Uptake, Internalization and Degradation of the Novel Plasminogen Activator Reteplase (BM 06.022) in the Rat

The catabolism of the novel plasminogen activator reteplase (BM 06.022) was described. For this purpose BM 06.022 was radiolabelled with l25I or with the accumulating label l25I-tyramine cellobiose (l25I-TC).

BM 06.022 was injected at a pharmacological dose of 380 μg/kg b.w. and it was cleared from the plasma in a biphasic manner with a half-life of about 1 min in the α-phase and t1/2of 20-28 min in the β-phase. 28% and 72% of the injected dose was cleared in the α-phase and β-phase, respectively. Initially liver, kidneys, skin, bones, lungs, spleen, and muscles contributed mainly to the plasma clearance. Only liver and the kidneys, however, were responsible for the uptake and subsequent degradation of BM 06.022 and contributed for 75% to the catabolism of BM 06.022. BM 06.022 was degraded in the lysosomal compartment of both organs. Parenchymal liver cells were responsible for 70% of the liver uptake of BM 06.022. BM 06.022 associated rapidly to isolated rat parenchymal liver cells and was subsequently degraded in the lysosomal compartment of these cells. BM 06.022 bound with low-affinity to the parenchymal liver cells (550 nM) and the binding of BM 06.022 could be displaced by t-PA (IC50 5.6 nM), indicating that the low-density lipoprotein receptor-related protein (LRP) could be involved in the binding of BM 06.022. GST-RAP, which is an inhibitor of LRP, could in vivo significantly inhibit the uptake of BM 06.022 in the liver.

It is concluded that BM 06.022 is metabolized primarily in the liver and the kidneys. These organs take up and degrade BM 06.022 in the lysosomes. The uptake mechanism of BM 06.022 in the kidneys is unknown, while LRP is responsible for a low-affinity binding and uptake of BM 06.022 in parenchymal liver cells.

Specific targeting of a lipophilic prodrug of iododeoxyuridine to parenchymal liver cells using lactosylated reconstituted high density lipoprotein particles

We recently reported the conversion of the water-soluble antiviral drug iododeoxyuridine (IDU) into the lipophilic prodrug dioleoyl-iododeoxyuridine (IDU-Ol2). The prodrug was incorporated into reconstituted high-density lipoprotein (NeoHDL) particles with physical and biological properties similar to those of native HDL. We also found, in initial experiments, that lactosylation of the prodrug-loaded NeoHDL increases its liver uptake. Because this offers the attractive perspective of using these particles for the delivery of drugs to the liver, we now analyze the characteristics and biological fate of lactosylated IDU-Ol2-loaded NeoHDL. The particles (containing approximately 25 prodrug molecules) have the same size and charge as native HDL, indicating that lactosylation does not cause aggregation or oxidative modification. At 10 min after intravenous injection of lactosylated [3H]IDU-Ol2-loaded NeòHDL into rats, only 13.5 +/- 2.8% of the dose was left in plasma and 75.9 +/- 2.4% of the dose was recovered in the liver. The relative specific uptake by the liver was 1-2 orders of magnitude higher than that of any other tissue. The hepatic uptake of lactosylated [3H]IDU-Ol2-loaded NeoHDL was much higher than that of free [3H]IDU ( < 20% of the dose). Both parenchymal liver cells and Kupffer cells express galactose-specific receptors. By isolating liver cells after injection of the prodrug-loaded particles, it was established that hepatic uptake occurred mainly (for 84.4 +/- 3.8%) in parenchymal liver cells. Preinjection with asialofetuin substantially reduced the liver uptake of lactosylated [3H]IDU-Ol2-loaded NeoHDL, which points to uptake by the asialoglycoprotein receptor. Subcellular fractionation of the liver indicated that lactosylated [3H]IDU-Ol2-loaded NeoHDL does not merely associate to cells, but is internalized and delivered to the lysosomes. In conclusion, we show that IDU can be specifically targeted to the parenchymal liver cell. Conversion of the water-soluble parent drug into a lipophilic prodrug that is incorporated into a lactosylated reconstituted HDL particle, is an approach that may also be used to deliver other water-soluble drugs to the parenchymal liver cells. This may lead to more effective therapy for liver diseases such as hepatitis B.

