New Antivirals: Could They Be Game Changers?

November 1, 2021

There have been recent reports of potentially effective oral antiviral drugs, including RNA replicase and viral protease inhibitors. If these prove safe and effective, this may overhaul the care of patients with COVID-19 by preventing severe COVID-19 in infected patients and, possibly in the future, prevent infection after contamination. It is likely that combining antivirals with different mechanisms of action will be necessary, as for many other viral infections. On the other hand, they may have some negative impact on preventing COVID-19 by reducing protection measures and increasing vaccine hesitancy. Thus, whatever the efficacy of the direct antivirals against infection by SARS-CoV-2, vaccination will remain key. What are these drugs, how do they act, and how effective are they?

Molnupiravir

The drug that has been receiving the most attention recently is molnupiravir, a ribonucleoside analog. This showed significant antiviral activity in Phase 2 trials against SARS-CoV2(1) and is beginning Phase 3 trials. Molnupiravir targets the viral RNA-dependent RNA polymerase (RDRP), and it was originally developed as a therapeutic for influenza. Unlike remdesivir, it is administered orally. In addition, remdesivir terminates growing viral RNA chains whereas molnupiravir incorporation does not terminate chain elongation(2). However, its structure allows it to resemble cytidine or uridine, depending upon which tautomeric form it assumes. This causes a mutational catastrophe for the virus, thus raising some concern about its mutagenic potential. One in vitro study showed that molnupiravir caused low levels of mutations in hamster cells. Merck, which has conducted trials, claims a low incidence of adverse events, but mutational effects may not manifest until much later. The drug has proven effective in preventing hospitalization or death in placebo-controlled trials. Due to this success, the trials were halted early. Further, there were more dropouts due to adverse effects in the placebo group than in the experimental group. In a hamster model, it was effective against the current variants of concern(3). Interestingly, when administered in combination with the anti-influenza drug favirapir (which also targets the RDRP), the activity of molnupiravir is further potentiated(4). As shown with other antivirals (i.e., neutralizing monoclonal antibodies), molnupiravir is most effective when given early in infection, before harmful immune sequelae can occur.

PF-07321332 (Pfizer)

Although it is not yet as far advanced as molnupiravir, PF-07321332 (Pfizer), an inhibitor of the viral 3CL protease, is currently in Phase 1 trials. This drug needs to be administered intravenously. Its development was facilitated by the elucidation of the 3D structure of the SARS-CoV-1 protease, which is similar to that of SARS-CoV2, followed by rational drug design. During its viral life cycle, SARS-CoV-2 proteins are synthesized as a polyprotein. A cleavage of this protein by the viral protease is vital to generate the mature and functional viral proteins. Inhibiting the viral protease interrupts this process. PF-07321332 was effective against SARS-CoV-2 in Vero cell culture and in a mouse infection model(5). It appears to have synergistic activity with remdesivir (an RNA chain terminator targeting the RDRP) by intravenous administration. Development of an orally available version would make this therapeutic more viable.

The drug that has been receiving the most attention recently is molnupiravir, a ribonucleoside analog. This showed significant antiviral activity in Phase 2 trials against SARS-CoV2(1) and is beginning Phase 3 trials.

PF-07321332 was effective against SARS-CoV-2 in Vero cell culture and in a mouse infection model(5). It appears to have synergistic activity with remdesivir (an RNA chain terminator targeting the RDRP) by intravenous administration.
Monoclonal antibodies have been successfully used prophylactically for early treatment of SARS-CoV-2 infection. A recent phase 3 trial of casirivimab gave 70% protection against hospitalization or death, while prophylactic administration gave 81% protection against the development of Covid-19.

Monoclonal Antibodies

Monoclonal antibodies have been successfully used prophylactically for early treatment of SARS-CoV-2 infection. A recent phase 3 trial of casirivimab gave 70% protection against hospitalization or death, while prophylactic administration gave 81% protection against the development of Covid-19. A new and interesting development is the creation of bispecific monoclonal antibodies that bind to two disparate regions on the spike protein and cross-links the trimer subunits(6). This allows neutralization of virus at lower antibody concentrations and should prevent the emergence of resistant variants. Recent studies have focused on the identification of broadly reacting monoclonal antibodies to various coronaviruses. Specifically, one antibody showed protection from disease against the E484K variant in a hamster model. However, monoclonal antibodies must be administered intravenously, requiring a hospital setting. Antibody treatment would be much more useful if it could be used without this requirement.

