Two new 19-hydroxy bufadienolides with cytotoxic activity from the skins of Bufo melanosticus

Two new 19-hydroxy bufadienolides, named bufohydrolide A (1) and bufohydrolide B (2), and four known analogs (3-6) were isolated from the aqueous extracts of the skins of Bufo melanosticus. Their structures were established by spectral data analyses, such as UV, IR, 1 D/2D NMR and mass spectra. Compounds 1-6 were evaluated for their cytotoxic activity against SMMC-7721, HT-29 and A549 cells. Noteworthily, all six isolates exhibited various levels of anti-proliferative activities with IC50 values ranging from 0.02 +/- 0.0002 to 25 +/- 0.5 mu M.