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Children with acute myeloid leukemia (AML) have a dismal prognosis due to a high relapse rate; however, the molecular basis leading to relapsed pediatric AML has not yet been fully characterized. To define the spectrum of alterations common at relapse, we performed integrated profiling of 136 relapsed pediatric AML cases with RNA sequencing (RNA-seq), whole-genome sequencing, and target-capture sequencing. In addition to well-characterized fusion oncoproteins, such as those involving KMT2A (n=36, 26.5%) or NUP98 (n=18, 13.2%), we also identified somatic mutations in UBTF (upstream binding transcription factor) in 12 of 136 cases (8.8%) of this relapsed cohort. Somatic alterations of the UBTF gene, which encodes a nucleolar protein that is a component of the RNA Pol I pre-initiation complex to ribosomal DNA promoters, have rarely been observed in AML. In our cohort, all alterations can be described as heterozygous in-frame exon 13 tandem duplications (UBTF-TD), either at the 3' end of exon 13 of UBTF or of the entire exon 13 (Fig. A). As we noticed limited detection in our pipeline as a result of complex secondary indels alongside the duplications, we established a soft-clipped read-based screening method to detect UBTF-TD more efficiently. Applying the screening to RNA-seq data of 417 additional pediatric AMLs from previous studies and our clinical service, we identified 15 additional UBTF-TDs, many of which have not been previously reported. At the amino acid level, UBTF-TDs caused amino acid insertions of variable sizes (15-181 amino acids), duplicating a portion of high mobility group domain 4 (HMG4), which includes short leucine-rich sequences. UBTF-TD AMLs commonly occurred in early adolescence (median age: 12.6, range: 2.4-19.6), and 19 of the total 27 cases had either normal karyotype (n=12) or trisomy 8 (n=7). UBTF-TD is mutually exclusive from other recurrent fusion oncoproteins, such as NUP98 and KMT2A rearrangements (Fig. B), but frequently occurred with FLT3-ITD (44.4%) or WT1 mutations (40.7%). The median variant allele fraction (VAF) of the UBTF-TD was 48.0% (range: 9.7-66.7%). In four cases with data at multiple disease time points, the identical UBTF-TDs were present at high allele fractions at all time points, suggesting that UBTF-TD is a clonal alteration. tSNE analysis of the transcriptome dataset showed that UBTF-TD AMLs share a similar expression pattern with NPM1 mutant and NUP98-NSD1 AML subtypes, including NKX2-3 and HOXB cluster genes (Fig. C) . Altogether, these findings suggest that UBTF-TD is a unique subtype of pediatric AML. To address the impact of UBTF-TD expression in primary hematopoietic cells, we introduced UBTF-TD and UBTF wildtype expression vectors into cord blood CD34+ cells via lentiviral transduction. UBTF-TD expression promotes colony-forming activity and cell growth, yielding cells with a persistent blast-like morphology (Fig. D). Further, transcriptional profiling of these cells demonstrated expression of HOXB genes and NKX2-3, similar to UBTF-TD AMLs in patients, indicating that UBTF-TD is sufficient to induce the leukemic phenotype. To investigate the prevalence of UBTF-TDs in larger de novo AML cohorts, we applied the above UBTF-TD screening method to the available de novo AML cohorts of TCGA (n=151, adult), BeatAML (n=220, pediatric and adult), and AAML1031 (n=1035, pediatric). We identified UBTF-TDs in 4.3% (45/1035) of the pediatric AAML1031 cohort, while the alteration is less common (0.9%: 3/329, p=0.002) in the adult AML cohorts (Fig. E). In the AAML1031 cohort, UBTF-TDs remain mutually exclusive with known molecular subtypes of AML and commonly occur with FLT3-ITD (66.7%) and WT1 (40.0%) mutations and either normal karyotype or trisomy 8. The presence of UBTF-TDs in the AAML1031 cohort is associated with a poor outcome (Fig. F, median overall survival, 2.3 years) and MRD positivity; multivariate analysis revealed that UBTF-TD and WT1 are independent risk factors for overall survival within FLT3-ITD+ AMLs. In conclusion, we demonstrate UBTF-TD defines a unique subtype of AMLs that previously lacked a clear oncogenic driver. While independent of subtype-defining oncogenic fusions, UBTF-TD AMLs are associated with FLT3-ITD and WT1 mutations, adolescent age, and poor outcomes. These alterations have been under-recognized by standard bioinformatic approaches yet will be critical for future risk-stratification of pediatric AML. Figure 1 Figure 1. Disclosures Iacobucci: Amgen: Honoraria; Mission Bio: Honoraria. Miller: Johnson & Johnson's Janssen: Current Employment. Mullighan: Pfizer: Research Funding; Illumina: Membership on an entity's Board of Directors or advisory committees; AbbVie: Research Funding; Amgen: Current equity holder in publicly-traded company.

Introduction: Relapse of pediatric acute myeloid leukemia (AML) remains a leading cause of childhood cancer mortality, and leukemias with activation of the Fms-like tyrosine kinase 3 (FLT3) are particularly susceptible to relapsed disease. Risk-directed therapy to prevent relapse is based both on genetic changes known to drive drug resistance, and measurable residual disease (MRD) at the end of induction therapy (EOI). In adult AML, resistance to type II FLT3-inhibitors, like sorafenib, is primarily driven by on-target FLT3 kinase domain (KD) mutations. However, the resistance mechanisms for pediatric leukemias, which are treated on combination therapies, have not been fully elucidated. MRD is considered the among the most predictive markers of future relapsed disease. It has been assumed that the major clone at the time of MRD assessment will predict the majority clone at relapse. However, this assumption has not been proven. The definition of the most specific genetic and MRD markers of relapse are essential to prognosticate and personalize therapy to prevent relapsed disease. Methods: We performed single cell sequencing (SCS) with a high-throughput DNA sequencing platform, Mission Bio Tapestri, on bone marrow or peripheral blood samples from 24 samples from 8 pediatric patients treated on COG AAML1031 with serial samples from diagnosis, EOI, and relapse. Results: We analyzed a total of 94,833 cells from 8 pediatric patients (median cells per patient 12,428) all treated on AAML1031. SCS revealed a sensitive and specific description of clonal evolution on the combination of sorafenib with cytotoxic chemotherapy. The FLT3 internal tandem duplication (ITD) was controlled by the therapy in only half of the patients. In five of the patients, the FLT3-ITD was present in multiple clones. The FLT3-ITD co-mutated with additional mutations (NRAS, SH2B3, WT1, TET2, or NPM1) in half of the patients. However, the presence of a co-mutation did not necessarily correlate with whether or not the ITD-containing clone persisted at the time of relapse. Of the leukemias whose relapse was not driven by FLT3, the most likely mutational driver of resistance was NRAS. Notably, however, despite the fact that FLT3 KD mutations make up the bulk of mutational resistance to type II FLT3i such as sorafenib in adult patients, there were no on-target FLT3 mutations found in any of these pediatric patients. Further, SCS allows for an unprecedented depth of analysis of the genetic complexity of pediatric AML. Phylogenic analysis revealed that the same mutations may arise independently in different cells (NPM1 W288fs, NRAS G60E). Additionally, the same gene may be mutated twice within the same cell (WT1, TET2). These data, consistent with our prior work, suggest that some leukemias may have a predilection to mutations within specific loci. Finally, although there is a standing assumption that the dominant MRD population will proliferate into relapsed disease, in 3/8 patients, the dominant MRD clone did not predict the dominant relapse clone. Conclusions: SCS allows for direct measurement of clonal hierarchy and evolution, phylogeny, co-mutational status, and zygosity, which can only be inferred through traditional bulk NGS. The mutational mechanisms of resistance seen in adult leukemias treated with sorafenib monotherapy are not necessarily relevant to the pediatric population; rather than on-target FLT3 mutations, off target mutations including NRAS are found. This corroborates prior findings that off-target RAS pathway mutations may drive resistance to FLT3i. Non-RAS off-target mutations found in this cohort do not necessarily predict sorafenib resistance, so may be passenger mutations. The lack of consistent resistance mutations suggests that other mechanisms of resistance such as epigenetic modifications may also drive resistance to combination chemotherapy with FLT3i in pediatric leukemia. Further, SCS exposes more genetic complexity in pediatric AML than has previously been appreciated: the same mutation may independently arise in more than one cell or the same cell may have multiple mutations within the same gene. Finally, the sensitivity of SCS reveals that the major clone at the time of MRD assessment is not necessarily the major clone at relapse. This suggests a benefit of more frequent MRD monitoring to track clonal evolution in real time. Disclosures Smith: Daiichi Sankyo: Consultancy; Revolutions Medicine: Research Funding; AbbVie: Research Funding; Amgen: Honoraria; FUJIFILM: Research Funding; Astellas Pharma: Consultancy, Research Funding.

