As SARS-CoV-2 spread across the globe early last year, the WHO initiated the “Solidarity Trial” to efficiently see if existing drugs could be repurposed to treat COVID-19 (asamonitor.pub/3kj5hmV; asamonitor.pub/3zaBsJb). Fifty-two nations are participating in the WHO Solidarity Trial. Unfortunately, the first four drugs evaluated – remdesivir, hydroxychloroquine, lopinavir, and interferon – all had the same outcome: little or no effect on hospitalized patients with COVID-19, as indicated by overall mortality, initiation of ventilation, and duration of hospital stay (N Engl J Med 2021;384:497-511). Although the U.S. is not a participant in the Solidarity Trial, several therapies have received emergency use authorizations (EUAs) from the U.S. Food and Drug Administration. Even though effective vaccines are now available in the U.S., effective antiviral treatment remains an essential component of managing the COVID-19 pandemic (asamonitor.pub/2Z45MsR).
Table 1 shows the antiviral therapies for COVID-19 that have been authorized for clinical use under the EUA regulations. It hasn't been smooth sailing for these drugs. As noted above, the WHO Solidarity Trial found that the remdesivir did not improve outcome. Bamlanivimab was found to be ineffective as a monotherapy and needed to be given in combination with a second monoclonal antibody. Given the role of autoantibodies in the pathogenesis of severe COVID-19 disease, it is not surprising that two of the drugs – baricitinib and tocilizumab – are powerful immune modulators.
“Massive investment in research and development by government, industry, and academia has identified potential therapies with great promise for treating moderate or severe COVID-19. SARS-CoV-2 will likely join the four other endemic coronaviruses and be with us forever. However, the dozen approved vaccines combined with the addition of at least a dozen highly effective therapies will likely bring the great pandemic of 2020 to an end in 2022.”
Table 2 lists several of the drugs used off-label to treat COVID-19. Dexamethasone was identified early in the course of the pandemic as an effective immune modulator for patients with moderate to severe disease and has been an off-label mainstay of therapy since. Hydroxychloroquine was found to be ineffective in the Solidarity Trial and seems to be of less interest now.
A great deal of misinformation has been put forward about ivermectin. A preprint published by Sapin Desai, founder of Surgisphere, reported a large reduction in COVID-19 mortality with ivermectin. The preprint was withdrawn along with the rest of Desai's COVID-19 studies for unreliable data (Nature 2020;582:160). Another preprint demonstrating ivermectin efficacy was withdrawn after data manipulation was identified by alert readers (Nature 2021;596:173-4; asamonitor.pub/3CgHmuc). Remarkably, this withdrawn preprint continues to be used in meta-analyses purporting to demonstrate benefit. As noted by Popp and colleagues, a common thread of meta-analyses purported to show benefit of ivermectin is the selective use of highly positive, low-quality studies (BMJ Evid Based Med August 2021). A recent narrative review advocating ivermectin went further, and evidently fabricated the epidemiologic data purporting to show benefit (Am J Ther 2021;28:e596-8). As usual, the highest-quality review is from the Cochrane Collaboration, and they report no compelling demonstration of benefit among well conducted trials (asamonitor.pub/3nzm9Yi).
Therapies under development
As reported in Nature News, “nanobodies” are small antibodies, derived from camelids (e.g., llamas and camels), that can attach to epitopes in protein folds too narrow to be accessed by human antibodies (Nature 2021;595:176-8). In another paper in Nature, Xu et al. created camelid-based nanobodies (in mice – go figure) targeting the highly conserved portion of the receptor binding domain (RBD) of the SARS-CoV-2 spike protein (Nature 2021;595:278-82). The region is inaccessible to human antibodies because it is somewhat buried in the surface. Also, the region is outside of the ACE2-binding portion of the spike protein, so mutations that increase binding affinity (e.g., most of them) do not confer resistance to binding (Nat Microbiol 2021;6:1188-98). Despite being outside of the ACE2-binding region, binding of these nanobodies to the spike protein blocked ACE2-RBD interaction and prevented infection.
A similar study by Koenig et al. in Science isolated four nanobodies from camelids (one alpaca and one llama) inoculated with formalin-inactivated SARS-CoV-2 and the receptor binding domain of the SARS-CoV-2 spike protein (Science 2021;371:eabe6230). The nanobodies neutralized the ability of the spike protein to initiate fusion with target cells. The researchers created three multivalent nanobodies, each incorporating two nanobody binding sites. Binding multiple sites makes it exceptionally difficult for SARS-CoV-2 escape mutations to evolve. Additionally, because nanobodies are small, soluble molecules, the authors note that nanobodies are inexpensive to manufacture and amenable to intranasal delivery. They speculate that an intranasal dose could accompany a COVID-19 test in outpatients with symptoms. Additionally, since nanobodies lack the Fc fragment present in antibodies, engagement by phagocytes bearing Fc-receptors is prevented, thus avoiding a potential mechanism for antibody-dependent enhancement (ADE).
