Abstract B31: A protein synthesis switch underlies initial survival of EGFR-mutant lung cancer to EGFR inhibitors

EGFR inhibitors (EGFRi) are effective at inducing transient tumor shrinkage in EGFR-mutant lung cancers. The efficacy of these drugs however is mitigated by the outgrowth of resistant cells: this is most often manifested by a secondary mutation in EGFR, T790M, which leads to reactivation of key intracellular signaling despite continued drug treatment. We recently demonstrated that T790M can occur both at low frequencies prior to initiation of EGFR inhibitor therapy, or alternatively arise de novo during treatment (Hata et al., Nat Med 2016). Since some cancers form T790M mutations de novo, one potential therapeutic strategy to thwart resistance is to identify the cells surviving initial therapy (referred to as persister cells or drug-tolerant cells [DTCs]) that eventually acquire the T790M mutation, and eliminate them prior to T790M acquisition. To this end, we hypothesized that some cells were refractory to EGFR inhibitor-induced apoptosis, surviving initial therapy and forming a reservoir of cells that could then eventually acquire T790M. We demonstrate that Western blots of lysates from EGFR-mutant lung cancers surviving initial therapy to the EGFR inhibitor gefitinib detect quick (<10 days) and selective upregulation of the anti-apoptotic protein MCL-1. We performed experiments with fluorescent-tagged exogenous MCL-1-expressing cells and demonstrate enrichment of MCL-1-expressing cells following gefitinib treatment by high-content imaging; these cells persisted as early survivors. Accordingly, exogenous expression of MCL-1 prevented gefitinib from shrinking EGFR-mutant lung cancer tumors, and the emergence of DTCs could be largely thwarted by co-incubation with MCL-1 specific inhibitors A-1210477 and S63845. Mechanistically, we report DTCs undergo a “translational switch” that manifests in increased cap-dependent mRNA translation of MCL-1, which corresponds to cellular upregulation of mTOR/eIF4 and downregulation of ribosomal proteins. These data reveal a novel mechanism in which lung cancer cells adapt to short-term pressures of apoptosis-inducing kinase inhibitors by shifting protein biosynthesis through cap-dependent translation of MCL-1 protein. Moreover, in EGFR-mutant lung cancer, MCL-1 is a key molecule governing the emergence of early DTCs to EGFR inhibitors and can be effectively co-targeted with clinically-emerging MCL-1 inhibitors, which may delay the acquisition of secondary mutations including T790M mutations, therefore prolonging therapy efficacy.

Citation Format: Kyung-A Song, Timothy L. Lochmann, Neha U. Patel, Jungoh Ham, Brad E. Windle, Hisashi Harada, Joel D. Leverson, Andrew J. Souers, Aaron N. Hata, Hiromichi Ebi, Anthony C. Faber. A protein synthesis switch underlies initial survival of EGFR-mutant lung cancer to EGFR inhibitors [abstract]. In: Proceedings of the Fifth AACR-IASLC International Joint Conference: Lung Cancer Translational Science from the Bench to the Clinic; Jan 8-11, 2018; San Diego, CA. Philadelphia (PA): AACR; Clin Cancer Res 2018;24(17_Suppl):Abstract nr B31.

Increased Synthesis of MCL-1 Protein Underlies Initial Survival of EGFR-Mutant Lung Cancer to EGFR Inhibitors and Provides a Novel Drug Target

Purpose: EGFR inhibitors (EGFRi) are effective against EGFR-mutant lung cancers. The efficacy of these drugs, however, is mitigated by the outgrowth of resistant cells, most often driven by a secondary acquired mutation in EGFR, T790M We recently demonstrated that T790M can arise de novo during treatment; it follows that one potential therapeutic strategy to thwart resistance would be identifying and eliminating these cells [referred to as drug-tolerant cells (DTC)] prior to acquiring secondary mutations like T790M Experimental Design: We have developed DTCs to EGFRi in EGFR-mutant lung cancer cell lines. Subsequent analyses of DTCs included RNA-seq, high-content microscopy, and protein translational assays. Based on these results, we tested the ability of MCL-1 BH3 mimetics to combine with EGFR inhibitors to eliminate DTCs and shrink EGFR-mutant lung cancer tumors in vivo Results: We demonstrate surviving EGFR-mutant lung cancer cells upregulate the antiapoptotic protein MCL-1 in response to short-term EGFRi treatment. Mechanistically, DTCs undergo a protein biosynthesis enrichment resulting in increased mTORC1-mediated mRNA translation of MCL-1, revealing a novel mechanism in which lung cancer cells adapt to short-term pressures of apoptosis-inducing kinase inhibitors. Moreover, MCL-1 is a key molecule governing the emergence of early EGFR-mutant DTCs to EGFRi, and we demonstrate it can be effectively cotargeted with clinically emerging MCL-1 inhibitors both in vitro and in vivo Conclusions: Altogether, these data reveal that this novel therapeutic combination may delay the acquisition of secondary mutations, therefore prolonging therapy efficacy. Clin Cancer Res; 24(22); 5658-72. ©2018 AACR.

