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.