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, [68Ga]NOTA-SFCVM and [68Ga]NOTA-SFCVHEM, 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. In vitro cellular experiments showed that the tracers could differentiate tumor cells with different expression levels of CTB. In vivo PET imaging further revealed that the tracers selectively accumulated in the CTB-positive tumors. Compared with [68Ga]NOTA-SFCVM, [68Ga]NOTA-SFCVHEM 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 [68Ga]NOTA-SFCVHEM holds potential for clinical use in the early diagnosis of CTB-related tumors.

组织蛋白酶B（Cathepsin B，CTB）在多种肿瘤中呈过表达状态，对其活性进行精准评估可为肿瘤早期诊断提供极具前景的技术手段。本研究开发了两种可被CTB激活的正电子发射断层扫描（positron emission tomography，PET）显像剂[68Ga]NOTA-SFCVM与[68Ga]NOTA-SFCVHEM，用于CTB的高灵敏度、高特异性检测。两种显像剂均可通过2-氰基-6-氨基苯并噻唑（2-cyano-6-aminobenzothiazole，CBT）与半胱氨酸（cysteine，Cys）之间的点击缩合反应形成环化产物，从而增强并延长肿瘤部位的PET信号。体外细胞实验结果显示，该类显像剂可区分CTB表达水平存在差异的肿瘤细胞；体内PET成像进一步证实，显像剂可选择性富集于CTB阳性肿瘤组织中。相较于[68Ga]NOTA-SFCVM，带有吗啉基团（morpholine group）以及组氨酸-谷氨酸-组氨酸-谷氨酸-组氨酸-谷氨酸序列（histidine–glutamate–histidine–glutamate–histidine–glutamate sequence）的[68Ga]NOTA-SFCVHEM对CTB的催化效率更高，肿瘤摄取量更高，且肝脏摄取量更低。上述研究结果表明，[68Ga]NOTA-SFCVHEM具备用于CTB相关肿瘤早期诊断的临床应用潜力。