Cotargeting BCL-2 and PI3K Induces BAX-Dependent Mitochondrial Apoptosis in AML Cells

Inhibitors targeting BCL-2 apoptotic proteins have significant potential for the treatment of acute myeloid leukemia (AML); however, complete responses are observed in only 20% of patients, suggesting that targeting BCL-2 alone is insufficient to yield durable responses. Here, we assessed the efficacy of coadministration of the PI3K/mTOR inhibitor GDC-0980 or the p110β-sparing PI3K inhibitor taselisib with the selective BCL-2 antagonist venetoclax in AML cells. Tetracycline-inducible downregulation of BCL-2 significantly sensitized MV4-11 and MOLM-13 AML cells to PI3K inhibition. Venetoclax/GDC-0980 coadministration induced rapid and pronounced BAX mitochondrial translocation, cytochrome c release, and apoptosis in various AML cell lines in association with AKT/mTOR inactivation and MCL-1 downregulation; ectopic expression of MCL-1 significantly protected cells from this regimen. Combined treatment was also effective against primary AML blasts from 17 patients, including those bearing various genetic abnormalities. Venetoclax/GDC-0980 markedly induced apoptosis in primitive CD34⁺/38^-/123⁺ AML cell populations but not in normal hematopoietic progenitor CD34⁺ cells. The regimen was also active against AML cells displaying intrinsic or acquired venetoclax resistance or tumor microenvironment-associated resistance. Either combinatorial treatment markedly reduced AML growth and prolonged survival in a systemic AML xenograft mouse model and diminished AML growth in two patient-derived xenograft models. Venetoclax/GDC-0980 activity was partially diminished in BAK^-/- cells and failed to induce apoptosis in BAX^-/- and BAX^-/-BAK^-/- cells, whereas BIM^-/- cells were fully sensitive. Similar results were observed with venetoclax alone in in vitro and in vivo systemic xenograft models. Collectively, these studies demonstrate that venetoclax/GDC-0980 exhibits potent anti-AML activity primarily through BAX and, to a lesser extent, BAK. These findings argue that dual BCL-2 and PI3K inhibition warrants further evaluation in AML.Significance: Combined treatment with clinically relevant PI3K and BCL-2 inhibitors may prove effective in the treatment of acute myeloid leukemia. Cancer Res; 78(11); 3075-86. ©2018 AACR.

Combined targeting of BCL-2 and BCR-ABL tyrosine kinase eradicates chronic myeloid leukemia stem cells

BCR-ABL tyrosine kinase inhibitors (TKIs) are effective against chronic myeloid leukemia (CML), but they rarely eliminate CML stem cells. Disease relapse is common upon therapy cessation, even in patients with complete molecular responses. Furthermore, once CML progresses to blast crisis (BC), treatment outcomes are dismal. We hypothesized that concomitant targeting of BCL-2 and BCR-ABL tyrosine kinase could overcome these limitations. We demonstrate increased BCL-2 expression at the protein level in bone marrow cells, particularly in Lin(-)Sca-1(+)cKit(+) cells of inducible CML in mice, as determined by CyTOF mass cytometry. Further, selective inhibition of BCL-2, aided by TKI-mediated MCL-1 and BCL-XL inhibition, markedly decreased leukemic Lin(-)Sca-1(+)cKit(+) cell numbers and long-term stem cell frequency and prolonged survival in a murine CML model. Additionally, this combination effectively eradicated CD34(+)CD38(-), CD34(+)CD38(+), and quiescent stem/progenitor CD34(+) cells from BC CML patient samples. Our results suggest that BCL-2 is a key survival factor for CML stem/progenitor cells and that combined inhibition of BCL-2 and BCR-ABL tyrosine kinase has the potential to significantly improve depth of response and cure rates of chronic-phase and BC CML.

Synthetic Lethality of Combined Bcl-2 Inhibition and p53 Activation in AML: Mechanisms and Superior Antileukemic Efficacy

Evasion of apoptosis is a hallmark of cancer. Bcl-2 and p53 represent two important nodes in apoptosis signaling pathways. We find that concomitant p53 activation and Bcl-2 inhibition overcome apoptosis resistance and markedly prolong survival in three mouse models of resistant acute myeloid leukemia (AML). Mechanistically, p53 activation negatively regulates the Ras/Raf/MEK/ERK pathway and activates GSK3 to modulate Mcl-1 phosphorylation and promote its degradation, thus overcoming AML resistance to Bcl-2 inhibition. Moreover, Bcl-2 inhibition reciprocally overcomes apoptosis resistance to p53 activation by switching cellular response from G₁ arrest to apoptosis. The efficacy, together with the mechanistic findings, reveals the potential of simultaneously targeting these two apoptosis regulators and provides a rational basis for clinical testing of this therapeutic approach.

