64Cu-1,4,7,10-Tetraazacyclododecane-N,N',N'',N'''-tetraacetic acid-E{E[c(RGDfK)2}2 (64Cu-DOTA-E{E[c(RGDfK)2}2) is an integrin-targeted molecular imaging agent developed for positron emission tomography (PET) imaging of tumor vasculature, tumor angiogenesis and osteoclasts (1). 64Cu is a positron emitter with a half-life (t½) of 12.7 h
Cellular survival, invasion, and migration control embryonic development, angiogenesis, tumor metastasis, and other physiologic processes (2, 3). Among the molecules that regulate angiogenesis are integrins which comprise a superfamily of cell adhesion proteins that form heterodimeric receptors for extracellular matrix (ECM) molecules (4, 5).These transmembrane glycoproteins consist of two noncovalently associated subunits, α and β (18 α- and 8 β-subunits in mammals), which are assembled into at least 24 α/β pairs. Several integrins, such as integrin αvβ3, have affinity for the arginine-glycine-aspartic acid (RGD) tripeptide motif, which is found in many ECM proteins. Expression of integrin αvβ3 receptors on endothelial cells is stimulated by angiogenic factors and environments. The integrin αvβ3 receptor is generally not found in normal tissue but it is strongly expressed in vessels with increased angiogenesis, such as tumor vasculature. It is significantly upregulated in certain types of tumor cells and in almost all tumor vasculature. Molecular imaging probes carrying the RGD motif that binds to the integrin αvβ3 can be used to image tumor vasculature and evaluate angiogenic response to tumor therapy (6, 7).
Various RGD peptides in both linear and cyclic forms have been developed for in vivo binding to integrin αvβ3 (8). It has been hypothesized that cyclic RGD peptide may have a faster rate of receptor binding or a slower rate of dissociation from the integrin αvβ3 than linear single-RGD peptides (1). Chen et al. (9) evaluated a cyclic RGD peptide [c(RGDyK)] labeled with 64Cu or 18F in nude mice bearing breast tumor. They used DOTA for c(RGDyK) conjugation with 64Cu. 64Cu-DOTA-c(RGDyK) showed prolonged tumor radioactivity retention but persistent liver radioactivity. Wu et al. (1) suggested that a multimeric RGD peptide with more than two repeating cyclic RGD units would further enhance the affinity of the receptor−ligand interactions through the phenomenon of a polyvalency effect. They also suggested that the increase in molecular size might also prolong circulation time and reduce tumor washout rate as well. Consequently, Wu et al. (1) developed a tetrameric RGD peptide, 64Cu-DOTA-E{E[c(RGDfK)2}2, and they showed that this PET radioligand appeared to have high integrin avidity and favorable biokinetics in nude mice bearing human glioma.