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Research on the association of the ribonucleoprotein enzyme, telomerase, with human cancer has expanded rapidly in recent years. Essentially all major types of cancer have been screened and the presence of telomerase activity has been detected in the vast majority of cases. In this article we provide a summary, in table form, of the current data.

Tumor regrowth after chemotherapy may be driven by growth of tumor ‘stem cells’. Telomerase, required for indefinite replication, is upregulated both in putative ‘stem cells’ and bulk tumor cells. Imetelstat, a lipidated 13-mer oligonucleotide, is a potent and specific inhibitor of telomerase. A randomized phase II study was conducted to assess whether imetelstat, given as maintenance therapy, prolongs PFS in advanced NSCLC: results for the primary and secondary endpoints are reported separately. NSCLC cell lines and other tumor cells with short telomeres appear to be more sensitive to imetelstat in vitro than those with long telomeres. A planned exploratory analysis to determine PFS as a function of tumor telomere length (TL) was performed. Tumor TL was assessed in archival tumor specimens from pts by quantitative PCR (qPCR). TL data were available for 57 of the 116 pts accrued in the clinical trial. PFS was evaluated in patients grouped into the shortest 1/2, shortest 1/3 and shortest 1/4 of TL. In 19 pts with the shortest 1/3 TL measured by qPCR, imetelstat maintenance increased PFS with a HR in favor of the imetelstat arm of 0.32 (95% CI 0.1 to 1.0), p=0.042 (un-stratified log rank). Median PFS was 4.0 months for the imetelstat-treated short TL sub-group and 1.5 months for the control short TL sub-group. In the 38 pts with the longest 2/3 TL HR was 0.83 (95% CI 0.36 to 1.9). Results in the group with the shortest 1/4 of TL were similar to the shortest 1/3 TL group, and in the shortest 1/2 group, results were consistent but attenuated, indicating that a smaller subset may contain patients with the most potential to benefit. In the control arm, short TL was associated with shorter median PFS (1.48 months) compared to patients with long TL (2.7 months), suggesting that short TL has a negative prognostic value. These findings suggest that imetelstat given as maintenance therapy prolongs PFS in pts with advanced NSCLC whose tumors have short telomeres as measured by qPCR. The data are consistent with the hypothesis that clinical benefit from telomerase inhibition is greater in patients with tumors possessing short telomeres. Prospective confirmation of these results in solid tumors and hematologic neoplasms is planned. Citation Format: Alberto Chiappori, Ekaterina Bassett, Bart Burington, Tatjana Kolevska, David R. Spigel, Steven Hager, Mark Rarick, Shirish Gadgeel, Normand Blais, Joachim Von Pawel, Lowell Hart, Hui Wang, Kevin Eng, Martin Reck, Joan Schiller. Improved progression-free survival (PFS) in patients with short tumor telomere length: Subgroup analysis from a randomized phase II study of the telomerase inhibitor imetelstat as maintenance therapy for advanced NSCLC . [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 2376. doi:10.1158/1538-7445.AM2013-2376

Telomerase is required for the unlimited lifespan of cancer cells. Pancreatic cancers are almost always positive for telomerase activity and inhibiting telomerase in these cells could limit their lifespan. In this project, we have characterized the biology of telomeres in a panel of 11 pancreatic cancer cell lines and have examined the effects of GRN163L (Imetelstat), a potent telomerase inhibitor, on these cells. GRN163L inhibited telomerase in all 11 pancreatic cancer cell lines, with IC50 ranging from 50 nM to 200 nM. Continuous exposure of AsPC-1, CAPAN1 and CD18 cells to 1 μM GRN163L resulted in telomere shortening, induction of crisis, and loss of the cultures. Crisis In these cells was accompanied by activation of a DNA damage response (γ-H2AX) and evidence of both senescence (SA-β-galactosidase) and apoptosis (Floating cells, sub-G1 DNA content, PARP1 cleavage). In L3.6 cells, continuous GRN163L exposure led to the emergence of a GRN163L-resistant sub-population of cells. Over the last year, these cells have been expanded at successively higher concentrations of GRN163L and are still dividing in the presence of 10 μM GRN163L. To shorten telomeres and limit the lifespan of these cells, an alternate approach was to use inhibitors of poly(ADP-ribose)polymerases (PARP) to block the parsylation of TRF1 and TRF2, thereby enhancing the telomerase-inhibitory activity of the Shelterin complex. Our results show that combining GRN163L with the general PARP inhibitor 3-aminobenzamide (3AB) was sufficient to shorten telomeres and limit the lifespan of the GRN163L-L3.6 resistant cells. These experiments were then repeated in the parental L3.6 cells to determine if 3AB could be used to block the development of GRN163L-resistance. Whereas 3AB alone was not sufficient to induce crisis, 3AB did synergize with GRN163L to accelerate the timing of crisis and block the emergence of GRN163L resistant cells. These results raise the possibility that inhibitors of poly(ADP-ribose)polymerases could be exploited in the clinic to accelerate the effects of anti-telomerase therapies. Citation Format: Katrina M. Burchett, Michel M. Ouellette. Inhibitors of telomerase and of poly(ADP-Ribose)polymerases synergize to limit the lifespan of pancreatic cancer cells. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 704. doi:10.1158/1538-7445.AM2013-704

