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In the MYF2001 trial, treatment of Janus kinase (JAK) inhibitor-relapsed/refractory intermediate-2 or high-risk myelofibrosis (MF) with imetelstat 9.4 mg/kg every 3 weeks demonstrated encouraging median overall survival of 29.9 months. To provide historical context, external real-world data (RWD) were collected from a study of 96 patients who had discontinued ruxolitinib and were subsequently treated with best available therapy (BAT) at Moffitt Cancer Center. A closely matched cohort was identified using the MYF2001 eligibility criteria, including patients with MF who had discontinued ruxolitinib due to lack or loss of response. Overall survival was measured from time of JAK inhibitor discontinuation to death or censored at last follow-up. To improve comparability, propensity score weighting approaches using average treatment effect for overlap population (ATO) and stabilized inverse probability treatment weighting (sIPTW) were used for 10 critical baseline covariates. Fifty-seven patients treated with imetelstat 9.4 mg/kg from MYF2001 and 38 patients treated with BAT from RWD were analyzed with improved balanced baseline covariates after propensity score adjustment, showing significantly lower risk of death with imetelstat compared with BAT (hazard ratio: 0.35; p = 0.0019). With sIPTW, results were similar. Results of sensitivity analyses were consistent with the primary analysis. In conclusion, treatment with imetelstat was associated with longer overall survival compared to BAT (30 vs 12 months, respectively) in closely matched patients with MF after JAK inhibitor failure, warranting further evaluation of imetelstat in this poor-prognosis patient population.

Acute myeloid leukemia (AML) is characterized by poor clinical outcomes due to high rates of relapse following standard-of-care induction chemotherapy. While many pathogenic drivers have been described in AML, our understanding of the molecular mechanisms mediating chemotherapy resistance remains poor. Therefore, we sought to identify resistance genes to induction therapy in AML and elucidated ALOX5 as a novel mediator of resistance to anthracycline based therapy. ALOX5 is transcriptionally upregulated in AML patient blasts in comparison to normal hematopoietic stem/progenitor cells (HSPCs) and ALOX5 mRNA, and protein expression is increased in response to induction therapy. In vitro, and in vivo genetic, and pharmacologic perturbation studies confirm that ALOX5 positively regulates the leukemogenic potential of AML LSCs, and its loss does not significantly affect the function of normal HSPCs. ALOX5 mediates resistance to daunorubicin (DNR) and promotes AML cell survival and maintenance through its leukotriene (LT) synthetic capacity, specifically via modulating synthesis of LTB4 and its binding to LTB receptor (BLTR). Our study reveals a previously unrecognized role of LTs in AML pathogenesis and chemoresistance, whereby inhibition of ALOX5 mediated LTB4 synthesis and function could be combined with standard chemotherapy, to enhance the overall therapeutic efficacy in AML.

PURPOSE Patients with myelofibrosis who are relapsed or refractory (R/R) to Janus-associated kinase inhibitors (JAKis) have poor clinical outcomes including dismal overall survival (OS) that ranges between 13 and 16 months. Imetelstat, a telomerase inhibitor, was evaluated in patients with intermediate-2 or high-risk myelofibrosis R/R to JAKi in a phase II multicenter study (ClinicalTrials.gov identifier: NCT02426086 ). PATIENTS AND METHODS Patients were randomly assigned to receive either imetelstat 9.4 mg/kg or 4.7 mg/kg intravenous once every 3 weeks. Spleen response (≥ 35% spleen volume reduction) and symptom response (≥ 50% reduction in total symptom score) rates at week 24 were coprimary end points. Secondary end points included OS and safety. RESULTS Study enrollment was closed early, and patients treated with 4.7 mg/kg were permitted to continue treatment with 9.4 mg/kg. At week 24, spleen and symptom response rates were 10.2% and 32.2% in the 9.4-mg/kg arm and 0% and 6.3% in the 4.7-mg/kg arm. Treatment with imetelstat 9.4 mg/kg led to a median OS of 29.9 months and bone marrow fibrosis improvement in 40.5% and variant allele frequency reduction of driver mutations in 42.1% of evaluable patients. Fibrosis improvement and variant allele frequency reduction correlated with OS. Target inhibition was demonstrated by reduction of telomerase activity and human telomerase reverse transcriptase level and correlated with spleen response, symptom response, and OS. Most common adverse events on both arms were grade 3 or 4 reversible cytopenias. CONCLUSION In this phase II study of two imetelstat doses, 9.4 mg/kg once every 3 weeks demonstrated clinical benefits in symptom response rate, with an acceptable safety profile for this poor-risk JAKi R/R population. Biomarker and bone marrow fibrosis assessments suggested selective effects on the malignant clone. A confirmatory phase III study is currently underway.

PURPOSE Patients with lower-risk (LR) myelodysplastic syndromes (MDS) who are RBC transfusion dependent and have experienced relapse after or are refractory to erythropoiesis-stimulating agent (ESA) have limited treatment options. High telomerase activity and human telomerase reverse-transcription expression in clonal hematopoietic cells have been reported in patients with MDS. Imetelstat, a first-in-class competitive inhibitor of telomerase enzymatic activity, targets cells with active telomerase. We report efficacy, safety, and biomarker data for patients with LR MDS who are RBC transfusion dependent and who were relapsed/refractory to ESAs. PATIENTS AND METHODS In this two-part phase II/III study (MDS3001), the primary end point was 8-week RBC transfusion independence (TI) rate, with key secondary end points of 24-week RBC TI rate, TI duration, and hematologic improvement-erythroid. RESULTS Data from the phase II part of the study are reported. Of 57 patients enrolled and treated (overall population), 38 were non-del(5q) and hypomethylating agent and lenalidomide naïve (subset population). The 8- and 24-week RBC TI rates in the overall population were 37% and 23%, respectively, with a median TI duration of 65 weeks. In the subset population, 8- and 24-week RBC TI rates were 42% and 29%, respectively, with a median TI duration of 86 weeks. Eight-week TI rate was observed across all subgroups evaluated. Cytogenetic and mutational data revealed a reduction of the malignant clones, suggesting disease modification activity. The most common adverse events were cytopenias, typically reversible within 4 weeks. CONCLUSION Imetelstat treatment results in a meaningful, durable TI rate across a broad range of heavily transfused patients with LR MDS who are ineligible for or relapsed/refractory to ESAs. Biomarker analyses indicated effects on the mutant malignant clone.

