Messenger RNA (mRNA) medicine is an emerging field that exploits the potential of mRNA molecules to encode and deliver therapeutic proteins or antigens to target cells. mRNA medicine has several advantages over conventional drugs, such as high specificity, versatility, safety, and scalability. mRNA medicine has shown promising results in various applications, such as cancer immunotherapy, infectious disease vaccination, and gene therapy. However, mRNA medicine also faces significant challenges in terms of delivery, stability, and immunogenicity. mRNA molecules are highly susceptible to degradation by nucleases and have poor membrane permeability. Moreover, mRNA molecules can trigger innate immune responses that may compromise their efficacy and safety. Therefore, efficient and safe delivery carriers are essential for the success of mRNA medicine. Several types of delivery carriers have been developed and investigated for mRNA delivery, such as lipid-based nanocarriers, polymeric nanocarriers, peptide-related carriers, and other typed carriers. Each type of carrier has its own advantages and limitations, depending on the physicochemical properties, biocompatibility, stability, immunogenicity, biodistribution, and targeting ability of the carrier. Moreover, some methods of cation-free administration have been explored to deliver mRNA without the need for carriers, such as electroporation, microinjection, and ultrasound.
Lipid nanoparticles (LNPs) are the most clinically advanced carriers for mRNA delivery. LNPs consist of cationic or ionizable lipids, helper lipids, cholesterol, and polyethylene glycol (PEG) lipids, forming a lipid bilayer that can encapsulate mRNA molecules through electrostatic interactions. LNPs can protect mRNA from degradation by nucleases, enhance cellular uptake by endocytosis, and facilitate endosomal escape by pH-dependent ionization of lipids. LNPs have shown significant progress in delivering mRNA for therapeutic purposes, particularly with the success of COVID-19 vaccines. LNPs can also deliver mRNA for cancer immunotherapy, infectious disease vaccination, and gene therapy. LNPs can be modified with targeting ligands, such as antibodies, peptides, or aptamers, to improve the specificity and efficiency of mRNA delivery to target cells or tissues.
Fig.1 Schematic representation of the different types of lipid-based nanocarriers. 1
However, LNPs also face some challenges and limitations, such as low biocompatibility, high immunogenicity, poor stability, and unfavorable biodistribution. LNPs can induce innate immune responses that may compromise their efficacy and safety. LNPs can also accumulate in the liver and spleen, leading to potential toxicity and off-target effects. Moreover, LNPs require careful optimization of their physicochemical properties, such as size, charge, composition, and ratio, to achieve optimal mRNA delivery performance.
Polymeric nanocarriers are nano-sized particles made of polymers that can encapsulate and deliver mRNA molecules to target cells. Polymeric nanocarriers are nanoscale delivery systems composed of synthetic or natural polymers that can encapsulate and protect mRNA from degradation, and facilitate its cellular uptake and endosomal escape. Polymeric nanocarriers can be classified into different types based on their structure and properties, such as dendrimers, micelles, polyplexes, and nanoparticles. Each type has its own advantages and disadvantages for mRNA delivery, such as size, charge, stability, biodegradability, immunogenicity, and toxicity.
mRNA-based therapies have potential applications in treating various diseases, such as cancer, infectious diseases, and genetic disorders. However, mRNA is unstable and easily degraded by enzymes in the body, and it also faces barriers such as the cell membrane and the endosome. Therefore, it needs a suitable delivery system to protect it and enhance its cellular uptake and release. Polymeric nanocarriers offer several advantages for mRNA delivery, such as biocompatibility, biodegradability, low immunogenicity, and tunable properties. They can also be modified with targeting ligands or stimuli-responsive groups to achieve specific and controlled delivery of mRNA. Polymeric nanocarriers have been widely explored for mRNA delivery in various preclinical and clinical studies, especially for cancer immunotherapy. Some examples of polymeric nanocarriers for mRNA delivery are polyethylenimine (PEI), poly(lactic-co-glycolic acid) (PLGA), and poly(beta-amino ester) (PBAE).
A peptide-related carrier is a type of delivery system that uses peptides or polypeptides to transport mRNA molecules into cells. Peptides are short chains of amino acids that can interact with cell membranes, endosomes, and nuclear pores, facilitating the intracellular delivery of mRNA. Peptides can also be modified or conjugated with other molecules to enhance their stability, specificity, and functionality.
One example of a peptide-related carrier is a self-assembled polymeric micelle based on vitamin E succinate modified polyethyleneimine copolymer (PVES). PVES can form nanoscale complexes with mRNA via electrostatic interaction and protect it from degradation. PVES can also transfect mRNA into various cell types with high efficiency and low toxicity. PVES has been used to deliver the SARS-CoV-2 mRNA vaccine in mice and induce potent antibody responses. PVES is a promising delivery carrier for mRNA therapy.
