Safety and efficacy of jaktinib in the treatment of Janus kinase inhibitor‐naïve patients with myelofibrosis: Results of a phase II trial

Abstract Myelofibrosis (MF) is associated with several constitutional symptoms. Currently, there are few therapeutic options for MF. Jaktinib, a novel, small‐molecule inhibitor of JAK, is currently being studied for its potential to treat MF. This phase 2 trial investigated efficacy and safety of jaktinib in the treatment of MF patients. The primary end point was the proportion of patients with ≥35% reduction in spleen volume (SVR35, proportion of patients with ≥35% reduction in spleen volume) at week 24. The secondary end points included improvement of anemia, rates of symptom response, and safety profile. Between January 8, 2019 and August 29, 2020, 118 patients were recruited and treated with either jaktinib 100 mg BID or 200 mg QD. At week 24, 54.8% (34/62) of patients in the 100 mg BID group and 31.3% (15/48) in the 200 mg QD group achieved SVR35 (p = .0199). Jaktinib treatment increased hemoglobin level to ≥20 g/L in 35.6% (21/59) of patients with hemoglobin ≤100 g/L at baseline. The proportion of patients who achieved a ≥50% improvement in total symptom score at week 24 was 69.6% (39/56) in the BID group and 57.5% (23/40) in the QD group. The most common ≥ grade 3 hematological treatment‐emergent adverse events (TEAEs; ≥ 10%) were anemia (100 mg BID: 24.2%, 200 mg QD: 28.8%), thrombocytopenia (16.7%, 11.5%), and neutropenia (3.0%, 11.5%). All non‐hematological TEAEs were mild. These results indicate that jaktinib can shrink the spleen, improve anemia, and other clinical symptoms with good tolerability.

Fedratinib, a highly selective JAK2 inhibitor, was found to be effective in phase II and phase III (JAKARTA I and JAKARTA II) clinical trials. Consequently, it was the second drug for MF approved by the USA Food and Drug Administration (US-FDA). 6,7 Pacritinib is the first and only JAK1-sparing inhibitor of JAK2 and IRAK1 which was recently approval by US-FDA for the management of cytopenic MF. Momelotinib has undergone several clinical trials, with two III phase trials competed, and some III phase trails on going. 8,9 In China, there are no effective drugs for the management of symptoms of MF patients.
Jaktinib is a deuterated compound of momelotinib that has been found to be a small-molecule inhibitor of JAK. Currently, it is being studied due to its potential to treat MF patients. Results from preclinical pharmacokinetic studies and the phase I trial in healthy Chinese volunteers revealed that jaktinib has a stable pharmacokinetic profile. 10 The pharmacological mechanism of jaktinib is similar to that of momelotinib. Compared with JAK1, jaktinib has higher inhibitory effect on JAK2 or TYK2. It can also inhibit ACVR1 like momelotinib does. To further evaluate the efficacy and safety of jaktinib, we con- status of 0 to 2; palpable splenomegaly (≥5 cm below the left costal margin); no JAK inhibitor exposure in the past; < 10% peripheral blood blasts; and a platelet count ≥75 Â 10 9 /L. Patients were excluded if they had received any anti-MF treatment within 2 weeks or six halflives (whichever is longer) prior to the first dose, and had a history of radiotherapy to the spleen within 12 months before screening, malignant tumors in the past 5 years, any significant clinical and laboratory abnormalities or serious diseases that the investigator considered would affect the safety assessment. Details of the inclusion and exclusion criteria, information about the trial design, and the statistical analysis plan are described in Supplementary 1 (Trial protocol). All patients provided written informed consent before undergoing any procedures. In the first stage of the study, 104 patients from 21 sites were randomly assigned to two groups using an interactive web response system at a 1:1 ratio: the open-label jaktinib 100 mg BID group and 200 mg QD group. A medium-term analysis showed that jaktinib at a dose of 100 mg BID showed good efficacy. Therefore, 14 more participants were enrolled in the 100 mg BID group in the second stage of the study. Treatment assignments were known to all members of the research team and the patients. Eligible participants were scheduled to undergo at least four treatment cycles for 24 weeks (6 weeks per cycle). Clinic visits were performed every 1 to 2 weeks during the first cycle, every 3 weeks during the 2nd to 4th cycles, every 6 weeks during 5th-8th cycles, and every 12 weeks after 48 weeks. Response was assessed once per cycle whereas safety was assessed during each  Table S1.

