Drug Repurposing for the SARS-CoV-2 Papain-Like Protease

As the pathogen of COVID-19, SARS-CoV-2 encodes two essential cysteine proteases that process the pathogens two large polypeptide translates ORF1a and ORF1ab in human host cells to form 15 functionally important, mature nonstructural proteins. One of the two enzymes, papain-like protease or PLpro, also possesses deubiquitination and deISGylation activities that suppresses host innate immune responses toward SARS-CoV-2 infection. Therefore, PLpro is a potential COVID-19 drug target. To repurpose drugs for PLpro, we experimentally screened 33 deubiquitinase and 37 cysteine protease inhibitors on their inhibition of PLpro. Our results showed that 15 deubiquitinase and 1 cysteine protease inhibitors exhibit potent inhibition of PLpro at 200 M. More comprehensive characterizations revealed 7 inhibitors GRL0617, SJB2-043, TCID, DUB-IN-1, DUB-IN-3, PR-619, and S130 with an IC50 value below 60 M and four inhibitors GRL0617, SJB2-043, TCID, and PR-619 with an IC50 value below 10 M. Among four inhibitors with an IC50 value below 10 M, SJB2-043 is the most unique in that it doesnt fully inhibit PLpro but has an outstanding IC50 value of 0.56 M. SJB2-043 likely binds to an allosteric site of PLpro to convene its inhibition effect, which needs to be further investigated. As a pilot study, the current work indicates that COVID-19 drug repurposing by targeting PLpro holds promises but in-depth analysis of repurposed drugs is necessary to avoid omitting allosteric inhibitors.