ALBERT

Description: Introduction: While children diagnosed with acute lymphoblastic leukemia (ALL) experience close to a 90% likelihood of cure, the outcome for certain high-risk pediatric ALL subtypes as well as adult ALL remains poor. Several standard-of-care drugs used in multi-agent treatment protocols for ALL (including vincristine, daunorubicin and methotrexate) are substrates for the ATP-dependent drug efflux pump P-glycoprotein (P-gp), encoded by the ABCB1 gene, although there are limited reports of ABCB1 gene expression being associated with poor outcome in ALL (Carrillo et al, Hematol. 22:286-91, 2017). A previously identified high-risk subtype of T-ALL (early T-cell precursor ALL, ETP-ALL) characterized by poor early response to conventional induction treatment, expresses significantly higher levels of the ABCB1 gene compared with typical T-ALL (1.97-fold; false discovery rate=0.0026; P=0.00029; Zhang et al, Nature 481:157-63, 2012). E7130 is a novel anti-microtubule drug that is a less potent substrate for P-gp compared with other anti-microtubule drugs such as vincristine, and it has shown significant preclinical activity against patient-derived xenograft (PDX) models of adult malignancies. Therefore, it was of interest for the Pediatric Preclinical Testing Consortium (PPTC) to test the in vivo activity of E7130 against its PDX models of pediatric ETP-ALL. Methods: ABCB1 mRNA expression in ALL PDXs was quantified by RNAseq (https://pedcbioportal.org) and qRT-PCR. P-gp protein expression was assessed by immunoblotting, while its activity was measured by the Rhodamine-123 efflux assay in the absence or presence of the P-gp inhibitor tariquidar. E7130 and vincristine were evaluated in vivo against 6 ETP-ALL PDXs. Each PDX was inoculated into 6-8 immune-deficient (NSG) mice per treatment group (2-5 x 106 cells/mouse). Engraftment and drug responses were evaluated by enumerating the proportion of human CD45+ cells in the peripheral blood (%huCD45+) at weekly intervals. E7130 was tested at 2 dose levels (0.09 and 0.135 mg/kg IV), while vincristine was evaluated at 1 mg/kg IP. Both drugs were administered weekly x 3. Events were defined as the %huCD45+ ≥25%. Drug efficacy was assessed by event-free survival of treated (T) and control (C) groups by T-C, T/C and stringent objective response criteria (Houghton et al, Pediatr Blood Cancer 49:928-40, 2007). Results: RNAseq analysis of the 6 ETP-ALL PDXs in the PPTC panel of 90 pediatric ALL PDXs revealed 3 with high ABCB1 expression (FPKM 7.1-13.6; ETP-2, -3 and -6) and 3 with low expression (FPKM 0-0.15; ETP-1, -4 and -5), which was confirmed by qRT-PCR and immunoblotting. Moreover, high levels of tariquidar-sensitive Rhodamine-123 efflux activity were confirmed in the 2 high ABCB1 expressing PDXs tested (ETP-2 and -3). E7130 was generally well tolerated in NSG mice, with maximum average weight loss of 2.7-17.6% in the groups treated with the highest dose compared with 3.2-12.7% in the vincristine treated groups. E7130 (0.09 mg/kg) significantly (P

Description: 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.