Pediatric acute myeloid leukemia (AML) is the deadliest malignancy in children. Despite maximally intensive therapy, inclusive of chemotherapy and hematopoietic stem cell transplant, approximately 20% of patients experience recurrent disease. These patients are also burdened with treatment-related toxicities. Significant improvements in survival in pediatric AML patients necessitate the incorporation of rational targeted therapies with reduced toxicity. Recent studies demonstrate that PRMT5 knockout or inhibition in syngeneic mouse models of KMT2A (MLL) rearranged leukemic cells increased disease latency (Serio et al., Oncogene, 37:450, 2018; Kaushik et al., Leukemia, 32:499, 2018), indicating that PRMT5 is a potential therapeutic target in pediatric AML. However, there are no reports testing the efficacy of PRMT5 in PDX models of pediatric AML. We evaluated the preclinical efficacy of C220, a potent and selective PRMT5 inhibitor (PRMT5i) (Pastore et al., Cancer Discovery, 10:1742, 2020) in three distinct patient-derived xenograft (PDX) models of KMT2A rearranged AML. Based on the model used for the study, 3-5 million AML cells were injected intravenously in NSG-B2m mice. Disease progression was monitored by evaluating the percentage of human cells in mouse peripheral blood at periodic intervals by flow cytometry. At 2-3 weeks post transplantation, when human cells were detectable in peripheral blood, mice were randomly assigned to control (n=4-5) or treatment (n=2) groups. C220 was administered daily p.o. at a dose of 15 mg/kg for seven days with a break of two days. Mice were dosed with 2-3 additional cycles (indicated in the figure by shaded areas) based on their health status. Mice were monitored daily for experimental endpoints that included body condition score and human cell percentages in peripheral blood. Kaplan-Meier survival plots were generated based on the time when mice were euthanized because they met experimental endpoints. Chronic dosing of C220 prolonged survival and delayed the rise in percentage of human AML cells in mouse peripheral blood in all 3 PDX models (Fig. 1B, D, F). In the NTPL-146 model (KMT2A-MLLT1 fusion), a 135-day improvement in median survival was observed with C220-treatment (Fig. 1A). In the DF-2 (KMT2A-MLLT10 fusion) and DF-5 (KMT2A-MLLT4 fusion) models, which showed a faster engraftment compared to NTPL-146, there was a 5.5-day and 18-day improvement in median survival respectively (Fig. 1C, E). The improvement in median survival was statistically significant in all models (*P Figure 1 Figure 1. Disclosures Gopalakrishnapillai: Geron: Research Funding. Zhang: Prelude Therapeutics: Current Employment. Ruggeri: Prelude Therapeutics: Current Employment, Current equity holder in publicly-traded company. Scherle: Prelude Therapeutics: Current Employment, Current equity holder in publicly-traded company. Barwe: Prelude Therapeutics: Research Funding.

Background Relapse free survival of pediatric AML remains only 60%. Current standard myelosuppressive therapy has been maximized, so novel therapies with minimal toxicities are needed to improve outcomes. Previously, we found atovaquone (AQ), an FDA-approved drug that treats pneumocystis jiroveci pneumonia (PJP), reduces AML burden - by suppressing oxidative phosphorylation (OXPHOS) - in xenograft mice. Clinically achievable concentrations of AQ for anti-PJP are 40-80µM, but the anti-leukemia effects are observed as low as 10µM (Stevens et al, Bld Adv, 2019). This makes AQ an ideal drug to incorporate into AML treatment. AQ is a daily administered oral medication, and plasma levels depend on patient compliance, absorption, and entero-hepatic recirculation, which can be compromised due to the patient population and adverse events (AE) of chemotherapy. Here we investigated the feasibility of incorporating AQ into standard pediatric AML treatment. Methods Patients with de novo AML were enrolled at two children's hospitals in the USA. Daily administration of AQ at established PJP prophylaxis dosing was combined with standard chemotherapy for AML, based on the Medical Research Council (MRC) backbone of cytarabine 100mg/m2 q12h x 10 days, and daunorubicin 50mg/m2/dose on days 1, 3, and 5. As it was unclear if our AQ dosing would provide adequate PJP prophylaxis, it was left to provider discretion to give additional PJP protection. AQ compliance, AEs (per NCI CTCAE v5), parent/caregiver ease of administration score (scale: 1-5, 1=very difficult, 5=very easy to administer) and peripheral blood/bone marrow pharmacokinetics (PK) were collected during Induction 1. Real time AQ plasma concentration results were not provided. To address feasibility of achieving adequate levels, all gastrointestinal (GI) AEs ≥ grade 2 were collected, in addition to grade 4 or greater AEs. Patients who took at least 85% of planned doses and missed less than 2 consecutive doses of AQ were eligible for analyses. Correlative biology studies assessed AQ induced apoptosis at 30uM, effects on OXPHOS and relevant signaling activities. Patient derived xenografts (PDX) were established and treated with AQ. This trial is registered with ClinicalTrials.gov (NCT03568994). Results A total of 26 pediatric AML patients enrolled (ages 8 months - 19.7 years, mean 10.7 years); 24 patients were evaluable for this study. Two patients had Grade ≥ 3 GI toxicities that prohibited enteral administration so they were excluded from AQ PK and ease of administration analyses. We found that 14/24 (58%) patients achieved plasma levels above the target anti-leukemia concentration (10µM) by day 11. At the end of induction, 19/24 (75%) patients achieved plasma levels above 10µM, but only 7/24 (29%) patients achieved adequate levels for PJP prophylaxis (40µM). Only 1 patient achieved levels above 40µM throughout the trial [FIG A]. Mean ease of administration score was 3.8. For the youngest patients (x ≤ 2.6years), the average score was 3.4 which was not significantly different from older patients (ANOVA, p > 0.05) [FIG B]. Ease of administration scores showed no association with plasma levels (Pearson's correlation, p > 0.05). Finally, correlative biology studies in patient samples demonstrated robust AQ-induced apoptosis, OXPHOS suppression, and prolonged survival in a PDX model receiving AQ [FIG C]. Conclusion Our data demonstrate the feasibility of combining AQ with traditional chemotherapy for pediatric AML. Patients of all ages were able to tolerate AQ and no AEs were attributable to AQ administration. The target anti-leukemic concentration of AQ in the plasma (> 10uM) was frequently achieved, but concentrations of > 40uM at standard dosing were rare. Low plasma levels of AQ did not correlate with the presence of GI related AEs or weight loss, so plasma levels should be monitored to ensure sufficient PJP prophylaxis. Our correlative biology results support suppression of OXPHOS as the primary mechanism of action by which AQ exerts its anti-leukemia effect, and AQ may be an active anti-leukemia agent for pediatric AML patients. Figure 1 Figure 1. Disclosures No relevant conflicts of interest to declare.

Introduction Together with considerable improvement in diagnostics and supportive care measures, treatment intensification has contributed to the improvement of pediatric acute myeloid leukemia (AML) survival outcomes. However, treatment-related toxicity is a consequence of intensified chemotherapy regimens, which has profound implications for both morbidity and mortality. Despite the known devastating impact of all types of toxicity on quality of life (QoL), studies presenting the different forms of non-infectious toxicity prevalent during the most recent Dutch pediatric AML protocols are lacking given that only prior protocols have been evaluated in this manner as of yet. Furthermore, the literature on non-infectious toxicity incidence in pediatric AML patients is relatively scarce. Therefore, the objectives of this study were to: 1) evaluate the cumulative incidence (C.I.) of severe, but non-lethal, non-infectious toxicity during AML treatment according to the last three consecutive protocols of the Dutch Childhood Oncology Group (DCOG), and 2) compare treatment-related toxicity frequencies between protocols. Methods A retrospective chart review was performed on 245 Dutch patients diagnosed with de novo AML (acute promyelocytic leukemia, myeloid leukemia of Down Syndrome, and secondary AML were excluded) and treated according to ANLL-97/AML-12 (1998-2005) n=118, AML-15 (2005-2010) n=60, or DB-AML-01 (2010-2013) n=67. Table 1 details protocol specifics, including drug dosages per course. Grade 3-4 toxicities, including hematological toxicity, cardiotoxicity, respiratory toxicity, mucositis, typhlitis, nephrotoxicity, hepatotoxicity, neurotoxicity, pain, allergic anaphylactic reaction, and elevation of alanineaminotransferase or bilirubin were defined according to Common Terminology Criteria for Adverse Event version 4.0, excluding infectious toxicity. Grade 5 toxicity (treatment-related mortality) was beyond the scope of this study. Intensive care unit (ICU) admission data was additionally assessed. Toxicity C.I.s were determined via competing events analyses. Relapse and death were considered competing events. Patients were censored at time of stem cell transplantation. Per-protocol comparisons were conducted via Chi-square test, due to lack of sufficient power required to calculate C.I.s. Results Median age at diagnosis was 6.0 years [interquartile range (IQR) 1.0-12.0], 58% were male. Mucositis was the most frequent form of non-hematological toxicity with a C.I. of 86.1% (standard error (S.E.) 3.5%), followed by hepatotoxicity (C.I. 27.4%, S.E. 2.9%), and respiratory toxicity (C.I. 24.6%, S.E. 3.0%) (Table 2). Eighty-three patients (C.I. 33.9%, S.E. 3.0%) were admitted to the ICU at least once, for a median of 6.0 days [IQR 2.0-15.0] (Table 2). Relatively more blood transfusions (both erythrocyte and thrombocyte) were administered to patients treated according to AML-15 (98.3%) and DB-AML-01 (97.0%) compared to AML-12 (86.4%) (p=0.01 and p=0.02, respectively). Relatively more patients suffered from severe mucositis during DB-AML-01 compared to AML-12 (43.4% vs. 25.4%, p=0.01). More patients treated according to AML-15 had bilirubin levels >3x upper limit of normal (ULN) compared to those treated according to AML-12 (13.3% vs. 3.4%, p=0.01). There were no differences in non-infectious toxicity frequency between AML-15 and DB-AML-01. Other forms of toxicity did not differ significantly between protocols. The percentage of patients admitted to the ICU at least once during treatment was 33.9% in AML-12, 26.7% in AML-15, and 40.3% in DB-AML-01. Conclusions The high incidence of severe short- and long-term toxicities during pediatric AML therapy poses substantial challenges for patients, families, and care providers. Mucositis was the most common form of non-hematological, non-infectious toxicity in the whole cohort and across all Dutch protocols. Toxicities were more prevalent during the more recent protocols (AML-15 and DB-AML-01) compared to AML-12. Therefore, our findings are important in that they substantiate the need to optimize pediatric AML care in a manner which decreases treatment-related toxicity and QoL impairment. High treatment-related morbidity rates stress the urge to improve supportive care and develop less toxic treatment, while maintaining efficacy. Figure 1 Figure 1. Disclosures Zwaan: Sanofi: Consultancy.

