Chemical Therapeutics Bcl-2 / Bcl-xL Inhibition Increases the Ef fi cacy of MEK InhibitionAlone and inCombinationwith PI 3 Kinase Inhibition in Lung and Pancreatic Tumor Models

Although mitogen-activated protein (MAP)–extracellular signal-regulated kinase (ERK) kinase (MEK) inhibition is predicted to cause cell death by stabilization of the proapoptotic BH3-only protein BIM, the induction of apoptosis is often modest. To determine if addition of a Bcl-2 family inhibitor could increase the efficacy of a MEK inhibitor, we evaluated a panel of 53 non–small cell lung cancer and pancreatic cancer cell lines with the combination of navitoclax (ABT-263), a Bcl-2/Bcl-xL (BCL2/BCL2L1) antagonist, and a novel MAPkinase (MEK) inhibitor, G-963. The combination is synergistic in themajority of lines,with an enrichment of cell lines harboring KRAS mutations in the high synergy group. Cells exposed to G-963 arrest in G1 and a small fraction undergo apoptosis. The addition of navitoclax to G-963 does not alter the kinetics of cell-cycle arrest, but greatly increases the percentage of cells that undergo apoptosis. The G-963/navitoclax combination wasmore effective than either single agent in the KRASmutant H2122 xenograft model; BIM stabilization and PARP cleavage were observed in tumors, consistent with the mechanism of action observed in cell culture. Addition of the phosphatidylinositol 3-kinase (PI3K, PIK3CA) inhibitor GDC-0941 to this treatment combination increases cell killing compared with doubleor single-agent treatment. Taken together, these data suggest the efficacy of agents that target theMAPK and PI3K pathways can be improved by combination with a Bcl-2 family inhibitor. Mol Cancer Ther; 12(6); 853–64. 2013 AACR. Introduction Recent advances in cancer drug therapies associated with targeting oncogenic "drivers" have shown tremendous potential for improving clinical benefit. However, a major limitation to the overall benefit from targeted therapy is the development of drug resistance. Resistance can occur because of mutations that render the drug target insensitive to the inhibitor or when cancer cells change their dependency on the pathway that is targeted. In the first example, resistance can be overcome by developing new drugs that effectively inhibit resistance-associated mutants, as in the example of dasatinib and nilotinib, which are effective on BCR/ABL mutants that confer resistance to imatinib (1). Another approach is to target multiple signaling pathways simultaneously, and thus prevent the cancer cell from changing its dependency to another signaling pathway. There is considerable preclinical evidence supporting this approach, for example, in the setting of combining inhibitors of mitogen-activated protein (MAP)–extracellular signal-regulated kinase (ERK) kinase (MEK; MAP kinase kinase, MAP2K) with inhibitors of PI3 kinase (phosphatidylinositol 3-kinase, PIK3CA) or (2–7) BRAF (8–10), and these combinations are beginning to show efficacy in the clinic (11). A third approach to enhancing the efficacy of targeted therapy is to simultaneously target downstream proteins that protect tumor cells from apoptosis and thus increase overall cell killing (12–14). This strategy relies on increasing cancer cell cytotoxicity cells to decrease the probability that a cell will survive to develop resistance. In addition, altering upstream dependencies will be of no avail because the signaling pathways ultimately converge on these downstream apoptotic pathways. Here, we have explored the combination of an MEK kinase inhibitor and a Bcl-2/Bcl-xL inhibitor to evaluate the potential utility of this combination treatment strategy. MEKtransduces signalsdownstreamof theEGFR,RAS, and RAF proteins, and activating mutations in any of these proteins can lead to enhanced MEK activity. MEK activation in turn produces prosurvival signals via inhibition of the proapoptotic BH3-only proteins BIM and Authors' Affiliations: Genentech, Inc., South San Francisco, California; and Argenta Discovery Ltd., Harlow, Essex, United Kingdom Note: Supplementary data for this article are available at Molecular Cancer Therapeutics Online (http://mct.aacrjournals.org/). Corresponding Author: Lisa D. Belmont, Genentech, Inc., 1 DNA Way, South San Francisco, CA 94080. Phone: 650-467-7462, Fax: 650-7425179; E-mail: belmont.lisa@gene.com doi: 10.1158/1535-7163.MCT-12-0949 2013 American Association for Cancer Research. Molecular Cancer Therapeutics www.aacrjournals.org 853 on June 21, 2017. © 2013 American Association for Cancer Research. mct.aacrjournals.org Downloaded from Published OnlineFirst March 8, 2013; DOI: 10.1158/1535-7163.MCT-12-0949