Cu-1,4,7-Triazacyclononane-1,4-diacetic acid-c(RGDyK)-Glu-6-aminohexanoic acid-bombesin[7-14]NH

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) that binds to GRP receptor (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 (CNS) and the gastrointestinal (GI) system. They also act as potential growth factors for both normal and neoplastic tissues (3). Specific BBN receptors (BBN-Rs) have been identified on CNS and GI 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 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. 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 (14). Because breast and prostate cancers express both GRPR and αβ, Jackson et al. (15) designed an RGD-BBN heterodimer in which c(RGDyK) and BBN[7-14]NH are connected with a glutamate linker (BBN on the Glu side chain γ-carboxylate group and RGD on the Glu side chain α-carboxylate group). A spacer, 6-aminohexanoic acid (6-Ahx), was put onto the glutamate α-amino group of RGD-Glu-BBN to increase the hydrophilicity and to relieve the steric hindrance. 1,4,7-Triazacyclononane-1,4-7-triacetic acid (NOTA) was used as a bifunctional chelator for labeling RGD-BBN to form Cu-NO2A-RGD-Glu-6-Ahx-BBN for use in positron emission tomography (PET) imaging of αβ and GRPR in nude mice bearing human tumors.