Abstract: Abstract 68 Early T-cell precursor acute lymphoblastic leukemia (ETP ALL) is characterized by an immature T-lineage immunophenotype (cCD3+, CD1a-, CD8- and CD5dim) aberrant expression of myeloid and stem cell markers, a distinct gene expression profile and very poor outcome. The underlying genetic basis of this form of leukemia is unknown. Here we report results of whole genome sequencing (WGS) of tumor and normal DNA from 12 children with ETP ALL. Genomes were sequenced to 30-fold haploid coverage using the Illumina GAIIx platform, and all putative somatic sequence and structural variants were validated. The frequency of mutations in 43 genes was assessed in a recurrence cohort of 52 ETP and 42 non-ETP T-ALL samples from patients enrolled in St Jude, Children's Oncology Group and AEIOP trials. Transcriptomic resequencing was performed for two WGS cases, and whole exome sequencing for three ETP ALL cases in the recurrence cohort. We identified 44 interchromosomal translocations (mean 4 per patient, range 0–12), 32 intrachromosomal translocations (mean 3, 0–7), 53 deletions (mean 4, 0–10) and 16 insertions (mean 1, 0–5). Three cases exhibited a pattern of complex rearrangements suggestive of a single cellular catastrophe (“chromothripsis”), two of which had mutations targeting mismatch and DNA repair (MLH3 and DCLRE1C). While no single chromosomal alteration was present in all cases, 10 of 12 ETP ALLs harbored chromosomal rearrangements, several of which involved complex multichromosomal translocations and resulted in the expression of chimeric in-frame novel fusion genes disrupting hematopoietic regulators, including ETV6-INO80D, NAP1L1-MLLT10, RUNX1-EVX1 and NUP214-SQSTM1, each occurring in a single case. An additional ETP case with the ETV6-INO80D fusion was identified in the recurrence cohort. Additionally, 51% of structural variants had breakpoints in genes, including those with roles in hematopoiesis and leukemogenesis, and genes also targeted by mutation in other cases (MLH3, SUZ12, RUNX1). We identified a high frequency of activating mutations in genes regulating cytokine receptor and Ras signalling in ETP ALL (67.2% of ETP compared to 19% of non-ETP T-ALL) including NRAS (17%), FLT3 (14%), JAK3 (9%), SH2B3 (or LNK; 9%), IL7R (8%), JAK1 (8%), KRAS (3%), and BRAF (2%). Seven cases (5 ETP, 2 non-ETP) harbored in frame insertion mutations in the transmembrane domain of IL7R, which were transforming when expressed in the murine cell lines, and resulted in enhanced colony formation when expressed in primary murine hematopoietic cells. The IL7R mutations resulted in constitutive Jak-Stat activation in these cell lines and primary leukemic cells expressing these mutations. Fifty-eight percent of ETP cases (compared to 17% of non-ETP cases) harbored mutations known or predicted to disrupt hematopoietic and lymphoid development, including ETV6 (33%), RUNX1 (16%), IKZF1 (14%), GATA3 (10%), EP300 (5%) and GATA2 (2%). GATA3 regulates early T cell development, and mutations in this gene were observed exclusively in ETP ALL. The mutations were commonly biallelic, and were clustered at R276, a residue critical for binding of GATA3 to DNA. Strikingly, mutations disrupting chromatin modifying genes were also highly enriched in ETP ALL. Genes encoding the the polycomb repressor complex 2 (EZH2, SUZ12 and EED), that mediates histone 3 lysine 27 (H3K27) trimethylation were deleted or mutated in 42% of ETP ALL compared to 12% of non-ETP T-ALL. In addition, alterations of the H3K36 trimethylase SETD2 were observed in 5 ETP cases, but not in non-ETP ALL. We also identified recurrent mutations in genes that have not previously been implicated in hematopoietic malignancies including RELN, DNM2, ECT2L, HNRNPA1 and HNRNPR. Using gene set enrichment analysis we demonstrate that the gene expression profile of ETP ALL shares features not only with normal human hematopoietic stem cells, but also with leukemic initiating cells (LIC) purified from patients with acute myeloid leukemia (AML). These results indicate that mutations that drive proliferation, impair differentiation and disrupt histone modification cooperate to induce an aggressive leukemia with an aberrant immature phenotype. The similarity of the gene expression pattern with that observed in the LIC of AML raises the possibility that myeloid-directed therapies might improve the outcome of ETP ALL. Disclosures: Evans: St. Jude Children's research Hospital: Employment, Patents & Royalties; NIH & NCI: Research Funding; Aldagen: Membership on an entity's Board of Directors or advisory committees.

