Khaled, Samer K.

PURPOSE Hematopoietic stem-cell transplantation–associated thrombotic microangiopathy (HSCT-TMA) is a serious complication with significant mortality and no approved therapy. HSCT-TMA results from endothelial injury, which activates the lectin pathway of complement. Narsoplimab (OMS721), an inhibitor of mannan-binding lectin-associated serine protease-2 (MASP-2), was evaluated for safety and efficacy in adults with HSCT-TMA. METHODS In this single-arm open-label pivotal trial ( NCT02222545 ), patients received intravenous narsoplimab once weekly for 4-8 weeks. The primary end point (response rate) required clinical improvement in two categories: (1) laboratory TMA markers (both platelet count and lactate dehydrogenase) and (2) organ function or freedom from transfusion. Patients receiving at least one dose (full analysis set [FAS]; N = 28) were analyzed. RESULTS The response rate was 61% in the FAS population. Similar responses were observed across all patient subgroups defined by baseline features, HSCT characteristics, and HSCT complications. Improvement in organ function occurred in 74% of patients in the FAS population. One-hundred-day survival after HSCT-TMA diagnosis was 68% and 94% in FAS population and responders, respectively, whereas median overall survival was 274 days in the FAS population. Narsoplimab was well tolerated, and adverse events were typical of this population, with no apparent safety signal of concern. CONCLUSION In this study, narsoplimab treatment was safe, significantly improved laboratory TMA markers, and resulted in clinical response and favorable overall survival.

Abstract Introduction: Light chain (AL) amyloidosis is a plasma cell disease characterized by the deposition of insoluble amyloid fibrils into organs leading to organ dysfunction and death. The analysis at 6 and 12 months of the ANDROMEDA study (NCT03201965) showed that the addition of subcutaneous (SC) daratumumab to the standard of care combination of bortezomib, cyclophosphamide, and dexamethasone (VCd) was superior to VCd alone, with higher rates of hematologic complete response (CR) and an acceptable safety profile. Based on these findings daratumumab with VCd (D-VCd) was approved for newly diagnosed AL amyloidosis in January 2021 in the US and June 2021 in the EU. Here, we present data from the analysis at 18 months of the ANDROMEDA study. Methods: ANDROMEDA is a randomized, open-label, active-controlled phase 3 study including patients with newly diagnosed AL amyloidosis with measurable hematologic disease, ≥1 involved organ, cardiac stage I-IIIA, estimated glomerular filtration rate ≥20 mL/min, and absence of symptomatic multiple myeloma. Patients were randomized (1:1) to D-VCd or VCd for 6 28-day cycles. Bortezomib (1.3 mg/m 2), cyclophosphamide (300 mg/m 2 up to 500 mg per week), and dexamethasone (20-40 mg) were administered weekly. SC daratumumab (1800 mg co-formulated with recombinant human hyaluronidase PH20 in 15 mL) was administered once weekly in cycles 1 and 2 and every 2 weeks in cycles 3 to 6. Patients in the D-VCd arm received only SC daratumumab after cycle 6, every 4 weeks (up to a total of 24 cycles from first dose). The primary endpoint was overall (ie, at any time) hematologic CR rate, defined here as normalization of free light chain (FLC) levels and ratio (FLCr) and negative serum and urine immunofixation, confirmed at a subsequent visit; normalization of uninvolved FLC level and FLCr were not required if involved FLC was lower than the upper limit of normal. Secondary endpoints included major organ deterioration progression-free survival (PFS), organ response rate, time to hematologic response, overall survival (OS), and safety. Results: A total of 388 patients were randomized to receive D-VCd (N=195) or VCd alone (N=193). At the May 2021 clinical cutoff, the median duration of