REVIEW A review of RGD-functionalized nonviral gene delivery vectors for cancer therapy J Park 1 , K Singha 1 , S Son 1 , J Kim 1 , R Namgung 1 , C-O Yun 2 and WJ Kim 1 The development of effective treatments that enable many patients suffering from cancer to be successfully cured is highly demanded. Angiogenesis, which is a process for the formation of new capillary blood vessels, has a crucial role in solid tumor progression and the development of metastasis. Antiangiogenic therapy designed to prevent tumor angiogenesis, thereby arresting the growth or spread of tumors, has emerged as a non-invasive and safe option for cancer treatment. Due to the fact that integrin receptors are overexpressed on the surface of angiogenic endothelial cells, various strategies have been made to develop targeted delivery systems for cancer gene therapy utilizing integrin-targeting peptides with an exposed arginine–glycine–aspartate (RGD) sequence. The aim of this review is to summarize the progress and prospect of RGD-functionalized nonviral vectors toward targeted delivery of genetic materials in order to achieve an efficient therapeutic outcome for cancer gene therapy, including antiangiogenic therapy. Cancer Gene Therapy (2012) 19, 741–748; doi:10.1038/cgt.2012.64; published online 28 September 2012 Keywords: RGD peptide; integrin; angiogenesis; nonviral vectors INTRODUCTION Cancer is one of the most notorious diseases in the world, and the mortality rate is significantly high. Conventional treatment options, including chemotherapy, radiotherapy, surgery and immunother- apy have shown adverse, nonspecific side effects, which impair successful treatment outcomes for many patients. Non-invasive and safe options for cancer treatment are, therefore, highly demanded. Gene therapy aiming at the treatment of human disorders by introducing genetic materials into specific target cells or tissues has gained significant attention in the medical community. 1,2 In particular, gene therapy for cancers has been the most widely studied, and many therapeutic strategies have been utilized, including immune system-targeted therapies (immune-stimulant genes and vaccines) and molecular-targeted therapies (suicide genes, anti-oncogenes and tumor suppressor genes). 3,4 Owing to the fact that cancers cannot grow beyond a certain size and spread without a blood vessel that provides sufficient supplement such as nutrients and oxygen, as well as pipelines to move out, angiogenesis plays a critical role in solid tumor progression and the development of metastasis. 5–7 Thus, antiangiogenic therapy designed to prevent tumor angiogenesis has emerged as a non-invasive and safe option for cancer treatment. 8,9 Furthermore, the fact that angiogenic endothelial cells overexpress integrin cell surface receptors that are known to bind arginine-glycine- aspartate (RGD)-containing peptides 10 makes them prime candidates for RGD-based targeted cancer gene therapy. Utilizing RGD motifs mounted by genetic or chemical manipulations, various vectors have been made for the development of targeted delivery systems, including viral vectors 11,12 and non-viral vectors. 13,14 Besides viral vectors known for inducing severe immune responses, 15 throughout this review, the progress in developing various RGD-modified nonviral vectors such as polymers, lipids and peptides for use in cancer gene therapy, and specifically antiangiogenic therapy, is summarized. More focus is made on the RGD-modified polymers. ANGIOGENESIS Angiogenesis is a process to form new capillary blood vessels from preexisting vasculature. This process is involved in a number of physiological and pathological events. In physiological conditions, it occurs during embryonic development, wound repairing and the menstrual cycle. In pathological conditions, unregulated angiogen- esis is seen, such as diabetic retinopathy, rheumatoid arthritis and cancer. 7 The construction of a vesicular network requires several sequential steps, including endothelial cell activation, degradation of the basement membrane, endothelial cell migration, proliferation and stabilization into mature blood vessels. 16 These processes are mediated by a wide variety of angiogenic inducers such as growth factors, chemokines, angiogenic enzymes, endothelial specific receptors and adhesion molecules. 16 The loss of control for cell proliferation is a characteristic of tumor growth. The cancerous cells divide rapidly, resulting in a small, spheroid tumor called in situ carcinoma. 17 Because of the lack of sufficient nutrients and oxygen, in situ carcinomas may remain dormant for many years. Once being switched to angiogenic phenotype, in situ carcinomas induce angiogenesis for tumor expansion, start to invade the surrounding tissues and enter the blood stream to spread throughout the body. Therefore, neovascularization or angiogenesis has a critical role in solid tumor progression and the development of metastasis. 16,17 RGD-INTEGRIN RECOGNITION Proteins containing a RGD sequence, together with the integrins that serve as receptors for them, constitute a major recognition system for cell adhesion. 10 Among others, avb3 integrin and 1 Department of Chemistry, BK21 Program, Polymer Research Institute, Pohang University of Science and Technology (POSTECH), Pohang, Republic of Korea and 2 Department of Bioengineering, College of Engineering, Hanyang University, Seoul, Republic of Korea. Correspondence: Professor WJ Kim, Department of Chemistry, BK21 Program, Polymer Research Institute, Pohang University of Science and Technology (POSTECH), San 31, Hyoja-dong, Pohang 790-784, Republic of Korea. E-mail: wjkim@postech.ac.kr Received 22 May 2012; revised 27 August 2012; accepted 27 August 2012; published online 28 September 2012 Cancer Gene Therapy (2012) 19, 741–748 & 2012 Nature America, Inc. All rights reserved 0929-1903/12 www.nature.com/cgt