Abstract: Acute leukemias are the most commonly occurring cancers among children, adolescents and young adults. Approximately 15% of newly-diagnosed patients present with T-lineage acute lymphoblastic leukemia (T-ALL). In contrast to B-precursor ALL, molecular lesions that deregulate homeobox (HOX) genes appear to be widely prevalent in T-ALL, but their impact on outcome is unclear. We hypothesized that the molecular lesions associated with HOXA -deregulated T-ALL might identify patients with high-risk disease. We performed gene expression profiling (GEP) on a cohort of 213 T-ALL diagnostic samples obtained from children and young adults enrolled on Children's Oncology Group (COG) treatment studies 9404 (n=50) and AALL0434 (n=163). We identified a cluster of 54 cases (25%) characterized by increased expression of HOXA3, 5, 7, 9, and 10 (FDR ≤ 0.05; fold change cut off 3). We screened our database for the presence of MLL-R, AF10-R (MLLT10) and other HOXA deregulating lesions using an analysis of cytogenetics, FISH, LDI-PCR and RNA sequencing. We identified 15 cases with MLL-R, including MLL-AF6 (n = 5), del3'MLL (n = 3), MLL-ENL (n = 5), MLL-AF17 (n = 1), and PICALM-MLL (n = 1). Five cases were confirmed to have PICALM-AF10 fusions and two cases showed DDX3X-AF10 lesions. Two cases harbored NUP98 fusions, two cases had inv(7)(p15q34), and one case each was identified for HOXA10-(3'UTR)TRBC, STAG2-LMO2, LOC338817-CCDC91. We could not identify fusion transcripts in 3 cases, but MLL -R and AF10-Rwere confidently excluded with RNA sequencing. Almost 90% of cases identified to have MLL-R or AF10-R had ≥ 8-fold over-expression of HOXA9/10. Because HOXA-deregulated T-ALL is characterized by a high degree of molecular heterogeneity, we hypothesized that subset analyses might identify lesions that were more likely to be associated with an inferior outcome. We found that chromosomal abnormalities involving the MLL gene, but not AF10, were associated with induction failure (IF) in T-ALL (P = 0.02, OR = 5.34). Since MLL- and AF10 -R leukemias also demonstrate features of undifferentiated leukemias, we discriminated early T-cell precursor (ETP) from non-ETP cases using a GEP developed by Coustan-Smith et al. (Lancet Oncol, 2009). Among the 26 cases that showed ETP features, eleven (42%) also co-expressed HOXA9/10 (≥ 8-fold increase over the median). We found an association between ETP-ALL and early treatment failure (P = 0.01, OR = 4.37), and next assessed whether ETP cases are enriched with translocations harboring MLL or AF10 genes. We found overlap between MLL-R and the ETP cases (P = 0.03, OR = 4.14). We confirmed that ETP-ALL and MLL-R are risk features for IF (P = 0.026, OR = 4.37), and that cases with MLL-R (n = 11) had an inferior EFS compared to those that did not (N = 89) (P = 0.0158). We extended these observations to assess the impact of ETP/MLL-R (n = 6 vs. non-MLL/non-ETP, n = 69) on EFS, and found a significant association with treatment failure (P = 0.0007). For T-ALL, MRD has emerged as a prognostic indicator of high-risk disease. While MLL-ENL (n = 5)cases did not fail therapy regardless of Day 29 MRD levels, all patients with Day 29 MRD 〉 0.1 and MLL-AF6 (n = 5) or FISH-identified del3'MLL (n = 3) either failed induction or relapsed. Patients with AF10 -R (N = 7) have been reported to have inferior EFS, but we observed that only patients with Day 29 MRD ≥10% failed treatment. Our findings show a heterogeneity of outcomes related to MLL-R, but those with MLL-AF6 and del3'MLL should be considered high-risk. We propose that cytogenetic testing including specific FISH should be performed on all T-ALL patients at diagnosis and relapse. The current development of epigenetic modifying therapies targeted against HOXA-deregulating lesions warrants further study in T-ALL. Disclosures Aplan: NIH Office of Technology Transfer: Patents & Royalties. Mullighan:Amgen: Honoraria, Speakers Bureau; Cancer Science Institute: Membership on an entity's Board of Directors or advisory committees; Incyte: Consultancy, Honoraria; Loxo Oncology: Research Funding. Hunger:Merck: Equity Ownership; Sigma Tau: Consultancy; Jazz Pharmaceuticals: Consultancy; Spectrum Pharmaceuticals: Consultancy.

Abstract: Abstract 3768 Background: Data from the phase 3, randomized multicenter ENESTnd trial have demonstrated the superiority of nilotinib over imatinib after 24 months (mo) of follow-up, with significantly higher rates of complete cytogenetic response (CCyR) and major molecular response (MMR), and significantly lower rates of progression to accelerated phase/blast crisis (AP/BC). The current subanalysis evaluated the efficacy and safety of nilotinib 300 mg twice daily (Nil300) and nilotinib 400 mg twice daily (Nil400) in older (≥ 65 years [yrs] at study entry) patients (pts) with newly diagnosed chronic myeloid leukemia (CML) in chronic phase (CP) with a minimum follow-up of 24 mo. Methods: In ENESTnd, 846 pts stratified by Sokal risk score were randomized 1:1:1 to Nil300 (n = 282), Nil400 (n = 281), or imatinib 400 mg once daily (n = 283). Pts with impaired cardiac function or ECOG performance status 〉 2 were excluded. Rates of CCyR and MMR by 24 mo, progression to AP/BC on treatment, and safety were evaluated according to age group ( 〈 65 vs ≥ 65 yrs) in the 2 nilotinib arms. Safety data are reported for any pt who received ≥ 1 dose of nilotinib (n = 279, Nil300; n = 277, Nil400). Results: 36 pts (13%) and 28 pts (10%) were ≥ 65 yrs old in the Nil300 and Nil400 arms, respectively. Of the pts aged ≥ 65 yrs, 51/64 (80%) had an ECOG performance status of 0 at baseline and 60/64 (94%) had intermediate or high Sokal risk scores. Of the older pts, 8 (22%) on Nil300 and 6 (21%) on Nil400 had type 2 diabetes at baseline. CCyR rates by 24 mo were 83% and 68% among older pts treated with Nil300 and Nil400, respectively, and 87% for