Eight new indole alkaloids (1-8) belonging to the rhazinilam-leuconolam-leuconoxine group, in addition to 52 other alkaloids, were isolated from the stem-bark extract of Leuconotis griffithii, viz., nor-rhazinicine (1), 5,21-dihydrorhazinilam-N-oxide (2), 3,14-dehydroleuconolam (3), and leuconodines A-E (4-8). The structures of these alkaloids were determined using NMR and MS analyses and in some instances confirmed by X-ray diffraction analyses. Alkaloids 1, 5, and 7 showed only moderate to weak cytotoxicity toward KB cells (IC50 12-18 μg/mL), while 8 showed moderate activity in reversing MDR in vincristine-resistant KB cells.
Four new bisindole alkaloids of the Strychnos-Strychnos type, leucoridines A-D (1-4), were isolated from the stem-bark extract of Leuconotis griffithii. Alkaloids 1-4 showed moderate cytotoxicity against drug-sensitive and vincristine-resistant human KB cells.
Seven new indole alkaloids of the Strychnos type, leuconicines A-G (1-7), and a new eburnan alkaloid, (-)-eburnamaline (8), were isolated from the stem-bark extract of two Malayan Leuconotis species. The structures of these alkaloids were established using NMR and MS analysis and in the case of 8 also by partial synthesis. Alkaloids 1-5 reversed multidrug resistance in vincristine-resistant KB cells.
From the aerial parts of Atriplex littoralis, three new flavonoid glycosides named atriplexins I-III (1-3), a known flavonoid glycoside, spinacetin 3-O-β-d-glucopyranoside (4), arbutin (5), and 4-hydroxybenzyl-β-d-glucopyranoside (6) were isolated. Their structures were elucidated on the basis of detailed spectroscopic analysis, including 1D and 2D NMR (COSY, NOESY, TOCSY, HSQC, HMBC) and HRESITOF MS data. The compounds were tested for in vitro protective effects on chromosome aberrations in peripheral human lymphocytes using a cytochalasin-B-blocked micronucleus (MN) assay in a concentration range of 0.8-7.4 μM of final culture solution. Chromosomal damage was induced by 2 Gy of γ-radiation on binucleated human lymphocytes, and the effects of the compounds were tested 2 to 19 h after irradiation. The frequency of micronuclei (MNi) was scored in binucleated cells, and the nuclear proliferation index was calculated. The highest prevention of in vitro biochemical and cytogenetic damage of human lymphocytes induced by γ-radiation was exhibited by 3 (reduction of MN frequency by 31%), followed by 4 and 6.
Proteins of the Bcl-2 family are key targets in anticancer drug discovery. Disrupting the interaction between anti- and pro-apoptotic members of this protein family was the approach chosen in this study to restore apoptosis. Thus, a biological screening on the modulation of the Bcl-xL/Bak and Mcl-1/Bid interactions permitted the selection of Knema hookeriana for further phytochemical investigations. The ethyl acetate extract from the stem bark led to the isolation of six new compounds, three acetophenone derivatives (1-3) and three anacardic acid derivatives (4-6), along with four known anacardic acids (7-10) and two cardanols (11, 12). Their structures were elucidated by 1D and 2D NMR analysis in combination with HRMS experiments. The ability of these compounds to antagonize Bcl-xL/Bak and Mcl-1/Bid association was determined, using a protein-protein interaction assay, but only anacardic acid derivatives (4-10) exhibited significant binding properties, with Ki values ranging from 0.2 to 18 μM. Protein-ligand NMR experiments further revealed that anacardic acid 9, the most active compound, does not interact with the anti-apoptotic proteins Bcl-xL and Mcl-1 but instead interacts with pro-apoptotic protein Bid.
