ALBERT

Description: Despite advances in understanding of the biology of acute myeloid leukemia (AML), cure remains elusive for the majority of patients. Pro-survival molecules of BCL-2 family play critical roles in leukemia transformation and chemoresistance. The anti-leukemia potency of selective BCL-2 inhibitor venetoclax (ABT-199/GDC-0199) has been demonstrated in AML models (Pan et al. Cancer Discovery 2014). However, venetoclax is often associated with resistance due to its poor inhibition of MCL-1. RAF/MEK/ERK (MAPK) pathway is commonly activated in AML, and can stabilize anti-apoptotic MCL-1 and inactivate the pro-apoptotic BIM. In this study, we evaluated the anti-leukemia effects of concomitant BCL-2 and MAPK blockade by venetoclax in combination with MEK1/2 inhibitor GDC-0973 (cobimetinib). First, anti-leukemia activity of cobimetinib and venetoclax was examined in 18 primary AML samples with diverse genetic alterations. The combination significantly enhanced cell death, as compared to the single agent treatment (Fig 1A). Cobimetinib inhibited cell proliferation in the majority of AML cases (34.2 ± 23.7%) and the cell growth suppression was more profound in the combination group (60.2 ± 28.8%, p

Description: Background: A role for the Chemokine (C-C motif) ligand 2 (CCL2) in attracting tumor-associated macrophages (TAMs), myeloid-derived suppressor cells (MDSC) and infiltrating monocytes has been described for many solid tumors in which they play an essential role in modifying the adaptive immune response, ultimately favoring tumor progression. Unfortunately, little is known about the importance of this mechanism for the progression of AML. We recently identified CCL2 as the most prominent chemokine produced by bone marrow (BM) mesenchymal stromal cells (BM-MSC) in response to the interaction with myeloid leukemia cells (PMID: 24599548). In addition, elevated CCL2 plasma levels have been reported in patients of AML (PMID: 17822317), ALL (PMID: 21298741) and CLL (PMID: 22397722) when compared to normal controls. In this study we assessed the effects of blocking the CCR2-CCL2 axis on the migration and signaling of hematopoietic cells as well as on the infiltration of immune-suppressive cells in leukemia-bearing mice. Results: We first studied the efficacy and potency of agents at inhibiting CCL2-mediated migration, using the human monocytic leukemia cell line THP-1. Migration towards human recombinant CCL2 (5 ng/ml) was significantly inhibited by as little as 1 nM of NOX-E36, a human-specific CCL2 Spiegelmer (NOXXON Pharma, Berlin). Spiegelmers are RNA-like molecules built from L-ribose units that are able to bind molecules such as peptides and proteins with an affinity in the pico-to nanomolar range. Similar results were obtained with a CCR2 antagonist (100 ng/ml; Santa Cruz). In anticipation of in vivo studies in mice, we next confirmed the ability of a mouse-specific CCL2 Spiegelmer (mNOX-E36) to inhibit migration and signaling pathway activation in murine hematopoietic cells. For this purpose, we cloned and overexpressed via lentiviral transduction the murine CCL2 receptor (CCR2) in Ba/F3 cells (a murine pro-B cell line). Stimulation of Ba/F3-CCR2 cells with 5 ng/ml of mouse recombinant CCL2 induced a ~2000 fold increase in migration of Ba/F3-CCR2 cells and was successfully blocked with mNOX-E36 in a concentration-dependent manner. Western blot analysis of protein lysates from mCCL2-stimulated cells (30 minutes treatment) indicated activation of AKT, ERK and p38-MAPK. The CCL2-induced phosphorylation of these molecules was completely abrogated by pre-treatment with mNOX-E36. Subsequently, we determined whether the expression of CCL2 by stromal cells in leukemia-resident organs triggers the infiltration of TAMs and possibly other immune-suppressive cells into those organs. We conducted preliminary in vivo studies in non-irradiated immunocompetent