Biophysical properties of chitosan/siRNA polyplexes: Profiling the polymer/siRNA interactions and bioactivity

Chitosans are naturally occurring polymers widely used in life science to mediate intracellular uptake of nucleic acids such as siRNA. Four chitosans of fungal origin (Agaricus bisporus; molecular weights MW=44, 63, 93 and 143kDa) were used in this study and profiled for size, viscosity and hydrodynamic radius using gel permeation chromatography (GPC). Polyplexes made of these chitosans and siRNA were developed and optimized for transfection efficacy in vitro. The characteristics of these polyplexes were low chitosan:siRNA ratios (4–8; N:P) similar positive zeta potential (20–30mV) and comparable particle sizes (about 150nm). Endogenous luciferase reporter gene down-regulation in human epithelial H1299 cells at nanomolar concentrations (37.5–150nM) was significantly stronger for the lower molecular weight chitosans. The impact of these low N:P polyplexes on the cellular viability was minimal also at 150nM. To help develop an understanding of these differences, an energetic profile of the molecular interactions and polyplex formation was established by isothermal titration calorimetry (ITC). The four polyplexes exhibited strong binding enthalpies delta Hbind(−84 to −102kcal/mol) resulting in nanomolar dissociation constants. Intracellular trafficking studies using rhodamine labeled siRNA revealed that polyplexes made from smaller MW chitosans exhibited faster cellular uptake kinetics than their higher MW counterpart. Transmission electron microscopy and small angle X-ray scattering studies (SAXS) revealed that the 44kDa derived polyplexes exhibited regular spherical structure, whereas the 143kDa chitosan polyplex was rather irregularly shaped. With regards to adverse effects these low N:P chitosan/siRNA formulations represent an interesting alternative to so far reported chitosan polyplexes that used vast N:P excess to achieve similar bioactivity. Novel bioactive chitosan/siRNA nanoparticles were devised using Agaricus bisporus derived chitosans at low N:P ratios (4–8) characterized by low toxicity and good bioactivity in vitro. [Display omitted]