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Myeloproliferative neoplasms research has entered a dynamic and exciting era as we witness exponential growth of novel agents in advanced/early phase clinical trials for myelofibrosis (MF). Building on the success and pivotal role of ruxolitinib, many novel agents, spanning a wide range of mechanisms/targets (epigenetic regulation, apoptotic/intracellular signaling pathways, telomerase, bone marrow fibrosis) are in clinical development; several are studied in registrational trials and hold great potential to expand the therapeutic arsenal/shift the treatment paradigm if regulatory approval is granted. Insight into MF pathogenesis and its molecular underpinnings, preclinical studies demonstrating synergism of ruxolitinib with investigational agents, urgent unmet clinical needs (cytopenias, loss of response to JAK inhibitors); and progressive disease fueled the rapid rise of innovative therapeutics. New strategies include pairing ruxolitinib with erythroid maturation agents to manage anemia (luspatercept), designing rational combinations with ruxolitinib to boost responses in both the frontline and suboptimal response settings (pelabresib, navitoclax, parsaclisib), treatment with non-JAK inhibitor monotherapy in the second-line setting (navtemadlin, imetelstat), novel JAK inhibitors tailored to subgroups with challenging unmet needs (momelotinib and pacritinib for anemia and thrombocytopenia, respectively); and agents potentially enhancing longevity (imetelstat). Beyond typical endpoints evaluated in MF clinical trials (spleen volume reduction ≥ 35%, total symptom score reduction ≥ 50%) thus far, emerging endpoints include overall survival, progression-free survival, transfusion independence, anemia benefits, bone marrow fibrosis and driver mutation allele burden reduction. Novel biomarkers and additional clinical features are being sought to assess new agents and tailor emerging therapies to appropriate patients. New strategies are needed to optimize the design of clinical trials comparing novel combinations to standard agent monotherapy.

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.

Hyperactive signaling of the Janus-Associated Kinase/Signal Transducers and Activators of Transcription (JAK/STAT) pathway is central to the pathogenesis of Philadelphia-chromosome-negative myeloproliferative neoplasms (MPN), i.e., polycythemia vera (PV), essential thrombocythemia (ET), and primary myelofibrosis (PMF) which are characterized by inherent biological and clinical heterogeneity. Patients with MPNs suffer from substantial symptom burden and curtailed longevity due to thrombohemorrhagic complications or progression to myelofibrosis or acute myeloid leukemia. Therefore, the management strategies focus on thrombosis risk mitigation in PV/ET, alleviation of symptom burden and improvement in cytopenias and red blood cell transfusion requirements, and disease course alteration in PMF. The United States Food and Drug Administration’s (USFDA) approval of two JAK inhibitors (ruxolitinib, fedratinib) has transformed the therapeutic landscape of MPNs in assuaging the need for frequent therapeutic phlebotomy (PV) and reduction in spleen and symptom burden (PV and PMF). Despite improving biological understanding of these complex clonal hematopoietic stem/progenitor cell neoplasms, none of the currently available therapies appear to modify the proclivity of the disease per se, thereby remaining an urgent unmet clinical need and an ongoing area of intense clinical investigation. This review will highlight the evolving targeted therapeutic agents that are in early- and late-stage MPN clinical development.

In a phase-2 study, the telomerase inhibitor imetelstat induced rapid hematologic responses in all patients with essential thrombocythemia who were refractory or intolerant to prior therapies. Significant molecular responses were achieved within 3-6 months in 81% of patients with phenotypic driver mutations in JAK2, CALR and MPL. Here, we investigated the dynamics of additional somatic mutations in response to imetelstat. At study entry, 50% of patients carried 1-5 additional mutations in the genes ASXL1, CBL, DNMT3A, EZH2, IDH1, SF3B1, TET2, TP53 and U2AF1. Three patients with baseline mutations also had late-emerging mutations in TP53, IDH1 and TET2. Most clones with additional mutations were responsive to imetelstat and decreased with the driver mutation, including the poor prognostic ASXL1, EZH2 and U2AF1 mutations while SF3B1 and TP53 mutations were associated with poorer molecular response. Overall, phenotypic driver mutation response was significantly deeper in patients without additional mutations (P = 0.04) and correlated with longer duration of response. In conclusion, this detailed molecular analysis of highly pretreated and partly resistant patients with essential thrombocythemia reveals a high individual patient complexity. Moreover, imetelstat demonstrates potential to inhibit efficiently co-incident mutations occurring in neoplastic clones in patients with essential thrombocythemia. (ClinicalTrials.gov number, NCT01243073. N Engl J Med 2015; 373:920-928, DOI: 10.1056/NEJMoa1503479.)

Purpose of review In this review, the focus is on the most recent improvements in diagnosis, prognostication and therapy of myelodysplastic syndromes (MDS) and on their relevance for clinical management. Recent findings Analytical methods to refine cytogenetic and molecular assessment of MDS have been proposed, improving prognostic stratification obtained from integration of clinical and genomic data. Novel agents with very different mode of action, as single drugs or added to HMA backbone, show promising clinical results in LR-MDS and HR-MDS. Luspatercept has obtained approval given the fact that in transfusion-dependent erythropoietic-stimulating agent resistant/relapsed LR-MDS induced nearly 50% of transfusion independence. Another investigational agent showing efficacy and possibly disease modifying activity in the same setting is the telomerase inhibitor imetelstat. Results from phase II study with azacytidine and pevonedistat indicate the concrete possibility to enhance durable responses compared with azacitidine single drug. In the same direction are the preliminary results of other agents with different mode of action: magrolimab, venetoclax, sabatolimab, as well as the targeted therapy with enasidenib and ivosidenib. New posttransplant maintenance strategies may concur to prolong response. Summary Better diagnosis and prognostic stratification may allow a more precise and personalized treatment of MDS with novel agent combinations leading to improved therapeutic algorithms.

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.

Myelofibrosis is a myeloproliferative neoplasm characterized by splenomegaly, constitutional symptoms, bone marrow fibrosis, and a propensity towards transformation to acute leukemia. JAK inhibitors are the only approved therapy for myelofibrosis and have been successful in reducing spleen and symptom burden. However, they do not significantly impact disease progression and many patients are ineligible due to coexisting cytopenias. Patients who are refractory to JAK inhibition also have a dismal survival. Therefore, non-JAK inhibitor-based therapies are being explored in pre-clinical and clinical settings. In this review, we discuss novel treatments in development for myelofibrosis with targets outside of the JAK-STAT pathway. We focus on the mechanism, preclinical rationale, and available clinical efficacy and safety information of relevant agents including those that target apoptosis (navitoclax, KRT-232, LCL-161, imetelstat), epigenetic modulation (CPI-0610, bomedemstat), the bone marrow microenvironment (PRM-151, AVID-200, alisertib), signal transduction pathways (parsaclisib), and miscellaneous agents (tagraxofusp. luspatercept). We also provide commentary on the future of therapeutic development in myelofibrosis.

