Rapid diagnostic imaging of cancer using radiolabeled liposomes

A novel tumor diagnostic imaging method was developed that allows tumor localization soon after administration of radiolabeled liposomes. Although previous studies showed that radiolabeled liposomes can reach various tumors in a short time, their blood clearance is slow, and the high blood background hinders early imaging. Therefore, we attempted to remove actively the liposomes from the circulation using the strong affinity between avidin and biotin. Liposomes that had biotin bound to their surface and were labeled with 111In, 67Ga, or 99mTc were administered to mice bearing sarcoma 180, followed by administration of avidin 2 or 4 h later. Avidin initiated liposomal aggregation, resulting in their rapid removal by the reticuloendothelial system. Consequently, their blood level was markedly reduced without any changes in tumor levels. The tumor-to-blood ratio reached about 13 at only 2.5 h after administration of 99mTc-labeled liposomes, versus 1.0 or less without postadministration of avidin. Increased liver accumulation was also observed, but it decreased gradually with time.