Mesenchymal-epithelial transition factor (c-Met) is a
receptor tyrosine kinase that has been shown to be overexpressed and mutated in a variety of
malignancies, such as
glioma. We have recently found that an (125)I-radiolabeled
Gly-Gly-Gly (GGG)- or
8-aminooctanoic acid (AOC)-containing c-Met binding
peptide (cMBP) specifically targets c-Met receptor in vivo and in vitro. In this report, cyanine
dye 5.5 (Cy5.5)-conjugated GGG- or AOC-containing cMBPs were evaluated in human
cancer cell xenografts in order to investigate the possibility of c-Met receptor targeting using an optical imaging system. The receptor binding affinity of Cy5.5-conjugated
peptides was tested in 96-well plates coated with a c-Met/Fc chimeric
protein. Optical imaging studies were performed in U87MG and Ramos bearing athymic mice. The binding affinities of Cy5.5-conjugated GGG- or AOC-containing cMBPs were determined to be 0.318 and 0.342 microM, respectively. Confocal images show that Cy5.5-conjugated
peptides bound mainly to the cell surface and that
peptide binding was clearly inhibited by free cMBP. Subcutaneous U87MG
tumors were clearly visualized with each of the two
fluorescent probes. Of the two, cMBP-AOC-Cy5.5 displayed higher
tumor uptake and
tumor-to-normal tissue ratios
at 10 min to 24 h postinjection in the U87MG
tumor model. For the in vivo blocking study, cMBP-AOC-Cy5.5 (4 nmol) was co-injected with cold cMBP (0.13 micromol) into the U87MG xenograft mice. Image-based tumoral uptake decreased up to approximately 35%. These results suggest that Cy5.5-conjugated cMBP could potentially be used to detect c-Met-positive
cancers in vivo. However, additional modifications to this optical imaging agent are needed to further improve its efficacy.