A single oncogenic enhancer rearrangement causes concomitant EVI1 and GATA2 deregulation in leukemia.

Chromosomal rearrangements without gene fusions have been implicated in leukemogenesis by causing deregulation of proto-oncogenes via relocation of cryptic regulatory DNA elements. AML with inv(3)/t(3;3) is associated with aberrant expression of the stem-cell regulator EVI1. Applying functional genomics and genome-engineering, we demonstrate that both 3q rearrangements reposition a distal GATA2 enhancer to ectopically activate EVI1 and simultaneously confer GATA2 functional haploinsufficiency, previously identified as the cause of sporadic familial AML/MDS and MonoMac/Emberger syndromes. Genomic excision of the ectopic enhancer restored EVI1 silencing and led to growth inhibition and differentiation of AML cells, which could be replicated by pharmacologic BET inhibition. Our data show that structural rearrangements involving the chromosomal repositioning of a single enhancer can cause deregulation of two unrelated distal genes, with cancer as the outcome.

Genetic risk classification for adults with AML receiving less-intensive therapies: the 2024 ELN recommendations.

The European LeukemiaNet (ELN) genetic risk classifications were developed based on data from younger adults receiving intensive chemotherapy. Emerging analyses from patients receiving less-intensive therapies prompted a proposal for an ELN genetic risk classification specifically for this patient population.

Clinical networking results in continuous improvement of the outcome of patients with acute promyelocytic leukemia.

ABSTRACT: The introduction of all-trans retinoic acid combined with anthracyclines has significantly improved the outcomes for patients diagnosed with acute promyelocytic leukemia (APL), and this strategy remains the standard of care in countries in which arsenic trioxide is not affordable. However, data from national registries and real-world databases indicate that low- and middle-income countries (LMIC) still face disappointing results, mainly because of high induction mortality and suboptimal management of complications. The American Society of Hematology established the International Consortium on Acute Leukemias (ICAL) to address this challenge through international clinical networking. Here, we present the findings from the International Consortium on Acute Promyelocytic Leukemia study involving 806 patients with APL recruited from 2005 to 2020 in Brazil, Chile, Paraguay, Peru, and Uruguay. The induction mortality rate has notably decreased to 14.6% compared with the pre-ICAL rate of 32%. Multivariable logistic regression analysis revealed as factors associated with induction death: age of ≥40 years, Eastern Cooperative Oncology Group performance status score of 3, high-risk status based on the Programa Español de Tratamiento en Hematologia/Gruppo Italiano Malattie EMatologiche dell'Adulto classification, albumin level of ≤3.5 g/dL, bcr3 PML/RARA isoform, the interval between presenting symptoms to diagnosis exceeding 48 hours, and the occurrence of central nervous system and pulmonary bleeding. With a median follow-up of 53 months, the estimated 4-year overall survival rate is 81%, the 4-year disease-free survival rate is 80%, and the 4-year cumulative incidence of relapse rate is 15%. These results parallel those observed in studies conducted in high-income countries, highlighting the long-term effectiveness of developing clinical networks to improve clinical care and infrastructure in LMIC.

Diagnosis and management of AML in adults: 2022 recommendations from an international expert panel on behalf of the ELN.

The 2010 and 2017 editions of the European LeukemiaNet (ELN) recommendations for diagnosis and management of acute myeloid leukemia (AML) in adults are widely recognized among physicians and investigators. There have been major advances in our understanding of AML, including new knowledge about the molecular pathogenesis of AML, leading to an update of the disease classification, technological progress in genomic diagnostics and assessment of measurable residual disease, and the successful development of new therapeutic agents, such as FLT3, IDH1, IDH2, and BCL2 inhibitors. These advances have prompted this update that includes a revised ELN genetic risk classification, revised response criteria, and treatment recommendations.

Molecular characterization of mutant TP53 acute myeloid leukemia and high-risk myelodysplastic syndrome.

