Cytotoxicity assay

Cytotoxicity was assessed by MTT assay as described previously [16]. Briefly, cells were plated into a 96-well plate at density of 5 × 10³ cells/well and treated with docetaxel and/or ABT-737 at indicated concentrations and time periods. A stock solution of 5-mg/mL MTT solution was prepared in PBS at pH 6.8 and kept at 4℃ in the dark. After 4-hour incubation at 37℃, a formazan solubilization buffer prepared by mixing 10% sodium dodecyl sulfate and 0.01 M HCl in PBS was added. Following an additional overnight incubation, the absorbance of each well was measured at wavelength of 570 nm in a microtiter plate reader (SpectraMax Plus 384, Molecular devices, Sunnyvale, CA, USA). Combination index (CI) was calculated by the following equation: CI = CAx/ICxA+CBx/ICxB, where CAx and CBx were the concentrations of agent A and agent B used in combination to achieve x% combinatory effect; ICxA and ICxB were concentrations for single agents to achieve the same effect. CI < 1, CI = 1, and CI > 1 indicated synergism, additive effect, and antagonism, respectively [17].