Flavonol Glycosides from Eugenia uniflora Leaves and Their In Vitro Cytotoxicity, Antioxidant and Anti-Inflammatory Activities

Abstract

In view of the extensive use of Eugenia uniflora leaves for the management of tumours and other chronic inflammatory diseases in traditional medicine, an activity-guided fractionation of its leaf ethanolic extract led to the isolation of two flavonol glycosides. Cytotoxicity study was based on the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-2H-tetrazolium bromide (MTT) viability assay against the non-tumourigenic human embryonic kidney (HEK-293) cells, and the cancerous liver (Hep-G2) and cervical (HeLa) cell lines. Antioxidant tests were carried out using 2,2-diphenyl-1-picrylhydrazyl (DPPH), nitric oxide (NO) and hydrogen peroxide (H2O2) radical scavenging assays, while an in vitro anti-inflammatory test was conducted using egg albumin denaturation (EAD) assay. Based on comprehensive spectroscopic and spectrometric evidence, the compounds were elucidated as myricitrin (1) and a newly described compound, 5,7-dihydroxy-3-(3,4,5-trihydroxy-6-methyltetrahydropyran-2-yloxy)-2-(2,4,5-trihydroxyphenyl)chromen-4-one, named “unifloratrin (2)”. The cytotoxicity of myricitrin (1) was comparable to 5-fluorouracil (standard drug), with a CC50 of 8.5 ± 2.2 µg/100 µL against HeLa cells. It also demonstrated better antioxidant activity, with an IC50 of 6.23 ± 1.09, 22.01 ± 2.59 and 30.46 ± 1.79 µM against DPPH, NO and H2O2 free radicals, respectively. At 20 µg/mL and an incubation time of 2 h, myricitrin was comparable to diclofenac (standard drug) in anti-inflammatory activity. This report may serve as a justification for the ethnomedicinal use of E. uniflora, while flavonol glycosides, such as myricitrin (1), could be further exploited as a candidate cytotoxic agent.