Description: Abstract 3565 Introduction: Birinapant is a small molecule mimetic of Smac that potently and specifically antagonizes multiple inhibitors of apoptosis proteins (IAPs). Birinapant rapidly degrades cIAPs and enables cytokines (TNFα, TRAIL) to activate the extrinsic apoptosis pathway, while it rapidly turns off the canonical NF-κB survival pathway, causing cancer cell death. Preclinical studies using adult cancer models have shown that birinapant causes tumor regressions as a single agent in selected models and that it has potent antitumor activity when combined with chemotherapies and death receptor ligands. Methods: Birinapant was evaluated against the 23 cell lines of the PPTP in vitro panel (including 1 AML and 5 ALL lines) using 96 hour exposure at concentrations from 1.0 nM to 3.0 μM, both as a single agent and in combination with TNFα (10 ng/mL) or TRAIL (10 ng/mL). Birinapant was tested against 2 PPTP solid tumor xenografts (rhabdomyosarcoma, Rh30; Ewing, CHLA-258), an anaplastic large cell lymphoma (ALCL) xenograft (Karpas-299), and 3 B-precursor ALL xenografts (ALL-2, ALL-17, & ALL-19) at a dose of 30 mg/kg administered by the intraperitoneal route using a Q3 day × 5 schedule. Gene expression data for the PPTP cell lines and xenografts was available using both Affymetrix U133 Plus 2.0 and Agilent SurePrint G3 arrays. Results: Birinapant demonstrated limited single agent activity (median relative IC50 (rIC50) 〉 3 μM), with only the AML cell line Kasumi-1 showing Relative In/Out% (Rel I/O%) values 〈 0% with rIC50 of 37 nM. Marked potentiation of birinapant was observed for a subset of cell lines with the addition of TNFα or TRAIL. The 5 ALL cell lines showed a median rIC50 value of 3.6 nM for birinapant in combination with TNFα, with Rel I/O% values between −50% and −100% (indicative of a profound cytotoxic effect). Four of 5 ALL cell lines showed little or no potentiation of birinapant effect with the addition of TRAIL. Among solid tumor cell lines, potentiation of birinapant effect was observed for selected rhabdomyosarcoma, rhabdoid tumor, Ewing sarcoma, and neuroblastoma cell lines with the addition of either TNFα or TRAIL. Birinapant was well tolerated in vivo. Birinapant induced significant differences in event-free survival (EFS) distribution compared to control in 3 of 3 (100%) of the B-precursor ALL xenografts, but in none of the solid tumor or ALCL xenografts. Objective responses were not observed for the solid tumor and ALCL xenografts, whereas for the ALL panel one xenograft (ALL-17) achieved a complete response (CR) and another (ALL-2) achieved a maintained CR, with treated animals remaining in remission at day 42, approximately 30 days after their last treatment with birinapant. Given the mechanism of action of Smac mimetics, TNFα expression was examined. TNFα expression was significantly higher for the PPTP ALL xenografts compared to the PPTP solid tumor xenografts and to 15 normal tissues. TNFα expression in ALL clinical specimens was examined using publicly available datasets, with the observation that its expression is significantly higher for high-risk B-precursor ALL compared to a set of normal tissues, but with a wide range of TNFα expression among ALL cases. Lymphotoxin A and B also show significantly elevated expression in ALL compared to normal tissues. Among the ALL xenografts tested with birinapant, the best responding xenograft (ALL-2) showed the highest TNFα expression. Karpas-299 also showed high TNFα expression, but the two solid tumor xenografts did not. Unlike the ALL cell lines for which exogenous TNFα potentiated birinapant in vitro activity, exogenous TNFα did not potentiate birinapant in vitro activity against Karpas-299. Conclusion: Birinapant showed little single agent in vitro activity against ALL cell lines, though it markedly potentiated the activity of exogenously added TNFα for these cell lines. In vivo, birinapant showed remission-inducing activity against 2 of 3 ALL xenografts, with one of these showing a maintained CR. TNFα is mechanistically associated with the activity of Smac mimetics, and the initial PPTP in vivo data for ALL xenografts are consistent with a relationship between TNFα expression and responsiveness to birinapant. The PPTP results suggest that birinapant may show high level activity against a subset of childhood ALL, and additional in vivo testing is ongoing to better identify predictive markers that can reliably select responsive cases. Disclosures: Chunduru: TetraLogic Pharmaceuticals: Employment, Equity Ownership. Graham:TetraLogic Pharmaceuticals: Employment, Equity Ownership. Geier:TetraLogic Pharmaceuticals: Honoraria. Houghton:TetraLogic Pharmaceuticals: Honoraria.

