Antitumor agents XXXVIII: Isolation and structural elucidation of novel germacranolides and triterpenes from Elephantopus mollis

Collective Synthesis of Highly Oxygenated (Furano)germacranolides Derived from Elephantopus mollis and Elephantopus tomentosus

Germacranolides, secondary metabolites produced by plants, have garnered academic and industrial interest due to their diverse and complex topology as well as a wide array of pharmacological activities. Molephantin, a highly oxygenated germacranolide isolated from medicinal plants, Elephantopus mollis and Elephantopus tomentosus, has exhibited antitumor, inflammatory, and leishmanicidal activities. Its chemical structure is based on a highly strained ten-membered macrocyclic backbone with an (E,Z)-dienone moiety, which is fused with an α-methylene-γ-butyrolactone and adorned with four successive stereogenic centers. Herein, we report the first synthesis of molephantin in 12 steps starting from readily available building blocks. The synthesis features the highly diastereoselective intermolecular Barbier allylation of the β,γ-unsaturated aldehyde with optically active 3-bromomethyl-5H-furan-2-one intermediate and ensuing Nozaki-Hiyama-Kishi (NHK) macrocyclization for the construction of the highly oxygenated ten-membered macrocyclic framework. This synthetic route enabled access to another germacranolide congener, tomenphantopin F. Furthermore, cycloisomerization of molephantin into 2-deethoxy-2β-hydroxyphantomolin could be facilitated by irradiation with ultraviolet A light (λmax=370 nm), which opened a versatile and concise access to the related furanogermacranolides such as EM-2, phantomolin, 2-O-demethyltomenphantopin C, and tomenphantopin C.

The specialized sesquiterpenoids produced by the genus Elephantopus L.: Chemistry, biological activities and structure-activity relationship exploration

The genus Elephantopus L. is a valuable resource rich in sesquiterpenoids with structural diversity and various bioactivities, showing great potential for applications in medicinal field and biological industry. Up to now, over 129 sesquiterpenoids have been isolated and identified from this plant genus, including 114 germacrane-type, 7 guaianolide-type, 5 eudesmane-type, 1 elemanolide-type, and 2 bis-sesquiterpenoids. These sesquiterpenoids were reported to show a diverse range of pharmacological properties, including cytotoxic, anti-tumor, anti-inflammatory, antimicrobial, and antiprotozoal. Consequently, some of them were identified as active scaffolds in the design and development of drugs. Considering that there is currently no overview available that covers the sesquiterpenoids and their biological activities in the Elephantopus genus, this article aims to comprehensively review the chemical structures, biosynthetic pathways, pharmacological properties, and structure-activity relationship of sesquiterpenoids found in the Elephantopus genus, which will establish a theoretical framework that can guide further research and exploration of sesquiterpenoids from Elephantopus plants as promising therapeutic agents.

Highly Oxidized Germacranolides from Elephantopus tomentosus and the Configurational Revision of Some Previously Reported Analogues

Highly oxidized germacranolides are mainly found in the genus Elephantopus, contain a characteristic ten-membered molecular core that is highly flexible, and exhibit potential cytotoxic properties. However, their configurations were assigned ambiguously in previous reports due to spectroscopic observation of macrocyclic systems. Herein, 17 highly oxidized germacranolides, including 12 new germacranolides (1-12), were isolated from Elephantopus tomentosus. Their structures were characterized by spectroscopic data analysis combined with X-ray crystallography and ECD calculations, and it was possible to propose configurational revisions of five previously reported analogues (13-17). Cytotoxic activities for 1-17 against two hepatocellular carcinoma cell lines (HepG2 and Hep3B) were tested, and compounds 1-10 and 13-16 generated IC₅₀ values of 2.2-9.8 μM. Furthermore, the observed cytotoxic activity of 1 was determined as being mediated by inducing the apoptosis of HepG2 and Hep3B cells via mitochondrial dysfunction.

