Potent Anti-SARS-CoV‑2 Activity by the Natural Product Gallinamide A and Analogues via Inhibition of Cathepsin L

Cathepsin L is a
key host cysteine protease utilized by coronaviruses
for cell entry and is a promising drug target for novel antivirals
against SARS-CoV-2. The marine natural product gallinamide A and several
synthetic analogues were identified as potent inhibitors of cathepsin
L with IC50 values in the picomolar range. Lead molecules
possessed selectivity over other cathepsins and alternative host proteases
involved in viral entry. Gallinamide A directly interacted with cathepsin
L in cells and, together with two lead analogues, potently inhibited
SARS-CoV-2 infection in vitro, with EC50 values in the nanomolar range. Reduced antiviral activity was observed
in cells overexpressing transmembrane protease, serine 2 (TMPRSS2);
however, a synergistic improvement in antiviral activity was achieved
when combined with a TMPRSS2 inhibitor. These data highlight the potential
of cathepsin L as a COVID-19 drug target as well as the likely need
to inhibit multiple routes of viral entry to achieve efficacy.