Her2-targeted pegylated stealth liposomal doxorubicin (PLD) retains its specific targeting ability to Her2-expressing tumor cells after in vivo circulation and extravasation to mouse malignant ascites

Abstract

13097 Background: Receptor-directed targeting of ligand-bearing liposomes to tumor cells may enhance therapeutic efficacy by intracellular delivery of a concentrated payload of liposomal drug. The goal of this study was to assess whether Her2-targeted PLD retains its binding ability to Her2-expressing target cells through circulation in the blood and extravasation to the ascitic fluid of mice with malignant ascites. Methods: PLD was grafted with a pegylated lipophilic conjugate of an anti-Her2 scFv antibody fragment (F5) at a ratio of 15 ligands per liposome. BALB/c mice were injected with J6456 lymphoma cells into the peritoneal cavity to generate malignant ascites. When abdominal swelling developed, Her2-targeted PLD and nontargeted PLD were injected into the mice i.v. at a dose of 15 mg/kg. The ascitic fluid was collected 48 hr later, ascitic tumor cells were removed, and the doxorubicin levels in the cell-free ascitic fluid and plasma were determined. Binding of the liposome-containing ascitic fluid was tested in vitro against Her2-expressing human tumor cell lines (N87, SKBR-3) and compared to the binding of shelf formulations (not passaged in vivo) of Her2-PLD and PLD, using as parameter the amount of cell-associated doxorubicin. Results: Plasma and ascitic fluid levels of Her2-PLD were only slightly below those of PLD indicating that the Her2 ligand did not cause any significant change in the clearance rate of PLD. Her2-PLD and PLD bound to an equal extent to J6456 cells in vivo. The in vitro binding of Her2-PLD from ascites to Her2- expressing cells was increased 10 to 20-fold above that of PLD from ascites, similarly to the 20-fold difference in binding between shelf Her2-PLD and PLD. Conclusions: Her2-targeted PLD demonstrates similar circulation time to that of nontargeted PLD. After in vivo passage, the targeted liposome retains most of its original binding capacity to Her2 expressing cells indicating that the ligand is stably maintained in association with the doxorubicin liposomal carrier. Targeting of PLD using this Her2 antibody fragment should provide an important means of selective drug delivery to tumors expressing the Her2 receptor. [Table: see text]