We have developed a new triantennary galactoside. in which the terminal galactose moieties are connected to the branching point of the cluster galactoside via a 20 Angstrom (2 nm) spacer [TG(20 Angstrom)]. In vitro binding studies have demonstrated that introduction of a 20 Angstrom spacer resulted in avid and specific binding of the triantennary galactoside to the asialoglycoprotein receptor on the parenchymal liver cell. Derivatization of this galactoside with a cholesterol moiety afforded a compound [TG(20 Angstrom)C] that lowered the serum cholesterol concentration when injected into rats. In the present study we have evaluated the direct effect of TG(20 Angstrom)C on the in vivo fate of high-density lipoprotein (HDL) and low-density lipoprotein (LDL). A direct association of TG(20 Angstrom)C with HDL and LDL was observed on mixing these components. Incorporation of TG(20 Angstrom)C into I-125-HDL and I-125-LDL significantly accelerated the serum decay and concomitantly stimulated the hepatic uptake of these lipoproteins in rats. The liver uptake of TG(20 Angstrom)C-loaded I-125-HDL or I-125- LDL could be inhibited by 81% and 82% respectively by preinjection of 150 mg of N-acetylgalactosamine, indicating that the enhanced liver uptake proceeded via galactose-specific receptors. More than 96% of the hepatic uptake of TG(20 Angstrom)C-loaded I-125-HDL could be attributed to the parenchymal cell. Surprisingly, the parenchymal cell also accounted for 93% of the liver association of TG(20 Angstrom)C-loaded I-125-LDL, suggesting that TG(20 Angstrom)C stimulates the uptake and processing of both lipoproteins by the asialoglycoprotein receptor on the parenchymal liver cell. This contrasts with earlier data indicating that a triantennary cluster galactoside provided with a 4 A spacer between the terminal galactose moieties and the branching point of the dendrite stimulated hepatic uptake of LDL via the Kupffer cells. The parenchymal cell is the only liver cell type that is capable of irreversibly removing cholesterol from the body in the form of bile acids. The above results imply that administration of TG(20 Angstrom)C not only facilitates the hepatic uptake of lipoprotein-derived cholesterol (esters) but also their elimination from the body. In addition, it might be possible to utilize TG(20 Angstrom)C as a targeting device to selectively deliver large drug carriers and possibly genes to the parenchymal liver cell.
