As a follow-up to an earlier GEITP email about “mRNA delivery, attempting to create vaccine production,” this excellent review [see attached] is very thorough and complete. 😊 Current clinical efforts include development of vaccines, protein replacement therapies, and treatment of genetic diseases. The latest advances in clinical translation of mRNA therapeutics have been made possible through novel developments in the design of mRNA manufacturing and intracellular delivery methods. Broad application of mRNA is still limited, however, by the need for improved delivery systems.
The translatability (process to create a protein from the messenger RNA) and stability of mRNA, as well as its ability to activate immune cells (immunostimulatory activity) are the key factors that require optimization for specific therapeutic application. Increased translation and stability can be affected by many regions of the RNA: 5’- and 3’-untranslated regions (UTRs) are required for recruiting RNA-binding proteins and microRNAs (miRNAs) in the intact cell, and these UTRs can profoundly affect translational activity. Modification of rare codons in protein-coding sequences — with synonymous (i.e. do not change the translated amino acid) frequently occurring codons (so-called ‘codon optimization’) — can result in order-of-magnitude increases in expression levels. Modification of the 5’-mRNA cap can also enhance mRNA translation, by inhibiting RNA decapping and improving resistance to enzymatic degradation. The importance of immunostimulation (by chemical modification of RNA bases) can depend on the application (and, in some cases, it may actually improve performance, as in the case of vaccines). Most importantly, methods and vehicles for intracellular delivery remain the most major barrier to the broad application of mRNA therapeutics.
Intracellular delivery of mRNA is generally more challenging than that of small oligonucleotides (proteins having relatively small numbers of amino acids) — because it requires encapsulation into a delivery nanoparticle — in part due to the significantly larger size of mRNA molecules (1000-15000; i.e. 1 to 15 kilobases, kb) as compared to other types of RNAs [e.g. small-interfering RNAs (siRNAs; 20-27 bases) and antisense oligonucleotides (ASOs; 21-28 bases)]. For those interested in more of the history of RNA therapeutics, and details of the mechanisms involved, please see [the attached] review. 😊
Mol Ther Apr 2019; 27: 710-728
Thanks Dan! Interesting review. I wish I had found this article earlier last week. In the last few years, we have performed safety assessments for a number these APIs (active pharmaceutical ingredients, i.e. drugs or mRNA molecules) — including silencing RNAs (siRNAs), antisense oligonucleotides (ASOs), and messenger RNAs (mRNAs). It is pretty incredible how fast these technologies are evolving.
It wasn’t that long ago that I was in the lab trying to transfect cells with siRNA, or transduce them to express a gene — sometimes with pretty mediocre results. Now I’m seeing a number of these new APIs in advanced clinical studies for the treatment of a range of disorders. Therapeutic doses tend to be in the low-microgram range, with as few as once- or twice-a-year administrations. The potency, target specificity, and long tissue half-life makes these drugs a game-changer for treatment of some diseases (and I’m sure they’ll be insanely expensive, once they arrive on the market). However, these same characteristics can also make these APIs risky for the workers that manufacture them. Last week we reviewed an mRNA vaccine for COVID-19 that is entering Phase 2/Phase 3 testing, with a pharmacologic dose in the nanogram range, and an occupational exposure limit (OEL) that is much lower — due to the many uncertainties among the sparse datasets in human pharmacology and toxicology
John, I totally agree that this is an interesting and highly informative review; it was “found” by one of our GEITP’ers (Ray D’Alonzo PhD) who referred it on to me. For me, this review updated my knowledge in this field — that has changed so rapidly during the past 2-3 decades. 😊