pts aged 〈 65 yrs in each nilotinib arm. By 24 mo, MMR was achieved by 72% and 61% of older pts on Nil300 and Nil400, respectively; in pts aged 〈 65 yrs, the respective rates were 71% and 67%. All 5 pts who progressed to AP/BC on treatment (2 on Nil300 and 3 on Nil400) were aged 〈 65 yrs. The frequency of grade 3/4 hematologic adverse events (AEs) was low in older pts; no pts had grade 3/4 neutropenia and only 1 older pt reported grade 3/4 thrombocytopenia in each nilotinib arm (Table). Transient, asymptomatic lipase elevations were reported in 11% and 16% of older pts treated with Nil300 and Nil400, and 7% of younger pts in each arm. Hyperglycemia occurred in 23% and 16% of older pts on Nil300 and Nil400, respectively, and 4% of younger pts in each arm; regardless of age, no pt discontinued study due to hyperglycemia. Among the 12 older pts with grade 3/4 hyperglycemia (8 on Nil300; 4 on Nil400), 9 pts had type 2 diabetes at baseline. There were no QTcF increases of 〉 60 msec from baseline in older pts and 3 in nilotinib-treated pts 〈 65 yrs old (1 on Nil300; 2 on Nil400). QTcF prolongation of 〉 500 msec did not occur in any pt treated with nilotinib on study. Periodic echocardiograms were done, and there were no decreases of 〉 15% in left ventricular ejection fraction from baseline in any pt treated with nilotinib on study. There were 4 cases of ischemic heart disease reported in older pts (1/35 [3%] on Nil300; 3/25 [12%] on Nil400) and 7 cases in pts 〈 65 yrs of age (4/244 [2%] on Nil300; 3/252 [1%] on Nil400). No sudden deaths occurred on study. Discontinuation occurred in approximately 25% of older and younger pts with Nil300, of which, 6% and 9%, respectively, were due to AEs/lab abnormalities. Discontinuation from study with Nil400 was 46% in older pts and 19% in younger pts; of which, 36% and 10% were due to AEs/lab abnormalities. Conclusions: Older pts treated with nilotinib demonstrated high rates of cytogenetic and molecular responses and low rates of progression. Nilotinib was generally well tolerated by older pts. In older pts, Nil300 had numerically higher rates of CCyR and MMR and was generally better tolerated (as evidenced by fewer AEs and discontinuations) vs Nil400. These data support the use of Nil300 in older pts with newly diagnosed CML-CP. Disclosures: Larson: Novartis Pharmaceuticals: Consultancy, Honoraria, Research Funding. Bunworasate:Novartis Pharmaceutical: Research Funding. Turkina:Novartis: Consultancy, Honoraria; BMS: Honoraria. Goldberg:Bristol Myers Squibb: Honoraria, Research Funding, Speakers Bureau; Novartis Pharmaceutical: Honoraria, Research Funding, Speakers Bureau; Ariad: Research Funding. Dorlhiac-Llacer:Bristol Myers Squibb: Research Funding; Novartis: Research Funding. Kantarjian:Novartis: Consultancy; Novartis: Research Funding; Pfizer: Research Funding; BMS: Research Funding. Saglio:Bristol-Myers Squibb: Consultancy, Speakers Bureau; Novartis Pharmaceutical: Consultancy, Speakers Bureau; Pfizer: Consultancy. Hochhaus:Ariad: Consultancy, Honoraria, Research Funding; Bristol Myers Squibb: Consultancy, Honoraria, Research Funding; Novartis Pharmaceutical: Consultancy, Honoraria, Research Funding; Merck: Consultancy, Honoraria, Research Funding. Hoenekopp:Novartis Pharmaceutical: Employment, Equity Ownership. Blakesley:Novartis Pharmaceutical: Employment. Yu:Novartis: Employment, Equity Ownership. Gallagher:Novartis: Employment, Equity Ownership. Clark:Bristol Myers Squibb: Honoraria, Research Funding; Novartis Pharmaceutical: Honoraria, Research Funding, Speakers Bureau. Hughes:Bristol Myers Squibb: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Novartis: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Ariad: Honoraria, Membership on an entity's Board of Directors or advisory committees.

Abstract: Background: Gilteritinib (ASP2215) is a novel, highly selective, potent oral FLT3/AXL inhibitor with preclinical activity against FLT3-ITD activating and FLT3-D835 resistance mutations. The objectives of this phase 1/2 study were to assess gilteritinib safety/tolerability, pharmacokinetic (PK) and pharmacodynamic (PD) profiles after single- and multiple-day dosing, and antileukemic effects in patients with R/R AML. Methods: This open-label study (NCT02014558) enrolled patients (≥18 yr) into 1 of 7 dose-escalation cohorts (20-450 mg once daily [QD]) or concomitant dose-expansion cohorts. While confirmed FLT3 mutation was not an inclusion criterion, each expanded dose level enrolled ≥10 patients with FLT3 mutations (FLT3mut+); 120 and 200 mg dose levels were further expanded with ≥40 FLT3mut+ patients. The choice to expand these dose cohorts was based upon FLT3 inhibition in correlative assays and clinical activity seen during dose escalation. Safety and tolerability were primary endpoints; blood samples were drawn from patients in the dose-escalation cohorts to evaluate gilteritinib PK parameters and PD effects. Antileukemic response rates (eg, complete remission [CR] , CR with incomplete platelet recovery [CRp], CR with incomplete hematological recovery [CRi] , overall response rate [ORR]) were secondary endpoints. Results: Patients (N=252; 129M:123F, median age 62 yr [range: 21-90]) enrolled between October 2013 and August 2015 received ≥1 dose of gilteritinib. The study population was heavily pretreated: 70% (n=177) had ≥2 prior AML therapies, 29% (n=73) had a prior stem cell transplant, and 25% (n=63) had prior TKI treatment with sorafenib most commonly used. Across the study, 194 patients had a locally confirmed FLT3 mutation (ITD, n=159; D835, n=13; ITD-D835, n=16; other, n=6). For all enrolled patients, progressive disease (n=75), lack of efficacy (n=44), adverse events (n=34), and death (n=29) were the most common reasons for treatment discontinuation. Seven deaths were considered possibly/probably related to treatment: pulmonary embolism, respiratory