Continued innovation promises new and improved mRNA-based therapeutics

It has been over 40 years since mRNA was first delivered using liposomes and shown to yield exogenous proteins in vitro. In that time, mRNA engineering technologies and drug delivery systems have evolved, culminating in the successful development of coronavirus 2019 (COVID-19) mRNA vaccines by Moderna and Pfizer/BioNTech. A recent review, published in Nature Medicine, discusses the technical challenges of developing mRNA-based therapeutics and explains how these are being overcome to help prevent and treat disorders, including infectious diseases, cancers, and genetic conditions.

Challenges for the clinical use of mRNA

For mRNA-based therapeutics to demonstrate clinical efficacy, they must undergo sufficient translation in vivo without causing unwanted immunostimulation. To achieve this, therapeutic mRNA must avoid nuclease-based degradation, macrophage phagocytosis, and clearance by renal filtration, as well as be free from impurities such as double-stranded RNA and RNA fragments. Therapeutic mRNA must also be able to pass through negatively charged cell membranes and reach ribosomes for translation. A key challenge for successful in vivo expression lies in circumventing detection by endosomal and cytosolic pattern recognition receptors, such as the Toll-like receptors (TLRs) expressed by immune cells, which can limit translation and reduce mRNA stability. Once delivered to a cell, a vast majority of exogenous mRNA remains trapped in endosomes, significantly reducing the potential activity for a therapy. Endosomal escape persists as a major hurdle for mRNA-based therapeutics to overcome.

Improving clinical efficacy through innovative mRNA design

Concerns around mRNA translatability, stability, and unwanted immunogenicity have largely been addressed through rational mRNA design. Effective strategies include optimizing the 5’ and 3’ untranslated regions (UTRs) to maximize translation efficiency, altering the length of the poly(A) tail for improved stability, and incorporating modified nucleosides, such as pseudouridine (ψ), 5‑methylcytidine, and N6‑methyladenosine, into mRNAs to limit PRR recognition. In addition, co-transcriptional capping can generate an endogenous cap structure, reduce unwanted immunostimulation and improve mRNA binding to ribosomes. In the review, the authors note that mRNAs with a natural cap-1 structure can be conveniently manufactured using TriLink’s CleanCap® technology.

Optimized mRNA delivery vehicles may expedite clinical translation

Cationic and ionizable lipid-based nanoparticles (LNPs) are the most clinically advanced mRNA delivery systems. Of these, the lower toxicity and increased circulation half-life of ionizable LNPs led to them being used in the two approved COVID-19 vaccines. Polymer-based nanoparticles are also being developed for mRNA delivery. Recently, these were loaded with chimeric antigen receptor mRNA and successfully used for T cell reprogramming in mouse models of human leukemia, prostate cancer, and hepatitis B-induced hepatocellular carcinoma. Additionally, hybrid lipid–polymer nanoparticles have shown promising utility for the systemic delivery of therapeutic mRNAs to tumors. These delivery systems have improved mRNA delivery substantially in recent years but are often still empirically designed and tested for each therapeutic.

Latest innovations and future perspectives

Ongoing developments in the field of mRNA-based therapeutics continue to drive innovation and find solutions. Self-amplifying mRNAs can lead to reduced dosage amounts. mRNA circularization can enhance stability. The production of biological membrane-based delivery vehicles can reduce unwanted immune responses and toxicities and possibly address the challenge of endosomal escape. Organ- or cell-specific mRNA delivery is being explored to broaden the scope and improve the specificity of existing technologies. Current research efforts include the development of novel mRNA vaccines (both for COVID-19 and other infectious diseases), the application of mRNA-based CAR-T cell therapy to the treatment of solid tumors, and the use of mRNA technologies for CRISPR-Cas9 gene editing. With a rejuvenated interest in mRNA therapeutics, these developments are progressing and suggest that the time is now for mRNA therapeutics to take center stage.

 

Featured technology: CleanCap®

Article reference: Huang X, Kong N, Zhang X, et al. The landscape of mRNA nanomedicine. Nat Med. 2022; 28(11):2273-2287

""