Lipid Nanocarriers for microRNA Delivery

Purpose: miRNAs can regulate about 60% of protein-coding genes and enable targeting of previously “undruggable” proteins and diseases, but their clinical use is limited by nuclease degradation, poor tissue targeting, inefficient cellular uptake, and inadequate cytosolic release. Lipid nanocarriers are a versatile, biocompatible platform that may overcome these delivery barriers.

Hypothesis: If miRNA is formulated into optimised lipid nanocarriers with appropriate lipid composition, size, charge, PEG coating, and targeting ligands, then it can be protected from degradation, delivered to target tissues and cells, internalised by endocytosis, escape the endosome, and reach the cytosol to modulate gene expression without unacceptable toxicity.

Aims: Review the development of therapeutic lipid-based nanoparticles for miRNA delivery. - Outline challenges and opportunities for miRNA-based therapies and the complexity of delivering functional miRNAs. - Address critical delivery issues: toxicity, disease-site targeting, cellular uptake, and endosomal escape. - Discuss current preclinical and clinical applications and provide an outlook for future clinical approaches.

Delivery system: Platform: lipid-based nanocarriers, including cationic lipoplexes, neutral/anionic lipoplexes, ionisable lipid nanoparticles/SNALPs, lipidoid nanoparticles, amphoteric liposomes (Smarticles®/NOV340), liposome–polycation–hyaluronic acid (LPH) hybrids, neutral lipid emulsions, solid lipid nanoparticles (SLNs), and niosomes. - Payloads: miRNA mimics, pre-miRNAs, miRNA inhibitors/antagomiRs, anti-miRNAs, and co-delivered siRNA or small-molecule drugs. - Representative lipids: DOTAP, DOTMA, DOSPA, DOGS, DC-Chol, DDAB, DOPE, cholesterol, DSPC, DOPC, DODAP, DODMA, DLinDMA, DLinDAP, DLin-2-DMAP, DLin-C-DAP, DLin-K-DMA, DLin-KC2-DMA, DLin-MC3-DMA, YSK05, and others. - PEGylation: PEG2000-PE typically at 3–5 mol% for steric stabilisation and prolonged circulation. - Targeting/functionalization: transferrin, folate, cyclic RGD, anisamide, anti-CD45.2, ephrin-A1, lactosylated ligands, scFv GC4, aptamers, and VCAM-1-targeting peptides. - Preparation methods: microfluidic rapid mixing, ethanol injection, thin lipid film rehydration/extrusion, and related scalable techniques.

Approach: Review and synthesis of preclinical and clinical literature. Cell models include A549, HepG2, U87, HUVEC, hMSCs, cancer stem cells, AML cells, and others. In vivo models include mouse tumour xenografts/orthotopic models, lung metastasis, AML, obesity/diabetes, hindlimb ischaemia, and calvarial bone regeneration. Clinical examples include the phase I MRX34 trial and the approved siRNA/LNP drug patisiran. 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: - SAXS, TEM/cryo-TEM, NMR, FRET, and density gradient centrifugation for structural characterisation. - DLS and zeta potential for size and surface charge. - Encapsulation efficiency and in vitro release assays. - Cytotoxicity, cellular uptake, and gene-silencing assays. - Biodistribution, tumour growth, and survival measurements in animal models. - Clinical safety, pharmacokinetics, and adverse-event monitoring.

Key results: Humans have approximately 2,000 miRNAs, estimated to regulate about 60% of protein-coding genes. - DOTAP/oligonucleotide lipoplexes showed interlamellar spacing of ~4.9 nm, DOTAP bilayer thickness of 3.72 nm, and an aqueous layer of ~1.2 nm. - Optimal intravenous nanoparticle size is generally 30–200 nm; larger particles are cleared by the RES, while very small particles undergo renal excretion. - PEG2000-PE at 3–5 mol% provides a stabilising polymer coat. - Pre-miR-133b lipoplexes showed ~30% accumulation in lung tissue. - miR-29b delivery downregulated CDK6 mRNA by ~57% and inhibited tumour growth by ~60% versus negative control. - Anisamide-targeted LPH nanoparticles silenced 80% of luciferase activity in B16-F10 lung metastasis after a single intravenous injection. - MRX34, the first miRNA mimic clinical candidate, entered phase I; 74 patients with solid tumours, including 14 with HCC, were enrolled. It was feasible/tolerable only with dexamethasone pretreatment, and the trial was halted in 2016 due to multiple immune-related adverse events. - Patisiran, an siRNA/LNP therapeutic using DLin-MC3-DMA/DSPC/cholesterol/PEG-lipid, was approved by the FDA in 2018. - Smarticles® are about 120 nm, slightly anionic at physiological pH, and become cationic in the lower-pH tumour environment.

Interpretation: The authors conclude that lipid-based nanoparticles are a promising and flexible platform for miRNA delivery. They emphasise that successful translation requires careful optimisation of lipid composition, particle size, charge, PEGylation, targeting, and endosomal escape. Microfluidics and combination treatments are highlighted as key future directions. They view miRNA therapeutics as still early-stage but advancing, with the first FDA-approved LNP ncRNA drug as a major milestone.

Limitations: Review article; no primary experimental data, effect sizes, n values, doses, or controls. - miRNA delivery still faces major challenges: toxicity, tissue-specific targeting, endosomal escape, stability, scale-up, cGMP manufacturing, quality control, and cost. - Clinical translation takes roughly 10–12 years from design to marketed nanopharmaceutical. - MRX34 was halted due to immune-related adverse events, and carrier-associated adverse effects remain a concern. - Endosomal escape mechanisms, including the proton-sponge hypothesis, remain controversial. - Many delivery insights come from siRNA rather than miRNA, and long-term safety/repeated dosing are not fully established.