Background: The reverse transcriptase enzyme telomerase is reactivated in 85-90% of malignancies and is a potential target for anti-cancer therapies due to its differential expression between normal and tumor cells. One such therapy involves targeting the template region of telomerase with an antagonistic oligonucleotide, imetelstat, which is currently in clinical trials for breast and other cancers. Imetelstat has been shown to be effective in inhibiting telomerase activity in breast cancer cell lines, as well as inhibiting breast tumor growth and decreasing lung metastases in an in vivo model. HER2 amplification (and/or overexpression) is associated with a more aggressive disease, a greater likelihood of recurrence, and decreased survival compared to breast cancers without HER2 amplification. Recent work has shown that HER2 overexpression increases both the normal and malignant stem/progenitor cell population. This increase in malignant stem cells, known as cancer stem cells (CSCs), drives tumorigenesis and invasion and may explain the aggressive phenotype and drug resistance of this disease. We hypothesize that CSCs have active telomerase and thus may be sensitive to imetelstat treatment, thereby enhancing the effects of therapy for HER2 positive cancers. Methods: HER2+/- breast cancer cells, grown as mammospheres in culture to enrich for CSCs, were analyzed by flow cytometry for CSC marker expression (CD49f+/EpCAM-, CD44+/CD24-, and ALDH1hi). Mammospheres were analyzed for spheroid forming efficiency, a functional measure of self-renewal potential, and telomerase activity in response to imetelstat treatment. Results: Imetelstat treatment resulted in inhibition of up to 90% of telomerase activity in mammospheres, similar to adherent cell cultures. The percentage of CSCs based upon analysis of marker expression decreased approximately 30% or 1.4-fold after 5 weeks of imetelstat treatment compared to untreated controls. Imetelstat pretreatment decreased spheroid counts by 84%. The spheroid formation efficiency decreased from 9.36% in the untreated to 1.5% after imetelstat pretreatment, a 6.24-fold decrease. Pretreatment with imetelstat also had a durable inhibitory effect on mammosphere telomerase activity, where over 60% of the enzyme's activity remained inhibited after one week without additional treatment. Conclusions: CSCs have active telomerase that can be inhibited using imetelstat. The percentage of CSCs as well as spheroid formation efficiency decreases following imetelstat treatment. The addition of imetelstat to therapy regimens may help decrease metastases and reduce tumor recurrence. Citation Format: Jillian Koziel, Sergei Gryaznov, Brittney-Shea Herbert. Targeting telomerase in HER2 positive breast cancer: role of cancer stem cells. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 2118. doi:10.1158/1538-7445.AM2013-2118

Tumor regrowth after chemotherapy may be driven by growth of tumor ‘stem cells’. Telomerase, required for indefinite replication, is upregulated in tumor progenitor cells. Imetelstat is a 13-mer oligonucleotide which is a potent and specific telomerase inhibitor. Progression-free survival (PFS) and the duration of responses after 1st-line chemotherapy for non-small-cell lung cancer (NSCLC) are short, which has led to an interest in developing active maintenance therapies. A randomized phase II study was conducted to assess whether imetelstat, given as maintenance therapy, prolongs PFS in NSCLC. Pts were eligible with advanced NSCLC not progressing after completing 4-6 cycles of platinum-based doublet induction chemotherapy, any NSCLC histology, performance status (PS) ECOG 0 or 1, and not scheduled to receive maintenance with pemetrexed or erlotinib. Pts were randomized 2:1 to imetelstat 9.4 mg/kg (d1 and 8 of a 21d cycle) or observation until progressive disease or unacceptable toxicity (concomitant use of bevacizumab was permitted). The 1o endpoint was PFS; safety/tolerability and objective response rate were 2o endpoints. 116 pts were enrolled between Jul 2010 to Apr 2012, with 114 completing a first visit (i.e. efficacy population). Baseline characteristics of age, gender, PS and number of induction cycles were well balanced. 18.4% had squamous histology; 31.6% received concomitant bevacizumab (bev). The median number of imetelstat maintenance cycles was 3. In the overall analysis of PFS, a non significant improvement in favor of the imetelstat arm was observed (HR = 0.77, P=0.295, 95% CI 0.48 - 1.25). Median PFS was 2.8m for imetelstat and 2.6m for control. In the subgroup of patients who did not receive bevacizumab, the HR was 0.59 (vs. 1.07 for the bevacizumab subgroup; interaction P=0.15). The objective response rate was 4.3% in the imetelstat arm and 2.8% in the control arm. Six month overall survival was 80% for imetelstat and 72% for control (HR = 0.86, P=0.642, 95% CI 0.44 - 1.66). Imetelstat was generally well tolerated although hematologic toxicity, predominantly neutropenia and thrombocytopenia, was increased in the imetelstat arm (grade 3/4 neutropenia 18% for imetelstat vs 0% for control, grade 3/4 thrombocytopenia 37% for imetelstat vs 0% for control). The most frequent non-hematologic toxicities were fatigue (imetelstat 42.1% vs control 18.4%), nausea (43.4% vs 7.9%), vomiting (25.0% vs 5.3%) and anaemia (19.7% vs 5.3%). The overall findings suggest that imetelstat has modest, but not clinically meaningful, activity as a maintenance therapy in pts with NSCLC. A pre-specified exploratory subgroup analysis of PFS by tumor telomere length is reported separately. Citation Format: Alberto Chiappori, Tatjana Kolevska, Bart Burington, David Spigel, Steven Hager, Mark Rarick, Shirish Gadgeel, Normand Blais, Joachim Von Pawel, Lowell Hart, Martin Reck, Joan Schiller. A randomized phase II study of the telomerase inhibitor imetelstat as maintenance therapy for advanced non-small cell lung cancer. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 4660. doi:10.1158/1538-7445.AM2013-4660

