Polymer-siRNA complexes (siRNA polyplexes) are being actively developed to improve the therapeutic application of siRNA. A major limitation for many siRNA polyplexes, however, is insufficient mRNA suppression. Given that modifying the sense strand of siRNA with 3′ cholesterol (chol-siRNA) increases the activity of free nuclease-resistant siRNA in vitro and in vivo, we hypothesized that complexation of chol-siRNA can increase mRNA suppression by siRNA polyplexes. In this study, the characteristics and siRNA activity of self assembled polyplexes formed with chol-siRNA or unmodified siRNA were compared using three types of conventional, positively charged polymers: (i) biodegradable, cross-linked nanogels (BDNG) (ii) graft copolymers (PEI-PEG), and (iii) linear block copolymers (PLL10-PEG, and PLL50-PEG). Chol-siRNA did not alter complex formation or the resistance of polyplexes to siRNA displacement by heparin but increased nuclease protection by BDNG, PLL10-PEG, and PLL50-PEG polyplexes over polyplexes with unmodified siRNA. Chol-CYPB siRNA increased suppression of native CYPB mRNA in mammary microvascular endothelial cells (MVEC) by BDNG polyplexes (35%) and PLL10-PEG polyplexes (69%) over comparable CYPB siRNA polyplexes but had no effect on PEI-PEG or PLL50-PEG polyplexes. Overall, these results indicate that complexation of chol-siRNA increases nuclease protection and mRNA suppression by select siRNA polyplexes. These results also suggest that polycationic block length is an important factor in increasing mRNA suppression by PLL-PEG chol-siRNA polyplexes in mammary MVEC.

Modifying the sense strand of nuclease-resistant siRNA with 3’-cholesterol (Chol-*siRNA) increases mRNA suppression after i.v. administration but with relatively low efficacy. We previously found evidence in vitro that suggests complexation of Chol-siRNA with PLL-PEG(5K), a block copolymer of poly-L-lysine and 5 kDa polyethylene glycol, may increase the efficacy of Chol-siRNA in vivo in a PLL block length-dependent manner. In this study, the extent that polyplexes of PLL10-PEG(5K), PLL30-PEG(5K), and PLL50-PEG(5K) protect complexed Chol-siRNA in high concentrations of murine serum and affect the activity of Chol-*siRNA in murine 4T1 breast tumor epithelial cells in vitro and in primary orthotopic tumors of 4T1 was compared. PLL-PEG(5K) required 3’-Chol to protect full-length siRNA from nuclease degradation in 90% (v/v) murine serum and protection was increased by increasing PLL block length and nuclease resistance of Chol-siRNA. Polyplexes of Chol-*siLuc suppressed stably expressed luciferase in 4T1-Luc cells to different levels in vitro where PLL30>PLL50>PLL10. In contrast, only polyplexes of Chol-*siLuc and PLL30-PEG(5K) or PLL50-PEG(5K) suppressed high levels of luciferase in primary orthotopic tumors of 4T1-Luc after i.v. administration, whereas polyplexes of Chol-*siLuc and PLL10-PEG(5K), inactive Chol-*siCtrl polyplexes of PLL-PEG(5K), or Chol-*siLuc alone had no detectable activity. As a whole, these results indicate that polyplexes of PLL-PEG(5K) increase the efficacy of nuclease-resistant Chol-siRNA in primary breast tumors after i.v. administration in a PLL block length-dependent manner. Thus, complexation of Chol-siRNA with PLL-PEG(5K) may be a promising approach to increase the efficacy of Chol-siRNA in a wide range of primary tumors, metastases, and other tissues but likely requires a PLL block length that balances polymer-related adverse effects, Chol-siRNA bioavailability, and subsequent activity in the target cell.

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