mRNA therapeutics: a limitless revolution in medicine

The term ‘mRNA’ has become commonplace globally. mRNA technologies have emerged as an innovative and effective approach to developing new drugs that can potentially transform existing therapies or target difficult‑to‑treat indications including respiratory, cardiac, metabolic and autoimmune diseases, as well as cancer. The success of mRNA vaccines against SARS-CoV-2 has quickly catapulted mRNA therapeutics as a disruptive, expanding drug category. Advances in mRNA technologies and cellular delivery systems coupled with their cost effectiveness, manufacturing simplicity and ability to target previously-undruggable pathways has limitless potential that could revolutionise medicine.

In addition to mRNA technologies, five other categories of RNA technologies exist: antisense oligonucleotides (ASOs), RNA interference (RNAi), micro RNAs (miRNA), small interfering RNA (siRNA) and aptamers. ASOs as a modality have the highest number of approved therapeutics followed by siRNAs and aptamers.^1,2 In 2021, ASO-based therapy casimersen was approved for Duchenne muscular dystrophy and inclisiran, a siRNA-based drug, was approved for familial hypercholesterolemia, a rare cardiovascular disease. An aptamer-based therapeutic defibrotide was approved in 2020 for unblocking clots in blood vessels inside the liver caused by a rare disease, veno-occlusive disease. While some RNA technologies have already seen regulatory approval, there are currently no approved mRNA‑based therapeutics on the market. However, several candidates are in preclinical and clinical development and the next decade may see an increase in regulatory approvals for this modality.

Benefits of mRNA-based technology

There are several critical advantages to mRNA therapeutics over other modalities such as DNA-, protein- and small molecule-based strategies. mRNA-based medicines can be modular with easy‑to-switch sequences providing custom molecules that can specifically target different proteins or genes. These have the potential to directly affect the underlying cause of a disease and halt or reverse its progression. Manufacturing time for mRNA therapeutics is also fast compared to antibodies or protein-based drugs and their predictable pharmacokinetics and lack of genome integration make them relatively safe. This functional versatility has wide applications in developing viral vaccines, protein replacement therapies, cancer immunotherapies and in cellular reprogramming.

New frontiers bring new challenges

As natural biological molecules, mRNAs are inherently highly unstable and vulnerable to degradation by RNA-cutting ribonuclease enzymes. When administered into the human body, naked mRNA molecules face a huge barrier to:

• diffuse across the cell membrane due to their hydrophilicity and negative charge
• evade endolysosomal degradation 
• be delivered directly to a specified tissue or organ.

These hurdles prevent intact therapeutic mRNAs from accumulating in the body or intended tissue and often trigger an immune response. To circumvent these shortcomings, mRNAs require chemical modification and suitable delivery systems to achieve maximum potency and efficient cellular uptake while having low toxicity and immunogenicity effects. Ethris, a German biotechnology company, has developed a proprietary technology platform called SNIM® RNA (stabilised non-immunogenic mRNA) for the design and delivery of mRNA that overcomes these obstacles. The company has also discovered a novel strategy to improve mRNA sequence selection for respiratory diseases and disorders, taking into consideration the required expression levels of the target protein for therapeutic benefit combined with the appropriate route of administration – systemic or inhalable.

Targeted entry of mRNA into human cells is a challenging process that requires special delivery systems to protect the therapeutic mRNA from degradation and to facilitate transport into cells. Selection of the right transport vehicle for the mRNA is also critical as it must limit the activation of host immune responses as well as withstand the barriers associated with systemic or inhalable routes of administration that can greatly influence organ distribution and therapeutic outcomes.³

The route of administration is usually determined by the properties of the delivery system and the therapeutic indication. Treatments must be designed with sophisticated and tailored delivery strategies and high-precision control of mRNA activity. In addition, the storage of mRNA therapeutics is important as this can affect the long-term stability of carrier-mRNA formulations.