Increased cell proliferation is a hallmark of acute lymphoblastic leukemia (ALL), and genetic alterations driving clonal proliferation have been identified as prognostic factors. To evaluate replicative history and its potential prognostic value, we determined telomere length (TL) in lymphoblasts, B-, and T-lymphocytes, and measured telomerase activity (TA) in leukocytes of patients with ALL. In addition, we evaluated the potential to suppress the in vitro growth of B-ALL cells by the telomerase inhibitor imetelstat. We found a significantly lower TL in lymphoblasts (4.3 kb in pediatric and 2.3 kb in adult patients with ALL) compared to B- and T-lymphocytes (8.0 kb and 8.2 kb in pediatric, and 6.4 kb and 5.5 kb in adult patients with ALL). TA in leukocytes was 3.2 TA/C for pediatric and 0.7 TA/C for adult patients. Notably, patients with high-risk pediatric ALL had a significantly higher TA of 6.6 TA/C compared to non-high-risk patients with 2.2 TA/C. The inhibition of telomerase with imetelstat ex vivo led to significant dose-dependent apoptosis of B-ALL cells. These results suggest that TL reflects clonal expansion and indicate that elevated TA correlates with high-risk pediatric ALL. In addition, telomerase inhibition induces apoptosis of B-ALL cells cultured in vitro. TL and TA might complement established markers for the identification of patients with high-risk ALL. Moreover, TA seems to be an effective therapeutic target, hence, telomerase inhibitors, such as imetelstat, may augment standard ALL treatment.

INTRODUCTION: Essential thrombocythemia (ET) and polycythemia vera (PV) belong to the BCR-ABL1-negative myeloproliferative neoplasms and are characterized by the clonal proliferation of hematopoietic stem and progenitor cells. The contribution of aberrant immune regulation within the bone marrow microenvironment to ET and PV pathogenesis as well as the underlying molecular landscape is becoming increasingly understood. AREAS COVERED: Authors searched PubMed and conference abstracts in August 2020 for preclinical and clinical studies to provide an overview of the immune pathobiology in ET and PV and the rationale for several novel agents. A discussion of recent clinical trials on interferon and ruxolitinib in ET and PV patients is provided followed by an outline of the future challenges in the field particularly for novel therapeutics and an increasingly individualized, molecularly-driven approach to treatment selection. Several novel agents are currently being actively evaluated and are reviewed herein as well. EXPERT OPINION: While hydroxyurea remains the first-line treatment for cytoreduction in most high-risk ET and PV patients, the disease-modifying potential of IFN is promising and could make it a preferred option for selected patients. Advances in molecular testing will enable a more individualized approach to prognostication and treatment selection.

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.

Simple Summary Myelofibrosis (MF) is an advanced form of a group of rare, related bone marrow cancers termed myeloproliferative neoplasms (MPNs). Some patients develop myelofibrosis from the outset, while in others, it occurs as a complication of the more indolent MPNs, polycythemia vera (PV) or essential thrombocythemia (ET). Patients with PV or ET who require drug treatment are typically treated with the chemotherapy drug hydroxyurea, while in MF, the targeted therapies termed Janus kinase (JAK) inhibitors form the mainstay of treatment. However, these and other drugs (e.g., interferons) have important limitations. No drug has been shown to reliably prevent the progression of PV or ET to MF or transformation of MPNs to acute myeloid leukemia. In PV, it is not conclusively known if JAK inhibitors reduce the risk of blood clots, and in MF, these drugs do not improve low blood counts. New approaches to treating MF and related MPNs are, therefore, necessary. Abstract Janus kinase (JAK) inhibition forms the cornerstone of the treatment of myelofibrosis (MF), and the JAK inhibitor ruxolitinib is often used as a second-line agent in patients with polycythemia vera (PV) who fail hydroxyurea (HU). In addition, ruxolitinib continues to be studied in patients with essential thrombocythemia (ET). The benefits of JAK inhibition in terms of splenomegaly and symptoms in patients with MF are undeniable, and ruxolitinib prolongs the survival of persons with higher risk MF. Despite this, however, “disease-modifying” effects of JAK inhibitors in MF, i.e., bone marrow fibrosis and mutant allele burden reduction, are limited. Similarly, in HU-resistant/intolerant PV, while ruxolitinib provides excellent control of the hematocrit, symptoms and splenomegaly, reduction in the rate of thromboembolic events has not been convincingly demonstrated. Furthermore, JAK inhibitors do not prevent disease evolution to MF or acute myeloid leukemia (AML). Frontline cytoreductive therapy for PV generally comprises HU and interferons, which have their own limitations. Numerous novel agents, representing diverse mechanisms of action, are in development for the treatment of these three classic myeloproliferative neoplasms (MPNs). JAK inhibitor-based combinations, all of which are currently under study for MF, have been covered elsewhere in this issue. In this article, we focus on agents that have been studied as monotherapy in patients with MF, generally after JAK inhibitor resistance/intolerance, as well as several novel compounds in development for PV/ET.

Telomeres are protective chromosomal ends that shield the chromosomes from DNA damage, exonucleolytic degradation, recombination, and end-to-end fusion. Telomerase is a ribonucleoprotein that adds TTAGGG tandem repeats to the telomeric ends. It has been observed that 85 to 90% of human tumors express high levels of telomerase, playing a crucial role in the development of cancers. Interestingly, the telomerase activity is generally absent in normal somatic cells. This selective telomerase expression has driven scientists to develop novel anti-cancer therapeutics with high specificity and potency. Several advancements have been made in this area, which is reflected by the enormous success of the anticancer agent Imetelstat. Since the discovery of Imetelstat, several research groups have contributed to enrich the therapeutic arsenal against cancer. Such contributions include the application of new classes of small molecules, peptides, and hTERT-based immunotherapeutic agents (p540, GV1001, GRNVAC1 or combinations of these such as Vx-001). Many of these therapeutic tools are under different stages of clinical trials and have shown promising outcomes. In this review, we highlight the current status of telomerase-based cancer therapeutics and the outcome of these investigations.

Despite the dramatic progress made in the treatment of patients with myelofibrosis since the introduction of the JAK1/2 inhibitor ruxolitinib, a therapeutic option that can modify the natural history of the disease and prevent evolution to blast-phase is still lacking. Recent investigational treatments including immunomodulatory drugs and histone deacetylase inhibitors benefit some patients but these effects have proven modest at best. Several novel agents do show promising activity in preclinical studies and early-phase clinical trials. We will illustrate a snapshot view of where the management of myelofibrosis is evolving, in an era of personalized medicine and advanced molecular diagnostics. Areas covered: A literature search using MEDLINE and recent meeting abstracts was performed using the keywords below. It focused on therapies in active phases of development based on their scientific and preclinical rationale with the intent to highlight agents that have novel biological effects. Expert commentary: The most mature advances in treatment of myelofibrosis are the development of second-generation JAK1/2 inhibitors and improvements in expanding access to donors for transplantation. In addition, there are efforts to identify drugs that target pathways other than JAK/STAT signaling that might improve the survival of myelofibrosis patients, and limit the need for stem-cell transplantation.

