Phase 1/1b study of azacitidine and hedgehog pathway inhibitor sonidegib in patients with myeloid neoplasms.

BACKGROUND: Myeloid neoplasms (myelodysplastic syndrome [MDS], myelofibrosis, and chronic myelomonocytic [CMML]) are aggressive hematological malignancies for which, despite recent approvals, novel therapies are needed to improve clinical outcomes. The hedgehog (HH) pathway is one of the main pathways for cancer stem cells survival and several HH inhibitors (HHi) are approved in clinical practice. METHODS: Sonidegib (SON), an oral HHi, was tested in this phase 1/1b trial in combination with azacitidine (AZA, 75 mg/m2 days ×7) in patients with newly diagnosed and relapsed/refractory (r/r) chronic MN or acute myeloid leukemia (AML). RESULTS: Sixty-two patients (28 [45%] newly diagnosed) were treated in this study, including 10 patients in the dose-finding component and 52 patients in phase 1b. SON 200 mg oral daily on days 1-28 each cycle was deemed the recommended dose for phase 1b. Out of 21 rrAML patients, two achieved response (one complete response/one morphologic leukemia-free state) with no responses seen in seven r/r MDS/CMML patients. In newly diagnosed AML/MDS, response was seen in six (three had complete remission, two had morphological leukemia-free status) of 27 patients. Median overall survival was 26.4 and 4.7 months for newly diagnosed MDS and AML, respectively. Safety was satisfactory with common (>20%) side effects including fatigue, constipation, nausea, cough, insomnia, and diarrhea. Only 7% of patients died in the study, and none of the deaths were deemed related to treatment. CONCLUSIONS: Our study shows that AZA + SON are a safe combination in a patient with MN. Similar to other hedgehog inhibitors, this combination yielded limited response rate in patients with myeloid neoplasms.

Mitophagy Promotes Resistance to BH3 Mimetics in Acute Myeloid Leukemia.

BH3 mimetics are used as an efficient strategy to induce cell death in several blood malignancies, including acute myeloid leukemia (AML). Venetoclax, a potent BCL-2 antagonist, is used clinically in combination with hypomethylating agents for the treatment of AML. Moreover, MCL1 or dual BCL-2/BCL-xL antagonists are under investigation. Yet, resistance to single or combinatorial BH3-mimetic therapies eventually ensues. Integration of multiple genome-wide CRISPR/Cas9 screens revealed that loss of mitophagy modulators sensitizes AML cells to various BH3 mimetics targeting different BCL-2 family members. One such regulator is MFN2, whose protein levels positively correlate with drug resistance in patients with AML. MFN2 overexpression is sufficient to drive resistance to BH3 mimetics in AML. Insensitivity to BH3 mimetics is accompanied by enhanced mitochondria-endoplasmic reticulum interactions and augmented mitophagy flux, which acts as a prosurvival mechanism to eliminate mitochondrial damage. Genetic or pharmacologic MFN2 targeting synergizes with BH3 mimetics by impairing mitochondrial clearance and enhancing apoptosis in AML. SIGNIFICANCE: AML remains one of the most difficult-to-treat blood cancers. BH3 mimetics represent a promising therapeutic approach to eliminate AML blasts by activating the apoptotic pathway. Enhanced mitochondrial clearance drives resistance to BH3 mimetics and predicts poor prognosis. Reverting excessive mitophagy can halt BH3-mimetic resistance in AML. This article is highlighted in the In This Issue feature, p. 1501.

A phase II study of combination daunorubicin, cytarabine (Ara-c), and nilotinib (TAsigna) (DATA) in patients newly diagnosed with acute myeloid leukemia with KIT expression.