Acute myeloid leukemia (AML) is a genetically heterogeneous hematologic malignancy characterized by uncontrolled proliferation of myeloid progenitor cells accompanied by impaired terminal differentiation. Despite intensive treatment regimes, the clinical outcomes remain poor, underscoring the need to decipher the underlying pathology and implement therapeutic interventions. Emerging evidence suggest myeloblasts could evolve machineries to evade T cell patrol and hinder immunotherapies. Here, we present a new mechanism driving immune escape in the context of pediatric AML, based on our discoveries of CD9 in hematopoietic stem cells (HSC; Leung et al, Blood, 2011) and acute lymphoblastic leukemia (ALL; Leung et al, Leukemia, 2020). We first examined CD9 expression and its prognostic impact in patient cohorts of childhood leukemia. The expression of cell surface CD9 on blasts of pediatric AML patients (13.2%, n=81) was significantly lower than that of pediatric ALL patients (90.4%, n=181, P Enforced CD9 expression in MV4-11 cells significantly suppressed proliferation (P A marked decrease of H3K9/27Ac occupancy in the CD9 locus was observed in AML than in ALL cells (4.8-14.2-fold, P Mechanistically, global transcriptome profiling of pediatric AML (n=31) revealed decreased stemness (NES: -1.7, P=0.01) and increased monocyte (NES: 1.8, P=0.034) gene signatures in CD9+ patient samples. Moreover, single-cell transcriptomic analyses of total bone marrow cells from MV4-11-tranplanted mice detected a significant enrichment of differentially regulated genes functioning in the antigen processing and presentation pathway. Concordantly, we observed a profound up-regulation of CD9 (9.4-51.1-fold, P Taken together, our data provided molecular, cellular and clinical evidence showing the plausible function of CD9 as a key driver intertwining differentiation and immunosurveillance in pediatric AML, and inspired a new combinatorial epigenetic/immunotherapy for this rare but aggressive malignancy. Disclosures No relevant conflicts of interest to declare.

Acute myeloid leukemia (AML) remains a therapeutic challenge with high mortality rates despite intensive and myeloablative therapies. While immunotherapies targeting CD19 have yielded remarkable outcomes in acute lymphoblastic leukemia, identifying similar antigen therapeutic targets in AML remains a challenge due to inherent heterogeneity associated with AML and overlapping immunophenotypes with normal hematopoietic stem and myeloid cell populations. Transcriptional heterogeneity within pediatric AML has primarily been linked to underlying fusion. Therefore, we integrated large transcriptomics and proteomics datasets from AML and normal tissues to identify potential targets expressed in leukemias, but not in normal bone marrow or other normal tissue types. To identify candidate therapeutic targets in pediatric AML, we leveraged transcriptome sequencing data from bone marrow aspirates or peripheral blood collected from 1,481 children, adolescents, and young adults with AML at the time of diagnosis. Patients were enrolled on one of four Children's Oncology Group trials spanning the past three decades: CCG-2961, AAML03P1, AAML0531, and AAML1031. We also leveraged transcriptome sequencing from normal bone marrow (NBM) and normal CD34+ hematopoietic stem and progenitor cells (HSPCs) in order to exclude targets that are highly expressed during normal hematopoiesis. Finally, we performed additional filtering based on proteomic databases to exclude targets that lack membrane localization (Human Protein Atlas, UniProt, and Ensembl) or that are highly expressed on normal tissue types (Human Proteome Map, Human Protein Atlas, and Proteomics DB databases) (Figure 1A). First, we computed the log expression ratio between AMLs and NBM/HSCPs for all protein coding genes. We next selected genes expressed greater than a threshold of two standard deviations above the mean in 50% or more of AMLs (Figure 1B). Additionally, we further selected genes on the basis of differential gene expression and absolute expression thresholds. This analysis was repeated for our entire pediatric AML cohort and the following AML subtypes: RUNX1-RUNX1T1, CBFB-MYH11, KMT2A-MLLT3, KMT2A-MLLT10, KMT2A-MLLT4, KMT2A-ELL, KMT2A-MLLT1, KMT2A-MLLT11, NUP98-NSD1, NUP98-KDM5A, and CBFA2T3-GLIS2. Candidate therapeutic targets were filtered based on membrane localization and normal tissue expression using the aforementioned proteomics databases (Figure 1C and 1D). Based on this algorithm, we identified a nonzero number of candidate therapeutic targets for each of our pediatric AML subtypes (Figure 1D). Intriguingly, we found no overlap between targets identified in our pediatric, adolescent, and young adult cohort and a previous similar analysis performed in AMLs diagnosed in older patients (PMID 29017060). This study demonstrates that by combining our large transcriptomics dataset with pre-existing proteomics datasets, we are able to identify a collection of candidate therapeutic targets in pediatric AML. Importantly, zero targets were identified that were inclusive to all pediatric AMLs within our cohort, which underscores the transcriptional heterogeneity that our group and others have previously identified in pediatric AML. Future preclinical and clinical studies will need to account for this heterogeneity by prioritizing targets on the basis of underlying molecular alteration. Our study comprises a platform and dataset of candidate targets for further functional validation and/or immunotherapy targeting studies in pediatric AML. Figure 1 Figure 1. Disclosures Shaw: T-Cell and/or Gene Therapy for Cancer: Patents & Royalties.

Background/Aims: Despite advances in chemotherapy-based treatment protocols, the outcomes of children with acute myeloid leukemia (AML) remain suboptimal. Implementation of targeted therapy based solely on genomics is challenging due to the complex mutational patterns and scarcity of pharmacologic agents for most lesions. In addition, pediatric and adult AML are genetically and biologically distinct, which poses a major hurdle for extrapolation of new agents approved for adult AML to the pediatric population. This study aims to adopt a functional approach that directly measure the response of patient-derived leukemic cells to targeted agents, and to establish the drug sensitivity pattern and identify candidates of immediate clinical relevance for precision usage in high-risk pediatric AML. Methods: A high-throughput drug screening, comprising 39 targeted agents (2 in Phase I, 10 in Phase II, 5 in Phase III, 22 FDA-approved) and 6 conventional chemotherapeutics, was performed on 30 pediatric AML samples collected at diagnosis or relapse using a serum-free, cytokine-supported culture system. A counter-screen of active drugs on cord blood hematopoietic stem cells was accomplished to reveal leukemia-selective activities. The robustness of the drug testing platform for predicting in vivo activities was validated in xenograft models. Genomic profiling was complementarily performed to identify the genetic markers and underlying mechanisms of drug sensitivity. Patients with refractory AML were treated with targeted agents based on drug profiling results, and assessed for clinical responses. Results: Unsupervised clustering revealed 5 distinct clusters of drug response: highly active compounds (IC50 2000 nM, 10 drugs). Targeted agents, including Bcl-2, HDAC, proteasome, HSP and survivin inhibitors, had substantially higher potency and selectivity over standard chemotherapeutic agents. New agents approved for adult AML were essentially inactive in pediatric AML. Drug sensitivity ex vivo accurately predicted in vivo single-agent and combinatorial activities with cytarabine in cell line- and patient-derived xenografts. Targeted resequencing of a 141-gene panel revealed novel mutations of prognostic relevance, such as KMT2C, in pediatric AML and their vulnerability to targeted agents. Whole-genome RNA-sequencing identified distinct gene expression signatures shaping the response to individual drugs. Administration of venetoclax to a child with refractory AML resulted in rapid blast clearance and achieved long-term remission. Complementary genomic profiling on serial specimens dictated the dynamic drug responses during disease evolution. Conclusions: Our study establishes a reliable drug testing platform and a pediatric-specific drug response profile of AML, which enables an evidence-based selection of targeted agents for patients without treatment options and endows therapies increasingly precise and personalized. The study also generates a valuable gene-drug-clinical dataset that could be leveraged to address the fundamental and translational biology of pediatric AML. It will ultimately impact the future design of clinical trials and protocols for managing this life-threatening malignancy. Disclosures No relevant conflicts of interest to declare.

