Isolation and characterization of an abietane diterpene from coleus neochilus with potent antiproliferative activity on a human breast cancer cell line

Abstract

Coleus spp. have diverse ethnobotanical applications, with their most prevalent use being attributed to their medicinal properties. We have previously described the antiproliferative effect of the ethanolic extract from C. neochilus (also known as “boldo rastrero”) on differ- ent human breast cancer cell lines. The present study is focused on the extraction and isolation of compounds responsible for this effect. The ethanol extract (BRET) was subjected to solvent-solvent ex- traction, yielding four different sub-extracts: hexane (BRHX), chlo- roform (BRCL), ethyl acetate (BRAE), and methanol-water (BRAM). Remarkably, incubation with BRAE (50 μg/ml) was more effective in reducing the viability of human breast cancer cell line MCF-7 than the other fractions (BRET: 23%, BRHX: 29%, BRCL: 52%, BRAE: 68%, and BRAM: 10% reduction; p<0.05) as determined by MTT assays. Given these results, BRAE was subjected to further frac- tionation. A column chromatography using silica gel and mixtures of dichloromethane and methanol as mobile phase rendered eleven fractions, among which seven demonstrated a significant reduction in cell viability (50 μg/ml, p<0.05). Notably, fractions 6 (F6) and 7 (F7) exhibited an even greater reduction in cell viability compared to BRAE (BRAE: 74%, F6: 80%, F7: 98%, reduction; p<0.05). Subsequently, a second separation was conducted on F7 using the same chromatographic conditions. This led to the isolation of an abietane diterpene identified by 1D and 2D NMR experiments as 7α,12β,17-triacetoxy-6β,19-dihydroxy-13β,16-spirocicloabi- et-8-ene-11,14-dione. This diterpenoid showed a significant reduc- tion in MCF-7 cell viability (25 μg/mL: 96%, 10 μg/mL: 26% reduc- tion; p<0.05). Therefore, this metabolite could be responsible, at least in part, for the antiproliferative activity of C. neochilus. To our knowledge, this compound has been isolated for the first time from C. neochilus and this is the first report of its antiproliferative effect on breast cancer cells.