Piliostigma thonningii (Schumach.) Milne-Redhead. (Leguminosae) is used for various medicinal purposes in African countries. Phytochemical investigation of P. thonningii yielded two compounds newly isolated from natural sources, 2β-methoxyclovan-9α-ol (1), and methyl-ent-3β-hydroxylabd-8(17)-en-15-oate (2), along with 14 known compounds (3-16). Compounds 1 and 4 (alepterolic acid) showed potential selectivity towards Trypanosoma brucei brucei with IC50 7.89 and 3.42 μM, respectively. Compound 2 showed activity towards T. brucei and Leishmania donovani Amastigote with IC50 3.84 and 7.82 μM, respectively. The structure activity relationship (SAR) of the isolated metabolites suggested that hydroxylation at C-2 enhances the antiprotozoal activity towards T. brucei in sesquiterpenes 1 and 3. Similarly hydroxylation at C-3 in labdane diterpenes elevates the antiprotozoal activity towards T. brucei.
Keywords: Leishmania donovani; Piliostigma thonningii; Sesquiterpene; Trypanosoma brucei; diterpene.