INTRODUCTION: Allogeneic hematopoietic stem cell transplantation (allo-HSCT) is a potentially curative therapy for high-risk acute myeloid leukemia (AML).The preparative regimen consisting of busulfan and cyclophosphamide (BuCy) is considered as one of the classical myeloablative conditioning regimens (MAC); however, it is associated with significant early and long-term toxicities, leading to a high rate of transplant-related mortality (TRM). P urpose: To compare toxicities and outcomes of BuCy and FluBu12 conditioning in the pediatric population. Methods: We retrospectively analyzed 71 pediatric high-risk AML patients in CR1/2 (n=51, 71,8%) and R/R disease (n=20, 28,2%) received allo-HSCT from MSD (n=16, 22,5%), MUD (n=43, 60,6%) and Haplo donor (n=12, 16,9%). BuCy and FluBu conditioning regimens were used in 47 (66,2%) and 24 (33,8%) patients respectively. Median age was 6 years (0-17). GVHD prophylaxis was PTCy±CNI±m-TOR inhibitor in 32 (45%) or ATG±CNI in 39 (55%) patients. The primary end points were TRM, relapse-free survival (RFS), graft-versus-host disease (GVHD) free, relapse free survival (GRFS) and overall survival (OS). Secondary end points included neutrophil engraftment, sinusoidal obstruction syndrome (SOS), acute and chronic GVHD. Patient were censored at the time of death or last follow-up. Probabilities of OS, RFS and GRFS were estimated using Kaplan-Meier curves. TRM was defined as any death that occurred in the absence of disease relapse; relapse was a competing risk for this event. Results: Engraftment rate was 91% and 87,5% in patients received BuCy and FluBu, respectively (p=0,4). There was a trend to lower day 100 TRM after FluBu (4,1% vs 14,9% after BuCy, p=0,07). Overall survival at 2 years did not differ as well (BuCy 48,9%, FluBu 56%, p=0,5). BuCy showed borderline higher RFS at 2 years (70,2% vs 52%, p=0,06). The composite endpoint GRFS did not differ between two study cohorts being 31,8% and 29,2% at 2 years for BuCy and FluBu (p=0,7). We observed a tendency towards a higher incidence of III0-IV0 aGVHD and cGVHD following BuCy when compared with FluBu: 57,1% vs 41% (p=0,06) and 46,5% vs 28,5% respectively (p=0,2). No significant differences were found between the BuCy and FluBu groups in risk of SOS (6% and 8%, respectively). Conclusions: The optimal conditioning regimen for children with AML is still a matter of debate. Our results suggest that FluBu represents a valid myeloablative regimen, able to provide lower TRM, aGVHD and cGVHD. This conditioning might become an alternative approach in patients with a high risk of severe post-transplant complications. Disclosures No relevant conflicts of interest to declare.

Sensitive and specific detection of measurable residual disease (MRD) after treatment in pediatric acute myeloid leukemia (AML) is prognostic of relapse and is important for clinical decision making. Mutation-based methods are increasingly being used, but are hampered by the limited number of common driver gene mutations to target as clone markers. Additional targets would greatly increase MRD detection power. However, even in cases with many AML-defining mutations, it is the limited accuracy of current molecular methods which establishes the lower bounds of sensitivity. Here we describe an ultrasensitive approach for disease monitoring with personalized hybrid capture panels targeting hundreds of somatic mutations identified by whole genome sequencing (WGS), and using extremely accurate Duplex Sequencing (DS) in longitudinal samples. In a pilot cohort of 13 patients we demonstrate detection sensitivities several orders of magnitude beyond currently available single locus testing or less accurate sequencing. With multi-target panels, overall power for MRD detection is cumulative across sites. For example, if a patient has MRD at a true frequency of 1/30,000, sequencing a single mutant site to 10,000x molecular depth would be unlikely to detect MRD. However, sequencing 10 sites each to 10,000x would effectively total 100,000x informative site depth, increasing power to >95%. However, standard sequencing assays are insufficiently accurate to achieve this theoretical limit of detection (LOD). DS enables accurate detection of individual variants to Marrow aspirates were collected from 13 uniformly treated pediatric AML patients at time of diagnosis (TOD), during treatment (end of induction, EOI), in remission (end of therapy, EOT), and at relapse. 9/13 patients relapsed. DNA from TOD was analyzed by WGS. Germline variants were excluded and somatic single nucleotide variants (SNVs) were targeted by a custom probe panel designed for each patient. An average of 170 SNVs were targeted per patient (range 53-200). More than 90% of the SNVs were noncoding. Longitudinal samples were then analyzed with DS, which compares sequences from both strands of each DNA molecule to eliminate technical noise and reveal biological mutation signal with extreme accuracy and sensitivity. A median of 82% of WGS SNVs were validated by DS in the TOD DNA, and the vast majority of those were also present at relapse. Relapsers had more SNVs at diagnosis than non-relapsers. EOT samples were sequenced to an average Duplex molecular depth of 29,400x, with a maximum of 61,283x. The figure below shows time course plots tracking SNVs at diagnosis, EOT and relapse for 2 patients. Among mutations validated in TOD samples, a median of only 8 (5%) were detected per EOT sample (range 0-66 mutations). MRD was detected in 8/9 relapsers. Targeting 1 or even 10 SNVs would therefore have missed MRD in the majority of these patients. Among relapsers, median EOT SNV VAF was 0.069%. The lowest single VAF detected per EOT sample ranged from 0.036% to 0.002%. The presence of an SNV at diagnosis and relapse implies that it must truly be present at EOT, whether or not it is detected. Therefore, if a small minority of leukemic mutations are detected at EOT, the true overall MRD frequency is much lower than the LOD at any single site. In the only relapser where MRD was not detected, targeted SNVs were present at diagnosis and relapse, so additional sequencing depth at EOT would eventually reveal ultra-low frequency mutations. Among patients that did not relapse by the end of the study, median VAF at EOI (the latest time point DNA available) was 0.0258%. Therefore, non-relapsers have a lower median VAF at EOI than relapsers do even later at EOT, potentially indicating very early on that treatment is more successful. This study shows excellent performance of DS-based assays for detecting MRD with patient-specific panels. We have demonstrated that among large panels of validated somatic SNVs present at time of diagnosis, a median of 5% are identified at EOT in eventual relapsers. DS detected MRD in 8/9 patients, and at a median VAF well below the limit of detection of any other sequencing technology. Comprehensive personalized hybrid selection panels coupled with DS represents a powerful option for MRD monitoring in pediatric AML and potentially other cancers. Figure Disclosures Higgins: TwinStrand Biosciences: Current Employment. Lo:TwinStrand Biosciences: Current Employment. Hipp:TwinStrand Biosciences: Current Employment. Valentine:TwinStrand Biosciences: Current Employment. Williams:TwinStrand Biosciences: Current Employment. Radich:TwinStrand Biosciences: Research Funding. Salk:TwinStrand Biosciences: Current Employment.

Background Cytogenetic and molecular aberrations are important prognostic factors in pediatric acute myeloid leukemia (AML). NUP98 translocations with more than 30 different partner genes have been identified in pediatric AML. The 2 most common fusions, NUP98-NSD1 and NUP98-KDM5A, have been shown to have distinct characteristics and are both associated with adverse outcomes. Although NUP98 fusions with less common fusion partners have been identified, the biological and clinical implications of these variants are unknown. Methods To determine the biological and clinical implications of the less common "other" NUP98 translocations (NUP98-X), we evaluated the clinical characteristics and transcriptome and genome sequencing data from 2396 children and young adults with AML within 4 consecutive Children's Cancer Group (CCG) and Children's Oncology Group (COG) trials CCG-2961, AAML03P1, AAML0531 and AAML1031. All NUP98-X translocations were confirmed by RNA sequencing. Results Of the 2396 patients screened, 164 patients (6.8%) had a NUP98 translocation. We identified 20 patients with a NUP98-X fusion (0.83%) and compared them with those with NUP98-NSD1 (n=110, 4.5%), NUP98-KDM5A(n=34, 1.4%), and a reference cohort without NUP98 translocations (n=2232). Translocation partners identified in the NUP98-X group were HOXA9 (n=4), HOXD13 (n=3), PHF15 (n=2), PHF23 (n=2), and single cases of BPTF, BRWD3, DDX10, HMGB3, HOXA13, KAT7, PRRX1, SET and TOP1. Besides the distinct characteristics of NUP98-NSD1 and NUP98-KDM5A, the NUP98-X group showed high inter-patient variance in clinical characteristics compared to our reference cohort. NUP98-X patients showed a clear bimodal age distribution with half of the patients being in the older age category and a similar number in the category We evaluated the impact of NUP98-X translocations in response to the initial induction therapy. The morphological complete remission (CR) rate after course 1 was 65% in the NUP98-X cohort versus 76% in the reference cohort (p=0.266). NUP98-NSD1 patients had an inferior CR rate (36%, p Conclusion NUP98-X translocated pediatric AML patients represent a rare cohort with a high variability in both translocation partners and other clinical characteristics. Despite this heterogeneity, the OS of these patients is comparable to high-risk pediatric AML patients, which justifies a high-risk stratification of these patients and emphasizes the need for developing new treatment strategies. Further research within large patient cohorts is needed to investigate the biologic and clinical characteristics of these rare translocations and potential differences between the different NUP98-X fusion partners. Disclosures Kaspers: AbbVie: Ended employment in the past 24 months; Helsinn Healthcare: Ended employment in the past 24 months; Boehringer Ingelheim: Membership on an entity's Board of Directors or advisory committees; Janssen R&D: Ended employment in the past 24 months. Cooper:Celgene: Other: Spouse was an employee of Celgene (through August 2019).

