Abstract #5501: In vitro synergy of the combination of Sorafenib and ABT-737 in melanoma cell lines

Introduction: ABT-737 is a small molecule inhibitor of Bcl-2, Bcl-xL and Bcl-w which has been shown to induce apoptosis in a variety of tumor types. Persistence of Mcl-1 expression has been proposed as a potential mechanism of resistance to ABT-737. Sorafenib is a multitargeted tyrosine kinase inhibitor of VEGFR2, PDGFR, B-raf, and C-raf and has been shown to downmodulate Mcl-1. We have previously shown that sorafenib can induce caspase-dependent apoptosis in A375 cells and caspase-independent apoptosis in A2058 cells through enhancement of the nuclear translocation of apoptosis inducible factor (AIF). We sought to evaluate the efficacy of ABT-737 in melanoma and determine if its effects may be additive to those of sorafenib. Experimental Methods: Varying doses of ABT-737 (0-10 µM) and sorafenib (5-20 µM) were used to treat A375 and A2058 melanoma cells which were analyzed for viability by propidium iodide staining as well as lysed for western blot analysis for Mcl-1, total and phosphorylated Erk, and PARP. Dependence of observed cytotoxicity on caspase activation was determined by concurrent treatment with the pan-caspase inhibitor Z-VAD. Results: Treatment with ABT-737 resulted in only modest single-agent lethality in both A375 and A2058 cell lines. However, concurrent treatment with sorafenib resulted in synergistic enhancement of lethality compared with either agent alone. This synergy was observed at sorafenib concentrations sufficient to downmodulate Mcl-1 but insufficient to inhibit Erk phosphorylation. Concurrent treatment with Z-VAD reverses the effects of the combination in the A375 cell line but not in the A2058 cell line, suggesting that ABT-737 may enhance the previously described effects of sorafenib on the nuclear translocation of AIF in this cell line. Conclusions: The combination of ABT-737 and sorafenib leads to synergistic cytotoxicity in melanoma cell lines. This effect appears independent of the inhibition of raf by sorafenib and is correlated instead with the downmodulation of Mcl-1. The variable dependence of this synergy on caspase activation suggests that the mechanism of lethality may differ between cell lines. Cellular fractionation assays are ongoing to explore the effects of the combination on nuclear translocation of AIF. Given the paucity of effective therapies in malignant melanoma, the combination of inhibitors of Bcl-2 family members with sorafenib may be an attractive combination to advance to clinical assessment. Citation Information: In: Proc Am Assoc Cancer Res; 2009 Apr 18-22; Denver, CO. Philadelphia (PA): AACR; 2009. Abstract nr 5501.