A Comparison of Plasmid DNA and mRNA as Vaccine Technologies

Liu, Margaret A.

For mRNA, the areas of continued Research and Development (R&D) efforts for improving the potency of mRNA vaccines are shown in Table 1 , which describes efforts similar to those for DNA vaccines. These focus on augmenting delivery of the mRNA and increasing potency via increased stability and greater expression of the protein. Alterations of the mRNA itself include changing the codon usage and the GC (guanine-cytosine) content [5] , along with modifications of the other regions, such as the 5 cap, the UTRs, and the poly-A tails. A more detailed description of the efforts to increase the mRNA potency and of delivery formulations that include lipids, nanoparticles, polymers, polycations, and various proprietary entities are presented and reviewed elsewhere with tabular and chemical descriptions [2, 11, 33, 34] . Cells generally take up mRNA by endocytosis, thus efforts are also being made to design delivery systems that increase the endosomal release of the mRNA into the cytoplasm [35] . Certain formulations, such as delivery of a particular encapsulated lipoplex mRNA vaccine, were found to specifically be taken up by dendritic cells via micropinocytosis [36] . As with DNA vaccines, possible immunomodulators added as recombinant proteins or encoded by mRNA are being evaluated [37] . Various routes of injection of mRNA are being explored, including i.m., i.d., s.c., i.v., and intranodal [2] , in addition to the ex vivo approach described. Delivery devices such as the gene gun (where mRNA is put onto gold particles) [38] and electroporation are also being explored. Optimize immune responses for the antigen (e.g., type of T helper response, subclass of antibody) • Decrease or select desired inflammatory effects of mRNA • Optimize the above for potency, safety, complexity of formulation, cost of manufacture, product stability Circular RNAs (circRNA) are endogenously expressed and are thought to play roles mainly for gene regulation, with potential activity as tumor antigens [39] . They can be exogenously constructed to produce proteins in cells [40] . These engineered circRNA molecules appear to be more stable and to result in more potent production of protein than linear mRNA. However, the mechanisms for their effects upon gene regulation and other activities are still being explored [41] .