treatment for D-VCd and VCd arms was 21.3 and 5.3 months, respectively. In the D-VCd arm, 149 patients (77.2%) received daratumumab monotherapy after completing 6 cycles of D-VCd; of those, 17 (11.4%) were still receiving treatment. The rates of deep hematological responses favored the D-VCd treatment arm (Table). At a median follow-up of 25.8 months, the rate of hematologic CR was significantly higher in the D-VCd arm vs the VCd arm (59.5% vs 19.2%; odds ratio [95% confidence interval (CI)], 6.03 [3.80-9.58]; P<0.0001). Similarly, more patients achieved a very good partial response or better (≥VGPR) (D-VCd vs VCd, 79.0% vs 50.3%; odds ratio [95% CI], 3.74 [2.39-5.86]; P<0.0001). Among patients who responded, the median time from randomization to ≥VGPR was shorter in the D-VCd arm (0.56 months) vs the VCd arm (0.82 months). Comparable to the cardiac response analysis at 6 months (D-VCd vs VCd, 42% vs 22%), greater cardiac response rates were achieved with D-VCd compared with VCd at 18 months (53% vs 24%). Similarly, renal response rates remained superior with D-VCd vs VCd alone at 18 months (58% vs 26% compared with 6 months [54% vs 27%]). A total of 79 deaths occurred (D-VCd, N=34; VCd, N=45). In the D-VCd arm, only 1 additional grade 3/4 treatment-emergent adverse event occurred over 18 months vs 12 months (119 [61.7%] vs 118 [61.1%] patients) and no additional infusion-related reactions were reported. OS will be analyzed and major organ deterioration PFS will be updated after approximately 200 events have occurred. Conclusions: These results demonstrate the sustained clinical benefits of D-VCd vs VCd in terms of hematologic and organ responses with longer follow-up, although it should be noted that many patients in the D-VCd arm received daratumumab monotherapy following 6 cycles of D-VCd, while patients in the VCd group stopped study treatment. Nevertheless, the study continues to support the use of D-VCd over VCd alone in patients with newly diagnosed AL amyloidosis. Following its recent approval, D-VCd represents a new SOC for patients with AL amyloidosis. Figure 1 Figure 1. Disclosures Comenzo: Prothena Biosciences: Consultancy, Research Funding; Takeda: Research Funding; Unum: Membership on an entity's Board of Directors or advisory committees, Research Funding; Karyopharm: Research Funding; Caelum: Consultancy, Research Funding; Sanofi-Aventis: Membership on an entity's Board of Directors or advisory committees; Janssen: Patents & Royalties: WO2016187546A1, Research Funding. Palladini: Pfizer: Honoraria; Siemens: Honoraria; Janssen Global Services: Honoraria, Other: advisory board fees. Kastritis: Pfizer: Consultancy, Honoraria, Research Funding; Takeda: Honoraria; Amgen: Consultancy, Honoraria, Research Funding; Janssen: Consultancy, Honoraria, Research Funding; Genesis Pharma: Honoraria. Minnema: Janssen: Consultancy; BMS: Consultancy; Kite/Gilead: Consultancy; Celgene: Other: Travel expenses; Alnylam: Consultancy; Cilag: Consultancy. Wechalekar: Amgen: Research Funding; Janssen: Consultancy; Takeda: Honoraria; Celgene: Honoraria; Caelum Biosciences: Other: Clinical Trial Funding; Alexion, AstraZeneca Rare Disease: Consultancy. Jaccard: Pfizer: Honoraria; Abbvie: Honoraria; Janssen: Membership on an entity's Board of Directors or advisory committees. Dispenzieri: Janssen: Consultancy, Research Funding; Sorrento Therapeutics: Consultancy; Oncopeptides: Consultancy; Alnylam: Research Funding; Pfizer: Research Funding; Takeda: Research Funding. Lee: Karyopharm: Consultancy; Celgene: Consultancy; Takeda Pharmaceuticals: Consultancy, Research Funding; Amgen: Consultancy, Research Funding; GlaxoSmithKline: Consultancy, Research Funding; Sanofi: Consultancy; Genentech: Consultancy; Legend Biotech: Consultancy; Janssen: Consultancy, Research Funding; Oncopetides: Consultancy; Bristol Myers Squibb: Consultancy; Regeneron: