Calculation of drug combination effects

The pharmacobiological interaction between VLX1570 and ibrutinib was examined in WM cells based on the Chou-Talalay principle²⁹ using the CompuSyn software program (ComboSyn Inc., Paramus, NJ, USA). WM cells were exposed to different concentrations of VLX1570 and ibrutinib for 24 h in quadruplicate and viability was examined using the CellTiter Glo assay (Promega Corp., Madison, WI, USA), according to the manufacturer's instructions. Cell viability data were expressed as the fraction of antiproliferative activity (Fraction affected, Fa) by the individual drugs or the combination in drug-treated cells from which a combination index (CI) value was derived. An additive effect is indicated by a CI of 1, an antagonistic effect is denoted by CI >1 and a synergistic effect is indicated by a CI <1.