Acute myeloid leukemia (AML) is a challenging cancer in terms of achieving and maintaining long-duration remissions. Many novel therapies have been added to the standard regimen (combining cytarabine and anthracycline "7 + 3") to achieve such goals. Nilotinib is an oral multikinase inhibitor that is active against KIT tyrosine kinase, an important stem cell target. In this trial, we combined nilotinib with 7 + 3 induction (daunorubicin 60 mg/m2), high-dose cytarabine consolidation, and subsequently, if the patient was a candidate, for 2 years' maintenance therapy in patients with AML and KIT (CD117) expression. Patients were allowed to proceed to allogeneic hematopoietic cell transplantation (HCT) if deemed necessary. Our primary goal was increased complete remission rate with this combination. Thirty-four patients (with a median age 58.5 years) were enrolled on a single-arm phase II bi-institutional study; 21 (62%) patients achieved remission. The complete remission rate was 78% in evaluable patients. Thirteen of 34 (38%) patients had allogeneic HCT, all thirteen of which are still alive (100%). Common (>20%) grade 3 non-hematological toxicities included febrile neutropenia, hypophosphatemia, elevated liver enzymes, and hypertension. Only one patient (3%) died in induction due to liver failure, which was thought secondary to daunorubicin. Our current study reveals good outcomes in patients who received HCT and may warrant a larger study to confirm our findings in that specific population.

The ABNL-MARRO 001 study: a phase 1-2 study of randomly allocated active myeloid target compound combinations in MDS/MPN overlap syndromes.

BACKGROUND: Myelodysplastic/myeloproliferative neoplasms (MDS/MPN) comprise several rare hematologic malignancies with shared concomitant dysplastic and proliferative clinicopathologic features of bone marrow failure and propensity of acute leukemic transformation, and have significant impact on patient quality of life. The only approved disease-modifying therapies for any of the MDS/MPN are DNA methyltransferase inhibitors (DNMTi) for patients with dysplastic CMML, and still, outcomes are generally poor, making this an important area of unmet clinical need. Due to both the rarity and the heterogeneous nature of MDS/MPN, they have been challenging to study in dedicated prospective studies. Thus, refining first-line treatment strategies has been difficult, and optimal salvage treatments following DNMTi failure have also not been rigorously studied. ABNL-MARRO (A Basket study of Novel therapy for untreated MDS/MPN and Relapsed/Refractory Overlap Syndromes) is an international cooperation that leverages the expertise of the MDS/MPN International Working Group (IWG) and provides the framework for collaborative studies to advance treatment of MDS/MPN and to explore clinical and pathologic markers of disease severity, prognosis, and treatment response. METHODS: ABNL MARRO 001 (AM-001) is an open label, randomly allocated phase 1/2 study that will test novel treatment combinations in MDS/MPNs, beginning with the novel targeted agent itacitinib, a selective JAK1 inhibitor, combined with ASTX727, a fixed dose oral combination of the DNMTi decitabine and the cytidine deaminase inhibitor cedazuridine to improve decitabine bioavailability. DISCUSSION: Beyond the primary objectives of the study to evaluate the safety and efficacy of novel treatment combinations in MDS/MPN, the study will (i) Establish the ABNL MARRO infrastructure for future prospective studies, (ii) Forge innovative scientific research that will improve our understanding of pathogenetic mechanisms of disease, and (iii) Inform the clinical application of diagnostic criteria, risk stratification and prognostication tools, as well as response assessments in this heterogeneous patient population. TRIAL REGISTRATION: This trial was registered with ClinicalTrials.gov on August 19, 2019 (Registration No. NCT04061421).

68Ga-Pentixafor PET/CT for Detection of Chemokine Receptor CXCR4 Expression in Myeloproliferative Neoplasms.

C-X-C motif chemokine receptor 4 (CXCR4) is an attractive target for cancer diagnosis and treatment, as it is overexpressed in many solid and hematologic malignancies. This study investigated the feasibility of CXCR4-directed imaging with PET/CT using 68Ga-pentixafor to visualize and quantify disease involvement in myeloproliferative neoplasms (MPNs). Methods: Twelve patients with MPNs (4 with primary myelofibrosis, 6 with essential thrombocythemia, and 2 with polycythemia vera) and 5 controls underwent 68Ga-pentixafor PET/CT. Imaging findings were compared with immunohistochemical stainings, laboratory data, and splenic volume. Results:68Ga-pentixafor PET/CT was visually positive in 12 of 12 patients, and CXCR4 target specificity could be confirmed by immunohistochemical staining. A significantly higher tracer uptake could be detected in the bone marrow of MPN patients (SUVmean, 6.45 ± 2.34 vs. 4.44 ± 1.24). Dynamic changes in CXCR4 expression determined by 68Ga-pentixafor PET/CT corresponded with treatment response. Conclusion:68Ga-pentixafor PET/CT represents a novel diagnostic tool to noninvasively detect and quantify the extent of disease involvement in MPNs.