Three phragmalin-type limonoids orthoesters and the structure of odoratone isolated from the bark of Entandrophragma candollei (Meliaceae)

The phytochemical exploration of the Entandrophragma candollei stem bark extract led to the isolation and identification of twenty compounds including three undescribed phragmalin-class limonoids named encandollens C-E (1-3), the undescribed protolimonoid 5 together with sixteen known compounds. The structures of all the isolated compounds were determined by interpretation of their spectroscopic and spectrometric data including HRMS, 1D and 2D NMR analyses. The assignment of the absolute and relative stereochemistry of the undescribed compounds was achieved using SC-XRD analyses as well as NOESY experiments. The previously reported structure of odoratone (5a) was corrected as 23 R,24 S-dihydroxy-22 S,25-epoxytirucall-7-en-3-one (5) based on its NMR and SC-XRD data. Prieurianin (4) exhibited high cytotoxic activity on KB3-1 cell lines with an IC₅₀ of 1.47 μM compared to the reference griseofulvin (IC₅₀ = 17-21 μM). The results of the in silico docking of compound 4 supported and delivered further insights on its cytotoxicity.

Elephantopus mollis Kunth extracts induce antiproliferation and apoptosis in human lung cancer and myeloid leukemia cells

ETHNOPHARMACOLOGICAL RELEVANCE

Elephantopus mollis Kunth (EM), which belongs to Asteraceae family, has been used as a folk medicine with diverse therapeutic properties. Previous studies reported that crude extracts of this plant could inhibit several cancer cell lines, including breast carcinoma MCF-7, liver carcinoma HepG2, colorectal carcinoma DLD-1, lung carcinoma NCI-H23, etc. AIM: In this study, the anticancer activity and associated molecular mechanism of EM which is distributed in Vietnam were investigated.

MATERIALS AND METHODS

The cytotoxicity of various EM extracts was evaluated on different cell lines by MTT assay. In addition, the effects of EM extracts on cell growth, cell morphology, nuclear morphology, caspase-3 activation, and mRNA expression levels of apoptosis-related genes were also examined.

RESULTS

Our results demonstrated that ethyl acetate extract (EM-EA) caused proliferative inhibition and apoptotic induction towards A549 lung cancer cells (IC₅₀ = 18.66 μg/ml, SI = 5.8) and HL60 leukemia cells (IC₅₀ = 7.45 μg/ml, SI = 14.5) while petroleum ether extract (EM-PE) showed high toxicity to HL60 cell line (IC₅₀ = 11.14 μg/ml, SI = 6.7). Notably, Raji lymphoma cells were also affected by these extracts (IC₅₀ < 20 μg/ml, SI > 4), which has not been reported yet. Furthermore, mechanisms of EM extracts were elucidated. The significant downregulation of PCNA mRNA level induced by EM-EA/PE extracts contributed to the cell-growth restraint. EM-EA extract might activate apoptosis in A549 cells through both extrinsic and intrinsic signaling pathways by causing a 1.55-fold increase in BID, 3.65-fold increase in BAK and 3.11-fold decrease in BCL-2 expression level. Meanwhile, with EM-EA-extract treatment, HL60 cells might encounter P53-dependent apoptotic deaths.

CONCLUSIONS

The combination of antiproliferation and apoptosis activation contributed to the high efficacy of EM extracts. These findings not only proved the anticancer potential of EM but also provided further insights into the mechanisms of EM extracts.

A new sesquiterpene lactone from Elephantopus tomentosus

A new sesquiterpene lactone, named tomenphantopin H (1), together with two known germacranolides, 2β-methoxy-2-deethoxy-8-O-deacylphantomolin-8-O-tiglinate (2) and 2-deethoxy-2-hydroxyphantomolin (3), was isolated from the whole plant of Elephantopus tomentosus Linn. The new compound was completely elucidated using a combination of 1D and 2D NMR techniques (COSY, HMQC, and HMBC) and HR-ESI-MS analyses. All compounds exhibited antibacterial activity.

