Liposomes for Use in Gene Delivery

Purpose: Liposomes are versatile, biocompatible vesicular carriers that can encapsulate and deliver DNA or RNA, but the relationship between lipid charge, lipid packing, and gene-delivery efficiency remains complex. This review discusses physical packing parameters and specific lipids used for gene delivery, classified by overall charge.

Hypothesis: As a review, there is no single experimental hypothesis. Central thesis: if liposomal lipid composition and charge are rationally matched to nucleic acid cargo and cellular barriers—using cationic and neutral helper lipids, and modifying surfaces with PEG or targeting ligands—then liposome-mediated gene delivery can be improved; anionic liposomes are generally less efficient unless divalent cations are used to overcome electrostatic repulsion.

Aims: Describe liposome formation, morphology, and the structure-packing parameter. - Review cationic, neutral, and anionic lipids used for gene delivery. - Discuss modifications for improved liposome-mediated gene delivery, including PEGylation and targeting ligands. - Compare lipid systems by transfection efficiency, cytotoxicity, and applicability.

Delivery system: Platform: Liposomes and lipoplexes (lipid–DNA/RNA complexes). - Cationic lipids: DOTMA, DOTAP, DC-Chol, DOSPA, DOGS. - Neutral helper lipids: DOPE, DOPC; cholesterol and galactosylated cholesterol derivatives. - Anionic lipids: phosphatidic acid, phosphatidylglycerol, phosphatidylserine; DOPG/DOPE mixtures. - Payloads: plasmid DNA, RNA. - Surface modifications: PEG, L-amino-acid-based polymers, transferrin, haloperidol. - Anionic lipoplex additives: divalent cations—Ca²⁺, Mg²⁺, Mn²⁺, Ba²⁺; Ca²⁺ was most effective.

Approach: Review of in vitro and in vivo literature. No primary experimental study. Includes multiple cell lines, such as CHO cells, primary hippocampal neurons, Hep G2 human hepatoma cells, and human breast cancer cells. Covers transfection efficiency, cytotoxicity, serum stability, and biodistribution. Not a systematic review.

Key methods: Structure-packing parameter: \(P = v/(a l_c)\), predicting spherical molecules, cylindrical molecules, flexible bilayers/vesicles, planar bilayers, or inverted micelles/hexagonal \(H_{II}\) phase. - Transfection reporter assays: luciferase, β-galactosidase, chloramphenicol acetyltransferase (CAT). - Biophysical characterization: circular dichroism, freeze-fracture electron microscopy, cryo-TEM. - Serum stability, cytotoxicity, and cellular uptake studies.

Key results: DOTMA facilitated up to 100-fold more efficient gene delivery than DEAE-dextran coprecipitation or calcium phosphate. - DC-Chol produced up to 2–4-fold greater CAT expression and a 4-fold reduction in cytotoxicity versus Lipofectin in some cell lines. - DOGS gave transgene expression more than 10-fold greater than calcium phosphate transfection, with no noticeable cytotoxicity. - DOTAP/DOPE and DOTMA/DOPE formulations were not statistically different in transfection activity or cytotoxicity. - DOPE generally yielded higher transfection efficiencies than DOPC in many cell types; the inverted hexagonal phase at low pH aids endosomal escape. - DOTAP without DOPE required a 2:1 N:P ratio to neutralize DNA, whereas DOTAP/DOPE 1:1 neutralized and complexed DNA at all charge ratios. - PEGylation increased transfection in the presence of serum and prolonged circulation, but inhibited endocytosis and proper complex dissociation depending on PEG mole percentage. - Anionic lipoplexes: Ca²⁺ was the most effective cation for DNA compaction; calcium concentrations >25 mM were detrimental due to aggregate lipoplexes ≥500 nm; optimal transfection occurred at particle sizes around 200 nm.

Interpretation: Liposomes are versatile supramolecular assemblies for gene delivery. Cationic and neutral helper lipids are typically used because DNA is polyanionic; anionic liposomes are less efficient but may offer biocompatibility advantages. Rational design based on lipid charge, packing parameter, and surface modification—especially PEGylation and targeting ligands—can improve transfection and in vivo performance. However, PEGylation can impair endocytosis and complex dissociation, so biodegradable or cleavable PEG strategies are desirable.

Limitations: This is a review, not a primary study. Knowledge of anionic lipofection remains limited, DNA entrapment in anionic liposomes is inefficient, and cytotoxicity data are inadequate. The mechanism of serum inactivation of lipoplexes is not fully explained. PEGylation can inhibit endocytosis and complex dissociation. Anionic lipoplexes require divalent cations and are sensitive to calcium concentration and aggregate size. No large-animal validation or long-term safety data are presented.