Preclinical Evaluation of Novel 64Cu-Labeled Gastrin-Releasing Peptide Receptor Bioconjugates for PET Imaging of Prostate Cancer

We report herein the preclinical evaluation of new [Cu-64] Cu-gastrin-releasing peptide receptor (GRPR) -targeting tracers, employing the potent peptide antagonist (D)Phe-Gln-TrpAla-VaI-Gly-His-Sta-Leu-NH2 conjugated to NOTA (in 1) or NODAGA (in 2) chelators via a 6-aminohexanoic acid linker. The Cu-1/2 metalated peptides were synthesized by reacting 1/2 with CuCl2 and were characterized by LC-ESI-MS and HR-ESI-MS. Cu1/2 exhibited high GRPR-binding affinities with IC50 values <3nM as measured in a competition assay using the GRPR-expressing human PC-3 prostate cancer cell line and[I-125] I-Tyr(4)-BBN as the competing ligand. Tracers [Cu-64]Cu-1 /2 were prepared in quantitative radiochemical yield (by radio-HPLC), and their identities were confirmed by coelution with their Cu-1/2 standards via comparative HPLC studies. Lipophilicity was measured in 1-octanol/PBS (pH 7.4), and the negative log D-7.4 values (<=-1) confirmed the anticipated hydrophilic character for [Cu-64]Cu-1/2. Both tracers demonstrated excellent in vitro stability, with >= 98% remaining intact through 24 h at physiological conditions (PBS, pH 7.4, 37 degrees C). Biodistribution in PC-3 tumor-bearing mice demonstrated good tumor uptake (%ID/g at 4 h: 4.34 +/- 0.71 for [Cu-64]Cu-1, 3.92 +/- 1.03 for [Cu-64]Cu-2) and rapid renal clearance (>= 87% ID at 4 h). Tumor uptake was receptor-mediated, as verified by parallel GRPR-blocking studies. Small-animal PET/CT imaging studies validated the biodistribution data. These preclinical data support that the [Cu-64]Cu-1/2 tracers show promise for further development as diagnostic PET imaging agents of GRPR-expressing tumors.

Automated summary

This study presents the preclinical evaluation of novel [Cu-64]Cu-GRPR-targeting tracers, demonstrating high binding affinities and favorable biodistribution for potential PET imaging of GRPR-expressing tumors. The synthesized tracers showed excellent stability and receptor-mediated tumor uptake, as validated by imaging studies, supporting their further development as diagnostic agents.