Cytotoxic and Apoptotic Effects of Ethyl Acetate Extract of Elephantopus mollis Kunth. in Human Liver Carcinoma HepG2 Cells Through Caspase-3 Activation

Previous cytotoxic (anticancer) evaluations of Elephantopus mollis were mainly focused on its elephantopin derivatives neglecting the combined effect of the phytochemicals in its traditionally used extracts. In this study, the cytotoxic mechanism of its extracts was investigated using methylene blue assay. The cytotoxic screening results revealed the ethyl acetate extract as the most potent extract by displaying prominent dose-dependent and time-dependent growth inhibitions in human liver carcinoma HepG2 cells with the lowest EC50 value of 9.38 ± 0.43 µg/mL after 72 hours of treatment. Acute exposure of the HepG2 cells to the ethyl acetate extract produced a significant regulation of caspase-3 with the peak expression at 8 hours of treatment ( P < .05). DNA fragmentation indicated by DeadEnd Apoptosis Detection System–labeled nuclei cells confirmed that the extract induced apoptotic cell death through caspase-3-dependent pathway in HepG2 cells.

Cytotoxic, apoptotic and anti-α-glucosidase activities of 3,4-di-O-caffeoyl quinic acid, an antioxidant isolated from the polyphenolic-rich extract of Elephantopus mollis Kunth

ETHNOPHARMACOLOGICAL RELEVANCE

The decoction of the whole plant of Elephantopus mollis Kunth. is traditionally consumed to treat various free radical-mediated diseases including cancer and diabetes.

AIM OF THE STUDY

This study was initiated to determine whether the most effective antioxidant compound isolated from the whole plant of Elephantopus mollis can also contribute to its claimed traditional values as anticancer and antidiabetes agents.

MATERIALS AND METHODS

An active antiradical phenolic compound (3,4-di-O-caffeoyl quinic acid) was isolated from the methanol extract (with the highest in polyphenolic content) and their antioxidant activities were compared using four different assays, that are DPPH, FRAP, metal chelating, and β-carotene bleaching tests. The compound was also evaluated for its cytotoxic activity, apoptotic induction and anti-glucosidase efficacies using methylene blue, DeadEnd™ assay and α-glucosidase assays, respectively.

RESULTS

The compound acted as a greater primary antioxidant than its methanol extract, by having higher ferric reducing activity (EC(50) 2.18±0.05 μg/ml), β-carotene bleaching activity (EC(50) 23.85±0.65 μg/ml) and DPPH scavenging activity (EC(50) 68.91±5.44μg/ml), whereas the methanol extract exhibited higher secondary antioxidant activity as a metal chelator with lower EC(50) value (49.39±3.68 μg/ml) than the compound. Cytotoxicity screening of this compound exhibited a remarkable dose-dependent inhibitory effect on NCI-H23 (human lung adenocarcinoma) cell lines (EC(50) 3.26±0.35 μg/ml) and was found to be apoptotic in nature based on a clear indication of DNA fragmentation. This compound also displayed a concentration-dependent α-glucosidase inhibition with EC(50) 241.80±14.29 μg/ml.

CONCLUSIONS

The findings indicate the major role of 3,4-di-O-caffeoyl quinic acid to antioxidant capacities of Elephantopus mollis extracts. The compound also exerted apoptosis-mediated cytotoxicity and α-glucosidase inhibitory effects and is thus a promising non toxic agent in treating cancer and type 2 diabetes mellitus.

Ethnopharmacology of Mexican asteraceae (Compositae)

Traditional herbal remedies have increased in popularity in Europe and the United States in recent years but have always been important to people living in rural Mexico and to their Mexican American/Chicano descendants in the United States. Mexican American patients will often be ingesting herbal teas at the same time that they are being treated for their ailments with antibiotics or antiinflammatory agents. The plant family Asteraceae (Compositae) has contributed the largest number of plants to this pharmacopoeia; the reasons for the importance of this family include its large number of species in Mexico and its wide array of natural products that are useful in the treatment of the maladies that have afflicted the inhabitants of rural Mexico. These natural products include sesquiterpene lactones, polyacetylenes, alkaloids, monoterpenes, and various phenolics such as flavonoids. In this review, we emphasize the sesquiterpene lactones, a large group of compounds with antiinflammatory properties and the ability to relax smooth muscles and thereby relieve gastrointestinal distress. These compounds also readily form adducts with glutathione or free thiols and can thereby affect the metabolism, activity, and toxicology of a wide array of pharmacological agents.