In Vivo Delivery of miRNAs for Cancer Therapy: Challenges and Strategies

Purpose: miRNAs can simultaneously regulate multiple cancer-related genes, making them attractive therapeutic agents or targets. However, efficient, specific, and safe systemic delivery of therapeutic miRNAs in vivo remains a major challenge due to rapid degradation, poor tumor penetration, immunotoxicity, endosomal trapping, off-target effects, and saturation of miRNA-processing enzymes. This review discusses these barriers and current strategies to deliver miRNAs for cancer therapy without inducing toxicity. ---

Hypothesis: No formal testable hypothesis is proposed. The central thesis is: if miRNA mimics or antagonists are chemically modified or packaged in rationally designed viral or non-viral carriers that overcome extracellular and intracellular delivery barriers, then miRNAs can be delivered systemically to tumors, silence multiple oncogenic targets, and produce effective anticancer therapy with reduced toxicity. ---

Aims: - Discuss limitations of current cancer therapies and advantages of miRNA-based therapy. - Review mechanisms of miRNA biogenesis and gene silencing. - Summarize roles of miRNAs in cancer, including cell proliferation, death, metabolism, metastasis, angiogenesis, diagnosis, and therapy. - Analyze key challenges in miRNA delivery: poor tumor penetration, rapid degradation/clearance, immunotoxicity, neurotoxicity, poor intracellular delivery, off-target effects, and insufficient/saturated miRNA-processing enzymes. - Review local and systemic in vivo miRNA delivery strategies, including modified oligonucleotides, viral vectors, and non-viral nanocarriers. - Provide future perspectives, including smart nanoparticles, stroma-targeting, and cell-based delivery. ---

Delivery system: Payloads: - miRNA mimics, miRNA antagonists (anti-miRs/AMOs), pre-miRNAs, siRNA cocktails, miRNA expression vectors. Chemical modifications of miRNA modulators: - 2′-O-methyl (2′-OMe), 2′-O-methoxyethyl, 2′-fluoro modifications. - Locked nucleic acid (LNA) modifications, including seed-targeting tiny LNAs. - Passenger-strand modifications: nucleotide analogs, backbone modifications, terminal modifications (inverted bases, biotin, alkyl groups). Viral delivery systems: - Lentiviral vectors, adenoviral vectors, adeno-associated viruses (AAVs). - Virus-derived exosomes for miRNA delivery. Non-viral delivery systems: - Inorganic nanoparticles: gold nanoparticles (e.g., AuNP-S-PEG), silica nanoparticles modified with antibodies (e.g., anti-GD2-silica nanoparticles). - Polymer-based nanoparticles: PLGA, PEGylated PLGA, PEI, PU-PEI, peptide-modified PLGA. - Lipid-based carriers: cationic lipoplexes (DOTMA/cholesterol), neutral lipid emulsions, LNP-DP1, solid lipid nanoparticles (SLNs), liposome-polycation-hyaluronic acid (LPH) nanoparticles modified with GC4 scFv. - Cell-based delivery: mesenchymal stem cells, neural stem cells, exosomes. Targeting ligands/moieties: - Antibodies (e.g., GC4 scFv), cell-penetrating peptides, GD2 antibody, hyaluronic acid, PEG, DSPE-PEG. Administration routes: - Local: intratumoral, intracranial, intranasal, topical, electroporation. - Systemic: intravenous, with passive EPR-based targeting and active ligand/receptor-mediated targeting. ---

Approach: Narrative review of published literature. No primary experimental groups. Model systems discussed include: - In vitro: cancer cell lines, cancer stem cells, lung cancer cells, hepatoma cells, colon carcinoma cells, lymphoma cells, glioblastoma cells. - In vivo: mouse models including hepatoma xenografts, NSCLC models, colon carcinoma xenografts, orthotopic breast tumors, chronic lymphocytic leukemia (CLL) mouse model, murine lymphoma, breast cancer metastasis, glioblastoma, and lung metastasis models. - Disease context: cancer therapy, including solid tumors and hematological malignancies. - Clinical context: LNA-antimir-122 entered Phase II for HCV; miRNA-126 has prognostic value in metastatic colorectal cancer; no approved miRNA cancer therapy yet. ---