failure, hemoptysis, intracranial bleed, ventricular fibrillation, septic shock, and neutropenia (all n=1). Maximum tolerated dose was determined to be 300 mg when 2 of 3 patients in the 450 mg cohort experienced diarrhea and/or hepatic transaminase elevation as dose-limiting toxicities. Diarrhea (16%) and fatigue (15%) were the most commonly reported treatment-related adverse events of any grade. Less than 5% of patients (11/252) had a maximum post-baseline QTcF interval 〉 500 msec. Gilteritinib concentrations were generally dose proportional and showed both a long-elimination half-life (45-159 h) and substantial accumulation (3.2-10 fold) by day 15. An exposure-related increase in the inhibition of FLT3 phosphorylation with increasing doses of gilteritinib was also observed. Gilteritinib showed strong antileukemic activity in FLT3mut+ patients (ORR=49%); response was observed less frequently in patients with wild-type FLT3 (ORR=12%). While CR, CRi, and CRp occurred at all doses, responses were enriched among FLT3mut+ patients with gilteritinib steady-state trough concentrations ≥100 ng/mL, which correlated with potent FLT3 inhibition in PD assays and corresponded to doses ≥80 mg. The ORR in 169 FLT3mut+ patients receiving ≥80 mg was 52% (Table); median overall survival in this patient population was ~31 wk (range: 1.7-61; Figure) and median duration of response was 20 wk (range: 1.1-55). Clinical responses occurred in FLT3mut+ patients with -ITD, -D835, and both mutations (ORR: 55%, 17%, and 62%, respectively) as well as in FLT3mut+ patients with or without prior TKI treatment (ORR: 42% vs 56%, respectively). Conclusions: This PD-driven, first-in-human study shows that gilteritinib was well tolerated and generated frequent, prolonged, clinically important responses in FLT3mut+ patients with R/R AML. Antileukemic responses were enriched in FLT3mut+ patients treated at doses that consistently and potently inhibited FLT3 phosphorylation. The survival of these patients appears better than expected for this patient population when treated with standard therapy. Our data suggest that FLT3 inhibition may improve survival in patients with FLT3mut+R/R AML; as such, phase 3 testing of oral gilteritinib 120 mg QD in patients with FLT3mut+R/R AML after first-line therapy is underway (NCT02421939). Disclosures Perl: Astellas US Pharma Inc.: Consultancy, Membership on an entity's Board of Directors or advisory committees; Daichi Sankyo: Consultancy, Membership on an entity's Board of Directors or advisory committees; Asana Biosciences: Consultancy, Membership on an entity's Board of Directors or advisory committees; Arog Pharmaceuticals: Consultancy, Membership on an entity's Board of Directors or advisory committees; Actinium Pharmaceuticals: Consultancy, Membership on an entity's Board of Directors or advisory committees. Altman:Janssen: Other: advisory board; BMS: Membership on an entity's Board of Directors or advisory committees; Spectrum: Other: advisory board; Ariad: Other: advisory board; Seattle Genetics: Other: advisory board; Syros: Other: advisory board. Cortes:ARIAD: Consultancy, Research Funding; BMS: Consultancy, Research Funding; Novartis: Consultancy, Research Funding; Pfizer: Consultancy, Research Funding; Teva: Research Funding. Smith:Astellas: Research Funding. Erba:Jannsen: Consultancy, Research Funding; Millennium Pharmaceuticals, Inc.: Research Funding; Agios: Research Funding; Juno: Research Funding; Incyte: Consultancy, DSMB, Speakers Bureau; Daiichi Sankyo: Consultancy; Ariad: Consultancy; Amgen: Consultancy, Research Funding; Astellas: Research Funding; Gylcomimetics: Other: DSMB; Seattle Genetics: Consultancy, Research Funding; Sunesis: Consultancy; Novartis: Consultancy, Speakers Bureau; Celator: Research Funding; Celgene: Consultancy, Speakers Bureau; Pfizer: Consultancy. Gill:Astellas: Employment. Goldberg:Bristol Myers Squibb, Novartis: Speakers Bureau; Novartis: Consultancy; COTA Inc: Employment; Pfizer: Honoraria; Neostem: Equity Ownership. Jurcic:Astellas: Research Funding. Larson:Astellas: Consultancy, Research Funding; Bristol-Myers Squibb: Consultancy. Liu:Astellas: Employment. Ritchie:Celgene: Consultancy, Other: Travel, Accomodations, Expenses, Speakers Bureau; Incyte: Consultancy, Speakers Bureau; Novartis: Consultancy, Other: Travel, Accommodations, Expenses, Research Funding, Speakers Bureau; Ariad: Speakers Bureau; Pfizer: Consultancy, Research Funding; Astellas Pharma: Research Funding; Bristol-Meyers Squibb: Research Funding; NS Pharma: Research Funding. Schiller:Incyte Corporation: Research Funding. Strickland:Celator: Research Funding; Cyclacel: Research Funding; Karyopharm Therapeutica: Research Funding; GlaxoSmithKline: Research Funding; Baxalta: Consultancy; Boehringer Ingelheim: Consultancy, Research Funding; Ambit: Consultancy; Alexion Pharmaceuticals: Consultancy; Astellas Pharma: Research Funding; CTI Biopharma: Consultancy; Daiichi Sankyo: Consultancy; Sunesis Pharmaceuticals: Consultancy, Research Funding; Abbvie: Research Funding; Sanofi: Research Funding. Wang:Incyte: Speakers Bureau; Immunogen: Research Funding. Stuart:Sunesis: Consultancy, Honoraria, Other: Travel, Accomodations, Expenses, Research Funding; Agios: Research Funding; Incyte: Research Funding; Bayer: Research Funding; Celator: Research Funding; Astellas: Research Funding. Martinelli:Ariad: Consultancy, Speakers Bureau; Pfizer: Consultancy, Speakers Bureau; BMS: Speakers Bureau; Roche: Consultancy, Speakers Bureau; MSD: Consultancy; Genentech: Consultancy; Novartis: Speakers Bureau; Celgene: Consultancy, Speakers Bureau; Amgen: Consultancy, Speakers Bureau. Bahceci:Astellas: Employment. Levis:Millennium: Consultancy, Research Funding; Astellas: Consultancy, Honoraria, Research Funding; Daiichi-Sankyo: Consultancy, Honoraria; Novartis: Consultancy, Honoraria, Research Funding.