Found in Translation: How Preclinical Research Is Guiding the Clinical Development of the BCL2-Selective Inhibitor Venetoclax

Since the discovery of apoptosis as a form of programmed cell death, targeting the apoptosis pathway to induce cancer cell death has been a high-priority goal for cancer therapy. After decades of effort, drug-discovery scientists have succeeded in generating small-molecule inhibitors of antiapoptotic BCL2 family proteins. Innovative medicinal chemistry and structure-based drug design, coupled with a strong fundamental understanding of BCL2 biology, were essential to the development of BH3 mimetics such as the BCL2-selective inhibitor venetoclax. We review a number of preclinical studies that have deepened our understanding of BCL2 biology and facilitated the clinical development of venetoclax.Significance: Basic research into the pathways governing programmed cell death have paved the way for the discovery of apoptosis-inducing agents such as venetoclax, a BCL2-selective inhibitor that was recently approved by the FDA and the European Medicines Agency. Preclinical studies aimed at identifying BCL2-dependent tumor types have translated well into the clinic thus far and will likely continue to inform the clinical development of venetoclax and other BCL2 family inhibitors. Cancer Discov; 7(12); 1376-93. ©2017 AACR.

Venetoclax Is Effective in Small-Cell Lung Cancers with High BCL-2 Expression

Purpose: Small-cell lung cancer (SCLC) is an often-fatal neuroendocrine carcinoma usually presenting as extensive disease, carrying a 3% 5-year survival. Despite notable advances in SCLC genomics, new therapies remain elusive, largely due to a lack of druggable targets.Experimental Design: We used a high-throughput drug screen to identify a venetoclax-sensitive SCLC subpopulation and validated the findings with multiple patient-derived xenografts of SCLC.Results: Our drug screen consisting of a very large collection of cell lines demonstrated that venetoclax, an FDA-approved BCL-2 inhibitor, was found to be active in a substantial fraction of SCLC cell lines. Venetoclax induced BIM-dependent apoptosis in vitro and blocked tumor growth and induced tumor regressions in mice bearing high BCL-2-expressing SCLC tumors in vivo BCL-2 expression was a predictive biomarker for sensitivity in SCLC cell lines and was highly expressed in a subset of SCLC cell lines and tumors, suggesting that a substantial fraction of patients with SCLC could benefit from venetoclax. Mechanistically, we uncover a novel role for gene methylation that helped discriminate high BCL-2-expressing SCLCs.Conclusions: Altogether, our findings identify venetoclax as a promising new therapy for high BCL-2-expressing SCLCs. Clin Cancer Res; 24(2); 360-9. ©2017 AACR.

Systems analysis of apoptotic priming in ovarian cancer identifies vulnerabilities and predictors of drug response

The lack of effective chemotherapies for high-grade serous ovarian cancers (HGS-OvCa) has motivated a search for alternative treatment strategies. Here, we present an unbiased systems-approach to interrogate a panel of 14 well-annotated HGS-OvCa patient-derived xenografts for sensitivity to PI3K and PI3K/mTOR inhibitors and uncover cell death vulnerabilities. Proteomic analysis reveals that PI3K/mTOR inhibition in HGS-OvCa patient-derived xenografts induces both pro-apoptotic and anti-apoptotic signaling responses that limit cell killing, but also primes cells for inhibitors of anti-apoptotic proteins. In-depth quantitative analysis of BCL-2 family proteins and other apoptotic regulators, together with computational modeling and selective anti-apoptotic protein inhibitors, uncovers new mechanistic details about apoptotic regulators that are predictive of drug sensitivity (BIM, caspase-3, BCL-X_L) and resistance (MCL-1, XIAP). Our systems-approach presents a strategy for systematic analysis of the mechanisms that limit effective tumor cell killing and the identification of apoptotic vulnerabilities to overcome drug resistance in ovarian and other cancers.High-grade serous ovarian cancers (HGS-OvCa) frequently develop chemotherapy resistance. Here, the authors through a systematic analysis of proteomic and drug response data of 14 HGS-OvCa PDXs demonstrate that targeting apoptosis regulators can improve response of these tumors to inhibitors of the PI3K/mTOR pathway.

