Cytotoxic properties of the anthraquinone derivatives isolated from the roots of Rubia philippinensis

Background: Cancer is one of the most frequently occurring diseases and is the second leading cause of death worldwide. In this study, anthraquinone derivatives (Compounds 1-5) were evaluated for their anti-cancer potential against various skin and breast cancer cell lines to assess whether these anthraquinone derivatives may serve as a lead for the augmentation of anti-cancer drug. Methods: Anthraquinone derivatives, 2-methyl-1,3,6-trihydroxy-9,10-anthraquinone-3-O-(6'-O-acetyl)-α-rhamnosyl(1→2)-β-glucoside (Comp 1), 2-methyl-1,3,6-trihydroxy-9,10-anthraquinone (Comp 2), and alizarin (Comp 3) were isolated from the dichloromethane fraction of the roots of Rubia philippinensis., whereas ethyl acetate fraction yielded xanthopurpurin (Comp 4) and lucidin-ω-methyl ether (Comp 5). Structures of all the isolated compounds were determined by spectral data analysis. All isolated compounds (Comp 1-5) were assessed for cytotoxicity by the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay against four different cancer cell lines, i.e. human melanoma (SK-MEL-5), murine melanoma (B16F10), and human breast adenocarcinoma (MCF7 and MDA-MB-231). Results: Significant activity of the compounds 4 and 5 was observed against the breast cancer cell line MDA-MB-231 with IC₅₀ values of 14.65±1.45 and 13.03±0.33 μM, respectively. Encouragingly, IC₅₀ values of 67.89±1.02 and 79.01±0.03 μM against normal kidney epithelial cells (MDCK) were also obtained for compounds 4 and 5, respectively, which indicated very low toxicity and favorable selectivity indices for compounds 4 and 5 in the range of 1.85 to 3.95 and 2.11 to 6.06 against skin cancer cell lines (SK-MEL-5, and B16F10), and breast cancer cell lines (MCF7 and MDA-MB-231), respectively. Conclusion: Our results suggested that the compounds 4 (xanthopurpurin) and 5 (lucidin-ω-methyl ether) showed high selective toxicity towards breast cancer cells at lower concentrations without showing toxicity towards normal cells, thus could be of potential as new lead molecules in cancer treatment.