Sci Rep. priming mediating viral entry. As the entry of SARS\CoV\2 into host cells is mandatory NF 279 for viral infection, it becomes an extremely attractive therapeutic intervention point. In this regard, this review will focus on the therapeutic targeting of the crucial steps of SARS\CoV\2 viral entry like S\protein/ACE2 interaction and S\protein priming by host cell proteases. In addition, this review will also give insights to the readers on several therapeutic opportunities, pharmacological targeting of the viral\entry facilitators like S\Protein, ACE2, cell surface HS, TMPRSS2, and CatB/L and evidence for those drugs currently ongoing clinical studies. (where the S\protein activation is mediated by the pH\dependent endosomal protease CatL) and (where the S\protein activation is mediated by TMPRSS2 for subsequent entry via host plasma membrane). Several evidence have been reported on the roles of these protease activators, indicating that both TMPRSS2 and endosomal cysteine protease CatB/L are crucial for SARS\CoV\2 entry. 4 , 11 , 24 , 25 , 26 Even though the high expression of both TMPRSS2 and cathepsins have been confirmed in the lung tissues, 27 the commonly used cell lines for performing viral assays may exhibit varying expression levels of both TMPRSS2 and cathepsins, which can potentially result in a dramatic impact on the viral\entry mechanism of SARS\CoV\2. 28 Since animal models of SARS\CoV\2 are still under optimization, the controversy on the expression levels of TMPRSS2 and cathepsins in cell lines and their ability to accurately mimic aspects NF 279 of the human infection still warrants investigation. Altogether, strategic selection of cell lines for the purpose of antiviral testing plays a crucial aspect in the excluding selection and screening of drugs, which could be efficiently tackled by SARS\CoV\2 by its redundant viral\entry pathways. 28 A detailed pictorial representation of the SARS\CoV\2 life cycle is given in Figure?1. Open in a separate window Figure 1 Life cycle of SARS\CoV\2. ACE2, angiotensin\converting enzyme 2; CatL, cathepsin L; E\protein, envelope protein; HSPG, heparan sulfate proteoglycans; M\protein, membrane glycoprotein; N\protein, nucleocapsid NF 279 protein; ORF1a, open\reading frame 1a; ORF1ab, open\reading frame 1ab; RER, rough endoplasmic reticulum; SARS\CoV\2, severe acute respiratory syndrome coronavirus\2; S\protein, spike protein; TMPRSS2, transmembrane serine protease COL5A1 2 3.?TARGETING THE VIRAL\ENTRY FACILITATORS 3.1. S\protein S\protein is a crucial structural protein of CoV, which is assembled into a unique corolla structure and exists as a trimer on the surface of the virus. NF 279 As mentioned earlier, S\protein plays a pivotal role in interactive binding to host cell receptors for facilitating the viral invasion and also acts as a determinant factor for host tropism. Since the structural integrity and cleavage activation of S\protein are key factors for both virulence and viral invasion, therapeutic strategies targeting S\protein can result in the development of effective antivirals and vaccines. 29 , 30 Among the two subunits of S\protein (S1 and S2), S1\subunit has diverged in sequence whereas S2\subunit acts as the most conserved region of the protein. C\terminal domain (CTD) and N\terminal domain (NTD) are the two subdomains of S\protein where both the subdomains can function as RBD. 31 In this approach of targeting S\protein, a pivotal target for neutralizing antibodies is the RBD. Apart from the high homology of the S\protein of the SARS\CoV\2 to that of SARS\CoV, remarkable alterations ( 85%) have been identified in the RBD antibody epitopes of SARS\CoV\2 when compared to SARS\CoV. This variation in the RBD antibody epitopes of SARS\CoV\2 necessitates the development of new monoclonal antibodies against SARS\CoV\2. 32 , 33 3.1.1. Pharmacological treatment In the line of antibodies against S\protein, were also reported for targeting S\protein. Further, has been reported to show neutralizing activity on both pseudotyped SARS\CoV and SARS\CoV\2 with an inhibitory concentration (IC50) of 0.8 and 0.1?g/ml, respectively. Also, recent.