Systems analysis of apoptotic priming in ovarian cancer identifies vulnerabilities and predictors of drug response

The lack of effective chemotherapies for high-grade serous ovarian cancers (HGS-OvCa) has motivated a search for alternative treatment strategies. Here, we present an unbiased systems-approach to interrogate a panel of 14 well-annotated HGS-OvCa patient-derived xenografts for sensitivity to PI3K and PI3K/mTOR inhibitors and uncover cell death vulnerabilities. Proteomic analysis reveals that PI3K/mTOR inhibition in HGS-OvCa patient-derived xenografts induces both pro-apoptotic and anti-apoptotic signaling responses that limit cell killing, but also primes cells for inhibitors of anti-apoptotic proteins. In-depth quantitative analysis of BCL-2 family proteins and other apoptotic regulators, together with computational modeling and selective anti-apoptotic protein inhibitors, uncovers new mechanistic details about apoptotic regulators that are predictive of drug sensitivity (BIM, caspase-3, BCL-X_L) and resistance (MCL-1, XIAP). Our systems-approach presents a strategy for systematic analysis of the mechanisms that limit effective tumor cell killing and the identification of apoptotic vulnerabilities to overcome drug resistance in ovarian and other cancers.High-grade serous ovarian cancers (HGS-OvCa) frequently develop chemotherapy resistance. Here, the authors through a systematic analysis of proteomic and drug response data of 14 HGS-OvCa PDXs demonstrate that targeting apoptosis regulators can improve response of these tumors to inhibitors of the PI3K/mTOR pathway.

Abstract 4033: Combined MEK and BCL-2/XL inhibition as a potential drug combination for the treatment of high-grade serous ovarian cancer

High-grade serous ovarian cancer (HGSOC) accounts for 70-80% of ovarian cancer deaths. Despite an initial response to platinum-based chemotherapy, treatment resistance eventually occurs in most patients. The overall aim of our studies is to identify synergistic drug combinations for the treatment of HGSOC and biomarkers that predict sensitivity for future translation in clinical trials. In particular, this study focuses on the vulnerabilities of patient-derived ovarian cancer cells to combined inhibition of the MEK pathway and the anti-apoptotic proteins BCL-2 and BCL-XL.

The Ras/MAPK pathway is activated in a subset of HGSOC by gene copy alterations. However, MEK inhibitors have not been evaluated in HGSOC. In this study, we used a collection of 14 primary samples derived from ascites cells of platinum-resistant HGSOC patients (Liu JF et al. 2016). We investigated the sensitivity of these tumor cells to the MEK inhibitor cobimetinib (GDC-0973). Treatment with GDC-0973 had no effects on cell viability in vitro in any of the patient-derived models. To investigate the response to MEK inhibition, we performed Reverse Phase Protein Array of the tumor cells treated with 0.3μM of GDC-0973 in vitro. We found that MEK inhibition by GDC-0973 reduced cell cycle progression markers and upregulated the pro-apoptotic protein BIM. Since more than half of the models express high levels of anti-apoptotic BCL-2 family proteins which neutralize BIM, we examined the effects of antagonism of BCL-2 pro-survival proteins in combination with GDC-0973. Treatment with a combination of GDC-0973 and the BCL-2/XL antagonist, navitoclax (ABT-263) significantly reduced cell number and increased cell death in 10 out of 14 patient-derived models. Protein levels of BIM following treatment with GDC-0973 correlated with sensitivity to the drug combination (R2=0.8 p<0.0001). Interestingly, the BIM levels after MEK inhibition correlate with BIM protein levels before the treatment, suggesting that baseline BIM levels could be used as biomarker of sensitivity. Levels of the pro-survival protein MCL-1 also played a critical role in patient-derived cells that were resistant to the drug combination. Indeed, treatment with a specific MCL-1 inhibitor, A-1210477, in combination with GDC-0973 and ABT-263 led to dramatic tumor cell killing in vitro compared to dual combination of GDC-0973 and ABT-263.

Finally, we tested the combination of GDC-0973 and ABT-263 in vivo and preliminary results indicate that the drug combination is well tolerated and is able to significantly reduce tumor growth in patient-derived xenograft models (n=4).

