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Patients with precursor B-cell acute lymphoblastic leukemia (pB-ALL) who have relapsed after allogeneic hematopoietic stem cell transplantation (allo-HSCT), have relapsed more than once, or are resistant upfront have a dismal prognosis. CD19-targeted chimeric antigen receptor (CAR) T cells have evolved as potent immune therapies. Tisagenlecleucel (Tisa-cel) is a commercially available autologous CD19-directed CAR T-cell product. We performed a retrospective study inviting all CAR T-cell centers in Germany to participate. Eighty-one patients with pB-ALL were included. Twenty-eight days after CAR T-cell infusion, 71 patients (87.7%) were in complete response, and 8 (9.9%) were in nonremission. At 2 years, the probabilities of event-free survival (pEFS), relapse-free survival (pRFS), and overall survival (pOS) were 45.3%, 51.7%, and 53.2%, respectively. pEFS was not different in patients without (n = 16, 55.0%) vs with prior allo-HSCT (n = 65, 43.4%). In patients treated after allo-HSCT, the time to relapse after allo-HSCT was a strong predictor of outcome. Patients relapsing within 6 months of allo-HSCT had a disappointing pEFS of 18.4% (pOS = 16.0%); the pEFS for those relapsing later was 55.5% (pOS = 74.8%). Our study provides real-world experience in pediatric, adolescent, and young adult patients with ALL treated with Tisa-cel, where most patients were treated after having relapsed after allo-HSCT. A total of 45.3% were rescued with a single dose of Tisa-cel. Our novel finding that patients with ALL after allo-HSCT had by far a better pEFS if relapse occurred beyond 6 months might be helpful in clinical decision-making and motivates studies to uncover the reasons.

Activating mutations in cytosolic 5′-nucleotidase II (NT5C2) are considered to drive relapse formation in acute lymphoblastic leukemia (ALL) by conferring purine analog resistance. To examine the clinical effects of NT5C2 mutations in relapsed ALL, we analyzed NT5C2 in 455 relapsed B-cell precursor ALL patients treated within the ALL-REZ BFM 2002 relapse trial using sequencing and sensitive allele-specific real-time polymerase chain reaction. We detected 110 NT5C2 mutations in 75 (16.5%) of 455 B-cell precursor ALL relapses. Two-thirds of relapses harbored subclonal mutations and only one-third harbored clonal mutations. Event-free survival after relapse was inferior in patients with relapses with clonal and subclonal NT5C2 mutations compared with those without (19% and 25% vs 53%, P < .001). However, subclonal, but not clonal, NT5C2 mutations were associated with reduced event-free survival in multivariable analysis (hazard ratio, 1.89; 95% confidence interval, 1.28-2.69; P = .001) and with an increased rate of nonresponse to relapse treatment (subclonal 32%, clonal 12%, wild type 9%, P < .001). Nevertheless, 27 (82%) of 33 subclonal NT5C2 mutations became undetectable at the time of nonresponse or second relapse, and in 10 (71%) of 14 patients subclonal NT5C2 mutations were undetectable already after relapse induction treatment. These results show that subclonal NT5C2 mutations define relapses associated with high risk of treatment failure in patients and at the same time emphasize that their role in outcome is complex and goes beyond mutant NT5C2 acting as a targetable driver during relapse progression. Sensitive, prospective identification of NT5C2 mutations is warranted to improve the understanding and treatment of this aggressive ALL relapse subtype.

Background: Children with high-risk 1st relapse B-ALL after frontline chemotherapy have a poor prognosis and are candidates for allogeneic hematopoietic stem cell transplant (alloHSCT) after achieving complete remission (CR). We previously reported that treatment with blinatumomab, a CD3/CD19-directed BiTE ® (bispecific T-cell engager) molecule, resulted in superior efficacy in event-free survival (EFS) and minimal residual disease (MRD) remission vs intensive multidrug chemotherapy as the 3rd consolidation block before alloHSCT in children with high-risk 1st relapse B-ALL in a phase 3 RCT (JAMA 2021;325:843-54).Here we provide FU of EFS, overall survival (OS), and safety as of Sep 2020, ie, beyond the July 2019 primary analysis. Methods: In this phase 3 RCT (Amgen NCT02393859), children >28 days and Results: Enrollment was terminated based on the interim analysis for benefit of blinatumomab (7/17/19 primary analysis). The following results reflect an updated analysis as of 9/14/20. Between Nov 2015-Aug 2019, 111 patients were randomized; 54 (49%) received blinatumomab and 57 (51%) received chemotherapy at 47 centers in 13 countries. Baseline characteristics were comparable in both arms (Table 1). After a median FU of 31 months, EFS was significantly superior with blinatumomab vs chemotherapy [63% vs 37%, stratified log-rank p CAR-T cell treatment after investigational therapy was reported for some patients; blinatumomab arm: 2/54 and chemotherapy arm: 9/57 patients (Table 2). Both blinatumomab arm patients had CD19+ disease after blinatumomab treatment, with OS of 22.1 and 12.8 months, and 1/2 alive at last FU. Of the 5 patients in the chemotherapy arm who later received blinatumomab, 3 had CD19+ disease after chemotherapy and 2 did not have CD19 expression data available; OS for these 5 patients ranged from 11.1-22.9 months with 2/5 alive at last FU. There were also 3 patients in the chemotherapy arm (but no exposure to blinatumomab) with CD19+ relapse who received CAR-T at relapse; OS for these patients ranged from 18.8-28.0 months and 2/3 were alive at last FU; a fourth patient with CD19-negative relapse who received chemotherapy only at relapse had OS of 24.8 months. Summary/Conclusion: In children with high-risk 1st relapse B-ALL, treatment with 1 cycle of blinatumomab vs chemotherapy before alloHSCT resulted in significantly superior EFS and improved OS. The incidence of CD19-negative relapse after blinatumomab was low. No evidence of a negative impact of blinatumomab followed by CAR-T cell therapy was noted in this small subset of patients. Figure 1 Figure 1. Disclosures Locatelli: Amgen: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Novartis: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Miltenyi: Speakers Bureau; Medac: Speakers Bureau; Jazz Pharamceutical: Speakers Bureau; Takeda: Speakers Bureau. Zugmaier: Amgen: Current Employment; Micromet/Amgen: Patents & Royalties: Patents 20190300609 and 20130323247 licensed; receives royalties of family members of international applications published as WO2010/052014; WO2010/052013; WO2011/051307; WO2012/055961; WO 2012/062596; WO2014/122251; and WO2015/181683; Amgen: Current equity holder in publicly-traded company. Rizzari: Sobi: Other: personal fees; Shire: Other: Grants and personal fees; Medac: Other: Grants and personal fees; Jazz Pharmaceuticals: Other: Personal fees; Amgen: Other: Personal fees. Morris: Amgen: Current Employment. Gruhn: AmgenGmbh: Honoraria, Membership on an entity's Board of Directors or advisory committees, Other: travel costs; Bellicum Pharma Gmbh: Membership on an entity's Board of Directors or advisory committees, Other: travel costs; EUSA Pharma Gmbh: Membership on an entity's Board of Directors or advisory committees; Jazz Pharmaceuticals: Honoraria, Other: travel costs; Novartis Pharma Inc.: Honoraria, Other: travel costs; pfizer: Honoraria; servier: Honoraria, Other: travel costs; Neovii Biotech GmbH: Other: travel costs; medac GmbH: Other: travel costs. Zeng: Amgen: Current Employment. Pilankar: Iqvia: Current Employment, Other: Iqvia provides support for Amgen. Kormany: Amgen: Current Employment. Moericke: Amgen: Other: Grants; Shire: Other: Personal fees. Peters: Amgen: Membership on an entity's Board of Directors or advisory committees, Other: Travel grants. Saha: Amgen: Other: Personal fees. von Stackelberg: Amgen: Consultancy, Honoraria; Novartis: Consultancy, Honoraria; Jazz: Consultancy, Honoraria; Miltenyi: Consultancy, Honoraria.

