Hydrophilic cationic star homopolymers based on a novel diethanol-n-methylamine dimethacrylate cross-linker for sirna transfection: Synthesis, characterization, and evaluation

Abstract

Four cationic hydrophilic star homopolymers based on the novel hydrophilic, positively ionizable cross-linker bis(methacryloyloxyethyl)methylamine (BMEMA) were synthesized using sequential group transfer polymerization (GTP) and were, subsequently, evaluated for their ability to deliver siRNA to mouse myoblast cells. The nominal degrees of polymerization (DP) of the arms were varied from 10 to 50. For the polymerizations, 2-(dimethylamino)ethyl methacrylate (DMAEMA) was employed as the hydrophilic, positively ionizable monomer. For comparison, four linear DMAEMA homopolymers were also synthesized, whose nominal DPs were the same as those of the arms of the stars. The numbers of arms of the star homopolymers were determined using gel permeation chromatography with static light scattering detection, and found to range from 7 to 19, whereas the hydrodynamic diameters of the star homopolymers in aqueous solution were measured using dynamic light scattering and found to increase with the arm DP from 13 to 26 nm. The presence of the hydrophilic BMEMA cross-linker enabled the solubility of all star homopolymers in pure water. The cloud points of the star homopolymers in aqueous solution increased with the arm DP from 23 to 29°C, while the cloud points of the linear homopolymers were found to decrease with their DP, from 42 to 32°C. The effective pK values of the DMAEMA units were in the range of 6.9 to 7.3 for the star homopolymers, whereas they ranged between 7.3 and 7.4 for the linear homopolymers. Subsequently, all star and linear homopolymers were evaluated for their ability to deliver siRNA to the C2C12 mouse myoblast cell line, expressing the reporter enhanced green fluorescent protein (EGFP). All star homopolymers and the largest linear homopolymer presented significant EGFP suppression, whereas the smaller linear homopolymers were much less efficient. For all star homopolymers and the largest linear homopolymer both the EGFP suppression and the cell toxicity increased with polymer loading. The siRNA-specific EGFP suppression, calculated by subtracting the effect of cell toxicity on EGFP suppression, slightly increased with star polymer loading for the two smaller stars, whereas it presented a shallow maximum and a decrease for the other two stars. Moreover, the siRNA-specific EGFP suppression also increased slightly with the DP of the arms of the DMAEMA star homopolymers. Overall, the EGFP suppression efficiencies with the present star homopolymers were at levels comparable to that of the commercially available transfection reagent Lipofectamine. © 2011 American Chemical Society.