Monitoring of Calr Allele Burden in Patients with Essential Thrombocythemia Treated with Imetelstat, a Telomerase Inhibitor, Reveals Rapid and Substantial Molecular Responses

Background: In essential thrombocythemia (ET), mutations in the calreticulin gene (CALR) are found in the majority of patients that are negative for mutations in the JAK2 and MPL genes. Patients with mutated CALR have a better prognosis and lower thrombosis risk than those with mutated JAK2. Recently, decreases in the CALR mutant allele burden have been observed with interferon alpha after long-term treatment of two- and four-years, respectively (NEJM 2014, 371;2:188-9, Cassinat B. et al.). From the clinical phase II ET study with imetelstat (IT), a first in class, potent, specific inhibitor of telomerase, we reported a substantial and rapid decrease in the JAK2V617F allele burden. 7/8 patients (88%) reached a partial molecular response (MR: >50% reduction from baseline), 6 within the first 3 months and 1 after 12 months.

Aims: We aimed to monitor molecular response to imetelstat therapy in ET patients with CALR mutations by serial measurements of CALR mutant allele burden.

Methods: The study enrolled patients with ET who had failed or were intolerant to ≥1 prior therapy, or refused standard therapy. During the induction phase, patients were treated with IT 7.5 mg/kg or 9.4 mg/kg IV weekly until attainment of platelet (plt) count ~250-300x10⁹/L. Maintenance phase was then commenced with dosing titrated to platelet count. CALR mutations were detected by Sanger sequencing and quantification of the allele burden was performed by fragment analysis.

Results: 18 patients with ET (10 patients with JAK2V617F, 5 patients with CALR and 2 patients with MPL mutations) were enrolled and were treated in the study. 4 of the 5 CALR positive patients achieved complete hematologic responses (CR: Plts < 400 x10⁹/L for 4 weeks) after a median of 6 weeks (range 5 to 14 weeks) and the 5th patient achieved a partial response after 19 weeks, with weekly imetelstat doses starting at 7.5 mg/kg in 2 patients and 9.4 mg/kg in 3 patients. CALR mutations consisted of three cases with type 1 (52-bp deletion; c.1092_1143del), one with type 2 (5-bp insertion; c.1154_1155insTTGTC) and one unknown mutation type (32-bp deletion; c.1092_1124del). Molecular monitoring of CALR allele burden at cycles 3, 6 and 9 demonstrated a rapid decrease in the CALR-mutated patients. 3 pts had a 35-50% reduction from baseline within 4 months and 1 pt had an 11% decrease within 8 months. One of these patients had a 48% reduction in 2 months and a second one had a deepening of response after 10 months to a 55% reduction. All 3 patients with CALR allele burden reductions of 35% or more also achieved hematologic CR.

Conclusions: In 4 of 5 patients with CALR-mutated ET, IT induced a rapid CR and in 3 patients hematologic CR was associated with a substantial decrease in the allele burden of 35-50% after 4 months which is more rapid than what has so far been seen with other treatments for ET. Overall 9/13 patients with JAK2 or CALR mutations reached a >35-50% decrease of the mutant clone within 4 months of treatment with IT, providing clinical confirmation of imetelstat’s inhibition of neoplastic clonogenic cell growth in vivo. This additional evidence of reduction in the clonal burden supports IT’s potential to modify the biology of MPNs long-term.