Evaluation of metabolically stabilized 18F-labeled bombesin analogs for imaging GRP receptors in prostate cancer

Objectives: According to GLOBOSCAN 2008 the worldwide prostate cancer incidence rates will increase to about 1.7 million new cases per year in 2030. In recent years, bombesin and bombesin analogs have attracted much attention as high affinity and selectivity ligands for the gastrin-releasing peptide (GRP) receptor. The GRP receptor was found to be overexpressed and implicated in a variety of human tumors including prostate cancer. Radiolabeled bombesin and bombesin analogues belong to an interesting class of new diagnostic probes for molecular imaging of GRP receptor-expressing prostate cancer. This study describes the synthesis and radiopharmacological evaluation of a high affinity and metabolically stabilized 18F-labeled bombesin analog for PET imaging of GRP receptors in prostate cancer. Methods: Three modified bombesin analogs bearing an aminovaleric (BBN-1, BBN-2), or an aminooctanoic acid linker (BBN-3) were tested in a competitive binding assay against 125I-[Tyr4]-BBN for their binding to the GRP receptor. A calcium release assay in human prostate cancer cells (PC3) was performed to determine agonistic or antagonistic behaviour. The derivative with the highest affinity to GRP (BBN-2) was selected to be conjugated with the prosthetic labeling agent N-succinimidyl-4-[18F]fluorobenzoate ([18F]SFB) in borate buffer (pH=8.2) for 30 min at 40°C to synthesize the desired 18F-labeled bombesin analog. Tumor-targeting of [18F]BBN-2 was evaluated in PC3 tumor-bearing male nude mice with biodistribution experiments (mean ± SD) and dynamic small animal PET studies. Results: The competitive binding assay revealed IC50-values between 8.7 and 16.7 nM for BBN-1, BBN-2 and BBN-3 against 125I-[Tyr4]-BBN versus 3.0 nM for I-[Tyr4]-BBN. All three stabilized bombesin analogs are GRP receptor antagonists. 18F-labeled [18F]BBN-2 was prepared in 30% radiochemical yield (based upon [18F]SFB) within 80 min including HPLC purification, evaporation of HPLC eluent and formulation in 0.9%