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Supplementary Materials and Methods. Supplementary Table S1. QC of Affymetrix U133_Plus_2.0 Data. Supplementary Table S2. Summary of the Objective Response Measure (ORM) scoring methodology. Supplementary Table S3. Correlation of xenograft with parent U133_Plus_2.0 expression. Supplementary Table S4. Probe sets where parent expression is always greater than xenograft. Supplementary Table S5. Probe sets where xenograft expression is always greater than parent. Supplementary Table S6. Complete summary of in vivo AZD1480 single agent responses for individual mice. Supplementary Table S7. In vitro Combination Indices (CIs) for AZD1480 and selumetinib. Supplementary Table S8. In vivo responses of JAK-mutated ALL xenografts to AZD1480/selumetinib combination treatment. Supplementary Table S9. Complete summary of in vivo AZD1480 single agent and AZD1480/selumetinib combination responses for individual mice. Supplementary Figure S1. Chemical structures of (A) AZD1480 and (B) selumetinib. Supplementary Figure S2. Relationship between high CRLF2 expression and engraftment potential of the P9906 cohort. Supplementary Figure S3. Probe set expression by range. Supplementary Figure S4. Heatmap of microarray gene expression data corresponding to the qRT-PCR data shown in Figure 1. Supplementary Figure S5. In vitro cytotoxicity of AZD1480 against ALL xenograft cells. Supplementary Figure S6. In vivo efficacy of single agent AZD1480 against ALL xenografts. Supplementary Figure S7. Time of drug exposure affects synergistic interactions of AZD1480 and selumetinib against JAK-mutated ALL xenografts. Supplementary Figure S8. The extent of inhibition of the JAK/STAT and MAPK signaling pathways depends on the length of exposure to AZD1480/selumetinib.

PR-104, a phosphate ester of the nitrogen mustard prodrug PR-104A, has shown evidence of efficacy in adult leukemia clinical trials. Originally designed to target hypoxic cells, PR-104A is independently activated by aldo-keto-reductase 1C3 (AKR1C3). The aim of this study was to test whether AKR1C3 is a predictive biomarker of in vivo PR-104 sensitivity. In a panel of 7 patient-derived pediatric acute lymphoblastic leukemia (ALL) xenografts, PR-104 showed significantly greater efficacy against T-lineage ALL (T-ALL) than B-cell-precursor ALL (BCP-ALL) xenografts. Single-agent PR-104 was more efficacious against T-ALL xenografts compared with a combination regimen of vincristine, dexamethasone, and l-asparaginase. Expression of AKR1C3 was significantly higher in T-ALL xenografts compared with BCP-ALL, and correlated with PR-104/PR-104A sensitivity in vivo and in vitro. Overexpression of AKR1C3 in a resistant BCP-ALL xenograft resulted in dramatic sensitization to PR-104 in vivo. Testing leukemic blasts from 11 patients confirmed that T-ALL cells were more sensitive than BCP-ALL to PR-104A in vitro, and that sensitivity correlated with AKR1C3 expression. Collectively, these results indicate that PR-104 shows promise as a novel therapy for relapsed/refractory T-ALL, and that AKR1C3 expression could be used as a biomarker to select patients most likely to benefit from such treatment in prospective clinical trials.

