Bioactivity-guided isolation and structural characterization of the antifungal compound, plumbagin, from Nepenthes gracilis

Abstract

Context: Despite several phytochemical studies of Nepenthes gracilis Korth (Nepenthaceae), the biological activities of this pitcher plant remain to be explored.

Objective: This study evaluates the antifungal activity of N. gracilis extracts, isolates, and characterizes its bioactive compound and evaluates the cytotoxicity of the isolated compound.

Materials and methods: Fresh leaves of N. gracilis were sequentially extracted. The fungistatic and fungicidal activities of the extracts were evaluated against six species of fungi of medical importance using a colorimetric broth microdilution method. The most active extract was fractionated by liquid–liquid partitioning and further purified by a preparative thin layer chromatography. Structural elucidation was carried out using FT-IR, GC-MS, and NMR. Cytotoxicity testing against rhesus monkey kidney epithelial cells (LLC-MK2) was assessed by a neutral red uptake (NRU) assay.

Results: The hexane extract, which showed the lowest minimum inhibitory concentration (MIC) and minimum fungicidal concentration (MFC), both at 20 μg/mL against Candida albicans, Issatchenkia orientalis, and Trichophyton mentagrophytes, was subjected to bioactivity-guided fractionation. The isolated compound exhibited potent activity with the MIC values ranging from 2 to 31 μg/mL against all the fungi. The active compound was identified as plumbagin (5-hydroxy-2-methyl-naphthalene-1,4-dione). The 50% cytotoxicity concentration (CC₅₀) of plumbagin was 0.60 μg/mL.

Discussion and conclusion: The selectivity indices of plumbagin against all the fungi were less than 1.0, indicating that plumbagin is more toxic to mammalian than fungal cells. This study provides information on the antifungal properties of N. gracilis leaf extracts, as well as the antifungal and cytotoxicity properties of plumbagin.

• Broth microdilution
• extraction
• fungicidal
• fungistatic
• Nepenthaceae
• naphthoquinone

Acknowledgements

The authors thank Universiti Tunku Abdul Rahman for a research grant (UTARRF Vote No. 6200/S07) which supported the M.Sc. candidature of Pei Shing Gwee.