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Nanoparticles (NPs) interact with complex protein milieus in biological fluids, and these interactions have profound effects on NP physicochemical properties and function. Surprisingly, most studies neglect the impact of these interactions, especially with respect to NP-mediated siRNA delivery. Here, the effects of serum on colloidal stability and siRNA delivery of a pH-responsive micellar NP delivery system were characterized. Results show cationic NP-siRNA complexes aggregate in ≥2% serum in buffer, but are stable in serum-free media. Furthermore, nonaggregated NP-siRNA delivered in serum-free media result in 4-fold greater siRNA uptake in vitro, compared to aggregated NP-siRNA. Interestingly, pH-responsive membrane lysis behavior, which is required for endosomal escape, and NP-siRNA dissociation, necessary for gene knockdown, are significantly reduced in serum. Consistent with these data, nonaggregated NP-siRNA in serum-free conditions result in highly efficient gene silencing, even at doses as low as 5 nM siRNA. NP-siRNA diameter was measured at albumin and IgG levels mimicking biological fluids. Neither albumin nor IgG alone induces NP-siRNA aggregation, implicating other serum proteins in NP colloidal instability. Finally, as a proof-of-principle that stability is maintained in established in vivo models, transmission electron microscopy reveals NP-siRNA are taken up by ductal epithelial cells in a nonaggregated state when injected retroductally into mouse salivary glands in vivo. Overall, this study shows serum-induced NP-siRNA aggregation significantly diminishes efficiency of siRNA delivery by reducing uptake, pH-responsive membrane lysis activity, and NP-siRNA dissociation. Moreover, these results highlight the importance of local NP-mediated drug delivery and are broadly applicable to other drug delivery systems.

The Effects of Biological Fluids on Colloidal Stability and siRNA Delivery of a pH-Responsive Micellar Nanoparticle Delivery System