• ≥18 years of age inclusive, at the time of signing the informed consent.

• Capable of giving signed informed consent, which includes compliance with the requirements of this protocol.

• Eastern Cooperative Oncology Group (ECOG) performance status of 0 to 2

• Diagnosis of MF, Post-Polycythaemia Vera Myelofibrosis MF (PPV-MF), or post-essential thrombocythemia MF (PET-MF)

• Dynamic International Prognostic Scoring System (DIPSS) risk category of intermediate-1, intermediate-2, or high

• Treated with ruxolitinib for ≥ 3 months with a stable dose ≥ 10 mg for a minimum of 8 weeks prior to Day 1. Furthermore, patients must show an unsatisfactory spleen reduction, such as a reduction of less than 25%, and spleen must be palpable ≥ 10 cm below the left costal margin on physical examination

• Did not receive experimental drug therapy for MF or any other drug considered as an effective treatment for MF (e.g., danazol, hydroxyurea, interferon products) with the exception of ruxolitinib, within 3 months of starting study drug (except in conditions where other effective treatments for MF were completed 6 months prior to starting ruxolitinib)

• Independent of spleen size, active symptoms of MF at the screening visit, as demonstrated by the presence of a Total Symptom Score (TSS) of ≥ 10 using the Screening Symptom Form.

• Peripheral blast count \< 10%

⁃ Act to avoid pregnancy or fathering children based on the criteria below:

∙ Women of non-childbearing potential (i.e., surgically sterile with a hysterectomy and/or bilateral oophorectomy OR ≥ 12 months of amenorrhea and at least 50 years of age).

‣ Women of childbearing potential who had a negative serum pregnancy test at screening and who agree to take appropriate precautions to avoid pregnancy (with at least 99% certainty) from screening through safety follow-up, at least 1 month after the last dose of study treatment. Permitted methods that are at least 99% effective in preventing pregnancy should be communicated to the patient and their understanding confirmed.

‣ Men who agree to take appropriate precautions to avoid fathering from screening through safety follow-up, at least 1 month after the last dose of study treatment. Permitted methods that are at least 99% effective in preventing pregnancy (see Appendix 3) should be communicated to the patient and their understanding confirmed.