Research Funding. Sanchorawala: Oncopeptide: Research Funding; Karyopharm: Research Funding; Sorrento: Research Funding; Pfizer: Honoraria; Proclara: Membership on an entity's Board of Directors or advisory committees; Regeneron: Membership on an entity's Board of Directors or advisory committees; Caelum: Membership on an entity's Board of Directors or advisory committees, Research Funding; Abbvie: Membership on an entity's Board of Directors or advisory committees; Prothena: Membership on an entity's Board of Directors or advisory committees, Research Funding; Janssen: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Takeda: Research Funding; Celgene: Research Funding. Gibbs: AbbVie: Consultancy; Janssen, Celgene, Amgen, Takeda, BMS and Pfizer: Consultancy, Honoraria. Mollee: Janssen, Pfizer: Research Funding; Amgen, BMS, Janssen, Caelum, EUSA, Pfizer, SkylineDx, Takeda: Membership on an entity's Board of Directors or advisory committees, Other: No personal fees received. Venner: BMS: Honoraria; Amgen: Research Funding; Celgene: Research Funding; Amgen: Honoraria; Takeda: Honoraria; Janssen: Honoraria; Sanofi: Honoraria; Pfizer: Honoraria. Schönland: Janssen: Honoraria, Other: Travel grants, Research Funding; Pfizer: Honoraria; Takeda: Honoraria, Other: Travel grants; Prothena: Honoraria, Other: Travel grants; Sanofi: Research Funding. Suzuki: Abie: Honoraria; Sanofi: Honoraria; Novartis: Honoraria; ONO: Honoraria; Takeda: Consultancy, Honoraria; Amgen: Consultancy, Honoraria, Research Funding; Celgene: Consultancy, Honoraria, Research Funding; Bristol-Myers Squibb: Honoraria, Research Funding; Janssen: Consultancy, Honoraria. Kim: BMS: Consultancy, Honoraria, Research Funding; Janssen: Consultancy, Honoraria, Research Funding. Cibeira: Janssen: Honoraria, Membership on an entity's Board of Directors or advisory committees; Celgene: Honoraria; Amgen: Honoraria, Membership on an entity's Board of Directors or advisory committees; Akcea: Honoraria, Membership on an entity's Board of Directors or advisory committees. Beksac: Oncopeptides: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Sanofi: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Janssen: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; BMS: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Takeda: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Amgen: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau. Libby: Genentech: Research Funding; BMS: Research Funding; GSK: Research Funding; Janssen: Consultancy, Research Funding. Valent: Caelum Biosciences: Other: Clinical Trial Funding; Celgene Corporation: Speakers Bureau; Takeda Pharmaceuticals: Speakers Bureau; Amgen: Speakers Bureau. Hungria: Amgen, BMS, Celgene, Janssen: Honoraria, Membership on an entity's Board of Directors or advisory committees, Other: Support for attending meetings/travel ; Abbvie: Honoraria; Sanofi: Honoraria, Other: Support for attending meetings/travel ; Takeda: Honoraria. Wong: Amgen: Consultancy; Genentech: Research Funding; Fortis: Research Funding; Janssen: Research Funding; GloxoSmithKlein: Research Funding; Dren Biosciences: Consultancy; Caelum: Research Funding; BMS: Research Funding; Sanofi: Membership on an entity's Board of Directors or advisory committees. Rosenzweig: Janssen: Consultancy, Speakers Bureau; Bristol Myers Squibb: Speakers Bureau; Akcea: Speakers Bureau; Takeda: Speakers Bureau; Onocopeptides: Speakers Bureau. Bumma: Amgen, Sanofi: Speakers Bureau; Janssen, Oncopeptides, Sanofi: Consultancy. Tran: Janssen: Current Employment, Current equity holder in publicly-traded company. Qin: Janssen: Current Employment. Khaled: Jazz: Honoraria; Astellas: Honoraria; Alexion: Honoraria, Speakers Bureau; Omeros: Honoraria; Janssen: Current Employment. Vermeulen: Janssen: Current Employment, Current equity holder in publicly-traded company.

