Design and Preclinical Evaluation of First 68Ga-Labeled Cathepsin D‑Targeted Radiotracers

Cathepsin D (CTSD), a lysosomal aspartic protease linked to cancer progression and poor patient outcomes, is underexplored as a diagnostic biomarker. This study developed the first CTSD-targeted radiotracers, identifying [68Ga]Ga-NOTA-FZCD-3 as optimal due to its strong binding to CTSD (dissociation constant = 0.65 μM), rapid clearance from the body (elimination half-life = 16.61 min), and high tumor-specific uptake in mouse models. Imaging analysis revealed that tumors with higher CTSD levels absorbed more of the tracer, particularly in the MDA-MB-175VII model, which showed the highest tumor uptake (3.63 ± 0.32%ID/g at 0.5 h) and excellent tumor/nontumor contrast (7.65 ± 1.14 at 1 h). The tracer remained stable in blood for over 2 h without causing toxicity. These findings highlight CTSD as a key cancer biomarker and demonstrate the potential of this radiotracer for improving early cancer detection and monitoring therapy response in patients.组织蛋白酶D（Cathepsin D，CTSD）是一种与癌症进展及患者不良预后相关的溶酶体天冬氨酸蛋白酶，其作为诊断生物标志物的研究尚未得到充分探索。本研究开发了首款靶向CTSD的放射性示踪剂，并筛选出[68Ga]Ga-NOTA-FZCD-3作为最优候选，该示踪剂与CTSD结合活性极强（解离常数为0.65 μM），可快速经体内清除（消除半衰期为16.61 min），且在小鼠模型中展现出高肿瘤特异性摄取能力。影像学分析结果显示，CTSD表达水平更高的肿瘤对该示踪剂的摄取量更多，其中MDA-MB-175VII模型的肿瘤摄取效果最为突出：在给药后0.5小时，其肿瘤摄取量可达3.63 ± 0.32 %ID/g，并在给药后1小时展现出优异的肿瘤/非肿瘤对比度（7.65 ± 1.14）。该示踪剂在血液中可稳定存在超过2小时，且未引发明显毒性。上述研究成果凸显了CTSD作为关键癌症生物标志物的重要价值，并证实了该放射性示踪剂在改善癌症早期检测及患者治疗反应监测方面的应用潜力。