Modulation of GSC sensitivity to TMZ by PARPi. Panel A. GCS and GBM cell lines were exposed to graded concentration of TMZ as single agent or in combination with a fixed dose of PARPi GPI 15427. Bars represent the TMZ IC50 values as single agent (black column) or in combination with PARPi (grey column) and are the mean (± SD) from three independent experiments. The table indicates the values of combination index (CI) for a TMZ concentration range of 62.5-250 (*1.9-7.8 μM in the case of #144 GSC line), evaluated according to the Chou-Talalay method to determine synergy. The DRI values refer to the fold decrease of TMZ IC50 obtainable when the methylating agent was combined with the PARPi. Panel B. Relationship between PARPi IC50 in GSC lines and the DRI (TMZ) in combination with a fixed dose of PARPi. The Spearman’s correlation coefficients and their significance levels are indicated.