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A Target Cell–Specific Activatable Fluorescence Probe for In vivo Molecular Imaging of Cancer Based on a Self-Quenched Avidin-Rhodamine Conjugate

¹ Molecular Imaging Program, Center for Cancer Research, National Cancer Institute; ² Nuclear Medicine Department, Warren Magnuson Clinical Center, NIH, Bethesda, Maryland; ³ Graduate School of Pharmaceutical Sciences, The University of Tokyo, Tokyo, Japan; and ⁴ Research Technology Program, SAIC-Frederick, Frederick, Maryland

Requests for reprints: Hisataka Kobayashi, Molecular Imaging Program, Center for Cancer Research, National Cancer Institute/NIH, Room 1B40, Building 10, MSC 1088, 10 Center Drive, Bethesda, MD 20892-1088. Phone: 301-451-4221; Fax: 301-402-3191; E-mail: Kobayash{at}mail.nih.gov.

A target cell–specific activation strategy for improved molecular imaging of peritoneal implants has been proposed, in which fluorophores are activated only in living targeted cells. A current example of an activatable fluorophore is one that is normally self-quenched by attachment to a peptide backbone but which can be activated by specific proteases that degrade the peptide resulting in "dequenching." In this study, an alternate fluorescence activation strategy is proposed whereby self-quenching avidin-rhodamine X, which has affinity for lectin on cancer cells, is activated after endocytosis and degradation within the lysosome. Using this approach in a mouse model of peritoneal ovarian metastases, we document target-specific molecular imaging of submillimeter cancer nodules with minimal contamination by background signal. Cellular internalization of receptor-ligand pairs with subsequent activation of fluorescence via dequenching provides a generalizable and highly sensitive method of detecting cancer microfoci in vivo and has practical implications for assisting surgical and endoscopic procedures. [Cancer Res 2007;67(6):2791–9]

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