C57BL/6 mice (n=5 per group) injected with syngeneic AML1/ETO9a-expressing primary murine leukemia cells (PMID: 19339691). After confirmation of leukemia engraftment by IVIS imaging, mice were treated with mNOX-E36 (14.4 mg/kg, s.c., three times per week) or vehicle control for 3 weeks. At this point, all animals were sacrificed and their tissues (spleens and BM from femurs) were collected for analysis. Although we did not observe differences in leukemia burden by imaging between vehicle and mNOX-E36 treated groups, flow cytometry analysis revealed an increase in the frequency of CD11b+ Ly6Clow MHC IIlow macrophages (2 to 7 fold increase) in spleens of mice engrafted with leukemia (vehicle-treated group) when compared to spleens collected from healthy mice. These MHC IIlow macrophages were previously identified as immunosuppressive M2-like macrophages as opposed to MHC IIhi macrophages which show a pro-inflammatory M1-like phenotype (PMID: 20570887). Importantly, CCL2 inhibitor mNOX-E36 abrogated this macrophage infiltration within the leukemia microenvironment. Conclusions: Our results indicate that blockade of the CCR2-CCL2 axis not only affects migration and signaling of treated cells in vitro, but also interferes with the infiltration of M2-like macrophages into spleens of leukemia-bearing mice. Current in vivo experiments using a combination of standard chemotherapy with mNOX-E36 in AML immunocompetent models are undergoing. We expect that in vivo modulation of CCL2 will improve response to chemotherapy of AML by reducing the marrow infiltration of infiltrating monocytes and tumor-associated macrophages, which would facilitate translation of this novel concept into clinical trials in AML. Disclosures Zuber: Boehringer Ingelheim: Research Funding; Mirimus Inc.: Consultancy, Other: Stock holder. Eulberg:NOXXON Pharma AG: Employment. Kruschinski:NOXXON Pharma AG: Employment.

Description: Heat shock factor 1 (HSF1) is best known as a key sensor of proteotoxic stress, but accumulating evidence also supports a major role for this transcriptional regulator in cancer biology. In a variety of human solid tumor cells, downregulation of HSF1 inhibits growth, induces cell death and limits metastatic potential. In breast cancers, nuclear accumulation of HSF1 and a tumor-specific gene expression signature reflecting HSF1 activation were found to be strongly associated with poor outcome (Mendillo et al, Cell 2012). In addition, we have recently reported, as a counter-intuitive reversal of the central dogma, that inhibition of protein translation represses the constitutive activation of HSF1 in cancers, and that HSF1 inhibition induced by the potent eIF4a inhibitor rohinitib (RHT) exerts profound, far-ranging anti-tumor effects (Santagata et al, Science 2013). Review of public databases supports targeting of HSF1 and eIF4a in AML: mRNA levels of HSPA8, one of the primary HSF1 targets, are correlated with poor prognosis in AML (Prognoscan, data from Metzeler et al, Blood 2008) and eIF4a mRNA levels were highest in AML among 12 cancer types (Oncomine, data from Ramaswamy et al, PNAS 2001). Here, we demonstrate that inactivation of HSF1 in acute myeloid leukemias (AMLs) by RHT exerts pronounced apoptogeniceffects with preferential activity against FLT3-ITD mutant cells in cell culture and in mice. First, we confirmed our previous finding of inactivation of HSF1 by RHT in AML. In OCI-AML3, MOLM-13 and MV4;11 cells, mRNA levels of HSPA8 were reduced by 70% after RHT treatment compared to untreated controls. OCI-AML3 cells were then infected with lentivirus encoding a reporter GFP-luciferase fusion protein the expression of which is driven by promoter elements from either the HSPA1A or HSPA6 genes; an approximately 50% reduction of reporter induction by heat shock was observed after RHT treatment compared to untreated controls. Next, treatment of 7 human AML