MF is a life-threatening myeloproliferative neoplasm. The JAKi ruxolitinib and fedratinib are the only FDA approved treatment options for MF. Despite benefits reported with ruxolitinib in the front-line setting, a high proportion of patients (pts) discontinue treatment (Abdelrahman 2015), and the median overall survival (OS) is 13-16 months (Kuykendall 2018; Newberry 2017; Schain 2019; Palandri 2019; Mcnamara 2019), highlighting a great unmet need for pts non-responsive to a JAKi treatment. Imetelstat, a first-in-class telomerase inhibitor, has shown meaningful clinical improvement in IMbark, a Phase 2 study in pts with Int-2 or high-risk MF who have relapsed after or are refractory to JAKi (Mascarenhas EHA 2020, ASH 2020). Treatment with imetelstat 9.4 mg/kg resulted in 32.2% symptom response (total symptom score [TSS] reduction ≥50%) at Week 24 and median OS of 28.1 months with overall study follow up of 42 months. Dose-dependent inhibition of telomerase with imetelstat resulted in on-target activity that correlated with clinical benefits; dose-dependent reduction in variant allele frequency of MF driver mutations indicated targeting of the underlying malignant clone. The Phase 2 results support continued study of imetelstat 9.4 mg/kg in a Phase 3 randomized controlled study. Study MYF3001 (IMpactMF; NCT04576156) is an open label, randomized (2:1), multicenter, Phase 3 study of imetelstat compared with BAT in ~320 pts with Int-2 or high-risk MF refractory to JAKi treatment. Pts will be randomized to receive imetelstat 9.4 mg/kg IV every 21 days or investigator selected BAT (including hydroxyurea, thalidomide, interferon, danazol, hypomethylating agents, chemotherapy, or other non-JAKi containing therapy as appropriate). Eligible pts will be stratified based on a) Int-2 or high-risk per Dynamic International Prognostic Scoring System; b) platelet count at entry (platelets ≥ 75 and < 150 x 10 9/L vs ≥ 150 x 10 9/L). Pts who meet progressive disease criteria and discontinue BAT may be eligible to crossover to imetelstat. The primary endpoint is OS, and one interim analysis is planned when >70% of death events have occurred. Secondary endpoints include symptom and spleen response rates at Week 24, progression-free survival, clinical response assessment per modified 2013 International Working Group - Myeloproliferative Neoplasms Research and Treatment criteria, time to and duration of response, reduction in degree of bone marrow fibrosis, safety, pharmacokinetics, and patient-reported outcomes. Biomarkers and mutation analyses will be performed to evaluate the impact of imetelstat on reduction/depletion of malignant clones. Approximately 180 sites are planned in North and South America, Europe, Middle East, Australia, and Asia. The study is open for enrollment. Disclosures Mascarenhas: Promedior: Consultancy, Membership on an entity's Board of Directors or advisory committees; Genentech/Roche: Consultancy, Membership on an entity's Board of Directors or advisory committees; PharmaEssentia: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Constellation: Consultancy, Membership on an entity's Board of Directors or advisory committees; Kartos: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Sierra Oncology: Consultancy, Membership on an entity's Board of Directors or advisory committees; Prelude: Consultancy; Geron: Consultancy, Research Funding; Geron: Consultancy; Galecto: Consultancy; Gilead: Consultancy, Membership on an entity's Board of Directors or advisory committees; Celgene/BMS: Consultancy, Membership on an entity's Board of Directors or advisory committees; Novartis: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; AbbVie: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Merck: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; CTI Biopharm: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Forbius: Research Funding; Incyte: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Roche: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Merus: Research Funding. Harrison: Gilead Sciences: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Sierra Oncology: Honoraria; Janssen: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Shire: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Incyte Corporation: Speakers Bureau; AOP Orphan Pharmaceuticals: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Constellation Pharmaceuticals: Research Funding; Galacteo: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Geron: 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; Abbvie: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Roche: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Promedior: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Keros: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; CTI BioPharma: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Celgene: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding, Speakers Bureau; Novartis: Membership on an entity's Board of Directors or advisory committees, Research Funding, Speakers Bureau. Kiladjian: Taiho Oncology, Inc.: Research Funding; AOP Orphan: Membership on an entity's Board of Directors or advisory committees; Novartis: Membership on an entity's Board of Directors or advisory committees; Incyte Corporation: Membership on an entity's Board of Directors or advisory committees; Bristol Myers Squibb: Membership on an entity's Board of Directors or advisory committees; PharmaEssentia: Other: Personal fees; AbbVie: Membership on an entity's Board of Directors or advisory committees. Komrokji: BMSCelgene: Consultancy, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Jazz: Consultancy, Speakers Bureau; Geron: Consultancy; Taiho Oncology: Membership on an entity's Board of Directors or advisory committees; PharmaEssentia: Membership on an entity's Board of Directors or advisory committees; Acceleron: Consultancy; AbbVie: Consultancy; Novartis: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees. Koschmieder: Abbvie: Other: Travel support; BMS: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Other: (e.g. travel support); Shire: Honoraria, Other; CTI: Membership on an entity's Board of Directors or advisory committees, Other; Alexion: Other: Travel support; AOP Pharma: Honoraria, Membership on an entity's Board of Directors or advisory committees, Other: (e.g. travel support), Research Funding; Sanofi: Membership on an entity's Board of Directors or advisory committees, Other: Travel support; Roche: Honoraria, Membership on an entity's Board of Directors or advisory committees; Incyte: Honoraria, Membership on an entity's Board of Directors or advisory committees, Other: (e.g. travel support); Pfizer: Honoraria, Membership on an entity's Board of Directors or advisory committees, Other: (e.g. travel support); Geron: Honoraria, Membership on an entity's Board of Directors or advisory committees, Other: (e.g. travel support), Research Funding; Novartis: Honoraria, Membership on an entity's Board of Directors or advisory committees, Other: Travel support, Research Funding; Image Biosciences: Other: Travel support; Baxalta: Membership on an entity's Board of Directors or advisory committees, Other; Janssen: Honoraria, Membership on an entity's Board of Directors or advisory committees, Other: (e.g. travel support), Research Funding; Ariad: Honoraria, Membership on an entity's Board of Directors or advisory committees, Other: (e.g. travel support); Bristol-Myers Squibb: Honoraria, Membership on an entity's Board of Directors or advisory committees, Other: Travel support, Research Funding; Celgene: Honoraria, Membership on an entity's Board of Directors or advisory committees, Other: (e.g. travel support); Karthos: Other: Travel support. Vannucchi: AbbVie: Membership on an entity's Board of Directors or advisory committees; Incyte: Honoraria, Membership on an entity's Board of Directors or advisory committees; BMS: Honoraria, Membership on an entity's Board of Directors or advisory committees; Novartis: Honoraria, Membership on an entity's Board of Directors or advisory committees. Berry: Geron Corp: Current Employment, Current equity holder in publicly-traded company. Sherman: Geron Corp: Current Employment, Current equity holder in publicly-traded company. Dougherty: Geron Corp: Current Employment, Current equity holder in publicly-traded company. Sun: Geron Corp: Current Employment, Current equity holder in publicly-traded company. Huang: Geron Corp: Current Employment, Current equity holder in publicly-traded company. Wan: Geron Corp: Current Employment, Current equity holder in publicly-traded company. Feller: Geron Corp: Current Employment, Current equity holder in publicly-traded company. Rizo: Geron Corp: Current Employment, Current equity holder in publicly-traded company. Verstovsek: Incyte Corporation: Consultancy, Research Funding; Protagonist Therapeutics: Research Funding; CTI BioPharma: Research Funding; NS Pharma: Research Funding; Ital Pharma: Research Funding; Promedior: Research Funding; PharmaEssentia: Research Funding; Roche: Research Funding; Gilead: Research Funding; Genentech: Research Funding; Celgene: Consultancy, Research Funding; Blueprint Medicines Corp: Research Funding; AstraZeneca: Research Funding; Novartis: Consultancy, Research Funding; Sierra Oncology: Consultancy, Research Funding; Constellation: Consultancy; Pragmatist: Consultancy. OffLabel Disclosure: Imetelstat, a first-in-class telomerase inhibitor, is undergoing clinical development but is not approved for treatment of MF

Introduction: Imetelstat is a first-in-class telomerase inhibitor that targets cells with high telomerase activity and human telomerase reverse transcriptase (hTERT) expression, characteristics observed in MDS patients (pts) across all disease stages. IMerge (MDS3001, NCT02598661) is a Phase 2/3 global study of imetelstat for red blood cell (RBC) transfusion dependent (TD) pts with non-del 5q LR-MDS ESA-R/R, who have limited treatment options. The results from the Phase 2 part indicated that imetelstat achieved durable transfusion independence (TI) with a manageable safety profile. Among 38 pts with median follow-up of 24 months, ≥8-week (w), ≥24-w, ≥1-year (y) TI rates were 42%, 32% and 29%, respectively (Steensma JCO 2020, Platzbecker ASH 2020). Aims: To assess the correlation between imetelstat's on-target activity with clinical benefits and safety data as of 10-May 2021 in the Phase 2 part of IMerge. Methods: Peripheral blood samples pre and after 1 and 2 cycles of imetelstat administration were collected to analyze hTERT level by Taqman RT-PCR assay. ≥50% hTERT reduction by imetelstat was considered optimal pharmacodynamic (PD) effect. Correlation analyses between the optimal PD and efficacy, including TI rates ≥8-w, ≥24-w, and ≥1-y, duration of the longest TI, and OS, as well as hematological and liver function lab parameters were performed. Results: hTERT analyses on 35/38 pts with matched pre- and post-1 to 2 cycles of treatment samples available demonstrated on-target activity/optimal PD effect of imetelstat in 54.3% (19/35) of pts. Pts who achieved optimal PD had significantly higher rates of TI compared to pts who did not achieve optimal PD: 63.2% (12/19) vs 25% (4/16) had ≥8-w TI (p=0.0411); 57.9% (11/19) vs 12.5% (2/16) had ≥24-w TI (p=0.0125); and 52.6% (10/19) vs 6.3% (1/16) had ≥1-y TI (p=0.0125) (Fig A). Pts who achieved optimal PD effect had a greater reduction in transfusions (both absolute change in transfusion units and % of change in transfusion burden) in the best 8-week interval compared to pts who did not. In addition, pts who achieved optimal PD had a significantly longer median TI duration (68.4 w, 95% CI 4.9, 92.4) compared to those who did not (5.5 w, 95% CI 2.3, 6.6) with a hazard ratio of 0.364 (95% CI 0.167, 0.794, log-rank p value=0.0087, Fig