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Background: The overexpression of gastrin-releasing peptide receptor (GRPR) has been found in early-stage of prostate cancer cells, leading it to an important theranostic target. GRPR antagonists maSSS/SES-PEG2-RM26 have been successfully labeled with Tc-99m for single-photon emission computed tomography, and [99mTc]Tc-maSSS-PEG2-RM26 has been evaluated in the clinical trial. Re-188, the homologous nuclide of Tc-99m, allows GRPR-targeted radiotherapy. 

Methods: GRPR antagonists maSSS/SES-PEG2-RM26 were labeled with Re-188 and evaluated in vitro using PC-3 cells. The biodistribution of 99mTc/188Re-labeled conjugates was compared in PC-3 tumor-bearing mice. 

Results: Peptides were successfully labeled with Re-188 with preserved binding specificity to GRPR, and demonstrated slow internalization in PC-3 cells. Biodistribution profiles of 99mTc/188Re-labeled peptides were quite similar, the accumulation of radioactivity was low in most healthy tissues and organs. The pancreas and tumor demonstrated high radioactivity uptake at 1 h pi, and the tumor uptake exceeded uptake in any other organs after 4 h. 

Conclusion: The homologous nuclide Re-188 could be conjugated with biological molecules through chelators identical to labeling with Tc-99m. Dosimetry estimation for Re-188 could be predicted based on their Tc-99m chemical analogs.