**5. Conclusions**

Taken together, our results show that *M. aquifolium* extracts and their active principles should be investigated further, either on their own or in combination with other anticancer drugs similar to DOX. It remains to be determined whether changes in MMP9 expression and DOX retention underlie these effects and which alkaloids are responsible for the anticancer activities against A549 cells. In addition, the synergistic effects of extracts with DOX in other systems remain to be confirmed.

**Author Contributions:** Conceptualization, A.D., B.K. and T.P.S.; methodology, A.D., B.K., I.Z.M. and A.K.; investigation, A.D., B.K. and J.A.S.; resources, G.Z. and K.Š.; data curation, A.D.; writing—original draft preparation, A.D. and B.K.; writing—review and editing, R.J., K.Š. and J.A.S.; supervision, T.P.S. and J.A.S.; project administration, T.P.S. and R.J.; funding acquisition, T.P.S., R.J. and J.A.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by the Ministry of Education, Science, and Technological Development of the Republic of Serbia (Grant No.451-03-68/2020-14/200026 and Grant No. 451-03-68/2020-14/200043).

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
