Proteolytic enzymes are key regulators of human physiological processes. It is also essential for the replication of viruses, parasites, and bacteria causing infectious diseases. Cysteine proteases are drug targets because they participate in many disease-related processes. The development of cysteine protease inhibitors with drug-like properties has many challenges. The papain-like protease (PLpro) from the coronavirus causes severe acute respiratory syndrome (SARS), SARS-CoV. Strains isolated from zoonoses are hosts of SARS-CoV. This causes SARS-CoV or related coronavirus will be transmitted from animals to humans in the future. The development of new antiviral drugs for SARS-CoV is an important guarantee for preventing future outbreaks and pandemics. Two cysteine proteases, PLpro and 3C-like protease (3CLpro), catalyze their own release and other nsps from the polyprotein. Thereby they can initiate virus-mediated RNA replication. GRL0617 is a selective and competitive noncovalent SARS-CoV papain-like protease/deubiquitinase inhibitor.
GRL0617 inhibits papain-like protease/deubiquitinase with an IC50, and a Ki of 0.6 μM, 0.49 μM, respectively. Specifically, GRL0617 is responsible for processing viral proteins into their functional units and can also cleave ubiquitin and ISG15 conjugates. In addition, GRL0617 plays an important role in helping SARS-CoV escape from the human immune system. Besides, GRL0617 can inhibit SARS-CoV replication in Vero E6 cells with EC50 of 15μM, and had no cytotoxicity. GRL0617 has a unique inhibitory mode. So it can bind to the S4-S3 subsite of the enzyme and induce a closed loop. Thus the catalytic action on the active site is closed. GRL0617 is a potent noncovalent cysteine protease inhibitor. Moreover, GRL0617 is useful in the development of pathogenic ubiquitinase without inhibiting host DUB. All in all, GRL0617 is a selective and competitive noncovalent papain-like protease/deubiquitinase inhibitor.