Study of Pelabresib add-on to Ruxolitinib in Japanese Adult Patients With Myelofibrosis

Key Inclusion Criteria: * Participants have diagnosis of primary myelofibrosis (PMF), post-polycythemia vera MF (Post-PV MF) or post-essential thrombocythemia MF (Post-ET MF) according to the International Consensus Classification (ICC) for Myeloid Neoplasms and Acute Leukemias 2022. * DIPSS risk category intermediate-1, intermediate-2 or high-risk at screening. * Participants currently treated with ruxolitinib monotherapy AND who are likely to benefit from the addition of pelabresib to ruxolitinib in the opinion of the investigator. * Receiving ruxolitinib at a stable dose (5 to 25 mg BID) for at least 8 weeks prior to the first dose of pelabresib. * Palpable spleen (spleen length below left costal margin \[LCM\] must be recorded) or documented splenomegaly by MRI or CT (image report must be recorded) at screening. * Platelet count ≥ 100 × 10\^9/L in the absence of growth factor support (including thrombopoietin mimetics/agonists) or platelet transfusions 4 weeks prior to the first dose of pelabresib. * Blasts \< 5% in peripheral blood. Assessment of blasts in peripheral blood is mandatory at screening. Key Exclusion Criteria: * Prior splenectomy at any time or splenic irradiation in the previous 6 months * Prior hematopoietic cell transplant or participants anticipated to receive a hematopoietic cell transplant within 24 weeks from the first dose of pelabresib. * Blasts ≥ 5% in bone marrow if results available at screening or history of accelerated phase or leukemic tran…