Polymeric Nanocarriers for Non-Viral Gene Delivery

Purpose: Viral vectors are efficient but carry risks of immunogenicity, inflammatory reactions, recombination, limited DNA cargo capacity, and manufacturing difficulties. Non-viral polymeric vectors are safer and easier to produce, but their transfection efficiency remains much lower than viral vectors because of multiple extracellular and intracellular barriers. This review summarizes these barriers and recent strategies to overcome them using polymeric nanocarriers.

Hypothesis: This is a review article and does not test a single formal hypothesis. Its central premise is that polymeric nanocarriers — including PLL, PEI, PDMAEMA, PAMAM, chitosan, PAEs, and PAAs — can be rationally modified to overcome biological barriers, reduce toxicity, and improve non-viral gene delivery efficiency.

Aims: Describe the current understanding of biological barriers to non-viral gene delivery. - Review recent progress in addressing these barriers. - Discuss the most widely used non-viral polymeric nanocarriers. - Highlight strategies developed over nearly two decades to improve gene transfer, including chemical modification, targeting, stimuli-responsive release, and biodegradable designs. - Provide conclusions and future perspectives for polymeric gene delivery.

Delivery system: Carrier class: Cationic polymeric nanocarriers forming polyplexes with nucleic acids. - Polymers reviewed: Poly(L-lysine) (PLL), polyethyleneimine (PEI), poly[2-(dimethylamino)ethyl methacrylate] (PDMAEMA), polyamidoamine (PAMAM) dendrimers, chitosan, poly(amino-co-ester)s (PAEs), and poly(amidoamine)s (PAAs). - Payloads: Plasmid DNA and small interfering RNA (siRNA). - Modifications: PEGylation; hydrophobic modification with lipids, cholesterol, stearic acid, palmitic acid, Tween 85, Pluronic; targeting ligands such as folate, transferrin, galactose, mannose, RGD peptide, antibodies, and cell-penetrating peptides; pH-sensitive, reduction-sensitive, and enzyme-sensitive linkages; disulfide cross-linking; nuclear localization signals; dexamethasone conjugation. - Formulations: Polyplexes, micelles, dendrimers, nanoparticles, ternary complexes, lipopolyplexes, and biodegradable cross-linked systems.

Approach: Review of in vitro and in vivo literature. In vitro models include COS-7, HepG2, HeLa, 293T, CT-26, B16-F10, C2C12, CHO, SkBr-3, and other cell lines. In vivo models include tumor xenografts, lung metastasis, glioma, and dystrophic mdx mice. No new primary experiments, group sizes, or doses are reported.

Key methods: Polyplex formation, DNA condensation, and gel electrophoresis. - Particle size and zeta potential measurements. - Cellular uptake and transfection efficiency assays. - Cytotoxicity and cell viability assays. - Endosomal escape and buffering capacity measurements. - In vivo gene expression, tumor growth inhibition, and biodistribution. - Fluorescence imaging and flow cytometry for cellular internalization. - siRNA-mediated gene silencing assays.

Key results: Optimal size: Non-targeting cationic vector–DNA complexes are most effective at 70–90 nm; microparticles of 1–10 µm are preferentially taken up by phagocytic antigen-presenting cells. - PLL: Low buffering capacity (pH 5.7–7.7) limits endosomal escape; palmitic acid substitution improved transfection; disulfide-linked PEG-PLL (mPEG-SS-PLL) gave >10-fold higher in vitro gene transfer than non-reducible controls. - PEI: Gold standard for polymeric gene delivery; branched PEI contains 25% primary, 50% secondary, and 25% tertiary amines; 5–25 kDa bPEIs are most suitable; high molecular weight and branching increase efficacy but also cytotoxicity. - PDMAEMA: Transfection efficiency increases with molecular weight in the range 43,000–915,000 g/mol; PDMAEMA/DNA ratios of 6/1 (w/w) for polymers >300 kDa gave highest transfection with acceptable cytotoxicity. - PAMAM: Low generations (G0–G3) show poor transfection and low cytotoxicity; high generations (G4–G8) show better transfection but cytotoxicity; degraded PAMAM (Superfect) and PAMAM-ABP improved delivery. - Chitosan: Degree of deacetylation >65% is needed for stable pDNA complexes; optimum N/P ratio around 5; optimal transfection pH 6.8–7.0; tocopheryl oligochitosan-based systems achieved >98% siRNA cellular uptake and outperformed Lipofectamine 2000 in silencing. - PAEs: Best PBAEs are linear, ~10 kDa, with hydroxyl side chains, primary amine end groups, and tertiary amines; C32 delivered DNA ~4-fold better than jetPEI intratumorally; hydrophobic terpolymers exceeded Lipofectamine 2000 in HeLa cells. - PAAs: Bioreducible PAAs showed up to 20-fold higher transfection efficiency than PEI-25kDa; hyperbranched PAAs gave up to 2-fold higher gene expression than linear PEI.

Interpretation: Non-viral polymeric gene vectors have advanced substantially, but transfection efficiency remains lower than viral vectors. Increasing cationic charge density can improve in vitro transfection but often increases cytotoxicity and hinders in vivo application. Coating positive polyplexes with anionic layers, incorporating targeting ligands, and designing biodegradable or stimuli-responsive polymers can improve safety and efficacy. Cationic polysaccharides and natural polymers are considered promising for future clinical translation because of their biodegradability and low toxicity.

Limitations: This is a review, not a primary study; no new experimental data or meta-analysis. - Many strategies are demonstrated only in vitro; in vivo efficacy remains limited. - Toxicity, immunogenicity, and long-term safety of polymeric carriers are not fully resolved. - Clinical translation of non-viral polymeric gene delivery is still limited. - Structure–function relationships are not fully understood across different cell types and disease models. - No large-animal validation or detailed clinical trial data are presented.