More recently, Güttler and colleagues published a library of 45 thermally stable and ultra-potent nanobodies (up to 95°C) (EMBO J August 2021). The isolated nanobodies displayed neutralized SARS-CoV-2 at concentrations ranging from 17 to 50 picomolar and were unaffected by any of the known variants.
Preclinical studies performed by Ralph Baric's research group evaluated an engineered human monoclonal antibody that targeted a highly conserved epitope in the ACE2 binding site (Science 2021;371:823-9). Interestingly, their antibody construct bound tightly against all clade 1 SARS betacoronavirus spike protein RBDs, suggesting likely efficacy in both this and future coronavirus pandemics. The authors found that binding was only affected by four specific mutations (D405, G502, G504, and Y505), mutations that are not observed in nature. Murine studies showed that the antibody provided broad protection in both SARS and COVID-19 models.
Research performed by Zhengli Shi's group at the Wuhan Institute of Virology identified two potent human neutralizing anti-SARS-CoV-2 antibodies (Nat Commun 2021;12:4887). These antibodies, termed nCoVmab1 and nCoVmab2, target the RBD of SARS-CoV-2. As with the nanobodies described earlier, these antibodies are ultra-potent with neutralizing concentrations in the picomolar and nanomolar levels, respectively. The more potent antibody, nCoVmab1, reduced viral loads with both prophylactic and therapeutic administration in genetically modified mice expressing “humanized” ACE2.
Preclinical studies performed at the Utrecht University evaluated two human monoclonal antibodies isolated from “humanized” genetically-modified mice (Nat Commun 2021;12:1715). The studied antibodies displayed significant cross-reactivity against the spike proteins of several betacoronaviruses, including SARS-CoV-1, SARS-CoV-2, MERS-CoV, and HC0V-OC43 (an endemic human coronavirus). Unlike antibodies that target the receptor binding domain, these antibodies targeted the stem helix portion of the S2 fusion subunit of the spike protein, preventing bound spike protein from fusing with the cell membrane. The antibodies also blocked infection with MERS-CoV, suggesting that the binding site is highly conserved.
When SARS-CoV-2 infects cells, the positive-sense RNA is transcribed into two polyproteins and one active protein: a protease. The job of the protease is to split the polyproteins into 11 small proteins vital to viral replication. This protease, called Mpro or 3CL protease, is exceptionally highly conserved among coronaviruses. Indeed, among the studied coronaviruses, 12 of the 13 amino acids that comprise the catalytic site are identical between SARS, MERS, and SARS-CoV-2.
A recent review in Current Opinion in Virology looked at the potential for developing Mpro inhibitors (Curr Opin Virol 2021;49:36-40). Interestingly, Pfizer started work on an Mpro inhibitor, PF-00835231, 15 years earlier as a candidate treatment for SARS. The program was stopped when the SARS pandemic waned from aggressive containment measures. Pfizer recently identified PF-07304814, a phosphate prodrug of PF-00835231, and recently completed a phase 1b clinical trial evaluating its clinical utility in a phase Ib study in patients hospitalized with COVID-19 (NCT04535167).
Pfizer is also developing a second-generation Mpro inhibitor, PF-07321332, that will be administered orally (asamonitor.pub/3lwADFV). The oral Mpro inhibitor shows potent in vitro pan-coronavirus protease inhibition against alphacoronaviruses (229E and NL63) and betacoronaviruses (SARS-CoV-2, SARS-CoV-1, MERS-CoV, HKU1, and OC43). As of this writing there are 9 studies from phase 1 through phase 3 of PF-07321332 reported on clinicaltrials.gov. Efficacy and safety results from these trials are expected by the end of this year (asamonitor.pub/3AgU63l).
Probenecid (used to treat gout) may have activity against SARS-CoV-2 as strongly suggested by a recent report in Nature Scientific Reports (Sci Rep 2021;11:18085). The data strongly suggest that one dose/day would easily achieve the necessary concentration to inhibit SARS-CoV-2. Quoting the authors, “The major advantages of probenecid are that it is an FDA-approved therapeutic drug that has been on the market for >50 years, it can be administered orally with favorable pharmacokinetics, it operates at the host cell level, is refractory to viral mutation, and has the potential to treat multiple other viruses.” Currently there are no registered trials at clinicaltrials.gov, but this will likely change since publication of the report.
In conclusion, science rocks. Massive investment in research and development by government, industry, and academia has identified potential therapies with great promise for treating moderate or severe COVID-19. SARS-CoV-2 will likely join the four other endemic coronaviruses and be with us forever. However, the dozen approved vaccines combined with the addition of at least a dozen highly effective therapies will likely bring the great pandemic of 2020 to an end in 2022.
Richard Simoneaux is a freelance writer with an MS in organic chemistry from Indiana University. He has more than 15 years of experience covering the pharmaceutical industry and an additional seven years as a laboratory-based medicinal chemist.