Objective: The 3C-like protease (3CLpro) of SARS-coronavirus mediates the proteolytic processing of replicase polypeptides 1a and 1ab into functional proteins, becoming an important target for the drug development. In this study, Isatis indigotica root extract, five major compounds of Isatis indigotica root, and seven plant-derived phenolic compounds were tested for anti-SARS CoV 3CLpro effects using cell-free and cell-based cleavage assays. Methods: Cell-free cleavage assay was carried out using trans-cleavage of substrate fusion protein by SARS-CoV 3CLpro, whereas cell-based cleavage assay was according to the cis-cleavage of the 3CLpro–substrate-luciferase fusion protein in Vero cells. Results: Cleavage assays with the 3CLpro demonstrated that IC50 values were in micromolar ranges for Isatis indigotica root extract, indigo, sinigrin, aloe emodin and hesperetin. Sinigrin (IC50 of 217 mM) was more efficient on blocking the cleavage processing of the 3CLpro than indigo (IC50 of 752 mM) and beta-sitosterol (IC50 of 1210 mM) in the cell-based assay. Only two phenolic compounds aloe emodin and hesperetin dose-dependently inhibited cleavage activity of the 3CLpro, in which the IC50 value was 366 mM for aloe emodin and 8.3 mM for hesperetin in the cell-based assay. In addition, MTT cell proliferation assay indicated that these compounds had no-effect on cell viability. Conclusion: Hesperetin with IC50 of 8.3 μM on the 3CLpro could be a potent inhibitor on SARS CoV. This study will be useful for development of anti-SARS drugs.
