Cationic Polymers
In gene therapy projects, the delivery of exogenous nucleic acids into cells generally requires a corresponding vector. These vectors need to be able to efficiently, safely and accurately carry specific genes into the nucleus of the desired cells. Some cationic polymers (CPs) have attracted the attention of biologists because of their ability to coagulate DNA, but the existing cationic polymers are difficult to put into clinical trials due to their physical and chemical defects. We have summarized some of the latest physicochemical and biological information about cationic polymers, and hope to provide you with new insights into the improvement and application development of this gene delivery system.
Cationic Polymers as Gene Carries
Diethylaminoethyl-dextran (DEAE-dextran) is generally considered to be the first cationic polymer for gene transfection, but its relatively low transfection efficiency, toxicity and non-biodegradability have prevented its development in gene therapy. For more than a decade, linear polylysine (pLL) has been widely used in the study of gene delivery. The non-uniformity of the chain length of commercially available pLL, and the major differences in the size distribution of the polymers resulting therefrom, are the main reasons for the development of polymers based on oligomers and synthetic polypeptides. In order to improve the solubility and stability of polymers and reduce non-specific interactions with biomolecules, scientists have continued to develop cationic copolymers with hydrophilic chain segments (pEG), mainly pLL-based block copolymers and comb copolymers. Recently, cationic polysaccharides such as CP and chitosan based on methacrylate have been introduced in some studies.
Figure 1. Cationic polymers for non-viral gene delivery to human T cells. (Olden, 2018)
DNA Condensation by Cationic Polymers
In many cases, plasmid DNA undergoes a large compression process in the presence of condensing agents (such as polyvalent cations and CP). After condensation, bare DNA coils, often with a hydrodynamic size (Rh) of hundreds of nanometers, are often compressed to Rh of only a few tens of nanometers, reducing their volume by 1,000 to 10,000 times. And unlike the high-level structure of proteins, DNA coils generally don't have unique compact structures. Over a wide range of DNA lengths, the DNA coils form a circular structure after coagulation. However, short DNA molecules (400 bp) do not form a torus, and large DNA strands (166 000 bp) form spherical spheres. DNA loops are usually observed when CP is used as a condensing agent, while spherical structures and loops can be detected when using pEG-pLL graft copolymers and pEG-pLL block copolymers, respectively. The cause of this morphological difference is unclear.
Advantages of Cationic Polymers
Cationic polymers have been shown to be a safe, predictable, biodegradable and non-toxic alternative to viral gene therapy, which relies on the incorporation of biopolymers into synthetic polymer-based carriers on target genes or other biomolecules swallow effect. Compared with liposomes, cationic polymer-based gene carriers (multimers) show good biodegradability, low toxicity, diverse structure and relatively high transfection efficiency. Currently, several important cationic polymers such as chitosan, PEI, polylysine, and polyurethane can be used as cationic carriers for gene delivery. Most of these cationic segments are derived from polyamines, which not only protects the DNA from degradation, but also promotes endocytosis of the carrier by the endosomal membrane.
Disadvantages and Optimization of Cationic Polymers
For cationic polymer carrier-based gene transfer, there are still many in vitro and in vivo barriers in practice to achieve the desired transfection efficiency. For in vitro transfection, polymeric nanoparticles do not have the magical structure of a virus entering a cell, but only depend on unpredictable endocytosis. Therefore, on the one hand, more detailed information about the mechanism of endocytosis through various technical methods is needed to increase the chance of entering cells. On the other hand, we need to conduct in-depth research on the size, surface potential and N/P ratio (ratio of polymer nitrogen to DNA phosphate) and integrate some targeting groups (such as nuclear localization signals) into the nanoparticles.
In addition, the escape of nanoparticles from organelles and the escape of DNA from complexes is one of the key factors affecting transfection efficiency. Another big problem with polymer nanoparticles is cytotoxicity, which is caused by poor biodegradability. Thus, the degradable moieties have been incorporated into the polymer, such as coating with human serum albumin, dextran, PEG and so forth. Out of these, the specific hurdle for transfection in vivo is that polymeric nanoparticles are foreign materials whose invasion will lead to the immune response by the body. The most effective solution is PEGylation, which helps avoid the clearance of the reticuloendothelial system.
Reference
- Olden, B. R.; et al. (2018). Cationic polymers for non-viral gene delivery to human T cells. Journal of controlled release. 282: 140-147.
