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The effect of macrocyclic chelators on the targeting properties of the 68Ga-labeled gastrin releasing peptide receptor antagonist PEG2-RM26
Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Pharmacy, Department of Medicinal Chemistry, Preclinical PET Platform.
Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Pharmacy, Department of Medicinal Chemistry, Preclinical PET Platform.
Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Pharmacy, Department of Medicinal Chemistry, Organic Pharmaceutical Chemistry.
Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Pharmacy, Department of Medicinal Chemistry, Preclinical PET Platform.
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2015 (English)In: Nuclear Medicine and Biology, ISSN 0969-8051, E-ISSN 1872-9614, Vol. 42, no 5, 446-454 p.Article in journal (Refereed) Published
Abstract [en]

Introduction

Overexpression of gastrin-releasing peptide receptors (GRPR) has been reported in several cancers. Bombesin (BN) analogs are short peptides with a high affinity for GRPR. Different BN analogs were evaluated for radionuclide imaging and therapy of GRPR-expressing tumors. We have previously investigated an antagonistic analog of BN (D-Phe-Gln-Trp-Ala-Val-Gly-His-Sta-Leu-NH2, RM26) conjugated to NOTA via a PEG2 spacer (NOTA-PEG2-RM26) labeled with 68Ga, 111In and Al18F. 68Ga-labeled NOTA-PEG2-RM26 showed high tumor-to-organ ratios.

Methods

The influence of different macrocyclic chelators (NOTA, NODAGA, DOTA and DOTAGA) on the targeting properties of 68Ga-labeled PEG2-RM26 was studied in vitro and in vivo.

Results

All conjugates were labeled with generator-produced 68Ga with high yields and demonstrated high stability and specific binding to GRPR. The IC50 values of natGa-X-PEG2-RM26 (X = NOTA, DOTA, NODAGA, DOTAGA) were 2.3 ± 0.2, 3.0 ± 0.3, 2.9 ± 0.3 and 10.0 ± 0.6 nM, respectively. The internalization of the conjugates by PC-3 cells was low. However, the DOTA-conjugated analog demonstrated a higher internalization rate compared to other analogs. GRPR-specific uptake was found in receptor-positive normal tissues and PC-3 xenografts for all conjugates. The biodistribution of the conjugates was influenced by the choice of the chelator moiety. Although all radiotracers cleared rapidly from the blood, [68Ga]Ga-NOTA-PEG2-RM26 showed significantly lower uptake in lung, muscle and bone compared to the other analogs. The uptake in tumors (5.40 ± 1.04 %ID/g at 2 h p.i.) and the tumor-to-organ ratios (25 ± 3, 157 ± 23 and 39 ± 4 for blood, muscle and bone, respectively) were significantly higher for the NOTA-conjugate than the other analogs.

Conclusions

Chelators had a clear influence on the biodistribution and targeting properties of 68Ga-labeled antagonistic BN analogs. Positively charged [68Ga]Ga-NOTA-PEG2-RM26 provided a low kidney radioactivity uptake, high affinity, high tumor uptake and high image contrast.

Place, publisher, year, edition, pages
2015. Vol. 42, no 5, 446-454 p.
National Category
Radiology, Nuclear Medicine and Medical Imaging
Identifiers
URN: urn:nbn:se:uu:diva-232120DOI: 10.1016/j.nucmedbio.2014.12.009ISI: 000353369000005PubMedID: 25684649OAI: oai:DiVA.org:uu-232120DiVA: diva2:746564
Funder
Swedish Cancer SocietySwedish Research Council
Available from: 2014-09-12 Created: 2014-09-12 Last updated: 2017-12-05Bibliographically approved
In thesis
1. Bombesin Antagonists for Targeting Gastrin-Releasing Peptide Receptor-Positive Tumors: Design, Synthesis, Preclinical Evaluation and Optimization of Imaging Agents
Open this publication in new window or tab >>Bombesin Antagonists for Targeting Gastrin-Releasing Peptide Receptor-Positive Tumors: Design, Synthesis, Preclinical Evaluation and Optimization of Imaging Agents
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis is focused on the development, preclinical evaluation, and optimization of radiotracers for the detection of gastrin-releasing peptide receptor (GRPR)-expressing tumors. The work is divided into three distinct parts: (1) the development of bombesin (BN) antagonist (RM26)-based imaging radiotracers for the detection of GRPR-expressing tumors using different positron emission tomography (PET) and single photon emission computed tomography (SPECT) radionuclides (68Ga, 18F and 111In), (2) the establishment of a method to monitor the ligand-G protein-coupled receptor (GPCR) interaction in real time without requiring purification and stabilization of the receptors, and (3) the evaluation of radiopeptide structure-related factors (length of mini-PEG linker and composition of chelator for metal labeling) affecting the in vitro and in vivo characteristics of RM26-based tracers.

We demonstrated the possibility of high-contrast in vivo imaging of GRPR-expressing xenografts despite the physiological expression of GRPR in abdominal organs. Fast radioactivity clearance from the blood and healthy organs, including receptor-positive organs, and long retention in the tumors resulted in high tumor-to-background ratios. A novel real-time assay for measuring the kinetics of the radiotracers targeting GPCR was evaluated. Living cells were used instead of purified receptors in this technology, bringing the developmental work one step closer to the true target environment (imaging in living systems). The comparative study of 68Ga-labeled NOTA-PEGn-RM26 with di-, tri-, tetra- and hexaethylene glycol chains demonstrated that the addition of only a few units of ethylene glycol to the spacer is insufficient to appreciably affect the biodistribution of the radiopeptide. Finally, a comparative study of 68Ga-labeled PEG2-RM26 analogs N-terminally conjugated to NOTA, NODAGA, DOTA or DOTAGA highlighted the influence of the chelator on the targeting properties of the radiopeptide.

The main conclusion that can be drawn from this thesis is that 68Ga-NOTA-PEG2-RM26 has favorable biodistribution properties, such as rapid clearance from blood and tissues with physiological GRPR expression levels and long retention in GRPR-expressing tumors, and that this radiopeptide is potentially suitable for initial clinical investigation.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2014. 66 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Pharmacy, ISSN 1651-6192 ; 191
Keyword
Bombesin, Gastrin-releasing peptide receptor (GRPR), Antagonist, Radionuclide molecular imaging
National Category
Pharmaceutical Sciences
Identifiers
urn:nbn:se:uu:diva-232123 (URN)978-91-554-9039-3 (ISBN)
Public defence
2014-10-31, Rudbecksalen, Dag Hammarskjölds väg 20, Uppsala, 09:00 (English)
Opponent
Supervisors
Available from: 2014-10-10 Created: 2014-09-12 Last updated: 2015-01-23

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Varasteh, ZohrehMitran, BogdanRosenström, UlrikaVelikyan, IrinaRosestedt, MariaLindeberg, GunnarSörensen, JensLarhed, MatsTolmachev, VladimirOrlova, Anna

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Varasteh, ZohrehMitran, BogdanRosenström, UlrikaVelikyan, IrinaRosestedt, MariaLindeberg, GunnarSörensen, JensLarhed, MatsTolmachev, VladimirOrlova, Anna
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