mRNA vaccine delivery strategy

mRNA vaccines have the advantages of rapid development, low-cost production, and safe management, and have strong potential to replace conventional vaccines. During the vaccination process, the formulation and delivery strategies of mRNA contribute to the effective expression and presentation of antigens and immune stimulation. mRNA vaccines have been delivered in many forms, including: encapsulated by delivery vectors, such as lipid nanoparticles, polymers, peptides, free mRNA in solution, and wrapped by dendritic cells in vitro. Appropriate delivery materials and formulation methods usually improve vaccine efficacy, which is also affected by the choice of an appropriate route of administration.

3 Delivery Strategies for mRNA Vaccines

Delivery Carriers of mRNA Vaccines

• Lipid-based delivery

Lipids, lipid-like compounds, and lipid derivatives have been widely used in the formulation of liposomes and lipid-derived nanoparticles (LNP) for the delivery of mRNA vaccines in vivo. LNP is usually defined as a synthetic or physiological lipid material.

There are two main reasons for developing LNP for the delivery of mRNA vaccines.

• LNP can encapsulate RNA molecules and protect RNA from enzymatic degradation. Most of the mRNA can be encapsulated by LNP.
• LNPs can effectively deliver mRNA molecules to the cytoplasm through a series of endocytosis mechanisms. The endocytosis process transports LNP loaded with mRNA to membrane-bound vesicles, such as endosomes and lysosomes. Finally, LNP helps transport mRNA cargo into the cytosol for protein expression.

Figure 1: parts of delivery methods for mRNA vaccines (Norbert Pardi, 2018).

• Polymer-based delivery

Polymeric materials (including polyamines, dendrimers, and copolymers) are functional materials capable of delivering mRNA vaccines. Similar to functional lipid-based carriers, polymers can also protect RNA from RNase-mediated degradation and promote intracellular delivery. However, the formulation of polymer-based mRNA nanoparticles tends to have a higher polydispersity. Studies have shown that structural modifications of polymer materials, such as the incorporation of lipid chains, hyperbranched groups, or biodegradable subunits can help to stabilize the formulation and improve safety.

• Peptide-based delivery

Various peptides are used as carriers to deliver mRNA vaccines. Cationic peptides contain many lysine and arginine residues, which provide positively charged amino groups and can therefore complex with nucleic acids through electrostatic interactions. The ratio of the positively charged amino groups on the peptide to the negatively charged phosphate groups on the RNA affects the formation of nanocomplexes. Increasing the ratio of charged amino groups to phosphate groups from 1:1 to 10:1 can provide smaller particle size, larger zeta potential, and higher encapsulation efficiency.

• Protamine

Protamine is a cationic peptide used in many early studies to deliver mRNA vaccines, and it is also the only peptide carrier evaluated in clinical trials of mRNA vaccines

• Cationic cell-penetrating peptides (CPPs)

• Virus-like replicon particle

Virus particles can package and deliver antigen-encoding self-amplified mRNA into the cytoplasm like viruses. The substance is called virus-like self-amplified mRNA particles, or virus-like replicon particles (VRP). The self-amplified mRNA will replicate itself and effectively express the specified antigen. The advantage of VRP comes from the effective cytoplasmic delivery of RNA payloads by viral vectors.

However, VRP-based mRNA vaccines have two challenges:

• Expansion of production scale;
• Production of antibodies against viral vectors.

Naked mRNA Vaccines

The mRNA vaccine can be delivered without any additional carrier, i.e. in naked form. This method is dissolve mRNA in a buffer, and then directly injects the mRNA solution. Some studies have shown that naked mRNA is internalized through macropinocytosis. This macropinocytosis pathway is highly active in macrophages and immature dendritic cells. Others speculate that the naked mRNA is taken up by mechanical force.

The naked mRNA vaccine has two outstanding features:

• Easy to store and prepare
• Inherent immunogenicity. Especially vaccines made from unmodified nucleotides has a dual advantage.

Dendritic Cells-Based mRNA Vaccines

Some special characteristics of DC make it a suitable vaccination target, including the directional migration of T cells in lymph nodes and the high expression of major histocompatibility complex (MHC) molecules, costimulators, and cytokines. In addition, DC may present complete antigens to B cells to trigger an antibody response. DC is also very suitable for mRNA transfection. For these reasons, DCs represent attractive targets for mRNA vaccine transfection in vivo and in vitro.

The delivery forms and delivery materials mentioned above have entered various stages of preclinical and clinical research. However, each delivery technology has its advantages and challenges.

Table 1: Summary of the delivery strategies of mRNA vaccines (Chunxi Zeng, 2020).

BOC RNA provides services to help develop mRNA vaccine from mRNA design, synthesis to delivery strategy slection. Learn more about our mRNA vaccine development capability.