A Study of Selinexor Monotherapy in Subjects with JAK Inhibitor-naïve Myelofibrosis and Moderate Thrombocytopenia

Eligibility Criteria

• * A diagnosis of MF or post-ET or post-PV MF according to the 2016 World Health Organization (WHO) classification of MPN, confirmed by the most recent local pathology report.
• * Measurable splenomegaly during the screening period as demonstrated by spleen volume of greater than equal to (\>=) 450 cubic square centimeter (cm\^3) by MRI or CT scan (results from MRI or CT imaging performed within 28 days prior to screening are acceptable).
• * Participants with DIPSS risk category of intermediate-1 with symptoms, or intermediate-2, or high-risk.
• * ECOG Performance Status less than or equal to (\<=) 2.
• * Platelet count of 50 to less than (\<) 100 x 10\^9/L without platelet transfusion within 7 days prior to the first dose of selinexor.
• * Absolute neutrophil count (ANC) \>=1.0 × 10\^9/L without need for growth factors within 7 days prior to the first dose of selinexor.
• * Adequate liver function as defined by the following: aspartate transaminase (AST) and alanine aminotransferase (ALT) \<= 2.5 × upper limit normal (ULN) and serum total bilirubin \<= 3×ULN.
• * Calculated creatinine clearance (CrCl) greater than (\>) 15 milliliter per minute (mL/min) based on the Cockcroft and Gault formula.
• * Active symptoms of MF as determined by presence of at least 2 symptoms with a score \>= 3 or total score of \>= 10 at screening using the MFSAF V4.0.
• * Participants must provide bone marrow biopsy samples (samples obtained up to 3 months prior to C1D1 are permitted) at screening and during the study.
• * Participants currently not a candidate for stem cell transplantation.
• * Participants must be willing to complete the MFSAF V4.0 daily during the study for evaluating the symptom response (i.e., TSS50).
• * More than 10% blasts in peripheral blood or bone marrow (accelerated or blast phase).
• * Previous treatment with JAK inhibitors for MF.
• * Previous treatment with selinexor or other XPO1 inhibitors.
• * Female participants who are pregnant or lactating.
• * Prior splenectomy, or splenic radiation within 6 months prior to C1D1.
• * History of myocardial infarction, unstable angina, percutaneous transluminal coronary angioplasty (PTCA) or coronary artery bypass graft (CABG), cerebrovascular accident (stroke or transient ischemic attack \[TIA\]), ventricular arrhythmias, congestive heart failure New York Heart Association (NYHA) class \> 2 within 6 months of C1D1.
• * Participants unable to tolerate two forms of antiemetics prior to each dose for the first two cycles.