Cell surface antigen expression in AML is becoming focus of investigation as it relates to prognostic impact as well as potential therapeutic implications. The CD117 (c-KIT) receptor tyrosine kinase is expressed on the cell surface of hematopoietic stem cells and its signaling is important in cell survival, proliferation and differentiation. CD117 is expressed in a majority of AML cases. In addition, mutations in c-KIT have been identified in a subset of patients, specifically those with CBF AML (inv(16)/t(16;16) and t(8;21)). As there is increasing interest in using tyrosine kinase inhibitors with anti-KIT activity based on surface CD117 expression as treatment in AML, we sought to evaluate the association of CD117 expression with biologic and clinical features in pediatric and young adult (YA) AML. A total of 1803 pediatric and YA patients (ages 0.01-29.2 years) treated on the Children's Oncology Group (COG) phase III trials AAML0531 (n=762) and AAML1031 (n=1041) with complete cyto/molecular and outcome data available were included in the analysis. The primary aim of AAML0531 evaluated the addition of gemtuzumab ozogamicin to a 5-course MRC-based backbone, while AAML1031 evaluated the addition of bortezomib to a similar 4-course backbone. Outcomes between the 2 primary randomized arms on each trial were similar and thus were included together for each trial. Multidimensional flow cytometry was used to determine the CD117 mean fluorescence intensity (MFI) of myeloid progenitor cells as defined by CD45 low and side scatter using a CD117-PE antibody with stable intensity detection since 2002. Patients were divided into quartiles based on CD117 expression and clinical characteristics and outcome were evaluated across the quartiles. CD117 MFI distribution was similar across both trials (Fig 1A). Analysis of CD117 expression with cytogenetics demonstrated that t(8;21) was significantly associated with higher CD117 (p Outcome analysis demonstrated that CD117 surface expression was not associated with CR, with similar CR rates across the quartiles (p=0.726). In a univariate analysis, overall survival (OS) was associated with higher CD117 expression (p We show here that CD117 expression was higher in patients with favorable risk CBF AML and inversely associated with KMT2A rearrangements, which are generally considered to be neutral or unfavorable depending on the specific translocation partner. Higher CD117 expression was associated with improved outcomes, which may in part be explained by the association with CBF and inverse association with KMT2A. Although patients with higher CD117 MFI generally have favorable risk genomic alterations, the strategy of c-KIT inhibition warrants further investigation in AML given the potential for facilitating de-escalation of conventional therapeutics with significant toxicity in future clinical trials if found to be safe and effective. Disclosures Eidenschink Brodersen: Hematologics, Inc: Employment. Pardo:Hematologics, Inc: Employment. Dai:Hematologics, Inc: Employment. Loken:Hematologics, Inc: Employment, Equity Ownership.

INTRODUCTION Pediatric acute myeloid leukemia (AML) requires intensive chemotherapy that results in prolonged periods of neutropenia, placing patients at significant risk for life threatening infection. Current supportive care guidelines strongly recommend hospitalization after chemotherapy completion until neutrophil recovery. While approximately 70% of United States (US) centers adhere to this recommendation, there are little data to support inpatient over outpatient management. Therefore, we compared clinical outcomes (Aim 1), patient experiences (Aim 2), and patient quality of life (QoL) (Aim 3), between the two strategies in a nationally representative sample of newly diagnosed AML patients METHODS Aim 1 employed retrospective, standardized chart abstraction for AML patients treated between 2011 and 2019 at 17 centers in the US. Patients were observed from their first course of chemotherapy through treatment completion, stem cell transplant, site transfer, relapse, or death. The unit of analysis was chemotherapy courses, the primary exposure was inpatient versus outpatient management, and primary outcomes were bacteremia and time to next treatment course. Only courses in which the patient met "discharge eligible" criteria were included (Figure 1). Analyses excluded Induction I given the low rate of outpatient management. Log-binomial regression was used to compare the bacteremia incidence and generalized linear regression was used to compare time to next course. Generalized estimating equations were used in overall analyses to account for correlation between courses from the same patient. For Aim 2, patients/ families at 9 centers underwent semi-structured interviews that were transcribed and analyzed using a conventional content analysis approach (InVivo). For Aim 3, parent proxies at 14 centers completed a PedsQL before the start of chemotherapy and then after neutrophil recovery but prior to the start of the subsequent course. Only one chemotherapy course per patient was considered. Only "discharge eligible" patients were included. Parent-proxy generic core QoL scores at the end of the chemotherapy cycle were compared by exposure group using analysis of covariance accounting for baseline scores. RESULTS 573 patients contributed 1188 treatment courses for Aim 1 (Figure 1). Mean age was 9 ±6 years, 47% were female, and AML was classified as low, intermediate, and high risk, respectively in 65%, 4%, and 31%, respectively. Overall rates of bacteremia were not significantly different in patients receiving outpatient versus inpatient management (23.8% vs 29.0%, RR 0.76, 95% CI: 0.56, 1.03; p=0.07). Days to next course also did not differ between the groups (30.2 vs. 31.6, adjusted difference -1.6, 95% CI: -3.2, 0.5; p=0.17). In Aim 2, 86 interviews in 57 families (39 inpatient, 18 outpatient) were performed. 86% of families receiving inpatient management expressed satisfaction and 85% receiving outpatient management expressed satisfaction. Dissatisfaction with inpatient management was driven by concerns for hospital acquired infections and separation from family. Dissatisfaction with outpatient management stemmed from stress of caring for a neutropenic child at home. Patients/families reporting satisfaction with outpatient management emphasized that the approach would not be appropriate for all families. For Aim 3, mean (±SD) parent proxy PedsQL score did not differ between outpatient (74.3 ±18.3) and inpatient management (70.3 ±19.2), adjusted mean difference 3.95, 95% CI: -5.57, 13.5. CONCLUSION In this nationally representative sample of AML patients, outpatient management had bacteremia rates similar to inpatient management, with no difference in time to the start of next treatment course or patient QoL. Semi-structured interviews revealed strong alignment between patient/family satisfaction and center discharge practice. However, families experiencing outpatient management note high stress in caring for profoundly neutropenic patients at home and that this strategy would not be suitable for all families. These clinical and patient-centered results suggest that outpatient management during neutropenia is a viable approach without excess risk for children with AML. However, implementation studies are needed to identify patient/family characteristics that portend a positive experience with an outpatient strategy. Disclosures Raetz: Pfizer: Research Funding. Rubnitz:AbbVie: Research Funding. Fisher:Pfizer: Research Funding; Astellas: Other: Data Safety Monitoring Board Chair for an antifungal study; Merck: Research Funding.

