*2.2. MTT Assay*

MDA-MB-231 cells were seeded in 96-well plates (2.5 × 10<sup>3</sup> cells/well) and treated with ISL (Sigma-Aldrich, St. Louis, MO, USA) for 24, 48, and 72 h. The cell survival rate was analyzed using an MTT (3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2H-tetrazolium bromide; Abcam, Cambridge, MA, USA) assay. At the end of incubation, serum-free DMEM medium with 0.5 mg/mL of MTT was substituted for conditional medium and incubated for an additional 3 h; subsequently, the media were removed. Crystal formazan was dissolved in 100 μL/well dimethyl sulfoxide (DMSO; ECHO Chemical Co. Ltd., Taipei, Taiwan). The optical density was measured by using a VERSA Max microplate reader (Molecular Devices, San Jose, CA, USA) at 570 and 630 nm as reference wavelengths.

#### *2.3. Lactate Dehydrogenase (LDH) Assay*

MDA-MB-231 cells were seeded in 96-well plates (2 × 10<sup>4</sup> cells/well) and treated with ISL for 24, 48, and 72 h. The medium LDH activity was detected using the LDH Cytotoxicity Assay Kit (Cayman Chemical Company, Ann Arbor, MI, USA). The procedure was performed according to the manufacturer's protocols. The absorbance was read at 450 nm with a VERSA Max microplate reader (Molecular Devices, San Jose, CA, USA).