Description: Key Points Simultaneous inhibition of Cdk9 and PI3K in human AML cells liberates Bak from both Mcl-1 and Bcl-xL, inducing Bak-dependent apoptosis. Dual inhibitors of Cdk9 and PI3K, such as PIK-75, have broad activity against malignant cells including human AML cells.

Description: Introduction While the overall cure rate for the most common pediatric cancer, acute lymphoblastic leukemia (ALL) now approaches 90%, infants (10 µM; JURKAT, IC50 〉10 µM). The in vitro sensitivity of BCP-ALL (n=3) and infant MLL-ALL (n=4) xenografts was consistent with wild-type p53 status, with IC50s of 0.11 - 2.2 µM. Exposure of ALL xenograft cells to nutlin-3 (10 µM, 6h) caused marked p53 up-regulation and nuclear translocation. Since we had previously shown that RG7112 administered as a single agent for 14 days induced significant regressions [Complete Responses (CRs) or greater] in 7/7 infant MLL-ALL xenografts in vivo, we assessed its efficacy in a 5-day combination treatment with an induction-type regimen (VXL) against two infant MLL-ALL xenografts (MLL-5 and MLL-14). The RG7112/VXL combination caused a Partial Response in MLL-5 compared with Progressive Disease for both RG7112 and VXL. The efficacy of RG7112/VXL was even more pronounced against MLL-14, causing a Maintained CR compared with CRs for both RG7112 and VXL, which met the criteria for Therapeutic Enhancement (the median EFS of RG7112/VXL-treated mice, 65.0 days, was significantly greater, P〈 0.0001, than that of RG7112, 22.2 days, and VXL, 28.5 days). Conclusions RG7112 induces significant regressions in a high proportion of infant MLL-ALL xenografts and enhances the efficacy of an induction-type regimen. The utility of targeting the p53-MDM2 axis in combination with established drugs for the clinical management of infant MLL-ALL warrants further investigation. This study was supported by NCI NO1CM42216. The authors thank Roche Pharmaceuticals, Inc., for providing RG7112. Disclosures: No relevant conflicts of interest to declare.

Description: Key Points ETP-ALL, a high-risk subtype of T-ALL, is characterized by aberrant activation of the JAK/STAT signaling pathway. The JAK1/2 inhibitor ruxolitinib demonstrates robust activity in patient-derived xenograft models of ETP-ALL.

Description: Acute lymphoblastic leukemia cells from 19 children, including 7 who remain in first complete remission (CR1), were engrafted into nonobese diabetic/severe combined immunodeficient (NOD/SCID) mice. High-level infiltration of bone marrow, spleen, and liver was observed, with variable infiltration of other organs. The immunophenotypes of xenografts were essentially unaltered compared with the original patient sample. In addition, sequencing of the entire p53 coding region revealed no mutations in 14 of 14 xenografts (10 from patients at diagnosis and 4 at relapse). Cells harvested from the spleens of engrafted mice readily transferred the leukemia to secondary and tertiary recipients. To correlate biologic characteristics of xenografts with clinical and prognostic features of the patients, the rates at which individual leukemia samples engrafted in NOD/SCID mice were analyzed. Differences in biologic correlates were encountered depending on stage of disease: a direct correlation was observed between the rate of engraftment and length of CR1 for samples harvested at relapse (r = 0.96; P = .002), but not diagnosis (r = 0.38; P = .40). In contrast, the in vivo responses of 6 xenografts to vincristine showed a direct correlation (r = 0.96; P = .002) between the length of CR1 and the rate at which the leukemia cell population recovered following vincristine treatment, regardless of whether the xenografts were derived from patients at diagnosis or relapse. This study supports previous findings that the NOD/SCID model of childhood ALL provides an accurate representation of the human disease and indicates that it may be of value to predict relapse and design alternative treatment strategies in a patient-specific manner.

