This is an adaptive design, randomized controlled, Phase 3 clinical trial in patients with glioblastoma multiforme (GBM) or gliosarcoma (GS), previously treated with surgery (if appropriate), standard of care chemo-radiation with temozolomide, +/- adjuvant temozolomide, and bevacizumab and now has progressive disease during or after bevacizumab. A total of up to 180 eligible patients with recurrent/progressive GBM or GS will be randomized to receive either the investigational drug (VAL-083) or "Investigator's choice of salvage therapy" as a contemporaneous control, in a 2:1 fashion. Up to 120 eligible patients will be randomized to receive VAL-083 at 40 mg/m2 IV on days 1, 2, and 3 of a 21-day treatment-cycle, for up to 12, 21-day treatment cycles or until they fulfill one of the criteria for study discontinuation. Up to 60 patients will be randomized to receive "Investigator's choice of salvage therapy", limited to temozolomide, lomustine, or carboplatin, until they fulfill one of the criteria for study discontinuation. The dose level for Investigator's choice salvage therapy (temozolomide, lomustine, or carboplatin), will be in accordance with the product label or institutional guidelines. In both study arms, interval medical histories, targeted physical exams, neurologic evaluations, complete blood counts, and other laboratory and safety assessments will be performed approximately every 21-days while receiving treatment. Tumor assessments are to be performed approximately every 42 ± 7 days while remaining on study. The study is estimated to last approximately 20 months.