Single-Arm Study of the Efficacy and Safety of Oral Rigosertib in Patients With Myelofibrosis (MF) and Anemia

Inclusion Criteria: 1. \>/= 18 years of age; 2. Diagnosis of primary myelofibrosis (PMF) or post-polycythemia vera (post-PV) MF or post-essential thrombocythemia (post-ET) MF based on the World Health Organization (WHO) criteria or the International Working Group-Myeloproliferative Neoplasms Research and Treatment (IWG-MRT) criteria, which must be confirmed by BM aspirate and/or biopsy within 6 weeks prior to Screening. Measurement of JAK2 V617F allele burden in Bone Marrow (BM) samples, if not done within 6 months prior to Screening, must be provided with the Screening BM biopsy/aspirate report (patients are eligible regardless of JAK2 mutation status); 3. Anemia or RBC-transfusion dependence defined as follows: a) Anemia: defined for the purpose of this protocol as 1) a hemoglobin level \<10 g/L on every determination over 84 days before study-entry, without red blood cell (RBC)-transfusions, or 2) a hemoglobin level \<10 g/L on a patient that is receiving RBC-transfusions periodically but not meeting criteria for transfusion-dependent patient as defined below. The baseline hemoglobin value for these subjects is the lowest hemoglobin level during the antecedent 84 days; b) RBC-transfusion-dependence: RBC-transfusion-frequency of \>/=2 units packed red blood cells (PRBC)/28 days averaged over 84 days immediately pre-study-entry. There must not be any consecutive 42 days without an RBC-transfusion during this interval. 4. Eastern Cooperative Oncology Group (ECOG) performance status of 0, 1 or 2; 5. Willing to adhere to the prohibitions and restrictions specified in this protocol (Notation: the subject's willingness to adhere to prohibitions and restrictions must be clearly communicated in the on-study note); 6. The patient must signed an informed consent form (ICF) indicating that s/he understands the purpose of, and procedures required for, the study and is willing to participate. Exclusion Criteria: 1. Ongoing clinically significant anemia due to factors such as known iron, vitamin B12, or folate deficiencies, auto-immune or hereditary hemolysis, or gastrointestinal (GI) bleeding; 2. Serum ferritin \< 50 ng/mL; 3. Any active malignancy within the past year, except basal cell or squamous cell skin cancer or carcinoma in situ of the cervix or breast; patients with history of prior malignancies should be free of disease for at least 3 years to be eligible for this study. 4. Uncontrolled intercurrent illness, including, but not limited to symptomatic congestive heart failure, unstable angina pectoris, or cardiac arrhythmia; 5. Active infection not adequately responding to appropriate therapy; 6. Direct bilirubin \>/= 2.0 mg/dL not related to hemolysis or Gilbert's disease; 7. Alanine transaminase (ALT) or aspartate transaminase (AST)\>/= 2.5 x the upper limit of normal (ULN); 8. Serum creatinine \>/= 2.5 mg/dL; 9. Ascites requiring active medical management including paracentesis; 10. Hyponatremia (defined as serum sodium level \< 130 mEq/L); 11. Female patients who are pregnant or lactating; 12. Patients of childbearing potential (ie, women of childbearing potential and men with female partners of childbearing potential) who are unwilling to follow strict contraception requirements (including 2 reliable methods in combination: 1 non-hormonal, highly-reliable method \[diaphragm, condoms with spermicidal foam or jelly, or sterilization\] plus 1 additional reliable method \[birth control pills, intrauterine device, contraceptive injections, or contraceptive patches\]) before entry and throughout the study, up to and including the 30-day non-treatment follow-up period; 13. Female patients of childbearing potential who have a positive blood or urine pregnancy test at Screening; 14. Major surgery without full recovery or major surgery within 3 weeks of Screening; 15. Uncontrolled hypertension (defined as a sustained systolic pressure \>/= 160 mmHg and/or a diastolic pressure \>/= 110 mmHg); 16. New onset seizures (within 3 months prior to Screening) or poorly controlled seizures; 17. Any other concurrent investigational agent or chemotherapy, radiotherapy, or immunotherapy; 18. Chronic use (\> 2 weeks) of corticosteroids (prednisone \>/= 10 mg/24 hr equivalent) within 4 weeks of Screening; 19. Investigational therapy within 2 weeks of Screening; 20. Psychiatric illness or social situation that would limit the patient's ability to tolerate and/or comply with study requirements.