An approved preparation of Silybum marianum contains constituents with activity against Plasmodium falciparum

Milk thistle (Silybum marianum (L.) Gaertn. (Asteraceae)) is a well-established medicinal plant with a long tradition and its pharmacological activities have been studied extensively. It is used as treatment and preventive agent against hepatic disorders/toxic liver damage.

“Protozoan” infections such as Malaria affect many millions of people worldwide, and constitute a global public health problem. Research to develop new efficient anti-protozoal leads is urgently required. One of our approaches in this direction is to search for potentially active constituents in legally approved and marketed herbal medicines [1]. In the present study, the antiparasitic activity of an approved and marketed preparation from milk thistle fruits (Silybi mariani fructus Ph. Eur.) was tested in vitro against Leishmania donovani, Tryopanosoma cruzi, Trypanosoma brucei rhodesiense and Plasmodium falciparum (Pf). It showed promising antiplasmodial activity with IC₅₀= 2 µg/mL and a good selectivity index (SI) 27.3 [1]. Because of the favourable biological data and the wide therapeutic window of milk thistle we considered it as a promising anti-plasmodial hit. The extract was characterized by UHPLC/ESI-QqTOF MS and fractionated by preparative-HPLC, affording 12 fractions. Among them, 4 pure constituents, namely, silybin A, silybin B, silychristin and silydianin as well as a mixture of isosilybins A and B were isolated and identified by comparison with literature data [2]. The activity of each compound and fraction was assayed in vitro against the NF54 strain of Pf. The IC₅₀ values of the isolated compounds were significantly higher than those of the total extract (IC₅₀ from 19.5 to > 50 µg/mL). The most active fraction, F11, showed IC₅₀ = 0.45 ± 0.02 µg/mL. The LC/MS characterization of the active fraction confirmed the presence of various compounds yet to be identified. Its high activity may be attributable to a yet unidentified constituent. Identification of the active principle is in progress.

Acknowledgements: We gratefully acknowledge the financial support by Apothekerstiftung Westfalen-Lippe. NLM is grateful for a fellowship from Heinrich-Böll-Stiftung. We thank Dr J. Sendker, IPBP, University of Münster, for performing UHPLC-MS analysis. This work is an activity within the Research Network Natural Products against Neglected Diseases (ResNet NPND; http://www.resnetnpnd.org/).

Keywords: Silybum marianum, neglected tropical diseases, Plasmodium falciparum.

References:

[1] Llurba-Montesino N, Kaiser M, Brun R, Schmidt TJ. Search for Antiprotozoal Activity in Herbal Medicinal Preparations; New Natural Leads against Neglected Tropical Diseases. Molecules 2015; 20: 14118 – 14138

[2] Kim NC, Graf TN, Sparacino CM, Wani MC, Wall ME. Complete isolation and characterization of silybins and isosilybins from milk thistle (Silybum marianum). Org Biomol Chem 2003; 1: 1684 – 1689