Bioactive α,β-conjugated 3-keto-steroids from the Australian brown alga Cystophora xiphocarpa

As part of our ongoing study of the specialised metabolites present in brown algae belonging to the Cystophora genus, eight new steroids including three pairs of diastereoisomers were isolated from Cystophora xiphocarpa (Harvey) (Sargassacea, Fucales). The metabolites identified by standard spectrometric methods are (16S,22S)-16,22-dihydroxyergosta-4,24(28)-dien-3-one and (16S,22R)-16,22-dihydroxyergosta-4,24(28)-dien-3-one, (16S,22S,24R)-16,22,24-trihydroxyporifera-4,28-dien-3-one and (16S,22S,24S)-16,22,24-trihydroxystigma-4,28-dien-3-one along with (16S,22S,24E)-16,22-dihydroxystigma-4,24(28)-dien-3-one and (16S,20S)-16,20-dihydroxyergosta-4,24(28)-dien-3-one. (16S,22S,24E)-16,22-Dihydroxystigma-4,24(28)-dien-3-one possessed the most potent cytotoxicity of the steroids in this series with cell growth inhibitions of GI₅₀ 8.7 ± 0.7 μM against colon cancer HT29, GI50 5.6 ± 0.8 μM against the breast cancer line MCF-7 and GI50 4.5 ± 0.2 μM against the ovarian cancer cell line A2780. (16S,22R)-16,22-dihydroxyergosta-4,24(28)-dien-3-one was found to be active against the ovarian cancer cell line A2780 with a GI₅₀ of 6.2 ± 0.1 μM.