Leukemia is the most frequently (30%) occurring type of pediatric cancer. Of these, approximately 80% are acute lymphoblastic leukemia (ALL) with acute myeloid leukemia (AML) cases making up the remaining 20%. Management of ALL has improved dramatically over the years with around 90% of patients surviving 5 years or more. Unfortunately, children with AML do not have such a promising prognosis with only 60% surviving 5 years or longer, plus a high risk of relapse, the side effects of the harsh chemotherapy regimen and a risk of not responding to induction therapy, provide a bleak outlook. Recently, Bolouri et al., (2017) identified the need for age-specific therapies for AML patients. To work towards age-specific treatment regimens we used the TARGET AML transcriptomic data, (https://ocg.cancer.gov/programs/target/data-matrix) and identified several therapeutically targetable pathways, in particular anomalies in apoptotic pathways amongst cytogenetically normal relapsed AML patients. Based on these apoptotic pathways we screened 80 apoptosis-inducing drugs from a compound library obtained from Selleckchem. The effect of three doses (0.01 µM, 0.1 µM and 1 µM) over 72 hrs of each drug was assayed using CellTox™ Green reagent (Promega) to investigate the cytotoxic potential of the compounds against three pediatric AML cell lines (Kasumi-1, MV4-11 and CMK) representative of a spectrum of cytogenetic and mutational anomalies. The cell lines showed varying responses in a time and dose-dependent manner with mutual and unique effects being highlighted, six compounds, including Navitoclax and TP-0903, overlapped in their effect on cell viability between the three cell lines. The cell lines also showed unique responses to many compounds, including Sunitinib and Obatoclax in the MV4-11 cell line. Based on these findings a dose of 0.1 µM was taken forward for the combination screen. To carry out the combination screen we used R to compile an algorithm, with the capability of determining all possible pairwise combinations for a given number of compounds. Several inputs were accommodated, including: the number of compounds to be used; the number of compounds in a well and the number of times the algorithm should run to get the best solution, with the best solution being the least number of wells to reduce redundant repeats of pairs. The output detailed which compounds should be in which well. With an input of 80 apoptotic-inducing compounds into the algorithm; the output was as follows: 3,160 possible pairwise combinations, 1,197 pairs used once, 827 pairs used twice, and so on with the largest number of repeats being 148 pairs used eight times. This output enabled us to cross validate our data, by analysing wells containing the successful combinations to corroborate them as 'hits' or as containing possible antagonistic pairings if they were deemed unsuccessful. Using the output of the algorithm, 160 wells were needed for all combinations to enable our Multiplex Screening for Interacting Compounds for pediatric AML (MuSICAL). CellTox™ Green was used to determine cytotoxicity up to 72 hours and the same three cell lines used as for the single agent screen. The MuSICAL screen identified some wells in common amongst the three cell lines (6.25%), although many of the positive wells (12.5%) were unique to each of the cell lines. Well 158, which included Nutlin-3, Embelin and Poziotinib, was common for the MV4-11 and the CMK cell lines, with the individual compounds in this well having no substantial effect on cell viability as single agents, but a combination of 2 or more compounds induced substantial cell death. The 10 drugs in well 158 were deconvoluted and screened, resulting in 45 pairwise combinations. Several pairwise combinations, at 0.1 µM each, induced cell death in the MV4-11 and CMK cell lines. The most effective pairwise combination was the combination of ABT-737, a Bcl-2 inhibitor and Purvalanol A, a CDK inhibitor, in the MV4-11 cell line. The effect of this combination was also a 'hit' in Well 6 validating the therapeutic potential. For the CMK cell line, the most effective combination was Poziotinib, a pan-HER inhibitor and SU9516, another CDK inhibitor. In conclusion, the MuSICAL screen has identified multiple apoptosis-inducing drug combinations across the 3 cell lines highlighting the potential for personalised treatment of pediatric AML with different cytogenetic and mutational backgrounds. Disclosures No relevant conflicts of interest to declare.

[Introduction] Infant AML has been defined as AML in patients aged [Methods] This study enrolled patients aged [Results] We first analyzed the distribution of fusion genes and FAB classification in 503 patients with clinical samples. Fusion genes were found in >50% of the patients in each age group. RUNX1-RUNX1T1, KMT2A-R, and CBFB-MYH11 were present in almost all age groups. RUNX1-RUNX1T1 was the most frequently identified fusion gene in all age groups excluding those Furthermore, we analyzed the prognosis of KMT2A-R and CBFB-MYH11 in all 723 patients and confirmed that these fusions had significantly different prognoses between patients In patients with KMT2A-R (n = 131), no molecular markers were identified as prognostic markers in combination with age; however, high WBC counts were related to poor prognosis regardless of age. Particularly, among patients 3-10,000/µL (n = 37) had a significantly poorer prognosis than those with WBC In patients with CBFB-MYH11 with clinical samples (n = 39), NRAS-mutated patients (n = 13) had significantly better 5y EFS and CIR than NRAS-wild-type patients (n = 26) (100% vs 60.5%, p = 0.012, and 0% vs 35.7%, respectively). Moreover, the combination of NRAS status with age identified the worst prognosis in NRAS-wild-type patients [Conclusions] Although age has not been used for the risk stratification of recent pediatric AML clinical trials, it might be a useful prognostic maker of this disease in combination with some molecular makers. Disclosures No relevant conflicts of interest to declare.

Recent studies of WT1 mutations in acute myeloid leukemia (AML) mostly report an association with unfavorable clinical outcome. We screened 842 patients treated on 3 consecutive pediatric AML trials for WT1 zinc-finger mutations. Eighty-five mutations were detected in 70 of 842 patients (8.3%). Mutations occurred predominantly in exon 7 (n = 74) but were also found in exons 8 (n = 5) and 9 (n = 6). Normal karyotype was observed in 35.3% of WT1mut patients, whereas 27.5% WT1mut patients harbored favorable risk cytogenetics. Patients with or without mutations had similar rates of complete remission after one course of induction chemotherapy. Overall survival (OS) for patients with WT1 mutations was 41% versus 54% for those without mutations (P = .016). Corresponding event-free survival (EFS) was also significantly worse for those with WT1 mutations (28% vs 42%; P = .01). However, FLT3/ITD was present in 36% of the WT1mut cohort; WT1mut patients without FLT3/ITD had similar OS (56% vs 56%, respectively; P = .8) and EFS (35% and 44%, respectively; P = .34) to patients who were wild type for both mutations. In current risk stratification schemes incorporating cytogenetics and FLT3/ITD status, the presence of WT1 mutations has no independent prognostic significance in predicting outcome in pediatric AML. The clinical trials are registered at www.clinicaltrials.gov as #NCT00002798 and #NCT00070174.

In this issue of Blood, Coenen et al report on a subset of pediatric acute myeloid leukemia (AML) with t(8;16)(p11;p13)/MYST3-CREBBP rearrangement demonstrating specific features with respect to cytomorphology and gene expression patterns, thus fulfilling the criteria of a distinct biological disease entity according to the World Health Organization (WHO) classification.1

Background: Pediatric acute myeloid leukemia (AML) comprises approximately 20% of pediatric leukemia, representing one of the major therapeutic challenges in pediatric oncology with a current overall survival rate of less than 70%. The pathogenesis of AML is heterogeneous and can be caused by various chromosomal aberrations, gene mutations/epigenetic modifications, and deregulated/overregulated gene expressions, leading to increased proliferation and decreased hematopoietic progenitor cell differentiation. Recurrent chromosomal structural aberrations [e.g., t(8;21), inv(16), and MLL-rearrangements] have been well established as diagnostic and prognostic markers of AML. Furthermore, recurrent mutations in FLT3, KIT, NPM1, and CEBPA have been reported in both adult and pediatric AML. Recently, massively parallel sequencing enabled the discovery of recurrent mutations in DNMT3A, TET2, and IDH, which are clinically useful for the prediction of the prognosis. However, these mutations are rare in pediatric AML, suggesting that other genetic alterations exist in pediatric AML. In contrast, recent reports have described NUP98-NSD1 fusion as an adverse AML prognostic marker and PRDM16 (also known as MEL1) as the representative overexpressed gene in patients harboring NUP98-NSD1 fusion. Intriguingly, PRDM16 overexpression occurs in nearly one-quarter of all children, with AML involving NUP98-NSD1-negative patients. Moreover, this overexpression is enriched in specimens with other high-risk lesions (e.g., FLT3-ITD, NUP98-NSD1, and MLL-PTD). Patients and Methods: To reveal a complete registry of gene rearrangements and other genetic lesions in pediatric AML with a normal karyotype, we performed transcriptome analysis (RNA sequencing) of 61 of 70 de novo pediatric AML patients with a normal karyotype using Illumina HiSeq 2000. We could not perform RNA sequencing in nine patients because of a lack of RNA quantity or quality. Among the 70 AML patients with a normal karyotype, 33 patients overexpressed PRDM16, which was found to be strongly associated with a poor prognosis in our previous studies. All patients were enrolled and treated with AML-05 in the study conducted by the Japan Pediatric Leukemia/Lymphoma Study Group (JPLSG). We also analyzed the known genetic mutations associated with these patients using the data derived from RNA sequencing. Results: A total of 144 candidate gene rearrangements, which were not observed in normal samples, were identified in 51 of 61 samples. Many of the recurrent gene rearrangements identified in this study involved previously reported targets in AML, including NUP98-NSD1, NUP98-JARID1A, CBFA2T3-GLIS2, MLL-MLLT10, and MLL-MLLT3. However, several gene rearrangements were newly identified in the current study, including MLL-SEPT6, HOXA10-HOXA-AS3, PRDM16-SKI, and CUL1-EZH2. We have also performed the validation of these novel gene rearrangements using Sanger sequencing. Most of these gene rearrangements were found in patients with a high expressionof PRDM16. In contrast, CEBPA mutations were frequently observed in patients with a low expression of PRDM16. Known gene alterations, such as FLT3-ITD and MLL-PTD, and mutations of the RAS, KIT, CEBPA, WT1, and NPM1genes were also detected using RNA sequencing. Conclusion: RNA sequencing unmasked a complexity of gene rearrangements and mutations in pediatric AML genomes. Our results indicate that a subset of pediatric AML represents a discrete entity that could be discriminated from adult counterparts, regarding the spectrum of gene rearrangements and mutations. In the present study, we identified at least one potential gene rearrangement or driver mutation in nearly all AML samples, including some novel fusion genes. These findings suggest that gene rearrangements in conjunction with mutations also play essential roles in pediatric AML. Disclosures Ogawa: Kan research institute: Consultancy, Research Funding; Takeda Pharmaceuticals: Consultancy, Research Funding; Sumitomo Dainippon Pharma: Research Funding.