Abstract: Introduction: ASP2215 is a highly selective inhibitor of AXL and FMS-like tyrosine kinase-3 (FLT3) receptors. ASP2215 is active against both FLT3 internal tandem duplication [ITD] and D835 mutations. Prior analyses of an open-label, dose-escalation/dose-expansion study in subjects with relapsed or refractory acute myeloid leukemia (R/R AML) show ASP2215 was well tolerated from 20-300 mg and associated with antileukemic activity in FLT3 mutation-positive (FLT3+) patients at ≥80 mg with minimal activity observed in wild-type FLT3 subjects. Here we describe the tolerability and potent activity of ASP2215 in a large cohort of FLT3+ patients. Methods: Patients (≥18 years) with R/R AML were assigned to treatment in dose-escalation cohorts or were randomized to an open dose level in the dose-expansion cohorts. Although FLT3 mutation was not an inclusion criterion, each expanded dose level enrolled ≥10 FLT3+ subjects; 120 mg and 200 mg dose levels were further expanded with ≥40 FLT3+ subjects. Tolerability was assessed by adverse event (AE) monitoring. Response assessment was based on modified Cheson criteria and duration of response and overall survival were calculated using Kaplan-Meier estimates. Results: As of June 19, 2015,215 patients with a median age of 61 yr (range: 21-90) had received ≥1 dose of ASP2215 (safety population). Across the safety population, 65% of subjects received ≥2 prior lines of AML therapy, 29% had a hematopoietic stem cell transplant prior to ASP2215 treatment, and 23% had prior tyrosine kinase inhibitor (TKI) treatment. Approximately 73% of patients were FLT3+, of which 137 had FLT3-ITD mutation, 7 were FLT3-D835+, and 9 had both FLT3-ITD and D835. Treatment-related AEs of all Grades, reported in ≥10% of the safety population were diarrhea (16%), fatigue (13%), and increased AST (11%); 〈 2% of subjects reported a Grade ≥3 QTc prolongation. Activity was assessed in the 133 FLT3+ patients treated with ASP2215 ≥80 mg. Overall response rate (ORR; composite complete [CRc] plus partial remission [PR] ) for all FLT3+ subjects was 55% (Table). Median overall survival for FLT3+ patients receiving ASP2215 ≥80 mg was ~29 weeks (95% CI: 22-32) and was similar for patients who achieved CRc or PR (Figure). Treatment with ≥80 mg ASP2215 was associated with an ORR of 60% in FLT3-ITD subjects; ORR for the other FLT3 populations was 29% (Table). No difference was observed in median ORR of ASP2215 (≥80 mg) in TKI-naïve patients (55%) and patients with prior TKI treatment (55%). Conclusions: ASP2215, a novel AXL/FLT3 TKI, was well tolerated in subjects with R/R AML and demonstrated a strong antileukemic activity in FLT3+ subjects. Importantly, the ASP2215 response rate in these FLT3+ patients was independent of prior TKI treatment. Even in this heavily pretreated population, the survival of R/R FLT3+ AML patients who received ≥80 mg ASP2215 was longer than prior reports of cytotoxic chemotherapy or other FLT3 inhibitors. Table. ASP2215 Response Assessment 80 mg 120 mg 200 mg 300 mg 450 mg Total All FLT3+ Subjects Population, n 12 52 57 10 2 133 CRc, n (%) 5 (42) 25 (48) 28 (49) 3 (30) 0 61 (46) PR, n (%) 3 (25) 3 (6) 3 (5) 3 (30) 0 12 (9) ORR, n (%) 8 (67) 28 (54) 31 (54) 6 (60) 0 73 (55) Subjects with FLT3-ITD Only Population, n 10 46 50 8 0 114 CRc, n (%) 4 (40) 23 (50) 26 (52) 3 (38) 0 56 (49) PR, n (%) 3 (30) 3 (7) 3 (6) 3 (38) 0 12 (11) ORR, n (%) 7 (70) 26 (57) 29 (58) 6 (75) 0 68 (60) Subjects with FLT3-D835 and Subjects with FLT3-ITD and FLT3-D835 Population, n 2 5 5 1 1 14 CRc, n (%) 1 (50) 1 (20) 2 (40) 0 0 4 (29) PR, n (%) 0 0 0 0 0 0 ORR, n (%) 1 (50) 1 (20) 2 (40) 0 0 4 (29) CRc, composite complete remission (complete remission + complete remission with incomplete platelet recovery + complete remission with incomplete hematologic recovery); ORR, overall response rate; PR, partial response. NR, no response. Subjects with non-evaluable data (N=8) were not included in this curve. Figure 1. Overall Survival by Best Overall Response Achieved with ASP2215 ≥80 mg Across All FLT3+ Subjects Figure 1. Overall Survival by Best Overall Response Achieved with ASP2215 ≥80 mg Across All FLT3+ Subjects Disclosures Altman: BMS: Other: Advisory board; Novartis: Other: Advisory board; Spectrum: Other: Advisory board; Ariad: Other: Advisory board; Seattle Genetics: Other: Advisory board; Astellas: Other: Participation in an advisory board December 2013. Off Label Use: ASP2215 is currently under investigation for the treatment of AML and is not yet approved.. Perl:Arog Pharmaceuticals: Consultancy; Asana Biosciences: Consultancy; Actinium Pharmaceuticals: Consultancy; Ambit/Daichi Sankyo: Consultancy; Astellas US Pharma Inc.: Consultancy. Cortes:Pfizer: Consultancy, Research Funding; BerGenBio AS: Research Funding; Novartis: Consultancy, Research Funding; Teva: Research Funding; BMS: Consultancy, Research Funding; Ariad: Consultancy, Research Funding; Astellas: Consultancy, Research Funding; Ambit: Consultancy, Research Funding; Arog: Research Funding; Celator: Research Funding; Jenssen: Consultancy. Levis:Arog: Research Funding; Ambit: Research Funding; Takeda: Research Funding; Astellas: Consultancy. Smith:Plexxikon: Research Funding; Astellas: Research Funding. Claxton:NCI: Research Funding; Medimmune, Inc: Research Funding; Ambit Biosciences Corp: Research Funding; Incyte Corporation: Research Funding; Merck Sharp & Dohme Corp: Research Funding; Astellas Pharma: Research Funding. Erba:Seattle Genetics: Consultancy, Research Funding; Amgen: Consultancy, Research Funding; Ariad: Consultancy; Celgene: Consultancy, Speakers Bureau; Celgene: Consultancy, Speakers Bureau; Incyte: Consultancy, Speakers Bureau; Incyte: Consultancy, Speakers Bureau; Novartis: Consultancy, Speakers Bureau; Novartis: Consultancy, Speakers Bureau; GlycoMimetics: Other: Data Safety and Monitoring Committees ; Jannsen (J & J): Other: Data Safety and Monitoring Committees ; Seattle Genetics: Consultancy, Research Funding; Millennium/Takeda: Research Funding; Amgen: Consultancy, Research Funding; Celator: Research Funding; Millennium/Takeda: Research Funding; Astellas: Research Funding; Sunesis: Consultancy; Celator: Research Funding; Pfizer: Consultancy; Astellas: Research Funding; Daiichi Sankyo: Consultancy; Sunesis: Consultancy; Ariad: Consultancy; Pfizer: Consultancy; GlycoMimetics: Other: Data Safety and Monitoring Committees ; Jannsen (J & J): Other: Data Safety and Monitoring Committees ; Daiichi Sankyo: Consultancy. Gill:Astellas Pharma US, Inc: Employment. Goldberg:Cyclacel: Research Funding; Celetor: Research Funding; Pfizer: Research Funding; Ambit: Research Funding; Astellas: Research Funding. Jurcic:Astellas Pharma: Research Funding. Larson:Astellas: Consultancy, Research Funding. Lui:Astellas Pharma US, Inc: Employment. Ritchie:Incyte: Speakers Bureau; Novartis: Speakers Bureau; Ariad: Other: Advisory Board; Celgene: Speakers Bureau; Onyx: Speakers Bureau. Sargent:Astellas Pharma US, Inc: Employment. Schiller:Sunesis: Honoraria, Research Funding. Strickland:Sunesis Pharmaceuticals: Other: Steering Committee and Advisory Board Participation; Alexion Pharmaceuticals: Other: Advisory Board Particpation; Amgen: Other: Advisory Board Particpation; Daiichi-Sankyo: Other: Advisory Board Particpation; Boehringer-Ingelheim: Other: Advisory Board Particpation. Wang:Immunogen: Research Funding. Stuart:Sunesis: Honoraria, Other: Advisory Board, Research Funding; Astellas Pharma, Inc: Research Funding. Baldus:Novartis: Research Funding. Martinelli:MSD: Consultancy; ARIAD: Consultancy; BMS: Speakers Bureau; Pfizer: Consultancy; Novartis: Speakers Bureau; Roche: Consultancy. Bahceci:Astellas Pharma Global Development: Employment.

