PROS1 and COVID-19: Considering that metalloproteinase inhibitors could thus possibly be used as prophylactic or therapeutic agents for COVID-19, we further demonstrated that marimastat (26) and prinomastat (27) (whose safety was previously confirmed in clinical trials, such as the clinical trial of CTS-1027 [https://clinicaltrials.gov/ct2/show/results/NCT01273064]) can preferentially block the TMPRSS2-independent fusion induced by the SARS-CoV-2 S protein (Fig. 2b; Fig. S1f).