| Study design
Dose adjustment). Patients in all arms were treated until disease progression or unacceptable toxicity.

| Study end points
The primary end point was a reduction of at least 35% in spleen volume (SVR35) at week 24 compared with baseline. The SVR35 was evaluated using MRI or CT scan by a blinded central reader. Main secondary end points were the proportion of participants who had SVR35 during the study period, time from the first dose to SVR35, duration of spleen response (duration from the time when SVR35 was achieved until the time when the spleen volume increased by 25% or more from the nadir), proportion of transfusion-dependent patients who converted to transfusion independent (transfusion dependent: requires red blood cell (RBC) transfusion of at least 2U within 30 days prior to administration of the drug, converted to transfusion independent: no RBC transfusion for more than 12 weeks and hemoglobin ≥85 g/L during the treatment phase), the proportion of transfusionindependent patients whose hemoglobin was ≤100 g/L at baseline and increased by ≥20 g/L after treatment, proportion of patients with at least 50% decrease in the times of RBC infusions (average times of blood transfusions per month after jaktinib treatment compared with the frequency of blood transfusions within 30 days prior to the first dose) during the entire treatment, proportion of patients whose MPN-SAF TSS decreased by ≥50% at week 24, absolute and percentage decrease in MPN-SAF TSS on prespecified visit, and safety.
Adverse events (AEs) were graded according to the Common Toxicity Criteria for Adverse Events (CTCAE) version 4.03. Overall survival (OS) was defined as the time from the first dose to death or last known alive date.

| Statistical analysis
A total of 100 patients were randomly assigned at a 1:1 ratio to the jaktinib 100 mg BID and jaktinib 200 mg QD in the first stage. With 50 patients in a treatment group, the half width of the 95% confidence interval (CI) was not greater than 15%, assuming the proportion of patients who had SVR35 at 24 weeks was 48% and the drop-out rate was below 10% based on published myelofibrosis studies on momelotinib and ruxolitinib. Having obtained good results at the dosage of 100 mg bid, we started the second stage of the study. To obtain a sample size that meets the requirements for a potential new drug application by China National Medical Products Administration, additional 36 patients were enrolled in the jaktinib 100 mg BID group in the second stage. This provided at least 80% probability to reject the null hypothesis that the SVR35 at 24 weeks was less than 23% at a significance level of two-sided 0.05 even if the data of patients receiving jaktinib 100 mg BID in stage 1 were not taken into account assuming the SVR35 at week 24 was 48% for jaktinib and the drop-out rate was 10%. During the study, the sponsor decided to initiate a randomized, active controlled phase III trial following an agreement with the regulatory agency. Therefore, participant recruitment at the second stage was terminated earlier.
Finally, 66 patients treated with jaktinib 100 mg BID and 52 patients treated with jaktinib 200 mg QD were enrolled, providing an adequate statistical power based on the aforementioned assumptions, despite the early termination.
Drug efficacy was determined in all patients who received at least one dose of jaktinib in accordance with the intention-to-treat (ITT) principle. However, for variables which need to be compared with baseline data, like SVR35, patients with missing baseline were excluded from the analyses. For the primary end point, data of patients who did not undergo the assessment of spleen volume at week 24 were imputed using the last observation carried forward (LOCF) method. Therefore, 13 patients with at least one post-baseline assessment were included to calculate the SVR35 by LOCF method.
Sensitivity analyses for the primary end point included analysis of the per-protocol analysis set (PPS), analysis using multiple imputation or observed case or non-responder imputation to handle missing data, and analysis based on the spleen volume were evaluated by the investigator. For binary end points, the Clopper Pearson method was used to calculate the 95% CIs, and the Fisher's exact test was used for between-group comparisons. The time-to-event end points including durability of splenic response, and OS were analyzed using the Kaplan-Meier method. Hazard ratios (HRs) and the corresponding 95% CIs were estimated using the Cox proportional hazards model.
Comparison of data of between-group treatments were performed using a two-sided log-rank test. For continuous end points, the values and changes from baseline were summarized for each treatment group, and no imputation was carried out for missing data. All statistical analyses were performed using SAS version 9.4. p < .05 was considered as significant (Supplementary 3, Table S2. Data set). The trial was registered with the ClinicalTrials.gov, registration number: NCT03886415.

