Jaktinib in patients (pts) with myelofibrosis (MF) who were intolerant to ruxolitinib (RUX): An open-label, single-arm phase 2b study.
Authors: Zhang Yi , Zhou Hu , Xiao Zhi-Jian , Duan Minghui , Gao Sujun , He Guangsheng , Jing Hongmei , Li Junmin , Ma Liangming , Zhu Huanling , Chang Chunkang , Du Xin , Hong Mei , Li Xin , Liu Qingchi , Wang Wei , Xu Na , Yang Haiping , Zhao Qingwei , Jin Jie 
Published: June 2023

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Background: As the first approved Janus kinase inhibitor (JAKi) for pts with symptomatic MF, RUX showed superiority in spleen volume reduction with increased anemia and thrombocytopenia. However, there are few treatment options after RUX intolerance. Jaktinib, an oral novel JAK and ACVR1 inhibitor showed promising efficacy in Jaki-naïve pts with intermediate or high risk MF with mild side effects. Here we present the results of MF pts who were intolerant to RUX and received jaktinib 100 mg bid from a phase 2b study. Methods: Pts who had primary or post-ET/PV MF and were previously treated with RUX for ≥28 days who either required RBC transfusions or required more frequent transfusions on RUX, or had grade 3/4 anemia, thrombocytopenia or hematoma when receiving RUX, and with palpable spleen ≥5 cm were included. The primary endpoint was the proportion of pts with spleen volume reduction of ≥ 35% from baseline (SVR35) at week (wk) 24, assessed centrally on the basis of MRI/CT images. Secondary endpoints included the best spleen response rate (defined as achieving SVR35 at any time), the proportion of pts with ≥ 50% reduction in MPN Symptom Assessment Form Total Symptom Score (MPN-SAF TSS), anemia response, safety, etc. Results: A total of 45 patients received jaktinib 100 mg bid. The full analysis set (FAS) for efficacy included 44 pts who had completed the 24-week treatments and evaluations or terminated prior to wk 24, and excluded one pt because of incorrect diagnosis of MF. The median baseline TSS was 9.5, hemoglobin (Hb) was 79.5 g/L, and platelets was 132.5×109/L. While on RUX, 21 pts (48%) required RBC transfusion and all pts experienced those grade 3 hematological toxicities. In FAS, the SVR35 rate at wk 24 was 43% (19/44). Over the study phase, the best spleen response rate was 55% (24/44). The median time to achieve the first SVR35 was 12 weeks. Response was maintained in 80% pts for at least 24 weeks and median duration was not reached. At wk 24, 62% of the evaluable pts achieved a ≥ 50% improvement in TSS from baseline. Of 31 pts with baseline Hb ≤ 100 g/L, 13 (42%) had a ≥20 g/L Hb increase. Of 19 pts who required RBC transfusion at baseline, 11 (58%) had a 50% decrease in RBC infusion frequency. Of 9 transfusion-dependent pts at baseline, one became transfusion-independent. In the safety set (n = 45), the median jaktinib exposure time was 280 days. The most common grade ≥3 TEAEs were anemia (31%), thrombocytopenia (22%), pneumonia (18%), neutropenia (16%) and leukopenia (16%). Treatment discontinuation due to TEAEs occurred in five pts (11%). A dose reduction or temporary interruption due to TEAEs were reported in 12 pts (27%). Conclusions: Jaktinib demonstrated promising activities in MF pts who was intolerant to RUX by substantially reducing spleen volume, ameliorating MF-related symptom burdens, and elevating Hb levels. Jaktinib could be a viable treatment for MF pts with anemia in this setting. Clinical trial information: NCT04217993 .