mRNA Nanocarriers
As an intermediate carrier, mRNA can transfer the genetic code in DNA to ribosomes for protein expression. It has great potential in vaccines, protein replacement therapy, and gene editing. Compared with traditional small molecule and protein drugs, mRNA-based therapies show some specific advantages in terms of safety, efficacy and preparation.
However, despite these potential advantages of mRNA, how to deliver it safely, efficiently, and stably within cells remains an important obstacle. In recent years, nanobiotechnology has made significant progress, providing important tools for the development of mRNA nanocarriers. Nanocarrier systems can be directly used to load, protect, and release mRNA in biological microenvironments, and can be used to stimulate mRNA translation to develop more effective intervention strategies. Therefore, designing a new generation of nanomaterials may trigger a revolution in mRNA technology.
Lipid Nanoparticles (LNP)
Lipid nanoparticles are the most clinically advanced mRNA carriers. LNPs offer many benefits for mRNA delivery, including formulation simplicity, modularity, biocompatibility, and large mRNA payload capacity. In addition to RNA drugs, LNPs typically include four components, ionizable lipids, cholesterol, accessory phospholipids, and PEGylated lipids, which together encapsulate and protect fragile mRNA.
Ionizable lipids and mRNA form nanoparticles in an acidic buffer, making the lipids positively charged and attracting RNA. Furthermore, they are positively charged in the acidic environment of endosomes, which facilitates their fusion with the endosomal membrane, releasing them into the cytoplasm.
DODAP and DODMA were the first ionizable lipids used for RNA delivery. By designing to improve the efficacy of DODMA, DLin-MC3-DMA was created. This is the first FDA-approved pharmaceutical formulation using ionizable lipids: the siRNA drug patisiran (Onpattro). In addition to delivering siRNA efficiently and safely, DLin-MC3-DMA is also used for the delivery of mRNA.
Currently, many groups in academia and industry use combinatorial reaction schemes to synthesize potential delivery materials. This approach has produced a number of effective lipids, including C12-200, 503O13, 306Oi10, OF-02, TT3, 5A2-SC8, SM-102, and ALC-0315.
Polymers and Polymer Nanoparticles
Although less clinically advanced than LNPs, polymers have similar advantages to lipids and can effectively deliver mRNA. Polymer-based delivery systems consist of three types of polymers, including cationic polymers, dendrimers, and polysaccharide polymers. Cationic polymers concentrate nucleic acids into complexes with different shapes and sizes that can enter cells via endocytosis.
Polyethyleneimine is the most extensively studied nucleic acid delivery polymer. Despite its excellent efficacy, its toxicity limits applications due to its high charge density. In addition, several less toxic biodegradable polymers have been developed. For example, poly(beta-aminoester) performs well in mRNA delivery, especially to the lungs.
Recently, a new type of lipid-containing polymers called charge-altered releasable transporters (CARTs) has been developed, which can effectively target T cells. It is very difficult to manipulate T cells. Therefore, CART is a very Attractive delivery materials with great potential in the fields of mRNA vaccines and gene therapy.
Liposome Multiplex
The liposome polyplex (LPP) nanodelivery platform is a bilayer structure with a polymer-wrapped mRNA molecule as the core, wrapped in a bilayer shell of phospholipids. As a non-viral gene delivery vehicle, LPP combines the advantages of polymers and liposomes, exhibiting excellent stability, reduced cytotoxicity, high gene transfection efficiency, and gradual release of mRNA molecules as the polymer degrades.
Virus-Like Nanoparticles
Virus-like nanoparticle (VLP) delivery systems are an intermediate between viral and non-viral vectors. These particles contain most of the components of a viral vector, such as the envelope and capsid, but no viral genome. Through virus engineering technology, specific recognition can be achieved between the mRNA structure and the viral capsid structural protein, thereby forming VLP-mRNA. VLP‐mRNA can efficiently infect cells with the help of transient expression of the viral capsid and the mRNA itself. Studies have shown that compared to viral systems that express Cas9 for long periods of time, Cas9 mRNA delivered by VLP-mRNA only persists for 72 hours. Therefore, the impact of off-target effects can be significantly reduced.
VLP delivery of Cas9 mRNA has been successfully developed to target VEGFa and reduce neovascular area by 63% in a mouse model of age-related macular degeneration. Sequencing results showed that VLP-mRNA did not induce off-target effects. These experimental results strongly support the clinical application potential of VLPs in delivering CRISPR gene therapy.
Biomimetic Cell Membrane-Coated Nanoparticles
Cell membrane-coated carriers can simulate the properties of cell membranes and combine the properties of natural cell membranes with those of nanomaterials, thereby significantly improving biocompatibility while achieving long circulation in the body and targeted delivery. A series of nanomedicine carriers have been developed using the membranes of immune cells, such as leukocytes, macrophages, neutrophils, etc. In addition, tumor cells and bacteria can also be used to prepare cell membrane-camouflaged nanocarriers. Considering that specific proteins on the surface of tumor cells and bacteria can activate the immune system and improve adhesion, these new drug carriers are more diverse than traditional nanocarriers in terms of functional delivery of mRNA drugs. In addition, the use of specific recognition proteins on the cell membrane can achieve targeted drug delivery to the diseased site, providing a new strategy to significantly improve the therapeutic effect.
Inorganic Nanomaterials
Inorganic nanomaterials as delivery carriers have unique physical and chemical properties, such as excellent storage stability, good biocompatibility, and easy preparation, making them an ideal platform for mRNA delivery. Currently, inorganic nanostructures, including quantum dots, silica nanoparticles (SNPs), gold nanoparticles (AuNPs), and carbon-based and magnetic iron oxide nanostructures, are the most popular types in nanomedicine.
Exosome-based Nanoparticles
Exosomes are intraluminal vesicles (ILVs) formed during the maturation of intravesicular bodies (MVBs). When MVBs fuse with the plasma membrane, ILVs are released into the periplasmic space, called exosomes. Exosomes contain a large number of specific proteins, lipids, DNA, mRNA, non-coding microRNAs and enzymes, which is a new mode of intercellular communication. Therefore, exosomes can function as messenger mRNA delivery systems.
Compared with synthetic nanoparticles, natural exosomes have excellent properties such as excellent biocompatibility and low immunogenicity. In addition, due to the small size of exosomes, they can inhibit the clearance of mononuclear phagocytes and have high permeability and retention in solid tumor sites, allowing drug accumulation at the target site. Exosomes themselves can pass the blood-brain barrier and deliver drugs into the brain. In particular, intranasal administration can promote the rapid delivery of drugs to brain lesions, providing the possibility of non-invasive treatment of brain diseases.
Exosomes have received increasing attention as carriers of mRNA vaccines, especially for tumor-targeted mRNA vaccines. DC-derived extracellular vesicles containing MHC on their surface can enhance the patient’s T cell immune response. In addition, compared with LNPs, exosomes have no side effects at any dose, both in vivo and in vitro. Exosomes have also shown better performance than LNPs in delivering functional mRNA to human cells.
In addition to serving as mRNA drug carriers, surface functionalization of exosomes can also enable targeted drug delivery. Modified exosomes can precisely deliver mRNA molecules to target cells or organs. Overall, exosomes are highly biocompatible and have great clinical application potential, opening up new avenues for mRNA drug delivery.