Abstract Background: Pracinostat, a potent oral Class I, II, IV histone deacetylase (HDAC) inhibitor, exhibited modest single-agent activity in relapsed AML. Inhibition of histone deacetylation and DNA hypermethylation induces re-expression of silenced genes in myeloid malignancies in a synergistic fashion. A pilot study of pracinostat with azacitidine (AZA) in myelodysplastic syndromes showed the combination to be active and well tolerated (Blood2012;120:3821). We conducted a Phase 2 study to evaluate pracinostat + AZA in patients ≥65 years with AML not eligible for induction chemotherapy. Minimum 1-year survival data were reported previously (Blood 2015;126:453). Here we report long-term survival and response analyses. Methods: Main eligibility: Age ≥65, untreated de novo or secondary AML, ≥20% bone marrow (BM) blasts, not candidate for intensive therapy due to co-morbidities and/or AML features, intermediate or high-risk cytogenetics, and no prior treatment with HDAC inhibitors or hypomethylating agents. Treatment consisted of pracinostat 60 mg orally 3 days/week on alternate days for 3 weeks and AZA 75 mg/m2 subcutaneously or intravenously daily for 7 days, with cycles repeated every 28 days until disease progression, lack of response or poor tolerability. The primary endpoint was a composite complete response rate (cCR) of CR + CRi+ MLFS by IWG criteria. Response was assessed at the end of Cycles 2, 4, 6 and then every 3 cycles or when clinically indicated. Secondary endpoints included overall response rate (cCR + PR), duration of response and overall survival (OS). Study registered under NCT01912274. Results: Between December 2013 and December 2014, 50 patients were enrolled at 15 U.S. sites. Baseline characteristics: Median age 75 years (range 66-84 years); 33 de novo and 17 secondary AML; 27 intermediate-risk and 21 high-risk cytogenetics and 2 not known; median BM blasts 40% (range 20%-89%), ECOG performance status 0-1 in 84% and 2 in 16%. The median number of treatment cycles was 6.5 (range 1-24+). A CR was achieved in 21 patients (42%), CRi in 2 (4%), and MLFS in 3 (6%) for a cCR rate of 52%. Median time to BM blasts <5% was 57 days (range 25-243 days) and exceeded 168 days (6 cycles) in 3 patients (12%). Median duration of cCR is 13.2 months (95% CI 10.9-21.5 months), with 6 of 26 patients (23%) continuing on therapy for a periods ranging from 20.5 to 29.5 months. With a median follow-up of 21 months, the median OS is 19.1 months (95% CI, 10.0-not reached). One-year and 2-year OS are 62% and 45%. Non-protocol therapies (NPT) were administered in 23 patients (46%). There was no effect on OS after censoring survival duration at the start of NPT. Response and OS by population subsets are summarized in Table 1. Non hematologic adverse events Grade ≥3 reported in >5% of patients: Fatigue 34%; anorexia 10%, cellulitis, pneumonia, asthenia and sepsis in 8%; urinary tract infection, nausea, back pain, hypoxia, hyponatremia, and syncope in 6%. Grade ≥ 3 hematologic toxicities in> 5% of patients: thrombocytopenia 46%; febrile neutropenia 44%; neutropenia 36%; anemia 30%; pancytopenia 6%. The 30-day and 60-day mortality rates were 2% and 10%. Conclusions: Pracinostat + AZA led to a high rate of responses in elderly patients with AML. Responses were durable and observed irrespective of age, cytogenetics risk, ECOG performance status, and de novo or secondary AML. Marrow remission was typically achieved early, but prolonged exposure was required in some patients to maximize response. The results compare favorably with historical single-agent AZA data in a similar AML population. A phase 3 study of AZA +/- pracinostat in untreated patients with AML unfit for standard induction chemotherapy is starting. Table 1. Response and Survival According to Baseline Characteristics Table 1. Response and Survival According to Baseline Characteristics Disclosures Atallah: Incyte: Consultancy; CTI biopharma: Consultancy; Novartis: Consultancy; BMS: Consultancy; Pfizer: Other: Grant review; Takeda: Research Funding; Ariad: Honoraria. Arellano:Cephalon: Research Funding. Odenike:Geron: Research Funding; Incyte: Honoraria, Membership on an entity's Board of Directors or advisory committees; Sanofi-Aventis: Honoraria, Membership on an entity's Board of Directors or advisory committees; Suneisis: Honoraria, Membership on an entity's Board of Directors or advisory committees; CTI/Baxter: Honoraria, Membership on an entity's Board of Directors or advisory committees; Spectrum: Honoraria, Membership on an entity's Board of Directors or advisory committees; Algeta: Honoraria, Membership on an entity's Board of Directors or advisory committees. Ghalie:MEI Pharma: Employment.