Nanobodies

Nanobodies able to bind the SARS-CoV2 spike protein (both within the receptor binding domain and outside the RBD) were identified from a yeast library of synthetic nanobody sequences. Their tiny size allowed them to be trimerized with gly-ser linkers, yielding a multivalent molecule with greatly increased potency. They were then subjected to saturation mutagenesis and affinity maturation, yielding at least one nanobody with picomolar neutralization potency and sub-picomolar affinity. Similar nanobodies have been isolated from a llama immunized with stabilized SARS-CoV-2 spike protein(7). Potency was increased by creating a bivalent molecule with two linked nanobodies. Because of the small size and stability, nanobodies are easily produced and stable to aerosolization. This makes it feasible to package them in inhalers or nebulizers. Their direct introduction to airway tissue (potential sites of infection) might this serve as effective prophylactics. As such, they could be a game changer, although the proof of the pudding will be in the tasting.

Addendum – November 19, 2021: Pavloxid

We recently posted a Perspective on New Antivirals, focusing in part on molnipiravir, a replicase inhibitor that can be taken as a pill and has shown a 50% efficacy in preventing hospitalization. More recently Pfizer has announced results with a second orally available drug that appears to be even more effective than molnupiravir, with ~90% prevention of hospitalization. Molnupiravir targets viral RNA replication, whereas the Pfizer drug, Pavloxid, targets the viral protease.  SARS-CoV-2 proteins are synthesized as a polyprotein. A cleavage of this protein by the viral protease is vital to generate the mature and functional viral proteins. This process can be blocked by the inhibitor, thus preventing maturation of a functional viral particle. There has been a concern about possible mutagenic potential of molnupiravir because of its mode of action. This is not a concern with protease inhibitors. The data from clinical trials of avloxid have not yet been submitted for peer review. However, it appears to hold considerable promise for the treatment of early stage of Covid-19 patients. Some scientists already suggest the possibility of emergence of resistant mutants after taking these antiviral drugs. However, amid this ongoing pandemic, availability of these antiviral drugs would be greatly beneficial for those countries where the access of COVID-19 vaccines is limited.

Conclusion

We expect to have more innovative therapeutics available for the treatment of COVID-19. Along with available COVID-19 vaccines, we will have a better chance to save lives and to efficiently mitigate the ongoing pandemic.

References

1. W. Fischer et al., Molnupiravir, an Oral Antiviral Treatment for COVID-19. medRxiv, 2021.2006.2017.21258639 (2021).
2. F. Kabinger et al., Mechanism of molnupiravir-induced SARS-CoV-2 mutagenesis. bioRxiv, 2021.2005.2011.443555 (2021).
3. R. Abdelnabi et al., Molnupiravir Inhibits Replication of the Emerging SARS-CoV-2 Variants of Concern in a Hamster Infection Model. J Infect Dis 224, 749-753 (2021).
4. R. Abdelnabi et al., The combined treatment of Molnupiravir and Favipiravir results in a marked potentiation of antiviral efficacy in a SARS-CoV-2 hamster infection model. bioRxiv, 2020.2012.2010.419242 (2021).
5. B. Boras et al., Discovery of a Novel Inhibitor of Coronavirus 3CL Protease for the Potential Treatment of COVID-19. bioRxiv, 2020.2009.2012.293498 (2021).
6. H. Cho et al., Bispecific antibodies targeting distinct regions of the spike protein potently neutralize SARS-CoV-2 variants of concern. Sci Transl Med, eabj5413 (2021).
7. D. Wrapp et al., Structural Basis for Potent Neutralization of Betacoronaviruses by Single-Domain Camelid Antibodies. Cell 181, 1004-1015 e1015 (2020).