B). Furthermore, median OS was 57.0 months (95% CI 34.6, -) in pts who achieved optimal PD vs 40.7 months (95% CI 26.9, -) in pts who did not, and the 4-year OS rate was 58.4% vs 31.0%, respectively, although not statistically significant. Pts who achieved optimal PD also had lower rates of Grade 3+neutropenia (52.6%), thrombocytopenia (57.9%), or liver function elevations (5.3%) compared to pts who did not achieve optimal PD (68.8%, 68.8% and 18.8%, respectively), although the differences were not statistically significant. Conclusion: Optimal PD effect of imetelstat treatment was demonstrated by ≥50% hTERT reduction. A significant correlation was observed between optimal PD effect of imetelstat and durable TI and improved 4-year OS rate, effectively linking imetelstat activity to clinical efficacy. Additionally, pts who achieved an optimal PD effect by imetelstat treatment did not have higher rates of cytopenias or liver enzyme elevations compared to pts without optimal PD effect. Enrollment is ongoing for the Phase 3 part of IMerge, a randomized (2:1) double-blind, placebo-controlled trial to compare efficacy of imetelstat vs. placebo. Figure 1 Figure 1. Disclosures Santini: Menarini: Membership on an entity's Board of Directors or advisory committees; Novartis: Honoraria, Membership on an entity's Board of Directors or advisory committees; Takeda: Membership on an entity's Board of Directors or advisory committees; Astex: Membership on an entity's Board of Directors or advisory committees; Gilead: Membership on an entity's Board of Directors or advisory committees; Geron: Membership on an entity's Board of Directors or advisory committees; BMS/Celgene: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding. Fenaux: JAZZ: Honoraria, Research Funding; Abbvie: Honoraria, Research Funding; Takeda: Honoraria, Research Funding; Novartis: Honoraria, Research Funding; Celgene/BMS: Honoraria, Research Funding; Janssen: Honoraria, Research Funding; Syros Pharmaceuticals: Honoraria. Raza: Celgene Inc: Research Funding, Speakers Bureau; Novartis: Speakers Bureau; Genoptix: Speakers Bureau; Kura Oncology: Research Funding; Janssen R&D: Research Funding; Syros Pharmaceuticals: Research Funding; Onconova Therapeutics: Research Funding, Speakers Bureau. Germing: Bristol-Myers Squibb: Honoraria, Other: advisory activity, Research Funding; Janssen: Honoraria; Jazz Pharmaceuticals: Honoraria; Novartis: Honoraria, Research Funding; Celgene: Honoraria. Font: CELGENE-BMS: Consultancy, Membership on an entity's Board of Directors or advisory committees, Other: Travel, accomodations, expenses; GILEAD: Membership on an entity's Board of Directors or advisory committees; Menarini: Membership on an entity's Board of Directors or advisory committees; Pfizer: Consultancy, Membership on an entity's Board of Directors or advisory committees; Novartis: Membership on an entity's Board of Directors or advisory committees, Other: Travel, accommodations, expenses, Speakers Bureau; Abbvie: Other: Travel, accomodations, expenses. Diez-Campelo: BMS: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding, Speakers Bureau; Takeda Oncology: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding, Speakers Bureau; Novartis: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding, Speakers Bureau. Patnaik: Kura Oncology: Research Funding; Stemline Therapeutics: Membership on an entity's Board of Directors or advisory committees; Stemline Therapeutics: Membership on an entity's Board of Directors or advisory committees. Sherman: Geron Corp: Current Employment, Current equity holder in publicly-traded company. Berry: Geron Corp: Current Employment, Current equity holder in publicly-traded company. Feller: Geron Corp: Current Employment, Current equity holder in publicly-traded company. Dougherty: Geron Corp: Current Employment, Current equity holder in publicly-traded company. Sun: Geron Corp: Current Employment, Current equity holder in publicly-traded company. Wan: Geron Corp: Current Employment, Current equity holder in publicly-traded company. Rizo: Geron Corp: Current Employment, Current equity holder in publicly-traded company. Huang: Geron Corp: Current Employment, Current equity holder in publicly-traded company. Platzbecker: Takeda: Honoraria; AbbVie: Honoraria; Janssen: Honoraria; Novartis: Honoraria; Geron: Honoraria; Celgene/BMS: Honoraria. OffLabel Disclosure: Imetelstat, a first-in-class telomerase inhibitor, is under clinical development but not approved for treatment of MDS

Background: Myelofibrosis (MF) is a life-threatening myeloproliferative neoplasm characterized by splenomegaly and debilitating symptoms, such as fatigue, pruritus, night sweats, fever, bone pain, and weight loss that impact quality of life (QoL). Imetelstat, a first-in-class telomerase inhibitor, demonstrated clinical benefits in terms of symptom response and potential improvement in overall survival in a pilot study in MF patients (pts) (Tefferi et al, NEJM 2015) and in IMbark, a Phase 2 study in MF pts relapsed or refractory (R/R) to a Janus associated kinase (JAK) inhibitor (Mascarenhas et al, ASH 2018 #685). Aims: We assessed the effects of imetelstat on MF symptom burden and QoL in IMbark, and the correlations of MF-related symptoms measured by modified Myelofibrosis Symptom Assessment Form (MFSAF) 2.0 and other patient-reported outcome (PRO) endpoints. Methods: IMbark (MYF2001; NCT02426086) is a two-dose, randomized, single-blinded, Phase 2 study of imetelstat in R/R intermediate-2/high-risk MF pts, who received imetelstat 9.4 mg/kg or 4.7 mg/kg IV every 3 weeks. Symptom response, one of the co-primary endpoints, was defined as ≥50% reduction in total symptom score (TSS) from baseline to Week 24 as measured by the modified MFSAF 2.0 e-diary. The TSS was calculated as the 7-day average of daily TSS, which is the summation of 6 individual symptom scores (night sweats, itchiness, abdominal discomfort, pain under ribs on left side, early satiety, bone or muscle pain). Correlation of TSS with other PROs was explored: European Organization for Research and Treatment of Cancer Quality of Life Questionnaire-Core 30 (EORTC QLQ-C30), Brief Pain Inventory (BP-I), and Patient Global Impression of Change (PGIC). Results: All 107 pts enrolled in IMbark were symptomatic, with baseline mean TSS scores of 25.7 in the 9.4 mg/kg arm (N=59) and 24.6 in the 4.7 mg/kg arm (N=48), indicating a high symptom burden. Pts in both arms had ≥96.5% mean compliance rates with completion of the e-diary. Treatment with imetelstat demonstrated a statistically significant dose-related improvement in TSS symptom response rate at Week 24 for 9.4 mg/kg vs 4.7 mg/kg (32.1% vs 6.3%, p=0.001) and at any time (52.5% vs 27.1%, p=0.010). A higher proportion of pts in the 9.4 mg/kg arm than in the 4.7 mg/kg arm achieved ≥50% reduction in 5 individual symptoms, including night sweats, itchiness, abdominal pressure, pain under left ribs, and early satiety. Based on EORTC QLQ-C30, improvements at Week 24 relative to baseline were seen in the imetelstat 9.4 mg/kg arm for global health status, all 5 function subscales, and all 3 symptoms. Notably for fatigue (a common MF symptom which was not measured in MFSAF v2.0), pts in the 9.4 mg/kg arm had improvement at Week 24 relative to baseline, compared to the pts in the 4.7 mg/kg arm (LS means -13.3 vs -3.1, p=0.042). The TSS symptom responses were shown to correlate with other PROs. Compared to pts in the 4.7 mg/kg arm, TSS symptom responders in the 9.4 mg/kg arm achieved significantly greater improvements in the global health status (p=0.004), fatigue (p=0.009), pain (p=0.033), and most of the functional scales in EORTC QLQ-C30. Pain scores per the modified MFSAF v2.0 correlated with the pain scores in EORTC QLQ-C30 and BP-I (correlation coefficients 0.5-0.6). More than 90% of TSS symptom responders in the 9.4 mg/kg arm also characterized their condition as having had either "very much improvement" (36.4%) or "somewhat improvement" (54.6%) in PGIC. Conclusion: These data show a dose-related, clinically meaningful improvement in overall and individual MF symptoms as measured by MFSAF 2.0 with imetelstat treatment in pts who are JAK inhibitor R/R. TSS symptom responders treated with imetelstat 9.4 mg/kg also had improvement in QoL as measured by EORTC QLQ-C30. The data further support the robustness of overall and individual symptom improvement in pts with MF as evidenced by the correlation of modified MFSAF v2.0 with other PROs such as EORTC QLQ-C30, PGIC, and BP-I. Disclosures Mascarenhas: Incyte, Kartos, Roche, Promedior, Merck, Merus, Arog, CTI Biopharma, Janssen, and PharmaEssentia: Other: Research funding (institution); Celgene, Prelude, Galecto, Promedior, Geron, Constellation, and Incyte: Consultancy. Komrokji:Geron: Honoraria; BMS: Honoraria, Speakers Bureau; Acceleron: Honoraria; Novartis: Honoraria; Incyte: Honoraria; JAZZ: Honoraria, Speakers Bureau; AbbVie: Honoraria; Agios: Honoraria, Speakers Bureau. Cavo:Jannsen, BMS, Celgene, Sanofi, GlaxoSmithKline, Takeda, Amgen, Oncopeptides, AbbVie, Karyopharm, Adaptive: Consultancy, Honoraria. Niederwieser:Novartis: Speakers Bureau; Amgen: Speakers Bureau; Cellectis: Membership on an entity's Board of Directors or advisory committees; Daiichi: Research Funding. Reiter:Gilead: Other: travel support ; Incyte: Consultancy, Other: travel support ; AOP: Consultancy, Other: travel support ; Celgene,: Consultancy, Other: travel support ; Abbvie: Consultancy, Other: travel support ; Deciphera: Consultancy, Other: travel support ; Blueprint: Consultancy, Other: travel support ; Novartis: Consultancy, Honoraria, Other: travel support , Research Funding. Scott:Agios, BMS: Honoraria; BMS, Novartis: Research Funding; Alexion, Incyte, Novartis, Regeneron: Consultancy. Baer:AbbVie: Other: Institutional research funding; Astellas: Other: Institutional research funding; Forma: Other: Institutional research funding; Incyte: Other: Institutional research funding; Kite: Other: Institutional research funding; Oscotec: Other: Institutional research funding; Takeda: Other: Institutional research funding. Hoffman:Dompe: Research Funding; Protagonist: Consultancy; Novartis: Membership on an entity's Board of Directors or advisory committees; Abbvie: Membership on an entity's Board of Directors or advisory committees; Forbius: Consultancy. Odenike:AbbVie: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Celgene: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Novartis: Consultancy, Membership on an entity's Board of Directors or advisory committees; Astex Pharmaceuticals, NS Pharma, Gilead Sciences, Janssen Oncology, Oncotherapy, Agios, CTI/Baxalta, Aprea: Other: Institutional research funding; Astra Zeneca: Research Funding; Incyte: Other: Institutional research funding; Impact Biomedicines: Consultancy, Membership on an entity's Board of Directors or advisory committees. Sherman:Geron Corp: Current Employment, Current equity holder in publicly-traded company. Dougherty:Geron Corp: Current Employment, Current equity holder in publicly-traded company. Feller:Geron Corp: Current Employment, Current equity holder in private company. Huang:Geron Corp: Current Employment, Current equity holder in publicly-traded company. Rizo:Geron Corp: Current Employment, Current equity holder in publicly-traded company. Sun:Geron Corp: Current Employment, Current equity holder in publicly-traded company. Wan:Geron Corp: Current Employment, Current equity holder in publicly-traded company. Vannucchi:AbbVie: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Celgene/BMS: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Blueprint: Membership on an entity's Board of Directors or advisory committees; Novartis: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Incyte: Membership on an entity's Board of Directors or advisory committees.

