Active removal of radioactivity in the blood circulation using biotin-bearing liposomes and avidin for rapid tumour imaging.

In order to shorten the delay between the administration of tumour-imaging agents and obtainment of scintigrams, rapid delivery of radionuclide to tumours followed by rapid clearance from the blood is required. We used liposomes with biotin bound on their surface (B-liposomes) as carriers for rapid delivery. For rapid blood clearance, we employed avidin in the expectation that the strong affinity between biotin and avidin would result in the aggregation of B-liposomes in the blood circulation, and that these aggregates would be taken up rapidly by the reticuloendothelial system, resulting in the rapid elimination of liposomes and the radionuclide encapsulated in them. When B-liposomes encapsulating gallium-67 deferoxamine were intravenously administered to mice bearing sarcoma 180, large amounts of 67Ga were delivered to tumours by 4 h after injection, though the 67Ga blood level remained high. On the other hand, administration of avidin 4 h after administration of the B-liposomes dramatically reduced the blood level so that it was only 5% of that in the non-treated group 1 h later. As a result, the tumour-to-blood ratio reached nearly 14 at 5 h after radionuclide administration, suggesting that rapid tumour-imaging will be feasible by means of this method.