Trypanocidal and antileukaemic effects of the essential oils of Hagenia abyssinica, Leonotis ocymifolia, Moringa stenopetala, and their main individual constituents.

Essential oils from three Ethiopian medicinal plants; Hagenia abyssinica (Rosaceae), Leonotis ocymifolia (Lamiaceae), and Moringa stenopetala (Moringaceae) were investigated for their chemical composition, trypanocidal, and cytotoxic activities. Twenty components were identified from the essential oil of H. abyssinica female flowers, ledol (58.57%) being the principal volatile oil component. Sixty-eight components were identified from the essential oil of L. ocymifolia aerial part, caryophyllene oxide (12.06%) being the major component. The essential oil of M. stenopetala seeds was dominated by isothiocyanates; benzyl isothiocyanate (54.30%) and isobutyl isothiocyanate (16.37%) were the major components. The trypanocidal (Trypanosoma b. brucei) and antileukaemic (HL-60) effects of the three essential oils were studied. The oil of M. stenopetala seeds and its main compound, benzyl isothiocyanate showed the most potent trypanocidal activities with IC(50) values of 5.03 μg/ml and 1.20 μg/ml, respectively. The oils of H. abyssinica and L. ocymifolia exhibited trypanocidal activities with IC(50) values of 42.30 μg/ml and 15.41 μg/ml, respectively. Individual components (28 compounds) of the essential oils bearing different functional groups were also studied for their structure-activity relationships using trypanosomes and human leukaemia cells. Cinnamaldehyde (IC(50)=2.93 μg/ml) (a representative for aldehydes), nerolidol (IC(50)=15.78 μg/ml) (an alcohol), cedrene (IC(50)=4.07 μg/ml) (a hydrocarbon), benzyl isothiocyanate (IC(50)=1.20 μg/ml) (a representative for mustard oils), 1,8-cineole (IC(50)=83.15 μg/ml) (an ether), safrole (IC(50)=18.40 μg/ml) (aromatics with allyl and/or methoxy side chains), carvone (IC(50)=12.94μg/ml) (a ketone), styrene oxide (IC(50)=3.76 μg/ml) (an epoxide) and carvacrol (IC(50)=11.25 μg/ml) (a phenol) showed the most potent trypanocidal activities from their respective groups. Of all essential oil components tested, carvone (selectivity index (SI)=17.46) and styrene oxide (SI=19.92) showed good selective indices for the parasite with minimal toxicity on the human leukaemia cells. These compounds could therefore serve as lead structures for the development of trypanocidal agents with higher potency.