Abstract The current view is that treatment failures of AML patients are due to persistence of leukemia stem cells (LSCs). The presence of FMS-like tyrosine kinase-3 (FLT3) Internal tandem duplication (ITD) is associated with poor prognosis. But, FLT3 tyrosine kinase inhibitors (TKI) demonstrate transient clinical activity in FLT3-ITD+ AML patients. Persistent FLT3-ITD+ AML LSC represent a source of relapse. There is a pressing need to target LSC and improve outcomes for FLT3-ITD+ AML patients. PRMT1, the predominant arginine methyltransferase, has been implicated in pathogenesis of AML rare subtype (i.e., acute megakaryoblastic leukemia). Herein, we show that PRMT1 protein expression is significantly increased in LSC-enriched AML CD34+CD38- cells relative to the counterparts of normal peripheral blood stem cells (PBSC) (AML n=9, normal n=8, p=0.0004,). Following PRMT1-knockdown (KD), AML CD34+ cells (n=15) demonstrated varying degrees of apoptosis while the survival of normal cells were not affected. Interestingly, we observed significant apoptosis-induction in a subset of samples bearing FLT3-ITD mutation (6 out of total 15) upon PRMT1-KD (ShCtrl 13.9±3.6%, ShPRMT1 32.5±4.6%, p<0.001). PRMT1-KD induced apoptosis was also more evident in cord blood (CB) CD34+ cells expressing FLT3-ITD (ShCtrl 19.4±1.3%, ShPRMT1 45.2±2.5%, p<0.001) relative to that of FLT3-WT (ShCtrl 27.47±1.8%, ShPRMT1 36.9±1.6%, p=0.0167). In 293T cells ectopically overexpressing FLT3-WT or FLT3-ITD, co-immunoprecipitation (co-IP) indicated greater interaction between PRMT1 and FLT3-ITD. Through RNA-Seq profiling two AML lines (MV4-11, OCI-AML3) plus one FLT3-ITD transduced CB CD34+ cells with PRMT1-KD, we obtained a differentially-regulated gene set as a "PRMT1 signature". This signature was enriched in FLT3-ITD+ AML relative to FLT3-WT AML according to ssGSEA analysis using two AML datasets (GSE14468, GSE10358), indicating PRMT1 may cooperate with FLT3-ITD regulating AML maintenance. Given that PRMT1 directly interacts with FLT3-ITD, we next asked whether PRMT1 catalyzes FLT3-ITD protein methylation. Through mass-spectrometry analysis of a FLT3-ITD+ AML specimen and in vitro methylation assay, we identified that PRMT1 catalyzes FLT3-ITD arginine (R) methylation (Me) at two conserved residues, 972 and 973. Using in-house R972/973 Me antibody, we validated the expression of FLT3 R-Me in FLT3-ITD AML speciemens (7 out of 7). To test R-Me function, we transduced MLL-AF9 (MA9) overexpressing murine c-Kit+ cells with methylation-deficient FLT3-ITD (R972/973K, arginine [R] to lysine [K]) construct, and found that MA9 cells expressing R972/973K underwent more apoptosis than that of WT FLT3-ITD (WT FLT3-ITD 9.7±1.1%, R972/973K 23.7±2.1%, p=0.003). The double transformed cells were further transplanted into recipients for leukemia development. Mice receiving MA9 cells expressing R972/973K exhibited longer survival (median survival: WT FLT3-ITD 36 days, R972/973K 50 days, p=0.002, n=6). Mechanistically, expression of R972/973K did not affect the total tyrosine phosphorylation level of FLT3-ITD. Additionally, FLT3-ITD R-Me expression persisted after a TKI (AC220) treatment. These facts indicated that FLT3-ITD R-Me function is independent of FLT3-ITD kinase activity. We then used a FLT3-ITD+ patient derived xenograft (PDX) model to assess the effects of TKI and PRMT1 inhibition. Following engraftment >1% in peripheral blood, we divided mice (n=24) into 4 groups and treated each with vehicle, MS023 (a type I PRMT inhibitor, ACS Chem Biol. 2016;11:772-781) (160 mg/kg/i.p), AC220 (10 mg/kg/i.g) or combination for 4 weeks. MS023 treatment downregulating FLT3 R-Me levels enhanced elimination of FLT3-ITD AML cells by AC220 treatment (AC220 24.6±13.4% vs combination 7.6±6.5%, p=0.02, n=6). At 16 weeks post-secondary BMT, significant AML burden in single drug treated transplants was observed, but less AML cells were detected in combination-treated transplants (AC220 52.3%, vs combination 25.4%, p<0.001, n=6). MS023 had little effect on long-term in vivo engraftment of CD34+ from a human CB specimen (vehicle 76.3±5.9%, MS023 72.2±3.4%, p=0.21, n=5). In summary, our study demonstrated PRMT1 overexpression contributes to AML stem/progenitor cell survival possibly through FLT3-ITD methylation, supporting further exploration into how PRMT1-mediated FLT3 methylation governs LSC survival. Disclosures Khaled: Alexion: Consultancy, Speakers Bureau; Daiichi: Consultancy; Juno: Other: Travel Funding.

