Biol. Pharm. Bull. 29(9) 1936—1940 (2006)

hematogenous metastasis of tumor. Recently, various antiangiogenic therapeutic modalities have accomplished remarkable progress. We previously proposed cancer antineovascular therapy (ANET): Indirect tumor regression is achieved through damaging neovessel endothelial cells by anticancer drugs, since neovessel endothelial cells are growing cells and are susceptible to anticancer drugs like as tumor cells. For this purpose, we isolated APRPG peptide specifically bound to tumor angiogenic vasculature from phage-displayed peptide library, and observed that APRPG-modified liposomes accumulated in tumor tissue higher than unmodified one in tumor-bearing mice. In addition, the liposomes encapsulating adriamycin (ADM) strongly suppressed tumor growth. ANET is expected to suppress both primary tumor and metastasis without acquiring drug resistance. In fact, ADM-resistant P388 tumor was susceptible to ADM encapsulated in APRPG-modified liposomes. The therapy is also expected for a broad spectrum of cancers. On the other hand, it is known that polyethyleneglycol (PEG)-coated liposomes have long-circulating characteristics through avoidance of uptake by reticuloendothelial system (RES) such as liver and spleen, because PEG-coating protects liposomes from opsonization and attack of lipoproteins by their surface aqueous layers. Furthermore, PEG-coated liposomes are expected to accumulate in tumor tissue through leaky vasculature of angiogenic vessels by enhanced permeability and retention (EPR) effect. Therefore, we synthesized APRPG peptide attached to PEG termini of PEG-disteaoylphosphatidylethanolamine (PEG-DSPE) to prepare angiogenesis-targeted liposomes with long circulating characteristic. We previously observed that ADM-encapsulated liposomes modified with APRPG-PEG caused more efficient tumor growth suppression than ADM-encapsulated liposomes modified with PEG alone in Colon 26 NL-17 carcinoma (C26 NL-17)-bearing mice, despite not so much different accumulation of both liposomes in the tumor. To clarify the advantage of angiogenesis-targeted long-circulating liposomes, we examined the in vivo trafficking of APRPG-PEG-modified liposomes as well as non-modified or PEG-modified ones in tumor-baring mice with positron emission tomography (PET) in the present study: The method is able to determine the real time liposomal trafficking non-invasively. Furthermore we investigated the intratumoral distribution of liposomes modified with APRPG-PEG by use of fluorescence-labeled liposomes, and observed that differential local distribution between APRPG-PEG-modified liposomes and those modified with PEG alone.