siRNA-aptamer chimeras on nanoparticles: preserving targeting functionality for effective gene silencing.

SiRNA-aptamer chimeras are emerging as a highly promising approach for cell-type specific delivery of siRNA due to the outstanding targeting capability of aptamers and the compatibility of chimeras with native ribonuclease (Dicer) processing. For efficient RNA interference (RNAi), however, additional challenges must be addressed, in particular how to get siRNA out of the endosome after cell entry and how to preserve aptamer targeting specificity when chimeras are combined with delivery carriers. Here, we report a rationally designed nanoparticle vector that simultaneously displays large surface area for high siRNA payload, exposed aptamer for specific targeting, proton sponge effect for endosome escape, and fluorescence for imaging and quantification. A key concept of this work is to graft chimeras onto nanoparticle surface via a two-step process: first immobilizing siRNA onto nanoparticle via noncovalent interactions to facilitate intracellular unpackaging and reduce nanoparticle surface charge (avoiding nonspecific electrostatic interactions between aptamers and nanoparticles) and then coupling siRNA and aptamer with retained conformation and high accessibility. Compared with conventional one-step adsorption of siRNA-aptamer chimeras onto nanoparticles with random orientations and conformations, which does not elicit much improved RNAi effect than nontargeted nanoparticle-siRNA complexes (∼6-8% improvement of the total cell population), under the same RNA concentration our approach shows selective gene silencing and enables 34% more silenced cells of the total cell population over nontargeted nanoparticle-siRNA complexes. This remarkable difference in RNAi efficiency using nanoparticle-chimera complexes is directly related to cell uptake discrepancy resulting from aptamer conformation on the nanoparticle surface (intact vs random).