PROS1 and infection: Furthermore, we used time-of-addition assays and found that both compounds lost most of their antiviral activity 5 h after infection by pseudotyped SARS-CoV-2 (Fig. 3d–g), strongly suggesting that the tested compounds may target the early stage of virus entry, possibly during the process of S protein activation by CTSL in the endocytic pathway, further highlighting cellular CTSL as their potential target.