by Reynald Castaneda in London.

Clover Biopharmaceuticals’ Covid-19 protein subunit vaccine could potentially have the upper hand versus Novavax’s and IMV’s due to the former’s use of Dynavax Technologies’ FDA-approved adjuvant. However, it is premature to predict a clear frontrunner among the three vaccines due to information gaps such as how the spike protein antigen is constructed and reproduction platforms to be used for manufacturing.

On Monday (25 May), Novavax’s stock jumped as high as 17% during trading hours on announcing it had enrolled the first participants in its Phase I/II trial. IMV’s stock rose 20% on 21 May after announcing plans to enter clinical development. Dynavax was up 14% on 24 March upon its collaboration announcement with Clover Biopharmaceuticals.

Chengdu, China-based Clover Biopharmaceuticals initially called its S-Trimer vaccine a recombinant protein subunit vaccine but has since described it as “native-like,” or resembling the native SARS-CoV-2 spike protein. A synthetic spike protein antigen, such as in Novavax’s NVX‑CoV2373, would have a leg up compared to a native version, as the latter is at risk of reconfiguration once administered.

Clover’s vaccine may be at an additional disadvantage, as its spike protein antigen is produced via a mammalian cell-culture based expression system, which is a more expensive process. While Novavax hasn’t announced its production process, IMV’s vaccine DPX-Covid-19 only featuring viral peptides could benefit on this front.

Yet, IMV’s vaccine not featuring the entire spike protein drew the least expert enthusiasm. Peptides by themselves are not immunogenic, and DPX-Covid-19 does not have an adjuvant. Clover’s vaccine, though, is coupled with Dynavax’s toll-like receptor 9 (TLR9) agonist adjuvant, which is part of an approved vaccine.

The shape of the spike protein- prominent proteins found on the virus’ outer surface – in SARS-Cov-2 is essential to determining how to target the virus.

Novavax’s Australia-based Phase I (NCT04368988) will recruit about 130 patients, and preliminary immunogenicity and safety data are expected in July. Clover’s Phase I will also be based in Australia but has not yet been scheduled, while IMV’s Phase I is slated to start in the summer.

Novavax, which has a $2.64bn market cap, has so far received $388m from the international foundation Coalition for Epidemic Preparedness Innovations, with Clover Biopharmaceuticals receiving $39.6m. IMV, which has a CAD 265.4m market cap, has submitted several grant applications in Canada, said a spokesperson.

Protein subunit vaccines are one of several vaccine types under investigation for Covid-19, with at least 60 assets from discovery to Phase I stage listed on the GlobalData database. While Moderna’s mRNA vaccine mRNA-1273 is the perceived frontrunner in the Covid-19 vaccine race, with positive interim Phase I data released on 18 May, this news service reported 16 April that Moderna has many long-term success obstacles ahead. Future Phase II immunogenicity data may not forecast Phase III protection results, and as reported on 14 May a registrational field trial expected to start in the summer is likely to require around 25,000 volunteers. A challenge trial could be considered for quick protection data, but such a trial design also has its own operational and ethical conundrums.

Clover Biopharmaceuticals adjuvant choice key for efficacy edge

Novavax is utilising its matrix-M saponin-based adjuvant with NVX‑CoV2373. However, a general issue with a saponin-based adjuvant is that it needs to be combined with another adjuvant, said David Dowling, PhD, principal investigator, Adjuvant Discovery and Development Laboratory, Harvard Institutes of Medicine, Boston. While a saponin-based adjuvant can enhance immune response, the second adjuvant is needed to polarise the immune system to prevent side effects such as antibody-dependent enhancement (ADE), he explained. ADE is a particular concern in Covid-19, as it was previously observed with SARS and MERS, he noted.

Clover’s S-Trimer vaccine has the advantage of being combined with an adjuvant, and in particular with Dynavax’s TLR9 agonist adjuvant CpG1018, as CpG1018 is part of an already approved vaccine, Dowling noted. Adjuvants face rigorous testing, which means a significant investment from companies, and thus they are not normally licensed out, added Gordon Dougan, PhD, professor, Department of Medicine, Cambridge University, UK.

Adjuvants from a branded product would have stronger safety data, which is critical as autoimmunity issues is a recurring concern with adjuvants, noted Dougan. Dynavax’s Heplisav-B has the same adjuvant as Clover’s S-Trimer and was approved in November 2017 for hepatitis B virus in adults. Novavax’s adjuvant is not part of an approved product, but the company announced positive topline results from its Phase III seasonal influenza trial (NCT04120194) of NanoFlu, which uses the same matrix-M adjuvant as NVX‑CoV2373. Phase III showed noninferior data versus Sanofi Pasteur’s (EPA:SAN) Fluzone in those 65 years and over. If an adjuvant is shown to work in older adults, who have weaker immune systems, it is also likely to work in younger adults, Dowling noted. Novavax’s Phase I NVX‑CoV2373 trial is recruiting participants aged 18 to 59 years.

That said, Dowling noted, it is hard to know if the adjuvant is mechanistically important to any vaccine, and so specific Covid-19 data are important. Before Clover Biopharmaceuticals announced its collaboration with Dynavax, it publicised it was working with GlaxoSmithKline for the latter’s adjuvant. GSK’s vaccine data shows this adjuvant is better suited in younger adults than older people, Dowling said.

