Discovery of juglone and its derivatives as potent SARS-CoV-2 main proteinase inhibitors

SARS-CoV-2 as a positive-sense single-stranded RNA coronavirus caused the global outbreak of COVID-19. The main protease (M^pro) of the virus as the major Enzyme processing viral polyproteins contributed to the replication and transcription of SARS-CoV-2 in host cells, and has been characterized as an attractive target in drug discovery. Herein, a set of 1,4-naphthoquinones with juglone skeleton were prepared and evaluated for the inhibitory efficacy against SARS-CoV-2 M^pro. More than half of the tested naphthoquinones could effectively inhibit the target Enzyme with an inhibition rate of more than 90% at the concentration of 10 μM. In the structure-activity relationships (SARs) analysis, the characteristics of substituents and their position on juglone core scaffold were recognized as key ingredients for Enzyme inhibitory activity. The most active compound, 2-acetyl-8-methoxy-1,4-naphthoquinone (15), which exhibited much higher potency in Enzyme inhibitions than shikonin as the positive control, displayed an IC₅₀ value of 72.07 ± 4.84 nM towards M^pro-mediated hydrolysis of the fluorescently labeled peptide. It fit well into the active site cavity of the Enzyme by forming hydrogen bonds with adjacent amino acid residues in molecular docking studies. The results from in vitro Antiviral activity evaluation demonstrated that the most potent M^pro inhibitor could significantly suppress the replication of SARS-CoV-2 in Vero E6 cells within the low micromolar concentrations, with its EC₅₀ value of about 4.55 μM. It was non-toxic towards the host Vero E6 cells under tested concentrations. The present research work implied that juglone skeleton could be a primary template for the development of potent M^pro inhibitors.