Longer-Term Follow up with TG101348 Therapy In Myelofibrosis Confirms Sustained Improvement In Splenomegaly, Disease-Related Symptoms, and JAK2V617F Allele Burden

A Phase I study of TG101348, a potent, oral JAK2-selective inhibitor in patients with high- or intermediate-risk primary, post-polycythemia vera (PV) and post-essential thrombocythemia (ET) myelofibrosis (MF) recently completed enrollment. Data from the dose escalation cohort (n=28; 30–800 mg administered as a single daily dose) showed dose-linear plasma exposure, with mean elimination T_1/2 at steady state ranging from 16 to 34 hours. The dose-limiting toxicity was reversible asymptomatic grade 3 or 4 amylasemia/lipasemia; the maximum tolerated dose was 680 mg. We now present updated results from an extension study in which subjects continued treatment beyond six 28-day cycles.

In the entire study, 59 subjects (median age 64 years) were treated. Forty four patients had PMF, 12 post-PV MF, and 3 post-ET MF; 86% were JAK2V617F-positive. Median palpable spleen size was 18 cm and 21 (36%) patients were transfusion-dependent at study enrollment. Forty three (73%) subjects continued treatment on the extension study; the median (range) cumulative exposure to TG101348 was 380 days (170-767); the number of cycles of treatment completed ranged from 7–29, and the proportion of subjects who completed 12, 18, and 24 cycles of treatment was 72%, 28%, and 14%, respectively. The median (range) treatment dose at data cutoff was 440 mg/day (120 to 680 mg/day; n = 35).

TG101348 was well tolerated. Non-hematologic, at least possibly related adverse events included nausea (69%; Grade 3=3%), diarrhea (64%; Grade 3=10%), and vomiting (58%; Grade 3=3%), that were dose-dependent; the symptoms were self-limited or controlled with symptomatic treatment and/or dose reduction. Also observed were asymptomatic increases in serum lipase (27%), AST/ALT (27%/25%), creatinine (24%), and alkaline phosphatase (17%). Grade 3/4 hematological adverse events possibly related to TG101348 included treatment-emergent anemia (35% of patients who were not transfusion dependent at baseline), thrombocytopenia (24%), and neutropenia (10%).

During cycles 1–6, 42% of patients achieved Clinical Improvement (CI) per the International Working Group for Myeloproliferative Neoplasms Research and Treatment (IWG-MRT) criteria for spleen response. The CI rate for the 43 subjects who continued treatment beyond 6 cycles was 65%, and the median time (range) to CI was 140 days (77-802). After 12 cycles, 36 (95%) of the 38 evaluable patients had some degree of spleen size reduction; 32 (85%), 26 (68%), and 10 (26%) of subjects had a minimum 25%, 50%, and 100% decrease in palpable splenomegaly.

Of the 48 JAK2V617F mutation-positive subjects, 23 (48%) had a baseline mutant allele burden of >20%. Of these, 20 subjects with a baseline median allele burden (range) of 62% (23% to 100%) completed at least 6 cycles of treatment. Sixteen subjects (80%) exhibited a median 62% (range 6% to 96%) decrease in JAK2V617F burden, including 9 (45%) with a ≥50% decrease; 4 subjects (20%) had increases (range 18% to 58%). Eighteen subjects (78%) completed 12 cycles of treatment; 13 (72%) had a median 50% decrease (range 29% to 82%), including 7 (39%) subjects with a ≥50% decrease; 5 subjects (28%) had no change or increases (range 0% to 22%) in JAK2V617F.

For patients with data at baseline and cycle 6, early satiety, fatigue, night sweats, cough, and pruritus were reported by 56%, 47%, 24%, 22%, and 19% of subjects, respectively. Improvement (complete resolution) rates observed at cycle 6 were: 75% (40%), 65% (29%), 89% (89%), 88% (75%), and 100% (57%) for the aforementioned symptoms, respectively.

Leukocytosis was present at baseline in 33 subjects; following 6 and 12 cycles of therapy, 57% (n=28) and 56% (n=25) achieved normal WBC counts. Thrombocytosis was noted at baseline for 10 (17%) subjects; following 6 and 12 cycles of therapy, 90% (n=10) and 88% (n=8) achieved normal platelet counts.

The current study provides proof-of-concept that selective JAK2 inhibition has anti-myeloproliferative/anti-clonal activity in myelofibrosis. The majority of subjects treated with TG101348 for more than 6 cycles experienced a rapid, significant, and durable decrease in splenomegaly, normalization of leukocyte/platelet counts, and improvement of disease-related symptoms, all within the context significantly decreased JAK2V617F burden.

Off Label Use: There is no FDA approved drug for Myelofibrosis.All the drugs discussed will be off-label. Stone:Novartis: Consultancy. Silverman:TargeGen: Consultancy. Shorr:TargeGen: Employment, Equity Ownership.