Description: Development of effective therapies against acute myeloid leukemia (AML) will likely require elimination of the leukemic stem cell (LSC), which maintains the AML hierarchy. Based on high expression of the IL-3 receptor α chain (CD123) on AML-LSC, we tested a neutralizing antibody against CD123 (7G3) for its in vitro and in vivo effects on AML cells. By flow cytometric analysis, high expression of CD123 on CD34+/38− AML-LSC was confirmed. In vitro, 7G3 inhibited IL-3-induced proliferation in 31/34 primary AML specimens tested. Moreover, exposure of primary AML cells to 7G3 in vitro caused inhibition of IL-3-induced tyrosine phosphorylation and downstream signaling of the IL-3 receptor βc chain in a concentration-dependent fashion. Ex vivo treatment of primary AML cells with 7G3 (10 μg/ml, 2 h) profoundly reduced leukemic engraftment of human CD45+ (huCD45+) cells in non-obese diabetic/severe-combined immunodeficient (NOD/SCID) mice in 9/10 evaluable samples (mean ± SEM, 89.7 ±1.9% inhibition relative to IgG2a control, P=0.013), establishing that the AML-LSC was targeted. The overall survival of engrafted mice was significantly improved (median survival of control IgG2a-treated group 11.5 weeks versus 24 weeks for the 7G3-treated group, n = 10 per group, P=0.019). Moreover, ex vivo 7G3 treatment reduced the short-term homing efficiency of AML cells to the bone marrow by 65% compared with IgG2a control-treated cells, measured at 24 h post inoculation. In contrast, ex vivo exposure to 7G3 had minor, if any, effects on the engraftment of normal bone marrow cells (23.5 ±8.9% inhibition relative to IgG2a control, n=5). Direct delivery of 7G3-treated AML cells into the bone marrow by intra-femoral injection did not completely restore engraftment, suggesting a specific biological role of 7G3 in inhibiting homing, lodgement and proliferation of AML-LSC. Despite some sample to sample heterogeneity, thrice-weekly administration of 7G3 to NOD/SCID mice with pre-established AML consistently reduced AML dissemination to peripheral blood, spleen and liver, and reduced infiltration of a single AML sample in the bone marrow. 7G3 treatment was also associated with down-regulation of CD123 expression on AML cells in vivo. AML cells harvested from 7G3-treated mice were unable to repopulate secondary recipient mice, with engraftment reduced to 5.7 ±4.1% (n=7) huCD45+ cells in the femoral bone marrow compared with 26.8 ±6.6% in IgG2a-treated controls (n=9, P=0.024), indicating that CD123 neutralization targeted AML-LSC self-renewal ability, specifically reducing the proportion of AML-LSC. These results suggest that blocking CD123 inhibits homing, lodgement and proliferation of AML-LSC in the bone marrow microenvironment and prevents subsequent organ infiltration, and support additional investigations into 7G3 and its potential as a novel therapy for AML.

Description: Continuous xenografts from 10 children with acute lymphoblastic leukemia (ALL) were established in nonobese diabetic/severe combined immunodeficient (NOD/SCID) mice. Relative to primary engrafted cells, negligible changes in growth rates and immunophenotype were observed at second and third passage. Analysis of clonal antigen receptor gene rearrangements in 2 xenografts from patients at diagnosis showed that the pattern of clonal variation observed following tertiary transplantation in mice exactly reflected that in bone marrow samples at the time of clinical relapse. Patients experienced diverse treatment outcomes, including 5 who died of disease (median, 13 months; range, 11-76 months, from date of diagnosis), and 5 who remain alive (median, 103 months; range, 56-131 months, following diagnosis). When stratified according to patient outcome, the in vivo sensitivity of xenografts to vincristine and dexamethasone, but not methotrexate, differed significantly (P = .028, P = .029, and P = .56, respectively). The in vitro sensitivity of xenografts to dexamethasone, but not vincristine, correlated significantly with in vivo responses and patient outcome. This study shows, for the first time, that the biologic and genetic characteristics, and patterns of chemosensitivity, of childhood ALL xenografts accurately reflect the clinical disease. As such, they provide powerful experimental models to prioritize new therapeutic strategies for future clinical trials.

