Peptides take on an increasingly important role as therapeutics in areas including diabetes, oncology, and metabolic, cardiovascular, and infectious diseases. In addition, many peptides such as insulin have been employed for many years. A challenge in the administration of peptide drugs is their often low hydrolytic stability, as well as other problems that are common to any drug treatment such as systemic distribution. There is a significant attention in the literature of protein drugs and their delivery strategies, but not many overviews are specifically dedicated to peptides. In this review, the different approaches to deliver peptides have been summarized where the focus is only on drug carriers based on organic materials. Initial discussion is on different methods of polymer-peptide conjugation before being followed by physical encapsulation techniques, which is divided into surfactant-based techniques and polymer carriers. Surfactant-based techniques are dominated by liposome, microemulsions and solid-lipid nanoparticles. The field widens further in the polymer field. The delivery of peptides has been enhanced using polymer-decorated liposomes, solid microspheres, polyelectrolyte complex, emulsions, hydrogels, and injectable polymers. The aim of this article is to give the reader an overview over the different types of carriers.

BackgroundAquaporins (AQP) are water channel proteins that enable fluid fluxes across cell membranes, important for homeostasis of the tissue environment and for cell migration. AQP1 knockout mouse models of human cancers showed marked inhibition of tumor-induced angiogenesis, and in pre-clinical studies of colon adenocarcinomas, forced over-expression of AQP1 was shown to increase angiogenesis, invasion and metastasis. We have synthesized small molecule antagonists of AQP1. Our hypothesis is that inhibition of AQP1 will reduce migration and invasiveness of colon cancer cells, and the migration and tube-forming capacity of endothelial cells in vitro.MethodsExpression of AQP1 in cell lines was assessed by quantitative (q) PCR, western blot and immunofluorescence, while expression of AQP1 in human colon tumour tissue was assessed by immunohistochemistry. The effect of varying concentrations of the AQP1 inhibitor AqB013 was tested on human colon cancer cell lines expressing high versus low levels of AQP1, using wound closure (migration) assays, matrigel invasion assays, and proliferation assays. The effect of AqB013 on angiogenesis was tested using an endothelial cell tube-formation assay.ResultsHT29 colon cancer cells with high AQP1 levels showed significant inhibition of migration compared to vehicle control of 27.9 % ± 2.6 % (p < 0.0001) and 41.2 % ± 2.7 (p <0.0001) treated with 160 or 320 μM AqB013 respectively, whereas there was no effect on migration of HCT-116 cells with low AQP1 expression. In an invasion assay, HT29 cells treated with 160 μM of AqB013, showed a 60.3 % ± 8.5 % decrease in invasion at 144 hours (p < 0.0001) and significantly decreased rate of invasion compared with the vehicle control (F-test, p = 0.001). Almost complete inhibition of endothelial tube formation (angiogenesis assay) was achieved at 80 μM AqB013 compared to vehicle control (p < 0.0001).ConclusionThese data provide good evidence for further testing of the inhibitor as a therapeutic agent in colon cancer.Electronic supplementary materialThe online version of this article (doi:10.1186/s13046-016-0310-6) contains supplementary material, which is available to authorized users.