Our studies provide significant evidence that combined inhibition of MEK and BCL-2/XL may be an effective drug combination for treatment of HGSOC and that the pro-apoptotic protein BIM may serve as a predictive biomarker to stratify patients that can benefit from these targeted therapies

Citation Format: Claudia Iavarone, Ioannis Zervantonakis, Laura M. Selfors, Sangeetha Palakurthi, Joyce F. Liu, Ursula A. Matulonis, Ronny I. Drapkin, Gordon B. Mills, Joel D. Leverson, Deepak Sampath, Joan S. Brugge. Combined MEK and BCL-2/XL inhibition as a potential drug combination for the treatment of high-grade serous ovarian cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 4033. doi:10.1158/1538-7445.AM2017-4033

Abstract 3082: Deficient NOXA in HER2-amplified breast cancer drives kinase inhibitor resistance

The purpose of this study is the development of a novel combination therapy that targets HER2-amplified breast cancer. About one quarter of breast cancers harbor amplification of HER2. HER2 is a transmembrane receptor tyrosine kinase (RTK) belonging to the ERBB family of receptors (ERBB1-4). Upon hetero- and homo-dimerization, HER2 activates several key intracellular pathways, regulating many cellular functions including proliferation and survival. HER2 inhibitors (HER2i) (e.g. the receptor tyrosine kinase (RTK) inhibitor, lapatinib) are now part of standard care for treating HER2-amplified breast cancers. However, despite their anti-cancer benefit, these drugs have limited efficacy as monotherapies, which contrasts to other RTK inhibitors in other RTK-driven cancers. To better understand this apparent dichotomy, in the present study, we evaluated potential modifiers of HER2i therapy. Here, we found that the pro-apoptotic NOXA, a member of the B-cell CLL/lymphoma 2 (BCL2) family which acts mainly via inhibitory binding of the pro-survival MCL-1, was markedly down-regulated in breast cancers compared to other cancers, and this was largely attributed to the HER2-amplified subset. Experimentally, overexpressing NOXA or silencing MCL-1 dramatically sensitizes HER2 amplified breast cancer cell lines to lapatinib via apoptosis. Consistently, pharmaceutical inhibition of MCL-1 sensitizes HER2-amplified breast cancers to lapatinib in vitro and in vivo. Mechanistically, disruption of MCL-1:BIM complexes and MCL-1:BAK underlie dual HER2i/MCL-1i therapy. Therefore, deficient NOXA expression constitutes a bonafide apoptotic block in HER2 amplified breast cancers, contributes to mitigated HER2i responses, and presents a rational combination therapy that may improve HER2i responses.

Citation Format: Konstantinos V. Floros, Kyung-A Song, Timothy L. Lochmann, Mark T. Hughes, Daniel A. Heisey, Hisashi Harada, Bin Hu, Jennifer Koblinski, Andrew J. Souers, Joel D. Leverson, Anthony C. Faber. Deficient NOXA in HER2-amplified breast cancer drives kinase inhibitor resistance [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 3082. doi:10.1158/1538-7445.AM2017-3082

Potential mechanisms of resistance to venetoclax and strategies to circumvent it

Background

Venetoclax (ABT-199), a first-in-class orally bioavailable BCL-2-selective inhibitor, was recently approved by the FDA for use in patients with 17p-deleted chronic lymphocytic leukemia who have received prior therapy. It is also being evaluated in numerous clinical trials for treating patients with various hematologic malignancies. As with any targeted cancer therapy, it is critically important to identify potential mechanisms of resistance, both for patient stratification and developing strategies to overcome resistance, either before it develops or as it emerges.

Methods

In order to gain a more comprehensive insight into the nature of venetoclax resistance mechanisms, we evaluated the changes in the BCL-2 family members at the genetic and expression levels in seven different venetoclax-resistant derived leukemia and lymphoma cell lines.

Results

Gene and protein expression analyses identified a number of different alterations in the expression of pro- and anti-apoptotic BCL-2 family members. In the resistant derived cells, an increase in either or both the anti-apoptotic proteins BCL-X_L or MCL-1, which are not targeted by venetoclax was observed, and either concomitant or exclusive with a decrease in one or more pro-apoptotic proteins. In addition, mutational analysis also revealed a mutation in the BH3 binding groove (F104L) that could potentially interfere with venetoclax-binding. Not all changes may be causally related to venetoclax resistance and may only be an epiphenomenon. For resistant cell lines showing elevations in BCL-X_L or MCL-1, strong synergistic cell killing was observed when venetoclax was combined with either BCL-X_L- or MCL-1-selective inhibitors, respectively. This highlights the importance of BCL-X_L- and MCL-1 as causally contributing to venetoclax resistance.

Conclusions

Overall our study identified numerous changes in multiple resistant lines; the changes were neither mutually exclusive nor universal across the cell lines tested, thus exemplifying the complexity and heterogeneity of potential resistance mechanisms. Identifying and evaluating their contribution has important implications for both patient selection and the rational development of strategies to overcome resistance.

Electronic supplementary material

The online version of this article (doi:10.1186/s12885-017-3383-5) contains supplementary material, which is available to authorized users.

