The elucidation and evaluation of antibacterial compounds from the Australian traditional medicinal plant, Planchonia careya

leaves of the tropical tree, Planchonia careya (Lecythidaceae), are used traditionally in the treatment of wounds and ulcers by the indigenous people of northern Australia. The purpose of this investigation was to isolate and identify some antibacterial compounds from the leaves of P. careya to validate the use of this species as a wound healing remedy, and also to evaluate the isolated active compounds for potential use as chemotherapeutic or topical antibacterial agents. Some of the comprising flavonoid glycoside constituents were also investigated as potential chemotaxonomic markers, since morphological similarities of this species to other taxa has led to many changes in the classification of P. careya. The comprising compounds of the crude aqueous and methanol leaf extracts were separated using activity-guided fractionation and this involved the use of various chromatographic media in conjunction with plate hole diffusion assays. The structures of the isolated antibacterial compounds were elucidated principally using Nuclear Magnetic Resonance (NMR) spectroscopy and Mass Spectrometry (MS), and the antibacterial activity was assessed using Minimum Inhibitory Concentration (MIC) assays. The cytotoxicity of the isolated compounds was assessed using monkey kidney epithelial (MA104) cells, to determine the selectivity of the compounds for prokaryotic cells relative to eukaryotic cells, as well as with human ovarian carcinoma (HeLa) cells to determine any antitumour activity. Two known antibacterial compounds, gallocatechin-(4a-8)-gallocatechin (prodelphinidin) (1) and (+)-gallocatechin (2), were isolated from the aqueous extract and the latter exhibited a significant (p = or less than 0.05) effect on the tumour cells relative to the MA104 cells. The benzene triol, pyrogallol (S1) was also obtained from the aqueous extract and may have been present as a degradation product. Analysis of the comprising flavonol glycosides from the aqueous extract revealed the known kaempferol 3-O-gentibioside and quercetin 3-O-glucoside as well as two novel acylated kampferol polyglycosides. The methanol extract yielded two fatty acids, 9(S)-hydroxyoctadeca-10E,12Z,15Ztrienoic acid (3), 9(S)-hydroxyoctadeca-10E,12Z-dienoic acid (a-dimorphecolic acid) (4), the triterpene, 2a,3b,24-trihydroxyolean-12-en-28-oic acid (hyptatic acid-A) (5), and a mixture of two known acylated triterpenes, 3-O-cis-p-coumaroyl-2a,19a-dihydroxyurs-12-en-28-oic acid (3-O-cis-coumaroyl tormentic acid) as well as the trans isomer (6ab). Compound S2, a mixture of 2a,3b,19a,24-tetrahydroxyurs-12-en-28-oic acid (24-hydroxyl tormentic acid) and an oleane-type triterpene was also obtained from the methanol extract. The mixture of 6ab demonstrated the greatest antibacterial activity and prokaryotic selectivity of the isolated compounds with a MIC against vancomycinresistant enterococci (VRE) of 59 μg/mL. The isolation of known antibacterial compounds 1-5 as well as 6ab, S1 and S2 from the crude leaf extracts of P. careya validates the use of this species as a treatment for sores and ulcers. Further, the activity of 6ab against the antibiotic-resistant strain, VRE, and the antitumour activity of 2, 4 and 5, suggests that there is still great potential for the discovery of potential therapeutic agents from plants. Finally, the presence of novel acylated kaempferol polyglycosides may assist in taxonomic classification of this and other Planchonia species.