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

Abstract

Bombesin (BN) analogs bind with high affinity to gastrin-releasing peptide receptors (GRPRs) that are up-regulated 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.

Trastuzumab cotreatment improves survival of mice with PC‐3 prostate cancer xenografts treated with the GRPR antagonist 177Lu‐DOTAGA‐PEG2‐RM26

“…In vitro binding specificity, cellular processing and kinetics of binding to and dissociation from living cells were evaluated using PC‐3 cells as previously described …”

“…In the present work, we have modified the same GRPR‐antagonist RM26 by coupling tri‐ and tetra‐aza macrocyclic chelators to its N‐terminus via a PEG 2 ‐linker and investigated the respective 177 Lu‐labeled radiopeptides for their efficacy in GRPR‐targeted radiotherapy of prostate cancer in preclinical models. Previous studies have revealed a pronounced effect of the radiometal‐chelate on the biodistribution of RM26 in mice . Based on the newly acquired in vitro and in vivo data we have concluded that the tetra‐aza chelators DOTAGA and DOTA resulted in 177 Lu‐radioligands with the most suitable properties for TRT, showing a significantly higher tumor uptake compared to the NODAGA‐carrying radioligand.…”

“…The effects of the PEG spacer's length on overall hydrophilicity were found to be minor . However, the use of different macrocyclic chelators had a profound influence on the biodistribution profile of the radiolabeled conjugates . In this regard, both the geometry and the net charge of the radionuclide–chelator complex were identified as important factors influencing the blood clearance, activity uptake in tumors and normal organs, and kidney activity retention.…”

“…5 However, the superior results reported for somatostatin antagonists 6 led to a change of paradigm in the development of bombesin analogs toward potent GRPR-antagonists. 7 We have recently investigated several constructs based on the GRPR-antagonist RM26 [(D-Phe 6 ,Sta 13 ,Leu 14 -NH 2 ) bombesin (6)(7)(8)(9)(10)(11)(12)(13)(14)]. 8,9 It is known that pharmacokinetics and targeting properties of small peptides and proteins may be altered by structural modifications.…”

Positron Emission Tomography Imaging of Prostate Cancer with Ga-68-Labeled Gastrin-Releasing Peptide Receptor Agonist BBN7–14 and Antagonist RM26

“…28 Recently, a nine-amino-acid analog of nonapeptide BBN 6–14 , d -Phe–Gln–Trp–Ala–Val–Gly–His–Sta–Leu–NH 2 (RM26) has been developed as an antagonist against GRPR and has been applied actively in preclinical studies. 29−32 Based on these observations, both BBN 7–14 and RM26 are considered high-quality candidates for further clinical translation.…”