Plectranthus species have been traditionally used to combat different types of cancer due to their anti-tumour properties . The cytotoxicity screenings have identified Plectranthus plants as potential sources of antitumor lead compounds. Abietane diterpenoids have been reported as the main constituents of some species in this genus and are responsible for its potential therapeutic value . These naturally occurring compounds display a vast array of biological activities including cytotoxic and antiproliferative activities against human tumour cells . The objective of this study was to evaluate the biological activity of sixteen Plectranthus spp. acetonic extracts and identify the bioactive compounds in the most effective extracts. P. mutabilis had the highest extraction yield (30.03%, dry weight % w/w). All extracts were screened for their general toxicity using the Artemia salina model . Thus, the antitumor activity of the five most toxic extracts was explored in three different cancer cell lines: HCT116, MCF-7 and NCI-H460. P. mutabilis with high cytotoxic activity was subjected to a bio-guided fractionation using the A. salina general toxicity assay. Column chromatography on silica or polyamide, with gradient systems of increasing polarity allowed to achieve the diterpenoid coleon U (1) and 8α,9α-epoxycoleon U quinone (2). The complete structure characterization was done mainly by 1D- and 2D-NMR, and comparison with literature data. Compound 1 showed moderate cytotoxicity on sensitive and resistant (ABCB1 overexpressing) human colon adenocarcinoma and normal cell lines showing slight selectivity towards resistant cells. Moreover, this compound is not an inhibitor of ABCB1 transporter based on the intracellular accumulation of the ABCB1 substrate rhodamine 123. Further phytochemical studies are ongoing.
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