A study of the antimalarial activity of combined plant extracts from medicinal plants used traditionally in Kenya.

Abstract

Introduction Medicinal plants have been in use in the management of disease and maintenance of health by societies for thousands of years. Emerging resistance to presently utilized single molecule drugs has heightened the need to research into plant sources of antimalarial drugs. Study objectives The study aimed to explore the antiplasmodial activity of combined plant extracts from six commonly used medicinal plants in Kenya and examine the safety profile of those with the best efficacy. Study methodology In vitro tests were carried out against chloroquine resistant P. falciparum W2 strain using the [3H] hypoxanthine uptake assay, while in vivo antiplasmodial and acute toxicity tests were carried out using Plasmodium berghei on mice. Cytotoxicity tests were done using the MTT based colorimetric assay on Vero 199 cells and selectivity index was used to assess safety of the extracts. Results and discussion Activity criteria in the in vitro assay for unpurified extracts were classified as follows: high at IC50 ≤10 μg/ml, moderate at 10–50 μg/ml, low at 50–10 μg/ml and inactive at >100 μg/ml. Schkuhria pinnata had the best activity of IC50 10.81 μg/ml in vitro of all the extracts tested. Extracts from Flueggea virosa, Boscia angistifolia and Zanthoxylum chalybeum had moderate activity of 19.03, 20.26 and 37.69 μg/ml respectively. Toddalia asiatica extract had low activity having IC50 above 50 μg/ml whereas Carissa edulis was found to be inactive having an IC50 above 100 μg/ml. The combinations of extracts involving Z. chalybeum showed the most activity with IC50’s below 10 μg/ml. Antagonism was observed with combinations such as F. virosa with S. pinnata, S. pinnata with B. angustifolia, B. angustifolia with T. asiatica, C. edulis with B. angustifolia and lastly, C. edulis with T. asiatica. xii

The combination of S. pinnata and T. asiatica extracts exhibited the best activity on in vivo tests having a parasite growth suppression of 81.3% at a dose of 500 mg/kg, which was comparable to 87.2% of Artemether at a dose of 10 mg/kg body weight as the positive control. Singly, the two plant extracts showed good activity 65.2 and 58.9% at doses of 250 mg/kg and 500 mg/kg respectively. On cytotoxicity testing, S. pinnata and T.asiatica had CC50 of 102.93±5.96 μg/ml and>1000 μg/ml respectively and selectivity indices of 9.52 and 15.24 respectively indicating good therapeutic potential due to their safety margins. Conclusion and recommendation The results obtained from both in vitro and in vivo tests support the use of the plants selected in traditional treatment of malaria. A combination of methanol fruit extract of T. asiatica and methanol whole plant extract of S. pinnata showed promising parasite growth suppression against P. berghei that was comparable with Artemether and was synergistic on in vitro testing against chloroquine- resistant W2 strain of P. falciparum. The combination of S. pinnata and T.asiatica extracts can be considered for formulation into a polyherbal antimalarial drug but would require further investigation such as effective dosage, chronic toxicity and phytochemical screening to determine the compounds responsible for activity.