Four new compounds, (+)- and (-)-ecarlottone (1), (±)-fislatifolione (5), (±)-isofislatifolione (6), and (±)-fislatifolic acid (7), and the known desmethoxyyangonin (2), didymocarpin-A (3), and dehydrodidymocarpin-A (4) were isolated from the stem bark of Fissistigma latifolium, by means of bioassay-guided purification using an in vitro affinity displacement assay based on the modulation of Bcl-xL/Bak and Mcl-1/Bid interactions. The structures of the new compounds were elucidated by NMR spectroscopic data analysis, and the absolute configurations of compounds (+)-1 and (-)-1 were assigned by comparison of experimental and computed ECD spectra. (-)-Ecarlottone 1 exhibited a potent antagonistic activity on both protein-protein associations with Ki values of 4.8 μM for Bcl-xL/Bak and 2.4 μM for Mcl-1/Bid.
The pressing need for SARS-CoV-2 controls has led to a reassessment of strategies to identify and develop natural product inhibitors of zoonotic, highly virulent, and rapidly emerging viruses. This review article addresses how contemporary approaches involving computational chemistry, natural product (NP) and protein databases, and mass spectrometry (MS) derived target-ligand interaction analysis can be utilized to expedite the interrogation of NP structures while minimizing the time and expense of extraction, purification, and screening in BioSafety Laboratories (BSL)3 laboratories. The unparalleled structural diversity and complexity of NPs is an extraordinary resource for the discovery and development of broad-spectrum inhibitors of viral genera, including Betacoronavirus, which contains MERS, SARS, SARS-CoV-2, and the common cold. There are two key technological advances that have created unique opportunities for the identification of NP prototypes with greater efficiency: (1) the application of structural databases for NPs and target proteins and (2) the application of modern MS techniques to assess protein-ligand interactions directly from NP extracts. These approaches, developed over years, now allow for the identification and isolation of unique antiviral ligands without the immediate need for BSL3 facilities. Overall, the goal is to improve the success rate of NP-based screening by focusing resources on source materials with a higher likelihood of success, while simultaneously providing opportunities for the discovery of novel ligands to selectively target proteins involved in viral infection.
The urgent need for new classes of orally available, safe, and effective antivirals─covering a breadth of emerging viruses─is evidenced by the loss of life and economic challenges created by the HIV-1 and SARS-CoV-2 pandemics. As frontline interventions, small-molecule antivirals can be deployed prophylactically or postinfection to control the initial spread of outbreaks by reducing transmissibility and symptom severity. Natural products have an impressive track record of success as prototypic antivirals and continue to provide new drugs through synthesis, medicinal chemistry, and optimization decades after discovery. Here, we demonstrate an approach using computational analysis typically used for rational drug design to identify and develop natural product-inspired antivirals. This was done with the goal of identifying natural product prototypes to aid the effort of progressing toward safe, effective, and affordable broad-spectrum inhibitors of Betacoronavirus replication by targeting the highly conserved RNA 2'-O-methyltransferase (2'-O-MTase). Machaeriols RS-1 (7) and RS-2 (8) were identified using a previously outlined informatics approach to first screen for natural product prototypes, followed by in silico-guided synthesis. Both molecules are based on a rare natural product group. The machaeriols (3-6), isolated from the genus Machaerium, endemic to Amazonia, inhibited the SARS-CoV-2 2'-O-MTase more potently than the positive control, Sinefungin (2), and in silico modeling suggests distinct molecular interactions. This report highlights the potential of computationally driven screening to leverage natural product libraries and improve the efficiency of isolation or synthetic analog development.
A polysaccharide, Ali-1, was isolated from the roots of Eurycoma longifolia, a popular traditional medicinal herb in Malaysia. The structure of Ali-1 was characterized by monosaccharide, methylation, and NMR data analyses. The average molecular weight of Ali-1 is 14.3 ku, and it is composed of arabinose (14.31%), xylose (57.69%), galacturonic acid (13.03%), and glucuronic acid (14.86%). The main chain comprises (1→4)-linked xylose residues. It has branch points in the main chain; (1→2,4)-linked xylose residues, 1,2-linked glucuronic acid residues, and 1,2-linked arabinose residues form the branches, and the branches are terminated with T-linked galacturonic acid residues and T-linked arabinose residues. Ali-1 significantly improves the pinocytic and phagocytic abilities of RAW264.7 cells and facilitates cytokine secretion according to an immunostimulation assay. These results demonstrate that Ali-1 has potential as a functional supplement for people with compromised immune systems.
