Isolation and characterization of filaricidal compounds from the stem bark of Voacanga africana, a plant used in the traditional treatment of onchocerciasis in Cameroon

Abstract

This study was carried out to isolate active ingredients from Voacanga africana Stapf (Apocynaceae) used as herbal medicine in Cameroon, and also to assess the efficacy of the compounds on Onchocerca ochengi and Loa loa worms. The compounds were isolated using solvent partitioning, column chromatography and fractional crystallization. The O. ochengi worms were isolated from cow skin while Loa loa was isolated from humans. Filaricidal activity was determined based on motility for adult male worms and microfilariae (Mfs), while adult female worm viability was assessed biochemically by the dimethylthiazol (MTT)/formazan assay. Cytotoxicity was assessed using monkey kidney epithelial cells. Auranofin was used as the positive control drug. Two compounds, voacangine (compound 1) and voacamine (compound 2) were isolated from the stem bark of Voacanga africana. Both compounds were found to inhibit the motility of both the microfilariae (Mfs) and adult male worms of O. ochengi in a concentration-dependent manner, but were only moderately active on the adult female worms upon biochemical assessment at 30 µM drug concentration. The IC50s for voacangine were 5.49 µM for Mfs and 9.07 µM for adult male worms; while for voacamine the values were 2.49 µM for Mfs and 3.45 µM for adult males. At 10 µM, voacamine showed 100% inhibition of Loa loa Mfs motility after 24 h. This is the first report of the anti-Onchocerca activity of voacangine (compound 1) and voacamine (compound 2) as well as activity of voacamine (compound 2) on L. loa. The results of this study support the traditional use of V. africana in the treatment of human onchocerciasis.   Key words: Voacanga africana, voacangine, voacamine, anti-Onchocerca activity.