Dynamic and Time-to-Event Analyses Demonstrate Marked Reduction in Transfusion Requirements for Janus Kinase Inhibitor-Naïve Myelofibrosis Patients Treated with Momelotinib Compared Head to Head with Ruxolitinib

Momelotinib (MMB) is a potent, selective, orally-bioavailable, small-molecule inhibitor of JAK1, JAK2 and ACVR1 being developed for the treatment of intermediate and high risk myelofibrosis (MF). Systemic inflammation integral to the pathogenesis of MF leads to increased ACVR1 activity which in turn increases secretion of hepcidin, resulting in perturbed iron homeostasis and an iron-restricted anemia (Physiol Rev. 2013;93:1721-41, Am J Hematol. 2014;89:470-9). MMB's inhibition of ACVR1, unique amongst the JAK inhibitor (JAKi) class, leads to a reduction of hepcidin, restoring iron homeostasis and RBC production and alleviating anemia and transfusion dependency (TD). Chronic, progressive anemia is the key hallmark feature of MF; anemia and TD are strongly predictive of reduced survival (Am J Hematol. 2013;88:312-6). MMB is the only clinical stage JAKi to possess potent ACVR1 inhibitory activity, resulting in improvement of anemia in contrast to ruxolitinib (RUX) which results in worsening.

The SIMPLIFY-1 (S1) trial, a double-blind, active-controlled Phase 3 study in which 432 patients received randomized treatment with MMB or RUX for 24 weeks was previously reported (JCO. 2017;35:3844-50). In addition to a significant reduction in splenomegaly and improvement in constitutional symptoms, the study demonstrated that patients in the MMB arm achieved nominal-statistical significance for all anemia endpoints tested, including a higher rate of transfusion independence (p<0.001) and lower rates of TD (p=0.019) at Week 24, compared to patients on RUX, consistent with MMB's pro-erythropoietic effect. Overall, a demonstrably decreased transfusion requirement was noted in patients who received MMB vs RUX.

Since transfusion burden is of significant concern to clinicians and patients, to better understand the dynamics of RBC transfusions we further examined the S1 data through statistical models utilizing a variety of novel anemia benefit endpoints including time until transfusion and overall intensity of transfusions across time. The proportions of patients with 0 and 4 transfusions were calculated and time-to-event analyses examining time-to-first and time-to-fifth units transfused also conducted. Since transfusions typically comprise 2 units, the fifth unit transfused represents a de facto third transfusion event. The number of units transfused were also considered to be recurrent events and examined with and without patients' baseline characteristics as covariates. Finally, a mixture model, based on a zero-inflated negative binomial (ZINB) distribution fit to the transfusion data, was employed to compare between the treatment groups the proportions of subjects with zero transfusion burden and the mean transfusion rates.

Kaplan-Meier estimates of the proportion of patients who did not require any units transfused during the 24 week randomized treatment period were 73% and 46% for MMB and RUX respectively (p<0.0001; Figure 1), while the proportion of patients requiring 4 or fewer units were 83% and 62% (p<0.0001). When examining units transfused as recurrent events, patients receiving MMB possessed a hazard ratio of approximately one-half that for patients on RUX (HR 0.522; p<0.0001) for models both with and without patients' baseline characteristics as covariates. The ZINB covariate model demonstrated that MMB increased the odds of having zero units transfused in the first 24 weeks by a factor of 9.3 (p<0.0001) vs RUX.

Taken together, the novel dynamic and time-to-event analysis methods described are relevant and informative additions to standard measures of transfusion burden in patients with MF. The results of these analyses allow more detailed description of MMB's differentiated anemia benefit as compared to RUX in a double-blind study of JAKi-naïve patients. These results when combined with additional data from the SIMPLIFY studies demonstrate that MMB is able to address the three hallmark features of MF, namely anemia, constitutional symptoms and splenomegaly, differentiating it from other JAK inhibitors. The benefit of MMB in reducing transfusion burden will be further evaluated in MOMENTUM, a future Phase 3 study of MMB in MF patients. In addition to assessment of constitutional symptoms, anemia and splenomegaly, MOMENTUM will provide opportunity to further evaluate associations between anemia benefit and patient reported measures of clinical benefit.

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