Radiofluorinated Smart Probes for Noninvasive PET Imaging of Legumain Activity in Living Subjects

Overexpression of legumain is closely associated with tumor proliferation, invasion, and metastasis. Because of its intrinsic properties, such as high sensitivity and resolution, positron emission tomography (PET) has become an effective imaging technique for early diagnosis, treatment response prediction, and monitoring. Herein, two legumain-targeting radiofluorinated smart probes (F-18-2 and F-18-3) as well as a control probe (F-18-1) were specifically designed for PET imaging of legumain activity in tumors.F-18-1, F-18-2, and F-18-3 were obtained with high radiochemical yield (RCY > 60%) and radiochemical purity (RCP > 99%) using a convenient one-step F-18-labeling method. The probes F-18-2 and F-18-3 exhibited high response to legumain activity and reductive environment and revealed comparable uptake in HCT116 cells (4.22% +/- 0.14% and 4.64% +/- 0.32% for F-18-2 and F-18-3, respectively; 8.46% +/- 0.33% and 9.05% +/- 0.24% for co-treatment of F-18-2 + 2 and F-18-3 + 3 at 1 h), while the control probe F-18-1 showed no response. PET imaging of tumor-bearing mice showed that the co-injection strategy (F-18-2 + 2 and F-18-3 + 3) resulted in higher tumor uptake (3.57% +/- 0.37% and 3.72% +/- 0.19% ID/g at 10 min, respectively) than the single injection strategy (2.59% +/- 0.19% and 2.60% +/- 0.46% ID/g for F-18-2 and 18 F-3, respectively). In addition, introduction of the trimeric histidine-glutamate (HEHEHE) tag to F-18-3 reduced the liver uptake by almost two-fold without any noticeable effect on the tumor uptake. All the results indicate that F-18-3 holds great potential applications in clinics for sensitive and specific PET imaging of legumain activity in tumors.