Introduction: Acute lymphoblastic leukemia (ALL) is the most common cancer affecting children and its cure rates are close to 90%. Nevertheless, relapsed ALL is still a major cause of cancer-associated deaths in children. Therefore, the multicenter trial Acute Lymphoblastic Leukemia-Relapse Study of the Berlin-Frankfurt-Münster Group (ALL-REZ BFM) 2002 was designed to improve the prognosis of children with relapsed ALL by introducing new chemotherapy elements. Little is known though about the prognostic relevance of deviations from the ALL protocols. This study aimed to investigate the characteristics of such protocol deviations and to determine their prognostic relevance for the relapsed disease. Methods: We performed a retrospective analysis of 686 children and adolescents up to 18 years of age with first relapse of ALL who were enrolled between 2002 and 2012 in the ALL-REZ BFM 2002 Study. Deviations were identified in terms of time and type. In order to investigate the prognostic importance of the time delay, the 90th percentile of delays between defined treatment elements (i.e. 15.5 days) was used as a cut-off point. Furthermore, deviations were categorized as avoidable (logistics, family/PI decisions) and non-avoidable (clear medical indication). The protocol deviation occurred during the course of therapy was therefore considered as a time-dependent variable. Disease-free survival, (DFS, induction failures excluded) and OS were analyzed using Kaplan-Meier estimate, log-rank test, Mantel-Byar test and Cox/logistic regression (univariate and multivariate analyses). Results: A total of 587 patients (86%) received the protocol therapy, whereas 99 patients (14%) underwent deviations during their treatment. Baseline characteristics were compared in both groups (Table 1). Deviations were categorized as: change of chemotherapy in 48% of the patients (n=47/99), stop of chemotherapy in 10 % (n=10/99), non-protocol compliant reaction to MRD findings in 28% (n=28/99) and change of radiation treatment in 14% (n=14/99). The cumulative incidence of protocol deviation was 6% and 15.5% at 3 and 9 months, respectively (Figure 1). The remission rate of patients with protocol deviation during induction was slightly lower compared to patients without such deviation (24/29, 83% vs. 579/657, 88%; P=0.38). The estimated 5-year probabilities of DFS and OS between the patients treated according to protocol therapy and those with deviations were significantly different (pDFS, Protocol therapy: 0.61 ± 0.02 vs. Deviation: 0.38 ± 0.05, P < 0.001; pOS, Protocol: 0.70 ± 0.02 vs. Deviation: 0.57 ± 0.05, P < 0.001; Figure 2, 3). In multivariate analyses protocol deviation, time point of relapse and immunophenotype turned out to be independent prognostic factors of the DFS (Table2). The hazard ratio of the protocol deviation was significant only in patients with the late relapse, whereas in patients with early/very early relapse no significant prognostic effect could be found [late (n=40): HR=2.53, 95%CI 1.53-4.21, P Moreover, DFS probabilities were comparable for avoidable (n=63) and non-avoidable (n=31) deviations (0.43 ± 0.07 vs. 0.40 ± 0.09, P=0.9; Non avoidable: HR=1.19, 95%CI=0.68-2.10, P= 0.53). In terms of the time-related deviations, the extent of the delays between induction and beginning of consolidation therapy was not significantly associated with the DFS [≤90th percentile (n=319/603); pDFS: =0.60 ± 0.02 vs. >90th percentile (n=36/603); pDFS: 0.56 ± 0.08, P=0.3]. Treatment delays during other phases of the protocol were also not related to the outcome. Conclusion: While protocol deviations did not significantly affect remission rates in this study, they were significantly related to inferior DFS. Since a relevant part of the protocol deviations could be classified as avoidable, strict protocol compliance should lead to improved outcomes. The prognostic impact of protocol deviations was in particular relevant in patients with late relapse with generally rather chemosensitive diseases and better prognosis. In this analysis, treatment delays did not influence the outcome. Figure 1 Figure 1. Disclosures von Stackelberg: Amgen: Consultancy, Honoraria; Novartis: Consultancy, Honoraria; Jazz: Consultancy, Honoraria; Miltenyi: Consultancy, Honoraria.

