Bioactivity-guided Isolation of Anticancer Constituents of Strophanthus hispidus DC. (Apocynaceae) Whole Plant

Background: Strophanthus hispidus DC; a medicinal plant in the Apocynaceae family was reported to be effective in the management of cancer through an ethnobotanical survey conducted in Ibadan, Nigeria. This study explicitly explored the cytotoxic potential of the whole plant of Strophanthus hispidus leading to the isolation of cytotoxic secondary metabolites. Methods: The crude extract of S. hispidus was prepared using the Soxhlet apparatus and concentrated using the rotary evaporator. The crude extract obtained from S. hispidus was then partitioned into n-Hexane, dichloromethane (DCM), ethyl acetate and butanol fractions for further study. Compounds were isolated by column chromatography, normal and reverse-phase high performance liquid chromatography, and structures were determined by spectroscopic methods. The crude extracts, fractions and isolated compounds were screened for cytotoxicity against breast (AU565, MCF-7), cervical (HeLa), and skeletal muscle (RD) with two normal cell lines: rat and human fibroblast (BJ and 3T3) for selectivity using the MTT assay. Doxorubicin and cycloheximide served as standards for the assay. Results: The methanol extract and DCM fraction showed significant cytotoxicity with IC50 values (Mean ± SEM) of 1.86 ± 0.01 μg/mL, 0.32 ± 0.01 μg/mL and 1.26 ± 0.06 μg/mL, 0.21 ± 0.06 μg/mL against AU565 and HeLa respectively. The bioassay-guided isolation of cytotoxic compounds from S. hispidus afforded two new compounds (1, 2) alongside six other known compounds (3-8). The compounds were identified as 16-hydroxy-17-(1-hydroxyethyl)-13,17-dimethyl-5H-cyclopentaaphenanthrene-4,6-dione (1), 2-(4-ethoxy-3-methoxyphenyl)-5-hydroxy-3,7,8- trimethoxy-4H-chromen-4-one (2); Stigmasterol (3); β-Sitosterol-β-D-glucoside (4); pityriacitrin (5); ursolic acid (6); helveticoside (7) and Urs-12-en-28-oic acid,2,3,23-trihydroxy-(2β,3β,4α) (8). Compounds 1, 4, 6, 7 and 8 displayed significant cytotoxicity and selectivity towards cancer and normal cell lines respectively. Conclusion: This study further confirms that S. hispidus is cytotoxic with the DCM fraction being the most significantly cytotoxic and selective to normal cells. This is the first report on some characterized compounds from S. hispidus. In addition, these secondary metabolites can serve as leads in cancer drug development.