Human cathepsin L is a cysteine endopeptidase and plays a key role for infection efficiency by activation of the S protein into a fusogenic state to escape the late endosomes. Targeting this protease with small molecules could interfere with virus‐cell entry and therefore be a possible intervention strategy for CoV infection. 114  Bates et al. identified MDL28170 (20; Figure 9) as an antiviral compound that specifically inhibited cathepsin‐L‐mediated substrate cleavage, with an IC50 value of 2.5 nM and EC50 value in the range of 100 nM. However, despite its potent inhibitory activity, no cytotoxicity data for 20 is currently available. 115