Abstract 4898 Despite improved response rates with the use of novel agents, the vast majority of patients with multiple myeloma (MM) experience disease relapse and progression. This clinical pattern suggests the cells responsible for tumor regrowth persist following treatment, and we previously demonstrated that highly tumorigenic and self-renewing MM cells are relatively resistant to a wide range of anti-myeloma agents. These MM CSCs are present in the peripheral blood circulation and express memory B cell surface antigens (CD19 and CD27) and the stem cell marker aldehyde dehydrogenase (ALDH). We have also studied cellular mechanisms regulating self-renewal and found that MM CSCs express increased levels of telomerase activity (TA). In preclinical models, the novel telomerase inhibitor imetelstat has been found to inhibit CSCs from a wide range of tumor types, and we reported that imetelstat reduced TA in MM CSCs resulting in the loss of self-renewal and clonogenic growth potential. In an open label phase II clinical trial initiated in February 2011, previously treated MM patients (pts) (n=6) with stable but detectable disease following treatment with an iMid or proteasome inhibitor received single agent imetelstat (7.5–9.4 mg/kg IV on days 1 and 8 every 21–28 days). Additional pts (n=4) receiving lenalidomide maintenance and whose disease was stable for at least three months have also been enrolled. Pts received imetelstat therapy until evidence of disease progression or toxicity. Median age is 62.5 years (range 53–68); median number of prior therapies is 1 (range 1–4) including 4 pts with prior autologous transplant and 1 pt with prior mini-allogeneic transplant; 8 pts with prior bortezomib use; 9 pts with prior iMiD use; and 4 pts with concurrent lenalidomide use with receipt of lenalidomide for a median of 11.38 months (range 4.51–19.21 months) prior to initiation of imetelstat. As of July 30, 2012, 6 pts remain on study. Four pts were discontinued from imetelstat therapy after receiving a median of 7 doses of imetelstat either due to disease progression (n=2) or hematologic toxicity (n=2). Cytopenias were the most frequently reported toxicity with 8 of 10 pts demonstrating grade 3–4 thrombocytopenia and neutropenia during cycle 2, often requiring dose reductions or holds in subsequent cycles. The frequency of circulating MM CSCs (CD19+CD27+ALDH+) was quantified by flow cytometry. Circulating MM CSCs could be detected prior to the initiation of imetelstat (mean 10.7 × 10e3 cells/ml, range 17–53 × 10e3) in 8 of the 9 pts who have completed at least 2 cycles of treatment with imetelstat. In the single remaining patient, circulating CSCs were assessed from Cycle 2 onwards. Over the course of treatment, the frequency of MM CSCs decreased significantly, on average 2 fold every 30 days (Fig 1), in 8 of the 9 patients studied despite no upgrades in clinical response as per IWG criteria. In two pts who received 4 and 6 cycles of single agent imetelstat respectively, standard responses detected as decreasing or plateaued serum M protein or light chain levels were sustained over 4 months following discontinuation (Fig 2). Moreover, these delayed responses occurred in the absence of any additional therapy. These findings demonstrate that imetelstat rapidly decreases circulating MM CSCs. In addition, several patients experienced delayed, but sustained, clinical responses as measured by standard criteria. Therefore, imetelstat may have therapeutic implications for MM and other malignancies driven by CSCs. Figure 1. Serial measurement of CSCs in patients (n=9) treated with imetelstat Figure 1. Serial measurement of CSCs in patients (n=9) treated with imetelstat Figure 2. Clinical response in Pts 003 and 004 based on changes in paraprotein level. Dashed line represents off study evaluation. Figure 2. Clinical response in Pts 003 and 004 based on changes in paraprotein level. Dashed line represents off study evaluation. Disclosures: Huff: Celgene: Scientific Advisory Board Other; Novartis: Consultancy. Jones:Cytomedix: Royalties, Royalties Patents & Royalties. Reddy:Geron Corp.: Employment. Nishimoto:Geron Corp: Employment. Stuart:Geron Corp: Consultancy, Employment; OncoMed Pharmaceuticals: Consultancy. Kelsey:Geron Corp: Employment. Matsui:Geron Corp.: Research Funding.

Abstract 179 Background: Myeloproliferative neoplasms (MPNs), such as essential thrombocythemia (ET), are driven by neoplastic progenitor cells. The JAK2 V617F mutation can be detected in approximately 50% of patients (pts) with ET, and the JAK2 V617F allele burden can be used to measure the treatment-induced molecular response (MR) over time. Telomerase is upregulated in neoplastic progenitor cells and sustains indefinite replication. Imetelstat is a first in class, potent, specific inhibitor of telomerase which selectively distributes to bone marrow and inhibits thrombopoiesis. In vitro studies demonstrate that imetelstat selectively inhibits spontaneous megakaryocytic colony-forming unit (CFU-Meg) growth from the blood of pts with ET but not from healthy individuals. Phase I studies have demonstrated that imetelstat inhibits telomerase activity in pts at doses of 7.5 mg/kg and above. Therefore, unlike conventional cytoreductive therapy and JAK2 kinase inhibitors, imetelstat may be uniquely able to selectively inhibit proliferation of neoplastic clonogenic cells in pts with ET and modify the biology and progression of the disease. Methods: A phase II study enrolled pts with ET who had failed or were intolerant to at least one prior therapy, or who refused standard therapy. Pts were treated with imetelstat 7.5 mg/kg or 9.4 mg/kg IV weekly. After attainment of best platelet response in the induction phase, maintenance dosing with imetelstat was commenced with dosing based upon platelet count. Primary endpoint was best overall hematologic response (HR) with complete response (CR) defined as platelet count Results: As of July 9, 2012, 13 pts were treated. Median age was 60 yrs (range 21–83) with a median of 2 prior treatments (range 1–3). Median years since initial diagnosis were 5.8 (range 0.3 to 24.9) and initial platelet count was 809 × 103/μl (range 601 to 1359 × 103/μl). Best overall HR was 100%, with 11 of 13 pts achieving a confirmed CR after a median of 6.1 weeks (range 5.1 to 14.1 wks). Twelve of 13 pts remain on maintenance therapy (median time on study 26.1 weeks) and despite transient elevations of platelets above best response, pts continue to be responsive to imetelstat. Four pts have reached 1 year of therapy and continue to be treated with ongoing HR. Dosing frequency on maintenance therapy was generally reduced with time. A substantial decrease in JAK2 V617F allele burden was demonstrated in all 5 JAK2 V617F-positive pts (mean allele burden reduction of 82%; range of 59–94%, see table below). Four pts who were eligible for MR assessment by LeukemiaNetcriteria (initial JAKV617F allele burden >10%) reached molecular partial responses (PR): one pt after 12 weeks, which has been maintained through 1 year, and 3 other pts at 24, 36 and 48 weeks of therapy. One additional pt with JAK2 V617F levels of 4.8% prior to therapy has also had a 75% reduction after 12 weeks of treatment. A reduction in the spontaneous growth of CFU-Meg was also observed in the 2 pts tested, with 93% and 96% reduction from baseline, respectively. Long-term administration of imetelstat was generally well tolerated. Common adverse events reported on therapy were mild to moderate gastrointestinal toxicities, reductions in neutrophil counts, and fatigue. Conclusions: Imetelstat rapidly induces and maintains hematologic responses in pts with ET who have failed or are intolerant to conventional therapies. Importantly, substantial MR is observed in all JAK2 V617F-positive pts and inhibition of the neoplastic clonogenic growth ex-vivo is demonstrated. The reduction in JAK2 V617F allele burden and cytokine-independent growth of CFU-Meg suggests that imetelstat has a relatively selective inhibitory effect on the growth of the neoplastic clone(s) which drive ET and has the potential to modify the underlying biology of MPNs. Additional data will be presented from this ongoing study. Disclosures: Baerlocher: Geron Corporation: Research Funding. Oppliger Leibundgut:Geron Corporation: Research Funding. Ayran:Geron Corporation: Employment. Blaney:Geron Corporation: Employment. Burington:Geron Corporation: Employment. Morfeld:Geron Corporation: Employment. Odenike:Sanofi Aventis: Honoraria, Membership on an entity's Board of Directors or advisory committees; Incyte Pharmaceuticals: Honoraria, Membership on an entity's Board of Directors or advisory committees. Reddy:Geron Corporation: Employment. Roeth:Geron Corporation: Research Funding. Stuart:OncoMed Pharmaceuticals: Consultancy; Geron Corporation: Consultancy, Employment.