Abstract Background: Combination studies with histone deacetylase (HDAC) inhibitors plus hypomethylating agents (HMA) have suggested beneficial clinical activity in higher risk MDS and AML, though exceptions have also been reported. Pracinostat is a potent oral HDAC inhibitor selective for class I, II and IV isoforms. A pilot phase Ib study of pracinostat in combination with azacitidine (AZA) in higher risk MDS demonstrated a complete response (CR)/CR with incomplete blood count recovery (CRi) rate of 89% (Proc ASH:3821, 2012). Preliminary data on 33 patients from a multi-center, open-label, single-arm Phase II study of Pracinostat in combination with AZA in elderly AML also reported a high CR/CRi rate (ASH 2014). Herein we report the latest survival and response results for this study. Methods: Eligibility includes previously untreated AML (≥20% bone marrow blasts), age ≥65 years, unsuitable for intensive therapy due to co-morbidities and/or AML related features, and intermediate or high-risk cytogenetics. Study therapy includes pracinostat, 60 mg p.o. 3 alternate days/week for 3 weeks plus AZA, 75 mg/m2) days 1-7 or days 1-5 and 8-9 either s.c. or i.v. with cycles repeated every 28 days until progressive disease, lack of response or intolerance. The primary endpoint is CR+CRi+ morphologic leukemia free state (MLFS) per IWG criteria. Response assessments occur at the end of cycle 1 or 2 then every other cycle or when clinically indicated. A Simon 2-stage statistical design is utilized with the following assumptions: null=0.10, alternate=0.25, a=0.10, power=0.90. Stage 1 n=27 and total stage 2 n=40. Secondary endpoints include overall response rate (ORR; CR+CRi+MLFS+partial response [PR]+PRi), duration of response and overall survival. Results: Between Dec 2013 and Dec 2014, 50 patients from 15 study sites were enrolled. At this time, 50 are evaluable for efficacy. Baseline disease characteristics for all patients include: median age 75 (range 66-84); 32 de novo AML, 13 evolved from AHD, 5 were treatment-related; 28 intermediate-risk and 20 high-risk cytogenetics and 2 unknown; baseline bone marrow blast counts ranged from 20% to 89% with a median of 40%. Thirty-one patients (62%) continue to be followed for survival (range: 8.5 to 18.5 months). Median overall survival has not been reached in the overall study population and neither in patients with high-risk cytogenics or those with AML secondary to MDS or prior anti-cancer therapy. The 1-year overall survival estimate is 60%. The primary endpoint of CR +CRi +MLFS has been observed in 27/50 evaluable patients (54%) to date, including 21/50 (42%) CR. The 60-day all-cause mortality rate is 10% (5/50). Treatment emergent adverse events (TEAEs) Grade ≥3 seen in >5% of patients: febrile neutropenia 30%; thrombocytopenia 22%; neutropenia 10%; cellulitis 10%; anemia 8%; fatigue 8%; sepsis 6%, and pancytopenia 6%. TEAE's leading to study therapy discontinuation: peripheral motor neuropathy (1), parainfluenza (1), atrial fibrillation/prolonged QTc (1), subdural hematoma after a fall (1), and sepsis (3). Conclusions: Pracinostat plus AZA produces a high rate of durable responses in this AML population. Median overall survival has not been reached; 1-year overall survival is estimated at 60%. Final response data and overall survival estimates will be presented at the meeting. Disclosures Off Label Use: Azacitidine is not approved for use in acute myelogenous leukemia.. Khaled:Sequenom: Research Funding. Arellano:Cephalon Oncology: Research Funding. Butler:MEI Pharma, Inc.: Employment. Ashby:MEI Pharma, Inc.: Employment. Medeiros:Celgene: Honoraria, Research Funding; Agios Pharmaceuticals: Honoraria.

