Silymarin is a traditional drug and food supplement employed for numerous liver disorders. The available studies indicate that its activities may be broader, in particular due to claimed benefits in some cardiovascular diseases, but the contributions of individual silymarin components are unclear. Therefore, we tested silymarin flavonolignans as pure diastereomers as well as their sulfated metabolites for potential vasorelaxant and antiplatelet effects in isolated rat aorta and in human blood, respectively. Eleven compounds from a panel of 17 tested exhibited a vasorelaxant effect, with half maximal effective concentrations (EC50) ranging from 20 to 100 µM, and some substances retained certain activity even in the range of hundreds of nM. Stereomers A were generally more potent as vasorelaxants than stereomers B. Interestingly, the most active compound was a metabolite-silychristin-19-O-sulfate. Although initial experiments showed that silybin, 2,3-dehydrosilybin, and 2,3-dehydrosilychristin were able to substantially block platelet aggregation, their effects were rapidly abolished with decreasing concentration, and were negligible at concentrations ≤100 µM. In conclusion, metabolites of silymarin flavonolignans seem to have biologically relevant vasodilatory properties, but the effect of silymarin components on platelets is low or negligible.

Department of Pharmaceutical Botany Faculty of Pharmacy in Hradec Králové Charles University Heyrovského 1203 500 05 Hradec Králové Czech Republic

Department of Pharmacology and Toxicology Faculty of Pharmacy in Hradec Králové Charles University Heyrovského 1203 500 05 Hradec Králové Czech Republic

Department of Pharmacology Toxicology and Therapeutic Chemistry School of Pharmacy and Food Science University of Barcelona Avda Joan XXII 27 31 08028 Barcelona Spain

Institute of Microbiology of the Czech Academy of Sciences Vídeňská 1083 142 20 Prague Czech Republic

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Chirality Matters: Biological Activity of Optically Pure Silybin and Its Congeners