Quantitative analysis of the targeting of mannose-terminal glucocerebrosidase. Predominant uptake by liver endothelial cells

Gaucher's disease is an inherited lysosomal storage disorder that is caused by a deficiency of glucocerebrosidase. The resulting accumulation of the substrate glucosylceramide in macrophages of liver, spleen, and bone marrow causes severe clinical symptoms. Gaucher's disease is treated by intravenous administration of a modified glucocerebrosidase (Alglucerase), which has exposed mannose residues to promote uptake by target macrophages. To evaluate the effectiveness of the targeting of Alglucerase, we studied the fate of the enzyme in the rat. Intravenously injected Alglucerase was rapidly cleared from the circulation (half-life 2.0 +/- 0.5 min). The liver was the main site of uptake, with 65.6 +/- 1.2% of the dose present at 10 min after injection. Smaller amounts ( < 3% of the dose) were taken up by spleen and bone marrow. Previous injection with mannan substantially increased the plasma half-life of the enzyme (14.8 +/- 3.2 min versus 1.7 +/- 0.3 min in solvent-preinjected controls) and uptake of the enzyme by liver, spleen and bone marrow was reduced by > 90%. These findings indicate that the enzyme is taken up by these organs via mannose-specific receptors. Subcellular fractionation of the liver indicated that the enzyme is internalized and transported to the lysosomes. By isolating various liver cell types after injection of the Alglucerase, it was found that endothelial cells are the main site of uptake of the enzyme: 60.8 +/- 3.4% of the total liver uptake. Parenchymal and Kupffer cells were responsible for 31.0 +/- 3.1% and 8.2 +/- 0.7% of the hepatic uptake, respectively. We conclude that Alglucerase is rapidly cleared from the circulation by mannose-specific receptors in liver, spleen, and bone marrow. However, less than 10% of the enzyme taken up by the liver is accounted for by Kupffer cells, the hepatic target cells for therapeutic intervention. It is suggested that alterations of the formulation of the therapeutic enzyme may lead to a higher uptake by Kupffer cells and other macrophages, and thus to a more (cost)effective therapy of Gaucher's disease.

Uptake, internalization and degradation of the novel plasminogen activator reteplase (BM 06.022) in the rat

The catabolism of the novel plasminogen activator reteplase (BM 06.022) was described. For this purpose BM 06.022 was radiolabelled with 125I or with the accumulating label 125I-tyramine cellobiose (125I-TC). BM 06.022 was injected at a pharmacological dose of 380 micrograms/kg b.w. and it was cleared from the plasma in a biphasic manner with a half-life of about 1 min in the alpha-phase and t1/2 of 20-28 min in the beta-phase. 28% and 72% of the injected dose was cleared in the alpha-phase and beta-phase, respectively. Initially liver, kidneys, skin, bones, lungs, spleen, and muscles contributed mainly to the plasma clearance. Only liver and the kidneys, however, were responsible for the uptake and subsequent degradation of BM 06.022 and contributed for 75% to the catabolism of BM 06.022. BM 06.022 was degraded in the lysosomal compartment of both organs. Parenchymal liver cells were responsible for 70% of the liver uptake of BM 06.022. BM 06.022 associated rapidly to isolated rat parenchymal liver cells and was subsequently degraded in the lysosomal compartment of these cells. BM 06.022 bound with low-affinity to the parenchymal liver cells (550 nM) and the binding of BM 06.022 could be displaced by t-PA (IC50 5.6 nM), indicating that the low-density lipoprotein receptor-related protein (LRP) could be involved in the binding of BM 06.22. GST-RAP, which is an inhibitor of LRP, could in vivo significantly inhibit the uptake of BM 06.022 in the liver. It is concluded that BM 06.22 is metabolized primarily in the liver and the kidneys. These organs take up and degrade BM 06.022 in the lysosomes. The uptake mechanism of BM 06.022 in the kidneys is unknown, while LRP is responsible for a low affinity binding and uptake of BM 06.022 in parenchymal liver cells.