Abstract: Introduction: The development of tyrosine kinase inhibitors (TKIs) has markedly improved the prognosis of patients (pts) with chronic myeloid leukemia (CML), with the perception by healthcare professionals that this is now a chronic disease to be managed. However, the need for continuous TKI therapy may result in ongoing toxicities, limits on fertility, and financial hardship. The H. Jean Khoury Cure CML consortium (HJKC3) is a collaborative effort of physicians and researchers at 17 academic centers. The HJKC3-001 2017 Patient Survey sought to define pts' expectations for treatment in CML to serve as a guidepost for future research in this area. Methods: Pts with CML were recruited by HJKC3 physicians, CML advocacy groups, and social media. An online survey platform (Qualtrics®) was used to obtain informed consent and administer the questionnaire. The anonymous survey was designed to gauge priorities for research in CML, understand patient definitions of cure, and elicit patient interest in future directions for CML therapy. Patient demographic and health characteristics were also collected. The data were analyzed using descriptive statistics. Results: Of the 458 pts who completed the survey, the median age of respondents was 54 years (range 18-81); 88% of pts identified as non-Hispanic white, 2% as non-Hispanic black, 2% as non-Hispanic Asian, 4% as Hispanic, and 4% other. Patients rated their overall health as poor (4%), fair (18%), good (40%), very good (28%) and excellent (9%). All but one respondent said that more research was needed for CML, with pts indicating their preferences for where they considered the need was greatest (Table 1). Overwhelmingly, 94% of respondents considered cure in CML as not taking any more pills. All but three respondents had received treatment with a TKI, with 26% (n=119) of pts having previously stopped their TKI medication for at least one month. When presented with the possibility of stopping all future treatment for CML with additional treatment, 97% of pts were willing to add another oral medication to their TKI while 89% of pts would accept intravenous treatment in addition to a TKIs. Half of the pts had discussed treatment discontinuation with their physician, with 45% considering this option in an attempt at treatment-free-remission. Of the pts that stopped taking their TKIs for at least one month, 65% did so because of side effects and another 10% because of cost. Conclusion: This survey demonstrates that pts do not consider disease control with life-long oral medication as cure; rather, cure requires the absence of treatment. Overwhelmingly, pts indicated the importance of continuing CML research with an ultimate goal of treatment-free cure. The advent of oral TKIs has been a tremendous success for pts with this disease. Nevertheless, it remains a source of disruption in pts' lives, particularly through side effects and costs. The HJKC3 was initiated with the goal of curing CML. Disclosures Atallah: Novartis: Consultancy; Jazz: Consultancy; Pfizer: Consultancy; BMS: Consultancy; Abbvie: Consultancy. Mauro:Bristol-Myers Squibb: Consultancy; Pfizer: Consultancy; Takeda: Consultancy; Novartis: Consultancy, Research Funding. Goldberg:COTA Inc.: Employment, Equity Ownership. Cortes:Daiichi Sankyo: Consultancy, Research Funding; Novartis: Consultancy, Research Funding; Pfizer: Consultancy, Research Funding; Astellas Pharma: Consultancy, Research Funding; Arog: Research Funding. Deininger:Pfizer: Consultancy, Membership on an entity's Board of Directors or advisory committees; Blueprint: Consultancy. Druker:ARIAD: Research Funding; Third Coast Therapeutics: Membership on an entity's Board of Directors or advisory committees; Patient True Talk: Consultancy; Amgen: Membership on an entity's Board of Directors or advisory committees; MolecularMD: Consultancy, Equity Ownership, Membership on an entity's Board of Directors or advisory committees; Novartis Pharmaceuticals: Research Funding; Henry Stewart Talks: Patents & Royalties; Leukemia & Lymphoma Society: Membership on an entity's Board of Directors or advisory committees, Research Funding; McGraw Hill: Patents & Royalties; Aptose Therapeutics: Consultancy, Equity Ownership, Membership on an entity's Board of Directors or advisory committees; Cepheid: Consultancy, Membership on an entity's Board of Directors or advisory committees; GRAIL: Consultancy, Membership on an entity's Board of Directors or advisory committees; Bristol-Meyers Squibb: Research Funding; Oregon Health & Science University: Patents & Royalties; Gilead Sciences: Consultancy, Membership on an entity's Board of Directors or advisory committees; Monojul: Consultancy; Vivid Biosciences: Membership on an entity's Board of Directors or advisory committees; Blueprint Medicines: Consultancy, Equity Ownership, Membership on an entity's Board of Directors or advisory committees; Millipore: Patents & Royalties; Fred Hutchinson Cancer Research Center: Research Funding; Beta Cat: Membership on an entity's Board of Directors or advisory committees; ALLCRON: Consultancy, Membership on an entity's Board of Directors or advisory committees; Aileron Therapeutics: Consultancy; Celgene: Consultancy. Larson:Novartis: Consultancy, Research Funding; Ariad/Takeda: Consultancy, Research Funding; Pfizer: Consultancy, Research Funding; BristolMyers Squibb: Consultancy, Research Funding. Lipton:Bristol-Myers Squibb: Consultancy, Research Funding; ARIAD: Consultancy, Research Funding; Novartis: Consultancy, Research Funding; Pfizer: Consultancy, Research Funding. Ritchie:Incyte: Consultancy, Speakers Bureau; NS Pharma: Research Funding; Bristol-Myers Squibb: Research Funding; Astellas Pharma: Research Funding; ARIAD Pharmaceuticals: Speakers Bureau; Novartis: Consultancy, Other: Travel, Accommodations, Expenses, Research Funding, Speakers Bureau; Pfizer: Consultancy, Research Funding; Celgene: Consultancy, Other: Travel, Accommodations, Expenses, Speakers Bureau. Shah:Bristol-Myers Squibb: Research Funding; ARIAD: Research Funding. Sweet:Celgene: Honoraria, Speakers Bureau; Jazz: Speakers Bureau; Celgene: Honoraria, Speakers Bureau; Agios: Consultancy; Phizer: Consultancy; Astellas: Consultancy; Astellas: Consultancy; Jazz: Speakers Bureau; Phizer: Consultancy; BMS: Honoraria; Novartis: Consultancy, Honoraria, Speakers Bureau; Agios: Consultancy; Novartis: Consultancy, Honoraria, Speakers Bureau; BMS: Honoraria.

