Pharmacokinetics of the hepatic transport of organic anions: influence of extra- and intracellular binding on hepatic storage of dibromosulfophthalein and interactions with indocyanine green.

The influence of intracellular and extracellular protein binding on the hepatic storage and biliary elimination of dibromosulfophthalein (DBSP) was studied in isolated perfused rat liver. Under first order kinetic conditions the amount of DBSP in the liver at a given plasma concentration (hepatic storage) was determined by extracellular binding to albumin and intracellular binding to the cytosolic Y and Z proteins as well as concentrative membrane transport from plasma into the liver. At higher doses, extensive binding of DBSP to intracellular organelles also occurred while liver cytosol/plasma concentration gradients of unbound DBSP were much lower. Hepatic storage increased with decreasing albumin concentration in the perfusate of isolated perfused rat livers. However, it was shown that this parameter is dose-dependent, and errors can be introduced in its calculation if nonlinearity of sinusoidal and canalicular transport processes as well as nonlinear protein binding are not taken into account. The influence of another organic anion, indocyanine green (ICG) on the hepatic storage, subcellular distribution, and elimination of DBSP was subsequently studied. At equimolar amounts the presence of ICG resulted in a 50% decrease in hepatic clearance and hepatic distribution volume of DBSP. It was inferred that these changes are due to an inhibition of carrier-mediated transport across the sinusoidal and canalicular membrane and preferential displacement from intracellular binding sites. In contrast DBSP in equimolar amount enhanced the initial disappearance rate and biliary excretion of ICG, probably due to increasing its free fraction in plasma. It is concluded that the level and mechanism of interaction of two drugs within the eliminating organ can be characterized by combining clearance studies with data on subcellular and extracellular binding.