Synthesis and Evaluation of ⁶⁸Ga- and ¹⁷⁷Lu-Labeled [Pro¹⁴]bombesin(8–14) Derivatives for Detection and Radioligand Therapy of Gastrin-Releasing Peptide Receptor-Expressing Cancer

The gastrin-releasing peptide receptor (GRPR) is overexpressed in a variety of cancers and represents a promising target for diagnosis and therapy. However, the extremely high accumulation in the pancreas observed for most of the clinically evaluated GRPR-targeted radiopharmaceuticals could limit their applications. In this study, we synthesized one GRPR antagonist (ProBOMB5) and two GRPR agonists (LW02056 and LW02057) by replacing the 4-thiazolidinecarboxylic acid (Thz¹⁴) residue in our previously reported GRPR-targeted tracers with Pro¹⁴. The ⁶⁸Ga and ¹⁷⁷Lu labeling were conducted in HEPES (2 M, pH 5.0) buffer and acetate (0.1 M, pH 4.5) buffer, respectively, and the radiolabeled products were obtained in a 24-57% decay-corrected radiochemical yield and >92% radiochemical purity. The binding affinities (<i>K</i>_i) of Ga-ProBOMB5, Ga-LW02056, Ga-LW02057, and Lu-ProBOMB5 were measured via <i>in vitro</i> competition binding assays and were 12.2 ± 1.89, 14.7 ± 4.81, 13.8 ± 2.24, and 13.6 ± 0.25 nM, respectively. The PET imaging and <i>ex vivo</i> biodistribution studies were conducted in PC-3 tumor-bearing mice at 1 h post injection. [⁶⁸Ga]Ga-ProBOMB5, [⁶⁸Ga]Ga-LW02056, and [⁶⁸Ga]Ga-LW02057 enabled clear tumor visualization in PET images. The tumor uptake values of [⁶⁸Ga]Ga-ProBOMB5, [⁶⁸Ga]Ga-LW02056, and [⁶⁸Ga]Ga-LW02057 were 12.4 ± 1.35, 8.93 ± 1.96, and 7.64 ± 0.55%ID/g, respectively, and their average pancreas uptake values were minimal (0.60-1.37%ID/g). Longitudinal SPECT imaging and <i>ex vivo</i> biodistribution studies were also conducted for [¹⁷⁷Lu]Lu-ProBOMB5 and clinically validated [¹⁷⁷Lu]Lu-RM2. Despite comparable tumor uptake at 1 h post injection ([¹⁷⁷Lu]Lu-ProBOMB5:8.09 ± 1.70%ID/g; [¹⁷⁷Lu]Lu-RM2:7.73 ± 0.96%ID/g), a faster clearance from PC-3 tumor xenografts was observed for [¹⁷⁷Lu]Lu-ProBOMB5, leading to a lower radiation-absorbed dose delivered to tumors. Our data demonstrate that [⁶⁸Ga]Ga-ProBOMB5 is a promising tracer for clinical translation for detecting GRPR-expressing tumor lesions. However, further optimizations are needed for [¹⁷⁷Lu]Lu-ProBOMB5 to prolong tumor retention for therapeutic applications.