PEG-PCL-PEI: characterization and analysis of a nucleic acid delivery system

In the last three decades, gene therapy had been supported as an innovative strategy for treating numerous types of hereditary and acquired disease. Although the promising potential, substantial challenges are still required for safe and efficient delivery systems as shown in Y. Liu et al. (2009). Considering the promising innovative therapeutic treatment, both viral and non-viral gene delivery techniques have been extensively studied as described by Douglas (2008) and Nayerossadat et al. (2012). Before long, the interesting promises of these methods were slowed down by safety studies. Investigation of viral vectors revealed mutagenicity problems, while for liposomal vectors unwanted negative interactions with the serum were shown in W. Li & Szoka (2007). Scientific investigations have shown that polymeric vectors could be convenient for gene delivery compared to the mechanism based on viruses and liposomes, especially including safety, immunogenicity, mutagenicity and cost of production. Important studies have been conducted along this line, and several publications have shown the important potential of the use of hyper-branched polyethylenimine (hy-PEI) as a cationic polymer for gene delivery both in vitro and in vivo. The PEI molecular structure is characterized by a protonable amino group at every third atom. The presence of this charge influences its inter and intra chemical-physical interactions, in particular, this is exploited as a desired property to favor and promote the endosomal escape, notably carried out by the proton-sponge effect. The important endosomal escape has aroused great interest in the prospect of internalization of any potential gene-delivery system containing PEI. Safety studies soon revealed a clear high cytotoxicity of unmodified hy-PEI. The cytotoxicity of hy-PEI was due to its intrinsic molecular structure that obviously influences its molecular weight, polydispersity, incubation time, and density of the cationic groups. Designed copolymers of Polyethylenimine (PEI) and Polyethylenglycole (PEG) proved a lower toxicity. PEI-PEG copolymers immediately demonstrated important strengths in their potential use as a genetic carrier. The architecture of the copolymers showed a better biocompatibility of the complex with biological fluids and membranes, in particular an important marked increasing of the circulation time and decrease in the aggregation and cytotoxicity. An important increase in hydrophilicity certainly allowed a better interaction with biological fluids, obviously minimized protein adsorption and decreased non-specific cell adhesion. The addition of the polycaprolactone (PCL) between the PEG-PEI forming a triblock copolymer PEG-PCL-PEI, as described in Y. Liu et al. (2009) showed important improvements. The use of this polymer has been consolidated in tissue and reconstruction engineering thanks to its known biodegradability, biocompatibility, and its flexible and resilient backbone structure. Peculiar properties are obtained from this PCL addition, contributing to the design of an innovative gene delivery system, with two different portions: a hydrophobic one given by PCL and a hydrophilic one consisting of PEI and mPEG. The PEI-PGL-PCL vector compared to previous systems could potentially reduces the cytotoxicity of PEI and promotes the proper uptake of nucleic acid / polymer complexes into cells. The project carried out in the past months involved the total screening of two PEI based copolymers as potential polyplex gene delivery systems.