Phytochemical studies and bioactivity of Centipeda and Eremophila species

Abstract

The aim of this study was to isolate and characterise biologically active compounds from endemic Australian plants. A total of 6 novel, and 26 known compounds have been isolated throughout the course of this work. A comprehensive investigation of the GC-MS chemical profile of C. cunninghamii leaf essential oil found that thymol (1) cis-chrysanthenyl acetate (4), myrtenyl acetate (2), myrtenol (3) and cis-chrsanthenol (5) were the major constituents. The essential oil and crude solvent extracts of C. cunninghamii possessed significant antioxidant and antiinflammatory activity. A 50% aqueous ethanol extract was demonstrated to possess multiple modes of anti-inflammatory action. The crude extract was found to significantly inhibit both COX-1 and COX-2 cyclooxygenases and was comparable to the positive controls; Ibuprofen and Celebrex respectively. The crude extract also exhibited anti-inflammatory activity in the nitric oxide (NO) and tumor necrosis factoralpha (TNF-α) assays, but did not show inhibition in the lipoxygenase (LO) assay. A total of seventeen compounds, of which 10, 6, 7, 8 and 9 are novel, have been identified from the aqueous-ethanolic extract of C. cunninghamii. Five flavonoids; axillarin (16), isokaempferide (17), 4’,5,7-trihydroxy-3,6-dimethoxyflavone (18), jaceidin (19), and 2’,4’,5,7-tetrahydroxy-6-methoxyflavone-3-O-β-glucopyranoside (10) were isolated from the flowers of C. cunninghamii. A series of caffeic acids were isolated as the major component of the stems, these included; chlorogenic acid (12) and its methyl ester (13), caffeic acid ethyl ester (11), isochlorogenic acid A (14) macroantoin G (15) and the novel 4ξ,5ξ-di-O-caffeoyl-2,6ξ-dihydroxyhept-2-ene-1,7- dioic acid (6) and its 1-methyl ester (7), 7-methyl ester (8) and 1,7-dimethylester (9) derivatives. Lastly, arnicolide C (20) a sesquiterpene lactone, 3-hydroxykaura-9(11),16- diene-18-oic acid (21) and 8-hydroxy-9,10-diisobutyryloxythymol (23) were characterised by spectroscopic methods. All of the compounds were evaluated for anti-inflammatory activity, as determined by the inhibition of prostaglandin E2 in 3T3 fibroblast cells. All compounds, inhibited PGE2 production to some extent, at a concentration of 31.25 μg/mL. The flavonoids 10 and 16-19 were the most active compounds. The caffeic acids 6-9, 12-14 and the thymol derivative 23 also significantly inhibited PGE2 production. The IC50 values were determined for the novel compounds; 10, 6, 7, 8 and 9, as 1.47, 2.48, 4.73, 5.54 and 1.26 μM, respectively. These novel compounds were more potent than the positive control, aspirin, which was found to inhibit PGE2 production by 42% at a concentration of 18 μM. Antioxidant activity, as determined by oxygen radical absorbance capacity (ORAC) has also been attributed to both the flavonoids; 10, 16-19 and caffeic acid compounds; 6-9 and 12-14. The antioxidant capacity of these compounds was found to be comparable to epicatechin, a major antioxidant constituent of green tea. A detailed analysis of the wood, leaf, branch and root oil of E. mitchellii was carried out by a combination of GC-FID, GC-MS, LC/MS and NMR spectroscopy. The wood, root, leaf and branch oils were found to be predominantly composed of sesquiterpenes. The three major compounds identified in the leaf oil, which accounted for 44% of the oil, were α-pinene (40), (+) spathulenol (15) and an unidentified sesquiterpene alcohol. The composition of the leaf oil was complex and chemically distinct from the wood and root oils, whereas the branch oil was found to exhibit a chemical composition that was intermediate between the leaf and the wood oil. After fractionation by preparative HPLC six components of the wood oil were characterized and accounted for 80% of the oil. The major constituents of the wood oil were; eremophilone (30), 9-hydroxy-7(11),9-eremophiladien-8-one (36), santalcamphor (35) and the novel 9-hydroxy-1,7(11),9-eremophilatrien-8-one (42). Two minor constituents, 8-hydroxy-10,11-eremophiladien-9-one (32) and 8-hydroxy-1,11- eremophiladien-9-one (33) were also isolated in this study. The two major constituents of the root oil of E. mitchellii were found to be eremophilone (30) and the zizaene, sesquithuriferone (43). These, together with the minor constituents 32, 33, 42, 35 and 36 accounted for 92% of the root oil. The insecticidal properties of E. mitchellii were evaluated against several species of termites Nasutitermes walkeri (Hill), Nasutitermes exitiosus (Hill) and Coptotermes acinaciformis (Froggatt). Bioassay-guided fractionation of E. mitchellii wood oil was undertaken to investigate the termiticidal metabolites. Of the major components, it has been determined that eremophilone (30) was the most active constituent of the wood oil followed by 8-hydroxy-1,11-eremophiladien-9-one (33), 9-hydroxy-7(11),9- eremophiladien-8-one (36) and santalcamphor (35). The methanolic extracts from a total of 36 species have been evaluated for cytotoxicity against P388D1 mouse lymphoblast cells. A collection of fifteen species of Eremophila from Western Australia and a further twenty from the Northern Territory were surveyed. Cytotoxicity was found to be largely non-selective across a range of human cancer cell lines, including MCF7 (mammary adenocarcinoma), Hep G2 (hepatocellular carcinoma), A2780 (ovarian carcinoma), A-375 (malignant melanoma) and PC-3 (prostate cancer). Fractionation of the leaf material of E. racemosa afforded the six major metabolites. Isolation and structural elucidation of these polar compounds revealed the cyanogenetic glycoside prunasin (65), the flavonoid luteolin (74), the furofuran lignans, phillygenin (75), its 4-O-β-D-glucoside phillyrin (76), pinoresinol-4-O-β-D-glucoside (77) and epipinoresinol-4-O-β-D-glucoside (78). Fractionation of the leaf material of E. maculata var. brevifolia afforded piceine (81) and epipinoresinol-4-O-β-D-glucoside (78). Quercetin (79) and nepetin (80) were isolated from the methanolic extracts of E. bignoniflora