Hepatobiliary Disposition of Troglitazone and Metabolites in Rat and Human Sandwich-Cultured Hepatocytes: Use of Monte Carlo Simulations to Assess the Impact of Changes in Biliary Excretion on Troglitazone Sulfate Accumulation

This study examined the hepatobiliary disposition of troglitazone (TGZ) and metabolites [TGZ sulfate (TS), TGZ glucuronide (TG), and TGZ quinone (TQ)] over time in rat and human sandwich-cultured hepatocytes (SCH). Cells were incubated with TGZ; samples were analyzed for TGZ and metabolites by liquid chromatography-tandem mass spectrometry. SCH mimicked the disposition of TGZ/metabolites in vivo in rats and humans; TGZ was metabolized primarily to TS and to a lesser extent to TG and TQ. In human SCH, the biliary excretion index (BEI) was negligible for TGZ and TQ, similar to 16% for TS, and similar to 43% for TG over the incubation period; in rat SCH, the BEI for TS and TG was similar to 13 and similar to 41%, respectively. Hepatocyte accumulation of TS was extensive, with intracellular concentrations ranging from 132 to 222 mu M in rat SCH; intracellular TGZ concentrations ranged from 7.22 to 47.7 mu M. In human SCH, intracellular TS and TGZ concentrations ranged from 136 to 160 mu M and from 49.4 to 84.7 mu M, respectively. Pharmacokinetic modeling and Monte Carlo simulations were used to evaluate the impact of modulating the biliary excretion rate constant (K-bile) for TS on TS accumulation in hepatocytes and medium. Simulations demonstrated that intracellular concentrations of TS may increase up to 3.1- and 5.7-fold when biliary excretion of TS was decreased 2- and 10-fold, respectively. It is important to note that altered hepatobiliary transport and the extent of hepatocyte exposure may not always be evident based on medium concentrations (analogous to systemic exposure in vivo). Pharmacokinetic modeling/simulation with data from SCH is a useful approach to examine the impact of altered hepatobiliary transport on hepatocyte accumulation of drug/metabolites.