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 limit of the normal range (ULN) (≤ 5 x ULN if liver is involved by extramedullary hematopoiesis as judged by the investigator or if related to iron chelator therapy that was started within the prior 60 days) * Calculated creatinine clearance of ≥ 45 mL/min * Direct bilirubin ≤ 2.0 x ULN * Life expectancy \> 24 weeks * Negative serum pregnancy test for female subjects (unless surgically sterile or greater than two years post-menopausal) * Males and females of childbearing potential must agree to use protocol-specified method(s) of contraception * Females who are nursing must agree to discontinue nursing before the first dose of MMB * Able to understand and willing to sign informed consent form (ICF) Key Exclusion Criteria: * Prior splenectomy * Splenic irradiation within 3 months prior to Randomization * Use of investigational agent within 28 days prior to Randomization * Prior treatment with MMB * Hematopoietic growth factor (granulocyte growth factor, erythropoiesis stimulating agent, thrombopoietin mimetic) within 28 days prior to Randomization * Uncontrolled inter-current illness, per protocol * Known positive status for human immunodeficiency virus (HIV) * Chronic active or acute viral hepatitis A, B, or C infection, or hepatitis B or C carrier * Presence of peripheral neuropathy ≥ CTCAE Grade 2 * Unwilling or unable to undergo a MRI or CT Scan per study protocol requirements Note: Other protocol defined Inclusion/Exclusion criteria may apply.