Poster Sessions

IMAG -26

Mikako Ogawa

M. Ogawa, N. Kosaka, Y. Urano, P. L. Choyke, H. Kobayashi

New activatable optical probes for in vivo tumor molecular imaging based on Fluorescence Resonance Energy Transfer (FRET)

In molecular cancer imaging, improved target-to-background ratios lead to more specific and sensitive detection of tumor. In this study, we developed activatable optical probes based on the fluorescence resonance energy transfer (FRET) quenching mechanism which were then bound to either a receptor-ligand or an antibody for in vivo tumor detection. With this targeted optical probe, the fluorescence is quenched outside the cancer cell by FRET, but is “turned on” only in the target cells after binding to the target receptor and following internalization. We selected TAMRA (fluorophore) and QSY7 (quencher) as a FRET pair, avidin-galactose lectin for a receptor-ligand system, and trastuzumab-HER2 for an antibody-antigen system. Therefore, the activatable probe consisted of TAMRA-QSY7 conjugated to either avidin (Av-TM-Q7) or trastuzumab (Traz-TM-Q7). The agents were evaluated using a SHIN3 (D-galactose receptor positive) ovarian cancer model or 3T3/HER2 (HER2 positive) cells/tumors, both in vitro and in vivo. The measured quenching capacity was 40-fold and 13-fold for Av-TM-Q7 and Traz-TM-Q7, respectively. In vitro microscopic studies demonstrated that both Av-TM-Q7 and Traz-TM-Q7 showed high fluorescent signal in the cell, indicating that these conjugates are activated after internalization. In vivo imaging demonstrated that tumors could be clearly detected by both Av-TM-Q7 and Traz-TM-Q7, and showed lower background signal than similar conjugates which use the self-quenching mechanism. Thus, both conjugates proved successful as “activatable” optical probes for tumor detection. FRET based targeted activatable optical probes have the capability to detect small tumors with high accuracy due to the higher quenching capacity compared to other activatable mechanisms.