Development of NGR-based anti-cancer agents for targeted therapeutics and imaging

Besides the common issue of drug-resistance, the conventional approaches for cancer diagnostics and treatment are constantly challenged by poor selectivity and limited access to neoplastic cells, which not only lead to the dose-limiting effect on the tumor region, but also bring side-effects to healthy cells/tissues. In recent years, a novel strategy has arisen to target the vasculature of tumors for drugdelivery and molecular imaging, based on the success of anti-angiogenic therapy. In addition to being easily accessible, the endothelial cells of tumor vasculature are also genetically stable and thus do not develop drug-resistance, making them ideal targets for chemotherapeutics and biomedical imaging. Among various ligands identified so far, the Asn-Gly-Arg (NGR) tripeptide can specifically target the neovasculature via interaction with the aminopeptidase N (APN / CD13) receptor which is highly up-regulated in the membranes of endothelial tumor cells. NGR-directed drug delivery as well as molecular imaging have therefore been undergone development, and appear to be intriguing approaches in current cancer research. Herein we highlight some recent developments of the NGR peptide based cancer therapy including drug-delivery and imaging studies, with future perspectives. Some of these agents have been under clinical trials, indicating promising future for the NGR-based drugs.