The neovasculature homing motif NGR: more than meets the eye

Abstract

IntroductionPhage display libraries are often used for discovering peptide sequences that interact with proteins differentially expressed in normal and pathologic tissues. 1,2 Indeed, in vivo panning of peptide-phage libraries in tumor-bearing animal models has proven useful for selecting peptides able to interact with proteins expressed within tumor-associated blood vessels and therefore to home to neoplastic tissues. 3 Among the ligands identified so far, cyclic and linear peptides containing the Asn-Gly-Arg (NGR) motif have been exploited for systemic, yet ligand-directed targeted delivery of therapeutic and imaging agents to angiogenic blood vessels, including cytotoxic drugs and cytokines, among other entities (such as viruses and nanoparticles). These findings highlight the value of NGR peptides in drug development. In this review we discuss the biochemical and biologic properties of NGR and NGR-derived compounds. Given that many native proteins contain the sequence NGR, we also address the emerging role of NGR as an unrecognized "molecular timer" due to the timedependent generation of isoAsp-Gly-Arg (isoDGR), a new integrinbinding motif that regulates a gain-of-function within the extracellular matrix protein fibronectin. 4 The discovery of the NGR motifIn vitro panning of several phage libraries against the ␣5␤1 integrin led to the selection of various RGD-containing peptides and also of the peptide NGRAHA. 5 These peptides (including NGRHA) inhibited cell attachment mediated by both ␣v␤3 and ␣v␤5 integrins. Moreover, 8 NGR-containing peptides were isolated upon screening of cyclic peptide libraries under similar experimental conditions. 6 Notably, one selected phage clone displayed the peptide CVLNGRMEC, which is similar to the sequence ALN-GREE found within the 9th type III repeat of human fibronectin. 7 Further studies based on in vitro selection of libraries on ␣v␤3, revealed several different peptides containing the NGR motif, such as NGRIPD, TNGRGP, NGRSFR, RSRNGR, NGRNTV. 8 In another line of investigation, in vivo phage-display screenings were performed to isolate tumor-homing peptides. Systemic administration of a phage library into nude mice bearing human breast carcinoma xenografts led to selection of a tumor vasculaturehoming phage carrying the sequence CNGRCVSGCAGRC. 3 Tumor homing was inhibited by co-injection with the CNGRC peptide (NGR-2C) indicating that this short cyclic loop is a functional tumor targeting peptide. Phage displaying the peptides NGRAHA or CVLNGRMEC, previously identified in vitro, also selectively localized to tumors. 3 The NGR receptor(s)In vivo phage display-based studies showed that also the peptide ACDCRGDCFC (RGD-4C), an ␣v␤3/␣v␤5 binding sequence, can bind to tumor neovasculature. 3,9 Cross-inhibition experiments of NGR-2C and RGD-4C-phage clones with synthetic RGD-4C and NGR-2C peptides showed that RGD-4C peptide does not compete the homing of NGR-2C-phage to tumors and vice versa. 3 This result suggests that NGR-2C and RGD-4C bind to distinct receptors in tumor ...

Critical role of indoleamine 2,3-dioxygenase in tumor resistance to repeated treatments with targeted IFNγ

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Cooperative effects of aminopeptidase N (CD13) expressed by nonmalignant and cancer cells within the tumor microenvironment

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CD13-positive bone marrow-derived myeloid cells promote angiogenesis, tumor growth, and metastasis

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