Description: Introduction Acute lymphoblastic leukaemia (ALL) in infants has poor overall survival despite being characterized by very few genetic aberrations per case. The most common genetic change, present in over 75% of cases, is the rearrangement of the mixed lineage leukaemia (MLL/KMT2A) gene (MLL-R) that also occurs in AML and mixed phenotype acute leukaemia (MPAL). Because infant ALL is rare and distinctive, most Australian and New Zealand patients are enrolled on specific clinical trials. In earlier infant ALL trials, MRD was not used for risk stratification firstly because of the difficulty of finding sensitive markers for specific immunoglobulin and T-cell receptor (Ig/TCR) gene rearrangements and the secondly because, in infant ALL, Ig/TCR markers are often present in sub-clones that can be lost at relapse due to clonal selection. MRD testing is performed in the current Interfant 06 trial preferentially using markers based on the genomic breakpoint sequences of MLL gene rearrangements. The treatment of infant ALL remains very challenging with relatively poor survival rates attributable to both toxicity and relapse. The objectives of this study were therefore to analyse MRD data for Interfant 06 patients enrolled at ANZCHOG centres and to perform a pilot experiment evaluating gene expression for key genes in infant ALL samples on a microfluidics platform, as a basis for identifying potential targeted therapies. Methods MRD was measured in bone marrow DNA from Interfant 06 patients enrolled since 2006, using sensitive Real-time Quantitative PCR (PCR-MRD) patient-specific assays to detect either MLL gene rearrangements (in 17) and/or conventional immunoglobulin and T-cell receptor (Ig/TCR) markers (21). Gene expression levels for 90 genes important in childhood cancers were measured by Taqman-based microfluidic assays in duplicate using cDNA from 2 micrograms of total RNA from 10 infant ALL samples (5 MLL, 5 non-MLL) at diagnosis (6) or relapse (4). Results Patient-specific PCR-MRD tests were developed for 27 out of 28 Australian and New Zealand infant ALL patients. 17/18 MLL-R ALL patients had MLL-R assays and 21/28 patients had Ig/TCR MRD tests, with only 1 (non-MLL) patient having no MRD markers. There was a wide range of MRD responses to induction therapy (Figure 1). Bone marrow MRD at the end of induction was high (〉1x10-3) in 44% of MLL-R ALL infants compared to 25% of non-MLL ALL infants and 15% of older children enrolled on ANZCHOG ALL8. In 8/13 MLL-R patients who had both types of marker, MRD levels were higher when measured by their MLL-R marker than by their Ig/TCR marker. In a set of 90 genes selected for expression analysis, higher levels were found for 17 genes in 2 or more of the 10 infant ALL samples evaluated. These more highly expressed genes included potential or known drug targets BCL2, ERBB2, ERBB4, ILRA2, CSF1R and PARP1. Conclusions The quantitation of MRD based on MLL rearrangements in ALL is effective and can also be used to monitor response to therapy in infant ALL as well as MLL-R cases of AML and MPAL. The combined application of MLL-R and Ig/TCR markers allowed 97% of infant ALL patients to be MRD monitored with a sensitive marker. In most patients with both type of MRD marker, higher levels of MRD were detected in end of induction samples using the MLL versus Ig/TCR tests. One interpretation is that Ig/TCR genes are rearranged after the MLL rearrangement in ALL sub-clones that are both more mature and more chemo-sensitive. This finding also confirms the current consensus that disease-related MLL-R markers provide better risk assessment than Ig/TCR markers. Our Fluidigm analysis has shown that quantitative measurement of multiple gene expressions is feasible on small RNA samples and can be used to rapidly screen for specific expression of genes coding for drug targets in ALL patients. Support: NHMRC Australia APP1057746, Sporting Chance Cancer Foundation. Figure 1. Comparison of MRD response to induction therapy in MLL-R infant ALL compared with non-MLL infant ALL (Interfant 06) and older children (ANZCHOG ALL8). Figure 1. Comparison of MRD response to induction therapy in MLL-R infant ALL compared with non-MLL infant ALL (Interfant 06) and older children (ANZCHOG ALL8). Disclosures No relevant conflicts of interest to declare.