Review An Update on Self-Amplifying mRNA Vaccine Development Anna K. Blakney 1,* , Shell Ip 2 and Andrew J. Geall 2 1 Michael Smith Laboratories, School of Biomedical Engineering, University of British Columbia, Vancouver, BC V6T 1Z4, Canada 2 Precision NanoSystems Inc., Vancouver, BC V6P 6T7, Canada; [email protected] (S.I.); [email protected] (A.J.G.) * Correspondence: [email protected] Abstract: This review will explore the four major pillars required for design and development of an saRNA vaccine: Antigen design, vector design, non-viral delivery systems, and manufacturing (both saRNA and lipid nanoparticles (LNP)). We report on the major innovations, preclinical and clinical data reported in the last five years and will discuss future prospects. Keywords: RNA; self-amplifying RNA; replicon; vaccine; drug delivery 1. Introduction: The Four Pillars of saRNA Vaccines In December 2019, the SARS-CoV-2 (severe acute respiratory syndrome coronavirus 2) virus emerged, causing a respiratory illness, coronavirus disease 2019 (COVID-19), in Hubei province, China [1,2]. The virus has spread globally, with the World Health Organization (WHO) declaring it a Public Health Emergency of International concern on 30 January 2020 and a pandemic officially on 7 March 2020 [3]. There is a strong consensus globally that a COVID-19 vaccine is likely the most effective approach to sustainably controlling the COVID-19 pandemic [4]. There has been an unprecedented research effort and global Citation: Blakney, A.K.; Ip, S.; Geall, coordination which has resulted in the rapid development of vaccine candidates and A.J. An Update on Self-Amplifying initiation of human clinical trials. This has included conventional vaccine technologies mRNA Vaccine Development. Vaccines 2021, 9, 97. https:// such as viral vectors and adjuvanted subunits, but we have witnessed a renaissance in doi.org/10.3390/vaccines9020097 the field of RNA vaccines and a shift towards synthetic RNA platforms (Figure1)[ 5,6]. In fact, one of the first vaccine to start clinical trials was a non-replicating mRNA vaccine Academic Editor: Norbert Pardi from Moderna, mRNA-1273 [7–9]; the first patient was vaccinated on 16 March at the same Received: 16 December 2020 time as a Chinese clinical trial was initiated with an adenovirus type-5 (Ad5) vector [10]. Accepted: 23 January 2021 Furthermore, the BioNTech/Pfizer vaccine, BNT162b2, was the first COVID-19 vaccine Published: 28 January 2021 to receive approval, first in the United Kingdom and then Canada, with an impressive 95% efficacy [11]. Since this time there have been several mRNA vaccine trials initiated, Publisher’s Note: MDPI stays neutral and publication of corresponding preclinical and clinical data, see Table1. with regard to jurisdictional claims in published maps and institutional affil- Table 1. Published preclinical and clinical trial data with mRNA COVID-19 vaccines. iations. Sponsor Type of mRNA Delivery System Preclinical Data Clinical Data Moderna bmRNA LNP [9,12][7,8] BioNTech/Pfizer bmRNA LNP [13][11,14–17] ICL saRNA LNP [18] Copyright: © 2021 by the authors. Arcturus saRNA LNP [19] Licensee MDPI, Basel, Switzerland. CureVac mRNA LNP [20] This article is an open access article Imperial College London (ICL), conventional non-amplifying messenger ribonucleic acid (mRNA), conventional distributed under the terms and base-modified non-amplifying mRNA (bmRNA) and self-amplifying messenger RNA (saRNA). conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Vaccines 2021, 9, 97. https://doi.org/10.3390/vaccines9020097 https://www.mdpi.com/journal/vaccines Vaccines 2021,, 9,, 97x 2 of 26 Figure 1. A comparison of vaccine platformsplatforms including including vaccines vaccines derived derived from from the the virus virus itself itself and and are are formulated formulated as as a part a part or wholeor whole modified modified version version of the of virusthe virus (left )( andleft) nucleic and nucleic acid vaccines, acid vaccines, such as such self-amplifying as self‐amplifying RNA vaccines RNA vaccines (right). Nucleic(right). Nucleicacid vaccines acid vaccines are derived are derived from knowledge from knowledge of the viralof the genome, viral genome, where where glycoproteins glycoproteins are encoded are encoded into nucleicinto nucleic acids acids and anddelivered delivered with with either either a synthetic a synthetic carrier carrier such such as a lipid as a nanoparticlelipid nanoparticle or an or inert an viralinert delivery viral delivery system system such as such adenoviruses. as adeno‐ viruses.The encoded The encoded antigen sequencesantigen sequences are then are expressed then expressed by the host by cells.the host cells. TableThe 1. Published use of mRNA preclinical vaccines and clinical for pandemic trial data response with mRNA has COVID been well‐19 vaccines. described previously in preclinical [12,21–33] and clinical settings [25], but this is the first time we have seen the platformsSponsor deployedType in of a mRNA real pandemic Delivery setting System [34]. ThePreclinical core principle Data behindClinical mRNA Data vaccinesModerna is to encode thebmRNA antigen in the mRNALNP and then to deliver[9,12] the transcript to[7,8] the host cellBioNTech/Pfizer cytoplasm using a non-viralbmRNA delivery system,LNP allowing antigen[13] expression[11,14–17] and induc- tion of anICL antigen-specific saRNA immune response.LNP This is especially advantageous[18] as a vaccine platformArcturus as mRNA vaccinessaRNA can be producedLNP for any pathogen with[19] a known protein target. mRNACureVac is made using a cell-freemRNA enzymatic transcriptionLNP reaction,[20] which allows rapid and Imperialscalable College manufacturing, London (ICL), as is conventional evident from non the‐amplifying swift pursuit messenger of RNA ribonucleic vaccines inacid the (mRNA), current conventional base‐modified non‐amplifying mRNA (bmRNA) and self‐amplifying messenger pandemic. Currently, there are three major types of RNA vaccines: conventional, non- RNA (saRNA). amplifying mRNA molecules (mRNA), base-modified, non-amplifying mRNA molecules (bmRNA), which incorporate chemically modified nucleotides, and self-amplifying mRNA The use of mRNA vaccines for pandemic response has been well described previ‐ (saRNA or replicons) that maintain auto-replicative activity derived from an RNA virus ously in preclinical [12,21–33] and clinical settings [25], but this is the first time we have vector. Self-amplifying RNA is beneficial compared to non-amplifying RNA as it maintains seen the platforms deployed in a real pandemic setting [34]. The core principle behind the advantages of mRNA vaccines, such as rapid development, modular design, and cell- mRNA vaccines is to encode the antigen in the mRNA and then to deliver the transcript free synthesis, but requires a lower dose of RNA due to the self-replicative properties. to the host cell cytoplasm using a non‐viral delivery system, allowing antigen expression This reduces the burden of manufacturing for both the drug substance and product and is and induction of an antigen‐specific immune response. This is especially advantageous as potentially advantageous in the context of pandemic response as it would enable a greater a vaccine platform as mRNA vaccines can be produced for any pathogen with a known percentage of the population to be vaccinated in a shorter amount of time. protein target. mRNA is made using a cell‐free enzymatic transcription reaction, which This review will explore the four major pillars required for design and development allows rapid and scalable manufacturing, as is evident from the swift pursuit of RNA vac‐ of an saRNA vaccine (Figure2): Antigen design, vector design, non-viral delivery systems, cinesand manufacturing in the current pandemic. 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