Eukaryotic chromosomes are capped with repetitive telomere sequences that protect the ends from damage and rearrangements. Telomere repeats are synthesized by telomerase, a ribonucleic acid (RNA)-protein complex. Here, the cloning of the RNA component of human telomerase, termed hTR, is described. The template region of hTR encompasses 11 nucleotides (5'-CUAACCCUAAC) complementary to the human telomere sequence (TTAGGG)n. Germline tissues and tumor cell lines expressed more hTR than normal somatic cells and tissues, which have no detectable telomerase activity. Human cell lines that expressed hTR mutated in the template region generated the predicted mutant telomerase activity. HeLa cells transfected with an antisense hTR lost telomeric DNA and began to die after 23 to 26 doublings. Thus, human telomerase is a critical enzyme for the long-term proliferation of immortal tumor cells.

Synthesis of DNA at chromosome ends by telomerase may be necessary for indefinite proliferation of human cells. A highly sensitive assay for measuring telomerase activity was developed. In cultured cells representing 18 different human tissues, 98 of 100 immortal and none of 22 mortal populations were positive for telomerase. Similarly, 90 of 101 biopsies representing 12 human tumor types and none of 50 normal somatic tissues were positive. Normal ovaries and testes were positive, but benign tumors such as fibroids were negative. Thus, telomerase appears to be stringently repressed in normal human somatic tissues but reactivated in cancer, where immortal cells are likely required to maintain tumor growth.

Abstract 3843 Background/Aims: Essential thrombocythemia (ET), a clonal myeloproliferative neoplasia, is characterized by elevation of platelets in peripheral blood (PB) and excessive proliferation of megakaryocytes (megs) in bone marrow. Most ET patients exhibit spontaneous growth of megakaryocytic colony-forming units (CFU-Meg) in vitro. One of the most important pathways involved in clonal expansion is reactivation of telomerase, an enzyme complex which is able to compensate for the loss of telomere repeats due to cell divisions. Over 90% of malignancies demonstrate up-regulation of telomerase, which is an essential component for their immortalization. Imetelstat sodium (GRN163L) is a potent and specific telomerase inhibitor that has demonstrated in vitro and in vivo activity against various tumor types and cancer stem cells. Our aims were to study the growth of CFU-Meg and their levels of telomerase activity (TA) from cord blood (CB), mononuclear cells (MNC) from healthy individuals and ET patients, as well as to test whether TA inhibition by imetelstat could suppress CFU-Meg formation. Agents which inhibit clonal expansion in vitro may have promising clinical activity. Methods: CB cells were enriched for CD34 expressing cells using a negative cell separation system. Cells were incubated with imetelstat (1–15μM) in serum free liquid medium, StemSpan® SFEM containing a cytokine formulation designed for the development of meg progenitor cells. CB cells were cultured for a total of 17 days; at various time point cells were enumerated, assessed by flow cytometry for differentiation markers (CD41) and for TA. Mononuclear cells (MNC) from healthy individuals and from 11 ET patients (WHO 2009 criteria) were isolated from PB and suspended in IMDM or plated into collagen ± cytokines (TPO, IL3, IL6, SCF, EPO) and treated with 0, 0.1, 1 and 10 μM imetelstat or a mismatch control, and incubated for several hours (cell suspensions) or 10–12 days (collagen plus 5% CO2) at 37° C. Megs were stained and the number of CFU-Meg was scored. In addition, TA was measured in CD34+ cells, megs and MNC by TRAP assay. Results: TA is low in CD34+ cells from CB of healthy individuals, but increases up to 9 times during early megakaryocytic differentiation (CD41a+CD34-) and peaks at day 3, followed by a decrease as the cells differentiate. Imetelstat (1–15 μM) significantly inhibited TA in a concentration-dependent manner in these progenitor cells (65% to 99%). Despite considerable TA inhibition, no significant inhibition in cellular growth and differentiation of megs was found in these CD34+ cell cultures from healthy CB donors. TA is low in MNC isolated from the PB of healthy individuals and of patients with ET. Imetelstat did not inhibit CFU-Megs in healthy individuals. In contrast, imetelstat significantly (p < 0.001) and concentration-dependently inhibited the spontaneous growth of CFU-Meg in each patient with ET (n=11, 7 JAKV617F-positive and 4 JAKV617F-negative). The growth of CFU-Meg (% of control) with various imetelstat concentrations in μM was: 100% (0 μM), 107% ± 8.6% (0.1 μM), 79% ± 11.8% (1 μM) and 33% ± 9.4% (10 μM). No correlation was found with the JAKV617F mutational status, other laboratory and clinical parameters, or with cytoreductive therapies. Conclusions: In summary, we can demonstrate telomerase activation with megakaryocytic proliferation and differentiation which precedes the decrease in cellular proliferation. Furthermore, we demonstrated concentration-dependent inhibition of TA by imetelstat in cells from healthy donors, but without inhibiting megakaryocytic proliferation. In contrast, TA inhibition by imetelstat results in a dose-dependent inhibition of spontaneous growth of CFU-Meg in ET patients. Inhibition was demonstrated at clinically relevant concentrations. This in vitro CFU-Meg inhibition is independent of the JAKV617F mutational status and of cytoreductive therapy. It appears that the transient inhibition of TA in normal megakaryocytic cells by imetelstat does not inhibit cellular proliferation, whereas TA inhibition by imetelstat in malignant cells significantly inhibited CFU-Meg proliferation. These findings suggest a specificity of imetelstat for malignant megakaryocytic cells. The impact of imetelstat's clinical activity is being explored in an ongoing phase 2 study in ET patients who have failed at least one prior therapy or who refuse standard of care. Disclosures: Go: Geron Corporation: Employment. Ninomoto:Geron Corporation: Employment. Kashani:Geron Corporation: Employment. Stuart:Geron Corporation: Employment. Oppliger Leibundgut:Geron Corporation: Service Contract; Novartis: Membership on an entity's Board of Directors or advisory committees, Service Contract; Bristol Myers Squibb: Membership on an entity's Board of Directors or advisory committees; Pfizer: Membership on an entity's Board of Directors or advisory committees. Baerlocher:Bristol Myers Squibb: Membership on an entity's Board of Directors or advisory committees; Novartis: Membership on an entity's Board of Directors or advisory committees; Geron Corporation: Research Funding, Service Contract; Pfizer: Membership on an entity's Board of Directors or advisory committees.

