Furin Inhibitors Block SARS-CoV-2 Spike Protein Cleavage to Suppress Virus Production and Cytopathic Effects

Development of specific antivirals is an urgent unmet need for SARS-coronavirus 2 (SARS-CoV-2) infections. This study focuses on host proteases that proteolytically activate the SARS-CoV-2 spike protein, critical for its fusion activity after binding to ACE2 receptor, as antiviral targets. We first validated cleavage at a putative furin substrate motif at SARS-CoV-2 spike by expressing it in VeroE6 cells and found prominent syncytium formation. Both cleavage and syncytium were abolished by treatment with the furin inhibitors decanoyl-RVKR-chloromethylketone (CMK) and naphthofluorescein but not by the TMPRSS2 inhibitor camostat. CMK and naphthofluorescein showed antiviral effects in SARS-CoV-2-infected cells by decreasing viral production and cytopathic effects. Further analysis revealed that, similar to camostat, CMK blocks virus entry, but it further suppresses the cleavage of spike and syncytium. Naphthofluorescein instead acts primarily by suppressing viral RNA transcription after viral entry. Therefore, furin inhibitors may become promising antivirals for prevention and treatment of SARS-CoV-2 infections. Funding: This study was supported by grants from the Ministry of Science and Technology, Taiwan (106-2622-B-002-002-CC2, 107-3017-F-002-002) and the “Center of Precision Medicine” from The Featured Areas Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education (MOE) in Taiwan. Conflict of Interest: The authors declare no competing interests.