A new 32-membered macrolactone antibiotic, named langkolide, was isolated from the mycelium of Streptomyces sp. Acta 3062. The langkolide structure was determined by HR-MS and 1D and 2D NMR as a 32-membered macrolactone connected from an overhanging polyketide tail to a naphthoquinone unit mediated by two carbohydrate moieties. The producing strain was isolated from a rhizosphere soil of Clitorea sp. collected at Burau Bay, Langkawi, Malaysia, and was characterized by its morphological and chemotaxonomic features in addition to its 16S rRNA gene sequence. It was identified as a member of the Streptomyces galbus clade. Langkolide exhibited various bioactivities including antimicrobial and antiproliferative activities. Furthermore, langkolide inhibited human recombinant phosphodiesterase 4 with an IC(50) value of 0.48 μM.
Four new chromone alkaloids, chrotacumines A-D (1-4), consisting of a 5,7-dihydroxy-2-methylchromone, an N-Me piperidine ring, and an ester side chain were isolated from Dysoxylum acutangulum, and their structures including absolute configurations were elucidated on the basis of spectroscopic data interpretation including 2D NMR, CD spectra, and X-ray analysis. The known compound rohitukine (5) showed moderate cytotoxicity against human HL-60 promyelocytic leukemia and HCT-116 colon cancer cells.
Five new indole alkaloids of the ibogan type (1-5), in addition to 12 other known iboga alkaloids, were obtained from the leaf and stem-bark extract of the Malayan species Tabernaemontana corymbosa, viz., 19(S)-hydroxyibogamine (1), 19-epi-isovoacristine (2), isovoacryptine (3), 3R/S-ethoxyheyneanine (4), and 3R/S-ethoxy-19-epi-heyneanine (5). The structures were determined using NMR and MS analysis and comparison with known related compounds.
Ten new bisindole alkaloids of the vobasinyl-ibogan type, viz., conodiparines A-F (1-6), conodutarines A and B (7, 8), and cononitarines A and B (9, 10), were obtained from the leaf extract of the Malayan species Tabernaemontana corymbosa. The structures were determined using NMR and MS analysis.
The EtOH extract of the leaves of Holarrhena curtisii yielded five new steroidal alkaloids: 17-epi-holacurtine (3), 17-epi-N-demethylholacurtine (4), holacurtinol (5), 3alpha-amino-14beta-hydroxypregnan-20-one (7), and 15alpha-hydroxyholamine (8), in addition to the known compounds, holacurtine (1), N-demethylholacurtine (2), and holamine (6). All eight compounds showed significant cytotoxic and leishmanicidal activities.
Ten new indole alkaloids, alstomaline (1), 10,11-dimethoxynareline (2), alstohentine (3), alstomicine (4), 16-hydroxyalstonisine (5), 16-hydroxyalstonal (6), 16-hydroxy-N(4)-demethylalstophyllal oxindole (7), alstophyllal (8), 6-oxoalstophylline (9), and 6-oxoalstophyllal (10), in addition to 21 other known ones, were obtained from the leaf extract of the Malayan Alstonia macrophylla. The structures were determined using NMR and MS analysis.
Natural products remain an important source of drug leads covering unique chemical space and providing significant therapeutic value for the control of cancer and infectious diseases resistant to current drugs. Here, we determined the antiproliferative activity of a natural product manzamine A (1) from an Indo-Pacific sponge following various in vitro cellular assays targeting cervical cancer (C33A, HeLa, SiHa, and CaSki). Our data demonstrated the antiproliferative effects of 1 at relatively low and non-cytotoxic concentrations (up to 4 μM). Mechanistic investigations confirmed that 1 blocked cell cycle progression in SiHa and CaSki cells at G1/S phase and regulated cell cycle-related genes, including restoration of p21 and p53 expression. In apoptotic assays, HeLa cells showed the highest sensitivity to 1 as compared to other cell types (C33A, SiHa, and CaSki). Interestingly, 1 decreased the levels of the oncoprotein SIX1, which is associated with oncogenesis in cervical cancer. To further investigate the structure-activity relationship among manzamine A (1) class with potential antiproliferative activity, molecular networking facilitated the efficient identification, dereplication, and assignment of structures from the manzamine class and revealed the significant potential in the design of optimized molecules for the treatment of cervical cancer. These data suggest that this sponge-derived natural product class warrants further attention regarding the design and development of novel manzamine analogues, which may be efficacious for preventive and therapeutic treatment of cancer. Additionally, this study reveals the significance of protecting fragile marine ecosystems from climate change-induced loss of species diversity.
