Lapachol and isomeric 5- and 8-hydroxy-2-(1′-hydroxyethyl)naphtho[2,3-b] furan-4,9-diones are effective antileishmanial constituents of Tabebuia avellanedae
The antileishmanial activity of a methanolic extract of Tabebuia avellanedae (Bignoniaceae) and some constituents against transgenic Leishmania major expressing green fluorescent protein (GFP) was examined using parasite retrieval assay and flow cytometry (FACS analysis). Leishmaniasis is a group of diseases caused by flagellate protozoan Leishmania spp. causing high morbidity and mortality in tropical to Mediterranean environments.
The methanol extract was sequentially extracted with n-hexane, dichloromethane, ethyl acetate, and n-butanol, yielding finally water-soluble extractives. The highest antileishmanial activity resided in the n-hexane and dichloromethane fractions with IC50 of 64µg/ml and 41µg/ml, respectively. Phytochemical analysis of the n-hexane fraction revealed the presence of lapachol which exhibited antileishmanial activity against both extra- and intracellular parasites, with IC50 values of 33µM and 115µM, respectively. Lapachol did not show any cytotoxicity on macrophages as host cells of Leishmania parasites (EC50 >300µM). From the dichloromethane fraction an inseparable mixture of isomeric 5- and 8-hydroxy-2-(1′-hydroxyethyl)naphtho[2,3-b] furan-4,9-diones was isolated by a combination of column chromatography and semi-preparative HPLC. This mixture was significantly more effective against both extra- and intracellular L. major parasites (IC50 of 4µM, ca. 28 fold more active than that of lapachol), while cytotoxic effects on macrophages were not evident at 8µM. The IC50 of amphotericin B, a reference compound, was 2.5µM and 0.2µM against promastigotes and amastigotes, respectively. It appears as if the presence of a furan ring increases the antileishmanial activity of naphthoquinones.