Background: Imetelstat is a first-in-class telomerase inhibitor currently in clinical development for hematologic myeloid malignancies. IMbark (MYF2001; NCT02426086) was a 2-dose (9.4 mg/kg or 4.7 mg/kg IV every 3 weeks), randomized Phase 2 study of imetelstat in intermediate-2/high-risk myelofibrosis (MF) relapsed or refractory to prior Janus kinase inhibitor treatment. Comparing the 9.4 mg/kg arm to the 4.7 mg/kg arm, the rate of symptom response (total symptom score [TSS] reduction ≥50%) at Week 24 was 32.2% and 6.3%, respectively (Mascarenhas et al, ASH 2018 #685), and median overall survival (OS) was 28.1 months (mos) (95% confidence interval [CI]: 22.8, 31.6) for the 9.4 mg/kg arm and 19.9 mos for the 4.7 mg/kg arm (95% CI: 17.1, 33.9) with an overall study follow up of 42 mos (Mascarenhas et al, EHA 2020, EP1107). Dose-dependent on-target pharmacodynamic (PD) activity of imetelstat was observed and it correlated with clinical responses and longer OS (Mascarenhas et al, EHA 2020 EP1098). Here we report the results of imetelstat exposure-response analyses from IMbark to further evaluate the benefit/risk profile and justify 9.4 mg/kg every 21 days as the optimal dosing regimen for the planned Phase 3 study of imetelstat in refractory MF. Methods: The average plasma concentration (Cavg) of imetelstat was used to define the exposure. The Cavg values of 107 patients (pts) were grouped into 4 quartiles (Q1-4) representing different levels of imetelstat exposure regardless of the protocol-specified dose arm assignment. Optimal PD effect of imetelstat was defined as ≥50% reduction in telomerase activity or human telomerase reverse transcriptase (hTERT) from baseline. The exposure-response relationships between exposure quartiles and PD effect, symptom response, OS, and laboratory safety parameters were assessed. Results: The association between the 4 exposure quartiles and dose levels showed that 89% of pts in Q1 (the lowest exposure quartile) were treated with 4.7 mg/kg, while 96% of pts in Q4 (the highest exposure quartile) were treated with 9.4 mg/kg. The pts in the 4.7 mg/kg arm who had higher exposure in Q3 and Q4 were mainly those with dose escalation to 9.4 mg/kg, and pts in the 9.4 mg/kg arm who had lower exposure in Q1-Q2 were mainly those with dose delay, reduction, or interruption. 51.9% of pts in Q1, 63% in Q2 (vs Q1, p=0.5826), 77.8% in Q3 (vs Q1, p=0.0861), and 80% in Q4 (vs Q1, p=0.0439) achieved the optimal PD effect, respectively, indicating exposure-dependent on-target activity of imetelstat. The symptom response rate for pts in exposure quartile Q1, Q2, Q3, and Q4 was 7.4%, 18.5% (vs Q1, p=0.4203), 18.5% (vs Q1, p=0.4203), and 38.5% (vs Q1, p=0.0091), respectively. Median OS was 18.9 mos in Q1, 23.6 mos in Q2 (hazard ratio [HR]=0.663; 95% CI: 0.350, 1.259; log-rank p=0.2097), 24.6 mos in Q3 (HR=0.656; 95% CI 0.348, 1.236; p=0.1920) and 30.6 mos in Q4 (HR=0.467; 95% CI: 0.236, 0.925; p=0.0260), respectively. The exposure-response analyses showed that pts in the highest exposure quartile were more likely to have better clinical outcome. The relationship between exposure quartiles and hematological and liver function safety parameters was assessed. There were no significant differences between each imetelstat exposure quartile group on the change from baseline in hemoglobin concentration, platelet count, and neutrophil counts; or in the rate of Grade 3+ neutropenia, thrombocytopenia, and elevations in alanine aminotransferase, alkaline phosphatase, aspartate aminotransferase, and bilirubin. Pts with higher imetelstat exposure had similar rates of Grade 3+ adverse events as pts with lower exposure, suggesting that the lower dose does not improve safety. Conclusions: In summary, the exposure-response analysis results indicated 9.4 mg/kg and 4.7 mg/kg covered the therapeutic window of imetelstat in MF pts. Imetelstat 9.4 mg/kg every 21 days treatment yielded higher exposure, leading to higher rate of pts achieving the optimal PD effect, consistently better clinical benefits, such as higher symptom response and significantly longer median OS, and had a similar safety profile as 4.7 mg/kg with regard to hematologic and liver function parameters. This exposure-response analysis of benefit/risk profile supports 9.4 mg/kg every 21 days as the optimal dosing regimen for the planned imetelstat Phase 3 study in refractory MF. Disclosures Mascarenhas: Celgene, Prelude, Galecto, Promedior, Geron, Constellation, and Incyte: Consultancy; Incyte, Kartos, Roche, Promedior, Merck, Merus, Arog, CTI Biopharma, Janssen, and PharmaEssentia: Other: Research funding (institution). Komrokji:Acceleron: Honoraria; Agios: Honoraria, Speakers Bureau; AbbVie: Honoraria; JAZZ: Honoraria, Speakers Bureau; Incyte: Honoraria; Novartis: Honoraria; BMS: Honoraria, Speakers Bureau; Geron: Honoraria. Cavo:Jannsen, BMS, Celgene, Sanofi, GlaxoSmithKline, Takeda, Amgen, Oncopeptides, AbbVie, Karyopharm, Adaptive: Consultancy, Honoraria. Niederwieser:Amgen: Speakers Bureau; Novartis: Speakers Bureau; Daiichi: Research Funding; Cellectis: Membership on an entity's Board of Directors or advisory committees. Reiter:Gilead: Other: travel support ; Incyte: Consultancy, Other: travel support ; AOP: Consultancy, Other: travel support ; Celgene,: Consultancy, Other: travel support ; Abbvie: Consultancy, Other: travel support ; Deciphera: Consultancy, Other: travel support ; Blueprint: Consultancy, Other: travel support ; Novartis: Consultancy, Honoraria, Other: travel support , Research Funding. Scott:Agios, BMS: Honoraria; Alexion, Incyte, Novartis, Regeneron: Consultancy; BMS, Novartis: Research Funding. Baer:AbbVie: Other: Institutional research funding; Astellas: Other: Institutional research funding; Forma: Other: Institutional research funding; Incyte: Other: Institutional research funding; Kite: Other: Institutional research funding; Oscotec: Other: Institutional research funding; Takeda: Other: Institutional research funding. Hoffman:Protagonist: Consultancy; Dompe: Research Funding; Forbius: Consultancy; Novartis: Membership on an entity's Board of Directors or advisory committees; Abbvie: Membership on an entity's Board of Directors or advisory committees. Odenike:AbbVie: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Celgene: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Impact Biomedicines: Consultancy, Membership on an entity's Board of Directors or advisory committees; Incyte: Other: Institutional research funding; Astra Zeneca: Research Funding; Astex Pharmaceuticals, NS Pharma, Gilead Sciences, Janssen Oncology, Oncotherapy, Agios, CTI/Baxalta, Aprea: Other: Institutional research funding; Novartis: Consultancy, Membership on an entity's Board of Directors or advisory committees. Sherman:Geron Corp: Current Employment, Current equity holder in publicly-traded company. Dougherty:Geron Corp: Current Employment, Current equity holder in publicly-traded company. Berry:Geron Corp: Current Employment, Current equity holder in publicly-traded company. Feller:Geron Corp: Current Employment, Current equity holder in private company. Sun:Geron Corp: Current Employment, Current equity holder in publicly-traded company. Wan:Geron Corp: Current Employment, Current equity holder in publicly-traded company. Rizo:Geron Corp: Current Employment, Current equity holder in publicly-traded company. Huang:Geron Corp: Current Employment, Current equity holder in publicly-traded company.

