The Antiprotozoal Activity of Sixteen Asteraceae Species Native to Sudan and Bioactivity-Guided Isolation of Xanthanolides from Xanthium brasilicum [*]

Abstract

In vitro screening of the dichloromethane extracts of 16 Asteraceae species native to Sudan for activity against major protozoan pathogens revealed that a Xanthium brasilicum Vell. [syn. X. strumarium var. brasilicum (Vell.) Baker in Mart.] extract was the most active against Trypanosoma brucei rhodesiense, the etiological agent of East African human trypanosomiasis (IC₅₀ = 0.1 µg/mL). This plant extract also exhibited noticeable activities against T. cruzi (Chagas disease), Leishmania donovani (Kala-Azar) as well as Plasmodium falciparum (Malaria tropica). Bioactivity-guided fractionation resulted in the isolation of four bioactive sesquiterpene lactones (STL) of the xanthanolide series (4,5-seco-guaianolide-type). They were identified by spectroscopic means as 8-epixanthatin (1), 8-epixanthatin 1β,5β-epoxide (2), and as the dimers pungiolide A (4) as well as pungiolide B (5). Two further modified xanthanolide sesquiterpene lactones, xanthipungolide (3) and 4,15-dinor-1,11(13)-xanthadiene-3,5β:12,8β-diolide (6) were isolated. While xanthipungolide turned out to be inactive against the tested parasites, the dinor-xanthanlide showed significant activity against T. brucei rhodesiense and L. donovani. All isolated compounds were previously known from other Xanthium species but this is the first report on their occurrence in X. brasilicum, and, most notably, on their antiprotozoal activity. As the most active single compound from this extract, 8-epixanthatin 1β,5β-epoxide showed IC₅₀ values of 0.09, 2.95, 0.16 and 1.71 µg/mL (0.33, 11.3, 0.6 and 6.5 µM) against T. brucei rhodesiense, T. cruzi, L. donovani and P. falciparum, respectively, while its cytotoxicity against rat myoblast cells used as control was determined at 5.8 µg/mL (22.1 µM). Besides assessment of their antiprotozoal activity, the structural assignments for the dimeric xanthanolides pungiolide A and B were reinvestigated and fully established.

sesquiterpene lactones - xanthanolides - Xanthium brasilicum Vell. - Asteraceae - Trypanosoma - Leishmania - Plasmodium

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1 This study is part of the doctoral thesis of A. M. M. Nour (University of Münster, 2007). Parts of the results were presented in abstract form: Nour AMM, Khalid SA, Abdallah WE, Kaiser M, Brun R, Schmidt TJ. Planta Med 2006; 72: 1004–1005