Purpose: Imetelstat is a first-in-class, lipidated 13-mer oligonucleotide thio-phosphoramidate telomerase inhibitor with anti-tumor activity. It is currently in Phase II clinical development. Its distribution in tissues, specifically tumor and bone marrow, was studied in a xenograft mouse model established using a human OVCAR-5 cell line. Data from the xenograft study indicated imetelstat tumor and bone marrow concentrations were sustained during the terminal phase. This suggests further study of imetelstat distribution in human tumor and bone marrow will be important in order to facilitate clinical understanding of drug concentration in the target tissues of hematological and solid tumor malignancies. Due to clinical sampling difficulties of these tissues, we aimed to develop a target tissue pharmacokinetic model to simulate imetelstat human tumor and bone marrow concentrations based on patient plasma levels. Experimental Design: Nude mice each received a tumor challenge of 3 million OVCAR-5 cells injected subcutaneously into contralateral flanks. Animals were randomized into 15 and 30 mg/kg treatment groups when tumors reached ∼200 mm3 in size, and were treated with imetelstat 3 times a week via IP bolus injection. Plasma, tumor and bone marrow samples were harvested at various time points post last drug treatment. Samples were subjected to a novel extraction procedure using a biotinylated capture probe before imetelstat concentration determination with ion-pairing reversed-phase LC/MS/MS. Human plasma data was obtained from solid tumor patients in a Phase I clinical trial after a 2-hr IV infusion of imetelstat at 9.4 mg/kg. Results: Imetelstat concentrations in plasma, tumor and bone marrow were quantified in xenograft mouse samples. Imetelstat distributes rapidly into bone marrow, and with a slight delay into the tumor tissue. For the 30 mg/kg dose group, based on preliminary data, imetelstat concentrations were determined up to 36 and 48 hrs in bone marrow and tumor, respectively, with 0.61 ug/mL in bone marrow and 1.05 ug/g in tumor. Concentrations in bone marrow and tumor were at higher levels than that in plasma when an equilibrium was established after 24 hrs following distribution between tissue and plasma. Tissue:plasma ratio during the terminal phase approximates 60 in bone marrow and 190 in tumor. This dataset was used in building a preclinical pharmacokinetic model to describe imetelstat tissue distribution in xenograft mouse. Imetelstat concentration time profiles in plasma and tissue were modeled simultaneously. Plasma profile was described by a two compartment open model, while tissue profiles were modeled with a first order transfer from central to tissue compartment. Using this model, together with human plasma data at 9.4 mg/kg, imetelstat tissue concentration profiles in human tumor and bone marrow were simulated. Conclusions: A target tissue pharmacokinetic model was established to describe imetelstat tumor and bone marrow distribution profiles in xenograft mouse. This model was applied toward the modeling and prediction of imetelstat target tissue concentrations in patients, and may be used to guide clinical trial dosing and dosing regimen(s). Detailed results of modeling and clinical implications will be determined when confirmatory studies are completed and will be presented at the meeting. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2011 Nov 12-16; San Francisco, CA. Philadelphia (PA): AACR; Mol Cancer Ther 2011;10(11 Suppl):Abstract nr A137.

Background: Activation of telomerase is essential for the indefinite replication potential of most cancer cells. Inhibition of telomerase is expected to lead to loss of telomere maintenance resulting in cell cycle arrest and/or cancer cell death, making telomerase inhibition an attractive anti-cancer approach. Cancer stem cells (CSCs) are rare cells in tumors implicated in cancer initiation and potentiation, as well as persistence or recurrence after standard treatment. Unlike normal tissue counterparts, all CSCs tested to date have upregulated telomerase activity. Recent identification of several non-canonical roles of telomerase components may expand the functionality of telomerase-inhibiting drugs. Imetelstat, a potent telomerase inhibitor currently in Phase II clinical trials, has been shown to reduce proliferative potential in multiple cancer models. Several recent studies have demonstrated that imetelstat depletes CSCs in various tumor types. The relatively rapid onset of CSC depletion could probably indicate a mechanism independent of telomere shortening. To understand the contributions of non-canonical pathways in the response of cancer cells to imetelstat treatment, we analyzed modulations of key signaling pathways in imetelstat-treated glioblastoma multiforme cancer cell lines (U87-MG and U118) and their CSC and bulk subsets. Results: Imetelstat inhibited telomerase activity, proliferative capacity and colony-forming potential in the U87 and U118 cell lines. Imetelstat treatment (2 weeks at 3uM) resulted in decreased expression of casein kinase 2 (CK2) subunits alpha and beta, and a reduction in phosphorylation of downstream CK2 substrates such as the DNA repair protein XRCC1. In addition, imetelstat reduced the transcriptional activity of beta-catenin, which is regulated by members of the CK2 family, in U87-MG cells. Cyclin D1, which is regulated by beta-catenin activity, was found to be down regulated by imetelstat treatment. These pathway modulations were not observed in T98G, a cell line resistant to imetelstat-mediated proliferation effects. Reducing CK2-alpha expression had an additive effect on cell growth inhibition in combination with imetelstat in U87-MG cells. Knock-down of the CK2-alpha, but not CK2-beta subunits, with siRNAs rapidly reduced the numbers of CSCs in U87-MG and U118 cell lines. Concomitant with the down-regulation of CK2-alpha signaling, imetelstat treatment depleted the number of CSCs in U78-MG and U118 cell lines. Preliminary data indicate a more marked reduction of CK2-alpha levels in the CSC subset of U87-MG cells compared to the bulk cells upon short term imetelstat treatment (1 week at 3uM). These in-vitro exposure concentrations of imetelstat are comparable to those attainable intratumorally in a xenograft mouse model. Conclusions: Our results suggest that CK2-alpha signaling and dysregulation of its downstream targets may play a key role in survival of CSCs as well as bulk tumor cells in U87 and U118 glioblastoma cell lines. These novel insights may help identify drugs that can act synergistically with imetelstat in treating cancer. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2011 Nov 12-16; San Francisco, CA. Philadelphia (PA): AACR; Mol Cancer Ther 2011;10(11 Suppl):Abstract nr A56.

