Synergistic effects of light-emitting probes and peptides for targeting and monitoring integrin expression

Abstract

mouse ͉ optical imaging ͉ RGD peptides ͉ tumor ͉ near-infrared A ngiogenesis, the formation of new blood vessels, is the cardinal feature of virtually all malignant tumors (1). Because of this commonality, probing tumor-induced angiogenesis and associated proteins is a viable approach to detect and treat a wide range of cancers. Angiogenesis is stimulated by integrins, a large family of transmembrane proteins that mediate dynamic linkages between extracellular adhesion molecules and the intracellular actin skeleton. Integrins are composed of two different subunits, ␣ and ␤, which are noncovalently bound into ␣␤ complexes (2-4). Particularly, the expression of ␣ v ␤ 3 integrin (ABI) in tumor cells undergoing angiogenesis and on the epithelium of tumor-induced neovasculature alters the interaction of cells with the extracellular matrix, thereby increasing tumorigenicity and invasiveness of cancers (5-9).Numerous studies have shown that ABI and more than seven other heterodimeric integrins recognize proteins and low molecular weight ligands containing RGD (arginine-glycineaspartic acid) motifs in proteins and small peptides (10). Based on structural and bioactivity considerations, cyclic RGD peptide ligands are preferentially used as delivery vehicles for molecular probes for imaging (8,(11)(12)(13) and treating (14-17) ABI-positive tumors and proliferating blood vessels. Until recently, most of the in vivo imaging studies were performed with radiopharmaceuticals because of the high sensitivity and clinical utility of nuclear imaging methods. Particularly, the use of small monoatomic radioisotopes does not generally interfere with the biodistribution and bioactivity of ligands. Despite these advantages, nuclear imaging is currently only performed in specialized centers because of regulatory, production, and handling issues associated with radiopharmaceuticals. Optical imaging is an alternative but complementary method to interrogate molecular processes in vivo and in vitro.Optical imaging for biomedical applications typically relies on activating chromophore systems with low energy radiation between 400 -and 1,500-nm wavelengths and monitoring the propagation of light in deep tissues with a charge-coupled device camera or other point source detectors (18). Molecular optical imaging of diseases with molecular probes is attractive because of the flexibility of altering the detectable spectral properties of the probes, especially in the fluorescence detection mode. The probes can be designed to target cellular and molecular processes at functional physiological concentrations. For deep-tissue imaging, molecular probes that are photoactive in the near-infrared (NIR) instead of visible wavelengths are preferred to minimize background tissue autof luorescence and light attenuation caused by absorption by intrinsic chromophores (19). In contrast to radioisotopes, the NIR antennas are usually large heteroatomic molecules that could impact the biodistribution and activity of conjugated bioactive ligands. However, previous s...

Hybrid PET-optical imaging using targeted probes

“…with three molecular probes: (i) the TAM-targeting nanoparticles described above ( 64 Cu-CLIO-VT680), (ii) a cathepsin imaging probe (excitation, 750 nm), and (iii) an α V β 3 integrin imaging probe (excitation, 635nm). The aim of this experiment was to compare the signal distribution of the PET candidate probe with that of independent markers of malignancy, such as high protease activity (25) and integrin expression (26,27). Spectrally resolved signals were observed in the tumors of all mice (Fig.…”

A systematic approach to the development of fluorescent contrast agents for optical imaging of mouse cancer models

“…Peptides targeting integrins, a family of cell surface receptors that broadly regulate tumor growth, metastasis, and angiogenesis, have been successful agents for imaging neovascular density [34], [35], [36], [37], [38], [39], [40], and [41]. Chen et al [37] and Achilefu et al [41] used an NIR peptide conjugate, arginine-glycine-aspartic acid (RGD), targeting α v β 3 integrin, to detect and image tumor xenografts and to monitor angiogenesis. Tumor uptake of the Cy5.5 RGD peptide was specifi c and could be blocked by preinjection with unlabeled RGD.…”

Integrin αvβ3-targeted gold nanoshells augment tumor vasculature-specific imaging and therapy

“…First, integrin α v β 3 is significantly upregulated on endothelium during angiogenesis and on fast-growing solid tumor cells but not on quiescent e ndothelium and normal tissues. [9][10][11][12] Second, RGD molecular probes have been developed for imaging integrin expression using different modalities, such as magnetic resonance imaging, 13 ultrasound, 14,15 optical imaging, [16][17][18][19] positron emission t omography (PET), [20][21][22] and single-photon emission computed tomography (SPECT). 22,23 Third, reagents that bind s electively to integrin α v β 3 can be designed by cyclizing peptides with selected sequences around the RGD and by synthesizing RGD mimics.…”