[¹⁸F]Fluorobenzyl-bombesin[7-14]-c(RGDyK)

[[¹⁸F]FB-BBN-c(RGDyK)]

Background

[PubMed]

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 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-NH₂. 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-R) 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 (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 α_vβ₃ 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 α_vβ₃ integrin is strong on tumor cells and activated endothelial cells, whereas expression is weak on resting endothelial cells and most normal tissues. Antagonists of α_vβ₃ are being studied as antitumor and antiangiogenic agents, and agonists of α_vβ₃ 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 α_vβ₃. Various ligands have been introduced for imaging of tumors and tumor angiogenesis (16).

Because prostate cancer expresses both GRPR and α_vβ₃, Li et al. (17) designed a BBN-RGD heterodimer in which BBN[7-14] and c(RGDyK) were connected with a glutamate linker. N-Succinimidyl-4-[¹⁸F]fluorobenzoate ([¹⁸F]SFB) was used to synthesize [¹⁸F]FB-BBN-RGD for tumor targeting.

Synthesis

[PubMed]

[¹⁸F]SFB was added to a solution of 100 nmol BBN-RGD (17). The mixture was heated for 30 min at 60°C. The product, [¹⁸F]FB-BBN-RGD, was purified with high-performance liquid chromatography with a decay-corrected yield of 12.0 ± 0.7% (n = 4) based on [¹⁸F]SFB and a radiochemical purity of >99%. [¹⁸F]FB-BBN and [¹⁸F]FB-RGD were synthesized using the same procedure. The specific activities of [¹⁸F]FB-BBN-RGD, [¹⁸F]FB-BBN, and [¹⁸F]FB-RGD were estimated to be ~100 MBq/nmol (2.7 mCi/nmol). The log P values were -0.92, 1.49, and -1.75 for [¹⁸F]FB-BBN-RGD, [¹⁸F]FB-BBN, and [¹⁸F]FB-RGD, respectively.

In Vitro Studies: Testing in Cells and Tissues

[PubMed]

Li et al. (17) performed in vitro inhibition studies of RGD, BBN-RGD, and FB-BBN-RGD in cultured U87MG cells with ¹²⁵I-echistatin for α_vβ₃ integrin binding. The 50% inhibition concentration (IC₅₀) values were 202, 428, and 282 nM, respectively. In vitro inhibition studies of BBN, BBN-RGD, and FB-BBN-RGD were also performed in cultured PC-3 cells with ¹²⁵I-BBN for GRPR binding. The IC₅₀ values were 20.7, 35.7, and 32 nM, respectively. Therefore, the BBN-RGD heterodimer exhibited comparable binding affinities for α_vβ₃ integrin with GRPR as the corresponding monomer. PC-3 cells express a high level of GRPR and a moderate level of α_vβ₃ integrin. PC-3 cells exhibited a 7% and 5% uptake of incubation dose of [¹⁸F]FB-BBN and [¹⁸F]FB-BBN-RGD within 30 min of incubation, respectively. On the other hand, PC-3 cells had low uptake of [¹⁸F]FB-RGD (<0.5%).

Animal Studies

Human Studies

[PubMed]

No publication is currently available.

NIH Support

R01 CA119053, R21 CA121842, R21 CA102123, P50 CA114747, U54 CA119367, R24 CA93862