Substantial heterogeneity within mutant TP53 acute myeloid leukemia (AML) and myelodysplastic syndrome with excess of blast (MDS-EB) precludes the exact assessment of prognostic impact for individual patients. We performed in-depth clinical and molecular analysis of mutant TP53 AML and MDS-EB to dissect the molecular characteristics in detail and determine its impact on survival. We performed next-generation sequencing on 2200 AML/MDS-EB specimens and assessed the TP53 mutant allelic status (mono- or bi-allelic), the number of TP53 mutations, mutant TP53 clone size, concurrent mutations, cytogenetics, and mutant TP53 molecular minimal residual disease and studied the associations of these characteristics with overall survival. TP53 mutations were detected in 230 (10.5%) patients with AML/MDS-EB with a median variant allele frequency of 47%. Bi-allelic mutant TP53 status was observed in 174 (76%) patients. Multiple TP53 mutations were found in 49 (21%) patients. Concurrent mutations were detected in 113 (49%) patients. No significant difference in any of the aforementioned molecular characteristics of mutant TP53 was detected between AML and MDS-EB. Patients with mutant TP53 have a poor outcome (2-year overall survival, 12.8%); however, no survival difference between AML and MDS-EB was observed. Importantly, none of the molecular characteristics were significantly associated with survival in mutant TP53 AML/MDS-EB. In most patients, TP53 mutations remained detectable in complete remission by deep sequencing (73%). Detection of residual mutant TP53 was not associated with survival. Mutant TP53 AML and MDS-EB do not differ with respect to molecular characteristics and survival. Therefore, mutant TP53 AML/MDS-EB should be considered a distinct molecular disease entity.

International Consensus Classification of Myeloid Neoplasms and Acute Leukemias: integrating morphologic, clinical, and genomic data.

The classification of myeloid neoplasms and acute leukemias was last updated in 2016 within a collaboration between the World Health Organization (WHO), the Society for Hematopathology, and the European Association for Haematopathology. This collaboration was primarily based on input from a clinical advisory committees (CACs) composed of pathologists, hematologists, oncologists, geneticists, and bioinformaticians from around the world. The recent advances in our understanding of the biology of hematologic malignancies, the experience with the use of the 2016 WHO classification in clinical practice, and the results of clinical trials have indicated the need for further revising and updating the classification. As a continuation of this CAC-based process, the authors, a group with expertise in the clinical, pathologic, and genetic aspects of these disorders, developed the International Consensus Classification (ICC) of myeloid neoplasms and acute leukemias. Using a multiparameter approach, the main objective of the consensus process was the definition of real disease entities, including the introduction of new entities and refined criteria for existing diagnostic categories, based on accumulated data. The ICC is aimed at facilitating diagnosis and prognostication of these neoplasms, improving treatment of affected patients, and allowing the design of innovative clinical trials.

Hematopoietic cell transplantation for older acute myeloid leukemia patients in first complete remission: results of a randomized phase III study.

Given the selection of elderly patients with AML in first complete remission (CR1) the advantage of consolidation with allogeneic hematopoietic cell transplantation (HCT) over chemotherapy is still unclear. Newly diagnosed AML patients in CR1 aged 60-75 years were registered and a donor search initiated. After one consolidation cycle, patients with a matched donor were randomized to HCT with fludarabine/lowdose total body irradiation and cyclosporine/mycophenolate mofetil immunosuppression or conventional non-HCT. Primary outcome was restricted mean leukemia-free survival (RM-LFS) up to five years. Between 2010 and 2017, 245 patients (median age 67 years) were registered at CR1. After one consolidation, 26.9% of patients failed inclusion criteria. Of the 179 (73%) patients still on study, 75.4% had an HLA identical donor. Ten ineligible patients were excluded, and 125 randomized to HCT (n=83) or non-HCT (n=42). The primary outcome RM-LFS up to 5 years was 24.5 months (95%CI:18.9-30.1) in the HCT and 15.6 months (95%CI:10.4-20.8) in the non-HCT arm (p=0.022) due to a decrease in cumulative relapse incidence from 91.1 (95%CI:80.7-100.0) after non-HCT to 37.8 (95%CI:27.2-48.4)% after HCT (p.

