Primary active transport of pravastatin across the liver canalicular membrane in normal and mutant Eisai hyperbilirubinemic rats.

We have previously demonstrated that the HMG-CoA reductase inhibitor pravastatin is efficiently taken up by the liver via the 'multispecific anion transporter' in an active manner. To further examine the fate of pravastatin within the liver, its biliary excretion was studied in a single-pass liver perfusion system and isolated liver canalicular membrane vesicles (CMVs) using normal (Sprague-Dawley rats; SDRs) and mutant Eisai hyperbilirubinemic rats (EHBRs). In the liver perfusion experiments, the outflowing drug concentration reached a steady state at 30 min and the extraction ratio was approximately 0.7 in both rat strains. Both the steady-state biliary excretion rate and bile flow rate of the EHBR group were 40% those of SDRs. At steady state, the fraction of unchanged drug in bile was 25-34% in both groups. The concentration ratios of unbound drug in cytosol versus that in sinusoid and in bile versus cytosol were, respectively, 11 and 87 in SDRs, and 13 and 94 in EHBRs. After correction for membrane potential (-40 mV in cytosol), the ratios became 49 and 19 in SDRs and 58 and 21 in EHBRs, respectively. The finding that all of these values were much larger than unity suggested that active transport occurred from liver to bile, as well as from plasma to liver, in both rat strains. Furthermore, ATP-dependent uptake of pravastatin was clearly observed in CMVs prepared from EHBRs as well as SDRs, whereas the stimulation by ATP of DNP-SG transport in CMV was observed only in SDRs. It was concluded that pravastatin is excreted into bile in high concentrations and a primary active transport mechanism which is maintained in EHBRs contributes to the biliary excretion of this drug.