Enhancement of BO-1978–induced toxic effects in EGFR mutant NSCLC cells upon gefitinib treatment. (A) Synergistic suppression of cell growth by combination treatment of EGFR mutant NSCLC with BO-1978 and gefitinib. Logarithmically growing PC9, PC9/gef B4, H1650, and H1975 cells were treated with BO-1978, gefitinib, or the combination for 72 hours. The cell growth was determined using an alamarBlue assay, as described in the Materials and Methods section. (B) Increased DNA damage marker (γH2AX) expression and suppression of DNA repair proteins (DNA-PK and Rad51) by gefitinib. PC9 and PC9/gef B4 cells were treated with BO-1978, gefitinib, or the combination for 24 and 72 hours. At the end of treatment, the cells were harvested, and γH2AX, DNA-PK, and Rad51 expression levels were analyzed by Western blotting.