Hyperacanthus genus (Rubiaceae): an underexplored source of bioactive compounds

The Hyperacanthus genus (Rubiaceae) contains species endemic to southern Africa and mainly to Madagascar [1–5] that remains chemically and biologically underexplored. This study aimed to describe chemical constituents and investigate biological activities of hree endemic species of Madagascar belonging to this genus: H. thouvenotii, H. poivrei and H. mangoroensis. Twelve compounds were isolated, and two other compounds such as oleanolic acid and chlorogenic acid were detected for the first time from Hyperacanthus genus.
The antiplasmodial and cytotoxic activities of the crude extracts and compounds were performed on Plasmodium falciparum 3D7 strain, on human melanoma A2058 and human breast cancer MDA‑MB‑231 cell lines, respectively. The dichloromethane extracts of these species showed a promising to moderate antiplasmodial activity with an IC50 ranging from 13.40 ± 1.61 to 19.71 ± 1.68 μg/mL and a selectivity index (SI) up to 3.87. 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 infusions of the three species 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. This is the first isolation of moretenol, 24-hydroxy α-amyrin and 24-hydroxy β-amyrin from plants belonging to the Rubiaceae family. These results suggest that Hyperacanthus species have a potential for exploitation as a source of antimalarial and antioxidant agents.