Cu-1,4,7,10-Tetraazacyclododecane-,,,-tetraacetic acid-PEGylated dimeric cyclic arginine-glycine-aspartic acid peptide

Cu-1,4,7,10-Tetraazacyclododecane-,,,-tetraacetic acid –PEGylated dimeric cyclic arginine-glycine-aspartic acid peptide {Cu-DOTA-PEG-E[c(RGDyK)]} is an integrin-targeted molecular imaging agent developed for positron emission tomography (PET) of tumor vasculature and tumor angiogenesis (1). Cu is a positron emitter with a physical half-life () of 12.8 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 αβ, have affinity for the arginine-glycine-aspartic acid (RGD) tripeptide motif, which is found in many ECM proteins. Expression of integrin αβ receptors on endothelial cells is stimulated by angiogenic factors and environments. The integrin αβ 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 (1). Molecular imaging probes carrying the RGD motif that binds to the integrin αβ 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 (RGDfK or RGDyK) have been developed for binding to integrin αβ (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 αβ than linear single RGD peptides (9). They also suggested that the increase in molecular size might prolong circulation time and reduce tumor washout rate. Chen et al. (10) prepared a cyclic RGD d-Tyr analog [Cu-DOTA-c(RGDyK)] and showed the cyclic peptide had prolonged tumor radioactivity retention but persistent liver radioactivity. To improve the pharmacokinetics and tumor retention of the radiolabeled peptide, Chen et al. (11) modified c(RGDyK) with monofunctional methoxy-polyethylene glycol (mPEG; molecular weight = 2,000) and showed that the modified PEGylated RGD peptide had faster blood clearance, lower kidney uptake, and prolonged tumor uptake. Using the same strategy, Chen et al. (12) inserted a heterobifunctional PEG linker (molecular weight = 3,400) between DOTA and c(RGDyK) to produce Cu-DOTA-PEG-c(RGDyK). The PEG moiety appeared to improve the kinetics of this PET radioligand. Chen et al. (13) also observed that a dimeric RGD peptide E[c(RGDyK)] had higher binding affinity than the monomeric analog. Subsequently, the same research group also reported the successful use of a heterobifunctional PEG to prepare Cu-lableled PEGylated E[c(RGDyK)] peptide (1).