Theranostic Perspectives in Prostate Cancer with the Gastrin Releasing Peptide Receptor Antagonist NeoBOMB1: Preclinical and First Clinical Results

We recently introduced the potent gastrin-releasing peptide receptor (GRPR) antagonist Ga-68-SB3 (Ga-68-DOTA-p-aminomethylaniline-diglycolic acid-DPhe-Gln-Trp-Ala-Val-Gly-His-Leu-NHEt), showing excellent tumor localizing efficacy in animal models and in patients. By replacement of the C-terminal Leu(13)-Met(14)-NH2 dipeptide of SB3 by Sta(13)-Leu(14)-NH2, the novel GRPR antagonist NeoBOMB1 was generated and labeled with different radiometals for theranostic use. We herein report on the biologic profile of resulting 67168Ga-, In-111-, and Lu-177-NeoBOMB1 radioligands in GRPR-expressing cells and mouse models. The first evidence of prostate cancer lesion visualization in men using 68Ga-NeoBOMB1 and PET/CT is also presented. Methods: NeoBOMB1 was radiolabeled with 67168Ga, In-111, and Lu-177 according to published protocols. The respective metalated species natGa-, natin-, and natLu-NeoBOMB1 were also synthesized and used in competition binding experiments against [1261-Tyr4]BBN in GRPRpositive PC-3 cell membranes. Internalization of Ga-67-, (111) In-, and Lu-177-NeoBOMB1 radioligands was studied in PC-3 cells at 37 C, and their metabolic stability in peripheral mouse blood was determined by high-performance liquid chromatography analysis of blood samples. Biodistribution was performed by injecting a 67Ga-, or Lu-177-NeoBOMB1 bolus (74, 74, or 370 kBq, respectively, 100 mu t, 10 pmol total peptide 40 nmol Tyro-BBN: for in vivo GRPR blockade) in severe combined immunodeficiency mice bearing PC-3 xenografts. PET/CT images with 68Ga-NeoBOMB1 were acquired in prostate cancer patients. Results: NeoBOMB1 and natGa-, natIn-, and natLuNeoBOMB1 bound to GRPR with high affinity (half maximal inhibitory concentration, 1-2 nM). Ga-67-, In-111-, and Lu-177-NeoBOMB1 specifically and strongly bound on the cell membrane of PC-3 cells displaying low internalization, as expected for receptor antagonists. They showed excellent metabolic stability in peripheral mouse blood (>95% intact at 5 min after injection). After injection in mice, all 3 (Ga-67-, and Lu-177-NeoBOMB1) showed comparably high and GRPR-specific uptake in the PC-3 xenografts (e.g., 30.6 3.9, 28.6 6.0, and >35 percentage injected dose per gram at 4 h after injection, respectively), clearing from background predominantly via the kidneys. During a translational study in prostate cancer patients, 68Ga-NeoBOMB1 rapidly localized in pathologic lesions, achieving high-contrast imaging. Conclusion: The GRPR antagonist radioligands Ga-67-, In-111-, and Lu-177-NeoBOMB1, independent of the radiometal applied, have shown comparable behavior in prostate cancer models, in favor of future theranostic use in GRPR-positive cancer patients. Such translational prospects were further supported by the successful visualization of prostate cancer lesions in men using Ga-68-NeoBOMB1 and PET/CT.