cell lines in culture showed that RHT induces marked anti-leukemia effects at low nanomolar concentrations (LD50s; 9.5 to 99.5 nM, IC50s; 4.7 to 8.8 nM, based on AnnexinV/PI-positivity as determined by flow cytometry at 72hr). The most pronounced cytotoxic effects were observed in FLT3-ITD+ cell lines (LD50s 〈 10 nM in MOLM13 and MV4;11 cells). Using two sets of isogenic cell lines (Ba/F3 and OCI-AML3 cells with FLT3-ITD or wild-type (wt) FLT3), we confirmed that RHT more potently kills FLT3-ITD cells (LD50s; 65.3 vs 20.1 nM in Ba/F3 cells). Furthermore, the combination of FLT3 inhibitor sorafenibwith RHT showed synergistic effects in cell culture (Combination Index: ED50 0.85, ED75 0.86, ED90 0.89). Immunoblot analysis showed higher phospho-HSF1 (Serine 326) in FLT3-ITD Ba/F3 cells than FLT3-wt cells, suggesting greater dependence of FLT3-ITD cells on HSF1 activation for survival. We also tested primary samples from 17 AML patients and bone marrow (BM) samples from 8 healthy donors. RHT potently induced apoptosis in AML cells, while relatively sparing normal BM cells (Figure 1A). Importantly, a similarly significant difference in sensitivity was also observed between AML and normal stem cells (CD45+CD34+CD38-). Moreover, the activity of RHT against the leukemic population was significantly higher in FLT3-ITD than in FLT3-wt cells (Figure 1B). We also evaluated the activity of RHT in a FLT3 mutant AML xenograft model using GFP-luciferase labeled MOLM-13 cells. Significantly decreased luciferase activity was detected by bioluminescence imaging and a dose-dependent reduction in GFP+ leukemic cells was seen in peripheral blood and BM by day 16 (Figure 2). Survival of the treatment groups was significantly prolonged (median; 18 vs 22.5 vs 24 days respectively, p 〈 0.0001). In conclusion, HSF1 function provides an attractive therapeutic target in AML. The eIF4a inhibitor RHT down-regulates HSF1 transcriptional function and exerts robust anti-leukemia activity in cell culture and in mice. Although the relative contributions of HSF1 inactivation and translation inhibition to the net anti-leukemic activity of RHT remain to be defined, promising features of this approach include its activity against AML stem cells, while sparing normal stem cells and its particularly potent cytotoxicity for poor-prognosis FLT3-ITD AMLs. Taken together, these preclinical findings strongly support further development of eIF4a inhibitors in the treatment of AML. Disclosures Ishizawa: Karyopharm: Research Funding. Konopleva:Novartis: Research Funding; AbbVie: Research Funding; Stemline: Research Funding; Calithera: Research Funding; Threshold: Research Funding.

Description: The clinical challenge posed by p53-deficiency in hematological malignancies needs novel therapeutic strategies. ONC201, a first-in-class small molecule, was discovered as a p53-independent activator of apoptosis with a benign preclinical safety profile (Allen et al, Sci Transl Med 2013). Here we report that ONC201 exerts anti-tumor effects in hematological malignancies and leukemia stem cells (LSC) via the induction of ATF4 in the integrated stress response (ISR). ONC201 induced p53-independent apoptosis in cell lines and primary patient samples from mantle cell lymphomas (MCL) and acute myeloid leukemias (AML), independent of genetic alterations that correlate with poor prognosis (e.g., TP53 mutation, FLT3-ITD, or complex karyotype). ONC201 also induced apoptosis in LSC, while sparing normal bone marrow progenitors, as measured in vitro and after transplantation. Specifically, we recovered unfractionated LSC-containing populations of AML cells (t(9;11)(p22; q23), CEBPA and ATM mutant) from secondarily-engrafted mice and cultured them in vitro for 48 hr in two groups, ONC201-treated (5 uM) or Control. For both groups, the same number of Trypan Blue-negative cells was then re-transplanted. The frequency of human