WP1130 increased the cytotoxic effects of cisplatin in breast cancer cells. Breast cancer cells were incubated with cisplatin alone or in combination with WP1130 for 48 h. The CCK-8 assay was performed to determine viability of MDA-MB-231 (A), MDA-MB-468 (B), and Bcap-37 (C) cells. Western blot analysis was performed to measure the protein expression of usp9x and Mcl-1 in MDA-MB-231, MDA-MB-468, and Bcap-37 cells exposed to cisplatin alone or in combination with WP1130 (2.5 µM). The protein bands were then quantified by densitometry with the imaging system (D). * vs. control, P < 0.05; # vs. cisplatin, P < 0.05.