Background: Inotuzumab ozogamicin (InO) was well tolerated and demonstrated anti-leukemia activity in heavily pre-treated pediatric patients (pts) with CD22-positive relapsed/refractory (R/R) acute lymphoblastic leukemia (ALL) in the Phase (Ph) I ITCC-059 study. With the established recommended phase 2 dose (RP2D) (1.8 mg/m2/course, as in adults) a consecutive Ph II study has been performed, sponsored by Erasmus MC and supported by Pfizer (NTR57360). Study design: Pts aged 1-18 years with R/R CD22+ BCP-ALL were included after informed consent was obtained. Key inclusion criteria included, M2/M3 marrow and adequate liver and kidney function. Overall response rate (ORR) was the primary endpoint, and included CR, CRp (ANC >500/µL but PLT ≤50.000/µL) and CRi (ANC ≤500/µL and/or PLT ≤50,000/µL). Secondary endpoints included safety, minimal residual disease (MRD) levels and durability of response. The study consisted of a single-stage design to test the null hypothesis (H0) ORR ≤30% and the alternative hypothesis of ORR >55%. With 25 pts, the study had 80% power to reject H0 at a significance level of 0.05. Central MRD analysis by flow cytometry was considered negative if Results: 32 pts were enrolled (Jun 2019-Apr 2020), including 2 screen failures and 2 pts who did not start treatment due to rapidly progressive disease. The median age of the 28 treated pts was 7.5 years (range 1-17); 19 pts were male (68%); 6 (21.5%) were primary refractory, 16 (57%) had ≥2nd relapse and 6 (21.5%) had 1st relapse post-HSCT (median 2 prior regimens, range 2-7). 50% of the pts had received a previous HSCT, 3 (11%) CAR T-cell therapy and 7 (25%) blinatumomab. Median baseline WBC was 3.1 x109/L (range 0.7-132). At time of data snapshot, 48 courses of InO were administered (range 1-4 per pt); one pt was still receiving InO. Disease response was not assessed in 1 pt (discontinued early due to sinusoidal obstruction syndrome (SOS)), 27 pts were evaluable for efficacy analyses. Twenty-two pts achieved a response (ORR 81.5%; 95% CI 61.9%, 93.7%), all after the first cycle (CR n=14, CRp n=1, CRi n=7). Hence, the primary objective was achieved (P-value When combining these results with pts treated at the RP2D in the Ph I study (n=13), 33/40 (82.5%) achieved a response (94% of whom achieved MRD-negativity). Of 9 primary refractory patients included in Ph I and II, 3 (33%) did not respond to InO. The median follow-up time was 7.3 months (mo). The EFS at 6 and 12 mo was 57.9% (95% CI: 40.3−83.3) and 24.8% (95% CI: 9.8−62.9); and OS at 6 and 12 mo was 61.6% (95% CI: 43.3−87.8) and 54.8% (95% CI: 35.9−83.6), respectively. The median EFS was reached at 6.34 mo (95% CI 2.53, NA). The cumulative incidence of non-response or relapse was 32% and 57% at 6 and 12 mo; 3 pts died in CR (2 due to HSCT complications, 1 due to neurological deterioration considered related to previous CNS leukemia and prolonged intrathecal treatment). Nine of 22 responding pts underwent HSCT (median 35 days after the last InO dose, range 20-72); 4 are still in CR. Three responders received consolidation with CAR T (52, 55 and 215 days after last Ino dose), all are still in CR. Four cases of VOD/SOS were reported. One case occurred during InO treatment (gr 3, resolved). Three of 9 transplanted pts (33%) developed post-HSCT SOS. None of these pts received prophylactic defibrotide. One pt, previously transplanted and treated with CAR T, received 1 course of InO prior to a second HSCT and developed SOS (gr 3), ongoing at the time of death due to multiorgan failure. The other 2 pts developed gr 2 SOS after 2 and 3 courses of InO before their first HSCT, both events resolved. All pts had at least 1 adverse event (AE) during the study and 19 pts reported at least 1 of gr 3-4 AE. The most common AE was fever. Detailed data will be presented at the meeting. Conclusion: InO was well tolerated in this Ph II study which confirmed remarkable activity in these heavily pretreated pts. The ORR was 81.5% with 95% MRD-negativity; 55% of pts remained alive after 1 year. Twelve pts proceeded to consolidation treatment (either HSCT or CAR T). In contrast with the Ph I cohort (where no pts developed SOS post 7 HSCTs), 3 cases of SOS were recorded here after 9 HSCTs. A Ph I cohort of this study combining InO with chemotherapy in R/R pediatric ALL is ongoing. Disclosures Brivio: Pfizer Inc.: Other: Erasmus MC received institutional funding for the study from Pfizer. Rossig:BMS: Honoraria; Celgene: Membership on an entity's Board of Directors or advisory committees; Roche: Honoraria, Membership on an entity's Board of Directors or advisory committees; Novartis: Membership on an entity's Board of Directors or advisory committees; Genetech: Membership on an entity's Board of Directors or advisory committees; Amgen: Membership on an entity's Board of Directors or advisory committees; EUSA Pharma: Membership on an entity's Board of Directors or advisory committees; Pfizer: Honoraria. Sleight:Pfizer Inc.: Current Employment. Reinhardt:Novartis: Membership on an entity's Board of Directors or advisory committees; CLS Behring: Research Funding; Celgene Corporation: Membership on an entity's Board of Directors or advisory committees, Research Funding; Janssen: Membership on an entity's Board of Directors or advisory committees; Roche: Research Funding; bluebird bio: Membership on an entity's Board of Directors or advisory committees. von Stackelberg:Morphosys: Membership on an entity's Board of Directors or advisory committees, Other: Personal fees, advisory committees and speakers bureau, Speakers Bureau; Roche: Membership on an entity's Board of Directors or advisory committees, Other: Personal fees, advisory committees and speakers bureau, Speakers Bureau; Jazz: Membership on an entity's Board of Directors or advisory committees, Other: Personal fees, advisory committees and speakers bureau, Speakers Bureau; Shire: Membership on an entity's Board of Directors or advisory committees, Other: Personal fees, advisory committees and speakers bureau, Speakers Bureau; Amgen: Membership on an entity's Board of Directors or advisory committees, Other: Personal fees, advisory committees and speakers bureau, Speakers Bureau. Zwaan:Servier, Sanofi, Daiichi Sankyo, Novartis, Janssen, Roche, Incyte, Pfizer, Celgene: Consultancy; Pfizer, Celgene, BMS: Research Funding; Jazz Pharmaceuticals: Other: Travel funding.

Despite progress with immunotherapy and targeted agents, treatment of refractory disease remains challenging, in particular for patients with T-ALL. Identification of genomic lesions defining actionable targets had limited impact on patient care so far. The complexity of biological systems highlights the need to develop complementary functional approaches. We and others have established platforms with a library of 120 drugs based on current treatment and (pre-)clinical development, to detect ex-vivo drug response phenotypes on leukemia samples at single cell resolution by high content image analysis. We demonstrated that drug response profiling (DRP) identifies dependencies not predicted by genetic alterations adding a functional information layer for clinicians. Here we report first correlations with clinical outcome using DRP in a non-interventional setting. From 2016 to 2019 we performed DRP in the framework of European ALL first- and second- line protocols upon request by treating centers. Here we analyze retrospectively treatment decisions and outcome for 23 T- and 50 BCP-ALL patients. To evaluate drug responses, we compared dose response curves of individual patients to data recorded for all patients. Sensitivity and resistance were defined based on the IC50 outlier analysis using cut-offs depending on distribution (normal gaussian vs. skewed). From 73 patients tested, clinical outcome data has been available for 36 BCP- and 15 T-ALL patients. NGS data provided by the INFORM registry has been available in 8 BCP- and 2 T-ALL patients. In first line BCP-ALL patients, ex-vivo Dexamethasone response predicted clinical response to prephase prednisone (d8) and minimal residual disease (MRD) reduction measured by flow cytometry at d15 of first line AIEOP BFM 2009 induction (Fig. 1). For refractory and relapsed ALL we observed an association of DRP and response to targeted agents in 14 out of 16 patients (87.5 %; Table 1). Data for the r/r BCP-ALL cohort is limited because most patients underwent CD19 and / or CD22-directed immunotherapy. Sensitivity and resistance to Calicheamicin correlated with clinical response to Inotuzumab, suggesting functional testing to be evaluated in future studies. In contrast, lack of correlation of ex-vivo sensitivity to MEK-inhibitors with presence of RAS-pathway alterations caution the exclusive use of molecular information to predict response to these agents. Most therapeutic decisions based on DRP information were made for patients with r/r T-ALL. Bortezomib ex-vivo sensitivity correlated with clinical responses in 5 T-ALL patients (Fig. 2). Both patients predicted to respond to Bortezomib and treated on Bortezomib + Venetoclax experienced good MRD response providing a bridge to stem cell transplantation (SCT). However, these patients relapsed after SCT emphasizing the need for additional consolidative therapeutic elements for heavily pretreated patients. In line with previous reports, we confirmed a T-ALL with high sensitivity to Dasatinib (IC50 1.9 nM; Fig. 3). Dasatinib monotherapy induced a molecular remission. A 2nd T-ALL showing an ex- vivo Dasatinib IC50 at 80 nM was refractory to treatment with Dasatinib + Daunoxome-FLAG. A 3rd ABL1-fusion positive T-ALL, ex-vivo resistant to Imatinib and Dasatinib, had only short- term response to Imatinib + chemotherapy. Finally, treating a T-ALL patient based on high sensitivity to the XPO1 inhibitor Selinexor as 4th line monotherapy led to significant decrease of PB blasts from d1 25 G/L to 0.7 G/L at d13 of treatment (Fig. 4). The patient experienced improved quality of life, minimizing need of hospitalization with stable disease for 3 months on maintenance with Selinexor. Given the promising preclinical data with this class of agents and current lack of established biomarkers, we propose that DRP should be evaluated for this class of agents. In conclusion, we established first associations between DRP and clinical response for various agents providing a rationale for the evaluation of DRP in prospective clinical trials. Integration of molecular and functional information may improve the selection of more specific treatment options for patients with resistant disease. The international BFM Study Group and ITCC Consortium are planning an international multiarm early clinical trial for treatment of r/r ALL patients that will include DRP for evaluation in order to improve selection of targeted therapy. Disclosures Cario: Jazz Pharmaceuticals: Consultancy, Other: travel support; Novartis: Consultancy, Other: travel support. Hrusak:Amgen: Other: MRD investigations funded by Amgen, Research Funding. Kulozik:Novartis: Consultancy, Honoraria; bluebird bio, Inc.: Consultancy, Honoraria. von Stackelberg:Morphosys: Membership on an entity's Board of Directors or advisory committees, Other: Personal fees, advisory committees and speakers bureau, Speakers Bureau; Roche: Membership on an entity's Board of Directors or advisory committees, Other: Personal fees, advisory committees and speakers bureau, Speakers Bureau; Jazz: Membership on an entity's Board of Directors or advisory committees, Other: Personal fees, advisory committees and speakers bureau, Speakers Bureau; Amgen: Membership on an entity's Board of Directors or advisory committees, Other: Personal fees, advisory committees and speakers bureau, Speakers Bureau; Shire: Membership on an entity's Board of Directors or advisory committees, Other: Personal fees, advisory committees and speakers bureau, Speakers Bureau. Jacoby:Novartis: Honoraria, Membership on an entity's Board of Directors or advisory committees; Lonza: Membership on an entity's Board of Directors or advisory committees. Bourquin:Servier: Other: Travel Support.