CD45+ cells 4 weeks after transplantation was 38.09 ± 2.59 % of tibial BM cells in untreated mice and 0.10 ± 0.05% in treated mice (n = 3 for each, p 〈 0.01). The survival of the treated group was dramatically prolonged (Figure 1). Gene Set Enrichment Analysis (GSEA) of gene expression profiling (GEP) data of Jeko-1 and Z-138 cells treated with ONC201 implicated up-regulated Endoplasmic Reticulum (ER) stress-related genes, such as targets of the ER stress-induced transcription factor CHOP (DDIT3; FDR q = 0.016), and ER component proteins (FDR q = 0.039). We confirmed the ONC201-induced increased mRNA levels of DDIT3, GADD34, DR5 and TRIB3, and increased protein levels of CHOP, ATF4 and IRE1-a in Jeko-1 cells. Knockdown of ATF4 and IRE1-a revealed that ATF4 is an essential protein for ONC201-induced apoptosis in most AML cells and MCL cells, while IRE1-a is only necessary for apoptosis in MCL cells. However, unlike other ER stress inducers, ONC201 did not cause phosphorylation of an eIF2a kinase PERK, a hallmark of classical ER stress. Importantly, ONC201 was also effective in lymphoma cells resistant to bortezomib, an ER stress inducer, supporting the notion that ONC201 uses a unique mechanism to trigger the ISR. In addition, ONC201 inhibited mTORC1 signaling, likely secondary to ATF4 activation via the induction of DDIT4, a negative regulator of mTORC1, which presumably increases the cytotoxicity of ONC201 by global translation inhibition. Investigating the type of apoptosis induced by ONC201, we examined protein levels of anti-apoptotic BCL-2 family members (BCL-2, BCL-XL and MCL-1) after ONC201 treatment. MCL-1 was reduced most notably after 12 hr. We then tested the effect of ONC201 on cells with overexpression or knockdown of BCL-2 family proteins. MCL-1 knockdown in OCI-AML3 cells increased their sensitivity to ONC201 only slightly, but ONC201 efficacy was dramatically reduced in BCL-2-overexpressing HL-60 cells, even more so than in BCL-XL-overexpressing HL-60 cells. Therefore, we investigated whether ONC201 sensitivity could be increased by ABT-199, a small-molecule BH3 mimetic that specifically inhibits BCL-2, and is known to be ineffective in cell lines with MCL-1 overexpression; accordingly, the combination of ONC201 and BCL-2 antagonist ABT-199 was highly synergistic (Figure 2). In conclusion, ONC201 induces p53-independent apoptosis and abrogates LSC function by ATF4 induction, via the ISR. By suppressing MCL-1, ONC201 can also increase the effectiveness of the Bcl-2 inhibitor ABT-199. These findings suggest that ONC201 may provide promising novel therapeutic strategies for TP53 -wild type and TP53 -mutant hematological malignancies. Phase I/II clinical trials have been initiated at the MD Anderson Cancer Center in leukemias and lymphomas to determine safety, efficacy and further characterize mechanism of action. Figure 1. Figure 1. Figure 2. Figure 2. Disclosures Ishizawa: Karyopharm: Research Funding. Allen:Oncoceutics, Inc: Employment, Equity Ownership. Orlowski:Janssen Pharmaceuticals: Membership on an entity's Board of Directors or advisory committees; Array BioPharma: Consultancy, Research Funding; Millennium Pharmaceuticals: Consultancy, Research Funding; Celgene: Consultancy, Research Funding; BioTheryX, Inc.: Membership on an entity's Board of Directors or advisory committees; Acetylon: Membership on an entity's Board of Directors or advisory committees; Genentech: Consultancy; Forma Therapeutics: Consultancy; Onyx Pharmaceuticals: Consultancy, Research Funding; Bristol-Myers Squibb: Consultancy, Research Funding; Spectrum Pharmaceuticals: Research Funding. Wang:Celgene: Research Funding. Konopleva:Novartis: Research Funding; AbbVie: Research Funding; Stemline: Research Funding; Calithera: Research Funding; Threshold: Research Funding. Andreeff:Oncoceutics, Inc.: Membership on an entity's Board of Directors or advisory committees.

