Efficacy of Momelotinib Versus Best Available Therapy in Anemic or Thrombocytopenic Subjects With Primary Myelofibrosis (MF), Post-polycythemia Vera MF, or Post-essential Thrombocythemia MF

Key Inclusion Criteria: * Palpable splenomegaly at least 5 cm below left costal margin * Confirmed diagnosis of PMF in accordance, or Post-PV/ET MF * Currently or previously treated with ruxolitinib for PMF or Post-PV/ET MF for at least 28 days, and characterized by * Requirement for RBC transfusion while on ruxolitinib treatment, OR * Dose adjustment of ruxolitinib to \< 20 mg twice daily at start of or during ruxolitinib treatment AND at least one of the following while on ruxolitinib treatment: * ≥ Common Terminology Criteria for Adverse Events (CTCAE) Grade 3 thrombocytopenia, OR * ≥ CTCAE Grade 3 anemia, OR * ≥ CTCAE Grade 3 hematoma (bleed) * High risk OR intermediate-2 risk as defined by Dynamic International Prognostic Scoring System (DIPSS), OR intermediate-1 risk as defined by DIPSS and associated with symptomatic splenomegaly, and/or hepatomegaly * If receiving myelofibrosis therapy, must be on a stable dose of the same regimen for at least 2 weeks prior to screen date and through the screening period * If not receiving myelofibrosis therapy, must remain off therapy for at least 2 weeks prior to screen date and through the screening period * Acceptable laboratory assessments obtained within 14 days prior to Randomization * Absolute neutrophil count (ANC) \> 0.75 x 10\^9/L in the absence of growth factor in the prior 7 days * Peripheral blood blast count \< 10% * Aspartate transaminase (AST) and alanine transaminase (ALT) ≤ 3 x the upper li…