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

We report herein the preclinical evaluation of new [Cu]Cu-gastrin-releasing peptide receptor (GRPR)-targeting tracers, employing the potent peptide antagonist Phe-Gln-Trp-Ala-VaI-Gly-His-Sta-Leu-NH conjugated to NOTA (in ) or NODAGA (in ) chelators via a 6-aminohexanoic acid linker. The Cu-/ metalated peptides were synthesized by reacting / with CuCl and were characterized by LC-ESI-MS and HR-ESI-MS. Cu-/ exhibited high GRPR-binding affinities with IC values <3 nM, as measured in a competition assay using the GRPR-expressing human PC-3 prostate cancer cell line and [I]I-Tyr-BBN as the competing ligand. Tracers [Cu]Cu-/ were prepared in quantitative radiochemical yield (by radio-HPLC), and their identities were confirmed by coelution with their Cu-/ standards via comparative HPLC studies. Lipophilicity was measured in 1-octanol/PBS (pH 7.4), and the negative log values (≤-1) confirmed the anticipated hydrophilic character for [Cu]Cu-/. Both tracers demonstrated excellent stability, with ≥98% remaining intact through 24 h at physiological conditions (PBS, pH 7.4, 37 °C). Biodistribution in PC-3 tumor-bearing mice demonstrated good tumor uptake (%ID/g at 4 h: 4.34 ± 0.71 for [Cu]Cu-, 3.92 ± 1.03 for [Cu]Cu-) 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]Cu-/ tracers show promise for further development as diagnostic PET imaging agents of GRPR-expressing tumors.