Other typed carriers are another type of carrier for mRNA delivery, which are composed of materials other than lipids, polymers, or peptides, such as extracellular vesicles, metal nanoparticles, and carbon nanotubes. Other typed carriers can protect mRNA from degradation, enhance cellular uptake, and facilitate endosomal escape by various mechanisms, such as membrane fusion, catalysis, or photothermal effect. Other typed carriers can also be modified with various functional groups, such as PEG, targeting ligands, or stimuli-responsive moieties, to improve the stability, specificity, and responsiveness of mRNA delivery. Other typed carriers have been explored for delivering mRNA for various purposes, such as cancer immunotherapy, infectious disease vaccination, and gene therapy. Other typed carriers can be classified into different categories, such as extracellular vesicles, gold nanoparticles, and mesoporous silica nanoparticles, depending on the origin, composition, and structure of the materials.
However, other typed carriers also have some challenges and limitations, such as low biocompatibility, high immunogenicity, poor scalability, and unfavorable biodistribution. Other typed carriers can induce cytotoxicity and immunogenicity due to their foreign nature, high surface reactivity, and residual impurities. Other typed carriers can also suffer from low mRNA loading, aggregation, and rapid clearance by the reticuloendothelial system. Moreover, other typed carriers require careful optimization of their physicochemical properties, such as size, charge, composition, and ratio, to achieve optimal mRNA delivery performance.
Several strategies have been developed to improve the delivery efficiency and safety of other typed carriers, such as conjugation of aptamers, hybridization of lipids and metals, and incorporation of organic/inorganic hybrid nanostructures. These strategies aim to enhance the mRNA loading, stability, cellular uptake, endosomal escape, and targeting ability of other typed carriers, as well as reduce their immunogenicity and toxicity.
Cation-free administration is a method of delivering mRNA without using cationic carriers, which are positively charged molecules that can bind to the negatively charged mRNA. Cationic carriers can cause toxicity and inflammation in the body, limiting the safety and efficacy of mRNA therapy. mRNA therapy is a type of gene therapy that uses mRNA to instruct cells to produce a desired protein, such as an antigen for a vaccine or a therapeutic enzyme for a disease.
Cation-free administration involves encapsulating mRNA in self-assembled polymeric micelles, extracellular vesicles, or other non-cationic nanosystems. These carriers can protect mRNA from degradation by enzymes in the blood and tissues, target specific cells by surface modification, and release mRNA in response to stimuli such as pH, temperature, or enzymes. Cation-free administration has been applied to various diseases, such as cancer, infectious diseases, and skin aging. For example, researchers have developed a self-assembled polymeric micelle based on vitamin E succinate modified polyethyleneimine copolymer (PVES) to deliver mRNA. PVES micelles showed high transfection efficiency, low cytotoxicity, and cancer cell targeting in vitro and in vivo. Mice injected with PVES micelles carrying mRNA encoding polo-like kinase 1 (PLK1), a protein involved in cell division and cancer progression, showed significant tumor growth inhibition and an improved survival rate.
Cation-free administration is a promising strategy for mRNA therapy, as it overcomes the limitations of cationic carriers and offers advantages such as biocompatibility, stability, targeting, and stimuli-responsiveness. Cation-free administration could potentially be used for a wide range of mRNA therapies that currently have no good methods of delivery.
| Type of carrier | Advantages | Limitations | Examples |
|---|---|---|---|
| Lipid-based nanocarrier | High stability, high transfection efficiency, low toxicity, easy modification | High immunogenicity, poor biodistribution, complex optimization | LNPs1, lipoplexes2, liposomes3 |
| Polymeric nanocarrier | High versatility, high biodegradability, easy modification | High toxicity, low transfection efficiency, poor stability, complex optimization | PEI4, PLGA5, chitosan6, dendrimers |
| Peptide-related carrier | High specificity, high efficiency, low toxicity, easy modification | Low stability, high immunogenicity, poor scalability, complex optimization | CPPs, self-assembling peptides, peptide amphiphiles |
| Other typed carriers | High functionality, high responsiveness, high biocompatibility, easy modification | High immunogenicity, high toxicity, poor scalability, complex optimization | Extracellular vesicles, gold nanoparticles, carbon nanotubes |
| Cation-free administration | No carrier toxicity, high transfection efficiency, high expression level | Low scalability, high invasiveness, low applicability, high cost | Electroporation, microinjection, ultrasound |
Table 1. Comparison of different types of carriers for mRNA delivery
The field of mRNA delivery is still in its infancy and faces many obstacles and uncertainties. However, it also offers many possibilities and promises for the future of medicine. Therefore, further research and development are needed to overcome the limitations and improve the performance of the existing delivery carriers and methods, as well as to explore new and innovative delivery carriers and methods.
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