Gene therapy holds tremendous potential for treating genetic disorders, cancer, infectious diseases, and beyond by delivering therapeutic genes, small interfering RNA (siRNA), short hairpin RNA (shRNA), and genome-editing tools into target cells. However, effective delivery remains a critical hurdle in translating gene therapy from bench to bedside. Cationic polymers have emerged as promising carriers for nucleic acid delivery due to their ability to condense DNA/RNA, protect cargo from degradation, facilitate cellular uptake, and promote endosomal escape.
Fig. 1 Cationic polymers are used as nucleic acid delivery vehicles (Cai X, et al. 2023).
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Synthetic Polymers
Synthetic polymers are artificial polymers produced by the chemical polymerization reaction of small molecules, or by modifying natural polymers. Different types of synthetic cationic polymers can be obtained depending on the reaction monomer and mechanism. There are conventional cationic polymers, as well as special and new types of cationic polymers. Some commonly used synthetic cationic polymers include DEAE-dextran, poly-amino acids like PLL and PLO, PAMAM, PPI, PEI, PDMAEMA, poly (β-amino ester), PHP, PAGA, PVL, aminated PAHA, polyphosphoester, and PLA.
Conventional Polymers
DEAE-dextran, a modified analog of dextran, was the first cationic polymer used for nucleic acid transfection. However, due to low efficiency and cytotoxicity, new candidates were needed. Poly-amino acids like PLL, PLO, and PAMAM show enhanced transfection efficiency. PPI dendrimers have been explored as gene-delivery agents with reduced cytotoxicity. PEI is known for high transfection efficiency and the proton sponge effect. Various modifications have been made to improve the biocompatibility and efficiency of these conventional polymers.
Polyesters
Poly (β-amino ester), PHP, PAGA, and PVL are examples of cationic polymers with applications in gene delivery. PBAEs are biodegradable and pH-responsive, made by reacting acrylates with amines through Michael addition reactions. PHP, derived from natural sources, is quickly degraded and stable for gene delivery. PAGA, a biodegradable poly-cation, shows promise in cancer therapy. PVL-based polymers are effective gene carriers with tunable properties. Aminated PAHA and PPE are also explored for gene delivery due to their biodegradability and low toxicity.
Polylactide (PLA)
PLA, a biodegradable polymer, can be used for gene delivery when modified with cationic compounds. Histone, gelatin, and protamine are naturally occurring cationic polymers with potential applications in gene transfection. Histone-mediated transfection is effective in delivering nucleic acids, and gelatin shows promise in reducing cytotoxicity. Protamine forms stable complexes with DNA and shows potential as an adjuvant for immune responses.
Natural Polymers
Natural polymers derived from living organisms exhibit remarkable biocompatibility and abundance, making them advantageous for various biological applications.
Histone
Histones, the primary protein components of chromatin, play essential roles in gene regulation by serving as spools for DNA winding. With positively charged residues and nuclear location signals, histones interact with nucleic acids through electrostatic interactions, facilitating their transfection into target cells. Histone-mediated transfection, termed histonefection, demonstrates effectiveness in delivering DNA, mRNA, and siRNA, particularly in primary cell lines. Despite concerns regarding interference with transcription, innovative strategies such as fibroblast-targeting DNA carriers and upconversion luminescence-guided nanoparticles have enhanced histone-based gene delivery for cancer treatment.
Gelatin
Extracted from animal collagen, gelatin is a biocompatible and biodegradable polymer with amphiphilic properties. Its amino acid sequences, including Arg-Gly-Asp (RGD), confer advantages in cell adhesion. The cationization of gelatin enhances its binding affinity to nucleic acids and cellular membranes, making it a promising transfection reagent for small RNA and plasmid DNA. Studies demonstrate the efficacy of cationic gelatin microspheres in tumor suppression and sustained gene expression, highlighting its potential in cancer therapy.
Protamine
Protamine, a polycationic peptide, exhibits a strong affinity for nucleic acids through electrostatic interactions. By adjusting complexation parameters, protamine can form nanoparticles with tunable sizes for various delivery requirements. Protamine not only facilitates nucleic acid delivery but also serves as an adjuvant to stimulate immune responses, enhancing its potential for anti-cancer vaccination. Intranasal delivery of mRNA-containing protamine complexes demonstrates efficacy in eliciting potent immune responses against tumor growth.
Cyclodextrins
Naturally occurring cyclic oligosaccharides, cyclodextrins, can form stable complexes with plasmid DNA, enhancing transfection efficiency. Derivatives of cyclodextrins further improve complex stability and reduce cytotoxicity, making them valuable in gene delivery. Cyclodextrin-based hydrogels offer sustained treatment at tumor sites while minimizing exposure to non-target tissues, showing promise in cancer therapy.
Chitosan
Derived from chitin, chitosan is a cationic polysaccharide with biocompatibility and biodegradability. Its mucosal adhesion properties enable diverse routes of administration, including intranasal delivery for brain cancer treatment. Strategies such as molecular weight reduction and chemical modifications enhance chitosan's solubility and efficacy in nucleic acid delivery. Nucleus-targeted co-delivery vectors based on chitosan demonstrate efficient gene and drug delivery into cancer cell nuclei, offering potential for targeted cancer therapy.
Applications of Cationic Polymers in Gene Therapy
Cationic polymers serve as versatile carriers for gene therapy, facilitating the delivery of plasmid DNA, mRNA, and siRNA for gene augmentation, suppression, and genome editing.
Gene Augmentation
In protein replacement therapies, cationic polymers deliver functional genes to restore protein production, offering potential treatments for monogenic recessive diseases. For instance, oral insulin gene delivery systems based on chitosan copolymers demonstrate sustained protection against hyperglycemia in diabetic mice, showcasing the therapeutic potential of cationic polymer-based vectors.
In vaccine development, cationic polymers provide alternatives to lipid nanoparticles for mRNA vaccines, enabling systemic delivery and immune response stimulation. Novel modifications, such as alkylated dioxophosphoryl oxides to cationic phospholipidated polymers, enhance mRNA delivery efficiency and immunogenicity, offering promising avenues for vaccine development beyond infectious diseases.
Gene Suppression
Cationic polymers facilitate the delivery of siRNA and shRNA for gene silencing, enabling targeted therapies for various diseases. Complexes formed by cationic polymers and chemotherapeutic agents demonstrate effective cascade therapy in tumor inhibition, highlighting the potential of RNA interference in cancer treatment.
Genome Editing
In CRISPR/Cas-mediated genome editing, cationic polymers serve as efficient delivery vehicles for Cas9 and sgRNA plasmids, enabling precise gene modifications. Recent advancements, such as the use of Cas13 for RNA editing, open new therapeutic avenues for treating diseases at the RNA level. Polymer-based delivery systems for Cas13a mRNA demonstrate efficacy in attenuating viral infections, showing promise for respiratory disease treatments.
Conclusion
Cationic polymers represent powerful tools in gene therapy, offering versatile platforms for delivering therapeutic nucleic acids. Continued research and innovation in polymer design and delivery strategies hold immense potential for advancing gene-based treatments and addressing a wide range of diseases in clinical settings.
Reference
- Cai X., et al. Cationic polymers as transfection reagents for nucleic acid delivery. Pharmaceutics. 2023, 15(5): 1502.
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