Optimising the self-assembly of siRNA loaded PEG-PCL-lPEI nano-carriers employing different preparation techniques

Amphiphilic cationic block copolymers consisting of poly(ethylene glycol), poly(ε-caprolactone) and poly(ethylene imine) spontaneously assemble to nano-sized particulate carriers, which can be utilised for complexation of nucleic acids (small-interfering RNA), representing a multifunctional vector system, designed for drug and gene delivery. Apart from polymer design and charge ratio, a more homogeneous complexation could lead to a more uniform charge distribution, subsequently increasing colloidal stability, RNA protection and consequently transfection efficiency. Microfluidic mixing techniques, bringing cationic polymer and nucleic acid together at a constant ratio during the entire mixing process, have the potential for a gentler complexation. In the present study carriers were prepared by a solvent displacement technique. In a first step complex size for addition of RNA during (addition to the aqueous or the organic phase) or after (classical pipetting or microfluidic mixing) carrier assembly was determined by dynamic light scattering. Suitable N/P ratios have previously been selected by measuring size and ζ-potential as a function of N/P. Subsequently, for the most promising techniques (loading after assembly), colloidal stability, the ability to protect RNA as well as transfection efficiency in vitro were compared. Finally, parameters for the superior microfluidic mixing process were optimised with the help of a central composite design. Generally, gentler loading leads to more homogeneous complexes. Hence, possibly due to a more consistent surface coating, loading after carrier assembly resulted in less aggregation. In comparison to bulk mixing, microfluidic assembly exhibited smaller diameters (179±11 vs. 230±97nm), less heterogeneity (PDI=0.205±0.028 vs. 0.353±0.161), enhanced RNA protection (RNA recovery=30.6±1.0 vs. 15.4±1.4%) as well as increased transfection performance (34.8±1.5 vs. 24.5±2.2% knockdown). Therefore, microfluidic complexation represents a reproducible alternative for formulating gene delivery carriers with superior colloidal stability, RNA protection and transfection efficiency.