Isolation and Identification of Secondary Metabolites in Rheum tataricum L.fil. Growing in Kazakhstan and Surveying of Its Anticancer Potential

Abstract

Rheum tataricum L.fil., known for its high tolerance to drought, salinity, and nutritional deficiency, is the least studied species of wild rhubarb. Extract of roots and rhizomes of R. tataricum has been traditionally used for the treatment of different diseases such as liver, kidney, womb, and bladder diseases and also relapsing fever. An ethanol extract of the roots of R. tataricum was prepared and further successively fractionated by extraction with tert-butyl methyl ether (TBME) and ethyl acetate (EtOAc). The obtained extract fractions were subjected to a series of chromatographic separations on silica gel for the isolation of its individual compounds. A total of 12 individual compounds, 2-Oβ-D-glucopyranoside of R-(4-hydroxyphenyl)-2-butanol (rhododendrin) 1, gallic acid 2, 2-O-β-D-glucopyranoside of S-4-(4-hydroxyphenyl)-2-butanol (epi-rhododendrin) 3, their aglycones (-)-(2R)-rhododendrol 4 and (+)-(2S)-rhododendrol 5, gallotannin β-glucogallin 6, chlorogenic acids (3,5-di-O-caffeoylquinic acid 7 and 5-O-caffeoyl-3-O-(p-coumaroyl) quinic acid 8), 4-(4-hydroxyphenyl)-2-butanon (raspberry ketone) 9 and three stilbenes (rhaponticin 10, desoxyrhaponticin 11 and resveratroloside 12), were isolated and characterized. The structure of desoxyrhaponticin 11 was confirmed by X-ray diffraction analyses. The results of in vitro biological assays (the MTT test) showed that ethanol extract Rheum tataricum was non-toxic against the normal epithelial VERO cells. The isolated compounds 1, 4, 11 and 12 exhibited cytotoxicity against a cervical cancer cell line (CaSki), breast adenocarcinoma (MCF7) and glioblastoma cell line (SNB-19) at low micromolar concentrations. Polyhydroxystilbenes 11 and 12 showed the best potency against adenocarcinoma cells (GI50 = 7–8 µM). The inhibition activity towards cancer cells was comparable to those of the standard drug doxorubicin. The available from R. tataricum secondary metabolites may serve as new leads for the discovery of anticancer drugs.