Background
Chitosan (CTS), also known as deacetylated chitosan, is a natural polycationic polysaccharide with the characteristics of non-toxicity, biodegradability, regeneration and low immunogenicity, and can be used for drug loading. However, the strong hydrogen bonding within and between CTS molecules gives it a regular molecular chain and good crystallization performance, and makes it only soluble in acidic aqueous solutions and very few complex organic solvents but not in neutral or alkaline aqueous solutions, which greatly restricts its application in the field of biomedicine and makes improving the water solubility of CTS a hot issue in the development of CTS-based biomaterials.
The -OH and -NH2 groups that exist in large quantities in the CTS structure have good reactivity, and their modification can give CTS many properties. For example, N, N, N-trimethyl chitosan ammonium chloride (TMC) can be obtained by trimethylation of -NH2, and the presence of a strongly hydrophilic quaternary ammonium salt structure in the TMC structure makes it more water-soluble than CTS. By introducing carboxyl groups into the CTS structure, it can not only destroy its interchain bonding force, but also give it the ability to absorb moisture and chelate metal ions. By introducing polyethylene glycol (PEG) structure into the sugar chain of CTS, it can not only destroy its original hydrogen bond structure, but also make it show better film-forming and hydrophilic and lipophilic properties. Among them, the polyethylene glycol modification of CTS can fully maintain the biocompatibility of CTS while achieving its solubility in aqueous solutions with a wider pH range through the selection of PEG polymerization degree and the control of grafting degree.
PEG is a linear nonionic polymer with a polyoxyethylene chain (PEO) as the backbone. It has the properties of hydrophilicity, lipophilicity, non-toxicity, high biocompatibility and non-immunogenicity. It has been approved by the US Food and Drug Administration for use in pharmaceutical preparations and other fields. PEG has flexibility and its ability to form hydrogen bonds with water, hydroxyl and amino groups, so that it can be used to modify polar polymers to improve their film-forming properties and moisture absorption and breathability. Active PEG containing reactive groups obtained by activating the terminal hydroxyl group of PEG can be used to introduce PEG structure into CTS sugar chain because it can undergo chemical bonding reaction with hydroxyl or amino group.
Preparation of PEG-modified CTS
The PEG modification of CTS mainly includes physical composite method and chemical modification method. The physical composite method mainly utilizes the hydrogen bonding between PEG and CTS or its derivatives to achieve the effective combination of CTS and PEG, while the chemical modification method mainly introduces the PEG structure to the sugar chain through the chemical bonding between PEG derivatives containing active groups and CTS or its derivatives to obtain CTS-PEG.
- Preparation of PEG-modified Chitosan by Physical Composite Method
The preparation of PEG-modified CTS by physical composite method is mostly achieved by uniformly mixing CTS dissolved in acidic aqueous solution with other solutions containing PEG. The products obtained mainly include CTS-PEG composite film, gel and microspheres.
- PEG Modification of Chitosan
The PEGylation modification of CTS mainly includes the grafting modification of CTS and its derivatives by PEG derivatives containing single active groups, the cross-linking modification of CTS and its derivatives by PEG derivatives containing double active groups, and the grafting effect between PEG and CTS derivatives containing active groups. Among them, the grafted modified products are divided into N-substituted, O-substituted and N, O-substituted products according to the position of the introduced PEG group. The PEG derivatives containing single active groups mainly include methoxy polyethylene glycol aldehyde (mPEG-CHO), methoxy polyethylene glycol acid (mPEG-CO2H), polyethylene glycol succinic acid monoester, terminal iodinated polyethylene glycol monomethyl ether, methoxy polyethylene glycol acrylate, etc. Cross-linking modification is mostly used to prepare biomaterials such as CTS-based microspheres or gels. The PEG derivatives containing double active groups mainly include polyethylene glycol diglycidyl ether and polyethylene glycol diacrylate.
Application of PEG-modified CTS in Various Fields
PEG-modified CTS not only retains the excellent properties of CTS, but also imparts better hydrophilicity and amphiphilicity, thus further expanding the application scope of CTS.
- Application of PEG-modified CTS in Drug Loading and Controlled Release
The derivatives produced by CTS after PEGylation not only have improved hydrophilicity, but also have some lipophilicity. Therefore, it can not only be used as a carrier for drug loading in a wider pH range to achieve controlled drug release, widen the "therapeutic window" of drugs and improve the drug utilization efficiency, but also reduce the side effects of drugs and improve the stability of drugs.
- Application of PEG-modified CTS in Tissue Engineering
CTS has good biocompatibility, degradability and non-toxicity, and is an ideal extracellular matrix material. CTS and its derivatives have been widely used in tissue engineering.
- Use of PEG-modified CTS in Antibacterial Materials
The protonatability of the amino group in the CTS structure makes it antibacterial, and the reactivity of the hydroxyl and amino groups in its structure makes it possible to form antibacterial and antimicrobial agents with wider applications through derivatisation and transformation. A variety of CTS-based antimicrobial and antibacterial materials, including CTS-PEG, have been developed.
- Application of PEG-modified CTS for Delivery of Bioactive Substances
Figure 1. The properties of the PEGylated chitosan/plasmid nanocomplex. (Kaifeng Chen, et al. 2019)
The purpose of gene therapy is to introduce nucleic acids into diseased cells to repair or replace them, and ultimately to treat diseases caused by genetic causes at the genetic level. Gene therapy has been successful in treating cancers, viral infections, cardiovascular diseases, etc. caused by genetic mutations, and compared to genetic diseases that cannot be treated by traditional methods, this method may have great potential. However, the inability to effectively achieve targeted delivery of genes to diseased cells, degradation during gene delivery and rapid clearance in the circulation, and low stability and toxicity of nucleic acid drugs limit the widespread application of gene therapy. As a natural cationic polymer with excellent biocompatibility, CTS can effectively bind to negatively charged DNA and prevent it from being degraded by nucleases, allowing it to be used as a non-viral gene delivery vehicle in gene therapy. However, CTS has the disadvantage of being slow to separate from the cytoplasm during gene delivery, which affects its efficacy. PEGylated polycationic compounds have improved biomolecule stability and can reduce non-specific interactions with biomolecules, giving them obvious advantages in loading nucleic acids and nucleic acid drugs and in the treatment of genetic diseases.
Reference
- Kaifeng Chen, et al. Cationic polymeric nanoformulation: Recent advances in material design for CRISPR/Cas9 gene therapy. Progress in Natural Science. 2019, 29(6).
