Comparison of sensitivity to various drugs between erlotinib- and gefitinib- resistant cell lines and their drug sensitive parental counterparts, PC9 and 11–18 cells.

a)IC50 values are calulated from logit regression line from triplicate assays. IC50 values (µM) for erlotinib, gefitinib, lapatinib, SU11274, BIBW2992 and cisplatin are 0.04, 0.03, 3.16, 3.97, 0.00021 and 2.07 in PC9, 9.96, 5.04, 14.54, 3.97, 0.41 and 2.89 in PC9/ER1, 0.87, 0.32, 2.57, 4.22, 0.35 and 5.37 in 11–18, 20.88, 8.32, 2.83, 4.64, 0.49 and 1.61 in 11–18/ER1-7, 95.7, 20.48, 4.11, 5.06, 0.74 and 9.13 in 11–18/ER2-1, >29.58, 11.52, 13.36, 3.38 and 2.41 in 11–18/GEF10-1, and >29.58, 10.24, 14.14, 3.80 and 1.61 in 11–18/GEF20-1 respectively. The relative resistance is defined as the IC50 value divided by the IC50 value of the parental PC9 or 11–18.b)nt, not tested.