Genome-wide studies have identified a high-risk subgroup of pediatric acute lymphoblastic leukemia (ALL) harboring mutations in the Janus kinases (JAK). The purpose of this study was to assess the preclinical efficacy of the JAK1/2 inhibitor AZD1480, both as a single agent and in combination with the MEK inhibitor selumetinib, against JAK-mutated patient-derived xenografts. Patient-derived xenografts were established in immunodeficient mice from bone marrow or peripheral blood biopsy specimens, and their gene expression profiles compared with the original patient biopsies by microarray analysis. JAK/STAT and MAPK signaling pathways, and the inhibitory effects of targeted drugs, were interrogated by immunoblotting of phosphoproteins. The antileukemic effects of AZD1480 and selumetinib, alone and in combination, were tested against JAK-mutated ALL xenografts both in vitro and in vivo. Xenografts accurately represented the primary disease as determined by gene expression profiling. Cellular phosphoprotein analysis demonstrated that JAK-mutated xenografts exhibited heightened activation status of JAK/STAT and MAPK signaling pathways compared with typical B-cell precursor ALL xenografts, which were inhibited by AZD1480 exposure. However, AZD1480 exhibited modest single-agent in vivo efficacy against JAK-mutated xenografts. Combining AZD1480 with selumetinib resulted in profound synergistic in vitro cell killing, although these results were not translated in vivo despite evidence of target inhibition. Despite validation of target inhibition and the demonstration of profound in vitro synergy between AZD1480 and selumetinib, it is likely that prolonged target inhibition is required to achieve in vivo therapeutic enhancement between JAK and MEK inhibitors in the treatment of JAK-mutated ALL. Mol Cancer Ther; 14(2); 364–74. ©2014 AACR.

Survivin is an essential component of the chromosomal passenger complex and a member of the inhibitor of apoptosis family. It is expressed at high levels in a large variety of malignancies, where it has been implicated in drug resistance. It was also shown previously that survivin is up-regulated during telomerase-mediated immortalization, which occurs at a relatively early stage during carcinogenesis. This study shows that up-regulation of survivin during immortalization of human myofibroblasts is an indirect consequence of the repression of p16(INK4a). Survivin and p16(INK4a) were functionally linked by assays that showed that either the up-regulation of survivin or repression of p16(INK4a) rendered telomerase-transduced MRC-5 myofibroblasts resistant to oxidative stress. Conversely, siRNA-mediated down-regulation of survivin activated caspases and enhanced the sensitivity of immortal MRC-5 cells to oxidative stress. The E2F1 transcription factor, which is negatively regulated by the pRB/p16(INK4a) tumor suppressor pathway, was implicated in the up-regulation of survivin. Using the ChIP assay, it was shown that E2F1 directly interacted with the survivin gene (BIRC5) promoter in cells that spontaneously silenced p16(INK4a) during telomerase-mediated immortalization. E2F1 binding to the BIRC5 was also enhanced in telomerase-transduced cells subjected to shRNA-mediated repression of p16(INK4a). Together, these data show that repression of p16(INK4a) contributes to the up-regulation of survivin and thereby provides a survival advantage to cells exposed to oxidative stress during immortalization. The up-regulation of survivin during immortalization likely contributes to the vulnerability of immortal cells to transformation by oncogenes that alter intracellular redox state.

Targeting the apoptosis pathway with BH3 mimetics such as ABT-263 (navitoclax) and ABT-199 is an appealing strategy for cancer therapy. While the Bcl-2/Bcl-w/Bcl-xL inhibitor navitoclax has shown promising activity in adults with lymphoid malignancies and in preclinical models of pediatric acute lymphoblastic leukemia (ALL), its application in pediatric patients has been limited by Bcl-xL-mediated thrombocytopenia. The specific Bcl-2 inhibitor ABT-199 has shown remarkable efficacy against adult chronic lymphocytic leukemia in recent clinical trials, although its activity against pediatric ALL is yet to be tested. The aim of this study was to evaluate the in vivo efficacy of ABT-199 against a large panel of pediatric ALL