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

Key Inclusion Criteria:

• A diagnosis of MF or post-ET or post-PV MF according to the 2016 World HealthOrganization (WHO) classification of MPN, confirmed by the most recent localpathology report

• Measurable splenomegaly during the screening period as demonstrated by spleen volumeof greater than or equal to (>=) 450 cubic square centimeter (cm^3) by MRI or CTscan (results from MRI or CT imaging performed within 28 days prior to C1D1 areacceptable)

• 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 greater than or equal to (>=) 50 x 10^9/L without platelettransfusion within 7 days prior to the first dose of selinexor

• Absolute neutrophil count (ANC) >=1.0 × 10^9/L without need for growth factorswithin 7 days prior to the first dose of selinexor

• Adequate liver function as defined by the following: aspartate transaminase (AST)and alanine transaminase (ALT) <= 2.5 × upper limit normal (ULN) and serum totalbilirubin <= 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 anaverage score >= 5 or total score of >= 12 at screening (at least 5 of 7 consecutivedays immediately preceding C1D1) using the MFSAF V4.0

• Must provide bone marrow biopsy samples (samples obtained up to 3 months prior toC1D1 are permitted) at screening and during the study

• Currently not eligible for stem cell transplantation

• Must be willing to complete the MFSAF V4.0 daily during the study for evaluating thesymptom response (i.e., TSS50)

Key Exclusion Criteria:

• 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

• Females who are pregnant or lactating

• Prior splenectomy, splenic radiation, or a splenic embolization within 6 monthsprior to C1D1

• History of myocardial infarction, unstable angina, percutaneous transluminalcoronary angioplasty (PTCA), coronary artery bypass graft (CABG), cerebrovascularaccident (transient ischemic attack [TIA]), ventricular arrhythmias, congestiveheart failure class > 2 per New York Heart Association (NYHA) within 6 months ofC1D1

• Unable to tolerate two forms of antiemetics prior to each dose for the first twocycles