Cytosolic delivery of nucleic acids The case of ionizable lipid nanoparticles

Purpose: Endosomal escape remains the major intracellular barrier for RNA therapeutics. Only a small fraction of RNA delivered by ionizable lipid nanoparticles (LNPs) reaches the cytosol, where siRNA, miRNA, mRNA, and CRISPR components must act. A deeper mechanistic understanding is needed to design next-generation LNPs with improved cytosolic delivery.

Hypothesis: If ionizable lipids with an optimal pKa become protonated in acidic endosomes and form non-bilayer, inverted hexagonal (HII) structures upon ion pairing with anionic endosomal lipids, then they can destabilize the endosomal membrane, promote fusion/disruption, and release nucleic acid cargo into the cytosol. Understanding and mimicking viral fusion mechanisms may further improve LNP endosomal escape.

Aims: Describe the journey of RNA-loaded LNPs across intracellular barriers, from extracellular space to cytosol. - Review the biophysical mechanisms of LNP-mediated endosomal escape using physico-chemical, cell-based, and computational evidence. - Compare LNP escape strategies with those used by enveloped viruses to deliver genetic material. - Identify analytical tools and nature-inspired design principles for future LNPs with improved cytosolic RNA delivery.

Delivery system: Platform: ionizable lipid nanoparticles (LNPs), typically composed of ionizable lipid, phospholipid, cholesterol, and PEG-lipid. - Payloads: siRNA, mRNA, antisense oligonucleotides (ASOs), miRNA, and CRISPR-Cas9 components (sgRNA/Cas9 mRNA). - Key ionizable lipids discussed: DODAP, DLin-DMA, DLin-KC2-DMA, DLin-MC3-DMA (MC3), lipid 5, SM-102, ALC-0315, and others. - Mechanism: ionizable lipids are positively charged at acidic pH for RNA condensation, near-neutral at physiological pH to reduce toxicity, and protonated in endosomes to drive membrane destabilization. - Viral-inspired components: GALA, KALA, HA2-derived peptides, TAT, and other fusogenic or pH-sensitive peptides. - Targeting/uptake: ApoE-mediated hepatocyte uptake via LDL receptor; targeting ligands can redirect uptake.

Approach: Review and synthesis of in silico, in vitro, and in vivo literature. Cited models include hepatocytes, cancer cell lines, primary cells, genetically modified haploid cells, non-human primates, and human clinical data (patisiran, COVID-19 mRNA vaccines, CRISPR-Cas9 LNP trials). 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: - Molecular dynamics simulations: coarse-grained Martini, all-atom CHARMM36 and OPLS force fields. - Cryo-EM, SAXS, and NMR for LNP structure and lipid phase behavior. - Confocal and electron microscopy with fluorescent or gold-labeled siRNA/mRNA. - Galectin-8 (Gal8) recruitment assays for endosomal disruption. - Live-cell imaging, real-time 3D single-particle tracking. - Membrane fusion assays with model endosomal vesicles. - In vivo imaging, biodistribution, and protein expression/knockdown assays.

Key results: Only 1–2% of siRNA delivered by MC3-LNPs was visualized in the cytosol in one key study; another estimated ~3.5% cytosolic release. - Endosomal escape occurs in a narrow time window from a hybrid early/late endosome compartment, not from late endosomes or lysosomes. - Late endosome/lysosome formation may still be required for efficient mRNA translation via mTORC1 signaling. - Lipid 5 LNPs achieved a 6-fold increase in cytosolic mRNA and 15% of internalized mRNA available for translation compared with MC3-LNPs. - DLin-MC3-DMA has a pKa of 6.44; MC3-based LNPs showed two orders of magnitude higher potency than first-generation DLin-DMA LNPs for hepatic FVII silencing. - Onpattro (patisiran) was FDA-approved in 2018; COVID-19 mRNA vaccines mRNA-1273 and BNT162b2 showed protection efficacies >94%. - Viral fusion proteins undergo pH-induced conformational changes to expose hydrophobic fusion peptides; GALA/KALA peptides improved endosomal escape in LNP/lipoplex systems. - Phytosterol-substituted LNPs (e.g., sitosterol-eLNPs) improved intracellular mRNA delivery and mobility.

Interpretation: The authors conclude that endosomal escape is the key limiting step for cytosolic RNA delivery. Promoting endosomal membrane instability—through optimized ionizable lipids, faceted LNP morphology, or virus-inspired fusogenic peptides—can improve RNA bioavailability. They argue that a multidisciplinary toolkit for quantifying endosomal escape and nature-inspired design will be essential for next-generation LNPs.

Limitations: Review article; no primary data, systematic meta-analysis, or new experiments. - Quantification of cytosolic RNA remains technically challenging and may underestimate or mislocalize cargo. - Computational models are simplified: often symmetric membranes, artificial dehydration, and incomplete electrostatic representation. - Only a limited number of computational studies address LNP endosomal escape directly. - MC3 is non-biodegradable, raising concerns for repeated dosing. - Extrahepatic delivery, organ tropism, and clinical translation remain unresolved. - High-throughput screening tools for cytosolic delivery are still needed.