Description: Pro-survival molecules including BCL-2 play critical roles in leukemia transformation and chemoresistance. ABT-199/GDC-0199 (venetoclax) is an orally available BH3-mimetic that binds with high affinity to BCL-2, but lacks affinity for BCL-XL and MCL-1. We have recently demonstrated anti-leukemia potency of venetoclax in acute myeloid leukemia (AML) models (Pan et al. Cancer Discovery 2014). However, venetoclax poorly inhibits MCL-1, causing resistance in leukemia cells that rely on MCL-1 for survival. The RAF/MEK/ERK (MAPK) cascade is a major effector pathway in AML that is activated by upstream mutant proteins such as FLT3, KIT and RAS. Additionally, the MAPK pathway regulates BCL-2 family proteins by stabilizing anti-apoptotic MCL-1 and inactivating pro-apoptotic BIM. In this study, we evaluated the anti-tumor effects of concomitant BCL-2 and MAPK blockade by venetoclax in combination with MEK1/2 inhibitor GDC-0973 (cobimetinib).. We initially examined activity of these agents in a panel of myeloid leukemia cell lines with diverse genetic alterations (Fig. 1A). The IC50 values of cobimetinib ranged from 〈 0.01 µM to 〉 1 µM after 72 hours of drug treatment but did not correlate with the basal level of p-ERK1/2. In 7 out of 11 cell lines, combination of the agents elicited synergistic growth inhibition. Notably synergism of venetoclax with cobimetinib was observed in venetoclax-resistant cell lines (MOLM14, OCI-AML3, NB4 and THP1). Ongoing analysis of pharmacodynamic markers include transcriptome assessment by RNA sequencing, functional proteomics by reverse phase protein array (RPPA) and quantification of BCL-2:BIM and MCL-1:BIM complexes using the electrochemiluminescent ELISA assay (Meso Scale Discovery, MSD-ELISA). The preliminary MSD data revealed that BCL-2:BIM complex was disrupted in most cell lines and accumulated following cobimetinib treatment, which may be due to the disruption of MCL-1:BIM complex by inhibition of MEK (Fig. 1B). In a long-term culture of primary AML blasts in serum-free stem cell growth medium supplemented with cytokines and StemRegenin 1 (SR1) to main the immature state of leukemia cells, the combination of venetoclax and cobimetinib induced distinct apoptotic cell death, with AML #1 sensitive to venetoclax but resistant to cobimetinib. Alternatively, AML #2 and #3 samples were resistant to venetoclax but sensitive to cobimetinib and the combination of both drugs (Fig. 1C). We next investigated signaling patterns and BCL-2 family protein expression in AML stem/progenitor cells using a 34-antibody panel and time-of-flight mass cytometry (CyTOF). In AML#1, BCL-2 was expressed in leukemia blasts, with enrichment in a progenitor AML population phenotypically defined as CD45dim CD34+ CD38+ CD123+ CD33+ (Fig. 1D). The high expression level of BCL-2 and low expression of MCL-1 and BCL-XL may account for sensitivity to venetoclax in AML#1. Both basal and G-CSF- or SCF-stimulated p-ERK was efficiently down-regulated by cobimetinib; however, G-CSF-evoked p-STAT3/5 and SCF-induced p-AKT were only slightly reduced (Fig. 1E). Notably we observed increased phosphorylation of STAT5 pathway upon treatment with cobimetinib, suggesting that active MAPK signals inhibit phosphorylation of the JAK-STAT pathway, as previously reported (Krasilnikov et al. Oncogene, 2003 and Lee at al. Cancer Cell, 2014). To test the efficacy of both compounds in vivo, we injected NSG mice with genetically engineered OCI-AML3/Luc/GFP cells. Bioluminescent imaging (BLI) demonstrated significantly reduced leukemia burden in treated groups compared to controls, more prominently in the cobimetinib single agent and venetoclax plus cobimetinib co-treated mice (Fig. 1F). The efficacy study is ongoing and median survival for cobimetinib and venetoclax co-treated mice has yet to be determined (Fig. 1G). In summary, our data demonstrates that combinatorial blockade of MAPK and BCL-2 pathways is synergistic in the majority of AML cell lines tested and can overcome intrinsic resistance to venetoclax. Ongoing studies will evaluate efficacy of this combination therapy in primary human AML xenografts and elucidate mechanisms of synergy. Disclosures Leverson: AbbVie: Employment, Equity Ownership. Dail:Genentech: Employment, Equity Ownership. Phillips:AbbVie: Employment, Other: Shareholder, Patents & Royalties. Chen:Abbvie: Employment, Equity Ownership. Jin:Abbvie: Employment, Equity Ownership. Jabbour:Pfizer: Consultancy, Research Funding. Sampath:Genentech: Employment, Equity Ownership. Konopleva:Novartis: Research Funding; AbbVie: Research Funding; Stemline: Research Funding; Calithera: Research Funding; Threshold: Research Funding.

