In vitro and in vivo activity of cynaropicrin against Trypanosoma brucei rhodesiense

As an outcome of a recent screen for new antiparasitic leads, cynaropicrine was identified as a potent inhibitor of Trypanosoma brucei rhodesiense [1]. This guajanolide sesquiterpene lactone found in Centaurea and Cynara species was more active against T. b. rhodesiense in vitro (IC50: 0.3µM) than against Plasmodium falciparum (IC50: 3.0µM) and Trypanosoma cruzi (IC50: 4.4µM). In vivo it decreased T. b. rhodesiense parasitaemia by 98% after 4 days when 10mg/kg/d when administered intraperitoneally. The mice had a 100% survival rate after 14 days (control: 0%). Preliminary structure activity studies with natural and semisynthetic derivatives showed the necessity of the 2-(hydroxymethyl), 2-propenoic acid side chain, for the preferential toxicity towards T. b. rhodesiense. The interaction of cynaropicrine with trypanothione -a trypanosomatid specific glutathione spermidine conjugate essential for parasite redox metabolism- was studied. Under physiological conditions trypanothione spontaneously forms stable bisadducts with cynaripicrin via a Michael addition of the thiols to the exocyclic double bonds at C-13 and C-3′, as was shown in NMR experiments. This adduct formation was also studied in STIB 900 strain and in NY-at1 (TbMRPA – efflux pump of trypanothione conjugates overexpressing) T. b. rodesiense strains. The antitrypanosomal activity of cynaropicrin shown in vivo and in vitro may be due to interaction with trypanothione metabolism. References: 1. Adams, M. et al. (2009) Nat. Prod. Commun. 9: 1377–1381.