Despite major advances in the treatment of patients with acute lymphoblastic leukemia in the last decades, refractory and/or relapsed disease remains a clinical challenge, and relapsed leukemia patients have an exceedingly dismal prognosis. Dysregulation of apoptotic cell death pathways is a leading cause of drug resistance; thus, alternative cell death mechanisms, such as necroptosis, represent an appealing target for the treatment of high-risk malignancies. We and other investigators have shown that activation of receptor interacting protein kinase 1 (RIP1)–dependent apoptosis and necroptosis by second mitochondria derived activator of caspase mimetics (SMs) is an attractive antileukemic strategy not currently exploited by standard chemotherapy. However, the underlying molecular mechanisms that determine sensitivity to SMs have remained elusive. We show that tumor necrosis factor receptor 2 (TNFR2) messenger RNA expression correlates with sensitivity to SMs in primary human leukemia. Functional genetic experiments using clustered regularly interspaced short palindromic repeats/Cas9 demonstrate that TNFR2 and TNFR1, but not the ligand TNF-α, are essential for the response to SMs, revealing a ligand-independent interplay between TNFR1 and TNFR2 in the induction of RIP1-dependent cell death. Further potential TNFR ligands, such as lymphotoxins, were not required for SM sensitivity. Instead, TNFR2 promotes the formation of a RIP1/TNFR1-containing death signaling complex that induces RIP1 phosphorylation and RIP1-dependent apoptosis and necroptosis. Our data reveal an alternative paradigm for TNFR2 function in cell death signaling and provide a rationale to develop strategies for the identification of leukemias with vulnerability to RIP1-dependent cell death for tailored therapeutic interventions.

Background: CAR T cells targeting CD19 are approved for patients with relapsed or refractory acute lymphoblastic leukemia (ALL), failing two or more prior therapies. The central nervous system (CNS) is a known sanctuary site of ALL. Despite observations of CAR T-cells trafficking to the CNS in patients, most clinical trials excluded patients with active CNS leukemia, partially for concerns of neurotoxicity. Methods: We conducted an international retrospective study of patients who were referred to CAR T-cell therapy for relapsed ALL with CNS involvement. Results: We reviewed data of 27 patients (16 males and 11 females) from 8 centers who were treated with CD19 CAR T-cells for ALL relapse involving the CNS. Fifteen patients had a prior bone marrow transplantation, and in seven total body irradiation was given. Four additional patients received previous cranial radiation. At time of CAR T-cell order or clinical trial inclusion, ten patients had an isolated CNS relapse, and 17 had a combined CNS and marrow relapse. The median number of cells in the CSF of patients at enrollment was 65 (range 0-5670), and 16 of 21 patients who had imaging performed showed leukemia-associated changes in brain MRI. All patients received bridging chemotherapy between enrolment and lymphodepletion, including intrathecal chemotherapy alone or in combination with systemic chemotherapy and/or radiation. At lymphodepletion, 15 patients had cleared CNS disease while 12 had active CNS blasts or imaging lesions. Following lymphodepletion with fludarabine and cyclophosphamide, patients received 2nd-generation CAR T cells: 15 patients treated with a CD19-4-1BB CAR and 12 with a CD19-CD28 CAR. Cytokine release syndrome (CRS) occurred in 20 patients: 12 patients (100%) treated with CD19-CD28 CARs and 8 with CD19-4-1BB CARs (53%, p=0.006). Immune-effector cell neurotoxicity (ICANS) was diagnosed in 11 patients, 9 of 12 following CD28-based CARs, and 3 of 15 following 41BB-based CARs (p=0.004). Patients with active CNS-disease prior to lymphodepletion also had a higher rate of ICANS (75% vs 20%, p=0.004) but not of CRS. One patient died of grade 5 ICANS on day +7 following CD28-based CARs for an isolated CNS relapse. Twenty-four of 26 evaluable patients (92%) had a complete remission following CAR T-cells. Nine patients were further referred to an allogeneic transplant, all following CD28-based CARs. Overall, six patients had relapsed - in four relapse involved bone marrow, and in two the CNS. Conclusions: In this cohort of patients with active CNS disease treated with CD19 CAR T-cells, we report a high remission rate in previously heavily pre-treated patients. Toxicities are significant, and associated mostly with the costimulatory domain of the CAR (CD28>41BB) and the presence of active CNS disease at lymphodepletion. This may be informative when designing future clinical studies of CAR T cell therapy for patients with CNS manifestations of ALL. Disclosures Jacoby: Novartis: Honoraria, Membership on an entity's Board of Directors or advisory committees; Lonza: Membership on an entity's Board of Directors or advisory committees. Ghorashian:Novartis: Honoraria; UCLB: Patents & Royalties; Amgen: Honoraria. De Moerloose:Novartis: Consultancy. Rossig:Celgene: Membership on an entity's Board of Directors or advisory committees; Novartis: Membership on an entity's Board of Directors or advisory committees; Roche: Honoraria, Membership on an entity's Board of Directors or advisory committees; Amgen: Membership on an entity's Board of Directors or advisory committees; BMS: Honoraria; EUSA Pharma: Membership on an entity's Board of Directors or advisory committees; Genetech: Membership on an entity's Board of Directors or advisory committees; Pfizer: Honoraria. von Stackelberg:Amgen: Membership on an entity's Board of Directors or advisory committees, Other: Personal fees, advisory committees and speakers bureau, Speakers Bureau; Jazz: Membership on an entity's Board of Directors or advisory committees, Other: Personal fees, advisory committees and speakers bureau, Speakers Bureau; Roche: Membership on an entity's Board of Directors or advisory committees, Other: Personal fees, advisory committees and speakers bureau, Speakers Bureau; Morphosys: Membership on an entity's Board of Directors or advisory committees, Other: Personal fees, advisory committees and speakers bureau, Speakers Bureau; Shire: Membership on an entity's Board of Directors or advisory committees, Other: Personal fees, advisory committees and speakers bureau, Speakers Bureau. Bourquin:Servier: Other: Travel Support. OffLabel Disclosure: We describe the use of commercial and investigational CAR T-cell products

Survival of patients with pediatric acute lymphoblastic leukemia (ALL) after allogeneic hematopoietic stem cell transplantation (allo-SCT) is mainly compromised by leukemia relapse, carrying dismal prognosis. As novel individualized therapeutic approaches are urgently needed, we performed whole-exome sequencing of leukemic blasts of 10 children with post-allo-SCT relapses with the aim of thoroughly characterizing the mutational landscape and identifying druggable mutations. We found that post-allo-SCT ALL relapses display highly diverse and mostly patient-individual genetic lesions. Moreover, mutational cluster analysis showed substantial clonal dynamics during leukemia progression from initial diagnosis to relapse after allo-SCT. Only very few alterations stayed constant over time. This dynamic clonality was exemplified by the detection of thiopurine resistance-mediating mutations in the nucleotidase NT5C2 in 3 patients' first relapses, which disappeared in the post-allo-SCT relapses on relief of selective pressure of maintenance chemotherapy. Moreover, we identified TP53 mutations in 4 of 10 patients after allo-SCT, reflecting acquired chemoresistance associated with selective pressure of prior antineoplastic treatment. Finally, in 9 of 10 children's post-allo-SCT relapse, we found alterations in genes for which targeted therapies with novel agents are readily available. We could show efficient targeting of leukemic blasts by APR-246 in 2 patients carrying TP53 mutations. Our findings shed light on the genetic basis of post-allo-SCT relapse and may pave the way for unraveling novel therapeutic strategies in this challenging situation.

