Are Genetic Vaccines the Right Weapon against COVID-19?

Abstract

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the causative agent of Coronavirus disease (COVID-19), emerged in China in December 2019. Investigators have made rapid efforts to design therapeutic strategies targeting this rapidly spreading pathogen. Although not as lethal as SARS-CoV or Middle East respiratory syndrome (MERS)-CoV, the virus is highly transmissible, often without symptoms, with an estimated reproductive number (R0) of 2.2.1Li Q. Guan X. Wu P. Wang X. Zhou L. Tong Y. Ren R. Leung K.S.M. Lau E.H.Y. Wong J.Y. et al.Early transmission dynamics in Wuhan, China, of novel coronavirus-infected pneumonia.N. Engl. J. Med. 2020; 382: 1199-1207Crossref PubMed Scopus (10808) Google Scholar On March 12, 2020, a mere 2 months since the outbreak was detected, World Health Organization (WHO) announced that COVID-19 had reached pandemic status. As a consequence, containment strategies have been implemented to slow viral transmission, but control of the infection will be challenging without a vaccine. The global scientific community and the vaccine industry, supported by international organizations such as Coalition for Epidemic Preparedness Innovations (CEPI) and the European Commission, have committed to developing a safe vaccine able to elicit a potent and long-lasting virus-specific immune response against SARS-CoV-2. The ideal target vaccine product profile should comprise some key elements. It should be consistently immunogenic with minimal side effects, such as undesired immunopotentiation in the form of increased infectivity.2Jiang S. Bottazzi M.E. Du L. Lustigman S. Tseng C.T. Curti E. Jones K. Zhan B. Hotez P.J. Roadmap to developing a recombinant coronavirus S protein receptor-binding domain vaccine for severe acute respiratory syndrome.Expert Rev. Vaccines. 2012; 11: 1405-1413Crossref PubMed Scopus (113) Google Scholar Vaccine production should be implemented in a timely and efficient manner and be relatively inexpensive and suitable for large-scale good manufacturing practice (GMP) manufacturing. Moreover, as requested by regulatory agencies during the first regulatory workshop on COVID-19 held in March 2020 under the umbrella of the International Coalition of Medicines Regulatory Authorities (ICMRA), vaccine design should include a careful assessment of possible immune complications, such as the possibility of antibody-dependent-enhancement (ADE) of infection, before being released to the public. To this aim, diverse platforms have been set up, but only a few can address these requirements. Conventional vaccines, such as inactivated, attenuated, or subunit vaccines, have been successful but have drawbacks, such as their strain specificity, and consequently are potentially associated with risks of viral interference and cross-immunity3Laurie K.L. Guarnaccia T.A. Carolan L.A. Yan A.W. Aban M. Petrie S. Cao P. Heffernan J.M. McVernon J. Mosse J. et al.Interval Between Infections and Viral Hierarchy Are Determinants of Viral Interference Following Influenza Virus Infection in a Ferret Model.J. Infect. Dis. 2015; 212: 1701-1710Crossref PubMed Scopus (78) Google Scholar and can be allergenic in some patient groups. Furthermore, vaccines based on viral proteins tend to elicit immune responses that are limited to the CD4+ T cell response or antibody-dependent mechanisms and lack a CD8+ T cell response. Besides this, the production of conventional vaccines can be expensive and time-consuming. Safety concerns, commonly associated with the use of whole virus as a vaccine platform, have been overcome by the development of replication-defective recombinant adenoviruses, which have proven safe for administration in humans and effective in inducing robust innate and adaptive immune responses. Third-generation adenoviral vectors have been employed to prevent or treat life-threatening infectious diseases such as Ebola, Zika, malaria, hepatitis C virus (HCV), and HIV4Kennedy S.B. Bolay F. Kieh M. Grandits G. Badio M. Ballou R. Eckes R. Feinberg M. Follmann D. Grund B. et al.PREVAIL I Study GroupPhase 2 placebo-controlled trial of two vaccines to prevent Ebola in Liberia.N. Engl. J. Med. 2017; 377: 1438-1447Crossref PubMed Scopus (177) Google Scholar,5López-Camacho C. Abbink P. Larocca R.A. Dejnirattisai W. Boyd M. Badamchi-Zadeh A. Wallace Z.R. Doig J. Velazquez R.S. Neto R.D.L. et al.Rational Zika vaccine design via the modulation of antigen membrane anchors in chimpanzee adenoviral vectors.Nat. Commun. 2018; 9: 2441Crossref PubMed Scopus (64) Google Scholar and tested in clinical trials for anticancer immunotherapy.6Zhang W.W. Li L. Li D. Liu J. Li X. Li W. Xu X. Zhang M.J. Chandler L.A. Lin H. et al.The first approved gene therapy product for cancer Ad-p53 (gendicine): 12 years in the clinic.Hum. Gene Ther. 2018; 29: 160-179Crossref PubMed Scopus (210) Google Scholar However, this vaccination strategy is hampered by issues such as pre-existing immunity in humans and challenges in construction. Therefore, newer vaccination approaches, such as genetic vaccines based on naked DNA or RNA, have emerged as promising alternatives owing to several beneficial features. First, they have a highly satisfactory safety profile without potential risk of integration or pathogenicity, and for this reason they are considered an ideal therapeutic strategy in cancer immunotherapy or for vaccinating immunocompromised people. Second, genetic vaccination can elicit both T cell activation and antibody production in response to even small amounts of expressed protein and, unlike whole virus vectors, can be more easily administered in multi-dose regimens without generating pre-existing immunity. Finally, the manufacturing process confers some advantages: both DNA and RNA are inexpensively and easily constructed directly from the genetic sequence of the desired antigen. Once established, the production process can be easily adjusted according to the histocompatibility leukocyte antigen (HLA) diversity in the field in order to include the most immunogenic antigens and modulators for a specific population. Hence, the use of nucleic acids in vaccine development programs is growing in a wide range of traditional pharmaceutical markets, such as cancers and allergies, as well as infectious diseases, and it is increasingly demonstrating its safety and efficacy in early and mid-stage human clinical trials.7Lee J. Arun Kumar S. Jhan Y.Y. Bishop C.J. Engineering DNA vaccines against infectious diseases.Acta Biomater. 