Synthesis and biological evaluation of novel shikonin ester derivatives as potential anti-cancer agents

Shikonin has previously been reported to function as a potent anti-cancer drug that induces cell apoptosis via diverse pathways. To optimize the effectiveness of its pro-apoptotic functions, shikonin was chosen as the best compound for obtaining shikonin ester derivatives (3a–3n) because it possesses an ester group on the side chain hydroxyl position, and the biological activity was evaluated as a potent anti-proliferating inhibitor for many cancer cell lines. Among these compounds, compound 3j exhibited better anti-cancer activities against human hepatocellular carcinoma cell line (HepG2) with an IC50 value of 0.759 μM which was better than that of shikonin which was 1.288 μM. The flow cytometry results showed that compound 3j inhibited the cell growth and caused the cell cycle to be arrested at the G2/M phase. Meanwhile, obvious apoptosis induced by 3j was observed using the Annexin V combined with propidium iodide assay, showing that 3j induced apoptosis of HepG2 cells in a dose and time dependent manner. To investigate the underlying mechanism in the process of apoptosis induced by 3j, the western blot technique was used and the cleavage of caspase-9 was observed but not that of caspase-8. Furthermore, the elevated expression of the p53 protein was positively correlated with the decreased Bcl-2 protein levels and increased Bax protein and the cytochrome C levels. This indicated that the mitochondrial apoptosis pathway directed by p53 responded in the process of apoptosis was induced by 3j. Based on these data, we conclude that compound 3j has the best anti-proliferating and pro-apoptotic effect among the 14 newly synthesized compounds, and it could be a potent candidate for cancer therapy.