Synthesis and Evaluation of 68Ga- and 177Lu-Labeled [diF-Pro14]Bombesin(6-14) Analogs for Detection and Radioligand Therapy of Gastrin-Releasing Peptide Receptor-Expressing Cancer

Background/Objectives: Overexpressed in various solid tumors, the gastrin-releasing peptide receptor (GRPR) is a promising target for cancer diagnosis and therapy. However, the high pancreas uptake of the current clinically evaluated GRPR-targeted radiopharmaceuticals limits their applications. In this study, we replaced the Pro(14) residue in our previously reported GRPR-targeted LW02056 and ProBOMB5 with 4,4-difluoroproline (diF-Pro) to obtain an agonist LW02060 (DOTA-Pip-[D-Phe(6),Tle(10),NMe-His(12),diF-Pro(14)]Bombesin(6-14)) and an antagonist LW02080 (DOTA-Pip-[D-Phe(6),NMe-Gly(11),Leu(13)(psi)diF-Pro(14)]Bombesin(6-14)), respectively. Methods/Results: The binding affinities (K-i) of Ga-LW02060, Ga-LW02080, Lu-LW02060, and Lu-LW02080 were measured by in vitro competition binding assays using PC-3 cells and were found to be 5.57 +/- 2.47, 21.7 +/- 6.69, 8.00 +/- 2.61, and 32.1 +/- 8.14 nM, respectively. The Ga-68- and Lu-177-labeled ligands were obtained in 36-75% decay-corrected radiochemical yields with >95% radiochemical purity. PET imaging, SPECT imaging, and ex vivo biodistribution studies were conducted in PC-3 tumor-bearing mice. Both [Ga-68]Ga-LW02060 and [Ga-68]Ga-LW02080 enabled clear tumor visualization in PET images at 1 h post-injection (pi). Tumor uptake values of [Ga-68]Ga-LW02060 and [Ga-68]Ga-LW02080 at 1 h pi were 16.8 +/- 2.70 and 7.36 +/- 1.33 %ID/g, respectively, while their pancreas uptake values were 3.12 +/- 0.89 and 0.38 +/- 0.04 %ID/g, respectively. Compared to [Lu-177]Lu-LW02080, [Lu-177]Lu-LW02060 showed higher tumor uptake at all time points (1, 4, 24, 72, and 120 h pi). However, fast tumor clearance was observed for both [Lu-177]Lu-LW02060 and [Lu-177]Lu-LW02080. Conclusions: Our data demonstrate that [Ga-68]Ga-LW02060 is promising for clinical translation for the detection of GRPR-expressing tumor lesions. However, further optimizations are needed for [Lu-177]Lu-LW02060 and [Lu-177]Lu-LW02080 to prolong tumor retention for therapeutic applications.