uPAR as a Glioma Imaging Target.

Gliomas account for 70% of primary brain tumors (1). Among the various types of glioma, glioblastoma is the most aggressive astrocytic tumor, being classified grade IV by the World Health Organization (2). Although CT and MRI are indispensable in providing morphologic information, functional imaging using PET plays an important role in grading tumors, delineating tumor boundaries, monitoring treatment, and discriminating recurrent tumor from treatment-induced changes (3). 18F-FDG is the best-established PET tracer for various malignancies; however, the high glucose metabolism of the brain prevents accurate evaluation of brain neoplasms using 18F-FDG PET. In addition, although higher-grade gliomas metabolize more glucose than lower-grade gliomas, even glioblastomas sometimes show lower uptake than the surrounding brain tissue, making 18F-FDG PET images difficult to interpret—especially in evaluating tumor expansion. In this context, other tracers for brain tumors have been extensively investigated over the past few decades. Among them, amino acid tracers such as 11C-methionine (4,5) and 18F-fluoroethyltyrosine (6,7) have been the most successful, followed by hypoxia imaging agents such as 18F-fluoromisonidazole (8) and nucleic acid analogs such as 18F-fluorothymidine (9). Now, urokinasetype plasminogen activator receptor (uPAR) has been added to the array of available tracers for imaging.