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.

Key words

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

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

References

• 1 World Health Organization .10 Facts on neglected tropical diseases web page. Available at http://www.who.int/features/factfiles/neglected_tropical_diseases/en/index.html. Accessed December 2007
• 2 Greenwood B M, Fidock D A, Kyle D E, Kappe S H, Alonso P L, Collins F H, Duffy D E. Malaria: progress, perils, and prospects for eradication.  J Clin Invest. 2008;  118 1266-1276
• 3 Yamey G. Public sector must develop drugs for neglected diseases.  BMJ. 2002;  324 698
• 4 Schmidt T J. Structure-activity relationships of sesquiterpene lactones. Atta-ur-Rahman Studies in natural products chemistry, Vol. 33. Amsterdam; Elsevier 2006: 309-392
• 5 Schmidt T J, Willuhn G, Brun R, Khalid S A. Antitrypanosomal activity of helenalin and some related sesquiterpene lactones.  Planta Med. 2002;  68 750-751
• 6 Kummer D A, Brennemann J B, Martin S F. Domino intramolecular enyne metathesis/cross metathesis approach to the xanthanolides. Enantioselective synthesis of (+)-8-epi-xanthatin.  Tetrahedron. 2006;  62 11437-11449
• 7 Yokotani-Tomita K, Kato J, Kosemura S, Yamamura S, Kushima M, Kakuta H, Hasegawa K. Light-induced auxin-inhibiting substance from sunflower seedlings.  Phytochemistry. 1997;  46 503-506
• 8 Bohlmann F, Singh P, Joshi K C, Singh C L. Xanthanolides from Xanthium indicum.  Phytochemistry. 1982;  21 1441-1443
• 9 Lanzetta R, Lama G, Mauriello G, Parrilli M, Racioppi R, Sodano G. Ichthyotoxic sesquiterpenes and xanthanolides from Dittrichia graveolens.  Phytochemistry. 1991;  30 1121-1124
• 10 Ahmed A A, Jakupovic J, Bohlmann F, Regaila H A, Ahmend A M. Sesquiterpene lactones from Xanthium pungens.  Phytochemistry. 1990;  29 2211-2215
• 11 De Riscala E C, Fortuna M A, Catalan C AN, Diaz J G, Herz W. Xanthanolides and a bis-norxanthanolide from Xanthium cavanillesii.  Phytochemistry. 1994;  35 1588-1589
• 12 Matile H, Pink J RL. Plasmodium falciparum malaria parasite cultures and their use in immunology. Lefkovits I, Pernis B Immunological methods. San Diego; Academic Press 1990: 221-234
• 13 Räz B, Iten M, Grether-Bühler Y, Kaminsky R, Brun R. The Alamar Blue assay to determine drug sensitivitiy to African trypanosomes (T. b. rhodesiense and T. b. gambiense) in vitro.  Acta Trop. 1997;  68 139-147
• 14 Buckner F S, Verlinde C L, La Flamme A C, Van Voorhis W C. Efficient technique for screening drugs for activity against Trypanosoma cruzi using parasites expressing beta-galactosidase.  Antimicrob Agents Chemother. 1996;  40 2592-2597
• 15 Cunningham I. New culture medium for maintenance of tsetse tissues and growth of trypanosomatids.  J Protozool. 1977;  23 325-329
• 16 Kim Y S, Kim J S, Park S-H, Choi S-U, Lee C O, Kim S-K, Kim Y-K, Kim S-H, Ryu S-Y. Two cytotoxic sesquiterpene lactones from the leaves of Xanthium strumarium and their in vitro activity on farnesyltransferase.  Planta Med. 2003;  69 375-377

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