Abstract IA34: Clinical proof of concept for the first-in-class BCL-2-selective inhibitor venetoclax (ABT-199/GDC-0199)

Many cancer cells maintain survival through over-expression of anti-apoptotic BCL-2 family proteins, which sequester high levels of their pro-apoptotic counterparts. While this makes BCL-2 proteins compelling targets for the development of cancer therapeutics, synthesizing small molecules capable of disrupting these protein-protein interactions has been a major challenge for the field. Structure-based drug design led to the discovery of navitoclax (ABT-263), a dual inhibitor of BCL-2 and BCL-XL. Navitoclax has shown promising activity in the clinic but its efficacy has been limited by thrombocytopenia caused by BCL-XL inhibition. To avoid this toxicity, we designed a BCL-2-selective inhibitor, venetoclax (ABT-199/GDC-0199), that maintains efficacy in hematologic malignancies while sparing platelets. In this presentation, I will review the preclinical work that led to the discovery of venetoclax, the translational studies that have pointed to utility for venetoclax in a variety of cancers, and the results from ongoing clinical studies, in which venetoclax has demonstrated clear signs of activity. I will discuss what we have learned from multiple iterations of translational work between the bench and the bedside, and how we are using a toolkit of highly selective BCL-2 family inhibitors to parse the roles of BCL-2 family proteins in cancer cell survival.

JDL is an AbbVie employee. The design, study conduct and financial support was provided by AbbVie and Genentech. AbbVie and Genentech participated in the data generation, interpretation of data, review and approval of this publication (A5107572).

Citation Format: Joel D. Leverson. Clinical proof of concept for the first-in-class BCL-2-selective inhibitor venetoclax (ABT-199/GDC-0199). [abstract]. In: Proceedings of the Fourth AACR International Conference on Frontiers in Basic Cancer Research; 2015 Oct 23-26; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2016;76(3 Suppl):Abstract nr IA34.

Abstract B48: Design of effective combination therapies for high-grade serous ovarian cancer using patient-derived xenograft models

Despite advances in understanding the genetics and molecular biology of high grade serous ovarian cancer, there have not been any significant improvements in the outcome of patients treated with approved therapies to date. The overall aim of our studies is to identify synergistic drug combinations for the treatment of high-grade serous ovarian (HGS-Ov) cancer and the biomarkers that predict sensitivity for future translation in clinical trials. In particular, this study focuses on the vulnerabilities of patient-derived ovarian cancer cells to inhibition of anti-apoptotic proteins and the design of novel combination therapies to overcome drug resistance and improve the response of selected patients to a particular set of targeted therapies.

Previous studies from our lab provided evidence that inhibition of BCL-2/XL can significantly enhance the sensitivity of tumor cells to targeted therapies in ovarian and breast cancer cell line models (Muranen T, et al. Cancer Cell 2012). In this study, we evaluated the efficacy of this combination treatment in ascites cells derived from 15 high-grade serous patient-derived ovarian cancer xenografts (PDXs). We found that inhibitors of PI3K/mTOR (GNE-493) and BCL-2/XL (ABT-737) act synergistically ex vivo, with an ~15 fold variability among the 15 patient-derived samples. A large scale in vivo experiment is ongoing to evaluate the efficacy of this combined treatment in six PDX models.

To identify biomarkers that predict drug sensitivity we performed proteomic Reverse Phase Protein Array (RPPA) and immunoblot analyses. We found that baseline levels of the pro-apoptotic protein BIM correlate with sensitivity to BCL-2/XL inhibition. This motivated us to examine the basis for low BIM expression. In three of the least sensitive PDX models, we found that low levels of BIM correlated with ERK activation based on increased ERK phosphorylation and that inhibition of MEK by PD-0325901 in these models caused upregulation of BIM, as predicted from previous published reports which demonstrated that phosphorylation of BIM by ERK causes its degradation (Ley R, et al. Biol Chem 2003). We then examined the sensitivity of these PDX models to ABT-737 and PD-0325901, MEK162, or CI-1040 ex vivo and observed strong synergy resulting in reduction of cell viability and increased cell death. These results suggest that low BIM/high phosphoERK is a potential biomarker for sensitivity of ovarian tumors to combined MEK and BCL-2/XL inhibition. We now plan to examine the efficacy of drug combinations that target MEK and BCL-2/XL in these PDX models in vivo.

Our studies promise to lead to the identification of new drug combination therapies for HGS-Ov cancer treatment and predictive biomarkers to stratify patients that can benefit from these targeted therapies.

Citation Format: Claudia Iavarone, Ioannis Zervantonakis, Hsing-Yu Chen, Sangeetha S. Palakurthi, Joyce Fu Liu, Ursula A. Matulonis, Ronny I. Drapkin, Gordon B. Mills, Joel D. Leverson, Deepak Sampath, Joan S. Brugge. Design of effective combination therapies for high-grade serous ovarian cancer using patient-derived xenograft models. [abstract]. In: Proceedings of the AACR Special Conference on Advances in Ovarian Cancer Research: Exploiting Vulnerabilities; Oct 17-20, 2015; Orlando, FL. Philadelphia (PA): AACR; Clin Cancer Res 2016;22(2 Suppl):Abstract nr B48.

