Coronavirus Disease 2019 (COVID-19) is a deadly emerging infectious disease caused by Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2). Because SARS-CoV-2 is easily transmitted through the air and has a relatively long incubation time, COVID-19 has rapidly developed into a global pandemic. As there are no antiviral agents for the prevention and treatment of this severe pathogen except for remdesivir, development of antiviral therapies to treat infected individuals remains highly urgent. Here, we showed that baicalein and baicalin exhibited significant antiviral activity against SARS-CoV-2, the causative agent of COVID-19 through in vitro studies. Our data through cell-based and biochemical studies showed that both compounds act as SARS-CoV-2 RNA-dependent RNA polymerase (RdRp) inhibitors directly and inhibit the activity of the SARS-CoV-2 RdRp, but baicalein was more potent. We also showed specific binding of baicalein to the SARS-CoV-2 RdRp, making it a potential candidate for further studies towards therapeutic development for COVID-19 as a selective non-nucleoside polymerase inhibitor.

The SARS-CoV-2 public health challenges have highlighted the urgent need for coronavirus-targeting life-saving therapeutics. Given the emergence of drug-resistant strains, the development of antivirals against viral proteins beyond the commonly targeted main protease or RNA-dependent RNA polymerase is critical. The SARS-CoV-2 nonstructural protein 13 (nsp13) is a highly conserved RNA helicase and an essential component of the viral replication–transcription complex (RTC). It unwinds double-stranded RNA to facilitate viral transcription and replication, making it a strong target for drug development. To identify nsp13 inhibitors, we used an ultra-high-throughput nucleic acid unwinding assay to screen a library of FDA-approved drugs and bioactive compounds. We identified forty inhibitors with IC₅₀ values ranging from 1.4 to 10 μM. Ten were further selected for biochemical and biophysical characterization. Four of these are bound to nsp13 without interacting with the nucleic acid substrate and without inhibiting the ATPase activity of nsp13. Hydrogen–deuterium exchange coupled with Mass Spectrometry (HDX-MS) studies show compound binding causes differential exchange in two regions of nsp13. Furthermore, these compounds have antiviral activity against infectious SARS-CoV-2 in multiple cell lines, with cytotoxicity affecting, in some cases, the apparent antiviral effect. Future optimization efforts could help develop therapeutics against SARS-CoV-2 and other potential coronavirus threats.