Isolation, characterization, and biological evaluation of a potent anti-malarial drimane sesquiterpene from Warburgia salutaris stem bark

Background: Malaria continues to be a global burden as the efficacy of most commercial anti-malarial drugs has been compromised by the evolution of parasite resistance. With the urgent need created for the development of alternative and more efficient anti-malarial drugs, this study focused on the evaluation of anti-malarial agents of the Warburgia salutaris stem bark. Methods: The stem bark was extracted with dichloromethane followed by silica gel column chromatography that led to the isolation of iso-mukaadial acetate, a drimanoid sesquiterpene. This compound was characterized by spectroscopic analysis (¹H NMR, ¹³C NMR, IR and MS), and its structure was confirmed by X-ray crystallography. In vitro anti-plasmodial activity was investigated using a chloroquine sensitive NF54 Plasmodium falciparum strain. Cytotoxicity was measured using the MTT assay on HEK239 and HEPG2 cell lines. Chloroquine-sensitive Plasmodium berghei was used to infect Sprague-Dawley rats for in vivo studies. The W. salutaris crude extract and iso-mukaadial acetate were administered orally at 0.5; 1.5, 2.5 and 5 mg/kg, chloroquine was used as the reference drug. Determination of percentage parasitaemia, haematological parameters, and rat body weights was done throughout the experimental study period. Results: The crude extract and iso-mukaadial acetate showed very good activity on the inhibition of parasite growth (IC₅₀ 0.01 ± 0.30 μg/ml) and (IC₅₀ 0.44 ± 0.10 μg/ml), respectively, with iso-mukaadial acetate having cytotoxicity activity of 36.7 ± 0.8 and 119.2 ± 8.8 (μg/ml) on HEK293 and HEPG2 cells, respectively. The crude extract and iso-mukaadial acetate reduced percentage parasitaemia in a dose-dependent manner in comparison to the control. There were no significant differences in the haematological parameters in all the experimental groups in comparison to control group. This study proves that W. salutaris contains components (including iso-mukaadial acetate) that exhibit anti-malarial activity. This study scientifically validates the use of this plant in folk medicine. Conclusions: This study proves that Warburgia salutaris contains components (including iso-mukaadial acetate) that exhibit anti-malarial activity and scientifically validates the use of this plant in folk medicine.