Imetelstat Significantly Reduces Leukemia Stem Cells in Patient-Derived Xenograft Models of Pediatric AML

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<0.05). Mice receiving standard chemotherapy consisting of cytarabine and daunorubicin or azacitidine showed prolonged survival compared to the untreated mice. Interestingly, sequential administration of imetelstat following chemotherapy treatment provided additional benefit over chemotherapy alone (P<0.01). Concurrent treatment of azacitidine and imetelstat further extended survival of these mice compared to azacitidine alone (P<0.05).

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<0.05). Neither chemotherapy nor azacitidine alone affected LSC population compared to untreated mice. However, imetelstat significantly reduced the LSC population when combined with chemotherapy or azacitidine compared to single agent (P<0.05). These results were confirmed by secondary transplantation in mice, which showed delayed engraftment of cells isolated from imetelstat treated mice (Fig. 1G, H).

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.

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