About Imetelstat

Originally known as GRN163L, imetelstat sodium (imetelstat) is a 13-mer N3’---P5’ thio-phosphoramidate (NPS) oligonucleotide that has a covalently bound 5’ palmitoyl (C16) lipid group. The proprietary nucleic acid backbone provides resistance to the effect of cellular nucleases, thus conferring improved stability in plasma and tissues, as well as significantly improved binding affinity to its target. The lipid group enhances cell permeability to increase potency and improve pharmacokinetic and pharmacodynamic properties. The compound has a long residence time in bone marrow, spleen and liver. Imetelstat binds with high affinity to the template region of the RNA component of telomerase, resulting in direct, competitive inhibition of telomerase enzymatic activity, rather than elicit its effect through an antisense inhibition of protein translation. Imetelstat is administered by intravenous infusion.


Imetelstat Bound to Telomerase

Preclinical Studies with Imetelstat

A series of preclinical efficacy studies of imetelstat have been conducted by Geron scientists and academic collaborators. These data showed that imetelstat:

  • Inhibits telomerase activity, and can shorten telomeres.
  • Inhibits the proliferation of a wide variety of tumor types, including solid and hematologic, in cell culture systems and rodent xenograft models of human cancers, impacting the growth of primary tumors and reducing metastases.
  • Inhibits the proliferation of malignant progenitor cells from hematologic cancers, such as multiple myeloma, myeloproliferative neoplasms and acute myelogenous leukemia.
  • Has additive or synergistic anti-tumor effect in a variety of cell culture systems and xenograft models when administered in combination with approved anti-cancer therapies, including radiation, conventional chemotherapies and targeted agents.

Clinical Experience with Imetelstat

Over 500 patients have been enrolled and treated in imetelstat clinical trials.

Phase 1

Six clinical trials evaluated the safety, tolerability, pharmacokinetics and pharmacodynamics both as a single agent and in combination with standard therapies in patients with solid tumors and hematologic malignancies:

  • Single agent studies of imetelstat were in patients with advanced solid tumors, multiple myeloma and chronic lymphoproliferative diseases. Combination studies with imetelstat were with bortezomib in patients with relapsed or refractory multiple myeloma, with paclitaxel and bevacizumab in patients with metastatic breast cancer, and with carboplatin and paclitaxel in patients with advanced non-small cell lung cancer (NSCLC).
  • Doses ranging from 0.5 mg/kg to 11.7 mg/kg were tested in a variety of dosing schedules ranging from weekly to once every 28 days.
  • The human pharmacokinetic profile was characterized in clinical trials of patients with solid tumors and chronic lymphoproliferative diseases. Single-dose kinetics showed dose-dependent increases in exposure with a plasma half-life (t1/2) ranging from 4-5 hours. Residence time in bone marrow is long (0.19-0.51 µM observed at 41-45 hours post 7.5 mg/kg dose).
  • Telomerase inhibition was observed in various tissues where the enzymes's activity was measurable.

Phase 2

Imetelstat was studied in two randomized clinical trials, two single arm proof-of-concept studies and an investigator sponsored pilot study:

Safety and Tolerability

The safety profile of imetelstat across the Phase 1 and 2 trials has been generally consistent. Reported adverse events (AEs) and laboratory investigations associated with imetelstat administration included cytopenias, transient prolonged activated partial thromboplastin time (aPTT; assessed only in Phase 1 trials), gastrointestinal symptoms, constitutional symptoms, hepatic biochemistry abnormalities, and infusion reactions. Dose limiting toxicities include thrombocytopenia and neutropenia.

A Focus on Hematologic Myeloid Malignancies

Early clinical data from the Phase 2 clinical trial in ET and the investigator sponsored pilot study in MF suggest imetelstat may have disease-modifying activity by suppressing the proliferation of malignant progenitor cell clones for the underlying diseases, and potentially allowing recovery of normal hematopoiesis in patients with hematologic myeloid malignancies.

Results from these trials were published in the New England Journal of Medicine:

Current Clinical Trials

Imetelstat is currently being tested in two clinical trials: IMbark, a Phase 2 trial in myelofibrosis (MF), and IMerge, a Phase 2/3 trial in myelodysplastic syndromes (MDS). 


IMbark is the ongoing Phase 2 clinical trial to evaluate two doses of imetelstat in intermediate-2 or high-risk MF patients who are refractory to or have relapsed after treatment with a JAK inhibitor.

Internal data reviews were completed in September 2016, April 2017 and March 2018. The safety profile was consistent with prior clinical trials of imetelstat in hematologic malignancies, and no new safety signals were identified. The data supported 9.4 mg/kg as an appropriate starting dose in the trial, but an insufficient number of patients met the protocol defined interim efficacy criteria and new patient enrollment was suspended in October 2016. As of January 2018, median follow up was approximately 19 months, and median overall survival had not been reached in either dosing arm. In March 2018, the trial was closed to new patient enrollment. Patients who remain in the treatment phase of the trial may continue to receive imetelstat, and until the protocol-specified primary analysis, all safety and efficacy assessments are being conducted as planned in the protocol, including following patients, to the extent possible, until death, to enable an assessment of overall survival.


IMerge is the ongoing two-part Phase 2/3 clinical trial of imetelstat in red blood cell (RBC) transfusion-dependent patients with lower risk MDS who are refractory or resistant to treatment with an erythropoiesis stimulating agent (ESA). Part 1 is a Phase 2, open-label, single-arm trial of imetelstat administered as a single agent by intravenous infusion, and is ongoing. Part 2 is designed to be a Phase 3, randomized, controlled trial, and has not been initiated.

Preliminary data as of October 2017 from the first 32 patients enrolled in the Part 1 (Phase 2) of IMerge were presented as a poster at the American Society of Hematology Annual Meeting in December 2017.

The data showed that among the subset of 13 patients who had not received prior treatment with either lenalidomide or a hypomethylating agent (HMA) and did not have a deletion 5q chromosomal abnormality (non-del(5q)), 54% achieved RBC transfusion-independence (TI) lasting at least 8 weeks, including 31% who achieved a 24-week RBC-TI. In the overall trial population, the rates of 8- and 24-week RBC-TI were 38% and 16%, respectively. Cytopenias, particularly neutropenia and thrombocytopenia, were the most frequently reported adverse events, which were predictable, manageable and reversible.

Based on the preliminary data from the 13-patient subset, Janssen expanded Part 1 of IMerge to enroll approximately 20 additional patients who were naïve to lenalidomide and HMA treatment and non-del(5q) to increase the experience and confirm the benefit-risk profile of imetelstat in this refined target patient population.

Additional information about the status of these clinical trials can be found on our Investors pages.