Solid lipid nanoparticles as nucleic acid delivery system Properties and molecular mechanisms

Purpose: Non-viral nucleic acid delivery requires carriers that protect DNA/RNA from nucleases and facilitate intracellular delivery. Solid lipid nanoparticles (SLNs) are promising, but the molecular mechanisms by which they assemble with nucleic acids, transfect cells, and release cargo remain poorly understood compared with cationic lipids and polymers.

Hypothesis: If cationic SLNs are formulated with an appropriate solid lipid matrix, cationic lipid/co-surfactant, and stabilizing surfactants, then they can bind and protect nucleic acids, deliver them via endocytosis, and release them intracellularly. However, SLN–nucleic acid complexes may differ mechanistically from lipoplexes, likely relying on surface adsorption, matrix stability, and lysosomal/endosomal processing rather than non-bilayer lipid phase transitions.

Aims: Review SLN composition and production methods for nucleic acid delivery. - Analyze physicochemical properties of SLN–nucleic acid complexes (SLNplexes). - Discuss molecular mechanisms of SLNplex assembly and transfection. - Summarize storage stability and lyophilization considerations. - Identify gaps in understanding structure–function relationships and intracellular processing.

Delivery system: Platform: solid lipid nanoparticles (SLNs), including cationic SLNs and nanostructured lipid carriers (NLCs). - Lipid matrix materials: compritol 888 ATO, behenic acid, cetyl palmitate, precirol ATO 5, stearic acid, Imwitor 900P, tricaprin, cholesteryl oleate, glyceryl trioleate, cholesterol, soya lecithin, glyceryl monostearate. - Cationic components: Esterquat 1, benzalkonium chloride, cetylpyridinium chloride, cetrimide/CTAB, DOTAP, DDAB, stearylamine, DC-Chol, 6-lauroxyhexyl lysinate. - Surfactants/co-surfactants: Pluronic F68, Tween 80, Span 85, taurocholate, glycolocholate, octanoic acid. - Payloads: plasmid DNA, antisense oligonucleotides, siRNA. - Production methods: warm oil-in-water microemulsion; hot high-pressure homogenization; solvent emulsification-evaporation; also coacervation, microchannel/microfluidic mixing, and hydrophobic ion pairing.

Approach: Review and synthesis of in vitro and in vivo literature. Cited models include prostate cancer cells and mice; in vivo examples include intradermal injection of siRNA-loaded SLNs showing sustained release for up to 10–13 days, and prolonged foreign protein expression in spleen and liver for approximately 7 days. As a review, it reports no primary experimental groups, n values, doses, or controls.

Key methods: No primary methods. The review discusses data generated by cited studies using: - Isothermal titration calorimetry (ITC), DSC, FTIR, Raman, circular dichroism. - TEM, SEM, AFM for morphology and ultrastructure. - Zeta potential, size, PDI, sucrose gradient centrifugation. - FRET-based lipid mixing assays, ³¹P NMR, molecular dynamics simulations. - Fluorescent labeling, live-cell imaging, particle tracking. - Transfection, gene expression, and siRNA knockdown assays.

Key results: Empty SLNs are typically 50–200 nm; after nucleic acid addition, SLNplexes can reach up to about 500 nm. For general in vivo use, ≤120 nm is preferred. - Cationic SLN formulations usually show zeta potential > +30 mV, which decreases with increasing DNA concentration. - SLNplex formation is spontaneous (ΔG < 0) with a positive enthalpy; entropy gain from counterion release is a major driving force. - DNA largely adsorbs to the SLN surface rather than being encapsulated in the solid core; surface-bound DNA can be nuclease-accessible, though some protection and transfection are still observed. - One SLN formulation with stearic acid, Pluronic F68, and DOTAP neither adopted inverted hexagonal structures nor showed lipid mixing; helper lipids such as cholesterol and DOPE had no effect on transfection. - SLNs showed prolonged release: siRNA release in vivo up to 10–13 days; siRNA-mediated knockdown lasted 5–9 days versus about 3 days for lipofectamine/liposomal formulations. - ODN release from SLNs was considerably slower than from lipofectamine lipoplexes. - Lyophilized cationic SLNs maintained physical integrity for at least 3 years; trehalose was an effective lyoprotectant, but rehydration sometimes increased size up to 100 nm and decreased zeta potential. - NLCs achieved high siRNA encapsulation, with release kinetics adjustable by oil content and, to a lesser extent, solid lipid composition.

Interpretation: The authors conclude that SLNs are promising non-viral nucleic acid carriers, but there is no clear correlation between SLN composition/structure and transfection potency. They argue that further mechanistic studies—especially on SLNplex structure, intracellular trafficking, endosomal escape, and cargo release—are essential for rational development. Advanced biophysical and imaging techniques should be used to clarify these structure–function relationships.

Limitations: Review article; no primary data, effect sizes, n values, doses, or controls. - Mechanistic understanding of SLN-mediated transfection remains limited. - Nucleic acids are mainly surface-adsorbed, making them potentially susceptible to nucleases. - Endosomal escape mechanism is unclear; SLNs may not rely on non-bilayer phase transitions like some lipoplexes. - Intracellular site and timing of cargo release remain enigmatic. - Helper lipids that enhance lipoplex transfection do not necessarily improve SLN transfection. - Lyophilization can alter particle size and zeta potential, requiring optimization. - Lack of standardized structure–function data and limited clinical translation.