IMV, on the other hand, announced animal model data showing that DPX-Covid-19’s peptide epitopes generated an antibody response after two doses without requiring an adjuvant. However, peptide vaccines are definitely likely to need an adjuvant, said Don Diamond, PhD, professor, Department of Hematology & Hematopoietic Cell Transplantation, City of Hope Comprehensive Cancer Center, California. It is unlikely that several peptides would be effective in activating the immune system, particularly in older adults, Dowling explained. Peptides are so small that the immune system may not recognise them, and they are not immunogenic by themselves, Dougan added.

Novavax’s Phase I trial is investigating adjuvanted and unadjuvanted versions of NVX‑CoV2373. Another adjuvant advantage is that the antigen amount needed for each vaccine would reduce. This means decreased manufacturing costs, with more total doses for the same number of antigens produced, Dowling explained.

An adjuvant would also reduce the ADE risk, as it directs the immune system to the right kind of response, Dowling noted. The IMV spokesperson said all of IMV’s assets are formulated on its DPX platform, which has had positive safety data so far. Most adverse events were Grade 1–2 and localised at the injection site, he added. IMV’s most advanced therapeutic vaccine is in Phase II and the most advanced prophylactic vaccine is in Phase I.

Protein/peptide construction, manufacturing critical for differentiation

Another avenue of differentiation between protein subunit vaccines could come down to the spike protein antigen. Novavax’s NVX‑CoV2373 features a recombinant version of SARS-CoV-2’s spike protein, whereas Clover’s S-Trimer vaccine has a native-like spike protein.

While only Novavax has noted that its vaccine has the prefusion configuration of the spike protein, this is likely to also be the case with Clover’s, as a postfusion configuration has its receptor-binding domains (RBDs) hidden, and RBD is the critical target in generating neutralising antibodies, said Andrew Ward, PhD, professor, Department of Integrative Structural and Computational Biology, Scripps Research, La Jolla, California. A prefusion configuration is also representative of an infectious virus, he added.

A native spike protein antigen, however, is at risk of converting from a prefusion to a postfusion configuration once administered, thus hiding the RBD, Ward noted. Nevertheless, there is still the possibility that enough RBD remains exposed even in the postfusion configuration, he added. Different RBDs can trigger different types or levels of neutralising antibodies, Dougan explained.

Another potential concern with a native spike protein antigen is that during the purification step to extract the spike protein, inserted sugars or lipids could lead to unwanted protein variations, noted Ciro Leonardo Pierri, PhD, spike protein researcher, Department of Biosciences, Biotechnologies and Biopharmaceutics, University of Bari, Italy. However, this could be irrelevant as long as the vaccine can still trigger neutralising antibodies, he added.

In contrast to Novavax’s and Clover’s vaccines, IMV’s DPX-Covid-19 only has 23 SARS-CoV-2 peptide epitopes as opposed to the entire spike protein. A potential advantage of this approach is that the vaccine could induce a more specific immune response, Pierri and Ward said. Considering how large the spike protein is, other types of antibodies could be generated, Pierri explained.

The RBD is only one-tenth of the entire spike protein, Ward added. The IMV spokesperson said the selected peptides have demonstrated in preclinical studies the ability to elicit a target-specific, robust and sustained immune response. Still, featuring the entire spike protein helps stimulate the immune system overall, and a domino effect would then lead the immune system to find the RBD, Ward said.

How the spike protein is replicated for manufacturing is one more point of distinction between the three companies’ strategies, Dougan said. As per the 16 April article, mRNA vaccines such as Moderna’s may have manufacturing edge over vaccines that carry viral proteins, as the latter approach has additional steps that include cell line and purification procedures. A company that uses a yeast platform may have the scale-up advantage, as it is cheaper versus mammalian cells, Dougan added. Clover Biopharmaceuticals, which did not respond to a request for comment, is producing its vaccine via a mammalian cell expression system.

When asked about its manufacturing approach, a Novavax spokesperson forwarded the media release from yesterday (27 May) stating it had acquired Praha Vaccines in a transaction worth $167m, which covers biologics manufacturing facilities and other assets in the Czech Republic. This facility would provide an annual capacity of over one billion doses of antigen in 2021 for NVX‑CoV2373. For IMV, the antigen peptides in DPX-Covid-19’s are synthetic and do not require biologic manufacturing, the company spokesperson said. Perhaps IMV’s approach is more scalable as it only features peptides, Ward added.

Nevertheless, none of these three protein subunit vaccines has human immunogenicity results yet. While Novavax’s NVX‑CoV2373 has preclinical data from microneutralising assays showing that the vaccine was able to induce neutralising antibodies, this is only the first milestone of many in indicating a vaccine can offer protection, said Diamond, who has a research focus on cancer and viral vaccines. Neutralising antibodies do not always correlate to protection, Dougan added, noting this disconnect is observed in rotavirus vaccines.

Although Clover’s S-Trimer vaccine has data showing it induced spike protein-specific antibodies the same as those in recovered Covid-19 patients, such a benchmark may be inappropriate, Dougan said. Antibodies induced by natural infection and vaccination are different, as the former is produced with moderation from the virus, he added.

Manasi Vaidya is a Senior Reporter for Clinical Trials Arena parent company GlobalData’s investigative journalism team. A version of this article originally appeared on the Insights module of GlobalData’s Pharmaceutical Intelligence Center. To access more articles like this, visit GlobalData.