Clinical significance of mitochondrial DNA content in acute promyelocytic leukaemia.

Although a growing body of evidence demonstrates that altered mtDNA content (mtDNAc) has clinical implications in several types of solid tumours, its prognostic relevance in acute promyelocytic leukaemia (APL) patients remains largely unknown. Here, we show that patients with higher-than-normal mtDNAc had better outcomes regardless of tumour burden. These results were more evident in patients with low-risk of relapse. The multivariate Cox proportional hazard model demonstrated that high mtDNAc was independently associated with a decreased cumulative incidence of relapse. Altogether, our data highlights the possible role of mitochondrial metabolism in APL patients treated with ATRA.

Ivosidenib or enasidenib combined with intensive chemotherapy in patients with newly diagnosed AML: a phase 1 study.

Ivosidenib (AG-120) and enasidenib (AG-221) are targeted oral inhibitors of the mutant isocitrate dehydrogenase (mIDH) 1 and 2 enzymes, respectively. Given their effectiveness as single agents in mIDH1/2 relapsed or refractory acute myeloid leukemia (AML), this phase 1 study evaluated the safety and efficacy of ivosidenib or enasidenib combined with intensive chemotherapy in patients with newly diagnosed mIDH1/2 AML. Ivosidenib 500 mg once daily and enasidenib 100 mg once daily were well tolerated in this setting, with safety profiles generally consistent with those of induction and consolidation chemotherapy alone. The frequency of IDH differentiation syndrome was low, as expected given the concurrent administration of cytotoxic chemotherapy. In patients receiving ivosidenib, the frequency and grades of QT interval prolongation were similar to those observed with ivosidenib monotherapy. Increases in total bilirubin were more frequently observed in patients treated with enasidenib, consistent with this inhibitor's known potential to inhibit UGT1A1, but did not appear to have significant clinical consequences. In patients receiving ivosidenib (n = 60) or enasidenib (n = 91), end-of-induction complete remission (CR) rates were 55% and 47%, respectively, and CR/CR with incomplete neutrophil or platelet recovery (CR/CRi/CRp) rates were 72% and 63%, respectively. In patients with a best overall response of CR/CRi/CRp, 16/41 (39%) receiving ivosidenib had IDH1 mutation clearance and 15/64 (23%) receiving enasidenib had IDH2 mutation clearance by digital polymerase chain reaction; furthermore, 16/20 (80%) and 10/16 (63%), respectively, became negative for measurable residual disease by multiparameter flow cytometry. This trial was registered at www.clinicaltrials.gov as #NCT02632708.

Flotetuzumab as salvage immunotherapy for refractory acute myeloid leukemia.

