Cancer Preventive Agents. 7. Antitumor-Promoting Effects of Seven Active Flavonolignans from Milk Thistle (Silybum marianum) on Epstein-Barr Virus Activation

Seven pure flavonolignans were isolated from an extract of milk thistle [Silybum marianum (L.) Gaertn (Asteraceae)], by semipreparative reverse-phase HPLC, and identified based on spectroscopic and LC-MS=IT-TOF data. All seven compounds were screened as potential antitumor-promoting agents by using the in vitro short-term 12-O-tetradecanoylphorbol-13-acetate (TPA)induced Epstein-Barr virus early antigen (EBV-EA) activation assay in Raji cells. They showed good inhibitory activity (87.7–94.9%) at 1000mol ratio=TPA. Silychristin A (1) and silychristin B (2) were slightly more potent than the well-known antitumor promoter b-carotene. Silychristins A (1) and B (2) and isosilybins A (6) and B (7) were more active than the clinically proven cancer prevention components silybins A (4) and B (5).