Quantitative Analysis of the Targeting of Mannose‐Terminal Glucocerebrosidase

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Abstract
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 (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.