Approximately 50% of acute myeloid leukemia (AML) patients do not respond to induction therapy (primary induction failure [PIF]) or relapse after <6 months (early relapse [ER]). We have recently shown an association between an immune-infiltrated tumor microenvironment (TME) and resistance to cytarabine-based chemotherapy but responsiveness to flotetuzumab, a bispecific DART antibody-based molecule to CD3ε and CD123. This paper reports the results of a multicenter, open-label, phase 1/2 study of flotetuzumab in 88 adults with relapsed/refractory AML: 42 in a dose-finding segment and 46 at the recommended phase 2 dose (RP2D) of 500 ng/kg per day. The most frequent adverse events were infusion-related reactions (IRRs)/cytokine release syndrome (CRS), largely grade 1-2. Stepwise dosing during week 1, pretreatment dexamethasone, prompt use of tocilizumab, and temporary dose reductions/interruptions successfully prevented severe IRR/CRS. Clinical benefit accrued to PIF/ER patients showing an immune-infiltrated TME. Among 30 PIF/ER patients treated at the RP2D, the complete remission (CR)/CR with partial hematological recovery (CRh) rate was 26.7%, with an overall response rate (CR/CRh/CR with incomplete hematological recovery) of 30.0%. In PIF/ER patients who achieved CR/CRh, median overall survival was 10.2 months (range, 1.87-27.27), with 6- and 12-month survival rates of 75% (95% confidence interval [CI], 0.450-1.05) and 50% (95% CI, 0.154-0.846). Bone marrow transcriptomic analysis showed that a parsimonious 10-gene signature predicted CRs to flotetuzumab (area under the receiver operating characteristic curve = 0.904 vs 0.672 for the European LeukemiaNet classifier). Flotetuzumab represents an innovative experimental approach associated with acceptable safety and encouraging evidence of activity in PIF/ER patients. This trial was registered at www.clinicaltrials.gov as #NCT02152956.

A 17-gene stemness score for rapid determination of risk in acute leukaemia.

Refractoriness to induction chemotherapy and relapse after achievement of remission are the main obstacles to cure in acute myeloid leukaemia (AML). After standard induction chemotherapy, patients are assigned to different post-remission strategies on the basis of cytogenetic and molecular abnormalities that broadly define adverse, intermediate and favourable risk categories. However, some patients do not respond to induction therapy and another subset will eventually relapse despite the lack of adverse risk factors. There is an urgent need for better biomarkers to identify these high-risk patients before starting induction chemotherapy, to enable testing of alternative induction strategies in clinical trials. The high rate of relapse in AML has been attributed to the persistence of leukaemia stem cells (LSCs), which possess a number of stem cell properties, including quiescence, that are linked to therapy resistance. Here, to develop predictive and/or prognostic biomarkers related to stemness, we generated a list of genes that are differentially expressed between 138 LSC+ and 89 LSC- cell fractions from 78 AML patients validated by xenotransplantation. To extract the core transcriptional components of stemness relevant to clinical outcomes, we performed sparse regression analysis of LSC gene expression against survival in a large training cohort, generating a 17-gene LSC score (LSC17). The LSC17 score was highly prognostic in five independent cohorts comprising patients of diverse AML subtypes (n = 908) and contributed greatly to accurate prediction of initial therapy resistance. Patients with high LSC17 scores had poor outcomes with current treatments including allogeneic stem cell transplantation. The LSC17 score provides clinicians with a rapid and powerful tool to identify AML patients who do not benefit from standard therapy and who should be enrolled in trials evaluating novel upfront or post-remission strategies.

Molecular Minimal Residual Disease in Acute Myeloid Leukemia.

BACKGROUND: Patients with acute myeloid leukemia (AML) often reach complete remission, but relapse rates remain high. Next-generation sequencing enables the detection of molecular minimal residual disease in virtually every patient, but its clinical value for the prediction of relapse has yet to be established. METHODS: We conducted a study involving patients 18 to 65 years of age who had newly diagnosed AML. Targeted next-generation sequencing was carried out at diagnosis and after induction therapy (during complete remission). End points were 4-year rates of relapse, relapse-free survival, and overall survival. RESULTS: At least one mutation was detected in 430 out of 482 patients (89.2%). Mutations persisted in 51.4% of those patients during complete remission and were present at various allele frequencies (range, 0.02 to 47%). The detection of persistent DTA mutations (i.e., mutations in DNMT3A, TET2, and ASXL1), which are often present in persons with age-related clonal hematopoiesis, was not correlated with an increased relapse rate. After the exclusion of persistent DTA mutations, the detection of molecular minimal residual disease was associated with a significantly higher relapse rate than no detection (55.4% vs. 31.9%; hazard ratio, 2.14; P<0.001), as well as with lower rates of relapse-free survival (36.6% vs. 58.1%; hazard ratio for relapse or death, 1.92; P<0.001) and overall survival (41.9% vs. 66.1%; hazard ratio for death, 2.06; P<0.001). Multivariate analysis confirmed that the persistence of non-DTA mutations during complete remission conferred significant independent prognostic value with respect to the rates of relapse (hazard ratio, 1.89; P<0.001), relapse-free survival (hazard ratio for relapse or death, 1.64; P=0.001), and overall survival (hazard ratio for death, 1.64; P=0.003). A comparison of sequencing with flow cytometry for the detection of residual disease showed that sequencing had significant additive prognostic value. CONCLUSIONS: Among patients with AML, the detection of molecular minimal residual disease during complete remission had significant independent prognostic value with respect to relapse and survival rates, but the detection of persistent mutations that are associated with clonal hematopoiesis did not have such prognostic value within a 4-year time frame. (Funded by the Queen Wilhelmina Fund Foundation of the Dutch Cancer Society and others.).

