Anti-proliferative withanolides from the Solanaceae: a structure-activity study

As part of our search for bioactive compounds from plant biodiversity, 29 withanolides (1, 3-6, 9, 12-18, and 20-35) were recently isolated from three members of the Solanaceae: Physalis longifolia, Vassobia breviflora, and Withania somnifera. Six derivatives (2, 7, 8, 10, 11, and 19) were prepared from these naturally occurring withanolides. All compounds (1-35) were evaluated for in vitro anti-proliferative activity against an array of cell lines [melanoma cell lines (B16F10, SKMEL28); human head and neck squamous cell carcinomas (HNSCC) cell lines (JMAR, MDA1986, DR081-1); breast cancer cell line (Hs578T), and non-malignant human cell line (MRC5)]. This led to the discovery of 15 withanolides, with IC50 values in the range of 0.067-17.4 µM, including withaferin A 1, withaferin A 4,27-diacetate 2, 27-O-glucopyranosylwithaferin A 3, withalongolide H 4, withalongolide C 5, withalongolide A 6, withalongolide A 4,27-diacetate 7, withalongolide A 4,19,27-triacetate 8, withalongolide B 9, withalongolide B 4-acetate 10, withalongolide B 4,19-diacetate 11, withalongolide D 16, withalongolide E 17, withalongolide G 21, and 2,3-dihydrowithaferin A 3-O-sulfate 22). In order to update the growing literature on withanolides and their activities, we summarized the distribution, structural types and anti-proliferative activities for all published withanolides to date. The structure-activity relationship analysis (SARA) confirmed the importance of the presence of a Δ²-1-oxo- functionality in ring A, a 5β,6β-epoxy or 5α-chloro-6β-hydroxy groupings in ring B, and nine carbon side chain with a lactone moiety for cytotoxic activity. Conversely, the SARA indicated that the -OH or -OR groups at C-4, 7, 11, 12, 14, 15, 16, 17, 18, 19, 20, 23, 24, 27, 28 were not contributors to the observed anti-proliferative activity within the systems analyzed.