Isolated conpounds and bioactivity screening of extracts from three endemic species to Madagascar belonging to Hyperacanthus genus (Rubiaceae)

The Hyperacanthus genus (Rubiaceae) contains species endemic to southern Africa and mainly to Madagascar that remain chemically and biologically underexplored. This study aimed to describe chemical constituents of three endemic species of Madagascar belonging to this genus : H. thouvenotii, H. poivrei and H. mangoroensis. The purpose is to investigate the biological activities of their crude extracts in order to valorize these plants in phytomedicine.
Isolation and characterization of chemical constituents of Hyperacanthus species were achieved using different chromatographic techniques followed by structure elucidation with spectroscopic techniques. Eleven known compounds such as α-amyrin, β-sitosterol, β-sitosterol-3-O-glucoside, stigmasterol, uvaol, ursolic acid, moretenol, 24-hydroxy α-amyrin, 24-hydroxy β-amyrin, vitexin and sorbitol were isolated for the first time from Hyperacanthus genus. The antiplasmodial and cytotoxic activities of the crude extracts were performed on 3D7 strain of Plasmodium falciparum and on human melanoma A2058 and human breast cancer MDA-MB-231 cell lines, respectively. The dichloromethane extract of H. thouvenotii showed a promising antiplasmodial activity with an IC50 value of 13.40±1.61 µg/mL and a selectivity index of SIMDA-MB-231/3D7 = 4.10. The dichloromethane extracts of H. poivrei and H. mangoroensis exhibited a moderate activity with an IC50 of 19.71±1.68 µg/mL ; SI >5 and 16.47±0.64 µg/mL ; SI > 6, respectively. The in vitro antioxidant activity was assessed by DPPH method and expressed by its IC50 and antioxidant activity index (AAI). All of the methanol extracts and infusion of H. poivrei have a very strong antioxidant activity with IC50 and AAI values ranging from 9.36±0.85 to 11.18±1.31 µg/mL and 4.59±0.40 to 3.86±0.45, respectively. In acute oral toxicity test, all the methanolic extracts did not cause any toxicity effects nor mortality at the different administered doses. Theoretical lethal dose is up to 2000 mg/kg body weight.
This is the first isolation of moretenol, 24-hydroxy α-amyrin and 24-hydroxy β-amyrin from plants belonging to Rubiaceae family. This is the first report on antiplasmodial, cytotoxic, antioxidant activities and acute oral toxicity of species belonging Hyperacanthus genus.