Alkaloids and Styryl lactones from Goniothalamus ridleyi King and Their α-Glucosidase Inhibitory Activity

Gonioridleylactam (1), a new compound, is a unique dimeric aristolactam isolated from the EtOAc extract of the twigs of Goniothalamus ridleyi King. The structure of gonioridleylactam (1) consists of two different aristolactams linked together with two methylenedioxy bridges at C-3/C-3' and C-4/C-4', generating a ten-membered ring of [1,3,6,8]tetraoxecine. A new natural product, gonioridleyindole (3-hydroxymethyl-1-methyl-1H-benz[f]indole-4,9-dione, 2), together with eight known compounds (3-10) were also isolated from this plant. Their structures were extensively characterized by spectroscopic methods and comparisons were made with the literature. Compounds 1-4, 7, and 9 were evaluated for their α-glucosidase inhibitory activity. Of these, 3,5-demethoxypiperolide (7) displayed the highest α-glucosidase inhibitory activity, with an IC₅₀ value of 1.25 µM.

Discovery of lead natural products for developing pan-SARS-CoV-2 therapeutics

The COVID-19 pandemic, caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), remains a global public health crisis. The reduced efficacy of therapeutic monoclonal antibodies against emerging SARS-CoV-2 variants of concern (VOCs), such as omicron BA.5 subvariants, has underlined the need to explore a novel spectrum of antivirals that are effective against existing and evolving SARS-CoV-2 VOCs. To address the need for novel therapeutic options, we applied cell-based high-content screening to a library of natural products (NPs) obtained from plants, fungi, bacteria, and marine sponges, which represent a considerable diversity of chemical scaffolds. The antiviral effect of 373 NPs was evaluated using the mNeonGreen (mNG) reporter SARS-CoV-2 virus in a lung epithelial cell line (Calu-3). The screening identified 26 NPs with half-maximal effective concentrations (EC50) below 50 μM against mNG-SARS-CoV-2; 16 of these had EC50 values below 10 μM and three NPs (holyrine A, alotaketal C, and bafilomycin D) had EC50 values in the nanomolar range. We demonstrated the pan-SARS-CoV-2 activity of these three lead antivirals against SARS-CoV-2 highly transmissible Omicron subvariants (BA.5, BA.2 and BA.1) and highly pathogenic Delta VOCs in human Calu-3 lung cells. Notably, holyrine A, alotaketal C, and bafilomycin D, are potent nanomolar inhibitors of SARS-CoV-2 Omicron subvariants BA.5 and BA.2. The pan-SARS-CoV-2 activity of alotaketal C [protein kinase C (PKC) activator] and bafilomycin D (V-ATPase inhibitor) suggest that these two NPs are acting as host-directed antivirals (HDAs). Future research should explore whether PKC regulation impacts human susceptibility to and the severity of SARS-CoV-2 infection, and it should confirm the important role of human V-ATPase in the VOC lifecycle. Interestingly, we observed a synergistic action of bafilomycin D and N-0385 (a highly potent inhibitor of human TMPRSS2 protease) against Omicron subvariant BA.2 in human Calu-3 lung cells, which suggests that these two highly potent HDAs are targeting two different mechanisms of SARS-CoV-2 entry. Overall, our study provides insight into the potential of NPs with highly diverse chemical structures as valuable inspirational starting points for developing pan-SARS-CoV-2 therapeutics and for unravelling potential host factors and pathways regulating SARS-CoV-2 VOC infection including emerging omicron BA.5 subvariants.

Macluracochinones A-E, antimicrobial flavonoids from Maclura cochinchinensis (Lour.) Corner

The phytochemical investigation of the fruit and leaf extracts of Maclura cochinchinensis (Lour.) Corner (Moraceae) resulted in the isolation and identification of four undescribed isoflavones (macluracochinones A-D) and one undescribed flavone (macluracochinone E), together with 24 known compounds. The structures of the undescribed compounds were elucidated using nuclear magnetic resonance (NMR) and high-resolution electrospray ionization time-of-flight mass spectrometry (HRESITOFMS) experiments. Gancaonin M, lupiwighteone, lupalbigenin, warangalone, auriculatin, and millexatin F displayed good antibacterial activities against Gram-positive bacteria with MIC values in the range of 1-8 μg/mL. Lupalbigenin showed strong activities against methicillin-resistant Staphylococcus aureus (MRSA) and S. aureus with the same MIC value of 1 μg/mL.

Nitric oxide production inhibitory activity of clerodane diterpenes from Monoon membranifolium

A new clerodane diterpene, 2β-methoxyhardwickiic acid (1), and four known compounds (2-5) were isolated from the twigs of Monoon membranifolium. The structure of the new compound was determined by extensive spectroscopic methods and ESITOFMS data. The absolute configuration of 1 was established by a comparison of its ECD spectrum and specific rotation with those of related previously reported compounds. All compounds were evaluated for their nitric oxide (NO) production inhibitory activities in RAW264.7 macrophage cells. Compounds 3 and 5 inhibited NO production with IC₅₀ values of 16.1 and 28.9 μM, respectively, which were better than that of standard compound, indomethacin (IC₅₀ = 32.2 μM).

Desmoschinensisflavones A and B, two rare flavones having a hybrid benzyl benzoate ester-flavone structural framework from Desmos chinensis Lour

Two rare flavones having a hybrid benzyl benzoate ester-flavone structural framework, desmoschinensisflavones A and B (1 and 2), together with 12 known compounds (3–14) were isolated from the fruit, leaf, and twig extracts of Desmos chinensis (red flower). The new structures were characterized by UV, IR, NMR, and HRESITOFMS data. Desmoschinensisflavones A and B have a distinctive skeleton of benzoate ester-flavones with a C-4′′ and C-6 and C-8 connection via a methylene group, respectively. Plausible biosynthesis pathways to compounds 1 and 2 are proposed based on an intermolecular nucleophilic 1,4-addition to ortho-quinone intermediates. Compounds 6–8 and 12 showed weakly antioxidant inhibition with IC₅₀ values in the range of 65.4–74.6 μM.

Two rare flavones having a hybrid benzyl benzoate ester-flavone structural framework, desmoschinensisflavones A and B (1 and 2), together with 12 known compounds (3–14) were isolated from Desmos chinensis (red flower).

Antioxidant, Cytotoxic and α-Glucosidase Inhibitory Activities of Compounds isolated from the Twig Extracts of Maclura fruticosa

The first phytochemical investigation of the twig extracts of Maclura fruticosa led to the isolation and identification of a new xanthone, maclurafruticosone (1), together with 14 known compounds (2–15). All compounds were elucidated using spectroscopic methods as well as through comparisons made with data reported in the literature. Some isolated compounds were evaluated for their antioxidant, α-glucosidase inhibitory and cytotoxic activities. Compound 4, 6 and 7 showed significant antioxidant activity against DPPH radicals with IC50 values ranging from 7.45–16.12 μM. Compound 4 also exhibited potent activity against ABTS•+ scavenging activity with an IC50 value of 0.55 ± 0.01 μM which was better than positive control (ascorbic acid, IC50 2.35 ± 0.17 μM). Compound 12 showed significant α-glucosidase inhibitory activity with an IC50 value of 0.02 ± 0.37 mM. Compounds 5, 6 and 12 showed weak cytotoxic activities against a colon cancer cell line with IC50 values ranging from 22.35–47.62 μM.