In vivo fate of phosphorothioate antisense oligodeoxynucleotides: Predominant uptake by scavenger receptors on endothelial liver cells

Systemically administered phosphorothioate antisense oligodeoxynucleotides can specifically affect the expression of their target genes, which affords an exciting new strategy for therapeutic intervention. Earlier studies point to a major role of the liver in the disposition of these oligonucleotides. The aim of the present study was to identify the cell type(s) responsible for the liver uptake of phosphorothioate oligo-deoxynucleotides and to examine the mechanisms involved. In our study we used ISIS-3082, a phosphorothioate antisense oligodeoxynucleotide specific for murine ICAM-1. Intravenously injected [H-3]ISIS-3082 (dose: 1 mg/kg) was cleared from the circulation of rats with a half-life of 23.3 +/- 3.8 min. At 90 min after injection (>90% of [H-3]ISIS-3082 cleared), the liver contained the most radioactivity, whereas the second-highest amount was recovered in the kidneys (40.5 +/- 1.4% and 17.9 +/- 1.3% of the dose, respectively). Of the remaining tissues, only spleen and bone marrow actively accumulated [H-3]ISIS-3082. By injecting different doses of [H-3]ISIS-3082, it was found that uptake by liver, spleen, bone marrow, and kidneys is saturable, which points to a receptor-mediated process. Subcellular fractionation of the liver indicates that ISIS-3082 is internalized and delivered to the lysosomes. Liver uptake occurs mainly (for 56.1 +/- 3.0%) by endothelial cells, whereas parenchymal and Kupffer cells account for 39.6 +/- 4.5 and 4.3 +/- 1.7% of the total liver uptake, respectively. Preinjection of polyinosinic acid substantially reduced uptake by liver and bone marrow, whereas polyadenylic acid was ineffective, which indicates that in these tissues scavenger receptors are involved in uptake. Polyadenylic acid, but not polyinosinic acid, reduced uptake by kidneys, which suggests renal uptake by scavenger receptors different from those in the liver. We conclude that scavenger receptors on rat liver endothelial cells play a predominant role in the plasma clearance of ISIS-3082. As scavenger receptors are also expressed on human endothelial liver cells, our findings are probably highly relevant for the therapeutic application of phosphorothioate oligodeoxynucleotides in humans. If the target gene is not localized in endothelial liver cells, the therapeutic effectiveness might be improved by developing delivery strategies that redirect the oligonucleotides to the actual target cells.