Abstract The presence of FMS-like receptor tyrosine kinase-3 internal tandem duplication (FLT3-ITD) mutations in patients with acute myeloid leukemia (AML) is associated with poor clinical outcome. FLT3 tyrosine kinase inhibitors (TKIs), although effective in kinase ablation, do not eliminate primitive FLT3-ITD+ leukemia cells, which are potential sources of relapse. Thus, understanding the mechanisms underlying FLT3-ITD+ AML cell persistence is essential to devise future AML therapies. Here, we show that expression of protein arginine methyltransferase 1 (PRMT1), the primary type I arginine methyltransferase, is increased significantly in AML cells relative to normal hematopoietic cells. Genome-wide analysis, coimmunoprecipitation assay, and PRMT1-knockout mouse studies indicate that PRMT1 preferentially cooperates with FLT3-ITD, contributing to AML maintenance. Genetic or pharmacological inhibition of PRMT1 markedly blocked FLT3-ITD+ AML cell maintenance. Mechanistically, PRMT1 catalyzed FLT3-ITD protein methylation at arginine 972/973, and PRMT1 promoted leukemia cell growth in an FLT3 methylation–dependent manner. Moreover, the effects of FLT3-ITD methylation in AML cells were partially due to cross talk with FLT3-ITD phosphorylation at tyrosine 969. Importantly, FLT3 methylation persisted in FLT3-ITD+ AML cells following kinase inhibition, indicating that methylation occurs independently of kinase activity. Finally, in patient-derived xenograft and murine AML models, combined administration of AC220 with a type I PRMT inhibitor (MS023) enhanced elimination of FLT3-ITD+ AML cells relative to AC220 treatment alone. Our study demonstrates that PRMT1-mediated FLT3 methylation promotes AML maintenance and suggests that combining PRMT1 inhibition with FLT3 TKI treatment could be a promising approach to eliminate FLT3-ITD+ AML cells.

Abstract Pracinostat, a potent oral pan-histone deacetylase inhibitor with modest single-agent activity in acute myeloid leukemia (AML), has shown synergistic antitumor activity when combined with azacitidine. This single-group, multicenter phase 2 study assessed the safety and efficacy of pracinostat combined with azacitidine in patients who were at least 65 years old with newly diagnosed AML and who were ineligible for standard induction chemotherapy. Patients received pracinostat 60 mg/d, 3 d/wk, for 3 consecutive weeks, plus azacitidine 75 mg/m2 daily for 7 days in a 28-day cycle. Primary endpoints were complete remission (CR), CR with incomplete count recovery (CRi), and morphologic leukemia-free state (MLFS) rates of the combination. Secondary endpoints included safety, progression-free survival (PFS), and overall survival (OS) of the regimen. Fifty patients (33 de novo, 12 secondary, and 5 therapy-related AML) were enrolled. Twenty-six patients (52%) achieved the primary endpoint of CR (42%), CRi (4%), and MLFS (6%). Median OS and PFS were 19.1 months (95% confidence interval [CI], 10-26.5 months) and 12.6 months (95% CI, 10-17.7 months), respectively, with a 1-year OS rate of 62%. Forty-three patients (86%) experienced at least 1 grade 3 or worse treatment-emergent adverse event with the combination, with infections (52%), thrombocytopenia (46%), and febrile neutropenia (44%) reported as the most common toxicities. The 30- and 60-day all-cause mortality rates were 2% and 10%, respectively. DNA sequencing revealed somatic mutations at baseline, and clearance rates correlated with response to treatment. Pracinostat plus azacitidine is a well-tolerated and active regimen in the frontline treatment of older patients with AML unfit for intensive therapy. A larger controlled trial is ongoing. This trial was registered at www.clinicaltrials.gov as #NCT01912274.