Effect of binding affinity for siRNA on the in vivo antitumor efficacy of polyplexes

To elucidate the effect of binding affinity for siRNA on the in vivo antitumor efficacy of polyplexes, five kinds of galactose modified trimethyl chitosan-cysteine (GTC) conjugate-based polyplexes were developed through adjusting the incorporated ionic crosslinkers. The resultant polyplexes exhibited similar particle size (135-170 nm) and zeta potential (30-35 mV). Their distinct binding affinities for siRNA were evaluated by gel retardation, heparin displacement, and in vitro siRNA release assays. GTC polyplexes with weak polymer-siRNA binding were structurally unstable and highly susceptible to nuclease degradation, resulting in poor cellular uptake. However, strong binding affinity for siRNA was correlated to delayed intracellular dissociation of polyplexes. The polyplexes with optimized binding affinity for siRNA provided enough protection of siRNA prior to releasing it efficiently in the cytoplasm, resulting in efficient and persistent gene knockdown in vitro. Furthermore, they had remarkable antitumor efficacy in vivo with regard to the tumor growth retardation, gene knockdown, angiogenesis inhibition, and apoptosis induction in QGY-7703 tumor bearing mice. Therefore, tailoring of the binding strength between the polymeric vector and its siRNA cargo in polyplexes may serve as a feasible tool for improving their therapeutic efficacy.