Biliary excretion of pravastatin in rats: contribution of the excretion pathway mediated by canalicular multispecific organic anion transporter.

The biliary excretion of pravastatin in normal rats and Eisai hyperbiliruminemic rats (EHBRs) was examined in vivo and in vitro using bile canalicular membrane vesicles (CMVs). In vivo, the total body clearances at steady-state (CLtot) for both rat strains decreased as the infusion rate increased. At the lowest infusion rate, CLtot for normal rats was 1.6 times higher than that for EHBRs. Under this set of conditions, the biliary excretion clearance (CLbile), defined as the biliary excretion rate at steady-state divided by the concentration in the liver (Cliver), for normal rats was 3-fold higher than that for EHBRs. The CLbile fell markedly with increasing Cliver for normal rats and the Michaelis constant (KM) for Cliver was 180 microM; in contrast, the degree of saturation was slight if any in EHBRs. In vitro, the uptake of pravastatin by CMVs prepared from normal rats exhibited clear ATP-dependence, whereas only a minimal effect of ATP was observed on the uptake by CMVs from EHBRs. Transport kinetic studies were performed over a wide range of pravastatin concentration (0.2-10,000 microM) with a tracer tritium-labeled pravastatin. Saturation was observed both in the ATP-dependent (KM: 220 microM) and ATP-independent (KM: 480 microM) uptake by CMVs prepared from normal rats. ATP-dependent uptake of 2, 4-dinitrophenyl glutathione, a typical substrate for the canalicular multispecific organic anion transporter (cMOAT), was inhibited by pravastatin in a concentration-dependent manner and the resultant inhibitory constant of pravastatin (170 microM) was comparable with the KM value of ATP-dependent pravastatin uptake itself. In conclusion, biliary excretion of pravastatin is mediated mainly by cMOAT in normal rats. This can explain the decrease in the biliary excretion of pravastatin in EHBRs.