Diphyllin elicits a doubled-pronged attack on the entry of SARS-CoV-2 by inhibiting cathepsin L and furin

The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) caused the coronavirus disease 2019 (COVID-19) pandemic, posing serious threats to global health. Effective broad-spectrum antiviral drugs for the treatment of COVID-19 are not sufficiently available. In the present study, we investigated the antiviral activity of the natural lignan diphyllin (PubChem CID 100492) against different SARS-CoV-2 variants and explored the underlying molecular mechanisms. We found that diphyllin dose-dependently inhibits the SARS-CoV-2 spike (S)-mediated entry into different types of cells. The potent inhibition was evident against spike proteins derived from the original SARS-CoV-2 and from variants of concern such as Alpha, Beta, Delta or Omicron. Accordingly, diphyllin also significantly inhibited the in vitro infection of a clinical SARS-CoV-2 virus isolate. Mechanistically, diphyllin simultaneously inhibited the endosomal entry of SARS-CoV-2 by neutralizing the endosomal acidification and reducing the activity of the cysteine protease cathepsin L (CTSL) as well as S-meditated cell surface entry by impairing furin activity. Collectively, our findings establish diphyllin as novel inhibitor of CTSL and furin proteases, resulting in a double-pronged attack on SARS-CoV-2 entry along endosomal as well as cell surface routes. Therefore, diphyllin has the potential to be advanced as an inhibitor of SARS-CoV-2 entry.