PET imaging of brain cancer with positron emitter-labeled liposomes

Since nanocarriers such as liposomes are known to accumulate in tumors of tumor-bearing animals, and those that have entrapped a positron emitter can be used to image a tumor by PET, we applied 18F-labeled 100-nm-sized liposomes for the imaging of brain tumors. Polyethylene glycol (PEG)-modified liposomes, which are known to accumulate in tumors by passive targeting and those modified with Ala-Pro-Arg-Pro-Gly, which are known to home into angiogenic sites were used. Those liposomes labeled with DiI fluorescence accumulated in a glioma implanted in a rat brain 1 h after the injection, although they did not accumulate in the normal brain tissues due to the protection afforded by the blood-brain barrier. Preformed liposomes were easily labeled with 1-[18F]fluoro-3,6-dioxatetracosane, and enabled the imaging of gliomas by PET with higher contrast than that obtained with [18F]deoxyfluoroglucose. In addition, the smallest tumor among those tested, having a diameter of 1 mm was successfully imaged by the liposomal 18F. Therefore, nanocarrier-based imaging of brain tumors is promising for the diagnosis of brain cancer and possible drug delivery-based therapy.