Antileishmanial Activity of Hydrolyzable Tannins and their Modulatory Effects on Nitric Oxide and Tumour Necrosis Factor-α Release in Macrophages in Vitro

Abstract

A series of 27 hydrolyzable tannins and related compounds was tested for antiparasitic effects against both extracellular promastigote and intracellular amastigote Leishmania donovani organisms. In parallel, the compounds were evaluated for their immunomodulatory effects on macrophage functions, including release of nitric oxide (NO), tumour necrosis factor-α (TNF-α) and interferon (IFN)-like properties using several functional assays. Of the series of polyphenols tested, only gallic acid (54 μM NO) and its methyl ester (32 μM NO) induced murine macrophage-like RAW 264.7 cells to release NO in appreciable amounts (IFN-γ/LPS 119 μM NO). The in vitro TNF-inducing potential of the polyphenols examined increased in the order of oligomeric ellagitannins (EC₅₀ > 25 μg/ml) < monomeric ellagitannins, gallotannins (EC₅₀ 8.5 to > 25 μg/ml) < C-glucosidic ellagitannins, dehydroellagitannins (EC₅₀ 0.6 - 2.8 μg/ml) at the host cell subtoxic concentration of 50 μg/ml. Furthermore, promastigotes of Leishmania donovani were assayed in the presence of these polyphenols and the results showed that none of the compounds was significantly toxic (EC₅₀ > 25 μg/ml) to the extracellular forms. In contrast, all polyphenols showed pronounced antileishmanial activities (EC₅₀ < 0.4 - 12.5 versus 7.9 μg/ml for Pentostam®) against intracellular amastigotes of L. donovani residing within RAW cells. Noteworthy, most compounds exhibited low cytotoxicity against the murine host cells (EC₅₀ >25 μg/ml). Furthermore, some ellagitannins and the majority of dehydroellagitannins induced potent interferon-like activities as reflected by inhibition of the cytopathic effect of encephalomyocarditis virus on fibroblast L929 cells. This is the first report on hydrolyzable tannins as a new class of natural products with leishmanicidal activity including their potential for inducing the release of NO, TNF and IFN-like activity in macrophage-like RAW cells.

Key words

Hydrolyzable tannins - antileishmanial activity - Leishmania donovani - nitric oxide - tumour necrosis factor - interferon

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