Antiplasmodial and antigonococcal bioactivity of Tabernaemontana elegans stapf. (Apocynaceae) stem bark fractions

Abstract

Ethnopharmacological relevance: The Vhavenda use Tabernaemontana elegans Stapf. (Apocynaceae) to treat malaria and gonorrhoea.

Materials and methods: The methanol crude extract of the stem bark was subjected to column chromatographic fractionation using a series of solvents with increasing polarity; hexane: dichloromethane, ethyl acetate: methanol, and distilled water. Thin layer chromatography was used to analyze fractions, and those with similar chemical profiles were combined to yield 11 fractions (A – K). Fraction F, G, I, and J was further subjected to column chromatography and fractionated with DCM, DCM and MeOH, and MeOH. Analysis using TLC was applied and fractions with comparable chemical profiles were pooled together to yield 27 fractions in total. The fractions were assayed for in vitro antiplasmodial and antigonococcal activities against Plasmodium falciparum NF54 and Neisseria gonorrhoeae ATCC 19424. Cytotoxicity was carried out on rat skeletal muscle cell lines L6, and selectivity indices were determined. The fractions were then subjected to 1H NMR and GC-MS analyses to tentatively identify classes of compounds and phytoconstituents for the observed antiplasmodial and antigonococcal biological activities and to dereplicate known antiplasmodial compounds. Metabolomics using GC-MS was employed to discriminate between selective and non-selective antiplasmodial fractions.

Results: Of the 27 assayed fractions, J4 (IC50 = 0.20 µg/ml, SI = 118) and J5 (IC50 = 0.10 µg/ml, SI = 165) had significant antiplasmodial activity. Compared to the methanol crude extract of the stem bark of T. elegans from which they were fractionated, J4 and J5 demonstrated remarkably higher antiplasmodial efficacy against P. falciparum. Fractions I1 and I4 had noteworthy antigonococcal activity with a minimum inhibitory concentration of 0.78 mg/ml. The two fractions showed reduced efficacy against N. gonorrhoeae when compared to the methanol crude extract from which they were fractionated. Analysis of the 1H NMR spectra of fractions J4 – J7 identified to reflect in the 0.5 – 1 ppm region. This section is represented by the class of compounds known as aliphatic hydrocarbons. The GC-MS data for fractions J4 – J7 revealed the presence of dodecane, pentadecane, and heptadecane phytoconstituents. All three compounds are alkanes (aliphatic hydrocarbon constituents). These constituents are alkanes with established antiplasmodial activity which can be tentatively attributed to the bioactivity observed in the stem bark fractions of T. elegans.

Conclusion: The fractions of the methanol crude extract of the stem bark of T. elegans are great candidates for malarial and gonococcal drugs as shown in this study.