Delivery is a significant challenge faced during the transition of siRNA initiated gene silencing from an in vitro setting to one of widespread clinical use. Much effort is being expended to develop cationic polymer systems that self-assemble with siRNA via electrostatic interactions with the phosphate backbone. These polymers protect the siRNA from nuclease degradation and facilitate cellular uptake, frequently through the endosomal pathway. Cytoplasmic localization is then commonly accomplished through inclusion of an endosomal escape agent, eventually leading to targeted gene silencing.

Unfortunately, there is not a clear understanding of the effects the various moieties play on the myriad of polymer chemistries currently being pursued for siRNA delivery. In order to better characterize the binding step of polymer/siRNA (nanoparticle; NP) complex formation, we are using a novel polymeric system that allows addition of virtually any moiety. Through our systematic studies, we will show that binding of siRNA by the NP is strongly dependent on the cationic charge density along the backbone and that incorporation of specific functionalities can influence that binding through physical and structural changes. The observed trends also appear to be important for biocompatibility (i.e., toxicity). Together, our results can be used to identify new polymers that are designed to meet the complex requirements for siRNA delivery.