Exploring phosphonium and ammonium chitosan polymers and their PEGylated analogs for high performance gene delivery

Among cationic polymers, chitosan (CS) is a promising non-viral vector due to its biocompatibility, biodegradability and safety profile. However, its application for gene delivery is hindered by its poor solubility at physiological pH. In this study, novel water-soluble chitosan derivatives containing triethylphosphonium chloride ((N-(1-carboxybutyl-4-triethylphosphonium chloride) chitosan = TEPB-CS)) or triethylammonium chloride ((N-(1-carboxybutyl-4-triethylammonium chloride) chitosan = TEAB-CS)) with 4-carbon spacers separating the cationic units from the chitosan backbone were synthesized by carbodiimide mediated coupling reactions. Two different degrees of substitution of quaternary phosphonium (43% and 21%) and ammonium (40.6% and 26%) were synthesized. The polymers with higher degrees of substitutions displayed higher solubility in aqueous media and were further grafted with mPEG (TEAB-CS-g-mPEG and TEPB-CS-g-mPEG) and used for DNA complexation, cytotoxicity, apoptosis, and gene transfection studies. Chemical characterization of the above polymers was performed by FTIR, 1H NMR, and 31P NMR, and the degree of substitution of the grafted phosphonium and ammonium and mPEG was calculated by 1H NMR. The polymers efficiently bound pDNA at weight ratio of polymer:pDNA of 2.8:1. From dynamic light scattering experiments, the mean diameters of the calf thymus DNA complexes with the polymers, in water and phosphate buffer (pH 7.4), were in the range of 140 – 330 nm. Zeta potential of these complexes in water became positive as the weight ratios of the polymers/DNA exceeded 2:1. Most of these polymers mildly affect the viability of HeLa cells with a final concentration of 10 µg/mL or higher. Cell viability was only modestly compromised in the presence of 100 µg/mL of TEPB-CS-g-mPEG, and this derivative was also found to display the highest transfection efficiencies among all four polymers. Apoptosis measurements by flow cytometry indicated that both TEAB-CS-g-mPEG and TEPB-CS-g-mPEG induce no perceptible and mild apoptosis with their concentration at 100 µg/mL, respectively. Together, the present study provides evidence that these PEGylated TEPB-CS and TEAB-CS are safe and effective vectors for efficient nucleic acid delivery.