This study investigates the extent of bone marrow (BM) involvement at diagnosis of apparent isolated extramedullary (AIEM) relapses of childhood acute lymphoblastic leukemia (ALL) and its relation to prognosis. Sixty-four children with first AIEM relapse treated in Germany, Czech Republic, or France were included. Real-time quantitative polymerase chain reaction using T-cell receptor and immunoglobulin gene rearrangements provided a sensitive measure of submicroscopic BM involvement, which was detectable at a level of 10−4 or higher in 46 patients and less than 10−4 in 11 patients, and was nondetectable (sensitivity: 10−4) in 7 patients. In the total cohort, the probability of event-free survival (pEFS) for children with BM involvement of 10−4 or higher was 0.30 (0.09 ± SE) versus 0.60 (± 0.12) for those with less than 10−4 (P = .13). The cumulative incidence of subsequent relapse was 0.24 (± 0.01) for patients with BM involvement less than 10−4 and 0.65 (± 0.01) for those with 10−4 or higher (P = .012). Restricted to central nervous system (CNS) relapses, pEFS was 0.11 (± 0.09) for patients with BM involvement 10−4 or higher and 0.63 (± 0.17) for those with less than 10−4 (P = .053). CNS relapses were associated with a higher (≥ 10−4: 80%) submicroscopic BM involvement than testicular relapses (≥ 10−4: 57%, P = .08). In summary, we show marked heterogeneity of submicroscopic BM involvement at first AIEM relapse diagnosis in children with ALL, and demonstrate its possible prognostic relevance.

Allogeneic stem cell transplantation (SCT) is frequently considered as treatment for relapsed childhood acute lymphoblastic leukemia (ALL). For patients without a matched sibling donor, SCT from unrelated donors (UD-SCT) has been increasingly performed during the past years. However, UD-SCT–related mortality and morbidity is still considerable, and the question remains as to which patients are at such high risk of recurrence that UD-SCT is indicated and, conversely, which patients do not require transplantation for long-term disease control. A matched-pair analysis was performed among patients treated according to Acute Lymphoblastic Leukemia Relapse Berlin-Frankfurt-Münster (ALL-REZ BFM) Study Group protocols after first relapse with chemotherapy or UD-SCT. Altogether 81 pairs were identified that could be matched exactly for site of relapse and immunophenotype, and as closely as possible for duration of first remission, age, diagnosis date, and peripheral blast cell count at relapse. No significant difference in the probability of event-free survival (pEFS) between UD-SCT and chemotherapy existed regarding 28 pairs with an intermediate prognosis (0.39 ± 0.10 vs 0.49 ± 0.11,P = .105), whereas the pEFS was significantly different in the 53 pairs with a poor prognosis (0.44 ± 0.07 vs 0.00 ± 0.00, P

To the Editor: In a recent issue of Blood, Loh et al[1][1] reported a low frequency of TEL-AML1 fusion [translocation t(12;21)(p13;q22)] in relapsed childhood acute lymphoblastic leukemia (ALL) treated initially on four different DFCI ALL consortium protocols from 1981 until 1995. The incidence of

Background: Relapse is the leading cause of mortality in children with acute lymphoblastic leukemia (ALL). Studies have shown that most ALL cases are polyclonal at diagnosis and that genetic changes in individual subclones influence sensitity to therapy and subsequent clonal evolution during therapy; but the molecular details remain to be worked out. Among different pathways enriched for mutations at relapse, purine metabolism is particularly interesting for two reasons: first, thiopurines are widely used in the ALL combination chemotherapy regimens, and are prodrugs that are converted by the purine salvage pathway to cytotoxic metabolites. Second, de novo nucleotide biosynthesis is often upregulated in cancer cells, and it is believed that sufficient nucleotide pools are required to maintain genomic stability, could bypass oncogene-induced senescence and promote tumor progression1. Therefore, we focus our current study on de novo purine biosynthesis in drug resistance and tumor relapse of childhood ALL. Methods and Results: Using whole-exome sequencing, we identified relapse-specific mutations in the phosphoribosyl pyrophosphate synthetase 1 gene (PRPS1), which encodes a rate-limiting purine biosynthesis enzyme, in 24/358 (6.7%) relapsed childhood B cell ALL (B-ALL) cases. Targeted sequencing identified mutations in additional genes in de novo purine biosynthesis pathway, providing further genetic evidence for its importance in relapsed ALL. All individuals with PRPS1 mutation relapsed early on-treatment (P Using various functional assays, we demonstrated that rather than causing a simple gain-of-function effect, the mutations in PRPS1 resulted in the disruption of the normal feedback inhibition of purine synthesis, in which the enzyme remained active despite an increased concentration of nucleoside analogs. PRPS1 mutants increased synthesis of the nucleoside inosine monophosphate, its metabolite hypoxanthine (HX) and de novo purine biosynthesis intermediates (e.g. AICAR, SAICAR) in Reh cells. Increased intracellular HX can competively inhibit the conversion of thiopurines into their active metabolites. Furthermore, inhibition of de novo purine biosynthesis in vitro, either by CRISPR-Cas9 genome editing of de novo purine synthesis pathway genes (GART, ATIC etc.) or treatment with a pathway inhibitor lometrexol (GART inhibitor) alleviated the metabolic disturbance and drug resistance induced by PRPS1 mutations. Using ultra-deep sequencing of unique serial remission samples before clinical relapse, we noticed that the PRPS1 mutant allele fraction increased drastically before clinical relapse, suggesting rapid clonal expansion occurs after the acquisition of a PRPS1 mutation. Interestingly, we also noticed that PPRS1 mutation coexist with RAS mutation in many relapse cases and at single cell resolution. Functional analysis revealed that tumor cells which harbored RAS and PRPS1 double mutations are more drug resistant than those with RAS or PRPS1 mutation alone. Previous studies have shown that oncogenic RAS mutation can also induce various stress responses including oncogene-induced senensence and DNA damage response (DDR), which all could impede tumor cell proliferation during relapse. In vitro, we found PRPS1 mutation can release the replication and metabolic stress caused by RAS mutation, in addition to their role in thiopurine resistance. The PRPS1 mutants not only increase the nucleotide pools but also elevate purine biosynthesis intermediate AICAR, which can activate AMPK and reduce the RAS mutant-induced DDR. We are currently working on in vitro and in vivo models (including patient derived xenograft models) to further test the double mutant's effects on tumor-reinitiation and clonal evolution during ALL relapse. Conclusions: We demonstrated that negative feedback-defective PRPS1 mutants can drive de novo purine biosynthesis, which can exert drug resistance and reduce genomic instability during tumor relapse. Our study highlights the importance of de novo purine biosynthesis in the pathogenesis of relapse, and suggests a diagnostic approach to predicting early relapse and a therapeutic strategy to circumventing resistance in ALL. 1 Li et al. Negative feedback-defective PRPS1 mutants drivee thiopurine resistance in relapsed childhood ALL. Nature Medicine,21(6): 563-571 (2015) Disclosures No relevant conflicts of interest to declare.

