Synthesis and Evaluation of Radiogallium-Labeled Peptide Probes for In Vivo Imaging of Legumain Activity

Legumain (LGMN), a lysosomal cysteine protease, is crucial for tumor progression, invasion, and metastasis, making it a promising target for cancer imaging and therapy. This study developed novel 67Ga-labeled 1,4,7-triazacyclononane-1,4,7-triacetic acid (NOTA)-conjugated LGMN-cleavable peptide probes ([67Ga]Ga-NOTA-LCPs) composed of polyarginine and polyglutamic acid sequences linked by LGMN-cleavable sites for nuclear medicine imaging of LGMN activity. The probes were synthesized via fluorenylmethoxycarbonyl solid-phase peptide synthesis and radiolabeled in high radiochemical yields. In vitro assays with HCT116 cells showed significantly higher uptake of [67Ga]Ga-NOTA-LCPs compared to non-cleavable controls, confirming efficient cleavage and cellular uptake. In vivo studies in tumor-bearing mice revealed rapid renal clearance, low non-specific binding, and favorable tumor-to-blood ratios, particularly for [67Ga]Ga-NOTA-LCP-1. These results demonstrate the potential of [67Ga]Ga-NOTA-LCPs as effective LGMN-responsive imaging agents, with further optimization needed to improve tumor specificity and reduce off-target accumulation.