Controlled Study of Rigosertib Versus Physician's Choice of Treatment in MDS Patients After Failure of an HMA

This is a Phase III, open-label, randomized, controlled, international study. Approximately 360 patients \< 82 years of age with MDS classified as RAEB-1, RAEB-2, or RAEB-t who received AZA or DAC for ≤ 9 months and/or ≤ 9 cycles over 12 months and had their last dose of AZA or DAC within 6 months prior to screening will be stratified by: * Very high risk (VHR) vs non-VHR per IPSS-R, and * Geographic region (North America vs Europe vs Asia; because approved products and standard of care may vary by region), and randomly assigned in a 2:1 ratio to one of the following 2 treatment groups: * Rigosertib 1800 mg/24 hr administered as a 72 hr CIV infusion on Days 1, 2, and 3 of a 2 week cycle for the first 8 cycles, and on Days 1, 2, and 3 of a 4-week cycle thereafter (N = approximately 240 patients); * Physician's Choice of alternative treatment, which may include any approved or standard-of-care therapy that the patient has not shown to be hypersensitive to, based on frequently used treatment for MDS, as per institutional guidelines, after receipt of HMAs (N = approximately 120 patients). The drugs used in the Physician's Choice arm should be used according to the recommendations, if clinically appropriate, provided in the corresponding Summary of Product Characteristics (SmPC) and Prescribing Information of these drugs. Experimental therapies are not allowed on the PC arm. Patients will be treated until 2006 IWG progression criteria are met (ie, 50% increase of BM blasts or worsening of cytopenias) or until an unacceptable toxicity or intolerance. For all randomized patients who discontinue study treatment, subsequent therapies with their start and end dates, as well as survival time after treatment discontinuation, will be documented at least monthly until death. Patients in the PC group who progress will not be allowed to cross over to rigosertib. All patients in both treatment groups will be allowed, as medically justified, access to RBC and platelet transfusions and to growth factors (granulocyte colony-stimulating factor (G-CSF), erythropoietin, and thrombopoietin).