Quarfloxin

We evaluated cell viability using MTT assay (ST316; Beyotime, Nanjing, China). MDA-MB-231 cells were evenly distributed at a density of 5000 cells per well in triplicate within 96-well plates. Following a 24 h incubation period, the cells were subjected to various concentrations of the compounds (0.1, 1, 10, 100, 1000 μM), which were initially dissolved by PBS to 10 mM. The positive control group received Quarfloxin (CX-3543) (A12380, Adooq Bioscience, Nanjing, China). The negative control group received an equivalent volume of DMEM. After an additional 48 h incubation, 5% MTT solution was added to each well (20 μm of MTT and 100 μL of medium) and incubated at 37 °C for 4 h. The optical density (OD) was measured using a multi-function microplate reader (INFINITE E PLEX, Tecan, Männedorf, Switzerland) at an absorbance wavelength of 490 nm. The cell viabilities were calculated using GraphPad Prism 8 software as follows: (OD experiment − OD background)/(OD negative − OD background) × 100%.

The following drugs were used: camptothecin (CPT), actinomycin D, cycloheximide, marizomib, chloroquine, Bmh21, BO-2, (all Sigma-Aldrich), aphidicolin (Santa Cruz), talazoparib (Axon Medchem), quarfloxin (CX-3543, Adooq Bioscience) and CX-5461 (Selleckchem).