A New Triterpene from the Plant of Uncaria Macrophylla

Our ongoing investigations on the stem bark of Uncaria macrophylla afforded a new ursolic triterpene, 3β,6β,19α-trihydroxy-urs-12-en-28-oic acid-24-carboxylic acid methyl ester (1), named uncariursanic acid, and three known ursolic triterpenes including 3β,6β,19α-trihydroxy-23-oxo-urs-12-en-28-oic acid (2), 3β,6β,19α-trihydroxy-urs-12-en-28-oic acid (3) and ursolic acid (4). Their structures were elucidated by extensive spectral methods, including 1D and 2D NMR and HR-ESI-MS. The cytotoxicities of the four compounds were evaluated against two cancer cell lines (MCF-7 and HepG2) by the MTT method, and only compound 4 exhibited potent activity.


Introduction
Uncaria, an important source of medicinal natural products from the Rubiaceae family, is widely distributed in tropical regions, including Southern Asia, Africa and South America [1]. Many species of OPEN ACCESS the Uncaria genus have been broadly used as drugs for curing wounds, ulcers, fevers, headaches, hypertension, and gastrointestinal illness [2,3]. Many studies concerning the chemical constituents of Uncaria species have been carried out, which have led to the isolation of a series of secondary metabolites including alkaloids, triterpenes and flavones [4][5][6][7]. In our ongoing research on the systematic isolation of phytochemical constituents, four triterpenes were isolated. Their structures have been identified by extensive spectral methods including 1D, 2D NMR and MS. This paper mainly deals with the isolation, structural elucidation of the new compound, as well as the cytotoxicities of the four triterpenes.
The four triterpenes were evaluated for their in vitro inhibitory abilities against two cancer cell lines (HepG2 and MCF-7). The results showed that only ursolic acid and cis-platin were effective in inhibiting the growth of HepG2 and MCF-7. The IC 50 values of ursolic acid against the two cell lines were 12.1 µg/mL and 15.1 µg/mL and cis-platin were 7.5 µg/mL, 8.2 µg/mL, respectively. However, compounds 1-3 showed no inhibitive activities against the two cancer cell lines in the experiment (IC 50 > 100 µg/mL).

Plant Material
The

Bioassays
Antitumor activity was assayed on HepG2 liver tumor cells and MCF-7 breast tumor cells using cis-platin as positive control. Cells were plated in the appropriate media on 96-well plates in a 100 μL total volume at a density of 6 × 104 cells/mL. The final concentrations of each compound were 0.625, 1.25, 2.5, 5.0, 10 μg/mL. The plates were incubated at 37 °C in 5% CO 2 for 72 h. Cell viability was determined based on the mitochondrial conversion of MTT to formazan.

Conclusions
A new triterpene, 3β,6β,19α-trihydroxy-urs-12-en-28-oic acid-24-carboxylic acid methyl ester (1), along with three known triterpenes were isolated from the stem bark of U. macrophylla. The isolation of the new compound was a new addition to the molecular diversity of U. macrophylla. Ursolic acid (4) exhibited antitumor activity.