**5. Conclusions**

In our study, we showed that phospho-TCTP is essential for correct mitosis. Reduction of phospho-TCTP levels by DHA causes mitotic aberration and increases microtubule density in trastuzumab-resistant breast cancer cells. In this way, DHA enhances the e ffect of T-DM1 as a poison for microtubules, leading tumour cells into an unstable state no longer compatible with viability. T-DM1 is already an FDA-approved agen<sup>t</sup> for advanced breast cancer. DHA is an antimalarial drug, already tested for its toxicity, and has been approved by the European Medicines Agency (EMA) for human use. Due to its low cost and high tolerability profile, it is currently undergoing clinical trials in various types of neoplasia. This study provides the rationale for the design of a new clinical protocol.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2073-4409/9/5/1260/s1, Figure S1: E ffect of trastuzumab and pertuzumab on cell growth of HER2+ BC cell lines, Figure S2: DHA induces an increase of ROS levels in MCF10A-pBabe, MCF10A-AATCTP and MCF10A-WTTCTP cells, Figure S3: In vivo efficacy of DHA with T-PM1 in HCC1954 xenografts, Figure S4: E ffect of two-drug combination on microtubule density in HCC1569 and HCC1954 cells.

**Author Contributions:** F.d.B. and M.L. contributed equally to this work. S.D., E.K.K. contributed equally to this work. Conceptualization, F.d.B. and M.L.; methodology, M.L., F.d.B., E.K.K., S.D.; validation, M.L., S.D., E.E.K., G.P., I.M., G.T.; formal analysis, M.L., S.D., E.K.K., I.M., G.P.; investigation, M.L., S.D., E.K.K., A.G., M.R., G.P.; I.M., G.T., S.B., F.D.G.; resources, M.L., G.P.; writing—original draft preparation, M.L.; writing—review and editing, M.L., F.d.B., F.B., C.V., G.P., A.G.; visualization, M.L., S.D., E.K.K., I.M.; supervision, M.L., F.d.B.; project administration, M.L.; funding acquisition, M.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was supported by Roche S.p.A. The sponsor had no role in the design, execution, interpretation, or writing of the study.

**Acknowledgments:** We thank Daniela Barilà for the helpful reading and suggestions on our manuscript, Antonio Rossi for assistance in retroviral transduction experiments, and Mauro Fabiani for technical support. Special thanks to Genentech, USA Inc. for providing us with T-DM1 under Material Transfer Agreement with MTA number OR-129895.

**Conflicts of Interest:** The authors declare no conflicts of interest.