| RESULTS
In this multicenter trial, patients were enrolled from January 8, 2019 to August 29, 2020 at 22 study centers. A total of 190 patients were assessed for eligibility among which 118 were enrolled in the study: 66 were assigned to receive jaktinib 100 mg BID and 52 were assigned to 200 mg QD (Figure 1). There were 71 (60.2%) males and 47 (39.8%) females, with a median age of 60. 5

| DISCUSSION
The present randomized, phase II study evidently showed that jaktinib   12 In the study on SIMPLIFY-2, it was found that 7% the patients previously treated with ruxolitinib were found to achieve SVR35 at week 24 after the change of ruxolitinib to momelotinib. 13 Besides, the studies of PERSIST-1 (JAKi Naïve) and PERSIST-2 carried out before the allowing the treatment with JAKi showed that between 19 and 18% of patients with MF achieved SVR35 at week 24 after treatment with pacritinib. 14,15 Nonetheless, the use of different prognostic scoring systems (IPSS /DIPSS/DIPSS-plus), the difference in population, duration of treatment, phase of trial, and supportive treatment may also affect the primary end point.
Based on the results of the present study, it was evident that jaktinib was well-tolerated. Anemia and thrombocytopenia are the frequent hematologic adverse events and the reasons for treatment discontinuation. 16,17 The incidences of grade 3 or higher grades of anemia and thrombocytopenia were 24.2% and 16.7% as well as 28.8% and 11.5% in the jaktinib 100 mg BID and 200 mg QD groups respectively.
Despite the effective potential of ruxolitinib to reduce the size of spleen, it has been previously found that 75% of patients experience anemia, whereas 25% of the patients would develop severe anemia. 18,19 In addition, it has been found that grade 3 or higher anemia and thrombocytopenia are found in the COMFORT I (45.2% and 42% respectively). However, although the side effects of fedratinib are mild and can be tolerated, the Wernicke encephalopathy needs special attention. 6,7 An ongoing clinical trial is investigating whether momelotinib can improve anemia (MOMENTUM trial, NCT03165734: Momelotinib vs. Danazol). Furthermore, it has been found that there is need for more attention to be geared on the momelotinib related peripheral neurotoxicity. A previous study has found that the incidence of peripheral neurotoxicity with momelotinib is between 10% and 40%. 20 However, although the neurotoxicity of jaktinib is currently low, the overall incidence of all grades is less than 10%, but there is still need for longer follow-up studies to verify its safety.
The current study had some limitations despite having reported a large data set on the efficacy and safety of jaktinib in JAK-naïve patients with MF in China. There is need for further studies to verify the conclusion on the potential of jaktinib to improve anemia. This is because our study included few patients with severe anemia and In conclusion, it was evident that jaktinib is safe and welltolerated in patients with MF in China. Furthermore, it was found that the 100 mg BID is significantly more effective in terms of splenic response as compared with that of the 200 mg QD. Therefore, the outstanding efficacy and mild side effects of jaktinib make it suitable for most patients with MF.

AUTHOR CONTRIBUTIONS
Jie Jin and Liqing Wu conceived and designed the study, all authors were involved in data collection and assembly. Yi Zhang, Hu Zhou, Zhongxing Jiang, Dengshu Wu, and Liqing Wu were involved in data analysis and interpretation. Yi Zhang and Jie Jin contributed to manuscript writing, and all the authors read as well as approved the final version of this manuscript for publication. All the authors had access to all study data and agreed to be accountable for the accuracy and integrity of the data. The corresponding author had the final responsibility to submit the manuscript for publication.