With the rapid development of new therapeutic agents, better strategies are needed to select patients with high risk or relapsed acute lymphoblastic leukemia (ALL) for experimental therapy. We developed a large-scale drug response profiling platform that is based on an established ALL co-culture system on bone marrow stromal cells, with the advantage of standardized conditions in serum-free medium. Comparing the profiles of 61 ALL samples that were enriched for relapsed and refractory ALL, we identified informative patterns of drug sensitivity and resistance comparing between clinical and cytogenetic ALL subtypes. We detected strong and selective activity for therapeutic agents that could be used in the clinic. These include a BCP-ALL with particular sensitivity to SMAC mimetics and a T-cell ALL subsets with very high sensitivity to dasatinib or topotecan. Here we focus on identification of ALLs subtypes that are critically dependent on BCL2, based on their response patterns to the BCL2-specific BH3-mimetic venetoclax (ABT-199). A subset of BCP-ALL, including MLL-AF4 and TCF3-HLF positive ALL cases were highly sensitive to venetoclax. As reported by others recently, we also detected a smaller subgroup of T-ALL that were highly sensitive to BCL2 inhibition and confirm that these are not restricted to early thymic precursor ALL. Sensitivity to venetoclax in vitro correlated with anti-leukemic activity in ALL xenografts in vivo. Moreover combination testing on our platform indicated synergistic activity of venetoclax with conventional anti-leukemic agents such as dexamethasone and vincristine, and with the new class BRD4 inhibitors. In vivo, combination of venetoclax with dexamethasone and vincristine completely prevented disease progression of very aggressive TCF3-HLF ALL cases compared to a transient delay in disease progression that we observed in the control arms with single agent venetoclax or dexamethasone + vincristine treatment. Based on our results, we propose that drug activity profiling should be evaluated prospectively in clinical phase I and II settings as a tool to develop more personalized approaches for patients with resistant disease that are at urgent medical needs. Disclosures No relevant conflicts of interest to declare.

BTZ is a proteasome inhibitor and could improve the poor prognosis of rALL (Messinger 2012). We developed a European phase II study in pediatric rALL, in which patients were randomised for 'early' or 'late' BTZ, in compliance with the Declaration of Helsinki. BTZ was given as iv push for 4 doses at 1.3 mg/m2/dose, in group 'early' on days 1, 4, 8 and 11 and in group 'late' on days 8, 11, 15 and 18. All patients also received DXM (10 mg/m2/day in 3 doses for 2 weeks) and VCR (1.5 mg/m2/dose (maximum 2 mg) as 1-hour infusion on days 8 and 15), plus intrathecal methotrexate (i.t. MTX, dose age-adjusted) on day 1. Cycles could be repeated in case of a good response. The number of 12 patients in each group provides a power of 80% to detect a statistically significantly lower absolute number of circulating leukemic blasts on day 8 of treatment in patients exposed to 'BTZ early' as compared to patients not yet exposed to BTZ ('BTZ late') of at least 30% (2-sided test, alpha Between October 2010 and September 2014, 29 patients with information on response and/or toxicity after cycle 1 enrolled, 17 boys and 12 girls, median 9.7 years of age (range, 1 - 17.5). Most had second or subsequent rALL (n=16). The primary endpoint, day 8 absolute blast count, was evaluable in 26 patients (n=2 not eligible for the primary endpoint in retrospect, n=1 no material received), and was not different between 'BTZ early' (n=13) and 'BTZ late' (n=13). Absolute blast count had decreased at day 8 in both 'BTZ early' (p =0.017) and 'BTZ late' (p =0.046) compared to day 1 blast count, but to the same extent (median 85-86% decrease). Regarding response after cycle 1, 25 patients were evaluable (n=2 discontinued treatment due to toxicity, n=2 no material received). Nine patients (36%) achieved a complete remission with incomplete blood count recovery (CRi), 6 (24%) a partial remission (PR), and 10 had a treatment failure (TF), for a total response rate of 60%. There was no relation between remission status and the administration of BTZ being early or late (p =0.7). There was no association between response and disease status, although numbers were small (p =0.8). Four patients with a relapse after allo-SCT achieved CRi, as well as 5 patients with a second or subsequent relapse. Grade 3 or 4 hematological toxicities were thrombopenia (n=29), leukopenia (n=23), neutropenia (n=25), and anemia (n=21). Most common grade 2-4 neurotoxicities were pain (n=9) and peripheral neuropathy (PN) (n=5). Thirteen patients received a second cycle, when one patient developed grade 2 PN and one patient with PN advanced from grade 2 to 3. There was remarkable intra- and inter-individual variability in peak plasma concentrations 15 min after the first administration of BTZ, ranging from 0.7 to 715 ug/L (median 12 ug/L, excluding 2 outliers above limit of quantification (2500 ug/L)). The area-under the plasma concentration curve (AUC) had a 25-fold interindividual variation: median 0.27 ug.h/L (range 0.07 - 1.77). Mean clearance in plasma was 1.1 L/h, with a interindividual variability of 95%. Median bone marrow concentrations of BTZ were 49 ug/kg (range: 1.6 - 106 ug/kg). In most patients, no BTZ was detected in the CSF, with a lower detection limit of 0.05 ng/ml. In 9 patients BTZ was detected at 0.07 - 5.2 ug/L. No correlation between AUC and concentration in CSF was found (r2 = 0.11). The 50% inhibitory concentration (IC50) of 20S activity in plasma was 10.7 ug/L. Plasma pharmacokinetic and pharmacodynamic parameters did not correlate with response to treatment. In conclusion, BTZ in combination with VCR and DXM and i.t. MTX is effective in a significant subset of pediatric rALL, with 60% of patients achieving PR or CRi. Because administration of BTZ early or late showed similar early treatment responses, randomized studies with more patients are necessary to determine the additive value of BTZ to conventional chemotherapy. Repetitive cycles could be given in several children without undue toxicity. In view of the observed toxicity, it might be wise to give these patients subcutaneous BTZ or novel proteasome inhibitors. BTZ does not or hardly penetrate the cerebrospinal fluid. This research is financially supported by the Dutch Foundation Children Cancer-free (clinical research support) and by Janssen Pharmaceuticals (clinical research support and free drug). Disclosures Kaspers: Janssen-Cilag: Research Funding. Off Label Use: Bortezomib off-label for pediatric ALL. Zwaan:Novartis: Consultancy, Membership on an entity's Board of Directors or advisory committees; Janssen: Research Funding.

