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Rapid Tumor Imaging by Active Background Reduction Using Biotin-bearing Liposomes and Avidin¹

Faculty of Pharmaceutical Sciences, Teikyo University, Suarashi 1091-1, Sagamiko, Tsukui, Kanagawa 199-01 [I. O-U., H. N.], and Division of PET Center, Tokyo Metropolitan Institute of Gerontology, Sakae-cho, Itabashi, Tokyo 173 [T. S., H. T., K. O., M. S.], Japan

² To whom requests for reprints should be addressed.

Tumor imaging with labeled liposomes is slow; although they reach the tumor quickly, their blood clearance is slow, and the high blood background hinders early imaging. We have developed a rapid tumor imaging technique based on the active removal of liposomes from the circulation by using the avidin-biotin system. ⁶⁷Ga- or ¹¹¹In-labeled liposomes with biotin molecules bound on the surface were administered to mice bearing sarcoma 180, and avidin was administered 2 h later. The strong affinity between biotin and avidin initiated the aggregation of liposomes, resulting in their rapid removal from the circulation by the reticuloendothelial system, and the blood level of radioactivity was dramatically reduced without any change of the tumor level. Consequently, the tumor:blood ratio reached 14–18 only 2.5 h after liposome injection. Increased accumulation in the liver was also observed. By this method, an acceptable tumor image could be obtained no more than 2 h after administration of labeled liposomes.

¹ This work was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture, Japan.

The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

Received 7/26/93. Accepted 11/15/93.

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