Study of antiplasmodial activity, cytotoxicity and acute toxicity of Zanthoxylum chalybeum ENGL, and Vernonia lasiopus o. Hoffman

Abstract

Malaria continues to cause heavy morbidity and mortality and it is the fifth leading cause of death globally. The disease causes over one million deaths annually and affects many more, particularly due to increasing multi-drug resistant strains of Plasmodium falciparum. Sustained investigations in both curative and prophylactic interventions have supported the ethno-pharmacological approach to identify novel compounds as a major channel towards achieving a solution. The medicinal plants Zanthoxylum chalybeum and Vernonia lasiopus are traditionally used for the treatment/prophylaxis of malaria by some of the local communities in Kenya. The aim of this study was to evaluate antiplasmodial activity and safety of the two selected plants. Their activity on combination was also investigated. Zanthoxylum chalybeum leaves, stem bark, root bark and Vernonia lasiopus aerial parts and roots were collected and extracted with methanol: dichloromethane (1:1) solvent mixture. The resulting crude extracts were each fractionated into six fractions by vacuum liquid chromatography using solvents of different polarities, and investigated for antiplasmodial activity (IC50) using the chloroquine (CQ) sensitive D6 and chloroquine (CQ) resistant W2 laboratory adapted Plasmodium falciparum strains. The safety profile was done by determining the in vitro cytotoxicity (CC50) on Vero 199 cells using the MTT based colorimetric assay and in vivo median lethal dose (LD50) in mice. The selectivity index (CC50/IC50) was used as an indicator of their antiplasmodial viability. The results showed that Z. chalybeum roots fraction 6 and Z. chalybeum stem bark fraction 6 are highly active and active respectively with IC50 values of 0.78 0µg/ml and 6.0µg/ml against the P. falciparum D6 strain. They had selectivity indices of >128.21and >16.67 respectively, and LD50 values of >5000mg/kg indicating safety. V. lasiopus crude root extract showed the most promise at IC50 of 13.1 µg/ml against D6 strain, with selectivity index > 7.63 and LD50 >5000mg/kg. Extracts of both Z. chalybeum and V. lasiopus were found to have antiplasmodial activity and favourable safety profile with the highly active Z. chalybeum roots fraction 6 being the most viable sample. Further chemical analysis is recommended for this fraction and the active samples, and they will hopefully be used as candidates for development as anti-malarial medicine. The crude roots extracts of the plants showed activity (IC50) of 1.11 µg/ml and 13.1 µg/ml respectively and this supports their use traditionally. Some samples classified as active also had good safety margin and may also be explored further. Detailed combination studies of the various samples may be carried out to as they may bear different pharmacologic properties whose interactions could result in beneficial outcomes such as potentiated activity and curtailing resistance. Key words: Plasmodium falciparum, Z. chalybeum, V. lasiopus, fractions, selectivity index, LD50.