  Presently, there are no approved drugs or vaccines to treat COVID-19 which
has spread to over 200 countries and is responsible for over 3,65,000 deaths
worldwide. Recent studies have shown that two human proteases, TMPRSS2 and
cathepsin L, play a key role in host cell entry of SARS-CoV-2. Importantly,
inhibitors of these proteases were shown to block SARS-CoV-2 infection. Here,
we perform virtual screening of 14010 phytochemicals produced by Indian
medicinal plants to identify natural product inhibitors of TMPRSS2 and
cathepsin L. We built a homology model of TMPRSS2 as an experimentally
determined structure is not available. AutoDock Vina was used to perform
molecular docking of phytochemicals against TMPRSS2 model structure and
cathepsin L crystal structure. Potential phytochemical inhibitors were filtered
by comparing their docked binding energies with those of known inhibitors of
TMPRSS2 and cathepsin L. Further, the ligand binding site residues and
non-covalent protein-ligand interactions were used as an additional filter to
identify phytochemical inhibitors that either bind to or form interactions with
residues important for the specificity of the target proteases. We have
identified 96 inhibitors of TMPRSS2 and 9 inhibitors of cathepsin L among
phytochemicals of Indian medicinal plants. The top inhibitors of TMPRSS2 are
Edgeworoside C, Adlumidine and Qingdainone, and of cathepsin L is Ararobinol.
Interestingly, several herbal sources of identified phytochemical inhibitors
have antiviral or anti-inflammatory use in traditional medicine. Further in
vitro and in vivo testing is needed before clinical trials of the promising
phytochemical inhibitors identified here.