Background: Plasma cells constitute the majority of tumor cells in multiple myeloma (MM) but lack the potential for sustained clonogenic growth. In contrast, clonotypic B cells can engraft and recapitulate disease in immunodeficient mice suggesting they serve as the MM cancer stem cell (CSC). These tumor initiating B cells also share functional features with normal stem cells such as drug resistance and self-renewal potential. Therefore, the cellular processes that regulate normal stem cells may serve as therapeutic targets in MM. Telomerase activity is required for the maintenance of normal adult stem cells, and we examined the activity of the telomerase inhibitor imetelstat against MM CSC. Moreover, we carried out both long and short-term inhibition studies to examine telomere length-dependent and independent activities. Methodology/Principal Findings: Human MM CSC were isolated from cell lines and primary clinical specimens and treated with imetelstat, a specific inhibitor of the reverse transcriptase activity of telomerase. Two weeks of exposure to imetelstat resulted in a significant reduction in telomere length and the inhibition of clonogenic MM growth both in vitro and in vivo. In addition to these relatively long-term effects, 72 hours of imetelstat treatment inhibited clonogenic growth that was associated with MM CSC differentiation based on expression of the plasma cell antigen CD138 and the stem cell marker aldehyde dehydrogenase. Short-term treatment of MM CSC also decreased the expression of genes typically expressed by stem cells (OCT3/4, SOX2, NANOG, and BMI1) as revealed by quantitative real-time PCR. Conclusions: Telomerase activity regulates the clonogenic growth of MM CSC. Moreover, reductions in MM growth following both long and short-term telomerase inhibition suggest that it impacts CSC through telomere length-dependent and independent mechanisms.

2598 Background: Following debulking chemotherapy, tumor regrowth may rely on proliferation of a cellular subcompartment with progenitor cell or “stem cell”-like properties. Self-renewal of this stem cell compartment requires the presence of telomerase, making inhibition of telomerase an appealing therapeutic strategy. Imetelstat, the first targeted telomerase inhibitor in clinical trials, is a 13-mer lipidated oligonucleotide that binds to the template RNA strand of telomerase. Methods: Pts with locally recurrent or MBC were treated with imetelstat in addition to P+B as in the previous E2100 study. Pts with prior adjuvant taxane therapy within 12 months were excluded. In a standard 3+3 dose-escalation design, imetelstat (160-375 mg/m2 2-hr IV infusion) was administered on days 1, 8, and 15 of a 28-day cycle. Study objectives were to define the maximally tolerated dose (MTD) and obtain preliminary safety and efficacy. Dose-limiting toxicities were determined by presence of hematologic toxicities and recei...

Numerous studies show that multiple tumor types have rare subpopulations of cells that are initiators of tumor growth, recurrence, and are implicated in tumor metastasis formation. Because of their clonogenic potential, these cells are referred to as tumor initiating cells (TICs) or cancer stem cells. TICs express ABC transporters and can reside in the host in a quiescent state within their preferred niche, which contributes to their resistance to therapies that are effective against bulk tumor cells. Patients having significant objective responses often develop recurrent resistant disease that is thought to be due to TIC outgrowth, and this is thought to contribute to a lack of clinically durable responses. Therefore agents targeting TICs should be included in current therapeutic strategies to assure more effective disease control in patients. Telomerase activation is a common phenotype for the majority of cancers and is essential for maintaining their immortal phenotype. Most tumors having elevated telomerase activity also have shorter telomere lengths than their normal tissue counterparts, and these characteristics make telomerase a promising therapeutic target for cancer. Tumor initiating cells, while distinct from the bulk tumor cell population, share the common traits of increased telomerase activity and relatively short telomeres, suggesting that inhibiting telomerase would be an effective modality for targeting these cells across multiple tumor types. Imetelstat is a potent and specific competitive inhibitor of telomerase currently in phase I clinical trials in solid tumor and hematological malignancies. We have carried out in vitro and in vivo studies demonstrating that imetelstat inhibits telomerase and is effective in targeting TICs from myeloma, melanoma, breast, pancreatic, pediatric glioma, neuroblastoma, prostatic, lung and glioblastoma multiforme tumor types. Our data show that while treatment of TICs with imetelstat is effective, the mechanisms of TIC inhibition may vary depending on the tumor type, and can include antiproliferative effects, induction of apoptotosis or senescence, terminal differentiation, and inhibition of clonogenicity in vitro, as well as inhibition of tumor engraftment and spontaneous metastasis formation in vivo. In summary, imetelstat, a first in class telomerase inhibitor currently in clinical trials, is a promising agent for targeting TICs with broad activity against multiple cancer stem cell types, but the mechanism of inhibition of TICs may vary depending on the specific tumor type. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 4291.

Telomerase activation is a hallmark of cancer and is critical for dysregulated cell proliferation and cell immortality. Increased telomerase activities were detected in essentially all epithelium derived cancers tested up-to-date and in many hematological malignancies as well, such as multiple myeloma and leukemia. Levels of telomerase activities in the malignant tissues are significantly higher compared to those detected in the normal, regenerating somatic tissues, such as liver and bone marrow cells. Given the differential activation states of telomerase in malignant and normal tissues, telomerase is considered an attractive cancer target by controlling telomere extension and cell immortality. Imetelstat is a first-in-class, lipidated oligonucleotide thio-phosphoramidate telomerase inhibitor. It is a potent and specific telomerase RNA template antagonist, currently in phase I clinical development. Objectives of this study were to demonstrate the in vivo telomerase activity inhibition in xenograft tumors and in bone marrow tissue in animal models, and to correlate the telomerase inhibition effect with mouse plasma drug exposure and xenograft tumor growth inhibition. Our preliminary results showed that treatment with imetelstat at 15-30 mg/kg tiw resulted in plasma concentrations greater than the IC50 for in vitro telomerase inhibition and a plasma exposure level comparable to the tolerable exposure in the clinic. Imetelstat treatment at 15-30 mg/kg tiw for 6 weeks inhibited OVCAR-5 xenograft tumor growth in vivo. Moreover, telomerase inhibition in the xenograft tumors and in the mouse bone marrow was demonstrated. Our results provided evidence of the specific target inhibition by imetelstat treatment in an in vivo system. Our data also suggested a pharmacodynamically efficacious dose level may have been achieved in the clinic. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 3202.

