Summary:
Plant-derived drugs play an important role in both traditional and modern medicine due to nature’s high potential of yielding therapeutically relevant bioactive compounds, and natural products are emerging as new lead structures for the development of novel drugs for the prevention and treatment of diseases of modern civilization. On the basis of modern bibliographic data, sesquiterpene lactones from Centaurea and Calea, phloroglucinols and essential oils from Hypericum, as well as diterpenoids from Euphorbia were studied, emphasizing on anti-inflammatory and cytotoxic activities. Plants from various families with different phylogenetic relationships and phytochemical profiles have been chosen. Centaurea, Hypericum, and Euphorbia have been referred from Hippocrates (5th century B.C.), and up to now, common uses have been reported for these genera in traditional medicine (including wound-healing, anti-inflammatory, diuretics, antipyretics). When selecting plants from regions with high diversity and endemism (Panama, Algeria, and Greece are biodiversity spots), the probability of finding novel (bioactive or not) substances is higher and demonstrates the potential of different plant species that have never been investigated before.
A total of 84 specialized natural products were isolated from endemic plants of cosmopolitan distribution: 26 from Calea jamaicensis, 12 from Centaurea papposa, 24 from Hypericum jovis, and 22 from Euphorbia deflexa. NMR metabolomic strategy was used to target the highly bioactive groups of interest e.g. sesquiterpene lactones, phloroglucinols, and diterpenes in the crude plant extracts and sub-fractions. The isolation procedure included a variety of chromatographic steps using column chromatography in different packing materials (e.g. silica gel, diaion, sephadex), preparative TLC, and HPLC equipped with RI or DAD detector using various columns (eg. RP-C18, RP-Biphenyl, NP). Their chemical structures were elucidated through a combination of HRESIMS, 2D NMR, and X-ray data. 28 sesquiterpene lactones, 14 phloroglucinols, 22 diterpenes, and 20 other compounds were elucidated, among them, 14 sesquiterpene lactones, 2 phloroglucinols, 16 diterpenes, and 3 chromenes are novel natural products and have not been described in the literature before; also the X-ray crystallographic data of 2 diterpenes were reported for the first time. In parallel, more than 120 volatile compounds were identified using GC-MS from the essential oils from Hypericum spp. (H. jovis, H. empetrifolium, H. amblyocalyx, H. triquetrifolium and H. perforatum), belonging mainly to the groups of monoterpene and sesquiterpene hydrocarbons.
Several sesquiterpene lactones were evaluated for their cytotoxic effects in vitro reporting IC50 ≤ 10 μM, thus considered to be active and could be potential anti-cancer drugs according to the National Cancer Institute. Carbetolide C afforded IC50 values for HeLa, SK-MEL-28 and HePG2 cells similar to that of parthenolide (2.9, 7.5, 12.6 μΜ and 2.1, 4.9, 10.0 μΜ, respectively), a well-known cytotoxic sesquiterpene lactone used as a positive control. It is noteworthy that the tested SLs revealed no toxicity in the non-cancerous endothelial HMEC-1 cell line, although Michael reactions of the lactone ring usually also involve non-specific toxicity. In the same cell line, sesquiterpene lactones showed strong inhibition of the TNF-α induced ICAM-1 expression, in a dose-dependent manner. Similar results were obtained from phloroglucinols in the ICAM-1 assay, as well as in the ex vivo COX/LOX assay, although only a few studies in the literature demonstrate anti-inflammatory effects from this group of compounds, suggesting the need for further research to explore their anti-inflammatory potential. Regarding diterpenes from Euphorbia plants, although abundant and extremely intriguing in their complicated structure elucidation, most of them presented very weak biological activities. Athough the wound healing activities of Hypericum perforatum infused oil is known, so far, no studies have been conducted on Hypericum essential oils. Besides several non polar ingredients of the essential oil are extracted to the infused oil and contribute to activity. To fulfill this gap, this study describes the investigation of five essential oils in vivo, and their significant wound healing effects gave a new insight into the healing properties of genus Hypericum. The metabolization of natural products is much less studied than their biosynthetic pathways, thus a final goal was the investigation of the in vitro metabolism of the drug-like group of sesquiterpene lactones using human liver microsomes, identified by LC-MS/MS. Sesquiterpene lactones resulted in changeable metabolism due to their structural diversity, though the highly active lactone ring remained the most common metabolic site.
As a concluding mark, high cytotoxic, anti-inflammatory, and wound healing effects were reported from natural products derived from Calea jamaicensis, Centaurea papposa, Hypericum jovis, H. empetrifolium, and Euphorbia deflexa. For these taxa, almost no records existed in the literature using modern techniques and investigating the biological activities of their constituents. Nature will still be a source of new drugs in the future, and the results of this study contribute to the natural product-based drug discovery, further supporting the strategic use of natural products as a starting point to investigate novel potential effective agents against a variety of human ailments.
