An international team of researchers has found that three commonly used antiviral and antimalarial drugs are effective in vitro at preventing replication of SARS-CoV-2, the virus that causes COVID-19. The work also underscores the necessity of testing compounds against multiple cell lines to rule out false negative results.
The team, which included researchers from North Carolina State University and Collaborations Pharmaceuticals, looked at three antiviral drugs that have proven effective against Ebola and the Marburg virus: tilorone, quinacrine and pyronaridine.
"We were looking for compounds that could block the entry of the virus into the cell," says Ana Puhl, senior scientist at Collaborations Pharmaceuticals and co-corresponding author of the research. "We chose these compounds because we know that other antivirals which successfully act against Ebola are also effective inhibitors of SARS-CoV-2."
The compounds were tested in vitro against SARS-CoV-2, as well as against a common cold virus (HCoV 229E) and murine hepatitis virus (MHV). Researchers utilized a variety of cell lines that represented potential targets for SARS-CoV-2 infection in the human body. They infected the cell lines with the different viruses and then looked at how well the compounds prevented viral replication in the cells.
The results were mixed, with the compounds' effectiveness depending upon whether they were used in human-derived cell lines versus monkey-derived cell lines, known as Vero cell lines.
"In the human-derived cell lines, we found that all three compounds worked similarly to remdesivir, which is currently being used to treat COVID-19," says Frank Scholle, associate professor of biology at NC State and co-author of the research. "However, they were not at all effective in the Vero cells."
"Researchers saw similar results when these compounds were initially tested against Ebola," says Sean Ekins, CEO of Collaborations Pharmaceuticals and co-corresponding author of the research. "They were effective in human-derived cell lines, but not in Vero cells. This is important because Vero cells are one of the standard models used in this type of testing. In other words, different cells lines may have differing responses to a compound. It points to the necessity of testing compounds in many different cell lines to rule out false negatives."
Next steps for the research include testing the compounds' effectiveness in a mouse model and further work on understanding how they inhibit viral replication.
"One of the more interesting findings here is that these compounds don't just prevent the virus from potentially binding to the cells, but that they may also inhibit viral activity because these compounds are acting on the lysosomes," Puhl says. "Lysosomes, which are important for normal cell function, are hijacked by the virus for entry and exit out of the cell. So, if that mechanism is disrupted, it cannot infect other cells."
"It's also interesting that these compounds are effective not just against SARS-CoV-2, but against related coronaviruses," Scholle says. "It could give us a head start on therapies as new coronaviruses emerge."
The work appears in ACS Omega and was supported in part by NC State's Comparative Medicine Institute and the National Institutes of Health. NC State undergraduates James Levi and Nicole Johnson, as well as Ralph Baric, from the University of North Carolina at Chapel Hill, contributed to the work. Other collaborating institutions included: Instituto Oswaldo Cruz and University of Campinas, both in Brazil; Utah State University; the University of Maryland; and SRI International.
Note to editors: An abstract follows.
"Repurposing the Ebola and Marburg Virus Inhibitors Tilorone, Quinacrine, and Pyronaridine: In Vitro Activity against SARS-CoV?2 and Potential Mechanisms"
Authors: Ana Puhl, Sean Ekins, Collaborations Pharmaceuticals; Frank Scholle, James Levi, Nicole Johnson, NC State University; et al
Published: March 12, 2021 in ACS Omega
Severe acute respiratory coronavirus 2 (SARS-CoV-2) is a newly identified virus that has resulted in over 2.5 million deaths globally and over 116 million cases globally in March, 2021. Small-molecule inhibitors that reverse disease severity have proven difficult to discover. One of the key approaches that has been widely applied in an effort to speed up the translation of drugs is drug repurposing. A few drugs have shown in vitro activity against Ebola viruses and demonstrated activity against SARS-CoV-2 in vivo. Most notably, the RNA polymerase targeting remdesivir demonstrated activity in vitro and efficacy in the early stage of the disease in humans. Testing other small-molecule drugs that are active against Ebola viruses (EBOVs) would appear a reasonable strategy to evaluate their potential for SARS-CoV-2. We have previously repurposed pyronaridine, tilorone, and quinacrine (from malaria, influenza, and antiprotozoal uses, respectively) as inhibitors of Ebola and Marburg viruses in vitro in HeLa cells and mouse-adapted EBOV in mice in vivo. We have now tested these three drugs in various cell lines (VeroE6, Vero76, Caco-2, Calu-3, A549-ACE2, HUH-7, and monocytes) infected with SARS-CoV-2 as well as other viruses (including MHV and HCoV 229E). The compilation of these results indicated considerable variability in antiviral activity observed across cell lines. We found that tilorone and pyronaridine inhibited the virus replication in A549-ACE2 cells with IC50 values of 180 nM and IC50 198 nM, respectively. We used microscale thermophoresis to test the binding of these molecules to the spike protein, and tilorone and pyronaridine bind to the spike receptor binding domain protein with Kd values of 339 and 647 nM, respectively. Human Cmax for pyronaridine and quinacrine is greater than the IC50 observed in A549-ACE2 cells. We also provide novel insights into the mechanism of these compounds which is likely lysosomotropic.