Lipid and polymeric carrier-mediated nucleic acid delivery

Purpose: Nucleic acid therapeutics—plasmid DNA, antisense oligonucleotides, and RNAi molecules—have broad therapeutic potential but face major biological barriers, including nuclease degradation, poor membrane penetration, endosomal entrapment, nonspecific biodistribution, and immune activation. The review addresses how lipid- and polymer-based carriers can be designed to overcome these barriers and improve nucleic acid delivery.

Hypothesis: As a review, no formal experimental hypothesis is tested. The central premise is that effective nucleic acid therapy requires a thorough understanding of biological barriers and the structure–activity relationships of lipid and polymeric carriers. If carrier components—cationic head group, hydrophobic anchor, linker, polymer architecture, and surface modifications—are optimized, then stability, cellular uptake, endosomal escape, cargo release, and target-cell specificity can be improved.

Aims: Review biological barriers to nucleic acid therapeutics. - Summarize commonly used cationic lipids, cationic polymers, and non-cationic lipids/polymers, including their characteristics, applications, advantages, and limitations. - Discuss design elements for effective delivery: liposomal systems, oligonucleotide/siRNA conjugates, and micro/nanoparticle systems. - Provide expert opinion on rational design and future translation of lipid/polymeric nucleic acid carriers.

Delivery system: Cationic lipids: DOTMA, DOTAP, DOSPA, DMRIE, DC-Chol, DOGS, DDAB, pyridinium lipids, SAINT-2, BGSC. Typical three components: cationic head group, hydrophobic domain, and linker. Helper/co-lipids: DOPE, cholesterol, DOPC. Cationic polymers: linear and branched PEI, chitosan, cyclodextrin-based cationic polymers, PAMAM and PPI dendrimers, poly(L-lysine), DEAE-dextran. Non-cationic lipids/polymers: DOPE, cholesterol, Pluronic/poloxamer, thiourea derivatives, PLGA. Conjugates: cholesterol-ODN/siRNA, PEG-siRNA/ODN, Gal-PEG-ODN, HPMA-ODN, Dynamic PolyConjugate-siRNA. Particulate systems: liposomes, PEGylated SPLP/SNALP, PLGA micro/nanoparticles. Payloads: plasmid DNA, antisense ODN, siRNA.

Approach: Review article; no original experimental groups, n, doses, or controls. It synthesizes published in vitro studies (CHO, HEK293, COS7, HeLa, HepG2, BHK-21, MES-SA, CT-26, 293T, etc.) and in vivo rodent studies (rats, mice, tumor models, HBV mouse model, liver fibrosis model, etc.).

Key methods: Summarizes techniques from cited studies: luciferase transfection assays, gene silencing/RNAi, particle size and zeta potential, phase behavior (Lα/HII), serum stability, cytotoxicity/viability, biodistribution/radioactivity, fluorescence/confocal imaging, and in vivo efficacy in disease models.

Key results: Pyridinium lipids with trans configuration showed higher transfection than cis counterparts; C16:1 unsaturated lipids outperformed C18:1 and C20:1; amide linkers generally gave higher transfection than ester linkers. - PEI: optimal DNA complexation molecular mass commonly 5–25 kDa; branched PEI gives higher in vitro transfection under serum-free conditions, while linear PEI is less serum-sensitive and more suitable for in vivo delivery. 25 kDa branched PEI/pDNA particles were ~180 nm, whereas linear PEI complexes aggregated under similar conditions. - Chitosan: particle size increased from ~100 to 500 nm as chitosan molecular mass increased from 7 to 540 kDa; high deacetylation (~80% or above) is generally used for efficient nucleic acid condensation. - PAMAM dendrimers: only ~1% of G3/G4 dendrimers remained in blood circulation, while >60% accumulated in liver at 1 h after intravenous injection in rats. - Vitamin A-coupled cationic liposomes delivering siRNA almost completely resolved liver fibrosis and prolonged rat lifespan. - PEGylated SNALP anti-HBV siRNA significantly inhibited HBV DNA and HBsAg expression in an HBV mouse model. - Cholesterol-conjugated apoB siRNA silenced apoB mRNA in liver and jejunum and reduced total cholesterol after systemic administration. - Gal-PEG-ODN enhanced hepatocyte uptake; hepatic localization was ~60% in hepatocytes vs ~40% in non-parenchymal cells, and pre-injection of Gal-BSA inhibited liver accumulation. - Dynamic PolyConjugate siRNA silenced apoB and ppara in mouse liver after systemic administration. - PLGA particles generally showed low nucleic acid encapsulation (<1 µg/mg); condensation with poly(L-lysine) or cationic polymers improved encapsulation and protection. - Anti-VEGF siRNA encapsulated in PLGA microspheres inhibited tumor growth in mice.

Interpretation: Cationic lipids and polymers are widely used and valuable nucleic acid carriers, but each has advantages and limitations. In vivo delivery is complex: carriers must protect cargo, cross biological barriers, escape endosomes, release nucleic acids at the right site, and avoid toxicity and nonspecific distribution. PEGylation, cleavable linkages, and targeting ligands are promising design strategies. siRNA differs from DNA in complexation and delivery behavior, so DNA-optimized conditions may not translate directly. The authors conclude that a thorough understanding of biological barriers and carrier structure–activity relationships is essential for effective nucleic acid therapy.

Limitations: Review only; no new primary data. Many cationic lipids/polymers are effective in vitro but inefficient in vivo. PEI and high-generation dendrimers can be cytotoxic; PLGA systems often suffer low encapsulation and nucleic acid inactivation; siRNA conjugates remain early-stage with limited therapeutic/side-effect data. Long-term safety, immune effects, scalable manufacturing, and clinical translation remain unresolved.