CLbile by approximately 4-fold, with only a minor effect on BEI, suggesting that quercetin had a more pronounced effect on uptake at the basolateral membrane rather than excretion across the canalicular membrane. Treatment of hepatocytes for 48 h with dexamethasone ( 10 muM) resulted in significant enhancement of CLbile, whereas rifampin (5 - 50 muM) increased both BEI and CLbile, indicating that the inducing effects of dexamethasone and rifampin were occurring at the basolateral and canalicular membranes, respectively. Total rhodamine 123 uptake in sandwich-cultured rat hepatocytes was partly saturable and was affected by the presence of typical Oatp1a4 substrates ( digoxin, quinine, d-verapamil, 17beta-estradiol- D-17beta-glucuronide). In summary, sandwich-cultured rat hepatocytes are a useful tool to study mechanisms of hepatobiliary drug disposition and to predict the potential for drug interactions in hepatic transport.