Actin cytoskeleton deregulation confers midostaurin resistance in FLT3-mutant acute myeloid leukemia.

The presence of FMS-like tyrosine kinase 3-internal tandem duplication (FLT3-ITD) is one of the most frequent mutations in acute myeloid leukemia (AML) and is associated with an unfavorable prognosis. FLT3 inhibitors, such as midostaurin, are used clinically but fail to entirely eradicate FLT3-ITD + AML. This study introduces a new perspective and highlights the impact of RAC1-dependent actin cytoskeleton remodeling on resistance to midostaurin in AML. RAC1 hyperactivation leads resistance via hyperphosphorylation of the positive regulator of actin polymerization N-WASP and antiapoptotic BCL-2. RAC1/N-WASP, through ARP2/3 complex activation, increases the number of actin filaments, cell stiffness and adhesion forces to mesenchymal stromal cells (MSCs) being identified as a biomarker of resistance. Midostaurin resistance can be overcome by a combination of midostaruin, the BCL-2 inhibitor venetoclax and the RAC1 inhibitor Eht1864 in midostaurin-resistant AML cell lines and primary samples, providing the first evidence of a potential new treatment approach to eradicate FLT3-ITD + AML.

Dose-response assessment by quantitative MRI in a phase 1 clinical study of the anti-cancer vascular disrupting agent crolibulin.

The vascular disrupting agent crolibulin binds to the colchicine binding site and produces anti-vascular and apoptotic effects. In a multisite phase 1 clinical study of crolibulin (NCT00423410), we measured treatment-induced changes in tumor perfusion and water diffusivity (ADC) using dynamic contrast-enhanced MRI (DCE-MRI) and diffusion-weighted MRI (DW-MRI), and computed correlates of crolibulin pharmacokinetics. 11 subjects with advanced solid tumors were imaged by MRI at baseline and 2-3 days post-crolibulin (13-24 mg/m2). ADC maps were computed from DW-MRI. Pre-contrast T1 maps were computed, co-registered with the DCE-MRI series, and maps of area-under-the-gadolinium-concentration-curve-at-90 s (AUC90s) and the Extended Tofts Model parameters ktrans, ve, and vp were calculated. There was a strong correlation between higher plasma drug [Formula: see text] and a linear combination of (1) reduction in tumor fraction with [Formula: see text] mM s, and, (2) increase in tumor fraction with [Formula: see text]. A higher plasma drug AUC was correlated with a linear combination of (1) increase in tumor fraction with [Formula: see text], and, (2) increase in tumor fraction with [Formula: see text]. These findings are suggestive of cell swelling and decreased tumor perfusion 2-3 days post-treatment with crolibulin. The multivariable linear regression models reported here can inform crolibulin dosing in future clinical studies of crolibulin combined with cytotoxic or immune-oncology agents.

Corrigendum to "Triazolo[4,5-d]pyrimidines as validated general control nonderepressible 2 (GCN2) protein kinase inhibitors reduce growth of leukemia cells" [Comput. Struct. Biotechnol. J. 16 (2018) 350-360].

[This corrects the article DOI: 10.1016/j.csbj.2018.09.003.].

Targeting Apoptosis in Acute Myeloid Leukemia: Current Status and Future Directions of BCL-2 Inhibition with Venetoclax and Beyond.

Acute myeloid leukemia (AML) is a disease of the hematopoietic system that remains a therapeutic challenge despite advances in our understanding of the underlying cancer biology over the past decade. Recent developments in molecular targeting have shown promising results in treating leukemia, paving the way for novel treatment strategies. The discovery of drugs that promote apoptosis in leukemic cells has translated to encouraging activity in clinical trials. B-cell lymphoma (BCL)-2 inhibition has been at the center of drug development efforts to target apoptosis in AML. Remarkable clinical success with venetoclax has revolutionized the ways we treat hematological malignancies. Several landmark trials have demonstrated the potent antitumor activity of venetoclax, and it is now frequently combined with traditional cytotoxic agents to treat AML. However, resistance to BCL-2 inhibition is emerging, and alternative strategies to address resistance mechanisms have become an important focus of research. A number of clinical trials are now underway to investigate a plurality of novel agents that were shown to overcome resistance to BCL-2 inhibition in preclinical models. Some of the most promising data come from studies on drugs that downregulate myeloid cell leukemia (MCL)-1, such as cyclin-dependent kinases (CDK) inhibitors. Furthermore, innovative approaches to target apoptosis via extrinsic pathways and p53 regulation have added new cytotoxic agents to the arsenal, including drugs that inhibit inhibitor of apoptosis protein (IAP) family proteins and murine double minute 2 (MDM2). This review provides a perspective on past and current treatment strategies harnessing various mechanisms of apoptosis to target AML and highlights some important promising treatment combinations in development.