Management of acute promyelocytic leukemia: updated recommendations from an expert panel of the European LeukemiaNet.

Since the comprehensive recommendations for the management of acute promyelocytic leukemia (APL) reported in 2009, several studies have provided important insights, particularly regarding the role of arsenic trioxide (ATO) in frontline therapy. Ten years later, a European LeukemiaNet expert panel has reviewed the recent advances in the management of APL in both frontline and relapse settings in order to develop updated evidence- and expert opinion-based recommendations on the management of this disease. Together with providing current indications on genetic diagnosis, modern risk-adapted frontline therapy, and salvage treatment, the review contains specific recommendations for the identification and management of the most important complications such as the bleeding disorder APL differentiation syndrome, QT prolongation, and other all-trans retinoic acid- and ATO-related toxicities, as well as recommendations for molecular assessment of the response to treatment. Finally, the approach to special situations is also discussed, including management of APL in children, elderly patients, and pregnant women. The most important challenges remaining in APL include early death, which still occurs before and during induction therapy, and optimizing treatment in patients with high-risk disease.

Azacitidine maintenance after intensive chemotherapy improves DFS in older AML patients.

The prevention of relapse is the major therapeutic challenge in older patients with acute myeloid leukemia (AML) who have obtained a complete remission (CR) on intensive chemotherapy. In this randomized phase 3 study (HOVON97) in older patients (≥60 years) with AML or myelodysplastic syndrome with refractory anemia with excess of blasts, in CR/CR with incomplete hematologic recovery (CRi) after at least 2 cycles of intensive chemotherapy, we assessed the value of azacitidine as postremission therapy with respect to disease-free survival (DFS; primary end point) and overall survival (OS; secondary end point). In total, 116 eligible patients were randomly (1:1) assigned to either observation (N = 60) or azacitidine maintenance (N = 56; 50 mg/m2, subcutaneously, days 1-5, every 4 weeks) until relapse, for a maximum of 12 cycles. Fifty-five patients received at least 1 cycle of azacitidine, 46 at least 4 cycles, and 35 at least 12 cycles. The maintenance treatment with azacitidine was feasible. DFS was significantly better for the azacitidine treatment group (logrank; P = .04), as well as after adjustment for poor-risk cytogenetic abnormalities at diagnosis and platelet count at randomization (as surrogate for CR vs CRi; Cox regression; hazard ratio, 0.62; 95% confidence interval, 0.41-0.95; P = .026). The 12-month DFS was estimated at 64% for the azacitidine group and 42% for the control group. OS did not differ between treatment groups, with and without censoring for allogeneic hematopoietic cell transplantation. Rescue treatment was used more often in the observation group (n = 32) than in the azacitidine maintenance group (n = 9). We conclude that azacitidine maintenance after CR/CRi after intensive chemotherapy is feasible and significantly improves DFS. The study is registered with The Netherlands Trial Registry (NTR1810) and EudraCT (2008-001290-15).