MLL-Rearranged Acute Lymphoblastic Leukemias Activate BCL-2 through H3K79 Methylation and Are Sensitive to the BCL-2-Specific Antagonist ABT-199

Targeted therapies designed to exploit specific molecular pathways in aggressive cancers are an exciting area of current research. Mixed Lineage Leukemia (MLL) mutations such as the t(4;11) translocation cause aggressive leukemias that are refractory to conventional treatment. The t(4;11) translocation produces an MLL/AF4 fusion protein that activates key target genes through both epigenetic and transcriptional elongation mechanisms. In this study, we show that t(4;11) patient cells express high levels of BCL-2 and are highly sensitive to treatment with the BCL-2-specific BH3 mimetic ABT-199. We demonstrate that MLL/AF4 specifically upregulates the BCL-2 gene but not other BCL-2 family members via DOT1L-mediated H3K79me2/3. We use this information to show that a t(4;11) cell line is sensitive to a combination of ABT-199 and DOT1L inhibitors. In addition, ABT-199 synergizes with standard induction-type therapy in a xenotransplant model, advocating for the introduction of ABT-199 into therapeutic regimens for MLL-rearranged leukemias.

Exploiting selective BCL-2 family inhibitors to dissect cell survival dependencies and define improved strategies for cancer therapy

The BCL-2/BCL-XL/BCL-W inhibitor ABT-263 (navitoclax) has shown promising clinical activity in lymphoid malignancies such as chronic lymphocytic leukemia. However, its efficacy in these settings is limited by thrombocytopenia caused by BCL-XL inhibition. This prompted the generation of the BCL-2-selective inhibitor venetoclax (ABT-199/GDC-0199), which demonstrates robust activity in these cancers but spares platelets. Navitoclax has also been shown to enhance the efficacy of docetaxel in preclinical models of solid tumors, but clinical use of this combination has been limited by neutropenia. We used venetoclax and the BCL-XL-selective inhibitors A-1155463 and A-1331852 to assess the relative contributions of inhibiting BCL-2 or BCL-XL to the efficacy and toxicity of the navitoclax-docetaxel combination. Selective BCL-2 inhibition suppressed granulopoiesis in vitro and in vivo, potentially accounting for the exacerbated neutropenia observed when navitoclax was combined with docetaxel clinically. By contrast, selectively inhibiting BCL-XL did not suppress granulopoiesis but was highly efficacious in combination with docetaxel when tested against a range of solid tumors. Therefore, BCL-XL-selective inhibitors have the potential to enhance the efficacy of docetaxel in solid tumors and avoid the exacerbation of neutropenia observed with navitoclax. These studies demonstrate the translational utility of this toolkit of selective BCL-2 family inhibitors and highlight their potential as improved cancer therapeutics.

Loss in MCL-1 function sensitizes non-Hodgkin's lymphoma cell lines to the BCL-2-selective inhibitor venetoclax (ABT-199)

As a population, non-Hodgkin's lymphoma (NHL) cell lines positive for the t(14;18) translocation and/or possessing elevated BCL2 copy number (CN; BCL2(High)) are exquisitely sensitive to navitoclax or the B-cell lymphoma protein-2 (BCL-2)-selective inhibitor venetoclax. Despite this, some BCL2(High) cell lines remain resistant to either agent. Here we show that the MCL-1-specific inhibitor A-1210477 sensitizes these cell lines to navitoclax. Chemical segregation of this synergy with the BCL-2-selective inhibitor venetoclax or BCL-XL-selective inhibitor A-1155463 indicated that MCL-1 and BCL-2 are the two key anti-apoptotic targets for sensitization. Similarly, the CDK inhibitor flavopiridol downregulated MCL-1 expression and synergized with venetoclax in BCL2(High) NHL cell lines to a similar extent as A-1210477. A-1210477 also synergized with navitoclax in the majority of BCL2(Low) NHL cell lines. However, chemical segregation with venetoclax or A-1155463 revealed that synergy was driven by BCL-XL inhibition in this population. Collectively these data emphasize that BCL2 status is predictive of venetoclax potency in NHL not only as a single agent, but also in the adjuvant setting with anti-tumorigenic agents that inhibit MCL-1 function. These studies also potentially identify a patient population (BCL2(Low)) that could benefit from BCL-XL (navitoclax)-driven combination therapy.

