Fig. 4. Lapatinib-resistant breast cancer cells are sensitive to Hsp90 inhibition.

a R172H/+;ERBB2 female mice were treated either with vehicle or 50 mg/kg lapatinib (three times a week starting at 8 weeks of age) until tumors acquired lapatinib resistance, i.e., lapatinib no longer suppressed tumor growth. At this point, previously vehicle-treated mice were continued on vehicle (Veh/Veh), while previously lapatinib-treated mice continued to be treated with either lapatinib alone (Lap/Lap) or with lapatinib together with 50 mg/kg ganetespib once a week (Lap/Lap + Ganet), as described in Results. Note that the initial tumor size in all three groups was on average comparable. Tumor size was measured and plotted every 5 days. The treatment has been ended and mice were sacrificed when tumors in Veh/Veh and Lap/Lap arms reached the size of 3.5 cm³. Note that while lapatinib-resistant tumors grew similarly to untreated tumors (did not respond to lapatinib), addition of ganetespib significantly suppressed tumor growth (wide-dash line). *p < 0.05, **p < 0.01, Student’s t-test. Top asterisks compare the Lap/Lap + Ganet group with Lap/Lap, bottom asterisks compare the Lap/Lap + Ganet group with the Veh/Veh group. n number of independent tumors. b, c Lapatinib-sensitive human BT474 (b) and murine 125R (c) cells have similar sensitivity to ganetespib as their corresponding lapatinib-sensitive cells (BT474 and 1251, 1252, 1253, respectively). Cells were treated with DMSO or 0.3 µM ganetespib for 48 h, followed by the cell viability assay. One representative experiment out of two independent experiments (each performed in triplicate) is shown; NS non-significant. d Ganetespib (indicated concentrations, 24 h) inhibits ERBB2 signaling (measured by phospho-ERBB2 and phospho-Erk) and destabilizes mutp53 and Mdm2 in both, lapatinib-sensitive BT474 and lapatinib-resistant BT474R cells. Western blot analysis, GAPDH is a loading control