Guadecitabine (SGI-110) in patients with intermediate or high-risk myelodysplastic syndromes: phase 2 results from a multicentre, open-label, randomised, phase 1/2 trial.

BACKGROUND: Guadecitabine is a next-generation hypomethylating agent whose active metabolite decitabine has a longer in-vivo exposure time than intravenous decitabine. More effective hypomethylating agents are needed for the treatment of myelodysplastic syndromes. In the present study, we aimed to compare the activity and safety of two doses of guadecitabine in hypomethylating agent treatment-naive or relapsed or refractory patients with intermediate-risk or high-risk myelodysplastic syndromes. METHODS: This phase 2 part of the phase 1/2, randomised, open-label study enrolled patients aged 18 years or older from 14 North American medical centres with International Prognostic Scoring System intermediate-1-risk, intermediate-2-risk, or high-risk myelodysplastic syndromes, or chronic myelomonocytic leukaemia. They were either hypomethylating agent treatment-naive or had relapsed or refractory disease after previous hypomethylating agent treatment as determined by the investigators' judgment. Eligible patients had Eastern Cooperative Oncology Group performance status of 0-2. Patients were randomly assigned (1:1) using a computer algorithm for dynamic randomisation to subcutaneous guadecitabine 60 or 90 mg/m2 on days 1-5 of a 28-day treatment cycle. Treatment was stratified by previous treatment with hypomethylating agents and neither patients nor investigators were masked. The primary endpoint was overall response (a composite of complete response, partial response, marrow complete response, and haematological improvement) assessed in all patients who received at least one dose of study drug. This study is registered with ClinicalTrials.gov, number NCT01261312. FINDINGS: Between July 9, 2012, and April 7, 2014, 105 patients were enrolled: 55 (52%) were allocated to guadecitabine 60 mg/m2 (28 patients were treatment-naive and 27 had relapsed or refractory disease after previous hypomethylating agent treatment) and 50 (48%) patients to 90 mg/m2 (23 patients were treatment-naive and 27 had relapsed or refractory disease). Three (3%) patients of 105 did not receive study treatment and were excluded from analyses. Median follow-up was 3·2 years (IQR 2·8-3·5). The proportion of patients achieving an overall response did not significantly differ between dose groups (21 of 53 [40%, 95% CI 27-54] with 60 mg/m2 and 27 of 49 [55%, 95% CI 40-69] with 90 mg/m2; p=0·16). 25 of 49 (51%, 95% CI 36-66) patients who were treatment-naive and 23 of 53 (43%, 30-58) patients with relapsed or refractory disease achieved an overall response. The most common grade 3 or worse adverse events in both groups, regardless of relationship to treatment, were thrombocytopenia (22 [41%] of 53 patients in the 60 mg/m2 group and 28 [57%] of 49 in the 90 mg/m2 group), neutropaenia (21 [40%] and 25 [51%]), anaemia (25 [47%] and 24 [49%]), febrile neutropaenia (17 [32%] and 21 [43%]), and pneumonia (13 [25%] and 15 [31%]). Seven (7%) of 102 patients died due to adverse events (three with 90 mg/m2 and four with 60 mg/m2), and all except one were in the relapsed or refractory cohort. Two deaths were deemed treatment related (septic shock with 60 mg/m2; pneumonia with 90 mg/m2). INTERPRETATION: Guadecitabine was clinically active with acceptable tolerability in patients with intermediate-risk and high-risk myelodysplastic syndromes. Responses and overall survival in the relapsed or refractory cohort offer the potential of a new therapeutic option for patients for whom currently available hypomethylating agents are not successful. We therefore recommend guadecitabine at a dose of 60 mg/m2 on a 5-day schedule for these patients. FUNDING: Astex Pharmaceuticals and Stand Up To Cancer.

