Preclinical Evaluation of (99m)Tc-Labeled GRPR Antagonists maSSS/SES-PEG(2)-RM26 for Imaging of Prostate Cancer

Background: Gastrin-releasing peptide receptor (GRPR) is an important target for imaging of prostate cancer. The wide availability of single-photon emission computed tomography/computed tomography (SPECT/CT) and the generator-produced (99m)Tc can be utilized to facilitate the use of GRPR-targeting radiotracers for diagnostics of prostate cancers. Methods: Synthetically produced mercaptoacetyl-Ser-Ser-Ser (maSSS)-PEG(2)-RM26 and mercaptoacetyl-Ser-Glu-Ser (maSES)-PEG(2)-RM26 (RM26 = d-Phe-Gln-Trp-Ala-Val-Gly-His-Sta-Leu-NH(2)) were radiolabeled with (99m)Tc and characterized in vitro using PC-3 cells and in vivo, using NMRI or PC-3 tumor bearing mice. SPECT/CT imaging and dosimetry calculations were performed for [(99m)Tc]Tc-maSSS-PEG(2)-RM26. Results: Peptides were radiolabeled with high yields (>98%), demonstrating GRPR specific binding and slow internalization in PC-3 cells. [(99m)Tc]Tc-maSSS-PEG(2)-RM26 outperformed [(99m)Tc]Tc-maSES-PEG(2-)RM26 in terms of GRPR affinity, with a lower dissociation constant (61 pM vs 849 pM) and demonstrating higher tumor uptake. [(99m)Tc]Tc-maSSS-PEG(2)-RM26 had tumor-to-blood, tumor-to-muscle, and tumor-to-bone ratios of 97 ± 56, 188 ± 32, and 177 ± 79, respectively. SPECT/CT images of [(99m)Tc]Tc-maSSS-PEG(2)-RM26 clearly visualized the GRPR-overexpressing tumors. The dosimetry estimated for [(99m)Tc]Tc-maSSS-PEG(2)-RM26 showed the highest absorbed dose in the small intestine (1.65 × 10(−3) mGy/MBq), and the effective dose is 3.49 × 10(−3) mSv/MBq. Conclusion: The GRPR antagonist maSSS-PEG(2)-RM26 is a promising GRPR-targeting agent that can be radiolabeled through a single-step with the generator-produced (99m)Tc and used for imaging of GRPR-expressing prostate cancer.