Background: Myelofibrosis (MF) is a life-threatening myeloproliferative neoplasm. Ruxolitinib, a Janus kinase 1 (JAK1)/Janus kinase 2 (JAK2) inhibitor and fedratinib, a JAK2/FLT3 inhibitor, are the only approved treatment options for MF. Despite the benefits reported with ruxolitinib in the front-line setting, a high proportion of patients discontinue treatment, the 1-, 2-, and 3-year discontinuation rates are 49%, 71%, and 86%, respectively (Abdelrahman et al, 2015). For patients who discontinue treatment with ruxolitinib, the median overall survival (OS) is dismal and ranges from 13 to 16 months (Kuykendall et al, 2018; Newberry et al, 2017; Schain et al, 2019; Palandri et al, 2019; Mcnamara et al, 2019). There remains a great unmet need for patients who are non-responsive to and have discontinued treatment with a JAK inhibitor. Imetelstat, a first-in-class telomerase inhibitor, has shown meaningful clinical improvement in symptom response and improved OS in IMbark, a Phase 2 study in patients with intermediate-2 or high-risk MF who have relapsed after or are refractory to JAK inhibitors (Mascarenhas et al, ASH 2018 #685; Mascarenhas et al, EHA 2020 #EP1107). Nineteen (32.2%) patients in the 9.4 mg/kg arm and 3 (6.3%) patients in the 4.7 mg/kg arm achieved symptom response (total symptom score [TSS] reduction ≥50%) at Week 24. As of clinical cutoff (7 February 2020), with an overall study follow up of 42 months, median OS was 28.1 months for the 9.4 mg/kg arm (95% confidence interval [CI]: 22.8, 31.6) and 19.9 months for the 4.7 mg/kg arm (95% CI: 17.1, 33.9). The improvement in OS for patients treated with 9.4mg/kg imetelstat was further supported by analyses of IMbark patients with closely matched real world controls (Kuykendall et al, EHA 2019 #PS1456). Taken together, these findings support continued study of imetelstat 9.4 mg/kg dose in a well-designed Phase 3 randomized controlled study in patients with refractory MF. Methods: Study MYF3001 is an open label, randomized (2:1), multicenter, Phase 3 study of imetelstat compared with best available therapy (BAT) in approximately 320 patients with intermediate-2 or high-risk MF (primary MF, post-essential thrombocythemia-MF, or post-polycythemia vera-MF) who are refractory to JAK inhibitor treatment. Approximately 214 patients will be randomized to Arm A to receive imetelstat, and approximately 106 patients will be randomized to Arm B to receive BAT. Eligible patients will be stratified based on: a) intermediate-2 or high-risk per Dynamic International Prognostic Scoring System (DIPSS); and b) platelet count at study entry (platelets ≥75 x 109/L and The primary endpoint of the study is OS. Secondary endpoints include symptom response rate at week 24, progression-free survival, spleen response rate at week 24, clinical response assessment per modified 2013 International Working Group - Myeloproliferative Neoplasms Research and Treatment (IWG MRT) criteria, time to and duration of response, reduction in the degree of bone marrow fibrosis, safety, pharmacokinetics, and patient-reported outcomes. One interim analysis based on OS is planned. Biomarkers relevant to the mechanism of action of imetelstat will be assessed to demonstrate target inhibition and their association with OS and clinical responses. Mutation analyses will be performed to evaluate the impact of imetelstat on reduction/depletion of malignant clones leading to disease modification. The study will be conducted at approximately 150 centers in North and South America, Europe, Asia, and Middle East. Disclosures Mascarenhas: Incyte, Kartos, Roche, Promedior, Merck, Merus, Arog, CTI Biopharma, Janssen, and PharmaEssentia: Other: Research funding (institution); Celgene, Prelude, Galecto, Promedior, Geron, Constellation, and Incyte: Consultancy. Harrison:Promedior: Honoraria; Celgene: Honoraria, Research Funding, Speakers Bureau; Gilead Sciences: Honoraria, Speakers Bureau; Sierra Oncology: Honoraria; Shire: Honoraria, Speakers Bureau; AOP Orphan Pharmaceuticals: Honoraria; Roche: Honoraria; Janssen: Speakers Bureau; Novartis: Honoraria, Research Funding, Speakers Bureau; CTI Biopharma Corp: Honoraria, Speakers Bureau; Incyte Corporation: Speakers Bureau. Mesa:Promedior: Research Funding; Genentech: Research Funding; Samus Therapeutics: Research Funding; Incyte: Research Funding; AbbVie: Research Funding; Bristol Myers Squibb: Research Funding; Novartis: Consultancy; CTI BioPharma: Research Funding; LaJolla Pharmaceutical Company: Consultancy; Sierra Oncology: Consultancy. Komrokji:Incyte: Honoraria; BMS: Honoraria, Speakers Bureau; Acceleron: Honoraria; Geron: Honoraria; Novartis: Honoraria; AbbVie: Honoraria; JAZZ: Honoraria, Speakers Bureau; Agios: Honoraria, Speakers Bureau. Koschmieder:Celgene/BMS: Honoraria, Membership on an entity's Board of Directors or advisory committees, Other: travel support, Research Funding; Janssen: Honoraria, Membership on an entity's Board of Directors or advisory committees, Other: travel support, Research Funding; Geron Corporation: Honoraria, Membership on an entity's Board of Directors or advisory committees, Other: travel support; Incyte/Ariad: Honoraria, Membership on an entity's Board of Directors or advisory committees, Other: travel support; Roche: Honoraria, Membership on an entity's Board of Directors or advisory committees, Other: travel support; CTI Biopharma: Honoraria, Membership on an entity's Board of Directors or advisory committees, Other: travel support; Novartis: Honoraria, Membership on an entity's Board of Directors or advisory committees, Other: travel support, Research Funding; Pfizer: Honoraria, Membership on an entity's Board of Directors or advisory committees, Other: travel support; Bayer: Honoraria, Membership on an entity's Board of Directors or advisory committees, Other: travel support; AOP Pharma: Honoraria, Membership on an entity's Board of Directors or advisory committees, Other: travel support, Research Funding; Promedior: Other. Vannucchi:AbbVie: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Celgene/BMS: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Blueprint: Membership on an entity's Board of Directors or advisory committees; Incyte: Membership on an entity's Board of Directors or advisory committees; Novartis: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau. Berry:Geron Corp: Current Employment, Current equity holder in publicly-traded company. Sherman:Geron Corp: Current Employment, Current equity holder in publicly-traded company. Dougherty:Geron Corp: Current Employment, Current equity holder in publicly-traded company. Sun:Geron Corp: Current Employment, Current equity holder in publicly-traded company. Huang:Geron Corp: Current Employment, Current equity holder in publicly-traded company. Wan:Geron Corp: Current Employment, Current equity holder in publicly-traded company. Feller:Geron Corp: Current Employment, Current equity holder in private company. Rizo:Geron Corp: Current Employment, Current equity holder in publicly-traded company. Verstovsek:PharmaEssentia: Research Funding; AstraZeneca: Research Funding; ItalPharma: Research Funding; Protagonist Therapeutics: Research Funding; CTI Biopharma Corp: Research Funding; Sierra Oncology: Consultancy, Research Funding; Novartis: Consultancy, Research Funding; Blueprint Medicines Corp: Research Funding; Genentech: Research Funding; Incyte Corporation: Consultancy, Research Funding; NS Pharma: Research Funding; Roche: Research Funding; Promedior: Research Funding; Gilead: Research Funding; Celgene: Consultancy, Research Funding.

