Selection of optimal chelator improves the contrast of GRPR imaging using bombesin analogue RM26

Mitran,Bogdan; Varasteh,Zohreh; Selvaraju,Ram  Kumar; Lindeberg,Gunnar; Sörensen,Jens; Larhed,Mats; Tolmachev,Vladimir; Rosenström,Ulrika; Orlova,Anna

doi:10.3892/ijo.2016.3429

Selection of optimal chelator improves the contrast of GRPR imaging using bombesin analogue RM26

Authors:
- Bogdan Mitran
- Zohreh Varasteh
- Ram Kumar Selvaraju
- Gunnar Lindeberg
- Jens Sörensen
- Mats Larhed
- Vladimir Tolmachev
- Ulrika Rosenström
- Anna Orlova
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Affiliations: Division of Molecular Imaging, Department of Medicinal Chemistry, Faculty of Pharmacy, Uppsala University, Uppsala, Sweden, Organic Pharmaceutical Chemistry, Department of Medicinal Chemistry, Faculty of Pharmacy, Uppsala University, Uppsala, Sweden, Department of Medical Sciences, Clinical Physiology, Uppsala University Hospital, Uppsala, Sweden, Science for Life Laboratory, Department of Medicinal Chemistry, Uppsala Biomedical Center, Uppsala University, Uppsala, Sweden, Department of Immunology, Genetics and Pathology, Faculty of Medicine, Uppsala University, Uppsala, Sweden
Published online on: March 9, 2016 https://doi.org/10.3892/ijo.2016.3429
Pages: 2124-2134

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Abstract

Bombesin (BN) analogs bind with high affinity to gastrin-releasing peptide receptors (GRPRs) that are upregulated in prostate cancer and can be used for the visualization of prostate cancer. The aim of this study was to investigate the influence of radionuclide-chelator complexes on the biodistribution pattern of the 111In-labeled bombesin antagonist PEG2-D-Phe-Gln-Trp-Ala-Val-Gly-His-Sta-Leu-NH2 (PEG2-RM26) and to identify an optimal construct for SPECT imaging. A series of RM26 analogs N-terminally conjugated with NOTA, NODAGA, DOTA and DOTAGA via a PEG2 spacer were radiolabeled with 111In and evaluated both in vitro and in vivo. The conjugates were successfully labeled with 111In with 100% purity and retained binding specificity to GRPR and high stability. The cellular processing of all compounds was characterized by slow internalization. The IC50 values were in the low nanomolar range, with lower IC50 values for positively charged natIn-NOTA-PEG2-RM26 (2.6±0.1 nM) and higher values for negatively charged natIn-DOTAGA-PEG2-RM26 (4.8±0.5 nM). The kinetic binding studies showed KD values in the picomolar range that followed the same pattern as the IC50 data. The biodistribution of all compounds was studied in BALB/c nu/nu mice bearing PC-3 prostate cancer xenografts. Tumor targeting and biodistribution studies displayed rapid clearance of radioactivity from the blood and normal organs via kidney excretion. All conjugates showed similar uptake in tumors at 4 h p.i. The radioactivity accumulation in GRPR-expressing organs was significantly lower for DOTA- and DOTAGA-containing constructs compared to those containing NOTA and NODAGA. 111In-NOTA-PEG2-RM26 with a positively charged complex showed the highest initial uptake and the slowest clearance of radioactivity from the liver. At 4 h p.i., DOTA- and DOTAGA-coupled analogs showed significantly higher tumor-to-organ ratios compared to NOTA- and NODAGA-containing variants. The NODAGA conjugate demonstrated the best retention of radioactivity in tumors, and, at 24 h p.i., had the highest contrast to blood, muscle and bones.

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