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