Bioassay-guided isolation and identification of antiproliferative compounds from Acokanthera schimperi (A.DC.) Schweinf and Gnidia involucrata Steudel ex A. Rich

The traditional Ethiopian medicinal plants Acokanthera schimperi (A.DC.) Schweinf. and Gnidia involucrata Steud. ex A.Rich were explored for their potential as sources of anticancer agents. Bioassay-guided fractionation yielded a previously unreported phorbol ester (5), alongside four known compounds (1–4) from G. involucrata and a pregnane-type steroid (6) from A. schimperi. Structural elucidation was accomplished through comprehensive NMR and HRMS analyses. The antiproliferative activities of these compounds were assessed against A-427, Dan-G, MCF-7, and SiSo cancer cell lines, revealing diverse levels of activity. Notably, the pregnane derivative (6) demonstrated potent activity, with GI₅₀ values ranging from 2.3 to 4.7 μM across the tested cell lines. Mechanistic investigations demonstrated that compound (6) inhibited mitotic spindle assembly and spindle-pole separation, leading to a pronounced G2/M phase cell cycle arrest in the SiSo cell line. In contrast, compound (5) exhibited potent antiproliferative activity against the Dan-G cell line (GI₅₀ = 0.3 ± 0.05 nM) and moderate to weak activity in the remaining cell lines, with GI₅₀ values ranging from 5.2 to >50 μM, inducing early apoptosis in Dan-G cells. These findings highlight compounds 5 and 6 as promising anticancer candidates and provide a scientific basis for the ethnomedicinal use of these plants in cancer therapy.