Evaluation of a novel GRPR antagonist for prostate cancer PET imaging: [⁶⁴Cu]-DOTHA₂-PEG-RM26

Gastrin releasing peptide receptors (GRPRs) are significantly over-expressed on a large proportion of prostate cancers making them prime candidates for receptor-mediated nuclear imaging by PET. Recently, we synthesized a novel bifunctional chelator (BFC) bearing hydroxamic acid arms (DOTHA₂). Here we investigated the potential of a novel DOTHA₂-conjugated, ⁶⁴Cu-radiolabeled GRPR peptide antagonist, [D-Phe⁶-Sta¹³-Leu¹⁴-NH₂]bombesin(6-14) (DOTHA₂-PEG-RM26) to visualize prostate tumors by PET imaging.

DOTHA₂-PEG-RM26 was conveniently and efficiently assembled on solid support. The compound was radiolabeled with ⁶⁴Cu and its affinity, stability, cellular uptake on PC3 prostate cancer cells were evaluated. The in vitro and in vivo behavior of [⁶⁴Cu]DOTHA₂-PEG-RM26 was examined by PET imaging using human PC3 prostate cancer xenografts and its behavior was compared to that of the analogous [⁶⁴Cu]NOTA-PEG-RM26.

The inhibition constant of ^natCu-DOTHA₂-PEG-RM26 was in the low nanomolar range (0.68±0.19 nM). The [⁶⁴Cu]DOTHA₂-PEG-RM26 conjugate was prepared with a labeling yield >95% and molar activity of 56±3 GBq/μmol after a 5-min room temperature labeling. [⁶⁴Cu]-DOTHA₂-PEG-RM26 demonstrated rapid blood and renal clearance as well as a high tumor uptake. Small animal PET images confirmed high and specific uptake in PC3 tumor. Both [⁶⁴Cu]-DOTHA₂-PEG-RM26 and [⁶⁴Cu]-NOTA-PEG-RM26 displayed similar tumor and normal tissue uptakes at early time point post injection.

[⁶⁴Cu]-DOTHA₂-PEG-RM26 allows visualization of prostate tumors by PET imaging. DOTHA₂ enables fast ⁶⁴Cu chelation under mild condition, and as such could be used advantageously for the development of other ⁶⁴Cu-labeled peptide-derived PET tracers.