Positron Emission\nTomography Imaging of Cathepsin\nB in Tumors with Activable Small Molecule Tracers

Cathepsin B (CTB) is overexpressed in several types of\ntumors,\nand precise evaluation of the CTB activity can offer a promising method\nfor the early diagnosis of tumors. In this study, two CTB-activated\npositron emission tomography (PET) tracers, <b>[</b><sup><b>68</b></sup><b>Ga]NOTA-SFCVM</b> and <b>[</b><sup><b>68</b></sup><b>Ga]NOTA-SFCVHEM</b>, were developed\nfor sensitive and specific detection of CTB. Both tracers undergo\na click condensation between 2-cyano-6-aminobenzothiazole (CBT) and\ncysteine (Cys) to form a cyclization product, thereby enhancing and\nprolonging the PET signal in tumors. <i>In vitro</i> cellular\nexperiments showed that the tracers could differentiate tumor cells\nwith different expression levels of CTB. <i>In vivo</i> PET\nimaging further revealed that the tracers selectively accumulated\nin the CTB-positive tumors. Compared with <b>[</b><sup><b>68</b></sup><b>Ga]NOTA-SFCVM</b>, <b>[</b><sup><b>68</b></sup><b>Ga]NOTA-SFCVHEM</b> containing a morpholine\ngroup and a histidine–glutamate–histidine–glutamate–histidine–glutamate\nsequence exhibited faster catalytic efficiency toward CTB, higher\ntumor uptake, and reduced liver uptake. These findings suggest that <b>[</b><sup><b>68</b></sup><b>Ga]NOTA-SFCVHEM</b> holds\npotential for clinical use in the early diagnosis of CTB-related tumors.