Decades since the discovery of messenger RNA, clinically effective mRNA-based therapeutics represent a game-changing disruptive technology that has finally been recognized by researchers and investors as well as the pharmaceutical and biotech community.
In 1956, scientists Elliot Volkin and Lazarus Astrachan of Oak Ridge National Laboratory (ORNL) in Tennessee discovered so-called DNA-like-RNA, later identified as messenger RNA by François Jacob and Jacques Monod. Alvin M. Weinberg, former ORNL director and a distinguished fellow of Oak Ridge Associated Universities, described the discovery as "next to the original discovery of the molecular structure of DNA, probably the most important event in the history of molecular biology."
Although RNA was historically overshadowed by DNA, it was investigated just as intensively. The genetic code was determined, the ribosomal machinery and transfer RNA were analyzed, splicing of RNA was elucidated, RNA was recognized as a catalytic enzyme and regulatory RNAs were discovered. In 2006, RNA was again in the spotlight: Professors Andrew Fire and Craig Mello were awarded with the Nobel Prize in physiology or medicine for the discovery of RNA interference, or RNAi, a mechanism of gene silencing by double-stranded RNA. Only a few years after the RNAi gene-silencing mechanism was first announced, the first RNAi therapeutics entered clinical trials.
So why did it take mRNA such a long time to be recognized for its therapeutic potential?
The biological role of mRNA is to transfer genetic information from the nucleus of the cell to the cytoplasm, where this information is translated into the corresponding protein. If applied externally, mRNA is taken up by the cell and remains in the cytoplasm, the "natural" location of the translation of "mature" mRNA to the corresponding protein. Thus, no nuclear localization or transcription is required, and the probability of genomic integration is nearly nonexistent. Additionally, mRNA degrades quite fast. Therefore, mRNA is considered a very safe biomolecule that allows transient protein expression of every protein in virtually all cell types. But mRNA is known for its instability, caused by the presence of ribonucleases, short RNAses. These are enzymes that are responsible for the degradation of single-stranded RNA. They are found in body fluids (tears, saliva, mucus and perspiration) and destroy mRNA in an unprotected environment. Therefore, despite all the advantages of mRNA, medical research for gene therapeutic approaches and expression of certain target molecules focused originally on the much-easier-to-handle molecule, DNA.
In 2000, researchers in the laboratory of Hans-Georg Rammensee and Günther Jung at the University in Tübingen, Germany, made a breakthrough discovery, as is so often the case in science, by chance. In a gene-therapy experiment that involved injecting DNA plasmids into mice, naked mRNA was used as a negative control. The common belief at this time was that mRNA would degrade so quickly that it would be impossible to induce an immune response. But surprisingly, the injection of naked, sequence-modified mRNA resulted not only in protein expression, but in the highest level of immune response: specific cytotoxic T lymphocytes and antibodies. Ingmar Hoerr, one of the researchers, first assumed that this must be a mistake, a mix-up. But after several independent repetitions with the same result, it was obvious: mRNA could be used as a minimal vaccine against diseases and as a prophylactic vaccine; mRNA vaccines do not need a vehicle such as a virus for delivery to the cells. The seed was planted for a completely novel medical approach.
Meanwhile, many advances in the last decade have made the clinical application of mRNA feasible. mRNA can be produced quickly, synthetically and at GMP quality for any sequence provided and can be stored at room temperature. Importantly, the mRNA is produced in a cell-free system by using naturally occurring nucleotides, the building blocks of nucleic acids. Additionally, very long-tailed mRNA molecules are feasible to produce using the same technology. By modifying the sequence of the mRNA, the stability can be improved and the translation levels are enhanced without changing the amino acid sequence of the corresponding protein. Today, a two-component mRNA immunotherapeutic containing free and protamine-complexed mRNA and a cationic peptide that protects mRNA against RNase-mediated degradation has been developed. mRNA cancer immunotherapies have shown effectiveness in early clinical trials for prostate cancer and lung cancer, inducing a balanced humoral and a T cell-mediated immune response.
The safety profile, ease of use, and intradermal (injected directly into the skin) applications also widen mRNA's use for prophylactic vaccines. Dr. Lothar Stitz, from the Friedrich Loeffler Institute in Greifswald, Germany, investigated the use of mRNA-based vaccines in the area of influenza. His study, recently published in Nature Biotechnology, showed that immune responses similar or superior to those triggered by commercially available vaccines were achieved in his animal models. The big advantage of mRNA-based vaccines: These vaccines can be produced easily and quickly and can match any sequence of a new influenza strain. Additionally, the vaccination with the conserved and intracellular antigen can further improve the protection. Immunization with a combination of conserved antigens is feasible and may form the basis for an optimized and more broadly protective vaccine.
Other therapeutic approaches that are currently being investigated include the in vivo production of intracellular proteins as well as secreted proteins. In order to utilize mRNA-based therapeutics for this and other approaches including recombinant therapeutic proteins and as a method for gene replacement therapy, researchers had to find ways to suppress the previously described mRNA's immune-stimulatory activity.
Katalin Kariko, of Penn Medicine in Philadelphia, suggested that the innate immune system detects RNA lacking nucleoside modifications, which are found on mammalian mRNA but not on bacterial mRNA, and showed that modifications of mRNA can suppress its immune-stimulatory activity and potentially widen the applications for mRNA as a therapeutic. Thus mRNA has recently become a more interesting field among members of the biotech community. In 2010 Derrick Rossi, of the Harvard Stem Cell Institute in Cambridge, MA, and colleagues showed that synthetically modified mRNA can be used as a safe (without genomic integration) strategy for efficiently reprogramming cells to pluripotency, and today chemically modified mRNAs that elude the body's innate immune response are investigated as a way to create therapeutic proteins. This approach is in preclinical development.
Finally, researchers have learned how to unlock the potential of mRNA to transform modern medicine. mRNA as a novel therapeutic with nearly limitless opportunity has gained traction from biotech and pharmaceutical companies and has begun to play an important part in the development of novel therapies to address human disease.
Ingmar Hoerr is co-founder and CEO of CureVac, an integrated biopharmaceutical company in the field of novel vaccines for infectious diseases and cancer. CureVac has secured about €65 million in equity financing in three financing rounds and currently employs more than 100 people. During his scientific career, Ingmar laid the foundation of CureVac's unique RNA-based technology platform, RNActive®, enabling the development of a new class of cancer vaccines. With a background in both biology and business management, he decided to set up CureVac in order to advance his findings to their full therapeutic and commercial potential. Ingmar Hoerr's scientific track record includes work in the laboratories of Professor Günther Jung and Professor Hans-Georg Rammensee, both renowned scientists in organic chemistry and immunology, respectively, as well as one year of field studies on leprosy and HIV in collaboration with the World Health Organization (WHO) at Madurai Kamaraj University, India. Ingmar Hoerr received his Ph.D. in biology from Tübingen University (1999) and a MBA from Danube University in Krems, Austria (2001).
