Acquired ABT-737 resistance is associated with changes in expression and interactions of Bcl-2 proteins. Four cell lines were rendered resistant to ABT-737 by sequentially increasing the ABT-737 concentration in their growth medium starting from 50nM up to 0.5μM (8226/S, KMS-18) or 2μM (KMS-11, U266). (A) Sensitivity (EC₅₀) of parental, control resistance (CR2), and resistant (ABTR) cell lines to ABT-737 was determined by annexin V/PI staining after 24 hours of treatment. All ABTR cell lines are significantly different from CR2 cells at all concentrations of ABT-737 (P < .001), with the exception of KMS18 at 3.2μM ABT-737 (P < .005). (B) Expression of Bcl-2 family members and (C) interaction between these proteins were determined as described in “Antibodies” and “Coimmunoprecipitation studies.” For all experiments, the cells were removed from the selecting concentration 24 hours before the experiment.