Key methods: Techniques highlighted across cited studies: - qPCR for miRNA and mRNA expression. - ELISA for cytokines and IFN-γ. - Flow cytometry for immune cell populations and miRNA uptake. - IVIS bioluminescence imaging for tumor delivery and gene expression. - Tumor growth measurement and survival analysis. - Histology/immunohistochemistry. - Magnetic resonance imaging and near-infrared fluorescence imaging. - MTS/cytotoxicity assays. - Luciferase reporter assays for miRNA function. - Western blot for target protein knockdown (e.g., survivin, MAPK pathway). - Blood chemistry for liver enzymes and cytokines to assess toxicity. ---

Key results: - Local let-7 delivery: intranasal lentiviral let-7a inhibited K-ras-dependent lung tumors; local synthetic let-7b reduced tumor size by 60–70% after four treatments at 3-day intervals. - Modified miR-143: systemic administration of passenger-strand-modified miR-143 produced 15% (low dose) to 50% (high dose) tumor growth inhibition in xenografted DLD-1 human colorectal tumors. - Neutral lipid delivery: systemic miR-34a or let-7 mimics in neutral lipid emulsion reduced tumor burden by 60% in a K-ras-activated autochthonous NSCLC mouse model without toxicity. - Peptide-modified PLGA nanoparticles: delivered miR-155 antagonists in a murine lymphoma model; reduced miR-155 levels by 65% vs 23% for unmodified nanoparticles; tumor growth was delayed to less than ~2-fold increase vs ~10-fold for scramble controls. - GC4-targeted LPH nanoparticles: delivered miR-34a to B16F10 lung metastasis, silenced survivin, inactivated MAPK pathway, induced apoptosis, and inhibited metastatic tumor load; pro-inflammatory cytokines and liver enzymes remained unchanged. - AAV-miR-26a: systemic delivery induced cell cycle arrest, apoptosis, and tumor growth inhibition in hepatocellular carcinoma with undetectable toxicity. - LNP-DP1/miR-122: restored miR-122, inhibited angiogenesis, and suppressed HCC tumor growth without systemic toxicity. - Tiny LNA anti-miR-21: systemic delivery repressed miR-21 function in orthotopic breast tumors. - Delivery barriers: unmodified miRNAs are quickly degraded/cleared; nanoparticles >100 nm increase RES clearance; endosomal trapping limits cytosolic delivery; TLR activation can cause immunotoxicity/neurotoxicity; off-target effects can silence tumor suppressors; processing enzyme saturation can reduce efficacy. ---

Interpretation: The authors conclude that miRNA-based therapy has great potential for cancer because miRNAs can regulate multiple oncogenic pathways simultaneously, target tumor cells and stromal cells, and show lower immunogenicity than plasmid DNA or protein drugs. However, clinical translation depends on safe, specific, and efficient in vivo delivery. Chemical modifications, viral vectors, and non-viral nanocarriers have shown promise, especially lipid- and polymer-based systems. Future strategies include “smart” stimuli-responsive nanoparticles, tumor-stroma-targeting delivery, and cell-based/exosome delivery. Personalized cancer medicine may be realized by designing specific miRNA mimic or antagonist sets based on individual patient miRNA expression profiles. ---

Limitations: - Poor tumor penetration: heterogeneous tumor perfusion and interstitial fibrosis limit delivery of miRNA-containing vesicles. - Rapid degradation and clearance: unmodified miRNAs are degraded by nucleases and cleared rapidly from blood. - Immunotoxicity: miRNAs can activate TLR7/8 and induce IFN-α, IFN-β, IL-1β, IL-6, TNF-α; sequence motifs (GU-rich, AU-rich) influence immune activation. - Neurotoxicity: miRNA-bound TLRs, e.g., let-7b via TLR7, may cause neurodegeneration. - Poor intracellular delivery: miRNAs are trapped in endosomes and degraded in lysosomes; endosomal escape strategies are needed. - Off-target effects: imperfect pairing with 3′ UTRs can cause unwanted silencing of tumor suppressor genes; combination strategies may be needed. - Insufficient or saturated miRNA processing enzymes: RISC/Dicer downregulation in cancer or hypoxia, competition with endogenous miRNAs, and enzyme saturation can reduce therapeutic efficacy. - Viral vector limitations: immunogenicity, insertional mutagenesis, difficulty scaling up, limited cargo capacity, potential pathogenic replication. - Non-viral limitations: lower transfection efficiency, shorter gene expression, cationic lipid toxicity, polymer systemic toxicity, poor loading efficiency, and insufficient specificity. - Clinical translation: no approved miRNA-based cancer therapy; most data are preclinical; delivery remains the major hurdle. - As a review: Not a systematic review or meta-analysis; no primary data.