AlexaFluor 700–Labeled regioselectively addressable functionalized template-[cyclo-(Arg-Gly-Asp-d-Phe-Lys)]

AlexaFluor 700 (A700)–Labeled regioselectively addressable functionalized template (RAFT)-[cyclo-(Arg-Gly-Asp-d-Phe-Lys)], abbreviated as A700-RAFT-RGD, is a tetrameric Arg-Gly-Asp (RGD)-based peptide conjugate synthesized by Briat et al. for tumor imaging by targeting integrin αβ (1). Angiogenesis is a process of neovascular development and growth from preexisting vessels. Different from the normal blood vessel system, the neovasculature is chaotic and irregular in tumors with abnormal expression of diverse vascular surface biomarkers. These biomarkers have fewer kinetic compartments that must be crossed for intravenously administered agents to reach them (2, 3). Integrin αβ is one of these biomarkers, and it has been intensively investigated as a target for imaging and antiangiogenic therapy. Integrin αβ is minimally expressed in normal blood vessels but is significantly overexpressed in newly sprouting vasculature in tumors (4, 5). Small peptides are one group of ligands frequently used for targeting purposes, such as RGD, Asp-Gly-Arg, His-Gly-Phe, and Arg-Arg-Leu (3). The RGD tripeptide sequence is an adhesive protein recognition site, presenting in the extracellular matrix and blood. Integrin αβ binds extracellular matrix proteins through the exposed RGD tripeptide sequence. In general, RGD peptides are less selective (binding with 8 of the 24 integrins), degrade rapidly , and have a relatively low binding affinity (3, 6). Development of multivalent RGD peptides is one approach to improve their properties. Boturyn et al. synthesized a RAFT-RGD platform based on a cyclic decapeptide scaffold that incorporates and presents in a spatially controlled manner two independent functional domains: a clustered ligand domain for integrin recognition and cell targeting, and a labeling domain for detection and characterization of this binding (7). This platform has been labeled with Cy5, Tc, and Cu and has been shown to efficiently accumulate in tumors in animal models (8-11). Although the results are promising, approximately 30%–50% of the total dose of the labeled RAFT-RGD has been found to remain trapped in the kidneys several days after injection, which is at least partially due to the tetramerization of RGD peptides (8, 12, 13). Although peptides can be reabsorbed, endocytosed, and degraded by the lysosomes of proximal tubular cells, radiometal-chelated amino acids may be retained in the tubular cells, leading to radiation-induced toxicity and a weak imaging contrast for tumors surrounding the kidneys. To reduce the renal accumulation of fluorescent and radiolabeled RAFT-RGD, Briat et al. evaluated the effect of Gelofusine on the renal reabsorption of these agents (1). Gelofusine is a gelatin-based plasma expander that consists of succinylated bovine gelatin molecules, a mixture of collagen-derived peptides. Infusion of Gelofusine has been reported to increase the renal excretion of megalin ligands and to decrease the retention of radiolabeled compounds in the cortical proximal tubules (14-16). The results by Briat et al. showed that Gelofusine significantly reduced the renal retention of labeled RAFT-RGD, while it increased the ratio of tumor to healthy tissue (1). The data obtained with In-DOTA-RAFT-RGD were summarized in the chapter on In-DOTA-RAFT-RGD in MICAD. This chapter summarizes the data obtained with A700-RAFT-RGD and Gelofusine.