Drug targeting to the liver with lactosylated albumins: Does the glycoprotein target the drug or is the drug targeting the glycoprotein?

Abstract

The isolated perfused rat liver preparation was employed to study hepatic disposition of the model drug-carrier conjugate fluorescein-lactosylated albumin (F-LnHSA) with special reference to the influence of the organic anion fluorescein on liver cell specificity of the endocytosed neoglycoprotein. Hepatic clearance of fluoresceinated neoglycoproteins was significantly faster than clearance of radioiodinated neoglycoproteins. Perfusate clearance of F-L7HSA and F-L25HSA could not completely be inhibited by a dose of 10 mg asialoorosomucoid that saturates the hepatocyte receptor-mediated endocytic process. From these data, we inferred an additional hepatic uptake mechanism, competing with the Ashwell-receptor-mediated internalization of galactose-terminated glycoproteins. Clearance experiments with fluoresceinated 125I-human serum albumin in the presence of the polyanionic probe dextran sulfate revealed a nearly complete (approximately 90%) inhibition of hepatic uptake, while also a pronounced effect was obtained with colloidal carbon. These data point to nonparenchymal cell uptake of fluoresceinated protein via interaction with scavenger receptors. In wash-out studies, it was shown that about 25% of ligand sequestrated by sinusoidal liver cells escaped degradation and recycled to the perfusion medium. Our results show that care should be taken in the use of neoglycoproteins as drug carriers to hepatocytes, since a load of only 2 to 3 moles fluorescein per mole neoglycoprotein considerably affects intrahepatic distribution. The relative contribution of nonparenchymal cell uptake by coupling of acidic drugs to the neoglycoproteins is very probably inversely related to the number of exposing galactose groups per molecule neoglycoprotein. This phenomenon of "inversed targeting" could therapeutically both be useful or detrimental, dependent on the spectrum of cell types that should be reached by the drug.

At physiologic albumin/oleate concentrations oleate uptake by isolated hepatocytes, cardiac myocytes, and adipocytes is a saturable function of the unbound oleate concentration. Uptake kinetics are consistent with the conventional theory.

“…While accumulating evidence has made it unlikely that the hepatocyte surface bears a discrete albumin receptor (26)(27)(28)(29)(30), aspects of the relevant data still seem inconsistent with the conventional model (31, 32). Cellmediated dissociation of albumin ligand complexes (33), the presence of a large unstirred water layer in the space of Disse (34) and dissociation limited availability of substrate (27, [35][36][37] are the principal alternative theories proposed to explain the observed kinetics.…”

Distribution of intravenously injected A-layer protein and lipopolysaccharide (LPS) from Aeromonas salmonicida in Atlantic salmon, Salmo salar L.

“…Furthermore, the isothiocyanate group in the FITC molecule binds to primary amino groups in the LPS and this binding results in a more anionic derivative. It is further documented that such a shift in charge may favour uptake of the ligand via the scavenger receptor (Van der Sluijs et al, 1986). Probably relevant to this question are recent data claiming that a FITC-conjugated invertase is taken up di#erently in heart endothelial cells of Atlantic cod than unconjugated invertase.…”

Summer Research Program - 1998 Summer Research Extension Program Volume 1 Armstrong Laboratory

“…When this binding is included in kinetic models, the assumption often made is that the chemical and protein are in binding equilibrium at all times, where the concentrations of free and bound chemical are determined by the equilibrium dissociation constant (K<i). While this assumption will be true for many chemicals, a state of binding equilibrium may not exist for some chemicals (Weisiger et al, 1984;Weisiger, 1985;van der Sluijs et al, 1987;Ott and Weisiger, 1997). A lack of binding equilibrium will cause the concentration of free chemical in the plasma to differ substantially from the concentration of free chemical predicted by using the binding equilibrium assumption.…”

“…Measurement of ko ff for a related compound, dibromosulfophthalein (DBSP), revealed a value of 0.047 s" 1 (van der Sluijs et al, 1987). Smaller values for ko ff would tend to promote the dissociation limited condition.…”

“…An estimate of k^ between 0.053 and 0.208 s" 1 has been made from application of a model to perfused liver data (Weisiger et al, 1984). The value of 0.114 s' 1 found in this work is within this range.Measurement of ko ff for a related compound, dibromosulfophthalein (DBSP), revealed a value of 0.047 s" 1 (van der Sluijs et al, 1987). Smaller values for ko ff would tend to promote the dissociation limited condition.…”