Genomic landscape and clonal evolution of acute myeloid leukemia with t(8;21): an international study on 331 patients.

Acute myeloid leukemia with t(8;21)(q22;q22) is characterized by considerable clinical and biological heterogeneity leading to relapse in up to 40% of patients. We sequenced coding regions or hotspot areas of 66 recurrently mutated genes in a cohort of 331 t(8;21) patients. At least 1 mutation, in addition to t(8;21), was identified in 95%, with a mean of 2.2 driver mutations per patient. Recurrent mutations occurred in genes related to RAS/RTK signaling (63.4%), epigenetic regulators (45%), cohesin complex (13.6%), MYC signaling (10.3%), and the spliceosome (7.9%). Our study identified mutations in previously unappreciated genes: GIGYF2, DHX15, and G2E3 Based on high mutant levels, pairwise precedence, and stability at relapse, epigenetic regulator mutations were likely to occur before signaling mutations. In 34% of RAS/RTKmutated patients, we identified multiple mutations in the same pathway. Deep sequencing (∼42 000×) of 126 mutations in 62 complete remission samples from 56 patients identified 16 persisting mutations in 12 patients, of whom 5 lacked RUNX1-RUNX1T1 in quantitative polymerase chain reaction analysis. KIT high mutations defined by a mutant level ≥25% were associated with inferior relapse-free survival (hazard ratio, 1.96; 95% confidence interval, 1.22-3.15; P = .005). Together with age and white blood cell counts, JAK2, FLT3-internal tandem duplicationhigh, and KIT high mutations were identified as significant prognostic factors for overall survival in multivariate analysis. Whole-exome sequencing was performed on 19 paired diagnosis, remission, and relapse trios. Exome-wide analysis showed an average of 16 mutations with signs of substantial clonal evolution. Based on the resemblance of diagnosis and relapse pairs, genetically stable (n = 13) and unstable (n = 6) subgroups could be identified.

Combining gene mutation with gene expression analysis improves outcome prediction in acute promyelocytic leukemia.

By combining the analysis of mutations with aberrant expression of genes previously related to poorer prognosis in both acute promyelocytic leukemia (APL) and acute myeloid leukemia, we arrived at an integrative score in APL (ISAPL) and demonstrated its relationship with clinical outcomes of patients treated with all-trans retinoic acid (ATRA) in combination with anthracycline-based chemotherapy. Based on fms-like tyrosine kinase-3-internal tandem duplication mutational status; the ΔNp73/TAp73 expression ratio; and ID1, BAALC, ERG, and KMT2E gene expression levels, we modeled ISAPL in 159 patients (median ISAPL score, 3; range, 0-10). ISAPL modeling identified 2 distinct groups of patients, with significant differences in early mortality (P < .001), remission (P = .004), overall survival (P < .001), cumulative incidence of relapse (P = .028), disease-free survival (P = .03), and event-free survival (P < .001). These data were internally validated by using a bootstrap resampling procedure. At least for patients treated with ATRA and anthracycline-based chemotherapy, ISAPL modeling may identify those who need to be treated differently to maximize their chances for a cure.

Effects of lenalidomide on the bone marrow microenvironment in acute myeloid leukemia: Translational analysis of the HOVON103 AML/SAKK30/10 Swiss trial cohort.

