Anti-malarial synergy of secondary metabolites from Morinda lucida Benth

The roots, stem and leaves of Morinda lucida are used in some African countries as treatment against different types of fevers including yellow fever, malaria, trypanosomiasis and feverish conditions during child birth.

To determine the in vitro cell toxicity and anti-malarial activity of the extracts of stem bark of M. lucida and to identify the secondary metabolites in the extract that may be responsible for this activity.

The cell toxicity studies of crude extract [dichloromethane (DCM): Methanol (MeOH) in a ratio of1:1 (v/v)] as well as compounds isolated from the same extract were carried out using human cervix adenocarcinoma cells (HeLa cells); while the anti-malarial activities of the same samples were performed against Plasmodium falciparum strain 3D7 using the parasite lactate dehydrogenase (pLDH) assay. The isolation of the active compounds was carried out using chromatographic techniques (column and thin layer chromatography) where as mass spectrometry (MS), Fourier transform infrared spectroscopy (FTIR) as well as 1D- and 2D- nuclear magnetic resonance (NMR) analyses were employed in the characterisation and identification of the isolated secondary metabolites.

The pLDH and cell toxicity assays for the crude extract and the fractions of M. lucida indicated that some fractions reduced the malaria parasite viability by approximately 50% at 100μg/mL and they were not significantly cytotoxic. An IC₅₀ done on the crude extract gave a value of 25μg/mL. The % cell viability for the crude extract in cell toxicity assay remained at 100%. Seven chemical constituents i.e. asperuloside (1), asperulosidic acid (2), stigmasterol (3a), β-sitosterol (3b), cycloartenol (3c), campesterol (3d) and 5,15-O-dimethylmorindol (4) were isolated from the DCM-MeOH extract of stem bark. The isolated compounds tested were not that active by themselves individually at 20μM but their activities were increased when the isolated compounds were combined. As seen when compounds 2, 3 and 4 (% viability: 93, 123 and 101 respectively) were combined yielding an IC₅₀ value of 17μM. Furthermore, this is the first report of compounds 1, 2, 3c, 3d and 4 isolated from M. lucida.

The crude extract completely suppressed the growth of P. falciparum. This indicates that the crude extract contains many compounds that might be acting in synergy. The observed activity of the crude extract and the samples containing a mixture of different compounds support the traditional use of M. lucida for the treatment of malaria.