Evolution of Bombesin Conjugates for Targeted PET Imaging of Tumors

Abstract

Bombesin receptors are under intense investigation as molecular targets since they are overexpressed in several prevalent solid tumors. We rationally designed and synthesized a series of modified bombesin (BN) peptide analogs to study the influence of charge and spacers at the N-terminus, as well as amino acid substitutions, on both receptor binding affinity and pharmacokinetics. This enabled development of a novel 64/67Cu-labeled BN peptide for PET imaging and targeted radiotherapy of BN receptor-positive tumors. Our results show that N-terminally positively charged peptide ligands had significantly higher affinity to human gastrin releasing peptide receptor (GRPr) than negatively charged or uncharged ligands (IC50: 3.2±0.5 vs 26.3±3.5 vs 41.5±2.5 nM). The replacement of Nle14 by Met, and deletion of D-Tyr6, further resulted in 8-fold higher affinity. Contrary to significant changes to human GRPr binding, modifications at the N-terminal and at the 6th, 11th, and 14th position of BN induced only slight influences on affinity to mouse GRPr. [CuII]-CPTA-[βAla11] BN(7–14) ([CuII]-BZH7) showed the highest internalization rate into PC-3 cells with relatively slow efflux because of its subnanomolar affinity to GRPr. Interestingly, [64/67Cu]-BZH7 also displayed similar affinities to the other 2 human BN receptor subtypes. In vivo studies showed that [64/67Cu]-BZH7 had a high accumulation in PC-3 xenografts and allowed for clear-cut visualization of the tumor in PET imaging. In addition, a CPTA-glycine derivative, forming a hippurane-type spacer, enhanced kidney clearance of the radiotracer. These data indicate that the species variation of BN receptor plays an important role in screening radiolabeled BN. As well, the positive charge from the metallated complex at the N-terminal significantly increases affinity to human GRPr. Application of these observations enabled the novel ligand [64/67Cu]-BZH7 to clearly visualize PC-3 tumors in vivo. This study provides a strong starting point for optimizing radiopeptides for targeting carcinomas that express any of the BN receptor subtypes.

N-Terminal Modifications Improve the Receptor Affinity and Pharmacokinetics of Radiolabeled Peptidic Gastrin-Releasing Peptide Receptor Antagonists: Examples of 68Ga- and 64Cu-Labeled Peptides for PET Imaging

“…charge (10,16). nat Ga-NOTA-MJ9 with a charge of 11 at the N terminus has the highest affinity, followed by nat Cu-NOTA-MJ9 and nat Ga-NODAGA-MJ9, which are neutral, whereas nat Cu-NODAGA-MJ9 (negatively charged) has a 10-fold-lower affinity than nat Ga-NOTA-MJ9.…”

“…Our aim was to develop antagonistic peptidebased radiotracers targeting the GRPr for clinical translation. Various radiolabeled GRPr agonists have been synthesized, evaluated, and applied in nuclear oncology (4,(16)(17)(18)(19)(20)(21)(22)(23). Innovative developments using agonists hybridized with other targeting units such as a v b 3 integrins or hypoxia-targeting agents such as 2-nitroimidazoles (25).…”

Evaluation of a 64Cu-labeled 1,4,7-triazacyclononane, 1-glutaric acid-4,7 acetic acid (NODAGA)-galactose-bombesin analogue as a PET imaging probe in a gastrin-releasing peptide receptor-expressing prostate cancer xenograft model

“…Various BBn analogues have been labeled with radiometals and suitable chelators and used for positron emission tomography (Pet) imaging in grPr-positive tumors (13)(14)(15). 99m tc-labeled BBn analogues have been synthesized with diaminedithiol (DaDt) and the dithiadiphosphine framework (P 2 S 2 -cooH) as chelators.…”

Dosimetry and First Clinical Evaluation of the New 18F-Radiolabeled Bombesin Analogue BAY 864367 in Patients with Prostate Cancer

“…PET/CT is a highly sensitive hybrid imaging modality that allows examination of the whole body (2). The poor performance of the most commonly used tracer in cancer, 18 F-labeled glucose ( 18 F-FDG), for the detection of PCa is due to the relatively slow growth of well-differentiated PCa and consequently low glucose metabolism (3,4).…”