Abstract: Abstract 2505 The 11q23 MLL gene is commonly rearranged by a diverse range of chromosomal translocations in B-progenitor acute lymphoblastic leukemia (ALL), acute myeloid leukemia and, in particular, in infants less than one year of age. These genomic lesions, commonly identified as MLL (MLL-R) rearrangements via fluorescence in situ hybridization (FISH), are highly variable but consistently encode for MLL fusion proteins that function as transcriptional deregulators of HOX genes, leading to sustained high expression of MEIS1 and many HOXA gene family members. The incidence of MLL-R in T-ALL has not been studied extensively, but has been reported to occur in approximately 1–3% of cases. Using the Affymetrix U133 Plus 2.0 microarray to assess gene expression, we hypothesized that over-expression of MEIS1, HOXA9 and HOXA10 could be use to screen for MLL-R in a cohort of 214 T-ALL cases from recent Children's Oncology Group (COG) studies. Fifty cases and 164 cases, respectively, were obtained from COG studies P9000/9404 and AALL03B1/AALL08B1/AALL0434; all cases underwent nucleic acid extraction, hybridization and profiling as previously described. Following RMA normalization and ROSE analysis for expression outliers, 27 cases (12.6%) showed ≥4-fold increase over the median expression of MEIS1 and 58 cases (27.0%) had ≥4-fold increase over the median expression of HOXA9/10. Interestingly, of the 27 cases with high MEIS1, 25 (92.6%) also had high expression of HOXA9/10 (Fisher's exact test P 〈 0.001). This signature of elevated expression of both MEIS1 and HOXA9/10 is a hallmark of MLL-R in pediatric ALL. For the 50 unselected P9000/9404 cases, 6 cases were identified to be possibly MLL-R based on elevated expression of both MEIS1 and HOXA9/10; however, no specific testing for MLL-R was required on those studies. Twenty-one of the 164 AALL0434 cases profiled demonstrated MEIS1 and HOXA9/10 over-expression; 7 cases had MLL-R confirmed by cytogenetics and/or FISH: MLL-AF6 (x2), del3'MLL, t(11;19)(q23;p13.3) (MLL-ENL; x3) and der(11)t(11;17)(q23;q21). Cases with HOXA9/10 over-expression alone showed one case each for t(11;19)(q23;p13.3), MLL-AF6 and a del3'MLL lesion, and one case without MEIS1 or HOXA9/10 over-expression showed a del3'MLL by FISH. To screen for MLL-R in this study, we also utilized the NG2 mAb, which has been previously shown to identify MLL-R in leukemic samples. Six cases with MLL translocations confirmed by cytogenetics showed detectable NG2 expression by flow cytometry. In our study of 214 T-ALL samples, 30 (14.0%) cases with MEIS1 and/or HOXA9/10 overexpression co-localized with 11 (5.1%) cases confirmed by cytogenetics to have MLL-R. Because of the higher-than-expected number of MLL-R in our series, we reviewed the cytogenetic information provided for 1088 patients currently enrolled onto AALL0434; MLL FISH was performed in 574 (53%) cases, in which 38 (6.62%) MLL-R cases were identified. Our results suggest that MLL-R is more common in T-ALL than previously appreciated. We propose that further confirmatory cytogenetic and molecular testing will identify additional cryptic/unusual translocations, and that MLL-Rs differentially activate MEIS1 and HOXA signaling pathways in T-ALL. Disclosures: No relevant conflicts of interest to declare.

Abstract: Inducing cell death by the sphingolipid ceramide is a potential anticancer strategy, but the underlying mechanisms remain poorly defined. In this study, triggering an accumulation of ceramide in acute myeloid leukemia (AML) cells by inhibition of sphingosine kinase induced an apoptotic integrated stress response (ISR) through protein kinase R–mediated activation of the master transcription factor ATF4. This effect led to transcription of the BH3-only protein Noxa and degradation of the prosurvival Mcl-1 protein on which AML cells are highly dependent for survival. Targeting this novel ISR pathway, in combination with the Bcl-2 inhibitor venetoclax, synergistically killed primary AML blasts, including those with venetoclax-resistant mutations, as well as immunophenotypic leukemic stem cells, and reduced leukemic engraftment in patient-derived AML xenografts. Collectively, these findings provide mechanistic insight into the anticancer effects of ceramide and preclinical evidence for new approaches to augment Bcl-2 inhibition in the therapy of AML and other cancers with high Mcl-1 dependency.

