Design and Preclinical Evaluation of First ⁶⁸ Ga-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.