Lactosaminated Fab fragments specific for low density lipoproteins/hepatocyte targeting and hypolipoproteinemic activity

Transport of sulfonated tetraphenylporphine by lipoproteins in the hamster

We have examined the transport and distribution properties of a bisulfonated tetraphenylporphine (TPPS-2A), an amphiphilic photosensitizer that spontaneously associates to lipoproteins. At different times after intravenous injection of TPPS-2A in hamsters, plasma was fractionated by density ultracentrifugation and porphyrin concentrations were measured in the different plasma (lipo)protein fractions. In order to mimic human lipoprotein composition hamsters were preinjected with 23 mg of apolipoprotein/kg human low density lipoprotein. In addition, the whole body distribution is described as detected by a novel method for the tissue quantification of TPPS-2A. At 5 min after injection into the penile vein, more than 50% of the injected dose appears to be associated with lung tissue, while only 30% is present in plasma and bound exclusively to plasma lipoproteins. After the initial phase, a more retarded decrease in plasma porphyrin concentration is observed. Between 5 min and 6 hr after administration, a redistribution of TPPS-2A from the lungs to the liver takes place. It is concluded that in the hamster, an animal model representing human lipoprotein composition, TPPS-2A is transported essentially exclusively by plasma lipoproteins. No depletion or accumulation of TPPS-2A in a particular plasma lipoprotein fraction could be observed, suggesting a continuous redistribution of the compound. The molecular skeleton of TPPS-2A may serve as a model for the development of new drugs that either have improved in vivo properties due to transport by lipoproteins or have a beneficial effect on the lipoprotein particle itself.

Enhanced hepatic uptake and processing of cholesterol esters from low density lipoprotein by specific lactosaminated Fab fragments

Reduction of the blood levels of low density lipoprotein (LDL) is important for lowering the incidence of atherosclerosis. In this study, LDL was directed to rat parenchymal liver cells by lactosaminated Fab fragments of anti-apolipoprotein B antibodies (LacFab). We followed the fate of intravenously injected complexes of LacFab and [3H]cholesteryl oleate-labeled LDL. Complexing of LacFab to LDL led to rapid disappearance of LDL from the circulation. At 30 minutes after injection, the liver contained 58.5 +/- 9.0% of the injected dose (at that time the liver contained only 5.7 +/- 2.2% of an injected dose of free LDL). Liver uptake was blocked by N-acetylgalactosamine but not by N-acetylglucosamine, which indicates that galactose-specific recognition sites are responsible for the LacFab-induced hepatic uptake. By isolating liver cells, it was found that parenchymal, endothelial, and Kupffer cells account for 87%, 3%, and 10% of the total hepatic uptake, respectively. Subcellular fractionation of the liver indicated that the complexes are rapidly internalized and transported to lysosomes. Within 1 hour after injection, virtually all the [3H]cholesteryl oleate of the internalized LDL was hydrolyzed; hydrolysis was followed by excretion of radioactivity into the bile. Compared with rats injected with native [3H]cholesteryl oleate-labeled LDL, eight times as much radioactivity was excreted into the bile during the first 4 hours after the injection of LacFab-complexed [3H]cholesteryl oleate-labeled LDL. Thus, LacFab induces enhanced hepatic uptake of LDL via galactose receptors on the parenchymal cells, followed by processing in lysosomes and excretion into the bile. In this way, LacFab induces an increased irreversible removal of LDL cholesterol from the body.

The human asialoglycoprotein receptor is a possible binding site for low-density lipoproteins and chylomicron remnants

Binding and internalization of chylomicron remnants from rat mesenteric lymph by HepG2 cells was inhibited by both excess remnants and low-density lipoprotein (LDL) to the same extent. Ligand blots revealed binding of remnants and LDL to the LDL receptor. Measures regulating LDL receptor activity greatly influenced the binding of remnants: ethinyloestradiol, the hydroxymethylglutaryl-CoA reductase inhibitor pravastatin and the absence of LDL all increased binding, whereas high cell density or the presence of LDL decreased binding. Also, asialofetuin, asialomucin, the neoglycoprotein galactosyl-albumin and an antibody against the asialoglycoprotein receptor all decreased substantially the binding of remnants. At high cell density, binding internalization and degradation of chylomicron remnants was inhibited by up to 70-80%, yet binding of LDL was inhibited by no more than 20-30%. In cross-competition studies, the binding of 125I-asialofetuin was efficiently competed for by asialofetuin itself or by the antibody, and also by LDL and remnants, yet remnants displayed an approx. 100-fold higher affinity than LDL. Likewise, remnants of human triacylglycerol-rich lipoproteins and asialofetuin interfered with each others' binding to HepG2 cells or human liver membranes. It is concluded that the LDL receptor mediates the internalization of chylomicron remnants into hepatocytes depending on its activity, according to demand for cholesterol. Additionally, the asialoglycoprotein receptor may contribute to the endocytosis of LDL, but predominantly of chylomicron remnants.