Background. The risk of early death (ED) due to bleeding and/or leukostasis is high in AML patients with initial hyperleukocytosis (white blood cell count [WBC] > 100 000/µl) and highest in those with hyperleukocytosis and mono- or myelomonocytic leukemia (FAB M4/M5). (Creutzig, et al 1987) Within the AML-BFM studies, emergency strategies for children with AML and high risk for bleeding and leukostasis included exchange transfusion (ET) or leukapheresis (LPh). In order to determine whether these interventions reduced the rate of ED, 1251 AML-BFM patients from the trials AML-BFM 98 and 04 were analyzed. Risk factors for ED and interventions performed were verified focusing on patients with hyperleukocytosis. Patients . 238 of 1251 (19%) AML-patients Results. The percentage of ED by bleeding/leukostasis increased with higher WBC counts and was highest in 105 patients with WBC > 200 000/µl (14.3%). The ED rates were even higher in patients with FAB M4/M5 and hyperleukocytosis >200 000/µl compared to others with WBC >200 000/µl (M4/M5 13/65 [20%] vs. others 2/40 [5%], p=0.04). Patients with WBC counts >200 000/µl did slightly better with ET or LPh compared to those without ET/LPh (ED rate 7.5 % vs 21.2 %, p=0.055). Patients with WBC between 100 000/µl and 200 000/µl received ET/LPh less frequently compared to those with WBC >200 000/µl (22/133 [17%] vs. 53/105 [50%]). ET/LPh was mainly given in case of clinical symptoms of bleeding or leukostasis or coagulopathies or insufficient reduction (or increase) of WBC counts despite low dose chemotherapy. 15/80 (19%) patients with FAB M4/5 and WBC 100 000-200 000/µl received ET/LPh and none of these patients died early. ET/LPh was even given in 14 patients with WBC 200 000/µl was the strongest independent risk factor for ED (hazard ratio =15.0, 95% confidence interval 4.9-46.3, p(chi) Conclusion. Our data confirm the high risk of bleeding/leukostasis in patients with hyperleukocytosis. Although we could only disclose a trend for a clinical benefit of ET/LPh in this retrospective analysis - probably due to some negative selection bias in patients with the intervention - we strongly advocate ET/LPh in AML patients with WBC >200 000/µl, and in particular in those with FAB M4/M5 subtypes or with clinical symptoms of bleeding or leukostasis or coagulopathies even with lower WBC (100 000/µl - 200 000/µl). Creutzig, U. et al. (1987) Early deaths due to hemorrhage and leukostasis in childhood acute myelogenous leukemia: Associations with hyperleukocytosis and acute monocytic leukemia. Cancer,60, 3071-3079. Disclosures No relevant conflicts of interest to declare.

Dysregulation of apoptotic pathways provides an indiscriminate mechanism for refractory acute lymphoblastic leukemia (ALL) to escape cell death induced by many chemotherapeutic compounds. Here we have assessed the potential of SMAC mimetic (SM) compounds to short circuit cell death resistance by blocking the pro-survival cellular inhibitor of apoptosis (cIAP) proteins. By screening a large set of patient-derived precursor B-cell ALL samples in an ex vivo model of the leukemia microenvironment we detect exquisite sensitivity to two different SM compounds, Birinapant and LCL161, in about one third of ALL samples. Strong ex vivo SM activity correlated with potent in vivo anti-leukemic efficacy against de novo refractory and relapsed ALL xenografts. Intriguingly, we find that although SM-sensitivity is independent of TNF and TNFR1 levels, expression of TNFR2 is highly predictive of response to SM in two separate cohorts of ALL samples, suggesting that TNFR2 expression may represent a promising biomarker for identifying SM-sensitive cells. Downstream, we employ a novel and powerful multi-colour Lenti-CRISPR approach to show that simultaneous disruption of both apoptotic and necroptotic genes is necessary to block SM-induced death. In contrast, disruption of RIP1 alone was adequate to block SM-induced apoptosis and necroptosis. Surprisingly, RIP1 loss had no significant impact on response to standard anti-leukemic therapies, supporting a view that the RIP1-dependent death pathway is not likely to be selected against in leukemia cells that have failed to respond to front line therapy. These results provide the first evidence that SM compounds can circumvent apoptotic escape in drug-refractory ALL through parallel activation of both RIP1-dependent apoptosis and necroptosis. Furthermore, our data strongly support further development of SM as anti-leukemic agents for treatment in resistant disease. Disclosures No relevant conflicts of interest to declare.

At acute lymphoblastic leukemia (ALL) relapse, about 40% of children can be salvaged with intensified multi-agent, high dose chemotherapy and in very high risk patients, with additional stem cell transplantation (SCT). To improve on this, the International BFM Study Group has led a consortium of 19 countries to develop the world's largest trial for relapsed ALL (IntReALL). Standard risk patients will be randomized to receive the ALL-REZ BFM 2002 or UK ALL R3 therapy and post induction will be randomised to receive the additional targeted anti-CD22 drug, Epratuzumab, during consolidation, to clear residual disease. Children with end of re-induction MRD positive bone marrow will undergo SCT following consolidation. Both ALL-REZ BFM 2002 and UK ALL R3 used MRD PCR-based quantification of clonal Ig/TCR rearrangements, with different cut offs (10-3 for ALL-REZ BFM 2002 and 10-4 for UK ALL R3) and this is the reference assay for IntReALL. However, flow MRD may also play a role for patients without PCR targets. Flow MRD relies on the discrimination of leukaemic blasts from hematogones and this can be hampered depending on the degree of haematopoietic regeneration which varies depending on the treatment protocol and is especially important after intensive induction treatment in relapsed protocols. Thus, prospective MRD quantification of patients entered onto the UKALLR3 and ALL-REZ BFM 2002 clinical trials was performed by a standardised, quality assured, 4-8colour Flow MRD assay in end of re-induction bone marrow aspirates, by laboratories in the IBFM FLOW consortium (n=221). Flow MRD in both treatment protocols was classed as a prospective biological add on study and not used for clinical decision making. Median MRD levels were 0.026 +/-9.9% SD for BFM versus 0.027+/-18% SD for UK protocols, with comparative MRD positivity rates of 45% versus 54%, respectively. Comparison with MRD levels as assessed by molecular analysis of antigen receptor gene rearrangements was performed in 170 samples (BFM,128; UK R3, 42). The Spearman rank correlation of all samples was 0.90 (p Although sample processing and quantification of MRD differ between PCR and FC MRD, in both re-induction protocols, there was good correlation of MRD levels assessed by flow cytometry and PCR, validating the use of Flow MRD as a method of choice in patients without PCR targets in the IntReALL trial. Flow MRD also has the advantage of enabling levels of CD22 to be assayed on MRD cells, prior to treatment with Epratuzumab. This research has received funding from the European Union's Seventh Framework Programme for research, technological development and demonstration under grant agreement no 278514 - IntReALL", Deutsche Kinderkrebsstiftung for its funding support of the ALL-REZ BFM 2002 clinical trial and the minimal residual disease studies by PCR and the Deutsche Jose Carreras Leukämiestiftung for support of the international principal investigator, Leukaemia and Lymphoma Research and North East Children's Cancer Research Fund, NT 13462-4, NV15-28525A, NV15-26588A, UNCE 204012. Disclosures No relevant conflicts of interest to declare.

