Investigating ethnopharmacology-based natural product leads for antimalarial drug discovery

Abstract

A collaborative study between the University of Salford and the National Institute of Pharmaceutical Research and Development (NIPRD), Nigeria, involving in vitro tests at the University of Salford and the use of facilities at NIPRD for invivo tests has led to the evaluation of a range of traditional “fever cures” for antimalarial efficacy. The main objective of this study is to identify a suitable antimalarial plant candidate towards the development of a phytopharmaceutical drug (a plant-based medicinal mixture/compound used in preventive or therapeutic medicine) for the treatment of malaria as a cheaper and more accessible alternative, inline with the WHO resolution (WHA 3049) urging nations to use their traditional systems of medicine as part of their healthcare systems. Traditionally, the mainstay of the antimalarial drug discovery process has been natural products. Their use however diminished over the past few decades due to several advances in molecular targets and technical difficulties encountered in high-thoughput screens of natural product leads. Natural products have played a very important role as a source of antimalarials (e.g. quinine and artemisinin derivatives). In this present study, fluorescent-based in vitro antimalarial assays including flow cytometry (FCM) and SYBR green microtitre assay (SG) were optimized to screen some aqueous plant extracts which were selected based on their ethnopharmacological usage. Giemsa light microscopy was used to validate the assays. Plasmodium berghei malaria animal model was also used to evaluate the anti-plasmodial activity of extracts in vivo. The results showed a strong antimalarial activity in all the six extracts. Bryocarpus coccineus and Bridelia ferruginea were chosen for further investigation due to their efficacy and the collaborative nature of the study. The IC50 values obtained in the in vitro antimalarial studies in the region of 70 µg/ml and 15 µg/ml. Bridelia ferruginea aqueous and methanolic extract was compared to determine any differences in IC50. In vitro comparison of the aqueous and methanolic extracts of the extract revealed an IC50 value in the region of 25.69 µg/ml for the aqueous extract and 15-16 µg/ml for the methanolic extract. Qualitative phytochemical screening of both extracts revealed the presence of various bioactive compounds including tannins, flavonoids, saponins and cardiac glycosides amongst others in both the aqueous and methanolic extract of B. ferruginea. Anthocyanins were found present in the methanolic extract only. Further investigation of the mechanism of action of the methanolic B. ferruginea extract showed that the extract inhibited β-haematin formation, indicating the inhibition of haemazoin formation in the parasite. Lastly, the methanolic extract was fractionated using HPLC analysis. Various resolved peaks were obtained and subsequent bioassays of the collected fractions revealed that antimalarial activity was distributed across the fractions. This may suggests that isolating a single active compound might not be advantageous, making a case for a phytopharmaceutical drugs