Tc-Regioselectively addressable functionalized template-[cyclo-(Arg-Gly-Asp-d-Phe-Lys)]

Integrins are a family of heterodimeric glycoproteins on cell surfaces that mediate diverse biological events involving cell–cell and cell–matrix interactions (1). 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 (2-7). 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 anti-tumor and anti-angiogenic agents, and agonists of αβ are being studied as angiogenic agents for coronary angiogenesis (6, 8, 9). 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 radiolabeled antagonists have been introduced to image tumors and tumor angiogenesis (10). Most of the cyclic RGD peptides are composed of five amino acids. Haubner et al. (11) reported that various cyclic RGD peptides exhibit selective inhibition of binding to the αβ integrin (50% inhibition concentration (IC) = 7–40 nM) but not to the αβ (IC = 600–4,000 nM) or αβ (IC = 700–5,000 nM) integrins. Various radiolabeled cyclic RGD peptides have been found to have high accumulation in tumors in nude mice (12). Hydrazinonicotinic acid (HYNIC) is a coupling agent for Tc-labeling of peptides that can achieve high specific activities without affecting the receptor-binding ability of the amino acid sequence. Tc is bound to the hydrazine group, and other coordination sites could be occupied by one or more coligands. Liu et al. (13) reported the success of radiolabeling the cyclo(Arg-Gly-Asp-d-Phe-Lys) (c(RGDfK)) tetramer linked by glutamic acid and conjugated with HYNIC, which displayed high tumor accumulation in nude mice bearing human tumor xenografts. Boturyn et al. (14) generated a versatile molecular regioselectively addressable functionalized template (RAFT) platform with a cyclic decapeptide [c(-Lys(Boc)-Lys(Alloc)-Lys(Boc)-Pro-Gly-Lys(Boc)-Lys(Alloc)-Lys(Boc)-Pro-Gly-)] that had two attachment sides. The upper side is linked to four copies of the c(RGDfK) peptide for integrin αβ targeting, and the bottom side is linked to Tc for single-photon emission computed tomography (SPECT) imaging or other labels for other imaging modalities. Sancey et al. (15) reported that Tc-RAFT-c(-RGDfK-) efficiently accumulated into tumors in mice. Tc-RAFT-c(-RGDfK-) is an integrin-targeted molecular imaging agent developed for imaging of tumor vasculature and tumor angiogenesis.