Chemical composition and amoebicidal activity of Croton pallidulus, Croton ericoides, and Croton isabelli (Euphorbiaceae) essential oils.

Acanthamoeba is a free-living amoebae genus that causes amoebic keratitis which is a painful sight-threatening disease of the eyes. Its treatment is difficult, and the search for new drugs is very important. Here, essential oils obtained from the aerial parts of Croton pallidulus, Croton isabelli, and Croton ericoides (Euphorbiaceae), native plants of Southern Brazil, were tested against Acanthamoeba polyphaga and analyzed by gas chromatography and gas chromatography-mass spectrometry. The essential oils of C. pallidulus and C. isabelli were characterized by the presence of sesquiterpenes: germacrene D (15.5 %), terpinen-4-ol (13.2 %), and β-caryophyllene (13.1 %) in C. pallidulus and bicyclogermacrene (48.9 %) in C. isabelli. The essential oil of C. ericoides presented mainly monoterpenes, β-pinene (39.0 %) being the main component. Laboratory tests were carried out to determine the effect of the essential oils against A. polyphaga trophozoites. The essential oil of C. ericoides was the most active, killing 87 % of trophozoites at the concentration of 0.5 mg/mL. The essential oil of C. pallidulus killed only 29 % of the trophozoites at the same concentration. The essential oil of C. isabelli presented the lowest activity, killing only 4 % of the trophozoites at the concentration of 10 mg/mL. The essential oils of the three species showed cytotoxic effect by the methyl thiazolyl tetrazolium (MTT) method in Vero cells. The oil of C. ericoides, which showed the highest amoebicidal activity, was the most cytotoxic on these mammalian cells.