Studies On Bioactive Secondary Metabolites From Four Plants Of Inula Genus

The Inula genus, which comprises ca.100species of the Compositae family, isdistributed throughout Asia, Europe, and Africa and can be found predominantly inthe Mediterranean. More than20Inula species are distributed in China and furtherdivided into five sections and11series in phytotaxonomy. A number of plants fromthe Inula genus are used as traditional herbal medicines throughout the world,especially in China. In the latest Chinese Pharmacopoeia, three Traditional ChineseMedicines that are used as expectorants, antitussives, diaphoretics, antiemetics, andbactericides Tumuxiang, Xuanfuhua, and Jinfeicao originate from four Inulaplants. Inula species have emerged as exceptionally rich sources of varioussesquiterpenoids, which accounted for anti-inflammatory, anti-proliferation,antibacterial, and hepatoprotective properties. In continuation of our research programfor bioactive sesquiterpenoids derived from the plants of Inula genus, we examined I.falconeri, I. hookeri, I. sericophylla, and I. wissmanniana, on which extensivephytochemical studies have not been previously conducted.178compounds were isolated from the whole plants of I. falconeri, I. hookeri,and I. wissmanniana via detailed phytochemical investigations with variouschromatographic techniques. The structures were elucidated by spectroscopic analysisto be84sesquiterpenes, seven dimeric sesquiterpenes,12monoterpenes, and75othercompounds which included20flavonoids,7lignans,6apocarotenoids, etc.Considering the great HPLC/DAD similarity between the crude extracts of I.sericophylla and I. hookeri, HPLC/DAD/ESI-MSnwas applied to analyse thesecondary metabolites from I. sericophylla, affording36terpenoids. The84sesquiterpenes comprised35new sesquiterpenoids and49knownsesquiterpene lactones which included nine defferent frameworks, mainly germacrane,xanthane, guaiane, pseudoguaiane, and eudesmane. Seven rare guaiane2,4-typeddimeric sesquiterpene lactones (IF-43IF-49), which three of them were structurallyequivalent to stereoselective and regioselective adducts of the same Diels-Alderreaction, together with biosynthetic analogues of1(2),4(5)-guaia-dienes (IF-16andIF-17) were obtained from I. falconeri, a plant endemic to the Himalayas. The firstnaturally occurring5,6-seco-and rearranged eudesmane sesquiterpenoids(IW-26IW-28), unprecedented C17-pseudoguaianolides (IF-41IF-44), and novel3,4-seco-eudesmane sesquiterpenoids (IW-25) were herein described. A survey ofliteratures also showed the new compounds IW-19, IW-24, and IF-50and knowncompound IF-37to be the first occurrence of eudesman-12,5-olide,4,5-seco-eudesman-12,5-olide,4,5-seco-caryophyllane derivative, chromolaevan-12,8-olide,respectively, from the plants of Inula genus.On the basis of theoretical and experimental reports, the biosynthetic pathwaysof novel secondary metabolites, including dimeric sesquiterpenes (IF-43IF-45),C17-pseudoguaianolides (IF-41IF-44), and eudesmane-derivatived sesquiterpenes(IW-21IW-23) were hypothesized in this study.On the basis of phytochemical investigations, the chemotaxonomic significanceof sesquiterpenes from seven Inula herbs were discussed via HPLC/DAD detectionfollowed by principal component analysis. Our experimental study supported the viewthat I. japonica Thunb. and I. lineariaefolia Turcz. were variants or subspecies of I.britanica L.. We also further suspected that I. hupehensis Ling, I. helianthus-aquaticaC. Y. Wu ex Ling, I. sericophylla Franch, and I. hookeri C. B. Clarke were evolvedfrom I. britanica L.. Considering the differences of chemical constituents between I.japonica Thunb. and I. lineariaefolia Turcz., the application of I. lineariaefolia Turcz.as Jinfeicao should be reconsidered.All the isolated sesquiterpenes and dimeric sesquiterpenes were assessed for theirinhibitory effects against LPS-induced NO production in RAW264.7macrophages.The guaianolides IF-17(IC50,0.22M), IF-24(IC50,0.11M), IF-26(0.29M), and IF-27(0.13M), pseudoguaianolides IF-28(0.07M), IH-31(0.29M), IH-37(0.28M), and IH-40(0.38M), eudesmanolide IF-40(0.11M), andeudesmane-derivatived sesquiterpene IW-26(0.38M) showed remarkable inhibitoryactivities, comparing with the positive control. These studies also led to a betterunderstanding of the structure-activity relationships from frameworks, lipophilicities,substituted groups, and structural rigidness for the sesquiterpenes.All the sesquiterpenes and dimeric sesquiterpenes were assessed for theircytotoxic activities against HepG2, HeLa, PC-3, and MGC-803cell lines by CCK-8assay. Some of the isolates, especiallly germacranolides and dimeric sesquiterpenesexhibited significant cytotoxicities against HepG2, PC-3, and MGC-803cells. Thedimeric sesquiterpene IF-45and novel eudesmane-derivatived sesquiterpene IW-26inhibited HepG2, PC-3, and MGC-803cells with IC50values at0.961.66and1.353.35M, respectively.