This translational study aimed at gaining insight into the effects of lenalidomide in acute myeloid leukemia (AML). Forty-one AML patients aged 66 or older of the Swiss cohort of the HOVON-103 AML/SAKK30/10 study were included. After randomization, they received standard induction chemotherapy with or without lenalidomide. Bone marrow biopsies at diagnosis and before the 2nd induction cycle were obtained to assess the therapeutic impact on leukemic blasts and microenvironment. Increased bone marrow angiogenesis, as assessed by microvessel density (MVD), was found at AML diagnosis and differed significantly between the WHO categories. Morphological analysis revealed a higher initial MVD in AML with myelodysplasia-related changes (AML-MRC) and a more substantial decrease of microvascularization after lenalidomide exposure. A slight increase of T-bet-positive TH1-equivalents was identifiable under lenalidomide. In the subgroup of patients with AML-MRC, the progression-free survival differed between the two treatment regimens, showing a potential but not significant benefit of lenalidomide. We found no correlation between the cereblon genotype (the target of lenalidomide) and treatment response or prognosis. In conclusion, addition of lenalidomide may be beneficial to elderly patients suffering from AML-MRC, where it leads to a reduction of microvascularization and, probably, to an intensified specific T cell-driven anti-leukemic response.

MBD4 guards against methylation damage and germ line deficiency predisposes to clonal hematopoiesis and early-onset AML.

The tendency of 5-methylcytosine (5mC) to undergo spontaneous deamination has had a major role in shaping the human genome, and this methylation damage remains the primary source of somatic mutations that accumulate with age. How 5mC deamination contributes to cancer risk in different tissues remains unclear. Genomic profiling of 3 early-onset acute myeloid leukemias (AMLs) identified germ line loss of MBD4 as an initiator of 5mC-dependent hypermutation. MBD4-deficient AMLs display a 33-fold higher mutation burden than AML generally, with >95% being C>T in the context of a CG dinucleotide. This distinctive signature was also observed in sporadic cancers that acquired biallelic mutations in MBD4 and in Mbd4 knockout mice. Sequential sampling of germ line cases demonstrated repeated expansion of blood cell progenitors with pathogenic mutations in DNMT3A, a key driver gene for both clonal hematopoiesis and AML. Our findings reveal genetic and epigenetic factors that shape the mutagenic influence of 5mC. Within blood cells, this links methylation damage to the driver landscape of clonal hematopoiesis and reveals a conserved path to leukemia. Germ line MBD4 deficiency enhances cancer susceptibility and predisposes to AML.

The DOT1L inhibitor pinometostat reduces H3K79 methylation and has modest clinical activity in adult acute leukemia.

Pinometostat (EPZ-5676) is a first-in-class small-molecule inhibitor of the histone methyltransferase disrupter of telomeric silencing 1-like (DOT1L). In this phase 1 study, pinometostat was evaluated for safety and efficacy in adult patients with advanced acute leukemias, particularly those involving mixed lineage leukemia (MLL) gene rearrangements (MLL-r) resulting from 11q23 translocations. Fifty-one patients were enrolled into 6 dose-escalation cohorts (n = 26) and 2 expansion cohorts (n = 25) at pinometostat doses of 54 and 90 mg/m2 per day by continuous intravenous infusion in 28-day cycles. Because a maximum tolerated dose was not established in the dose-escalation phase, the expansion doses were selected based on safety and clinical response data combined with pharmacodynamic evidence of reduction in H3K79 methylation during dose escalation. Across all dose levels, plasma pinometostat concentrations increased in an approximately dose-proportional fashion, reaching an apparent steady-state by 4-8 hours after infusion, and rapidly decreased following treatment cessation. The most common adverse events, of any cause, were fatigue (39%), nausea (39%), constipation (35%), and febrile neutropenia (35%). Overall, 2 patients, both with t(11;19), experienced complete remission at 54 mg/m2 per day by continuous intravenous infusion, demonstrating proof of concept for delivering clinically meaningful responses through targeting DOT1L using the single agent pinometostat in MLL-r leukemia patients. Administration of pinometostat was generally safe, with the maximum tolerated dose not being reached, although efficacy as a single agent was modest. This study demonstrates the therapeutic potential for targeting DOT1L in MLL-r leukemia and lays the groundwork for future combination approaches in this patient population. This clinical trial is registered at www.clinicaltrials.gov as NCT01684150.