Redox-based control of the transformation and activation of siRNA complexes in extracellular environments using ferrocenyl lipids.

We report physical characterization and biological evaluation of complexes of small interfering RNA (siRNA) formed using a cationic lipid [bis(11-ferrocenylundecyl)dimethylammonium bromide (BFDMA)] containing redox-active ferrocenyl groups at the end of each hydrophobic tail. We demonstrate that control over the redox state of BFDMA can be used to influence key physical properties and control the activities of lipoplexes formed using siRNA-based constructs. Specifically, lipoplexes of siRNA and reduced BFDMA lead to high levels of sequence-specific gene silencing in cells, but lipoplexes formed using oxidized BFDMA do not. Lipoplexes of oxidized BFDMA can be activated in situ to induce high levels of silencing by addition of a chemical reducing agent, demonstrating a basis for external control over the activation/delivery of siRNA in cellular environments. Differences in activity arise from the inability of oxidized BFDMA to promote efficient internalization of siRNA; these differences also correlated to significant differences in the nanostructures of these lipoplexes (determined by cryo-TEM) and their ζ potentials as a function of oxidation state. These results are considered in view of recent studies characterizing the nanostructures, properties, and behaviors of lipoplexes formed using BFDMA and macromolecular plasmid DNA. We find that several key structural features and aspects of redox control observed for lipoplexes of plasmid DNA are maintained in complexes formed using smaller and more rigid siRNA. The ability to transform BFDMA in complex media presents opportunities to exert control over the nanostructures and behaviors of siRNA lipoplexes in ways not possible using conventional lipids. The approaches reported here could thus prove useful in both fundamental and applied contexts.