Targeting Mitochondrial Structure Sensitizes Acute Myeloid Leukemia to Venetoclax Treatment.

The BCL2 family plays important roles in acute myeloid leukemia (AML). Venetoclax, a selective BCL2 inhibitor, has received FDA approval for the treatment of AML. However, drug resistance ensues after prolonged treatment, highlighting the need for a greater understanding of the underlying mechanisms. Using a genome-wide CRISPR/Cas9 screen in human AML, we identified genes whose inactivation sensitizes AML blasts to venetoclax. Genes involved in mitochondrial organization and function were significantly depleted throughout our screen, including the mitochondrial chaperonin CLPB. We demonstrated that CLPB is upregulated in human AML, it is further induced upon acquisition of venetoclax resistance, and its ablation sensitizes AML to venetoclax. Mechanistically, CLPB maintains the mitochondrial cristae structure via its interaction with the cristae-shaping protein OPA1, whereas its loss promotes apoptosis by inducing cristae remodeling and mitochondrial stress responses. Overall, our data suggest that targeting mitochondrial architecture may provide a promising approach to circumvent venetoclax resistance. SIGNIFICANCE: A genome-wide CRISPR/Cas9 screen reveals genes involved in mitochondrial biological processes participate in the acquisition of venetoclax resistance. Loss of the mitochondrial protein CLPB leads to structural and functional defects of mitochondria, hence sensitizing AML cells to apoptosis. Targeting CLPB synergizes with venetoclax and the venetoclax/azacitidine combination in AML in a p53-independent manner.See related commentary by Savona and Rathmell, p. 831.This article is highlighted in the In This Issue feature, p. 813.

Targeting an RNA-Binding Protein Network in Acute Myeloid Leukemia.

RNA-binding proteins (RBPs) are essential modulators of transcription and translation frequently dysregulated in cancer. We systematically interrogated RBP dependencies in human cancers using a comprehensive CRISPR/Cas9 domain-focused screen targeting RNA-binding domains of 490 classical RBPs. This uncovered a network of physically interacting RBPs upregulated in acute myeloid leukemia (AML) and crucial for maintaining RNA splicing and AML survival. Genetic or pharmacologic targeting of one key member of this network, RBM39, repressed cassette exon inclusion and promoted intron retention within mRNAs encoding HOXA9 targets as well as in other RBPs preferentially required in AML. The effects of RBM39 loss on splicing further resulted in preferential lethality of spliceosomal mutant AML, providing a strategy for treatment of AML bearing RBP splicing mutations.

E6201, an intravenous MEK1 inhibitor, achieves an exceptional response in BRAF V600E-mutated metastatic malignant melanoma with brain metastases.

Malignant melanoma (MM) exhibits a high propensity for central nervous system dissemination with ~50% of metastatic MM patients developing brain metastases (BM). Targeted therapies and immune checkpoint inhibitors have improved overall survival for MM patients with BM. However, responses are usually of short duration and new agents that effectively penetrate the blood brain barrier (BBB) are needed. Here, we report a MM patient with BM who experienced an exceptional response to E6201, an ATP-competitive MEK1 inhibitor, on a Phase 1 study, with ongoing near-complete response and overall survival extending beyond 8 years. Whole exome and transcriptome sequencing revealed a high mutational burden tumor (22 mutations/Megabase) with homozygous BRAF V600E mutation. Correlative preclinical studies demonstrated broad activity for E6201 across BRAF V600E mutant melanoma cell lines and effective BBB penetration in vivo. Together, these results suggest that E6201 may represent a potential new treatment option for BRAF-mutant MM patients with BM.

Transcriptional Silencing of MCL-1 Through Cyclin-Dependent Kinase Inhibition in Acute Myeloid Leukemia.

