Article
Novel influenza virus inhibitors from the bark of Burkea africana
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Published: | October 8, 2018 |
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Burkea africana Hook is a flat-topped tree belonging to the family of Leguminosae and widely distributed in the tropical and subtropical African regions south to South Africa. Its bark decoctions are used e.g. for the treatment of colds and coughs. Therefore, bark extracts of B. africana were included in a study aiming to identify novel anti-influenza natural lead compounds.
From the tannin-depleted bark extracts, microfractions and pure compounds thereof were obtained in several chromatographic steps and phytochemically characterized. Their cytotoxicity and anti-influenza A activity were analyzed in Madin Darby canine kidney (MDCK) cells.
The tannin-depleted bark extract of B. africana exerted a promising antiviral activity in cytopathic effect (CPE) inhibition assays with influenza virus A/Hong Kong/68 (HK/68; IC50 of 5.5 µg/mL) in MDCK cells. Eight new triterpene saponins with four so far undescribed aglycone structures were isolated and structurally identified.
Three lupane-type triterpene saponins comprising a disaccharide chain showed only weak or no anti-influenza activity, whereas the presence of a branched trisaccharide moiety conferred anti-influenza virus activity with IC50s of 1.05 µM and 1.89 µM against HK/68 and influenza virus A/Jena/8178/09 (Jena/8178), respectively. Two oleanane-type triterpene saponins comprising a branched trisaccharide moiety and two oleanane-type triterpene saponins with linear trisaccharide residue or branched tetrasaccharide moiety also selectively inhibited the CPE of both influenza viruses in the low micromolar range.
Since after oral intake the sugar moieties of triterpene saponins are most likely removed in the gastrointestinal tract, the aglycones of the eight triterpene saponin were obtained by hydrolysis. Most aglycones showed better compatibility and lower or no antiviral activity compared to their glycosidic counterparts. Intriguingly, the sapogenins of the most potent compounds, namely the so far unknown 21-p-methoxycinnamoyloxy-maslinic acid and 21-p-methoxycinnamoyloxy-oleanolic acid, lost their cytotoxicity in MDCK cells but demonstratedan anti-influenza activity against both tested strains in the range of 7–11 µM.
In conclusion, the study has given access to so far undiscovered triterpene saponins from B. africana bark. The antiviral results might help to explain the rationale behind the traditional use of this herbal remedy in Southern Africa and warrant further studies on the antiviral mechanism of action.