Exploring the role of polymer structure on intracellular nucleic acid delivery via polymeric nanoparticles

Purpose: Viral vectors raise safety concerns, and non-viral polymeric vectors still require optimization for clinical translation. This review addresses how polymer structural features govern intracellular nucleic acid delivery — including cargo protection, cellular uptake, endosomal escape, unpacking, and nuclear delivery — to enable rational design of safer and more effective polymeric gene-delivery nanoparticles.

Hypothesis: This is a review article and does not test a single formal hypothesis. Its central premise is that understanding polymer structure–function relationships — especially cationic amine content, hydrophobicity, molecular weight, and degradable linkages — enables rational engineering of polymeric nanoparticles that overcome intracellular barriers for DNA and RNA delivery.

Aims: Discuss how structural elements of natural and synthetic polymers aid transport of nucleic acid cargo across intracellular barriers. - Describe chemical properties that facilitate each delivery step: nucleic acid binding/protection, cellular uptake, endosomal escape, cargo unpacking, vector degradation, intracellular trafficking, and nuclear uptake. - Highlight specific polymer structures and design principles, including PBAEs, PEI, PLL, chitosan, poly(amido amine)s, polyurethanes, polyorthoesters, and disulfide-containing polymers. - Emphasize the need for balance and moderation in polymer design and the dependence on cargo type and cell type.

Delivery system: Vector type: Polymeric nanoparticles (polyplexes) formed by electrostatic complexation of cationic polymers with anionic nucleic acids. - Payloads: Plasmid DNA (pDNA), siRNA, miRNA, shRNA, mRNA. - Natural polymers: Poly(L-lysine) (PLL), chitosan, histone proteins, cationic peptides. - Synthetic polymers: Poly(ethyleneimine) (PEI; linear and branched), poly(β-amino ester)s (PBAEs), poly(amido amine)s (PAAs), polyurethanes, polyorthoesters, imine-containing polyamines, PLGA, cyclodextrin-containing polymers, epoxide-containing block copolymers, alkyl-amine-containing polymers. - Targeting/functional ligands: Folic acid, epidermal growth factor (EGF), transferrin, MC1SP peptide, cell-penetrating peptides (GALA, KALA, TAT), nuclear localization signals (NLS), DNA-targeted sequences (DTS). - Degradable linkages: Esters, disulfides, imines, orthoesters, MMP-cleavable peptides. - Particle properties: Size ~25–300 nm; zeta potential neutral to positive; cationic surface for cell interaction.

Approach: Review of in vitro and in vivo literature. In vitro models include HEK293T, human glioblastoma cells, human breast cancer cells, CT26, HeLa, COS-7, NIH 3T3, and others. In vivo contexts include murine hepatocytes and tumor models. No new primary experiments are reported.

Key methods: Polymer/nucleic acid binding affinity measurements. - Particle size and zeta potential characterization. - Transfection and gene knockdown assays. - Cellular uptake pathway analysis (clathrin-, caveolae-, macropinocytosis-mediated). - Confocal microscopy for intracellular trafficking. - Microfluidics-assisted polyplex formulation. - High-throughput combinatorial polymer synthesis and screening. - Principal component analysis of physicochemical properties. - Endosomal buffering capacity measurements. - In vivo biodistribution and gene expression.

Key results: Binding affinity is biphasic: Optimal PBAE transfection occurred with binding constants per amine of 1–6 × 10⁴ M⁻¹; too low or too high binding reduced efficacy. - Microfluidic formulation: Produced polyplexes 40–50% smaller than bulk mixing, transfected 6–31% more cells, and gave 1.9–6.8-fold higher total exogenous gene expression. - Buffering capacity: PBAEs had 1.4–4.6 mmol H⁺/g vs PEI 6.2 mmol H⁺/g, but lower cytotoxicity allowed higher weight ratios, yielding ~5-fold higher total buffering capacity than PEI. - PEI acetylation: Acetylating 43% of primary amines increased transfection up to 21-fold over branched PEI, correlated with weakened DNA binding. - Degradable PEI: Crosslinking 800 Da PEI with ester-containing diacrylates matched 25 kDa branched PEI in size, charge, binding, and MW, but gave 16-fold higher transfection and less toxicity. - Disulfide-linked PEI: Comparable siRNA delivery to branched PEI with significantly reduced cytotoxicity. - KALA peptide: Induced 100% endosomal leakage over pH 4.5–8. - NLS-containing polyplexes: Achieved 8-fold greater transfection than without NLS. - Hydrophobicity and molecular weight: Key drivers of uptake and transfection; moderate hydrophobicity optimized delivery, while excessive hydrophobicity caused cytotoxicity. - Uptake pathways: Clathrin-mediated endocytosis enabled the most efficient transfection; caveolae-mediated uptake sometimes led to inefficient intracellular delivery.

Interpretation: Effective polymeric gene delivery requires simultaneous consideration of nucleic acid binding, cellular uptake, endosomal escape, cargo release, intracellular trafficking, and nuclear uptake. Polymer design requires balance: sufficient positive charge for binding but not so strong as to prevent release; sufficient hydrophobicity for uptake but not toxicity; degradability for release and reduced toxicity but not so fast as to lose particle stability. These attributes are significantly dependent on cargo type (DNA vs. siRNA) and cell type. Although no U.S. FDA-approved polymeric nanoparticle systems for DNA or siRNA delivery exist, many clinical trials are ongoing, and improved structure–function understanding should enable next-generation polymeric gene-delivery nanomedicines.

Limitations: This is a review, not a primary study; no new experimental data or meta-analysis. - Focus is primarily on intracellular delivery and trafficking; tissue-scale and systemic barriers (clearance by mononuclear phagocytic system, colloidal stability, serum aggregation, tissue targeting) are only briefly addressed. - Many discussed systems remain preclinical; clinical translation is limited. - Optimal polymer properties are cargo- and cell-type-dependent, limiting universal design rules. - Long-term safety, large-animal validation, and detailed pharmacokinetics/biodistribution are not comprehensively covered.