Purpose: The in vivo imaging of a novel peptide-Cy5.5 fluorescent optical probe targeted to matrix metalloproteinases was assessed using CW fluorescence imaging accomplished via an ICCD camera.
Methods and Materials: In vitro binding studies were performed by incubating the optical contrast agent with cells of SKBr-3, HT1080, and PC-3 tumors on cover slips in 96-well plates over a period of 5-60 min. After washing steps, cells were examined using a fluorescence microscope. In vivo imaging was performed on nude mice with subcutaneous human SKBr-3 breast and HT1080 sarcoma tumors and on mice with intratibia human PC-3 prostate tumors. Unconjugated Cy5.5 was used as a control. Fluorescence images were obtained at 24 hrs after intravenously administration of each contrast agent at an injected dose of 15 nmol per mouse.
Results: The peptide dye targeted to MMPs bound to cells of all three lines as early as 5 min. The size of KBr-3 and HT1080 tumors at the time of contrast injection was 6-8 mm in average diameter. Images acquired one day after the injection of targeted optical probe clearly delineated the tumors. In contrast, images with Cy5.5 afforded only weak signal. Moreover, intratibia PC-3 tumors were detectable as early as 3 days after tumor inoculation. Histological examinations of the excised bone tissues revealed extensive osteolytic activity and local cell proliferation. Interestingly, X-ray evaluation of the mice failed to reveal tumors at the inoculation site. Mice injected with saline did not show contrast enhancement 24 after injection of the optical probe, indicating that the observed optical signal following inoculation of PC-3 tumors was not a result of injection trauma.
Conclusion: Our data suggest that optical imaging with contrast agent targeted to MMPs may be used to detect tumors at the early stage of tumor progression.