Dual-Modality Imaging of Prostate Cancer with a Fluorescent and Radiogallium-Labeled Gastrin-Releasing Peptide Receptor Antagonist

Methods: We designed and synthesized an IRDye 650 and DOTA-conjugated GRPr antagonist, HZ220 (DOTA-Lys(IRDye 650)-PEG-[D-Phe, Sta]-BN(6-14)NH), by reacting DOTA-Lys-PEG-[D-Phe, Sta]-BN(6-14)NH (HZ219) with IRDye 650 N-hydroxysuccinimide (NHS) ester. Receptor-specific binding of gallium-labeled HZ220 was characterized in PC-3 prostate cancer cells (PC-3), and tumor uptake in mice was imaged with PET/CT and fluorescence imaging. Receptor binding affinity, in vivo tumor uptake, and biodistribution were compared with the GRPr antagonists HZ219, DOTA-PEG-[D-Phe, Sta]-BN(6-14)NH (DOTA-AR), and DOTA-(4-amino-1-carboxymethyl-piperidine)-[D-Phe, Sta]-BN(6-14)NH (DOTA-RM2).

Results: After hydrophilic-lipophilic balance cartridge purification, Ga-HZ220 was obtained with a radiochemical yield of 56% ± 8% (non-decay-corrected), and the radiochemical purity was greater than 95%. Ga-HZ220 had a lower affinity for GRPr (inhibitory concentration of 50% [IC], 21.4 ± 7.4 nM) than Ga-DOTA-AR (IC, 0.48 ± 0.18 nM) or Ga-HZ219 (IC, 0.69 ± 0.18 nM). Nevertheless, Ga-HZ220 had an in vivo tumor accumulation similar to Ga-DOTA-AR (4.63 ± 0.31 vs. 4.07 ± 0.29 percentage injected activity per mL [%IA/mL] at 1 h after injection) but lower than that of Ga-DOTA-RM2 (10.4 ± 0.4 %IA/mL). The tumor uptake of Ga-HZ220 was blocked significantly with an excessive amount of GRP antagonists. IVIS spectrum imaging also visualized PC-3 xenografts in vivo and ex vivo with a high-contrast ratio. Autoradiography and fluorescent-based microscopic imaging with Ga-HZ220 consistently colocated the expression of GRPr. Ga-HZ220 displayed a higher kidney uptake than both Ga-DOTA-AR and Ga-DOTA-RM2 (16.9 ± 6.5 vs. 4.48 ± 1.63 vs. 5.01 ± 2.29 %IA/mL).

Conclusion: Ga-HZ220 is a promising bimodal ligand for noninvasive PET imaging and intraoperative optical imaging of GRPr-expressing malignancies. Bimodal nuclear/fluorescence imaging may not only improve cancer detection and guide surgical resections, but also improve our understanding of the uptake of GRPr ligands on the cellular level.