[F]Fluorobenzyl-PEG-Glu-c(RGDyK)-bombesin[7-14]

The amphibian bombesin (BBN or BN, a peptide of 14 amino acids) is an analog of human gastrin-releasing peptide (GRP, a peptide of 27 amino acids), which binds to GRP receptors (GRPR) with high affinity and specificity (1, 2). Both GRP and BBN share an amidated C-terminus sequence homology of seven amino acids, Trp-Ala-Val-Gly-His-Leu-Met-NH. BBN-Like peptides have been shown to induce various biological responses in diverse tissues, including the central nervous system and the gastrointestinal system. They also act as potential growth factors for both normal and neoplastic tissues (3). Specific BBN receptors (BBN-R) have been identified on central nervous system and gastrointestinal tissues and on a number of tumor cell lines (4). The BBN-R superfamily includes at least four different subtypes, namely the GRPR subtype (BB2), the neuromedin B (NMB) receptor subtype (BB1), the BB3 subtype, and the BB4 subtype. The findings of GRPR overexpression in various human tumors, such as breast, prostate, lung, colon, ovarian, and pancreatic cancers, provide opportunities for tumor imaging by designing specific molecular imaging agents to target the GRPR (5, 6). Integrins are a family of heterodimeric glycoproteins on cell surfaces that mediate diverse biological events involving cell–cell and cell–matrix interactions (7). Integrins consist of an α and a β subunit and are important for cell adhesion and signal transduction. The αβ integrin is the most prominent receptor affecting tumor growth, tumor invasiveness, metastasis, tumor-induced angiogenesis, inflammation, osteoporosis, and rheumatoid arthritis (8-13). Expression of the αβ integrin is strong on tumor cells and activated endothelial cells, whereas expression is weak on resting endothelial cells and most normal tissues. Antagonists of αβ are being studied as antitumor and antiangiogenic agents, and agonists of αβ are being studied as angiogenic agents for coronary angiogenesis (12, 14, 15). A peptide sequence consisting of Arg-Gly-Asp (RGD) has been identified as a recognition motif used by extracellular matrix proteins (vitronectin, fibrinogen, laminin, and collagen) to bind to a variety of integrins, including αβ. Various ligands have been introduced for imaging of tumors and tumor angiogenesis (16). Because prostate cancer expresses both GRPR and αβ, Liu et al. (17) designed an RGD-BBN heterodimer in which BBN[7-14] and c(RGDyK) were connected with a glutamate linker (BBN on the Glu side-chain γ-carboxylate group and on the Glu side-chain α-carboxylate group). A spacer, 11-amino-3,6,9-trioxaundecanoic acid (PEG), was placed on the glutamate α-amino group of Glu-RGD-BBN to increase the hydrophilicity and to relieve the steric hindrance. -Succinimidyl-4-[F]fluorobenzoate ([F]SFB) was used to synthesize [F]FB-PEG-Glu-RGD-BBN for tumor imaging .