**5. Conclusions**

In summary, multifloroside (**4**), together with other three 10-oxyderivatives of oleoside secoiridoids (**1**–**3**), were evaluated for their anti-cancer activities in *vitro*. These four compounds differ in their inhibition potency against human cancer cell lines A431 and A549, and interestingly, only multifloroside exhibited the most potent inhibitory activity against A431 cells. Further studies on multifloroside sugges<sup>t</sup> that it can inhibit cell colony formation, arrest the cell cycle in the S-phase, and increase the

level of ROS and MMP but cannot exert significant changes in cell apoptosis. These findings can help us understand the structure-activity relationships behind the tumor-inhibitory e ffect of **1**–**4** against the two tumor cell lines and have important implications for the potential use of multifloroside in the treatment of human epidermoid carcinoma. In a subsequent study, we will identify and design novel derivatives of multifloroside, investigate their activities in vitro and in vivo, and provide additional information on the ability of these compounds to protect against epidermoid carcinoma cancer.

**Author Contributions:** Y.Z. and Y.Y. designed the experiments. X.Z. and Y.L. conducted the experiments. X.Z., Z.F. and Y.Z. drafted the manuscript. Z.F., Y.G. and H.S. helped to conduct some experiments. X.D. conducted some of the data analysis and assisted in figures making. Y.Y. contributed to the discussion section and revised the manuscript. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by grants from the National Natural Science Foundation of China (81571596, 81601044 and 81771279), the Fundamental Research Funds for the Central Universities (GK201906010, GK201701009 and GK201603110), and the Postdoctoral Science Foundation of Shaanxi Normal University.

**Conflicts of Interest:** The authors declare no conflict of interests.
