Triterpenes from Euphorbia hirta and their cytotoxicity

To investigate the chemical constituents of the stems, leaves and roots of Euphorbia hirta, and to test for the cytotoxic and antimicrobial potentials of the major constituents of the plant.

The compounds were isolated by silica gel chromatography and their structures were elucidated by NMR spectroscopy. The cytotoxicity tests were conducted using the 3-(4, 5-dimethylthiazol-2-yl)-2, 5-diphenyltetrazolium bromide (MTT) assay, while the antimicrobial tests employed the agar well method.

The air-dried stems of E. hirta afforded taraxerone 1, a mixture of 25-hydroperoxycycloart-23-en-3β-ol (2a) and 24-hydroperoxycycloart-25-en-3β-ol (2b) (sample 2) in a 2 : 1 ratio, and another mixture of cycloartenol (3a), lupeol (3b), α-amyrin (3c) and β-amyrin (3d) (sample 3) in a 0.5 : 4 : 1 : 1 ratio. The air-dried leaves of E. hirta yielded sample 2 in a 3 : 2 ratio, sample 3 in a 2 : 3 : 1 : 1 ratio, phytol and phytyl fatty acid ester, while the roots afforded sample 2 in a 2 : 1 ratio, sample 3 in a 2 : 1 : 1 : 1 ratio, a mixture of cycloartenyl fatty acid ester 4a, lupeol fatty acid ester 4b, α-amyrin fatty acid ester 4c and β-amyrin fatty acid ester 4d (sample 4) in a 3 : 2 : 1 : 1 ratio, linoleic acid, β-sitosterol and squalene. Compound 1 from the stems, sample 2 from the leaves, and sample 3 from the stems were assessed for cytotoxicity against a human cancer cell line, colon carcinoma (HCT 116). Sample 2 showed good activity with an IC50 value of 4.8 μg·mL(-1), while 1 and sample 3 were inactive against HCT 116. Sample 2 was further tested for cytotoxicity against non-small cell lung adenocarcinoma (A549). It showed good activity against this cell line with an IC50 value of 4.5 μg·mL(-1). Antimicrobial assays were conducted on 1 and sample 2. Results of the study indicated that 1 was active against the bacteria: Pseudomonas aeruginosa and Staphylococcus aureus, but was inactive against Escherichia coli and Bacillus subtilis. Sample 2 was active against the bacteria: Pseudomonas aeruginosa, Staphylococcus aureus and Escherichia coli and fungi: Candida albicans and Trichophyton mentagrophytes. It was inactive against Bacillus subtilis and Aspergillus niger.

The triterpenes: 2a, 2b, 3a, 3b, 3c and 3d were obtained from the stems, roots and leaves of E. hirta. Taraxerol (1) was only isolated from the stems, the leaves yielded phytol and phytyl fatty acid esters, while the roots afforded 4a-4d, linoleic acid, β-sitosterol, and squalene. Triterpene 1 and sample 2 were found to exhibit antimicrobial activities. Thus, these compounds are some of the active principles of E. hirta which is used in wound healing and the treatment of boils. The cytotoxic properties of sample 2 imply that triterpenes 2a and 2b contribute to the anticancer activity of E. hirta.