Clinical trials of imetelstat therapy have indicated that this telomerase inhibitor might have disease-modifying effects in a subset of patients with myelofibrosis (MF). The mechanism by which imetelstat induces such clinical responses has not been clearly elucidated. Using in vitro hematopoietic progenitor cell (HPC) assays and in vivo hematopoietic stem cell (HSC) assays, we examined the effects of imetelstat on primary normal and MF HSCs/HPCs. Treatment of CD34+ cells with imetelstat reduced the numbers of MF but not cord blood HPCs (colony-forming unit–granulocyte/macrophage, burst-forming unit–erythroid, and colony-forming unit–granulocyte/erythroid/macrophage/megakaryocyte) as well as MF but not normal CD34+ALDH+ cells irrespective of the patient’s mutational status. Moreover, imetelstat treatment resulted in depletion of mutated HPCs from JAK2V617F+ MF patients. Furthermore, treatment of immunodeficient mice that had been previously transplanted with MF splenic CD34+ cells with imetelstat at a dose of 15 mg/kg, 3 times per week for 4 weeks had a limited effect on the degree of chimerism achieved by normal severe combined immunodeficiency repopulating cells but resulted in a significant reduction in the degree of human MF cell chimerism as well as the proportion of mutated donor cells. These effects were sustained for at least 3 months after drug treatment was discontinued. These actions of imetelstat on MF HSCs/HPCs were associated with inhibition of telomerase activity and the induction of apoptosis. Our findings indicate that the effects of imetelstat therapy observed in MF patients are likely attributable to the greater sensitivity of imetelstat against MF as compared with normal HSCs/HPCs as well as the intensity of the imetelstat dose schedule.

Key Points Clinically relevant imetelstat concentrations significantly inhibit CFU-MEG formation from MNCs of ET patients and reduce hTERT expression. However, similar concentrations of imetelstat do not inhibit cytokine-induced CFU-MEG from MNCs of healthy donors.

Background Telomerase activity is required for both initiation and maintenance of tumorigenesis and over 90% cancers overexpress telomerase. Therefore, telomerase targeting has emerged as a potential strategy for cancer treatment. In agreement with this, several telomerase inhibitors are being tested for cancer treatment and have shown some promise. However, because of the variability in response between the cancer patients, it is important to identify biomarkers that allow for distinguishing cancers that are responsive to telomerase inhibition from the cancers that are not. Therefore, in this study we performed experiments to identify a biomarker that can be used to predict telomerase inhibition induced tumor growth inhibition. Methods In our study, we have performed transcriptome-wide gene expression analysis on multiple ovarian and colon cancer cell lines that were treated with telomerase inhibitor imetelstat and were responsive to telomerase inhibition-induced tumor growth attenuation. Results We demonstrate that telomerase inhibition by telomerase inhibitor imetelstat results in decreased expression of interleukin 8 (IL8) in all telomerase responsive cancer cell lines. This phenomenon is of general occurrence because we find that multiple ovarian and colon cell lines show decrease in IL8 mRNA and protein levels after telomerase inhibition. Additionally, we find loss of IL8 phenocopy Telomerase inhibition mediated growth inhibitory effect in cancer cells. Conclusion Taken together, our results show that IL8 is a biomarker that predict telomerase inhibition mediated growth attenuation of cancer cells and its loss phenocopy telomerase inhibition. Therefore, IL8 expression can be utilized as a biomarker for telomerase targeted cancer therapies to potentially predict therapeutic response. Electronic supplementary material The online version of this article (10.1186/s12885-018-4633-x) contains supplementary material, which is available to authorized users.

Introduction Acute myeloid leukemia (AML) is the deadliest malignancy in children. Despite the use of maximally intensive therapy, 20% of patients experience recurrent disease. These patients are also burdened with significant treatment-related toxicities. To improve survival in pediatric AML, novel targeted therapies that are more effective and less toxic are needed. Telomerase inhibition has been shown to be effective in reducing leukemic burden and eradicating leukemia stem cells (LSCs) in syngeneic mouse models of AML and in patient-derived xenograft (PDX) models of adult AML (Bruedigam et al., 2014). Recent transcriptome analyses demonstrate that the genomic landscape of pediatric AML is distinct from adult AML (Bolouri et al., 2018). In fact, mutations in the telomerase complex components are infrequent in pediatric AML unlike adult AML patients (Aalbers et al., 2013). However, similar to what is seen in adult patients, Aalbers et al. identified that telomere lengths in pediatric AML cells were shortened compared to normal leukocytes, and pediatric AML patients with the shortest telomere length tend to have shorter overall survival. Furthermore, the 5-year survival rate was 88% for pediatric AML patients who had lower telomerase activity, and 43% for those patients with higher telomerase activity, suggesting telomerase activity could be an important prognostic factor in pediatric AML patients (Verstovsek et al., 2003). Imetelstat is an oligonucleotide that specifically binds with high affinity to the RNA template of telomerase and is a potent, competitive inhibitor of telomerase enzymatic activity (Asai et al., 2003; Herbert et al., 2005). In this study, we evaluated if imetelstat has anti-leukemia activity in pediatric AML PDX models. Results The PDX lines tested in this study were derived using samples from pediatric AML patients who were 1-14 years old, representing different FAB subtypes. Mouse passaged pediatric AML PDX lines (n=6) were treated ex vivo with imetelstat or mismatch oligo control and the viability of LSC (CD34+CD38low population) was determined at 48 or 96 h by staining with BV785-human CD45, APC-human CD34, Pacific blue-human CD38, FITC conjugated annexin V and propidium iodide (PI). Imetelstat treatment significantly increased apoptosis/death (PI+/annexin V+) of the LSC population in a dose-dependent manner in all PDX lines evaluated (Fig. 1A, B), while it had limited activity on LSCs in normal pediatric bone marrow samples (n=4). The efficacy of imetelstat either alone or in combination with chemotherapy or azacitidine was evaluated in two distinct PDX models of pediatric AML in vivo. Mice engrafted with both NTPL-377 and DF-2 lived longer when treated with imetelstat than the untreated mice (Fig. 1C, D, n=5 each, P At the end of the in vivo studies, the percentage of LSC population was evaluated in the bone marrow of mice post euthanasia. There was a significant reduction of LSC population in mice treated with imetelstat compared to those treated with the mismatch oligo (Fig. 1E, F, P Conclusions Imetelstat treatment of pediatric AML PDX samples showed significant dose- and time-dependent effects on the viability of the LSCs to induce cell apoptosis/death. These results were corroborated in vivo in two distinct PDX models which showed reduced LSC population and increased median survival in mice with imetelstat treatment. Combining imetelstat with chemotherapy or azacitidine further enhanced activity against LSCs, suggesting imetelstat could represent an effective therapeutic strategy for pediatric AML. Figure 1 Figure 1. Disclosures Barwe: Prelude Therapeutics: Research Funding. Huang: Geron Corp: Current Employment, Current equity holder in publicly-traded company. Gopalakrishnapillai: Geron: Research Funding.