Acute myeloid leukemia (AML) is the most common adult acute leukemia. Survival remains poor, despite decades of scientific advances. Cytotoxic induction chemotherapy regimens are standard-of-care for most patients. Many investigations have highlighted the genomic heterogeneity of AML, and several new targeted therapeutic options have recently been approved. Additional novel therapies are showing promising clinical results and may rapidly transform the therapeutic landscape of AML. Despite the emerging clinical success of B-cell lymphoma (BCL)-2 targeting in AML and a large body of preclinical data supporting myeloid leukemia cell (MCL)-1 as an attractive therapeutic target for AML, MCL-1 targeting remains relatively unexplored, although novel MCL-1 inhibitors are under clinical investigation. Inhibitors of cyclin-dependent kinases (CDKs) involved in the regulation of transcription, CDK9 in particular, are being investigated in AML as a strategy to target MCL-1 indirectly. In this article, we review the basis for CDK inhibition in oncology with a focus on relevant preclinical mechanism-of-action studies of CDK9 inhibitors in the context of their therapeutic potential specifically in AML.

Triazolo[4,5-d]pyrimidines as Validated General Control Nonderepressible 2 (GCN2) Protein Kinase Inhibitors Reduce Growth of Leukemia Cells.

Cellular stress signals activate adaptive signaling pathways of the mammalian integrated stress response (ISR), of which the unfolded protein response (UPR) is a subset. These pathways converge at the phosporylation of eIF2α. Drug-like, potent and selective chemical inhibitors (valid chemical probes) targeting major ISR kinases have been previously identified, with the exception of GCN2. We synthesized and evaluated a series of GCN2 inhibitors based on a triazolo[4,5-d]pyrimidine scaffold. Several compounds potently inhibited GCN2 in vitro and displayed good selectivity over the related kinases PERK, HRI, and IRE1. The compounds inhibited phosporylation of eIF2α in HEK293T cells with an IC50 < 150 nM, validating them as chemical probes for cellular studies. These probes were screened against the National Cancer Institute NCI-60 human cancer cell line panel. Uniform growth inhibition was observed in the leukemia group of cell lines. Growth inhibition in the most sensitive cell lines coincided with high GCN2 mRNA expression levels. Oncomine analysis revealed high GCN2 expression accompanied by lower asparagine synthetase (ASNS) expression in patient-derived acute lymphoblastic leukemias with B-Cell origins (B-ALL) as well. Notably, asparaginase, which depletes amino acids and triggers GCN2 activity, is a licensed, first-line B-ALL treatment. Thus, we hypothesize that leukemias exhibiting high GCN2 expression and low ASNS expression may be susceptible to pharmacologic GCN2 inhibition.

Safety, pharmacokinetics, and preliminary efficacy of E6201 in patients with advanced solid tumours, including melanoma: results of a phase 1 study.

BACKGROUND: This phase 1 first-in-human study aimed to determine the maximum-tolerated dose (MTD), dose-limiting toxicities, and safety of E6201, and to establish recommended dosing in patients with advanced solid tumours, expanded to advanced melanoma. METHODS: Part A (dose escalation): sequential cohorts received E6201 intravenously (IV) over 30 min (once-weekly [qw; days (D)1 + 8 + 15 of a 28-day cycle]), starting at 20 mg/m2, increasing to 720 mg/m2 or the MTD. Part B (expansion): patients with BRAF-mutated or wild-type (WT) melanoma received E6201 320 mg/m2 IV over 60 minutes qw (D1 + 8 + 15 of a 28-day cycle) or 160 mg/m2 IV twice-weekly (D1 + 4 + 8 + 11 + 15 + 18 of a 28-day cycle; BRAF-mutated only). RESULTS: MTD in Part A (n = 25) was 320 mg/m2 qw, confirmed in Part B (n = 30). Adverse events included QT prolongation (n = 4) and eye disorders (n = 3). E6201 exposure was dose-related, with PK characterised by extensive distribution and fast elimination. One patient achieved PR during Part A (BRAF-mutated papillary thyroid cancer; 480 mg/m2 qw) and three during Part B (2 BRAF-mutated melanoma; 1 BRAF-WT melanoma; all receiving 320 mg/m2 qw). CONCLUSIONS: An intermittent regimen of E6201 320 mg/m2 IV qw for the first 3 weeks of a 28-day cycle was feasible and reasonably well-tolerated in patients with advanced solid tumours, including melanoma with brain metastases, with evidence of clinical efficacy.

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.