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

Study Drug Administration: If participant is found to be eligible to take part in this study, participant will take rigosertib capsules by mouth 2 times each day (1 morning dose, 1 afternoon dose) for 48 weeks. Participant will take participant's morning dose on an empty stomach. Participant's afternoon dose should be taken at about 3:00 pm (±1 hour), at least 2 hours after lunch. Participant should drink at least a ½ gallon of water each day. Rigosertib should be refrigerated. Participant will also be given a study drug diary. Participant should write down each dose of study drug that participant takes, when participant takes it, and if participant misses or vomit any doses. If participant misses or vomits a dose of rigosertib, participant should not make up the dose. Participant should take the next scheduled dose as planned. Every 4 weeks for 12 weeks, then every 8 weeks thereafter, participant should bring any leftover rigosertib capsules and participant's study drug diary with participant to the clinic. Study Visits: On Cycle 1 Day 1: * Participant will have a physical exam. * Blood (about 2-3 teaspoons) and urine will be collected for routine tests. * Blood (about 2 teaspoons) will be drawn for pharmacokinetic (PK) testing about 1 hour after participant's dose of study drug. PK testing measures the amount of study drug in the body at different time points. * Participant will complete a questionnaire about participant's quality of life. It should take about 10-15 minutes. * If the doctor thinks it is needed, participant will have a computed tomography (CT) or magnetic resonance imaging (MRI) scan to check the status of the disease. Each week, blood (about 2-3 teaspoons) will be drawn for routine tests. If the study doctor approves it, participant may have these weekly blood draws at a local lab closer to participant's home. The results from these routine tests will be sent to the study doctor. On Day 1 of Week 4, blood (about 2 teaspoons each time) will be drawn for PK testing before and 1 hour after participant's dose of study drug. At Weeks 24 and 48 (± 1 week): * Participant will have a bone marrow aspirate/biopsy performed to check the status of the disease. This sample may also be used for cytogenetic testing. * If participant had a CT/MRI scan performed during Week 1, participant will have another CT/MRI scan to check the status of the disease. * If participant can become pregnant, urine will be collected for a pregnancy test. Every 4 weeks until Week 12 and then every 8 weeks after that: * Participant will have a physical exam. * Blood (about 2-3 teaspoons) and urine will be collected for routine tests. * Participant will complete a questionnaire about participant's quality of life. Length of Study: Participant may continue taking the study drug for up to 48 weeks. Participant will no longer be able to take the study drugs if the disease gets worse, if intolerable side effects occur, or if participant is unable to follow study directions. If the doctor thinks it is in participant's best interest, participant may be able to continue taking the study drug after 48 weeks. The study doctor will discuss this with participant. Participation on the study will be over after the end-of-study visit. End-of-Study Visit: About 4 weeks after participant's last dose of study drug, participant will have an end-of-study visit. At this visit: * Participant will have a physical exam. * Blood (about 2-3 teaspoons) and urine will be collected for routine tests. This routine blood and urine collection will also include a pregnancy test if participant can become pregnant.