Prognosis of refractory and relapsed ALL is poor and its improvement requires the introduction of novel agents with a new mechanism of action. Bortezomib (BTZ) as proteasome inhibitor is such an agent, and has been shown to be safe as single agent in phase I studies in children with either solid tumors (Blaney 2004) or leukemias (Horton 2007). Recently, Messinger (2012) reported in a single-arm study that BTZ can be combined safely with vincristine (VCR), dexamethasone (DXM), pegylated asparaginase (PEG-ASP) and doxorubicin and that the combination was remarkably effective in B-cell precursor ALL. BTZ has been reported to sensitise malignant cells to other agents, both in vitro for leukemias as well as for multiple myeloma patients in vivo. In patients with ALL, the sensitizing effect of BTZ regarding the efficacy of glucocorticoids (GC) has not been addressed yet. Considering the lack of any clinical experience with BTZ in children, we developed a European multicenter feasibility and phase II study in refractory or relapsed ALL, in which all patients receive BTZ, with day 8 peripheral blast count as primary endpoint. The study is performed in compliance with the Declaration of Helsinki. Patients are randomised for BTZ to be administered “early”, or “late”. Bortezomib is then given as iv push for 4 doses at 1.3 mg/m2/dose, thus in group “early” on days 1, 4, 8 and 11 and in group “late” on days 8, 11, 15 and 18. In addition, all patients receive DXM (10 mg/m2/day in 3 doses for 2 weeks, orally or iv) and VCR (1.5 mg/m2/dose with a maximum of 2 mg as 1-hour infusion on days 8 and 15), as well as one intrathecal administration of methotrexate (MTX, dose age-adjusted) on day 1. No ASP and no anthracycline is given. Cycles can be repeated in case of a good response. Eligible patients have 2nd or greater relapsed ALL, 1strelapsed ALL after allogeneic stem cell transplantation (allo-SCT) in CR1, or refractory first relapsed ALL, and bone marrow (BM) involvement and at least 100 leukemic cells per µl blood. Exclusion criteria include symptomatic CNS leukemia. It is planned to evaluate 24 fully evaluable randomised patients. This analysis, not being an official interim-analysis and not evaluating the primary endpoint of the study, focusses on haematological responses with this modestly intensive regimen. BM M1 indicates 15% blasts. Between October 2010 and March 2014, a total of 21 eligible patients with information on response and/or toxicity after cycle 1 has been enrolled, 11 boys and 10 girls, median 9.8 years of age (range, 1.2 – 17.5). Most had second relapsed ALL (n=11), others first relapsed ALL following allo-SCT in CR1 (n=8) or refractory first relapsed ALL (n=2). Regarding efficacy, 17 patients were evaluable (1 patient no material received, 2 patients discontinued treatment due to toxicity, and 1 patient had early progression which led to treatment discontinuation). Day 22 BM showed M1 in 5, M2 in 6, and M3 in 6 patients. There was no relation between day 22 BM status and the administration of BTZ being early or late. There was no association between response and disease status, being either refractory or relapsed, although numbers are small. Two patients with a relapse after allo-SCT achieved BM M1, as well as 3 patients with a second or subsequent relapse. Four patients (23.5%) achieved hematological remission after cycle 1, while 11 out of 17 patients (65%) had a good initial response (day 22 BM M1 or M2). A total of 10 patients received a 2ndcycle and 5 out of them achieved a complete remission, 4 of whom already had a day 22 BM M1. Eight out of 21 (38%) patients suffered from grade 3 and/or 4 toxicity during cycle 1, and most frequently reported were pain (n=4), peripheral neuropathy and fatigue (n=2 each). After 2 cycles, grade 3/4 toxicity was reported in 3 out of 10 patients: one with peripheral neuropathy, one with peripheral neuropathy, haemorrhage and hematuria, and one with raised ALAT. Future analyses will focus on sensitisation to GC by BTZ and on the efficacy of BTZ in relation to PK and PD. Meanwhile, BTZ in combination with VCR, DXM and intrathecal MTX is effective in a significant subset of pediatric relapsed and refractory ALL, and repetitive cycles could be given in several children without undue toxicity. This research is financially supported by the Dutch Foundation Children Cancer-free (clinical research support) and by Janssen Pharmaceuticals (clinical research support and free drug). Disclosures Baruchel: Janssen-Cilag: Consultancy.

Introduction: Acute lymphoblastic leukemia (ALL) is the most common malignant disease in childhood. Patients (pts) with relapse after allogeneic hematopoietic stem cell transplantation (alloHSCT) and/or multiple prior salvage therapies and/or refractory disease have a particularly poor prognosis. Blinatumomab is an investigational bispecific T-cell engager (BiTE®) antibody construct that redirects T cells to CD19+ B cells resulting in serial lysis. In phase 2 clinical studies, blinatumomab has demonstrated antitumor activity in adults with relapsed/refractory non-Hodgkin lymphoma and ALL. We evaluated efficacy and toxicity of blinatumomab in pediatric pts with relapsed/refractory ALL who have a particularly poor prognosis. Methods: Eligible pts were 2-18 years of age and had B-cell precursor ALL that was refractory, in ≥2nd bone marrow relapse, or in any marrow relapse after alloHSCT. This was a multicenter, open-label study using a Simon 2-stage design with a phase 1 and a phase 2 part. In the phase 1 part, a stepwise dose of 5→15 µg/m²/day was established as the recommended phase 2 dose (von Stackelberg et al. Blood. 2013;122:70). Blinatumomab was given by continuous intravenous infusion for 4 weeks (5 µg/m²/day for the first week and 15 µg/m²/day thereafter), followed by two treatment-free weeks (one cycle) for up to five cycles. The primary endpoint for the phase 2 part of the study was rate of complete remission (CR) within the first two cycles of treatment, including CR with incomplete hematologic recovery. Secondary endpoints included relapse-free survival (RFS), rate of advancement to alloHSCT, overall survival (OS), and incidence and severity of adverse events (AEs). Minimal residual disease (MRD) response ( Results: 39 pts received blinatumomab at a dose of 5→15 µg/m²/day. The median (range) age was 9 (2–16) years; 62% of pts were male. Sixteen (41%) pts had one and 16 (41%) pts had two or more prior salvage therapies, respectively; seven (18%) were either primary refractory or had refractory relapse. Twenty-five (64%) pts had received prior alloHSCT; of those, six (16%) had one relapse and 19 (50%) had two or more relapses. Of the 39 pts, 34 (87%) had relapsed within 6 months prior to study entry. The median (range) time from last prior relapse to the relapse at study entry was 2.1 (0–13.7) months. 69% of pts had bone marrow blast infiltration of ≥50%. 19 (49%) pts started and completed one cycle of blinatumomab treatment (two cycles, 4 [10%] pts; three cycles, 2 [5%] pts). During the first two treatment cycles, 12 pts achieved CR, (31%; 95% CI, 17%–48%), mainly during the first cycle. Two additional pts (5%) had blast-free hypoplastic or aplastic bone marrow. Among pts with CR in the first two cycles, five (42%) had complete MRD response. Median RFS for CR responders was 5.6 (95% CI, 2.6–12.1) months. Among all 39 pts, median OS was 4.3 (95% CI, 3.6–8.1) months with 6 months of study follow-up time. Six (50%) of the 12 pts with CR proceeded to alloHSCT. An additional five pts who did not respond to blinatumomab received alloHSCT after blinatumomab treatment was stopped. All pts experienced AEs, mostly flu-like symptoms consistent with the mechanism of action of blinatumomab. AEs regardless of causality occurring in >20% of pts were pyrexia (74%), anemia (33%), nausea (31%), headache (28%), hypertension (26%), increased alanine aminotransferase (23%), and cough (21%). The most common grade ≥3 AEs included anemia (26%), pyrexia (21%), increased alanine aminotransferase (18%), increased aspartate aminotransferase (18%), and febrile neutropenia (15%). Cytokine-release syndrome occurred in three (8%) pts, two of whom (5%) had grade 3 events. Conclusions: Blinatumomab showed promising antileukemia activity in heavily pretreated pediatric relapsed/refractory B-cell precursor ALL pts, with a median time from last relapse of 2.1 months. Half of the pts who responded within the first two cycles were able to receive alloHSCT following blinatumomab-induced remission, suggesting that blinatumomab may open a window for alloHSCT in those pts who are resistant to salvage chemotherapy. Disclosures Gore: Amgen Inc.: Travel Support Other. Off Label Use: This presentation will discuss the off-label use of blinatumomab, as this agent is not approved for use by the FDA, EMA or any other regulatory authorities.. Zugmaier:Amgen Inc.: Equity Ownership; Amgen Research (Munich) GmbH: Employment. Hu:Amgen Inc.: Employment, Equity Ownership. Mergen:Amgen Inc.: Equity Ownership; Amgen Research (Munich) GmbH: Employment. Fischer:Amgen Inc.: Equity Ownership; Amgen Research (Munich) GmbH: Employment. von Stackelberg:Amgen Inc.: Consultancy, Honoraria.