Biflavonoids from Torreya nucifera displaying SARS-CoV 3CL ^pro inhibition

Graphical abstract

Inhibitory activity appeared to be associated with the presence of an apigenin moiety at position C-3′ of flavones, as biflavonoid had an effect on SARS-CoV 3CL ^pro inhibitory activity.

Abstract

As part of our search for botanical sources of SARS-CoV 3CL ^pro inhibitors, we selected Torreya nucifera, which is traditionally used as a medicinal plant in Asia. The ethanol extract of T. nucifera leaves exhibited good SARS-CoV 3CL ^pro inhibitory activity (62% at 100 μg/mL). Following bioactivity-guided fractionation, eight diterpenoids ( 1– 8) and four biflavonoids ( 9– 12) were isolated and evaluated for SARS-CoV 3CL ^pro inhibition using fluorescence resonance energy transfer analysis. Of these compounds, the biflavone amentoflavone ( 9) (IC ₅₀ = 8.3 μM) showed most potent 3CL ^pro inhibitory effect. Three additional authentic flavones (apigenin, luteolin and quercetin) were tested to establish the basic structure–activity relationship of biflavones. Apigenin, luteolin, and quercetin inhibited 3CL ^pro activity with IC ₅₀ values of 280.8, 20.2, and 23.8 μM, respectively. Values of binding energy obtained in a molecular docking study supported the results of enzymatic assays. More potent activity appeared to be associated with the presence of an apigenin moiety at position C-3′ of flavones, as biflavone had an effect on 3CL ^pro inhibitory activity.