Self-assembled mRNA vaccines

Purpose: mRNA vaccines are promising but naked mRNA is fragile, susceptible to enzymatic degradation, poorly taken up by cells due to electrostatic repulsion, and can trigger innate immune responses. Self-assembly offers a versatile approach to prepare delivery vehicles with customizable properties. The review discusses design and self-assembly of mRNA vaccines, materials commonly used, physicochemical characteristics, routes of administration, and clinical utility, with emphasis on COVID-19 vaccines.

Hypothesis: No formal experimental hypothesis. Central thesis: self-assembly is an effective and versatile approach for preparing mRNA delivery vehicles that protect mRNA, facilitate cellular entry and endosomal escape, and elicit potent immune responses; careful design of mRNA structure and delivery materials can optimize vaccine efficacy, as demonstrated by the clinical success of LNP-based COVID-19 mRNA vaccines.

Aims: Describe design and properties of mRNA, including structural elements and modified nucleotides. - Explain immunological activity of mRNA vaccines, including antigen presentation, T cell priming, and B cell activation. - Discuss importance of administration routes (IM, SC, ID, IV, IN). - Review mechanisms of intracellular delivery and endosomal escape. - Summarize materials for mRNA delivery, including lipid-based and polymer-based systems and their self-assembly. - Discuss desired physicochemical properties (size, charge, shape, surface composition) for optimal delivery. - Highlight clinical utility of mRNA vaccines, especially for COVID-19.

Delivery system: mRNA types: non-replicating mRNA; self-amplifying RNA (saRNA) with alphavirus-derived nsP1–4 replicase; trans-amplifying RNA. - Lipid nanoparticles (LNPs): ionizable lipids (DODMA, DLinDMA, DLin-KC2-DMA, DLin-MC3-DMA, L319, YSK12-C4, CL4H6, Moderna Lipid 5, Acuitas ALC-0315, BAMEA-O16B, ssPalmO-Phe), lipidoids (C12-200, C14-113, cKK-E12, OF-02, OF-Deg-Lin, TT3, 98N12-5), structural lipids (DSPC, cholesterol, DOPE, DGTS), PEG-lipids (DMG-PEG, ALC-0159). - Polymer-based systems: PEI, PLL, PAMAM, PBAE, PACE, PLA, PLGA, PDMAEMA, chitosan, dextran, hyperbranched PBAEs. - Payloads: mRNA encoding SARS-CoV-2 spike protein (prefusion-stabilized or RBD), influenza HA, RSV F protein, HIV gp120, Zika prM-E, rabies glycoprotein, Cas9 mRNA, Factor IX, therapeutic antibodies (Trastuzumab, ACE2), cancer antigens. - Routes: intramuscular, subcutaneous, intradermal, intravenous, intranasal, nebulized. - Targeting/functionalization: mannose, PEG, ApoE-mediated uptake, LN-targeting, PECAM-1 antibody.

Approach: Review of preclinical and clinical literature; no primary experiments. In vitro systems include DCs, HEK293, hepatocytes, adipocytes, JAWSII DC line. In vivo models include mice, rats, cynomolgus macaques, and nonhuman primates. Clinical trials include BNT162b2 (NCT04368728, phase III, ~43,000 participants), mRNA-1273 (NCT04470427, phase III, ~30,000 participants), ARCoV, COVAC1, LUNAR-COV19, CVnCoV, ChulaCoV19, and cancer immunotherapy trials (NCT02410733, NCT02316457).

Key methods: Nanoparticle characterization: DLS for size and polydispersity, cryo-TEM for morphology, SAXS/SANS for internal structure, NMR, DSC, HPLC. - mRNA characterization: capping efficiency, poly(A) tail length, modified nucleotide incorporation. - Cellular uptake and endosomal escape: galectin-8 recruitment assay, fluorescence microscopy, flow cytometry. - Immune response: antigen-specific antibody titers, neutralization assays, CD4⁺/CD8⁺ T cell responses, germinal center B cells, T follicular helper cells. - Biodistribution and trafficking: PET-CT, intravital microscopy, smFISH, RNAScope. - Clinical endpoints: efficacy against COVID-19, adverse events, storage stability.

Key results: BNT162b2: 95.0% efficacy in phase III (8 COVID-19 cases in vaccine group vs 162 in placebo); 94.6% efficacy including prior infection; ~52% efficacy between first and second dose. - mRNA-1273: 94.1% efficacy in phase III (11 cases vs 185 in placebo); 63% reduction in asymptomatic infection after first dose; 100% efficacy against severe COVID-19. - Moderna produced clinical-grade mRNA within 27 days of spike sequence release; phase I, II, III trials initiated within 66, 140, and 199 days, respectively. - LNP-delivered 0.1 μg mRNA enhanced in vivo transfection 10-fold greater than 1.0 μg naked mRNA. - Endosomal escape efficiency: MC3-LNPs ~2.5%; Lipid 5-LNPs ~15%. - ~70% of internalized siRNA-LNPs cleared by exocytic recycling. - DLin-KC2-DMA SNALPs: in vivo activity at 0.01 mg/kg in mice; TTR silencing ED50 ~0.3 mg/kg in nonhuman primates. - Moderna Lipid 5 was 3-fold more potent than MC3; Acuitas ALC-0315 was ~6-fold more potent than MC3. - Optimal pKa for IM delivery: 6.6–6.9; for IV delivery: 6.2–6.5. - Storage: BNT162b2 requires −60°C to −80°C; mRNA-1273 requires −15°C to −25°C.

Interpretation: Self-assembly is a powerful approach for mRNA vaccine design, enabling customizable delivery vehicles. LNPs are the leading delivery system, with demonstrated clinical success during the COVID-19 pandemic. The ~95% efficacy and favorable safety profiles of BNT162b2 and mRNA-1273 validate the mRNA vaccine platform. The authors conclude that the pandemic has paved the way for scale-up and resolved distribution barriers, and that RNA therapeutics and nanomedicine are ready for a “full-scale industrial revolution.”

Limitations: Review article; no primary data. - Naked mRNA is fragile, susceptible to degradation, and poorly taken up by cells. - mRNA can trigger innate immune responses via TLRs and RLRs. - Endosomal escape remains inefficient (~2.5–15%). - ~70% of internalized LNPs cleared by exocytic recycling. - Optimal pKa varies by administration route, complicating universal design. - Long-term storage stability issues; BNT162b2 requires ultra-cold storage. - PEG-lipids can induce anti-PEG antibodies and accelerated blood clearance (ABC) upon repeated dosing. - Polymeric systems have batch-to-batch variability and lower gene transfer efficiency than LNPs. - Long-term effects of mRNA vaccines not yet evaluated. - Most IN administration evaluations limited to rodent models. - Potential for off-target effects and unintended immune activation. - Manufacturing complexity of ionizable lipids and LNPs requires multi-step synthesis and microfluidic mixing.