Clonogenic survival analysis of MB cells treated with etoposide in the presence of ABCB1 inhibitors. The left panel shows the effect of increasing etoposide concentrations on clonogenic survival (100% representing solvent control). The middle and right panels represent clonogenic survival in the presence of etoposide (IC50 concentrations derived from dose–response data) in combination with VPL (10 and 20 μM) or vardenafil (5 and 10 μM) respectively. (A) MED1 clonogenic survival is significantly inhibited in cells co-treated with etoposide and VPL or vardenafil (unpaired student’s t-test, ٭٭٭P < 0.0001 in all cases). (B) There was also a significant increase in etoposide cytotoxicity in DAOY cells when combined with VPL and vardenafil (unpaired student’s t-test, ٭٭P < 0.001; ٭P = 0.02). (C) No significant inhibition was observed in the MED3 cell line. Data represent mean ± SEM of n ≥ 3 experiments, each experiment was performed in duplicate.