Background: Myelofibrosis (MF) is a clonal stem cell disease characterized by marrow fibrosis and a heterogeneous disease phenotype with a variable degree of splenomegaly, cytopenias, and constitutional symptoms that significantly impact quality of life and survival. Both cytogenetic abnormalities and gene mutations in MF carry prognostic significance. Development of novel agents for treatment of MF to target the underlying malignant clones remains a significant area of unmet need. IMbark (MYF2001; NCT02426086) is a randomized Phase 2 study of imetelstat (9.4 mg/kg or 4.7 mg/kg) in intermediate-2/high-risk MF relapsed/refractory to prior Janus kinase inhibitor treatment. Comparing the 9.4 mg/kg arm to the 4.7 mg/kg arm, the rate of symptom response (total symptom score [TSS] reduction ≥50%) was 32.2% and 6.3%, respectively (Mascarenhas et al ASH 2018 #685), and median overall survival (OS) was 28.1 months (mos) (95% confidence interval [CI]: 22.8, 31.6) and 19.9 mos (95% CI: 17.1, 33.9) with an overall study follow up of 42 mos (Mascarenhas et al, EHA 2020, EP1107). Dose-dependent target inhibition as evaluated by reduction of telomerase activity and human telomerase reverse transcriptase level correlated with clinical responses and longer OS; also, reduction in variant allele frequency (VAF) of driver mutations by imetelstat was reported (Mascarenhas et al EHA 2020 #EP1098). Aims: To evaluate imetelstat activity on depletion of malignant cells (cytogenetically abnormal clones and mutation burden); to assess the relationships between baseline cytogenetic status and change in mutation VAF with clinical benefits such as spleen volume reduction, symptom response, bone marrow fibrosis improvement, and OS. Methods: Cytogenetic analyses were performed on bone marrow aspirates. Peripheral blood samples pre and post imetelstat administration were collected and granulocytes were isolated to analyze mutations and VAF by next-generation sequencing (NGS) using the Illumina TruSight Myeloid Sequencing Panel. Bone marrow fibrosis was assessed by a central pathologist. Results: 83 patients (pts) had baseline cytogenetic results available; 45 (54.2%) had an abnormal karyotype, including 32 (71.1%) with a sole abnormality, the most prevalent being deletions 13q- (24.4%) and 20q- (8.8%), and 5 (11.1%) with a complex karyotype. Spleen and symptom responses were observed in pts with or without abnormal karyotype. Of the 24 pts that had identified cytogenetic abnormalities at baseline and with matched post-treatment cytogenetic results available, 5 (20.8%) achieved ≥50% reduction in their cytogenetically abnormal clones, and interestingly they all had sole deletion 13q-. 49 pts had matched pre- and post-imetelstat treatment (3-14 mos) NGS data and had at least one mutation at baseline. 46.2% of pts (12/26) in the 9.4 mg/kg vs 17.4% (4/23) in the 4.7 mg/kg arm (p=0.0321) achieved ≥20% VAF reduction in any of the mutated genes present at baseline. In addition, pts who achieved ≥20% VAF reduction had higher rates of spleen response (12.5% vs 3%), symptom response (31.3% vs 24.2%) or bone marrow fibrosis improvement (54.4% vs 25%) compared to pts who did not have VAF reduction regardless of dose level (Fig. 1A). Furthermore, a prolonged median OS was seen (Fig. 1B) in pts who achieved ≥20% VAF reduction (31.6 mos, 95% CI: 21.5, NE) vs those with no VAF reduction (22.8 mos, 95% CI: 17.1, 31.6). The 3-year survival rate was 45.5% for pts who achieved ≥20% VAF reduction vs 14.9% for pts with no VAF reduction. Conclusions: Clinical responses to imetelstat treatment were observed regardless of baseline cytogenetic status, and a subset of pts achieved ≥50% reduction in cytogenetically abnormal clones. Significant dose-dependent reductions of mutation burden by imetelstat were noted and correlated with improved overall clinical benefits including higher rates of spleen and symptom responses, bone marrow fibrosis improvement, and prolonged OS. Together with the clinical data that suggest improvement in median OS in these pts, the data presented here further demonstrate that imetelstat has disease-modifying activity by targeting malignant cells, as evidenced by depletion of cytogenetically abnormal clones and reduction in mutation burden. These results will be confirmed in the planned Phase 3 study in refractory MF. Figure Disclosures Mascarenhas: Celgene, Prelude, Galecto, Promedior, Geron, Constellation, and Incyte: Consultancy; Incyte, Kartos, Roche, Promedior, Merck, Merus, Arog, CTI Biopharma, Janssen, and PharmaEssentia: Other: Research funding (institution). Komrokji:Novartis: Honoraria; BMS: Honoraria, Speakers Bureau; Agios: Honoraria, Speakers Bureau; JAZZ: Honoraria, Speakers Bureau; Acceleron: Honoraria; AbbVie: Honoraria; Geron: Honoraria; Incyte: Honoraria. Cavo:Jannsen, BMS, Celgene, Sanofi, GlaxoSmithKline, Takeda, Amgen, Oncopeptides, AbbVie, Karyopharm, Adaptive: Consultancy, Honoraria. Niederwieser:Novartis: Speakers Bureau; Cellectis: Membership on an entity's Board of Directors or advisory committees; Amgen: Speakers Bureau; Daiichi: Research Funding. Reiter:Gilead: Other: travel support ; Incyte: Consultancy, Other: travel support ; AOP: Consultancy, Other: travel support ; Celgene,: Consultancy, Other: travel support ; Abbvie: Consultancy, Other: travel support ; Deciphera: Consultancy, Other: travel support ; Blueprint: Consultancy, Other: travel support ; Novartis: Consultancy, Honoraria, Other: travel support , Research Funding. Scott:Alexion, Incyte, Novartis, Regeneron: Consultancy; BMS, Novartis: Research Funding; Agios, BMS: Honoraria. Baer:Astellas: Other: Institutional research funding; Forma: Other: Institutional research funding; Incyte: Other: Institutional research funding; Kite: Other: Institutional research funding; Oscotec: Other: Institutional research funding; Takeda: Other: Institutional research funding; AbbVie: Other: Institutional research funding. Hoffman:Protagonist: Consultancy; Forbius: Consultancy; Novartis: Membership on an entity's Board of Directors or advisory committees; Abbvie: Membership on an entity's Board of Directors or advisory committees; Dompe: Research Funding. Odenike:Novartis: Consultancy, Membership on an entity's Board of Directors or advisory committees; Astex Pharmaceuticals, NS Pharma, Gilead Sciences, Janssen Oncology, Oncotherapy, Agios, CTI/Baxalta, Aprea: Other: Institutional research funding; Astra Zeneca: Research Funding; Impact Biomedicines: Consultancy, Membership on an entity's Board of Directors or advisory committees; Incyte: Other: Institutional research funding; AbbVie: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Celgene: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding. Sherman:Geron Corp: Current Employment, Current equity holder in publicly-traded company. Dougherty:Geron Corp: Current Employment, Current equity holder in publicly-traded company. Feller:Geron Corp: Current Employment, Current equity holder in private company. Berry:Geron Corp: Current Employment, Current equity holder in publicly-traded company. Sun:Geron Corp: Current Employment, Current equity holder in publicly-traded company. Wan:Geron Corp: Current Employment, Current equity holder in publicly-traded company. Rizo:Geron Corp: Current Employment, Current equity holder in publicly-traded company. Huang:Geron Corp: Current Employment, Current equity holder in publicly-traded company.

