*4.27. Proliferation*

MDA-MB-231 and HS578T cells were seeded in 6-well plates and treated with different concentrations (10 nM, 100 nM, 250 nM, 500 nM, 1 μM, and 2 μM) of compound 28 after 24 h. The number of dead and viable cells was measured by use of a CASY cell counter (OMNI Life Science, Bremen, Germany) after 72 h.

#### *4.28. Staining*

Analysis of subcellular localization of compound AS101 was performed in MDA-MB-231 cells using the mitochondrial targeting compound BioTracker™ 488 Green Mitochondria Dye (Sigma-Aldrich Chemie GmbH, Taufkirchen, Germany) for comparison. Cells were seeded in a μ-Plate 96 Well Black plate (ibiTreat: #1.5 polymer coverslip bottom, ibidi GmbH, Gräfelfing, Germany) at a cell density of 50,000 per well. After 24 h, cells were treated with 100 nM AS101 for 6h or 100 nM BioTracker488 for 30 min, followed by rinsing and supplementation with RPMI 1640 w/o Phenol-red (Pan-Biotech GmbH, Aidenbach, Germany). Live-cell imaging was performed on an Axio Observer 7 (Carl Zeiss Microscopy Deutschland GmbH, Oberkochen, Germany) using the settings for Ex/Em as follows: BioTracker (475 nm/514 nm), AS101 (555 nm/592); Scale bar: 50 μm.

**Author Contributions:** Conceptualization, R.C.; methodology, T.M., M.B. and A.G.; software, N.H.; validation, N.H., T.M., M.B., A.G. and R.C.; formal analysis, N.H.; investigation, N.H., S.B., T.M., M.B. and A.G.; resources, R.C., M.B. and T.M.; data curation, R.C.; writing—original draft preparation, R.C., N.H., M.B., T.M. and A.G.; writing—review and editing, R.C., N.H. and M.B.; visualization, R.C.; supervision, R.C.; project administration, R.C.; funding acquisition, R.C., T.M. and M.B. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.

**Acknowledgments:** We would like to thank D. Ströhl, Y. Schiller, and S. Ludwig for the NMR spectra and T. Schmidt for the MS measurements. IR, UV/Vis spectra, and optical rotations were recorded by M. Schneider and S. Ludwig; microanalyses were performed by M. Schneider. We would also like to thank J. Block and G. Thomas for their excellent technical assistance. We thank J. Dittmer from the Department of Gynecology (Martin Luther University Halle-Wittenberg) for providing breast cancer cell lines. Additionally, we would like to thank A. Navarette Santos of the Center for Basic Medical Research, who aided in flow cytometry.

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