[18F]FB-NH-mini-PEG-E{E[c(RGDyK)]2}2 ([18F]FPRGD4) is an integrin-targeted molecular imaging agent developed for positron emission tomography (PET) of tumor vasculature and angiogenesis (1). 18F is a positron emitter with a physical half-life (t½) of 110 min.
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 (RGDfK or RGDyK) have been developed for in vivo binding to integrin αvβ3 (8). To improve the pharmacokinetics and tumor retention of the radiolabeled peptide, a dimer analog was synthesized as [18F]FB-[c(RGDyK)]2, which showed improved tumor localization and predominant renal excretion (9). Alternatively, Chen et al. (10) modified c(RGDyK) with monofunctional methoxy-polyethylene glycol (mPEG; molecular weight = 2,000 kDa) 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. (11) inserted a heterofunctional PEG (molecular weight = 3,400 kDa) molecule between the [18F]fluorobenzoyl component and the RGD peptide to produce [18F]FB-PEG-c(RGDyK) for imaging of brain tumor angiogenesis. The PEGylated [18F]FB-c(RGDyK) analog appeared to improve tumor retention and in vivo kinetics compared with [18F]FB-c(RGDyK). These improvements might be attributed to a number of possible causes that include shielding of antigenic and immunogenic epitopes, shielding receptor-mediated uptake by the reticuloendothelial systems, preventing the recognition and degradation by proteolytic enzymes, and increasing the apparent size of the peptide. Wu et al. (12) reported 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. However, the attempt of synthesis of 18F-labeled tetrameric RGDyK peptiode (18F-FRGD4) proved to be difficult and gave very low yield (<2%) because of increased molecular size and steric hindrance (1). On the basis of these observations, Wu et al. (1) inserted a mini-PEG linker to improve the labeling yield and successfully prepared [18F]FPRGD4 by radiolabeling PEGylated tetrameric RGD peptide with reasonable yield. The in vivo kinetics of this PEGylated probe was studied in three different tumor models.