Calculation of combination index (CI)

Drug interactions were analysed using CalcuSyn 2.0 software (BIOSOFT, Cambridge, UK), as described by Bijnsdorp et al (16). CIs were extrapolated from drug cytotoxicity values derived from the quantification of the sub-G1 (apoptotic) population of cells. CalcuSyn software employs the Chou-Talalay method for drug combination, which is based on the median-effect equation, derived from the mass-action law principle (17). The resulting CI values depict synergy (<1), additive effect (=1) and antagonism (>1).