Synthetic mRNA, also known as in vitro transcribed (IVT) mRNA, is a type of nucleic acid that can be artificially synthesized and delivered into cells to express a desired protein. The concept of synthetic mRNA was first proposed in the 1970s, but it was not until the 1990s that the first successful experiments of synthetic mRNA delivery and expression were reported. Since then, synthetic mRNA has emerged as a promising technology for various applications in vaccines and therapeutics, especially in the context of the COVID-19 pandemic. Synthetic mRNA offers several advantages over conventional gene-expressing systems such as plasmid DNA and viral vectors, such as higher efficiency, lower immunogenicity, greater versatility, and easier scalability.
Synthetic mRNA consists of four main components: the 5' cap, the 5' untranslated region (UTR), the open reading frame (ORF), and the 3' UTR. The 5' cap is a modified nucleotide that protects the mRNA from degradation and facilitates its recognition by the ribosome. The 5' UTR is a sequence of nucleotides that regulates the translation efficiency and stability of the mRNA. The ORF is the coding sequence that determines the protein to be expressed. The 3' UTR is another sequence of nucleotides that influences mRNA stability and localization. The poly(A) tail is a string of adenine nucleotides that is added to the 3' end of the mRNA to enhance its stability and translation.
There are various methods and strategies to synthesize, modify, and optimize synthetic mRNA. The most common method is enzymatic transcription, which uses a DNA template and a bacteriophage RNA polymerase to produce mRNA. Alternatively, chemical synthesis can be used to generate short or modified mRNA fragments. The synthetic mRNA can be further modified by adding different types of caps, such as the natural cap, the anti-reverse cap analog (ARCA), or the N1-methyl-pseudouridine cap. The synthetic mRNA can also be modified by replacing some of the nucleosides with modified ones, such as pseudouridine, N1-methyl-pseudouridine, or 5-methylcytidine. These modifications can improve the stability, translation, and immunogenicity of the synthetic mRNA.
There are various challenges and solutions to deliver synthetic mRNA into cells, such as using lipid nanoparticles, cationic lipids, or liposomes as carriers. Lipid nanoparticles are the most common and effective delivery system for synthetic mRNA, as they can protect the mRNA from degradation, enhance its cellular uptake, and release it into the cytoplasm. Cationic lipids are positively charged lipids that can form complexes with negatively charged mRNA and facilitate its delivery into cells. Liposomes are spherical vesicles that can encapsulate mRNA and deliver it into cells. However, these delivery systems also have some limitations, such as toxicity, immunogenicity, and biodistribution issues.
There are different synthetic mRNA platforms and their features, such as self-amplifying mRNA, non-replicating mRNA, and modified mRNA. Self-amplifying mRNA is a type of mRNA that contains replication machinery derived from RNA viruses, such as alphaviruses or flaviviruses. This allows the mRNA to replicate and amplify itself in the cytoplasm, resulting in higher and longer protein expression. Non-replicating mRNA is a type of mRNA that does not contain any replication machinery and thus relies on the cellular machinery for translation. This results in lower and shorter protein expression, but also lower immunogenicity and toxicity. Modified mRNA is a type of mRNA that has been modified by nucleoside modification or other methods, such as codon optimization or RNA folding. This results in improved stability, translation, and immunogenicity of the mRNA.
One of the most promising applications of synthetic mRNA is the development of vaccines for various infectious diseases. Synthetic mRNA vaccines work by delivering mRNA into cells, where it is translated into antigens that stimulate the immune system. Synthetic mRNA vaccines can induce both cellular and humoral immunity, as well as memory responses, by presenting mRNA-encoded antigens to the major histocompatibility complex (MHC) class I and II molecules. Synthetic mRNA vaccines have several advantages over traditional vaccines, such as faster and cheaper production, higher specificity and diversity, and a lower risk of infection or integration.
Fig.1 Schematic representation of the production and purification steps of a mRNA vaccine manufacturing process. (Rosa SS, 2021)
The most notable example of synthetic mRNA vaccines is the COVID-19 vaccines developed by Pfizer-BioNTech and Moderna, which have shown remarkable efficacy and safety in clinical trials and mass vaccination campaigns. These vaccines use modified mRNA that encodes the spike protein of the SARS-CoV-2 virus, which is the main target of neutralizing antibodies. The modified mRNA is encapsulated in lipid nanoparticles that protect it from degradation and facilitate its delivery into cells. The Pfizer-BioNTech and Moderna vaccines have demonstrated more than 90% efficacy in preventing COVID-19 and have been authorized for emergency use in many countries.
Besides COVID-19, synthetic mRNA vaccines are also being developed for other infectious diseases, such as influenza, shingles, and respiratory combination vaccines. Synthetic mRNA vaccines for influenza can encode multiple strains of the virus, such as seasonal or pandemic strains, and provide broad and long-lasting protection. Synthetic mRNA vaccines for shingles can encode the varicella-zoster virus glycoprotein E, which is the main antigen that induces immunity against the virus. Synthetic mRNA vaccines for respiratory combination vaccines can encode multiple antigens from different pathogens, such as respiratory syncytial virus, parainfluenza virus, and human metapneumovirus, and provide a single-shot solution for preventing respiratory infections.
Another potential application of synthetic mRNA is the development of therapeutics for various diseases. Synthetic mRNA therapeutics work by delivering mRNA into cells, where it is translated into proteins that can replace, edit, or modulate the function of defective or missing genes. Synthetic mRNA therapeutics have several advantages over conventional gene therapy, such as higher specificity, lower immunogenicity, and reversibility.
One of the most promising areas of synthetic mRNA therapeutics is cancer treatment. Synthetic mRNA can be used to express tumor antigens, immune modulators, or gene-editing enzymes that can target and eliminate cancer cells. For example, BioNTech has developed a personalized cancer vaccine that uses synthetic mRNA to encode neoantigens, which are unique mutations found in each patient's tumor. The synthetic mRNA vaccine can elicit a strong and specific immune response against the tumor and has shown promising results in clinical trials for melanoma and other cancers.
Another promising area of synthetic mRNA therapeutics is rare genetic diseases. Synthetic mRNA can be used to express functional proteins that are deficient or mutated in patients with inherited disorders. For example, Translate Bio has developed a synthetic mRNA therapy for cystic fibrosis, a disease caused by mutations in the CFTR gene that affect the function of the lungs and other organs. Synthetic mRNA therapy can deliver normal CFTR mRNA into the lung cells, where it is translated into functional CFTR protein that can restore normal fluid balance and mucus clearance. The synthetic mRNA therapy has shown positive results in preclinical studies and is currently in phase 1/2 clinical trials.
Other potential areas of synthetic mRNA therapeutics include cardiovascular diseases, metabolic diseases, and immunotherapy. Synthetic mRNA can be used to express proteins that can enhance the function of the heart, liver, or pancreas or modulate the immune system to treat autoimmune diseases or transplant rejection. Synthetic mRNA therapeutics have the potential to revolutionize the field of medicine and provide new hope for patients with unmet medical needs.
In conclusion, synthetic mRNA is a novel and versatile technology that has shown great potential in the development of vaccines and therapeutics for various diseases. Synthetic mRNA technology has the potential to revolutionize the field of medicine and provide new hope for patients with unmet medical needs.