High Contrast PET Imaging of GRPR Expression in Prostate Cancer Using Cobalt-Labeled Bombesin Antagonist RM26

High gastrin releasing peptide receptor (GRPR) expression is associated with numerous cancers including prostate and breast cancer. The aim of the current study was to develop a ⁵⁵Co-labeled PET agent based on GRPR antagonist RM26 for visualization of GRPR-expressing tumors. Labeling with ⁵⁷Co and ⁵⁵Co, stability, binding specificity, and in vitro and in vivo characteristics of ⁵⁷Co-NOTA-PEG₂-RM26 were studied. NOTA-PEG₂-RM26 was successfully radiolabeled with ⁵⁷Co and ⁵⁵Co with high yields and demonstrated high stability. The radiopeptide showed retained binding specificity to GRPR in vitro and in vivo. ⁵⁷Co-NOTA-PEG₂-RM26 biodistribution in mice was characterized by rapid clearance of radioactivity from blood and normal non-GRPR-expressing organs and low hepatic uptake. The clearance was predominantly renal with a low degree of radioactivity reabsorption. Tumor-to-blood ratios were approximately 200 (3 h pi) and 1000 (24 h pi). The favorable biodistribution of cobalt-labeled NOTA-PEG₂-RM26 translated into high contrast preclinical PET/CT (using ⁵⁵Co) and SPECT/CT (using ⁵⁷Co) images of PC-3 xenografts. The initial biological results suggest that ⁵⁵Co-NOTA-PEG₂-RM26 is a promising tracer for PET visualization of GRPR-expressing tumors.