Highly branched HK peptides are effective carriers of siRNA

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Abstract
Background Both viral and nonviral carriers have been used to carry small interfering RNA molecules (siRNA) to their cytosolic mRNA target. To date, few peptide carriers have been developed that have proved effective for siRNA delivery. Our previous branched carriers composed of histidine and lysine were useful for transfection of plasmids. In this study, we determined if these and more highly branched HK polymers were effective carriers of siRNA. Methods Several branched polymers were synthesized on a Ranin Voyager synthesizer. These polymers were then screened for their ability to transfer siRNA into SVR‐bag4 cells, MDA‐MB‐435 cells, and C6 cells. After one polymer, H3K8b, was identified as an effective carrier of siRNA, additional polymers were synthesized to determine the essential domains for siRNA transport. The size/zeta‐potential of HK : siRNA complexes were measured with the N4 submicron particle size analyzer and the Delsa 440 SX zeta‐potential analyzer, respectively. Toxicity of the highly branched polymers in complex with siRNA was investigated by flow cytometry. Results In an endothelial cell line (SVR‐bag4) that stably expressed β‐galactosidase (β‐gal), an siRNA in complex with the H3K8b polymer inhibited β‐gal expression by more than 80%. In contrast, the polymer H2K4b, which was an effective carrier of plasmids, was not an efficient carrier of siRNA. The size and surface charge did not distinguish effective from ineffective HK carriers of siRNA. By modifying H3K8b, we then determined what properties of H3K8b augmented siRNA delivery. The histidine‐rich domain and the length of the terminal arms of H3K8 were important for siRNA delivery. The modestly more effective analog of H3K8b containing an integrin ligand, H3K8b(+RGD), was able to inhibit markedly intracellular β‐gal expression. Furthermore, we determined that H3K8b(+RGD) in complex with a luciferase‐targeting siRNA inhibited luciferase expression in MDA‐MB‐435 cells. At its optimal concentration for inhibiting its target, H3K8b(+RGD) : siRNA complex had minimal toxicity. In contrast, carriers of siRNA such as Oligofectamine and Lipofectamine 2000 were significantly more toxic. Conclusions Both the degree of complexity and the sequence specificity are important factors to be considered for developing the HK carrier of siRNA. In particular, we found that certain branched HK polymers (H3K8b, H3K8b(+RGD), and similar structural analogs) with eight terminal branches and a histidine‐rich domain were effective carriers of siRNA. Copyright © 2005 John Wiley & Sons, Ltd.