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: In cancer evolution, development of telomere maintenance mechanisms represents a critical feature to achieve replicative immortality and prevent cell death after shortening below a critical threshold. Several studies have indicated that excessive telomere shortening or aberrant telomeric repeats are present in cancer genomes, however, their precise role in myeloid malignancies remains largely unexplored. We stipulated that immature leukemia-initiating cells (LIC) would display higher telomerase activity and thus longer telomeres while in mature LIC lower telomerase activity would be associated with shorter telomeres due to excessive division rate. Additional lesions may also lead to disturbed differentiation programs resulting in altered expression of telomerase. Taking advantage of a vast cohort (N=1648) of patients with myeloid neoplasia analyzed by whole genome sequencing (WGS, mean. coverage 100x) we determined telomere length (TL). Aim: We set out to hypothesize that WGS data can be used to reliably measure the TL and that if systematically applied to well defined patient cohorts, we could test whether preservation of TL or its excessive shortening are prognostically or biologically relevant to clinical outcome. Methods: Our cohort included patients with AML (N=732), MDS (N=701) and MPN (N=102 incl. CML, ET, PMF, PV). Healthy subjects (N=11) and patients with the clonal non-malignant HSC disorder PNH (N=102) served as controls. To retrieve TL and telomere repeat heterogeneity via singleton analysis from WGS data we used two independent bioinformatic tools TelomereHunter and Telseq (1A). Results: First, using bidirectional cross-confirmation with independent bioinformatics pipelines, reliability of WGS-based TL measurements (1B) was confirmed and, as anticipated, age-dependency of TL in healthy controls was reproduced (1C). Next we investigated age as a possible confounder of TL shortening in patients. While in PNH age had an expected impact, patients with AML or MDS exhibited TL decoupled from its inherent age-dependence (r=.00034, P=.72). In general, we noted TL shortening as an overarching feature of myeloid proliferation irrespective of entity (1D). Furthermore, mutations in the promotor region of TERT were not correlated with TL. In AML or MDS, neither morphological nor genetic subclassification where associated with differences in TL (1E), with the exception of a small subset of AML and MDS patients (n=103, n=97), harboring abnormally long telomeres compared to healthy subjects. These were significantly associated with mutations in TP53 (AML P=1.91x10-6, mean TL 558.4; MDS P=.55x10-7, mean TL 414.5) but not with any other common mutation. This is surprising, since TP53 mutations are thought to protect cells from telomeric catastrophe following excessive shortening. Moreover, comparing the VAF of TP53mut AML samples with the corresponding measured TL, revealed a positive correlation (r=.2281, P=.021), irrespective of the blast count by cytomorphology (1F). Singleton analysis from all 1648 patients revealed that in AML and MDS patients, alternative telomere repeat- variants (chiefly TTTGGG) were increased pointing towards a possible implication of alternative telomere extension (1G). Lastly, outcome analysis revealed that in AML, the presence of longer telomeres (compared to controls) was associated with worse prognosis (P=.0035) (1H). Multivariate analysis, which included age, karyotype, TP53 status and gender, revealed that this was due to the strong association with TP53 mutations (n=630 vs n=103). The implications of our study extend beyond the realm of telomere biology and leukemia pathophysiology. Therapies targeting the telomere machinery, e.g. imetelstat, may offer an opportunity for application beyond MPN, its current indication. Ongoing studies are currently investigating its use in MDS. Based on our findings we argue that treatment may be rationally evaluated in TP53mut AML. Conclusion: Our findings provide a rationale for (1) the use of WGS to investigate TL, (2) the fundamental discovery that TL shortening is a common principle in myeloid neoplasms, (3) TP53 mutations being accompanied with increased TL and (4) the exploitation of TL to identify subgroups eligible for treatment with imetelstat. Moreover, WGS-based TL analysis in leukemias can contribute to our understanding of TL maintenance mechanisms in leukemic genome. Disclosures Maciejewski: Novartis, Roche: Consultancy, Honoraria; Alexion, BMS: Speakers Bureau.

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 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.

Myelodysplastic syndromes (MDS) are characterized by ineffective hematopoiesis with varying degrees of dysplasia and peripheral cytopenias. MDS are driven by structural chromosomal alterations and somatic mutations in neoplastic myeloid cells, which are supported by a tumorigenic and a proinflammatory marrow microenvironment. Current treatment strategies for lower-risk MDS focus on improving quality of life and cytopenias, while prolonging survival and delaying disease progression is the focus for higher-risk MDS. Several promising drugs are in the horizon, including the hypoxia-inducible factor stabilizer roxadustat, telomerase inhibitor imetelstat, oral hypomethylating agents (CC-486), TP53 modulators (APR-246 and ALRN-6924), and the anti-CD47 antibody magrolimab. Targeted therapies approved for acute myeloid leukemia treatment, such as isocitrate dehdyrogenase inhibitors and venetoclax, are also being studied for use in MDS. In this review, we provide a brief overview of pathogenesis and current treatment strategies in MDS followed by a discussion of newer agents that are under clinical investigation.

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.

Imetelstat (GRN163L) is a potent and selective inhibitor of telomerase. We have previously reported that GRN163L could shorten telomeres and limit the lifespan of CD18/HPAF and CAPAN1 pancreatic cancer cells. Here, we examined the effects of GRN163L on two other pancreatic cancer cell lines: AsPC1 and L3.6pl. In both lines, chronic exposure to GRN163L led to an initial shortening of telomeres followed by a stabilization of extremely short telomeres. In AsPC1 cells, telomere attrition eventually led to the induction of crisis and the loss of the treated population. In L3.6pl cells, crisis was transient and followed by the emergence of GRN163L-resistant cells, which could grow at increasing concentrations of GRN163L. The Shelterin complex is a telomere-associated complex that limits the access of telomerase to telomeres. The telomerase inhibitory function of this complex can be enhanced by drugs that block the poly(ADP-ribosyl)ation of its TRF1 and/or TRF2 subunits. Combined treatment of the GRN163L-resistant L3.6pl cells with GRN163L and 3-aminobenzamide (3AB), a general inhibitor of poly(ADP-ribose) polymerases, led to additional telomere shortening and limited the lifespan of the resistant cells. Results from this work suggest that inhibitors of telomerase and poly(ADP-ribose) polymerases can cooperate to limit the lifespan of pancreatic cancer cells.

The unprecedented success of the Janus kinase (JAK) 1/2 inhibitor ruxolitinib in myelofibrosis provided much-needed impetus for clinical drug development for the Philadelphia chromosome negative myeloproliferative neoplasms (MPN). The survival benefit conferred by this agent, along with its marked efficacy with regard to spleen volume and symptom reduction, have made ruxolitinib the cornerstone of drug therapy in myelofibrosis. However, there remain significant unmet needs in the treatment of patients with MF, and many novel classes of agents continue to be investigated in efforts to build upon the progress made with ruxolitinib. These include inhibitors of histone deacetylases (HDACs) and DNA methyltransferases, phosphatidylinositol-3-kinase isoforms, heat shock protein 90, cyclin-dependent kinases 4/6, and Hedgehog signaling, among others. In parallel, other JAK inhibitors with potential for less myelosuppression or even improvement of anemia, greater selectivity for JAK1 or JAK2, and the ability to overcome JAK inhibitor “persistence” are in various stages of development. First-in-class agents such as the activin receptor IIA ligand trap sotatercept (for anemia of myelofibrosis), the telomerase inhibitor imetelstat, and the anti-fibrotic agent PRM-151 (recombinant human pentraxin-2) are also in clinical trials. In polycythemia vera, a novel interferon administered every 2 weeks is being developed for frontline therapy in high-risk individuals, and inhibitors of human double minute 2 (HDM2) have shown promise in preclinical studies, as have HDAC inhibitors, e.g., givinostat (both in the laboratory and in the clinic). Ruxolitinib is approved for second-line therapy of polycythemia vera, and is being developed for essential thrombocythemia.

