The experimental drug TEMPOL may be a promising oral antiviral treatment for COVID-19, according to a new study of cell cultures by a team of researchers that includes Penn State scientists. TEMPOL can limit SARS-CoV-2 infection by impairing the activity of a viral enzyme called “RNA replicase.” A paper describing the work, which was led by researchers at the National Institutes of Health’s Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD), appears June 3 in the journal Science.
The study team was led by Tracey A. Rouault, M.D., head of the NICHD Section on Human Iron Metabolism. It discovered TEMPOL’s effectiveness by evaluating a more basic question on how the virus uses its RNA replicase, an enzyme that allows SARS-CoV-2 to replicate its genome and make copies of itself once inside a cell.
The researchers found that RNA replicase (specifically a subunit of the enzyme called “nsp12”) requires iron-sulfur clusters for structural support. Their findings indicate that the SARS-CoV-2 RNA replicase requires two iron-sulfur clusters to function optimally. Earlier studies had mistaken these iron-sulfur cluster binding sites for zinc-binding sites, likely because iron-sulfur clusters degrade easily under standard experimental conditions.
Identification of the correct cofactor was enabled by the Penn State team, led by Professor of Chemistry and of Biochemistry and Molecular Biology Carsten Krebs and post-doctoral scholar Debangsu Sil, using a technique called Mössbauer spectroscopy.
“The virtue of this technique is that it allows for identification and quantification of all iron-containing species contained in a sample,” said Krebs.
Identifying this characteristic of the RNA replicase also enables researchers to exploit a weakness in the virus. TEMPOL can degrade iron-sulfur clusters, and previous research from the Rouault Lab has shown the drug may be effective in other diseases that involve iron-sulfur clusters. In cell culture experiments with live SARS-CoV-2 virus, the study team found that the drug can inhibit viral replication.
Based on previous animal studies of TEMPOL in other diseases, the study authors noted that the TEMPOL doses used in their antiviral experiments could likely be achieved in tissues that are primary targets for the virus, such as the salivary glands and the lungs.
The study team plans on conducting additional animal studies and will seek opportunities to evaluate TEMPOL in a clinical study of COVID-19.
"Why this enzyme requires iron-sulfur clusters, which are most often associated with electron transport, remains an intriguing question for the future," said Professor of Chemistry and of Biochemistry and Molecular Biology J. Martin Bollinger, Jr., a member of the research team at Penn State.
NIH authors on the study include researchers from the National Cancer Institute, the National Institute of Allergy and Infectious Diseases, and the National Institute of Neurological Disorders and Stroke. Authors from the Penn State are funded by NIH’s National Institute of General Medical Sciences.