patient-derived xenografts (PDXs), and to identify biomarkers predictive of ABT-199 response. Immune-deficient (NOD/SCID or NSG) mice inoculated with PDXs derived from patients with infant ALL harboring rearrangement of the Mixed Lineage Leukemia (MLL) oncogene (infant MLL-ALL, n = 4), B-cell precursor ALL (BCP-ALL, n = 6), BCP-ALL categorized as Ph-like (n = 4), T-cell ALL (T-ALL, n = 4) or early T-cell precursor ALL (ETP-ALL, n = 2) were treated with ABT-199 (100 mg/kg x 21 days, p.o.) or vehicle control. Responses were assessed by time to event measurements or stringent objective response criteria modeled after the clinical setting. ABT-199 significantly delayed the progression of 12/20 PDXs by between 0.4 and 28 days, and elicited objective responses in 6/20 (30%) of PDXs (4 complete responses and 2 partial responses). No objective responses were observed in the T-ALL or ETP-ALL PDXs. By comparison, navitoclax administered on the same dose and schedule as ABT-199 elicited objective responses in 19/31 (61%) of PDXs derived from the same pediatric ALL subtypes. Analysis of basal gene and protein expression revealed that high Bcl-xL and low Bcl-2 expression were significantly associated with in vivo ABT-199 resistance. Moreover, the Bcl-2/Bcl-xL gene (P = 0.03) and protein (P = 0.002) expression ratios were significantly elevated in PDXs that responded to ABT-199 in vivo compared with non-responders. Notably, Mcl-1 expression at the gene or protein level showed no significant correlation with in vivo ABT-199 sensitivity. In conclusion, the inferior objective response rate observed for ABT-199 (30%) compared with navitoclax (61%) indicates that pediatric ALL PDXs are less dependent on Bcl-2 for cell survival compared with adult chronic lymphocytic leukemia. Moreover, Bcl-xL appears to be a more significant ABT-199 resistance factor than Mcl-1 in pediatric ALL. While ABT-199 has the potential to exert significant efficacy in the treatment of aggressive and chemoresistant pediatric ALL, the prospective identification of patients who might benefit from such treatment is of paramount importance. Supported by NCI NO1CM42216. Citation Format: Santi Suryani, Kathryn Evans, Jennifer Richmond, Alissa Robbins, Lauryn Bracken, Raushan Kurmasheva, Peter J. Houghton, Malcolm A. Smith, Richard B. Lock. Evaluation of the Bcl-2 inhibitor ABT-199 in xenograft models of acute lymphoblastic leukemia by the pediatric preclinical testing program. [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 3276. doi:10.1158/1538-7445.AM2015-3276

Abstract 3562 Relapsed/refractory pediatric acute lymphoblastic leukemia (ALL) remains a continuing challenge to treat with currently available therapies, and new treatments are urgently required for the management of these high-risk cases. Activating mutations in the pseudokinase or kinase domains of Janus kinases (JAKs) 1, 2 and 3 are present in approximately 10% of high-risk pediatric ALL and are associated with high expression of cytokine receptor-like factor 2 (CRLF2) and poor outcome. These mutations can lead to continuous activation of JAKs, resulting in constitutive activation by phosphorylation of downstream signaling, including the signal transducer and activator of transcription (STAT), AKT, and mitogen-activated protein kinase (MAPK) pathways. The availability of specific JAK inhibitors, developed primarily for the treatment of JAK-mutated myeloproliferative diseases (MPDs), represents an opportunity to improve the treatment options for JAK-mutated pediatric ALL. AZD1480, a potent ATP-competitive small-molecule JAK2 inhibitor that also exhibits inhibitory activity against JAK1, is in solid tumor clinical trials. The purpose of this study was to gain a greater understanding of a potential role for AZD1480 in the treatment of JAK-mutated pediatric ALL either as a single agent or in rational drug combinations, using a preclinical model of xenografts established in immune-deficient mice from direct patient explants. As part of the Pediatric Preclinical Testing Program (PPTP) we previously showed that AZD1480 administered at 10 mg/kg twice daily × 5 then at 15mg/kg once daily × 2 via oral gavage for an intended three weeks significantly delayed the progression of only one in five JAK-mutated xenografts, with no tumor regressions observed. We now show that the relative insensitivity of