Abstract: The somatic hotspot mutation SF3B1K700E is characteristically found in myelodysplastic syndrome with ring sideroblasts (MDS-RS) and frequently occurs as an isolated mutation. However, our understanding of how this mutation drives MDS pathogenesis remains limited. To explore the downstream consequences of the SF3B1K700E mutation and its role in disease pathogenesis, we generated a panel of isogenic SF3B1K700E and SF3B1WT induced pluripotent stem cell (iPSC) lines from 3 MDS-RS patients with isolated SF3B1K700E mutation (3 SF3B1K700E and 3 SF3B1WT lines per patient, total 18). Upon hematopoietic differentiation, SF3B1K700E cells exhibited lower growth and colony-forming ability, compared to SF3B1WTcells, recapitulating hallmark phenotypes of MDS cells. To investigate the effects of the SF3B1K700E mutation on the transcriptome and chromatin landscape, we performed RNA- and ATAC- sequencing in purified CD34+/CD45+ hematopoietic stem/progenitor cells (HSPCs) derived from the panel of the 18 isogenic SF3B1K700E and SF3B1WT iPSC lines. Principal component analysis (PCA) and hierarchical clustering based on gene expression grouped the iPSC lines primarily by genotype (SF3B1K700E vs SF3B1WT) and secondarily by genetic background. To assess the impact of the SF3B1K700E mutation at the exon, transcript and gene level, we developed an analytical framework integrating differential splicing with differential transcript usage and differential gene expression analyses. We thus discovered 59 splicing events linked to 34 genes (most statistically significant events that also mapped to differentially used transcripts and differentially expressed genes). This SF3B1K700E splicing signature includes genes previously reported as mis-spliced in SF3B1K700E cells (e.g BRD9, ABCB7), as well as novel genes. We tested this signature against a published dataset of primary MDS patient samples (Pellagatti et al.). PCA based on the inclusion level of the splicing events of our signature separated SF3B1-mutated MDS patients from patients without splicing factor mutations (SF-WT) or healthy individuals. Furthermore, it identified one patient erroneously annotated as SF-WT that clustered together with the SF3B1-mutated patients, who had a, previously overlooked, 6bp in-frame deletion spanning the K700E hotspot. By comparing the chromatin accessibility profiles of SF3B1K700E and SF3B1WT iPSC-HSPCs to those defined in primary human cell types along the hematopoietic hierarchy (Corces et al.), we found that the chromatin landscape of SF3B1K700E HSPCs resembled more this of megakaryocyte-erythroid progenitor cells (MEPs) and erythroid cells, whereas that of SF3B1WT HSPCs resembled more granulocyte-monocyte progenitors (GMPs) and monocytes. This finding may underlie the more prominent involvement of the erythroid lineage in the pathology and clinical presentation of MDS-RS. To interrogate transcriptional programs in SF3B1K700E mutant cells, we performed transcription factor (TF) motif enrichment analysis. Motifs enriched in ATAC-Seq peaks more accessible in SF3B1K700E cells that were linked to genes upregulated in SF3B1K700E cells, included motifs of several TFs with known roles in hematopoiesis (GATA, ETS, STAT, AP-1). Unexpectedly, motifs of the TEAD TFs were also enriched. The TEAD family of TFs are best known as effectors of the Hippo signaling pathway, with important roles in various biological processes and malignancies, albeit no clear links to adult hematopoiesis or hematologic disease. TEAD2 and TEAD4 were upregulated in SF3B1-mutant, compared to the WT, iPSC-HSPCs and TEAD transcriptional activity, measured with a luciferase reporter construct, was higher in SF3B1K700E, compared to SF3B1WTiPSC-HSPCs. We did not find expression or activation of YAP or TAZ, which bind to DNA as a complex with TEAD upon Hippo pathway activation. These results support a Hippo-independent increase of TEAD expression and activity in SF3B1K700E cells. In summary, we generated a panel of isogenic patient-derived iPSCs that allowed us to comprehensively characterize the transcriptome and chromatin landscape of SF3B1K700E HSPCs in an isogenic system, derive a splicing signature of SF3B1K700E and identify the TEAD TF as a new transcriptional regulator of SF3B1K700Emutant HSPCs. G. Asimomitis, A.G. Deslauriers: shared 1 st authorship E. Papaemmanuil, E.P. Papapetrou: shared senior authorship Disclosures Deslauriers: Novo Nordisk A/S: Current Employment. Hellström-Lindberg: Celgene: Research Funding. Papaemmanuil: Isabl Technologies: Divested equity in a private or publicly-traded company in the past 24 months; Kyowa Hakko Kirin Pharma: Consultancy.

Abstract: SF3B1K700E is the most frequent mutation in myelodysplastic syndrome (MDS), but the mechanisms by which it drives MDS pathogenesis remain unclear. We derived a panel of 18 genetically matched SF3B1K700E- and SF3B1WT-induced pluripotent stem cell (iPSC) lines from patients with MDS with ring sideroblasts (MDS-RS) harboring isolated SF3B1K700E mutations and performed RNA and ATAC sequencing in purified CD34+/CD45+ hematopoietic stem/progenitor cells (HSPCs) derived from them. We developed a novel computational framework integrating splicing with transcript usage and gene expression analyses and derived a SF3B1K700E splicing signature consisting of 59 splicing events linked to 34 genes, which associates with the SF3B1 mutational status of primary MDS patient cells. The chromatin landscape of SF3B1K700E HSPCs showed increased priming toward the megakaryocyte- erythroid lineage. Transcription factor motifs enriched in chromatin regions more accessible in SF3B1K700E cells included, unexpectedly, motifs of the TEA domain (TEAD) transcription factor family. TEAD expression and transcriptional activity were upregulated in SF3B1-mutant iPSC-HSPCs, in support of a Hippo pathway-independent role of TEAD as a potential novel transcriptional regulator of SF3B1K700E cells. This study provides a comprehensive characterization of the transcriptional and chromatin landscape of SF3B1K700E HSPCs and nominates novel mis-spliced genes and transcriptional programs with putative roles in MDS-RS disease biology.