Effects of the Incorporation of a Hydrophobic Middle Block into a PEG−Polycation Diblock Copolymer on the Physicochemical and Cell Interaction Properties of the Polymer−DNA Complexes

Abstract

One-component homopolymers of cationic monomers (polycations) and diblock copolymers comprising poly(ethylene glycol) (PEG) and a polycation block have been the most widely used types of polymers for the formulation of polymer-based gene delivery systems. In this study, we incorporate a hydrophobic middle block into the conventional PEG−polycation architecture and investigate the effects of this hydrophobic modification on the physicochemical and cell-level biological properties of the polymer−DNA complexes that are relevant to gene delivery applications. The ABC-type triblock copolymer used in this study consists of (A) PEG, (B) hydrophobic poly(n-butyl acrylate) (PnBA), and (C) cationic poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) component polymers. The properties of the triblock copolymer/DNA complexes are compared with those of two other more conventional DNA carriers derived, respectively, using a PDMAEMA homopolymer and a PEG−PDMAEMA diblock copolymer that had comparable molecular weights for individual blocks. In aqueous solution, the PEG−PnBA−PDMAEMA polymer forms positively charged spherical micelles. The electrostatic complexation of these micelles with plasmid DNA molecules results in the formation of stable small-sized DNA particles that are coated with a micelle monolayer, as confirmed by agarose gel electrophoresis, dynamic light scattering (DLS), and cryogenic transmission electron microscopy (cryo-TEM). Proton nuclear magnetic resonance (¹H NMR) spectroscopy measurements indicate that the whole micelle−DNA assembly (named “micelleplex” for convenience) is shielded predominantly by the PEG chains. DLS and optical microscopy imaging measurements indicate that compared with PDMAEMA−DNA polyplexes, the micelleplexes have a significantly lower tendency to aggregate under physiological salt concentrations and show reduced interactions with negatively charged components in serum such as albumin and erythrocytes. While the micelleplexes are comparable to the PEG−PDMAEMA-based DNA polyplexes in terms of their stability against aggregation under high salt concentrations and in the presence of the albumin protein, they have a slightly higher tendency to interact with erythrocytes than the diblock copolymer polyplexes. Agarose gel electrophoresis measurements indicate that relative to the PEG−PDMAEMA polyplexes, the micelleplexes provide better protection of the encapsulated DNA from enzymatic degradation and also exhibit greater stability against disintegration induced by polyanionic additives; in these respects, the PDMAEMA homopolymer-based polyplexes show the best performance. In vitro studies in HeLa cells indicate that the PDMAEMA polyplexes show the highest gene transfection efficiency among the three different gene delivery systems. Between the micelleplexes and the PEG−PDMAEMA polyplexes, a higher gene transfection efficiency is observed with the latter system. All three formulations show comparable levels of cytotoxicity in HeLa cells.