The sensitivity of BC cells to OX was measured using a CCK-8 assay kit (Dojindo Molecular Technologies, Inc.). In short, BT474/OX or MCF-7/OX cells (5 × 104 cells/well) were seeded into 96-well plates and exposed to OX in a concentration gradient from 0 to 20 μmol/L for 48 h. Then, the cells were cultured for another 2 hours after the 10 μL CCK-8 reagent (Beyotime, China) was added to the plates. The absorbance was detected using a microplate reader (BioTek China) at 450 nm.