<i>In vitro</i> and <i>in silico</i> studies of the SARS-CoV-2 main protease inhibition and antioxidant activities of isolated compounds from <i>Marrubium alysson</i> L.

A phytochemical investigation of the aerial parts of <i>Marrubium alysson</i> L. yielded 10 compounds: β-sitosterol (1), glycerol monopalmitate (2), β-sitosterol glucoside (3), chrysoeriol (4), apigenin (5), (apigenin-7-<i>O</i>-(3'',6''-<i>E-p</i>-dicoumaroyl)-β-d-glucoside) (6), apigenin-7-<i>O</i>-(3''-<i>E-p</i>-coumaroyl)-β-d-glucopyranoside (7), apigenin-7-<i>O</i>-(6''-<i>E-p</i>-coumaroyl)-β-d-glucoside (8), apigenin-7-<i>O</i>-β-glucopyranoside (9) and verbascoside (10). Their structures were established using 1D and 2D NMR spectroscopic techniques. Compounds 1, 2, 6, 7 and 8 are reported from <i>M. alysson</i> L. for the first time in this study. The <i>in vitro</i> inhibitory activities against the SARS-CoV-2 main protease (M^pro) were evaluated using fluorogenic substrate assay. Compound 6 showed the highest inhibitory activity against the SARS-CoV-2 main protease (M^pro) with IC₅₀ = 8.349 ± 0.35 μM, comparing favourably with the reference tipranavir (IC₅₀ = 3.231 ± 0.14 μM). The antioxidant activities were determined using <i>in vitro</i> ABTS radical scavenging assay; it is noteworthy that compounds 6, 7 and 8 have potent antioxidant activity compared with l-ascorbic acid, while compound 10 has radical scavenging activity with IC₅₀ = 25.58 ± 0.12 μM, more potent than the reference l-ascorbic acid (IC₅₀ = 30.43 ± 0.14 μM). Molecular modelling studies of compound 6 showed that it is perfectly engaged in a wide range of hydrogen bonding with multiple residues, including Met49, Glu47, Thr24, Thr26, Gly143, and Gln 189 in the active site of CoV-2-3CL protease. This is reflected by its promising binding affinity, which explains the observed biochemical activity of compound 6 for inhibiting SARS-CoV-2 main protease (M^pro). Assessments of the pharmacokinetics, drug likeness and medicinal chemistry friendliness of the isolated compounds were also conducted.