Description: Background : Tissue transglutaminase (TG2) is a multi-domain, multi-functional enzyme with diverse biological functions including calcium-dependent posttranslational modification of proteins, extracellular matrix formation, integrin-mediated signaling, and signal transduction. In many cancer types, increased TG2 expression has been associated with malignancy and resistance to chemotherapy. Previous studies using reverse phase proteomic arrays (RPPA) on 511 AML samples indicated in many cases that TG2 expression was elevated in samples from patients with relapsed AML when compared to samples collected at AML diagnosis (PMID: 23576428). In addition, elevated TG2 expression correlated with increased expression of proteins involved in cell-adhesion and the regulation of apoptosis. These findings suggest that the expression of TG2, in combination with other makers, may be suitable to predict survival outcome in AML patients and that targeting TG2 might sensitize AML cells to standard chemotherapy. Results : Our preliminary data demonstrated that the expression of TG2 mRNA, as determined by cDNA array analysis and validated by qRT-PCR, is induced (〉30 fold increase) in OCI-AML3 leukemia cells when co-cultured with normal donor-derived Bone Marrow-derived Mesenchymal Stromal Cells (BM-MSC) in hypoxic conditions (1% pO2) for 48hs. Increase in mRNA correlated with an increase in protein level as determined by Western Blot. In order to test the hypothesis that the expression of TG2 in leukemia cells is related to resistance to chemotherapy and poor prognosis we down-regulated or over-expressed TG2 by lentiviral transduction with TG2-shRNA or TG2-over-expressing vector, respectively, in OCI-AML3 cells. Our in vitro studies showed that although the over-expression of TG2 in OCI-AML3 cells did not have a significant impact on resistance to chemotherapy, down-regulation of TG2 had a moderate effect on sensitizing cells to AraC (13% increase in apoptosis; P = 0.0006). To further investigate these findings we performed in vivo experiments in NGS mice engrafted with OCI-AML3 stably-transduced with either scramble-shRNA or TG2-shRNA. After confirmation of engraftment by bioluminescence imaging, leukemia-bearing mice were treated with a combination of AraC and doxorubicin. Consistent with the in vitro data, the down-regulation of TG2 rendered OCI-AML3 cells more susceptible to chemotherapy and prolonged the survival of leukemia-bearing mice suggesting a role for TG2 in resistance to chemotherapy (median survival of mice transplanted with scramble-shRNA cells, 44 days; compared to mice transplanted with TG2-shRNA cells, 51 days; P = 0.0027). Conclusions : Our data indicate that TG2 is up-regulated in response to microenvironmental signals like hypoxia and cell-cell contact with stromal cells. Silencing of TG2 in AML cells sensitized cells to chemotherapy in vitro and in vivo and extended the survival of the leukemia-bearing mice suggesting that targeting TG2 in combination with standard chemotherapy in AML could be developed into a novel therapeutic approach. Disclosures Konopleva: Cellectis: Research Funding; Calithera: Research Funding.

