A series of endosomolytic mixed micelles was synthesized from two diblock polymers, poly[ethylene glycol-b-(dimethylaminoethyl methacrylate-co-propylacrylic acid-co-butyl methacrylate)] (PEG-b-pDPB) and poly[dimethylaminoethyl methacrylate-b-(dimethylaminoethyl methacrylate-co-propylacrylic acid-co-butyl methacrylate)] (pD-b-pDPB), and used to determine the impact of both surface PEG density and PEG molecular weight on overcoming both intracellular and systemic siRNA delivery barriers. As expected, the percent PEG composition and PEG molecular weight in the corona had an inverse relationship with mixed micelle zeta potential and rate of cellular internalization. Although mixed micelles were internalized more slowly, they generally produced similar gene silencing bioactivity (∼ 80% or greater) in MDA-MB-231 breast cancer cells as the micelles containing no PEG (100 D/no PEG). The mechanistic explanation for the potent bioactivity of the promising 50 mol% PEG-b-DPB/50 mol% pD-b-pDPB (50 D) mixed micelle formulation, despite its relatively low rate of cellular internalization, was further investigated as a function of PEG molecular weight (5 k, 10 k, or 20 k PEG). Results indicated that, although larger molecular weight PEG decreased cellular internalization, it improved cytoplasmic bioavailability due to increased intracellular unpackaging (quantitatively measured via FRET) and endosomal release. When delivered intravenously in vivo, 50 D mixed micelles with a larger molecular weight PEG in the corona also demonstrated significantly improved blood circulation half-life (17.8 min for 20 k PEG micelles vs. 4.6 min for 5 kDa PEG micelles) and a 4-fold decrease in lung accumulation. These studies provide new mechanistic insights into the functional effects of mixed micelle-based approaches to nanocarrier surface PEGylation. Furthermore, the ideal mixed micelle formulation identified (50 D/20 k PEG) demonstrated desirable intracellular and systemic pharmacokinetics and thus has strong potential for in vivo therapeutic use.
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