Fig. 1

Development of SKBR3-derived cell lines as in vitro models for acquired and intrinsic drug-resistance. SKBR3 cells were continuously cultured for 7 months in the absence or the presence of 10 µg/mL trastuzumab. a Cells were seeded at a density of 2000 cells/well and treated with 10 µg/mL trastuzumab or equal volume of H₂O (vehicle), 24 h after seeding and relative cell viability was assessed over 192 h with a MTS assay. Two cell lines continuously cultured in the absence of trastuzumab (SV₁, SV₂) retained trastuzumab-sensitivity (i), two cell lines cultured in the presence of trastuzumab (RT₁, RT₂) acquired trastuzumab-resistance (ii) and two cell lines continuously cultured in the absence of trastuzumab (RV₁, RV₂) developed intrinsic resistance to trastuzumab (iii). Viable cell number of all samples are expressed relative to control cells (no trastuzumab) at 192 h. Normalised cell viability = [Absorbance of sample (t) − Average absorbance of control (t = 0)]/[Average absorbance control (t = 192 h) − Average absorbance control (t = 0)], (n = 3 ± SD). b Relative viable cell numbers of all cell lines were compared at the end of the protocol (192 h after start of treatment). Trastuzumab sensitive cell lines (SV₁, SV₂) showed a significant decreased cell viability in the presence of trastuzumab compared to vehicle control cells, whereas the relative viable cell number of acquired resistant cell lines (RT₁, RT₂) and intrinsic resistant cell lines (RV₁, RV₂) was not significantly altered by trastuzumab (n = 3 ± SD). Statistical analyses were performed using multiple t tests with Holm Sidak correction (**p ≤ 0.01). White bars present vehicle controls and solid coloured bars represent trastuzumab (10 µg/mL) treated cells