Introduction: Myelofibrosis (MF) is a myeloproliferative neoplasm associated with peripheral blood cytopenias, marrow fibrosis, splenomegaly systemic symptoms and evolution to a refractory form of acute myeloid leukemia. Treatment with a JAK1/2 inhibitor significantly improves the symptom burden and reduces the degree of splenomegaly. JAK1/2 therapy, however, results in a modest prolongation of survival, but does not halt disease progression or reduce the risk of leukemic transformation. Novel agents with the potential to alter the progressive disease course of MF are urgently needed.Areas covered: The pre-clinical rationale for the use of the telomerase inhibitor, imetelstat, in patients with advanced MF and essential thrombocythemia (ET) will be reviewed as well the results of the initial clinical trials with this agent.Expert opinion: Results from a single institution pilot study of imetelstat therapy have been reported and indicate a signal of activity. This approach appears to have the potent...

During the last decade, substantial advances have been made in the understanding of the complex molecular, immunological and cellular disturbances involved in the initiation as well as evolution of myelodysplastic syndromes (MDS). In 85% of the mainly frail and older patient population, anemia is present at the time of diagnosis and is thus a major therapeutic challenge. High rates of primary resistance to erythropoiesis-stimulating agents (ESAs), the currently only approved standard therapy to treat anemia in lower-risk MDS, demand the development of novel and efficient drugs with a good safety profile. Luspatercept, a ligand trap of activin receptor II, is able to promote late stage erythropoiesis even in patients failing prior ESA treatment. The presence of ring sideroblastic phenotype defines a subgroup of patients with higher response rates. Additionally, recent developments in clinical research using HIF-1 or telomerase modulation by roxadustat or imetelstat are promising. Other areas of translational research involve targeting the inflammasome by anti-inflammatory drugs in order to improve anemia. These efforts will hopefully pave the way for new targeted treatment options for anemic low-risk MDS patients.

Engaging a telomere maintenance mechanism during DNA replication is essential for almost all advanced cancers. The conversion from normal and premalignant somatic cells to advanced malignant cells often results (85%-90%) from the reactivation of the functional ribonucleoprotein holoenzyme complex, referred to as telomerase. Modulation of the human telomerase reverse transcriptase (hTERT) appears to be rate limiting to produce functional telomerase and engage a telomere maintenance mechanism. The remaining 10% to 15% of cancers overcome progressively shortened telomeres by activating an alternative lengthening of telomeres (ALT) maintenance mechanism, through a DNA recombination pathway. Exploration into the specific mechanisms of telomere maintenance in cancer have led to the development of drugs such as Imetelstat (GRN163L), BIBR1532, 6-thio-dG, VE-822, and NVP-BEZ235 being investigated as therapeutic approaches for treating telomerase and ALT tumors. The successful use of 6-thio-dG (a nucleoside preferentially recognized by telomerase) that targets and uncaps telomeres in telomerase positive but not normal telomerase silent cells has recently shown impressive effects on multiple types of cancer. For example, 6-thio-dG overcomes therapy-resistant cancers in a fast-acting mechanism potentially providing an alternative or additional route of treatment for patients with cancer. In this perspective, we provide a synopsis of the current landscape of telomeres and telomerase processing in cancer development and how this new knowledge may improve outcomes for patients with cancer.

Introduction The Janus kinase (JAK)1/2 inhibitor ruxolitinib provides rapid, sustained and often dramatic benefits to patients with myelofibrosis, inducing spleen shrinkage and ameliorating symptoms, and improves survival. However, the drug has little effect on the underlying bone marrow fibrosis or on mutant allele burden, and clinical resistance eventually develops. Furthermore, ruxolitinib-induced cytopenias can be challenging in everyday practice. Areas covered The developmental therapeutics landscape in MF is discussed. This includes potential partners for ruxolitinib being developed with an aim to improve cytopenias, or to enhance its disease-modifying effects. The development of other JAK inhibitors with efficacy post-ruxolitinib or other unique attributes is being pursued in earnest. Agents with novel mechanisms of action are being studied in patients whose disease responds sub-optimally to, is refractory to or progresses after ruxolitinib. Expert opinion The JAK inhibitors fedratinib, pacritinib and momelotinib are clearly active, and it is expected that one or more of these will become licensed in the future. The activin receptor ligand traps are promising as treatments for anemia. Imetelstat has shown interesting activity post-ruxolitinib, and azactidine may be a useful partner for ruxolitinib in some patients. Appropriately, multiple pre-clinical and clinical leads are being pursued in this difficult therapeutic area.

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.

Telomerase activation is critical in many cancers including central nervous system (CNS) tumors. Imetelstat is an oligonucleotide that binds to the template region of the RNA component of telomerase, inhibiting its enzymatic activity. We conducted an investigator-sponsored molecular biology (MB) and phase II study to estimate inhibition of tumor telomerase activity and sustained responses by imetelstat in children with recurrent CNS malignancies. In the MB study, patients with recurrent medulloblastoma, high-grade glioma (HGG) or ependymoma undergoing resection received one dose of imetelstat as a 2-h intravenous infusion at 285 mg/m(2), 12-24 h before surgery. Telomerase activity was evaluated in fresh tumor from surgery. Post-surgery and in the phase II study, patients received imetelstat IV (days 1 and 8 q21-days) at 285 mg/m(2). Imetelstat pharmacokinetic and pharmacodynamic studies were performed. Of two evaluable patients on the MB trial, intratumoral telomerase activity was inhibited by 95 % compared to baseline archival tissue in one patient and was inevaluable in one patient. Forty-two patients (40 evaluable for toxicity) were enrolled: 9 medulloblastomas, 18 HGG, 4 ependymomas, 9 diffuse intrinsic pontine gliomas. Most common grade 3/4 toxicities included thrombocytopenia (32.5 %), lymphopenia (17.5 %), neutropenia (12.5 %), ALT (7.5 %) and AST (5 %) elevation. Two patients died of intratumoral hemorrhage secondary to thrombocytopenia leading to premature study closure. No objective responses were observed. Telomerase inhibition was observed in peripheral blood mononuclear cells (PBMCs) for at least 8 days. Imetelstat demonstrated intratumoral and PBMC target inhibition; the regimen proved too toxic in children with recurrent CNS tumors.

