Pharmacokinetics of the hepatic transport of organic anions: Influence of extra- and intracelluar binding on hepatic storage of dibromosulfophthalein and interactions with indocyanine green

Abstract

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.

Hepatocyte Heterogeneity in Bile Formation and Hepatobiliary Transport of Drugs

“…Binding to the cytosolic proteins is both saturable and concentration-dependent [69]. Increasing the dose of organic anions results in higher binding to intracellular membranes and organelles.…”

“…Increasing the dose of organic anions results in higher binding to intracellular membranes and organelles. These structures have such a high binding capacity that the unbound frac tion of organic anions in the cytosol remains almost constant when increasing hepatic content [69].…”

“…oleate) and cholephilic dyes (e.g. rose bengal, BSP and DBSP) are avidly bound to serum albumin in the general circu lation [55, 68,69]. Despite their very high albumin binding, these ligands are effec tively removed from the circulation by the liver.…”

“…For organic anions two types of cyto solic binding proteins have been identified. Glutathione-S transferase B (also called li-gandin) constitutes 4-5% of the cytosolic protein and is the major binding protein [68,69,82]. Fatty acid-binding protein (also called Z protein) [55,129,130] binds not only organic anions such as bilirubin and fatty acids but also uncharged compounds like sex steroids.…”

“…After primary uptake, extensive binding of drugs to proteins [55, [67][68][69] and organelles [76,77] within the cell occurs. For organic anions two types of cyto solic binding proteins have been identified.…”