Potential solutions to overcome challenges with mRNA therapeutics

Many of these challenges have been overcome with recent advances in RNA biology and bioinformatics that have facilitated the rapid advancement of RNA therapeutics. Major milestones include molecular design improvements and optimisation within the mRNA molecule.

Chemical modifications in the mRNA’s structural elements such as the 5’ cap, 3’ poly(A) tail, 5’ and 3’ untranslated regions (UTRs) and/or the use of nucleoside analogues can dramatically affect its stability, half-life, translation efficiency and immunogenicity. Optimising mRNA sequence length and codon composition have also been shown to greatly stabilise the molecule.⁴ The discovery that adding synthetic structures to mRNA can further reduce immunogenicity and toxicity while improving potency and cell penetration has been a tremendous advancement in the field. While mRNA technologies still face long-term safety and efficacy evaluations, recent developments in new materials and delivery formulations are becoming effective solutions to enhancing therapeutic efficacy.

Delivery riders aplenty for mRNA therapeutics

mRNA delivery can be mediated by viral and non‑viral vectors. Several non-viral delivery platforms for mRNA therapeutics are being tested in clinical studies with materials derived from lipids, lipidoids (lipid-like materials), polymers, protein derivatives, etc. Lipid nanoparticles (LNPs) have been the most thoroughly investigated due to their versatile composition.⁵ LNPs’ positive charge (cationic) helps entrap the negatively charged mRNA within it and their chemical formulation has been the main reason for their success in delivering small molecules, siRNA and mRNA in humans.

New delivery system materials such as ionisable lipids and lipidoids are being used to overcome some of the challenges of conventional cationic lipids. The pH sensitivity of ionisable lipids makes them beneficial for mRNA delivery and can improve their biocompatibility. The first drug to be approved with an ionised lipid was Onpattro® (patisiran) for the treatment of hereditary transthyretin protein amyloidosis in 2018.⁹ Lipidoids containing other lipid-like materials can improve nanoparticle properties such as particle stability, efficacy of delivery, tolerability and biodistribution. To illustrate the development of this technology, Ethris is employing a proprietary formulation of lipidoid nanoparticles and a unique delivery platform to provide highly versatile, multi-route, multi-cargo delivery options for its drug candidates for the treatment of respiratory viral infections and rare pulmonary diseases while also developing a next‑generation of mRNA vaccines that are mucosal, multivalent and mutation-agnostic (Ethris’ ‘Triple-M’ concept).

Polymers comprise the second largest group of delivery vehicles after lipids that form stable complexes with nucleic acids and offer a versatile, scalable, and easily adjustable platform for efficient delivery while minimising immune responses and cellular toxicity. Another emerging nucleic acid delivery system indicating early promise is a lipid‑polymer hybrid nanoparticle (LPN).⁷ This system has shown its ability to effectively deliver siRNA to cells and is thermodynamically favourable. Several lipid-polymer combinations are currently being investigated to create stable particles for mRNA delivery.

In the early 2000s, cell-penetrating peptides were being studied as potential new delivery tools. Since then, a renewed momentum is emerging for their use in nucleic acid delivery as stable peptide‑based nanoparticles (PBNs).⁸ Although in early development, they still need to be optimised for targeted cell delivery and improved half-life.

Navigating the right route for delivering mRNA

Intramuscular accines are just one way of delivering mRNA therapeutics

Various administration routes are being evaluated to mediate mRNA transport and expression, including tracheal inhalation, intravenous, intraperitoneal and intramuscular injections. Topical administration of mRNA therapeutics is also being explored to achieve local therapeutic effects such as the supplementation of proteins in specific human tissues like the heart, eyes and brain.⁶ Choosing the right route for the mRNA‑carrier formulation can significantly impact tissue and organ distribution, target protein expression and therapeutic outcomes and is often determined by the properties of the carrier and the focus indication.