Introduction: Myeloproliferative neoplasms (MPN) including blast crisis (BC) CML and myelofibrosis (MF) are characterized by the clonal proliferation of hematopoietic cell lineages, mostly in the marrow. BC CML gives rise to TKI resistant myeloid progenitors that activate Wnt/β-catenin signaling pathway. However, BCR-ABL TKI resistant BC CML exhibits a robust telomerase activity and presents at very low or undetectable level in normal cells, and telomerase plays a pivotal role in cancer cell growth and may serve as an ideal target for anticancer therapeutics. In addition, β-catenin had been reported to transcriptionally regulate human telomerase reverse transcriptase (TERT). Telomerase is composed of the enzymatic reverse transcriptase protein TERT and the RNA template TERC. Thus, we investigated the capacity of imetelstat, a telomerase inhibitor, which binds to the TERC subunit with high affinity to prevent self-renewal of malignant progenitors. Recent clinical trials showed early signs of efficacy in MF. But, it's role in selectively inhibiting leukemia stem cell (LSC) self-renewing in CML has not been elucidated. Methods and Results: Progenitors from BC CML were compared with chronic phase (CP) CML and primary normal samples, RNA-seq results revealed upregulation of TERT suggesting a role for TERT activation in BC CML progenitor transformation. Human MPN progenitor co-culture experiments revealed that combined treatment with dasatinib at 1 nM, and imetelstat at 1 or 5 uM significantly inhibited (p < 0.001, ANOVA) in vitro replating of BC CML compared with aged bone marrow progenitors. Also, imetelstat (5 mM) significantly reduced replating of MF compared with normal progenitor samples. Treatment of humanized MPN mouse models, established with 5 different BC CML and 4 different MF samples, at 30 mg/kg of imetelstat, three times a week for 4 weeks resulted in a significant reduction of malignant progenitors and human CD45+ cells (p < 0.001, t test) in BC CML, and a significant reduction of human CD45+ cells (p Conclusions: Niche responsive interactions between the telomerase complex and the Wnt/β-catenin self-renewal pathway sensitize β-catenin activated MPN progenitors to imetelstat treatment in both the in vitro co-culture and in vivo humanized MPN mouse models thereby providing a strong rationale for studies assessing eradication of malignant progenitors using imetelstat. Citation Format: Wenxue Ma, Larisa Balaian, Cayla Mason, Raymond Diep, Jessica Pham, Qingfei Jiang, Jeremy Lee, Sheldon Morris, Phoebe Mondala, Ping Chen, Thomas Whisenant, Mary Donohoe, Benjamin Heyman, Edward Ball, Fei Huang, Catriona Jamieson. Imetelstat inhibits RNA-editing mediated myeloproliferative neoplasm stem cell self-renewal [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 3792.

Telomere maintenance mechanisms (i.e., telomerase activity (TA) and the alternative lengthening of telomere (ALT) mechanism) contribute to tumorigenesis by providing unlimited proliferative capacity to cancer cells. Although the role of either telomere maintenance mechanisms seems to be equivalent in providing a limitless proliferative ability to tumor cells, the contribution of TA and ALT to the clinical outcome of patients may differ prominently. In addition, several strategies have been developed to interfere with TA in cancer, including Imetelstat that has been the first telomerase inhibitor tested in clinical trials. Conversely, the limited information available on the molecular underpinnings of ALT has hindered thus far the development of genuine ALT-targeting agents. Moreover, whether anti-telomerase therapies may be hampered or not by possible adaptive responses is still debatable. Nonetheless, it is plausible hypothesizing that treatment with telomerase inhibitors may exert selective pressure for the emergence of cancer cells that become resistant to treatment by activating the ALT mechanism. This notion, together with the evidence that both telomere maintenance mechanisms may coexist within the same tumor and may distinctly impinge on patients’ outcomes, suggests that ALT may exert an unexpected role in tumor biology that still needs to be fully elucidated.

Telomeres and telomerase have become attractive targets for the development of anticancer therapeutics due to their involvement in cancer cell immortality. Currently, several therapeutics have been developed that directly target telomerase and telomeres, such as telomerase inhibitors and G-quadruplex stabilizing ligands. Telomere-specific oligonucleotides that reduce telomerase activity and disrupt telomere architecture are also in development as novel anticancer therapeutics. Specifically, GRN163L and T-oligos have demonstrated promising anticancer activity in multiple cancers types via induction of potent DNA damage responses. Currently, several miRNAs have been implicated in the regulation of telomerase activity and may prove to be valuable targets in the development of novel therapies by reducing expression of telomerase subunits. Targeting miRNAs that are known to increase expression of telomerase subunits may be another strategy to reduce carcinogenesis. This review aims to provide a comprehensive understanding of current oligonucleotide-based anticancer therapies that target telomeres and telomerase. These studies may help design novel therapeutic approaches to overcome the challenges of oligonucleotide therapy in a clinical setting.

Imetelstat, a novel telomerase inhibitor, failed to improve significantly median PFS and OS as maintenance therapy (±bevacizumab) in advanced NSCLC. Telomere length (TL) biomarker results were consistent with the hypothesis that telomerase inhibition is of greater benefit to patients with tumors possessing shorter telomeres; the patients with shorter TL had a trend toward longer median PFS and OS. Background Continuation or ‘switch’ maintenance therapy is commonly used in patients with advancd non-small-cell lung cancer (NSCLC). Here, we evaluated the efficacy of the telomerase inhibitor, imetelstat, as switch maintenance therapy in patients with advanced NSCLC. Patients and methods The primary end point of this open-label, randomized phase II study was progression-free survival (PFS). Patients with non-progressive, advanced NSCLC after platinum-based doublet (first-line) chemotherapy (with or without bevacizumab), any histology, with Eastern Cooperative Oncology Group performance status 0–1 were eligible. Randomization was 2 : 1 in favor of imetelstat, administered at 9.4 mg/kg on days 1 and 8 of a 21-day cycle, or observation. Telomere length (TL) biomarker exploratory analysis was carried out in tumor tissue by quantitative PCR (qPCR) and telomerase fluorescence in situ hybridization. Results Of 116 patients enrolled, 114 were evaluable. Grade 3/4 neutropenia and thrombocytopenia were more frequent with imetelstat. Median PFS was 2.8 and 2.6 months for imetelstat-treated versus control [hazard ratio (HR) = 0.844; 95% CI 0.54–1.31; P = 0.446]. Median survival time favored imetelstat (14.3 versus 11.5 months), although not significantly (HR = 0.68; 95% CI 0.41–1.12; P = 0.129). Exploratory analysis demonstrated a trend toward longer median PFS (HR = 0.43; 95% CI 0.14–1.3; P = 0.124) and overall survival (OS; HR = 0.41; 95% CI 0.11–1.46; P = 0.155) in imetelstat-treated patients with short TL, but no improvement in median PFS and OS in patients with long TL (HR = 0.86; 95% CI 0.39–1.88; and HR = 0.51; 95% CI 0.2–1.28; P = 0.145). Conclusions Maintenance imetelstat failed to improve PFS in advanced NSCLC patients responding to first-line therapy. There was a trend toward a improvement in median PFS and OS in patients with short TL. Short TL as a predictive biomarker will require further investigation for the clinical development of imetelstat.

Cancer stem cells (CSCs) are thought to be responsible for tumor progression, metastasis, and recurrence. HER2 overexpression is associated with increased CSCs, which may explain the aggressive phenotype and increased likelihood of recurrence for HER2(+) breast cancers. Telomerase is reactivated in tumor cells, including CSCs, but has limited activity in normal tissues, providing potential for telomerase inhibition in anti-cancer therapy. The purpose of this study was to investigate the effects of a telomerase antagonistic oligonucleotide, imetelstat (GRN163L), on CSC and non-CSC populations of HER2(+) breast cancer cell lines. The effects of imetelstat on CSC populations of HER2(+) breast cancer cells were measured by ALDH activity and CD44/24 expression by flow cytometry as well as mammosphere assays for functionality. Combination studies in vitro and in vivo were utilized to test for synergism between imetelstat and trastuzumab. Imetelstat inhibited telomerase activity in both subpopulations. Moreover, imetelstat alone and in combination with trastuzumab reduced the CSC fraction and inhibited CSC functional ability, as shown by decreased mammosphere counts and invasive potential. Tumor growth rate was slower in combination-treated mice compared to either drug alone. Additionally, there was a trend toward decreased CSC marker expression in imetelstat-treated xenograft cells compared to vehicle control. Furthermore, the observed decrease in CSC marker expression occurred prior to and after telomere shortening, suggesting that imetelstat acts on the CSC subpopulation in telomere length-dependent and -independent mechanisms. Our study suggests addition of imetelstat to trastuzumab may enhance the effects of HER2 inhibition therapy, especially in the CSC population.

