Direct measurement of the extravasation of polyethyleneglycol-coated liposomes into solid tumor tissue by in vivo fluorescence microscopy

The extravasation of liposomes of different sizes into solid tumors after i.v. injection was visualized by in vivo fluorescence microscopy in mouse neuroblastoma C-1300-bearing mice. Liposomes composed of distearoylphosphatidylcholine/cholesterol (1/1 molar ratio) and 6 mol% distearoylphosphatidylethanolamine derivative of poryethyleneglycol (PEG) were prepared. The PEG-coated liposomes were fluorescently labeled with 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI) as a liposome marker or with doxorubicin (DXR) as an aqueous-phase marker. Liposomes with an average diameter of 100-200 nm showed the greatest tumor accumulation. With time after injection of DiI-labeled liposomes, the tumor interstitial fluorescence intensity increased. Most fluorescent spots were located outside and around the vessel wall, indicating extravasation of intact liposomes. The perivascular distribution was heterogeneons. We also obtained the same fluorescence localization pattern with DXR released from extravasated liposomes after injection of DXR-encapsulated liposomes. No fluorescence from extravasated liposomes was detected in normal s.c. tissue; the fluorescent spots were observed only in the vessel wall. Our results indicate that small-size long-circulating liposomes are able to traverse the endothelium of blood vessels in tumors and extravasate into interstitial spaces. Moreover, encapsulated drug was released from extravasated liposomes in the tumor.