Background: Current treatment options for red blood cell (RBC) transfusion-dependent (TD) patients with lower risk (LR) myelodysplastic syndromes (MDS) relapsed after or refractory to erythropoiesis-stimulating agents (ESAs) have limited efficacy and durability; new approaches are needed. Imetelstat is a 13-mer lipid-conjugated oligonucleotide that targets the RNA template of human telomerase and is a competitive inhibitor of telomerase enzymatic activity (Asai et al, Cancer Res 2003; Herbert et al, Oncogene 2005). Preclinical, in vivo xenograft models (Dikmen et al, Cancer Res 2005; Hochreiter et al, Clin Cancer Res 2006) and preliminary clinical data from a pilot study conducted at Mayo Clinic (Tefferi et al, Blood Cancer Journal 2016) supported initiation of a study in TD LR MDS patients. A Phase 2 study of imetelstat, IMerge, demonstrated an 8-week RBC transfusion independence (RBC-TI) rate of 42%, 24-week RBC-TI rate of 29%, and 68% erythroid hematologic improvement (HI-E) rate in 38 heavily TD patients (median prior RBC transfusion burden 8 units / 8 weeks over the 16 weeks pre-study period) with LR MDS. Responses were durable with median duration of 8-week RBC-TI of 85.9 weeks by Kaplan Meier estimates (range 8.0-140.9) (Steensma ASH 2018, Fenaux EHA 2019). These Phase 2 results provided further evidence of potential clinical benefit of imetelstat treatment in TD LR MDS, and supported initiation of a Phase 3 trial. Methods: IMerge is two-part, Phase 2/3 study (ClinicalTrials.gov: NCT02598661). The Phase 2 portion of the study described above is closed for enrollment. The Phase 3 portion of the study is open for enrollment of adult patients with International Prognostic Scoring System (IPSS) low or intermediate-1 risk, non-del(5q) MDS, who are TD, are relapsed after or refractory to ESAs, and have not received treatment with lenalidomide or hypomethylating agents. The study is a randomized (2:1) double-blind, placebo-controlled trial to compare efficacy of imetelstat vs. placebo that will enroll approximately 170 patients and will be conducted at approximately 90 centers in North America, Europe, Asia and Middle East. Imetelstat will be administered as 2-hour IV infusion every 4 weeks at 7.5 mg/kg. The primary endpoint of the study is to assess the rate of RBC-TI lasting ≥8 weeks. Secondary endpoints include safety, rate of RBC-TI ≥24 weeks, time to RBC-TI start, RBC-TI duration, rate of HI-E, the amount and relative change in RBC transfusions, rate of CR or PR, overall survival, progression of MDS, pharmacokinetics and effect of treatment on quality of life. Biomarkers relevant to the mechanism of action of imetelstat will be assessed to demonstrate target inhibition and their association with clinical responses. Cytogenetics and mutation analyses will be performed to evaluate the impact of imetelstat on reduction/depletion of malignant clones leading to disease modification. Disclosures Platzbecker: Novartis: Consultancy, Honoraria, Research Funding; Celgene: Consultancy, Honoraria, Research Funding; Abbvie: Consultancy, Honoraria. Steensma:Astex: Consultancy; Arrowhead: Equity Ownership; Summer Road: Consultancy; Onconova: Consultancy; Aprea: Research Funding; Pfizer: Consultancy; Stemline: Consultancy; H3 Biosciences: Other: Research funding to institution, not investigator.. Santini:Celgene Corporation: Honoraria, Membership on an entity's Board of Directors or advisory committees; Novartis: Honoraria, Membership on an entity's Board of Directors or advisory committees; Johnson & Johnson: Honoraria; Acceleron: Membership on an entity's Board of Directors or advisory committees; Amgen: Membership on an entity's Board of Directors or advisory committees; Menarini: Membership on an entity's Board of Directors or advisory committees. Germing:Novartis: Honoraria, Research Funding; Celgene: Honoraria, Research Funding; Jazz Pharmaceuticals: Honoraria; Amgen: Honoraria. Font:Celgene: Consultancy, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Novartis: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Pfizer: Membership on an entity's Board of Directors or advisory committees. Díez-Campelo:Novartis: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Celgene Corporation: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding. Patnaik:Stem Line Pharmaceuticals.: Membership on an entity's Board of Directors or advisory committees. Sherman:Geron Corporation: Employment, Equity Ownership. Dougherty:Geron Corporation: Employment, Equity Ownership. Feller:Geron Corporation: Employment. Sun:Geron Corporation: Employment, Equity Ownership. Wan:Geron Corporation: Employment, Equity Ownership. Huang:Geron Corporation: Employment, Equity Ownership. Rizo:Geron Corporation: Employment, Equity Ownership. Fenaux:Celgene Corporation: Honoraria, Research Funding; Aprea: Research Funding; Astex: Honoraria, Research Funding; Jazz: Honoraria, Research Funding.

TPS7056 Background: Current treatment options for red blood cell (RBC) transfusion-dependent (TD) patients (pts) with lower risk (LR) myelodysplastic syndromes (MDS) relapsed after or refractory to erythropoiesis-stimulating agents (ESAs) have limited efficacy and durability; new approaches are needed. Imetelstat is a first-in-class telomerase inhibitor that targets cells with short telomeres and active telomerase, characteristics observed in MDS pts across all disease stages. IMerge (MDS3001) is a Phase 2/3 global study of imetelstat for TD pts with non-del(5q) LR MDS post ESA therapy. The results from Phase 2 part indicated that imetelstat achieved durable RBC transfusion independence (RBC-TI) and the most frequently reported adverse events were manageable and reversible grade ≥3 cytopenias. Among 38 pts with median follow-up of 24 months, 8-week, 24-week and 1-year TI rates were 42%, 32% and 29%, respectively; these responses were seen across different LR MDS subtypes. Median TI duration was 20 months and the longest TI was 2.7 years. A high and durable hematologic improvement-erythroid (HI-E) rate of 68% for a median duration of 21 months were also achieved. Reduction of variant allele frequency of mutations by imetelstat treatment was observed in some pts and correlated with clinical benefits (Platzbecker et al EHA 2020; Steensma et al JCO 2020). These results support the Phase 3 part of the trial. Methods: IMerge is two-part, Phase 2/3 study (ClinicalTrials.gov: NCT02598661). The Phase 3 part of the study is open for enrollment to adult pts with International Prognostic Scoring System (IPSS) low or intermediate-1 risk, non-del(5q) MDS who are TD, are relapsed after or refractory to ESAs, and have not received treatment with lenalidomide or hypomethylating agents. The study is a randomized (2:1) double-blind, placebo-controlled trial to compare efficacy of imetelstat vs placebo that will enroll approximately 170 pts and will be conducted at approximately 120 centers in North America, Europe, Asia and Middle East. Imetelstat is administered as 2-hour IV infusion every 4 weeks at 7.5 mg/kg. The primary endpoint of the study is to assess the rate of RBC-TI lasting ≥8 weeks. Secondary endpoints include safety, rate of RBC-TI ≥24 weeks, time to RBC-TI start, RBC-TI duration, rate of HI-E, the amount and relative change in RBC transfusions, rate of CR or PR, overall survival, progression of MDS, pharmacokinetics, and quality of life. Biomarkers relevant to the mechanism of action of imetelstat will be assessed to demonstrate target inhibition and their association with clinical responses. Cytogenetics and mutation analyses will be performed to evaluate the impact of imetelstat on reduction/depletion of malignant clones leading to disease modification. The study is currently recruiting pts. Clinical trial information: NCT02598661.

Background: Current treatment options for red blood cell (RBC) transfusion-dependent (TD) patients with lower risk (LR) myelodysplastic syndromes (MDS) relapsed after or refractory to erythropoiesis-stimulating agents (ESAs) have limited efficacy and durability; new approaches are needed. Imetelstat is a 13-mer lipid-conjugated oligonucleotide that targets the RNA template of human telomerase and is a competitive inhibitor of telomerase enzymatic activity (Asai et al, Cancer Res 2003; Herbert et al, Oncogene 2005). Preclinical, in vivo xenograft models (Dikmen et al, Cancer Res 2005; Hochreiter et al, Clin Cancer Res 2006) and preliminary clinical data from a pilot study (Tefferi et al, Blood Cancer J 2016) supported initiation of a study in TD LR MDS patients. The Phase 2 part of IMerge demonstrated an 8-week RBC transfusion independence (RBC-TI) rate of 42%, 24-week RBC-TI rate of 32%, with median duration of TI being 88 weeks. The responses were seen across different subtypes of LR MDS (Platzbecker et al, EHA 2020, S183). No new safety signal was identified. These results support the Phase 3 part of the trial. Methods: IMerge is two-part, Phase 2/3 study (ClinicalTrials.gov: NCT02598661). The Phase 2 portion of the study described above is closed for enrolment. The Phase 3 portion of the study is open for enrollment of adult patients with International Prognostic Scoring System (IPSS) low or intermediate-1 risk, non-del(5q) MDS, who are TD, are relapsed after or refractory to ESAs, and have not received treatment with lenalidomide or hypomethylating agents. The study is a randomized (2:1) double-blind, placebo-controlled trial to compare efficacy of imetelstat vs. placebo that will enroll approximately 170 patients and will be conducted at approximately 130 centers in North America, Europe, Asia and Middle East. Imetelstat will be administered as 2-hour IV infusion every 4 weeks at 7.5 mg/kg. The primary endpoint of the study is to assess the rate of RBC-TI lasting ≥8 weeks. Secondary endpoints include safety, rate of RBC-TI ≥24 weeks, time to RBC-TI start, RBC-TI duration, rate of hematologic improvement-erythroid (HI-E), the amount and relative change in RBC transfusions, rate of complete response or partial response, overall survival, progression of MDS, pharmacokinetics, and effect of treatment on quality of life. Biomarkers relevant to the mechanism of action of imetelstat will be assessed to demonstrate target inhibition and their association with clinical responses. Cytogenetics and mutation analyses will be performed to evaluate the impact of imetelstat on reduction/depletion of malignant clones leading to disease modification. Disclosures Platzbecker: AbbVie: Consultancy, Honoraria; Novartis: Consultancy, Honoraria, Research Funding; BMS: Consultancy, Honoraria; Takeda: Consultancy, Honoraria; Geron: Consultancy, Honoraria; Amgen: Honoraria, Research Funding; Janssen: Consultancy, Honoraria, Research Funding. Fenaux:Abbvie: Honoraria, Research Funding; Novartis: Honoraria, Research Funding; BMS: Honoraria, Research Funding; Jazz: Honoraria, Research Funding. Steensma:CRISPR: Current equity holder in publicly-traded company; Aprea Therapeutics: Research Funding; Takeda: Consultancy; BMS/Celgene: Consultancy; Onconova: Consultancy; Arena: Current equity holder in publicly-traded company; H3 Biosciences: Research Funding; Astex Pharmaceuticals, Otsuka: Consultancy; Arrowhead Pharmaceuticals: Current equity holder in publicly-traded company. Font:Abbvie: Other: Travel, accommodations, expenses; Menarini: Membership on an entity's Board of Directors or advisory committees; Novartis: Membership on an entity's Board of Directors or advisory committees, Other: Travel, accommodations, expenses, Speakers Bureau; Celgene: Consultancy, Membership on an entity's Board of Directors or advisory committees, Other: Travel, accommodations, expenses; Pfizer: Consultancy, Membership on an entity's Board of Directors or advisory committees. Diez-Campelo:Takeda: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding, Speakers Bureau; Celgene BMS: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding, Speakers Bureau; Novartis: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding, Speakers Bureau. Thepot:astellas: Honoraria; novartis: Honoraria; sanofi: Honoraria; celgene: Honoraria. Jang:Bristol Myers Squibb: Research Funding; Abbvie: 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; Amgen: Honoraria, Research Funding. Sherman:Geron Corp: Current Employment, Current equity holder in publicly-traded company. Dougherty:Geron Corp: Current Employment, Current equity holder in publicly-traded company. Sun:Geron Corp: Current Employment, Current equity holder in publicly-traded company. Huang:Geron Corp: Current Employment, Current equity holder in publicly-traded company. Wan:Geron Corp: Current Employment, Current equity holder in publicly-traded company. Rizo:Geron Corp: Current Employment, Current equity holder in publicly-traded company. Berry:Geron Corp: Current Employment, Current equity holder in publicly-traded company. Feller:Geron Corp: Current Employment, Current equity holder in private company. Santini:Takeda: Membership on an entity's Board of Directors or advisory committees; Novartis: Consultancy, Honoraria; Acceleron: Consultancy; Takeda: Consultancy, Honoraria; Menarini: Consultancy, Honoraria; Pfizer: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees; BMS: Consultancy, Honoraria; Johnson & Johnson: Honoraria.