Overview Finding specific targeted agents for cancer therapy remains a challenge. One of the hallmarks of cancer is the limitless proliferation (immortalization) of cancer cells, which correlates with the activation of the ribonucleoprotein enzyme complex called telomerase. As 85–90% of primary human cancers express telomerase activity, while most normal cells do not, telomerase is a unique and almost universal therapeutic target for cancer. Although there have been several approaches developed for telomerase-targeted therapies, there is still no agent that has been approved. This chapter focuses on the pros and cons of the different approaches to target telomerase and summarizes preclinical and clinical results. We will also review novel telomerase-mediated telomere uncapping approaches that target telomerase-expressing cancer cells, but not normal cells. Keywords: telomere; cancer therapy; G-quadruplex; immunotherapy; vaccine; targeted gene therapy; small-molecule inhibitor; Imetelstat; GRN163L; 6-thio-2′-deoxyguanosine

Pediatric ependymomas are highly recurrent tumors resistant to conventional chemotherapy. Telomerase, a ribonucleoprotein critical in permitting limitless replication, has been found to be critically important for the maintenance of tumor-initiating cells (TICs). These TICs are chemoresistant, repopulate the tumor from which they are identified, and are drivers of recurrence in numerous cancers. In this study, telomerase enzymatic activity was directly measured and inhibited to assess the therapeutic potential of targeting telomerase. Telomerase repeat amplification protocol (TRAP) (n = 36) and C-circle assay/telomere FISH/ATRX staining (n = 76) were performed on primary ependymomas to determine the prevalence and prognostic potential of telomerase activity or alternative lengthening of telomeres (ALT) as telomere maintenance mechanisms, respectively. Imetelstat, a phase 2 telomerase inhibitor, was used to elucidate the effect of telomerase inhibition on proliferation and tumorigenicity in established cell lines (BXD-1425EPN, R254), a primary TIC line (E520) and xenograft models of pediatric ependymoma. Over 60 % of pediatric ependymomas were found to rely on telomerase activity to maintain telomeres, while no ependymomas showed evidence of ALT. Children with telomerase-active tumors had reduced 5-year progression-free survival (29 ± 11 vs 64 ± 18 %; p = 0.03) and overall survival (58 ± 12 vs 83 ± 15 %; p = 0.05) rates compared to those with tumors lacking telomerase activity. Imetelstat inhibited proliferation and self-renewal by shortening telomeres and inducing senescence in vitro. In vivo, Imetelstat significantly reduced subcutaneous xenograft growth by 40 % (p = 0.03) and completely abolished the tumorigenicity of pediatric ependymoma TICs in an orthotopic xenograft model. Telomerase inhibition represents a promising therapeutic approach for telomerase-active pediatric ependymomas found to characterize high-risk ependymomas. Electronic supplementary material The online version of this article (doi:10.1007/s00401-014-1327-6) contains supplementary material, which is available to authorized users.

Purpose: Imetelstat is a covalently-lipidated 13-mer thiophosphoramidate oligonucleotide that acts as a potent specific inhibitor of telomerase. It binds with high affinity to the template region of the RNA component of human telomerase (hTERC) and is a competitive inhibitor of telomerase enzymatic activity. The purpose of this study was to determine the recommended phase II dose of imetelstat in children with recurrent or refractory solid tumors. Experimental Design: Imetelstat was administered intravenously more than two hours on days 1 and 8, every 21 days. Dose levels of 225, 285, and 360 mg/m2 were evaluated, using the rolling-six design. Imetelstat pharmacokinetic and correlative biology studies were also performed during the first cycle. Results: Twenty subjects were enrolled (median age, 14 years; range, 3–21). Seventeen were evaluable for toxicity. The most common toxicities were neutropenia, thrombocytopenia, and lymphopenia, with dose-limiting myelosuppression in 2 of 6 patients at 360 mg/m2. Pharmacokinetics is dose dependent with a lower clearance at the highest dose level. Telomerase inhibition was observed in peripheral blood mononuclear cells at 285 and 360 mg/m2. Two confirmed partial responses, osteosarcoma (n = 1) and Ewing sarcoma (n = 1), were observed. Conclusions: The recommended phase II dose of imetelstat given on days 1 and 8 of a 21-day cycle is 285 mg/m2. Clin Cancer Res; 19(23); 6578–84. ©2013 AACR.

