After one year of COVID-19 Pandemic and Hundreds of Suggested Drugs, Will Cathepsin L Inhibitors be the Solution?

Cysteine cathepsins are defined as lysosomal enzymes which are member of the papain family. Cysteine cathepsins (Cts) prevalently exist in whole organisms varying from prokaryotes to mammals and possess in their active site greatly conserved residue of cysteine. Cts are engaged in the digestion of cellular protein, activation of zymogen, and remodeling of extracellular matrix (ECM). Host cells are entered by SARS-CoV-2 via endocytosis. Cathepsin L and phosphatidylinositol 3-phosphate 5-kinase are crucial in terms of the endocytosis by cleaving the spike protein, which permits viral membrane fusion with endosomal membrane, and succeeded by the releasing of viral genome to the host cell. Thereby, inhibition of cathepsin L may be advantageous in terms of decreasing infection caused by SARS-CoV-2. Coordinate inhibition of multiple Cts and lysosomal function by different drugs and biological agents might be of value for some purposes such as parasite or viral infections and anti-neoplastic applications. It has been found that Zn2+ deficiency or dysregulation leads to an exaggerated activity of Cysteine cathepsin increasing the autoimmune/inflammatory response. At this purpose Zn2+ metal can be safely combined with a drug that increases the anti-proteolytic effect of endogenous Zn2+ lowering the excessive activity of some CysCts. Biguanide derivatives complex with Zn2+ have been found to be promising inhibitors of CysCts protease reactions. Molecular docking studies of Cathepsin L Inhibited by Metformin-Zn+2 complex have been performed showing two strong key interactions ( Cys-25&His-163) and an extra H-bond with Asp-163 compared to the co-crystallized Zn+2 (PDB ID 4axl).There are studies using Quercetin intravenously( along with corn smut) in patients with Covid 19 with positive outcome" Biological Disruptive Therapies as Counter Measures for Unpredictable Biological Insults, a Case for Successful Treatment of COVID-19