Anti-Vancomycin-resistant Enterococcus faecium and E. faecalis Activities of (-)-Gossypol and Derivatives from Thespesia garckeana

The root extract of Thespesia garckeana yielded three known oxidatively coupled sesquiterpenoids, namely (-)-gossypol (1) and two of its derivatives (-)-6-methoxygossypol (2) and (+)-6,6′-dimethoxygossypol (3), and the stem bark afforded (E)-docosyl-3-(3,4-dihydroxyphenyl) acrylate (4), stigmasterol (5) and betulinic acid (6). The structures of the isolated compounds were determined on the basis of full spectral data (1D and 2D NMR and HRMS) and comparison with literature values. Compound 1 showed potent antibacterial activity against vancomycin-resistant Enterococcus faecium (VRE) with IC₅₀/MIC/MBC values of 1.71/4.82/19.31 μM, respectively, whereas the reference standard vancomycin was found to be inactive. The mono- and di-methoxylated derivatives of this compound, (-)-6-methoxygossypol (2) and (+)-6,6′-dimethoxygossypol (3), were less active with respective IC₅₀/MIC/MBC values of 2.73/4.70/9.40 μM and 6.14/18.32/18.32 μM against this microbe. Compound 2 was more potent than 1 against the low level VRE strain with IC₅₀/MIC/MBC values of 4.34/9.40/9.40 μM (vs 5.23/19.31/19.31μM for 1). This compound also showed interesting activities against Candida glabrata with an IC₅₀ value of 2.97 μM, but was less active against methicillin-resistant S. aureus (MRSA) exhibiting an IC₅₀ value of 17.33 μM. Compound 1 demonstrated modest activity against the other microbes tested including C. glabrata, S. aureus and MRSA with IC₅₀ values of 0.73, 9.15 and 8.99 μM, respectively.