Efficiency of polyethylenimines and polyethylenimine-graft-poly (ethylene glycol) block copolymers to protect oligonucleotides against enzymatic degradation

Enzymatic instability of oligonucleotides (ON) is one of the major drawbacks of this new class of therapeutic agents. The development of safe, efficient delivery systems capable of stabilizing and protecting these molecules within the formulation, as well as during application, is a challenge in modern gene therapy. In the present study, polyethylenimine (PEI) of different molecular weights and PEGylated PEI block copolymers (PEI-g-PEG) were investigated with regard to their protective properties when complexes with chemically unmodified DNA (d-ON) and RNA (r-ON) oligonucleotides. PEI/ON complexes were incubated with different amounts of serum or nucleases. The influence of pH on the stability was studied and the integrity of the ON was determined by gel electrophoresis. The amount of stable ON within the gels was quantified via densitometric analysis. PEI homopolymers ranging from 800 to 2 kDa protected both types of ON very efficiently, whereas PEI 0.8 kDa demonstrated a slight decrease in protection. The PEGylated PEI derivatives generally protected ON as efficiently as the PEI homopolymers. In particular, the PEI-g-PEG derivative containing 100 PEG chains of 550 Da yielded the highest protection efficiency for both d-ON and r-ON. In general, the highest protection could be achieved at pH 6.7. The ratio of polymer and ON (N/P ratio) also had a great impact on ON stability with higher N/P ratios achieving a better protection. In conclusion, PEIs showed advantageous protective properties for ON. The results of this study offer indications for a rational design of PEI derivatives for the protection and the delivery of ON.