INTRODUCTION Acute myeloid leukemia (AML) is a hematopoietic malignancy that comprises almost 25% of pediatricleukemiasand is characterized by genetic and epigenetic alterations that lead to impairment of differentiation and clonal expansion. Despite intensive chemotherapy, more than half of children with AML either fail to achieve a complete remission (CR) or relapse after initial response.As such, the availability of a predictor of treatment failure at diagnosis may allow early institution of alternative therapies to improve outcome. MicroRNAs (miRNAs), a class of small non-coding RNAs that regulate the translation of their target mRNAs, are frequentlydysregulatedin cancers, and thus may serve as robust biomarkers of patient outcome. METHODS To identifymiRNAbiomarkers associated with treatment failure and candidate therapeutic targets, we performed a comprehensive sequence-based characterization of the pediatric AMLmiRNAexpression landscape usingmiRNA-seqdata from 637 primary samples. AmiRNAexpression-based model for EFS separating patients into low, intermediate, andhigh riskgroups was produced using penalized Cox regression. The model was designed usingmiRNAexpression data obtained from a training cohort, which consisted of two-thirds of our study cohort (n=425), and then tested on the remaining one-third of our study cohort (n=212). The training and test cohorts were derived by random selection. RESULTS A 36-miRNA EFS predictive model was generated. This model was comprised of16miRNAsthat were over-expressed and 20 that were under-expressed in patients who experienced an event (death, relapse or IF). Among the 36miRNAtranscripts were miR-155, miR-335, miR-139 and miR-375, which have been previously individually associated with survival in pediatric AML. To demonstrate the potential clinical utility of the model, we determined 2 miRNAmodel score thresholds in the training cohort to separate patients into low, intermediate and high miRNAmodel score risk groups. The miRNAmodel score groupings were significantly associated with EFS in both the training cohort and test cohort (Figure 1A; P Furthermore, to demonstrate the strength of our predictive model, we evaluated the clinical significance of the model in each of the low, standard and high CM risk cohorts. In this analysis, our model was capable of further stratifying patients in each of the 3 CM risk cohorts into 3 distinct miRNAmodel score-based risk categories (Figure 1B, P CONCLUSIONS We present amiRNAexpression-based predictor of outcome in pediatric AML, comprised of 36miRNAtranscripts. Our predictive model was developed and tested on a large cohort of primary patient samples (n=637), and demonstrated that diagnosticmiRNAexpression profiles can identify risk groups in patients independent of other CM risk factors. Moreover, this model is applicable to RNA from samples that are routinely obtained for diagnosis, and thus has the potential to impact clinical practice. Figure 1. Stratifying patients using the miRNA-based EFS predictive model. A)Kaplan-Meier plots displaying EFS differences between patients in variousmiRNA model score groups. Patients with highmiRNA model scores had the poorest outcomes, while patients with the lowmiRNA model scores had superior outcomes. B)Kaplan-Meier curves depicting EFS of patients when classified using ourmiRNA model score groupings. ThemiRNA model score groupings were effective in further stratifying patients within the low, standard and high conventional CM risk group classifications. Figure 1. Stratifying patients using the miRNA-based EFS predictive model. A)Kaplan-Meier plots displaying EFS differences between patients in variousmiRNA model score groups. Patients with highmiRNA model scores had the poorest outcomes, while patients with the lowmiRNA model scores had superior outcomes. B)Kaplan-Meier curves depicting EFS of patients when classified using ourmiRNA model score groupings. ThemiRNA model score groupings were effective in further stratifying patients within the low, standard and high conventional CM risk group classifications. Disclosures No relevant conflicts of interest to declare.

Becton et al report the results of a trial in children with acute myeloid leukemia (AML) who were randomized to receive consolidation therapy with or without cyclosporine-A (CsA).[1][1] The authors measured P-glycoprotein (P-gp) expression in vitro by flow cytometry, using MRK16 antibody staining. P

Pediatric cytogenetically normal acute myeloid leukemia (CN-AML) is a heterogeneous subgroup of myeloid clonal disorders that do not harbor known mutations. To investigate the mutation spectrum of pediatric CN-AML, we performed whole-transcriptome massively parallel sequencing on blasts from 7 CN-AML pediatric patients. In 3 patients we identified a recurrent cryptic inversion of chromosome 16, encoding a CBFA2T3-GLIS2 fusion transcript. In a validation cohort of 230 pediatric CN-AML samples we identified 17 new cases. Among a total of 20 patients with CBFA2T3-GLIS2 fusion transcript out of 237 investigated (8.4%), 10 patients (50%) did not belong to the French-American-British (FAB) M7 subgroup. The 5-year event-free survival for these 20 children was worse than that for the other CN-AML patients (27.4% vs 59.6%; P = .01). These data suggest that the presence of CBFA2T3-GLIS2 fusion transcript is a novel common feature of pediatric CN-AML, not restricted to the FAB M7 subtype, predicting poorer outcome.

Introduction: Black patients with acute myeloid leukemia (AML) have inferior overall survival relative to White patients. Few studies have evaluated differences in induction mortality and none has assessed the contribution of severity of illness at presentation to the disparity in survival. Our primary objectives were to compare induction mortality and acuity of presentation among Black relative to White patients and to assess whether any disparity in induction mortality is the consequence of differences in presentation acuity. In addition, we explored the interaction between Black race and public insurance on induction mortality with use of single referent models. Methods: Using a retrospective cohort of children (ages 0 to 18 years) from 2004 to 2014 with new-onset AML diagnosed and treated at free-standing pediatric hospitals who contribute inpatient information to the Pediatric Health Information System administrative database (PHIS), we evaluated inpatient mortality over two courses of standard induction chemotherapy. We examined race (Black versus White) as the primary exposure and insurance was considered with race using a common reference group. We also considered Intensive Care Unit (ICU)-level resource use during the first 72 hours following the initial AML admission as a surrogate for acuity at presentation and a potential mediator of the association between race and induction mortality. Results: 1,122 patients (183 Black, 939 White) with AML who received standard induction chemotherapy were included. Induction mortality for Blacks was substantially higher than for Whites (cHR= 2.31, 95% CI: 1.01, 5.42). Blacks also had a significantly higher risk of requiring any ICU-level care within the first 72 hours after initial presentation compared with Whites (cHR= 1.52, 95% CI: 1.02, 2.24).The association between race and induction mortality was attenuated following adjustment for ICU-level care within the initial 72 hours after admission, (aHR=1.42, 95% CI: 0.67, 2.99). Publicly insured patients experienced greater induction mortality than privately insured patients regardless of race. Induction mortality rates for Black and White patients were more similar among the privately insured and were increasingly disparate among the publicly insured. Conclusion: Our findings suggest that Black patients with AML present with more acute illness at initial diagnosis, accounting for up to 63% of the relative excess induction mortality. Identifying factors impacting acuity of illness at presentation and associated with public insurance may help to identify opportunities for intervention and thus narrow the current racial disparities in pediatric AML survival. Table 1. Inpatient Induction Mortality and ICU level Care by Race Outcome, Follow-up Period Overall (N=1122) n (%) Black (n=183) n (%) White (n=939) n (%) cHR (95% CI) aHRa (95% CI) Induction Death 27 (2.4%) 8 (4.4%) 19 (2.0%) 2.31 (1.01, 5.42) 1.42 (0.67, 2.99) Any ICU Level Care in first 72 hrs 135 (12.0%) 31 (16.9%) 104 (11.1%) 1.52 (1.04, 2.24) ICU involving >1 system in first 72 hrs 47 (4.2%) 18 (9.8%) 29 (3.1%) 3.35 (1.84, 6.12) Any ICU Level Care in Induction 237 (21.1%) 48 (26.2%) 189 (20.1%) 1.30 (0.99, 1.71) 1.09 (0.74, 1.61) ICU involving >1 system in Induction 99 (8.8%) 22 (12.0%) 77 (8.2%) 1.42 (0.85, 2.38) 0.92 (0.54, 1.57) a adjusted for ICU acuity score within the first 72 hours of index admission Figure 1. Independent and joint effects of Black race and public insurance on induction mortality Figure 1. Independent and joint effects of Black race and public insurance on induction mortality Disclosures Wilkes: Alex's Lemonade Stand Foundation: Research Funding; Healthcare Research and Quality: Research Funding. Fisher:Merck: Research Funding; Pfizer: Research Funding. Epstein:Medicus Economics: Consultancy. Aplenc:Sigma Tau: Consultancy.