BACKGROUND: Patients with TD lower-risk (LR)-MDS relapsed or refractory to ESA have limited treatment options. Imetelstat is a first-in-class telomerase inhibitor that targets cells with short telomere lengths and active telomerase, characteristics observed in some MDS patients. IMerge is an ongoing global study of imetelstat in RBC TD patients with LR-MDS (IPSS Low or Int-1). In the first 32 patients enrolled, 8-week TI rate was 34%, with 24-week TI of 16%, and HI-E of 59%. The most frequently reported adverse events were reversible grade ≥3 cytopenias (Fenaux et al EHA 2018 Abstr S1157). Higher response rates were observed in patients (n=13) who were LEN and HMA naïve without del(5q). We report here results in an additional 25 LEN and HMA naïve patients without del(5q), with longer term follow-up of the 13 initial patients meeting the same criteria. METHODS: IMerge is a phase 2/3 trial (NCT02598661) that includes LR-MDS patients with a high transfusion burden (≥4 units / 8 weeks) who are relapsed/refractory to ESA or have sEPO >500 mU/mL. The additional 25 were required to be LEN and HMA naïve and lack del(5q). Imetelstat 7.5 mg/kg was administered IV every 4 weeks. In addition to the key endpoints noted above, secondary endpoints include safety, time to and duration of TI. Biomarkers are also being explored, including telomerase activity, hTERT, telomere length, and genetic mutations. RESULTS: Overall, for the 38 LEN/HMA naïve and non-del(5q) patients, median age was 71.5 years and 66% were men. 63% of patients were IPSS Low and 37% Int-1. Median prior RBC transfusion burden was 8.0 (range 4-14) U, and 71% had WHO 2008 RARS or RCMD-RS. 9/37 (24%) patients with evaluable sEPO levels had baseline level >500 mU/mL. As of July 2018, with a median follow-up of 25.8 months for the initial 13 patients, and 5.2 months for the 25 recently included patients, the 8-week RBC-TI rate was 37% (14/38). Durability of 24-week TI responses was demonstrated, with a median duration of 10 months and the longest ongoing response now >2 years. Among the patients achieving durable TI, all showed a Hb rise of ≥3.0 g/dL compared to baseline during the transfusion-free interval. Response rates were similar in RARS/RCMD-RS (33% [9/27]) and other patients (27% [3/11]), and those with baseline EPO levels >500 mU/mL (33% [3/9]) and ≤500 mU/mL (32% [9/28]). Reversible grade ≥3 neutropenia and thrombocytopenia were each reported in 58% of the patients. Liver function test (LFT) elevations were mostly grade 1/2. Reversible grade 3 LFTelevations were observed in 3 (8%) patients on study. An independent Hepatic Review Committee deemed the observed LFT elevations were not imetelstat-related hepatic toxicities. SUMMARY / CONCLUSIONS: In this cohort of 38 non-del(5q) LR-MDS patients with a high RBC transfusion burden who were ESA relapsed/refractory and naïve to LEN/HMA, single-agent imetelstat yielded a TI rate of 37%, with a median duration of 10 months and limited side effects. Durable responses were characterized by transfusion independence >24 weeks and accompanied by Hb rise. Updated data will be presented. Disclosures Steensma: Takeda: Consultancy; Syros: Research Funding; Otsuka: Membership on an entity's Board of Directors or advisory committees; Onconova: Consultancy; Novartis: Membership on an entity's Board of Directors or advisory committees; Kura: Research Funding; Janssen: Consultancy, Research Funding; H3 Biosciences: Research Funding; Celgene: Research Funding; Amphivena: Membership on an entity's Board of Directors or advisory committees; Acceleron: Consultancy. Platzbecker:Celgene: Research Funding. Van Eygen:Janssen: Consultancy, Research Funding; Roche: Research Funding; Amgen: Research Funding. Raza:Kura Oncology: Research Funding; Onconova: Research Funding, Speakers Bureau; Celgene: Research Funding; Novartis: Speakers Bureau; Geoptix: Speakers Bureau; Janssen: Research Funding; Syros: Research Funding. Santini:Amgen: Membership on an entity's Board of Directors or advisory committees; Novartis: Honoraria; AbbVie: Membership on an entity's Board of Directors or advisory committees; Otsuka: Consultancy; Janssen: Honoraria, Membership on an entity's Board of Directors or advisory committees; Celgene: Honoraria, Research Funding. Germing:Celgene: Honoraria, Research Funding; Janssen: Honoraria; Novartis: Honoraria, Research Funding. Font:Celgene: Membership on an entity's Board of Directors or advisory committees. Samarina:Janssen: Research Funding. Díez-Campelo:Novartis: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding, Speakers Bureau; Celgene: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding, Speakers Bureau. Bussolari:Janssen: Employment, Equity Ownership. Sherman:Janssen: Employment, Equity Ownership. Sun:Janssen: Employment, Equity Ownership. Varsos:Janssen: Employment, Equity Ownership. Rose:Janssen: Employment, Equity Ownership. Fenaux:Roche: Honoraria; Otsuka: Honoraria, Research Funding; Jazz: Honoraria, Research Funding; Janssen: Honoraria, Research Funding; Celgene: Honoraria, Research Funding.

Background: Preclinical studies have shown the combination of imetelstat (GRN163L), an inhibitor of telomerase, and T results in synergistic growth inhibition and restoration of T sensitivity in T-resistant cells (Clin Can Res 12(10):3184–92, 2006). Here we translate those findings to the clinic with the first-in-man phase I trial of imetelstat+ T in patients (pts) with T-refractory HER2+ metastatic disease. Methods: T (6 mg/kg q3 wk) was administered with increasing doses of imetelstat (240/300/375 mg/m2 q3 wk) using a standard 3+3 dose escalation design. Maximum Tolerated Dose (MTD) was based on toxicity observed during cycle 1. Responding or stable pts continued treatment until progression. Tumor biopsy and bone marrow aspirate for biologic correlates were obtained at baseline and prior to cycle 2. Limited pharmacokinetics (PK) for T and imetelstat were included. Results: Ten pts were enrolled; median age was 54 (28–64). Patients were extensively pre-treated with the number of prior regimens ranging from 3 to >12. Prior cytotoxic therapies included anthracyclines (n = 9), taxanes (n = 9), vinorelbine (n = 8), capecitabine (n = 8), gemcitabine (n = 6), ixabepilone (n = 4), and eribulin (n = 4). Prior anti-HER2 targeted therapies included trastuzumab (n = 10), capecitabine (n = 8), T-DM1 (n = 6), and pertuzumab (n = 1). Therapy was well tolerated; no treatment related grade 4 toxicities were observed. Myelosuppression was limited to one pt with Grade 2 anemia and one each with Grade 2/3 thrombocytopenia. MTD was not reached. There were no objective responses; 2 pts. in cohort 3 had SD. Serial tumor biopsies and bone marrow aspirates have been analyzed for 7 pts (cohort 1: 001–004; cohort 2: 005–007; cohort 3: analysis ongoing). Tumor hTERT (p = ns) and phosphorylated HER2 (p = 0.03) decreased after treatment in nearly all pts. Bone marrow S-phase did not change consistently with treatment. Conclusion: The combination of trastuzumab + imetelstat is well tolerated and results in decreases in HER2 phosphorylation similar to that seen in preclinical models. Additional biologic correlates and PK analyses are ongoing. Further study of this combination in less heavily pre-treated patients is planned. Citation Information: Cancer Res 2012;72(24 Suppl):Abstract nr P5-18-13.

Imetelstat (GRN163L) is a specific telomerase inhibitor that has demonstrated clinical activity in patients with myeloproliferative neoplasms (MPN) and in patients with solid tumors. The antitumor effects were associated with the development of thrombocytopenia, one of the common side effects observed in patients treated with imetelstat. The events underlying these adverse effects are not apparent. In this report, we investigated the potential mechanisms that account for imetelstat's beneficial effects in MPN patients and the manner by which imetelstat treatment leads to a reduction in platelet numbers. Using a well-established system of ex vivo megakaryopoiesis, we demonstrated that imetelestat treatment affects normal megakaryocyte (MK) development by exclusively delaying maturation of MK precursor cells. By contrast, additional stages along MPN MK development were affected by imetelstat resulting in reduced numbers of assayable colony-forming unit MK and impaired MK maturation. In addition, treatment with imetelstat inhibited the secretion of fibrogenic growth factors by malignant but not by normal MK. Our results indicate that the delay observed in normal MK maturation may account for imetelstat-induced thrombocytopenia, while the more global effects of imetelstat on several stages along the hierarchy of MPN megakaryopoiesis may be responsible for the favorable clinical outcomes reported in MPN patients.

Telomerase is a ribonucleoprotein that catalyses the addition of telomeric repeat sequences (having the sequence 5'-TTAGGG-3' in humans) to the ends of chromosomes. Telomerase activity is detected in most types of human tumours, but it is almost undetectable in normal somatic cells. Therefore, telomerase is a promising therapeutic target. To date, the known inhibitors of telomerase include nucleoside analogues, oligonucleotides and G-quadruplex stabilizers. This review highlights recent advances in our understanding of telomerase inhibitors, the relationships between telomerase inhibitors, cancer, and fields such as inflammation.This review summarizes new patents published on telomerase inhibitors from 2010 to 2015.The review provides a brief account of the background, development, and on-going issues involving telomerase inhibitors. In particular, this review emphasizes imetelstat (GRN163L) and some typical G-quadruplex stabilizers that participate in telomerase inhibition. Overall, the research scope of antineoplastic is becoming broader and telomerase inhibitors have been shown to be a promising therapeutic target. Therefore, novel antineoplastic agents with greater activity and higher specificity must be developed.