Evaluation of a technetium-99m labeled bombesin homodimer for GRPR imaging in prostate cancer
Multimerization of peptides can improve the binding characteristics of the tracer by increasing local ligand concentration and decreasing dissociation kinetics. In this study, a new bombesin homodimer was developed based on an ε-aminocaproic acid-bombesin(7–14) (Aca-bombesin(7–14)) fragment, which has been studied for targeting the gastrin-releasing peptide receptor (GRPR) in prostate cancer. The bombesin homodimer was conjugated to 6-hydrazinopyridine-3-carboxylic acid (HYNIC) and labeled with 99mTc for SPECT imaging. The in vitro binding affinity to GRPR, cell uptake, internalization and efflux kinetics of the radiolabeled bombesin dimer were investigated in the GRPR-expressing human prostate cancer cell line PC-3. Biodistribution and the GRPR-targeting potential were evaluated in PC-3 tumor-bearing athymic nude mice. When compared with the bombesin monomer, the binding affinity of the bombesin dimer is about ten times lower. However, the 99mTc labeled bombesin dimer showed a three times higher cellular uptake at 4 h after incubation, but similar internalization and efflux characters in vitro. Tumor uptake and in vivo pharmacokinetics in PC-3 tumor-bearing mice were comparable. The tumor was visible on the dynamic images in the first hour and could be clearly distinguished from non-targeted tissues on the static images after 4 h. The GRPR-targeting ability of the 99mTc labeled bombesin dimer was proven in vitro and in vivo. This bombesin homodimer provides a good starting point for further studies on enhancing the tumor targeting activity of bombesin multimers.
KeywordsGRPR Bombesin homodimer Radiolabeled Imaging Prostate cancer PC-3 99mTc HYNIC SPECT
This work was made possible by a financial contribution from CTMM, project PCMM, project number 03O-203. We thank Chao Wu for technical assistance on microSPECT images reconstruction and D.F. Samplonius for technical assistance on cell culturing, and J. W. A. Sijbesma for assisting with animal experiments. All animal experiments were approved by the local animal welfare committee in accordance with the Dutch legislation and carried out in accordance with their guidelines.
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