High ERG gene expression is an unfavorable prognostic marker in pediatric acute myeloid leukemia Response

Abstract 2494 Introduction. Acute myeloid leukemia (AML) is an heterogeneous disease with known specific recurrent genetic aberrations. The continuous and increasing identification of new genetic mutations has permitted to identify new subgroups with different prognosis. In the present work we evaluated the incidence of rare genetic abnormalities in pediatric AML such as del(4)(q12)FIP1L1-PDGFRA, t(16;21)(p11;q22)FUS-ERG, t(8;16)(p11;p13)MOZ-CBP, t(11;17)(q23;q12–21)MLL-AF17, t(4;11)(q35;q23)MLL-ArgB2, t(5;11)(q35;p15.5)NUP98-NSD1, t(3;5)(q25;q34)NPM1-MLF1, and MLLPTD. Methods. We selected 306 patients with AML other than acute promyelocytic leukemia, negative for known recurrent genetic abnormalities involving MLL, CBF-beta and FLT3 genes. RNA was extracted from fresh bone marrow at diagnosis, and multiplex RT-PCR was employed. Sequencing by Sanger method was applied to all positive cases to characterize breakpoints of fusion. The Kaplan-Meier method was used for estimating the probability of event-free survival (EFS). Results. We identified one patient each positive for t(16;21)(p11;q22)FUS-ERG, t(11;17)(q23;q12–21)MLL-AF17, and t(4;11)(q35;q23)MLL-ArgB2, respectively, this suggesting that these rearrangements are rare in pediatric AML. 2/306 patients had del(4)(q12)FIP1L1/PDGFRA, and 4/306 the t(8;16)(p11;p13)MOZ-CBP; both these anomalies should be investigated in larger cohorts for definining their prognostic value. Interestingly 6/306 (2%) patients had the t(3;5)(q25;q34)NPM1-MLF1, 6/306 (2%) the MLLPTD, and 8/306 (2.6%) were found to carry the t(5;11)(q35;p15.5)NUP98-NSD1. Since the t(5;11) fusion was recently associated to FLT3ITD, we enlarged the screening to 42 de novo AML harbouring FLT3ITD mutation enrolled in the AIEOP-LAM 2002 protocol finding that 6 of them (14%) had the NUP98-NSD1 fusion gene. We documented a poor EFS for patients with t(5;11)NUP98-NDS1 (n=12) as compared to patients negative for molecular lesions and enrolled in the LAM 2002-AIEOP protocol (25% vs 53.1% at 3 years, p Conclusions. We provide evidence that NUP98-NSD1 may be considered a recurrent translocation in pediatric AML with poor prognosis. Being cryptic to conventional karyotyping, we confirmed the need of using molecular approaches for a proper identification of this anomaly. We also suggest that the NUP98-NSD1 fusion gene be considered for a better evaluation of the FLT3ITD+ patients. Disclosures: No relevant conflicts of interest to declare.

Woods et al[1][1] emphasized allogeneic bone marrow transplantation (BMT) as treatment of choice for children with acute myeloid leukemia (AML) in first remission. They supported this recommendation with the results of the randomized Children's Cancer Group (CCG) 2891 study comparing 3 aggressive

Abstract 2288 Poster Board II-265 Background. More than 80% of children with de novo acute myeloid leukemia (AML) achieve complete remission (CR) after aggressive induction chemotherapy. However, post-remission treatment plays a crucial role in the final outcome of these patients. In this regards, possible post-remission treatments include high dose chemotherapy, as well as autologous or allogeneic hematopoietic stem cell transplantation (HSCT). In the last years, several groups have compared allogeneic HSCT with chemotherapy or autologous HSCT in adults as well and children with AML in first remission. These studies have demonstrated that allogeneic HSCT is superior to the other forms of post-remission therapy in children with AML not carrying favourable cytogenetic characteristics. We herein report the long-term results of a prospective multicenter study aimed at evaluating the safety and efficacy of an original combination of 3 alkylating agents, busulfan (BU), cyclophosphamide (CY) and melphalan (L-PAM) as conditioning regimen for patients affected by de novo AML other than the FAB M3 subtype and given allogeneic HSCT from a matched family donor in first haematological CR. Patients and methods. Seventy consecutive patients (40 males and 30 females), affected by de novo AML in first CR, received an allogeneic HSCT from a HLA-matched family donor after a conditioning regimen combining BU (4 mg/Kg/day for 4 days), CY (60 mg/Kg/day for 2 days) and L-PAM (140 mg/m2). BU dosage was adjusted after the pharmacokinetic study performed following the first administration, in order to maintain a concentration to the steady state (Css) comprised between 600 and 900 ng/mL. Transplants were performed in one of the transplant centers of the Italian Association for Pediatric Hematology / Oncology (AIEOP) between 1992 and 2002. Median patient age at HSCT was 7 years (0.5 – 17). Two patients had FAB M0 AML, 25 M1, 17 M2, 12 M4, 21 M5, 1 M6 and 2 M7 AML. Children with M3 AML were excluded from the study. Five patients with CBF anomalies, either isolated or associated with loss of sex chromosome, were considered to have favourable cytogenetics. The median interval between diagnosis and HSCT was 3.9 months (2.9 - 9). All patients were given bone marrow stem cells. GVHD prophylaxis consisted of cyclosporine-A (Cs-A) alone in 89% of patients, the remaining children receiving a combination of Cs-A with short-term methotrexate. Results. Median follow-up for surviving patients is 9 years (range, 6 – 16 years). The conditioning regimen was well tolerated, and no patient died for causes directly attributable to the myeloablative treatment. All patient engrafted, the median time to reach neutrophil and platelet recovery being 13 (7 - 28) and 21 (13 – 115) days, respectively. The cumulative incidence (CI) of grade II-IV acute GVHD was 58% (48 – 71), while that of grade III-IV acute GVHD was 14% (8 – 25). Chronic GVHD incidence was 27% (18 – 39). Ten-years survival probability was 77% (67 - 87), while 10-years disease-free survival (DFS) was 76% (65 - 86). The cumulative incidence of relapse was 17% (10 – 29), while the cumulative incidence of transplant-related mortality was 7% (3 – 17). Results improved over time, DFS being 59% (41 – 78) for patients transplanted before 1997 and 86% (75 – 96) for those transplanted after 1997. No other variables influenced the probability of both survival and DFS. Conclusions. Our study, conducted on a significant number of children affected by AML in first CR and with a very long follow-up demonstrate that allogeneic HSCT from a matched family donor can cure a large proportion of patients. The combination of dose-adjusted BU, CY and L-PAM was safe and well tolerated, with an incidence of TRM of only 7% and no death directly attributable to the conditioning regimen. Furthermore, the combination of 3 alkylating agents showed a good anti-leukemic activity, the probability of leukaemia recurrence being only 17%. Disclosures: No relevant conflicts of interest to declare.

Introduction: The presence of disease in the central nervous system (CNS) in pediatric acute myeloid leukemia (AML) is often thought to confer a worse prognosis. This study examined the outcome of children with AML who had CNS disease at diagnosis. Methods: Patients enrolled on Children’s Oncology Group protocols 2891 (N=836)and 2961 (N=901) being treated for de novo AML were classified for the presence of CNS disease at diagnosis as: CNS1 (less than 5 WBC in the CSF with no blasts), CNS2 (less than 5 WBC in the CSF with blasts) or CNS3 (5 or more WBC in the CSF with blasts). CNS disease at diagnosis was then analysed for survival, patient characteristics and outcome using univariate analysis. Patients on these protocols were treated with intrathecal chemotherapy and not radiation therapy for their CNS disease. Results: For both AML protocols, the overall survival and event free survival were highest in patients with CNS2 disease but this was not significantly different from the survival of the CNS1 and CNS3 patients. Overall survival from 2891 study entry for all de novo patients Overall survival from 2891 study entry for all de novo patients Overall survival from 2961 study entry for all de novo patients Overall survival from 2961 study entry for all de novo patients Patients with CNS 2 and 3 disease were of significantly lower median age than CNS1 patients (p=0.001 in 2891 and p=0.005 in 2961). There was a significantly higher WBC count at diagnosis (p=0.001 in both studies), presence of hepatomegaly (p=0.001 in both studies), presence of splenomegaly (p=0.003 in 2891 and p=0.001 in 2961), FAB M2 morphology (p=0.001 in both studies) and FAB M4 morphology (p=0.001 in both studies) in patients with CNS2 and 3 disease compared to CNS 1 patients. Also, CNS2 and 3 patients had a significantly higher incidence of abnormal chromosome 16 (p=0.002 in 2891 and p=0.001 in 2961). In protocol 2891 significantly more CNS2 and 3 patients had hyperdiploid cytogenetics compared to CNS1 patients (p=0.016), and in protocol 2961 there were significantly more patients with t(8;21) in the CNS1 group (p=0.007). In terms of overall outcome, there was a significantly higher incidence of isolated CNS relapse in patients with CNS3 disease at diagnosis in the 2891 protocol (p=0.001), and in patients with both CNS2 and 3 disease at diagnosis in the 2961 protocol (p=0.001). Conclusion: Patients with CNS disease at diagnosis have similar survival to those without CNS disease, although they have an increased incidence of isolated CNS relapse. More aggressive CNS directed therapy may be warranted in this patient population.

To find poor prognostic factors other than karyotypes is desired to improve the treatment outcome in pediatric acute myeloid leukemia (AML). Total three hundread and eighteen patients were enrolled in Japanese Childhood AML Cooperative Treatment Protocol, AML99, from January 2000 to December 2002. We performed the mutation analysis of FLT3 and MLL in 158 patients (13 of 29 with FAB-M3, 10 of 49 with Down syndrome, 135 of 240 with other type of AML) Patients with FAB-M3 and patients with Down syndrome were treated on different protocol. Seventeen (12.6%) of 135 patients had FLT3-internal tandem duplication (ITD). The differences between AML patients with or without FLT3-ITD were significant in 4-year overall survival (OS) (35.3% vs. 86.1%, p