Description: Background and rationale: Philadelphia chromosome-like acute lymphoblastic leukemia ("Ph-like ALL") is a subtype of high-risk B-precursor ALL (B-ALL), which carries a high risk of relapse with conventional chemotherapy(Roberts et al, N Engl J Med. 2014). Rearrangements in CRLF2, leading to overexpression of cytokine receptor for thymic stromal lymphopoietin (TSLP), are present in approximately 50% of Ph-like ALL and are associated with hyperactive JAK/STAT and PI3K/mTOR signaling (Harvey et al, Blood 2010;Tasian et al, Blood 2014).In addition,JAK2 fusion proteins, such as PAX5-JAK2 represent a novel class of JAK2-driven cellular transformation in B-ALL (Dagmar et al, Blood 2015). Our prior studies in Ph+ B-ALL established that combining tyrosine kinase inhibitors (TKIs) with second generation ATP-competitive mTOR kinase inhibitors (TOR-KIs) provides greater anti-leukemia efficacy compared to TKIs in Ph+ ALL (Janeset al, Nat. Med. 2013). In this study, we investigated anti-leukemia efficacy and intracellular signaling networks upon combination of type I or type II JAK2 inhibitors and TOR-KIs in JAK2-driven Ph-like ALL models. Methods. The human B-precursor Ph-like ALL cell lines MUTZ5 (which harborsIGH-CRLF2 translocation and JAK2 R683G mutation), MHH-CALL-4 (IGH-CRLF2 translocation and JAK2 I682F),Reh (ETV6-RUNX1 B-precursor ALL cell line)and mouse Arf-null PAX5-JAK2-MIG + IK6-MIR(IL7-dependent primary Arf-/- pre-B cells expressing the dominant negative Ikaros isoform IK6 with PAX5-JAK2 fusion protein) were studied. Signal transduction inhibitors (STIs): JAK2 type I inhibitor ruxolitinib and type II inhibitor NVP-BBT594 (Andraos et al., Cancer Discovery 2012); allosteric mTOR inhibitor rapamycin or mTOR-KI AZD2014. Effects on intracellular signaling were determined using phospho-flow cytometry and Westernblot analysis. Anti-leukemia effects were quantified using CellTiter-Glo viability assay and annexin V flow cytometry. Results. In vitro stimulation of CRLF2-rearranged cells with TSLP robustly induced JAK/STAT signaling (Fig 1D). JAK2 inhibition with ruxolitinib or BBT594 efficiently inhibited TLSP-induced STAT5, AKT, ERK and S6 activation, yet failed to affect4E-BP1 activation. The TOR-KI AZD2014 but not rapamycin fully inhibited phosphorylation of 4E-BP1, consistent with efficient inhibition of TORC1, and caused profound cell cycle arrest and growth inhibition of Ph-like cells. Combination of ruxolitinib and AZD2014 further inhibited cell proliferation, yet did not induce apoptotic cell death. Recent studies indicate persistence of JAK2-mutated cells upon chronic exposure to type I JAK2 inhibitors, through an adaptive resistance mechanism involving JAK2 heterodimerization and reactivation of JAK-STAT signaling (Koppikar et al., Nature 2012). We therefore compared the in vitro efficacy of ruxolitinib and BBT594, a type II JAK2 inhibitor that retains the ability to bind inactive JAK2 in Ph-like ALL cells. In MUTZ-5 but not in MHH-CALL-4 cells, ruxolitinib increased JAK2 activation loop phosphorylation (p-JAK2-Tyr1008) despite suppression of p-STAT5; in contrast, BBT594 diminished bothp-JAK2 and p-STAT5 in both cell lines. Unexpectedly, BBT594 induced apoptotic cell death in all JAK2-driven Ph-like ALL cell lines MUTZ5, MHH-CALL-4 and Arf-null PAX5-JAK2+IK6, but not in REH cells. Combination of BBT594 with AZD2014 further inhibited phosphorylation of JAK2, AKT, 4E-BP1 and eIF4E, and synergistically induced apoptosis and reduced cell viability in Ph-like ALL cell lines(combination index: MUTZ5, 0.71; MHH-CALL-4, 0.57; Arf-nullPAX5-JAK2+ IK6, 0.81). Of importance, BBT594 and AZD2014 combination induced apoptosis in five JAK2-mutant Ph-like ALL xenograft primary samples. In summary, these results suggest that efficient blockade of JAK2/STAT5 with type II JAK2 inhibitors translates into cell death of mutant JAK2-driven Ph-like ALL cells. Furthermore, concomitant blockade of TORC1 signaling with TOR-KI reduces B-ALL cell proliferation through potent inhibition of 4E-BP1 and causes synthetic activity, providing avenues for novel rationally designed combinatorial regimens in this subset of Ph-like B-ALL. The in vivo studies to test these hypotheses are ongoing using patient-derived xenografts. Disclosures Jabbour: Pfizer: Consultancy, Research Funding. Tasian:Incyte: Consultancy; Gilead: Research Funding. Mullighan:Amgen: Honoraria, Speakers Bureau; Cancer Science Institute: Membership on an entity's Board of Directors or advisory committees; Incyte: Consultancy, Honoraria; Loxo Oncology: Research Funding. Konopleva:Novartis: Research Funding; AbbVie: Research Funding; Stemline: Research Funding; Calithera: Research Funding; Threshold: Research Funding.