Drug synergy and combination index

All drug concentrations were simultaneously combined in a non-constant ratio or constant ratio, and the combination index (CI) values were calculated by the Chou-Talalay method with CompuSyn software [19,20]. In general, combinations with a CI value < 1.0 are considered synergistic, and the closer the index is to 0, the stronger the synergism [20]. The non-constant ratio experiments were carried out with a fixed concentration of CDK4/6is (palbociclib and abemaciclib, 0.5–1 µM) and variable concentrations of PARPis (olaparib and rucaparib, 0.5–10 µM). The constant ratio (1:5) combination experiments were carried out by serial dilution (1:2) of the mixture of palbociclib (2 µM) and olaparib (10 µM).