Cytotoxic Bufadienolides from the Leaves of Melianthus major

Melianthus major is a medicinal plant endemic to South Africa. Its leaf extract led to the isolation of five new bufadienolides, 2β-acetoxy-3,5-di-O-acetylhellebrigenin (1), 2β-acetoxy-3-O-acetylhellebrigenin (2), 2β-acetoxy-14-deoxy-15β,16β-epoxymelianthugenin (4), 2β-acetoxy-14-deoxy-15β,16β-epoxymelianthusigenin (5), and 2β-hydroxymelianthusigenin (6), and four known analogues. The structures of the compounds were elucidated using NMR and HRESIMS data analyses. The relative configurations were defined by single-crystal X-ray crystallography and NOESY correlations. The isolated compounds exhibited strong cytotoxicity against MCF-7 breast cancer cells and sensitive CCRF-CEM and multidrug-resistant CEM/ADR5000 leukemia cells. Compound 1 showed the most potent activity, with IC₅₀ values of 0.1 μM toward CCRF-CEM and CEM/ADR5000 and 0.3 μM toward MCF-7.