Positron Emission Tomography Imaging of Cathepsin B in Tumors with Activable Small Molecule Tracers

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