Nanoscale Res Lett (2009) 4:982–992 degradation of the polymer can be used as a tool to release the plasmid DNA into the cytosol. Recently, the use of nano- particles prepared with biocompatible and biodegradable poly (D, L-lactide-co-glycolide) (PLGA) or poly (D, L-lactide) (PLA) polymers have attracted much attention due to their favorable physicochemical characteristics in terms of safety, stability, the relative ease of large-scale production, and lack of intrinsic immunogenicity that make them suitable candi- dates for gene delivery application [3, 4]. These biodegrad- able polymers undergo bulk hydrolysis thereby providing sustained delivery of the therapeutic agent depending on the polymer molecular weight and copolymer [5]. The degra- dation products, lactic acid and glycolic acid, are removed from the body through citric acid cycle. The degradation time of PLGA/PLA can be altered from days to years by varying the polymer molecular weight, the lactic acid to glycolic acid ratio in copolymer. And recent studies dem- onstrated rapid escape of PLGA nanoparticles from the endo-lysosomal compartment into cytosol following their uptake [6]. Therefore, PLGA/PLA nanoparticles might be a promising and suitable candidate for gene delivery application. Although all features makes PLGA/PLA attractive to many researchers involved in DNA delivery,it should be noticed that simple PLGA/PLA nanoparticles shows, indeed, several drawbacks as an ideal gene delivery system [7]. First, because of the large size and hydrophilic char- acter of DNA, encapsulation of plasmid DNA in hydro- phobic PLGA/PLA nanoparticles is a challenge. Second, the rate of DNA release from PLGA/PLA nanoparticles is often too slow, and thus also subsequent antigen production in case of DNA vaccination, which may prevent an optimal immune response. Third, PLGA/PLA nanoparticles tend to bind serum protein (such as albumin) in systemic appli- cation due to the hydrophobic surface of PLGA/PLA nanoparticles, which lead to opsonization and clearance by the reticuloendothelial system (RES), limiting their thera- peutic applications. Based on these considerations, it’s necessary to modify the structure of PLGA/PLA in order to improve its hydrophilicity, the gene loading efﬁciency, release behavior, and stability both in vitro and in vivo. Polyethylene glycol (PEG) is a commonly used modiﬁer, which is expected to be a good candidate as the soluble polymeric modiﬁer in organic synthesis or a pharmacologi- cal polymer due to its high hydrophilicity, low cytotoxicity and high cell permeability [8]. Reportedly, the PLA and PLGA nanoparticles with either adsorbed or grafted PEG layers exhibit prolonged blood circulation times and reduced uptake by the reticuloendothelial system in comparison with their uncoated counterparts [9]. It has been proved that PEG corona could diminish non-speciﬁc interactions with serum proteins and decrease the uptake of cationic polymer/DNA complex by the macrophages in the liver and spleen leading 983 to an increased blood circulation time, which ultimately improve the transfection efﬁciency of polycationic poly- mers. Furthermore, the hydrophilicity of PLA nanoparticles could be improved after modiﬁcation with PEG, and the afﬁnity between polymers and DNA would be accordingly enhanced which is favorable to load gene into the nanopar- ticles. In addition, the microenvironment formed by PEG is beneﬁcial to the activity of protein or gene agents in storage or administration. In addition, although PEGylated PLA or PLGA polymer have come out for a while as drug delivery systems [10, 11], there are few reports on gene delivery systems [12, 13]. Therefore the feasibility and suitability of PEGylated PLA or PLGA polymer as gene delivery system remains to be adequately investigated and addressed. Usually plasmid DNA is encapsulated into PLGA/PLA particles using the common water-in-oil-water (W/O/W) double emulsion/solvent evaporation method in order to achieve a better protection of plasmid and a more precise control of the release process [14, 15]. However, the double emulsion/solvent evaporation method can not guarantee the integrity of DNA under ultra-sonication or high speed homogenization, which is necessary in the encapsulation process to obtain smaller particle size [16, 17]. One strat- egy to solve the problem is to adsorb the plasmid DNA onto the surface of cationic PLA/PLGA particles which are modiﬁed to display a positively charged surface by inclu- sion of cationic surfactants such as cetyltrimethyl ammo- nium bromide (CTAB) in formulations [18, 19]. It was reported that the cationic gene loaded particles substan- tially improved the immune responses generated by DNA, both in mouse and macaque models [20]. Based on these considerations, the main goal of the present work was to develop a novel cationic PEGylated PLA (PLA-PEG) nanoparticles and explore its applicability and feasibility as a nonviral vector for gene transport. For this purpose, cationic PLA-PEG nanoparticles with appropriate positive charge and particle size was prepared applying an easily performed nanoprecipitation technique and using CTAB as a cationic surfactant and Tween 80 as a cosurfactant. The cationic gene loaded PLA-PEG nano- particles was then obtained by adsorbing plasmid DNA onto the surface of cationic PLA-PEG nanoparticles through electrostatic interactions. The physicochemical properties and biological properties of the nanoparticles were investigated and characterized, respectively. Materials and Methods Materials Poly(ethylene glycol)-block-polylactide copolymer (PLA- PEG, PEG average Mn 3,000 Da, PLA average Mn 123