Clearance of systemic hematologic tumors by venetoclax (Abt-199) and navitoclax

The Bcl-2 family inhibitors venetoclax and navitoclax demonstrated potent antitumor activity in chronic lymphocytic leukemia patients, notably in reducing marrow load and adenopathy. Subsequent trials with venetoclax have been initiated in non-Hodgkin's lymphoma and multiple myeloma patients. Traditional preclinical models fall short either in faithfully recapitulating disease progression within such compartments or in allowing the direct longitudinal analysis of systemic disease. We show that intravenous inoculation of engineered RS4;11 (acute lymphoblastic leukemia) and Granta 519 (mantle cell lymphoma) bioluminescent reporter cell lines result in tumor engraftment of bone marrow, with additional invasion of the central nervous system in the case of Granta 519. Importantly, apoptosis induction and response of these systemically engrafted tumors to Bcl-2 family inhibitors alone or in combination with standard-of-care agents could be monitored longitudinally with optical imaging, and was more accurately reflective of the observed clinical response.

Abstract 6: Quantitative assessment of BCL-2:BIM complexes as a pharmacodynamic marker for venetoclax (ABT-199)

The BCL-2-selective inhibitor venetoclax (ABT-199) binds with high affinity to the BH3-binding groove of BCL-2, thereby competing for binding with the BH3-only protein BIM (Souers et al., 2013). Venetoclax is currently being evaluated in clinical trials for CLL, AML, multiple myeloma and NHL. To facilitate these studies, we developed and validated a 384-well electrochemiluminescent ELISA (MSD, Gaithersburg, MD,USA) that quantifies expression of BCL-2, BCL-XL, and MCL-1protein alone or in complex with BIM. We subsequently quantified expression of BCL-2 and BCL-2:BIM complexes in 16 hematologic tumor cell lines. We found the EC50 of venetoclax in these tumor cell lines to correlate strongly with baseline BCL-2:BIM complex levels. This correlation was superior to the correlation between venetoclax EC50 and absolute BCL-2 expression. We also applied the assay to measure disruption of BCL-2:BIM complexes in vivo. Treatment of the Non-Hodgkin's Lymphoma (NHL) xenograft model SU-DHL-4 with a BCL-2-selective inhibitor resulted in disruption of tumor BCL-2:BIM complexes that aligned with serum and tumor concentrations of inhibitor. Collectively, these data demonstrate that quantifying BCL-2:BIM complexes offers an accurate means of assessing target engagement by venetoclax and, potentially, predicting its efficacy. The utility of this assay is currently being assessed in clinical trials.

Citation Format: Sha Jin, Paul Tapang, Donald J. Osterling, Wenqing Gao, Daniel H. Albert, Andrew J. Souers, Joel D. Leverson, Darren C. Phillips, Jun Chen. Quantitative assessment of BCL-2:BIM complexes as a pharmacodynamic marker for venetoclax (ABT-199). [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 6. doi:10.1158/1538-7445.AM2015-6

MCL-1 Is a Key Determinant of Breast Cancer Cell Survival: Validation of MCL-1 Dependency Utilizing a Highly Selective Small Molecule Inhibitor

Hyperexpression of antiapoptotic BCL-2 family proteins allows cells to survive despite the receipt of signals that would ordinarily induce their deletion, a facet frequently exploited by tumors. Tumors addicted to the BCL-2 family proteins for survival are now being targeted therapeutically. For example, navitoclax, a BCL-2/BCL-XL/BCL-W inhibitor, is currently in phase I/II clinical trials in numerous malignancies. However, the related family member, MCL-1, limits the efficacy of navitoclax and other chemotherapeutic agents. In the present study, we identify breast cancer cell lines that depend upon MCL-1 for survival and subsequently determine the mechanism of apoptosis mediated by the MCL-1 selective inhibitor A-1210477. We demonstrate that apoptosis resulting from a loss in MCL-1 function requires expression of the proapoptotic protein BAK. However, expression of BCL-XL can limit apoptosis resulting from loss in MCL-1 function through sequestration of free BIM. Finally, we demonstrate substantial synergy between navitoclax and MCL-1 siRNA, the direct MCL-1 inhibitor A-1210477, or the indirect MCL-1 inhibitor flavopiridol, highlighting the therapeutic potential for inhibiting BCL-XL and MCL-1 in breast cancer.

Chemical parsing: Dissecting cell dependencies with a toolkit of selective BCL-2 family inhibitors

The BCL-2/BCL-XL inhibitor navitoclax has shown promise for the treatment of cancer but on-target toxicities have limited its utility. Recently, the generation of selective BCL-2 family inhibitors has enabled a careful dissection of BCL-2 biology, and early work indicates that these molecules have improved therapeutic profiles for the treatment of cancer.