Schwarzinicines A-G (1-7), representing the first examples of 1,4-diarylbutanoid-phenethylamine conjugates, were isolated from the leaves of Ficus schwarzii. The structures of these compounds were determined by detailed analysis of their MS, 1D and 2D NMR data. Compounds 1-4 exhibited pronounced vasorelaxant effects in the rat isolated aorta (Emax 106-120%; EC50 0.96-2.10 μM). However, compounds 1 and 2 showed no cytotoxic effects against A549, MCF-7, and HCT 116 human cancer cells (IC50 > 10 μM).
Six new linear peptides, pterulamides I-VI (1-6), were isolated from the fruiting bodies of a Malaysian Pterula species. The structures were elucidated by MS and 2D NMR experiments, and the absolute configurations of the constituent amino acids established using Marfey's method. The pterulamides are mainly assembled from nonpolar N-methylated amino acids and, most interestingly, have non-amino-acid N-terminal groups, among them the unusual cinnamoyl, (E)-3-methylsulfinylpropenoyl, and (E)-3-methylthiopropenoyl groups. Furthermore, pterulamides I-V are the first natural peptides with a methylamide C-terminus. Pterulamides I and IV are cytotoxic against the P388 cell line with IC50 values of 0.55 and 0.95 microg/mL (0.79 and 1.33 microM), respectively.
Melicope pteleifolia has long been consumed as a popular vegetable and tea in Southeast Asian countries, including Malaysia and southern mainland China, and is effective in the treatment of colds and inflammation. In the search for active metabolites that can explain its traditional use as an antipyretic, six new phloroacetophenone derivatives (3-8) along with seven known compounds (1, 2, and 9-13) were isolated from the leaves of M. pteleifolia. Their chemical structures were confirmed by extensive spectroscopic analysis including NMR, IR, ECD, and HRMS. All compounds isolated from the leaves of M. pteleifolia (1-13) have a phloroacetophenone skeleton. Notably, the new compound 8 contains an additional cyclobutane moiety in its structure. The bioactivities of the isolated compounds were evaluated, and compounds 1, 6, and 7 inhibited tumor necrosis factor-α-induced prostaglandin E2. Moreover, the major constituent, 3,5-di-C-β-d-glucopyranosyl phloroacetophenone (1), was found to be responsible for the antipyretic activity of M. pteleifolia based on in vivo experiments.
Schwarzinicines A-D, a series of alkaloids recently discovered from Ficus schwarzii, exhibit pronounced vasorelaxant activity in rat isolated aorta. Building on this finding, a concise synthesis of schwarzinicines A and B has been reported, allowing further investigations into their biological properties. Herein, a preliminary exploration of the chemical space surrounding the structure of schwarzinicine A (1) was carried out aiming to identify structural features that are essential for vasorelaxant activity. A total of 57 analogs were synthesized and tested for vasorelaxant activity in rat isolated aorta. Both efficacy (Emax) and potency (EC50) of these analogs were compared. In addition to identifying structural features that are required for activity or associated with potency enhancement effect, four analogs showed significant potency improvements of up to 40.2-fold when compared to 1. Molecular dynamics simulation of a tetrameric 44-bound transient receptor potential canonical-6 (TRPC6) protein indicated that 44 could potentially form important interactions with the residues Glu509, Asp530, Lys748, Arg758, and Tyr521. These results may serve as a foundation for guiding further structural optimization of the schwarzinicine A scaffold, aiming to discover even more potent analogs.