The 2019 coronavirus disease pandemic (COVID-19) has mobilized efforts worldwide, and several ongoing clinical trials aimed at developing a drug-based treatment for its control. Cathepsin L is an endosomal cysteine protease that mediates the cleavage of the S1 subunit of the coronavirus surface spike glycoprotein. This cleavage is necessary for coronavirus entry into human host cells and viruses/host cell endosome membrane fusion. Therefore, cathepsin L is a potential target for the treatment of COVID-19 patients. In this report, we describe a previously unknown inhibitory effect of two FDA-approved drugs, saquinavir and nelfinavir, on human cathepsin L activity. Whether the pivotal role for cathepsin L in Sars-Cov-2 infection described in vitro can be translated to humans, our results support immediate clinical trials of saquinavir or nelfinavir as a potential treatment for COVID-19 patients.
