N-[2-(4-[18F]Fluorobenzamido)ethyl]maleimide-sulfhydryl-cyclic-arginine-glycine-aspartic acid peptide ([18F]FBEM-SRGD) 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 physiological 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. 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). Chen et al. (9) evaluated 64Cu-labeled and 18F-labeled cyclic RGD peptide [c(RGDyK)] monomers in nude mice bearing breast tumors. [18F]FB-c(RGDyK) showed high tumor accumulation but also rapid tumor washout with unfavorable biliary excretion (10). 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 (11). Radiofluorination can generally be achieved through the functional groups of amino, carboxylic acid, and sulfhydryl. Labeling of RGD peptide with 18F in most fluorinated peptide studies used 18F-synthons such as N-succinimidyl 4-[18F]fluorobenzoate to form a stable amide bond by reacting with primary amino groups of RGD peptides at the N terminus or the lysine side chain (1). One of the main concerns with this approach is the potential interference with the biological activities of these peptides. Cai et al. (1) suggested that 18F-labeling of RGD peptides via the carboxylic acid group at the C terminus or internal glutamic/aspartic acid side chain might minimize the potential interference. They reported the use of a thiol-reactive 18F-synthon, 18F-FBEM, as the prosthetic group for labeling a sulfhydryl-functionalized c(RGDyK) monomeric peptide (SRGD) and a E[c(RGDyK)]2 dimeric peptide (SRGD2). The maleimide group in 18F-FBEM allows for a thiol-specific Michael addition reaction.