Background: Telomerase maintains telomere length (TL) of dividing cells, and is essential for cell immortalization. Telomerase is upregulated in tumors, particularly in cancer progenitor cells. Imetelstat (GRN163L), the first targeted telomerase inhibitor in clinical trials, is a 13-mer lipidated oligonucleotide that binds to the template RNA strand of telomerase. With weekly dosing, thrombocytopenia was dose limiting and MTD was 4.8 mg/kg. To increase drug exposure and maintain tolerability, we tested an intermittent dosing schedule of imetelstat. Methods: Patients (pts) with advanced solid cancers were given imetelstat as a single agent by 2 hr. i.v. infusions on D1 and D8 of 21-Day cycles (Cy), starting at 4.8 mg/kg. A 3+3 dose escalation was used. Pre-treatment granulocyte TL (PMN TL), a surrogate of hematopoietic progenitor TL, was measured by Flow-FISH. Results: 31 pts have been treated, with 4 currently on study. Mean number of prior cytotoxic regimens reported for 26 pts was 3.6 and 16 pts had prior irradiation. Dose escalation proceeded to 11.7 mg/kg, which was judged to be in excess of the MTD due to cytopenias. Three pts had been previously treated with 9.4 mg/kg without DLT, and the 9.4 mg/kg cohort was subsequently expanded to 12 pts, maintaining q21 D dosing. To date, severe myelosuppression has been observed in only 1 pt, and all 12 completed Cy 1. Three pts had doses held in Cy 2 for thrombocytopenia. Eight pts discontinued treatment after completing 1 to 4 Cy, 6 for PD, 1 for a hypersensitivity reaction and 1 for an unrelated SAE. Four pts are currently on treatment and have completed 2 to 3 Cy. Hypersensitivity reactions were observed in 5 of 31 pts dosed to date and were associated with biochemical evidence of complement activation; one was managed with secondary prophylaxis with steroids and antihistamines, and routine primary prophylaxis was subsequently instituted for all patients. Transient ( Preliminary analysis of pre- and post-treatment hair follicle samples for pharmacodynamic activity suggests a trend for telomerase inhibition. As previously noted*, thrombocytopenia correlated with short baseline PMN TL among 17 pts. Conclusions: Intermittent dosing of imetelstat at 9.4 mg/kg was well tolerated for multiple cycles. Drug exposures were in a range associated with antitumor activity in preclinical models and preliminary evidence of a pharmacodynamic effect has been observed. Hematologic toxicity is doselimiting, and may be mechanism-based. Citation Information: Mol Cancer Ther 2009;8(12 Suppl):C169.

Abstract 2647 Poster Board II-623 Background/Aims: Reflecting their proliferative history, malignant cells generally have shorter telomeres compared to their normal counterparts. We have reported that leukemic cells from patients with T-prolymphocytic leukaemia (T-PLL) have exceptionally short telomeres (telomere lengths around 1 kb) compared to other malignancies, and high levels of telomerase activity (Roeth et al., Leukemia, 2007). We have also previously presented (Roeth et al., ASCO 2008) cytotoxic effects of the telomerase inhibitory agent imetelstat sodium (GRN163L), which is currently in clinical trials. The objective of this study was to further characterize the relationship between mean telomere length (TL), the single chromosome telomeric size distributions (single TL), telomerase levels and imetelstat-induced telomerase inhibition and cytotoxicity in T-PLL, B-CLL, and normal cells. Methods: Cells were obtained from peripheral blood of normal donors and patients with T-PLL and B-CLL and cryopreserved. TL was measured by flow-FISH (which yields a multicellular average measurement across all chromosomes) and the distribution of individual telomere lengths by single telomere length analysis (STELA, XpYp specific). Telomerase activity (TA) was assessed by TRAP. Effects of imetelstat upon cell survival, apoptosis and TA were studied in short-term cell cultures. Apoptosis was measured by Annexin V-PI double staining. Results: Average (±SD) TL of the T-PLL cells was 1.8 ± 0.73 kb (n=13) which was shorter than for B-CLL cells (ZAP-70+/CD38+ CLL: 2.46 ± 1.08 kb (n=30); ZAP-70neg/CD38neg CLL: 5.06 ± 1.76 kb (n=29), Roeth et al, Br. J. Haem., 2008). T-PLL cells were found to have a narrow range of single chromosome telomere lengths, while both subtypes of B-CLL cells demonstrated a broader distribution of telomere lengths. No attrition of mean telomere length was observed over time in two patients with T-PLL who were followed over time, despite continued clinical progression. In contrast, we have observed that most patients with B-CLL do have decreases in telomere length over the course of their disease. Leukemic T-PLL cells exhibited high levels of telomerase activity, whereas there was little or no detectable telomerase activity in normal, unstimulated T-cells and most B-CLL cells. Imetelstat (GRN163L, Geron Corporation), a potent inhibitor of telomerase, induced significant and dose-dependent inhibition of telomerase activity in T-PLL cells after 2 hours of culture at 1μM and 10μM. Imetelstat significantly reduced cell viability of T-PLL cells in vitro after 7-10 days of culture (viability as % of control (mean ± SD) with 1, 3, 10 μM imetelstat: 69.9 ± 27.3, 51.7 ± 30.2, 37.1 ± 27.6). No significant cytotoxicity of imetelstat was observed in CLL samples, or in unstimulated and stimulated T-cells from 5 normal donors. Leukemic cells from one T-PLL patient early in the course of the disease did not exhibit loss of viability when exposed to imetelstat at levels up to 10 μM; however, when cells from this patient were tested later during the progression of the disease significant cytotoxicity was observed. Conclusions: In summary, leukemic T-PLL cells exhibit extremely short telomere lengths, narrower ranges of single chromosome telomere size distributions, and higher telomerase activity compared to B-CLL cells and normal T cells. These observations may explain why inhibition of telomerase activity in T-PLL cells by imetelstat is associated with rapid and dose-dependent cell death. Disclosures: Baerlocher: Geron Corporation: Research Funding. Bassett:Geron Corporation: Employment, Equity Ownership. Elias:Geron Corporation: Employment, Equity Ownership. Go:Geron Corporation: Employment, Equity Ownership. Roeth:Geron Corporation: Research Funding.