Abstract 2610 Megakaryocytopoiesis consists of a succession of events in which MK progenitors initially proliferate and acquire lineage-specific markers, followed by polyploidization and cytoplasmic maturation. MK maturation culminates in the formation of cytoplasmic extensions (i.e. proplatelets) that leads to platelet shedding into the circulation. Panobinostat (LBH589) is a histone deacetylase inhibitor that has antiproliferative and cytotoxic effects on several types of cancer cells including blood cells from patients with hematological malignancies. One of the major adverse events associated with LBH589 treatment is thrombocytopenia. In this study, we hypothesize that the effects of LBH589 on thrombopoiesis might occur by targeting acetylation of histone and/or non-histone proteins resulting in defective platelet production. To test this hypothesis we investigated the effects of LBH589 on megakaryocytopoiesis in MK cell lines (i.e. HEL JAK2V617F positive cells) and in primary human MK. First, we tested the effects of LBH589 on the ability of human CD34+ cells to generate MK colony forming units (CFU-MKs). Neither CFU-MK or CFU-MIX derived colony formation was reduced in the presence of LBH589. To evaluate the effects of LBH589 on parameters of MK maturation, MK were generated in vitro from peripheral blood-derived CD34+ cells by employing an expansion culture system containing SCF and TPO for 6 days followed by 8 additional days incubation in the presence of TPO. These studies were pursued in the presence or absence of LBH589. Treatment with LBH589 did not significantly influence the number of CD61+ MK (i.e. control = 55.8%; 2.5nM LBH589 = 45.2%, p value=0.109; 5nM LBH589=38.5%, p value=0.095, of viable 7-AAD−/CD61+ cells) or the degree of polyploidization (i.e. control = 17.4%; 2.5nM LBH589 = 14.4%86.7, p value=0.157; 5nM LBH 589=12.8%, p value=0.116, cells with >4N DNA content). Culture-derived platelets were analyzed phenotypically and quantitated by means of dual labeling with anti-CD41 antibodies and with thiazole orange (TO) in order to identify new reticulated platelets. The percentage of CD41+/TO+ platelets derived from MK generated in the presence of LBH589 was significantly reduced (i.e. 2.5nM LBH589=11%, p value 0.046 and 5nM LBH589=9%, p value=0.011, CD41+/TO+ cells) as compared with MK generated in the absence of LBH589 (18.5% CD41+/TO+ cells). These findings were consistent with the observation of significant numbers of proplatelet-bearing MKs in control cultures but not in LBH 589-treated cultures. Collectively, these data suggest that LBH589 impairs platelet production while having a minimal effect on MK commitment, cytoplasmic maturation or polyploidization. To better understand the mechanisms responsible for such effects on thrombopoiesis, RNA extracted from control MK and from MK treated in vitro with LBH589 was analyzed by real time quantitative PCR to evaluate GATA-1 and NF-E2 expression. GATA-1 and NF-E2 mRNA levels were unchanged after treatment with LBH589. We found, however, that LBH589 induced a 4.8 to 7.5-fold increase in histone H3 acetylation. These data suggest that the negative impact of LBH589 on MK maturation was not mediated by its effects on chromatin but rather was possibly due to its effects on acetylation of nonhistone proteins. We demonstrated that LBH589 treatment increased acetylation of tubulin, a non-histone cytoplasmic protein that is a component of the microtubule (MT) cytoskeleton. The later stages of MK maturation are highly dependent on MT which represent the structural scaffold for proplatelet extension and enables the transport of cytoplasmic organelles into nascent platelets. The changes in the acetylation status of tubulin are critical for proper MT function and are mediated by HDAC6 which we found by Western blot analysis to be inhibited by LBH589 treatment. Based on these findings we suggest that LBH589-induced changes in tubulin acetylation result in aberrant MT function which in turn, leads to defective proplatelet and platelet formation. These nonhistone protein modifications might serve as a drug target for the development of novel agents (LBH589) to treat patients with extreme thrombocytosis due to underlying myeloproliferative neoplasms. Disclosures: Iancu-Rubin: Novartis: Research Funding. Hoffman:Novartis: Research Funding.

Post-natal vasculogenesis has been reported to be derived from a hierarchy of circulating endothelial progenitor cells (EPC). Some of these EPC are of myeloid origin while others have a more robust proliferative potential and are solely of endothelial cell (EC) origin. A number of groups have hypothesized that EC dysfunction might contribute to the hypercoagulable state associated with polycythemia vera (PV) by orchestrating the recruitment of blood elements to sites of injury or by regulating vascular tone. The JAK2V617F mutation is present in >95% of patients with PV. In addition, some individuals with normal blood counts who develop splanchnic vein thrombosis, including Budd-Chiari syndrome (BCS) and portal vein thrombosis (PVT) have been reported to have JAK2V617F positive hematopoiesis (45% and 34%, respectively) indicating that this thrombotic tendency might precede the development of PV. We explored whether this activating mutation is present in EC in the vessels of patients with BCS. We tested this hypothesis by studying EC in venules of liver biopsy specimens of patients with BCS with (n=2) or without PV (n=1) and PVT (n=2) without PV using laser capture microdissection (LCM) followed by nested PCR or RT-PCR in order to determine if EC were JAK2V617F positive and were of hematopoietic or EC origin. EC from the hepatic venules of BCS and PVT patients were captured by LCM from hematoxylin and eosin stained sections of archival formalin-fixed paraffin-embedded liver biopsy tissue specimens. EC were identified by their fusiform nuclei and their location along the lining of the hepatic venules. Hepatocytes were identified by their morphology as having round centrally placed nuclei and abundant cytoplasm with a trabecular arrangement. At least 10 EC and 10 hepatocytes from each biopsy specimen were captured and DNA or RNA was extracted. The JAK2V617F mutation was detected using nested allele-specific PCR. The hepatocytes of each patient contained exclusively wild type JAK2 while the EC of the two BCS patients with PV were homozygous for the JAK2V617F. The EC of the other BCS patient and 2 PVT patients who did not have PV contained exclusively wild type JAK2. The EC identity of these cells in the PV patients was confirmed by the presence of the EC transcripts (VE-Cadherin and VEGF-R2) and the absence of hematopoietic cell transcripts (CD45 and CD14). These findings indicate that hepatic venule EC in BCS patients with PV are JAK2V617F positive. The presence of JAK2V617F in both endothelial cells and hematopoietic cells belonging to such patients suggests that PV originates in an adult hemangioblast like cell in such patients. Further studies are required to understand the consequence of JAK2 mutation in EC as related to their propensity to thrombosis in PV.