| In order to identify anti-proliferative compounds from selected Latin American plants, approximately 200 plant extracts were screened by the MTS assay (3-[4,5-dimethylthiazol-2yl]-5-[3-carboxymethoxyphenyl]-2-[4-sulfophenyl]-2 H tetrazolium). Vassobia breviflora (Solanaceae), was identified as the most active of the species tested in this study. Following a bioassay-guided approach, withaferin A was isolated and characterized. This withanolide-type steroidal lactone showed anti-proliferative activities with IC50 values from 0.5 to 2.2 muM against head and neck squamous cell carcinoma (HNSCC) MDA1986, JMAR, UM-SCC-2 and JHU011. A mechanistic study showed that withaferin A induced apoptosis and cell death in HNSCC cells as well as a shift from G0/G1 to G2/M arrest in cell cycle studies. Western data demonstrated that the anti-proliferative action of withaferin A could be in part explained through degradation of total Akt levels as well as decrease in activation of Akt levels. In order to establish anticancer structure activity relationships, three analogues were semi-synthesized from withaferin A, and ten related withanolides, including a new chlorinated withanolide 6&agr;-chloro-5beta,17&agr;-dihydroxywithaferin, were isolated from Withania somnifera (Solanaceae). All structures were elucidated on the basis of spectroscopic methods (IR, MS, and 1D/2D NMR). X-ray crystallography confirmed the absolute configuration of the new withanolide. A method for large-scale isolation of withaferin A from W. somnifera was also developed and presented. |