Background— Cells exhibiting dysregulated growth may express telomerase reverse transcriptase (TERT), the dual function of which consists of maintaining telomere length, in association with the RNA template molecule TERC, and controlling cell growth. Here, we investigated lung TERT in human and experimental pulmonary hypertension (PH) and its role in controlling pulmonary artery smooth muscle cell (PA-SMC) proliferation. Methods and Results— Marked TERT expression or activity was found in lungs from patients with idiopathic PH and from mice with PH induced by hypoxia or serotonin-transporter overexpression (SM22-5HTT + mice), chiefly within PA-SMCs. In cultured mouse PA-SMCs, TERT was expressed on growth stimulation by serum. The TERT inhibitor imetelstat and the TERT activator TA65 abrogated and stimulated PA-SMC growth, respectively. PA-SMCs from PH mice showed a heightened proliferative phenotype associated with increased TERT expression, which was suppressed by imetelstat treatment. TERC −/− mice at generation 2 and TERT −/− mice at generations 2, 3, and 4 developed less severe PH than did wild-type mice exposed to chronic hypoxia, with less distal pulmonary artery muscularization and fewer Ki67-stained proliferating PA-SMCs. Telomere length differed between TERC −/− and TERT −/− mice, whereas PH severity was similar in the 2 strains and across generations. Chronic imetelstat treatment reduced hypoxia-induced PH in wild-type mice or partially reversed established PH in SM22-5HTT + mice while simultaneously decreasing TERT expression. Opposite effects occurred in mice treated with TA65. Conclusions— Telomerase exerts telomere-independent effects on PA-SMC growth in PH and may constitute a treatment target for PH.

Telomerase activation enables replicative immortality in the majority of cancers including acute myeloid leukemia (AML). Imetelstat competitively inhibits telomerase activity with recently reported clinical efficacy in myelodysplasia and myelofibrosis. Here we show final results on a randomized preclinical trial in AML patient-derived xenografts (PDX; n = 30 individual AML; each treated with imetelstat vs. vehicle control; n = 6 per condition). Using drug sensitivity scores (DSS) to define in vivo PDX responses, we provide integrated phenotypic, cytogenetic, genomic and transcriptomic analysis of samples to understand AML response to imetelstat. We use this mechanistic insight to rationally sequence induction chemotherapy with imetelstat to optimize in vivo responses, even in initially poorly responding samples. Methods: An AML PDX cohort was established using bone marrow (BM) or peripheral blood (PB) samples collected from 30 AML patients and transplanted into NOD/SCID/IL2gR-/-/hIL3,CSF2,KITLG (NSGS; n = 12 recipients / AML patient sample). AML onset was defined by reconstitution of BM and spleen with CD45+ CD33+ donor cells, circulating blasts, anemia (HCT < 35%) or thrombocytopenia (PLT < 400 x 10^6/ml). After initial engraftment was confirmed by PB chimerism (> 1%), recipients were treated with imetelstat (15 mg/kg I.P.) or control every 48 - 72h. Results: Across the entire cohort, survival was improved for imetelstat- vs. PBS-treated PDX (Median survival PBS: 88d, Imetelstat: 138d; Hazard ratio PBS: 2.87; 2.20 to 3.74; Cox proportional hazards model: p < 0.0001), however there were clear differences in the quality and duration of response to imetelstat treatment. In order to quantify the sensitivity of each individual AML patient sample to therapy, DSS were derived using the area under the curve of PB donor chimerism in PBS versus imetelstat-treated recipients (Mean DSS entire cohort: 3.21 ± 0.36). DSS positively correlated with survival benefit (R square: 0.87), and values higher than 2.3 gained at least 30 days of survival benefit. Based on DSS, AML patient samples were grouped into "Sustained" (17 AML patient samples; 57%) or "Poor" responders (13 AML patient samples; 43%) with DSS 2.3 as cut-off value. European LeukemiaNet 2017 adverse prognostic samples were trend-wise enriched in poor vs. sustained responders to imetelstat (83.3% poor vs. 16.7% sustained response; p = 0.06, Fisher's exact test). RNA sequencing and mutational profiling was performed on all baseline AML samples. Differential expression analysis revealed a gene signature specifically expressed in sustained compared to poor responders (p < 0.05; log2FC > 1.5; 109 annotated genes) that was functionally annotated in replicative stress including nucleosome (p = 0.01), transcription factor activity (p = 0.04), and sequence-specific DNA binding (p = 0.04). We therefore examined whether the induction of replicative stress could sensitize response to imetelstat therapy. Seven independent AML patient samples with poor response to imetelstat monotherapy were transplanted into NOD.Rag1-/-Il2Rg-/-/ hIL3,CSF2,KITLG (NRGS). Mice received standard induction chemotherapy intravenously as a cytarabine (Arac; 50 mg/kg) and doxorubicin (Doxo; 1.5 mg/kg) 5+3 dosing regimen followed by continuous imetelstat therapy vs. imetelstat alone, standard induction chemotherapy alone, or vehicle control (n = 6 / group, n = 24 / AML sample). Combination therapy significantly prolonged overall survival (Median survival: Control: 38.5d; Imetelstat: 41d; Arac+Doxo: 61d; Arac+Doxo + Imetelstat: 71d post-start of treatment; Hazard ratio Arac+Doxo alone vs. Arac+Doxo+Imetelstat: 2.34; 1.19 to 4.586; Cox proportional hazards model: p < 0.001). In summary, imetelstat prolonged overall survival in a large cohort of AML PDX. In 57% (17 out of 30 individual AML patient samples), sustained responses to imetelstat were associated with marked improvements in survival and a baseline replicative stress-annotated transcriptional signature. The rational sequencing of standard induction chemotherapy to induce replicative stress provides proof-of-concept to sensitize imetelstat-resistant AML patient samples and suppress expansion and prevent relapse in AML. Disclosures Lane: novartis: Consultancy; janssen: Consultancy, Research Funding.

Leukocyte telomere length (TL) has been suggested as a marker of biological age in healthy individuals, but can also reflect inherited and acquired hematopoietic dysfunctions or indicate an increased turnover of the hematopoietic stem and progenitor cell compartment. In addition, TL is able to predict the response rate of tyrosine kinase inhibitor therapy in chronic myeloid leukemia (CML), indicates clinical outcomes in chronic lymphocytic leukemia (CLL), and can be used as screening tool for genetic sequencing of selected genes in patients with inherited bone marrow failure syndromes (BMFS). In tumor cells and clonal hematopoietic disorders, telomeres are continuously stabilized by reactivation of telomerase, which can selectively be targeted by telomerase-specific therapy. The use of the telomerase inhibitor Imetelstat in patients with essential thrombocythmia or myelofibrosis as well as the use of dendritic cell-based telomerase vaccination in AML patients with complete remissions are promising examples for anti-telomerase targeted strategies in hematologic malignancies. In contrast, the elevation in telomerase levels through treatment with androgens has become an exciting clinical intervention for patients with BMFS. Here, we review recent developments, which highlight the impact of telomeres and telomerase targeted therapies in hematologic dysfunctions.