Intramuscular injection is the most used administration route for vaccination. This method’s effectiveness is due to the immune cells in the skin and muscles that internalise and express mRNA‑encoded antigens. However, this method has so far been unable to achieve sterile immunity by preventing virus transmission from vaccinated individuals. The development of intranasal mucosal vaccines could provide the solution and is the rationale for Ethris’ next-generation therapeutics and vaccines. To deliver mRNAs directly to the respiratory tract, the company has developed drug candidates with superior thermostability and high resistance to mechanical manipulation for use with a nebuliser. This route of administration provides two additional layers of protection. Direct application can activate memory B and T cells in the respiratory mucosa, eliciting an effective barrier to infection. If an infection still occurs, resident immune cells can encounter antigens earlier and respond more quickly than systemic memory cells, thus impeding viral replication and reducing viral shedding and transmission.

The mRNA therapeutics revolution is just beginning

mRNA-based therapeutics have made significant scientific strides in recent decades and continue to develop as an emerging field in drug development. mRNA-based medicines were making considerable headway in several therapeutic areas even before COVID-19 took centre stage, and the additional interest and investment now flowing into the field will help to further accelerate this progress.

Small biotech companies with transformative ideas are part of the RNA revolution, having the agility to test and advance new product candidates quickly, making them well placed to address unmet clinical needs. Several opportunities and some challenges surround mRNA-based therapeutics, but it goes without saying that mRNA will become a powerful and versatile modality to develop transformative therapies spanning diverse disease indications in the next decade. 

About the authors

Carsten Rudolph, PhD is the Chief Executive Officer and co‑founder of Ethris and the lead inventor of its SNIM® RNA Technology. His deep expertise lies in delivering mRNA specifically to the lungs. He is the inventor of 15 patents/applications and has authored over 120 scientific publications. Carsten is affiliated with the Dr. von Hauner Children’s Hospital, part of the Ludwig Maximilian University in Munich. He obtained his pharmaceutical degree from the Freie Universität Berlin.

Christian Plank, PhD is the Chief Technology Officer and co-founder of Ethris. He is the author of more than 170 publications and co-inventor of numerous patents in the field of nucleic acid delivery. Christian is a Professor at the Technical University of Munich and obtained his PhD in biochemistry from the University of Vienna, Austria.

References

1. Sasso JM, et al. (2022) The Progress and Promise of RNA Medicine – An Arsenal of Targeted Treatments. J. Med. Chem. 65, 6975−7015. PMID: 35533054
2. Zogg H, et al. (2022) Current Advances in RNA Therapeutics for Human Diseases. Int J Mol Sci. Mar 1;23(5):2736. PMID: 35269876
3. Damase TR, et al. (2021) The Limitless Future of RNA Therapeutics. Front. Bioeng. Biotechnol. 9, 628137. PMID: 33816449
4. Wadhwa A, et al. (2020) Opportunities and Challenges in the Delivery of mRNA-based Vaccines. Pharmaceutics 12, 102. PMID: 32013049
5. Hou X, et al. (2021) Lipid nanoparticles for mRNA delivery. Nat Rev Mater 6, 1078–1094. PMID: 34394960
   Qin S, et al. (2022) mRNA-based therapeutics: powerful and versatile tools to combat diseases. Sig Transduct Target Ther. 7, 166. PMID: 35597779
6. Mukherjee A, et al. (2019) Lipid–polymer hybrid nanoparticles as a next-generation drug delivery platform: state of the art, emerging technologies, and perspectives. Int J Nanomedicine. 14:1937-1952. PMID: 30936695
7. Boisguérin P, et al. Peptide-Based Nanoparticles for Therapeutic Nucleic Acid Delivery. Biomedicines. 2021 May 20;9(5):583. PMID: 34065544
8. US Food and Drug Administration News Release. 2018. Available at: https://www.fda.gov/news-events/press-announcements/fda-approves-first-its-kind-targeted-rna-based-therapy-treat-rare-disease