Synthesis and radiopharmacological evaluation of a high-affinity and metabolically stabilized 18F-labeled bombesin analogue for molecular imaging of gastrin-releasing peptide receptor-expressing prostate cancer

Introduction: Bombesin (BBN) and BBN analogues have attracted much attention as high-affinity ligands for selective targeting of the gastrin-releasing peptide (GRP) receptor. GRP receptors are overexpressed in a variety of human cancers including prostate cancer. Radiolabeled BBN derivatives are promising diagnostic probes for molecular imaging of GRP receptor-expressing prostate cancer. This study describes the synthesis and radiopharmacological evaluation of various metabolically stabilized fluorobenzoylated bombesin analogues (BBN-1, BBN-2, BBN-3). Methods: Three fluorobenzoylated BBN analogues containing 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 potency to the GRP receptor. Intracellular calcium release in human prostate cancer cells (PC3) was measured to determine agonistic or antagonistic profiles of fluorobenzoylated BBN derivatives. Bombesin derivative BBN-2 displayed the highest inhibitory potency toward GRP receptor (IC50 = 8.7 ± 2.2 nM) and was subsequently selected for radiolabeling with fluorine-18 (18F) through acylation with N-succinimidyl-4-[18F]fluorobenzoate ([18F]SFB). The radiopharmacological profile of 18F-labeled bombesin [18F]BBN-2 was evaluated in PC3 tumor-bearing NMRI nude mice involving metabolic stability studies, biodistribution experiments and dynamic small-animal PET studies. Results: All fluorobenzoylated BBN derivatives displayed high inhibitory potency toward the GRP receptor (IC50 = 8.7–16.7 nM), and all compounds exhibited antagonistic profiles as determined in an intracellular calcium release assay. The 18F-labeled BBN analogue [18F]BBN-2 was obtained in 30% decay-corrected radiochemical yield with high radiochemical purity N95% after semi-preparative HPLC purification. [18F]BBN-2 showed