2018; 80: 31-47Crossref PubMed Scopus (128) Google Scholar,8Liu M.A. A Comparison of Plasmid DNA and mRNA as Vaccine Technologies.Vaccines (Basel). 2019; 7: 37Crossref PubMed Scopus (280) Google Scholar Nevertheless, these vaccination strategies still present some drawbacks, and differences between DNA and RNA must be taken into account. As for immunogenicity, a number of factors can increase DNA potency, such as the use of immunostimulants (cytokines and immunostimulatory molecules), tailored delivery routes and devices (with intramuscular injection followed by electroporation having been found to be the most effective in inducing strong immune responses), and different combination strategies (e.g., DNA prime followed by viral vector, peptide, or recombinant protein heterologous boosts). Conversely, over the past decade, vaccine developers have striven to increase RNA stability, improve its cellular delivery through encapsulation into nanoparticles, and reduce its constitutive reactogenicity by using modified nucleosides and controlling the onset of eventual toxicities. Challenges remain for RNA-based strategies, such as further improving stability, reducing toxicity (due to intrinsic inflammatory activity), and increasing protein translation, necessitating additional clinical studies. Additionally, in order to avoid the use of any animal or cellular materials, researchers are exploring alternative manufacturing strategies, such as the use of PCR-generated linear DNA fragments.9Allen A. Wang C. Caproni L.J. Sugiyarto G. Harden E. Douglas L.R. Duriez P.J. Karbowniczek K. Extance J. Rothwell P.J. et al.Linear doggybone DNA vaccine induces similar immunological responses to conventional plasmid DNA independently of immune recognition by TLR9 in a pre-clinical model.Cancer Immunol. Immunother. 2018; 67: 627-638Crossref PubMed Scopus (26) Google Scholar As shown in Table 1, genetic vaccines, DNA-based ones in particular, are an ideal vaccination platform for infectious diseases: both DNA and RNA can be developed and manufactured rapidly, ensuring a reproducible and standard production process, whatever the target disease or gene insert, but still with some intrinsic limitations to overcome, such as lower immunogenicity, in comparison to protein- and viral-based conventional vaccines.10Lurie N. Saville M. Hatchett R. Halton J. Developing Covid-19 Vaccines at Pandemic Speed.N. Engl. J. Med. 2020; 382: 1969-1973Crossref PubMed Scopus (1071) Google ScholarTable 1Some Candidate Viral-Based and Genetic Vaccines against COVID-19 in Clinical/Preclinical EvaluationPlatformType of Candidate VaccineDeveloperCurrent Stage of Clinical Evaluation/Regulatory StatusDNADNA electroporationInovio Pharmaceuticalsphase 1 (NCT04336410)RNALNP-encapsulated mRNAModerna/NIAIDphase 2 (IND submission)phase 1 (NCT04283461)RNA3 LNP-mRNAsBioNTech/Fosun Pharma/Pfizerphase 1/2 (2020-001038-36)Non-replicating viral vectoradenovirus type5 vectorCanSino Biological/Beijing Institute of Biotechnologyphase 2 (ChiCTR2000031781)phase 1 (ChiCTR2000030906)Non-replicating viral vectorChAdOx1University of Oxfordphase 1/2 (NCT04324606)DNADNA/PCR electroporationTakis/Applied DNA Sciences/Evvivaxpreclinical evaluationDNADNA electroporationKarolinska Institute/Cobra Biologics (OPENCORONA Project)preclinical evaluationRNAmRNACurevacpreclinical evaluationNon-replicating viral vectorAd26Janssen Pharmaceutical Companiespreclinical evaluationNon-replicating viral vectorreplicationdefective simianadenovirusReiThera/LEUKOCARE/Univercellspreclinical evaluationNon-replicating viral vectororal vaccine platformVaxartpreclinical evaluationFrom WHO,11WHO DRAFT landscape of COVID-19 candidate vaccines – 20 April 2020.https://www.who.int/blueprint/priority-diseases/key-action/novel-coronavirus-landscape-ncov.pdf?ua=Date: 2020Google Scholar updated on May 5, 2020. Open table in a new tab From WHO,11WHO DRAFT landscape of COVID-19 candidate vaccines – 20 April 2020.https://www.who.int/blueprint/priority-diseases/key-action/novel-coronavirus-landscape-ncov.pdf?ua=Date: 2020Google Scholar updated on May 5, 2020. The last decade has witnessed the outbreak of several new human pathogens, including Ebola, Chikungunya, Zika, SARS-CoV, MERS-CoV, and more recently the novel coronavirus (SARS-CoV-2). The continuous spread of such diseases at regular intervals poses a significant threat to human health and the economy, and there is an urgent need for a vaccine technology that is able to protect against rapidly arising or mutating pathogens. Genetic vaccination represents an ideal vaccine target product profile to be developed in response to an unexpected pandemic outbreak, so, in the future, every effort must be addressed to develop a vaccine platform that can be designed and produced in large scale in a timely fashion to be ready for a “disease X.” A.C., E.M., G.R., F.P., and L.A. have an interest in developing vaccines and therapeutics to treat SARS-CoV-2. A patent has been filed at Takis for a genetic vaccine under development. G.C. declares no conflict of interest.

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