Thermosensitive polymers as carriers for DNA delivery

Abstract

Copolymers of 2-(dimethylamino)ethyl methacrylate (DMAEMA) and N-isopropylacryl amide (NIPAAm) of various monomer ratios and molecular weights were evaluated as carrier systems for DNA delivery. All copolymers, even with a low DMAEMA content of 15 mol%, were able to bind to DNA at 25°C. Light-scattering measurements indicate that complexation is accompanied by precipitation of the (co)polymer in the complex caused by a drop of the lower critical solution temperature of the (co)polymer. The (co)polymer/plasmid ratio at which complexes with a size of around 200 nm were formed increased with increasing NIPAAm content of the copolymer and was independent of molecular weight of the (co)polymer. However, complexes containing (co)polymers of low molecular weight or high NIPAAm content prepared at 25°C aggregated rapidly when the temperature was raised to 37°C, whereas complexes containing (co)polymers of high molecular weight or lower NIPAAm content were relatively stable at 37°C. The zeta potential of the complexes was also independent of molecular weight of the (co)polymer and increased with increasing (co)polymer/plasmid ratio until a plateau value was reached. The (co)polymer/plasmid ratio at which this plateau was reached increased with increasing NIPAAm content. The plateau values decreased from around 26 mV to around 13 mV when the NIPAAm content of the copolymer was increased from 0 to 85 mol%. The cytotoxicity of the complexes strongly decreased with increasing NIPAAm content and was independent of molecular weight of the (co)polymer. The transfection efficiency of complexes with poor stability was in general much lower than that of complexes with good stability. The transfection efficiency as a function of the (co)polymer/plasmid ratio showed a bell-shaped curve. The (co)polymer/plasmid ratio at which the transfection efficiency was maximal increased with increasing NIPAAm content, while the maximum transfection efficiency strongly decreased with increasing NIPAAm content of the copolymer. The results of this study show that the formation of stable (co)polymer/plasmid complexes with a size of around 200 nm is a prerequisite for efficient transfection. Furthermore, the transfection efficiency and cytotoxicity strongly decreased with decreasing zeta potential. Therefore, besides the size, the zeta potential can also be used as a characteristic to predict the behavior of this type of (co)polymer/plasmid complexes in transfection. Copolymers of DMAEMA and NIPAAm provided with a homing device may be interesting carrier systems for gene targeting because these copolymers can condense DNA to small particles, and the resulting complexes show a low cytotoxicity and aspecific transfection.