JAK-mutated ALL xenografts to AZD1480 is a cell-intrinsic phenomenon, since 6/7 JAK1- or JAK2-mutated xenografts exhibited ex vivo IC50 values >2 μM following 72 h drug exposures, as assessed by mitochondrial function cell viability (MTT) assay. In order to gain a greater understanding of the underlying mechanisms for the lack of AZD1480 single-agent efficacy against JAK-mutated xenografts we analyzed intracellular signaling pathways and their responses to AZD1480 treatment. In contrast with “Typical” B-cell precursor (BCP)-ALL xenograft cells, JAK-mutated xenografts exhibited constitutive JAK pathway activation, as assessed by increased levels of phospho-JAK1 (pJAK1), pJAK2, pSTAT1/3/5, pAKT, pMAP2K1/2 (MEK1/2) and phospho-extracellular signal-regulated kinase 1/2 (pERK1/2). Ex vivo exposure of JAK-mutated xenografts to 1 μM AZD1480 caused rapid (within 1 h) and sustained (up to 24 h) decreases in pSTATs, but minimal reduction in pMEK1/2 and pERK1/2. These results indicate that AZD1480 alone selectively inhibits JAK downstream signaling pathways, which may be insufficient to delay leukemia progression in vivo or induce cell death ex vivo. Moreover, they provide a rationale for dual targeting of the JAK and MAPK pathways to elicit synergistic anti-leukemic cell killing in JAK-mutated ALL. Ex vivo exposure of two JAK2-mutated xenografts to 1 μM of the MEK1/2 inhibitor AZD6244 (selumetinib) caused a profound decrease in pERK1/2, and the combination of 1 μM each of AZD1480 and AZD6244 resulted in reductions of both pSTATs and pERK1/2. Moreover, fixed-ratio MTT cytotoxicity assays using these two JAK2-mutated xenografts demonstrated very strong synergy between AZD1480 and AZD6244, with Combination Indices for each xenograft of 0.36 and 0.098 at the ED50; 0.23 and 0.015 at the ED75; and 0.15 and 0.002 at the ED90. This strong synergistic effect was observed despite AZD1480 and AZD6244 exerting minimal cell killing activity against the xenograft cells when used as single agents. In conclusion, our data indicate that AZD1480 is unlikely to exert significant single-agent activity in the treatment of JAK-mutated pediatric ALL, and that future efforts focusing on dual targeting of the JAK/STAT and MAPK offer a potential pathway to achieving clinical efficacy. Disclosures: No relevant conflicts of interest to declare.

Summary To design rational therapies for JAK2-driven hematological malignancies, we functionally dissected the key survival pathways downstream of hyperactive JAK2. In tumors driven by mutant JAK2, Stat1, Stat3, Stat5, and the Pi3k and Mek/Erk pathways were constitutively active, and gene expression profiling of TEL-JAK2 T-ALL cells revealed the upregulation of prosurvival Bcl-2 family genes. Combining the Bcl-2/Bcl-xL inhibitor ABT-737 with JAK2 inhibitors mediated prolonged disease regressions and cures in mice bearing primary human and mouse JAK2 mutant tumors. Moreover, combined targeting of JAK2 and Bcl-2/Bcl-xL was able to circumvent and overcome acquired resistance to single-agent JAK2 inhibitor treatment. Thus, inhibiting the oncogenic JAK2 signaling network at two nodal points, at the initiating stage (JAK2) and the effector stage (Bcl-2/Bcl-xL), is highly effective and provides a clearly superior therapeutic benefit than targeting just one node. Therefore, we have defined a potentially curative treatment for hematological malignancies expressing constitutively active JAK2., Graphical Abstract, Highlights • Bcl-2 and Bcl-xL are the key survival factors downstream of oncogenic JAK2 • Combined targeting of JAK2 and Bcl-2/Bcl-xL is highly efficacious in vivo • Combination therapy is well tolerated in preclinical models of JAK2-driven ALL • Combination therapy can overcome and circumvent resistance to JAK2 inhibitors, Better therapeutic options are required for mutant JAK2-driven hemopoietic malignancies to overcome intrinsic and acquired therapy resistance. Johnstone and colleagues functionally dissect signaling pathways downstream of hyperactive JAK2 and define the STAT5-Bcl-2/Bcl-xL axis as crucial for tumor cell survival and development of acquired JAK inhibitor resistance. Combined inhibition of this oncogenic JAK2 signaling network at two critical points—JAK2 and Bcl-2/Bcl-xL—proved highly efficacious in vivo and was able to circumvent and overcome acquired resistance to single-agent JAK inhibitors.